Geological Survey of Japan, Japan

The mineralogy of periplatform carbonates is well documented in the literature. However, little is written about the grain-size properties for carbonate rocks. This fundamental property forms a controlling factor for other derived physical properties such as bulk density, porosity, and permeability. Thus, grain-size distribution and sorting might also steer fluid flow through the sediments, and combined with the mineralogy, might affect the development of the initial diagenetic pattern, which is significant in the interpretation of ancient depositional environments and transport conditions. This study, therefore, documents grain-size variations in conjunction with carbonate mineralogy for periplatform oozes of Sites 1003 (mid-slope) and 1006 (basin) on the leeward side of the Great Bahama Bank. The results reveal some distinct differences between glacial periods (glacials) and interglacial periods (interglacials) through both time and space. The delta (super 18) O and aragonite stratigraphy shows an almost complete sedimentary record for Site 1006, which is supported in the upper section by the U/Th dates assigned to interglacial Stages 1, 5, 9, and 11. However, Site 1003 stratigraphy indicates that large hiatuses exist within the sedimentary record and that there is evidence for diagenetic overprinting. This interpretation is further supported by the U/Th dates provided. Glacials are represented by sediment dominated by high-Mg calcite (HMC) and low-Mg calcite (LMC). The HMC probably originates from erosion of magnesium-calcite micrite cements formed in the upper slope deposits or HMC cements formed during early diagenesis. Detrital dolomite is also present at the distal site (Site 1006). Quartz also occurs preferentially during these periods. Although the grain-size distribution shows dominance by silts and clays (i.e., fine fraction [<63 mu m]) the percentage of the coarse fraction (>63 mu m) increases markedly during glacials. The latter fraction shows an increased dominance by the coarse (500-1000 mu m) to very coarse (>1000 mu m) sand-size fractions. Interglacials, in contrast, are dominated by aragonite, mainly in the form of fine-grained, bank-top -derived aragonite needles. This is supported by the grain-size distribution, which again shows dominance by silts and clays (<63 mu m). Dolomite is present at Site 1003, originating from early diagenesis. The coarse fraction (>63 mu m) is dominated by the very fine (63-125 mu m) to medium (250-500 mu m) sand-sized particles. Therefore, the fine-grained interglacial deposits will have a low diagenetic potential because of restricted fluid flow and low permeability, whereas the glacials will show the reverse pattern where the coarse-grained sediment facilitates early diagenesis. The diagenetic potential of the sediment on the leeward side of the Great Bahama Bank, therefore, varies through both time (between glacial and interglacials) and space (decreasing in potential with increasing distance from the platform). The composition of the coarse grains (63 mu m) exported from the platform during glacial and interglacials forms the key link in understanding the mineralogy and grain-size data, and thus is the main topic of work in progress.