The timing and environment of tepee formation in a Marinoan cap carbonate

Abstract

Thin carbonate units capping Marinoan-age glacial diamictites contain atypical tepee-like structures that have been interpreted as evidence of seafloor dolomite cementation. At Parachilna Gorge in the Adelaide Fold–Thrust Belt, a Marinoan-age cap dolomite contains paired tepee and growth fault structures that demonstrate the tepees grew 3.5–7.1 m below the sediment–water interface, in the early diagenetic zone. Although generally much larger, such tepees and their associated sheet veins have an external morphology and internal structure identical to that of inter-tidal tepee structures formed through expansive crystallisation. On the other hand, cap carbonate-hosted tepees lack the size, chaotic internal bedding, carbonate crust and concretionary morphologies, and expected carbon isotope geochemistry to be cold seep structures. Expansive crystallisation during early diagenesis seems the most likely means of forming tepees in cap carbonates. The only cement common to all cap carbonate tepees is microcrystalline dolomite. The crystallisation of this mineral phase drove both tepee growth and the fluid overpressure fracturing that formed the sheet veins. The same microcrystalline dolomite is the component of cap carbonates routinely selected for isotopic analysis, with the resulting δ 13C values assumed to be representative of coeval marine bicarbonate. However, the Parachilna Gorge tepees suggest that the microcrystalline dolomite contains both primary and early diagenetic components. Their separation is not achieved by current analytical methods.