Ostracod faunas from the Halul, Laffan and Nahr Umr Formations of offshore Abu Dhabi, U.A.E.

The Halul and Laffan (Coniacian), and Nahr Umr (Albian) Formations of offshore Abu Dhabi are dated using palynomorphs, foraminifera and nannofossils and their ostracod faunas are illustrated and compared with similar assemblages in the Middle East.


INTRODUCTION
This is the first occasion on which the Cretaceous ostracod faunas of Abu Dhabi have been illustrated, despite their use for many years as biostratigraphic indices in the S. Arabian Gulf area and despite numerous references to them in unpublished oil company reports. Ostracods with regional stratigraphic value are found at three levels in the Cretaceous of offshore Abu Dhabi (see Fig. 1 for location area). The Halul, Laffan and Nahr Umr Formations each yield distinctive ostracod faunas which compare well with time equivalent assemblages in other parts of the Middle East. The age significance of each of these faunas in Abu Dhabi has been determined by calibration of their occurrences against nannofossil, palynomorph and foraminifera1 distributions (see Fig. 2).        The faunas of the Halul and Laffan Formations illustrated herein are best compared with those described by Grosdidier (1973) from Iran. The Nahr Umr faunas are more widely distributed along the mid-Cretaceous southern Tethyan shelf and are known from many N. African, Levantine and Middle Eastern localities. The distribution of these faunas is reviewed by Athersuch (1985, in press).
Plates 1-4 depict most of the ostracods recovered from the three formations in offshore Abu Dhabi. None of the species have been formally described, and previously undescribed species (prefixed ABU) have been left in open nomenclature as, due to the proprietary nature of the material, the exact provenance of individual specimens cannot be revealed. For the Halul and Laffan Formations, most specimens were recovered from core samples, a few from ditch cuttings. The Nahr Umr material comprised ditch, cuttings alone.

The Halul Formation
The Halul Formation has been dated as Coniacian in age throughout the region on the basis of the planktonic foraminifera Whiteinella archaeocretacea, Marginotruncana renzi and Dicarinella imbricata and the dinoflagellate cysts Xenascus ceratioides, Corongera oceanica, Hystrichosphaera pulchrum and Canningia reticulata. The lower part of this formation, which is developed in a marine 'reefal' lime-mud facies, is characterised by the ostracoda Brachycythere spp. (including B . IRJlO Grosdidier) and "Mehesella" spp. (including 'M'. IRC23 Grosdidier). The former species has a Coniacian age range in Iran, but 'M' sp. IRC23 is apparently restricted to the Santonian there (Grosdidier, 1973).

The Laffan Formation
The Laffan Formation is dated as Coniacian by the occurrence of the dinoflagellate cyst Cannosphaeropsis utinensis and by the nannofossil Micula staurophora. Shallow marine shales within the formation yielded large and diverse assemblages of ostracods. Over 30 species were recorded in the offshore Abu Dhabi area. Many of these are previously undescribed forms, but some are known from the Coniacian Laffan Formation of Iran (Grosdidier, 1973 Grosdidier. In addition, Cytherella posterosulcata Al-Abdul-Razzaq , previo us1 y recorded from the Santonian and Coniacian throughout the Middle East (Athersuch, 1985) was also recovered.
There are insufficient data to determine precisely the palaeoenvironmental significance of most of the species recovered from the Laffan Formation. However, it is presumed that the great variation in faunas from one locality to another observed in the offshore Abu Dhabi area reflects facies control. In this respect it is

Explanation of Plate 3
All specimens are x75     interesting to note that most of the 'indeterminate' taxa recorded herein are from the flanks of a basin which existed in the offshore area during Laffan times. This suggests that endemic faunas developed in marginal marine areas at this time. In basinal areas, taxa with regional affinities were more frequently recovered.

Nahr Umr Formation
The transgressive marine shales of the Nahr Umr are dated as Albian in the offshore Abu Dhabi area on the basis of the nannofossils Prediscosphaera cretacea, Parhabdolithus achlyostaurion and the palynomorphs Subtilisphaera cheit, Xiphophoridium alatum, Dinopterygiurn cladoides. The occurrence of the planktonic foraminifer Planomalina buxtorfi (latest Albian) in the overlying Mauddud equivalent also supports an age no younger than Albian for this formation. This age assignment is consistent with the age previously determined for this formation in other parts of the Middle East.

ACKNOWLEDGEMENTS
I would like to thank the Ministry of Petroleum, Abu Dhabi and the management of ADMA-OPCO, Abu Dhabi for permission to publish this paper. I acknowledge with gratitude the British Petroleum Co. p.1.c. for providing financial and logistical support.