A clay mineral investigation of six cores from the Gulf of Mexico

Abstract

Samples were studied from each color change along six gravity cores from nearshore to deep-sea areas in the Gulf of Mexico. Analytical methods and techniques used to characterize the sediments were X-ray diffraction, differential thermal analysis, cation exchange capacity, particle size distribution and fractionation of the clay-size material with the supercentrifuge. Fractionation of clay-size material expedites clay mineral identification and semi-quantitative estimates of abundance. A 5-g, clay-size sample passed five times through a supercentrifuge should remove the < 0.2 μ fraction when the proposed sample preparation method is employed. Duplicate fractionations usually agree within 3% and fractionation efficiency is unaffected by the mineral suite. The clay minerals and semi-quantitative estimates of their abundance suggest that the sum of source conditions has remained constant during and since Pleistocene time represented by the cores in this study. Montmorillonite and kaolinite are not more abundant in warm water than in cold water sediments and illite and chlorite are not more abundant in cold water than in warm water sediments from the Gulf of Mexico. Sand-size material is usually a small weight percent of a sample. Silt and clay-size materials are approximately equal except for the cores farthest from the Mississippi Delta where clay-size material dominates. In the clay-size fraction, montmorillonite is generally more abundant than illite, while kaolinite comprises less than 20% and chlorite less than 7%. Relative contents of quartz may be obtained by using differential thermal analysis. Quartz of similar particle size as occurs in the sample is used to obtain the standard curve. Quartz and feldspar exist only in the 2–0.2 μ portion of the clay-size fraction. Possible worm fecal pellets are present in the sand-size fraction of the two cores farthest from shore. The effects of differential settling of clay minerals have not been recognized.