FURTHER RECORDS ON
SUBSURFACE BIOSTRATIGRAPHY OF
WESTERN IRAQI DESERT
(BLOCK 7)

The subsurface biostratigraphy of a part of the Western Iraqi Desert (Block 7) is described in the 1978, 1979 and 1981 Records. An addition to the mentioned Records is presented here - a short review of relatively shallow boreholes B 7-7, B 7-10, B 2-7, B 4-7, B 15-7, B 18-7, B 19-7 and Key Hole-17/7, previously unreported and drilled under the same Project . The new data contribute to the clarification of regional stratigraphy and the relationship of respective formations.

An earlier version of this paper was published in 1986 under the "RADOVI Geoinstituta, Beograd, knjiga 19, p. 123-132, 4 pl. (I-IV)".

This new English version has been revised by H. Stewart Edgell.

BIOSTRATIGRAPHIC REVIEW OF BOREHOLES

Key Hole-17/7

Location:   Hasaiba, 8.8 km west of pumping station T-1; total depth drilled: 707.6 m.

More than 120 metres were drilled in the Euphrates Formation. Foraminiferal grainstone and packstone from 129 to 170 metres are close correlative with the Oligocene Anah Formation (peneroplids, operculinids, different miliolids, etc.)

The 170-468 m depth interval of nummulite occurrence consists of various limestones: chalky, marly, more or less dolomitized and recrystallized, shaley, locally sandy and glauconitic. Gray compact silty marly limestones and calcareous shales occur for the most part between 220-240 metres and 407-437 metres. They contain Frondicularia, Nodosaria, other lagenids, minute rotalids, and planktonic assemblage: globigerinids and rare Globorotalia .

Planktonic foraminifers (x100); 246 m, Cuisian ?, KH-17/7

Fine grained and sandy calcarenite (oil saturated), marly limestones, calcareous shales and dolomite between 468 and 550.7 metres contain rotalids, miliolids, other benthic and planktonic foraminifers. In the 496th metre Frondicularia sp. (aff. nakkadyi Futyan) , Nodosaria, globigerinids and Globorotalia are present .

Frondicularia sp. (aff. nakkadyi Futyan), (cca 40x), 496 m, KH-17/7

Globorotalia sp. (x90); 496 m, KH-17/7

Frondicularia sp. (x90); 496 m, KH-17/7

In the sequence of 550.7-558.5 m foraminiferal - dasycladacean grainstone, packstone and dolomitized limestone occur rich with Alveolina (Glomalveolina), Rotalia, Lokhartia, other rotalids, miliolids, etc. .

To the depth of 669 metres foraminiferal - dasycladacean grainstone, wackestone and packstone are found (rotalids, miliolids, different dasyclads, metazoan debris).

Rotalids (x60), Glomalveolina beds; 558.5 m, Middle Palaeocene, KH-17/7

Rotalids (x80), Glomalveolina beds; 558.5 m, Middle Palaeocene, KH-17/7

The lowermost interval, 669-707.6 m, consists of compact fine grained phosphatic sandy limestone, mainly phosphatic sandstone, sandy argillaceous sediments with globigerinoidal assemblages and some benthic foraminifers. Between 705 and 707.6 m: globotruncanids (ex Glt. gr. ventricosa and other forms), Globigerinoides?, Rugoglobigerina, large globigerinoidal and other forms (? Ruseifaella Hamam), small anomalinids, Omphalocyclus fragments   .

Undetermined foraminifer (?Ruseifaella Hamam) (x150), 707 m, Maastrichtian, KH-17/7

Globotruncana spp. (x200); 705 m, Maastrichtian,  KH-17/7

Globotruncana ex gr. ventricosa (x200); 705 m, Maastrichtian,  KH-17/7

Stratigraphic comment. Nummulites at 181st metre may be Lutetian, according to Pavlovec (1977), or somewhat older. This may be part of the third Dammam Formation unit (Chabd - Shawiya - Huweimi nummulitic) which should be underlain by two older Dammam units of Cuisian age (or their equivalents). Nummulites from the 371st metre are not determined, while those between 399th and 406th metres are leached out (only nummulitic ghosts are left or voids filled by sparry calcite and gypsum ). Nummulites from 441st metre are supposed by Pavlovec to be most likely Upper Ilerdian. The oldest nummulites in KH-17/7 within the 468th metre are indeterminable.

External moulds and nummulitic ghosts filled by sparry calcite and gypsum (x40); 405.5 m, Uppermost Ilerdian or Lower Cuisian, KH-17/7

Globorotalia specimens in the thin section from 496th metre are close correlative with those from the pusilla zone of the Middle Palaeocene (Luterbacher, 1964, Fig. 127).

Marker beds with Glomalveolina (primaeva zone) are of the Thanetian age (Hottinger, 1960, table 1).

Foraminiferal - dasycladacean member of the column, between beds with Glomalveolina and sandy phosphatic sequence with planktonic foraminifers, is accordingly equivalent to parts of the Middle and the Lower Palaeocene.

For sandy - phosphatic - argillaceous sediments between 669 and 707.6 m only two facts can be given: The globigerinal assemblage from the top of the interval indicates the Palaeocene, and globotruncanids in 705th and 707th metres the Maastrichtian age. The bottom of the sequence is probably in the Digma facies due to its phosphatic character. The Palaeocene part of the same sequence may be provisionally termed as the phosphatic facies of the Aaliji Formation.

The Palaeocene - Lower Eocene series in Key Hole-17/7, i.e. in the north-east belt of the Block, can not be termed the Umm Er Radhuma Formation. It includes elements of the Aaliji Formation and its phosphatic facies, and some shallow-water sediments never known within the Umm Er Radhuma (nummulitic limestone). That is way that the introduction of a new formation seems justified.

Wadi Well B 7-7

Location: Wadi Ghadaf, about 37 km west from Tel Bardan village; total depth drilled: 150 m.

Ž. Lešević infers from the visual core inspection that 95 metres of the column belong to the Euphrates Formation. The Dammam Formation in this interpretation covers chalky and sandy limestones between 95th and 120th metres. To TD the borehole was drilled through nummulitic and chalky limestones. In the 120th metre, Pavlovec recognized Cuisian species. For nummulites which occur below this level Pavlovec assumes an Upper Ilerdian age (emphasizing that species in the 147th metre are identical to those in 441st metre of the KH-17/7). Besides nummulites, the sample from 138th metre contains other foraminifers, dasyclads (? Cylindroporella ), crinoidal skeleton and microgastropods.

Unknown dasyclad (? Cylindroporella) (x75) in the Ilerdian limestone with nummulites; 138 m, well B 7-7

Structural Hole B 7-1O

Location: Wadi Ghadaf, about 40 km north from Qasra village; total depth drilled: 200 m.

The uppermost part of the borehole to 106 m belongs to the Dammam Formation . In the interval 106-137 m, very fossiliferous limestone in alternation with marly and chalky limestone occurs, which is closely similar to those from the Palaeocene (). Fossils are not noted in sandy and dolomitic limestones between the 137th and 152nd  metres. At the end of the borehole (152-200 m), dolomitized and recrystallized limestones with orbitoid - Omphalocyclus assemblage are found (Harta Formation).

Porous Lower Palaeocene limestone (x22,5); 106 m, well B 7-10

Productive Well B 2-7

Location: Tel Rufha, 25 km north from Wadi Ubaiyth; total depth drilled: 441,7 m.

The well is located in sediments of the Dammam Formation, about 50 metres of the uppermost column part is attributed to it. Nummulites between 3rd and 24th metres are not determined (Cuisian??, or, probably Lutetian species: Pavlovec, 1977).

The interval from the 62rd to 276th metres is close correlative with foraminiferal - dasycladacean member in other boreholes (probably Lower - Middle Palaeocene).

Dolomitized grainstone - packstone, marly limestone and calcareous shales between the 276th and 326th metres contain pelecypods, benthic (Frondicularia, Nodosaria, rotalids) and some planktonic foraminifers. In the 311st metre rare Globigerinoides (?) (), Rugoglobigerina, and one peculiar form (globotruncanid?) shown on are found. The attribution of the lowermost part of this sequence to the Cretaceous should not be ruled out.

The 326-368 m depth interval consists of dark gray silty marly limestones and calcareous shale. In the 358.8 metre, very rare planktonic foraminifers - Gansserina gansseri (Bolli) and some bulloid forms () are found.

Alveolina (Glomalveolina) sp. (x60); 59 m, well B 2-7, Middle Palaeocene

Planktonic foraminifers (x130); 311 m, well B 2-7, Lowermost Palaeocene?

Planktonic foraminifers (x130); 311 m, well B 2-7, Lowermost Palaeocene?

Bulloid Globotruncana cf. Gl. fornicata cesarensis Gandolfi (x200); 394 m, well B 2-7, Maastrichtian

In the lower part of borehole (368-441.7 m) orbitoid bearing sediments occur (dolomitized and recrystallized slightly porous limestones, marly and chalky limestones). In thin sections from the 394th to 417th metres the following fossils were found:

• Clypeorbis mamillata (Schlumberger)

• Lepidorbitoides socialis (Leymerie)

• Pseudorbitoides ?

• Sulcoperculina ex gr. dickersoni (Palmer)

• Marsonella trochus (d’Orbigny)

• Frondicularia sp.

• rare bulloid Globotruncana

• globigerinoids

• some heterohelicids

• Stomiosphaera

• crinoidal skeleton, etc. ( ).

The foraminiferal assemblage in more than 70 lowermost metres of the B 2-7 indicates a Maastrichtian age. The borehole is located in the Unstable Shelf area (Henson, 1951, tab. 1) characterized by interfingering of the Harta Formation and the Shiranish Formation . I recognized the lower division (368-441.7 m) as the Harta Formation where the open sea influence is evident, while the upper division (326-368 m) is a tongue of the basinal Shiranish Formation overlying the Harta.

Clypeorbis mamillata (Schlumberger) x40); 394 m, well B 2-7, Maastrichtian

Clypeorbis mamillata (Schlumberger) x40); 394 m, well B 2-7, Maastrichtian

Structural Hole B 4-7

Location: 55 km west from Kussaiba, between Wadi Hauran and Rutbah - Ramadi road; total depth drilled: 263 m.

The uppermost part of the column - foraminiferal limestone - seems to be of Oligocene age. Nummulites, same as orbitoids, are not noted in core . Noteworthy is a finding from the lower mostly dolomitized sequence of the borehole. An Aptian assemblage is recognized in the less dolomitized portion of rocks between the 212nd and218th metres:

• Salpingoporella dinarica Radoičić

• Cylindroporella sp.

• other dasyclad fragments,

• Debarina hahounerensis Fourcade, Raoult & Vila

• Cuneolina laurentii Sartoni & Crescenti

• Pseudotextulariella sp.

• Nezzazata sp.

• Trochammina sp.

• Nautiloculina

• poorly preserved orbitolinids

• other minute foraminifers, etc.

Discussion. It should be remembered that facially identical shallow­water lagoonal Aptian sediments are know from the KH-7/7. Aptian very dolomitized limestones occur in the central Block 7 area marked on the “Late Berriasian – Aptian” facies distribution map as “area of erosion or non-deposition”, west of clastic belt (Buday, 1980, p. 390, Fig. 12). Data from KH-7/7 and B 4-7 (the presence of neritic Shu’aiba Formation ) greatly modify our knowledge of the facies distribution in the Western Iraqi Desert area during the Aptian time.

Wadi Well B 15-7

Location: North from T-1 pipeline, 30 km NE from Adnan Experimental Farm; total depth drilled: 300 m.

Limestone between 59.5 and 76 m look like Oligocene. Nummulites and operculinids occur through the sequence of recrystallized limestone to 132nd metre, but are not determined (Dammam Formation, according to Lešević).

Upper Senonian porous recrystallized orbitoid bearing sediments are found from the 278th metre to the bottom of borehole (Harta Formation).

Productive Well B 18-7

Location: As Sagra, 40 km northeast from Nukhaib Police Station; total depth drilled: 374 m.

Sediments to the depth of 21st metre are of the Dammam Formation, and from the 21st to 203rd metres of the Umm Er Radhuma Formation .

Limestone at 205-206 m contains indeterminable rudists. Downward, to the depth of 357 m, the borehole was drilled in completely or partly dolomitized and very porous orbitoid limestone, in which orbitoids are leached out (Harta Formation). In the next twenty metres of dolomitized limestone fossils are not noted.

The interval 357-374 m consists of foraminiferal grainstone, packstone and wackestone, partly dolomitized and recrystallized. At the 370th metre, a relatively abundant foraminiferal fauna consists of Tetraminouxia, Cuneolina, Raadshoowenia (), Nezzazata, Trochammina, cf. Biconcava, nubecularids, miliolids and other foraminifers (very fine rudistid debris are also present). This interval is most probably of the Campanian age (equivalent to part of the Sa’di Formation).

Raadshoowenia sp. (x20); 370 m, well B 18-7, Campanian

Structural Hole B 19-7

Location: Close to village of Habbariya, north from Nukhaib Police Station; total depth drilled: 348 m.

No indicative data has been collected in the upper part of the borehole to 247th metre (Palaeocene?).

In the 247th metre very recrystallized indeterminable rudists are found. Downward, to 299th metre, the hole is drilled through orbitoid bearing sediments (mainly external mould) which, in Block 7 area, are attributed to Harta and/or Tayarat Formation .

The lowermost depth interval (299-348 m) consists of dolomitized and recrystallized grainstone - wackestone. Locally, fine rudistid fragments and poorly preserved foraminifers are noted. Assemblage in the thin section from 326.2 m is composed of benthic foraminifers Minouxia (Tetraminouxia ?), Nezzata, Moncharmontia, Raadshoowenia, cf. Biconcava, nubecularids, Pyrgo, and other miliolids, then Thaumatoporella and fine mollusca debris (Campanian - ? Santonian).

Discussion. It is evident that the lowermost interval of the column in short-spaced holes B 18-7 and B 19-7 includes facially identical Campanian (or Campanian - ? Santonian) foraminiferal fauna of a protected shelf area. In the southern Iraq, the widely distributed Sa’di Formation is mentioned to be underlying the Harta. Campanian sediments in boreholes B 18-7 and B 19-7, however, are facially differing from the type Sa’di , but they must be a part or the equivalent to a part of it.

According to Darmoian (fide Buday, 1980, p. 182) the Harta Formation is Upper Campanian - Lower Maastrichtian and the Tayarat Formation is Upper Maastrichtian. Buday, however, gives the Harta Formation a broader sense and takes the Tayarat as a facies of the Harta. Buday’s interpretation seems to be more acceptable. Tayarat is a Harta's tongue that made the greatest progress in the Maastrichtian time on the Rutbah Uplift. This facies is not recognized in Block 7 boreholes. A characteristic of the Tayarat Formation is, among others, the presence of Loftusia, which are absent in series of Block 7 (Loftusia almost always occur in the belt of terrigenous influx).

Orbitoid beds of the Harta are identified all over area (missing in boreholes B 7-12 and B 4-7 due to erosion). The problem concerning the precise age of the orbitoid sequence has been discussed earlier (Radoičić, 1979, pp. 103-105). Actually, this is primarily the problem of the lower Harta boundary to which Buday (1980, p. 184) refers as often being unclearly defined. Orbitoid bearing limestones have the greatest thickness in the boundary belt Stable/Unstable Shelf (in the B 18-7: 130 metres; in the KH-7/7: 120 metres). Outside this line, toward the basinal Shiranish area, the effect of the open sea environment is notable.

Remarks. - While subsurface series in Block 7 boreholes were not studied in detail (see: 1981, p. 115), the relatively small number of analysed selective samples gave more than satisfactory result. Complete sets of drill-cores are deposited with the Station at 167 km on the Ratawi - Rutbah road. If preserved, certain intervals of considered boreholes should be examined more thoroughly.

Series with interfingering neritic and pelagic facies are very indicative of the stratigraphy. This is the case with the more than 500 metres thick Paleogene series of the KH-17/7, and a detailed study of KH-17/7 cores should be very important (the more so that the type section of a new formation may be defined).

REFERENCES

Bellen, R.V.C. van, Dunnington, H.V., Wetzel, R. and Morton, D.M. - 1959: Lexique stratigraphique international, Asia, Irak; Inter. Geol. Cong., Comm. Str., Vol. 3, pt. 10, Paris.

Buday, T. - 1980: The Regional Geology of Iraq. Stratigraphy - Paleogeography, Vol. 1, Baghdad.

Futyan, A.I. - 1976: Late Mesozoic and early Cainozoic benthic Foraminifera from Jordan; Palaeont., Vol. 19, 3, London.

Hamam, K.A. - 1978: Ruseifaella jordanensis n. gen. n. sp. (Foraminifera) du Maestrichtien de Jordanie; Rev. Micropal., Vol. 21., 1, Paris.

Henson, F.R.S. - 1951: Observation on the Geology and Petroleum Occurrences of the Middle East; 3rd World Petr. Congress, Proc. Sect. 1, Hague.

Hottinger, L. - 1960: Über paleocene und eocene Alveolinen; Ecl. Geol. Helv., Vol. 53/1, Basel.

Luterbacher, H. - 1964: Studies in some Globorotalia from the Paleocene and Lower Eocene of the Central Apennines; Ecl. Geol. Helv., Vol. 57/2, Basal.

Naqib, K.L. Al. - 1967: Geology of the Arabian Peninsula. Southwestern Iraq; Geol. Surv. Prof, paper, 560-G, Washington.

0wen, R.M.S. and Nasr, S.N. - 1958: The stratigraphy of the Kuwait - Basra area. Habitat of oil, a Symposium, Am. Ass. Petr. Geol., Tulsa.

Pavlovec, A. - 1977: Poročilo o preiskavi numulitin iz Iraka. Unpublished Report, Ljubljana.

Radoičić, R. - 1978: Clypeina ? alrawii, n. sp., a Dasycladacea from the Upper Cretaceous of Iraq; Bull. Acad. serbe Sci. et Arts, T. 61, Cl. de Sci. math. et nat., 17, Beograd.

Radoičić, R. - 1979: Salpingoporella ubaiydhi, a new Senonian Dasyclad and some data about subsurface biostratigraphy of Western Iraqi Desert; Bull. Acad. serbe Sci. et Arts, T. 68, Cl. de Sci. math. et nat., 19, Beograd.

Radoičić, R. - 1981: Some new data about biostratigraphy of the Western Iraqi Desert (Block 7); Bull. Acad. serbe Sci. et Arts, T. 75, Cl. de Sci. math. et nat., 21, Beograd.

 © Rajka Radoičić