ABSTRACT

Alteration of feldspars in the youngest of the Gulf Coast Cenozoic sands and sandstones is dominated by dissolution and albitization. Volumetrically significant amounts of alteration are only observed below burial depths of about 4.5 km in sands of Pliocene and Miocene age. Only trivial amounts of plagioclase dissolution were observed in Pleistocene units. In general, plagioclase exhibits minor amounts of secondary dissolution at all depths, and greater amounts in the deepest samples. Potassium feldspar is subject to very little dissolution to depths of about 3.5 km; by 4.5 km K-feldspar removal is virtually complete. Albitization affects only plagioclase and appears to be operative, in these sediments, over temperatures of 110°C-140°C.

Compared to older Cenozoic units elsewhere around the Gulf of Mexico feldspar dissolution and albitization in Neogene sands have advanced to a lesser degree, at least in the sense that they affect a smaller proportion of the total section. Comparison of pre-alteration feldspar composition, temperatures of alteration, and geothermal gradients for Gulf Coast sandstones of different ages suggests that the main controls on feldspar alteration are temperature, pre-alteration plagioclase composition, and possibly the amount of fluid flow. Time per se seems to be a factor of negligible importance, at least over time spans greater than 106 yr. Thus, the lesser volume of Neogene sand affected by feldspar dissolution and albitization can be attributed primarily to the lower geothermal gradients of the northern Gulf.

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