Zarqa Revisited: Ice Advance Megafacies and Its Significance in Relation to the Sequence Stratigraphy of the Late Ordovician Glaciogenic Succession in Saudi Arabia

J. Melvin¹

Abstract

During the outcrop mapping exercise carried out by BRGM in Saudi Arabia in the 1980s and 1990s, the uppermost Ordovician sediments in the Kingdom were assigned to the Zarqa Formation and the overlying Sarah Formation. The Zarqa Formation was subsequently discarded as a formal lithostratigraphic unit upon the realization that in many areas it occurred not only below but also within the Sarah Formation. It was thenceforth considered only to represent a major lithofacies within the redefined Sarah Formation and consequently has since tended to be ignored almost completely by stratigraphers working the Lower Paleozoic. There is a real need nonetheless to characterize the Zarqa facies and to recognize its sequence stratigraphic importance in the context of the Late Ordovician glaciation in Saudi Arabia. At outcrop, the Zarqa facies is a grey-green, silty very fine-grained sandstone within which commonly occur pebble to boulder-size clasts of older lower Paleozoic sedimentary rocks, as well as granitic and other basement lithologies deriving from the Arabian Shield. Some of the latter are heavily striated. The Zarqa facies generally displays a strongly faulted and sheared fabric. It commonly contains mounded piles, several meters in lateral extent and up to a few meters thick, of randomly-oriented, deformed slabs of bedrock (including various pre-existing depositional facies of the Sarah Formation as it was originally defined). Subsurface core material demonstrates that Sarah Formation sandstones and diamictites commonly sit upon a heavily deformed substrate which, although palynologically assignable to earlier-deposited formations, is considered here to be Zarqa facies of the Sarah Formation. The characteristics of the Zarqa facies strongly suggest that it represents an ice-contact, pro-glacial to subglacial setting. As such it represents conditions of glacial advance or re-advance (i.e. lowered sea level) and can therefore be usefully applied in considering the sequence stratigraphy of the Late Ordovician glaciogenic succession in Saudi Arabia.

AAPG Search and Discovery Article #90188 ©GEO-2014, 11th Middle East Geosciences Conference and Exhibition, 10-12 March 2014, Manama, Bahrain