The paragenetic history of a Marinoan cap carbonate

Abstract

Carbonates that cap late Neoproterozoic glacial diamictites contain large, atypical tepee-like structures that have been interpreted as evidence for seafloor dolomite cementation. The Nuccaleena Formation in the Adelaide Fold–Thrust Belt is the stratotype Marinoan-age cap dolostone. At Parachilna Gorge the Nuccaleena Formation contains paired cogenetic tepees and growth faults that demonstrate these structures grew well below the sediment–water interface. These tepees, and their associated sheet veins, were the result of pore fluid overpressure driven by expansive, early diagenetic dolomite cementation. Fluid overpressure resulted in two forces: 1) macroscale lateral compression that overcame the lithostatic weight to buckle sediments upward, forming the tepee structure, and 2) macro- to microscale extension that overcame the lithologic tensile strength to form sheet veins. The physical processes of fluid overpressure within the Parachilna Gorge cap carbonate are in many ways analogous to the fluid overpressure dynamics in mud volcanoes, and were sufficiently high to generate veins extending 17 m below the cap carbonate. Overpressure-driven dewatering removed pore fluid, clay and organic matter from the dolomicrospar matrix into dewatering structures and fabrics. The latter are hierarchically arranged from small linear clay fabrics to clay-lined dendritic arrays to clay-choked curvilinear veins to large sheet veins. Packets of red clay laminae were deposited when pore fluids vented to the sea floor. The early diagenetic chemical evolution of the Parachilna Gorge cap carbonate was equally dynamic, in which an initial reducing trend was supplanted by an oxidation trend brought about by tepee-sheet vein deformation. Once established the sheet vein network also reinvigorated diffusional processes that likely enhanced early diagenetic cementation of the cap dolostone. The detailed paragenetic history also suggests that calcite not dolomite was the initial carbonate precipitated from the post-glacial Adelaidean sea. The Nuccaleena Formation exhibits the dolomicrospar, isotopic signatures and tepee-like structures that distinguish Marinoan-age cap carbonates. At Parachilna Gorge these typical features are a product of early diagenesis, suggesting that other Marinoan-age cap carbonates may also be early diagenetic in origin.