Impact of upwelling on heterozoan, biosiliceous, and organic-rich deposits: Jurassic (oxfordian) Hanifa Formation, Saudi Arabia

The Middle East contains some of the world's most prolific source rocks and carbonate hydrocarbon reservoirs. Deposition of these strata were controlled by a range of local, regional, and global geologic processes. To better understand part of one Middle East hydrocarbon system, this study describes and interprets the sedimentology and total organic content (TOC) of an illustrative outcrop of the Upper 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, grouped into four lithofacies associations, interpreted to represent deposition on a carbonate ramp flanking the Arabian intrashelf basin. The bottom third of the section includes abundant biosiliceous deposits (mainly sponge spicules, which can constitute up to 50% of the grains) and early silicification; these strata also include a dominantly heterozoan grain association (crinoids, echinoderms). Strata higher in the section include a progressive decrease in abundance of biosiliceous grains and increase in abundance of photozoanassemblage organisms, but include elevated TOC (up to several %). Although the bottom third of the section reflects deposition in a paleo-equatorial setting, these strata, dominated by a biosiliceous and heterozoan association and with elevated TOC, are interpreted to indicate conditions other than warm, shallow, oligotrophic conditions. Instead, these patterns are interpreted to reflect the influence of cooler and nutrientrich water ultimately derived from monsoonal upwelling, equatorial upwelling, or both. In this conceptual model, upwelled water drives radiolite deposition off the Tethyan continental shelf, but subsequently the upwelled water moves westward into the Arabian intrashelf basins and onto adjacent shelves. These regional processes may explain organic-rich source rocks in nearby subsurface Hanifa Formation equivalents, and likely have analogs in other paleo-equatorial systems.