THE CHALLENGE OF DEVONIAN-MISSISSIPPIAN CARBONATE MUD MOUNDS

Carbonate mud mounds were unusually large and abundant during the Late Devonian-Early Mississippian (LDEM), ~350 Ma ago. A sediment baffling origin has been suggested, but a suitable source of off-mound carbonate mud has been difficult to identify. Late Devonian atmospheric p_CO2 reduction and p_O2 increase were sufficiently large to induce CO₂ concentrating mechanisms (CCM) in cyanobacterial phytoplankton. CCM act to maintain photosynthesis, and include bicarbonate transport into cells. This in turn promotes extracellular pH rise that can cause water column precipitation of fine-grained carbonate (whitings) if aquatic carbonate saturation state is elevated. It is proposed that imported off-mound whiting mud substantially augmented LDEM on-mound carbonate production. Increase in benthic calcified cyanobacteria, and organic carbon rich sediments plus large positive δ¹³C_PDB excursions, support elevated carbonate saturation state and CCM induction at this time. Seawater carbonate saturation may have risen due to Late Devonian demise of reef communities.

Typical features of LDEM mud mounds that are consistent with current-driven accumulation of fine-grained carbonate include (i) layered structure; (ii) geometries such as orientation, asymmetry, progradation and amalgamation, (iii) grainstone haloes; (iv) presence of current-reliant filter feeders (bryozoans, crinoids, sponges); (v) formation over a wide depth range; and (vi) internal collapse structures (stromatactis and slumps). Carbonate mud derived from phytoplanktic whitings can be rich in organic matter. This could have promoted microbial lithification (e.g., by bacterial sulfate reduction, BSR) that included widespread development of clotted-peloidal microfabric. Off-mound carbonate mud production mediated by cyanobacterial oxygenic photosynthesis could therefore have been augmented by on-mound syndepositional lithification mediated by BSR mineralization of whiting organic matter.

This reasoning, based on LDEM conditions, should not be applied to carbonate mud mounds in general or even to all LDEM mounds. Nonetheless, support for an off-mound mud source at a time when mounds were exceptionally abundant calls for reappraisal of concepts of on-mound origin that have long dominated interpretations of carbonate mud mounds.