ABSTRACT

Non-opaque heavy minerals have been examined from contemporaneous sandstone-shale pairs at six horizons in Tertiary sediments of the Texas Gulf coastal Plain. The total weight of heavy minerals in the shales is much less than in their associated sandstones but the same grain sizes of heavy minerals are present in both rock types. Sediment in the coarse and medium silt sizes contains the same variety of minerals as the very fine sand size.

In these relatively young shales the variety of minor minerals (those forming 0.1-5.0 percent of the non-opaque suite) is only slightly larger than in the contemporaneously deposited sandstones. however, the number of species present only in trace amounts (one or two grams among the 5400 grains counted) is significantly larger in the shales than in contemporaneously deposited sandstones. Also, the sandstones contain much larger percentages of altered grains than do the shales. These results apparently are due to the greater permeability of the sandstones and suggest that shales rather than sandstones should be examined in heavy-mineral studies. unstable species post-depositionally dissolved in sandy beds may be preserved in contemporaneously deposited shales.