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

SOUTHGATE, PETER N., and JOHN H. SHERGOLD, Bureau of Mineral Resources, Canberra, Australia

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. Both transgressive systems tracts are characterized by a systematic lateral and vertical shift in facies. During the initial stages of onlap parasequences composed of economically significant phosphorites and phosphatic limestones accumulated in near-shore peritidal, lagoonal, and locally restricted shallow-water environments 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 basin are the shallow-water, laterally equivalent facies of the weakly phosphatic and glauconitic dolomitic limestones and organic matter-rich shales, characterisitic of the deeper water-condensed section.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)