I . : .. IBRARY COPY 

GEOLOGY OF THE 

ARABIAN PENINSULA 

Bahrain 

,... ..... 

.....,-,~-~ 

GEOLOGICAL SURVEY PROFES IONAL PAPER 560-E 
Geology of the Arabian Peninsula 

Bahrain 

By R. P. WILLIS 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 590-E 

A review of the geology of Bahrain as shown on 

USGS Miscellaneous Geologic Investigations Map 

l-270 A, "Geologic Map of the Arabian Peninsula,'' I963 

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

STEWART L. UDALL, Secretary 

GEOLOGICAL SURVEY 

William T. Pecora, Director 

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washin~ton, D.C. 20402 - Price 15 cents (paper cover) 
FOREWORD 

This volume, "The Geology of the Arabian Pe1finsula,n 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 governments 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. InN ovember of the following year, 1954, Director Wrather approv~d 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 Aram'co'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 King-· 

dom 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 bilingu~l-in 

Aral;>ic 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 utiliz.ing 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 I-270 A). 

III 
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 P~ninsula. 

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 subcon tin en t. 

c:J-a-~ 

Arabian-American Oil Go. (Retired) . 

w. D. STON, JR., 

Former Chief, Foreign Geology Branch, 

U.S. Geological Survey. 
CONTENTS 

Foreword------------------------------------------

Abstract------------------------------------------- Introduction ______________________________________ _ 

Stratigraphy ______________________________________ _ 

General----------------------------------------

Lower Eocene-Rus Formation __________________ _ 

Lower and middle Eocene-Dammam Formation __ _ Sharks tooth shale _________________________ _ 

Brown crystalline limestone _________________ _ 

Page 

III 

E1 

1 

1 

1 

3 

3 

3 

3 

Stratigraphy-Continued 

Lower and middle Eocene-Dammam Formation-Continued 

Orange marL _____________________________ _ 

White limestone ___________________________ _ ~iocene ______________________________________ _ 

Recent----------------------------------------

Structure-----------------~------------------------

ILLUSTRATIO:N" 

Page 

FIGURE 1. Geologic map of Bahrain__________________________________________ E2 

v 

Page 

E3 

3 

4 

4 

4 
GEOLOGY OF THE ARABIAN PENINSULA 

BAHRAIN 

BY R. P. WILLIS 1 

ABSTRACT 

The Sheili:hdom of Bahrain is a group of islands in the 

Persian Gulf between the Qatar Peninsula and Saudi Arabia. 

The discovery well was completed as a producer from the middle 

Cretaceous in June 1932. The principal outcropping rocks on 

Bahrain, the main island, are of Eocene age, with Miocene and 

younger rocl{s on the periphery. The anticlinal structure is 

clearly discernible in the rimrock that encircles the central part 

of Bahrain Island. 

INTRODUCTION 

The Sheikhdom of Bahrain (fig. 1) consists of a 

group of low-lying islands between the Qatar Peninsula 

and Saudi Arabia at approximately lat 26° N., long 

50°30' E. The 1nain island of Bahrain is about 48.3 

kilometers (30 1niles) long and 16.1 kilometers (10 

miles) wide and reaches a maximum altitude of 134.1 

meters ( 440 ft) at Jabal ad Dukhan near its geographical center. 

The principal subsidiary islands are: Al Muharraq 

and Sitt·ah to the northeast, U mm N a'san to the west, 

and ]Juwar to the southeast near the Qatar coast. 

Numerous other small islands are within the Bahrain 

territorial boundary. 

The geology of Bahrain was first mentioned in 1908 

in a memoir by Guy S. Pilgrim, Indian Geological Survey, on the geology of the Persian Gulf region. This 

study was followed in 1928 by private work by R. 0. 

Rhoades of the Gulf Oil Corp. The discovery well was 

drilled in 1931-32 by the Standard Oil Co. of California 

and was completed as a producer from the middle 

Cretaceous in June 1932. The present rate of oil production is 45,000 barrels per day from 165 wells. 

Acknowledgments are made to the Bahrain Petroleum Co., Ltd., and American Overseas Petroleum Ltd., 

for permission to publish this discussion, and to those 

who, in the past, contributed numerous reports to the 

company files on the geology of Bahrain. These reports 

1 Of the Bahrain Petroleum Co., Ltd. 

were drawn on extensively during the preparation of 

this report. 

STRATIGRAPHY 

GENERAL 

The principal outcropping rocks on Bahrain Island 2 

are of early and middle Eocene age, with Miocene and 

younger rocks evident along the periphery. Recent deposits are concentrated in the southern and southwestern 

parts of the island in the Ra's al Barr and Ra's al J aza'ir 

areas. 

Eocene and Miocene rocks are also exposed in the 

]Juwar Islands to the southeast, but the other small 

islands of the Bahrain Group are limited to Pleistocene ( ~) and Recent deposits, with the exception of 

Umm Na'san, which appears to have some Miocene deposits. The dominant rock types within the Eocene 

and Miocene of Bahrain are limestone, dolomitic limestone, and chalk, with subsidiary marls and shales. 

Evidence of wind sculpturing can be seen throughout 

the islands; erosional remnants and striations have a 

predominant north-northwesterly orientation. A prominent topographic feature of Bahrain is the rimrock of 

Eocene limestone which forms a complete ring around 

the central part of the island. This rimrock in some 

places supports a cliff·face in :excess of 30.5 m (100ft). 

The only water gap is on the west. 

An al\:,oular unconformity is indicated between middie Eocene and younger rocks by variations in thickness 

of the uppermost Eocene unit and by a change in lithology of the Miocene rocks to a 1nore sandy appearance. 

The major basin of deposition during Eocene time 

appears to have been north-northwest of Bahrain in the 

Al Burqan-As Saffaniyah area, but the Miocene trough 

was farther to the east. 

2 Since publication of the "Geologic 1\lap of the Arabian Peninsula," 

I-270 A, by the U.S. Geological Survey nnd Arabian American 011 Co. 

in 1963, new fieldwork has been completed. Resulting revisions nre 

shown in figure 1. 

El 
E2 GEOLOGY OF THE ARABIAN PENINSULA 

~----~--------------------~5~0~3~0~'-------------------------------5~0~ 40' 

AI Mul}arraq 

Umm Na'san Island 

0 

FIGURE 1.-Geologic map of Bahrain. 

EXPLANATION 

w:QsbjJ 

Sabkha deposits 

Gravel, 

-

sand, and silt 

~{ R ~ Dam For~:i~·~~:::~~uivalent 

Rus Formation 

)I 

>-

c:: 

<( 

i= 

c:: 

w 

I-

\ 
BAHRAIN E3 

The Bahrain area is near the eastern edge of the 

Arabian Shelf, which was fairly stable. The prevalent 

environment of deposition was shallow water that was 

normal marine to restricted marine. Sedimentation was 

typical of a ~helf area, and individual units extended 

for great distances laterally. 

LOWER EOCENE-RUS FORMATION 

The type section of the Rus Fo1~mation is· in the Ad · 

Damman area of Saudi Arabia, and it is appro~imately 

56.1 m ( 184 ft) thick. This is the oldest formation outcropping on Bahrain, and it occurs o~ly i~ the central 

part of Bahrain Island within the rimrock area, where 

it reaches a total thickness (surface and subsurface) of 

about 67.1 m (220ft). · 

The exposed part of this formation consists principally of chalk- and chert-ben.ring dolomitic limestones. 

N uinerous quartz geodes can be found in many localities 

and a few have been known to contain petroleum. The 

only other surface indication of the presence of hydrocal'bons is an inactive oil seep in the south-central part 

of the island on a possible fault trend. 

Some anhydr!tic beds have been ·observeq. in the Rus 

Formation; erratic dips plus the existence of closed 

topographically low areas and lost circulation in all 

drilling operations suggest that much of the erosion 

within the rimrock area has been magnified by.slumping 

of the strata above anhydritic beds that are probably 

leached. 

The base of the Rus Formation is not exposed on Bahrain, but information derived from drilling indicates 

that the contact with the underlying Umm er Radhuma 

Formation is conformable. 

Little. or no paleontologi~ work has been done on the 

Eocene and younger rocks of Bahrain, but similar stratigraphic position and lithology have made possible a 

close correlation with Saudi Arabia to the west. 

The ·Rus Formation on Bahrain is termed the "Zone 

C aquifer" and is the source of a limited supply of fresh 

water, the product of loc~l rains, that floats on top of 

the usually saline water of the formation. The thickness of this fresh-water "cap" may reach 3-4.6 m (10-15 

ft) in some localities. 

The limestones of the Rus are used extensively in the 

production of lime for the local construction industry. 

LOWER AND MIDDLE EOCE~E-DAMMAM FORMATION 

SHARKS TOOTH SHALE 

The basal sharks tooth shale member of the Dammam 

Formation immediately overlies the "chalky beds" of the 

Rus Formation and averages between 9.1 and 15.2 m 

(30-50 ft) in thickness. It consists of gray to yellow 

shale and marl and subsidiary ·thin dolomitized limestones. The name given to this interval is derived from 

the lowest clay-shale bed, in which fossil shark teeth 

have been found. The upper part of the member is 

sometimes referred to as the Al veolina zone. 

The sharks tooth shale member is believed to correlate with the Midra Shale of Saudi Arabia. 

BROWN CRYSTALLINE LIMESTONE 

The brown crystalline limestone member of the Dammam Formation averages 33.5 m ( 110 ft). in thickness. 

Tho rock is brown to buff massive hard 'porous finely 

crystalline dolomitic limestone. The lower part contains 

bands of nodular chert which may be largely responsible 

for the development of the rimrock that surrounds the 

central basin of the island. The upper contact with the 

overlying orange marl is sharp and conformable. 

The brown crystalline limestone member of Bahrain 

is correlated with the Khobar Member qf Saudi Arabia, 

and is the main fresh-water aquifer of Bahrain; it carries the designatimi of "Zone B aquifer." Salinity increases and static head decreases to the southeast. 

ORANGE MARL 

The orange marl unit serves as an impermeable barrier between the overlying Zone A and underlying Zone . 

B aquifers, and is easily recognized in drilling operations by its distinctive color and i'ts rather sharp contacts with contiguous members. The thickness ranges 

£rom 6.1 to 19.8 m (20-65 ft), the average being approximately 12.2 m ( 40 ft) . 

The rock consists of limonite-stained yellow-orange 

and brown slightly dolomitic marl; some thin marly 

limestone is interbedded in the upper part. The equivalent unit in Saudi Arabia is the Alat Marl. 

WHITE LIMESTONE 

The white limestone, designated the "Zone A aquifer," 

ranges from 6.1 to 62.5 m ( 20-205 ft) in thiclmess. Pronounced thickening is noted downdip beneath the 

Eocene-Miocene unconformity. 

. This upper member of the Dammam Formation is 

composed of white very porous finely crystalline commonly chalky and in part dolomitized limestone, and 

is correlated with the Alat Limestone on the Arabian 

Peninsula. 

Local silicification of the limestone at the surface has 

resulted in a resistant layer in the form of a prominent 

scarp in the Biiri area of Bahrain Island and prominent 

cliffs along the shore of Huwar Island. 

The white limestone is a major source of fresh water 

on Bahrain; however, as is true of the Zone B aquifer, 

the static head decreases to the southeast and the salinity 

increases, so that in the southern. half of the island, 
E4 GEOLOGY OF THE ARABIAN PENINSULA 

practically no fresh water is available from these 

aquifers.· 

MIOCENE 

No formal names have been given to the Miocene and 

younger rocks on Bahrain. The Miocene comprises a 

sequence of clay, marl, shale, and sandy limestone and 

ranges in thickness from 0 to almost 61 m (200ft). An 

abrupt increase in thickness is noted down flank on the 

Bahrain anticline. 

The basal unit of the Miocene is generally a thin sandy 

limestone and is considered as part of the Zone A 

aquifer, whose main component is the underlying white 

limestone of Eocene age. The thin sandy limestone is 

overlain by approximately 21.3 m (70 ft) of soft gray 

clay and shale, which in turn is overlain by light-colored 

soft porous sandy limestone and mad. Miocene outcrops of cream to brown limestone and marl have been 

recognized on Umm Na'san. 

Crossbedded calcareous sandstone composed of masses 

of Milliolidae occurs at several localities on Bahrain 

and adjacent islands; it has been considered by some 

workers to be Miocene and by others to be as young as 

Pleistocene.1 On ]juwar Island a calcareous sandstone 

overlying known Eocene strata has been described. On 

Umm Na'san Island a similar sandstone is seen to rest 

unconformably on the Miocene beds, and on Bahrain 

such sandstone occurs in gaps in the rimrock, resting 

unconformably on the Eocene strata. These sandstones 

are believed to be old dune deposits. 

The Miocene of Bahrain is believed to be the equivalent of the Dam and Hadrukh Formations of Saudi 

Arabia and the Lower Fars of Iran. 

RECENT 

Recent beach sands are found on all islands of Bahrain, but the most noteworthy deposits occur on Bahrain 

Island in the Ra's al J aza'ir and Ra's al Barr areas. 

In these areas extensive salt flats and marshes (Sabkha 

deposits) make any surface operations extremely difficult. Beachsand deposits have not been .mapped in 

detail and, as a result, it has not been possible in figure 

1 to differentiate them from other unconsolidated surficial deposits of gravel, sand, and silt that cover extensive parts of the islands. 

There is also much evidence of Recent limestone and 

calcareous-mud deposition in the shallow near-shore 

water of Bahrain; much of this limestone is used in local 

construction work. 

STRUCTURE 

Bahrain is on the Interior Platform (Rasa Structural Terrace), which is a prominent feature of the 

Arabian Peninsula; the formation of individual structures appears to have. resulted from vertical uplift 

rather than compressive stresses. The only evidence 

that compressive stresses were active at all is the somewhat steeper west flank of the Bahrain anticline. 

The Bahrain anticlinal axis is oriented in a northsouth direction and the structure is fairly simple-no 

faults visible at the surface. Most faults observed in 

the subsurface have displacements of less than 15.2 m 

(50 ft), and all are classified as normal tension faults. 

Early Eocene time was characterized by a shallowwater restricted marine environment as indicated by 

the presence of anhydrite in the Rus Formation. Increased subsidence during middle Eocene time brought 

more normal marine conditions, and limestone and shale 

were deposited. Local and regional uplift near the close 

of middle Eocene initiated a period of erosion in the 

Bahrain area, and no upper Eocene or Oligocene sediments have been recognized. Continued local. uplift 

exceeded the regional tilting, and Miocene sediments 

were deposited in the relatively low areas around the 

main anticline, unconformably onlapping the Eocene 

deposits. Further local uplift in the late Miocene and 

Pliocene caused erosion of some of the Miocene sediments and gave the islands their present configuration. 

Recent beach-sand accumulations added the flat saltmarsh areas in the south and southwest parts of the 

main island. The post-Eocene uplift made possible 

the le~ching of anhydrite from the Rus Formation, and 

the leaching caused slumping along the structural axis. 

This slumping, together with the pr·evailing northerly 

winds, completed the geologic sculpturing of Bahrain. 

U.S. GOVERNMENT PRINTING OFFICE: 1967 0-238-236