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

sula,'' I963

UNITED

STATES

of the

Arabian Penin-

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
Page

Page

Foreword-----------------------------------------Abstract------------------------------------------Introduction ______________________________________ _
Stratigraphy ______________________________________ _
General---------------------------------------Lower Eocene-Rus Formation __________________ _
Lower and middle Eocene-Dammam Formation __ _
Sharks tooth shale _________________________ _
Brown crystalline limestone _________________ _

Stratigraphy-Continued
E1
Lower and middle Eocene-Dammam Forma1
tion-Continued
1
Orange marL _____________________________ _
1
White limestone ___________________________ _
3
______________________________________ _
III

~iocene

3
3
3

E3
3
4

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

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

ILLUSTRATIO:N"
Page

FIGURE 1. Geologic map of Bahrain__________________________________________

E2
v

GEOLOGY OF THE ARABIAN PENINSULA

BAHRAIN
BY R. P.
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
5030' 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 Sittah 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.

WILLIS

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 cliffface 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
I-270 A,
in 1963,
shown in

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

by the U.S. Geological Survey nnd Arabian American 011 Co.
new fieldwork has been completed. Resulting revisions nre
figure 1.

El

E2

GEOLOGY OF THE ARABIAN PENINSULA

0

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

AI Mul}arraq

EXPLANATION

w:QsbjJ

Umm Na'san Island

Sabkha deposits

Gravel, sand, and silt

~{ Dam For~:i~~~:::~~uivalent
R

)I
>c::
<(

i=
c::
w

I-

Rus Formation

FIGURE

1.-Geologic map of Bahrain.

E3

BAHRAIN

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.

stones. 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.

LOWER EOCENE-RUS FORMATION

BROWN CRYSTALLINE LIMESTONE

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 lime-

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 sur-

ficial 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 prevailing northerly
winds, completed the geologic sculpturing of Bahrain.

U.S. GOVERNMENT PRINTING OFFICE: 1967

0-238-236