Sediment-pore fluid interactions during diagenesis of modern and ancient mudrocks.
Hover, Victoria Christa
1996
Abstract
0Following deposition, marine sediments undergo major phases of organic matter decomposition. Input of CO$\sb2$, H$\sb2$S and other acids modifies pH and redox conditions of pore fluids and promotes mineral dissolution and precipitation during early burial near the sediment-water interface, and during deeper burial within thick sediment accumulations. This study investigates sediment-pore fluid interactions in various modern and ancient diagenetic settings using high resolution scanning electron microscopy (SEM) and scanning transmission/analytical electron microscopy (STEM/AEM) techniques to characterize diagenetic minerals combined with detailed pore water geochemistry. Three modern marine environments were studied by sampling the upper meter of sediment and characterizing sediment components and pore water composition: (1) clay-rich sediments of the Mississippi Delta plain estuary (Louisiana), (2) hypersaline salt pan deposits of the Salina Ometepec (Baja, Mexico), and (3) tropical carbonate sediments of the Bahama Banks and Florida Bay. The Mississippi Delta plain study documents K$\sp+$-exchange between smectite-rich sediments and pore waters, soon after burial. K$\sp+$-content increases in smectite and decreases in sediment pore waters with increasing salinity. Similar uptake by smectite near river mouths may account for $\sim$25% of the K$\sp+$-input to the oceans. Early diagenesis in hypersaline evaporative pan deposits (Salina Ometepec) involves early formation of K$\sp+$ and Mg$\sp{2+}$-rich smectite via alteration of K$\sp+$ and Al$\sp{3+}$-rich smectite or direct precipitation in marine brines. Mg-rich smectite formed in such environments is a likely precursor for trioctahedral mixed-layer corrensite and chlorite assemblages common to deeply buried evaporate sediments. Modern biogenic carbonate mud from the Bahama Banks and Florida Bay platforms undergo early dissolution and precipitation of metastable aragonite and Mg-calcite minerals. There is net dissolution of carbonate, but also a significant component of reprecipitation. Biogenic submicron crystallites with excess surface free energy undergo Ostwald ripening processes, which produces compositionally similar overgrowths so does not result in obvious changes in bulk sediment mineralogy. Burial diagenesis of Devonian-age organic-rich shales was investigated in several structurally distinct intracratonic basins (USA). Matrix clays are illite-rich mixed-layer illite/smectite (I/S). The I/S has fabric, disordered microstructure, and composition typical of authigenic I/S formed in younger burial sequences. Preservation of burial diagenetic textures implies that these shales have remained effectively closed following the I/S transformation. Known late Paleozoic fluid flow events in some of the mid-continent sedimentary basins have, thus, not perturbed the shale system.Subjects
Ancient Antrim Shale Corrensite Diagenesis Interactions Mexico Mississippi Delta Modern Mudrocks Pore Fluid Sediment Smectite
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