Westphal, Hildegard (1997) Sediment input and diagenesis of periplatform carbonates on a leeward slope of Great Bahama Bank = Sedimenteintrag und Diagenese von Periplattformkarbonaten auf einem leewärtigen Hang der Großen Bahama Bank. (PhD/ Doctoral thesis), Christian-Albrechts-Universitat zu Kiel, Kiel, Germany, XII, 163 pp.

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Abstract

Periplatform carbonates of flat-topped, steep-sided carbonate platforms are well known to record the varying environmental conditions that prevail on the platform top at the time of deposition. The depositional record in slope sediments of carbonate ramps sensu lato is less well understood. The present study describes and interprets the compositional signatures of Pliocene periplatform sediments from the core CLINO, located on the leeward paleo-slope of Great Bahama Bank (present-day margin). During the Pliocene, the morphology of Great Bahama Bank evolved from a distally-steepened ramp into a steep-sided, flat-topped platform. Thereby, the Pliocene deposits offer the opportunity to compare slope sediments of the different morphology types. Compositional analyses revealed that the Upper Pliocene periplatform sediments, that were deposited on the slope of a steep-sided platform, clearly record the repetitive submergence and subaerial exposure of the platform top. Highstand deposits are characterized by a predominance of fine-grained sedimentary matrix exported from the platform interior (aragonite needles). The lowstand deposits, in contrast, typically consist of coarse-grained skeletal material and cortoids, whereas fine-grained material from the platform interior is absent. The interpretation of finegrained highstand and coarse-grained lowstand deposits is substantiated by examining palynomorph assemblages that offer a direct proof of the conditions that prevailed on the platform top. Sea-level lowstand assemblages contain terrestrial pollen, while the highstand assemblages are dominated by dinoflagellate cysts. The selected Upper Pliocene interval thus represents a straight-forward image of highstand shedding. The selected Lower Pliocene interval, that was deposited when the platform exhibited a distallysteepened ramp morphology, shows a more complex picture. Composition of the sediments is rather uniform and does not reveal similarly clear cyclicities as seen in the Upper Pliocene. This is partly accounted to the ramp morphology being less sensitive to sea-level fluctuations, because facies belts are able to migrate upslope and downslope, following the changing sea-level. Additionally, however, sea-level fluctuations in the Lower Pliocene were overlain by an overall sea-level rise that could have obscured the signatures of short-term sea-level drops. Therefore, no final decision could be taken on the influence of the morphology on the composition of slope sediments. As indicated by stable isotopes compositions, the selected parts from the core CLINO are devoid of meteoric influenc during diagenesis. It was found that many textural features seen under the light microscope and under the scanning electron microscope, frequently thought to be indicative for meteoric diagenetic environments, also occur in the marine shallow-burial environment. The applicability of these textural patterns as indicators for specific diagenetic environments, therefore, is restricted. Diagenesis of the periplatform sediments was found to be strongly influenced by sea-level induced compositional changes where those are present (Upper Pliocene). As seen in mineralogical and in scanning electron microscopic examinations, the coarse-grained lowstand deposits are diagenetically more mature than the fine-grained highstand intervals. This pattern is thought reflect the high primary permeability of the coarse-grained lowstand layers. These lowstand deposits contain amounts of dolomite that require an import of magnesium ions from an external source by fluid flow. Reflux of evaporated fluids from the platform top as a probable source is supported by fluid inclusions in sparry cements that show strongly elevated salinities. Fine-grained highstand deposits from the Upper Pliocene interval and the overall fine-grained Lower Pliocene sediments, in contrast, appear much more immature. They still contain metastable aragonite in varying amounts locally exceeding 90%. The low initial permeability seems to have restricted fluid flow that could have driven diagenetic alterations. The fine-grained intervals are characterized by an alternation of cemented (uncompacted) and uncemented (compacted) layers. As seen in the scanning electron microscope, the uncompacted layers are lithified by microspar cements that enclose fine-grained sedimentary material such as aragonite needles. Observations in the Pliocene material, and the comparison to Paleozoic limestones, led to the postulation of a model for the lithification of micritic limestones by microspar precipitation. The source of the carbonate cement, that forms the microspar cements in the uncompacted layers, is thought to be located in the intercalated compacted layers. The compacted layers are passively enriched in insoluble constituents (such as palynomorphs and organic carbon), indicating that carbonate has been removed. These compacted layers could correspond to the marl layers in many limestone-marl alternations that are described in the literature.

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