Southwestern Ifaq

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 560-G
Geology of the 

Arabian Peninsula

Southwestern Iraq

By K. M. AL NAQIB

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 560-G

A review of the geology of southwestern Iraq 

as shown on USGS Miscellaneous Geologic 

Investigations Map 1-2 ^O A, "Geologic Map 

of the Arabian Peninsula"

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1967
DEPARTMENT OF THE INTERIOR 

DONALD PAUL HODEL, Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

First printing 1967 

Second printing 1985

For sale by the Distribution Branch, U.S. Geological Survey 

604 South Pickett Street, Alexandria, VA 22304
FOREWORD

This volume, "The Geology of the Arabian Peninsula," is a logical consequence of the 

geographic and geologic mapping project of the Arabian Peninsula, a cooperative venture 

between the Kingdom of Saudi Arabia and the Government of the United States. The ArabianAmerican Oil Co. and the U.S. Geological Survey did the fieldwork within the Kingdom of 

Saudi Arabia, and, with the approval of the go'vernnients of neighboring countries, a number of 

other oil companies contributed additional mapping to complete the coverage of the whole of 

the Arabian Peninsula. So far as we are aware, this is a unique experiment in geological 

cooperation among several governments, petroleum companies, and individuals.

The plan for a cooperative mapping project was originally conceived in July 1953 by the 

late William E. Wrather, then Director of the U.S. Geological Survey, the late James Terry 

Duce, then Vice President of Aramco, and the late E. L. deGolyer. George Wadsworth, then 

U.S. Ambassador to Saudi Arabia, and Sheikh Abdullah Sulaiman, then Minister of Finance 

of the Government of Saudi Arabia, lent their support to the plan. In November of the following year, 1954, Director Wrather approved the U.S. Geological Survey's participation and 

designated G. F. Brown responsible for the western Arabian shield region in which he had 

previously worked under U.S. foreign-aid programs. In January 1955 F. A. Davies, Chairman, 

Board of Directors, Arabian-American Oil Co., approved Aramco's participation and appointed 

the late R. A. Bramkamp, chief geologist, responsible for compilation of the area within the 

Kingdom where the sediments crop out. This responsibility fell to L. F. Ramirez following 

the death of R. A. Bramkamp in September 1958.

R. A. Bramkamp and G. F. Brown met in. New York in February 1955 and planned the 

program, including scales of maps, areas of responsibility, types of terrain representation, 

and bilingual names. Thus there was established a cooperative agreement between the Kingdom of Saudi Arabia, the U.S. Department of State, and the Arabian-American Oil Co. to 

make available the basic areal geology as mapped by Aramco and the U.S. Geological Survey.

The agreement specified publication of a series of 21 maps on a scale of 1:500,000, each 

map covering an area 3° of longitude and 4° of latitude. Separate geologic and geographic 

versions were to be printed for each of the quadrangles; both versions were to be bilingual in 

Arabic and English. A peninsular geologic map on a scale of 1:2,000,000 was to conclude the 

project.

High-altitude photography, on a scale of 1:60,000, of the Kingdom of Saudi Arabia was 

initiated during 1949 by the Aero Service Corp. and completed in 1959. Both third-order 

vertical and horizontal control and shoran were utilized in compiling the photography. This 

controlled photography resulted in highly accurate geographic maps at the publication scale 

which then served as a base for the geologic overlay. The topography of the sedimentary 

areas was depicted by hachuring and that of the shield region by shaded relief utilizing the 

airbrush technique.

The first geographic quadrangle was published in July 1956 and the last in September 

1962. While preparation of the geographic sheets was in progress, a need arose for early 

publication of a 1:2,000,000-scale peninsular geographic map. Consequently, a preliminary 

edition was compiled and published in both English and Arabic in 1958. The second edition, 

containing additional photography and considerable new topographic and cultural data, was 

published in 1963. The first of the geologic map series was published in July 1956 and the 

final sheet in early 1964. The cooperative map project was completed in October 1963 with 

the publication of the 1:2,000,000-scale "Geologic Map of the Arabian Peninsula" (Miscellaneous Geologic Investigations Map 1-270 A).

m
IV FOREWORD

As work on the quadrangles progressed, geologists, companies, and governments working 

in areas adjacent to the Kingdom of Saudi Arabia were consulted by Aramco and invited 

to participate in the mapping project. The number of cooperating participants was expanded 

to 11, which included the operating oil companies in the peninsula and which are identified 

elsewhere in this text; the Overseas Geological Surveys, London; the Government of Jordan; 

F. Geukens, who had worked in Yemen; and Z. R. Beydoun, who had studied the Eastern 

Aden Protectorate. With the close cooperation of the authors, the new data were added to 

data already plotted on the base map of the Arabian Peninsula.

As the geological coverage of the peninsular map grew, the need for a text to accompany 

the map became apparent to both the U.S. Geological Survey and the Aramco geologists. 

Exploratory conversations were begun by Aramco with companies working in the other countries 

of the Arabian Peninsula for their participation in the preparation of a monograph on the 

geology of the Arabian Peninsula. Each author prepared a description of the geology of the 

area for which he was responsible, as shown in the sources of geologic compilation diagram 

on the peninsular map. The U.S. Geological Survey undertook the publishing of the volume 

as a professional paper, and the Government of Saudi Arabia was to finance its printing. 

It was early agreed that there would be no effort to confine the contributions to a standard 

format and that no attempt would be made to work out an overall correlation chart other 

than shown on the "Geologic Map of the Arabian Peninsula." Thus, the individual style 

of authors of several nationalities is preserved.

Cooperation and relations have been of the highest order in all phases of the work. The 

project would not have been possible without the full support of the U.S. Department of State, 

the Kingdom of Saudi Arabia, and all contributors. In fact, the funds which made publication of this volume possible were contributed by the Saudi Arabian Government.

The data provided by the maps and in the professional paper provide information for 

an orderly scientific and economic development of a subcontinent.

x"

O. A. S&GER,

Arabian-American Oil Co. (Retired).

W. D. J^rfsTON, JR., V/ 

Chief, Foreign Geology Branch,

U.S. Geological Survey.
CONTENTS

Foreword_ _______________________________________

Abstract.__________________________________________

Introduction.______________________________________

Acknowledgments, ______________________________

General geomorphology____________________________

Mesopotamian alluvial plain_._--______-_---_-___

Sand-dune belt _________________________________

Ad Dibdibba gravel and sand region. _____________

Al Hajara limestone region_______________________

Al Widyan country. ____________________________

Stratigraphy.______________________________________

General features._______________________________

Philosophy of nomenclature._________________

Time units represented______________ ________

General distribution of systems and dominant 

rock types _______________________________

Unconformities. _ ___________________________

Regional sedimentary pattern and history____

Mesozoic. _____________________________________

Cretaceous._ _______________________________

Ratawi Formation._____________________

Zubair Formation.______________________

Shu'aiba Formation..___________________

Nahr Umr Formation. __________________

Mauddud Formation,___________________

Rutbah Sandstone Formation.___________

Ahmadi Formation,_____________________

Rumaila Formation.____________________

Kifl Formation.________________________

M'sad Formation.______________________

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Stratigraphy Continued 

Mesozoic Continued

Cretaceous Continued

Mishrif Formation.___.__-_--_-____

Khasib Formation. .____--____-_-__

Tanuma Formation_____----__-__-_

Sa'di Formation________----___-.__

Hartha Formation. ________________

Safawi Formation________________

Shiranish Formation._______.--__-.

Tayarat Formation.. ______________

Cenozoic---------------_-------_-----_--__

Tertiary ______________________________

Paleocene and Eocene._____________

Umm er Radhuma Formation. __ 

Rus Formation._______---__-__

Dammam Formation__________

Miocene and Pliocene._____________

Zahra Formation.. ____________

Euphrates Limestone Formation. 

Ghar Formation.____-_-_-_____

Lower Fars Formation _________

Dibdibba Formation___________

Quaternary.__________________________

Hammar Formation_.__--_________

Structure._________---__--_____--_--_-_-___--_

Structural province of southwestern Iraq_____

Structural history__________________________

Graben system.___________________________

Summary ____________________________________

Bibliography. _________________________________

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ILLUSTRATIONS

{Plates are in pocket]

PLATE 1. Generalized geologic map of central and southern Iraq.

2. Schematized regional correlation of rock units, southern Iraq.

3. Tentative subsurface stratigraphic correlation in southwestern Iraq-Kuwait.

4. Diagrammatic regional geologic section across southern Iraq.

Page

FIGURE 1. Tectonic map of Iraq_____-__________-_-_---__-_--_--------_-------- G50

GEOLOGY OF THE ARABIAN PENINSULA

SOUTHWESTERN IRAQ

By K. M. Ai,

ABSTRACT

Southwestern Iraq covers all Iraqi territory south of lat 

32° N. and west of the Euphrates River. Slightly to the north, 

the area's oldest rocks (Lower Triassic?) crop out in the 

Ga'ara depression where they are succeeded by Middle and 

Upper Triassic, Rhaetic, and Lower and Middle Jurassic 

rocks. The Middle Jurassic sequence is unconformably overlain by Cenomanian sandstones and limestones; the gap in 

sedimentation diminishes away from the Ga'ara area. The 

Cenomanian deposits are succeeded by Upper Cretaceous limestone. The Cretaceous sedimentation in southwestern Iraq 

was due to periodic epeirogenetic withdrawals, which are reflected in a series of sedimentary cycles characterized by 

strong diachronism of the sediments. A sequence of Paleocene 

and Eocene deposits covers most of the area in long scarpy 

strips. On the east side the Miocene limestones, evaporites, 

and elastics overstep the Eocene beds. The Miocene limestone 

overstepping, however, seems to be more extensive and covers 

large areas in the west Oligocene deposits are not recognized 

in the area, indicating a major pre-Miocene unconformity. The 

Pliocene and Pleistocene sequence covers the southeastern 

part of the area, the transition from Miocene to Pliocene deposition seems to be gradational. Descriptions of 27 formations have been made in some detail.

Structurally, the area constitutes a small part of the Interior 

Homocline of the Arabian Peninsula. It is simple, flat-lying 

country with very gentle dips to the east and northeast. There 

are a few depressions in the central parts which may be indicative of deeply buried graben systems accentuated by the 

solution-collapse effects. There is evidence of relatively 

strong movements during the Cenomanian and early Miocene 

times, which influenced the realms of sedimentation more than 

the epeirogenetic movements during Cretaceous and Tertiary 

times. In southern Iraq these movements are still active 

and influence the retreat of the Persian Gulf. Minor linear 

structural elements, oriented north-northwest and north, occur 

in the area of Tertiary outcrops. These were possibly initiated on deeply buried old faults which were activated by recurrent disturbances.

Oil was found in commercial quantities in the sand reservoirs of the Zubair Formation, in the Zubair and Rumaila 

fields, after considerable effort, energy, and investment by 

the Iraq Petroleum Company, Ltd., and its associate the 

Basrah Petroleum Company, Ltd.

1 Of the Iraq Petroleum Company, Ltd.

INTRODUCTION

The area of present review covers a territory of more 

than 82,000 square kilometers lying south of lat 32° N., 

and is limited by the Euphrates and Shatt al Arab 

Eivers on the east and by the Iraq-Saudi Arabia and 

Iraq-Kuwait borders on the west and south.

During the early 19th century many scientists and 

scholars who were interested in the archaeology of 

southern Iraq, a classic area, attempted to explain 

their findings geologically. Beke (1835) described 

the geological evidence of the advance of the land at 

the head of the Persian Gulf and commented on the 

alluvium of Babylonia and Chaldaea. Ainsworth 

(1838) gave the geological results of the Euphrates 

Expedition which was conducted by Colonel Chesney.

With increased demand for oil, interest in the 

petroliferous area of Mesopotamia and in its geology 

grew. In 1905, H. Bucking, C. Porro, and Kissling 

reported on the seepage areas of the Euphrates and 

Tigris Eivers. Two years later, P. Grosskoff reported 

on the petroleum deposits of the area for the Turkish 

Civil List. Geological interest increased and authorities such as Pascoe (1922) made more serious and 

prolonged surveys and evaluations of the general area. 

G. W. Halse in 1918 examined the bituminous districts 

west of Euphrates River.

These surveys in southwestern Iraq, however, were 

only reconnaissance in nature. Greater attention to the 

geology of western Iraq was paid by A. H. Noble and 

L. D. Evans, who spent parts of 1919 and 1920 conducting a general geological reconnaissance of Iraq, the primary aims of which were the establishment of a stratigraphic section for the area and the investigation of 

the most important structures from the point of view 

of petroleum.

With the 1925 Oil Concession agreement, the Turkish Petroleum Company, Ltd. (later renamed the Iraq 

Petroleum Company, Ltd.), commenced an intensive 

geological reconnaissance in Iraq. In the course of

Gl
G2 GEOLOGY OF THE ARABIAN PENINSULA

this, H. de Boeckh, who had made some investigation 

in western Iraq in 1924, revisited the area in the spring 

of 1925 with S. L. James, G. M. Lees, and F .D. S. 

Kichardson. During 1925-29 A. C. Trowbridge, E. 

W. Shaw, A. H. Noble, Bourchier, and H. De Boeckh 

undertook general geological reconnaissance and inspection of the possible oil prospects in western Iraq. 

Exploratory activities were cut back in 1929 after 

a decision was made to devote all available resources 

to the development of the Kirkuk field, where oil had 

been struck in October 1927.

Interest in the western desert was not confined to 

petroleum exploration alone. Geologists were intermittently active in investigating potential groundwater supply. An excellent work reporting on the 

progress of hydrological inquiries in the area was 

made by W. A. Macfadyen and S. D. Bekhor during 

April and May 1934. Macfadyen (1938) described 

the geology of the area in some detail, discussing both 

stratigraphy and structure in relation to water 

resources.

Interest in the petroleum possibilities of southern 

Iraq led to the incorporation, on July 22, 1938, of the 

Bashrah Petroleum Company, Ltd., an associate of 

Iraq Petroleum Company, Ltd. A week later a convention was made with the government of Iraq, whereby the Company was granted a 75-year oil concession 

covering southern Iraq, which extended from the 33d 

parallel southward to the borders of Iran, Kuwait, and 

Saudi Arabia. The area of the present review, which 

comprises most of the southern desert west of the 

Euphrates River, together with the District 'of Brasa, 

occupies the western and southern part of this concession. The convention became operative on November 

30, 1938, on which date its ratifying law was published 

in the Iraqi Government Gazette. Article 4 of the 

convention required the Company to conduct a detailed 

geological survey before July 30, 1939.

Company geological field parties and a torsionbalance survey party under contract from the Torsion 

Exploration Co., Texas, began operations in the area 

in January and March of 1939, respectively. This 

early geological investigation varied in scope and 

nature; it embraced reconnaissance geologic mapping 

as well as detailed mapping of selected areas and stratigraphic as well as structural studies. It included 

systematic aerial reconnaissance as well as topographic 

surveying. The geological field parties operated continuously until May 1941, when exploration was temporarily suspended because of civil disturbances within 

Iraq and because of the general stringencies imposed 

by world war. By this date a considerable area of the 

western desert of Iraq had been examined by geologists. The early investigators in this field included 

T. F. Williamson, D. C. Kogers, W. T. Foran, H. L. 

Thompson, H. Hotchkiss, H. Huber, H. Badoux, and 

F. Kubba under the general direction of the Chief 

Geologist, N. E. Baker. In September 1940 the torsion-balance party was replaced by a gravimeter party 

from the Mott Smith Corp., Texas, which left Iraq 

in April 1941 at the time of the revolt.

During the war years attention was chiefly paid to 

solving difficult, detailed problems of stratigraphy, 

partly through laboratory studies of samples collected 

prior to 1941. Fieldwork was on a restricted scale 

because of the shortage of staff.

During the years following World War II, geological exploration activities continued in the southern 

desert. Geological field parties engaged in detailed 

structural mapping, stratigraphic investigations, and 

reconnaissance surveying. These were supported by 

photogeological and topographical surveys and by a 

limited program of structural drilling of shallow holes 

with an M. I. Failing rig. The structure drilling was 

restricted to selected areas such as Luqait, Shagra, 

Abu Ghar, Takhadid, Dimana, and Galaib and had as 

its objectives the investigation of gravity anomalies, 

the confirmation of localized structures, and the clarification of facies changes. Field investigations were 

discontinued when the Iraqi Government ordered suspension of the Company's exploratory activities in 

April 1961 as a result of the breakdown of oil negotiations. During the period 1946-61 a great many Company geologists, under the successive guidance of N. E. 

Baker, F. E. Wellings, and C. E. Thiebaud, contributed to the geological knowledge of the area 

through field investigations, subsurface studies, laboratory and stratigraphic correlations, and paleontological studies. These contributors, whose work is 

contained for the most part in unpublished reports of 

the Company, include C. Andre, A. S. Abbo, R. V. 

Browne, R. C. van Bellen, Z. R. Beydoun, M. Chatton, 

J. M. Cox, E. J. Daniel, H. V. Dunnington, E. K. 

Elliott, G. F. Elliott, H. A. Field, T. Fothergill, D. 

Glynn Jones, A. S. Halliwell, E. Hart, F. R. S. Henson, J. M. Hudson, R. G. S. Hudson, J. P. Jaccard, 

A. V. James, K. D. Jones, F. A. Al Kasim, A. P. 

Leggat, J. McGinty, J. M. Macleod, S. Marchant, S. 

C. McKinley, G. Medaisko, D. M. Morton, K. M. Al 

Naqib, S. Nasr, R. M. V. Rabanit, R. M. Ramsden, 

M. Roy, I. Rolleigh, A. Th. C. Rutgers, V. M. Safar, 

H. M. Saleh, A. S. Sayyab, A. H. Smout, T. P. Storey, 

W. Sugden, K. F. M. Thompson, F. J. Venn, P. 

Walmsley, R. Wetzel, and E. Williams-Mitchell. NonCompany geologists who published reports on the 

geology of the region are included in the bibliography.
SOUTHWESTERN IRAQ G3

Geophysical work by a combined gravity and magnetic survey party recommenced after the war years 

in November 1945, the first area to be investigated 

being Zubair. In the following year, geophysical activities were increased to a scale greater than at any 

time since 1940. Three geophysical survey parties 

(one seismic and two gravity and magnetic) of the 

Robert Ray Co. were engaged on extensive exploratory work in the western desert during 1948 and 1949. 

Seismic parties of the Independent Exploration Co., 

Ltd., London, using reflection and refraction techniques, were also deployed in this area. In 1951 two 

double gravity-magnetic geophysical parties of the 

Robert Ray Co., and one party of Seismograph Services, Ltd,, operated intermittently in the area and were 

employed in different parts of Iraq according to season. The geophysical surveys were supplemented by 

topographic as well as aerial surveys. The geophysical 

investigation continued at about this intensity through 

ensuing years. In June 1956 the Company, in addition 

to the contractors' crews, organized its own seismic 

crew which started operation near Karbala.

After the Suez crisis and sabotage of the Syrian 

pipelines in 1956, exploration activity was reduced during 1957. However, during the following years, until 

the suspension of exploration by order of the Iraqi 

Government in April 1961, full-scale exploratory activities were carried on. By 1964 the gravity survey 

had covered the entire area of the present review, while 

seismic-survey coverage was largely confined to particular areas for example, Basra, Qasr Shagrah district, and Shawiyat.

The results of the early geophysical surveys suggested the existence of some potential oil-bearing structures. Accordingly, test wells were drilled on the 

most promising of them at Zubair No. 1 (Feb. 14,1948, 

to Mar. 13, 1948), Nahr Umr No. 1 (Mar. 23, 1948, to 

Feb. 13, 1949), and Ratawi No. 1 (Nov. 30, 1948, to 

Mar. 18, 1950). The first two wells penetrated oil 

sands, Zubair No. 1 between 10,200 and 11,100 feet and 

Nahr Umr No. 1 below 8,000 feet. In September 1948 

the Iraq Petroleum Company informed the Government that oil in commercial quantities had been discovered. More wells were drilled at both places, but 

the second well at Nahr Umr encountered water; to 

produce oil for export at the earliest possible time, all 

the drilling efforts were concentrated at Zubair.

Drilling activities were greatly intensified during 

1950, and the transition from exploratory operations 

to the development of the Zubair field was accomplished. By the end of the year the laying and welding 

of the entire Zubair-Fao 12-16-inch pipeline was completed, and work on terminal and field installations

232-387 67 2

and facilities was progressing well. At the end of 1951 

the first jetty (No. 3) at Fao was completed, together 

with berthing and mooring dolphins. The M. V. Niso 

was the first ship to berth at the jetty. She sailed with 

a cargo of 11,043 tons of oil on December 21, 1951. 

The official inauguration of production from the Zubair field took place on January 10,1952. In February 

of the same year, a 50-50 profit-sharing agreement, 

negotiated in 1951, ws approved by the Iraqi Chamber 

of Deputies and ratified by the Senate, retroactive to 

January 1951.

Prior to the commencement of production, 12 wells 

had been completed in the Zubair field. The increasing 

exploratory activities of the Company were rewarded 

by the discovery in 1953 of the Rumaila field, 20 miles 

west of Zubair. Drilling on a geophysical prospect 

began on February 21, 1953, and oil was found in the 

first well, under conditions similar to those prevailing 

at Zubair.

With this discovery, attention in the following years 

was directed to the development of the Rumaila field. 

For the purposes of immediate production, however, 

reliance was mainly placed on Zubair, where 38 wells 

had already been completed. The Company switched 

the emphasis of its activities from Zubair to the development of the Rumaila field in 1955, with the object 

of increasing the production rate to 8 million tons per 

year.

In 1953 a 24-inch pipeline was laid between Zubair 

and Fao. As the development progressed as planned, 

some effort reverted to exploratory drilling. The exploratory well Rachi No. 1 (June 11, 1956, to Jan. 11, 

1957) was drilled to a depth of 12,959 feet, but the results were disappointing. Exploration activities were 

intensified, and the western desert received further attention which continued until the suspension of exploration in April 1961.

During this period the following wells were drilled 

west of the Euphrates River: Samawa No. 1 (Aug. 31, 

1958 to Nov. 9, 1959); drilled to a depth of 12,602 

feet, was plugged back owing to disappointing results; 

Tuba No. 1 (Mar. 3, to Dec. 27, 1959), on reaching 

depth of 11,940 feet, was completed as an observation 

well; and Nahr Umr No. 3 (Mar. 2 to Oct. 11, 1958), 

with a total depth of 10,109 feet, found oil in the Nahr 

Umr sands. North-northwest of An Najaf, two wells 

Kifl No. 1 (Sept. 21, 1959, to July 8, 1960) and Kifl 

No. 2 (Jan. 23, to Feb. 28, 1961) were drilled to total 

depths of 10,684 and 6,293 feet, respectively, some oil 

having been found in the Zubair Formation in the first 

well.

On the deep wells Ideal 100 rigs were used. However, an H 525 Rambler rig was used to drill shallow
G4 GEOLOGY OF THE ARABIAN PENINSULA

stratigraphic and structural test wells such as Shawiya 

No. 1, Ghalaisan No. 1. Safawi No. 1, and Ubaid No. 

1, which did not exceed 6,600 feet in depth. Shawiya 

No. 1 (Feb. 15, to Apr. 20,1960), which was completed 

at a depth of 5,853 feet, found the Zubair Formation 

waterlogged. Except for the presence of some gas 

in the Nahr Umr Formation, the findings were similar 

in Ubaid No. 1 (Nov. 2, to Dec. 12, 1960), which was 

completed at a depth of 6,050 feet. The results of the 

wells Ghalaisan No. 1 (May 15, to July 25, 1960; 6,533 

ft) and Safawi No. 1 (Aug. 17, to Oct. 9, 1960; 6,000 

ft) were also disappointing, but oil in good quantity 

was found in the Zubair Formation in Luhais No. 1 

(Jan. 17, to Apr. 15, 1961). This well was plugged at 

a depth of 9,646 feet after the general suspension of 

exploration in April 1961.

By 1964 there were 39 wells in each of the two oil 

fields of Kumaila and Zubair. The productive length 

of the Rumaila fields has proved to be at least 60 

kilometers.

Oil production from these fields is maintained by an 

elaborate modern system of pipelines, degassing stations, tank farms, and oil-terminal facilities. A 16-inch 

pipeline between Rumaila and Zubair was commissioned in October 1957. To handle an envisaged increase in production rate, the deep-water terminal of 

Khor al Amaya was constructed in the Arabian Gulf 

during 1959-62, and a 30- to 32-inch pipeline between 

Zubair and Foa was laid parallel to the 24-inch pipeline. 

The old 12- 16-inch pipeline was realined and converted 

for gas transmission.

ACKNOWLEDGMENTS

The writer greatly appreciates and thanks the 

management of Iraq Petroleum Company, Ltd., and 

associated companies for the opportunity to prepare 

and the permission to publish this paper. Special 

thanks are due to Dr. C. E. Theibaud, Manager, Exploration Division, and to Mr. H. V. Dunnington, Chief 

Geologist, for their interest and encouragement; to 

Mr. G. F. Elliott for reading the manuscript and making valuable suggestions; and to Mrs. I. Barber for 

the painstaking effort of typing the manuscript. 

Thanks are also due Mr. H. A. Field for valuable and 

constructive suggestions and discussion on the region. 

The author's grateful appreciation is extended to the 

management of the Arabian-American Oil Co., especially to Mr. O. A. Seager, without whose enthusiasm 

and sustained advocacy this work would not have been 

completed.

The author has drawn freely on unpublished Company reports of many colleagues and on papers by 

authors mentioned in the bibliography. Contributions 

from all these sources are gratefully acknowledged.

GENERAL GEOMORPHOLOGY

Southwestern Iraq as a whole is relatively flat, though 

there are very gradual and gentle rises in altitude from 

east to west and from south to north. The rise in altitude along the Shatt al Arab and Euphrates Rivers is 

gentle, between Al Faw (Fao) and Bahr al Milh, west 

of Karbala, amounting to only 245 feet (74.6 meters), 

whereas southwestward from Al Basrah on the Shatt 

al Arab to the Iraq-Saudi Arabia Neutral Zone, the 

rise in altitude is five times as great in about one third 

of the distance. In the extreme west, at Jabal Unazah 

(Aneiza), where the borders of Iraq, Saudi Arabia, 

and Jordan intersect, altitudes are as much as 3,084 

feet (940.1 m) above sea level. Even here, the region 

has a general east-northeast slope, toward the Euphrates the only perennial river.

The region has been subdivided by various authors 

into a number of physiographic areas (Macfadyen, 

1938; Mitchell, 1956). For purposes of the present review the following subdivisions are recognized:

Mesopotamia!! alluvial plain covers the area between 

the Euphrates and Tigris Rivers and comprises flat 

country surfaced by Recent alluvium (or locally by the 

marine Hammar Formation).

Sand dune belt, according to Macfadyen (1938), 

forms an area 10-15 kilometers wide on the east border 

of the southern desert of Iraq, and separates it from 

the Euphrates River. It extends from south of An 

Najaf to the Wadi Batin in the extreme south.

Ad Dibdibba gravel and sand region occupies the 

southern part of Iraq and is bounded roughly to the 

north and west by Jaliba, Abu Ghar, Al Busayyah, 

Jahmah, B'ir Juraybl'at, and Al Waqisa. Outlying 

areas of physiographic similarity are to be found near 

An Najaf and Karbala, in the north.

Al Hajara limestone region includes most of the area 

under review. The surface is largely made up of limestone outcrops, at some places obscured and littered 

with residual flints and cherts.

Al Widyan country covers a large area south of the 

Ga'ara in lat 33°30' N, and west of long 43° E. It 

consists of a vast plain gently rising westward so as to 

appear almost flat. The geomorphic expression of the 

region is largely a function of the geologic framework 

of the area and particularly of the lithologic characteristics of the constituent outcrops. The different physiographic belts or areas are discussed below in some 

detail.

MESOPOTAMIA!! ALLUVIAL PLAIN

In southwestern Iraq the Mesopotamian alluvial 

plain extends west of the Euphrates River as a narrow 

belt and consists of mudflats and sabkhas, which receive the drainage of the numerous shaibs and wadis
SOUTHWESTERN IRAQ G5

during the wet season. The sabkhas are soft rather 

saline mudflats, which in many places hold moisture 

even through the dry season; they are, as a result, very 

difficult to cross. They cover extensive areas between 

Ur Junction and As Samawah, and hinder transport 

from the desert to the railway. In the south, sabkhas 

also occur along the margins of Hawr al Hammar; they 

extend northward as far as An Najaf. Their magnitude and characteristics vary from place to place.

SAND-DUNE BELT

The rather mobile sand-dune belt covers the eastern 

part of the territory. It may transgress to any of the 

other adjacent physiographic zones, the Mesopotamian 

plain, Al Hajara, or the Dibdibba.

AD DIBDIBBA GRAVEL AND SAND REGION

The Ad Dibdibba zone is distinctive in its physiography ; it is largely flat, low, and scrub covered. The 

Wadi Batin, cut into the Dibdibba plain, forms a conspicuous negative feature which defines part of the 

Iraq-Kuwait boundary. In southwestern Iraq the Dibdibba extends westward from the Shatt al Arab and 

Euphrates (Hawr al Hammar) Rivers in the east to 

the vicinity of Qasr Shaqrah and Al Busayyah and 

into the Iraq-Saudi Arabia Neutral Zone. Here the 

terrain is rather flat, having but a few slightly higher 

isolated elongated hills. The country is covered with 

gravel and sand. The gravels are of various igneous 

and quartz pebbles, and the coarse sands are in places 

cemented into hard grit.

South-southwest of Az Zubair village and very close 

to the Iraq-Kuwait border, the Jabal Sanam stands 

out as a solitary and isolated hill. It has an altitude 

of more than 500 feet in a low-lying region whose altitude elsewhere ranges between 50 and 150 feet above 

sea level. This prominent and impressive feature is a 

piercement salt dome (the only one in Iraq), the detailed geology of which is still obscure. The drainage 

system of Ad Dibdibba is apparently controlled by the 

Boliya-Ar Rachi topographic feature. The surface is 

sandy and gravelly with good porosity; and because 

the rainfall is generally slight, drainage lines have not 

been significantly incised, except for the long, deep, 

and very straight Wadi Al Batin, which runs northeastward. The other shaibs and wadi courses on the 

east side of the Boliya feature show a gradual swing 

in direction northeast to north toward Hawr al Hammar, as evidenced in Shaib ash Shich, Shaib al Ubtain 

(Butayn), and Shaib al Adhbi. Most of the courses 

west of the Boliya-Ar Rachi feature end in gentle 

closed depressions, forming a kind of internal drainage basin, such as that at Khor Ash Shihiyat.

AL HAJARA LIMESTONE REGION

The Al Hajara region takes its name from the 

weathered rocky limestone terrane of Huweimi and 

Shawiya outcrops that extends between Shabicha and 

Nisab (Ansab), where travel by vehicles is extremely 

difficult. In this region Ad Dibdibba morphology gives 

way to a topography which is controlled by a series of 

scarps that radiate around a point just to the south of 

the Iraq-Saudi Arabia Neutral Zone. These scarps and 

alinements, of variable form and size, are from east to 

west as follows: Al Busayyah, At Traq, At Takhadid, 

Al Waqisah (Wagsa), Al Qasas, and Al Batn. Each 

scarp line passing northward takes on a general northnorthwest trend extending toward Wadi Al Ghadaf, 

north of the area. The scarps coincide with different 

beds of the Dammam and Umm er Radhuma Formations. Their general distribution can clearly be appreciated from the geologic map (pi. 1). In the Hajara 

region there are a number of broad depressions which 

form internal drainage systems.

Westward from the Euphrates Limestone Formation, the outcrops are almost flat. The topography 

changes in the area of Dammam limestones. The main 

feature in the southern part, beginning north of Aim 

Ghanzir (lat 30° IT N., long 45° 25' E.), is the big 

scarp of At Traq, trending generally southward; local 

embayments face towards the east (for example, Faydat az Zahra, near Shibrumiya, Ad Dayir). Near 

Kashmat al Muharh, the trend of the scarp swings due 

east, toward Tuqaiyid, and from there it again trends 

southward and then disappears east of the north tip 

of the Neutral Zone. The difference in altitude between the scarp and the low-lying ground west of it is 

as much as 50 meters.

West of At Traq, a higher ridge trends north-northwest. Toward the north it disappears in the vicinity 

of Zum, and in the south it plunges underneath the 

Dibdibba Formation. Another prominent feature is 

Chabd (Kabd), which is a flat-topped hill farther west, 

rising 38 meters above the surrounding low-lying country and characterized by the steep cliffs which delimit 

it. Some distance west of this is another scarp that 

trends north-northwest.

Still farther west, there are two dominant topographic features: the Wagsa-Jil Raschid scarp and the 

Al Batn scarp. Both are alined roughly north-northwest and face westward, forming three distinct parallel strips or zones, which are gently tilted to the northeast. The most easterly zone extends from Nisab (Ansab) to Ash Shabakah (Shabicha), and then to the 

west edge of the very rocky area known as Al Hajara. 

The second strip, between the Batn and Wagsa scarps, 

is named Al Basita; this is also rocky in parts, but is
G6 GEOLOGY OF THE ARABIAN PENINSULA

not marked by any very distinct drainage system. The 

zone below the Batn scarp, known locally as Hamrat 

al Batn, is more dissected than the Basita area; its 

drainage is poorly defined, and the terrain is dotted 

with numerous small depressions, each having its own 

catchment.

Farther north, all these scarps and features tend to 

lose their distinctive characteristics. West and southwest of An Najaf and Karbala the area becomes rather 

tabular. Hills (mesas) capped by silicified limestone, 

occur in the eastern belt and are generally small, being 

some 10-20 meters above the plain or wadi floor. Old 

wadis, dating from pre-Zahra or even pre-Fars times, 

cut only slightly into the stratigraphic succession and 

reveal minor scarps from time to time, although collapse structures, slumping, and bending of beds generally smooth out the morphology.

Two major scarps dominate the topography of this 

part of the region: The first of these is the Upper Fars 

scarp, which forms the first major scarp on the west 

side of the Mesopotamian structural valley. Its average height is 50 meters; it has a roughly northward 

trend; and it extends far beyond the limits of the area 

considered. The second is the Batn az Zawr scarp, 

which like the Upper Fars scarp is due to differential 

erosion of regionally tilted rocks. Near the top of the 

Dammam chalks is a bed of chert, 0.5 meter thick, so 

competent and resistant to erosion that it virtually 

controls the steep scarp face which extends for a long 

distance, and it can be considered as the extension of 

the Wagsa scarp. This scarp forms the eastern boundary of an extensive Miocene Zahra-Euphrates embayment of deposition that transgressed over all formations in the area, from Miocene to Cretaceous. Erosion 

of these later sedimentary rocks is responsible for the 

varied irregular hummock and plain topography of 

the broad area to the south of Nukayb (Nukhaib).

A number of wadis, including Wadi Arar, Wadi Hamir, and Safawiyat (Shaib) Al Ubaydat, which have 

their origin in Saudi Arabia drain into gentle depressions south of Nukhayb, such as Khabra al Qatah and 

Faydat al Adyan.

In the extreme southwestern part of Al Hajara, north 

of Al Busayyah toward Abu Ghar, the Euphrates 

limestones form numerous isolated hills, especially in 

the region of Al Afaif. The Lower Fars Formation, 

which occurs between the Euphrates limestones and 

Dibdibba beds, generally forms a pronounced ridge. 

The drainage here flows toward the north into the 

sand-dune belt on the west bank of the Euphrates 

River. The principal intermittent water courses are 

the Shaib Abu Ghar with its tributaries, Shaib Juda 

and Shaib Luwaihadh. Some distance farther west,

Shaib Sudayr drains ia big part of the territory covered 

by Tuqaiyid limestone, which underlies the Euphrates 

Limestone Formation. The drainage system in the 

western desert is characteristically subdivided between 

those shaibs which drain into the Euphrates River and 

those with interior drainage into inland depressions, 

mudflats, tand lakes.

The Tuqaiyid, Barbak, and Ghanimi sedimentaryrock units extend north-northwestward from the Al 

Busayyah area toward al Khirr, where they overlie the 

Euphrates limestone. The regions covered by these 

units are generally flat, the dip and slope commonly 

toward the northeast. The drainage lines have the 

same trend. These include Shaib al Ghanimi, with its 

tributaries Shaib al Arjawi and Shaib Abu Finajin, 

Shaib Ashali, Shaib al Haisam, Rijlat As Salhubiya, 

Shaib Firk (Firch), Shaib Huwaymi, Shaib Hisb, and 

Wadi Al Khirr. Most of these lead northeastward to 

the mudflats and sabkhas bordering the Euphrates 

River.

Beyond An Najaf, the drainage from the desert is 

interrupted by the long Upper Fars sandstone ridge of 

Al Lisan Tar as Sayyid, which trends northward from 

Tuqtuqanah. The presence of this ridge has given rise 

to the salt lakes of Bahr Al Milh, which lie east of 

Shithatha.

Numerous wadis that cross the area from west to east 

drain into these salt lakes, following a general easterly 

direction. These include the Wadi al Ubaiyih and 

Wadi al Ghadaf, which have their origin in Saudi 

Arabia to the west. In the central parts of Al Hajara 

there are several depressions into which numerous wadis 

drain in the rainy season. The Ghanimi depression, 

which is drained by Shaib al Ghanimi, forms a conspicuous inland drainage system southwest of Al 

Qusayr. To the south of Ghanimi, the drainage flows 

mostly into a number of isolated systems each emptying 

into a depression with no outlet, such as Jahama, 

Tukayyid, and Faydat as Zahra. Farther west, in the 

neighborhood of As Salman, there is a group of very 

deep depressions completely surrounded by cliffs, some 

of which have a relief of as much as 50 meters. These 

have no surface outlet, owing to the flatness of the country, and form several topographic closed basins of varying diameter, which may be as much as 15 kilometers. 

Extensive pools form in some of these depressions after 

rains. Northwest of this area, the vicinity of Ash 

Shabakah (Shabicha) and Baraibir is marked by a 

few important depressions. Several solution holes, such 

as Ashuriyah and Chebritiyah, occur in the area.

AL WIDYAN COUNTRY

Al Widyan area, within the study region considered, 

includes outcrops of the Umm er Radhuma, Tayarat,
SOUTHWESTERN IRAQ G7

and M'sad Limestones west of the Zahra and Euphrates limits. The topography is more undulating westward, beyond the Tertiary-Cretaceous contact. The 

country is deeply dissected, with broad, deeply incised 

wadis, many more than 1 kilometer across, bounded by 

steep scree-covered and slumped walls attaining altitudes of 40-50 meters. The eastern part of the area 

is drained chiefly by the Wadi al Ubayyid and its tributaries Wadi Tibaland and Wadi al Mira and by 

the Wadi al Ghadaf and its tributaries, which all flow 

generally eastward, ultimately ending in the Bahr al 

Milh, west of Karbala. The western part of this area 

is drained by Wadi al Hauran, which flows northeastward. Within this catchment several solution holes of 

various sizes are irregularly distributed and are limited 

mostly to the Paleocene beds. These include Wadi al 

Ghadaf (E. 1188, N. 1195), Jelta (E. 202, N. 1173), 

Kursa (E. 225, N. 1160), and Wishwashia. Al Umchaimin, in the Al Hamand plain, to the west of the 

area), is a similar feature (Merriam and Holwerda, 

195T).

The origin and mode of formation of these solution 

holes may vary widely, and some may be attributed to 

the combined effect of several different mechanisms. 

For example, for the Al Umchaimin, which is a large 

depression, Merriam and Holwerda (1957) postulated 

meteoritic origin, whereas Mitchell (1958) argued for 

a possible volcanic origin. The majority of the geologists of the Basrah Petroleum Company, however, 

considered it best explained in a less dramatic manner. 

They maintained that the initial dissolution probably 

began along fracture surfaces and so lead to the development of a rudimentary underground drainage pattern. Evidence of this pattern remains in the small 

tunnels now partly blocked by collapse debris. The 

vertical drainage through fractures then apparently 

augmented the underground streams and swallow holes 

were formed. This stage is possibly represented by 

holes at Kursa and Wishwashia. Continued dissolution finally resulted in the foundering of the roof and 

in the formation of a large collapse structure of irregular outline, as at Jelta and Wadi al Ghadaf. Theoretically it would seem that this kind of topographic 

feature can originate in different ways: the effect of 

climate on geological structures and rock facies, volcanic activity, or meteoritic impact.

STRATIGRAPHY

GENERAL FEATURES

PHILOSOPHY OF NOMENCLATURE

The incompleteness of the geological record is not 

peculiar to the systems in Iraq. Establishment of the 

stratigraphic sequence in virgin territory depends primarily on the background of the investigators and the 

amount and type of data available. Through the 

efforts of a great many geologists of the Iraq Petroleum Company and its associated companies, a great mass 

of geologic, stratigraphic, and paleontologic information and material has been accumulated. After 33 

years of investigation the progressive synthesis had 

reached a stage, toward the end of the last decade, at 

which the broad patterns of the sedimentation in Iraq 

were fairly well established. A statement of the stratigraphic synthesis was published by Dunnington, Wetzel and Morton for Paleozoic and Mesozoic units and 

by Bellen for Tertiary units (Bellen and others, 1959) 

in the Stratigraphical Lexicon for Iraq sponsored by 

the Commission on Stratigraphy, International Geological Congress.

The system of rock-unit classification and naming 

employed by Iraq Petroleum Company geologists is advocated in the code of rules compiled by Ashley and 

others (1939) as supplemented in recent papers and 

rulings of the Commission on Stratigraphy.

It has been generally realized that the sedimentation 

realms were dominated by the proximity of the Arabian shield on one side and by the Zagros (Tethys) 

geosyncline on the other and that the sedimentary 

characteristics followed the natural shore-to-basin pattern from the Arabian shield east and northeastward. 

Furthermore, the sedimentary patterns showed clear 

diachronism following epeirogenic cycles in western 

and central Iraq and diastrophic and orogentic movements in northern and northeast Iraq. Of course, an 

attempt has also been made to define the basic units of 

the sedimentary cycles. However, because of gaps in 

knowledge and especially because of paucity of information on the deep subsurface stratigraphy of central 

Iraq, separate stratigraphic nomenclatures were developed for northern and southern Iraq.

Description of some of the rock units of southwestern Iraq was published by Owen and Nasr (1958), and 

all the formations except for the newly defined Safawi 

and lately defined Kifl Formations were redescribed 

and expanded by Bellen, Dunnington, Wetzel, and 

Morton (1959).

The present report is chiefly based on the above 

works, supplemented by later unpublished works of a 

number of geologists, including especially M. C. Chatton and E. Hart. Some of the original descriptions of 

individual formations have been revised, redescribed, 

and expanded, where necessary, to take into account 

the findings of recent investigations and especially of 

recent deep wells in the southern and western areas.
G8 GEOLOGY OF THE ARABIAN PENINSULA

TIME UNITS REPRESENTED

In western Iraq the oldest rocks exposed, the Nijili 

Formation and the overlying Ga'ara Sandstone Formation, are of Middle Triassic age; the Ga'ara is followed 

unconformably by the Upper Triassic Mulussa Formation, which is succeeded upward by the Rhaetic( ?) Zor 

Hauran Formation and the Liassic (pre-Toarcian) 

Uba'id Formation. The Bathonian-Bajocian Muhaiwir Formation overlies the Uba'id Formation with 

probable unconformity. A major unconformity exists 

between the Middle Jurassic Muhaiwir and the overlying Middle Cretaceous Rutbah Sandstone. All the 

Middle Triassic, Jurassic, and Lower and middle Cretaceous units are exposed in the Ga'ara depression and 

Wadi Hauran, which are to the north of the area of 

review. No detailed account will therefore be given 

of them.

In the area of present consideration, the exposed 

rocks range in age through middle and late Cretaceous, Paleocene, Eocene, Miocene, and Pliocene Epochs 

to Pleistocene and Recent. With the exception of 

the Al Hamad plain in the extreme western part of 

Iraq, where the Cretaceous units are succeeded westward by Paleocene formations, the general sequence 

of the exposed formations is in ascending stratigraphic 

order to the east. The Oligocene Epoch is not represented by any rock unit in the area, owing to the great 

unconformity between the Eocene and Miocene Epochs, 

though there was some tendency in the past to assign 

an Oligocene age to the Tuqaiyid beds of the Dammam Formation.

In the subsurface sections of the area, the oldest sequence found is the Butmah Formation in the BPC 

(Basrah Petroleum Company) well Samawa No. 1, 

which was completed 'at a total depth of 12,602 feet. 

This formation is Early Jurassic in age.

GENERAL DISTRIBUTION OF SYSTEMS AND DOMINANT 

ROCK TYPES

The surface occurrences of the different systems and 

their general areal distribution in the defined area are 

illustrated on the geological map (pi. 1). This map 

shows the presence of a major stratigraphic gap between the Bathonian and Bajocian Muhaiwir Formations, and the overlying Cenomanian Rutbah Sandstone Formation in the Ga'ara area, a little to the 

north of the area that is considered in detail. This 

hiatus becomes less important away from the Ga'araKhleisia high. The sequence is almost complete in the 

wells of the Basrah area, but there are successive convergences northwestward, beneath the pre-Cenomanian 

unconformity. Thus the Cretaceous System is only 

partly represented in outcrop, the oldest exposed rock

units being the Cenomanian and lower Turonian Rutbah Sandstone and M'sad (Mishrif) Limestone, which 

come to the surface only in the region of the Ga'ara 

depression, especially in the Wadi Al Hauran area.

The Upper Cretaceous (upper Campanian and Maestrichtian) is represented by the Tayarat limestones, 

which include subsidiary marls and thin shales. They 

cover most of the Al Widyan region, where between 

the Upper Cretaceous and middle Cretaceous (Cenomanian) there is an extensive hiatus. The missing 

rock units include those of the middle Turonian and 

lower Campanian cycle, which are widespread in the 

subsurface in the eastern parts of the region. These 

rocks include the basinal Khasib limestones, neritic 

Tanuma shales, and Sadi subbasinal chalks and limestones. Also, the upper Campanian Hartha limestones 

and the related Safawi evaporites, which are well 

represented in the subsurface section in western Iraqi 

wells, wedge out between the westernmost well and the 

outcrop belt.

The Paleocene series embraces the Umm er Radhuma 

Formation, which consists of two main limestone units 

in the surface exposures. These trend north-northwestward along the western Iraqi-Saudi Arabian 

border. In the Al Widyan area the Umm er Radhuma 

Hmestones are exposed both east and west of the Cretaceous outcrops. The Umm er Radhuma is very extensive in the subsurface beneath the western desert.

The overlying Eocene Dammam limestones crop out 

along parallel belts that are also oriented north-northwestward. These limestones also have a widespread 

subsurface distribution in the area. The evaporitic 

Rus Formation is present in the Basrah area as a 

lateral f acies variant at the base of the Dammam.

The Oligocene series is not represented in the area.

The re]ations of the rock units of the Miocene series 

are complex. The lower Miocene is perhaps represented 

by the Euphrates Limestone, which crops out along 

the east margin of the area and in the Al Busayyah 

region. The stratigraphic position of the Zahra beds 

is rather problematical, but they are generally included 

in the Miocene. They occupy old depressions and wadi 

floors, occurring as isolated outcrops in the southern 

and central areas and as extensive sheets in the northern and western parts of the area where they overlie 

rocks of both Paleocene and Eocene ages. The middle 

Miocene Lower Fars evaporites cover the Basrah area 

and follow the general distribution of the Euphrates 

Limestone. The Lower Fars evaporites are underlain 

by the elastics of the Ghar Formation, the relationship 

between the two rock units being one of f acies change 

by intergradation, the incidence of evaporitic components diminishing westward.
SOUTHWESTERN IRAQ G9

In the north the Lower Fars evaporites are succeeded 

by Upper Fars sandstones and siltstones in the region 

of Karbala, where they underlie the Recent Mesopotamian alluvium, whereas in the south the Lower Fars 

is directly followed by the rudites of the Dibdibba 

Formation, which covers the Dibdibba physiographic 

region of southern Iraq. In the Basrah area the Dibdibba Formation dips under the Eecent marine sediments and alluvium of the Hammar Formation.

The 27 formations recognized in the area are described below, in ascending stratigraphic order, beginning with the Lower Cretaceous Ratawi Formation. 

The relations of these units to each other and to correlative units in other areas are illustrated on plate 2.

The schematized regional correlation of the current 

rock units (pi. 2) is based on plates 4 and 6 in Bellen, 

Dunnington, Wetzel, and Morton (1959), or work by 

Naqib (1960), and on unpublished diagrams by M. 

Chatton, E. Hart, and the writer. It illustrates the 

general horizontal and vertical distribution of the 

different rock units in broad terms and their relation 

to counterparts in Arabia. A broad and general tentative subsurface stratigraphic correlation 'of the rock 

units in the area is shown on plate 3.

UNCONFORMITIES

The geological column of Iraq contains numerous 

erosional and nondepositional hiatuses. Most of the 

unconformities which are recognized and accorded importance in the area are disconformities in which two 

units of stratified rock are parallel but separated by an 

erosion surface having appreciable relief. Discernible 

angular discordance is very rare, except for localized 

and surficial unconformities of the Zahra beds on irregular topography in the western and southern desert 

areas. Some of the major disconformities involve very 

considerable convergences, with progressive cutout of 

hundreds or thousands of feet of sediments, but the distances over which the convergences are distributed are 

so large that the angularity is not visible.

In Iraq the geological sequence, from Triassic time 

to possibly late Miocene, consists of units that preserve 

their general characteristics over large regions in which 

the more significant variations are related to paleogeographical position within a basin. The recognizable erosional unconformities relate to epeirogenic 

movements.

According to Dunnington, Wetzel, and Morton (Bellen and others, 1959), the Upper Triassic (Mulussa 

Formation) is unconformable on the Middle Triassic 

(Ga'ara Sandstone). Though there is no apparent 

angular discordance or convergence, there is a wellmarked erosional surface indicating subaerial exposure 

at the top of the Ga'ara Sandstone.

Only one major unconformity of any significant regional extent has been detected in the Jurassic succession, this being the well-marked break between the 

Middle Jurassic and Upper Jurassic. There is no 

recognizable unconformity or sedimentary break at the 

Cretaceous-Jurassic boundary. According to Dunnington, Wetzel, and Morton (in Bellen and others, 1959), 

the subsurface sections, especially in the north, show 

an erosional unconformity lying within the Berriasian, 

though unpublished work by E. Hart and M. C. Chatton has cast doubt on the existence of this break.

The Cretaceous sequence is divided by a number of 

widespread unconformities of Albian-Aptian, Cenomanian-Albian, Turonian-Cenomanian, and upper 

Campanian-lower Campanian ages. Near the area of 

review the most spectacular of these is the onlap of the 

Cenomanian over eroded extremities of units as old as 

Middle Triassic. The Cretaceous rock succession terminates at a widespread major erosional unconformity 

(disconformity) of post-Maestrichtian (or very late 

Maestrichtian) date, which is followed by Paleocene or 

younger sediments, the Danian being unrepresented.

The unconformities in the Paleocene and Eocene succession may be of paraconf ormity type. The most important gap recognized is between the transgressive 

Eocene (Danunam Formation) and Paleocene(?) and 

Lower(?) Eocene (Umm er Radhuma Formation).

A major discontinuity exists between the Eocene 

units and the overlying Miocene sequence, the Oligocene being unrepresented throughout the entire area of 

study. The southwestern shoreline of the Oligocene 

sea lay to the northeast of the Euphrates, and the 

Eocene rock units were subaerially exposed over the 

whole area through Oligocene time.

In the Miocene and Pliocene succession, unconformities on both large and small scales can be detected. 

These include the disconformity between the middle 

Miocene (Lower Fars) and lower Miocene (Euphrates 

Limestone), the disconformity between the Pliocene 

(Dibdibba Formation) and middle Miocene (Lower 

Fars), and the commonly angular unconformity of the 

Zahra beds on the underlying older rocks, as well as 

paraconf ormities and diastems within each of the main 

units. Finally, the Dibdibba Formation is unconformably overlain by the Recent Hammar Formation.

The present-day erosion surface exposes a sequence 

ranging from Triassic to Recent. The topography on 

the whole is flat with gentle east-northeasterly dip and 

slope. Intermittent wadis cross the region in this general slope direction. Interior drainage basins and depressions locally interrupt the general pattern, which
G10 GEOLOGY OF THE ARABIAN PENINSULA

is largely dictated by the dip slopes and strike escarpments of the almost flat-lying sedimentary sequence.

REGIONAL, SEDIMENTARY PATTERN AND HISTORY

The individual formations from the surface and subsurface type or reference sections have been treated 

independently in some detail elsewhere, but in any 

stratigraphic work it is not sufficient to describe specific rock types or individual sedimentary environments 

without discerning the broader sedimentary patterns. 

The reconstruction of these patterns is primarily base,d 

on good correlation by fossil content (biostratigraphic 

units) but also on equivalences in lithology (rock 

units). Thus a pattern advanced today may later be 

modified or completely altered in the light of new 

evidence that leads to a completely different history 

and picture. With proper correlation and the knowledge of mode of sedimentation, it may be possible to 

reach a reasonable pattern which could convey the 

actual history of sedimentation in time and space. The 

pattern presented here is, of course, subject to the 

validity of the correlation and to a proper assessment 

of the mode of sedimentation and modifications introduced during subsequent geological history.

Southwestern Iraq occupies a position between the 

relatively stable Arabian shield in the west and the 

highly mobile Zagros belt in the east. The Zagros 

mountain belt has risen on the site of the great "Tethys 

geosyncline" which persisted from at least Permian 

until middle Miocene time.

In this work only the period from Early Cretaceous 

to Recent is treated because the present information on 

older rocks in the area is scanty. The Cretaceous to 

Recent stratigraphic sequence is similar to rocks normally found on stable continental platforms. It is 

made up of units that are rather widespread. Facies 

changes are not unusual, but they are generally gradual. The type of sedimentary rocks and erosional unconformities in the successive geological periods indicate that the area was the scene of periodic epeirogenic 

movements; some tectonism, as evinced by broad swells, 

depressions, and perhaps block faulting, accompanied 

regression and transgression of the seas. The western 

shorelines during these geological periods followed the 

margins of the Arabian shield, the axis of the basin or 

geosyncline being far away to the east in western Iran 

and eastern Iraq. The effects of geosynclinal depositional conditions upon sedimentation in the platform 

region must have been very small. The Arabian shield, 

and to a lesser extent the Ga'ara-Khleisia high, must 

have been the primary source of the sediment that was 

deposited over western Iraq. In recent unpublished 

work on the region, great stress has been put on the

importance of diachronism as a factor controlling the 

sedimentary pattern. Dunnington (1958) and Dunnington, Wetzel, and Morton (in Bellen and others, 

1959) recognized the prevalence of diachronous contacts of many of the Mesozoic rock units, and Bellen 

(1956; Bellen and others, 1959) dealt in some detail 

with the age relationships of facies-dictated Tertiary 

rock units in which diachronous lateral passage is a 

characteristic feature. Lately, M. C. Chatton and 

E. Hart, in as yet unpublished studies, have endeavored 

to simplify the correlation problems between northern 

and southern Iraq by recognizing a more extensive contribution of diachronism of rock facies to the sedimentary pattern. Their work affects chiefly the Cretaceous 

sedimentation cycles, which are briefly defined as 

follows:

1. Tithonian (Upper Jurassic) to Aptian cycle includes the following formations, in order, from 

the shore to the center of the basin: Zubair, Ratawi, Yamama, Qamchuqa, Sarmord, Chia Gara, 

and Balambo.

2. Albian cycle includes the following formations 

from shore to basin: Burgan, Nahr Umr, Nahr 

Umr Shales, Qamchuqa, and Balambo.

3. Cenomanian to lower Turonian cycle includes the 

following basic formations from shore to basin: 

Rutbah Sandstone, Admadi, Mishrif, Rumaila, 

Dokan, and Balambo. The Kifl Formation is considered as the lagoonal-evaporitic facies on the 

shelf at the terminal phase of the cycle.

4. Middle Turonian to lower Campanian cycle includes the following basic recognized formations 

from shelf to basin: Khasib, Tanuma, Sa'di, Kometan, and Balambo.

5. Upper Campanian to Maestrichtian cycle includes 

the following formations from shelf to basin: 

Saf awi, Hartha and Tayarat, Shiranish, and Tanjero. The Bekhme and Aqra Limestones may be 

considered to be the respective equivalents of the 

Hartha and Tayarat Formations in northern 

Iraq.

The Tertiary group of sediments in Iraq follows 

broadly similar sedimentary patterns; however, owing 

to lack of adequate information the general picture 

seems rather confused for the Miocene and Pliocene 

series. The main cycles follow.

1. Paleocene cycle may extend into the lower Eocene. 

The main formations composing this cycle from 

shelf to basin include: Upper Radhuma, Aaliji, 

Sin jar, Khurmala, and Kolosh.

2. Eocene cycle is chiefly confined to the middle and 

upper Eocene, and it includes the following for-
SOUTHWESTERN IRAQ Gil

mations from shelf to basin.: Dammam, Bus, Jaddala, Avanah, Pila Spi, and Gercus (red beds).

3. Oligocene cycles are characterized by three reefcontrolled cycles that are recognizable in central 

and northern Iraq; however, they have not been 

found in southwestern Iraq.

4. Miocene cycles:

a. Lower Miocene cycle (s) may be more than 

two cycles. An older cycle includes the 

Euphrates Limestone, Dhiban Anhydrite, 

and Sevikagni Formations. A second cycle 

includes the Jeribe Limestone, 

b. Middle Miocene cycles include the evaporitic 

Lower Fars sequence and Ghar Formation. 

The Zahra Formation is problematical and 

difficult to include in any of the recognized 

cycles with any certainty. There may be 

many cycles, as evidenced by the rhythms 

of the Lower Fars sediments, particularly 

in the region of full thickness in northern 

and eastern Iraq.

5. Pliocene and Pleistocene cycles: The Dibdibba, 

Bakhtiari, Upper Fars, and Lower Fars Formations. These formations certainly represent more 

than one cycle, but lateral and vertical relationships remain somewhat obscure.

The above Mesozoic and Tertiary and Quaternary 

cycles constitute merely the main patterns of sedimentation over the whole of Iraq. These broad patterns, 

however, should not rule out the possibility of minor 

patterns due to localized cycles, or local movements 

and environments. Indeed, local divergences from this 

simple pattern are by no means rare: many minor 

cycles do in fact occur within the major cycles, sometimes perhaps to the extent of obscuring the broad 

and generalized (and therefore somewhat oversimplified) outline.

In the general sedimentary province, southwestern 

Iraq lay in the shelf zone from Triassic to Eecent 

time. During this long interval the area underwent a 

series of periodic epeirogenic movements with fluctuating intensities which resulted in periodic emergence 

or submergence of the land. Thus almost all the sedimentary rocks that occupy the area have characteristics 

of the shelf f acies. For a general understanding of the 

sedimentary history, brief recourse is made to the older 

outcrop systems of the Ga'ara area.

During Triassic time the sea covered most of the 

area; its shorelines must have been not far west of the 

Ga'ara, as (according to Dunnington, 1958) the elastics 

of the lower parts of the Nijili show continental features. During Middle Triassic time the Nijili saliferous marls and shales and their overlying Ga'ara sand232-387 67 3

stones and quartzites were deposited. These were succeeded unconformably, though without angular discordance, by the Upper Triassic Mulussa limestones.

These were subsequently covered by the Ehaetic Zor 

Hauran gypsiferous marls, shales, and marly or 

oolitic limestones and by the dolomitized and recrystallized Liassic Uba'id limestones, which were succeeded 

by alternations of limestones and evaporites, indicating intermittent barred-basin conditions during Liassic time. This alternating evaporite sequence is represented in the Butmah shales and limestones, Adaiyah 

anhydrites, Mus limestones, and Alan anhydrites. The 

Liassic sequence has not yet been recognized in outcrop, though it is well developed in subsurface sections 

in the area, as in Samawa well No. 1. According to 

Dunnington, Wetzel, and Morton (in Bellen and 

others, 1959), however, it is possible that Liassic 

equivalents do occur at outcrops in the Wadi Hauran 

area. Toward the basin, during Bajocian and Bathonian times, bituminous black limestones, dolomites, 

and shales of the Sargelu Formation were laid down; 

westward, however, the neritic Muhaiwir limestones of 

the Ga'ara area were deposited. The Middle Jurassic 

sedimentation was terminated by a major regional 

unconformity (Bellen and others, 1959). During Late 

Jurassic time, as a result of a marine transgression 

cycle over the area, the Najmah limestones and Gotnia 

evaporites were deposited. Later they were succeeded 

by the shallow-water Upper Jurassic and Lower Cretaceous Sulaiy Formation.

According to M. Chatton (unpub. data, 1960), sedimentation during the Tithonian-Aptian cycle was continuous, but involved a single major regression and 

transgression of the sea. It seems that Yamama 

calcarenites and oolitic limestones were first deposited 

on Sarmord-type sediments (as in wells Samawa No.l 

and Kifl No.l) with which they intergrade. The overlying Eatawi neritic shales and limestones (with subordinate sandstones, Zubair sands, neritic shales, and 

subordinate limestones) were being deposited contemporaneously in their respective belts of sedimentation. 

The oscillatory migrations of the belts are distinctly 

observable in the ratios of sand to shale components 

that make up the Zubair Formation, and apparent 

overlap of one formation on the other may be best 

interpreted as being due to diachronism. The Shu'aiba 

limestones represent the rapid transgression of the sea 

at the close of the cycle. In most localities shales similar to the Eatawi were deposited between the Zubair 

sands proper and the Shu'aiba limestone.

During Albian time, after a brief period of emergence evidenced in the unconformity between the Nahr 

Umr Formation and the Shu'aiba Formation, the Bur-
G12 GEOLOGY OF THE ARABIAN PENINSULA

gan sands and Nahr Umr shales and sands were 

deposited during a transgressive-regressive phase, 

which was followed by a rapid Mauddud transgression. The Mauddud neritic limestone sedimentation 

ended with epeirogenic withdrawal and emergence.

The Cenomanian cycle followed the same general 

pattern, with the additional complication that some 

kind of high must have formed to account for the 

growth and deposition of the Mishrif bioherms and 

biostromes in the Basra area. These are distributed in 

an arcuate belt, trending north-northwest. As might 

be expected, the sediments on the two sides of the 

biohermal belt are different.

In the Ga'ara area the Cenomanian cycle is represented by the Rutbah sandstones, whereas in Kuwait 

and southern Iraq the corresponding sedimentary 

sequence comprises the thin sandy Wara Formation, 

followed by a thick sequence of rock units comprising 

in turn the neritic shales and marls of the Ahmadi 

Formation, subbasinal Rumaila chalks and limestone, 

and Mishrif neritic limestones. The cycle was terminated locally by the deposition of the lagoonal 

evaporitic Kifl Formation. This indicates that an 

enclosed basin covered the western part of the region 

during latest Cenomanian and (perhaps) early Turonian times.

Clastic and neritic limestone sediments of the middle 

Turonian and lower Campanian cycle limestones are 

not recognized in the area. The Khasib subbasinal 

limestones, the Tanuma neritic shales, and the Sa'di 

basinal chalky limestone are represented in the eastern 

subsurface sections only. This may indicate that the 

sea during this period had transgressed over such a 

large area that the clastic components could not reach 

the region. The greatest recognized thickness of the 

Khasib, Tanuma, and Sa'di Formations is found in 

the Rumaila-Rachi area. It may be that the sand and 

neritic limestone sedimentation belts of the Turonian 

and lower Campanian cycle may lie as yet unknown in 

the area between the westernmost subsurface sections 

(Ghalaisan well No.l, Samawa well Nb.l) and the 

easternmost outcrops (Wadi Hauran and Rutbah 

areas).

Some indications of the imposition of a new tectonic 

framework within the depositional basin are apparent 

in the sediments of the Turonian and lower Campanian cycle. During the succeeding upper Campanian 

and Maestrichtian cycle, this framework was firmly 

established. A major uplift of the Middle Cretaceous 

and older basinal sediments occurred on the site of the 

present Zagros mountain belt northeast of the area of 

study, and a deep trough, trending northwest, sank at 

the foot of this uplift to accomodate vast volumes

of flyschlike elastics derived from the newly emerged 

highlands. At the same time major east-west troughs 

formed in the Sin jar area of northwestern Iraq and 

northeastern Syria and at Anah, on the Euphrates 

River, north of the Ga'ara high. In the Basrah area 

some warping of the platform on east-west swells and 

troughs is evidenced in the thickness and facies distribution of the Upper Cretaceous rock units.

These rock units fall into two subcycles, or phases, 

in southwestern Iraq. The lower subcycle consists 

largely of the neritic limestones and shales of the 

Hartha Formation, with its newly recognized evaporatic lagoonal equivalent, the Safawi Formation. The 

upper subcycle was inaugurated by widespread deposition of globigerinal marls, formerly named the Qurna 

Formation but now identified with the Shiranish Formation of northern Iraq. It terminated with the 

deposition of the neritic Tayarat limestone over the 

whole area.

The energetic tectonism of northeastern Iraq at 

this time was apparently in the south and west. The 

relations of the rock units here imply that toward 

the close of the Hartha sedimentation came a period of 

quiescence; and in the extreme west, lagoonal basins 

formed, as evinced by the Safawi Formation. The 

subsequent deposition of Shiranish marls implies 

deepening, but there may be a minor unconformity 

between the two phases. The second phase, wherein 

the basinal Shiranish marls were deposited over much 

of the area (except for the extreme west where Tayarat limestones were being laid down), was manifestly 

transgressive. Toward the end of the Maestrichtian, 

the area apparently underwent a regressive period 

whereby the neritic Tayarat limestones migrated eastward in the wake of the Shiranish belt of sedimentation. This cycle was terminated by the major Cretaceous-Tertiary unconformity.

The Cretaceous-Paleocene transition period was one 

of widespread regression, which exposed most if not all 

of the region, resulting in a major and extensive unconformity, after which sedimentation recommenced with 

the deposition of neritic and lagoonal Umm er Radhuma limestones (Paleocene and lower Eocene?) over 

most of the area.

Bellen (Bellen and others, 1959) state that the 

proximity of the Ga'ara high resulted in a garland of 

neritic reef and shoal deposits (the Umm er Radhuma 

Formation) east of it. This facies has been found to 

extend to Kifl well Nb.l, to cover almost the entire 

area of discussion, and to extend west and southwestward into Saudi Arabia, where the proximity of the 

Arabian shield has been the deciding factor in the 

mode of sedimentation of this cycle. In the transi-
SOUTHWESTERN IRAQ G13

tion belt the neritic shoal facies of Umm er Radhuma 

pass eastward, with intergradation, into basinal Aaliji 

marls and marly limestones. There is also strong 

evidence for shallowing of the sea and gradual northeasterly retreat, resulting in the migration of the 

neritic facies line toward the basin.

This regression was followed by a gradual advance 

of the sea that permitted the formation of semibarred 

restricted basins in which the evaporitic Rus Formation was deposited in the central and southern parts 

of the area during early stages of the early Eocene 

sedimentation. Good evidence of this marine advance 

exists in the relationship of the open sea basinal sediments of Aaliji and the overlying Rus evaporites in the 

Kifl region. Northward the neritic Dammam Formation passes into the basinal Jacldala Formation.

The Eocene cycle was terminated by a period of 

uplift during which the uppermost part of the Dammam Formation was eroded over an extensive area, 

including parts of the Arabian Peninsula. Bellen 

(1959) stated that after the deposition of the Eocene 

a regression of some importance occurred, followed by 

a transgression of smaller extent, introducing the first 

Oligocene cycle.

Three Oligocene cycles are recognized in northern 

and central Iraq where the sedimentation was controlled by immigating and emigrating organic reefs 

(Bellen, 1956) back reef, reef, fore reef, and basinal 

sediments) : (1) Anah Limestone, Azkand Limestone, 

and Ibrahim Formation, (2) Bajawan Limestone, 

Baba Limestone, and Tarjil Formation, and (3) 

Shurau Limestone, Sheikh Alas Limestone, and Palani 

Formation.

The Oligocene series has not been recognized in outcrops nor in subsurface sections in the area including 

the Afaq well No.l area. However, there are occurrences of back-reef and reef Shurau in Awasil well 

No.5: the back-reef and reef Shurau Limestone, the 

back-reef and reef Bajawan Limestone, and fore-reef 

Baba Limestone in Falluja well No.l; the basinal 

Palani, fore-reef Sheikh Alas, back-reef and reef limestone of the Shurau, fore-reef Baba Limestone, and 

Bajawan back-reef and reef in Musaiyib well No.l; 

and the basinal Palani and Tarjil, fore-reef Baba 

Limestone, and back-reef and reef Bajawan Limestone 

in Dujaila.

These widely scattered occurrences suggest that backreef and reef Shurau of the first cycles trended northwestward from the area of Awasil well No.5 to 

Musaiyib well No.l and Dujaila well No.l in an arcuate belt and that the lithological belts of the three 

superimposed cycles apparently shifted repeatedly

basinward. Although it may be possible to visualize 

the present westerly limits of these cycles, their original extent cannot be detected because the Ogliocene 

Series is absent in southern Iraq, probably removed by 

erosion during a period of emergence prior to Miocene 

transgression.

The Miocene transgression resulted in the deposition 

of the Euphrates Limestone Formation over a wide 

area presently covering the northern and eastern parts 

of the study region. The Ghar elastics suggest that 

it belongs to this early cycle and forms the nearshore deposits; this attribution is, however, arguable. 

The evaporitic (Dhiban) equivalent of the Euphrates 

was found in Musaiyib No.l, northeast of the area. 

This evaporitic unit succeeded the Euphrates Limestone and according to Bellen (Bellen and others, 

1959), was precipitated from sea water after the 

access to the sea arm, probably in the south, became 

wholly or partly closed.

This cycle was succeeded by the transgressive Jeribe 

limestones which have been found in wells Awasil No.5, 

Fallujah No.l, Kifl No. 1, Musaiyib No.l, and Afaq 

No.l. The transgression may have reopened access to 

the gulf, a condition that may have brought- the evaporitic precipitation to an end and introduced lagoonal 

sedimentation once again. It is difficult to determine 

the westerly extent of the Jeribe Limestone. Its areal 

distribution suggests that before the deposition of the 

Lower Fars evaporites there had been a short period of 

emergence and erosion of some of the Jeribe Limestone in the west. The position of the Zahra Formation is rather obscured by the lack of knowledge of the 

stratigraphic relationships of several formations.

The Lower Fars evaporites were laid down in a 

restricted basin in the form of a barred gulf or arm 

of the sea, Where the sedimentation was governed by 

events near the exit of the gulf in which the sediment 

was deposited. Restriction of the basinal exit, according 

to Bellen (Bellen and others, 1959), caused the deposition of anyhydrite and salt. Its opening resulted in 

limestone and in siltstone and shale sedimentation.

After the deposition of the Lower Fars, rapidly 

rising mountains in northeastern Iraq produced large 

quantities of detritus. This material was at first deposited in a marine environment and formed the 

clastic Upper Fars Formation of late Miocene age. 

Representatives of this formation are seen in the eastern part of the depositional area. Meanwhile, less 

clastic Middle Fars limestones and siltstones were deposited in the southern part of the depositional area. 

The large quantity of mountain-derived detritus rapidly forced the sea to retreat. A continental environ-
G14 GEOLOGY OF THE ARABIAN PENINSULA

ment spread southward; and sands, sandstones, gravels, 

and finally thick conglomerates (the lower and upper 

formations of the Bakhtiari Group of Pliocene age) 

spread over the marine Upper and Middle Fars Formations.

In the area of the Ga'ara high, the Lower Fars 

Formation is missing. The western known limit of 

this formation lies a little to the west of the Euphrates 

River in the Awasil area. In the south the position 

of the formation is not clear. The clastic Ghar Formation may be a somewhat detrital facies of the basal 

beds of the Lower Fars Formation, whereas the Zahra 

Formation may have been deposited as lacustrine, in 

places ^A&ra-bearing, limestone in landlocked depressions at the end of the period of Lower Fars deposition. Further fieldwork is needed to clarify the stratigraphy in this area.

In the south, land-derived detritus subsequently 

formed the thick Dibdibba Formation, roughly comparable in lithology to the rocks of the Bakhtiari 

Group of the north. Part of the Dibdibba may represent part or all of the Middle and Upper Fars Formation of northern Iraq. The Middle Fars, Upper Fars, 

lower part of the Bakhtiari, and upper part of the 

Bakhtiari are all absent from the area of the Ga'ara 

high. Alluvium in the form of lacustrine, estuarine, 

and fluviatile deposits overlies the upper part of the 

Bakhtiari Group and the Dibdibba Formation. In 

the southern part of Iraq, however, a small subrecent 

transgression is present between the Dibdibba Formation and the alluvium. This has been recently 

defined as the Hammar Formation.

MESOZOIC

The Mesozoic Era is represented in western Iraq by 

sedimentary units of Triassic, Rhaetic, Jurassic, and 

Cretaceous ages, though there are several important 

gaps in the geological sequence. Triassic and Jurassic units crop out in the Ga'ara area, which lies slightly 

beyond the study region. Jurassic units have been 

found in some deep wells in the area, but information 

on pre-Cretaceous units is generally scanty. Descriptions of the formations are therefore confined to Cretaceous and post-Cretaceous units.

CRETACEOUS

Of the 43 established rock units of the Cretaceous 

period in Iraq, 18 occur in southwestern Iraq; surface 

and subsurface sections of these units are described in 

some detail. The general time gaps in the geological 

history of the Cretaceous Period have been dealt with 

in the section on "Unconformities." The Cretaceous 

sedimentary rocks are as much as 10,000 feet thick.

RATAWI FORMATION 

TYPE SECTION, THICKNESS, AND EXTENT

The type section of the Ratawi Formation in BPC 

well Ratawi No.l (lat 30°33'22" N., long 47°05'45" 

E.; alt 107.9 ft.; completed Mar. 18, 1950). The formation lies between drilled depths of 10,870 and 11,585 

feet (715 ft thick) and takes its name from this well. 

It is not exposed in the western desert but is found in 

BPC wells Nahr Umr No. 3, Zubair Nos. 24, 38, and 

others; Rumaila No. 4; Samawa No. 1; Ghalaisan No. 

1; and Safawi No. 1. It is also found in Kuwait, in 

the Kuwait Oil Co. wells Burgan No. 113, Magwa No. 

1, and Umm Gudair No. 1. This formation decreases 

in thickness northward and westward, mainly owing 

to a change of facies; thickness in wells Samawa No. 

1, Ghalaisan No. 1, and Safawi No. 1 are 285, 219, and 

146 feet, respectively.

DETAILED LITHOLOGIC DESCRIPTION

The Ratawi Formation consists of greenish-black 

shales that are slightly pyritic, massive in the upper 

part, interbedded in the lower part, and contain stringers and beds of buff pyritic pseudo-oolitic detrital 

fossiliferous limestones. The upper shaly beds contain some fine quartz silt.

The Ratawi Formation becomes thinner and less 

shaly to the northwest, for example, in Samawa well 

No. 1 where it is divisible into three units:

Thickness' 

(feet)

3. Limestone, compact, slightly porous, dolomitized, detrital, and dolomite with thin shale intercalations 29

2. Sandstone, porous, fine-grained, calcite-cemented, interbedded with thin shales and sandy dolomites 40

1. Limestone, generally tight, finely detrital, commonly 

characterized by pyritic mud rimming the constituent fragments; locally silty and argillaceous __ 216

At Ghalaisan well No.l the sequence is dominantly 

calcareous. It comprises 219 feet of buff pyritic finegrained limestones some of which are leached and 

vugular. Some of the limestones have a chalky matrix 

and locally grade to buff-gray or gray indurated marls. 

Minor shale intercalations occur at the top of the 

sequence and also near the base. There is a considerable and progressive reduction in thickness westward. 

At Safawi well No.l the sequence comprises 146 feet 

of marly gray to white indurated limestones, shelly 

algal limestones, and arenaceous limestones. Northward and westward this formation grades into Yamama or Garagu Formations.

DIAGNOSTIC FOSSILS

Pseibdocyclammina lituus (Yokoyama), 0street, rectangularis Sowerby, and Terebratula cf. squamosa 

Mantell are diagnostic of the Ratawi Formation. The
SOUTHWESTERN IRAQ G15

lower part of the Eatawi, containing detrital limestone intercalations, has a fauna which includes 

Cyclammina greigi Henson at the top, and Pseudocyclammina lituus (Yokoyama), P. cf. lituus, P. cf. kelleri Henson, Trocholina spp., CristeUaria sp., and 

algal, bryozoan, and stromatoporoid fragments. Macro fossils include Exogyra sinuata Sowerby, Terebratula cf. T. squamosa Mantell, Anomia sp., Pecten 

(Synocyclonema) cf. P. alpinus Mayer, Wenus pilatina Mayer, Cylindrites sp., and Kingena spp., including K cf. Zeilleria tamarindus (Sowerby) in Douville.

The shaly part carries locally abundant microfauna, 

with Choffatella decipiens Schlumberger, Cyclammina 

spp., Trocholina spp., and Pseudocyclammina sp., 

including P. cf. lituus (Yokoyama) and P. cf. kelleri 

Henson. Abundant Cyclammina greigi Henson occurs 

at the base of this interval.

The thin pellety limestone which marks the boundary with the overlying Zubair Formation contains 

abundant debris of the alga Lithocodium aggregatum 

Elliott.

In Samawa well No. 1 the formation contains 

Cyclammina sp., N'autiloculina sp., Pseudocyclammina 

hedbergi Maync, P. kelleri Henson, P. lituus (Yokoyama), Trocholina conica (Schlumberger), T. altispira Henson, T. ardbica Henson, textularids, valvulinids, algae, gastropods, and ostracodes.

AGE

The age of the Eatawi Formation is Neocomian 

(Owen and Nasr, 1958), probably Hauterivian and 

Valanginian by regional correlation (Dunnington and 

others, 1959). The lower part of the Eatawi is considered to be of Valanginian age, largely from correlation with the better dated Garagu Formation. 

The age of the upper shaly division is probably 

Hauterivian, on the basis of the foraminiferal assemblages. This attribution is strengthened by the presence 

of Lima carteroniana d'Orbigny in the immediately 

overlying Zubair Formation; this form is especially 

abundant in the Hauterivian of other Tethyan regions 

(E. G. S. Hudson, unpub. data).

CONTACTS AND EQUIVALENT UNITS IN ADJACENT AREAS

The contacts of the Eatawi Formation are easily 

detected on electric self-potential and resistivity logs. 

The base of the underlying Yamama Formation is set 

at the top of a thick sequence of fine-grained marly 

commonly pellety limestone, which has been variously 

treated in unpublished work by P. M. V. Eabanit, S. 

N. Nasr, W. Sugden, and others. The upper parts of 

these limestones should perhaps best be referred to the

Yamama Formation of Qatar (W. Sugden, unpub. 

data), and Owen and Nasr (1958) did identify them 

with the Yamama of Saudi Arabia.

The age equivalents of the upper shaly measures at 

the type section of the Eatawi are probably represented by sandstone in the Zubair Formation in 

Samawa well No. 1. The boundary with the Zubair 

Formation is diachronous.

The Eatawi Formation is conformably overlain by 

the Zubair Formation. The contact is placed at the 

top of a thin pellety limestone that contains abundant 

algal debris. In Samawa well No. 1 the Eatawi and 

Yamama contact is recognized by the first appearance 

of pellety limestone.

The lower part of the Eatawi Formation, containing 

limestone intercalations, probably corresponds approximately in stratigraphic position to the Garagu Formation of northern and central Iraq. The upper part of 

the formation is represented by coarse-grained sandstones, silts, and silty shales in the wells of the Awasil 

area, which have been identified as Zubair Formation. 

Hence, the upper part of the Eatawi Formation is 

equivalent in age to the lower part of the Zubair Formation of Awasil, Fullujah, and Mileh Tharthar wells.

ZUBAIR FORMATION 

TYPE SECTION, THICKNESS, AND EXTENT

The type section of the Zubair Formation is in BPC 

well Zubair No. 24 (lat 30°22'00" N., long 47°36'46" 

E.; alt 42.53 ft; completed Aug 22, 1953). The formation lies between drilled depths of 10,368 and 11,645 

feet (1,277 ft thick). In the vicinity of the type area in 

the Zubair oil field, this formation has an average 

thickness of 1,250 feet. It is widespread in the subsurface sections of southwestern Iraq, where it has 

been penetrated in BPC wells Ghalaisan No. 1, Kifl 

No. 1 and 2, Luhais No. 1, Nahr Umr No. 3, Eachi No. 

1, Eatawi No. 1, Eumaila wells, Safayi No. 1, Samawa 

No. 1, Shawiya No. 1, Tuba No. 1, Ubaid No. 1, and 

Zubair wells and in the Mosul Petroleum Co. well 

Awasil No. 5.

In a belt extending from Luhais well No.l (incomplete thickness) to Samawa well No.l (1,581 ft) and 

Kifl well No.l (1,612 ft), the formation shows slight 

thickness variation. Westward from this belt, the 

formation abruptly decreases in thickness to 785 feet 

in Ghalaisan well No.l; therefore, the maximum thickness is to be expected northeast of a line between wells 

Kifl No.l and Samawa No.l.

DETAILED LITHOLOGIC DESCRIPTION

The Zubair Formation is heterogeneous and exhibits 

rapid lateral and vertical variations, though it has
G16 GEOLOGY OF THE ARABIAN PENINSULA

a widespread distribution. In the type locality it consists of the following broad, main lithologic units:

Thickness 

(feet)

6. Shale, black, pyritic, fissile, with rare intercalations 

of gray siltstone; four beds (10-15 ft thick) of very 

fine grained gray cemented sandstones, calcareous 

in places and grouped into two beds separated by 

shaly partings in upper half of unit; 18 ft of sandy 

limestone toward top is separated from overlying Shu'aiba limestone by a shale band 286

5. Sandstone, light-brown, oil-stained, naturally white, 

fine-grained, with subangular grains; contains 

subordinate shale alternating with siltstone; comprises possibly 9 sandstone beds; grades abruptly 

into overlying shales; forms "Third Pay" of 

Zubair field and "Main Pay" of Rumaila field 428

4. Shale, black to greenish-black, subsidiary gray calcareous silt intercalations, especially toward 

middle ________________________ _ 144

3. Sandstone, light-brown, oil-stained, very fine grained, 

current-bedded; in places contains thin shaly and 

carbonaceous or lignitic foliations; intercalated in 

places with gray siltstone, greenish-gray mudstone, 

and hard greenish-black fissile resinous shale containing plant remains; grades to underlying and 

overlying shale through siltstone; constitutes 

"Fourth Pay" of Zubair field _____________ 239

2. Shale, greenish-black, fissile, pyritic, hard; some 

intercalations of gray locally calcareous siltstone 

in lower half ________________________ 127

1. Sandstone, white, in places oil-stained, very fine 

grained, locally silty, separated from underlying 

Ratawi Limestone by 10 ft of black shale; grades 

upward into shale through siltstone _________ 53

At Samawa well No.l the Zubair Formation, apart 

from a thin shale band at the top, is represented 

throughout by characteristically weakly cemented or 

loose crossbedded quartz sands; streaks of lignitic shale 

are common but insignificant in amount. Silt and 

silty shale intercalations are chiefly confined to the 

lower part of the formation. Northward at Kifl well 

No.l, the Zubair comprises buff, gray, or white fine to 

medium unfossiliferous sand and sandstone with 

pyritic calcareous cement that alternates with thin 

bands of silt stone, shales, and rare lignite.

The Zubair Formation interfingers basinward laterally and vertically into the Eatawi Formation. The 

occurrence and relationship of the two formations were 

dependent on their position on the shield and in the 

basin. The influence and significance of this phenomenon is indicated in the shale units of the Zubair Formation in the type area.

DIAGNOSTIC FOSSILS AND AGE

Fossils diagnostic of the Zubair Formation are 

Orbitolina cf. 0. discoidea Gras, Chaffatella decipiens

Schlumberger, Cristellaria spp., Guneolina sp., Trocholina sp., Exogyra sp., C'erithium sp., and Lima carteroniana d'Orbigny (at base).

According to recent investigation and stratigraphic 

reinterpretation by M. Ohatton, the age of this formation ranges from Valanginian to Barremian over southwestern Iraq. Owing to the strong diachronism of the 

formation, its age varies according to the paleogeographic position and sedimentary history. In southern 

Iraq this formation is of Hauterivian and Barremian 

ages only. Though the full Valanginian to Barremian 

sequence has been found in wells Samawa No. 1 and 

Kifl No. 1, farther north the sequence of Zubair Formation may extend into Aptian as it does at Awasil 

well No. 5. Elsewhere, these age limits may be expanded, extended, or restricted.

CONTACTS AND EQUIVALENT UNITS ADJACENT TO SOUTHWESTEBN

IEAQ

The contact of the Zubair Formation with the underlying Katawi Formation is comformable, gradational, 

and diachronous and is placed at the highest Ratawi 

limestone bed underlying the shale and siltstone of the 

Zubair Formation. In wells Kifl No. 1 and Awasil No. 

5 the Zubair Formation overlies the Yamama Formation. The contact with the overlying Shu'aiba Formation is conformable and is placed at the top of black 

fissile shales. In Saf awi well No. 1, where the Shu'aiba 

Formation is missing, the Nahr Umr Formation overlies the Zubair Formation directly, presumably with 

unconformity.

In Saudi Arabia the Zubair Formation may pass 

into the Buwaib and Biyadh sandstones, predominantly into the Biyadh.

ECONOMIC ASPECTS

Commercial quantities of oil have been discovered 

in the sands and sandstones of the Zubair Formation in 

several localities in southwestern Iraq, including the 

Zubair oil fieid, Rumaila oil fieid, Luhais well No.l, 

and Kifl well No.l. Present oil production of the Basrah Petroleum Company comes from the two oil fields. 

The details and reservoir characteristics follow.

Zubair field

The "Third Pay" oil occurs in sands of the Zubair 

Formation under a cover of dark shales and units of the 

formation itself. The reservoir section is thus a sandstone shale complex, some 320 feet thick, in which four 

main and significant members can be distinguished, 

averaging about 200 feet in total thickness. The oil is 

undersaturated by roughly 2,000 psi and, therefore, no 

primary gas cap is present. Water has been struck on 

both flanks at approximately the same elevation. The
SOUTHWESTERN IRAQ G17

quality of the oil varies with structural position, so 

that crestal wells have a gas-oil ratio of approximately 

800 cu ft. per bbl (cubic feet per barrel) and low flank 

wells, of 600 cu ft per bbl. Oil gravities show a similar 

trend, suggesting some gravitational segregation of oil 

within the reservoir. The average value is 36° API. 

Porosity averages 20 percent permeability, 250-400 

millidarcies. A spacing of 1 well to 866 acres on a 

diamond grid, has been adopted and pressure interference on a small scale has been noted.

The wells average 11,000 feet in depth and are completed by selective-shape charge perforation of the 

S^-inch oil string which is cemented through the producing section. Initial flowing pressures average 1,600 

to 1,800 psi at 5,000 barrels per day. A feature of production has been the large salt content of wells high in 

the structure. No water has been detected, but the oil 

may contain as much as 1,000 pounds of salt per 1,000 

barrels. It is believed that this salt is derived from 

water-saturated siltstones associated with oil sands; a 

desalting plant is being installed. The reservoir mechanism to date has been expansion of the undersaturated 

oil. This process has resulted in a steep pressure decline. A poorly effective water drive may be operative 

on one flank.

The "Fourth Pay" oil occurs in sands of the Zubair 

Formation, but is separated from the "Third Pay" by 

some 220 feet of black shale and siltstone. It consists 

of a thinner but similar sandstone-shale complex, containing, however, a better quality oil of average gravity, 42° API. A large primary gas dome is present. 

Sand characteristics are less favorable than the "Third 

Pay", and high-pressure drawdowns are associated 

with relatively low production rates.

Rumalla field

The reservoir section at Rumaila is in the same geological formation as at Zubair but contains a much 

higher proportion of sand and the shale and siltstone 

members are noticeably absent. Porosity averages 25 

percent and permeability, 1,000 millidarcies. The oil 

is even more undersaturated than at Zubair; that is, 

saturation pressure is 2,500 psi below initial reservoir 

pressure. A horizontal oil-water level was proved to 

exist throughout the reservoir.

Some variation of quality with structural position 

is present but is not nearly so marked as at Zubair. The 

gas-oil ratio averages TOO cu ft per bbl, and the gravity 

of the crude, 36° API. The field has only recently 

gone on production and, so far, by oil expansion. 

From the characteristics of the reservoir sand, a water 

drive may be expected.

SHU'AIBA FORMATION 

TYPE SECTION, THICKNESS, AND EXTENT

The type section of the Shu'aiba Formation is in 

BPC well Zubair No. 3 (lat 30°23'01" N., long 47° 

43'29" E.; alt 51.9 ft; completed Feb. 21, 1951). The 

formation lies between drilled depths of 9,962 and 

10,132 feet (170 ft thick) and forms a transgressive 

limestone unit which is fairly widespread in the subsurface sections of southern Iraq wells. It has not yet 

been found in the Cretaceous outcrops in the western 

desert. In the subsurface sections throughout the Al 

Basrah area, its thickness ranges between 150 and 350 

feet. West of the Euphrates River it thins to about 

100 feet between As Samawah and An Najaf. Farther 

west the thickness decreases to about 80 feet in the vicinity of Ash Shawiyat and Al Ubaid. In Ghalaisan 

ash Shabicha the thickness is not more than 30 feet. 

The Shu'aiba Formation disappears by pinching out 

in the Safawi well east of Birkat az Zafiri on the 

Iraqi-Saudi Arabian border.

DETAILED LITHOLOQIC DESCBIPTION

The Shu'aiba Formation consists mainly of creamcolored orbitolinid, detrital, in places chalky, pseudooolitic limestones and recrystallized skeletal limestones. 

Some argillaceous, sandy, and shaly intercalations 

occur toward the base.

DIAGNOSTIC FOSSILS AND AGE

Orbitolina cf. O, discoidea Gras, Choffatella deciplens Schlumberger (at base only), and globigerinids 

in upper parts are diagnostic forms in the Shu'aiba 

Formation, which is Aptian in age.

CONTACTS AND EQUIVALENT UNITS IN AEEAS ADJACENT TO 

SOUTHWESTERN IBAQ

The contact with the underlying Zubair Formation 

is conformable and gradational and is taken at the top 

of the first shale bed below Orbitolina cf. O. discoidea 

limestone. Normally the contact is easily picked on 

the electric logs; however, owing to the gradational 

nature of the contact, it is sometimes difficult to detect 

on the logs. The top of the shale is taken as a marker 

and called the Z/l marker (see section on "Zubair Formation").

The contact with the overlying Nahr Umr Formation may be unconformable, and it indicates a sharp 

lithological change from the porous elastic limestones 

of the Shu'aiba Formation to the silts and shales of the 

basal Nahr Umr Formation. The contact has been designated as the SH/1 marker and can easily be located 

at a sharp negative deflection of the selfpotential curve 

of the electric log, which coincides approximately with 

a substantial increase of rate of penetration. Above
G18 GEOLOGY OF THE ARABIAN PENINSULA

this marker there are local beds of limestone which 

some authors include in the Shu'aiba Formation; however, for the present purpose they have been considered 

as reworked limestone deposited in the wake of a regressive sea.

The Shu'aiba Formation passes laterally into the 

Zubair Formation in Iraq and Kuwait. This explains 

in part the westward thinning from the Basrah area. 

It is possibly represented in part by the Biyadh Formation, with which a gradational relationship is postulated.

ECONOMIC ASPECTS

The Shu'aiba Formation is of no direct economic 

significance, although its equivalent, which has good 

porosity, contains oil in the Abu Hasa area, Abu Dhabi. In Iraq the formation is marked by a zone in 

which circulation, is lost; thus drilling is hazardous.

NAHR UMR FORMATION 

TYPE SECTION, THICKNESS, AND EXTENT

The type section of the Nahr Umr Formation is in 

BPC well Nahr Umr No. 2 (lat 30°44'15" N., long 

47°41'45" E.; alt 21.7 ft; completed Mar. 4, 1950). 

The formation lies between drilled depths of 8,688 and 

9,321 feet (633 ft thick) and is named after the well. 

It is extensive in the subsurface, and has been found 

in all deep subsurface sections in the Basrah area, in 

the western desert, and in central Iraq and also in the 

wells of Nahr Umr, Zubair, Tuba, Eumaila, Eatawi, 

Eachi, Luhais, Ubaid, Samawa, Shawiya, Safawi, 

Ghalaisan, Kifl, Awasil, and Nafatah.

In its greatest development, the formation trends 

northward, parallel to the Euphrates Eiver. The maximum thickness of about 1,100 feet is in the subsurface 

sections of Zubair field. This thickness is comparable 

to that recorded in Kuwait (average thickness 1,150 

ft). This area of great thickness is seemingly confined 

to a narrow belt, to the east of which the formation 

is sharply reduced in thickness, whereas to the west the 

change is gradual. The thickness of this formation in 

the Eumaila-Luhais area ranges between 900 and 800 

feet. Farther west, in the Ubaid Ghalaisan area, the 

thickness diminishes to between 400 and 600 feet. 

Northward there is a general reduction in thickness; 

in the Samawa-Shawiya area the formation ranges 

between 200 and 500 feet. Farther north in the Awasil 

and Nafatah areas the formation thins to between 200 

and 300 feet.

DETAILED LITHOLOGIC DESCRIPTION

In the type locality the Nahr Umr Formation can 

be subdivided from core and electric-log evidence into

three distinct lithologic units. These units from top to 

bottom are:

Thickness 

(feet)

3. Shale, hard, homogenous, black, interbedded with black 

to green-gray shale containing streaks of fossiliferous skeletal black limestones some of which contain molluscan shells and sponge spicules 144

2. Limestone, marly and in places dolomitic, orbitolinid, 

in part globigerinal, with streaks and intercalations 

of greenish-black shale; predominantly black shale 

rich in small molluscs and in places containing pyritized slightly calcitic ostracodes, with bands of 

detrital ?orbitolinid skeletal limestones. (Dair Limestone Member) ___________ 118

1. Sandstone, fine- to medium-grained, brown; loosely 

or well-cemented with well-rounded quartz grains 

or, in places, glauconite; a few intercalations of 

black siltstones, shale containing fossil plants, 

amber, and lignite; streaks of chalky partly detrital 

limestone _______________ 371

These units maintain their general characteristics 

throughout a large area including Samawa, Eatawi, 

Eachi, Kifl, Eumaila, and Zubair. Westward and 

northwestward, however, the shales interdigitate and 

intergrade laterally into sandstone, and distinguishing 

the Nahr Umr Formation from the Zubair Formation 

becomes difficult when one reaches the Safawi area, 

where the underlying Shu'aiba is missing. Eastward, 

marl, shale, and limestone replace this sandstone and 

its terrigenous components. Southward, in Kuwait, 

the Nahr Umr Formation is lithologically divisible 

into two members which have been called the Third 

and the Fourth Sands.

In central Iraq the Nahr Umr is recognized only in 

the subsurface sections of the Mosul Petroleum Co. 

wells Awasil No.5, Fallujah No.l, Nafatah No.l, and 

Mileh Tharthar No.l. In these wells the thickness of 

the formation ranges between 200 and 300 feet, and 

the formation comprises coarse- to medium-grained 

sandstones and subordinate shales. Correlation of the 

unit, as found in the wells of the Awasil area, with 

the Nahr Umr Formation of the Basrah area, rests 

upon lithology and upon homotaxy relative to the preceding Shu'aiba Formation and succeeding Mauddud 

Formation. In the Awasil and Nafatah areas the unit 

was originally named the "Nafatah Sand" (H. Huber, 

unpub. data), but this name has never been sustained 

by publication and is now obsolete.

The view was prevalent at one time that the Nahr 

Umr Formation was directly equivalent to the Eutbah 

Sandstone Formation of the Eutbah-Wadi Hauran 

region of western Iraq, and the name Eutbah (or Eutba) Sand was originally applied to the unit in Basrah 

and also to the sequence of formations (Wara, Maud-
SOUTHWESTERN IRAQ G19

dud, and Nahr Umr) which are now included within 

the Burgan subgroup of Kuwait (Owen and Nasr, 

1958). The name Rutba Sand is a collective name 

that includes the current Ahmadi, Wara, Mauddud, 

and Nahr Umr Formations of the Kuwait succession. 

The term Rutbah (or Rutba) Sandstone is not now 

applied in the subsurface sections of southern Iraq and 

Kuwait, as it is now thought that the Rutbah Sandstone Formation of western Iraq is of Cenomanian 

age and is separated from the Mauddud Formation and 

underlying Nahr Umr sands by a widespread erosional 

unconformity. (The Wara Formation of the Basrah 

area may be directly equivalent to and continuous with 

the Rutbah Sandstone Formation.)

The Dair Limestone Member thins westward and 

southward, being replaced by shales and sands. It is 

not discernible in the Kuwait area.

DIAGNOSTIC FOSSILS AND AGE

Orbitolina cf. 0. discoidea Gras, Haplopkragmoides 

sp., Cythereis sp., and lExagyra sp. are diagnostic of 

the Nahr Umr Formation. The prominent Dair Limestone Member contains Orbitolina cf. 0. discoidea Gras, 

and 0. cf. 0. concava (Lamarck), algae, and a molluscan fauna including Plicatula cf. P. auressensis 

Coquand, Orbiculoidea sp., Neithea dutrugei (Coquand), and Exogyra cf. E. dieneri (Blanckenhorn). 

The formation is Albian in age.

CONTACTS

The contact of the Nahr Umr Formation with the 

underlying Shu'aiba Formation is conformable and 

gradational; it is placed at the base of the lowest 

bedded shales of the Nahr Umr Formation and the top 

of the shale-streaked limestones that make up the highest division of the Shu'aiba Formation. However, in 

the Awasil area the Nahr Umr Formation rests upon 

dolomitized Shu'aiba Formation that includes coarsegrained sandstones in direct contact with coarsely 

granular dolomites that are entirely free from detrital 

quartz. An emergent episode and depositional break 

has thus been inferred between the two formations. 

This contact becomes undistinguishable in the extreme 

west, where, owing to absence of the Shu'aiba Formation, it is difficult to separate the Nahr Umr Formation 

from the underlying Zubair Formation. The overlying 

Mauddud Formation is both conformable and gradational ; the contact is placed at the base of the limestone 

of the Mauddud Formation and at the top of a black 

shale section.

EQUIVALENT UNITS IN AREAS ADJACENT TO SOUTHWESTERN IEAQ

Between the Awasil area and Makhul well No.l, 

the Nahr Umr Formation passes laterally into the

limestone-anhydrite-shale-marl sequence of the Jawan 

Formation, which is between recognizable Mauddud 

Formation and Shu'aiba Formation. This sedimentary 

sequence, which is of near-evaporitic f acies in its upper 

parts, contains a marine macrof auna that includes the 

Albian index fossil Globiconcha altispira Whitfield 

near the base; correlative beds in the Jawan Formation 

at Jawan have yielded Knemiceras syriacum (von 

Buch), confirming Albian age for the basal Jawan 

Formation and supporting Albian age for the lower 

part of the laterally equivalent Nahr Umr. Thin siltstones and silty shales and limestones in the lower 

part of the Jawan of Makhul well sections may be 

regarded as far-reaching tongues of Nahr Umr Formation, lying far to the east or northeast of the recognized 

limits of the unit. The Nahr Umr Formation has been 

recognized in Kuwait and correlates very well with 

that in Iraq. In Saudi Arabia its equivalent is included in the Wasia Formation.

ECONOMIC ASPECTS

The upper shaly units of the Nahr Umr Formation 

form caprocks for any accumulation of oil in the lower 

sandstones, as they do at the type locality. The Nahr 

Umr Formation, the Wara Formation, and to some 

extent the intervening Mauddud Formation, together 

compose the main reservoir for oil in the Burgan-Magwa-Ahmadi area.

The sand to shale ratio of the unit varies considerably between the various oil fields of the Basrah-Kuwait area. Generally speaking, the sand content 

increases souUiwestward and southward. In the Burgan field, for example, this unit is 1,150 feet thick and 

is made up almost entirely of sand, whereas in its type 

locality at Nahr Umr well No.2, the sand to shale ratio 

is 40:60.

MAUDDUD FORMATION

The Mauddud Formation was defined from the subsurface section of Dukhan well No. 1, in Qater, by 

F. R. S. Henson in 1940, but the definition was revised 

and amended by W. Sugden in 1958, and a reference 

section in the Basrah area was described by Owen and 

Nasr (1958).

REFERENCE SECTION, THICKNESS, AND EXTENT

The reference section of the Mauddud Formation is 

in BPC well Zubair No. 3 (lat. 30°23'01" N., long 47° 

43'29" E.; alt 51.9 ft; completed Feb. 21, 1951). The 

formation lies between the drilled depths of 8,517 and 

8,923 feet, based on the sharp change in lithology as 

well as the distinct change in the self-potential curve 

on electric logs. The limits of the formation were put 

at 8,457-8,910 feet by Owen and Nasr (1958); however,
G20 GEOLOGY OF THE ARABIAN PENINSULA

those are based solely on electric logs without lithologic 

evidence.

The formation is 406 feet thick and is fairly extensive throughout southwestern Iraq. It is recorded in 

the wells at Nahr Umr, Zubair, Tuba, Rumaila, Ratawi, Rachi, Luhais, Ubaid, Samawa, Shawiya, Safawi, 

Ghalaisan, and Kin1 . From the Shatt al Arab River 

the thickness of the formation decreases southwestward 

from 570 to 430 and 350 feet at the Zubair and Rumaila 

fields and to 380 and 320 feet at Ratawi and Rachi. 

At Luhais No. 1 it further decreases in thickness to 

291 feet. Northward the decrease is also noticeable 

at Samawa and Kifl, where 90 to 95-foot thicknesses 

were recorded. These are less than at Shawiya (111 

ft) and in the extreme west at Ghalaisan (125 ft). At 

Safawi and Ubaid there is a great decrease in thickness 

to about 35 feet. These decreases have generally been 

attributed to erosion marked by the unconformity between this formation and the overlying Ahmadi Formation.

DETAILED LITHOLOGIC DESCRIPTION

The Mauddud Formation consists mainly of creamcolored microdetrital pseudo-oolitic orbitolinid locally 

recrystallized limestones and a few thin blue shales; 

tight, marly, chalky limestones are intercalated especially in the lower half and middle. The limestones 

are spotted with pyrite. Glauconite has been noted 

toward the base. These limestones become more porcelaneous northward.

DIAGNOSTIC FOSSILS

Diagnostic fossils of the Mauddud Formation are 

Iraqia simplex henson, Trocholina altispira Henson, 

T. ardbica Henson, Orbitolina cf. 0, concava (Lamarck, Rdbanitina basraensis Smout, Archeolithothamnium sp., and IBairdia sp.

A. S. Sayyab (unpub. data, 1956) studied a suite of 

Cretaceous ostracodes from the Persian Gulf area, including the Cretaceous section from Zubair well No.5. 

He described a rich characteristic fauna, mostly composed of hitherto undescribed species, from the Mauddud Formation.

In the Mauddud Formation in the western desert 

wells, the fauna includes rare Cyclammina sp., Oligostegina sp., Globigerina sp., Cuneolina sp., Pseudochrysalidina sp., Gumbelina sp., rotalids, gastropods, 

bryozoans, and algae.

AGE

The Mauddud Formation is Albian in age. Owen 

and Nasr (1958) considered the age of the Mauddud 

Formation in the Basrah-Kuwait wells as Cenomanian. 

Smout (1956) concurred, though admitting the possibility of Albian age, which was favored by Dunnington, Wetzel, and Morton (in Bellen and others, 1959), 

who made the following statement:

In the Basrah and Kuwait wells, according to Owen and 

Nasr (1958), the Mauddud is represented by organic, detrital, 

sometimes pseudo-oolitic, cream-colored limestones with occasional green or bluish shale streaks. It ranges in thickness 

from only 6 feet, within the Burgan field, to a maximum of 

over 500 feet in parts of the Zubair field. It overlies Albian 

Nahr Umr Formation conformably, and is overlain by the 

Wara Formation, which comprises black silty shales and siltstones (Basrah) and sandstone (Kuwait). The Wara underlies the Cenomanian Ahmadi shale and the contact of the Wara 

with the Mauddud suggests slight disconfonnity (Owen and 

Nasr, 1958).

The fauna of the Mauddud in the subsurface sections 

previously mentioned includes Orbitolina cf. O. concava (Lamarck), 0. concava var. qatarica Henson 

(Awasil and Nafatah), T. lenticularis Henson, and 

Cyclammina spp. Orbitolina cf. 0. discoidea Gras is 

recorded from the lower part of the formation at Makhul, Awasil, and other wells.

It is manifest that the Mauddud has been correctly 

identified in the Basrah-Kuwait area and in northern 

Iraq, and it is equally clear that the formation must 

have been deposited more or less synchronously over 

the whole region from Qatar to Makhul well 1. The 

acceptance of Albian age for the formation in Qatar 

(W. Sugden, unpub. data, 1958) is incompatible with 

the Cenomanian attribution made by Owen and Nasr 

(1958) and accepted by Smout (1956).

The evidence assembled from northern Iraq unequivocally favors a late Albian age. The cooccurrence of 

Orbitolina cf. 0. concava (Lamarck) and 0. cf. O. 

discoidea Gras is sufficient to demonstrate pre-Cenomanian age for the lower part of the formation at 

Makhul, and the constancy of the associated fauna 

throughout the formation is strong evidence that the 

age range of the formation is not great. Orbitolina 

cf. 0. concava is not restricted to the Cenomanian, as 

suggested by F. R. S. Henson (unpub. data, 1948), but 

in northern Iraq, at least, it seems to be indicative of 

Albian age. Specimens attributed to Orbitolina cf. O. 

concava occur below a middle and upper Albian ammonite fauna at Naokelekan. Hence the absence of Cenomanian forms, as Praealveolina spp. and others may 

be more significant in determining between Albian and 

Cenomanian age than is the presence of Orbitolina cf. 

0. concava.

Orbitolina conclava var. qatarica certainly lived on 

into Cenomanian time, for this form is found with a 

normal Cenomanian fauna of Praealveolina spp. in the 

Rumaila Formation of Basrah and 6ther places (Owen 

and Nasr, 1958).
SOUTHWESTERN IRAQ G21

Smout (1956) commented that the type locality of 

Iraqia simplex Henson is either Cenomanian or so high 

in the Lower Cretaceous that a Cenomanian occurrence would not be surprising. In fact, the type locality of this species lies about TOO feet below the top of 

the Qamchuqa limestone in the Rowanduz Gorge. In 

that section the top of the Quamchuqa is now considered to be of Albian age, and Iraqia simplex ranges 

here through late and early Albian, in association 

with Orbitolina cf. 0. concava (Lamarck) and Orbitolina cf. 0. discoidea Gras. Hence the presence of 

Iraqia simplex in the Maududd Formation of Basrah 

wells is supporting evidence for Albian rather than 

Cenomanian age.

In Awasil and Nafatah the Mauddud is separated 

from the Mahilban by an erosional unconformity, 

though the Mahilban is of sufficiently late Cenomanian 

age to allow an early Cenomanian age for the Mauddud. It is tempting, however, to correlate the Mahilban-Mauddud unconformity, at Awasil, with the slight 

disconformity reported by Owen and Nasr (1958) 

between the Wara Formation and the Mauddud Formation in Basrah and Kuwait.

The Wara Formation (Owen and Nasr, 1958) may 

be early Cenomanian or even perhaps late Albian, 

because, according to R. G. S. Hudson (unpub. data), 

the overlying Ahmadi Formation contains Turritella 

amotzi Shalem, T. 'blanckerihomi Shalem, Corbula sp. 

juv., Exogyra conica (J. Sowerby), IE. luynesi (Lartet) and Aspidiscus (Helladastrae) juv. cf. A. semhae 

Kossmat. Early Cenomanian age is probable for this 

assemblage (the late Cenomanian age, suggested by 

fragmentary Neolobites sp. from within the Ahmadi, 

and by Metoicoceras sp. (undescribed species) from the 

basal Oythereis ~bahraini limestone, is ruled out by the 

occurrence of Aspidiscus sp.).

CONTACTS WITH ADJACENT UNITS

The contact between the Mauddud and the underlying Nahr Umr Formation is conformable and gradational and is placed at the top of a black shale section.

The Ahmadi Formation overlies the Mauddud, perhaps with slight disconformity. In Kuwait, the Wara 

Formation overlies the Mauddud. The Wara has been 

observed in certain Zubair wells and in Rachi well 

No. 1. In the Awasil and Nafatah area the Maddud 

is unconformably overlain by Mahilban. The Mauddud Formation does not crop out in Saudi Arabia nor 

in southwestern and western Iraq.

ECONOMIC ASPECTS

The Mauddud Formation is the "Main Pay" limestone or "Second Pay Limestone" of the Bahrain 

field, and it produces oil in parts of the Burgan field

of Kuwait. In the Basrah area these limestones as 

a rule are barren of oil.

RUTBAH SANDSTONE FORMATION 

TYPE SECTION, THICKNESS, AND EXTENT

The type section of the Rutbah Sandstone Formation is at the northeast end of a promontory lying 

between the Wadi Ubeila and Rutbah, and about 4 

kilometers northwest of the settlement of Rutbah, 

after which the formation is named. The base of the 

formation is at approximately lat 33 0 04:'20" N., long 

40°12'50" E.

The formation has a thickness of 23 meters, according to Dunnington, Wetzel, and Morton (in Bellen and 

others, 1959). This formation is widely exposed 

around Rutbah and as isolated outliers overlying the 

Mulussa limestone plateau to the east of Rutbah. It 

occurs also in the Wadi Hauran, east and west of 

Muhaiwir. The Rutbah Sandstone Formation transgresses an extensive eroded surface which exposes the 

Bathonian Muhaiwir Formation in the eastern Wadi 

Hauran and Ga'ara Sandstone of probable Middle 

Triassic age in the western part of the Ga'ara depression area. The formation pinches out abruptly to the 

north and northwest between Ga'ara Sandstone and 

the M'sad Formation. It thickens considerably to the 

east where it overlies the Bathonian Muhaiwir Formation and is unconformably overlain by Miocene limestone in the vicinity and east of Muhaiwir.

The Rutbah Sandstone Formation at one time was 

considered to be of Aquitanian (Oligocene) age in the 

Muhaiwir area, the confusion arising through misdating of the coral fauna of the Muhaiwir Formation 

as Oligocene. The name "Hauran sandstone" (Hauran Quartzite) was informally applied to the sandstone 

during this period of misunderstanding, and, though 

never defined or published, this name has been fairly 

widely used for sandy sediments, which occur below 

the lower Miocene limestones in other areas. The unpublished "Hauran sandstone" is a junior synonym 

for the Rutbah Sandstone Formation; and its use is 

now abandoned; it should not be confused with the 

"Zor Hauran Formation", which is now formally defined as the rock unit which intervenes between the 

(Liassic?) Uba'id Formation and the (Upper Triassic?) Mulussa Formation, in the Wadi Hauran, below 

the Rutbah Sandstone Formation, west of Muhaiwir.

The name Rutbah (or Rutba) sand (or sandstone) 

was formerly applied to the Nahr Umr Formation of 

southern Iraq, and also to those units of the Kuwait 

succession which are now collectively designated as the 

Burgan subgroup (Owen and Nasr, 1958). For the 

most part, such usage was confined to unpublished
G22 GEOLOGY OF THE ARABIAN PENINSULA

reports of oil companies, but Barber (1948) included 

in his publication a stratigraphic table for the Kuwait 

succession which groups the Ahmadi, Wara, Mauddud, 

and Nahr Umr Formations of current classifications as 

subdivisions of the Rutbah. sand.

DETAILED LITHOLOGIC DESCBIPTION

Varicolored, white and ferruginous coarse to fine 

sands and sandstones, locally cemented to quartzites, 

make up the formation. Basal parts are possibly of 

continental origin; the upper parts are marine. No 

diagnostic fossils have been found.

AGE

The Rutbah Sandstone Formation is pre-Cenomanian or early Cenomanian, as it is overlain by the upper 

Cenomanian M'sad Formation. It is Upper Triassic 

to post-Triassic or Upper Triassic, as it is underlain 

unconformably by Upper Triassic Mulussa Formation. 

Its age is not ascertained within these units at the type 

locality.

CONTACTS AND EQUIVALENT UNITS IN ADJACENT AEEAS

The Rutbah is underlain by the Mulussa Formation; 

the contact is unconformable, erosional, and discordant. 

Overlying the Rutbah is the M'sad Formation, with 

transitional, gradational contact which is placed at the 

base of the first definite limestone bed above the continuous sandstone of the Rutbah sandstone.

According to Dunnington, "Wetzel, and Morton (in 

Bellen and others, 1959), the Rutbah Formation is 

homotaxial with the "gres lignitifere" of Zumoffen 

(1926) and with the so-called Nubian sandstone of 

the Lebanon and other places. It has been loosely 

equated with the Nahr Umr Formation of the subsurface sections, with which it may be continuous, but 

it is much more probable that these two units are 

genetically distinct and that the Rutbah Sandstone 

Formation overlies the eroded outcrops of the Nahr 

Umr in the area between Muhaiwir and Awasil.

Whereas the Nahr Umr Formation is considered to 

be of early Albian age, grading upward into the late 

Albian Mauddud Formation, the Rutbah is probably 

of Cenomanian age at its type locality, where it is 

overlain by the upper Cenomanian M'sad Formation. 

The M'sad Formation is probably approximately correlative in age with the Mahilban Limestone of the 

Awasil area, and the Rutbah Sandstone Formation at 

Rutbah is probably the same age as part of the hiatus 

between the Mauddud and Mahilban Formations at 

Awasil. This interpretation is preferable to the earlier 

held concept that the Rutbah and Nahr Umr Formations compose a single diachronous sandstone deposit.

The Rutbach may also be equated with the Wasia 

Formation of Saudi Arabia.

ECONOMIC ASPECTS

The Rutbah Sandstone Formation forms a good 

aquifer in western Iraq, where a number of Iraq 

Petroleum Company water wells along the pipeline 

draw their water from this formation.

AHMADI FORMATION 

EEFEEENCE SECTION, THICKNESS, AND EXTENT

The reference section of the Ahmadi Formation is 

in BPC well Zubair No. 3 (lat 30°23'01" N., long 

47°43'29" E., alt 51.9 ft, completed Feb. 21, 1951) 

between drilled depths of 8,070 and 8,517 feet (447 ft 

thick). The formation is widespread in the subsurface sections of southwestern Iraq in the wells of the 

Nahr Umr, Rumaila, and Zubair fields and in wells 

Tuba No.l, Ratawi No.l, Rachi No.l, Luhais No.l, 

Samawa No.l, Kifl No.l and 2, Shawiya No.l, Ubaid 

No.l, Safawi No.l, and Ghalaisan No.l. Thus far the 

greatest thicknesses of the formation are in the Basrah 

area and are between 400 and 550 feet. This thickness 

trend extends to Samawa well No.l (437 ft thick) and 

almost to Luhais well No.l (382 ft thick). Over Ash 

Shawiyat and Ubaid areas the thickness decreases to 

about 300 feet. In the extreme west at wells Safawi 

No.l and Ghalaisan No.l, recorded thicknesses are 185 

and 249 feet, respectively. At Kifl well No.l a thickness of only 56 feet is recorded.

DETAILED LITHOLOGIC DESCEIPTION

The Ahmadi Formation at the reference subsurface 

section can be subdivided into three lithologic units, 

which are from top to bottom:

Thickness 

(feet)

3. Shale, greenish-gray, fissile, ostracode-bearing, interbedded with gray ostracode-bearing marl and streaks 

of arenaceous detrital limestone containing Cythereis sp. and Cytherella sp. ________________ 163

2. Limestone, gray, marly with streaks of green shale

(Tuba Limestone Member) _______________ 177

1. Shale, light-blue to gray, pyritic, fissile, with streaks 

of gray soft locally calcareous marls containing 

Haplophragmoides, Cythereis sp., and Cytherella 

sp. 107

A similar sequence has been recognized in wells 

Luhais No.l and Samawa No.l. However, this formation changes its character westward where it becomes 

more like the Wara Formation. At Safawi well No.l 

it becomes predominantly red, brown, and green fissile 

somewhat silty shale containing glauconite toward the 

base. Northward in the Kifl area, the sequence is 

highly dolomitized and predominantly calcareous.
SOUTHWESTERN IRAQ G23

Glauconitization at the base of Ahmadi may be indicative of shallowing, and a possible unconformable contact with the underlying Mauddud Formation was 

thus postulated. Southward the Ahmadi Formation 

has been considered to grade laterally into the Wara 

Formation of Kuwait, and northward into the Rutba 

Sandstone Formation.

The neritic Tuba Limestone Member in the Basrah 

area increases in thickness eastward toward the basin 

by replacing the shaly components, and the f acies becomes more and more Oligosteginal till it cannot be 

distinguished from that of the Eumaila. In the more 

basin ward areas the upper shales of the Ahmadi merge 

into the neritic Mishrif Formation.

DIAGNOSTIC FOSSILS AND AGE

Turritella spp., Corbula sp., Exogyra conica (J. 

Sowerby), Exogyra luynesi (Lartet), Neolobites sp., 

Parasmilia sp., Metoicoceras sp., Aspidiscus (Helladastraea) juv. cf. Aspidiscus semhae Kossmat, 

and species of Haplophragmoides, Fldbellina, Vaginulina, AmmoboLGulites, Gunibelina, Lenticulina, Frankeina, PlaplopTiragmium have been recognized at most 

places, but elsewhere Praealveolina cretacea (d'Archiac), Pseudocyclammina cf. P. hedbergi Maync and 

Meandro psina sp. have been recognized and indicate 

Mishrif influence.

In 1956, Sayyab described a suite of ostracodes from 

the Ahmadi Formation of the Zubair well No. 5. This 

suite includes C. ovata Roemer and a large and characteristic assemblage of undescribed new species, including some new genera.

The formation is Cenomanian; Sayyab (1956) 

favors Albian age for this formation on the basis of 

the ostracode fauna, and W. Sugden (unpub. data, 

1958) argues for late Albian age for the presumably 

correlative lower parts of the Khatiyah Formation of 

Qatar.

In most of the wells in southern Iraq, the Ahmadi 

Formation lies unconformably over the Mauddud 

Limestone, although it is underlain by Wara Formation in places; however, recognition of the Wara in 

these wells is difficult. The base of the Ahmadi Formation is placed at the base of a shale (at times a 

gray dense limestone), the M/l marker. The Ahmadi 

is overlain by the Rumaila Formation, with which a 

conformable and intergraduational contact has been 

postulated. Equivalent units in adjacent areas are 

the Khatiyah, Wasia, Rutbah Sandstone, and Wara 

Formations, in part.

ECONOMIC ASPECTS

The Ahmadi Formation constitutes the caprock that 

retains oil within the underlying sands of the Burgan

Subgroup in the Kuwait oil fields. In the Basrah oil 

fields area of southern Iraq, although the formation is 

present, there is not thus far any indication that it 

functions as a caprock. The Ahmadi here is the cause 

of a great deal of caving in drilling wells, if a wellcontrolled drilling-fluid system is not maintained.

RUMAILA FORMATION

TYPE SECTION, THICKNESS, AND EXTENT

The type section of the Rumaila Formation is in 

BPC well Zubair No. 3 (lat 30°23'01" N., long 47° 

43/29// E.; alt 51.9 ft; completed Feb. 21, 1951), where 

is lies between drilled depths of 7,605 and 8,070 feet 

(465 ft thick). This formation has widespread distribution in southwestern and central Iraq, and owing 

to facies changes at the expense of its thickness the 

formation varies considerably from one place to another in the oil fields of southern Iraq. Generally, it 

ranges in thickness between 140 and 500 feet. Except 

for local variations, the general thinning is gradual 

and westward. At wells Samawa No.l and Shawiya 

No.l the thickness decreases to slightly more than 200 

feet. In the south between wells Ubaid No.l and 

Safawi No.l, the thickness ranges between 170 and 

180 feet. Westward the thickness increases slightly, 

as compared with the thickness at wells Samawa No.l 

and Shawiya No.l, to 239 feet at Ghalaisan well No.l. 

At Kifl well No.l the equivalents of this formation 

attain the exceptional thickness of about 900 feet.

DETAILED LITHOLOQIC DESCEIPTION

Lithologically the Rumaila Formation can be subdivided into two units as follows:

Thickness 

(feet)

2. Marl, buff to brown, chalky, foraminiferal, oligosteginal; intercalations of crystalline leached-out limestones ; chalky to dense limestones; shale streaks 265

1. Limestone, cream-colored, dense, cryptocrystalline; 

Oligosteginal; some is marly with some streaks of 

shale _____________________________ 200

The lithology of this formation undergoes considerable lateral and vertical variation, depending on the 

interdigitation of chalky marls and limestones. One 

such manifestation is observed in wells Kifl No.l and 

Awasil No.5, where the formation has been subdivided 

into three formations Mahilban, Fahud, and Maotsi. 

The Rumaila interdigitates with the overlying Mishrif 

Formation and the underlying Ahmadi Formation, 

and in the Rutba area it is postulated to merge into 

the exposed M'sad Formation.

DIAGNOSTIC FOSSILS AND AGE

Diagnostic fossils in the Rumaila Formation are 

Oligostegina Globigerina spp., Orbitolina conea/oa var.

232-387 67-
G24 GEOLOGY OF THE ARABIAN PENINSULA

qatarica Henson, Gumbelina sp., Nummoloculina sp., 

Begia sp., miliolids, valvulinids, and rotalids. The formation is Turonian and Cenomanian in age.

CONTACTS AND EQUIVALENT UNITS IN ADJACENT AREAS

The Ahmadi Formation underlies the Rumaila Formation conformably. The contact is placed at the top 

of the black fissile ostracode-bearing shales. The Rumaila may rest directly on the Mauddud limestones in 

central Iraq. It is overlain by the Mishrif Formation; 

the contact is conformable, diachronous, and intergradational at the top of Oligostegina-\)Q&T:'mg limestone. 

In the area of wells Samawa No. 1, Ghalaisan No. 1, 

and Kifl No. 1, the Rumaila Formation is directly 

overlain by the lagoonal evaporitic Kifl Formation.

The Rumaila Formation is composed of subbasinal 

f acies and may pass into the more basinal Dokhan f acies. In facies and age the Rumaila Formation is 

equivalent to the Mahilban, Fahud, and Maotsi Formations of central Iraq. M. Chatton (unpub. data) 

recommended adoption of the name Rumaila Formation to replace the three formation names. This formation is not differentiated from the Mishrif Formation 

in wells of southeastern Kuwait, where these two formations are replaced by the Magwa Formation. The 

Magwa Formation has been eroded from the crestal 

areas of the Burgan, Magwa, and Ahmadi structures, 

but occurs in progressively increasing thicknesses down 

the flanks.

KIFL FORMATION

The Kifl Formation has been defined by M. Chatton 

(unpub. data) to include the evaporite facies of the 

Cenomanian-lower Turonian sedimentary cycle in 

southern Iraq.

TYPE SECTION, THICKNESS, AND EXTENT

The type section of the Kifl Formation is in BPC 

well Kifl No. 1, (Iat44°ll'14.55" N. long 32°15/08.04/ ' 

E.; alt 141 ft; completed July 8, 1960). The formation lies between drilled depths of 4,738 and 4,830 

feet (92 ft thick).

The unit has been found in the BPC wells Ubaid 

No. 1, Shawiya No. 1, Samawa No. 1, Ghalaisan No. 

1, Kifl No. 2, and the Mosul Petroleum Co. wells at 

Nafatah and Awasil. The distribution of this formation is not limited to the region already mentioned but 

extends to other parts of the country. In southern 

Iraq, however, the formation passes laterally by interdigitation into the Mishrif Formation, within a 

zone extending from east of Ubaid to Shawiya and 

Samawa. Northeastward the two facies are interbedded. At Kifl itself, Kifl evaporites indicate downward gradation into the Rumaila chalks.

DETAILED LITHOLOGIC DESOBIPTION, FOSSILS, AND AGE

The Kifl Formation is predominantly white crystalline anhydrite with streaks of green marl and chalky 

pelletal limestone locally having an anhydritic matrix; 

cream-colored porcelaneous anhydritic marly limestone 

occurs at the base. The Kifl contains uncommon miliolids and textularids. Its age is probably late Cenomanian and early Turonian.

CONTACTS WITH ADJACENT UNITS

The Kifl is underlain by the Rumaila Formation 

(formerly called Maotsi Formation); the contact is 

conformable, gradational, and is placed at the change 

from continuous white, creamy Oligostegina-be&ring 

chalky limestone below to anhydritic marly limestone. 

The Khasib Formation overlies it. The contact is unconformable, with a sedimentary break, and is placed 

at the change from anhydrite below to shale above. 

No equivalent units in adjacent areas are recorded.

ECONOMIC ASPECTS

The Kifl Formation has no known economic significance ; it may induce trapping of oil in the porous section of the Mishrif Formation.

M'SAD FORMATION 

TYPE SECTION, THICKNESS, AND EXTENT

The type section of the M'sad Formation is in Wadi 

M'sad al Rutbah, which runs due north for 20 miles 

from Jebel Tarayat (lat 32°46' N., long 40°lY' E.) to 

join the Wadi Hauran at Rutbah (lat 33°02' N., long 

40°07' E.). The formation is about 211 feet thick. It 

is widely exposed in the area north of Rutbah; sections have been measured at An Nadhara, Khasm Mulussa, Wadi Semhat, Ras Semhat, Rutbah, Wadi Mulussa, and "Hill 2TO."

The occurrence of sands and sandy limestones within 

the unit is a feature of this heterogeneous formation 

in the area of the type locality. One thin sand bed, 

remarkably constant over the area of exposure, separates the main, limestone-dominated upper part of the 

formation from the thin, basal limestone unit.

DETAILED LITHOLOGIC DESCRIPTION"

The M'sad Formation consists of alternating shallow 

marine limestones, shell breccias, microdetrital limestones, chalky buff and white limestones, pink marls, 

and sandy marls and sands; a thin sandstone tongue 

occurs near the base.

In the vicinity of the type locality the M'sad Formation grades conformably into and perhaps interdigitates laterally with the underlying Rutbah Sandstone 

Formation. It underlies an erosional unconformity 

H. H. Boesch, (unpub. data) over which the Maestrich-
SOUTHWESTERN IRAQ G25

tian Tayarat Formation transgresses. The Turonian, 

lower Senonian, and Campanian Stages are not represented by sediments in the Rutbah-Jebel Tayarat area, 

though formations of these ages enter the succession in 

the area to the east, southeast, and south.

DIAGNOSTIC FOSSILS AND AGE

No fossils are recorded from the type locality of the 

M'sad Formation. The limestones of this unit elsewhere carry a rich if ill-preserved macrofauna from 

which Nerinea cochleaeformis Conrad was identified 

by A. Keller and H. H. Boesch (unpub. data). Rudists, including Caprinula sp. and Eoradiolites liratus 

Conrad, were also recorded by A. Keller (unpub. data) 

but have not been confirmed in postwar collections. 

Recent collections, identified by R. G. S. Hudson and J. 

Robinson, include Nerinea cretacea Conrad and N. cf. 

N. gemmifera Conrad, indicating Cenomanian age, 

probably late Cenomanian.

The associated microfauna, which is represented 

throughout the formation, both above and below the 

thin sandstone previously mentioned, includes Meandropsina cf. M. vidali Schlumberger, Cuneolina cf. C. 

cylindrica Henson, Dicyclina cf. D. qatarica Henson, 

rare Praealveolina sp., Begia spp., Pseudochrysalidina 

conica (Henson), and Taberina cf. T. bingistani (Henson). The microfauna is probably of late Cenomanian 

rather than early Cenomanian age and is unlikely to 

be younger than Cenomanian.

The age of the formation has not been ascertained at 

the type locality. Elsewhere in the vicinity of the section the formation is middle Cretaceous, early Cenomanian at the base and probably early Cenomanian 

throughout.

CONTACTS AND EQUIVALENT UNITS IN ADJACENT AREAS

The underlying unit of the M'sad Formation is the 

Rutbah Sandstone Formation; the contact is gradational and conformable and is placed at the base of the 

lowest limestone bed above the continuous Rutbah.

The M'sad Formation is overlain by the Tayarat 

Formation; the contact is unconf ormable owing to an 

erosional break in sequence which omits sediments of 

Turonian to intra-Maestrichtian ages. The contact is 

placed at the base of the localized basal conglomerate 

of the Tayarat Formation.

On the basis of age, microfauna, and microfacies, 

the M'sad Formation is considered to be correlative 

with the Mahilban limestone of the Awasil area and 

the Mishrif Formation (part) of southern Iraq, from 

both of which it may be distinguished by its heterogeneity. Distinction will be difficult in areas where 

the sand and marl components are subordinate, and in

intermediate areas the name M'sad and Mahilban Formation may be necessary.

MISHRIF FORMATION 

TYPE SECTION, THICKNESS, AND EXTENT

The type section of the Mishrif Formation is in 

BPC well Zubair No. 3 (lat 30°23'01" N., long 47° 

43'29" E.; alt 51.9 ft; completed Feb. 21, 1951). The 

formation is between drilled depths of 7,204 and 7,605 

feet (401 ft thick).

In the Basrah fields this formation ranges between 

350 and 550 feet in thickness. Its eastern extension is 

unknown. Thickness trends follow a northerly direction, as if the Mishrif were deposited over a shoal extending basinward from the Arabian shield across the 

Kuwait-Basrah region, bordered on the east by the 

main Balambo basin and on the west by a subsidiary, 

shallower "Rumaila" basin. Thus, geographically, 

rocks of Mishrif type may be restricted to a narrow 

belt. To the west the Mishrif is only a few feet thick 

at wells Samawa No. 1 and Safawi No. 1 and is absent 

at Ghalaisan well No. 1.

According to Dunnington, Wetzel, and Morton (in 

Bellen and others, 1959), in the Rutbah area of the 

western desert the Mishrif is probably represented in 

outcrop by the correlative M'sad Formation which carries a microfauna similar to that of the Mishrif Formation. The rudist Eoradiolites liratus Conrad, reported here by A. Keller, is believed to occur also in 

the subsurface Mishrif Formation (W. Sugden and E. 

Hart, unpub. data, 1957). The Mishrif Formation is 

not represented in southeastern Kuwait, where equivalent sedimentary rocks form the upper part of the 

Magwa Formation.

DETAILED LITHOLOGIC DESCRIPTION

The Mishrif facies, which is thickly deposited 

throughout the Basrah area, consists of neritic sedimentary rocks, mostly limestones and, in places, shales. 

The limestones are variable in texture and may be 

oolitic or pelletal, shelly, and coquinoid types which 

contain algae, rudists, bryozoans, corals, and miliolids; 

in places there are indications of offshore deposition.

The following is a detailed lithologic sequence in 

the type section, based on reinterpretation of the electric logs:

TMclcness 

(feet)

8. Limestone, fine-grained, limonitic, fresh-water, con 

taining Charophytes and marl; interbedded with 

black shale having some light-buff streaks 50

7. Limestone, white-buff, fine-grained, fractured or stylolitic, marly, partly pseudo-oolitic and microbreccious 

with streaks of greenish-black shale and pale-buff 

marl toward base ____________________ 26
G26 GEOLOGY OF THE ARABIAN PENINSULA

6. Marl, whitish-buff, algal, and greenish-black shale 12 

5. Limestone, brown and white, dense, detrital, containing gastropods, rotalids, and sponge spicules; partly 

pseudo-oolitic and contains rare chalky streaks and 

thin green shale intercalations _____ _ 68 

4. Limestone, cream to white or brownish-buff, porous, 

partly very shelly and foraminiferal; contains bands 

of rudists, Cisalveolina sp., Begia sp., Dicyclina sp., 

Dictyoconus sp., miliolids, and others ________ 136 

3. Marl, white to buff, chalky, oil-stained in places____ 16 

2. Limestone, porous, oil-stained, brown to light-buff; 

leached-out fossils; contains calcite veins and floods 

of miliolids, Begia sp., Cisalveolina sp., Dicyclina 

sp. Taberina sp., Praealveolina sp. and others ___ 58 

1. Limestone, buff to brown, algal, contains miliolids or

Cisalveolina sp.; very dense at base _________ 35

Total 401

At Samawa well No.l the formation is very thin 

and barely recognizable, consisting of 32 feet of tight, 

fine dolomite containing some miliolids.

According to Dunnington, Wetzel, and Morton (in 

Bellen and others, 1959) the Chara limestone which 

occurs at the top of the Mishrif in near-crestal wells 

in the Zubair field is missing in most other areas. The 

limonitization of the upper beds of the Mishrif and the 

abrupt facies change from algal limestone to globigerinal marly limestones strongly suggests nondeposition and an emergent episode, with erosional termination of the Mishrif. On the other hand, close correlation between wells and fields in the Basrah area does 

not confirm any significant erosional convergence below 

the base of the Khasib Formation. In Kuwait, however, the Mishrif and Kumaila Formations and the 

upper parts of the older Ahmadi Formation were 

eliminated at the erosional unconformity which preceded the Upper Cretaceous transgression, and the hiatus narrows, significantly, to the limonitized surface 

atop the Mishrif as the Basrah area is approached 

(Owen and Nasr, 1958, fig. 5).

DIAGNOSTIC FOSSILS AND AGE

Fossils found in the Mishrif Formation include 

Chara sp., Permocalculus ireane Elliott, Cisalveolina 

sp., C. fallax Keichel, C. lehneri Keichel, Praealveolina, 

cretacea (d'Archiac), P. cretacea var. tennis Keichel, 

MuUispirina iranensis Keichel, Dicyclina qatarica Henson, Taberina bingistani (Henson), PseudocJirysalidina 

conica (Henson), Begia spp., Coxites zubairensis 

Smout, and TrocJiolina spp.

Recent stratigraphic work in Iraq by M. Chatton 

and E. Hart (unpub. data) dates the Mishrif Formation as Cenomanian and early Turonian. Owen and 

Nasr (1958) attributed this formation to the Turonian; 

however, according to Dunnington, Wetzel, and Morton (in Bellen and others, 1959) :

A. H. Smout (1956) has described numerous species of the 

foraminiferal genus, Begia, from the Mishrif formation of the 

Basrah area. He argues for Turonian age for the rich faunas 

of the lower part of the unit, but the Alveolinidae are of Cenomanian rather than Turonian age in Iran, and the rudist 

evidence from Rumaila wells and macrofossil evidence as to 

age of the correlative M'sad Formation are strongly in favour 

of a Cenomanian attribution also. The upper part of the 

formation could be Turonian, but the Permocalculus flora near 

to the top of the unit matches that from the Mahilban Formation of the Awasil area (G. F. Elliott, unpub. data) which is 

separated by unconformity from the presumably Turonian Fahad Limestone.

CONTACTS AND EQUIVALENT UNITS IN ADJACENT AREAS

The contact of the Mishrif Formation with the underlying Rumaila Formation is conformable and is placed 

at the change from oligosteginal-globigerinal limestone 

below to neritic limestones, with miliolids, Begia spp., 

and larger Foraminifera above; thus, the contact is 

one of normal lateral and vertical gradation. The contact with the overlying Khasib Formation is disconforniable, owing to a considerable sedimentary hiatus 

(Bellen and others, 1959). The Mishrif Formation 

commonly is conformably overlain by a thin evaporitic 

lagoonal Kifl Formation, as in some of the western 

desert subsurface sections. In the extreme west, for 

example at Safawi well No.l, where the Khasib is absent, the Tanuma shales rest directly but unconformably on a few feet of the Mishrif Formation.

The Mishrif Formation may be equated with at 

least the upper part of the Wasia Formation of Saudi 

Arabia, though the Wasia is a different facies.

ECONOMIC ASPECTS

According to staff members of the Iraq Petroleum 

Company (oral commun., 1956), medium oil (26° API) 

can be produced from the limestones of the Mishrif 

Formation in the Zubair and Rumaila oil fields of the 

Basrah Petroleum Company, where it constitutes what 

is called the "Second Pay."

KHASIB FORMATION

TYPE SECTION, THICKNESS, AND EXTENT

The type section of the Khasib Formation is in BPC 

well Zubair no. 3 (lat 30°23'01" N., long 47°43'29" E.; 

alt 51.9 ft; completed Feb. 21, 1951). The formation 

lies between drilled depths of 7,040 and 7,204 feet (164 

ft thick). It has been found in all subsurface sections 

in southern and western Iraq except in well Safawi 

No.l. To date (1964) these sections include wells in the 

Nahr Umr, Zubair, and Rumaila fields; in the wells 

Rachi No.l, Ratawi No.l, Luhais No.l, Samawa No.l, 

Kifl No.l and 2, Ubaid No.l, Ghalaisan No.l, and 

Shawiya No.l; and in the wells in northeastern Kuwait. The formation was not identified, however, in
SOUTHWESTERN IRAQ G27

southeastern Kuwait, where it probably has equivalents 

in the lower part of the Gudair Formation that are

 complete. The Gudair thins toward the structural 

uplift of Burgan, and equivalents of the Khasib and 

Tanuma Formations are probably eliminated by progressive overlap (Owen and Nasr, 1958). The thickness distribution and variation is complicated by 

facies interrelation with the overlying Tanuma Formation. The Khasib thickens basinward to the northeast to as much as 300 feet, whereas a belt, 200 feet 

thick, may follow the Euphrates River very closely 

between wells Kifl No.l and Nahr Umr No.3. In the 

Basra fields the thickness ranges between 17 and 195 

feet. At wells Ratawi No.l and Rachi No.l thicknesses of 140 and 118 feet respectively, were recorded. 

Westward, at wells Ubaid No.l and Ghalaisan No.l, 

the thickness decreases to 95 and 77 feet, respectively, 

and in Safawi well No.l the formation's identity is

 completely lost.

DETAILED LITHOLOGIC DESCRIPTION

Lithologically, the type section of the Khasib Formation can be subdivided into two units as follows:

Thickness 

(feet)

2. Limestone and chalk, light-gray, very marly, slightly

dolomitic, with subordinate streaks of black shale _ 95

1. Shale, dark-gray or greenish-gray, fissile, alternating 

with globigerinal-gumbelinid and dolomitic fissile 

buff marly limestones _________________ 69

DIAGNOSTIC FOSSILS AND AGE

Fossils found in the Khasib Formation are Globigerina sp., Gumbelina, spp., and Oligostegina. Sayyab 

(1956) identified three new ostracode species from the 

Khasib Formation of Zubair well No. 5. Other microfossils, which include Globotruncana lapparenti subspp. and G. leupoldi Bolli from Zubair well No. 1 and 

G. stuarti (de Lapparent) from Nahr Umr well No. 1, 

were recognized in this formation by Dunnington, Wetzel, and Morton (in Bellen and others, 1959). The 

Khasib Formation is Senonian (early?), according to 

Owen and Nasr (1958).

Since Owen and Nasr did not recognize any break 

within the Mishrif Formation, or within the overlying 

sequence of the Khasib, Tanuma, and Sa'di Formations, the interpretation that the Khasib-Mishrif contact involves no hiatus in sedimentation implies an 

older age than late Campanian for the base of the 

Khasib. The Mishrif Formation is considered to be 

not younger than Turonian (Owen and Nasr indicated 

Turonian age, but Cenomanian age for at least the 

lower part of the Mishrif was argued by others. See 

section on "Mishrif Formation.") Recently, however, 

M. Chatton and E. Hart (unpub. data) established the

age of this unit as middle Turonian and early Coniacian.

CONTACTS AND EQUIVALENT UNITS IN ADJACENT AREAS

The contact of the Khasib Formation with the 

underlying Mishrif Formation is disconf ormable, being 

between oligosteginal shales above and limonitic limestones with Charophytae below. In Ubaid No. 1 and 

Kifl No. 1 and 2, the Khasib Formation rests directly 

on the Kifl evaporites. Owen and Nasr (1958) indicated that the contact between the Khasib and the 

underlying Mishrif is disconformable in the Basrah 

area and unconformable on structural highs in Kuwait.

In the Basrah area this unconformity is represented 

by a thinning of sediments and by a fresh-water limonitic tfAara-bearing limestone at the top. This limestone is overlain conformably by the transgressive 

Khasib Formation of the Aruma Group. The contact with the overlying Tanuma Formation is conformable at the change from black fissile shales above to 

gray marly limestones below.

In northeastern Iraq the Khasib passes into the 

basinal lower limestones of middle Turonian and lower 

Coniacian Kometan Formation and Gulneri shales. It 

is represented in southeastern Kuwait in the lower part 

of the Gudair Formation.

ECONOMIC ASPECTS

The Khasib Formation has no economic value. It 

causes some trouble in drilling wells by caving and 

sloughing.

TANUMA FORMATION 

TYPE SECTION, THICKNESS, AND EXTENT

The type locality of the Tanuma Formation is in 

BPC well Zubair No. 3 (lat 30°23'01" N., long 47° 

43'29" E.; alt 51.9 ft; completed Feb. 21, 1951). The 

formation lies between drilled depths of 6,900 and 

7,040 feet (140 ft thick). It occurs in all the following 

deep subsurface sections in southwestern Iraq: wells in 

the Zubair, Rumaila, and Nahr Umr fields and Ratawi 

No.l, Luhais No.l, Ubaid No.l, Samawa No.l, Shawiya No.l, Safawi No.l, Ghalaisan No.l, and Kifl Nos. 

1 and 2. According to Dunnington, Wetzel, and Morton (in Bellen and others, 1959), this formation has 

not been formally recognized in southeastern Kuwait, 

where equivalents are probably represented by the lower part of the Gudair Formation, though cut out by 

onlap convergence in areas of structural uplift; however, it is recognized in northeastern Kuwait (Fox, 

1957) in a stratigraphic position similar to that of 

the type area. In southwestern Iraq the greatest thickness of Tanuma occurs along a trend extending from 

wells Rachi No.l (190 ft) to Ratawi No.l (170 ft),
G28 GEOLOGY OF THE ARABIAN PENINSULA

Samawa No.l (163 ft), and Kifl No.l (130 ft). Westward from the Basrah area there is gradual thinning 

of the formation from wells Ubaid No.l (54 ft) to 

Shawiya No.l (49 ft), except at Safawi No.l (83 ft) 

where an observable thickness anomaly is due to a complete change of facies of the underlying Khasib Formation. Northwestward there is appreciable thinning 

toward Ghalaisan well No.l (15 ft).

DETAILED LITHOLOGIC DESCRIPTION

The Tanuma Formation consists of black shale with 

streaks of detrital limestone. The shales are fissile and 

the limestones are gray, marly, microcrystalline, pyritespotted, and detrital; glauconite and dolomite crystals occur throughout. An oolitic limestone streak 

appears in the upper part of the formation; the ooliths 

have a core of pyrite or glauconite.

DIAGNOSTIC FOSSILS AND AGE

In the type locality of the Tanuma Formation the 

detrital limestones contain Monolepidorbis sp., Cristellaria sp., ostracodes, and bryozoans. A. S. Sayyab 

in 1956 described some ostracodes, mostly new species, 

from the Tanuma Formation of Zubair well No. 5. 

The age of this formation was considered as late Senonian by Owen and Nasr (1958), but the fauna, according to Dunnington, Wetzel, and Morton (in Bellen 

and others, 1959), seems to justify a late Campanian 

age. Kecently, however, M. Chatton and E. Hart 

(unpub. data) have considered the age of this formation as late Coniacian. Here it should be remembered 

that because of the diachronous sedimentation of the 

formation one may expect extension of the total age 

limits.

CONTACTS AND EQUIVALENT UNITS IN ADJACENT AREAS

The Tanuma Formation is underlain by the Khasib 

Formation; the contact is conformable and is placed 

at the change from black fissile shales above to gray 

marly limestones below. The contact with the overlying Sa'di Formation is conformable and gradational 

at the change from globigerinal marly limestone above 

to black fissile shales below. The Tanuma Formation 

is recognized in the northern Kuwait subsurface sections. To the southeast it passes into the Gudair Formation. The Tanuma may be correlated with one of 

the Aruma shales in Saudi Arabia.

ECONOMIC ASPECTS

The Tanuma Formation may be good caprock for 

the trapping of oil in the Mishrif limestones and to a 

lesser extent in porous parts of the Khasib Formation. 

Owing to the tendency of the shales to cave, there may 

be considerable difficulty in drilling operations.

SA'DI FORMATION 

TYPE SECTION, THICKNESS, AND AGE

The type section of the Sa'di Formation is in BPC 

well Zubair No. 3 (lat 30°23'01" N., long 47°43'29" E.; 

alt 51.9 ft; completed Feb. 21, 1951). The formation 

lies between drilled depths of 6,013 and 6,900 feet (887 

ft thick). No outcrop in southwestern Iraq has so far 

been designated as Sa'di Formation, although the unit 

reaches a thickness of 1,279 feet and widespread subsurface distribution, which seems to be in the Rumaila 

area. Wesward, this thickness decreases to from 500- 

to 600 feet between wells Samawa No. 1 and Ubaid 

No.l. Further decreases in thickness to 400 ft, were 

observed near the Iraqi-Saudi Arabian border. A similar decrease occurs northward, where at Kifl well 

No. 1. Further decreases in thickness to 400 ft were 

thickening is recognized at Awasil well No.5 (475 ft) 

and at Nafatah weU No.l (575 ft). The Sa'di Formation, because of nondeposition and erosional cutout, is 

not represented in the general Ga'ara-Rutbah area. 

The true thickness variation cannot be evaluated realistically without paleogeographic and paleogeologic 

reconstruction of the conditions which prevailed during 

and after the formation of the Sa'di sediments.

DETAILED LITHOLOGIC DESCRIPTION

The Sa'di Formation can be subdivided into two 

units in the type locality as follows:

Thickness 

(feet)

2. Limestone, white to buff, chalky, marly, slightly dolomitic, fossiliferous with fossil contents decreasing 

upward; rare green calcareous shale streaks ____ 562

1. Marl, greenish-gray to gray, with rare dolomite crystals and pyrite; globigerinal; ostracode bearing; 

contains some calcareous dark-gray to greenish-gray 

shale intercalations; upper 190 ft of this unit is recognized as Hamrina marl _____________ 325

These general characteristics persist westward to the 

vicinity of Ubaid well No.l, and they are also obvious 

in Kifl well No.l. "Basinwards, as in Nahr Umr No.2 

well," according to Dunnington (1958), "the sequence 

becomes practically all marl with only a few marly 

limestone beds." Southward toward Kuwait the Sa'di 

in Bahra well No.l is between 5,025 and 5,600 feet 

thick, but thins considerably toward Burgan and eventually disappears.

DIAGNOSTIC FOSSILS AND AGE

The Sa'di Formation is very fossiliferous and contains abundant microfauna and flora, especially in the 

lower part. The fossil generaly listed are Bairdia sp., 

Gytfierella sp., Brachythere sp., Nodosaria spp., Gldbigerina spp., O. cretacea d'Orbigny, Oligostegina, Textularia sp., Palmula sp., Marginulina sp., Gristellaria
SOUTHWESTERN IRAQ G29

«p., Globotruncana angusticarinata Gandolfi, G. forni-

 cata Plummer, G. lapparenti lapparenti Brotzen, G. 

lapparenti coronata Bolli, G. sigali Reichel, G. margi- 

.nata (Reuess), and Praeglobotruncana concavata 

(Brotzen). Elsewhere, the list may be augmented by 

the algae Archaeolithamnium digitatum, Pfender Disxocladella undulata (Raineri) Pia, and Trinocladus 

tripolitanus Raineri.

The Sa'di formation is Santonian and early Campanian in age. Owen and Nasr (1958) considered this

 unit to be late Senonian age. Recently, however, M. 

Ohatton and E. Hart (unpub. data), because of the 

'discovery of Praeglobotruncana concavata (Brotzen) 

associated with the other fossils, advocated an age of 

late Coniacian to early Santonian for the Sa'di; the 

upper part of the Sa'di being younger, they maintained 

that its age extends to early Campanian. This may 

well be the case, at least at some localities, owing to 

the diachronic sedimentary history of the formation.

CONTACTS AND EQUIVALENT UNITS IN ADJACENT AREAS

The contact of the Sa'di Formation with the underlying Tanuma Formation is conformable at the top of 

black calcareous shale and at the base of white chalky 

limestone. The contact with the overlying Hartha 

^Formation is an erosional unconformity corresponding to that noted between the Bahra and Gudair Formations in Kuwait (Owen and Nasr, 1958). Westward and southward the subbasinal Sa'di Formation 

passes into the neritic Aruma and Gudair Formations 

in Saudi Arabia and Kuwait.

ECONOMIC ASPECTS

The Sa'di Formation has no known economic value. 

'Owing to the swelling nature of the rock constituents, 

it may cause stuck-pipe problems during drilling op-

 erations.

HARTHA FORMATION

TYPE SECTION, THICKNESS, AND EXTENT

The Hartha Formation type section is in BPC well 

Zubair No. 3 (lat 30°23'01" N. long 47°43'29" E.; alt 

.51.9 ft; completed Feb. 21, 1951). The formation lies 

between drilled depths of 5,590 and 6,013 feet (423 ft 

thick).

The distribution of this formation in the subsurface 

^sections of southwestern Iraq is extensive: Its thickness ranges between 370 and 765 feet in the Basrah 

area; gradually increases westward to 1,048 feet in the 

Ash Shawiya area; and, together with tongues of the 

.Safawi Formation, reaches a maximum of 1,210 feet 

farther west. From the Kifl area northward toward the 

'Ga'ara massif, the Hartha Formation thins out. Owing 

to the absence of the intervening Qurna Formation, it

is difficult in some places to differentiate the Hartha 

Formation from the overlying Tayarat Formation, except possibly by detailed micropaleontologic studies. 

On the east side of the Ga'ara massif the Hartha 

Formation is only 210 feet thick at Nafatah well No.l. 

According to Owen and Nasr (1958), this formation 

thins out toward northern Kuwait but shows little 

facies change. They maintained that this lithologic 

unit has not been precisely delimited in low structural 

wells in Kuwait, and is missing in structurally high 

areas of southern Kuwait, owing to erosion or nondeposition. "In the Burgan-Ahmadi area erratic variation in thickness from 50 to 300 feet suggests that the 

earliest deposits of this formation were laid down in 

hollows eroded at the top of the underlying Gudair 

Formation, and the lithology suggests a transgressive 

phase in this area."

DETAILED LITHOLOGIC DESCRIPTION

The Hartha Formation consists of the following 

lithologic units:

Thickness 

(feet)

4. Limestone, thin-bedded, gray, pyrite-spotted, algal,

glauconitic, alternating with dark-gray fissile shale. 35

3. Limestone, buff to gray, recrystallized, globigerinal, 

algal, with some chalky matrix; passes upward into 

very marly limestone __ _ _ 86

2. Dolomite, light-brown, crystalline, with shale streaks

toward the base ____ 214

1. Marl, light-buff, ostracode-bearing, alternating with 

green calcareous shale which in places passes into 

argillaceous pyritic mottled glauconitic limestones 88

This lithologic sequence varies from place to place 

and changes laterally according to the paleogeographic position of the formation; for example, in the western desert there is a general tendency toward increase 

of dolomite content and interfingering of the Safawi 

anhydrite.

DIAGNOSTIC FOSSILS AND AGE

Fossils recognized in the Hartha Formation are 

Globotruncana cf. G. stuarti (do Lapparent), Cosinella 

sp., Valvulammina sp., Ammobaculites sp., Monolepidorbis sp., Pseudedomia complanata Eames and Smout, 

Brachycythere spp., and Bryozoa. In sections other 

than the type section, the Hartha has yielded Omphdlocyclus macropora (Lamarck) and Orbitoides media in association with Monolepidorbis spp. from close 

to the base of the formation. On the basis of this association, the lower part of the Hartha and the unconformity at its base may be dated fairly closely as being 

of earliest Maestrichtian or latest Campanian age, 

probably the latter.

Elsewhere the following fossils were recognized: 

Archaecyclus midorientalis Eames and Smout, Sid-
G30 GEOLOGY OF THE ARABIAN PENINSULA

erolites heracleae (Ami), Rotalia skourensis Pfender, 

Ogivalia parisiensis d'Orbigny, Dicyclina schlumbergeri Munier-Chalmas, and Cuneolina cylindrica Henson. The formation is Maestrichtian and late Campanian in age.

CONTACTS AND EQUIVALENT UNITS IN ADJACENT AEEA8

The contact of the Hartha Formation with the 

underlying Sa'di Formation is an erosional unconformity in the Basrah area and also in Kuwait, where 

it overlies the Gudair Formation. The contact with 

the overlying Shiranish Formation (formerly Qurna) 

was stated to be disconformable by Owen and Nasr 

(1958), and was placed at the top of white chalky 

glauconitic limestone and at the bottom of soft gray 

marl. This relationship has recently been contested 

by M. Chatton (unpub. data), who believes that the 

contact is conformable. In the extreme west at Safawi 

well No. 1 this formation is overlain conformably by 

the newly defined Safawi Formation. Northwestward, 

where the intervening formation is missing, the Hartha 

is overlain directly by the Tayarat Formation.

In southeastern Kuwait the term Bahra Formation 

has been used to include the combined lateral equivalents of the Hartha and the overlying Shiranish Formations, which are apparently less distinguishable in that 

area than in the northern Iraq fields, or in northern 

Kuwait, owing to the lateral southward passage of the 

Shiranish marls into limestones. The Hartha Formation may have its equivalent within the Aruma Formation of Saudi Arabia.

SAFAWI FORMATION

The Safawi Formation is newly defined to include 

the evaporitic facies of the upper Campanian and 

Maestrichtian cycles of sedimentation. As exploratory 

drilling operations progressed westward from the Basrah area, an appreciable amount of evaporite stringers, 

increasing in thickness westward, was observed in the 

Hartha Formation. In the BPC well Safawi No.l 

these evaporites are so thick that they merit definition 

as a formation.

TYPE SECTION, THICKNESS, AND EXTENT

The type section of the Safawi Formation is in BPC 

well Safawi No. 1, (lat 30°2'45" N. long 43°45'50" E.; 

alt 1,362 ft; completed Oct. 9, 1960). The formation 

lies between drilled depths of 2,058 and 2,290 feet (232 

ft thick).

This unit, thus far, occurs in the subsurface sections 

interfmgering with the Hartha Formation in the wells 

Ghalaisan No.l, Kifl Nos. 1 and 2, Shawiya No.l, 

Ubaid No.l, and Awasil No.5. The occurrence of 

evaporites in a similar stratigraphic position was observed in the well Makhul No.l, and defined as Dibs 

Anhydrite Member of the Pilsener Limestone Formation by Dunnington, Wetzel, and Morton (in Bellen 

and others, 1959).

LITHOLOGIC DESCEIPTION AND AGE

The Safawi Formation is predominantly white, compact hard crystalline anhydrite, with subordinatebrown hard crystalline dolomite and buff to gray soft 

marls. No fossils have been found in it thus far. A 

Maestrichtian and late Campanian age is assigned to> 

the formation on the basis of its stratigraphic relations with the underlying Hartha and overlying Tayarat Formations.

CONTACTS AND EQUIVALENT UNITS IN ADJACENT ABEAS

The lower contact of the Safawi Formation with, 

the Hartha Formation is conformable, and the unit 

grades into Hartha Formation laterally eastward. Thecontact with the overlying Tayarat Formation is not 

well defined owing to the lack of cores; slight unconformity is postulated because of the transgressive nature of the Tayarat.

The Safawi Formation is equated with the Dibs 

Anhydrite Member of the Pilsener Formation in Makhul well No. 1; the formation has not been recorded in 

Saudi Arabia.

SHIRANISH FORMATION

In the Basrah area the Shiranish Formation was. 

formerly known as the Qurna Formation; the synonymy is now recognized.

EEFEBENCE SECTION, THICKNESS, AND EXTENT

The reference section for the Shiranish Formation 

is in BPC well Zubair No. 3, (lat 30°23'01" N., long47°43'29" E.; alt 51.9 ft; completed Feb. 21, 1951). 

The formation lies between drilled depths of 5,202 and 

5,590 feet (388 ft thick). It occurs in all deep subsurface sections in the Basrah area and in Kuwait. 

According to Owen and Nasr (1958), the Shiranish 

Formation ranges in thickness between 250 and 4504 

feet, being thinnest in the Kumaila field. There is a 

general thickening of the formation northwestward, 

and in the Samawa area it attains a thickness of 5504 

feet. Westward from that area it thins to about 450" 

feet in the area of Ghalaisan well No. 1. It passes 

latterally westward into the neritic Tayarat Formation.

DETAILED LITHOLOGIC DESCEIPTION

The Shiranish Formation consists of buff or ash-gray 

globigerinal marl, in some places dolomitic, and somemarly and detrital limestone beds; it is glauconitic 

toward the base and contains a rich microfauna.
SOUTHWESTERN IRAQ G31

According to Owen and Nasr (1958), the Shiranish 

marls in the Basrah area pass in southern Kuwait 

into white to gray dense marly macrocrystalline limestone, which in many places contain gray chert nodules. 

This calcareous facies has not always been distinguished from the ad j acent formations. Similar changes 

have been observed west of Ubaid well No.l; at Safawi well No.l it appears that all the Shiranish marls 

have passed into chalky and marly limestone within 

the Tayarat Formation.

DIAGNOSTIC FOSSILS AND AGE

Fossils found in the Shiranish Formation are Cytherella spp., Bairdia spp., Nodosaria sp., Globotrunoana spp., Cristettaria spp., Gyroidina naranjoensis 

White, Anomalia sp., Marssonella oxycona (Keuss), 

Gaudryina sp., Bolimna inc rassata Keuss, Buliminella 

laems (Beissel), Oibicides beaumontianus (d'Orbigny), 

Bolivinoides draco (Marsson), Textularia cretosa 

Plummer, Gumbelina sp., and others. The formation 

is Maestrichtian in age.

CONTACTS AND EQUIVALENT UNITS IN ADJACENT AEEAS

The Hartha Formation underlies the Shiranish with 

slightly disconformable contact. The overlying Tayarat Formation is conformable and gradational; the 

contact is placed at the junction of dolomitic limestones above and globigerinal marls below. In the 

Kifl area the Shiranish is directly overlain by the 

Aaliji Formation with unconformable contact. In 

southeastern Kuwait the Shiranish Formation passes 

into the Bahra Formation and in Saudi Arabia into 

the Aruma formation.

TAYARAT FORMATION 

TYPE SECTION, THICKNESS, AND EXTENT

The type section of the Tayarat Formation is the 

scarp face and crest of Jebel Tayarat (Tayarat, not 

Ayarat), 21 miles nearly due south of Kutbah, lat 

32°46' N., long 40° IT E. A reference section in the 

BPC well Zubair No. 3 lies between drilled depths of 

4,482 and 5,202 feet. The formation is about 98 feet 

(incomplete) thick at the type locality.

According to Macfadyen (1938), this formation has 

a, widespread outcrop distribution in the Wadian area, 

south of Kutbah. H. A. Field and K. D. Jones (unpub. 

data, 1958) described good, but incomplete, sections at 

Abu Ghar (E. 1183, N. 1156), Jelta solution hole (E. 

1202, N. 1173), Wadi Ithayna (E. 1195, N. 1183), Al 

Jandali (E. 1156, N. 1065), and Wadi Tubal west of 

Abu Nufuq (E. 1193, N. 1184). (Grid coordinates are 

shown on pi. 1.) In this general area Field and Jones 

assigned a minimum thickness of 250 feet for the incomplete Tayarat. South of the Widyan and Kifl areas,

this formation was found in all deep subsurface sections 

and in some of the shallow structural holes in the southern desert west of the Euphrates Kiver.

Owen and Nasr (1958) maintained that in the Basrah-Kuwait area this formation is essentially in the 

same facies as in the type locality, but it is thicker 

than in outcrop section. Its greatest thickness, 970 

feet, seems to be localized in the Rachi well No.l area, 

decreasing westward to about 600 feet in the area of 

wells Ubaid No.l and Shawiya No.l. It also decreases in thickness northward from Kachi well No.l; 

in the wells Samawa No.l and Ghalaisan No.l, thicknesses of 344 and 296 feet respectively, were recorded.

Dunnington, Wetzel, and Morton (in Bellen and 

others, 1959) stated:

Passing northwards from Jebel Tayarat through Jebel Thoba, 

and towards the western rim of the Ga'ara depression, the 

Tayarat limestone is cut out progressively, by the erosion of 

its upper parts, and also perhaps by onlap convergence. In 

the section of Hill 270 (Lat 33°18'00" N., long 40°10'30" E.), 

and Khasm Mulussa (lat 33°18'30" N., long 40°07' E.), the 

formation is absent, the Palaeocene Umm er Radhuma formation resting discordantly on eroded M'sad formation. The convergence increases northwards, so that, at Ras Semhat, the 

Umm er Radhuma rests directly on Triassic Ga'ara sandstone. 

The details of the erosional convergence from Tayarat to An 

Nadhara have not been studied.

Owing to the formation's abrupt local lithologic 

changes and closeness to the Ga'ara massif, further 

study is needed to elucidate the detailed geological 

history.

DETAILED LITHOLOGIC DESCEIPTION

The Tayarat Formation consists of rubbly porous 

limestone that is white, buff, and pink; it is rather 

chalky, fossiliferous, recrystallized, dolomitized, locally sandy, and is conspicuously more massive at the base.

W. T. Foran (unpub. data, 1940) described the composite Upper Cretaceous section in the Widyan area 

as follows:

Thickness 

(feet)

Umm er Radhuma white chalky limestones with silicified limestone nodules (flint?).

Unconformity, especially in Wadi Tibal region.

6. Limestone, purplish-gray, fossiliferous, contains Loftusia ; yellow marls locally passing to sandstone; 

gray to white granular gastropodal limestones; 

and soft yellow limestone 27.2

5. Marl, yellow, varying locally to shale, especially

toward southeast; numerous corals in upper part. 19.7

4. Sandstone, yellow, has marly matrix to west, passing 

into current-bedded ferruginous oolite with 

rhombohedral calcite in Wadi Ghadfat at Tawil; 

unit thins out and passes to normal marly shale in 

region of Wadi Tibal, where it is overlain disconformably by Umm er Radhuma limestones; is 

comparable with yellow sandstone bed (2 m thick)
G32 GEOLOGY OF THE ARABIAN PENINSULA

near middle of Tayarat section east of type locality _________________________ 9.5

3. Marl, yellow, varying to shale ____________ 19.7

2. Limestone, gray, hard, dolomitic, rudistid, with several gray marly intercalations (1.4 m) having 

gritty marly zones, especially toward top and 

base; contains Omphalocyclus macropora (Lamarck), Eoradiolites, and corals __________32.8-98.4

1. Marl, yellow, with some thin limestone intercalations __________________ __________ 48.5

?Unconformity.

M'sad soft yellow limestones containing Nerinea sp.

W. T. Foran (unpub. data, 1940) called unit 2 the 

Tibal dolomitic limestone; units 3, 4, and 5, the Tawil 

marl; and unit 6, the Tayarat limestone. However, 

F. R. S. Henson (unpub. data, 1940) and H. A. Field 

and K. D. Jones (unpub. data, 1958) included all these 

units in the Tayarat Formation.

Macfadyen (1938) pointed out the presence of a fish 

bed, in presumably Cretaceous sandy limestones at Gur 

Ayarat, where abundant teeth but fewer bone fragments and vertebrae were found.

DIAGNOSTIC FOSSILS AND AGE

Fossils in the Tayarat Formation include Loftusia 

morgani Douville and Omphalocyclus macropora (Lamarck). According to H. A. Field and K. D. Jones 

(unpub. data, 1958), the Tayarat limestones contain 

corals, rudists (including Eoradiolitesl], gastropods, 

lamellibranchs, and algae. They maintained that generic identification is often difficult as most of the 

original structures have been destroyed by recrystallization and some by dolomitization. In addition to the 

fossils recognized in the type section, the following 

microfossils have been determined from the subsurface 

sections: Lepidorbitoides socidlis (Leymerie), Orbitoides media (d'Archiac), 0. apiculata, Ogivalia parisiensis d'Orbigny, Rotalia skourensis Pfender, R. trochidiformis Lamarck, Globotruncana spp., Dictyoconus 

sp., Dicyclina schlumbergeri Munier-Chalmas, and 

Pseudochrysalidina sp.

For complete faunal assemblages of the Tayarat 

Formation, reference to the equivalent Aqra Limestone 

of northern Iraq is recommended. (See Bellen and 

others, 1959.) The formation is Maestrichtian in age.

CONTACTS

The lower contact of the Tayarat Formation with 

the M'sad Formation is an erosional unconformity 

placed at the base of a localized eonglomerate. In 

southwestern Iraq, with the exception of the extreme 

western region in the vicinity of Sawafi well No.l, 

where it is underlain by Safawi anhydrites, this formation overlies the Shiranish Formation (formerly Qurna) conformably with lateral interdigitation.

The upper contact is not present in the type locality, 

owing to the complete erosion of the overlying formation ; however, in the Al Widyan area of western Iraq, 

though the structural grain of the country is flat, there 

is strong evidence of an overlap relationship between 

the Paleocene rocks and the underlying Upper Cretaceous Tayarat Formation.

Macfadyen (1938) described this area as follows: 

"Nearly the whole of the Wadian (Al Widyan) is 

covered with flat-lying limestones occasionally conglomeratic. These are believed to be wholly of Eocene 

and Cretaceous age. Since the lithology is generally 

similar, and recognizable fossils mostly leached out, the 

Eocene-Cretaceous boundary appears to unmappable 

without a great expenditure of time and labour."

According to W. T. Foran (unpub. data, 1940), the 

contact of the Cretaceous with the overlying Paleocene 

formations is readily recognizable throughout the exposed area; he placed the contact at the level of the 

lowest appearance of Loftusia and the lowermost chalk 

containing bluish opaline quartz geodes. One or both 

of these are commonly near the contact. Later investigators such as H. A. Field and K. D. Jones (unpub. 

data, 1958) agreed with this view and have placed the 

contact a short distance above the last appearance of 

Loftusia, at the first recognizable break below the chalk 

containing nodular cherts and flints. In the deep subsurface sections of southwestern Iraq, the Tayarat 

Formation is generally overlain by the Umm er Radhuma Formation.

EQUIVALENT UNITS IN ADJACENT ABEAS

According to Dunnington, Wetzel, and Morton (in 

Bellen and others, 1959), "Formations closely comparable with Tayart are known at outcrop in Saudi 

Arabia (Aruma formations; R.A. Bramkamp and M. 

Steineke 1952; W.H. Thralls and R.C. Hasson, 1957,, 

and so forth) and in well sections in Arabia, and in 

Qatar (Sismisima formation, W. Sugden 1958, MS).""

ECONOMIC ASPECTS

The Tayarat limestones are porous and occupy an 

extensive area in Al Widyan. Macfadyen (1938) 

stated that this area is an almost flat-lying Eocene and 

Cretaceous limestone terrane that has very gentle slope 

and drainage to the east. The annual rainfall averagesless than 10 millimeters, and in summer and autumn 

this area constitutes the most waterless part of Iraq. 

A very considerable proportion of the rainfall probably percolates deeply underground through solution 

channels in the limestone, and the ground water isthus beyond reach of shallow wells; however, deep* 

drilled wells near the underground solution channels
SOUTHWESTERN IRAQ G33

would probably yield water. A detailed account of 

this subject was given by Macfadyen (1938, 127-135). 

Minor quantities of phosphatic material may be found 

in the fossiliferous and chert beds in the area south 

and west of Rutbah.

CENOZOIC 

TERTIARY

The Tertiary System includes the Paleocene, Eocene, 

Oligocene, Miocene and Pliocene series. Rocks of Oligocene age are not recognized in southwestern Iraq 

because of a major break due to nondeposition, preMiocene erosion, or both. Tertiary rocks there are composed of more than 6,500 feet of sediments.

PALEOCENE AND EOCENE

Rocks of Paleocene and Eocene age are represented 

by four formations that are described in succeeding 

discussion. The upper Eocene is not represented, perhaps because of pre-Miocene erosion.

TJMM EB EADHUMA FOBMATION

Reference sections and thickness

The Umm er Radhuma Formation was originally 

named in Saudi Arabia. Owen and Nasr (1958) referred to a supplementary type section from the Basrah 

area in BPC well Zubair No. 3 (lat 30°23'01" N., long 

47°43'29" E., alt 51.9 ft, completed Feb. 21, 1951). 

The unit occurs here at drilled depths of between 2,980 

and 4,482 feet. The formation crops out in the western 

desert of Iraq, where for the convenience of field mapping the exposed part has been subdivided into the 

Basita Beds and the Ghurra Beds.

The type location of the Basita Beds is at the section 

of a scarp about 50 kilometers west-southwest of Waqisa wells and 2 kilometers southwest of triangulation 

point S 85. The type location of the Ghurra Beds is 

about 8 kilometers southwest of Aydahah, near the 

track at Saddat al Batn. The formation is 1,502 feet 

thick (458 m) ; it has been found in all deep surface 

sections west of the Euphrates and Shatt al Arab 

Rivers, more specifically in the wells of Nahr Umr, 

Zubair, and Rumaila oil fields, Tuba No. 1, Ratawi 

No. 1, Rachi No. 1, Luhais No. 1, Ubaid No. 1, Samawa No. 1, Shawiya No. 1, Ghalaisan No. 1, and Safawi 

No. 1. The thickness of this formation is extremely 

variable; the greatest thickness (1,683 ft) was recorded 

in the Ubaid well No. 1. The thickness appears to decrease both westward and northward, for at wells 

Shawiya No. 1 and Ghalaisan No. 1 the thickness is 

1,202 and 975 feet, respectively. At the extreme western wall, Safawi No. 1, where the formation is exposed, 

the thickness in the subsurface section is only 858 feet.

Extent

According to Bellen (Bellen and others, 1959), the 

Umm er Radhuma Formation is exposed over the 

western part of Iraq, west of a line joining Nisab 

(Ansab), Ash Shabakeh, and Al Lasaf. For field-mapping purposes, H. Huber (unpub. data, 1941) recognized two units there, mainly on the basis of their 

topographic expression as scarps in the otherwise 

rather flat desert.

The Umm er Radhuma outcrops form a strip 20 

kilometers wide that extends northwestward from 

Qalib ar Rufayat-Nisab (Ansab), with a few inliers 

over Ar Ruwagarea, through Habbas and Jal Al Batn. 

About 25 kilometers southeast of Al Jumaymah, 

another strip of the Umm er Radhuma Formation 

crops out under the Zahra Formation and follows the 

general northwesterly trend. Between Jumaymah and 

Jal al Batn, the Umm er Radhuma Basita Beds commonly crop out as inliers in the erosion depressions 

within the Zahra Formation. Farther north, owing to 

the absence of the Zahra Formation, the two strips of 

the outcrops form one major belt more than 45 kilometers in width in the region of Hagai al Hawara, 

especially between Al Lu'a'ah in the east, and Birkat 

al Al Aqabah in the west. The region is strewn with 

outliers of Zahra Formation. Between As Shubrum 

and Aydahah, several outliers of the Dammam Formation "VVagsa Beds occur directly on the Umm er Radhuma Basita Beds. Southwest of the Jal al Batn scarp 

the Basita Beds are folded and fluted and contain 

minor contortions from the foot of the scarp itself 

along a belt 2 5 kilometers wide. In this area the 

Basita limestone has an extensive outcrop. Farther 

north, the width of the outcrop on the Iraqi side constricts to 30 kilometers between Al Jill and Athamin, 

mainly because the greater part of the formation extends into Saudi Arabia. At Athamin as well as Al 

Athman, outliers of Wagsa Beds of the Dammam 

Formation can be recognized on the Umm er Radhuma. 

From Al Jill toward Wagisuh (Wagsa) there is a 

sharp westward swing, or curve, in the distribution of 

the outcrop, resulting in the Basita plateau. In this 

area the Umm er Radhuma is generally confined between the Hagai al Basita-Ash Shaba-Hagai Al Wagsa feature and Al Ghurra. This nearly continuous 

outcrop ends near Chebritiya, north of where the formation occurs as isolated inliers, seen either within 

the Zahra Formation on the Dammam Formation. 

These inliers, which are of variable dimensions, occur 

in the area of Wadi al Khirr, Al Ghurabiya, Sahb 

Nauwman, Guwairat al Halib, and Ruwaiyat as Sahal. 

From this extensive outcrop, northward from Al Makmin (Machmi), only a narrow belt of the Umm er Rad-
G34 GEOLOGY OF THE ARABIAN PENINSULA

huma crops out along the Az Zawr feature to Wadi 

Al Ubayyid, where it follows this wadi eastward. In 

this region the Umm er Radhuma disappears under 

the Dammam Formation on one side and under the 

Zahra Formation on the other; however, it reappears 

west of a line from Faidha al Adyan, Aglat al Khail, 

and Abu Infuq. Here it covers a large territory south 

of the Wadi Tibal where dissection causes the Upper 

Cretaceous Tayarat Formation to form the wadi floors, 

especially in the Wadi al Jaudali, Shaib al Ubaydat, 

Wadi al Ubayyid and Wadi al Mira. The general 

outcrop distribution elsewhere is clearly indicated on 

the geological map (pi. 1).

In the type locality of the Ghurra Member, the section is only 118 feet (36 m) thick; however, in Safawi 

well No.l the unexposed part has a greater thickness. 

A true thickness measurement for the Basita Beds is 

difficult to make, owing to the width of the exposure 

and its relation to the overlying formation. Denudation has removed the highest beds from most of the 

Basita plain, but the most complete section is probably 

that of Al Batn at the type locality, where its thickness is only 49 feet (15 m). Here, as the overlying 

Dammam Formation Wagsa Beds are unconformable, 

the top of the section may show the highest existing 

Basita Beds. According to Bellen (Bellen and others, 

1959), the thickness of a composite section near Aydahah, in the Batn scarp near the frontier with Saudi 

Arabia, is 705 feet (215 m).

W. T. Foran (unpub. data, 1940) reported a thickness of 348 feet (105 meters) for the Umm er Radhuma Formation. In the Wadi Ghadaf the thickness 

was estimated by H. A. Field and K. D. Jones to be more 

than 50 meters. They found representative sections at 

Al Hisichiyat (E. 1166, N. 1160) and Al If Hada 

(E. 1140, N. 1110).

Bellen (Bellen and others, 1959) stated:

This formation also occurs at surface in western central Iraq. 

Along the western rim of the Ga'ara depression it transgresses 

over the pre-Tertiary outcrops of the Maestrichtian Tayarat 

Formation and the Cenomanian M'sad Formation and Rutbah 

Sandstone Formation onto deeply eroded Middle Triassic 

Ga'ara Sandstone. It is absent from the southwestern parts 

of the Ga'ara and from the Wadi Hauran.

Detailed litnologic description

The Umm er Kadhuma Formation consists of anhydritic and dolomitic limestone, which is mostly dull 

white or buff, macrocrystalline, and porous. Chert occurs in the higher part of the formation (Bellen and 

others, 1959).

In the outcrops at the type location, the Ghurra Beds 

comprise limestone that is white to gray, partly very 

hard and recrystallized, saccharoidal, partly leached,

and well bedded and that contains oblate siliceous concretions and chert nodules and layers at some levels.

The Basita Beds at the type locality consist of gray 

marly hard thick to massive-bedded limestones alternating with white and yellow saccharoidal well-bedded 

limestones and gray dense recrystallized dolomitic thinbedded limestones containing chert nodules. Hard 

roughly concentric limestone concretions as much as 75 

centimeters in diameter occur sporadically on the Basita Beds between Aydahah and Nisab. Their origin is 

not fully understood.

In the Al Widyan country the Umm er Radhuma 

Formation consists mostly of alternating soft friable 

chalks, sporadic dolomitic chalks, bedded chalky limestones, and thin porcelaneous silicified limestones containing subordinate marl, shale, and chert horizons. 

Chert, flint, quartz vugs, and geodes are distributed 

generally throughout the sequence, but are better developed to the south. Some shell banks occur in the 

limestones, for example, 2 kilometers north of Wishashia (E. 1202, N. 1167). The upper chalky zone contains Operculma libyca Schwager. Southward, the 

formation becomes a fissile yellow shale overlying the 

Operculina horizon and the chalky zone includes several irregular zones of ferruginous and siliceous limestone. In the Al Widyan area a zone composed of large 

oblate spheroids of siliceous limestone occurs about 20 

meters from the base. Owing to the differential hardness, these also strew the plain after erosion of their 

soft chalky crust. Eastward, the Umm er Radhuma 

passes into basinal Aaliji facies, as in the Kifl wells.

Diagnostic fossils and agre

Fossils in the Umm er Radhuma Formation are rare, 

but one horizon at the top contains Alveolina dolioliformis Schwager, A. oblong a d'Orbigny, and A. ovoidea d'Orbigny. Operculina libyca Schwager has also 

been reported. R. G. S. Hudson (unpub. data, 1951) 

recorded Pseudomilthat (Lucina) gigantea Deshayes 

and ITurritella cf. T. carinifera Deshayes.

The fossils of the Ghurra Beds are both nondiagnostic and scant; they consist chiefly of Valvulina sp., miliolids, rare Alveolina sp., molluscs, ostracodes, and 

algae. In contrast the Basita Beds contain a great 

variety of fossils. The following were originally determined by F. R. S. Henson and R. G. S. Hudson: 

Alveolina spp., A. primaeva Reichel, Bigenerina sp., 

Dictyoconoidesl, Dictyoconus gr. walnutensis Carsey, 

miliolids, Nummulites spp., Operculina libyca Schwager, peneroplids, Saudia spp., S. cf. discoidea Henson, 

Spirolina sp., Textularia, Valvulina sp., ostracodes, 

Lucina sp., Cardita sp., C. aegyptiaca Frass, ILima 

sp., Dentalium^ Worbula sp., Area sp., Tellina sp.,
SOUTHWESTERN IRAQ G35

Venus sp., Anomia sp., Turritella sp., Valuta sp., Conoclypeus aff. C. delanouei de Loriol, Echinolampas sp., 

and various other molluscs and echinoids, as well as 

fish fragments and algae. The formation is Paleocene 

and early ? Eocene in age.

Contacts and equivalent units

The Upper Cretaceous Tayarat Formation underlies 

the Umm er Radhuma with major unconformity. In 

the outcropping areas "in western central Iraq, along 

the western rim of Ga'ara depression it transgresses 

over the pre-Tertiary outcrops of the Maestrichtian 

Tayarat Formation, and the Cenomanian M'sad Formation and Rutba Sandstone Formation, onto deeply 

eroded Middle Triassic Ga'ara Sandstone" (Bellen and 

others, 1959). For further comments about the contacts, see the section on "Tayarat Formation."

The Rus Formation overlies the Umm er Radhuma 

conformably in the supplementary type section. In the 

outcrops of western Iraq, where the Rus Formation is 

absent, the Dammam Formation Wagsa Beds overlie 

the Umm er Radhuma Formation unconformably.

The Umm er Radhuma Formation is recognized in 

Kuwait and Saudi Arabia, the type section of the 

formation being within Saudi Arabia.

Economic aspects

Ground water. The areas where the Umm er Radhuma limestones crop out are very dry and arid. From 

data of Macfadyen (1938) it is inferred that the water 

supply in these areas is in direct relation to the rainfall and that in deep wells the supply is due to percolation downward by means of solution holes such as 

those of Chebretiya, Kusra (E. 1225, N. 1160), Jelta 

(E. 1202, N. 1173), Wishashia (E. 1202, N. 1167), Mussir al Awaij (E. 1205, N. 1228), and Wadi Ghadaf 

(E. 1188, N. 1195) with storage in such cavities or in 

some porous horizon at depth. He also maintained 

that because of apparent scarcity of faulting and folding in the desert, the geologic structure is generally of 

little help in locating water. The varying quality of 

the waters, where not due to concentration by evaporation, is due to the heterogeneity of the rocks and to the 

rainfall being in contact with somewhat variable soluble constituents on its way down from the surface. The 

lithologic character of the strata is extremely variable, 

and gives rise to some marked quantitative and qualitative differences in the analyses of the waters. For full 

details, reference to Macfadyen's (1938) work is 

recommended.

Sulfur and gypsum. Macfadyen (1938) referred to 

the occurrence of native sulfur and gypsum at Chebretiya, some 60 kilometers west of Ash Shabakah (Shabicha), where he described the occurrence and formation of solution structures.

A dry wadi leads into a double swallow hole, and two vast, 

roughly circular chambers being 35 metres and 60 metres deep, 

respectively, and each 50 metres in diameter. They are situated close together, so that the chambers intersect, though 

at the surface they are a short distance apart, the second, 

deeper, chamber being partially roofed over. At the base of 

the first a great mass, some 30X20X25 metres in size, of an 

admixture of white secondary gypsum and native sulphur, 

which latter has been worked by the Bedouin for ages, the 

marks of their tools being seen everywhere. The mass must 

presumably contain something of the order of 1,000 metric 

tons of free sulphur. Traces of malachite are also found 

associated. The whole phenomenon seems to have been caused 

by an acid spring, or the oxidation of underlying pyrites, which 

has resulted in the presence of free sulphuric acid. The gypsum tastes strongly acid, and is reported to rot the clothes of 

those who work there, a report fully confirmed by the rotting 

effect on the cotton bags and paper in which the samples collected were stored. The swallow holes were clearly dissolved 

out of the limestone seen in the walls, the portion converted 

into gypsum having now vanished in solution. A channel five 

metres square, unexplored, leads out underground from the 

base of the second chamber, in which there is neither gypsum 

nor sulphur deposited. A rough section was measured in the 

walls as follows:

0-3 m. Weathered white shale.

3-60 m. A series of rather soft limestones, marly limestones, 

and white chalk. Conspicuous bands of flint at 9 m. and 12 m. 

At 12-13.8 m. a band of small calcite nodules, simulating casts 

of larger f oraminifera, geodes of quartz crystals, and chalcedony 

are common, particularly in the lower part. No fossils were 

recognized.

The strata are nearly flat bedded, but a very slight westerly 

dip was seen in the section of the walls of the chambers.

Phosphates. Some traces of phosphates may be 

found in the Umm er Radhuma Formation in the 

fossiliferous horizons and in horizons containing chert 

and flint.

BUS FOEMATION

Reference section, thickness, and extent

The type locality of the Rus Formation is in Saudi 

Arabia at lat 26°19'.5 N., long 50°10'.0 E. A supplementary section in Iraq was given by Owen and Nasr 

(1958) in BPC well Zubair No.3 (lat 30°23'01" N., 

long 47°45'29" E.; alt 51.9 ft; completed Feb. 21,1951) 

and between drilled depths of 2,673 and 2,980 feet (307 

ft, or 94 m, thick); however, the formation is thicker 

elsewhere in the Basrah area, especially between the 

Zubair field and Luhais well No.l, where evaporites 

ranging in thickness between 300 and 700 feet have 

been drilled. Northward, this thick Rus f acies extends 

to wells Samawa No.l and Shawiya No.l, where it is 

represented by 200 to 350 feet of evaporites.

This formation does not cropout in the western 

desert, and it seems to change its facies and pass into 

limestone east of wells Ubaid No.l and Ghalaisan No.l,
G36 GEOLOGY OF THE ARABIAN PENINSULA

where no Rus Formation has been recognized. The 

stratigraphic position of the Rus Formation equivalent 

in outcrop is difficult to define. Bellen (Bellen and 

others, 1959) has equated the Huweimi (chalk), Shabicha and Sharaf units, and the Wagsa unit of the 

Dammam Formation with this formation. This would 

imply that the Rus evaporites pass westward into the 

Dammam limestones. None of these units has been 

correlated as yet with any of the limestone beds within 

the Rus Formation in the subsurface sections. Bellen 

(Bellen and others, 1959) maintained that none of them 

is recognizable in wells east of the outcrop area.

Detailed litnologic description

The Rus Formation is predominantly massively 

bedded white and blue tinted crystalline anhydrite 

with alternations of thin unfossiliferous limestone, blue 

shale, and marl. Westward, this evaporitic sequence 

passes laterally into the Dammam Formation.

Age

The Rus Formation is probably early Eocene (Bellen 

and others, 1959) in age; no diagnostic fossils have 

been found. The exact age of the formation is difficult 

to demonstrate. In the subsurface sections the Rus 

Formation is sandwiched between the overlying Dammam Formation and the underlying Umm er Radhuma Formation.

According to Bellen (Bellen and others, 1959), the 

base of the Dammam Formation is of middle Eocene 

age in subsurface sections but contains a lower Eocene 

fauna in outcrop. He maintained that it is perfectly 

possible that the two lower units of the Dammam 

Formation in the field (the Wagsa Beds and the Sharaf, Shabicha and Huweimi Chalk Beds) change facies 

toward the offshore regions into part of the Rus Formation as found in the wells. Thus the age of the Rus 

Formation can indeed be early Eocene. Similar age 

attribution has been given to this formation in Saudi 

Arabia (Steineke and others, 1958).

Contacts and equivalent units

The lower contact of the Rus Formation with the 

Umm er Radhuma Formation is possibly conformable, 

and the upper contact with the Dammam Formation 

is unconformable, according to Bellen (Bellen and 

others, 1959). However, this may be locally conformable, and a gradational contact is advocated on the basis 

of the lateral facies passage of the two formations.

DAMMAM FOBMATION

Type and reference sections

The type locality of the Dammam Formation is 

in Saudi Arabia at lat. 26°17'03" N., long 50°07'07" 

E. The supplementary type section chosen by Owen

and Nasr (1958) from Al Basrah (Basra) area is in 

the BPC well Zubair No. 3 (lat 30°23'01" N., long 

47°43'29" E.; alt 51.9 ft; completed Feb. 21, 1951). 

The formation lies between drilled depths of 1,935 and 

2,673 feet (738 ft thick).

Outcrops of this formation in western Iraq were 

divided into 10 field units for mapping convenience by 

H. Huber and R. M. Ramsden (unpub. data, 1944-45). 

According to Bellen (Bellen and others, 1959), these 

units have no status as formations or even as members, 

but it is essential and convenient for future investigation to mention their type localities, which are in descending stratigraphic order as follows:

Tuqaiyid Beds, about 5.6 miles (9 km) west of Tukayyid water 

well on the Tukayyid scarp.

Ghanimi Beds, east of triangulation point C 26, near Khasmn 

ad Duda on the At Traq scarp.

Barbak Beds, in prominent hills at Barbak and Diud in the 

southeastern part of the Galaib area.

Rudhuma Beds, in the cliff to the northwest of Chabd, west of 

Abu Rudhuma.

Chabd Beds, at the north end of the Chabd "structure."

Shawiya Beds, in a small wadi, north-northwest of triangulation point 5.2, to the north of the Ash Shawiyat depression.

Huweimi Beds, 4.3 miles (7 km) west of Ash Shabakah (Shabicha) police post, along the Nukhayb track.

Shabicha Beds, at triangulation point S 25 about 4.3 miles (7 

km) west of As Shabakah (Shabicha) police post, along the 

Nukhayb track.

Sharaf Beds, northeast of the Jil water wells along the track 

leading to Sulman.

Wagsa Beds, in the vicinity of Waqisah (Wagsa) water wells. 

The greatest thickness of the Wagsa Beds is on the white 

cliffs east and north of Wagsa wells, from which the name 

is taken. At the prominent point S 86, 5 km south of the 

wells, there is a section nearly 30 meters thick.

Thickness and extent

The Dammam Formation is 738 feet (225 m) thick 

in Zubair well No. 3. This formation is extensive in 

subsurface and surface distribution west of the Euphrates and Shatt al Arab Rivers. Its thickness in the 

Al Basrah oil fields and in the Basrah Petroleum Company exploration wells Luhais No. 1 and Rachi No. 1 

ranges between 600 and 800 feet. A slightly greater 

thickness (835 ft) was found in Ratawi well No. 1. The 

Company's other exploration wells, such as Samawa 

No.l, Shawiya No.l, Ghalaisan No.l, and Ubaid No.l, 

which were drilled directly into the exposed Dammam 

Formation, penetrated incomplete sections of less than 

400 feet, excluding the 350- and 245-foot Rus equivalent 

section of wells Shawiya No.l and Samawa No.l, respectively.

The 10 units of the Dammam Formation cover a 

territory exceeding 200 kilometers in width, in which 

they generally stand out as scarps and recognizable 

topographic features. They crop out in ascending se-
SOUTHWESTERN IRAQ G37

quence toward the east and trend generally northnorthwest, almost parallel to the Euphrates Eiver. 

Minor irregularities in this pattern are due to local 

topographic depressions such as those found in the As 

Salman and Ash Shabakah regions, where younger 

rocks have escaped erosion and thus occur in anomalous 

position.

Detailed llthologlc description

In the reference section, the Dammam Formation 

consists of cream to gray limestones that are recrystallized, leached out (leaving a honeycomb appearance), 

very porous, and fossiliferous (abundant Nummulites 

spp.). Blue silty waxy shale streaks or partings occur 

toward the base; toward the top, the cream to gray 

limestone passes upward into dark-gray to black limestone, which in places is spotted by pyrite, is porous to 

fairly dense, and is variably recrystallized and dolomitized.

Because this formation crops out in the western 

desert in broad, flat-lying, and extensively denuded 

exposures, the width of which may exceed 200 kilometers, selection of one surface reference section is 

difficult; no measured thickness can convey the true 

thickness of the entire formation at one particular 

point. The great lengths of typical sections on the 

ground, the distances between sections, the possibility 

of lateral and vertical variations, and the possibilities 

of minor intraformational breaks and of extensive 

differential erosion of the various beds that make up 

this formation make thickness assessment very difficult. 

However, for the purpose of field studies, 10 distinct 

but informal units were recognized in the field. Their 

type sections are scattered throughout the territory, 

and their measured thicknesses are valid only for the 

respresentative sections; therefore, their cumulative 

thickness cannot be a measure of the true thickness for 

the Dammam Formation in any of the type sections, 

except perhaps by sheer coincidence.

The different beds, which generally stand out as 

scarps on the plain, crop out in ascending order eastward and in belts of variable width that, apart from 

the few exceptions previously mentioned, generally 

trend north-northwest. The lithologic descriptions of 

the informal field units, in ascending stratigraphic 

order, are as follows:

Wagsa Beds: Limestone succession, white, chalky, intercalated

with some yellow more marly beds, particularly in the lower

few meters; thickness, 98 ft (23 m). 

Sharaf Beds: Limestone, white and gray, saccharoidal, partly

vuggy, fossiliferous, well-bedded, becoming massive bedded;

recrystallized and siliceous with increase of quartz upward;

weathers reddish brown and black toward top; thickness, 52

ft (16m).

Shabicha Beds: Limestone, gray and white, recrystallized, massive, with thin-bedded intercalations; dense to vuggy; fossiliferous ; chert nodules in upper half; weathers grayish brown; 

thickness, 131 ft (40 m).

Huweimi Beds : Limestone, white and gray, saccharoidal, vuggy, 

recrystallized, leached-out, nummulitic, well-bedded, with 

sporadic thick to massive beds; thickness, 102 ft (31 m).

Shawiya Beds: Limestone, white, friable, detrital, succeeded 

upward by white siliceous nummulitic coarsely recrystallized 

hard limestone, which weathers red brown and black, thickness, 13ft (4 m).

Chabd Beds: Limestone, white, soft in the upper part; well 

bedded with sporadic and local massive or thick beds; weathers gray; thickness, 39 ft (12 m).

Rudhuma Beds: Limestone, mainly gray and white, well-bedded, 

saccharoidal, some chert nodules; sporadically vuggy and recrystallized. According to H. Huber (unpub. data, 1944), 

this unit contains a thin bed of gypsum in vicinity of As Salman-As Samawah track near Faydat Ash Shaykh. This possibly indicates influence of the Rus Formation; thickness, 

33 ft (10 m). Possible disconformity between this unit and 

overlying Barbak unit in southern part of Galaib "structure."

Barbak Beds: Limestone, white and gray, medium hard; some 

marly limestone incalated with siliceous detrital material; 

also bryozoan and algal hard ringing limestone; weathers 

reddish brown and black; gray and brown chert nodules 

throughout; thickness, 81 ft (25 m).

Ghanimi Beds: Limestone, white, generally thin-bedded, some 

massive beds, especially toward base; includes dense to vuggy, commonly saccharoidal limestone; followed by soft leached 

out and friable saccharoidal limestones, some of which are 

highly siliceous and weather dark reddish brown and black; 

microbreccias and rare stringers of halite and gypsum occur in the limestones, especially near Abu Khanzir; thickness, 

52 ft (16 m).

Tuqaiyid Beds: Limestone, gray and white, massive, recrystallized, very hard, passing upward into white thin-bedded 

saccharoidal sporadically silicifled limestones.

A number of these informal units were subsequently 

combined by R. M. Ramsden and C. A. Andre (unpub. 

data, 1953) into four informal units, as follows:

1. Tuqaiyid, Ghanimi, Barbak, and Rudhuma unit, 

consisting of bryozoan-peneroplid limestones and 

shelf limestones, with a rich fauna containing 

Coskinolina balsillei Davies, Dictyoconus kohaticus Davies, Opertorbitolites douvillei Nuttall, 

Orbitolites complanatus Lamarck, Peneroplis damesini Henson, P. dusenburyi Henson, Praerhapidionina huberi Henson, Rhapidionina urensis 

Henson, var. minima Henson, R. williamsoni 

Henson, Somalina danieli Henson var., and Spirolina cylindracea Lamarck.

2. Chabd, Shawiya and Huweimi (nummulitic) unit, 

consisting of nummulitic limestone containing 

Alveolina elliptica (Sowerby), Dictyoconus kohaticus Davies, Halkyardia minima (Liebus), 

Nummulities discorbinus (Schlotheim), N. gizehensis (Forskal), Orbitolites complanatus La-
G38 GEOLOGY OF THE ARABIAN PENINSULA

marck, Peneroplis duseriburyi Henson, Rhapidionina urensis Henson, R. urensis Henson var. 

minima Henson, and Somalina danieli Henson.

3. Huweimi (chalk), Shabicha and Sharaf unit, in 

which chalks alternate with chalky limestones 

that contain badly preserved lamellibranchs. The 

chalks are white or pink and have a fresh-water 

appearance.

4. Wagsa unit is a chalky, moderately fossiliferous 

limestone, with Operculina libyca Schwager occurring as a prominent fossil.

Attempts to correlate these field units with the subsurface Dammam Formation have been made by many 

geologists; however, none of their conclusions can be 

considered satisfactory because of lack of adequate 

information and study materials. A most plausible 

correlation was advanced by Bellen (Bellen and others, 

1959). He stated that in the subsurface sections east 

of the area of outcrops, the base of the Dammam 

Formation contains middle Eocene fauna consisting of 

Alveolina elliptica (Sowerby), Nummulites discorbinus (Schlotheim), N. gizehensis Forskal, and N. lucasanus d'Archiac. This fauna occurs in the rocks that 

overlie the Rus Formation directly in the well sections 

but not in the base of the Dammam Formation in the 

field. There the basal limestones at the base of the 

Wagsa Beds contain a lower Eocene fauna with Operculina libyca Schwager. The Wagsa Beds in the field 

appear to lie unconformably on the Umm er Radhuma 

Formation without intervention of the Rus Formation. 

In well sections the middle Eocene fauna of the Dammam Formation occurs higher up in the limestone f acies of the Huweimi Beds. We also may argue that 

the limestones containing Operculina libyca fauna of 

the surface exposures grade toward the offshore area 

into strata of the Rus Formation as found in the wells, 

and so does the second field unit of the Dammam Formation (Huweimi chalk, Shabicha, and Sharaf Beds) 

which has no apparent equivalent of Dammam Formation in the wells sections in which the Rus facies is 

developed. Bellen (Bellen and others, 1959) maintained that both the offshore facies were subsequently 

covered by the middle Eocene nummulitic limestones, 

which are normally found in the field at the base of the 

third unit of the Dammam Formation, the Huweimi 

(limestone), Shawiya, and Chabd Beds. This unit correlates very well with the base of the Dammam Formation in the wells. The above interpretation, advanced 

by Bellen, finds further support in the subsequently 

drilled wells of Samawa No.l, Shawiya No.l and Ghalaisan No.l. In Samawa well No.l the sequence consists 

of the following three units:

Thickness 

(feet)

3. Dolomite, buff, leached-out, vacuolar, and dolomitic 

limestones with gastropods and lamellibranchs at 

certain horizons _______ ___ 91 

2. Limestone, white, recrystallized, locally chalky 90 

1. Limestone, white, recrystallized, in places chalky and

containing Nummulites spp ___ _____ 200

At Samawa well No.l the Rus Formation itself comprises 245 feet of crystalline anhydrite with marl limestone and marl intercalations. A. Sayyab (unpub. 

data, 1960) considered this sequence to be very similar 

to that of other subsurface sections in the Al Basrah 

area, except for the absence of the upper bryozoanmiliolid limestone, which has possibly been eroded. 

Furthermore, the nummulitic facies of the above-mentioned unit No.l which directly overlies the Russ Formation, according to H. A. Halliwell (unpub. data, 

1962), can possibly be correlated with Huweimi (limestone), Shawiya, and Chabd Beds.

Farther west, in the wells Shawiya No.l and Ghalaisan No.l, which were spudded directly into the Huweimi (limestone), Shawriya, and Chabd Beds, the Rus 

Formation shows indication of gradual change of the 

facies. In Shawiya well No.l, stringers of anhydrite 

persist in an otherwise completely dolomitic limestone 

sequence with a thickness of 350 feet, but in Ghalaisan 

well No.l the facies change from evaporite to limestone seems to be complete where the evaporites have 

not been leached out. If this is true then it is another 

possible explanation for the absence of the Rus Formation in the surface sections. Bellen (Bellen and others, 

1959) maintained that it is possible that percolating 

ground water may have removed the anhydrite, so that 

the apparent unconformity is not an original one. 

Facies change from Wagsa Beds and lower part of the 

Huweimi (chalk) Beds to Rus Formation remains a 

possibility because the correlation of upper Huweimi 

part of the Beds with the lower part of the Dammam 

Formation in the wells is probable.

Diagnostic fossils and age

The fossils contained in the rock samples from the 

reference section of the Dammam Formation are on 

the whole badly preserved; they include such forms 

as Lockhartia Jiunti Ovey var. pustulosa Smout and 

Nummulites discorbinus (Schlotheim). However, a 

more numerous fauna determined from the surface and 

subsurface sections, includes algae, silicified wood, fish 

remains, ostracodes, lamellibranchs, Meretrix (Cordiopsis) incrassata Sowerby, Pecten sp., Ostrea sp., 0. 

(Pycnodonta brongniarti) Bronn., cf. 0. multiscostata 

Deshayes, gastropods, echinoids, sponge spicules, bryozoans, Alveolina spp., A. aff. elongata d'Orbigny, A.
SOUTHWESTERN IRAQ G39

elliptica (Sowerby), A. elliptica (Sowerby) var. nuttalli Davies, A. globosa, Articulina sp., A. iraqii Henson, Assilina sp., Bigenerina sp., B. cf. truncata d'Orbigny, Coskinolina sp., C. balsiUiei Davies, Dictyoconoides spp., D. kohaticus Davies, D. cookil (Carter), 

Dictyoconus sp., D. indicus\ Davies, D. aegyptiensis, 

Discorbis sp., Halkyardia sp., H. minima (Liebus), 

Linderina sp., L. buranensis Nuttall and Brighton, 

L. brugesi, Lituola sp., Lituonella sp., Lockhartia tipperi Davies, Meandropsina sp., miliolids, Nummulites 

spp., N. beaumonti d'Arcliiac and Haime, N. discorbinus\ Sch.loth.eim, N. gizehensis (Forskal), N. lucasanus 

d'Archiac, N. tsubaticious Douville, Operculina sp., 0. 

libyca Schwager, Opertorbitolites douvillei Nuttall, 

Orbitolites spp., 0. complanatus Lamarck, Peneroplis 

spp., P. aff. carinatus, P. damesini Henson, P. dusen- 

~buryi Henson, Praerhapidionina huberi Henson, Pyrgo 

sp., Rhapidionina spp., R. macfadyeni ?Henson, R. 

urensis Henson, R. urensis Henson var. minima Henson, Rhipidionina sp., R. williamsoni Henson, Rotalia 

sp., Sakesaria sp., Saudi sp., S. discoidea Henson, Siderina sp., Sismondia sp., Somalina sp., S. danieli Henson, Spirolina sp., S. huberi Henson, S. cylindracea 

Lamarck, Textularia, Valvulina sp., and V. kitchini 

Henson.

The Dammam Formation is middle Eocene, according to Owen and Nasr (1958); however, absence of 

the upper Eocene has not been proved in Saudi Arabia 

(Sander, 1952; Steineke and others, 1958). Bellen 

(Bellen and others, 1959) maintained that the possibility of the base of the formation being late early 

Eocene cannot be excluded.

Contacts and equivalent units

The contact of the Damman Formation with the 

underlying Rus Formation is possibly a minor disconf ormity. There is slight indication of an unconformity 

between the Rus and the Dammam Formations in well 

sections, where a gray-green shale approximately 1 

feet (2.5 m) thick occurs near or at the base of the 

Dammam (Bellen and others, 1959). No such unconformity is known to have been found in the field, but 

the lower part of the Huweimi (chalk) Beds contains 

white or pinkish chalks of fresh-water appearance (R. 

M. Ramsden and C. Andre, unpub. data, 1953). On the 

other hand, the upper part of the Huweimi (limestone) 

Beds is definitely representative of marine deposition.

Within these chalks of fresh-water appearance, however, are chalky limestones containing lamellibranchs. 

This disturbing fact has not been properly investigated in the field as yet. To date there is little reason 

to doubt the existence of a major facies change between 

lower and upper parts of the Huweimi Beds. In exposures in western Iraq, the Wagsa Beds of the Dammam Formation rest directly on the Umm er Radhuma Formation, presumably with unconformity.

The contact with the overlying Ghar Formation is 

unconf ormable. In the eastern sector of this area the 

Dammam Formation is overlain trangressively with 

great unconformity by the younger units: Euphrates, 

Zahra, Lower Fars, and Dibdibba Formations, and Recent alluvium. The Zahra and Euphrates Formations 

rest directly on the Dammam throughout an extensive 

sector in the northern part of the area.

The type locality of this formation is in Saudi Arabia 

at lat 26°ir03" N., long 50°07'07'/ E. The typical 

Dammam extends into Kuwait. North and northeastward from the study region the formation passes laterally into the basinal Jaddala Formation (as in wells 

Awasil No.5, Anah No.2, and Falluja No.l).

Economic aspects

Owen and Nasr (1958) stated that:

due to its natural porosity, enhanced by leaching of fossils, the 

Dammam Formation is an excellent aquifer, and losses of circulation while drilling through this formation are a common 

occurrence. In Rumaila and Zubair fields, saline sulphurous 

waters are encountered. This is also the case in the BurganMagwa-Ahmadi area of southeastern Kuwait. However, westward from Burgan the quality of the water progressively improves and in the vicinity of Abdulla the Kuwait Oil Company 

has developed a supply of industrially usable water carrying 

some 3,700 ppm of total dissolved solids, some 1,400 ppm of 

calcium sulphate.

The Dammam limestones cover an extensive part of 

western Iraq, including most of Al Hajara physiographic belt. A considerable number of wells have been 

dug and drilled in the area. Many of these wells 

reach into the Dammam. Most of them are in surface 

depressions, or not far from the base of scarps, or in 

some wadi system. Macfadyen (1938) stated that:

A comparison of altitude and depth of water, taking into account so far as possible the topography and geological structure, does not appear to show any definite relationship between 

the water wells. Chemical analyses show wide variations, 

though the general type of water is, with few exceptions, similar throughout the area in both shallow and deep wells and 

springs.

It may be said with a measure of confidence that the evidence is sufficient to show that there is no true water table in 

the strong part of the southern desert, i.e., the Hajara, so that 

a well sited for convenience cannot be expected a priori to find 

water at any known depth. All ground-water supplies seem to 

be due to local accumulation of rainwater, and successful wells 

are those sited in or close to natural drainage lines, or in actual 

storage features, such as depressions with no outlet.

The water supply is thus in very direct relation to the rainfall, and in deep wells the supply is due to percolation downwards by means of fissures or solution holes, and storage in 

such cavities or in some porous horizon at depth. Thus, wells 

are best located in close relation to favourable surface features.
G40 GEOLOGY OF THE ARABIAN PENINSULA

Faulting and folding appear to be rare in this desert, so that 

generally little help is available from the geological structure of 

the rocks. The varying quality of the waters, where this is 

not due to concentration by evaporation, is due to contact with 

more or less soluble constituents in the strata on its way down 

from the surface. The strata vary from place to place, and so 

give rise to some marked quantitative differences in the analyses of the waters in question.

MIOCENE AND PLIOCENE

The Miocene and Pliocene are represented by five 

formations having a maximum aggregate thickness of 

more than 3,350 feet. The stratigraphic correlation of 

the units is rather tentative because of lack of adequate 

information. The correlation adopted is that generally 

accepted by geologists of the Iraq Petroleum Company.

ZAHEA FOBMATION

The Zahra Formation type section is in the small 

anticline that lies northwest of the depression Faydat 

Az Zahra, which lies 4.3 miles (7 km) south of Al 

Busayyah-As Salman track and 40 miles (65 km) west 

of Al Busayyah.

The Zahra Formation is 328 feet (100 m) thick at the 

type locality; elsewhere, the thickness of the formation 

is extremely variable, and it has a widespread but 

sporadic occurrence. Between the type locality and As 

Samawah, the Zahra Formation occurs as isolated outliers on the Dammam Formation, predominantly occupying the old topographic depressions and former 

wadi floors. Typical outcrops are the Zahra outliers of 

Shaib Abu Al Hazair, 30 kilometers southwest of 'Ayn 

Sa'i; Faydat Abu Sidaira and Shaib At Tayyar, 40 

kilometers southwest of As Samawah; and Shaib Al 

Ash'ali, 10 kilometers west of Ayn Hamud.

To the west of this main belt of outcrop, in many 

topographic depressions, are beds that closely resemble 

the Zahra Formation. Examples occur in the As Salman group of depressions, such as Lahab and As Salman itself, where characteristic limestones and marls 

appear. Another good example occurs at Ash Shabakah (Shabicha) where the marls and limestones, lying 

on eroded surfaces, are slightly folded. Extensive outliers can be found in Shaib Al Ghanimi between Al 

Burga and Ar Eimtha, on At Traq between Faydat Al 

Uba'id and Khashm and Dimana, and between Faydat Umm Jalib and Faydat Ar Eudhaima, Traq Al 

Galaib, and Ash Shawiyyat. The Zahra beds may 

occur at any altitude in relation to the present drainage system. In the two last-mentioned localities, the 

beds cap some of the highest points of the two structures there, in places attaining altitudes of 20 to 30 

meters above the valley floor, whereas in Ash Shafoakah 

depressions the top of the exposed Zahra beds is in 

places as much as 30 meters lower than the Eocene

limestones. Northward from the type location, towards 

Galaib, the Zahra Formation thins considerably to only 

a few meters of white limestone. To the northeast, in 

the valley of the Shail Al Ghanimi, however, a thickness of more than 100 meters occurs in the normal 

facies of white limestones and red marls, indicating 

apparent eastward thickening. In the Zahra-Kabd 

(Chabd) area, eroded ridges of Dammam (Ghanimi 

and Chabd Beds) project through the Zahra beds 

which here commonly show false or exaggerated dips.

Along the southwestern Iraqi border, the Zahra Formation is extensive, commencing in the area of As 

Sadawiyat, 15 kilometers northwest of Nisab, and following the strip between Johka and Faydat al Batn 

and the border. In the region of Faydat Al Aghsas 

and Al Jumaima numerous inliers of Umm er Eadhuma (Basita Beds) occur within the Zahra beds. The 

Zahr Formation extends into Saudi Arabia, near Birkat Al Amya, and reenters Iraq about 12 kilometers 

east of Al Lifiyah, where it continues northwestward 

rather irregularly. Farther north, it occupies the wadi 

floors of the Shaib Hisb, of Ghar as Saigal, and of Wadi 

al Khirr toward Al Lasaf; west of this line, and in the 

general area, it occurs as irregular patches, especially 

in the vicinity of Al Ashuriyah, Al Jamiliyah, and Bir 

Samit.

The Al Makim-Al Barrit-Al Lasaf region is extensively covered by Zahra beds containing numerous 

large inliers of Umm er Eadhuma beds. The formation forms most of the gravel plain between Wadi al 

Ghadaf, Wadi al Ubayyi$, and Wadi Tibal. It also 

covers part of the area west of Tar al Bahr. Westerly 

extensions of the formation occur in the Wadi 'Ar'ar, 

Wadi Hamir in the vicinity of Al Artawiya-Makhot, 

Shaib Ubaydat, east of Al Ubaydat, Shaib ash Shaykh, 

Wadi al Ubayyid, and Nukhaib; the formation extends 

farther north, where it covers the Habbariyah area.

Detailed lltnologlc description

The Zahra beds consist of gray, white, and red porcelaneous hard locally sandy limestones, which contain 

occasional Chara, fresh-water molluscs, and ostracodes 

and are interbedded with red sandy marl. Oblong 

cavities filled with a red sandy material are common 

in the limestones, especially in the west margin between 

Shaib Hisb and Wadi Al Khirr, and have been explained as being fossil cavities of roots of old reeds. 

According to T. F. Williamson and D. C. Eoger 

(unpub. data, 1939), the formation as a whole becomes 

markedly thicker southward, until in the extreme south, 

near the Neutral Zone, it becomes a complete succession of grits and marls containing only a few thin 

fresh-water limestones. However, H. Huber (unpub.
SOUTHWESTERN IRAQ G41

data, 1944:), is in favor of including the sands and 

grits in the locality near the Neutral Zone in the Dibdibba Formation.

The succession may consist of strata of white and 

reddish-brown marls, sandy marls, sands, irregular 

pinkish-white chalky concretional limestones, and conglomeratic or brecciated river deposits such as those at 

Saigal and As Salman.

In the Wadi Tibal and Wadi Al Ubayyid area the 

formation consists mostly of fresh-water limestones 

and coarse conglomeratic river deposits composed of 

recrystallized limestones of the Umm er Radhuma and 

Tayarat Formations.

Diagnostic fossils and age

Fossils in the Zahra Formation are rather rare; they 

occur mostly in the limestones and include Ohara sp., 

Melanoides tuberculata Mueller, Lymnaea, Planorbis 

sp., Vivipara, and ostracodes. Some of the limestones 

show tubular holes probably made by reed stalks.

The age of this formation is still an open question, 

but opinion favors a Miocene (Fars) age. According 

to R. M. Ramsden and C. Andre (unpub. data, 1953), 

the formation overlies marls containing Ostrea latimarginata Vredenburg in the Shaib Hisb area; however, it could also be equivalent to the Euphrates Limestone, and thus be of early Miocene age.

Contacts and equivalent units In adjacent areas

The Zahra Formation may rest on any of the older 

units; the base of the type section is obscured by debris 

and cannot be seen, but a few kilometers to the southwest the white limestone beds rest on the brown Dammam limestones.

In the As Salman area, H. Huber (unpub. data, 

1944) found, at the base of the Zahra, a coarse-grained 

red sandstone, 13 meters thick, succeeded by wellbedded limestones containing sporadic marly intercalations. These bedded limestones contain numerous Planorbis sp., Lymnaea sp., Ostracoda^ and Chara, but 

locally they may vary to gray cavernous limestones 

composed entirely of stalks and seeds of CJiara. At 

Ukhaidir and in the Shaib Hisb the bedded limestones 

overly marls containing Ostrea latimarginata Vredenburg (indicative of Lower Fars), and near Takayyid 

(Tuqaiyid) the Zahra Formation rests on a pebble bed 

which has been taken to represent the Ghar Formation. Along the southwest border of Iraq the Zahra 

rests unconformably on beds of either Umm er Kadhuma or Dammam Formation. The Zahra Formation 

unconformably underlies Recent gravels and sand in 

the Rudhuma-Chabd area. It may be partly equivalent 

to the Hofuf Formation of Saudi Arabia and may also

be equated with the Euphrates and Ghar Formations 

of southwestern Iraq.

EUPHRATES LIMESTONE FORMATION

Type and reference sections

The type section of the Euphrates Limestone Formation is near Wadi Fuhaimi at lat 34°15/58// N., long 

42°08/09// E., outside the study region. In southwestern Iraq, however, C. Andre and P. J. Walmsley 

(unpub. data, 1952) referred to good exposures in Shaib 

Hish, 30 kilometers southwest of An Najaf; Wadi Al 

Muhari, southwest of As Shinafiyah and Sha'ib Firk 

(Firch); and Sha'ib Abu Marls, southwest of As Samawah. In the Al Busayyah area, a section just north 

of the Abu Ghar police post was described by T. F. 

Williamson and D. C. Rogers (unpub. data, 1939).

Thickness and extent

In the type section, which is outside of the study 

region, the thickness of the Euphrates Limestone Formation is only 26 feet (8m). The Euphrates crops out 

over a wide area west of the Euphrates River where 

it overlies the older rocks transgressively, thus forming the eastern part of the western desert of Iraq, west 

of Karbala. The outcrop may exceed 100 kilometers in 

width. The Euphrates has a thickness of 15 meters in 

Wadi Al Ubayyid, less than 30 kilometers west of Qasr 

al Ukhaidhir; slightly south of Khan ar Runbah, itattains a 20-meter thickness. In this general area the 

width of outcrop decreases to about 50 kilometers, except for the few westerly isolated, scattered outliers. 

In the Kifl well No.l the Euphrates-Jeribe Limestone 

Formation is 143 feet thick. These outcrops extend 

as far south as As Samawah, where they apparently 

disappear under the younger rocks; however, the Euphrates reappears farther south in the area of Makhfar (Shurtat) al Qusayr, from where it extends southwestward, swinging southward from the vicinity of 

Qal'at Abu Ghar, to an area south of Qulban Layyah, 

where the outcrop width may be as much as 50 kilometers. H. Huber (unpub. data, 1944) estimated a 

thickness of 30 to 35 meters for the Euphrates in the 

vicinity of Qal'at Abu Ghar.

Detailed litnologic description

In the type section the Euphrates Limestone Formation consists mainly of shelly, chalky well-bedded recrystallized limestones. West of Karbala the Euphrates consists chiefly of pseudo-oolitic generally soft 

marl-limestones, with chalky aspects and a few recrystallized bands.

In As Samawah and Makhfar Shurtat al Qusayr 

areas, the Euphrates consists of white limestones 

(some with pinkish tints) and marly, sandy limestones,
G42 GEOLOGY OF THE ARABIAN PENINSULA

having a few siliceous intercalations and thick beds 

of loosely consolidated rounded sand, indicating the 

gradual influence of the Ghar elastics. Farther south 

this influence is strongly manifested in the section at 

a small hill just north of the Qal'at Abu Ghar police 

post, where the following section was described by T. 

F. Williamson and D. C. Rogers (unpub. data, 1939):

Thickness 

(meters) 

6. White, sandy, marly, fossiliferous limestone, with

abundant lamellibranchs, Clausinella sp Vz 

5. White calcareous grit __ _ 1 

4. White calcareous marl grading into above _ ____ 4 

3. Pale greenish-white sand ______ _ 4 

2. White, sandy calcareous marls, slightly and locally

ironstained __ ___ ___ _ ____ 3 

1. White marly limestone ___________________ %

South of Al Busayyah the Euphrates perhaps passes 

completely into the clastic Ghar Formation.

Diagnostic fossils and age

At the type section of the Euphrates Limestone Formation the following fossils were recognized: Dendritina sp., Rotalia beccarii Linn, miliolids, gastropods, 

lamellibranchs, and a rare Bryozoa, Cellepora sp. (Bellen and others, 1959).

In this area the fossils are rare and poorly preserved, 

they include a fauna of lamellibranchs including 

Clausinella sp. and Ostrea latimarginata Vredenburg 

and also gastropods and the foraminifer Peneroplis 

farsensis. The formation is early Miocene, according 

to Bellen (Bellen and others, 1959).

Contacts and equivalent units

In the type section the Euphrates Limestone Formation overlies the Oligocene Anah Limestone unconf ormably; however, in the southwestern desert this formation, being transgressive, overlaps older formations. 

The base of the Euphrates southwest of Karbala is a 

marked Clausinetta-luma,cho[[Q horizon. In some 

places, the base is slightly conglomeratic, sandy, and 

commonly current bedded, as in the Sha'ib Abu Maris, 

or marly and chalky as at Shaib Hisb. The Euphrates 

rests unconformably on the Ghanimi to Shawiya beds 

northwest of Shaib Hisb, whereas to the southeast they 

may lie on the Ghanimi and Barbuk beds.

According to Bellen (Bellen and others, 1959), the 

Euphrates in the type area is covered by Jeribe Limestone Formation. In the western desert however, according to C. Andre and P. J. Walmsley (unpub. data, 

1952), the Euphrates underlies the Lower Fars Formation and passes upward and laterally grades into it. 

The Lower Fars Formation west of the Karbula-An 

Najaf area is marked by a line of sabkha deposits 

along the west bank of the Euphrates River. It may

be directly overlain by the Dibdibba Formation, as in 

the area of Qulban Layyab.

The Euphrates Limestone Formation is equated with 

the Ghar Formation, which is considered to be the 

clastic facies of the Euphrates, and with the Serikagni Formation, which is considered to be the basinal 

facies of the Euphrates. The Zahra Formation may 

well be the fresh-water facies of the Euphrates. These 

relationships are rather complex and require verification by additional investigations.

Economic aspects

Water supplies. Macfadyen (1938) pointed out the 

presence of a narrow and sharply defined zone of 

springs of economic importance along the east border 

of the southern desert. They lie mainly between the 

Euphrates and the overlying Lower Fars Formation 

and other younger formations, including the Recent 

alluvium. These springs include Ayn Ubaira, Ar Eahhallya group (lat 32°46' K, long 43°23' E.; alt 90 m 

above sea level), Ayn Fashash, Ayn al Bakhaiti, Ayn 

Zanga, Ayn Umm Shilla, Ayn Faidhia, Ayn Soda, 

Ayn ed Din (lat 32°41' N., long 43°23' E.), Ayn Umm 

al Khers (lat 32°37' N., long 43°26' E.), Shithatha 

group (lat 32°34' N., long 43°29' E.), Ayn Erbid, Ayn 

al Hamra, Ayn Abeid, Ayn Barkat, Khan ar Rahbah 

(lat 31°44' N., long 44°19' E.), Ayn Hassan, Ayn al 

Gayiani, and Ayn Seyid (lat 31°00' N., long 45°27' E.).

According to Macfadyen (1938),

The total of water from these springs is not less than 150 

cusecs (425 centicumecs), and this appears to be a permanent 

and invariable supply. It is very unequally distributed, over 

100 cusecs (283 centicumecs) being yielded by the Shithatha 

group alone, while other of the springs are very small.

The quality of the water varies slightly from spring to spring, 

but the analyses form a homogeneous group of saline water, 

characterized by moderately high magnesium, moderate calcium sulphate, high sodium chloride, and low carbonates. It 

is interesting to observe the approach of the Euphrates water 

towards this composition at the top water season in September. 

The springs often contain a little sulphurated hydrogen. The 

water is therefore not of the best quality, but it is drinkable 

and it is used extensively for irrigation of date and other fruit 

gardens. The temperature of water varies from 24° to 27 °C, 

most springs being about 26°C.

These springs are replenished from rainfall in the 

southern desert that drains underground toward the 

Euphrates Kiver, down both dip and slope; water of 

the Euphrates basin that makes its way southward 

along the strike of porous strata in the Euphrates 

limestone or lower fars beds; and, to a much lesser 

degree, by rainfall in the northeastern Iraqi mountains 

that makes its way beneath the Mesopotamian plain 

in the Euphrates Formation. (For further details see 

Macfadyen, 1938.)
SOUTHWESTERN IRAQ G43

Hydrocarbons. Because the * Euphrates Limestone 

Formation is extensively exposed in the area, the 

chance of finding accumulations of hydrocarbons in it 

is rather remote, though oil and gas in good quantities 

have been found in it elsewhere, as in the Jambur and 

Bai Hassan oil fields of northern Iraq.

QHAB FORMATION

Type section, thickness, and extent

The type locality of the Ghar Formation is in BPC 

well Zubair No. 3, (lat 30°23'01" N., long 47°43'29" 

E., alt 51.9 ft; completed Feb. 21, 1951). The formation occurs between drilled depths of 1,513 and 1,935 

feet. It is 422 feet (129 m) thick. This formation has 

been recognized in the Basrah Petroleum Company oil 

fields of Zu'bair and Eumaila, in Nahr Umr wells, and 

in the BPC wells Tuba No.l, Eachi No.l, Eatawi Nol, 

and Luhais No.l. In southern Iraq its thickness 

ranges between 300 and 500 feet; it shows thinning 

and change of facies eastward, northward, and westward by gradual passage to marly or silty sandstone. 

At Kifl well No.l this interval is represented by the 

Jeribe limestone, whereas in the outcrop area it is 

possible that the Ghar sandstones pass into the Lower 

Fars marls and limestones and into the Euphrates 

limestones, which cover a considerable territory.

According to Owen and Nasr (1958), "the thickness of the Ghar Formation increases in a southwesterly direction towards what is believed to be the source 

area. Eepresentative thicknesses in the southern Iraq 

fields are Nahr Umr No.l, 350 feet; Zubair No.3, 422 

feet; and Eumaila No.7, 544 feet."

Detailed lithologic description

The Ghar Formation consists of: coarse sand, containing pink, black, and colorless gravel pebbles and 

locally having a calcareous cement; sandy limestone; 

marl; and streaks of gypseous blue clayey shale. In 

the outcrop area the entire Ghar formation seems to 

pass laterally into the Euphrates Limestone Formation, or it could be included with the Dibdibba Formation, in the absence of the intervening Lower Fars 

Formation, because of their close lithologic familiarities.

In this last respect Owen and Nasr (1958) stated, 

"that the division of the Kuwait Group into Dibdibba, Lower Fars, and Ghar Formations depends essentially on the presence of the Lower Fars beds, and where 

these are absent or only questionably developed, it has 

been found practically impossible to separate accurately the two clastic formations of the Dibdibba and Ghar. 

When this occurs, reference can usually be made only 

to the Kuwait Group."

The age of this formation is unknown, for fossils are 

absent, but it is considered to be early Miocene, though 

Owen and Nasr (1958) assigned to it an Oligocene and 

early Miocene age.

Contacts and equivalent units

The contact of the Ghar Formation with the underlying Dammam Formation is unconformable and is 

marked by the absence of proved Oligocene strata. The 

contact with the overlying Lower Fars Formation is 

gradational, without evidence of unconformity; in the 

absence of the intervening Lower Fars Formation, the 

Dibdibba Formation may overlie the Ghar Formation 

unconformably?, but it has been very difficult to 

recognize or separate them with great accuracy.

From stratigraphic considerations, the Ghar Formation seems to be equatable with the Hadruth Formation of Saudi Arabia.

Economic aspects

Eegarding the geologic occurrence of oil and gas in 

Iraq, the staff of the Iraq Petroleum Company (unpub. 

data, 1958) stated that very viscous sulfurous heavy 

oil (20° API) occurs in the lower limestone beds of 

the Lower Fars Formation, the upper Sands of the 

underlying Ghar Formation, and at Nahr Umr. This 

oil is locally called "First Pay."

LOWER FARS FORMATION

The heterogeneous Lower Fars Formation is the 

basal (evaporitic) division of Pilgrim's (1908) tripartite "Fars Series," according to Owen and Nasr (1958). 

The name Lower Fars was first published by Busk and 

Mayo (1918). In Iraq, equivalents of the Lower and 

Middle Fars were called the Hamrin Stage by Pascoe (1922), but the Middle Fars is so poorly developed 

outside Iran that it has long been included as an Upper Fars division, the term "Lower Fars" being arbitrarily used for the evaporitic Miocene below the highest bedded anhydrite.

Reference section, thickness, and extent

BPC well Nahr Umr No.l (lat 30°45' K, long 

47°40' E.; alt 21.5 ft) was completed Feb. 10, 1949; 

the Lower Fars Formation lies between drilled depths 

of 1,090 and 2,093 feet (1,003 ft thick). This formation has been found in the subsurface sections of the 

Basrah oil fields at Nahr Umr, Zubair, and Eumaila 

and at wells Tuba No.l, Eatawi No.l, Eachi No.l, and 

Luhais No.l. According to Owen and Nasr (1958) it 

ranges in thickness from 1,003 feet at Nahr Umr to 

600 feet at Zubair and 300 feet at Eumaila. However, 

the thickness shows great local variations; for example, westward the thickness of Lower Fars at Luhais 

well No.l is about 390 feet, and a similar thickness was
G44 GEOLOGY OF THE ARABIAN PENINSULA

found at Kifl well No.l, north of the area of present 

discussion. To the south in Kuwait the Lower Fars, 

according to Owen and Nasr (1958), is readily recognizable in the Kuwait Oil Company's well Raudhatain 

No.l between drilled depths of 345 and 704 feet. 

Farther east in Kuwait, in the Jal-es-Zor escarpment 

along the north side of Kuwait Bay, is a 125-foot unit 

of Lower Fars equivalent. In the western desert this 

formation crops out in the area of Qasr Shaqrah, Dafina, Qasr Naba, Qasr Abu Ghar, Al Busayyah, Al 

Buwaib, and Khashmat ibn Hallaf. Isolated outliers 

of the Lower Fars lying above the Euphrates occur at 

Sha'Ib Abu Marls, 30-35 kilometers west and southwest of As Samawah. Farther north, outliers of Lower 

Fars Formation are found at Al Muhair and Quwairat 

al Habbusa, southwest of As Shinafiyah, and at Khan 

ar Rahbah and Qarat as Saba. Extensive outcrops 

spread west of An Najaf in the area around Az Zigla 

and Gur al Hibari.

In the Gur al Hibari area the Lower Fars Formation has been subdivided into two units; the lower is 

yellow limestone with some white chalk bands and 

green and red marls, and the upper is selenite. The 

lower unit is considered to be much more persistent. 

In the Qasr al Ukhaidir area the thickness of the 

lower unit does not exceed 130 feet, and the width of 

continuous outcrop is not great, but outlying hills 

of it occur almost as far west as the Nukhaib police 

post, some 130 kilometers west of where the formation disappears under the overlying sandstones and 

marls. As for the upper unit, T. F. Williamson and 

D. C. Rogers (unpub. data, 1939), after a study of the 

north-trending cliff which passes east of the Bahr al 

Milh, maintained that the sequence is a lateral variant 

of the basal small section of the Upper Fars Formation; this is clearly exhibited at the Nuqtah Cliff section on the track between Shithatha and Karbala 

where the basal beds of the Upper Fars Formation 

contain much crystalline selenite. Northward these 

beds show such an increase of gypsum that 15 kilometers northwest of the Tazaza village, distinct white 

beds of selenite can be distinguished. These attain 

thicknesses of as much as 5 feet (l1/^ ni) and stand 

out as hard bands among marls and a sandstone sequence. South of the Shithatha-Karbala track only 

the lower unit is recognizable. It forms numerous outliers in the area of Wadi al Ubayyid. Even along the 

base of the scarp toward An Najaf there is comparatively little continuous outcrop. Here the formation dips gently to the northeast, with the result that 

only a few isolated outliers of limestones and green

marls can be seen resting on the Euphrates Limestone 

Formation. These are present in the low ground from 

near the Euphrates river to As Samawah, beyond 

which there are practically no further occurrences 

until the group emerges again from the sabkhas near 

Makhf ar al Qusayr. From here southward the Lower 

Fars appears to be insignificant, and its name has, for 

the sake of uniformity, been applied to the rather 

marly fossiliferous beds above the Euphrates and below the more definitely sandy beds of the Upper Fars 

and Dibdibba Formations. In places the Lower Fars 

is very thin. It consists mainly of greenish marls 

with abundant selenite crystals, and sporadic "oyster 

beds" occur containing very numerous Ostrea latimarginata Vredenburg, pectens, and Clausinella sp. 

One good example of these "oyster beds" can be seen 

just west of the track from Al Busayyah to An Nasiriyah at a point about 10 kilometers north of Qasr 

Naba. At Al Busayyah, Laiyah, and farther south, 

the grits and marls rest directly on the Euphrates 

Limestone Formation.

Detailed lithologic description

The Lower Fars Formation is an evaporitic sequence; 

it comprises the lithologic units given below:

Thickness 

(feet)

4. Mudstone, gray and red, with subordinate streaks of

nodular gypsum; some red-blue marl and loamy grit. 258

3. Limestone, buff, sandy, porous, shelly, fossiliferous 

(Clausinellid), in part oolitic or cherty and porcelaneous, alternating with gray clayey mudstones, 

shales, marls, and rare streaks of selenite 321

2. Selenite beds alternating with gray-blue mudstones

and brown-blue marls _____ ___ 249

1. Limestone, pinkish-white, sandy, in part oolitic or very 

shelly, recrystallized with rare streaks of blue mudstone especially toward base 175

To the south, in Kuwait, the Lower Fars consists 

mainly of anhydrites, gypsum, clays, marls, and shallow-water limestones containing Ostrea latimarginata, 

Clausinella sp., and Quinqueloculina, sp. Farther south 

into Kuwait, in the Val-es-Zor escarpment along the 

north side of Kuwait Bay, there is ia 125-foot unit of 

fine-grained sand, silt, and clay carrying Ostrea latimarginata, 0. vestita, lamellibranchs, and other fossil 

forms. This unit is locally called the Zor Formation.

According to T. F. Williamson and D. C. Rogers 

(unpub. data, 1939), the thickness of the Lower Fars 

Formation in this southwestern region is insignificant 

in comparison with that attained in the Iranian and 

north Iraqi foothills.

Westward from the Euphrates River, the Lower 

Fars Formation becomes generally and gradually less
SOUTHWESTERN IRAQ G45

evaporitic and passes into clastic sedimentary rocks 

laterally and vertically.

Diagnostic fossils and age

The fossils of the Lower Fars Formation include 

Ostrea latimarginata Vredenburg, Clausinella sp., pectens, gastropods, Quinqueloculina sp. and other miliolids, ostracodes, and other forms. Owens and Nasr 

(1958) considered the age of this formation to range 

from early to middle Miocene.

Contacts and equivalent units

The underlying contact of the Lower Fars Formation with the Ghar Formation is conformable and 

gradational in the Basrah area; in the Karbala, An 

Najaf, and As Samawah areas the Lower Fars Formation overlies the Euphrates Limestone Formation 

conformably.

The contact with the overlying Dibdibba Formation 

is gradational (Owen and Nasr 1958). Northward, 

the Upper Fars replaces the Dibdibba Formation 

gradually. Here the Lower Fars Formation is considered as being equivalent to the Dam Formation in 

Saudi Arabia and the Zor Formation in Kuwait.

Economic aspects

In the Basrah area the basal limestones of the Lower 

Fars Formation and the upper sandstones of the Ghar 

Formation generally contain very viscous sulfurous 

heavy oil (20° API). The two zones constitute what 

is called the "First Pay." The Lower Fars anhydrites 

and shale can act as caprock under favorable circumstances for the accumulation of oil.

DIBDIBBA FOEMATION

Reference section, thickness, and extent

Macfadyen (1938) mentioned Birjisiya (lat 30°22' 

N., long 47°38' E.) as the type locality for the Dibdibba Formation. Since his account, however, the 

thickness of the material exposed in the hand-dug 

water well at Birjisiya has been far exceeded in nearby 

Zubair, where numerous wells of the Basrah Petroleum 

Company have penetrated deeper than 1,000 feet (305 

m). Owens and Nasr (1958) did not specify a type 

locality but merely mentioned the presence of the Dibdibba Formation at the surface and in all drilled wells 

on the Dibdibba plain. In the BPC well Zubair 

No. 3 this formation lies between the ground surface 

and a drilled depth of 935 feet.

It is as much as 1,160 feet (354 m) thick in the 

northernmost wells of the Zubair oil field. The formation has extensive distribution in southern Iraq, for 

it covers the area between the Shatt al Arab Kiver in 

the east and Haur al Hammar and the Euphrates

River in the north. Westward, the outcrop trends 

from Tall al Muqaiyir Dafina, Qasr Shaqrah, to Al 

Amghar, where it extends farther west to Jan Khirsan, Al Buwaib, Adam, Abu Radham, Tukayyid, and 

the Neutral Zone. Southward, it extends into Kuwait 

and Saudi Arabia. The region covered by this formation is known as Ad Dibdibba.

This formation shows gradual decrease in thickness 

westward. In the Rumaila field it decreases to some 

500 feet; farther west, to about 160 feet in the vicinity 

of Luhais well No.l. Northeast of Abu Ghar, a thickness of 196.8 feet (60 m) has been noted. Still farther 

west the formation disappears completely. According 

to Dunnington (1958), "thinning takes place also 

towards the south where over 700 feet (243 metres) occur in the southern wells of the Zubair oil field." Northward and eastward this formation disappears under 

the Recent alluvium.

Northward, the Dibdibba Formation passes into the 

Upper Fars Formation which crops out as a scarp between Ur Junction and Al Shararain. Here the 

Upper Fars consists of about 8 meters of sandy marl 

containing selenite, which dips gently to the northeast. 

These marls are practically surrounded by sabkha. 

Farther northwest, in the sandhills south of As Samawah, the few Upper Fars sandstone inliers outcrop 

from the sabkha. To the southeast of An Najaf, very 

little is seen of this formation, as the Lower Fars 

Formation extends under the recent deposits of the 

Euphrates River valley. A thickness of at least 154 

feet (50 m) for this formation was measured by T. F. 

Williamson and D. C. Rogers (unpub. data 1939) in 

the scarp east of Ukhaidhir. Farther north, this formation forms the prominent scarp, which overlooks 

the Bahr al Milh and Abu Dibs from the east, and extends eastward toward the Euphrates. The cliff extends south-southwest and then turns sharply to the 

east to An Najaf. The base of the formation follows 

roughly the line of the foot of the cliff. In this area 

it comprises a series of brown, red, yellow, and green 

marls containing some sandstone bands and, at the 

base, some disseminated selenite crystals.

Detailed lltnologlc description

The composition of the Dibdibba Formation is, 

"mainly sand and gravel of igneous rocks, including 

pink granite, various liver-coloured and slate-grey intrusives, dolerites, etc., and white quartz pebbles. Not 

infrequently the rock is cemented to a hard grit." 

(Macfadyen, 1938).

Similar lithological sequence occurs in the subsurface sections of the Zubair field where the Dibdibba
G46 GEOLOGY OF THE ARABIAN PENINSULA

Formation consists chiefly of coarse sands grading into 

gravel with subordinate arenaceous limestone beds and 

gypsum streaks.

In the northern part of the Dibdibba area, in a low 

ridge southwest of Un Junction, coarse- and mediumgrained Upper Fars sandstones grade southward into 

the pebbly Dibdibba Formation.

Between the Tukayyid scarp, Jalib Al Amghar, and 

the north tip of the Neutral Zone, a great thickness of 

pebbly sandstones, grits, and fresh-water limestones occurs. H. Huber (unpub. data, 1940) included in the 

Dibdibba Formation the loose sands and loosely cemented sandstones that occur in the depression between 

the Tukayyid (At Traq) scarp and the Neutral Zone 

and the Eocene beds in the west. In these sandstones 

Huber observed some pebbly beds. In several places 

along the Tukayyid scarp from Khashmat al Masasah 

to Jalib al Amghar, as well as along the north boundary of the Neutral Zone, the Dibdibba sandstones 

and grits contain intercalations of fresh-water limestones. Some of the limestones, especially to the west 

of the Tukayyid well, are dark greenish brown and contain coarse rounded sand. The limestones are well 

bedded ( + 0.20 m) and very hard and contain Chara 

sp., Planorbis sp., and Radix.

Several small hills in the vicinity of the Tukayyid 

well are composed of a bedded, at places sandy, dense 

gray and white limestone; at many places it shows 

reddish tints and contains remains of Chara. Some 

dark gray thick-bedded hard limestones containing 

Chara, also occur. Between this well and Al Karatha, 

similar Chara, bearing limestones are interbedded with 

sandstones and grits on top of the scarp along the 

track leading to Jalib Al Amghar and to Maghaizil 

and along the northwest border of the Neutral Zone. 

Some conglomerates occur in the vicinity of Ad Dayir.

K. M. Kamsden and C. Andre (unpub. data, 1953) 

stated that the exposed part of this formation can be 

divided in ascending order, into two recognizable units: 

Chilawa Sandstone and Kafaiya Gravel.

Chilawa Sandstone. The Chilawa Sandstone consists of bedded unfossiliferous sandstones and grits. A 

type section at Chilawa near triangulation point 130 m 

(coordinates 1.672,401.44 E.; 88.7,596.94 N.) exposes approximately 180.4 feet (55 m) of gritty white, pink, and 

green sandstone. The white sandstones have a calcareous 

cement and form terraces and erosional plains. The 

pink sandstones are softer than the white and probably 

contain some calcium sulfate and grains of sulfur. The 

green sandstones are generally uncemented and slightly marly. Small brown quartz pebbles are scattered

throughout the section. This unit forms the upper 

part of the Al Busayyah-Qasr Shawrah scarp and 

passes eastward across the Dhafir plains to the Boliya 

axis. The exposures can be seen in the scarps at Twai 

al Hashash, Khabra or Kachi, and Jarishan to the 

south. The unit can also be found dispersed along and 

adjacent to the Al Basrah-Jalibra track, north and 

south of Hawr al Hammar, where it forms the main 

exposed part of the Dibdibba Formation.

Rafaiya Gravel. The Rafaiya Gravel is thick in the 

area between the Boliya axis and the west flank of the 

Zubair anticline, where it forms a layer of well-rounded 

exotic pebbles. These pebbles are composed of igneous, 

metamorphic, sedimentary, and conglomeratic rocks, 

the character and source of which have not yet been 

investigated in detail. Individual cobbles may be as 

large as 15 centimeters in diameter. The lateral and 

vertical limits of this unit are difficult to define in the 

east. To the west the gravel is not found west of the 

Boliya axis, perhaps in part because of erosion. At 

Boliya it forms a single layer of pebbles overlying 

gypseous and sandy earth which is a few feet thick. 

At Rafaiya (grid coordinates 1732 E., 932 N.) the 

type section of the unit, moderately coarse gravel lies 

at the surface; the pebble layers can also be found in 

the walls of the large waterhole. In the south, extensive tracts of coarse gravel, mainly quartz, are found 

in the Adan region. The true correlation of these still 

remains to be decided.

Diagnostic fossils and age

Ostracodes, Chara sp., Planorbis sp., and Radix are 

found in the Dibdibba Formation in isolated surface 

limestones in places west of Basrah, and according to 

Hudson and others (1957) their age is not known. 

Dunnington, Wetzel, and Morton (in Bellen and others, 

1959) stated, "This formation probably ranges from 

"late" Miocene through Pliocene to early Pleistocene, 

but no fossil evidence has been produced to support or 

add precision to this range." On the age of Dibdibba 

Formation Hudson and others (1957) wrote, "Direct 

evidence of age is not available; comparison with other 

areas in the Middle East suggests that the Dibdibba 

Formation is late Miocene or Miocene and Pliocene."

Contacts and equivalent units

The contact of the Dibdibba Formation with the 

underlying Lower Fars Formation is conformable and 

gradational. The Upper Fars Formation has not been 

recognized in the subsurface sections of southern Iraq, 

where equivalents are probably included within the 

lower parts of the Dibdibba Formation. Wherever the 

Lower Fars Formation shows a change of facies into
SOUTHWESTERN IRAQ G47

a clastic type, the differentiation of it from either the 

overlying Dibdibba Formation or the underlying Ghar 

Formation is very difficult. In the western outcrops, 

according to T. F. Williamson and D. C. Rogers 

(unpub. data, 1939), the Dibdibba Formation overlies 

the Euphrates Limestone Formation and the Lower 

Fars Formation. In the extreme southwest where the 

Dibdibba cannot readily be distinguished from the 

clastic variant of Lower Fars, if present, or from Ghar 

Formation, it rests unconformably with great overlap 

on the Dammam Formation.

The Hammar Formation and Quaternary gravels 

and alluvium overlie the Dibdibba; the contact is unconformable. Hudson and others (1957) stated:

The solid rock underlying the Mesopotamian plain and outcropping west of Har Al Hammar is that of the Dibdibba Formation (Macfadyen, 1938, p. 103; Mitehell, 1956, p. 400). This 

succession is of sand and pebble beds, often cross-bedded, 

occasionally calcareous or gypsiferous and cemented, and with 

some silt and clay. To the west of Zubair the Dibdibba beds 

are capped by pebbles and boulder beds of igneous, metamorphic, or sedimentary rocks, * * * [and] above these older rocks 

there is a surprisingly small thickness, rarely exceeding 100 

feet, of unconsolidated Recent sediments, in which the succession seems to be fairly constant, being one of marine beds, followed by alluvium, variously estuarine, lacustrine, or fluviatile. The marine beds are here called the Hammar Formation.

In southern Iraq the Dibdibba Formation is considered to grade eastward and northward into Upper 

Fars Formation and the Bakhtiari Group beneath the 

Mesopotamian Alluvia and to crop out in central, eastern, and northern Iraq as distinctly recognizable units. 

The equivalent of this formation in Saudi Arabia may 

be found as gravel and sandstone in the Hofuf Formation.

Economic aspects

The Dibdibba Formation comprises gravels and 

coarse-grained sandstones which can make a good aquifer. In the Dibdibba region of southern Iraq the 

water supply is chiefly dependent on the rainfall; owing 

to the high porosity of the Dibdibba sandstone and 

sands, much of the water may sink into the ground and 

be stored or may drain into local surface depressions 

in which some water gradually percolates into the 

ground and some evaporates. The underground drainage is toward the Euphrates and Shatt al Arab Rivers, 

down both dip and topographic slope. The movement 

of the ground water in the Dibdibba region will certainly be affected and influenced by the general ground 

water of the Mesopotamian basin. (See Macfadyen, 

1938.) Water in great quantities can be found stored 

in this formation, but its quality may be highly 

variable.

The gravelly components of the formation locally 

are manually collected and sold for concrete aggregates and for surfacing the roads.

QUATERNARY

The Quaternary is represented by Pleistocene and 

Recent (Holocene) sediments, which may not exceed 

300 feet in thickness. The marine Hammar Formation only has been described below. The top of the Recent sediments in the area is constantly acted upon by 

factors of current denudation or deposition.

HAMMAR FORMATION 

TYPE SECTION, THICKNESS, AND EXTENT

The type section of the Hammar Formation is in 

BPC well Zubair No. 31 (lat 30°31'00" N., long 

47°36'34" E.; alt 20.3 ft, completed Oct. 24, 1953) between 20 and 41 feet of drilled depth. The formation 

is 21 feet thick. This Recent marine formation occurs 

in the subsurface sections on the southern limits of 

Hawr al Hammar in the northern sector of the Zubair 

oil field, where the type section is located.

Eastward across the Shatt al Arab, north of Bandar 

Shapur, Thomas (in Lees and Falcon, 1952) reported 

a succession of marine shelly silts overlain by fresh- or 

brackish-water silts containing ostracodes. The brackish-water silts total at least 20 feet in thickness, and 

the lower marine silts are possibly 60 feet thick. On 

the northeast side of Hawr Al Hammar, this formation 

was found in the Nahr Umr wells, close to the Shatt 

al Arab, where it consists of 32 feet of shell marl containing lamellibranchs, gastropods, bryozoans, and 

other forms in a fauna almost identical with that of 

the Hammar Formation in the type section. According to Hudson and others (1957), this formation extends as far south as Al Faw (Fao), in the extreme 

south of Iraq, where a number of water wells were 

drilled in soft silts to a depth of 30 feet. They stated 

that "the general succession was one of a lower marine 

silt with abundant shells and an upper silt with ostracodes and occasional crab debris, probably estuarine. 

There was a slight difference between the succession in 

the various wells, the greatest thickness of the estuarine silt being 20 feet (well No.3) and that of the 

marine silts being 30 feet (well No.2)." The fauna of 

the marine silts is that of the Hammar Formation.

The only evidence of the northward extension of this 

formation, according to Hudson and others (1957), is 

that of W. A. Macfadyen (in Lees and Falcon, 1952) 

who reported 35 feet of alluvium with marine Formaminifera at Al Amarah, 95 miles north-northwest of Al 

Basrah. Mitchell (1958) discovered a Recent marine 

fauna near An Najaf at altitudes of 40.7 to 41.3 meters 

above present sea level.
G48 GEOLOGY OF THE ARABIAN PENINSULA

DETAILED LITHOLOGIC DESCRIPTION

In the type section the Hammar Formation consists 

of two main units:

Thickness 

(feet)

2. Clay, gray, with thin washes of shells consisting predominantly of &bra cadabra; local crab debris; the 

lowest foot consists of shell marl containing an 

abundant and varied fauna, including abundant echinoid debris _______________________ 7

1. Sand, coarse and very coarse, poorly graded, rarely 

cemented, and some silt; some of the sand is windblown; abundant small shells of marine organisms 

concentrated in thin bands and washes _______ 14

DIAGNOSTIC FOSSILS AND AGE

The Hammar Formation is of Eecent age. Its 

fauna was described fully by Hudson and others 

(1957), who stated that:

The common fossils, in order, are: Lamellibranchs Pitar 

belcheri, Brachidontes variabilis, Corbula sulculosa, Abra cadabra ; Gastropods Minolta adyma, Hinia idyllia. The forms 

identified are listed below."

Gastropoda Anachis (Zafra) melatoma (Melvill and Standen), 

Calyptraea chinensis (Linne), Cerithium amirantium E. A. 

Smith, Chrysallida callista (Melvill) Chrysallida irenae 

Eames and Wilkins, Crassispira (Clathrodrillia) lucida (G. 

and H. Nevill), Cuma carinifera (Lamarck), Cyclostrema supremum Melvill, Fossarus stoliczkanus Nevill, Gemmula multiseriata (E. A. Smith), Gibberula (Cystiscus) mazagonica 

(Melvill), Hamarilla bicarinata Eames and Wilkins, Hinia 

idyllia (Melvill and Standen), Lienardia soror (E. A. Smith), 

Mangelia pettyi (E. A. Smith), Meioceras sp. Melanella 

(Polygireulima) bivittata (H. and A. Adams), Minolia edyma Melvill, Mitrella blanda (G. B. Sowerby), Natica (Cochlis) tranquilla Melvill and Standen, Obtortio (Alabina) 

(Fenella) purpureoapicata (Preston), Odostomia eutropia 

Melvill, Pyrunculus abbreviates Eames and Wilkins, Retusa 

inconspicua (A. Adams), Refusal, Siphonium cf. maximum 

(G. B. Sowerby), Stenothyra minima (G. B. Sowerby), Strioterebrum (Noditerebra) edgari (Melvill), Triphora acuta 

(Kiener), Turbonilla (Chemnitzia) galactodes Melvill, Turritella terebra Lamarck.

Lamellibranchia: Abra cadabra Eames and Wilkins, Abra Telline, Barbatia fusca (Bruguiere), Brachidontes variabilis 

(Krauss), Corbula sulculosa A. Adams, Cuna coxi Eames 

and Wilkins, Leionucula layardi (A. Adams), Mactra olorina 

Philippi, "Ostrea", Pholas manillensis Philippi, Pitar belcheri (G. B. Sowerby), Placuna, Tellina (Psammotaea) methoria Melvill, Tellina, Yoldia tropica Melvill. 

Scaphopoda: Dentalium quadriapicale G. B. Sowerby, Dentalium. 

Bryozoa: Acanthodesia savartii (Audouin), Mucronella trispinosa (Johnston), Mucronellal.

Anthozoa: Paracyathus sp. nov., corals indet, Pennatulacea. 

Cirripedia: Balanus.

Decapoda: Callianassa sp. nov., crab fragments, echinoid plates 

and radicles.

The fauna of the Hammar formation is without doubt of 

Recent age, all forms existing in the present seas, some of them 

now living in the shallow coastal waters of the Red Sea, 

Trucial Coast, Oman Coast, and Persian Gulf.

CONTACTS AND EQUIVALENT UNITS

The Hammar Formation is underlain, probably unconformably, by the Dibdibba Formation. It is overlain by Eecent lacustrine or estuarine alluvium which 

consists of 20 feet of buff and red silt and some sand 

and pebbles. The top 3 feet of the alluvium contains 

abundant salt and gypsum crystals. Toward the middle the silt contains the brackish-water ostracodes 

Cyprideis litoralis (Brady) and, more rarely, Limnocyihere. Thus, the silt is considered as estuarine and 

the remainder of the alluvium as fluviatile or lacustrine.

The equivalents of this Eecent marine formation 

are expected along the coastal line of Arabia and over 

Arabia to be manifested in different and highly variable lithologic and facies forms.

ECONOMIC ASPECTS

No economic value has so far been attributed to the 

Hammar Formation, although some of the higher 

parts in the Eecent alluvium that are scattered along 

the highway between Al Basrah and Al Amarah have 

been utilized for brick making in the Brick Kiluf.

STRUCTURE 

STRUCTURAL PROVINCE OF SOUTHWESTERN IRAQ

Iraq, in general both geologically and geographically, has been influenced by the proximity of the Precambrian Arabian shield (Furon, 1963) and the Zagros 

province of the Tethyan geosyncline. Tectonically, it 

can be subdivided into three major interrelated units, 

each with its distinct and characteristic tectonics and 

geologic history. These units from northeast to southwest are: (1) the major and complex overthrust or 

"nappe" belt, (2) the autochthonous folded belt, and 

(3) the unfolded area (Dunnington, 1958; Naqib, 

1960).

An elaborate analysis of the tectonic construction 

of the Middle East in general, including northern 

Iraq, has been made by Henson (1951), who stressed 

the important role played by pre-Miocene vertical 

movements in the unfolded area and in the folded 

zone. Henson considered that folding due to late 

Tertiary compression was superimposed on and molded 

against an earlier deep-seated block-faulted framework, inherited from the pre-Miocene history of the 

region. Other accounts by Boeckh and Viennot (1929), 

Lees and Eichardson (1940), and Lees (1950a, b, 1951, 

1953) have accepted uncomplicated compressional folding, with a broad "normally folded zone" intervening 

between the "nappe front" and the unfolded "foreland."
SOUTHWESTERN IRAQ G49

A rather different quadripartite subdivision was introduced by Mitchell (1959), who included Iraq in 

(1) the Stable Shelf, (2) the buckled Labile Shelf of 

the Foreland, (3) the autochthonous border folds, and 

(4) the Iranides of his Orthogeosynclines of southwest 

Asia.

The tripartite division into unfolded, folded, and 

"nappe" zones, on the dominantly northwest-southeast 

trend of the Zagros, is appropriate for post-Miocene 

time and is harmonious with existing concepts of a 

Cretaceous and Pliocene geosyncline developing on the 

same Zagros trend, with its axis migrating more or 

less progressively southwestward until the orogeny in 

Pliocene time. Conditions in Jurassic and pre-Jurassic 

time were somewhat different. The basinal trend ran 

more nearly north to south than northwest to southeast 

in the known Jurassic stages, and the Jurassic and 

Cretaceous transition was probably a time of tectonic 

disturbance, including widespread uplift and tilting 

of fault-bounded blocks on the west margins of the 

basin. Several deduced faults of this age are directed 

north to south, across the Zagros trend. The axes of 

thickest basinal deposition for the Jurassic stages underlie thin accumulations of shallow-water Lower Cretaceous sediments. Figure (1) shows the major tectonic zones of Iraq and their characteristic elements.

The "nappe" zone is confined to a narrow belt in 

the remote northeastern part of Iraq and consists of 

an area of highly complex tectonics featured in extensive overthrusts; the rocks generally show some 

low-grade regional metamorphism (slates, phyllites, 

and schists). The area has undergone extensive igneous 

activity, including extrusion of basalt, spilite, and andesite, accompanied by some explosive vulcanism, evidenced by pyroclastics and tuffs and also by intrusion 

of dolerite, mafic rocks, and granite. The topography 

of this area contrasts sharply with that of the simplefold mountainous country of the zone of autochthonous 

folding farther southwest. It follows a surprisingly 

linear northwest-southeast trend from Rayat in northern Iraq to Bandar Abbas in southwestern Iraq.

The second unit forms a wider belt that extends in a 

general northwest direction in the northeastern part of 

Iraq and falls into three arcuate segments that bulge 

southwestward. This zone has an average width of 

about 100 miles (160 km), and the alinements of individual folds and the belt itself follow the Zagros swing 

from east-west in the north to northwest-southeast in 

the south. The segments of the autochthonous folded 

belt have been overridden by the linearly trending overthrust belt (Gansser, 1955) in the northeast as if they 

had differential lateral displacement along lines of

crustal weakness perhaps with north-south Caledonian 

trend.

The boundary between the folded and unfolded 

zones is abrupt. The southernmost and westernmost 

anticlines of the folded zone Jebels Sin jar, Sheikh 

Ibrahim, Sadid, Maklml, and Hamrin North and Hamrin South are surprisingly large folds, tens of miles 

long, and in some places rise several thousands of feet 

out of the adjacent synclines. The boundary follows a 

more or less arcuate line, from northwest-southeast in 

the southeast to east-west in the northwest, but it is 

offset sharply in the area west of Qaiyarah. Here its 

course is picked out by the north-south alinement of 

the pitching ends of five rather small and low anticlines. The line plunges below alluvium in the plains 

east of Bagdad or is offset northward, near Naft 

Khaneh, to the foot of the exposed Iraq-Iran frontier 

folds.

In this unit the anticlinal folds are simple and 

sharp, generally without faulting and subordinate 

thrusting. The structures are en echelon and are separated by broad synclines. In this zone, the intensity of 

folding, as reflected in the magnitude of the amplitude 

of the fold system, diminishes gradually southwestward 

from the region of high complexity in the northeast 

to a stable and tectonically undisturbed zone in the 

west. Thus, indeed, according to Dunnington (1958), 

there is an overall tendency for anticlines to increase 

in amplitude and tightness as the "nappe front" is 

approached, the most northeasterly folds being themselves thrust and locally overturned. There are, however, many departures from the superficially natural 

expectation that the folds should diminish in strength 

with increasing distance from the thrust front. Thus, 

Hamrin North, Maklml, and Sin jar are much more 

developed and much more elevated than the anticlines 

that occur immediately to the north and northeast, 

whereas flat, low domes appear between steep, narrow 

neighboring anticlines in the middle of the zone. Other 

oddities include the great elevational differences between adjacent anticlines of otherwise similar dimensions, the pitching down of successive anticlines along 

linear features, and the appearance of marked asymmetry in some folds and of opposed asymmetry in some 

pairs of adjacent folds. These and other anomalies 

substantiate the opinion of Henson (1951) that the 

late Tertiary folding in the folded zone was intimately 

affected by a preexisting complex of faults, which left 

residual features to buttress or deflect folding and 

which predisposed the region to react in irregular and 

complex fashion to simple tangential pressures.

The unfolded zone includes an extensive area of central, southern, and western Iraq; the Jebels Sin jar,
G50 GEOLOGY OF THE ARABIAN PENINSULA

43° 44°

42

45°

37°- 37°

41

36°

39

34°

32°

31°

Persian 

Gulf

Oil pipeline

44"

0 20 40 60 80 100120 140 KILOMETERS

I i I I I I I I I

0 20 40 60 80 100 MILES 

lil I I I I

FIGURE 1. Tectonic map of Iraq.
SOUTHWESTERN IRAQ G51

Makhul, and Hamrin mark the northern and eastern 

limits. The present discussion covers only the southwestern part of this area. The structural unit itself 

is characterized by relative absence of surface structural features except for a few folds of small amplitude and structural depressions many of which may be 

of only superficial origin.

In the unfolded zone, but northwest of the area discussed, a single broad dome with an east-west alinement occurs in the Ga'ara depression north of the 

Wadi Hauran. This dome, which exposes Lower Triassic (?) rocks in its center, is on the flank of a very 

broad uplift, which may reflect basement arching. 

Farther north, a second prominent east-west fold, 

narrow but gentle, is known at Anah on the Euphrates; 

its fault-trough setting strongly indicates a fault origin for the anticline itself. Gentle folds may occur 

between the Tigris and Euphrates Rivers in front of 

the autochthonously folded zone under the flat-lying 

Pliocene, Pleistocene, and Recent sediments. A few 

minor folds, trending northwest, occur in the neighborhood of Hit and Ramadi.

The unfolded area considered falls within the Interior Homocline of the Arabian 'tectonic framework 

defined by Aramco geologists. Here, with the exception 

of the Miocene and post-Miocene rock units, exposures 

of rocks from the Cretaceous System upward shift 

east in almost parallel strips, and their structural features appear in general to be of the simplest nature. 

They are almost flat over wide areas, with slight dip, 

measureable only in a few meters per kilometer, toward 

the east or northeast. The salient features of the area 

are Jabal Sanam in the Basrah area, depressions which 

may be structural basins, and some disturbed scarps. 

The structural profile across the area (pi. 4) portrays 

the general structural relationships of the different 

systems in surface and subsurface. At the surface the 

Cretaceous exposures do not convey any impression of 

diastrophism.

The Batn-Ghurra escarpment of Paleocene and lower Eocene (?) rocks is irregular and roughly alined in 

a north-northwest direction. It is extensive and variable with uncertain origin. In the Al Makmin (Machi) 

area it shows slight sagging. The Batn scarp in general has a very gentle east dip, and in certain places 

(for example, Lifiya) it consists of a simple typical 

erosional scarp with folded Basita Beds about 2 kilometers to the west. Reversals in dip over the scarp 

can be seen in places, for example near 'Aydaha, where 

closely folded beds occur more than 1 kilometer west 

of the scarp. In many places, beds of the Zahra Formation are also involved in local folding.

The Waqisah sca:-p in the area of Al Jill-Wagish, 

according to F. J. Venn (unpub. data, 1957), is of 

erosional origin. It shows well-bedded horizontal limestones commonly wi:h more than one scarp because of 

differential erosion. They show no "draping" toward 

the foot of the scarj > except for occasional but random 

collapse of the scarp edges.

The At Traq escarpment is generally alined northnorthwest between 1 ukayyid and Abu Khanzir, though 

there are several swings from the general trend along 

its length. There a?e definite signs of faulting on the 

scarp, ias at Khashm ad Duda and Khashm ad Dimand. 

At the south end of Khashm al Mahasa there are no obvious signs of faultii ig, but the "scarp face" is essentially the steepening soi .th flank of an anticline. The feature is made up oi folded very weathered Tuqaiyid 

Limestone and overlying Zahra Beds. The area here 

looks slightly compl icated, perhaps because of the cover of Zahra Beds. I linor anticlinal features can be observed west of the At Traq scarp, as at Kabd (Chabd), 

Uba'id, and Faydai Ar Radhuma.

Slightly northwest of the At Traq scarp is the 

Galaib scarp, where surface geology indicates a northnorthwest faulting trend and general lineation in a 

similar direction, wl den is manifested also in the trend 

of the escarpment 1 o the northwest of Galaib. This 

is considered to be a fault scarp having a possible 

throw of 100 feet in Ash Shawyyat area. Eighteen miles north of h< re, in the area of Shaikhiya-Dhiriyat, is a series of purallel folds which show two axial 

trends introducing the additional north-south trend 

which may be relaled to the structural grain of the 

Salman area.

In the central part of the area between the Waqisah scarp in the w^st and At Traq belt in the east 

is a series of depress ions which, considered as a group, 

show alinement in a north-northeast direction, convergent with the trend seen from Tufcayyid through 

At Traq, Chabd, Galaib, and Ash Shawyyat. The As 

Salman depression forms one of the largest of this 

group. This is enclosed by high escarpments with repeated north-northe ist trend surrounded by a horizontal plateau. About 100 kilometers northwest of the 

As Salman group o c depressions is the Ash Shabakah 

(Shabicha) depress .on, which shows small anticlinal 

and synclinal folds in the Zahra Beds.

East of the At Ti aq group of scarps, Ghanimi anticlines, trending north-northwest and south-southwest, 

and Abu Ghar, ta ge itle north-south fold, are the dominant features. Be:ween Abu Ghar and Al Basrah 

there is a slight topographic feature, which may be 

structural expression, and which can be seen in the 

Shagra and Boliya areas.
G52 GEOLOGY OF THE ARABIAN PENINSULA

Apart from the slight structural features of Ratawi, 

Rumaila, and Zubair there is no significant feature 

in the Al Basrah area except the dominating Jabal 

Sanam, which rises high above the surrounding flat 

country. Here, according to Boeckh and Viennot 

(1929), the old rocks project from below the Cenozoic 

cover. They stated that the rocks show a great resemblance to the Cambrian of the Persian Gulf.

The structural manifestations previously mentioned 

have been attributed to several different factors by various authors. Some stressed the role of diastrophism 

and epeirogenic movements, and others emphasized 

the role of solution collapse. According to Dunnington (1958), "gravimetric data and slight structural 

manifestations in the flat-lying Tertiary cover provide 

evidence of important buried structural elements 

throughout the 'unfolded area'." Lateral variations 

in sedimentary thicknesses over small distances are 

deductible, and it is concluded that most of these 

variations are controlled by pre-Miocene faulting of 

various dates, the relief due to faulting having been 

erased at the surface by subsequent erosion and deposition. The tectonic pattern suggested by the evidence 

is a complex one, resulting from interplay of vertical 

movements on fault trends which are oriented predominantly north to south, east to west, northwest to southeast, and northeast to southwest, the importance of individual trends showing marked local variations. 

Some of the prominent detected structural features are 

continuous for some tens or even hundreds of miles. 

Some pass beyond the limits of the region considered 

into Saudi Arabia. A few transgress directly across 

the limit of the unfolded area into the folded zone.

Where directional characters can be read into the 

complications, one or more of the four main tectonic 

trends discerned in the unfolded area is generally 

found to be involved. As a few important tectonic 

features on these trends pass directly from folded into 

unfolded terrain, it is 'argued that the folded zone 

was probably traversed prior to folding by a tectonic 

network comparable with the network that is now detectable in the unfolded area.

STRUCTURAL HISTORY

The Arabian shield dates from pre-Cambrian time. 

Since then, during various geological periods, the sea 

has transgressed and regressed over its margins. Western Iraq has undergone this sedimentary history as 

part of the fringing region (shelf) of this shield, the 

structural history of the area being reflected in the 

mode of sedimentation. From Triassic time this was 

controlled by the presence of the Ga'ara-Khleisia feature in the north and the Arabian shield proper in the

southwest. The Triassic sediments indicate epicontinental conditions. The various unconformities in the 

geological sequence, considered with the nature of the 

sediments, point to several epeirogenic movements 

which caused the advance and retreat of the sea over the 

area. Duration and extent of these movements are 

highly varied. The main ones occurred during late 

Bathonian and early Callovian times. The emergence 

was followed by a major transgression heralding the 

deposition of the Upper Jurassic Najmah limestones. A 

minor movement during late Berriasian and early Valanginian times has also been postulated to account for 

the deposition of the Yamama limestones on the Sulaiy 

Formation.

During the rest of Cretaceous time the area underwent a series of sedimentary cycles caused by marine 

transgressions and regressions. There are indications 

in the Cenomanian cycle that certain structural positive features, probably arcuate and extending from 

the Rumaila area, gave rise to the Mishrif reef. It was 

during the middle Turonian and lower Campanian 

cycle that the east-west-trending Anah graben trough 

formed. This trough continued subsiding during Maestrichtian time to accommodate the great thickness of 

sediments accumulated.

The Ga'ara high must have been near shore during 

the Cenomanian time, from the evidence of the Rutbah sandstones, and a slight transgression followed, 

culminating in the deposition of the Mishrif (or M'sad) 

Formation.

Late Cretaceous time was mainly a period of transgression, in which neritic limestones, including tongues 

of basinal sediments from the east side, were deposited.

The Eocene deposition was followed by a long period 

of emergence, which was ended by the deposition of 

the Euphrates limestones over most of the eastern parts 

of the area, while the western parts perhaps remained 

above sea level.

Further transgression during middle Miocene time 

led to the deposition of the elastics of the Ghar and 

lacustrine Zahra deposits and evaporites of the Lower 

Fars. During this period a more extensive area was 

covered, as evinced by the extensive Zahra beds.

During late Miocene and Pliocene times the Dibdibba elastics were deposited, the material derived possibly 

from a rapidly rising mass of the Arabian shield.

Since then, the central part of the area seems to 

have undergone relatively more uplift than the remainder, as can be seen in the distribution of the 

Zahra beds. Rejuvenation of the wadi system exposure 

area of Upper Cretaceous rocks may be considered as 

an indication of a sudden uplift and readjustment.
SOUTHWESTERN IRAQ G53

GRABEN SYSTEM

There is no direct indication of a graben system in 

the area; however, gravity-survey results, sedimentary 

history, and some seismic investigations demonstrate 

that even the flat-lying western desert area is underlain at depth (and locally at not very great depth) by 

buried structural elements and grabenlike faults of 

very considerable throw and shift. Some of these can 

be traced for very great distances into and across 

neighboring countries and some through a series of 

complex gravity features far into the zone of late 

Tertiary folding. The major tectonic elements of 

southwestern Iraq are shown in figure 1.

According to Dunnington (1960), the ages and 

trends of movement of these faults, and the nature of 

the faults themselves, remain for the most part obscure. Some, trending approximately east to west, 

may be identified with fair certainty as delimiting 

major troughs of subsidence which originated during 

Senonian time; thus, the east-west-trending Anah anticlinal structure has been formed on the site of a 

graben trough which dented and broke the Ga'araKhleisia block during early Late Cretaceous time. 

Others, including some of north-south alinement, appear 

from subsurface evidence outside the area considered 

here, to have moved significantly in very Late Jurassic 

or very Early Cretaceous times, though they may have 

had a much older existence and may have been slightly dislocated in later times. There is the possibility of 

As Salman depressions being basically manifestations 

of a deep buried graben system, though greatly influenced and accentuated by subsequent solution collapse 

phenomena. A similar argument is also applicable to 

the other depression and solution features of the area. 

However, with the present degree of knowledge and 

the inadequacy of available data, it would be dangerous 

to give rein to speculation and imagination.

SUMMARY

The study region comprises more than 82,000 square 

kilometers of southwestern Iraq and lies in the northeast side of the Arabian Peninsula. We owe much if 

not all our knowledge of the geology of this area to the 

exploration for oil, over almost a quarter of a century, 

by the Iraq Petroleum Company and its associate, the 

Basrah Petroleum Company.

In western Iraq near the Ga'ara area the oldest rocks 

are considered to belong to the Lower Triassic(?). 

They are succeeded by the Rhaetic, Liassic, and Bajocian-Bathonian sequence. In the same area a major 

erosional gap exists between the Middle Jurassic and the 

middle Cretaceous. This gap narrows away from the 

Gra'ara massif. Thus, it is possible to recognize successive occurrences of the intervening formations basinward.

The Cretaceous System shows strong diachronism 

and a history of sedimentation characterized by sedimentary cycles, which were controlled by the epeirogenic withdrawals over the Arabian platform. The 

basic near-shore to basin sequences are hardly recognizable there, the sediments having been laid down 

close to the old shorelines. The main Cretaceous cycles are: Tithonian (Upper Jurassic) to Aptian, Aptian, Cenomanian to lower Turonian, Middle Turonian 

to lower Campanian, and upper Campanian to Maestrichtian.

Similar cycles are detectable in the Tertiary, but 

their significance can only be appreciated if other 

areas of Iraq are considered. In particular, the understanding of Miocene stratigraphy is in a state of flux 

and requires a great deal of investigation. The Oligocene Series is not recognized in the area.

Structurally, the area lies in the Interior Homocline 

of the Arabian Peninsula. It is flat-lying country 

with gentle dips to the east and northeast. Minor 

northwest- and north-trending linear structural features occur in the Tertiary outcrop area. These may 

be due to recurrent disturbance along old lines of 

weakness, accompanied by the dissolution of the evaporites in the Paleocene and Eocene sequence. Solutioncollapse structure is well developed in many places.

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*U.S. GOVERNMENT PRINTING OFFICE: 1986-^91-409:40002