A model for the development of phosphatic and calcareous lithofacies in the Middle Cambrian Thorntonia Limestone, northeast Georgina Basin, Australia

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Abstract
Chemical sediments of the Thorntonia Limestone were deposited during a major, gradually transgressive, but rapidly regressive sea level cycle that spanned the Ordian Stage of the early Middle Cambrian. In the northeastern parts of the Georgina Basin the Cambrian sea invaded an irregular landscape and proceeded to drown basement rocks with a series of small scale transgressive pulses, represented by repeating sets of three generalized shallowing‐upward cycles. Sediments deposited within the cycles are represented by seven lithofacies within two mineralogical suites of rocks. One, a mixed mineralogy suite (Suite A), contains: (1) the mud dominant, peloid and bioclast lithofacies, (2) a coated peloid, phosphate pavement lithofacies, and (3) the skeletal particle dominant lithofacies. The other, a calcareous mineralogy suite (Suite B), comprises: (1) the peloid and oncolite grainstone dominant lithofacies, (2) the peloid boundstone dominant lithofacies, (3) the thin bedded and fenestrate mudstone lithofacies, and (4) a flat and domal laminate stromatolite lithofacies. Generalized cycles mP and gP contain phosphatic carbonates of the mixed mineralogy suite and where complete culminate in emergence and the formation of phosphatic hardgrounds, phoscrete profiles, desiccated mudstone phosphorites and stromatolitic phosphorites. These phosphatic sediments accumulated in peritidal environments where the sea floor geometry was influenced by the irregular palaeotopography of the basement. Generalized cycle gC is characterized by rocks of the calcareous suite that accumulated in peritidal environments above a sea floor of negligible relief. Although the two mineralogical suites are spatially separated, they may be temporally equivalent. The coexistence of similar peritidal environments characterized by a different suite of chemical sediments provides a model for palaeotopographic controls on processes of phosphogenesis.