S.M. Awramik Karen Morrison D.G. Hadley Preston Cloud 1979 <p>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 <i>Obruchevella parva</i> as well as two size classes of spheroidal unicells of uncertain affinity. Also present is the conical stromatolite <i>Conophyton</i> 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.</p><p><i>Conophyton</i> has heretofore been found only in strata older than about 680 m.y. (except for puzzling records in modern hot springs) while <i>Obruchevella</i> 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.</p><p>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.</p><p><br data-mce-bogus="1"></p> application/pdf 10.3133/ofr791186 en U.S. Geological Survey Earliest Phanerozoic or latest Proterozoic fossils from the Arabian Shield, Kingdom of Saudi Arabia reports