ABSTRACT

Mud-mounds are an important Phanerozoic reef type generally formed in deeper water on carbonate shelf-to-basin slopes. They commonly contain radiaxial calcite spar-filled depositional cavities (stromatactis) but lack abundant metazoan framebuilders. Mud-mounds are often flanked by steeply dipping bioclastic grainstones and sometimes attained widths and depositional reliefs of over a hundred meters. They occasionally occur as basal cores on which developed shallow-water metazoan-constructed reefs.

Stromatactoid mud-mounds are composed of a framework of submarine-cemented, crudely reticulate masses or a succession of laminar crusts, surrounded by cement-filled cavities. These cavities developed when unconsolidated sediment around the framework was removed by winnowing. Mud-mound frameworks of all ages show strong geometric and petrographic similarities to shallow-water cryptalgal structures, especially thrombolites (unlaminated stromatolites) which occur in many Phanerozoic shallow-water reefs. Both thrombolites and mud-mound frameworks are largely composed of lime mudstone, cemented on the sea floor, and contain fenestral fabric and diagnostic cryptalgal microstructure. Their depositional surfaces were irregular, not bored or burrowed, and rarely encrusted. Stromatolitic lamina ion, although commonly not well developed in the subtidal zone anyway, occasionally occurs in mud-mounds.

It is proposed that mud-mounds accumulated from organic (probably blue-green algal) binding of locally derived lime mud and bioclasts deposited relatively slowly from suspension. Patchy distribution of algal mats resulted in areas of both bound and unbound sediment. Sporadic floodlike episodes of rapid sedimentation, however, temporarily smothered the living algal mat and also left layers of unbound sediment. Periods of winnowing washed out unconsolidated unbound sediment and promoted synsedimentary cementation of resulting stromatactoid cavities and algal-bound framework.