adipec-0918 lower cretaceous stratigraphy of abu … (oman, qatar, etc.). a new stratigraphic...

Copyright 2000, ADIPEC

This paper was selected for presentation at the 9th Abu Dhabi International PetroleumExhibition and Conference held in Abu Dhabi, U.A.E., 15-18 October 2000.This paper was selected for presentation by the ADIPEC Program Committee followingreview of information contained in an abstract submitted by the author(s). Contents of thepaper, as presented, have not been reviewed by the Society of Petroleum Engineers andare subject to correction by the author(s). The material, as presented, does not necessarilyreflect any position of the ADIPEC or its members. Permission to copy is restricted to anabstract of not more than 300 words. Illustrations may not be copied. The abstract shouldcontain conspicuous acknowledgment of where and by whom the paper was presented.Write ADIPEC Coordinator, GEC, P.O. Box 5546, Abu Dhabi, U.A.E., Fax 009712-4446135.

Abstract

This paper presents a reappraisal of the published papers onstratigraphic nomenclatures of the Lower Cretaceous rocks.Thought it focuses on the Abu Dhabi offshore areas, it alsodeals with coeval strata from the neighbouring emirates andcountries (Oman, Qatar, etc.). A new stratigraphicclassification emerged from the review of the currently usedoperational units in Abu Dhabi. Accordingly, the KahmahAllogroup (the best substitute for the Thamama Group) asdefined herein comprises 7 alloformations/synthems whichare from top to bottom: Shu'aiba, Hawar, Kharaib,Lekhwair, Zakum, Belbazem (new name), and Bu Haseer(new name); Bab and - revised - Habshan are excluded fromthis allogroup and respectively transferred to Wasi'a andSahtan allogroups. This new classification which is based on‘unconformity-bounded units’ is supported by bothbiostratigraphic and sequence stratigraphic evidence. Thisrevised stratigraphic framework suggests a newinterpretation of regional basin history.

Introduction

Some believe the stratigraphy of the Middle East oilprovince is “all done”. However, an in depth examination ofthe extant stratigraphic scheme reveals it merely consists ofgroup of a jigsaw puzzle of units as illustrated in theexhaustive compilation recently published by A.S.ALSHARHAN and A.E.M. NAIRN [1997]. The LowerCretaceous stratigraphy presented here is no exception. Partof the problem arises from the use of lithostratigraphicconcepts for correlation since "rock"-stratigraphy isbasically facies correlation.

In the late 60’s - early 70’s, some petroleum companiesoperating in the Arabian Gulf oil province (Abu Dhabi,Dubai, Oman, Qatar, Sharjah) arranged “StratigraphicLiaison Meetings” to standardise the regional stratigraphicnomenclature. They came up with a practical schema for theso-called “Thamama Group”, the Abu Dhabi chapter ofwhich was issued by T.H. HASSAN et al. [1975]: Table 1.

In the ensuing discussion, it will be demonstrated that many

operational units defined at the time fall into the category of‘unconformity-bounded units’, as their boundariescommonly correspond either to a ‘sequence boundary’ (SB)or to a ‘transgressive surface’ (TS), i.e. an unconformityonto the shelf which passes basinward into its correlativeconformity. Unconformity-bounded units are the referencerock-unit on which is based allostratigraphy, a classificationwhich was not introduced until recently into the Code ofStratigraphic Nomenclature. In the subsurface, suchboundaries, which are usually easily picked from well-logsare traceable far away from the reference locality; cores orcuttings provide additional physical evidence for these keybounding surface features (erosional surface, hard ground,karst, etc.). Now, almost thirty years later, it is time toreview these early attempts of regional stratigraphicstandardisation.

Current regional stratigraphic nomenclatures

Most rock units described from Middle East outcrops areformally defined as they fulfil the requirements of the Codeof Stratigraphic Nomenclature. On the contrary most unitsintroduced for the subsurface correspond to informal termswhich have been perpetuated in published or unpublishedlater reports.

In Abu Dhabi and according to T.H. HASSAN et al. [1975],the “Thamama Group” in Abu Dhabi includes the sectionfrom the base of the Nahr Umr Fm down to the top of theHith Fm. The group consists from top to base of:

• the “Shu'aiba Fm” which locally comprises the BabMember in its upper part,

• the “Kharaib Fm” comprising the Hawar Member in itsupper part,

• the “Lekhwair Fm” comprising the Zakum Member in itslower part, and

• the Habshan Fm, which supposedly passes graduallyeastward and downward into the Salil Fm, which in turnpasses downward into the Rayda Fm.

In Saudi Arabia, the Thamama Group is bracketed by theWasi’a Group above and the Hith Anhydrite below.According to R. W. POWERS [1968], the upper boundaryof the Thamama Group falls at different stratigraphic levelsdepending on whether we are dealing with outcrop orsubsurface sections. In outcrop, the type-Thamamacomprises, from top to base, the Biyadh Sandstone, and theBuwaib, Yamama, and Sulaiy formations; which in thesubsurface, beds not represented on outcrop include theShu'aiba Fm and the upper part of the Biyadh Fm. As statedby the latter author, “it is quite possible (…) that the type-

ADIPEC-0918

Lower Cretaceous Stratigraphy of Abu Dhabi and the United Arab Emirates - AReappraisalBruno R.C. GRANIER, ADMA-OPCO

527 [Bruno R.C. GRANIER] [0918]

Biyadh includes beds assigned to the Wasi’a (…) insubsurface sections”. This is a typical case of what has beencalled an ‘obscure unconformity’ in the various versions ofthe Code of Stratigraphic Nomenclature (“sand-on-sandrelationship” as quoted from R. W. POWERS [1968]); thisunconformity should be found in the Biyadh type-section,and the Wasi’a should be significantly extended downward.

In Qatar, according to W. SUGDEN and A.J. STANDRING[1975], the name “Thamama Group” covers the Shu’aiba,Hawar, Kharaib, Ratawi, Yamama, and Sulaiy in descendingorder.

In Oman, the Kahmah Group [GLENNIE et al., 1974],which has been considered either as a synonym for, or alateral equivalent of, the so-called “Thamama Group”, issubdivided into 6 formations which are from top to bottomthe Shu’aiba, Kharaib, Lekhwair, Habshan, Salil, and Raydaformations.

Review of the extant "Lower Cretaceous"stratigraphic units in the United Arab Emirates

Amongst the very first papers published on thebiostratigraphy of the Middle East, R.G.S. HUDSON andM. CHATTON [1959] work should be highlighted. Theydivided G.M. LEES’s ‘Musandam Limestone’ [1928] into a“series of groups, generally of formational rank and lettereda-p”; these subdivisions have been combined into 2 majorunits, i.e. a ‘Lower Musandam Limestone’ (groups a-f) andan ‘Upper Musandam Limestone’ (groups g-p). Accordingto these authors, the upper part, which is our concern,consists from base to top of:

• Group g, limestone with breccias (58.5 m), ? Sequanian[=Upper Oxfordian] - Tithonian;

• Group h, porcellaneous limestones with Calpionella(127.5 m), ? Berriasian;

• Group i, porcellaneous and massive, brown, finelycrystalline limestones, with Calpionella andPseudocyclammina (111 m), ? Valanginian;

• Group j, buff-coloured shelly limestone withNautiloculina oolithica (38 m), Valanginian;

• Group k, light-buff porcellaneous limestone, oftenfinely shelly, with Pseudocyclammina lituus (27 m),Valanginian;

• Group l, mainly white-weathering nodular limestone,usually radiolarian or miliolid (35 m), Hauterivian-Barremian;

• Group m, mainly white-weathering chalky limestonewith Choffatella decipiens (30 m), Hauterivian-Barremian;

• Group n, fine-grained shelly limestones withDictyoconus arabicus, occasional radiolarian cherts (54m), Hauterivian-Barremian;

• Group o, molluscan limestone with Heterastermusandamensis (38 m), Aptian;

• Group p, limestone with breccias, with Orbitolina spp.(48 m), Aptian.

By and large, these informal outcrop units correspond to oil-

bearing formations in the subsurface. In the followingsections, attempt to draw these correlations first to member-or formation- ranked units, then to units of higher rank, i.e.units with a group status will be highlighted:

• Rayda and Salil formations,• Habshan Formation,• Zakum Member,• Lekhwair Formation,• Kharaib and Hawar formations,• Shu’aiba Formation, Bab Member, and Sabsab

Formation,• Nahr Umr Formation,• Thamama and Kahmah groups.

1- Rayda and Salil formations

The Rayda and Salil formations correspond to formallithostratigraphic units based on mountain sections exposedin Jebel Akhdar, Oman. First mentioned by F. SCHERER[1969, unpublished report], they were not effectivelypublished until T.H. HASSAN et al. [1975]; M.W.HUGHES CLARKE [1988] and E.A. HAAN et al. [1990]provided additional information on the type-sections. Bothunits are facies-driven: the Rayda facies is a porcellaneouslime mudstone with Calpionellids, equivalent to ‘group g’and ‘group h’, while the Salil facies covers argillaceouslimestones and calcareous shales, probably equivalent to‘group i’ in R.G.S. HUDSON and M. CHATTON [1959].

A prominent erosional hiatus exists in between the Raydadeposits and the underlying Upper Jurassic limestones of theSahtan Group [HAAN et al., 1990; TOLAND andPEEBLES, 1993]. Distinctive facies are developed at thebase of the Rayda immediately above this unconformity,which is referred to as the pre-Kahmah unconformity. First,R.G.S. HUDSON and M. CHATTON [1959] introduced thename “Ashhab Limestone” to cover “characteristicallybreccia-conglomerates with associated porcellaneouslimestones” from Jebel Hagab, in Ras Al Khaimah.Unfortunately, they misinterpreted these beds whichalternatively they called ‘group g’ as “solution breccias”and, though the subordinate limestones bear Calpionellids,these authors erroneously ascribed to that unit aquestionable Late Oxfordian - Tithonian age. In the type-area, i.e. in Jebel Akhdar, E.A. HAAN et al. [1990] reportthe local occurrence of a “condensed horizon” withlimestones pebbles. In Wadi Hagil, a locality close toHUDSON and CHATTON’s, C. TOLAND and R.G.PEEBLES [1993] also give a description of “unstratifiedclast-supported conglomerates” above the pre-Kahmahunconformity.

Microfossils.- R.G.S. HUDSON and M. CHATTON [1959]found Calpionella alpina LORENZ in their AshhabLimestone from Jebel Hagab. They also mentionedCalpionella in their ‘group h’ and C. cf. alpina in their‘group i’, both groups constituting their “RadiolarianLimestones”. Besides, C. TOLAND and R.G. PEEBLES[1993] identified various Calpionellid assemblages ranging

[0918] [Lower Cretaceous Stratigraphy of Abu Dhabi and the United Arab Emirates - A Reappraisal] 528

from Zone B to Zone D (C. alpina LORENZ, C. ellipticaCADISCH, Calpionellopsis simplex (COLOM),Tintinnopsella carpathica (MURGEANU et FILIPESCU),and Remaniella cadischiana (COLOM)) from the samefacies in the neighbouring Wadi Hagil, all in Ras AlKhaimah. These observations seem credible although theyare not yet substantiated by figured specimens. T.C.CONNALLY and R.W. SCOTT [1985] also reported theoccurrence of Calpionellids in the Amoco wells Sajaa N° 1,2, and 3, onshore Sharjah. Finally, in onshore Abu Dhabi,J.E. de MATOS [1994] quoted C. alpina LORENZ and C.elliptica CADISCH in ADCO wells L1-A and Jumaylah N°1; he also illustrated some Calpionellid sections which herefers to Calpionella alpina [op. cit.: Pl. 5.6, fig. 6-7]. Thereis no evidence for Calpionellid Zone A in the Rayda facies;it is likely that the base of the Rayda Fm is not older thanEarly Berriasian.

2- Habshan Formation

The Habshan Fm is an informal lithostratigraphic unit basedon ADCO (ex ADPC) well Bab N° 2, in onshore AbuDhabi; the ADMA well Zakum N° 1 is the reference sectionfor the offshore. The name “Habshan” was introduced byT.H. HASSAN et al. [1975] as a substitute for the double-barrelled name “Yamama/Sulaiy” which was usedpreviously in Abu Dhabi.

Microfossils.- Microfossils originally described fromequivlant interval in Saudi Arabia include Bramkampellaarabica REDMOND, Pseudocyclammina sulaiyanaREDMOND, P. cylindrica REDMOND, Everticyclamminaeccentrica REDMOND, and E. elegans REDMOND. In apaper published in “Revue de Micropaléontologie” [1965,vol. 8, Nº 1], W. MAYNC comments on C.D. REDMOND’s“unfortunate attitude of splitting up a taxon merely onaccount of very slight variations”.

The recent finding of Campbelliella striata (CAROZZI) andAnchispirocyclina lusitanica (EGGER) in core samplesfrom Zakum wells suggest a Tithonian age for the lower andmiddle parts of the Habshan. The Dasyclad algaCampbelliella striata (CAROZZI) ranges from theKimmeridgian to the Tithonian [GRANIER andDELOFFRE, 1994]. The Foraminifer Anchispirocyclinalusitanica (EGGER), which was also coined from the type-Sulaiy in Saudi Arabia by R.-W. POWERS et al. [1966]under the name “Iberina lusitanica (EGGER)”, is a markerfor the Tithonian [BASSOULLET and FOURCADE, 1979].

Among the Cretaceous microfossils recently identified fromthe Zakum field, I will mention Arenobulamina sp., a formknown from the Late Berriasian, and Montsalevia elevataZANINETTI et al., a form known from the Late Berriasianto the Early Valanginian; both were found in the uppermostpart of the Habshan.

It is clear that the base of the Thamama Group (i.e. the topof the Hith Fm) is not time-equivalent to the base of theKahmah Group (i.e. the base of the Rayda Fm): it is older.

In addition, although it is desirable that lithostratigraphicunits should not contain major unconformities, this is not thecase with the Habshan Fm, at least in onshore Abu Dhabiwhere this unit needs to be properly redefined. Mostunconformities found in the Tithonian section of the type-Habshan are probably intercepted by the pre-Kahmahunconformity. It is therefore suggested to split the type-Habshan into at least two formations, one that covers thelower and median parts bearing Tithonian microfossils andthe other corresponding to the upper part bearing Berriasianmicrofossils. The use of "Habshan" as a formation name ispresently restricted to the lower and median parts of the typesection; two new formation names, “Bu Haseer” and“Belbazem”, are introduced to cover respectively the lowerpart and the upper part of its Cretaceous section. The new“Bu Haseer” unit displays distinctive facies associations,notably brackish environment facies with Charophytes andSalpingoporella (Hensonella) dinarica RADOICIC.

3- Zakum Member

The Zakum Member is first mentioned by T.H. HASSAN etal. [1975]. It is an informal lithostratigraphic unit based onADMA well Zakum N° 1 in offshore Abu Dhabi and shouldcorrespond to the lower unit of the so-called “Lekhwair Fm”in Abu Dhabi. It is bracketed by two unconformities; thelower one corresponds to the pre-Buwaib unconformity,which recorded the drowning of the eastern Arabiancarbonate platforms. This interval covers both the so-called“Chrysalidina zone” or “Valvulinella zone”, and the sectionimmediately below down to the pre-Buwaib unconformity.According to R.-W. POWERS [1968], the latterbiostratigraphic zone “near the middle of the Buwaib Fm”provides highly reliable correlations over most of SaudiArabia and neighbouring countries; it is identified by thepresence of either the fossil marker, Paravalvulina arabica(HENSON), or Pseudocyclammina lituus (YOKOYAMA).R.G.S. HUDSON and M. CHATTON [1959: p. 89] reportthis association from their ‘group j’ and ‘group k’ in theMusandam Peninsula.

Microfossils.- F.R.S. HENSON [1948a: Pl. XV, fig. 6-7; Pl.XVII, fig. 1-2] described Dukhania arabica from QGPC (exQPC) well Dukhan N° 2 in Qatar where the type-level islocated in “limestones and shales with Pseudocyclamminalituus”. Topotypes were illustrated by R. SCHROEDER etal. [1975: Pl. II, fig. 1-2] under Pseudochrysalidina (?)arabica (HENSON). This Foraminifer was also coined asChrysalidina, Valvulinella, Broeckinella, Paravalvulina or“Valvulines spéciales”. It is supposedly unknown fromdeposits older than the Valanginian (? Late Berriasian).

On the other hand, the large Foraminifer Pseudocyclamminalituus (YOKOYAMA), which was also coined by R.-W.POWERS [1968: p. 56] under the name “Cyclammina sp.”from the type-Buwaib in Saudi Arabia, is not known fromdeposits younger than the Early Valanginian [GRANIER,1987]. Some specimens from the Sulaiy or Yamamaformations quoted as Pseudocyclammina aff. lituus(YOKOYAMA) do not expressly refer to that species, but toa much smaller form called Pseudocyclammina sulaiyana


REDMOND, a synonym for “Pseudocyclammina lituusforme α” sensu W. MAYNC [1959; MAYNC, 1965].Genuine Pseudocyclammina lituus have rarely beenillustrated from Middle East sections. W. MAYNC [1959:Pl. 4, fig. 9] and F.R.S. HENSON [1948: Pl. IX, fig. 3; Pl.XIII, fig. 7] figured some specimens from QGPC (ex QPC)Dukhan wells in Qatar. T.C. CONNALLY and R.W.SCOTT [1985] figured specimens from the Amoco wellSajaa N° 2 [op. cit., Fig. 9.H] and 3 [op. cit., Fig. 9.A], inonshore Sharjah; unfortunately, they erroneously ascribedthem to the Habshan Fm, instead of the Zakum Member,strata where this species is commonly associated with high-spired Trocholina sp.

Amongst the other microfossils, another Foraminiferrecently identified from the Zakum field is Montsalevia?salevensis (CHAROLLAIS et al.); this form is known fromthe Early Valanginian only.

Finally, in addition to Foraminifers such as fairly commonhigh-spired Trocholina sp., there are some Dasyclad algae:Cylindroporella cruciformis GRANIER et BRUN andHolosporella arabica GRANIER et BRUN were originallydescribed from this interval in offshore Abu Dhabi (TOTALwell Abu Al Bu Khoosk N° 41).

The above fossil assemblage strongly suggests an EarlyValanginian age can be ascribed to the type-Zakum fromoffshore Abu Dhabi and time equivalent units.

4- Lekhwair Formation

The Lekhwair Fm is an informal lithostratigraphic unitbased on a PDO well from the Lekhwair field, in onshoreOman. It could be either well N° 6 according to T.H.HASSAN et al. [1975] or, most probably, the well N° 7according to M.W. HUGHES CLARKE [1988;MOHAMMED et al., 1997]. First mentioned by F.SCHERER [1969, unpublished report], it was noteffectively published until M.W. HUGHES CLARKE[1988]. The Lekhwair is probably equivalent to ‘group l’and ‘group m’ in R.G.S. HUDSON and M. CHATTON[1959].

In Abu Dhabi, the so-called “Lekhwair Fm” covers both theZakum Member and a time-equivalent to the type-Lekhwair(sometimes referred to as the Nasr Formation in unpublishedreports [SIMMONS, 1994: p. 185]).

Microfossils.- F.R.S. HENSON [1948: Pl. XIII, fig. 9, 11 &15-17] described Cyclammina greigi, later coined asEverticyclammina, from QGPC (ex QPC) well Dukhan N° 2in Qatar. The type-level is “just above the horizon ofPseudocyclammina lituus”, i.e. it probably as old asValanginian.

5- Kharaib and Hawar formations

The Kharaib and Hawar formations are informallithostratigraphic units based on QGPC (ex QPC) well

Kharaib N° 1, in onshore Qatar. First mentioned by W.SUGDEN [1953, unpublished report], they were noteffectively published until W. SUGDEN and A.J.STANDRING [1975]. They are probably equivalent to‘group n’ and ‘group o’ in R.G.S. HUDSON and M.CHATTON [1959].

Macrofossils.- From the scree of their ‘group o’, R.G.S.HUDSON and M. CHATTON [1959] collected some largefossils among which were Echinids, such as “Heterastermusandamensis LEES” and “H. aff. couloni AGASSIZ”.Genuine H. couloni AGASSIZ are exclusively characteristicof the Hauterivian-Barremian interval. On the other hand,according to B. CLAVEL and A. VIRGONE (personalcommunications), the figures and description given by G.M.LEES [1928b: p. 642-643, Pl. XLIV, fig. 4.a-b] point towarda synonymy with H. oblongus BRONGNIART, a marker forthe Early Aptian; in H.V. DUNNINGTON et al. [1959], thesame species found from the Sarmord Fm in North Iraq iscoined as “Heteraster oblongus BRONGNIART var.musandamensis LEES”.

Microfossils.- Dictyoconus arabicus was originallydescribed by F.R.S. HENSON [1948: Pl. I, fig. 6-8; Pl. XIV,fig. 1-12] from QGPC (ex QPC) well Dukhan N° 3 in Qatar,possibly from the lower part of the Kharaib Fm. A variety of“Orbitolina discoidea GRAS”, O. discoidea var. delicatawas also described by F.R.S. HENSON [1948: Pl. II, fig.13-14] from the same layer in well Dukhan N° 2; it hasproven to be a junior synonym of the early Palorbitolinalenticularis (BLUMENBACH) [SCHROEDER, 1963;GUSIC, 1981]. Most early reports refer to obsolete names,such as “Orbitolina cf. discoidea GRAS” which might coverseveral taxa among which Valserina sp. and Palorbitolinalenticularis (BLUMENBACH).

Additional fossils originally described from the Kharaib Fmin onshore Oman (Fahud field) include the Dasyclad algaCylindroporella sugdeni ELLIOTT 1957.

On the basis of an accurate biostratigraphic scale based onOrbitolinids [SCHROEDER in CHAROLLAIS et al., 1992],it is inferred that the top of the Lekhwair is Hauterivian, theKharaib is time-equivalent to most of the Barremian, and theHawar to the Early Aptian pro parte. In ADMA Umm Shaiffield, offshore Abu Dhabi, the Kharaib can be split into 4third-order transgressive-regressive cycles [GRANIER, AL-SUWAIDI and AZIZ, nearing completion; SCHROEDERand GRANIER, nearing completion]:

• Kharaib 1 cycle with advanced Valserina sp. couldcorrespond to CHAROLLAIS et al. cycle Ba 1;

• Kharaib 2 and 3 cycles with early Palorbitolina sp. couldrespectively correspond to their cycles Ba 2 and Ba 3;

• Kharaib 4 cycle with P. lenticularis could correspond totheir cycle Ba 4;

• Hawar cycle could correspond to their cycles Bd 1 or Bd2.

The fact that the Hawar Fm has been alternately lumped


together with the overlying Shu’aiba Fm (in Oman and Iraq)or with the underlying so-called “Kharaib Fm” (in AbuDhabi) or so-called “Ratawi Fm” (in Qatar) is due to amisunderstanding of its genesis: it is an unit on its own. Inthe ADMA Umm Shaif field, the base of the Hawar Fmconsists of transgressive beach deposits which cut throughthe underlying shallow-water carbonates; in some locations,relicts of a pre-existing karstic surface are still preserved;therefore this lower boundary is associated to a sea-levelfall. The rest of the unit consists predominantly of offshorecarbonate-sand deposits with abundant Palorbitolinalenticularis. The maximum flooding is possibly reached atthe base of the uppermost shale interval that lies on top of aglauconitic carbonate-sand layer. The top of the Hawar Fmcoincides with an abrupt break in sedimentation from thelatter shale interval to very shallow-water carbonates above.As far the lower boundary, the upper one also records aforced regression. The Hawar is an unit on its own, i.e. anunconformity-bounded unit.

6- Shu’aiba Formation, Bab Member, and SabsabFormation

The Shu’aiba Fm is an informal lithostratigraphic unit basedon INOC (ex BPC) well Zubair N° 3, in South Iraq. Firstmentioned by P.M.V. RABANIT [1951, unpublishedreport], it was not effectively published until R.M.S. OWENand S.N. NASR [1958]; it was subsequently amended firstby H.V. DUNNINGTON et al. [1959], then by K.M. ALNAQIB [1967]. It is equivalent to ‘group p’ in R.G.S.HUDSON and M. CHATTON [1959].

6.a- Shu’aiba Formation

According to R.M.S. OWEN and S.N. NASR [1958], theShu’aiba type-section lies between drilled depths 9,870 ft /3,008.4 m to 10,132 ft / 3,088.2 m, and “is made up ofdolomitic limestones which are coarsely crystalline, porous,and cavernous, with recrystallized Rudistae and with rareOrbitolina discoidea and Choffatella decipiens” (p. 1276).The original description by P.M.V. RABANIT [1951,unpublished report] was significantly different with, fromtop to bottom:“9870’-9962’: Crystalline limestone with large thick shelledglobigerinids and traces of glauconite.9962’-10125’: Dominantly fine grained limestone the topmore chalky and the lower part more argillaceous.10125’-10132’: Pseudo-oolitic limestone with clear calcitematrix and angular sand grains.The fauna of this limestone formation is on the whole ratherpoor with the exception of a rich zone Orbitolina discoideaat the base.”

Such a succession, from well Zubair N° 3, in South Iraq, isquite similar to the one found in most wells offshore AbuDhabi. It is likely that this original description of the type-Shu’aiba covered an interval ranging from the top of theBab Member down to the base of the Hawar Fm.

H.V. DUNNINGTON et al. [1959] reverted to the original

description, besides K.M. AL NAQIB [1967] significantlyrevised the top of the unit by lowering it 92 ft / 28.1 m.According to him, the top is picked at a drilled depth of9,962 ft / 3,036.4 m, and the part bearing planktonicForaminifers is excluded. Therefore, it is likely that, in itslatest version, the type-Shu’aiba still includes the HawarFm, whereas it excludes the Bab Member.

Macrofossils.- Ammonites identified by C.W. WRIGHT[1959, unpublished report; BANNER and WOOD, 1964;HASSAN et al., 1975; AZER and TOLAND, 1993] fromADMA well Umm Shaif N° 2 comprise one ancyloceratinidspecimen; those identified by M. HOWARTH [1992,unpublished report; AZER and TOLAND, 1993] fromUmm Shaif N° 3 comprise cf. Pseudohaploceras sp. andDiadochoceras sp. These observations point toward aMiddle Aptian (Gargasian) age.

6.b- Bab Member

First mentioned by T.H. HASSAN et al. [1975], the BabMember, which should corresponds to the upper unit of theso-called “Shu’aiba Fm” in Abu Dhabi, is an informallithostratigraphic unit based on ADCO (ex ADPC) well BabN° 2, in onshore Abu Dhabi.

Macrofossils.- Ammonites identified from this interval byC.W. WRIGHT [1959, unpublished report; BANNER andWOOD, 1964; HASSAN et al., 1975; AZER and TOLAND,1993] in ADMA well Umm Shaif N° 2 compriseCheloniceras (Epicheloniceras) sp. and Colombiceras cf.caucasicum. T.H. HASSAN et al. [1975; ALSHARHAN,1985a] also reported Pseudosaynella fimbriata,Pseudohaploceras sp., Diadochoceras sp., Gargasiceras sp.,and Dufrenoyia sp., possibly from Bab N° 2, but there areuncertainties with respect to their formation ascription (arethey from the Shu'aiba or the Bab?). Except for theoccurrence of a representative of the genus Dufrenoyia thatis contrary to expectations, these observations point towarda Middle Aptian (Gargasian) age. New ammonite findingsare presently under evaluation [BUSNARDO andGRANIER, nearing completion]; so far the results agreewith this age.

6.c- Sabsab Formation

The Sabsab Fm is an informal lithostratigraphic unit basedon QGPC (ex QPC) well Dukhan N° 27, in West Qatar. Itwas not effectively published until W. SUGDEN and A.J.STANDRING [1975]. Described as “an oolitic, pelletylimestone containing abundant abraded Orbitolinidae”, itwas erroneously “believed to infill a channel”.

7- Nahr Umr Formation

The Nahr Umr Fm is an informal lithostratigraphic unitbased on INOC (ex BPC) well Nahr Umr N° 2, in SouthIraq. First mentioned by D. GLYNN JONES [1948,unpublished report], it was not effectively published untilR.M.S. OWEN and S.N. NASR [1958]. In its present


definition it is the lowermost unit of the Wasi'a Group.

8- Thamama and Kahmah groups

In Saudi Arabia, the finding of presumed Early Cretaceousfossils in the upper part of the Riyadh Fm, above the HithAnhydrite, led M. STEINEKE [1940, unpublished report] tobreak out the supposed Lower Cretaceous beds and placethem in a new formation, the Thamama. In an abstract oftheir talk, M. STEINEKE and R.A. BRAMKAMP [1952]raised this unit to group status and included within it theBuwaib (“limestone, with shale”), Yamama (“fragmentallimestone”), and Sulaiy (“limestone”) rock-units, which theyelevated to formation rank. R.A. BRAMKAMP et al. [1956]while mapping the southern Tuwayq quadrangle followedthis classification. Two years later, while mapping thenorthern Tuwayq quadrangle, R.A. BRAMKAMP and L.F.RAMIREZ [1958] expanded the group to include theoverlying Biyadh Sandstone. Finally, the Shu’aiba Fm,which was first described from a subsurface section in SouthIraq but which was also known from subsurface sections inSaudi Arabia, was lumped into the group by R.M.S. OWENand S.N. NASR [1958] as it lies below the so-called “pre-Wasi’a” unconformity. In a footnote, F.R.S. HENSON[OWEN and NASR, 1958] advises that “The significance ofthis group is that it comprises the marine section between anunconformity, present at outcrop (Dakl al Hith) at the top ofthe Jurassic Hith Anhydrite, and the next recognisedunconformity at the top of Aptian limestone”. This is thepresent view of this controversial group, which is per se anunconformity-bounded unit.

The Kahmah Group is a formal lithostratigraphic unit basedon mountain sections exposed in Jebel Akhdar, in Oman[GLENNIE et al., 1974]. Per definition, it is a synonym forthe 'Upper Musandam Limestone’ of R.G.S. HUDSON andM. CHATTON [1959]. This group is bracketed by twomajor unconformities, one at the bottom of the Nahr UmrFm, the other at the top of the Sahtan Group. Therefore itcould also be equated to an unconformity-bounded unit.

On the basis of their respective boundaries, both groupscannot be considered as possible synonyms or lateralequivalents:

• the upper boundary of the type-Thamama, i.e. the upperboundary of the type-Biyadh (intra- Albian orCenomanian), is younger than the upper boundary of thetype-Kahmah, as the latter equates to the upper boundaryof the so-called “Thamama” in subsurface sectionsThamama, i.e. the upper boundary of the type-Shu’aiba(intra- Middle Aptian);

• the lower boundary of type-Thamama, i.e. the lowerboundary of the type-Sulaiy (intra- Tithonian), is olderthan the lower boundary of the type-Kahmah, i.e. thelower boundary of the type-Rayda (close, if not equal, tothe Tithonian - Berriasian boundary).

Some would eventually prefer to revert to original

understanding of the Thamama that is a group comprisingonly the Buwaib, Yamama, and Sulaiy formations.Alternatively, its passing may in particular help diminish theconfusion that prevails over its upper limit.

New allostratigraphic nomenclature

A classification in terms of ‘unconformity-bounded units’emerges from the preceding review of the operational unitsin Abu Dhabi. Though changes occur in the classificationand the unit status, there is no real need to introduce newnames; in a few cases only, unit rank has been changed orunit boundary revised.

The Kahmah Allogroup/Supersynthem as defined hereincomprises from top to bottom (Fig. 1):

• a Shu’aiba Alloformation/Synthem, Early to MiddleAptian (Bedoulian – Gargasian) in age. In the type-section (Zubair N° 3), its lower boundary needs to beslightly raised in order to exclude the bottom part, time-equivalent to the Hawar; its upper boundary as redefinedby K.M. AL NAQIB [1967] is unchanged.

• a Hawar Alloformation/Synthem, Early Aptian(Bedoulian) in age. Its lower and upper boundariesremain unchanged in the type-section (Kharaib N° 1).

• a Kharaib Alloformation/Synthem, Barremian in age. Itslower and upper boundaries are unchanged in the type-section (Kharaib N° 1).

• a Lekhwair Alloformation/Synthem, Valanginian toHauterivian in age. Its lower and upper boundariesremain unchanged in the type-section (Lekhwair N° 7).

• a Zakum Alloformation/Synthem, Early Valanginian inage. Its lower and upper boundaries are unchanged in thetype-section (Zakum N° 1).

• the Belbazem and Bu Haseer Alloformations/Synthems,both Berriasian in age, are a new units introduced tocover the Cretaceous part of the former Habshan. Thelower boundary of the Bu HaseerAlloformation/Synthem is readily mappable in basinareas (Ras Al Khaimah, Oman) where it corresponds tothe pre-Kahmah unconformity. The upper boundary ofthe Belbazem Alloformation/Synthem is chosen in orderto coincide with the upper boundary of the formerHabshan in both ADMA well Zakum N° 1 and its formertype-section (Bab N° 2); it corresponds to the pre-Buwaib unconformity.

The revised Habshan, Tithonian in age, now covers theJurassic part of the former Habshan. It is excluded from theKahmah allogroup as their sedimentation took place beforethe pre-Kahmah unconformity. The lower boundary of therevised Habshan is unchanged in both ADMA well ZakumN° 1 and the former type-section (Bab N° 2). However, itsupper boundary is significantly lowered in ADMA wellZakum N° 1, but still needs to be properly identified in itsformer type-section in Bab No 2.

The Bab Alloformation/Synthem, Middle Aptian


GRANIER B. [1987] - Le Crétacé inférieur de la Costa Blancaentre Busot et Altea, Alicante (Espagne) :Biostratigraphie, Sédimentologie, Evolution tectono-sédimentaire. Mémoires des Sciences de la Terre,Université Pierre et Marie Curie, Paris, 87-49, part I(text): 281 p., part II (plates).

GRANIER B. [1994] - Early Valanginian unconformities inCentral Atlantic and Western Mediterranean areas.Géologie Méditerranéenne, Marseille, XXI, p. 81-85.

GRANIER B. and DELOFFRE R. [1994] - Inventaire des AlguesDasycladales fossiles. II° partie - Les AlguesDasycladales du Jurassique et du Crétacé. Revue dePaléobiologie, Geneva, 12/1 [1993], p. 19-65.

GUSIC I. [1981] – Variation range, evolution, and biostratigraphyof Palorbitolina lenticularis (BLUMENBACH)(Foraminiferida, Lituolacea) in the Lower Cretaceous ofthe Dinaric Mountains in Yugoslavia. Paläont. Z.,Stuttgart, 55/¾, p. 191-204.

International Subcommission on Stratigraphic Classification(Amos SALVADOR, Chairman) [1987] - Unconformity-bounded stratigraphic units. Geological Society ofAmerica Bulletin, 98, p. 232-237.

MAYNC W. [1959] - Biocaractères et analyse morphométrique desespèces jurassiques du genre Pseudocyclammina(Foraminifères). Pseudocyclammina lituus (Yokoyama).Revue de Micropaléontologie, Paris, 2/3, p. 153-172.

SCHROEDER R. [1975] – Les Orbitolines de l'Aptien :définitions, origine et évolution. Géobios, Lyon, Mém.spéc. 2, p. 289-299.

SEPTFONTAINE M. [1980] – Les Foraminifères imperforés desmilieux de plate-forme au Mésozoïque : Déterminationpratique, interprétation phylogénétique et utilisationbiostratigraphique. Revue de Micropaléontologie, Paris,23/¾, p. 169-203.

SEPTFONTAINE M. [1988] – Vers une classification évolutivedes Lituolidés (Foraminifères) jurassiques en milieu deplate-forme carbonatée. In: Benthos 86. Revue dePaléobiologie, Genève, vol. spéc. 2, p. 229-256.

About the author

Bruno R.C. Granier joined TOTALFINA ELF in 1991 as aSenior Carbonate Sedimentologist at its Scientific andTechnical Research Centre, Saint-Rémy-lès-Chevreuse,where he co-ordinated studies undertaken on Angola andLibya, and was also responsible for carbonate studies onFrance, Qatar, and Yemen. Previously, from 1987 to 1991,he worked in turn for the CNRS, GSC, SCOP6, SNEA(P),and TOTAL. Bruno holds a B.Sc. in Sedimentary Geology(1981) from P. Sabatier University - Toulouse, and both aM.Sc. in Geodynamics (1983) and a Ph.D. in Stratigraphy(1987) from P. et M. Curie University - Paris. Since 1995 hehas been seconded to ADMA-OPCO in Abu Dhabi(U.A.E.), as a Petrography Expert. So far Bruno hascontributed to more than sixty publications in the field of

sequence stratigraphy, carbonate sedimentology, diagenesis,rock typing, and micropalaeontology. He is also the editorfor several geology-driven web sites.


Figure 1: The new allostratigraphic classification in Abu Dhabi and the neighbouring emirates. The so-called "Thamama" includesthe section from the base of the Nahr Umr Fm down to the top of the Hith Fm, that is the Kahmah Allogroup as well as parts of theSahtan and Wasi'a allogroups.

Figure 2: Regional basin history, phase one, somewhere in Belbazem (Berriasian) times. This figure illustrates the main geologicalfeatures which characterise phase one (karsts, mega-breccias, neptunian dykes). Bu Haseer deposits including some mega-brecciaslie above the pre-Kahmah unconformity.


Figure 3: Regional basin history, phase three. This figure illustrates the geometrical relationships between the Shu'aiba sequence,the first Bab sequence, the second Bab LST, and the Sabsab TST. Figure caption: 1- Sabsab deposits, 2- Rudist build-ups, 3- firstBab sequence, 4- Bacinella build-ups.

Table 2: The new allostratigraphic classification in the Abu Dhabi reference wells. All the values included in this table are eitheralready published depth data (A from FOX & BROWN 1968, B from HASSAN et al. 1975, C from ALSHARHAN 1985, 1987, Dfrom AZER & TOLAND 1993) or values derived from correlations on already published well logs. The ADMA well Zakum N° 1which was already selected as the reference section for the offshore Abu Dhabi by T.H. HASSAN et al. [1975] is my firstreference too for the lower part of so-called “Thamama Group” as many logs from this well have already been published [Soniclog from 6,900’ to 9,250’ in A.F. FOX and R.C.C. BROWN, 1968; Neutron log from 7,950’ to 8,250’ and Gamma Ray log from7,950’ to 91,50’ in T.H. HASSAN et al., 1975]. The ADCO well Bab N° 2 will be my second reference as its main logs have alsoalready been published [Gamma Ray and Sonic logs from 7,800' to 11,000' in ALSHARHAN, 1987; Neutron log from 7,900’ to8,100’ in ALSHARHAN, 1987]. Finally, the ADMA well Umm Shaif N° 88 will be my third reference for the upper part as itsmain logs have also already been published [Gamma Ray, Neutron, and Density logs from 5,500’ to 6,000’ in AZER andTOLAND, 1993].

Table 1: The extant lithostratigraphic classification in Abu Dhabi and the neighbouring emirates [HASSAN et al., 1975, modified].

adipec-0918 lower cretaceous stratigraphy of abu … (oman, qatar, etc.). a new stratigraphic...

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