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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 52 (1968)

Issue: 3. (March)

First Page: 524

Last Page: 525

Title: Clay-Mineral Facies in Upper Jurassic Rocks in Northeastern Texas and Adjacent Parts of Louisiana and Arkansas: ABSTRACT

Author(s): Kendell A. Dickinson

Article Type: Meeting abstract

Abstract:

X-ray analyses of rotary-drill cuttings and cores of Upper Jurassic rocks show a distinct relation between clay-mineral suites and environments of deposition.

Offshore marine shale and lagoonal-mudflat evaporitic mudstone contain chlorite and illite in a ratio of about 1:7. The chlorite is predominantly a high-iron variety in the marine shale and a low-iron variety in the lagoonal evaporitic mudstone. In most nonmarine mudstone there is no chlorite, but the rock contains kaolinite and illite in a ratio of about 1:7. Nearshore marine and transitional nonmarine shale and mudstone contain chlorite, kaolinite, and illite in ratios of 1:1:12. The degree of crystallinity of illite in the lagoonal-mudflat evaporitic mudstone is greater than in any of the shale or other mudstone.

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Although differentiation of clay suites of sandstone according to environment is not so pronounced, the pattern generally conforms with that found in shale and mudstone. However, significant differences exist in the relative amounts and in the degree of crystallinity of the minerals. The chlorite:kaolinite:illite ratio is about 2:1:3 for nonmarine sandstone and 1:1:4 for nearshore marine sandstone. All species of clay minerals in the sandstone are more crystalline than are those in the shale and mudstone.

Chlorite probably enters the depositional basin in a cation-depleted condition and accepts Mg++ in the oxidizing environment of evaporitic mudflats and lagoons and Fe++ in the reducing environment that occurs after burial in the normal marine environment. The absence of chlorite in the nonmarine shale and of kaolinite in the offshore marine environment is the likely result of differential flocculation during sedimentation. The greater degree of crystallinity of the illite from mudstone in evaporitic rocks probably results from the increase in the availability of cations, particularly K+, in the evaporite brine. The complete lack of expandable-lattice clays may result from deep burial and accompanying incipient metamorphism. All processes, except differ ntial flocculation, probably did not occur until temperature and pressure rose to a significant level in response to increased depth of burial.

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