Abu-Jaber N S; Jawad Ali A; Shinaq R

4 (3/4), 381-393
The Upper Cretaceous Amman Formation and its stratigraphic equivalents are exposed in Jordan and adjacent areas. The formation consists of two subdivisions: a lower silicified limestone unit (30-100 m thick) and an upper phosphorite unit. The silicified limestone was deposited on a shallow carbonate platform along the fringes of the Tethyan seaway. Previous basinward studies have postulated a sill separating the open marine environment from the depositional site of the silicified limestone. If deposition occurred behind a sill, it must have been leaky because open marine waters are indicated by the faunal assemblage. The silica responsible for chertification of these and similar deposits has previously been attributed to four potential sources: silica-rich lakes, marine upwelling, fluvial input, and exhalation of subsurface fluids. The possibility of lacustrine sedimentation is negated by the great mass of silica present in this region (˜10¹⁹g SiO2), by the great regional extent of these deposits, by the lack of silica-rich rocks in the surrounding areas at the time of deposition, and by the dominance of marine fossils. The estimated average rate of silica deposition during the time of formation of these deposits (˜10¹²g SiO2/year) is comparable to the modern supply of silica by upwelling along the Chilean- Peruvian continental margin and to the fluvial input in modern Amazon subaqueous delta However, there is no sedimentologic or stratigraphic evidence of an Amazon-scale river having been present at the time of chert formation. Previous studies of the faunal assemblages within this unit reveal low diversity, presumably reflecting stressed conditions. Such stress is uncharacteristic of modern marine upwelling regimes. Moreover, the average ratio of chlorine/bromine in the chert is 30% smaller than the Cl/Br ratio in normal seawater. REE data reveal a slightly negative Ce anomaly which probably reflects the imprint left by carbonate and phosphate protoliths. Given this and previously published isotopic and tectonic data, the possibility of a deep-seated source of silicifying fluids merits consideration.