--> Abstract: Thermohaline Diagenesis and Lithification of Upper Oligocene Mudstones, West Hackberry Field, Louisiana Gulf Coast, by K. M. McManus and J. S. Hanor; #91012 (1992).

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ABSTRACT: Thermohaline Diagenesis and Lithification of Upper Oligocene Mudstones, West Hackberry Field, Louisiana Gulf Coast

MCMANUS, KATHLEEN M., and JEFFREY S. HANOR, Louisiana State University, Baton Rouge, LA

Our study of alteration of sediments along the western flank of the West Hackberry salt dome, Louisiana Gulf Coast, documents depth-related compositional changes in adjacent Upper Oligocene mudstones. There is a gradual increase in detrital feldspar, a decrease in calcite content, and pronounced increases in authigenic analcime, siderite, and smectite with depth over a depth range of 1.8 to 2.2 km and a temperature range of 75 degrees to 85 degrees C. The less than 2 um fraction consists primarily of smectite and mixed-layer illite/smectite with only minor amounts of kaolinite and illite.

Precipitation of authigenic analcime and siderite are the primary mechanisms for mudstone lithification. The analcime is enriched in silica (Si/Al = 2.54) with respect to stoichiometric analcime (Si/Al = 2.00). Lithification of the mudstones was apparently synchronous with the burial cementation of interbedded sands, where analcime and siderite fill fractures, intergranular pore space, and intraskeletal pores and replace earlier calcite cement and detrital framework sand grains. The precipitation of analcime is presumably the result of salt dome-derived NaCl brines permeating the entire Upper Oligocene section adjacent to the dome and reacting with volcaniclastic grains in a warm, high (Na+/H+) environment. The relative increase in smectite abundance with increasing analcime content i the mudstones is consistent with alteration of volcaniclastic material. The decrease in calcite appears to reflect changes in original depositional environment.

 

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)