Upper Cenomanian Foraminifers from the 

Southern Part of the San Juan Basin, 

New Mexico

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Chapter N

Upper Cenomanian Foraminifers from the 

Southern Part of the San Juan Basin, 

New Mexico

By MARY ALICE CAREY

A multidisciplinary approach to research studies of sedimentary 

rocks and their constituents and the evolution of sedimentary 

basins, both ancient and modern

U.S. GEOLOGICAL SURVEY BULLETIN 1808 

EVOLUTION OF SEDIMENTARY BASINS SAN JUAN BASIN
U.S. DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Any use of trade, product, or firm names in this publication is for descriptive 

purposes only and does not imply endorsement by the U.S. Government

UNITED STATES GOVERNMENT PRINTING OFFICE: 1992

For sale by

Book and Open-File Report Sales

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Box 25425

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Library of Congress Cataloging-in-Publication Data

Carey, Mary Alice.

Upper Cenomanian foraminifers from the southern part of the San Juan Basin, 

New Mexico : a multidisciplinary approach to research studies of sedimentary 

rocks and their constituents and the evolution of sedimentary basins, both ancient 

and modern / by Mary Alice Carey.

p. cm. (Evolution of sedimentary basins San Juan Basin ; ch. N) 

(U.S. Geological Survey bulletin ; 1808-N)

Includes bibliographical references.

Supt. of Docs, no.: I 19.3:R1808N

1. Foraminifera, Fossil San Juan Basin (N.M. and

Colo.) 2. Paleontology Cretaceous. I. Title. II. Series. III. Series: U.S. 

Geological Survey bulletin ; 1808-N. 

QE75.B9 no. 1808N 

[QE772]

557.35 dc20 91-42291 

[563'.12'097895] CIP
CONTENTS

Abstract Nl 

Introduction Nl 

Method of study N3 

Stratigraphy N3 

Micropaleontology N4

Whitewater Arroyo Tongue N4

Lower Member of the Mancos Shale at Laguna N6

Hartland Member-lower member, Chama Basin N7 

Significance of planktonic foraminifers N8 

Faunal and environmental interpretations N9 

Paleogeographic theories Nil

Late Cenomanian stratigraphy and ammonite zones N12 

Conclusions N13

Taxonomic notes on selected species N14 

References cited N16 

Appendix Description of localities N17

PLATES 

[Plates follow text]

1-3. Scanning electron microscope photomicrographs of selected specimens from study 

area.

FIGURES

1. Cross section showing intertonguing rock units of Dakota Sandstone and 

Mancos Shale N2

2. Index map showing sample localities in study N2

3. Chart showing stratigraphic nomenclature of middle Cenomanian to lower 

Turanian rocks in parts of New Mexico and southeastern Colorado N3 

4-10. Charts showing distribution of foraminifers:

4. Hyde Mine N4

5. Casamero Lake N5

6. Cubero N5

7. L-Bar N6

8. Type section near Two Wells N16

9. Laguna N7

10. Heron Reservoir N7

11. Chart showing foraminiferal zones and stratigraphic nomenclature as related to 

the Laguna and Heron Reservoir localities N8

12. Maps showing distribution of foraminifers in the Western Interior during latest 

Graneros time and middle Hartland time Nil

13. Sketch showing the general outcrop boundary of the Twowells Tongue and 

deltaic headland that served as source for offshore bars and shoreface sands of 

the Twowells N12

Contents III
14, 15. Maps showing general positions of shorelines:

14. Late Cenomanian N13

15. Middle Cenomanian N13

16. Chart showing foraminiferal zones and subzones, ammonite zones, and 

stratigraphic nomenclature of study area N14

IV Contents
EOLUTION OF SEDIMENTARY BASINS SAN JUAN BASIN

Upper Cenomanian Foraminifers from the 

Southern Part of the San Juan Basin, New Mexico

By Mary Alice Carey

Abstract

The Cenomanian Whitewater Arroyo Tongue of the 

Mancos Shale is the upper shale in an intertonguing sequence 

consisting of five Dakota Sandstone units and two Mancos 

Shale units. This sequence is best developed in the vicinity of 

Laguna, New Mexico, where the Whitewater Arroyo is 90 ft 

thick. Overlying the Whitewater Arroyo is the Twowells 

Tongue of the Dakota, which separates the Whitewater Arroyo 

from the lower member of the Mancos in the southern part of 

the San Juan Basin. These two tongues, the Whitewater Arroyo 

and the Twowells, are the most laterally extensive units of the 

Dakota-Mancos sequence and thus provide a basis for regional 

environmental interpretation.

Foraminifers recovered from six outcrops of the Whitewater Arroyo Tongue and from one outcrop of the basal part of 

the lower member of the Mancos Shale show that the Whitewater Arroyo represents a shallower, nearshore, better 

oxygenated bottom environment than does the lower member. 

Although both planktonic and arenaceous and calcareous 

benthic foraminifers were recovered from the Whitewater 

Arroyo, some nearshore localities in the middle Cenomanian 

Seboyeta bay produced only arenaceous specimens. The 

greatest abundance and diversity of foraminifers in the Whitewater Arroyo assemblage at the Laguna section indicate a 

northerly flow of southern ocean waters and an increase in 

vertical circulation of the sea that produced an oxygenated sea 

floor. In contrast, the dominance of planktonic foraminifers in 

the lower member of the Mancos at Laguna indicates a deeper 

water, open-marine environment and oxygen-reduced sea 

floor.

The Hartland Member-lower member of the Mancos 

Shale in the Chama Basin, an inferred equivalent stratigraphic 

unit to the Whitewater Arroyo, Twowells, and lower member 

of the Mancos in the San Juan Basin, produced an abundance 

of planktonic foraminifers dominated by Rotalipora cushmani, 

R. greenhornensis, and several species of Hedbergella, which

Manuscript approved for publication October 1, 1991.

characterize a deeper water environment and record a major 

incursion of the southern ocean into the Western Interior. The 

foraminiferal fauna of the Hartland Member-lower member of 

the Mancos in the Chama Basin correlates with that of the 

Hartland Shale Member of the Greenhorn Limestone of the 

Great Plains.

Differences in the foraminiferal assemblages between the 

Whitewater Arroyo Tongue and the lower member of the 

Mancos imply that a variety of bottom conditions and watermass circulation patterns existed within the Whitewater Arroyo 

sea and that an abrupt change in water depth occurred at the 

beginning of deposition of the lower member of the Mancos, a 

change caused by rapid subsidence and (or) rapid sea-level 

rise.

INTRODUCTION

Despite considerable studies on middle and late 

Cenomanian foraminifers from the central, most openmarine part of the Cretaceous seaway (Richer, 1969; Eicher 

and Diner, 1985; Eicher and Worstell, 1970), little work has 

been done to correlate these foraminiferal faunas with those 

of the more shoreward facies of the seaway. A complex 

intertonguing rock sequence of Dakota Sandstone and 

Mancos Shale units exposed in west-central New Mexico 

represents a segment of the shoreward facies and is coeval 

with the middle and late Cenomanian and Turonian 

Greenhorn Limestone of southeastern Colorado and central 

Kansas. The foraminifers from this intertonguing sequence 

document the early and ephemeral transgressive-regressive 

pulses of the seaway prior to the major incursion at or near 

the Cenomanian-Turonian boundary.

The geologic units of concern in this report are the 

Whitewater Arroyo Tongue of the Mancos Shale, the 

Twowells Tongue of the Dakota Sandstone, and the lower 

member at the base of the Mancos Shale, all in the southern 

part of the San Juan Basin of west-central New Mexico. The 

Whitewater Arroyo is the upper shale in an intertonguing

Upper Cenomanian Foraminifers, New Mexico N1
sequence consisting of five Dakota Sandstone and two 

Mancos Shale units (fig. 1). Overlying the Whitewater 

Arroyo is the Twowells, which separates the Whitewater 

Arroyo from the lower member of the Mancos. These two 

tongues, the Whitewater Arroyo and the Twowells, are the 

most laterally extensive units of the Dakota-Mancos 

sequence in the basin and provide a basis for regional 

paleoenvironmental and paleogeographic interpretations.

Middle and late Cenomanian foraminiferal faunas and 

related microfaunas from the Whitewater Arroyo and the 

lower member of the Mancos provide biostratigraphic 

information that is useful in correlating sections 

characterized by marginal-marine rocks and in placing time 

lines through paralic paleoenvironments. Even though these 

faunas are limited in abundance, diversity, and preservation, 

they permit detailed paleogeographic reconstruction, 

including water depth, salinity, water-mass influence, and

120 mi

39'

W

Gallup Grants Laguna

Lower member of Mancos

Whitewater Arroyo 

Tongue of Mancos Shale

Mesa

Tongue of 

Mancos Shale

Figure 1. East-west schematic cross section showing intertonguing rock units of the Dakota Sandstone (dark gray) and 

Mancos Shale (light gray) in the west-central part of the San 

Juan Basin of New Mexico. Morrison Formation underlies 

Dakota Sandstone.

0 50 100 150 MILES 

I i i I

Figure 2. Sample localities in study: six localities of the 

Whitewater Arroyo Tongue in the San Juan Basin (large shaded 

area, northwestern New Mexico) and one locality of the 

Hartland Member-lower member of the Mancos Shale in the 

Chama Basin (small shaded area, Heron Reservoir east of San 

Juan Basin). Whitewater Arroyo type section is about 1 .2 miles 

west of Two Wells. Description of localities given in appendix.

subfacies limits of the Cretaceous seaway in the Western 

Interior, and contribute evidence of proximity to the 

shoreline and landmass or submarine barriers.

This report discusses the stratigraphic distribution, 

significance, and interpretation of planktonic and benthic 

foraminifers recovered from the Whitewater Arroyo Tongue 

and the lower member of the Mancos at six localities in the 

southern part of the basin (fig. 2). The six localities 

represent a variety of environments within the Whitewater 

Arroyo, from nearshore to open ocean. An additional 

locality of the Hartland Member-lower member of the 

Mancos in the Chama Basin provides important faunal 

evidence for correlation and environmental interpretation 

between the San Juan Basin, Chama Basin, and Great Plains 

for middle and late Cenomanian time.

Acknowledgments. The author thanks the following 

people and groups for their assistance and contributions to 

this report: Jennie L. Ridgley, U.S. Geological Survey 

(USGS) (Denver), for suggesting the initial research idea; 

William V. Sliter, USGS (Menlo Park), and William A. 

Cobban and William M. Aubrey, USGS (Denver), for their 

constructive reviews of the manuscript; Jean K. Dillinger, 

USGS (Denver), for her physical and professional field 

support; Douglas G. Wolfe and Don L. Eicher, University of 

Colorado, for specimen identification; Elisabeth M. Brouwers, USGS (Denver), for her assistance with ostracode 

identification; the Pueblo of Laguna Tribal Council for

N2 Evolution of Sedimentary Basins San Juan Basin
permission to access, study, and collect on their land; and 

Parley Fleming, USGS (Denver), for his professionalism in 

processing rock material.

METHOD OF STUDY

Thirty-five samples were collected from seven 

localities in New Mexico: Whitewater Arroyo type section 

on the north side of Whitewater Arroyo near Two Wells, 

Hyde Mine, Casamero Lake, Cubero, Laguna, L-Bar, and 

Heron Reservoir (fig. 2, appendix). Most of the samples 

collected were spot samples and were taken at 5-, 10-, or 

15-ft stratigraphic intervals, depending on the total 

thickness of the section. Complete sections of the Whitewater Arroyo Tongue, lower member, and Hartland Member-lower member were collected wherever possible.

Splits of 150 g from each shale sample were prepared 

for foraminiferal study using the standard kerosene 

technique (Kummel and Raup, 1965), which involves oven 

drying, a 15-hour soak in kerosene, decanting, and a 2-hour 

boil in Calgon and hot water. After mechanical breakdown 

in the kerosene process, each sample was sieved using 

ASTM sieves 20 and 230. The fine fraction (<62 (im) was 

dried and, then split if necessary. The sample portion to be 

examined was dry sieved using sieves 20, 60, 80, and 100.

STRATIGRAPHY

Middle Cenomanian through middle Turanian 

stratigraphic nomenclature in the southern San Juan Basin is 

cumbersome due to complex facies changes in the 

stratigraphic sequence (figs. 1, 3). The following clear and 

concise statements on the stratigraphy of the San Juan Basin 

by Cobban and Hook (1984, p. 258-259) are followed in 

this report.

In east-central New Mexico, the standard Great 

Plains nomenclature has long been applied: Dakota 

Sandstone (oldest), Graneros Shale, Greenhorn 

Limestone, and Carlile Shale (youngest). In recent years, a 

Thatcher Limestone Member has been recognized in the 

Graneros Shale (personal observations); the Greenhorn 

Limestone has been divided into Lincoln Limestone 

Member (oldest), Hartland Shale Member, and Bridge 

Creek Limestone Member (youngest); and the Carlile 

Shale has been divided into Fairport Member (oldest), 

Blue Hill Member, Codell Sandstone Member, Juana 

Lopez Member, and an upper unnamed shaly member*** 

In west-central New Mexico, tongues of sandstone 

largely of a western source complicate the stratigraphy 

and necessitate a different nomenclature. The Dakota 

Sandstone is intertongued extensively with the Mancos 

Shale, and the sequence is, from oldest to youngest, 

[Encinal Canyon Member (of the Dakota) where present 

(Aubrey, 1986),] Oak Canyon Member (of the Dakota),

05

0) 

tO

w

c to 

1 8. D 

1-

Cenomanian (part)

Substage

(5 

1

03

Q. 

Q.

D

Middle

NEW MEXICO

East-west-central

Mancos 

Shale 

(part)

Dakota 

Sandstone

Mancos 

Shale

Dakota 

Sandstone

Mancos 

Shale

Dakota 

Sandstone 

(part)

Bridge 

Creek 

Limestone 

Member 

(part)

Unnamed 

shale

Twowells 

Tongue

Whitewater 

Arroyo 

Tongue

Paguate 

Tongue

Clay 

Mesa 

Tongue

Cubero 

Tongue

Oak 

Canyon 

Member 

(part)

Northeast

Greenhorn 

Limestone 

(part)

Graneros 

Shale

Bridge 

Creek 

Limestone 

Member 

(part)

Hartland 

Shale 

member

Lincoln 

Limestone 

Member

Upper 

shale

Thatcher 

Limestone 

Member

Lower 

shale

Dakota Sandstone 

(part)

COLORADO

Southeast

Greenhorn 

Limestone 

(part)

Graneros 

Shale

Bridge 

Creek 

Limestone 

Member 

(part)

Hartland 

Shale 

Member

Lincoln 

Limestone 

Member

Upper 

shale

Thatcher 

Limestone 

Member

Lower 

shale

Dakota Sandstone 

(part)

Figure 3. Stratigraphic nomenclature of middle Cenomanian 

to lower Turonian rocks in parts of New Mexico and 

southeastern Colorado. Modified from Cobban and Hook 

(1984).

Cubero Tongue (of the Dakota), Clay Mesa Tongue (of the 

Mancos), Paguate Tongue (of the Dakota), Whitewater 

Arroyo Tongue (of the Mancos), and Twowells Tongue (of 

the Dakota) (Landis etal., 1973). Where the Oak Canyon, 

Cubero, Clay Mesa, and Paguate cannot be differentiated, 

the sequence consists of the main body of the Dakota 

overlain by the Whitewater Arroyo Tongue of the Mancos, and that in turn, by the Twowells Tongue of the 

Dakota (Landis etal., 1973, Fig. 2); or an unnamed shale 

tongue of Mancos may separate the Paguate Tongue from 

the main body of the Dakota (Hook etal., 1980, Fig. 2); 

or the Twowells Tongue may be separated from a basal 

tongue of the Dakota by a thick lower unnamed tongue of 

Mancos Shale.

The shale unit that overlies the Twowells at Laguna 

will be referred to in this report as the lower member of the 

Mancos. Dane (1948) named this unit the Graneros shale 

member of the Mancos, Landis and others (1973) referred to 

it as main body Mancos, and Cobban and Hook (1989)

Upper Cenomanian Foraminifers, New Mexico N3
proposed to call it the Rio Salado Tongue of the Mancos 

Shale. Because of the local outcrop situation at Laguna, 

none of the published nomenclature is applicable to the 

Laguna stratigraphic section. Cobban and Hook (1984, 

p. 259) continued,

Middle Cenomanian-upper Turanian rocks in eastern Colorado are included in the Graneros Shale, Greenhorn Limestone, and Carlile Shale. The sequence is best 

exposed and documented in the Pueblo area (Cobban 

and Scott, 1972). The Graneros consists of the Thatcher 

Limestone Member separating a lower unnamed shale 

member barren of molluscan fossils from an upper 

unnamed shale member containing molluscan fossils. 

The Greenhorn consists of, from oldest to youngest, 

Lincoln Limestone Member, Hartland Shale Member, and 

Bridge Creek Limestone Member. 

At Heron Reservoir (fig. 2) in the Chama Basin, just 

east of the San Juan Basin, 63 ft of well-exposed sandstone 

on the south side of the dam, has been documented on the 

basis of ammonites as a Paguate equivalent (W.A. Cobban, 

U.S. Geological Survey, oral commun., 1989). At lake level 

and northeast of the dam (see appendix), a 25-ft-thick 

outcrop of medium-gray, fissile shale was assigned by 

Ridgley (in press) to the Hartland Member-lower member 

of the Mancos Shale, which occupies a stratigraphic 

position between the base of the Bridge Creek and the top of 

the Dakota Sandstone below. In the Chama Basin, this shale 

is 120-55 ft thick. Caution must be applied to interpretations of the strata at the reservoir because the Hartland 

Member-lower member shale is part of the southeastdipping flank of the Laguna dome to the north; its lower 

contact is covered up slope and its upper contact is below 

waterline.

MICROPALEONTOLOGY

Whitewater Arroyo Tongue

The Whitewater Arroyo benthic foraminiferal faunas 

recovered from four localities Hyde Mine, Casamero 

Lake, Cubero, and L-Bar are characteristic of nearshore, 

inner shelf, oxygenated environs; they are low in abundance 

and diversity, and their preservation is poor. The faunas 

from the other two localities of the Whitewater Arroyo 

Laguna and the type section near Two Wells reflect a 

normal, open ocean, shelf environment and comprise 

planktonic foraminifers, though limited in diversity, and 

arenaceous and calcareous benthic foraminifers.

The Hyde Mine benthic fauna (fig. 4) is low in 

abundance and diversity and indicates very shallow, nearshore conditions. The arenaceous foraminiferal fauna is 

represented by species of Haplophragmoides, Reophax,

Sampled interval

 t->

0

 

 t->

o

OS

 

 

 

o

00

 

o Tt

 

 1-1

Tt **

Microfossils

Haplophragmoides spp. 

Reophax sp. 

Ammobaculites spp. 

Trochammina spp.

Lingula sp. 

Bivalves

Gastropod 1

Gastropod 2

Gastropod 3

Spines

Gypsum

Leaf chitin

Resin

Arenaceous 

benthics

Figure 4. Distribution of foraminifers and other microfossils 

recovered from the complete 44-ft-thick section (base=0 ft) of 

the Whitewater Arroyo Tongue of the Mancos Shale at the 

Hyde Mine locality near Gallup, New Mexico (fig. 2).

Ammobaculites, and Trochammina. Casts of Lingula, an 

inarticulate brachiopod, are present throughout the Hyde 

Mine section and indicate quiet waters. Small bivalves, 

three genera of gastropods, resin, and leaf chitin are also 

present. The Casamero Lake fauna (fig. 5) is also entirely 

arenaceous benthic. Note that the Hyde Mine and Casamero 

Lake localities are at a similar latitude (fig. 2). Both faunas 

reflect a lack of influence from southern ocean waters 

(absence of planktonics), and the environments were 

probably hyposaline (Richer and Diner, 1985). The 

Casamero Lake fauna has the lowest abundance and 

diversity of benthic foraminifers in the study, and the 

localitiy is probably closest to the Whitewater Arroyo 

shoreline. Reophax, Haplophragmoides, Ammobaculites, 

and Trochammina were the only genera recovered from the 

Casamero Lake section.

The Cubero fauna is 99 percent arenaceous and has 

only three calcareous benthics: one specimen each of 

Palmula sp. (pi. 1, fig. 27) and Astacolus sp. (pi. 2, fig. 2) 

from the lower part (15 ft) of the section and a questionable 

specimen of Bullopora! sp. (pi. 2, fig. 1) from the basal 

sample (0 ft). Figure 6, a distribution diagram of the Cubero 

section, shows the limited fauna recovered. The Cubero 

fauna has almost no calcareous benthics and probably

N4 Evolution of Sedimentary Basins San Juan Basin
Sampled interval

o o

 

ID

 

0 

<N

 

ID 

<N

Microfossils

Ammobaculites fragmentarius 

Ammobaculites spp. 

Haplophragmoides spp. 

Trochammina spp. 

Reophax sp.

W

IfSE 

§1

<

Ostracode 

Gypsum 

Prisms 

Gastropod (turritellid-type)

Figure 5. Distribution of foraminifers and other microfossils 

recovered from the complete 28-ft-thick section (base=0 ft) of 

the Whitewater Arroyo Tongue of the Mancos Shale near 

Casamero Lake, New Mexico (fig. 2).

represents a very shallow water environment characterized 

by moderately oxygenated waters and hyposaline 

conditions, similar to the Hyde Mine and Casamero Lake 

environments. Throughout the Cubero section, sediment is 

mostly composed of very clean, fine-grained, angular 

quartz; minor amounts of mica, both black and white; 

carbonaceous coaly material; and chitinous leaf debris. The 

upper half of the section contains tiny iron oxide pellets but 

no pyrite, and calcareous prisms were found only at the 

60-ft level.

The L-Bar locality, represented by only two shale 

samples from the exposed upper 15 ft of the Whitewater 

Arroyo, is about 7 miles northeast of the Laguna locality 

(fig. 2). The two shale samples produced only arenaceous 

and calcareous benthic foraminifers. The arenaceous fauna 

is dominated by species of Reophax, Ammobaculites, Trochammina, and Haplophragmoides (fig. 7). Lenticulina was 

the only calcareous benthic recovered at L-Bar. Specimen 

preservation is extremely poor in both samples, and 

abundance is very low; 53 specimens were recovered in the 

lower sample and 19 in the upper sample. No faunal 

correlation can be made between the shale at L-Bar and that 

at Laguna because the L-Bar locality exposes the upper part 

of the Whitewater Arroyo and the Laguna locality the basal 

part. The examined part of the Whitewater Arroyo at L-Bar 

contains a very shallow nearshore fauna, not an unexpected 

fauna for a diminishing marine sedimentary environment.

The Whitewater Arroyo type section on the north side 

of Whitewater Arroyo near Two Wells, New Mexico (fig. 

8), produced two planktonic genera Hedbergella (three

Sampled interval

0 in o

CO

in * 0

(0

IDf- ID

00

Microfossils

Reophax spp. 

Reophax inordinatus

Trochammina spp. 

Ammobaculites spp.

A. variablis

A. goodlandensis

A. fragmentarius

Textularia sp.

Gaudryina sp.

Bullopora? sp.

Astacolus sp.

Palmula sp. ~

(0

_o

!E

c CD

JD

CO

D

O 

CJ>

O

co

CD

<

in3 o>

O 0

£ =

"3 £

«S

10 .Q

Spines

Fish tooth

Ostracode

Gypsum

Incertae sedis

Figure 6. Distribution of foraminifers and other microfossils 

recovered from the complete 85-ft-thick section (base=0 ft) of 

the Whitewater Arroyo Tongue of the Mancos Shale near 

Cubero, New Mexico (fig. 2).

species) and Globigerinelloides (one species). Hedbergellids dominate the planktonic specimens and are well 

represented in the upper part of the section. Some tests of 

both Hedbergella and Globigerinelloides are replaced with 

iron oxide or are represented by iron oxide internal molds. 

These iron oxide tests imply that bottom conditions near 

Two Wells were reducing. A few calcareous benthics were 

also found at the 30-ft level, but the most abundant and 

diverse forms in the 46-ft-thick section are arenaceous. 

Species of Trochammina, Ammobaculites, Reophax, Textularia, and Haplophragmoides represent the total benthic 

assemblage below the 30-ft level. The planktonic fauna of 

the Whitewater Arroyo near Two Wells infers that this 

environment was deeper water than that of nearby Hyde 

Mine and Casamero Lake. The Two Wells area, which is 

south of the Hyde Mine and Casamero Lake localities, was 

apparently influenced by warmer southern waters of the 

Tethyan ocean.

At the Laguna Jocality, the Dakota-Mancos sequence 

is the most complete and well exposed of the localities 

studied and the Whitewater Arroyo foraminiferal fauna the 

most abundant and diverse. Landis and others (1973) 

formally named and described the Oak Canyon Member, 

Cubero Tongue, Clay Mesa Tongue, and Paguate Tongue 

(fig. 1) at this locality.

At Laguna, 5.5 miles directly east of the Cubero 

locality, the Whitewater Arroyo fauna changes abruptly.

Upper Cenomanian Foraminifers, New Mexico N5
Sampled

interval

M 

0

.

t

4-*

M 

CO

Microfossils

Ammobaculites sp.

Haplophragmoides sp.

Reophax sp.

Trochammina sp.

Lenticulina sp.

Svenia sp.

CO

_o

!c

4-1

c

CD 

.Q

CO

D

O 

CD

U

CD

C 

CD

<

CO

3 CO

O 0

££

CD ? 

« S

CD JD

0

Fish tooth

Prisms

Figure 7. Distribution of foraminifers and other microfossils 

recovered from the upper 15 ft of the Whitewater Arroyo 

Tongue of the Mancos Shale at the L-Bar section about 3 miles 

east of the town of Moquino, New Mexico, and just north of 

the Laguna Indian Reservation (fig. 2). Top of section=0 ft.

Planktonics, limited in diversity, are present in significant 

numbers and are dominated by hedbergellids. Two species 

of Hedbergella (H. amabilis and H. delrioensis), one 

species of Heterohelix (H. moremani), and one species of 

Rotalipora (R. greenhornensis) represent the planktonic 

fauna. The Laguna assemblage produced several planktonic 

and benthic foraminiferal species (fig. 9) that have also been 

found in the Hartland Shale Member of the Greenhorn 

Limestone (Eicher and Worstell, 1970) of the Great Plains 

near Pueblo, Colorado (285 miles to the northeast). Only 

two specimens of Rotalipora greenhornensis were 

recovered from the Whitewater Arroyo at the Laguna 

locality; however, no other deeper water planktonic forms 

that characterize the Great Plains fauna, such as Praeglobotruncana, Schackoina, or Rotalipora cushmani, were 

found in the Whitewater Arroyo during this study.

Lower Member of the 

Mancos Shale at Laguna

The fauna of the late Cenomanian lower member of 

the Mancos Shale at Laguna (fig. 9) reflects an abrupt 

environmental change at the Twowells-lower member 

contact. The lower member fauna implies a deeper, more

Sampled interval

o

.

4-" 

M 

O

.

.

14-

0 

<N

14-

0 

CO

.

M 

0

Microfossils

Haplophragmoides spp. 

Reophax inordinatus 

Reophax spp. 

Trochammina spp. 

Ammobaculites spp. 

Ammobaculoides mosbyensis 

Textularia sp.

Arenaceous benthics

Svenia sp.

^ CO

Lenticulina? sp. ° .2 

Lenticulina sp. B o §

CD JD

Citharina sp. °

Hedbergella amabilis 

H. planispira 

H. delrioensis 

Globigerinelloides bentonensis

Planktonics

Ostracode 

Spines 

Prisms

Figure 8. Distribution of foraminifers and other microfossils 

recovered from the complete type section of the Whitewater 

Arroyo Tongue of the Mancos Shale on the north side of 

Whitewater Arroyo, NWV* sec. 12, T. 12 N., R. 19 W., near 

Two Wells, New Mexico (fig. 2). Base=0 ft.

normal open-marine environment than that of the Whitewater Arroyo by having a greater percentage of planktonic and 

calcareous benthic genera and the first and only occurrence 

of Rotalipora cushmani (one iron-oxide-replaced test) in the 

San Juan Basin study area. A significant decrease in 

arenaceous genera at the contact implies that bottom waters 

were less oxygenated. Haplophragmoides coronata is the 

only arenaceous benthic foraminifer in the lower member 

fauna; however, eight calcareous benthic genera were 

identified. The abundance of bivalves, the occurrence of the 

ostracode Cythereis eaglefordensis, and the increase in 

variety of calcareous benthic and planktonic foraminifers 

imply that the environment of the lower member was 

hospitable to calcareous organisms and highly influenced by 

a northerly flow of warm southern waters.

N6 Evolution of Sedimentary Basins San Juan Basin
1

03 

Hi 

(0

.Q

03 

O

(0

O

CN

*

*

03 

U) 

(0

O

(0

o

CO

03

03

O 

O

<e

§

2

*;

js.^

!O

3

3

(0

1o

03

O

(0

t±

o

tn

1O

3

o

(0

t;

in

*

tn

1O

»

o

(0

^

o

Microfossils

Ammobaculites fragmentarius

Ammobaculites sp.

Ammodiscus cretaceus

Trochammina sp.

Reophax sp.

Textularia sp.

Verneuilinoides perplexus

Bimonilina reciprocata

Haplophragmoides coronata

Lenticulina sp.

Svenia sp.

Citharina kochii

Citharina sp.

Psilocitharella sp.

Gavelinella modesta

Gavelinella sp.

Praebulimina sp.

Lingulogavellinella sp.

Ramulina aculeata

Neobulimina albertensis

Rotalipora greenhornensis

Hedbergella amabilis

H. delrioensis

Hedbergella spp.

Archaeoglobigerina sp.

Heterohelix moremani

Guembelitria harrisi

Rotalipora cushmani

Hedbergella portsdownensis

CO 

U

!E

c

CD 

JD

CO

3

O

(U 

0

CD

C

CD

CO

_o

c

CD

JD 

CO

O 

CD

CD 

0

CD

CJ

CO

_o'E

o

5

c

CD

Q_

Cythereis eaglefordensis (ostracode)

Figure 9. Distribution of foraminifers recovered from the (1) 

lower 30 ft of the Whitewater Arroyo Tongue of the Mancos 

Shale and the (3) lower 10 ft of the lower member of the 

Mancos Shale near Laguna, New Mexico (fig. 2). The Twowells Tongue of the Dakota Sandstone (2) separates the shale 

units.

Hartland Member-Lower 

Member, Chama Basin

Though not within the structural boundary of the San 

Juan Basin, an inferred equivalent stratigraphic unit to the 

Whitewater Arroyo, Twowells, and lower member of the 

Mancos sequence is the Hartland Member-lower member of 

the Mancos Shale (Ridgley, in press) at the Heron Reservoir

in the Chama Basin (fig. 2). The unit crops out above the 

reservoir at water level. Six shale samples from the upper 

part of the unit were collected at 5-ft intervals beginning at 

lake level (upper part of outcrop) (see appendix). The lower 

contact is covered higher in the topographic slope, and the 

upper contact is below the reservoir waterline. The fauna is 

99 percent planktonic and is abundantly represented by 

Rotalipora greenhornensis and R. cushmani (fig. 10); thus 

the fauna is in the Rotalipora cushmani Zone of middle to 

late Cenomanian age (fig. 11) (Sliter, 1989). Several species 

of Hedbergella are present, but specific identification of 

many of the specimens was difficult due to moderately poor 

preservation and recrystallization. Heterohelix moremani is 

common in five of the six samples, but its abundance is 

overshadowed by the abundance of rotaliporids recovered 

from the Chama Basin site. Although very rare, Globigerinelloides bentonensis and Clavihedbergella simplex (pi. 3, 

figs. 13, 14) are present at the 5- and 15-ft levels and 0- and 

10-ft levels, respectively. Clavihedbergella simplex is 

unique to this locality. Eight calcareous specimens six 

Lenticulina, one Svenia, and one Psilocitharella represent 

the benthic fauna of the section. The ostracode Cythereis 

eaglefordensis is also present but is not as abundant as at the 

Laguna locality (fig. 9).

Sampled interval

Microfossils

Svenia spp. 

Lenticulina sp. 

Psilocitharella sp.

D CO

0.2

CD.C

CD C 

_O CD

(D-D 

CJ

Rotalipora greenhornensis 

R. cushmani 

Heterohelix moremani 

Hedbergella delrioensis 

H. portsdownensis 

Hedbergella spp. 

Clavihedbergella simplex 

Globigerinelloides bentonensis

Ostracode sp. 1 

Cythereis eaglefordensis 

Prism

Figure 10. Distribution of foraminifers and other microfossils 

recovered from an incomplete 25-ft-thick section of the Hartland Member-lower member of the Mancos Shale at the 

Heron Reservoir (fig. 2) near Tierra Amarilla, New Mexico, in 

the Chama Basin. Top of section=0 ft.

Upper Cenomanian Foraminifers, New Mexico N7
0

D) 

CD -1 i

C/D

c

CD .-^

c t

2 S.

D --

H

Cenomanian (part)

.Q D) 

D CD 

W to

0 

O

i_ 

0 

Q. 

Q.

D

Middle

FORAMINIFERAL ZONES 

ANDSUBZONES

Helvetoglobotruncana 

helvetica

Whiteinella 

archaeocretacea

Rotalipora cushmani

Dicarinella 

algeriana

Rotalipora 

greenhornensis

Rotalipora reicheli

LAGUNA

Mancos Shale

Not 

studied

Bridge Creek 

Limestone Member

Lower 

member

Twowells Tongue 

of Dakota Sandstone

Whitewater Arroyo Tongue 

of Mancos Shale

Paguate Tongue 

of Dalcota Sandstone

Clay Mesa Tongue 

of Mancos Shale

Cubero Tongue 

of Dakota Sandstone

Oak Canyon Member 

of Dakota Sandstone

HERON 

RESERVOIR

Mancos Shale

P

Not 

studied

--?- -H» 

Hartland Memberlower member

(only upper 

part exposed)

aquate equivalent

Undivided 

Dakota 

Sandstone

Figure 11. Foraminiferal zones and subzones (Sliter, 1989) and stratigraphic nomenclature in the 

southern San Juan Basin (modified from Cobban and Hook, 1989) as related to the Laguna and Heron 

Reservoir localities of this study. (Only the lower 20 ft of the lower member of the Mancos Shale is 

exposed at Laguna.)

The overwhelming abundance of rotaliporids in the 

Hartland Member-lower member at the Heron Reservoir 

locality is certainly unique to this study and yet characteristic of the late Cenomanian sea. Eicher and Diner (1985) 

stated that Rotalipora was a distinctive, widespread, rapidly 

evolving genus at this time; however, the rotaliporid 

assemblage was not present at the studied localities of the 

Whitewater Arroyo in the San Juan Basin.

SIGNIFICANCE OF 

PLANKTONIC FORAMINIFERS

Modern planktonic foraminifers are rare in the nearshore environment of the continental shelf and increase in 

abundance, diversity, and size with increasing water depth 

toward the open sea. They are generally restricted to 

open-marine environments of normal oceanic salinity and 

clear water. Spinose symbiont (algae)-bearing species require sunlit, near-surface waters, whereas nonspinose 

symbiont-barren species occupy deeper waters (Leckie, 

1987) that support their full life cycles.

The application of modern and established foraminiferal distribution patterns is useful in the reconstruction of 

Cretaceous paleoenvironments. For example, in the Western

Interior of the United States Eicher (1969) and Eicher and 

Worstell (1970) showed that the globular morphotypes 

(Hedbergella and Heterohelix) were the first planktonics to 

appear during transgression of the Cretaceous (Cenomanian 

and Turonian) epicontinental sea and the last to disappear 

during regression. Sliter (1972), in a study of planktonic 

foraminifers from the eastern Pacific margin, proposed that 

the increase in eurytopic hedbergellid and heterohelicid 

abundance and diversity in shallow waters implies that these 

types are opportunistic, mostly epipelagic species that 

dominate adverse environments of shelf waters. Leckie 

(1987) observed planktonic species distribution in modern 

shelf environments and applied the distribution patterns to 

mid-Cretaceous planktonic assemblages to derive consistent 

stratigraphic indicators of generalized paleowater depths. 

His criteria were based on test morphology: inflated, 

globose, spinose morphotypes inhabited near-surface 

waters, whereas flatter keeled forms occupied deeper 

habitats. Leckie (1987) believed that the nonspinose 

globorotaliid morphotypes (Rotalipora and Praeglobotruncana) keeled genera were the deepest dwelling of the 

mid-Cretaceous planktonic foraminifers.

Leckie (1987) proposed three faunal groups 

containing characteristic genera.

N8 Evolution of Sedimentary Basins San Juan Basin
1. Epicontinental sea fauna (ESF): shallow epeiric 

or marginal sea (<300 ft); fauna probably inhabited nearsurface waters. Characteristic globose genera, Gubkinella, 

Guembelitria, and Heterohelix, are present in greatly 

reduced abundance in open-ocean pelagic sections. Large 

proportions of small specimens of Hedbergella spp. and 

Globigerinelloides spp. are also associated with epicontinental sea fauna. This fauna! assemblage is believed to 

have had the widest and highest tolerance to the 

environment in the mid-Cretaceous.

2. Shallow-water fauna (SWF): shallow water, open 

marine, pelagic (<300 ft). Major generic components are 

Hedbergella and Globigerinelloides. Associated genera 

include species of Clavihedbergella, Schackoina, and 

Ticinella.

3. Deep-water fauna: deep water, open marine, 

(>300 ft). Believed to be the most sensitive to the 

environment in the mid-Cretaceous. Characteristic genera 

are keeled forms such as species of Planomalina, Rotalipora, and Praeglobotruncana.

Applying Leckie's (1987) generic characteristics to 

the Whitewater Arroyo Tongue, lower member of the 

Mancos Shale, and Hartland Member-lower member 

faunas, the Laguna assemblage is a shallow-water fauna in 

which Hedbergella amabilis, H. delrioensis, and Heterohelix moremani are the dominant planktonic forms. The 

Laguna fauna, characterized by arenaceous and calcareous 

benthics and planktonics, reflects a mid- to outer-shelf, 

hospitable, open-marine environment and water depths of 

less than 300 ft. The lower member faunal assemblage at 

Laguna, in which planktonic genera, specimen abundance, 

and calcareous benthics increase, indicates a warmer water 

mass and sufficiently oxygenated bottom waters to support 

both arenaceous and calcareous benthics. Water depth is 

still less than 300 ft and represents the mid- to outer-shelf. 

The dominance of rotaliporids in the Hartland Memberlower member of the Mancos Shale in the Chama Basin 

reflects a water depth of probably greater than 300 ft.

FAUNAL AND ENVIRONMENTAL 

INTERPRETATIONS

Faunal data recovered from the Whitewater Arroyo 

Tongue of the Mancos Shale in the southern San Juan Basin 

provide a basis for limited regional environmental 

interpretations. The faunal assemblages from the northern 

localities of Hyde Mine and Casamero Lake represent a 

very shallow water, nearshore environment characterized by 

moderately oxygenated waters and low salinity; the 

shoreline probably was nearby and to the north. The faunal 

assemblage from Cubero contains very little calcareous 

material; three calcareous foraminifers and Inoceramus 

prisms from one sample comprise the entire calcareous 

contribution. The Cubero environment probably was a

protected shallow-water realm close to shore as evidenced 

by coaly material and chitinous leaf remains that are present 

throughout the section. Moderately oxygenated waters and 

hyposaline conditions persisted at Cubero. The limited 

faunal assemblage from L-Bar, dominated by arenaceous 

foraminifers, represents a very shallow nearshore 

environment transitional into the regressive sandstone 

environment of the overlying Twowells Tongue.

The locality of the Whitewater Arroyo Tongue near 

Two Wells, south of the Hyde Mine and Casamero Lake 

localities, probably was influenced by northerly flowing 

warm waters of the Tethyan ocean and slightly deeper 

waters than the two northern localities. Planktonics are 

present in the upper part of the Two Wells section, and no 

evidence of terrestrial influence such as leaf chitin or resin 

was found.

The abrupt change in the Whitewater Arroyo fauna 

between the Cubero and Laguna localities is interpreted as 

an abrupt environmental change. At Laguna, 5.5 miles east 

of Cubero, the fauna reflects a normal marine foraminiferal 

assemblage of the mid-shelf. The presence of both arenaceous and calcareous benthics and three genera of planktonics suggests that the mid-shelf sea floor was well 

oxygenated and that warm southern waters of the Tethyan 

ocean flowed northward and brought the planktonics to 

Laguna.

The ostracode, Cythereis eaglefordensis Alexander 

(pi. 2, fig. 13), is common in both the Whitewater Arroyo 

Tongue and the lower member of the Mancos Shale at the 

Laguna locality. It is common and widespread in the Gulf 

Coastal Plain and Western Interior of the United States. 

Hazel (1969) used it as a guide fossil for the latest 

Cenomanian and placed it at the "top of the Cenomanian in 

the Rotalipora cushmani-R. greenhornensis zone" (Hazel 

followed Pessagno's 1967 zonation), which is approximately equal to the Rotalipora cushmani Zone of Sliter 

(1989). (The "top" of the Cenomanian is in the Whiteinella 

archaeocretacea Zone (Sliter, 1989) (fig. 11).) Because the 

ostracode Cythereis eaglefordensis and the foraminifer R. 

greenhornensis are the only age-diagnostic species in the 

Whitewater Arroyo, the Whitewater Arroyo fauna is placed 

in the R. cushmani Zone of middle and late Cenomanian age 

(Sliter, 1989) (fig. 11). Based on morphology, E.M. Brouwers (U.S. Geological Survey, written commun., 1989) 

stated that the species is "deeper water-neritic but not inner 

shelf." The ostracode data support the environment 

suggested by the foraminiferal data, mid-to outer-shelf and 

normal open marine water less than 300 ft deep.

The Laguna fauna exhibits the greatest diversity of 

the six Whitewater Arroyo sections and therefore is used for 

comparison with the coeval Great Plains faunas. The 

Whitewater Arroyo and the Twowells together are generally 

accepted as coeval with the Lincoln Member (late middle to 

early late Cenomanian) of the Greenhorn Limestone of the

Upper Cenomanian Foraminifers, New Mexico N9
Great Plains. Eicher and Worstell (1970) and Eicher and 

Diner (1985) reported that no benthics were recovered from 

the Lincoln Member. Eicher and Diner (1985, p. 63) stated 

that,

In the uppermost sample of the Graneros and lowest 

samples of the Lincoln Member, the foraminifera consist 

only of tiny specimens of the ubiquitous genera, Hedbergella and Heterohelix.

The foraminiferal distribution charts of Eicher and 

Worstell (1970) illustrate that little faunal change occurred 

between deposition of the lowest samples and the upper 

sample of the Lincoln Member, except for a few rare 

additions of planktonic genera. The Laguna Whitewater 

Arroyo planktonic foraminifers consist only of the same two 

genera, Hedbergella and Heterohelix (fig. 9); the only 

exception is the two specimens ofRotalipora greenhornensis. The Whitewater Arroyo also contains both arenaceous 

and calcareous benthics, and this major difference between 

the two coeval middle to late Cenomanian age assemblages 

can be explained by the geographic locations of the two 

faunas within the seaway. The Whitewater Arroyo is in the 

shallower "mixed" benthic biofacies of the western margin, 

and the Lincoln is in the "no benthics fades" in the deeper 

central part of the seaway to the east, as proposed by Eicher 

and Diner (1985) (fig. 12). The data of Eicher and Worstell 

(1970) and Eicher and Diner (1985) are for localities much 

farther to the east and north of San Juan Basin (Pueblo and 

Fort Collins, Colorado, and western Kansas), where deeper 

and warmer waters contributed to a hospitable planktonic 

environment.

The next significant faunal change observed by 

Eicher and Worstell (1970) and Eicher and Diner (1985) is 

first recorded in the upper part of the Lincoln Member and 

continues through the Hartland Shale Member, which 

overlies the Lincoln Member. Eicher and Diner (1985, p. 

63) stated that,

In the upper Lincoln and Hartland these [Hedbergella and 

Heterohelix] are joined successively by larger species of 

Hedbergella and species of Globigerinelloides, Rotalipora, Praeglobotruncana, Schackoina, Clavihedbergella, 

Guembelitria and /4naf/c/ne//a.***The foramininferal 

fauna of the Lincoln and Hartland Members of the 

Greenhorn Formation [Limestone] consists solely of 

abundant planktonic specimens and contrasts markedly 

with the comparatively scanty, arenaceous benthic fauna 

that dominates the underlying Graneros.

At Laguna, at the conformable contact between the 

Twowells Tongue of the Dakota and the overlying lower 

member of the Mancos, an abrupt faunal change is observed 

in the lower member. The faunal change, noted by an 

increase in calcareous benthic and planktonic generic 

diversities and a decrease in arenaceous benthic diversity 

(fig. 9), records the appearance of many of the same species 

(both planktonics and calcareous benthics) that are present 

in the uppermost Hartland. The Hartland fauna of the Great 

Plains (except the uppermost part) consists only of

planktonic foraminifers as does that of the Lincoln. 

According to Eicher and Worstell (1970) and Eicher and 

Diner (1985), only two faunal differences are observed 

between the Lincoln and the Hartland. (1) Rotalipom is 

present near the top of the Lincoln and continues through 

the Hartland, and (2) Schackoina and Praeglobotruncana 

are present about midway through the Hartland and 

continue across the Cenomanian-Turonian boundary.

Based on the presence of Rotalipora, Hedbergella, 

Heterohelix, Archacoglobigerina, and Guembelitria, the 

lower member of the Mancos at Laguna is considered 

coeval with the uppermost Lincoln and lower Hartland of 

the Great Plains. Other than the presence of Rotalipora and 

calcareous benthic species (also in the uppermost Hartland) 

in the Graneros fauna of the Great Plains, no finer 

distinction can be made regarding the assignment of this 

fauna. The lower member of the Mancos occupied the 

shallower part of the seaway to the west and is a timeequivalent stratigraphic unit to the Hartland Shale Member 

of the Greenhorn Limestone, which occupied the deeper 

part of the seaway to the east (fig. 12).

The foraminiferal fauna of the Hartland Memberlower member in the Chama Basin consists solely of 

planktonics and calcareous benthics (fig. 10). Rotalipora 

cushmani and R. greenhornensis overwhelmingly dominate 

the assemblage. Hedbergella spp. abundances decrease and 

Clavihedbergella simplex is unique to this locality in this 

study. The presence of C. simplex, Heterohelix moremani, 

and Globigerinelloides bentonensis places the fauna in the 

upper R. cushmani Zone (Sliter, 1989). Because none of the 

distinctive genera characteristic of the Dicarinella algeriana Subzone of late Cenomanian age is present, the author 

is reluctant to assign the fauna to the subzone. Benthics are 

rare, which indicates poorly oxygenated bottom waters in 

the Western Interior in Lincoln and Hartland time as 

reported by Eicher and Diner (1985). Applying Leckie's 

(1987) criteria based on test morphology, the Hartland 

Member-lower member of the Mancos assemblage at the 

Heron Reservoir locality in the Chama Basin represents a 

sensitive, open-marine fauna and a water depth probably 

greater than 300 ft.

The dominance and abundance of rotaliporid species 

and the presence of Clavihedbergella simplex indicate that 

the Hartland Member-lower member of the Mancos in the 

Chama Basin probably is time equivalent to the Hartland 

Shale Member of the Greenhorn Limestone of the Great 

Plains and to the lower planktonic zone of Eicher and 

Worstell (1970) recovered at Pueblo, Colorado, 145 miles to 

the northeast. On species content, it is in closer association 

with the Hartland fauna at Pueblo than with the lower 

member fauna at Laguna, 140 miles to the southwest. The 

geographic distances between the localities of Pueblo, 

Heron Reservoir, and Laguna and the distribution of 

foraminifers in the middle Hartland sea as proposed by

N10 Evolution of Sedimentary Basins San Juan Basin
Figure 12. Distribution of foraminifers in the Western Interior during (A) latest Graneros time and (B 

and (Q middle Hartland time. Modified from Eicher and Diner (1985).

Eicher and Diner (1985) in figure 12 imply that the fauna of 

the lower member at Laguna inhabited a shallower biofacies 

than did the Hartland faunas at Pueblo and Heron Reservoir.

PALEOGEOGRAPHIC THEORIES

New theories and paleogeographic models have been 

proposed since the work of Landis and others (1973) on the 

Dakota and Mancos sequence at Laguna. Noon (1980)

proposed a deltaic Four Corners Headland (fig. 13) that 

would serve as the source for the fluvial facies of the Dakota 

Sandstone in the Four Corners area as well as for the 

offshore sands of the Twowells Tongue of the Dakota. The 

deltaic model supports generally accepted concepts 

concerning the Whitewater Arroyo Tongue of the Mancos 

Shale and the Twowells Tongue: (1) the shoreline positions 

of uooban and Hook (1984) (fig. 14) for the Whitewater 

Arroyo and Twowells; (2) the time-transgressive nature of

Upper Cenomanian Foraminifers, New Mexico N11
the top of the Twowells, which is younger to the southwest 

(in San Juan Basin) (Hook and others, 1980; Esterly, 1985; 

D.G. Wolfe, University of Colorado, oral commun., 1989); 

and (3) a southwest source area for the Twowells as 

proposed by Landis and others (1973). Assuming that the 

Four Corners protodeltaic headland existed in early 

Cenomanian time (Encinal Canyon time), it could have 

contributed to the landmass in the area of southwestern 

Colorado and northwestern New Mexico (fig. 15). As the 

seaway transgressed westward, it encroached the headland 

as defined by Noon (1980) (fig. 13). In early late 

Cenomanian time (middle to late Whitewater Arroyo time), 

a barrier may have existed along the Colorado-New Mexico 

State line that formed the northern boundary of Seboyeta 

bay and maintained a nearshore, marginal-marine foraminiferal environment in northwestern New Mexico. 

Perhaps a submarine remnant of Seboyeta bay, which was 

established in middle Cenomanian time (Oak Canyon and 

Cubero time) (Cobban and Hook, 1984), existed even 

though the actual surface configuration of the bay had 

disappeared by Whitewater Arroyo time and the shoreline 

was far to the west (fig. 14).

Wolter (1987) built part of his sedimentation theory 

of the Twowells on the Four Corners Headland model of 

Noon (1980) and coined a sedimentary structural term, the 

"Acoma ridge," for a structure in the Acoma Basin between 

Grants and Acoma and from Interstate Highway 1-40 south 

for more than 25 miles (fig. 13). As further defined by 

Wolter, the Acoma ridge trends north-south, is 6 miles wide, 

and has a maximum thickness of 105 ft. During the early 

and middle Cenomanian, the Acoma ridge in Seboyeta bay 

was protected from the southward-flowing tidal currents of 

the open shelf by the Four Corners Headland. Wolter (1987) 

stated that the funneling effect of the bay, the open tidal 

communication with the proto-Gulf of Mexico, and the 

shallow, wide shelf all enhanced tidal currents of the bay 

and deposition of the Acoma ridge (Twowells) in a northsouth-trending erosional low.

Fauna! evidence implies that the Laguna area was at 

the eastern edge of Seboyeta bay in its late phase and was 

greatly influenced by southward-flowing currents (Wolter, 

1987) coming off the eastern promontory of a deltaic 

headland (Noon, 1980) along the Colorado-New Mexico 

State line and by the northerly flowing waters from the 

Tethyan ocean. These southward-flowing currents carried 

large amounts of sediment, nutrients, and species of 

foraminifers from a deeper carbonate realm to the east 

(fig. 12).

The 6-mile width of the Acoma ridge is remarkably 

close to the 5.5-mile distance between the Cubero and 

Laguna sections, and the ridge would have formed an 

excellent barrier if it existed in the pre-Twowells Seboyeta 

bay. Conversely, the Cubero locality is 8.5 miles east of the 

Acoma ridge and north of Interstate Highway 1-40 as plotted 

by Wolter. Hence, present-day geographic distances

109° 108° 107°

37°

36°

35°

34°

COLORADO

x&

Acoma ridge o 

0 Laguna /Albuquerque

°Acoma /

3. Q i

"^

Nonmarine

General outcrop

boundary of 

Twowells Tongue

20 40 MILES

Figure 13. Sketch showing deltaic Four Corners Headland 

model of Noon (1980, from D.E. Owen, 1979, unpublished 

sketch), location of Acoma ridge, and general outcrop 

boundary of the Twowells Tongue of the Dakota Sandstone 

(Wolter, 1980). The headland served as a source for the 

offshore bars and shoreface sands of the Twowells. The same 

river system that supplied sediments for the headland probably 

provided the fluvial facies of the Dakota Sandstone in 

northwestern New Mexico.

between Cubero, Laguna, and the Acoma ridge do not 

provide evidence to explain the faunal differences between 

the Cubero and Laguna Whitewater Arroyo.

LATE CENOMANIAN STRATIGRAPHY AND 

AMMONITE ZONES

Middle Cenomanian geography of the San Juan 

Basin, particularly Seboyeta bay, played a major role in 

forming the depositional geometry of upper Cenomanian 

strata, the upper part of the Whitewater Arroyo Tongue of 

the Mancos Shale, the Twowells Tongue of the Dakota 

Sandstone, and the lower member of the Mancos Shale.

During early middle Cenomanian time, the Oak 

Canyon Member and Cubero Tongue of the Dakota 

Sandstone were deposited in Seboyeta bay (Hook and

N12 Evolution of Sedimentary Basins San Juan Basin
108° 106° __104° 114°

39°

112° 110° 108° 106° 104°

0 50 100 150 MILES 

i i i i

Figure 14. General position of late Cenomanian shorelines 

during deposition of the upper part of the Whitewater Arroyo 

Tongue of the Mancos Shale, the Twowells Tongue of the 

Dakota Sandstone, and the main body of the Mancos Shale as 

defined by the geographic distribution of zone indicator 

ammonite species in the San Juan Basin. Modified from 

Cobban and Hook (1984). Solid circles indicate sample 

localities (fig. 2).

others, 1980) (fig. 1). The molluscan fauna of these two 

units is in the ammonite zone of Conlinoceras tarrantense 

(fig. 16). Deposition of the Clay Mesa Tongue in the middle 

middle Cenomanian shows a considerable southward 

transgression of the sea to El Paso, Texas, and reveals that 

Seboyeta bay still existed but with little southern boundary. 

In the study area, the Clay Mesa Tongue is in the ammonite 

zone of Acanthoceras amphibolum. Cobban and Hook 

(1989) reported that ammonites are uncommon and list only 

three species in the Clay Mesa. A third transgression to the 

west in late middle Cenomanian deposited the Whitewater 

Arroyo Tongue. Fossil molluscs are abundant and in great 

variety in shallow-water sandstones such as the Paguate 

Tongue of the Dakota Sandstone in Seboyeta bay (Cobban, 

1977). The molluscan fauna in the Paguate at Laguna places 

the sandstone in the ammonite zone of Plesiacanthoceras 

wyomingense (Cobban and Hook, 1989). Ammonites from 

the Whitewater Arroyo Tongue reveal that it ranges in age 

from the late middle Cenomanian zone of P. wyomingense 

into the early late Cenomanian zone of Calycoceras canitaurinum (Cobban and Hook, 1989). Shoreline positions of 

the middle Cenomanian changed the least in the Casamero 

Lake area, and encroachment was minor in the Four Corners 

area. Figure 15 shows the general position of middle 

Cenomanian shorelines based on the occurrences of ammonite species (Cobban and Hook, 1984).

In the late Cenomanian the sea continued to 

transgress to the west and encroached the Four Corners area

37° - _ _ _ _ _ JLJTAH 

ARIZONA

35°

33°^

Paguate

PuebloO

COLO_

NEW 

MEXICO

SEBOYETA 

O Albuquerque - 

BAY

50 100 150 MILES 

i i i

Figure 15. General position of middle Cenomanian shoreline 

during deposition of the Oak Canyon Member and Cubero and 

Paguate Tongues of the Dakota Sandstone and the Clay Mesa 

Tongue of the Mancos Shale as defined by the geographic 

distribution of zone indicator ammonite species. Modified 

from Cobban and Hook (1984). Solid circles indicate sample 

localities (fig. 2).

for the first time during this stage. During early late 

Cenomanian the sea flooded the areas of southwestern 

Colorado and northwestern New Mexico, removed the 

northern boundary of the Seboyeta bay, and deposited the 

upper part of the Whitewater Arroyo and Twowells 

Tongues. The Twowells molluscan fauna is in the zones of 

Calycoceras canitaurinum and Metoicoceras mosbyense of 

early late Cenomanian age. The lower member of the 

Mancos at Laguna, which immediately overlies the Twowells, marks the beginning of a major advance of the 

Greenhorn sea into southeastern Utah and northeastern 

Arizona. Foraminifers from this basal unit imply that water 

depth changed abruptly and document a large-scale 

incursion of the Greenhorn sea. The oyster Pycnodonte 

newberryi (Stanton) paves the ground as float on top of the 

Two wells at its contact with the lower member of the 

Mancos. These molluscs may be weathering out of the 

Bridge Creek Limestone Member, which is about 15 ft 

above the contact. At Laguna, the Bridge Creek Limestone 

Member marks the base of the Sciponoceras gracile ammonite zone of middle late Cenomanian age (Cobban and 

Hook, 1989) (fig. 16).

CONCLUSIONS

The Whitewater Arroyo Tongue of the Mancos Shale 

in the southern San Juan Basin is latest middle to early late

Upper Cenomanian Foraminifers, New Mexico N13
0

D) 

CO *J

C/D

Turonian (part)

Cenomanian (part)

 0

-Q O) 

D CD

Wto

0)

o

_1

1_

0 

Q. 

Q.

D

Middle

FORAMINIFERAL ZONES 

AND SUBZONES

Helvetoglobotruncana 

helvetica

Whiteinella 

archaeocretacea

Rotalipora cushmani

Dicarinella 

algeriana

Rotalipora 

greenhornensis

Rotalipora reicheli

LACUNA

0)

Mancos Sha

Not 

studied

Bridge Creek 

Limestone Member

Lower 

member

Twowells Tongue 

of Dakota Sandstone

Whitewater Arrow Tongue 

of Mancos Shale

Paguate Tongue 

of Dakota Sandstone

Clay Mesa Tongue 

of Mancos Shale

Cubero Tongue 

of Dakota Sandstone

Oak Canyon Member 

of Dakota Sandstone

AMMONITE ZONE

Vascoceras birchbyi

Pseudaspidoceras 

flexuosum

Neocardioceras juddii

Burroceras clydense

Sciponoceras gracile

Metoicoceras mosbyense

Calycoceras canitaurinum

Plesiacanthoceras 

wyomingense

Acanthoceras amphibolum

Conlinoceras tarrantense

Figure 16. Foraminiferal zones and subzones (Sliter, 1989), stratigraphic nomenclature (Cobban and 

Hook, 1989), and ammonite zones (Cobban and Hook, 1989) in the southern San Juan Basin as related 

to the study area. (Only the lower 20 ft of the lower member of the Mancos Shale is exposed at Laguna.)

Cenomanian age. Foraminifers in the northwestern part of 

Seboyeta bay of the Cretaceous seaway represent a shallow, 

nearshore, marginal-marine environment influenced somewhat by the Tethyan ocean to the south. The fauna reflects 

the existence of a land barrier to the north, in about the 

position of the Colorado-New Mexico State line. The 

foraminifers at Laguna characterize an open-ocean, normalmarine, mid- to outer-shelf environment that implies a 

diminishing influence of the northern land barrier on the 

foraminiferal environment. The Twowells Tongue of the 

Dakota Sandstone is early late Cenomanian in age, and the 

lower member of the Mancos at Laguna is middle late 

Cenomanian in age. The foraminifers of the lower member 

reflect an abrupt change in water depth (deeper and 

transgressive) and environment (reduced oxygen bottom 

waters) at the base of the unit, possibly caused by rapid 

subsidence or rapid sea-level rise.

The fauna from the Hartland Member-lower member 

in the Chama Basin characterizes much deeper, openmarine conditions than the Whitewater Arroyo fauna and 

shows little association in species duplication and environmental elements with the Whitewater Arroyo fauna. The 

assignment of the Hartland Member-lower member fauna at 

the Heron Reservoir as coeval with the Hartland Shale 

Member fauna of the Greenhorn Limestone of the Great 

Plains implies that the Lincoln-equivalent fauna probably 

exists up flank (above the 25-ft sample) of the Hartland

Member-lower member and that the Bridge Creekequivalent fauna is probably present in the shale below the 

reservoir waterline.

TAXONOMIC NOTES ON 

SELECTED SPECIES

Bimonilina redprocata Loeblich and Tappan

(Plate 2, figure 10)

A single specimen of this form was recovered from 

the Whitewater Arroyo Tongue of the Mancos Shale at the 

20-ft level of the Laguna locality. Identification remains in 

question because the aperture of Bimonilina is described as 

"slitlike." Specimen is magnificent.

Heterohelix moremani (Cushman)

(Plate 3, figure 1)

This small species consisting of globular chambers 

represents a major planktonic constituent of the lower 

member of the Mancos at the Laguna locality. It has less 

lobate chambers than Heterohelix globulosa, the Campanian to Maastrichtian species (Pessagno, 1967) that Eicher 

and Worstell (1970) stated is present in the Cenomanian

N14 Evolution of Sedimentary Basins San Juan Basin
Great Plains fauna. Heterohelix moremani is also present in 

the Hartland Member-lower member of the Mancos at the 

Heron Reservoir locality but cannot compete in dominance 

with Rotalipora spp.

Guembelitria harrisi Tappan

(Plate 3, figure 2)

All specimens of this rare and extremely small 

species are damaged. It is present only in the lower member 

of the Mancos in the sample from the 5-ft level of the 

Laguna locality.

Hedbergella amabilis Loeblich and Tappan

(Plate 3, figure 3)

The illustrated specimen, beautifully preserved as an 

internal mold, shows the globose periphery of the chambers 

and the rapid increase of chamber size, both of which 

differentiate this species from H. delrioensis. It is a common 

species at most localities where planktonic foraminifers are 

present.

Hedbergella portsdownensis (Williams-Mitchell)

(Plate 3, figures 4-6)

This species is recognized by its lobate periphery, 5-6 

chambers in the final whorl, and the distinctive overhang of 

the last chamber onto the umbilical side. It is most common 

in the lower member of the Mancos at the Laguna locality 

but also is present in the Hartland Member-lower member 

of the Mancos at the Heron Reservoir locality.

Considerable discussion surrounds this genus and 

species. Some foraminiferal researchers regard Hedbergella 

portsdownensis to be synonymous with Whiteinella brittonensis. Richer and Worstell (1970) wrote that "both Hermes 

(1969, p. 50-51) and Pessagno (1967, p. 282) have pointed 

out the close similarity of H. portsdownensis [from 

Germany]***and H. brittonensis" [from Texas]. Later in 

the same discussion, Eicher reported that he "has seen 

numerous specimens from the Cenomanian of northern 

Europe [that are referred to as] H. portsdownensis and these 

appear to be identical to the numerous representatives of//. 

brittonensis from the Greenhorn." Eicher and Worstell used 

Hedbergella portsdownensis in their systematic paleontology. Pessagno (1967), in his systematic remarks, wrote 

that "Hedbergella brittonensis Loeblich and Tappan*** 

seems suspiciously similar to Hedbergella portsdownensis 

(William-Mitchell, 1948) and may be a junior synonym of 

this latter specie***Hedbergella brittonensis appears to 

have been derived from a Hedbergella delrioensis stock."

Loeblich and Tappan (1988, p.-462) stated that, at 

the generic level, Hedbergella has an "***umbilicalextraumbilical arch with a narrow lip or flap [and

Whiteinella has the same arch but is] bordered by a 

wide apertural flap or porticus that extends into the 

umbilicus***."

Because the specimens recovered from the lower 

member of the Mancos at the Laguna locality and from the 

Hartland Member-lower member of the Mancos at the 

Heron Reservoir locality appear to be identical to those in 

photomicrographs in Eicher and Worstell (1970), the same 

systematics are retained herein.

Hedbergella delrioensis (Carsey)

(Plate 3, figures 7-9, 11, 12)

This is a common species in both the Whitewater 

Arroyo and the lower member at the Laguna locality and in 

the Hartland Member-lower member L at the Heron 

Reservoir locality. Both localities represent open-ocean 

areas. Chambers are less lobate and increase in chamber size 

is less pronounced than in Hedbergella amabilis, Recrystallized specimens from the Heron Reservoir locality 

prevent examination of details.

Clavihedbergella simplex (Morrow)

(Plate 3, figures 13, 14)

This species is rare and limited to the Hartland 

Member-lower member of the Mancos at the Heron 

Reservoir locality. The few small specimens recovered have 

4-5 chambers in final whorl.

Archaeoglobigerina

(Plate 3, figure 15)

The aperture location and hispid texture of the test 

characterize this genus. It is confined to the base of the 

lower member of the Mancos at the Laguna locality.

Rotalipora cushmani (Morrow)

(Plate 3, figures 16-19)

Figure 16 of plate 3 is the single iron-oxide-replaced 

representative of this species from the San Juan Basin. 

Although the species is abundant at the Heron Reservoir 

locality in the Hartland Member-lower member, the 

specimens are recrystallized and details are obscured. 

Species variation exists between the San Juan Basin (lower 

member at Laguna) specimen and the Chama Basin (Heron 

Reservoir) specimens (pi. 3, figs. 17-19), but no conclusion 

can be drawn from a single specimen for comparative 

purposes.

Upper Cenomanian Foraminifers, New Mexico N15
Rotalipora greenhornensis (Morrow)

(Plate 3, figures 21-23)

This distinctive species is the most abundant form in 

the Hartland Member-lower member of the Mancos at the 

Heron Reservoir locality in the Chama Basin. Two 

specimens were also recovered from the 20-ft interval of the 

White water Arroyo Tongue at the Laguna locality.

REFERENCES CITED

Aubrey, W.M., 1986, The nature of the Dakota-Morrison 

boundary, southeastern San Juan Basin: American 

Association of Petroleum Geologists Studies in Geology 22, 

p. 93-104.

Cobban, W.A., 1977, Fossil mollusks of the Dakota Sandstone and 

intertongued Mancos Shale of west-central New Mexico, in 

Fassett, J.E., ed., San Juan Basin III (northwestern New 

Mexico): New Mexico Geological Society Annual Field 

Conference, 28th, Guidebook, p. 213-220.

Cobban, W.A., and Hook, S.C., 1984, Mid-Cretaceous molluscan 

biostratigraphy and paleogeography of southwestern part of 

western interior, United States, in Westerman, G.E., ed., 

Jurassic-Cretaceous biochronology and paleogeography of 

North America: Geological Association of Canada Special 

Paper 27, p. 257-271.

____ 1989, Mid-Cretaceous molluscan record from west-central 

New Mexico: New Mexico Geological Society Field 

Conference, 40th, Southeastern Colorado Plateau, 

Guidebook, p. 247-264.

Cobban, W.A., and Scott, G.R., 1972, Stratigraphy and ammonite 

fauna of the Graneros Shale and Greenhorn Limestone near 

Pueblo, Colorado: U.S. Geological Survey Professional 

Paper 645, 108 p.

Dane, C.H., 1948, Geologic map of part of eastern San Juan Basin, 

Rio Arriba County, New Mexico: U.S. Geological Survey 

Oil and Gas Investigations Preliminary Map 78.

Dane, C.H., Landis, E.R., and Cobban, W.A., 1971, The Twowells 

Sandstone Tongue of the Dakota Sandstone and the Tres 

Hermanos Sandstone as used by Herrick (1900), western 

New Mexico: U.S. Geological Survey Professional Paper 

750-B, p. 17-22.

Eicher, D.L., 1969, Cenomanian and Turonian planktonic foraminifera from the western interior of the United States: 

International Conference on Planktonic Microfossils, 1st, 

Geneva, 1967, Proceedings, v. 2, p. 163-174.

Eicher, D.L., and Diner, Richard, 1985, Foraminifera as indicators 

of water mass in the Cretaceous Greenhorn sea, Western 

Interior, in Fine-grained deposits and biofacies of the 

Cretaceous Western Interior seaway Evidence of cyclic 

sedimentary processes: Society of Economic Paleontologists and Mineralogists, Midyear Meeting, Golden, 

Colorado, 1985, Field Trip Guidebook 4, p. 60-71.

Eicher, D.L., and Worstell, Paula, 1970, Cenomanian and Turonian foraminifera from the Great Plains, United States: 

Micropaleontology, v. 16, no. 3, p. 269-324.

Esterly, D.R., 1985, Stratigraphy of Twowells Tongue of Dakota 

Sandstone, west-central New Mexico and adjacent Arizona: 

Bloomington, Indiana University, M.S. thesis, 113 p.

Hazel, J.E., 1969, Cythereis eaglefordensis Alexander, 1929 A 

guide fossil for deposits of latest Cenomanian age in the 

Western Interior and Gulf Coast regions of the United 

States: U.S. Geological Survey Professional Paper 650-D, 

p. D155-D158.

Hermes, J.J., 1969, Late Albian foraminifera from the Subbetic of 

southern Spain: Geologie en Mijnbouw, v. 48, no. 1, 

p. 35-65.

Hook, S.C., Cobban, W.A., and Landis, E.R., 1980, Extension of 

the intertongued Dakota Sandstone-Mancos Shale 

terminology into the southern Zuni Basin: New Mexico 

Geology, v. 2, p. 42-44, 46.

Kummel, Bernhard, and Raup, David, 1965, Handbook of 

paleontological techniques: San Francisco, W.H. Freeman, 

852 p.

Landis, E.R., Dane, C.H., and Cobban, W.A., 1973, Stratigraphic 

terminology of the Dakota Sandstone and Mancos Shale, 

west-central New Mexico: U.S. Geological Survey Bulletin 

1372-J, 44 p.

Leckie, R.M., 1987, Paleoecology of mid-Cretaceous planktonic 

foraminifera A comparison of open ocean and epicontinental sea assemblages: Micropaleontology, v. 33, no. 

2, p. 164-176.

Loeblich, A.R., Jr., and Tappan, Helen, 1988, Foraminiferal genera 

and their classification: New York, Van Nostrand Reinhold, 

970 p.

Noon, P.L., 1980, Surface to subsurface stratigraphy of the Dakota 

Sandstone San Juan Basin, New Mexico and Colorado: 

Bowling Green, Bowling Green State University, M.S. 

thesis, 133 p.

Owen, D.E., 1966, Nomenclature of Dakota Sandstone 

(Cretaceous) in San Juan Basin, New Mexico and Colorado: 

American Association of Petroleum Geologists Bulletin, v. 

50, p. 1023-1028.

Pessagno, E.A., Jr., 1967, Upper Cretaceous planktonic foraminifera from the western Gulf Coastal Plain: Palaeontographic 

Americana, v. 5, no. 37, p. 245^45.

Ridgley, J.L., in press, Geologic map of the Rio Chama Wilderness 

Study Area, Rio Arriba County, New Mexico: U.S. 

Geological Survey Miscellaneous Field Studies Map 

MF-2046, scale 1:24,000.

Sliter, W.V., 1972, Upper Cretaceous planktonic foraminiferal 

zoogeography and ecology Eastern Pacific margin: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 12, 

p. 15-31.

____ 1989, Biostratigraphic zonation for Cretaceous planktonic 

foraminifers examined in thin section: Journal of Foraminiferal Research, v. 19, no. 1, p. 1-19.

Wolter, N.R., 1987, The depositional history of the Cenomanian 

Twowells Tongue of the Dakota Sandstone, New Mexico: 

Baton Rouge, Louisiana State University, M.S. thesis, 

132 p.

N16 Evolution of Sedimentary Basins San Juan Basin
APPENDIX DESCRIPTION OF LOCALITIES

[Localities shown in figure 2]

San Juan Basin

Whitewater Arroyo type section. NE1A sec. 17, T. 

12 N., R. 19 W., Vanderwagen 71/2-minute quadrangle, 

McKinley County, New Mexico. About 1.2 miles west of 

Two Wells, New Mexico. The entire section of the Whitewater Arroyo Tongue of the Mancos Shale is well exposed 

and accessible. It is overlain by 30 ft of the Twowells 

Tongue of the Dakota Sandstone. Owen (1966) described an 

80-ft-thick type section, and Dane and others (1971) measured a 46-ft-thick section at this same locale from which 

five shale samples were collected. The upper 6 ft of the 

latter section is sandy and transitional and was not collected 

for this study.

Hyde Mine. NEViNWVi sec. 12, T. 15 N., R. 18 W., 

Gallup East 7 1/2-minute quadrangle, McKinley County, 

New Mexico. About 1.5 miles due north of 1-40 at the 

Hogback; actual road distance from 1-40 is 3 miles. The 

entire section of the Whitewater Arroyo is nicely exposed 

and easily accessed. The Whitewater Arroyo is 54 ft thick; 

the upper 10 ft of the section is a transition zone with the 

Twowells and was not collected. Five shale samples cover 

the sampled section, although eleven were collected.

Casamero Lake. NElA sec. 30, T. 15 N., R. 11 W., 

Casamero Lake 71/2-minute quadrangle, McKinley County, 

New Mexico. Though not a prominent outcrop, a complete 

section (28 ft) of the Whitewater Arroyo is well exposed 

and is overlain by 15 ft of Twowells. A thin, 2-in.-thick, 

dark-brown, iron-stained, fossiliferous bed caps the top of 

the Twowells. The Mancos above the Twowells has been 

eroded from the valley floor. Five samples were collected 

from this locality.

Cubero.-^SEV&WA sec. 21, T. 10 N., R. 6 W., 

Laguna 7 1/2-minute quadrangle, Cibola County (formerly 

included in Valencia County), New Mexico, on the Laguna 

Indian Reservation. The 85-ft-thick outcrop of the Whitewater Arroyo is excellent and complete. A 20-ft-thick 

transition zone is between the medium-gray shale of the 

Whitewater Arroyo and the weathered yellowish-gray of the 

48-ft-thick cliff-forming Twowells. The transition zone is 

light to medium gray and increases upward in silt content. It 

has the same cliff-face habit as the Twowells above. 

Mancos is eroded on top of the Twowells. Seven samples

were studied from this locality, although nineteen were 

collected.

Laguna. NEViSEVi sec. 20, SWViNWVi sec. 21, T. 

10 N., R. 5 W., Laguna 7V2-minute quadrangle, Cibola 

County (formerly included in Valencia County), New 

Mexico, on the Laguna Indian Reservation. Access is time 

consuming but not difficult through section 29 to the south. 

Of the 90 ft of the Whitewater Arroyo measured by Landis 

and others (1973), 55 ft is covered by very large slope debris 

from the overlying Twowells, which is 74 ft thick. The 

Whitewater Arroyo at Laguna is represented by the lower 

31 ft of exposed shale. The remaining 4 ft of Landis and 

others' section is bentonite and siltstone. The lower 20 ft of 

the lower member of the Mancos is well exposed and was 

sampled. The top of the section is incomplete at this 

locality. The Laguna locality is the best exposure of the 

intertonguing sequence of the Dakota and Mancos. Landis 

and others (1973) stated that "the Dakota-Mancos rock 

sequence***is developed to its fullest degree and is well 

exposed in the area***of Laguna." It is the type section for 

several of the units in the Dakota and Mancos sequence 

described by Landis and others (1973).

L-Bar. SEViSWV* sec. 24, T. 11 N., R. 5 W., 

Moquino 7V2-minute quadrangle, Cibola County (formerly 

included in Valencia County), New Mexico, just north of 

the Laguna Indian Reservation. Access is excellent but only 

the upper few feet of the Whitewater Arroyo are exposed 

below a very steep cliff face of 79 ft of Twowells. Because 

L-Bar is not a complete section, it is a check point and not 

a reference section. Only the upper 15 ft of the Whitewater 

Arroyo are represented by two shale samples.

Chama Basin

Heron Reservoir. No township and range are on the 

map. Heron Reservoir 7!/2-minute quadrangle, Arriba 

County, New Mexico. Approximately lat 36°42'50"N., long 

106°38'52"W. Outcrop is at lake level on a small peninsula. 

Six shale samples were collected from about 25 ft of 

section, from the upper part (lake level) toward the base of 

the section. The upper contact is below water and the lower 

contact is covered above. Ridgley (in press) assigned this 

unit to the Hartland Member-lower member of the Mancos 

Shale. Caution must be applied to interpretations of the 

strata at the reservoir because the Hartland Member-lower 

member is part of the southeast-dipping flank of Laguna 

dome to the north.

*U.S. GOVERNMENT PRINTING OFFICE: 1992-673-049/46059 Upper Cenomanian Foraminifers, New Mexico N17

PLATES 1-3

Contact prints of the plates in this report are available, at cost,

from U.S. Geological Survey Photographic Library, Federal Center,

Denver, Colorado 80225
PLATE 1

[All figures are scanning electron microscope photomicrographs. Bar scale for each specimen is 200 Jim unless otherwise noted; 

all specimens are from the Whitewater Arroyo Tongue of the Mancos Shale unless otherwise noted. Distance above base of 

section is given)

FIGURE 1. Gaudryina sp. Cubero section, 60 ft. 

2, 3. Reophax inordinatus Young.

2. Type section, 20 ft.

3. Cubero section, 60 ft. 

4, 8, 15. Ammobaculites fragmentarius Cushman.

4. Casamero Lake section, 10 ft. a, Side view, b, Apertural view, 40 Jim. 

8. Cubero section, 75 ft. 

15. Laguna section, 20 ft.

5. Ammodiscus cretaceous (Reuss). Laguna section, 20 ft, 100 Jim.

6. Ammobaculites variablis Tappan. Cubero section, 30 ft.

7. Ammobaculites sp. Casamero Lake section, 0 ft. 

9, 10. Ammobaculites goodlandensis Cushman and Alexander.

9. Cubero section, 30 ft.

10. Cubero section, 75 ft.

11. Ammobaculoides mosbyensis Eicher. Type section, 0 ft. Damaged aperture, 100 \im. 

12, 13. Textularia spp.

12. Cubero section, 75 ft.

13. Type section, 10 ft, 100 |im. 

14, 16, 18. Haplophragmoides coronata (Brady).

14, 16. Lower member of the Mancos Shale, Laguna section, 5 ft.

18. Type section, 40 ft. a, Side view, 100 Jim. b, Reverse side view with

light-colored piece of debris, 100 Jim. 

17. Verneuilinoides perplexus (Loeblich). Laguna section, 20 ft, 100 Jim.

19. Ramulina aculeata (d'Orbigny). Lower member of the Mancos Shale, Laguna section, 

5 ft, 100 |im.

20. Citharina sp. Lower member of the Mancos Shale, Laguna section, 5 ft.

21. Psilocitharella sp. Lower member of the Mancos Shale, Laguna section, 0 ft.

22. Citharina kochii (Roemer). Type section, 30 ft, 100 Jim. 

23-25. Svenia spp.

23, 24. Hartland Member-lower member of the Mancos Shale, Heron Reservoir

section, top of section, 100 Jim. 

25. Type section, 30 ft, 100 |im.

26. Praebulimina sp. Lower member of the Mancos Shale, Laguna section, 5 ft, 40 Jim.

27. Palmula sp. Cubero section, 15 ft.

28. Neobulimina albertensis (Stelch and Wall). Lower member of the Mancos Shale, 

Laguna section, 10 ft, 100 |im.
U.S. GEOLOGICAL SURVEY BULLETIN 1808-N PLATE 1

SELECTED SPECIMENS FROM STUDY AREA
PLATE 2

[All figures are scanning electron microscope photomicrographs. Bar scale for each specimen is 100 Jim unless otherwise noted; 

all specimens are from the Whitewater Arroyo Tongue of the Mancos Shale unless otherwise noted. Distance above base of 

section is given]

FIGURE 1. Bullopordl sp. Cubero section, 0 ft.

2. Astacolus sp. Cubero section, 15 ft.

3. Unknown foraminifer. Cubero section, 45 ft.

4. Lenticulina sp. A. Type section, 30 ft. a, Side view, b, Edge view. 

5-8. Gavelinella sp.

5, 7, 8. Lower member of the Mancos Shale, Laguna section, 10 ft. 

5, 7. Umbilical view.

8. Spiral view. 

6. Lower member of the Mancos Shale, Laguna section, 5 ft. Spiral view.

9. Gavelinella modesta Eicher and Worstell. Laguna section, 20 ft.

10. Bimonilina reciprocata Loeblich and Tappan. Laguna section, 20 ft. a, Side view, 

200 Jim. b, Enlargement of aperture, 20 Jim.

11. Lingulogavelinella sp. Internal mold. Lower member of the Mancos Shale, Laguna 

section, 5 ft. a, Spiral view, b, Umbilical view.

12. Lenticulina sp. B. Type section, 40 ft. a, Side view, b, Oblique view of aperture and 

side.

13. Cythereis eaglefordensis Alexander. Ostracode. Lower member of the Mancos Shale, 

Laguna section, 0 ft. a, Exterior view, 200 Jim. b, Interior view, 200 jim.
U.S. GEOLOGICAL SURVEY BULLETIN 1808-N PLATE 2

SELECTED FORAMINIFERS FROM STUDY AREA
PLATE 3

[All figures are scanning electron microscope photomicrographs. Bar scale for each specimen is 100 urn unless otherwise noted. 

Distance above base of section given except for Heron Reservoir samples, which are described by distance below top of section]

FIGURE 1. Heterohelix moremani (Cushman). Lower member of the Mancos Shale, Laguna 

section, 5 ft.

2. Guembelitria harrisi Tappan. Lower member of the Mancos Shale, Laguna section, 

5 ft, 40 Jim.

3. Hedbergella amabilis Loeblich and Tappan. Internal mold. Whitewater Arroyo

Tongue, type section, 30 ft. 

4-6. Hedbergella portsdownensis (Williams-Mitchell).

4. Lower member of the Mancos Shale, Laguna section, 5 ft, 40 Jim. 

5, 6. Lower member of the Mancos Shale, Laguna section, 10 ft. 

7-9, 11, 12. Hedbergella delrioensis (Carsey).

7-9, 12. Lower member of the Mancos Shale, Laguna section, 10 ft.

11. Hartland Member-lower member of the Mancos Shale, Heron Reservoir

section, 0 ft (below top).

10. Hedbergella sp. Lower member of the Mancos Shale, Laguna section, 0 ft. 

13, 14. Clavihedbergella simplex (Morrow).

13. Hartland Member-lower member of the Mancos Shale, Heron Reservoir 

section, 0 ft (below top).

14. Hartland Member-lower member of the Mancos Shale, Heron Reservoir

section, 10 ft (below top).

15. Archaeoglobigerina sp. Lower member of the Mancos Shale, Laguna section, 0 ft. 

16-19. Rotalipora cushmani (Morrow).

16. Lower member of the Mancos Shale, Laguna section, 5 ft. 

17-19. Hartland Member-lower member of the Mancos Shale, Heron Reservoir

section, 0 ft (below top).

20. Small cluster of sample taxa from Hartland Member-lower member of the Mancos 

Shale, Heron Reservoir section, 10 ft (below top). Note both spiral and umbilical 

views of Rotalipora greenhornensis (Morrow) and Heterohelix moremani (Cushman), 400 [im.

21-23. Rotalipora greenhornensis (Morrow). All specimens are from Hartland Memberlower member of the Mancos Shale, Heron Reservoir section, 10 ft (below top).

21. Spiral view. 

22, 23. Umbilical view.
U.S. GEOLOGICAL SURVEY BULLETIN 1808-N PLATES

SELECTED FORAMINIFERS FROM STUDY AREA



SELECTED SERIES OF U.S. GEOLOGICAL SURVEY PUBLICATIONS

Periodicals

Earthquakes & Volcanoes (issued bimonthly). 

Preliminary Determination of Epicenters (issued monthly).

Technical Books and Reports

Professional Papers are mainly comprehensive scientific reports 

of wide and lasting interest and importance to professional scientists and 

engineers. Included are reports on the results of resource studies and of 

topographic, hydrologic, and geologic investigations. They also include 

collections of related papers addressing different aspects of a single scientific topic.

Bulletins contain significant data and interpretations that are of 

lasting scientific interest but are generally more limited in scope or geographic coverage than Professional Papers. They include the results of 

resource studies and of geologic and topographic investigations; as well 

as collections of short papers related to a specific topic.

Water-Supply Papers are comprehensive reports that present significant interpretive results of hydrologic investigations of wide interest 

to professional geologists, hydrologists, and engineers. The series covers 

investigations in all phases of hydrology, including hydrology, availability of water, quality of water, and use of water.

Circulars present administrative information or important scientific information of wide popular interest in a format designed for distribution at no cost to the public. Information is usually of short-term interest.

Water-Resources Investigations Reports are papers of an interpretive nature made available to the public outside the formal USGS publications series. Copies are reproduced on request unlike formal USGS 

publications, and they are also available for public inspection at depositories indicated in USGS catalogs.

Open-File Reports include unpublished manuscript reports, maps, 

and other material that are made available for public consultation at depositories. They are a nonpermanent form of publication that maybe cited in other publications as sources of information.

Maps

Geologic Quadrangle Maps are multicolor geologic maps on topographic bases in 7 1/2- or 15-minute quadrangle formats (scales mainly 1:24,000 or 1:62,500) showing bedrock, surficial, or engineering 

geology. Maps generally include brief texts; some maps include structure 

and columnar sections only.

Geophysical Investigations Maps are on topographic or planimetric bases at various scales, they show results of surveys using geophysical techniques, such as gravity, magnetic, seismic, or radioactivity, which 

reflect subsurface structures that are of economic or geologic significance. Many maps include correlations with the geology.

Miscellaneous Investigations Series Maps are on planimetric or 

topographic bases of regular and irregular areas at various scales; they 

present a wide variety of format and subject matter. The series also includes 7 1/2-minute quadrangle photogeologic maps on planimetric 

bases which show geology as interpreted from aerial photographs. The 

series also includes maps of Mars and the Moon.

Coal Investigations Maps are geologic maps on topographic or 

planimetric bases at various scales showing bedrock or surficial geology, 

stratigraphy, and structural relations in certain coal-resource areas.

Oil and Gas Investigations Charts show stratigraphic information for certain oil and gas fields and other areas having petroleum potential.

Miscellaneous Field Studies Maps are multicolor or black-andwhite maps on topographic or planimetric bases on quadrangle or irregular areas at various scales. Pre-1971 maps show bedrock geology in relation to specific mining or mineral-deposit problems; post-1971 maps 

are primarily black-and-white maps on various subjects such as environmental studies or wilderness mineral investigations.

Hydrologic Investigations Atlases are multicolored or black-andwhite maps on topographic or planimetric bases presenting a wide range 

of geohydrologic data of both regular and irregular areas; the principal 

scale is 1:24,000, and regional studies are at 1:250,000 scale or smaller.

Catalogs

Permanent catalogs, as well as some others, giving comprehensive 

listings of U.S. Geological Survey publications are available under the 

conditions indicated below from the U.S. Geological Survey, Books and 

Open-File Reports Sales, Box 25286, Denver, CO 80225. (See latest 

Price and Availability List.)

"Publications of the Geological Survey, 1879-1961" may be purchased by mail and over the counter in paperback book form and as a set 

microfiche.

"Publications of the Geological Survey, 1962-1970" may be purchased by mail and over the counter in paperback book form and as a set 

of microfiche.

"Publications of the U.S. Geological Survey, 1971-1981" may be 

purchased by mail and over the counter in paperback book form (two 

volumes, publications listing and index) and as a set of microfiche.

Supplements for 1982, 1983, 1984, 1985, 1986, and for subsequent years since the last permanent catalog may be purchased by mail 

and over the counter in paperback book form.

State catalogs, "List of U.S. Geological Survey Geologic and Water-Supply Reports and Maps For (State)," may be purchased by mail and 

over the counter in paperback booklet form only.

"Price and Availability List of U.S. Geological Survey Publications," issued annually, is available free of charge in paperback booklet 

form only.

Selected copies of a monthly catalog "New Publications of the 

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a free subscription to the monthly catalog "New Publications of the U.S. 

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Note.-Prices of Government publications listed in older catalogs, 

announcements, and publications may be incorrect. Therefore, the prices 

charged may differ from the prices in catalogs, announcements, and publications.