Subsurface alteration of plagioclase, potassium feldspar, and a variety of nonopaque heavy minerals has substantially modified the detrital composition of Plio-Pleistocene sediments in the northern Gulf of Mexico. Albitization of plagioclase and dissolution of unstable grains are the two principal mechanisms of grain alteration. Grain alteration increases progressively with depth and has taken place without the influence of significant amounts of meteoric fluids. The degree to which strictly subsurface grain alteration has affected these young sediments suggests that grain assemblages in older rocks should be viewed most skeptically with regard to their state of preservation.

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