THE CHALLENGE OF DEVONIAN-MISSISSIPPIAN CARBONATE MUD MOUNDS
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.