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Pliocene and Pleistocene sands that underlie the Louisiana shelf are lithic arkoses and feldspathic litharenites. The composition of this detrital material is similar to that of Eocene and Oligocene sandstones of the Texas Gulf Coast. Among rock fragments, grains of volcanic and low-rank metamorphic origins dominate. Untwinned plagioclase is the dominant feldspar. Calcium content of plagioclase in unalbitized sands is as great or greater than that observed in unalbitized sandstone samples from the Eocene and Oligocene of Texas.
Despite a primary detrital composition that is potentially as reactive as detrital assemblages in the older units, Pliocene-Pleistocene sands have comparatively lesser amounts of cementation and grain alteration. An interesting reflection of the lesser degree of grain alteration is the relatively more unstable and complex assemblage of heavy minerals present in the younger sands.
In addition to detrital composition, the depositional setting of the Pliocene-Pleistocene clastics was also broadly similar to that of other major wedges of Tertiary sediment in the Gulf Coast basin. Thus, differences in stages of diagenesis are believed to be the result of different physical and/or chemical environments present during burial.
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