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abstract

Rim cements of high magnesium calcite, with exceptionally high magnesium carbonate content, are presently lithifying quartz sands at the surface along the South Louisiana Gulf Coast. Cements accumulate at the margin of a marsh environment on the leeside of a coastal beach complex. Based on X-ray diffraction analyses, surface cements range from 35 to 50 mole % magnesium carbonate. Cements show a well defined calcite lattice structure with no indication of dolomite ordering. Surface cements are associated with evaporites and blue-green algae but are not restricted to areas of algal activity.

Surface crusts are periodically buried by storm action and preserved in the subsurface. X-ray diffraction analyses of buried samples indicate a magnesium carbonate content of 17 to 33 mole %. Scanning electron microscopy indicates that cements are “fiber-bundles” with individual fibers constructed of crystallites with an average diameter of 0.3μm

Sandstone clasts, found along beaches of South Louisiana, are interpreted to be ripped-up fragments of older surface crusts. Clasts are cemented by high magnesium calcite with 8 to 17 mole % magnesium carbonate. Cements appear as blades with well defined trigonal terminations. These blades are also constructed of crystallites, averaging 0.3 μm in diameter. The surface crusts, buried crusts and rip-up clasts can be categorized into three cement types based on composition, crystal morphology and environment of occurrence.

The three cement types reported here are considered to be representative of diagenetic stages involving dissolution and precipitation. After burial, magnesium carbonate content decreases while the δC13 values become more negative. Isotope analyses yield average δC13 values of −1.0%c, −4.0%c, and −37.4%c PDB for surface, buried, and rip-up-clast cements, respectively. Oxidation of organics is probably the controlling mechanism of the dissolution/precipitation process. Extreme negative δC13 values of the rip-up-clast cements and a progressive decrease in values with time and depth of burial support this interpretation.

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