Carbonate sedimentation and diagenesis of Pleistocene limestones in the Bermuda Islands

• Main Author: Gross, M. Grant
• Format: Others
• Published: 1961
• Online Access: https://thesis.library.caltech.edu/1362/1/Gross_mg_1961.pdf; Gross, M. Grant (1961) Carbonate sedimentation and diagenesis of Pleistocene limestones in the Bermuda Islands. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/E4V7-ZV93. https://resolver.caltech.edu/CaltechETD:etd-04122006-135611 <https://resolver.caltech.edu/CaltechETD:etd-04122006-135611>

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. The Recent carbonate sediments in the Bermuda Islands consist largely of skeletal fragments of marine organisms. Benthonic foraminifera and lithothamnoid algae contribute high-Mg calcite (>10 mol % MgCO3). Low-Mg calcite (<5 mol % MgCO3) appears to be derived from erosion of Pleistocene limestones. Aragonite, which makes up 30 to 70% of the Recent sediments, is derived from molluscs and Halimeda. The Pleistocene limestones are composed of fragments of the same types of marine organisms which make up the Recent sediments. The limestones vary from unconsolidated, unaltered carbonate sands to well consolidated, crystalline limestones. Lithification of the limestones is caused by the removal through solution of aragonitic skeletal fragments and the deposition of secondary calcite. Obliteration of the internal structure and the formation of replacement mosaics is observed in some of the fragments. The high-Mg calcite phase, which is present in the poorly consolidated limestones, is absent in highly altered limestones which contain only low-Mg calcites. No limestones were found which contained only a single Mg-calcite of intermediate composition (between 5 and 10 mol % MgCO3). Mg-calcites containing between 5 and 10 mol % MgCO3 were found only in the fragments of Homotrema. The SrCO3 originally present in the calcites and aragonites is removed during diagenesis of the limestones. No dolomites were found in the Bermuda limestones, which suggests that the Mg and Sr removed during diagenesis is not retained in the limestones. Dolomite containing 57 mol % CaCO3 was found in limestones recovered from drilling operations on Plantagenet Bank, off Bermuda. With the exception of Halimeda fragments, the skeletal fragments making up the Recent sediments have [...] values between 0 and -1% and positive [...] values. Most of the Recent sediments have [...] and [...] values in this range. Fresh water carbonates deposited on the islands have [...] values from -3 to -5% and [...] values from -4 to -9%. The [...] and [...] values of the diagenetically altered lime stones exhibit change from values similar to the Recent sediments to [...] values similar to those of the fresh water carbonates. This change in [...] values is shown to result from the precipitation of secondary calcite in the limestones and is interpreted as indicating that the secondary calcites have the same origin as the fresh water carbonates. The diagenetic environment of the Bermuda limestones was characterized by temperatures less than 30°C and essentially surface pressures. Most of the Pleistocene limestones have probably been in the fresh water zone since their deposition. The mineral assemblage and Sr and Mg content of the limestones is discussed in terms of the equilibrium mineral assemblage at these temperatures and pressures. Cursory studies were made of limestones from Bikini and Eniwetok Atolls and mid-Ordovician limestones from Oklahoma in order to test the usefulness of the [...] and [...] values for distinguishing different diagenetic environments. The data from the analyses of the Bikini and Eniwetok Atoll samples substantiate the geological evidence that portions of these limestones have been subaerially exposed during the Pleistocene epoch. The data from the mid-Ordovician limestone indicate that the diagenetic environment was different from that of the Bermuda limestones.