Sequence Stratigraphy of Cambrian Economic Phosphorites and Organic Matter-Rich Shales, Georgina Basin, Australia

Abstract

Sequence stratigraphic relationships in clastic- and carbonate-dominated sedimentary systems are comparatively well known. In these systems maximum flooding surfaces are often characterized by organic matter-rich sediments, glauconite, and phosphatic hardgrounds. This model explains the repeated occurrence of minor amounts of phosphate in the sedimentary record. However, it does not account for the large reserves of sedimentary phosphorite found in phosphate deposits of Cambrian, Ordovician, Permian, Cretaceous, and Miocene age. In Australia, economic phosphorites and their associated organic matter-rich shales, of Middle Cambrian age, accumulated in an intracratonic setting in the Georgina basin. The economic phosphorites occur in the lowermost two stratigraphic sequences of the basin and coincide with a period of major inundation of the continent. In each sequence the phosphorites are restricted to the upper parts of retrogradational parasequence sets of the transgressive system tract. In deeper water, drowned-platform and -ramp settings, micritic limestones, phosphatic hardgrounds, and organic matter-rich shales with benthonic and planktonic faunal components accumulated in dysaerobic environments. Further down the depositional slope and in subsequent parasequences, laminated organic matter-rich shales with TOC contents up to 16%, lacking benthonic but hosting planktonic faunal elements, accumulated in anaerobic conditions. Thus, economically significant phosphate deposits in the Georgina basinmore » are the shallow-water, laterally equivalent facies of the weakly phosphatic and glauconitic dolomitic limestones and organic matter-rich shales, characteristic of the deeper water-condensed section.« less