The carbonate beach sequence can be divided into a backshore (dolomite), an upper foreshore (mollusc lime grainstone), a lower foreshore (mollusc lime packstone) and an offshore zone (pellet lime packstone). This sequence represents a prograding beach-supratidal complex. The early diagenetic history can be divided into two phases. (1) The beach-backshore accretion phase is characterized by intertidal and submarine cementation with aragonite cements under marine phreatic and vadose conditions. A meteoric phreatic lens within the beach was the site of concurrent grain solution and precipitation of calcite cements. (2) The post-backshore accretion phase is characterized by dolomitization of the supratidal, and wholesale silicification of the upper foreshore by reflux waters rom the prograding supratidal. Late post-burial solution took place after the sequence was uplifted, probably during Balcones faulting (Miocene-Pleistocene).

The beach proper is characterized by secondary mollusc moldic porosity. The highest porosity-permeability values (39% and 460 md.) are found in the base of the upper foreshore and top of the lower foreshore zones. The backshore dolomites have intercrystalline porosity averaging 33% with a permeability of 18 md. The offshore pellet packstones have intercrystalline porosity averaging 29% and 36 md. The concentration of secondary porosity and permeability in the middle of the beach is a function of early cementation and silicification of the upper foreshore that created a permeability barrier at the top of the sequence while the normal depositional fabrics of the offshore zone had the same effect at the base of the sequence, effectively restricting the major flow of a later groundwater s stem to the middle of the beach. Total rock history, therefore, including both deposition and diagenesis, is the key to effective pore system analysis.

FROM 24TH IGC, P. 124-136.

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