Lowstand wedges in carbonate platform slopes (Quaternary, Maldives, Indian Ocean) (original) (raw)
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Marine Geology, 2002
Numerous small-scale depositional cycles are present in the Miocene sediments of seismic sequence along the margin of Great Bahama Bank (ODP Leg 166). These cycles consist of decimetre- to metre-scale alternations between light-grey and dark-grey/black wackestones/packstones. The light-grey layers are well cemented and nearly uncompacted. They contain planktonic and benthic foraminifera, and bioclasts. Bioturbation in these layers is moderate. The dark-grey wackestones/packstones are uncemented, strongly compacted and normally strongly bioturbated. The main components are planktonic foraminifera and fine-grained bioclasts. The dark layers are rich in aragonite and organic carbon and contain around 80% carbonate. The light layers show low aragonite and organic carbon contents combined with carbonate values of up to 97.5%. Light δ13C and slightly negative δ18O values were observed in the dark uncemented layers while the cemented intervals show heavy δ13C values and slightly more positive δ18O values. The carbon isotope signal between the dark and the light layers shows variations of up to 1.45‰. Both δ13C and δ18O co-vary with carbonate content. The Formation MicroScanner images reflect the changes in carbonate mineralogy. The Natural Gamma-Ray Tool shows that variations in siliciclastic content of the examined succession displays a different frequency than the cyclic alternations in carbonate mineralogy. The internal stacking pattern of the cycles is closely tied to sea level. The dark layers are deposited during rising sea level, while the light layers reflect sediment production and export during highstand and falling sea level. These cycles thus represent a rather continuous sediment shedding pattern that is clearly related to the ramp morphology and differs from the highstand shedding pattern typical for rimmed flat-topped platforms. Most measured parameters suggest that primary sediment composition played an important role in the cementation process. The primary differences in composition were enhanced during shallow burial diagenesis. The initially high content of metastable carbonate phases in the periplatform sediments triggered rapid cementation of the primary aragonite-rich layers leading to the formation of the light beds. Dissolution of metastable high-Mg calcite and aragonite was followed by in situ precipitation of more stable dolomite and calcite. Diagenesis was enhanced by the coarse grain size of the primary aragonite-rich layers. In contrast the more pelagic, dark calcite-rich layers were only slightly affected by diagenesis and could preserve their aragonite content. The uncemented layers subsequently were subject to strong mechanical compaction, whereas the cemented beds stayed nearly uncompacted.
Facies Patterns of Holocene Shallow Isolated Carbonate Platforms: Sediment Factories and Dynamics
Purposes To better understand sedimentologic variability on isolated platforms, this project examines the sedimentology of several isolated platforms from across a range of wave, tide, and geochemical settings, examining the influence of these variables on facies distribution and character. Collectively, data on facies body dimensions (presented last year) and this study provide insights into the nature and controls on sedimentology and geomorphology (e.g., facies bodies and their depositional phi/k) in Holocene systems, and offer new conceptual models for variability that might be present in ancient analogs. Project Description Deposits of shallow, isolated carbonate platforms form an important component of the stratigraphic record throughout geological history (Wilson, 1975; Lukasik & Simo, 2008). For geologists seeking to understand these ancient systems, Holocene platforms have provided a unique opportunity to explore fundamental questions concerning the possible nature and cont...