Depositional model for a prograding oolitic wedge, Upper Jurassic, Iberian basin

Abstract

Facies architecture and bedding patterns of the Kimmeridgian Pozuel Formation (Iberian Basin) evidence that this 50–70-m thick oolitic-grainstone unit conforms to the Infralittoral Prograding Wedge (ILPW) model instead of the classic models used for interpreting oolitic grainstones sandbodies on carbonate ramps or platforms (i.e., bank-margin shoal complexes, beaches and beach ridges).Ten lithofacies have been distinguished in the Pozuel Formation: 5–10° dipping clinobedded oolitic grainstone foresets passing to tabular oolitic packstones-grainstones, which interfinger the muddy basinal bottomsets. Landwards, the clinobeds pass into subhorizontal topsets composed of trough cross-bedded to structureless oolitic grainstones; oolitic-skeletal grainstones with stromatoporoids and coral-stromatoporoid-microbial mounds. Siliciclastic lithofacies and oncolitic/peloidal packstones occur at the innermost position. These lithofacies stack in strike elongated, 5–20-m thick, 0,5–2 km dip-oriented wide, aggradational-progradational packages with complex sigmoid-oblique geometries.Lithofacies, depositional geometries and stacking pattern permit to summarize the main characteristic of such Upper Jurassic oolitic infralittoral prograding wedge potentially to be applied in other oolitic sandbodies both in outcrops and subsurface: 1) sediment production within the wave action zone, 2) grainstone-dominated textures, 3) prograding basinward onto basinal muds, 4) laterally (strike) extensive, paralleling the shoreline, 5) variable thickness, commonly of few tens of meters, 6) broadly sigmoidal to oblique internal architecture, with topsets, foresets and bottomsets, 7) dip of foresets close to the angle of repose, 8) topsets deposited in shallow-water, extending through the shoreface, from the shoreline down to the wave base, 9) mounds, either microbial or skeletal, may occur in the topsets.The coated-grains factory was along the high-energy, wave-dominated outer platform (topset beds), from where the mud was winnowed and the grains transported both landward to the platform interior, and seaward to the platform edge, from were the grains cascaded down the slopes as grain flows and mass flows, forming clinobeds. This genetic model can be applied to other grain-dominated lithosomes, some of them forming hydrocarbon reservoirs, e.g., the Jurassic Hanifa Formation and some Arab-D (e.g., Qatif Field) in Arabia, the Smackover Formation in northern Louisiana and south Arkansas, the Aptian Shuaiba Formation (e.g., Bu Hasa Field) and the Cenomanian Mishrif Formation (e.g., Umm Adalkh Field) of the Arabian Gulf.