Masters Theses

Date of Award

8-1991

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Otto C. Kopp

Committee Members

Richard Arnseth, Steve Driese

Abstract

Early diagenesis of argillaceous sediment is generally marked by the development of mixed-layer illite/smectite (I/S). The % illite layers in I/S increase with depth and this increase is associated with an increase in the stacking order of the constituent layers. A general equation for the alteration of a smectite layer to one of illite is:

Smectite + Al3+ + K+ --> illite + Si4+ This study investigated the influence of chemical composition and detrital mineralogy on the development of I/S in response to burial diagenesis. Samples for this study came from two argillaceous sequences from the southern Mississippi Embayment: 1) the Yazoo Formation (Upper Eocene), and 2) the Midway Group (Paleocene). Yazoo samples included both surface samples and well cuttings from a maximum depth of 1,670 m. Midway samples consisted of cuttings from two wells from depths ranging from 2,713 to 3,780 m. The chemical compositions and mineralogies of the samples were determined for the bulk, 5-63μm, <2μm, and <0.25μm (mineralogy only) size fractions by X-ray fluorescence and X-ray diffraction techniques. The composition of the Yazoo Formation varies with depth and along strike. The Yazoo becomes more siliceous and aluminous, and less calcareous with depth, and more calcareous and less aluminous toward the SE. The only abundant K-bearing phase in the Yazoo Formation is illite. Mixed-layer I/S in both the <0.25μm and <2μm size fractions of the Yazoo Formation is randomly interstratified and contains 20 to 46% illite layers. I/S in the subsurface Yazoo is not demonstrably more illitic than that in the surface Yazoo. A plot of K2O(<2μm)/K2O(5-63μm) depth does not indicate a depth-related transfer of potassium from the 5-63μm size fraction to the <2μm fraction. The absence of additional illite layers in I/S appears to be due to the lack of soluble potassium. The more deeply-buried Midway samples are more potassic and aluminous, and less calcareous than the Yazoo samples. The Midway Group contains randomly interstratified, IS-ordered, and ISII-ordered I/S, which suggests that the unit contains some detrital I/S which may not represent the thermal history of the host sediment during burial. A plot of K2O(<2μm)/K2O(5-63μm) vs depth suggests that potassium has been transferred from the 5-63μm fraction to the <2μm fraction, presumably by the dissolution of silt-sized K-feldspar and the uptake of potassium associated with illitization of smectite layers. The ratio of K2O(<2μm)/K2O(5-63μm) correlates with the % illite layers in I/S, suggesting that illitization in the Midway Group is governed by the availability of potassium. Although increasing temperature with depth is thought to be the principal driving force for illitization, the results of this study support the view that potassium availability is an equally important factor. In addition, the role of grain size and its effect on fluid flow may influence the extent to which illitization takes place.

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