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Abstract:

The major controls on changes in carbonate productivity, as well as platform or bank growth and the resultant facies distribution, are interpreted here to be short-term eustatic changes superimposed on longer term tectonic changes (i.e., relative changes in sea level). Carbonate platforms associated with sea-level highstands are characterized by relatively thick aggradational-to-progra-dational geometry. They are bounded below by the top of a transgressive unit and above by a sequence boundary. Two types of highstand platform, keep-up and catch-up, are differentiated here. (1) A keep-up carbonate highstand platform is interpreted to represent a relatively rapid rate of accumulation that is able to keep pace with periodic rises in relative sea level. A keep-up carbonate is characterized at the platform margin by grain-rich, mud-poor lithofacies and nonpervasive submarine cementation. keep-up platforms display a mounded/oblique stratal configuration at the platform/bank margin and in places on the platform. (2) A keep-up carbonate highstand platform is interpreted to represent a relatively slow rate of accumulation that is characterized by micrite-rich parasequences and pervasive early submarine cementation at the platform margin. A keep-up carbonate displays a sigmoid depositional profile at the platform/bank margin.

At the formation of a type 1 sequence boundary, where the rate of eustatic fall is interpreted to be greater than subsidence at the platform/bank margin, two major processes occur: (1) local-to-regional slope front erosion and (2) subaerial exposure of the shelf and major seaward movement of the regional meteoric lens. At a large-scale type 1 sequence boundary, sea level may fall from 75 to 100 m or more and for an extended period of time. When this occurs, the meteoric lens becomes established over the shelf for a long time, and its influence extends well into the subsurface. If there is sufficient rainfall and a permeable section with mineralogically unstable grains, significant solution will occur over the shelf in the shallow portion of the underlying highstand carbonate platform. Precipitation of phreatic cements will occur deeper or downdip in the section. At a small-scale type 1 sequence boundary, where sea level falls less than about 100 m and for a short period of time, the meteoric lens becomes less well established. It remains in a shallow position on the shelf, causing less extensive solution. Mixing and hypersaline dolomitization may be important processes during the late highstand and continuing through the formation of either a large- or small-scale type 1 sequence boundary. At a type 2 sequence boundary, in which the rate of eustatic fall is interpreted to be less than the rate of subsidence at the platform/bank margin, the inner-platform peritidal and outer-platform shoal areas will be exposed. The dominant meteoric effect will be in the inner-platform areas.

During sea-level lowstands, three types of carbonate deposits are recognized: (1) allochthonous material derived from erosion of the slope (i.e., debris sheets and allodapic carbonate sands); (2) autochthonous wedges deposited on the upper slope during type 1 sea-level lowstands; and (3) type 2 platform/bank margin wedges. In addition, given the appropriate climatic and hydrographic conditions (i.e., evaporation exceeds influx, and basin is restricted), evaporite lowstand wedges may occur associated with either type 1 or type 2 sequence boundaries. During evaporitic lowstands, hypersaline dolomitization, evaporite replacement, and solution may occur in associated carbonate highstand platforms. Siliciclastic lowstand deposition will occur in areas where an updip-source terrain is available.

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