Controls and Effects of Subaerial Exposure on Cementation and Development of Secondary Porosity in the Subsurface of Great Bahama Bank
David K. Beach, 1995. "Controls and Effects of Subaerial Exposure on Cementation and Development of Secondary Porosity in the Subsurface of Great Bahama Bank", Unconformities and Porosity in Carbonate Strata, David A. Budd, Arthur H. Saller, Paul M. Harris
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Cementation trends and porosity profiles across multiple subaerial unconformities demonstrate how induration created during initial subaerial exposure played an important role in controlling fluid flow in shallow subsurface Pliocene-Pleistocene carbonate rocks on Great Bahama Bank (GBB). This control over fluid flow helped govern loci of dissolution and cementation during shallow burial of these metastable carbonates. Its role varied between meteoric vadose and phreatic, and mixing-zone diagenetic environments. Early induration also resulted in preferential preservation of subaerial unconformities in the subsurface.
This study of cementation and porosity trends also revealed gradual changes in diagenetic maturity of the rocks and progressive evolution of the pore systems with increasing depth of burial. Subsurface cementation and secondary porosity development occurred primarily during emergence and subaerial exposure of the bank top. Three diagenetic stages were recognized, and were related to changing diagenetic environments regulated by changing Pliocene-Pleistocene sea level and slow bank subsidence. Stage I, dominated by vadose diagenesis, commenced with initial subaerial exposure of metastable sediments, and ended with development of an indurated surface breached locally by vertical solution pipes. In Stage II, with shallow burial (surface to variably 12 to 20 m) and under ephemeral freshwater phreatic conditions, metastable carbonate sediments completed alteration to low-Mg calcite, and porosity inverted from primary interparticle and intra-particle to moldic. Relatively uniform cementation by equant calcite also occurred. In Stage III (depths to 150 to 200 m), subjection of deeper subsurface rocks to prolonged episodes of corrosive bank-wide freshwater phreatic and mixing-zone conditions during bank emergence resulted in extensive dissolution. Because GBB is comprised of carbonate rock and lacks siliciclastic aquitards, freshwater lenses and underlying mixing zones fluctuated freely with changing sea level. This allowed shallow-meteoric and mixing-zone processes to modify rocks and porosity at considerable depths within the subsurface during sea level lowstands.
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Looking for insights and methods useful for predicting and identifying subaerial exposure surfaces and associated porosity? This memoir addresses four major topics: Detection of unconformities; Modification of porosity during exposure; Preservation of porosity during burial; and Influence of unconformities on subsequent depositional and diagenetic patterns.