Micropalaeontology and sedimentology of an inter-evaporite Arab-C carbonate exposure, Riyadh, Saudi Arabia

Abstract

Alternating carbonate and evaporite successions of strata characterise the Late Jurassic, Kimmeridgian–Tithonian, Arab Formation of Saudi Arabia. Outcrops of the Arab Formation are poorly exposed and are mostly studied from subsurface samples, due to dissolution of the evaporites and the collapse of intervening carbonate strata. The type section for this formation is in the subsurface, where a more complete succession is available for study where four carbonate and evaporite members, namely D, C, B and A in ascending stratigraphic order, have been identified. The Arab has important hydrocarbon associations, due to its excellent porosity, permeability and proximity to underlying source rocks and overlying evaporite seals. This study provides new insights into the internal palaeo environment and depositional cyclicity of the Arab C, with implications to improve understanding of microfacies and porosity variations of the reservoir equivalent. The studied section provides a rare datapoint for the proximal palaeoenvironment of the Arab C carbonates that are mostly studied in the subsurface far to the east. Arab Formation carbonates are exposed along road cuts in the city of Riyadh. One recent exposure, 27 ft (8.2 m) thick, provides new micropalaeontological, sedimentological and palaeoenvironmental information for this stratigraphic unit that is otherwise elusive as an exposure. The new section is located next to the flanks of a sinkhole within gently folded strata and implies that the exposure must represent one of the supra-Arab D evaporite members, for which regional mapping indicates assignment to the C member of the Arab Formation. The Arab C consists of a succession of low diversity microfossiliferous wackestones and packstones, in which the red alga Permocalculus cf. iranae and rhaxellid sponge spicules are well represented. The exposure extends the stratigraphic range of Bramkampella arabica from the Sulaiy Formation down section into the older Arab C. Arab C microfacies are interpreted to represent deposition in a normal marine salinity, carbonate platform setting, probably within a shelf lagoon, mostly below fair-weather wavebase and within storm wavebase. Within the succession is a distinctive grainstone bed containing the benthonic foraminifera Everticyclammina kelleri, Kurnubia jurassica, Nautiloculina oolithica, Pseudocyclammina lituus and Redmondoides lugeoni and is considered to represent the top of one high frequency sequence (HFS). Immediately above the grainstone additional strata in the measured section form the base of a second HFS.