Condensation origin for Neoproterozoic cap carbonates during deglaciation

Abstract

Neoproterozoic deglacial stratigraphy is commonly characterized by a sharp contact separating glacial sediments from laminated capping carbonates. This stratigraphic relation is generally assumed to have time significance and to reflect an abrupt shift from icehouse to greenhouse conditions. In contrast to this, sequence stratigraphic field studies of an Ediacaran (ca. 635 Ma) glacial to postglacial transition in the Amadeus Basin of central Australia reveal a complex deglacial stratigraphy, in which more than 175 m of conglomerate, sandstone, marl, and carbonate at the basin margin, and portions of four unconformity-bounded sequences, pass basinward into no more than 3 m of laminated dolomicrite of typical cap carbonate facies. The unconformities, which are characterized by as much as several tens of meters of erosional relief (oblique sections of incised valleys), separate intervals of contrasting sediment provenance, and are confidently mapped on the basis of both criteria. Comparable unconformities are absent in the overlying Neoproterozoic succession, which is >2 km thick and encompasses many tens of millions of years. The Amadeus Basin cap carbonate was thus deposited during a protracted interval of multiphase (cyclical) transgression more similar to Phanerozoic cyclical sea-level rise than to the single catastrophic deglaciation and instantaneous precipitation invoked by popular current models to explain the classic cap carbonate. The superposition of carbonate on glacial facies in distal sections evidently records condensation in the absence of siliciclastic sediment rather than abrupt shifts between glacial and tropical conditions. Facies lithologically similar to cap carbonates may be less obvious in Phanerozoic successions because of a secular change in carbonate composition to reefal and deep-sea pelagic deposits.