Introduction to the revised Mesozoic stratigraphy and nomenclature for Yemen

Abstract

Until the early 1980s the Mesozoic stratigraphy of Yemen was based on measured surface sections exposed mainly in the faulted and dissected shoulders of the Gulf of Aden and Red Sea Rifts or on the high Yemen Plateau. On the basis of correlations between areas, the broad facies belts, palaeogeographies and tectonic frameworks for the Mesozoic epochs were established. The arealy extensive Tertiary limestone tablel and and Tertiary volcanics, however, masked more local underlying pre-Cenozoic major structural features.Extensive oil exploration drilling backed by regional and/or detailed geophysical surveys (increasingly based on seismic surveys) from the late 1970s, have gradually unraveled a complex Mesozoic tectonic history which had given rise to the formation of a number of NW-SE and E-W trending rift basins and depressions; these were initiated mainly in the Late Jurassic and Early Cretaceous, principally by rejuvenation of ancient basement-controlled fractures during further Gondwanan break-up which led to the separation of India (with eastern Gondwana) from Afro-Arabia. These basins exhibit considerably thicker and rapidly varying stratigraphic sequences which reflect successive phases of rifting, subsidence and depositional settings in time, with the type and provenance of basin-fill helping to detail the developing palaeogeography for each stage. The precise relationships of these varied intra- and inter-basin facies are not yet properly understood.The discovery of commercial oil and gas in several of these Mesozoic rift basins accelerated competitive exploration activity by a host of operating companies leading to the proliferation of informal locally-applicable in-house subsurface lithostratigraphic nomenclature schemes. These, in the extreme cases, either ignore published surface nomenclature, or else utilize existing surface-defined formation names which then become over-loaded with undifferentiated coeval but depositionally and environmentally different units without due regard to internationally accepted rules. This confusion is further compounded by detailed local academic research from within and outside Yemen, with little or no access to subsurface data, and by service companies utilizing limited client-supplied data which is then extrapolated on a province-wide or a regional basis; proliferation takes place through data exchange between operators or through circulation of non-exclusive reports, or by means of localized publications.A Stratigraphic Commission was appointed to address this problem and to carry out nomenclature clean-up and simplification. After intensive review of all utilised nomenclature and with full cooperation from the operating companies, its principal findings are summarised in graphic form (in Figure 4 and Figure 5) which include, for example: retention but revision; correction and expansion of the formalised nomenclature in Beydoun and Greenwood (1968) which will apply to all of Yemen; one new formation, previously unrecognised until recent subsurface and outcrop studies (the Saar Formation) has been added; formalisation of several new members in the Madbi Formation (previously informally assigned formation rank); a new member to the Qishn formation, the Sa'af Member (previously informally called the Furt formation); correction to and reassignment of the Nayfa Formation type section to the Ma'abir Member of the Madbi Formation, the type Nayfa provisionally, now being at Mintaq salt dome. Full descriptions of these and of all the revised Mesozoic nomenclature together with that for the Palaeozoic and Cenozoic will be published later in International Lexicon of Stratigraphy format.