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Post-Depositional Subaerial Weathering Effects on the Mineralogy of an Upper Cretaceous Sand in Southeastern United States
Norman C. Hester
The Upper Cretaceous Cusseta Sand, which crops out in east-central Alabama and west-central Georgia, occurs frequently as cuestas. The lithologies range from a clayey sand to a fine- to a coarse-grained, moderately well sorted sand. Because of their topographic position above the water table, their high permeability, and an extensive period of subaerial exposure, these sediments have been subjected to intensive leaching.
Compositions and distributions were determined for heavy and light minerals, and clay minerals from channel samples at five selected outcrop profiles. In the unweathered lower portion of each outcrop profile, the dominant nonopaque heavy minerals are rutile, tourmaline, zircon, staurolite, kyanite, garnet, epidote, and sillimanite. Ilmenite is the most prominent opaque heavy mineral. Among the light minerals, potassium feldspar accounts for as much as 6% of the sand fraction. The predominant clay minerals are montmorillonite and kaolinite in order of importance.
In all of the sections, the following changes take place from bottom to top: (1) reduction in the percentages of feldspars and some heavy minerals, particularly garnet; (2) ilmenite is partly altered to leucoxene; (3) the X-ray diffraction peak for the 17 A montmorillonite shortens and broadens. These changes in mineralogy are not the result of changes in provenance, changes in source-area climate, or abrasion during transport, but are attributed to intense post-depositional subaerial weathering.
In the light of post-depositional subaerial weathering and intrastratal solution re-evaluation of the significance of mineral suites from Cretaceous and Tertiary sediments in the eastern and southeastern United States is required for interpretations of their provenance and reconstruction of the paleoclimates.
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