Earliest Phanerozoic or latest Proterozoic fossils from the Arabian Shield, Kingdom of Saudi Arabia

Abstract

We report here the first biologically definable fossils from pre-Saq (pre-Middle Cambrian) rocks of the Arabian Shield. They include the distinctive helically coiled tubular filaments of the oscillatorialean blue-green alga Obruchevella parva as well as two size classes of spheroidal unicells of uncertain affinity. Also present is the conical stromatolite Conophyton and unidentified stromatolites. All occur in cherty limestones of the Jubaylah group, northern Saudi Arabia, a non-marine to locally marine taphrogeosynclinal sequence that fills depressions along the northwest-trending Najd faults. Conophyton has heretofore been found only in strata older than about 680 m.y. (except for puzzling records in modern hot springs) while Obruchevella is so far known only from rocks between about 680 and 470 m.y. old. Thus it appears that the Jubaylah group is close to the Proterozoic-Phanerozoic transition. The simple spheroidal nannofossils are not diagnostic as to age. Their relationships within what appears to be early diagenetic chert suggest a classical algal-mat association. The brecciated and micro-channeled appearance of much of the fossiliferous rock, its locally dolomitic nature, and the prevalence of cryptalgalaminate favor a very shallow, locally turbulent, and perhaps episodically exposed marine or marginal marine setting. The Jubaylah group lies unconformably beneath the Siq Sandstone (basal member of the Saq Sandstone) of medial Cambrian age, rests nonconformably on crystalline basement, and has yielded a K-Ar whole-rock age (on andesitic basalt) of ~540 m.y. To judge from the fossils, however, that age may be as much as 100 m.y. or more too young.