Study of diagenesis in upper Miocene sandstones of coastal Louisiana documents depth-related cementation and geochemical changes of primary detrital mineralogy. Samples were collected from depths of 8,000-20,000 ft (2,600-6,500 m) in an area roughly corresponding to the upper Miocene depocenter in the Terrebonne trough of southeast Louisiana. Sandstones are primarily subarkoses and sublitharenites with minor amounts of feldspathic litharenites and lithic arkoses. Plagioclase feldspar (oligoclase/andesine composition) composes approximately 60% of the detrital feldspar. Dominant rock fragments are siltstone or mudstone, silicified volcanic rock, and chert.

Authigenic minerals and cements occur in the following order: dolomite, chlorite grain coats, albite overgrowths on plagioclase and K-feldspar, quartz, calcite, kaolinite, and ankerite. Calcite composition remains nearly constant with depth, but ankerite composition differs both with depth and within individual samples. In general, the mineralogy and order of cements resemble that of the lower Tertiary sandstones of the Texas Gulf Coast; however, in the upper Miocene, the volume of each cement is much less and the depth of first occurrence is greater.

Feldspars have reacted substantially with pore fluids. With increasing depth, feldspar becomes more sodic because of albitization and dissolution of calcic plagioclase. At approximately 20,000 ft, 75% of the plagioclase is nearly pure albite. Alteration of K-feldspar is not common above 17,000 ft; below 17,000 ft occurrences are rare because of dissolution.

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