Strontium (87Sr/86Sr) and calcium isotope ratios (44Ca/40Ca

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Miocene Dam Formation, QatarGeoArabia, Vol. 12, No. 3, 2007Gulf PetroLink, Bahrain

Strontium (87Sr/86Sr) and calcium isotope ratios (44Ca/40Ca-44Ca/42Ca) of the Miocene Dam Formation in Qatar: tools for stratigraphic correlation and environment analysis

Harald G. Dill and Friedhelm Henjes-Kunst

ABSTRACT

The Dam Formation in Qatar is a series consisting of calcareous (calcite, dolomite) and evaporitic sediments (gypsum, celestite) that developed under subtidal through supratidal conditions passing towards younger and older series in an environment of deposition more akin to modern beach deposits. In the present study 87Sr/86Sr ratios, δ44/40Ca and δ44/42Ca data are discussed together with δ13C and δ18O values obtained during an environmental analysis carried out previously. Rather uniform isotope curves of the Sr, Ca and O isotopes for tidal deposits are replaced by more oscillating ones when these tidal-influenced regimes became substituted for by a more wave-dominated regime. Calcium isotope ratios still at its infancy and not fully understood seem to provide a new tool in carbonate petrography when it comes to an interpretation of the environment of deposition and calcification of dolomitic series. The Sr isotopes not only indicate an influx of more primitive Sr from the hinterland but also allow for a refinement of the stratigraphy, which yields a late Aquitanian to early Burdigalian age of sedimentation for the Dam Formation in Qatar.

INTRODUCTION

Holocene carbonate and evaporite sequences in the Arabian Gulf, located mainly along the coasts of the United Arab Emirates (UAE) and Kuwait, have been studied by sedimentologists and ecologists alike (e.g. Shinn, 1983; Sheppard et al., 1992; Saleh et al., 1999; Alsharhan and Kendall, 2002, and references cited therein). In contrast, the sabkhas of the Qatar Peninsula have not been as extensively investigated (Figure 1a). The Qatar Peninsula is the surface expression of the Qatar Arch, a deep structural trend that projects northwards from the Arabian Peninsula into the Arabian Gulf (Cavelier, 1970; Figure 1a). It is covered mainly by Quaternary sandy dunes, aeolianites and calcareous coastal sediments that rest upon Miocene and Eocene calcareous and evaporitic rocks. Despite the great number of outcrops, the investigation of Qatar’s geology is primarily limited to biostratigraphic studies of the calcareous Cenozoic sediments (El Beialy and Al-Hitmi, 1994; Al-Hinai et al., 1997; Al-Saad and Ibrahim, 2002).

In southwest Qatar, the prominent Dukhan Anticline hosts the NNW-trending onshore Dukhan giant oilfield (Sugden, 1962; Foster and Beaumont, 1991; Dill et al., 2003, 2005) (Figure 1b). It presents an excellent locality to study, not only Holocene sabkha sequences, but also the Neogene offshore and continental sediments. Accordingly a research project was conducted to study the sedimentary petrography, mineralogy and chemistry of these sediments. Supplementary palaeontological data were obtained by the study of body and ichnofossils (Dill et al., 2005).

Based upon the palaeontological and sedimentological data, a palaeoecological-palaeoenvironmental analysis of the evaporite-bearing series was successfully concluded (Dill et al., 2005); however the biostratigraphic age of the sediments was not possible. In places, the diversity of species of the macrofossil assemblages is low while the number of individuals is considerably high. This pattern implies a strong environmental stress, and in many beds fossils are absent due to the inhospitable conditions. To circumvent these palaeontological limitations, 87Sr/86Sr ratios of the marine sedimentary rocks were determined and compared with the sea-water ratios (De La Rocha and DePaolo, 2000). Trends of Sr-isotope ratios measured in marine series may be used to both constrain geochronological estimates and to refine the interpretation of palaeohydrological conditions in nearshore environments.

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Dill and Henjes-Kunst

Fahud Salt Basin

Ghaba Salt Basin

South Oman Salt Basin

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THE STUDY AREA AND ITS GEOLOGICAL SETTING

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QATAR

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Figure 1: Overview of the geological setting and the position of the study area on the Qatar Peninsula. (a) The regional setting of the Arabian

Gulf region with the major salt basins and main structural elements. The green arrow points to the position of the working area in Qatar.

(b) Satellite image showing the study area at Al Nakhsh (red framed area) on the Qatar Peninsula. The Dukhan Anticline, which hosts the most prominent onshore oil field in Qatar extends in a NNW-SSE direction along the western coast of Qatar.

In this paper we present plots of the Sr isotopes together with oxygen, carbon and sulphur isotopes as a function of depth and environment. Also for the first time for these Arabian Gulf Cenozoic rocks, 44Ca/40Ca and 44Ca/42Ca isotope ratios have been determined (see DePaolo, 2004; and Fantle and DePaolo, 2005, for overview of the geological application of Ca-isotope methods). These ratios may assist sedimentologists during environmental analysis. To evaluate the strengths and weaknesses of these methods, the Ca-isotope ratios are discussed in relation to the classical methods of environmental analysis.


63

Miocene Dam Formation, Qatar

ANALYTICAL METHODS

Twenty samples were investigated for their Sr and Ca isotope compositions and their Sr concentrations. Sr and Ca isotope analyses were performed at the Federal Institute for Geosciences and Natural Resources, Hannover (BGR). Approximately 50 mg of the sample powders were weighed into Teflon beakers and dissolved in 5 ml ~1.6 M distilled acetic acid at temperatures of 100–140°C on a hot plate for about 32 hours. This procedure was applied in order to dissolve the carbonates but avoid leaching of silicates in the residue. The leachate was separated and recovered from the sample solutions during several steps of centrifuging and washing with ultra-pure water. Different aliquots of the leachate were spiked for determination of Sr concentration using a single Sr spike on the one hand, and for Ca-isotope determination using a double spike enriched in 43Ca and 48Ca, on the other. The leachates were dried and then converted to chlorides. Sr and Ca fractions for mass spectrometrical isotope determination were obtained by standard cation-exchange techniques. An international seawater salinity standard (IAPSO) used as a Sr- and Ca-isotope reference material was treated in a similar manner.

Sr (approximately 200 ng) and Ca (approximately 4 µg) were loaded on Re filaments and run on a double-filament assembly using a Thermo Triton multicollector mass spectrometre in static (Sr) and dynamic (Ca) modes. During Ca-isotope measurement, 40K interference at 40Ca were monitored via 41K but was generally found to be negligible. All Ca samples were measured in replicate (n = 2, 3), the mean of which are reported here. Sr-isotopic ratios were normalised to 86Sr/88Sr = 0.1194. In the course of this study, repeated measurements of the NIST 987 Sr-isotope standard yielded a mean value for 87Sr/86Sr of 0.710248 ± 16 (2 SD). For IAPSO we obtained a Sr concentration of 7.66 ± 0.02 (2 SD) ppm and a 87Sr/86Sr = 0.709181 ± 8 (2 SD; n = 3). The latter value corresponds within error limit to a 87Sr/86Sr ratio of 0.709175 for modern sea water (e.g. Howarth and McArthur, 1997). Because of the young age of the investigated sediments and their very low Rb/Sr ratio as indicated by XRF analysis (Dill et al., 2005), no age correction of the 87Sr/86Sr ratio for decay of 87Rb was applied. Ca-isotope ratios were calculated from mass spectrometric raw data according to the procedure described by Heuser et al. (2002) and are reported in the common delta notation:

δ 44/40Ca = [(44Ca/40Ca)sample/(44Ca/40Ca)standard-1]*1000 and

δ 44/42Ca = [(44Ca/42Ca)sample/(44Ca/42Ca)standard-1]*1000 in per mil and as the difference to the respective values determined for the NIST SRM915a clinical carbonate standard at the BGR (δ44/40CaNIST 915a and δ44/42CaNIST 915a) (Coplen et al., 2002; Hippler et al., 2003). In the course of this study, we obtained δ44/40CaNIST 915a and δ44/42CaNIST 915a values of 1.88 ± 0.20‰ and 0.89 ± 0.09‰ (2 SD; n = 6), respectively for IAPSO, which agree within error to values determined for this material at other laboratories (e.g. Hippler et al., 2003; Schmitt et al., 2003a). Procedural blanks for Sr and Ca are less than 0.1% of the relevant sample concentration and are therefore negligible. Uncertainties are reported as 2 sigma standard deviation (2 SD) and are 25 ppm for 87Sr/86Sr. Although replicate Ca measurements of samples yielded in part uncertainties that are < 0.10‰ and < 0.05‰ for δ44/40Ca and δ44/42Ca, respectively, we assume that the uncertainties quoted above for repeated determinations of homogenous reference materials are more representative of the overall errors in Ca isotope determination. In all calculations, the IUGS-recommended constants (Steiger and Jäger, 1977) were used. The analytical results are presented in Table 1.

LITHOFACIES AND DEPOSITIONAL ENVIRONMENT OF THE MIOCENE DAM FORMATION IN QATAR

The Neogene Dam Formation was subdivided by Dill et al. (2005) into seven members named after type localities on the Qatar Peninsula (Figure 2). In the following paragraphs an overview of the environment of deposition is given based on Dill et al. (2005).

The Lower Salwa Member is a silicate-dolomite-calcite sequence. Fine-grained siliciclastics at the base indicate a deeper marine environment. Calcitic clay-rich marlstone, forming the top stratum indicate an intertidal to beach environment. Bright grey tints among the rock colours are unambiguous redox indicators for well-oxygenated conditions. Trace fossils are ubiquitous in the Lower Salwa Member;


64


their very complex surface tracks and trails belong to the Cruziana Facies of Seilacher (1967). The ichnofossils are Planolites sp. and Thalassinoides sp., both of which are observed on bedding planes of sedimentary rocks that formed in a subtidal environment between 10–100 m water depths.

Part of the Middle Salwa Member has also been interpreted as a restricted platform sedimentary unit. The top strata, however, are interpreted as a beachrock (intertidal environment) very much like the lithologies in the Lower Salwa Member. Red and green rock colours observed in this member indicate varying oxidising and reducing conditions. The basin began deepening during the passage into the Middle Salwa Member. The state of oxygenation deteriorated (dysaerobic reducing conditions), so that part of the environment is described as lagoonal. The water depth in the basin reached a maximum at c. 20 m. In the shallow-marine basin-and-swell topography of the Middle Salwa Member a shift from a microtidal to a mesotidal regime occurred. The basin received a strong terrigenous input from the northwest to north during the deposition of the Middle Salwa Member.

The Upper Salwa Member consists of two coarsening- or shallowing-upward sequences. Trace fossils reappear in the Upper Salwa Member with a burrow morphology most likely attributed to the ichnofossil assemblages of the Callianassa Facies sensu Miller and Curran (2001). The fauna had their habitat in the subtidal to lower intertidal or shoreface environments.

The Lower Al Nakhsh Member encompasses three fully-developed coarsening- or shallowing-upward sequences, each starting with bioclastic calcareous rocks and ending up with stromatolites. Locally, the calcareous sediments are intercalated with some gypsum lenses, or peppered with gypsum concretions. Similar cyclic entities were denominated by Pratt (2002) as peritidal cycles. Tidal channels are indicated in the sedimentary record by the bioclastic pure limestones in the lower section of each cycle (subtidal).

Most cycles encountered in the evaporite-bearing facies of the Middle Al Nakhsh Member are topped by a seam of gypsum. Fully developed cycles may be denominated as brining-upward cycles reflecting a shallowing-upward trend in a supratidal-dominated regime sensu Warren (1999).

The red bed facies in the Upper Al Nakhsh Members with gypsum-bearing coarsening-upward cycle represents the maximum regression following the supratidal regime of the Middle Al Nakhsh Member. It is the most landward (inland sabkha) equivalent of the Middle Al Nakhsh Member. It passes into mottled argillaceous calcrete, which evolved on top of shoals in the sabkha or may grade into arenaceous aeolian deposits.

Table 1 Sr and Ca isotope data of calcareous sediments from the Miocene Dam Formation in Qatar (Arabian Peninsula). In addition, the analytical results for the IAPSO salinity standard used as a Sr and Ca isotope standard in the course of the study are reported. For analytical details see text. asl = above sea level.

Position in sectionSample

Sr 87Sr/86Sr δ44/40 CaNIST 915a δ

44/42 CaNIST 915a

2468

21242836404246485663738084858689

84.8 434.1 0.708352 1.08 0.5184 295.4 0.708498 0.91 0.4482 209.7 0.708447 0.69 0.3080 732.4 0.708471 0.71 0.3773 235.2 0.708536 0.78 0.3972 376.7 0.708537 0.89 0.4470 673.4 0.708499 0.85 0.4361 305.8 0.708480 1.01 0.4659 184.0 0.708497 0.86 0.4257 126.4 0.708394 0.80 0.3753 299.2 0.708472 0.97 0.4651 1035.5 0.708425 0.91 0.4144 646.7 0.708382 0.76 0.3840.2 654.7 0.708416 0.78 0.4035.4 202.8 0.708386 0.77 0.3730 452.2 0.708386 0.75 0.3526.6 1038.7 0.708359 0.93 0.4924.4 1766.7 0.708364 0.86 0.3824 158.6 0.708396 0.85 0.4021 175.7 0.708217 0.81 0.46

7.653 0.709176 1.88 0.92 7.654 0.709184 1.85 0.89 7.673 0.709183 1.89 0.91 1.89 0.89 2.02 0.96 1.74 0.83

(asl) (ppm) (‰) (‰)

IAPSO Salinity standardaliquot 1aliquot 2aliquot 3aliquot 4aliquot 5aliquot 6


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The calcarenites of the Abu Samrah Member were deposited in a high-energy near-shore marine environment with its flow strength increasing towards younger series as shown, for example, from the eastern coastal plains of the USA (Katuna et al., 1997). The onset of the Abu Samrah Member, marked by a hardground, is equivalent to a transgressive plane. Tidal flats or mudflats evolved in a microtidal regime. In the Abu Samrah Member the marine setting eventually turned from a tide-dominated into a wave-dominated beach environment. A change from wave-dominated to tide-dominated coastal sediments has been reported from environments in the Arabian Gulf in Abu Dhabi (Kirkham, 1998) and the Kuwait-Saudi Arabian Coast (Lomando, 1999). All carbonate and siliciclastic sediments younger than the Middle Salwa were subjected to strong dolomitisation, excluding the uppermost part of the Abu Samrah Member. The calcareous beds immediately beneath the unconformity, which is overlain by fluvial gravely sediments of the Pliocene Hofuf Formation, were named beach rocks.

STRONTIUM ISOTOPES AND THE AGE OF THE FORMATION

The 87Sr/86Sr isotope ratio of the samples varies only within a narrow range from approximately 0.70822 for sample 89 from the base of the section, to maximum values of approximately 0.70850–0.70854 for samples in the upper part of the section between about 60–70 m above sea level (Figures 3 and 4, Table 1). There is an almost steady increase in the Sr-isotope ratio from the base to the top of the section with only a few outliers at approximately 20 m, 57 m above sea level and within 80–85 m asl (see below) (Figure 4). With respect to the assumed Miocene stratigraphic age of the sediments the Sr-isotopic data fit to the marine Sr-isotope curve for the time interval of approximately 22–18 Ma (Howarth and McArthur, 1997). In Figure 3 the marine Sr-isotope curve for that time interval is eye-fitted (dashed line) to the Sr-isotope data of the section using samples 84.6, 84, 80, 73 and 63 from the lower part, and samples 40, 36, 28, 24 and 21 from the upper part as reference points.

It is evident that most samples fit the marine Sr isotopes in that time interval and thus suggest a late Aquitanian to early Burdigalian stratigraphic age for the section. It is also clear that there are some outliers along the curve: samples from the lowest part (< 25 m above sea level; Lower Salwa), from approximately 54 m asl (Lower Al Nakhsh) and from the uppermost part (80–85 m asl; Abu Samrah) have significantly lower Sr-isotopic ratios compared to the respective parts of the marine Sr-isotope curve (Figure 3). We interpret these outliers to be due to a significant amount of strontium of non-marine origin in the sample. The lower Sr-isotope ratios may indicate an influx of more primitive Sr from the hinterland. Interestingly, the outliers in the Sr-isotope pattern apparently match outliers to more negative values in the δ13C and δ18O curves along the section signalling an impact of meteoric fluids on the calcareous rocks (Dill et al., 2005) (Figure 4).

CALCIUM ISOTOPES AND THE ENVIRONMENT OF DEPOSITION

The Ca-isotope composition of the samples is relatively uniform with mean δ44/40CaNIST 915a and δ44/

42CaNIST915a values of 0.85 ± 0.20‰ and 0.41 ± 0.10‰, respectively (Figure 5). Compared to the statistical uncertainties obtained for the homogeneous NIST915a and IAPSO reference materials (0.20‰ and 0.09‰, respectively) the Ca-isotope data of the samples indicate that there are no significant Ca-isotope variations throughout the sampled section. Furthermore, the Ca-isotope ratios do not show an overall increase or decrease from the base to the top of the section. Nevertheless, there are some obvious disturbances in both Ca-isotope curves, which in part match those of the other isotope curves (87Sr/86Sr, δ13C and δ18O): at the base of the section, between 50 m and 55 m and in the uppermost part of the section. In these sections, the Ca-isotope budget may be influenced by a contribution from other non-marine sources. Judging from the similarity of the δ44/40CaNIST 915a and δ44/42CaNIST 915a curves of the samples, no significant contribution of more radiogenic 40Ca coming from old continental sources is evident for those parts of the section for which a supratidal or freshwater environment may be invoked (see supratidal subenvironments) (Figure 4).

Calcareous sediments of marine origin generally show negative δ44/40Ca and δ44/42Ca values relative to seawater of appropriate age (see Fantle and DePaolo, 2005; Heuser et al., 2005; DePaolo, 2004 for compilation of earlier studies). Thus, they are enriched in the lighter (40Ca, 42Ca) over the heavier Ca isotopes (for instance 44Ca) relative to seawater due to mass-fractionation processes during their formation. The exact nature, extent and controls of these fractionation processes are still not well


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b 0.

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48 47 46 45

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ch

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ls -

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l fla

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86 85 84

83 82 81 80 79 78 77 76 75 74 73 72 71 70 6929 30

70 69 68 67

66

65 64

63

62 61 60

HofufFm Abu Samrah Member Middle Al Nakhsh Member Middle Al Nakhsh MemberUpper ANM

Tida

l fla

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ls

Tida

l fla

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Tida

l fla

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alA

l Nak

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Al N

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h 10

Al N

akhs

h 11

Al N

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h 12

Al N

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h 13

Sup

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tratid

alch

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Al N

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h 14

Al N

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h 15

Al N

akhs

h 16

Pal

aeos

ol

Sup

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al

mar

shAl

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17

Al N

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h 18

Bea

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(pro

xim

al)

Dur

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Har

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Aeo

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depo

sits

-pal

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ol

Abu

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1

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rah

2B

each

ridg

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ista

l)

Abu

Sam

rah

3Ti

dal c

hann

el/

delta

Abu

Sam

rah

4Ti

dal f

lat

Abu

Sam

rah

5Ti

dal f

lat/

chan

nel

Bea

chro

ckA

bu S

amra

h 6

Fluv

ial

Fe FeSr Sr Sr Sr

Sr

SrSr Fe

b 0.

5

LLH

b 0.

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5

LLH

H 1

.5

root

s

g-b

0.5LL

H-S

HLL

H

b-H

<0.5

g-b

0.5

H 1

H 0

.1 -1

LLH

>S

Hb

1

LLH

g-b

1

LLH

B 0

.5 L

LH

b 1

SH

SH

>LLH

b 0.

5

g-b

0.5

g-b

0.5

g 0.

5S

hark

teet

h


Ooi

d-pe

lloid

Ooi

d-pe

lloid

Stru

ctur

es/

Text

ures

Foss

ils/

Bio

geni

cSt

ruct

ures

Cyc

loth

em/

Uni

tEn

viro

nmen

t

Min

Max

33 32 31 30 29 28 27 26 25 24 23 22 21 20 19

Middle Salwa Member

18 17

Fe

Sal

wa

1a

Sal

wa

2

Sal

wa

3

Sal

wa

4

Sal

wa

5

b 1

b 1

b 3 H

Fe

Fe

Sal

wa

1b

Lower Salwa Member

Res

trict

ed

plat

form

ope

nm

arin

e w

ell

oxyg

enat

ed

Tida

l del

ta(d

ista

l)

Sur

f zon

e/B

rack

ish

Res

trict

edpl

atfo

rm/

Lago

onal

-dy

saer

obic

Res

trict

edpl

atfo

rm/

Lago

onal

-po

or

oxyg

enat

ion

Lago

onal

-dy

saer

obic

Inte

rtida

l-be

ach

Inte

rtida

l-be

ach

Inte

rtida

l-be

ach

Sur

f zon

e

Sha

rk te

eth

Sha

rk te

eth

Sha

rk te

eth

Verti

cal b

urro

ws

Sha

rk te

eth

Sha

rk te

eth

44 43 42 41 40 39 38 37 36 35 34

Upper Salwa Member Lower Al Nakhsh Member

Sub

tidal

to

low

er

inte

rtida

l S

alw

a 6

Sal

wa

7

G-B

0.1

-0.5B1

H b

0.5

Envi

ronm

ent

Thin

ly b

edde

d, m

arly

inte

rbed

s

Wav

e rip

ples

Goe

thite

Foss

ils

Trou

gh c

ross

bed

ding

Cel

estit

e

gG

astro

pod

bBi

valv

e (p

elec

ypod

)

Her

ringb

one

cros

s be

ddin

g

Hal

ite (s

olut

ion

cast

s)

GH

igh

dive

rsity

Plan

ar c

ross

bed

ding

Dol

omite

gLo

w d

iver

sity

1cm

Aver

age

diam

eter

Pelle

t (al

gal+

feca

l)

Cal

cite

Ooi

d

Sulp

hate

Silic

icla

stic

s

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sum

cry

stal

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g-up

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d cy

cle

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e st

ruct

ures

Fini

ng-u

pwar

d cy

cle

Intra

clas

ts/ R

ip-u

p cl

asts

Mud

cra

cks

Seab

ed li

thifi

catio

n

Ichn

o fo

ssils

(bur

row

s)

Fe Sr

8 7 6

5b

4

4a

3b

3a 2b5a

2c

2a

1b 1a1c

STR

ATIG

RA

PHY,

LIT

HO

LOG

Y A

ND

PA

LAEO

GEO

GR

APH

Y O

F TH

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IOC

ENE

DA

M F

OR

MAT

ION

IN Q

ATA

R

Figu

re 2

: Lit

hol

ogs

of th

e m

emb

ers

of th

e M

ioce

ne

Dam

For

mat

ion

in s

outh

wes

t Q

atar

an

d t

hei

r d

epos

itio

nal

en

viro

nm

ents

. All

dep

th-r

elat

ed d

ata

are

give

n i

n

met

res,

an

d a

ll d

imen

sion

s in

th

e li

thol

og a

re g

iven

in

cen

tim

etre

s. T

he

wed

ge

den

otes

th

e d

irec

tion

in

wh

ich

a t

extu

re o

r st

ruct

ure

fad

es o

ut.

LL

H =

lat

eral

ly

lin

ked

hem

isp

her

oid

s, S

H =

ver

tica

lly-

stac

ked

hem

isp

her

oid

s (m

odifi

ed f

rom

D

ill e

t al.,

200

5).


68


c

a

0.7078

0.7080

0.7082

0.7084

0.7086

0.7088

0.7090

0.7092

0 5 10 15 20 25 30 35Numerical Age (Ma)

Sr (

ppm

) 87

Sr/8

6 Sr R

atio

87

Sr/8

6 Sr R

atio

Miocene 5.32 to 23.8 Ma

Studied Section

0

200

400

600

800

1,000

1,200

1,400

1,600

1,800

2,000

0.70820

0.70830

0.70840

0.70850

0.70860

2 sigma error (± 20 ppm)

10 20 30 40 50 60 70 80 90

10 20 30 40 50 60 70 80 90

b

STRONTIUM GEOCHEMISTRY AND THE AGE OF FORMATION OF THE MIOCENE DAM FORMATION

Figure 3: Strontium geochemistry and the age of formation.(a) Whole-rock-strontium

concentration given in ppm of samples taken along a reference section of the Dukhan Anticline (for location see Figure 1) plotted as a function of altitude above sea level (for reference see Figure 2).

(b) Strontium isotope ratios (87Sr/86Sr) of samples plotted in the same way as Figure 4d.

(c) The evolution of 87Sr/86Sr ratios for the seawater from 35 Ma to the Present (Howarth and McArthur, 1997). The shaded area along the time scale gives stratigraphic age of the Miocene. The thin horizontal lines bracket the age of deposition of the section series under study based on the 87Sr/86Sr ratios.

understood. It is assumed that calcareous sediments of chemical origin show slightly different fractionation behaviour as compared to carbonates formed by biomineralisation (Schmitt et al., 2003a). In addition, δ44/40Ca and δ44/42Ca values of chemically formed carbonates and phosphorites of identical stratigraphic age may vary due to mineral-dependant kinetic mass-fractionation (Gussone et al., 2003; Schmitt et al., 2003b). The Ca-isotope composition of palaeo-seawater itself is a function of the complex evolution of its Ca-elemental budget in the past (Fantle and DePaolo, 2005; Heuser et al., 2005; DePaolo, 2004 for overview of earlier studies).

For the early Miocene, models suggest a significant decrease in the δ44/40CaNIST 915a value of seawater from approximately +2.0‰ (22 Ma) to +1.4‰ (18 Ma) (De La Rocha and DePaolo, 2000; Schmitt et al., 2003a; Schmitt et al., 2003b; DePaolo, 2004; Heuser et al., 2005). Ca dissolved in modern river waters is isotopically lighter in δ44/40Ca by about 0.7–1.7‰ compared to modern seawater (Zhu and MacDougall, 1998; Schmitt et al., 2003a; DePaolo, 2004). The lowest δ44/40Ca values of -1.4 to -1.7‰ (relative to seawater) are reported for Ganges tributaries, which also show high 87Sr/86Sr ratios. This suggests that their low δ44/40Ca values may in part be due to excess 40Ca accumulated from radioactive decay of 40K in chemically strongly fractionated igneous rocks in the hinterland. Analyses of rainwater and groundwater also reveal δ44/40Ca values that are by 0.9–1.5‰ lower compared to modern seawater (Schmitt et al., 2003a).


69


17

20

25

30

35

40

45

50

55

60

65

70

75

80

84

Low

er S

alw

aM

iddl

e Sa

lwa

Upp

er S

alw

a

Lo

wer

Al N

akhs

h

Mid

dle

Al N

akhs

h

Abu

Sam

rah

UAN

Supratidal intertidal

Supratidal-aeolian

duricrustsPu

re li

mes

tone

Mar

ly li

mes

tone

Cla

yey

mar

lsto

neM

arls

tone

Lim

y m

arls

tone

Mar

ly c

lays

tone

Cla

ysto

ne

Dep

th (m

)

Beachtidal flats/ Channelsduricrusts

Supratidalintertidal subtidal

(channels)

Subtidal to lower intertidal

Restricted platform,intertidal

beachtidal delta lagoonal

Restricted platform,

intertidal beach

c. 19

c. 21

MIDDLE MIOCENE - PLIOCENE

HOFUF FORMATION

Aqu

itani

an-B

urdi

galia

n

c. 21.6

c. 20

c. 18.9

SB-in

tra-

Dam

eros

iona

l sur

face

MFSNg 10

MFS Ng 20 ?

SB

Sulphate

Calcite Siliciclastics

Dolomite

Dill et al. (2005)

Dill et al. (2005)

Dill et al. (2005)

10 15 20 25 0 2.5-2.5 5 -10 -5 0 5 0.7083 0.7085 0.50 1.00 0.00 0.50 1.00

δ34S(‰)

a b c d e f

δ13C(‰) δ44/40CaNIST 915A

δ44/42CaNIST 915A

δ18O (‰) 87Sr/86Sr

Represent the marine Sr-isotope curve for the time interval of approximately 22 to 18.9 Ma (eye-fitted to the sample data. Compare text for details).

STRATIGRAPHY, LITHOLOGY, ISOTOPE VARIATION AND PALAEOGEOGRAPHY OF THE MIOCENE DAM FORMATION IN QATAR

Figure 4: Stratigraphy (UAN: Upper Al Nakhsh), lithology, isotope variation, the environment of deposition and age of Dam Formation. Outline of the environment of deposition (simplified, for more details see Figure 2).


70


a

b

0.2

0.5

1.0

0.5

1.0

1.5

10 20 30 40 50Altitude Above Sea level (metre)

60 70 80 90

2 sigma error (± 0.18 ‰)

2 sigma error (± 0.18 ‰)

δ44/40

Ca N

IST

915A

(‰)

δ44/42

Ca N

IST

915A

(‰)

CALCIUM ISOTOPE DATA

Figure 5: Calcium isotope data. (a) δ44/40Ca NIST 915a values of samples plotted as a function of altitude asl (for reference see Figure 2); and (b) δ44/42Ca NIST 915a values of samples plotted as a function of altitude asl (for reference see Figure 2).

Marine carbonates are isotopically lighter than seawater by about 1.3‰ (biogenic carbonates) to 1‰ (phosphorites) (De La Rocha and DePaolo, 2000; Schmitt et al., 2003b; DePaolo, 2004). Therefore, for early Miocene carbonates formed in a marine environment δ44/40CaNIST 915a values of approximately 1.0–0.1‰ are expected. The investigated samples for which Sr isotope data give evidence of a formation age of 22–18 Ma before present show δ44/40CaNIST 915a values of on average 0.85 ± 0.20‰. The Ca-isotope data are therefore in line with a formation of the sediments in a marine environment without significant contributions from other sources to their calcium budget.

EARLY MIOCENE SERIES ALONG THE NORTHEASTERN MARGIN OF THE ARABIAN PLATFORM

The section through the southern part of the Dukhan Anticline (Dill et al., 2005) takes the centre-stage for the correlation of early Miocene sedimentary sequences along the north-eastern margin of the Arabian Platform. It is correlated to reference sections in Dhofar, Oman (Roger et al., 1987); the western regions of the United Arab Emirates (Ditchfield et al., 1999; Whybrow et al., 1999); the Dammam region, Saudi-Arabia (Weijermars, 1999); and south-western Iran (Motiei, 1993) (Figure 6). Cavalier (1970) suspected a middle to late Miocene age for the Dam Formation in Qatar. A late Aquitanian to early Burdigalian age of sedimentation may be concluded from the Sr isotopes presented in this paper. Hence, early Miocene sedimentary sequences along a SE-NW section can be correlated and treated in more detail as to their lithology, environment of deposition and their sequence stratigraphic key elements (Figure 6).

In Dhofar, Oman, the Mughsayl Formation represents the early Miocene (Roger et al., 1987). The onset of turbiditic calcareous rocks was dated as early as late Stampian (early Oligocene) and lasted until the middle Burdigalian when conglomeratic limestones of the Adawnib Formation came to rest unconformably on top of the Mughsayl Formation. The slumped calcareous sediments of the Mughsayl Formation were laid down in a pelagic environment of deposition, whereas the hanging- wall rocks of the Adawnib Formation were apparently deposited in a marginal marine environment.

In the UAE, only the upper part of the Dam Formation is exposed and it consists of dolomitic claystones and hardgrounds. The strontium-isotope record published by Peebles (1999) for Abu Dhabi suggests a Burdigalian age. During the Neogene mainly clastic series formed. They were defined as the Shuwaihat Formation (lower part) and Baynunah Formation (upper part). Whybrow


71


Stratigraphic section not considered for correlation

30° 50°40°

30°E 50° 60°40°

30°

20°

40°N

30°

20°

40°

RED

SEA

CaspianSea

MediterraneanSea

ARABIANPLATE

LUTBLOCK

AFRICANPLATE

ALBORZ

Anatolia

TURKISHPLATE

LEVANTPLATE

Ow

enB

asin

NW Iran

Partides

SANANDAJ-SIRJAN ZONE

CENTRALIRAN BLOCKS

Makran

Dry landShallow marine(epicontinenatl)Deep marine (oceanic)Fault zone

Reference section

Northwest Southeast

Dolomite

Calcite

Sulphate

Siliciclastics

UAEQatar

Age

(Ma)

Age

(Ma)Saudi

ArabiaIran

50 m

19

20

21

1520

Ng 10

Ng 10

?

Motiei (1993)in Sharland et al. (2001)

Weijermars(1999)

Dill et al. (2005)

Whybrow et al. (1999);

Ditchfield (1999)

?

AQ

UIT

AN

IAN

-BU

RD

IGA

LIA

NSt

age

Asm

ari

Had

rouk

hD

am

Dam D

am

Mug

hsay

l

?

22 Ma

OmanRoger

et al. (1987)

1

1

1

23

4

5

23

4

5

4

2 3 4 5

ArabianPlate

Form

atio

n

Form

atio

n

Form

atio

n

Form

atio

n

Form

atio

n

Shu

wai

hat -

ba

ynna

h

Unconformity (SB) A

daw

nib

CORRELATION OF LOWER MIOCENE SEDIMENTARY SEQUENCES ALONG THE NORTHEAST BOUNDARY OF THE ARABIAN PLATFORM

N0 500

km

N0 500

km

Figure 6: Correlation of Aquitanian to Burdigalian sedimentary sequences along the northeastern boundary of the Arabian Platform. The sequence stratigraphic element of the maximum flooding zone (MFS Ng 10) was positioned according to Sharland et al. (2001). Its position in the NW-SE cross-section is based on data obtained during this study and a re-interpretation of data from the literature (the vertical lining denotes stratigraphic section not considered for correlation). The palaeogeographic map is based on data from Dercourt et al. (2000) supplemented with data from literature as far as the northeastern boundary of the Arabian Platform.


72


a

b

AEOLIAN SANDSTONE OF THE UPPER AL NAKHSH MEMBERFigure 7: Red, fine- to medium-grained sandstones (a) with planar cross bedding are separated from subjacent sandstones, and (b) of the same lithology by an uneven reaction surface. The red bedsets on top of the reaction surface display large-scale trough cross-stratification with a tangential basal contact. Sand ripples are common. They occur only near the tangential basal contacts of foresets, immediately above the first-order bounding surfaces. The sandstone of the Upper Al Nakhsh Member is aeolian by origin. The locality is on the southern Dukhan Anticline.

et al. (1999) found an erosional surface in the Shuwaihat Formation with a relief of at least 6 m that was subsequently covered by the overlying sediments. Aeolian cross-stratification are distinctive features of the Shuwaihat Formation, which is apparently coeval with the Hofuf Formation that unconformably overlies the Dam Formation in Qatar and Saudi Arabia (Figures 4 and 6). Due to the lack of faunal or floral remains this siliciclastic sequence can only be assigned a middle Miocene to Pliocene age. Aeolian sediments recorded from Abu Dhabi have also been encountered in the section of the southern Dukhan Anticline (Figures 4 and 7). In addition to these continental sediments, pedological and hydrological processes in the reaches of a fluctuating ground-water level gave rise to argillaceous dolomitic calcretes (“dolcretes”) (Figure 8). Dissolution of highly soluble compounds (halite?) in the subsurface has given rise to dolines and caused a pervasive karstification at this site (Figure 9). A relative increase in relief on a rather small scale resulted from differential salt dissolution at depth and halokinetic processes along the northeast limb of the Dukhan Anticline. The uppermost part of the Dam Formation in Qatar (Upper Al Nakhsh and Abu Samrah Members) could not be chronologically constrained by means of Sr isotopes and, hence, its age remains conjectural. Aeolian sediments, duricrust and prominent karst relief in the Qatari reference section suggest that the uppermost Dam Formation correlates to the lowermost Shuwaihat Formation in the UAE, where Whybrow et al. (1999) described significant erosional features.

In Saudi Arabia, the Dam Formation is said to be middle Miocene in age (Powers, 1968; Weijermars, 1999). In the type section it overlies the Hadroukh Formation while in the Dammam region, it unconformably overlies the Ypresian-Lutetian Dammam or Ypresian Rus formations. The Hadroukh Formation is considered by Weijermars (1999) as Aquitanian to Burdigalian (23.7–20 Ma) in age. This means that the lower Dam Formation in Qatar, which is rife with fine-grained siliciclastic rocks, is coeval with the uppermost Hadroukh Formation on the Dammam Peninsula in Saudi Arabia.

The Dam Formation is stratigraphically equivalent to the Middle Asmari Formation in Iran (Motiei, 1993, in Sharland et al., 2001). According to these authors, the Middle Asmari Formation consists in the lower half of siliciclastics and carbonates and in the upper half of dolomites and carbonates. The dolomitic section is correlative with evaporitic series in the Lower and Middle Al Nakhsh members in Qatar.

The sedimentary sequence taken for reference in Oman marks the transition from deep towards shallow marine. Stratigraphically equivalent series from Saudi Arabia, Qatar and the western UAE are typical of shallow marine and strongly influenced by the uplift of the Qatar Arch (Figures 1, 4 and 6). Erosional surfaces or discontinuities of local scale may be due to differential uplifts over the various salt-cored anticlines (e.g. Dukhan Anticline, Dammam Dome) in this part of the Arabian Platform. The drill section in Ab-Teymur-1 in Iran (Motiei, 1993) represents a deeper part on the northern shelf of the Arabian Platform.


73


Figure 8: A red bed facies varying in lithology from marly claystones through clayey marlstones developed near the upper boundary of the Upper Al Nakhsh Member (Al Nakhsh 16-see Figure 2) on the southern Dukhan Anticline. This lithofacies does not contain any evaporite seams but red and brown argillaceous matter instead. Veinlets, nodular structures and brecciation in the lower part of the section suggest that these sediments were laid down in a supratidal environment. Pedological and hydrological processes in the reaches of a fluctuating ground-water level gave rise to argillaceous dolomitic calcretes (“dolcretes”). Varying shades of purple, red and brown at the base are indicating changing redox conditions. See 15-cm yardstick placed in the red series for scale (lower half of Figure).

ARGILLACEOUS DOLOMITIC CALCRETES NEAR THE UPPER BOUNDARY OF THE UPPER AL NAKHSH MEMBER

Figure 9: Alternating red and grey siltstones, clays and marls formed in place of gypsum seams at the passage from the Middle (a), into the Upper Al Nakhsh Members (b) on the northeast limb of the Dukhan Anticline. The only evidence for evaporitic conditions in the Miocene basin are some veinlets with selenite, which randomly intersect the massive red beds. To the right an entrance to a cave may be recognised on the photograph. Dissolution of highly soluble compounds (halite?) in the subsurface has given rise to dolines and caused a pervasive karstification at this site. A relative increase in relief on a rather small scale resulted from differential salt dissolution at depth and halokinetic processes along the northeast limb of the Dukhan Anticline. These processes brought about argillaceous mud flats and groundwater-induced calcretes instead of evaporites. See hammer for scale. Locality: southern Dukhan Anticline.

b

a

PASSAGE FROM THE MIDDLE INTO THE UPPER AL NAKHSH MEMBER


74


The most pronounced sequence boundary to be traced on a regional scale separates the shallow-marine carbonate sequences of early Miocene age (Dam Formation) from the siliciclastic series of the Hofuf Formation and lithological equivalent series found at outcrop in the UAE. The maximum flooding surface MFS Ng10 sensu Sharland et al. (2001) cannot be identified in all the studied reference sections. According to the present study, this MFS has to be drawn within the siliclastic-dominated Middle Asmari Formation in Iran, the marly Lower Dam Formation (Middle Salwa Member) in Qatar and in the calcareous Mughsayl Formation in Oman. If MFS Ng20 exists at all in the Dam Formation of Qatar, it has to be “squeezed” between the sequence boundary truncating the Dam Formation and the “intra-Dam erosional surfaces” (Figure 4). In both cases, the MFS Ng 10 and Ng 20 coincide with a “relative low” in the chemologs, illustrating the distribution of isotope ratios (Figure 4).

CONCLUSIONS

Calcareous and evaporitic sediments (gypsum, celestite) of the Dam Formation in Qatar reflect deposition under subtidal through supratidal conditions, which towards the base and the top of the series are replaced by an environment of deposition more akin to a modern beach. A rather uniform isotope curve of Sr, Ca and O isotopes for the tidal deposits is replaced by a more oscillating one when these tidal-influenced regimes became substituted for by a more wave-dominated regime. Small-scale disturbances of the isotope ratios in the Lower Al Nakhsh Member are correlative with the onset of the evaporite series in the section under study. The Sr isotopes do not only indicate an influx of more primitive Sr from the hinterland, but also allow for a refinement of the stratigraphy, which yields a late Aquitanian to early Burdigalian age of sedimentation for the Dam Formation in Qatar. Calcium isotope ratio studies, still in their infancy and not fully understood, seem to provide a new tool in carbonate petrography when interpreting the environment of deposition and calcification of dolomitic series. The isotope ratios need to be tested to determine if they can assist in positioning the planar architectural elements of sequence stratigraphy. The present study is promising in that way but not yet a proof, and needs further geochemical support.

ACKNOWLEDGMENT

The first author is grateful for the support provided by the Scientific and Applied Research Centre (SARC), which provided transportation in the field and to S. Nasir (Sultan Qaboos University, Muscat, Oman) and H. Al-Saad (University of Qatar). We acknowledge the laboratory support by S. Gerlach and P. Macaj (both BGR). We thank two anonymous reviewers for their important suggestions that have improved the manuscript. The final design and drafting by Nestor Niño Buhay is appreciated.

REFERENCES

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ABOUT THE AUTHORS

Harald G. Dill is involved in international technical training with the German Federal Institute for Geosciences and Natural Resources (BGR) and is an Associate Professor at Hannover University. He studied Geology and Mineralogy at Würzburg, Aachen, and Erlangen universities and received a Diploma in Geology in 1975 and a Doctorate in Mineralogy in 1978. Harald conducted research at Bayreuth University before joining BGR in 1979. In 1982, he became Lecturer in Applied Geology at Mainz University where he was awarded his Doctor Rerum Naturalium Habilitatus in 1985. From 1986 to 1991, he was assigned to the Continental Deep Drilling Program of the Federal Republic of Germany. In 1991, Harald was appointed Associate Professor in Economic Geology at Hannover University. In the same year he rejoined BGR in the Department of Economic Geology and International Cooperation training geologists in sedimentology and the geology of non-metallic mineral deposits through international cooperation schemes. Harold lectures in Economic Geology and Sedimentology at Hannover University, elsewhere in Germany, and abroad. He has published over 200 papers and abstracts on the sedimentology and economic geology of metallic and non-metallic deposits in South America, Asia, and Central Europe. His interest lies in the capture of digital data in the field and the study of heavy minerals as well as mineral and energy deposits, mainly within sedimentary rocks.

[email protected]

Friedhelm Henjes-Kunst is a Research Scientist at the Department of Geochemistry and Mineralogy of the German Federal Institute for Geosciences and Natural Resources (BGR), specializing in isotope geochemistry and geochronology of igneous, metamorphic and sedimentary rocks. He studied mineralogy at the universities of Clausthal-Zellerfeld and Braunschweig and received a Diploma in 1976 and a Doctorate in 1980 both in Mineralogy. Afterwards, Friedhelm was involved in research projects at the universities of Münster, Karlsruhe and Freiburg covering petrology, geochemistry and isotope geochemistry of granitoids, rift-related volcanics and mantle-derived rocks. In 1990, he joined the BGR in the isotope geochemistry section. Since then, his major research interests were assigned to the Continental Deep Drilling Program of the Federal Republic of Germany and to the Polar Geoscience Program of the BGR. Friedhelm joined six expeditions to Antarctica and one to the Canadian High-Arctic. Since 2001 he has been involved in the investigation of ore deposits. His major interests are application of unconventional isotope methods to investigate formation and age of ore deposits.

[email protected]

Manuscript submitted September 2, 2006Revised December 15, 2006

Accepted December 28, 2006Press version proofread by authors April 29, 2007


Documents

Strontium (87Sr/86Sr) and calcium isotope ratios (44Ca/40Ca