Sedimentary and Faunal Associations of Protected Carbonate Shelves and Platforms: ABSTRACT

Protected carbonate shelves and platforms exhibit faunal and lithologic uniformity through time and space. Organic reefs of varied origin (stromatoporoid, rudist, coral) have sedimentologically and faunally similar backreef and lagoonal deposits. Backreef deposits, by definition, are dependent on organic reefs for their origin. Lagoonal deposits, however, primarily reflect restriction of circulation and are independent of the type of circulation barrier (reefs, islands, shoaling water). Thus, backreef deposits indicate presence of reefs, but lagoonal deposits reveal only the environmental character of the shelf or platform. Backreef deposits--skeletal sands and gravels originating through breakdown and transport of reef organisms--reflect relatively high-energy nearreef environments. Though generally characterized simply as debris deposits, these materials form well-defined sedimentary bodies. Transport by sheet flow across the reef or by tidal currents in interreef channels results in formation of skeletal sand banks, islands, downwind offreef drape, coarse rubble piles, and tidal deltas. Reef migration and lagoonward expansion of the reef flat may incorporate these deposits in the reef mass, accounting in part for the large amount of loose debris in reefs. More detailed investigation of lateral and vertical relations in modern environments should lead to recognition of these sedimentary bodies in ancie t rocks. Lagoonal deposits reflect restriction of circulation, and lagoons commonly are characterized as sediment traps for fine-grained carbonates. Geometry, lithology, and faunal content of lagoonal deposits reflect depth, lagoonal circulation, and ecology of lagoonal organisms. Facies belts paralleling bathymetric contours in atoll lagoons, and paralleling strike of carbonate-shelf lagoons reflect the effect of depth. The generally simple pattern of lagoonal deposition is modified by sedimentary accumulations such as mud mounds, islands, and their intertidal and supratidal facies, tidal deltas, and oolite bars. Climate and tectonics influence relative abundance of different types of lagoonal sediments. Increased restriction, arid climate, and low coastal relief lead to formation of vast sup atidal sabkhas, lagoonal evaporites, and reflux dolomitization. Exchange with the open sea promotes organic productivity and formation of skeletal sediments. Terrigenous influx results in complex terrigenous-carbonate facies. Sparse paleontologic data reveal consistent faunal associations in lagoonal and backreef deposits of diverse origins and ages. Foraminifera, mollusks, and algae dominate these faunas. Ostracods also are characteristic. Devonian reefs are unique in that lagoonal deposits are dominated by abundant fragile branching stromatoporoids. The striking similarity of lagoonal faunas through time should provide a superb framework for ecologic and evolutionary studies. Detailed resolution of carbonate subenvironments has been attempted most commonly in relatively thin, shelf-carbonate sequences. More detailed resolution of backreef and lagoonal subenvironments and sediment bodies is obtainable by more thorough definition and by application of available recent models. Investigation of vertical and lateral variation in lagoonal carbonates should provide useful data on shelf history. Effects of sea-level change, climatic variation, growth and destruction of barriers, and variation of sediment sources may be revealed more explicitly in these relatively uncomplicated sediments than they would in the complexly varying facies of the shelf or platform edge. Consideration of recent models suggests possible reinterpretation of development in several ancient carbonate complexes. End_of_Article - Last_Page 872------------