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Heterozoan, Biosiliceous and Organic-Rich Deposits of Jurassic (Oxfordian) Hanifa Formation, Saudi Arabia


The Middle East includes some of the world's most prolific source rocks and carbonate hydrocarbon reservoirs whose character is controlled by a range of variables. The purpose of this study is to describe and interpret the sedimentology and total organic content (TOC) of an illustrative outcrop of the Late Jurassic Hanifa Formation in Saudi Arabia, a unit correlative to nearby prolific subsurface source and reservoir strata. The 109-m thick Hanifa section includes nine lithofacies and four lithofacies associations, generally interpreted to represent deposition on a carbonate ramp within the Arabian intrashelf basin. The bottom third of the section includes an abundance of biosiliceous sediment (mainly sponge spicules, which can constitute up to 50% of the grains) and early silicification; these strata also include a dominantly heterozoan association (crinoids, echinoderms). Strata higher in the section include progressively decreasing abundance of biosiliceous sediment, and common photozoan organisms, but elevated TOC (up to several %). Occurring in an equatorial setting, the biosiliceous sediment, dominantly heterozoan association, and elevated TOC implicate conditions other than warm, shallow, oligotrophic settings. These patterns instead are interpreted to reflect the influence of cooler and nutrient-rich water, upwelled by monsoonal upwelling, equatorial upwelling, or both. This upwelled water drove the radiolite deposition flanking the Tethyan continental shelf, and subsequently moved westward into intrashelf basins and adjacent shelves. Thus, although the area lay near the paleo-equator, the sedimentologic patterns are distinctly non-tropical, except in the uppermost part of the succession. These regional processes may explain organic-rich source rocks in nearby subsurface Hanifa Formation deposits, and likely have analogs in other paleo-equatorial systems.