MICROSCOPIC TEXTURES, MACROSCOPIC SHAPES AND DEPOSITION RATES OF PELOIDAL FACIES IN THE BASAL EDIACARAN CAP DOLOSTONE

Basal Ediacaran cap dolostones from various paleomargins are petrographically and stratigraphically distinct and contain morphologically unique sedimentary structures, including giant ripples, tube-hosting stromatolites and corrugated stromatolites. The unusual macroscopic morphologies of tube-hosting and corrugated stromatolites and giant ripples extend over meters to tens of meters of stratigraphy and are typically interpreted as records of uncommonly high carbonate deposition rates during the deglaciation. However, current estimates of these rates range from 2-10 kyr to many hundreds of kyrs. Typical textures in stromatolites, ripples and other mechanically laminated facies of cap dolostones are inclusion-rich, clotted, or formed by partially lithified, deformable macropeloids, indicating that cap dolomites precipitated primarily around organic nuclei, at rates that did not outpace microbial growth. The textures and shapes of giant ripples and tube-hosting stromatolites, respectively, suggest that the former were shaped by strong orbital velocities, and the latter by weak orbital velocities and currents. Both ripples and stromatolites were also repeatedly colonized by microbial mats, indicating recurring periods of weak flow that lasted longer than one month. Studies of clotted textures in modern microbial carbonates constrain the maximum accumulation rates of basal Ediacaran clotted dolostones to 1 m/kyr, suggesting that ~10 m thick cap dolostone facies accumulated over more than 10 kyr, longer than predicted by standard ice-sheet melting models. The commonplace microscopic textures in the unusual sedimentary structures within basal Ediacaran cap dolostones may not indicate high oversaturation of oceans with respect to carbonate minerals, but instead suggest episodic carbonate deposition in shallow areas teeming with organic-rich, slowly lithifying particles and microbial mats.