The Wadi ad Dawasir “delta”, central Saudi Arabia: A relative sea-level fall of Early Bathonian age

Abstract

ABSTRACT The fluvial and deltaic deposits formerly described as the Wadi ad Dawasir “delta” have been reinterpreted on the basis of a new examination of biostratigraphic data, notably ammonites and brachiopods studied by the authors and nautilids studied by the late H. Tintant (the term “delta” as used in other documents is shown here in quotes because of insufficient evidence to confirm it as a true delta). Direct dating of the fluvial and deltaic deposits is not possible in the absence of age-indicative fossils, but the entire “delta” complex can be placed in the Lower Bathonian on the basis of Bathonian ammonite and brachiopod faunas found in underlying strata, and a Lower Bathonian nautilid found in the uppermost proximal beds. An Early Bathonian age of the “delta” revises the Middle Callovian age assumed by previous authors based on their admittedly lithostratigraphic correlation with marine strata of the Dhruma Formation. The Early Bathonian age is consistent with the alternative interpretation, which correlates the “delta” with the discontinuity in the marine sequence between units D5 and D6 of the Dhruma Formation, with a stratigraphic gap of Lower (in part) and Middle Bathonian. The nautilids and brachiopods from above and “apparently at the front” of the “delta” together with the ammonites, nautilids, brachiopods and calcareous nannoplankton from units D6 and D7, indicate a complete sequence from the Upper Bathonian (unit D6) to Upper Callovian (units T2 and T3 of the Tuwaiq Mountain Limestone) with transgressive overlap of the “delta” during Middle and Late Callovian. The Wadi ad Dawasir “delta” is here interpreted as resulting from a limited relative sea-level fall, causing progradation of a shelf deltaic wedge over the downlap surface associated with the D5–D6 discontinuity. Similar deltaic or/and paralic deposits have been described in the Lower Bathonian of northern Sinai, presumably of the same age, Negev and in the Bathonian of southern Tunisia, which indicate an event of wide extent (eustatic, climatic or tectonic) along the northern Gondwanan margin.