Mineralogy and Incipient Diagenesis in Sediments of Pigmy Basin, Northern Gulf of Mexico
S. V. Stearns, T. T. Tieh, B. J. Presley
Clay-rich sediments from the Pigmy basin, a blocked canyon-type intraslope basin in the northern Gulf of Mexico, were examined for mineralogic variations and for pore-fluid chemistry. The objective was to identify incipient diagenesis in these shallowly buried clayey sediments. Sediment and pore-fluid samples from hole 619 of DSDP Leg 96, representing a vertical sequence of materials with a maximum burial depth of 150 m below the sea floor, were used in this study.
The sediments are composed, on the average, of less than 1% sand, 37% silt, 48% clay, and 14% carbonate minerals. Analysis of the sand and silt fractions by optical microscopy, x-ray diffraction, and scanning electron microscopy methods showed the predominance of quartz, a minor amount of feldspar, and little variation in mineralogy throughout the sequence. Occasional minor dissolution textures were observed in the silt grains, and no alteration of the sand was noted. Mineralogic composition and uniformity thereof in these fractions suggest that the sediments were derived from a constant source, probably the Mississippi River.
Incipient diagenesis, however, is evident in depth-related variations in clay mineralogy, amount of exchangeable cations in clays and bulk samples, as well as in chemistry of pore fluids. With increasing burial in the 150-m interval, smectitic clays have partly transformed to illite as revealed by x-ray diffraction peak intensities, total K2O, and CEC analysis of the clay fractions. Closely associated with this change is a gradual decrease in the amount of ammonium acetate-exchangeable K+, Mg++, Ca++, and Si+4 in pore fluids squeezed from these sediments also decrease with depth. Therefore, we concluded that incipient diagenesis, induced primarily by compaction, results in not simply dewatering and reorientation of clay minerals, ut also in a gradual increase in the fixation of cations onto clay minerals and, thus, in a decrease in the smectite component.
AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.