Figure

Mud mounds are an important Phanerozoic reef type generally formed in deep water on carbonate shelf-to-basin slopes. They commonly contain spar-filled depositional cavities (stromatactis) but lack abundant metazoan frame-builders. Mud mounds are flanked by steeply dipping bioclastic grainstones and commonly attained widths and depositional relief of hundreds of meters. They may occur as basal zones in many metazoan-constructed reefs.

Stromatactoid mud mounds are composed of a succession of submarine-cemented crusts separated by cavities that developed in intercalated, unlithified mud. Crusts from mud mounds of all ages show strong similarities to thrombolites (unlaminated stromatolites) which occur in many Phanerozoic shallow-water reefs; both are largely composed of micrite, cemented on the sea floor, contain fenestral fabric and cryptalgal microstructure, and their depositional surfaces are rugose, not burrowed, encrusted, or bored. Both thrombolite and crust morphologies are probably related to environmental factors: thrombolites in shallow-water reefs are columnar and branching whereas crusts in deeper water are more tabular. Irregular sediment loading and distribution of algal mats probably caused rugose depo itional surfaces. Stromatolitic lamination, although not commonly developed in the subtidal zone, occasionally occurs and corroborates a cryptalgal origin.

It is proposed that mud mounds accumulated from successive floodlike episodes of sedimentation off carbonate shelves. Rapid sedimentation at the onset of each flood smothered the living algal mat which re-established itself as sedimentation began to subside. Winnowing between sediment floods washed out unbound sediment and promoted synsedimentary cementation of resulting stromatactis and algal-bound crusts.

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