Mineralogy and Diagenesis of Miocene and Pleistocene Sediments, Northeastern Gulf of Mexico

Steven P. Conner, Miles E. Denham, Thomas T. Tieh

Cored Miocene and Pleistocene sediments from the northeastern Gulf of Mexico (ranging in depth from 1,400 to 2,000 m) consist of interbedded sands, silts, and muds with varying degrees of consolidation. Samples selected from these sediments were studied for their mineralogy and diagenesis.

The Miocene sediments represent various subenvironments of the ancestral Mississippi River delta complex; Pleistocene sediments are marine shelf deposits. Sands from both Miocene and Pleistocene cores are fine to very fine-grained feldspathic litharenites with abundant (^approx25%) matrix fines and minor (< 7%) carbonate clasts and shell fragments.

Carbonate diagenesis is a continuing process in these sediments. Clayey sands contain disseminated siderite crystallites formed during organic matter fermentation. In clean sands, scattered thin (< 15 cm) bands of poikilotopic carbonate-cemented sand are present in the middle of thicker unconsolidated intervals. Cemented bands are thicker in the Miocene section, having grown during burial through dissolution of shell fragments and reprecipitation at the margin of cemented zones. Diagenetic events evident in the Miocene core illustrate processes that could occur in the Pleistocene sediments with continued burial. These are: (1) dolomitization of some calcite cements, (2) precipitation of Fecarbonate as crystallites, rims on dolomite cements, and overgrowths on siderite crystallites, and (3) zeolite formation on amorphous silica substrates. Amorphous silica is associated with limited dissolution of feldspars or lithic fragments.

Because of their shallow depth of burial and limited temperature increase, these sediments are at an early stage of diagenesis. Cementation zones are limited in extent, but are growing with continued burial. Mudstones are poorly consolidated and highly smectitic; although not altered to illite, smectite layers show the initial steps of the clay transformation.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.