506 SEDIMENTARY ENVIRONMENTS/Carbonate Shorelines and Shelves Figure Cross section through the Florida shelf region normal to the coast and shelf margin, showing the main sedimentary environments (After Bosence DWJ and Wilson RCL (2003) Carbonate depositional systems In: Coe A (ed.) The Sedimentary Record of Sea level Change, pp 209 233 Milton Keynes and Cambridge: The Open University and Cambridge University Press.) The photographs illustrate the fundamental control that organisms have on carbonate sediment production and accumulation (A) Mangrove roots stabilizing the shoreline of Florida Bay (foreground m across) (B) Thalassia seagrass bed; the grass leaves (20 30 cm high) act as a substrate for carbonate secreting organisms, whose debris is trapped and stabilized within the grass beds (C) Porites corals (branches are cm across) are common reef builders in the back reef zone Note also the encrusted seagrass blades (D) The shallow zone of the shelf margin barrier reef is mainly constructed by the elk horn coral, Acropora palmata (branches are 15 20 cm across) trees are adapted to live in coastal waters of variable salinity and are unusual in having aerial root systems, which have a useful sedimentological function in that they trap and bind sediment so as to resist coastal wave erosion (Figure 7A) The occurrence of fully marine organisms in semi-isolated shallow coastal waters of Florida Bay is restricted by the variable salinities, dissolved oxygen levels, and fine carbonate sediment suspended within the bay waters Seagrass (e.g Thalassia) beds (Figure 7B) and the calcareous organisms that encrust the grass blades (foraminifera, serpulid worms, and bryozoans) are common, as are other skeletal organisms such as molluscs, foraminifera, and calcareous green algae (Penicillus and Udotea) When these organisms die their skeletal debris accumulates as shelly muds, which would form bioclastic mudstones and wackestones (see Sedimentary Rocks: Limestones) if lithified into limestones Shells are concentrated on beaches and storm ridges as bioclastic gravels and sands Identifying the origin of the carbonate mud is a complex problem because lime mud may be produced by the breakdown of the lightly calcified tissue of green algae, by organisms that graze or bore into rocky carbonate shorelines and skeletal carbonate material (known as bioerosion), or by chemical precipitation directly from seawater Mud production has been extensively studied in South Florida, and mineralogical (calcite and aragonite) and geochemical (calcium, magnesium, and iron levels, etc.) analyses of the muds show that they are similar to the carbonate minerals of the skeletal organisms This implies that most of the mud is produced by the breakdown of skeletons of algae and benthic invertebrates Once formed, the mud is reworked by burrowing organisms (mainly shrimps and worms) that digest what they can from the mud and defecate the remainder as sedimentary faecal pellets or peloids Within Florida Bay the peloidal muds are swept by storm waves into shallow-water mud mounds (piles of mud that accumulate from the seafloor to sealevel), which are stabilized by seagrass (Thalassia) (Figure 7B) These mounds have distinct windward erosional margins on their north-east sides and accretionary Thalassia-stabilized leeward (south-westerly) margins Mounds therefore migrate in a southwesterly direction towards the Gulf of Mexico Between the mounds there may be a thin cover of bioclastic (molluscan, algal, and foraminiferan) sands and muddy sands or alternatively areas may be bare of