Formation and stratigraphic facies distribution of early Jurassic iron oolite deposits from west central Nevada, USA

Abstract

The stratigraphic context and geochemical proxy potential are described for a newly reported early Jurassic marine iron ooid deposit in west central Nevada, U.S.A. Sedimentary analysis of the Ferguson Hill Member in the Sinemurian-aged Sunrise Formation was conducted in the Gabbs Valley Range of Nevada, U.S.A. Iron ooids were examined by transmitted light microscopy and electron scanning microscopy and found to consist of chamosite/berthierine and amorphous silica in cortical layers of the Sunrise Formation. Sedimentological characteristics of the uppermost beds of the Ferguson Hill Member show iron ooids are associated with transgressive deposits on the earliest-known first post-extinction metazoan carbonate factory identified in Eastern Panthalassa. The Ferguson Hill Member iron ooids are unique among most iron ooid deposits for being distributed grains within the carbonate. The formation of such a deposit required concentrations of iron and silica and fluctuating oxidation/reduction conditions sufficient to produce chamosite/berthierine and silica while not completely shutting down the shallow marine carbonate factory. For this reason, iron ooids observed in shallow carbonate sections serve as a useful indicator of shallow water geochemical conditions in the sedimentary record.