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The Keg River bioherms consist mainly of mechanically deposited carbonate sediments and of stromatolites. True skeletal boundstone is rare. The salinity of the contemporaneous sea fluctuated repeatedly between normal marine and highly supersaline. Accordingly, fossiliferous marine carbonates are intercalated with essentially nonfossiliferous evaporitic carbonates. Parts of these sediments were intermittently at or above sea level.
Intensive penecontemporaneous cementation by high magnesium calcite and, possibly, aragonite occurred on the bioherms at times of near-normal salinity. Unconsolidated fossiliferous carbonate sediments and soft algal mats became solidly lithified, wave-resistant rocks. The bioherms grew as cementation welded most loose carbonate particles firmly to the reef body. Abundant early fractures opened after cementation. These fractures commonly were filled with internal sediment of evaporitic facies. Early diagenetic anhydrite is present in evaporitic carbonates mainly as replacement, displacement nodules, and pore cement. Dolomitization of bioherms occurred mainly during evaporitic episodes at subtidal to supratidal sites. Unconsolidated evaporitic carbonate sediments selectively were dolomi ized completely and lithified. The dolomitization produced finely crystalline dolomite in evaporitic carbonates and medium to coarsely crystalline dolomite in previously lithified fossiliferous carbonates. Temporary subaerial exposure caused little or no cementation by meteoric low-magnesium calcite. Late diagenetic processes after effective burial include: (1) conversion of all metastable calcium carbonate to low-magnesium calcite; (2) cementation and replacement by minor amounts of anhydrite, calcite, and dolomite; (3) moderate fracturing; (4) weak stylolitization; and (5) deposition of large amounts of carbonaceous material in pores.
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