On Wondersella athersuchi, a new stratigraphically significant hedbergellid foraminiferan from the Cretaceous Shuaiba Formation of the Middle East

Wondersella athersuchi, gen. et sp. nov., is a uniquely streptospiral hedbergellid apparently confined to the latest Aptian in pelagic micritic limestones of the Middle East. This taxon probably characterised highly stratified, oceanic-marginal environments of central, palaeoequatorial Tethys.


INTRODUCTION
Recent micropalaeontological studies of the Lower Cretaceous Thamama Group sediments from the Middle East have revealed several new taxa. This paper deals with one new planktonic genus and species which is stratigraphically restricted to the highest part of the Shuaiba Formation equivalent, and has, to date, been found in cores from the ADMA-OPCO Well Zakum-1 and the ADMA Well Umm Shaif-3, offshore Abu Dhabi (Fig. 1).
The nomenclature of the stratigraphic units adopted for this publication (Fig. 2) are those of Hassan et al. (1975). Supporting data have been taken from extensive proprietary stratigraphic and biostratigraphic studies of the offshore Abu Dhabi area, and datings have been subjected to minor revision as shown on Fig. 2.

STRATIGRAPHY AND MICROPALAEONTOLOGY
The type section of the Shuaiba Formation is the subsurface section of BPC Well Zubair No. 3, South Iraq (Dunnington et al., 1959); the interval 6856 -7084 ft. in the Well Zakum-1 is the reference equivalent section in offshore Abu Dhabi. The upper contact with the Nahr Umr shales (Fig. 2) is sharply defined and is probably unconformable throughout the Umm Shaif-Zakum offshore a r a . The lower limit of the Shuaiba Formation is the contact between basal porous limestones with the argillaceous limestones of the underlying Kharaib Formation. The Shuaiba, which is equivalent to the upper and middle parts of the Thamama Zone I succession of ADMA, can be divided into two members. The upper member comprises biolclastic wackestones together with pyritic shales, micrites and bituminous mudstones, and has a high gamma ray response. The upper member equates with the Bab Member of Hassan et al. (1975). The lower member comprises predominantly chalky foraminifera1 grainstones with a clean gamma ray signature. The Shuaiba thins from over 300 ft. to less than 150 ft. in the central offshore Abu Dhabi area by loss of the highest beds beneath the basal Nahr Umr unconformity. In Zakum-1 the thickness is 238 ft., and most or all of the highest beds appear to be present.
On the basis of the contained foraminifera, regional correlations with ammonite dated sequences and its relative stratigraphic position, the Shuaiba Formation in the offshore Abu Dhabi area may be dated as Aptian to earliest Albian in age, as suggested by Hassan et al. (1975). In Zakum-1, the highest Shuaiba Formation contains Choffatella decipiens (Valanginian-Cenomanian), Buccicrenata hedbergi (Barremian-Albian) .
The new taxon Wondersellu irrhersuchi is confined to a relatively deep water carbonate micrite facies (PI. 4) near the base of the Bab Member (Fig. 2). Its type horizon of occurrence is a bed about 2ft thick, cored at 7036 -7037ft BRT in Zakum-1 (Fig. l), about 9ft above the top of Thamama Zone IA of Hassan et al. (1975, Fig . 3); it also occurs 60km away in the nearby Umm Shaif field (US-3, core at 5857ft BRT), 5ft above the top of Thamama Zone IA, a precisely equivalent horizon.    (Banner & Blow, 1967) whereas in Wondersella it is abrupt, terminating a well developed regular trochospire with its true, deep umbilicus. This taxon is distinguished from Hedbergella at generic level because, if it were not, the amended diagnosis for Hedbergella would become too broad to retain that genus usefully.

Explanation of
The external appearance of typical W . athersuchi is reconstructed in Fig. 3; in other, wholly conspecific forms, the final chambers may be more ventrally directed (extremes shown on P1. 3, figs. 5, 5a, 6). Remarks. The extraordinary shape of the later chambers in W . athemuchi cannot be due to post-deposition-a1 compression and distortion. The sediments in which the assemblages occur, although microbedded, contain abundant tests in all orientations (Pl. 4) and yet it is ; however, in none of these is streptospirality and umbilical closure attained, and the phylogenetic separation of these forms from Wondersella is likely to preclude even ho.mology between the terminal structures. In any case, the significance of sac-like terminal chambers in G. quadrilobatus is itself unknown: attempts to correlate the distribution of G. quadrilobatus f. sacculifer in the Gulf of Elat with seawater temperature, density and nutrition have met with no success (Reiss & Halicz, 1976) and no explanation could be offered for the frequency of individuals with sac-like chambers (Reiss, 1977). The abundance of Wondersella within its stratigraphic interval (representing at least 30,000 years of geological time in the Zakum Section) suggests that it is not an abnormal phenotype of another (unknown) Hedbergella, but an opportunistic incursion of an ecologically (and, therefore, palaeobiogeographically) restricted species. The benthic foraminifera1 fauna of the Wondersella horizon is poor both in numbers and diversity; the presence of pyrite, limonite and fish debris (Pl. 4, fig. 3) also suggests oxygen-deficient bottom waters, confirmed by lack of metazoan debris and bioturbation in the microbedded micrite (Pl. 4). It is probable that the waters were, during this phase of shelf deepening, strongly density-stratified, perhaps due to incursions of highly saline waters into this subtropical gulf and on its deepening carbonate shelf. It may be that Wondersella athersuchi was a species characterising warm, highly saline outer shelf waters of low turbidity in central, equatorial Tethys during Latest Aptian time.