Microfabrics of Carbonate Muds in Reefs

Reef frameworks are prominent, rigid, skeletal structures that generate numerous variously connected cavities as they build. Fine sediment enters the structure and descends to a base level of accumulation in hollows and on ledges. In the inner recesses sediment settles out of suspension, and where it only partially fills cavities, geopetal fabrics are created. Once deep inside the structure the internal sediment is usually trapped in its final resting place—where it is protected from reworking by currents and infauna but yet lies in a zone of high hydraulic energy. Synsedimentary lithification of this sediment is aided by its immobility and the high flux of supersaturated seawater. Cavities are progressively sealed by organic growth, sedimentation and cementation, and the pile of internal sediments may show a grain-size grading that parallels the decrease in cavity throat size. Multiple generations of internal sediments are created when sediment supply to cavities is in phases controlled by flushing of turbid water during storms, by framework fracturing, and bioerosion by framework-penetrating boring organisms. The main components of internal sediments of small cavities are silt-sized skeletal debris—especially spicules and sponge-bored chips, microcrystalline Mg-calcite crystals, and peloids that most probably originated by nucleation on fine mobile nuclei within cavity fluids. The peloidal fabric is characteristic of reefs because the rigid roofs of cavities protect the internal sediment from mechanical and chemical compaction, and since the early lithification preserves the loose packing of these delicate aggregates of microcrystalline calcite.In areas where cementation dominates sedimentation, micrite crusts of unusual surface relief are formed around reef components. These draped or pendant crusts may show internal wavy laminations or a clotted fabric, related to their modes of accretion.