Mineralogy and geochemistry of paleosols in the Javelina and Black Peaks formations (late Cretaceous-Paleocene), Big Bend National Park, Texas
Vines, Carol Marie McNally
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A conformable section of Upper Cretaceous and Lower Tertiary sedimentary rocks is exposed on Dawson Creek on the west side of Big Bend National Park, Texas. In this study the upper units of the Javelina Formation and the Black Peaks Formation are analyzed. Through the comparison of selected samples within this section, insights are provided regarding the mineralogy, geochemistry, and depositional history of these formations. All these strata contain presumed soil horizons which are approximately 60-75% smectite clay, while the remaining percentages are made up of detrital grains of monocrystalline quartz, plagioclase feldspar, calcite, and anatase. Iron oxide and manganese oxide permeate all specimens. Micromorphologic features include soil fabrics, voids, illuvial channels and skew planes, cutans, and glaebules. Elemental enrichment and depletion trends are correlated with modern soils. The Dawson Creek sediments display a massive blocky structure with dispersed carbonate nodules and appear similar to modern soils. The mineralogical composition of these deposits indicates that prolonged weathering and leaching did not occur. Micromorphological features indicate translocation and illuviation of material did occur. The similarity in texture, grain size and mineralogy suggests that color differences within these strata did not result form original stratification but occurred due to soil forming processes. This is confirmed by ratios of immobile trace elements. Also, major element abundances differ in each color-banded interval that correspond with color differences. Most of the features observed In the mudstones of the Javelina and Black Peaks Formations are compatible with their interpretation as buried soils (paleosols), however the results of the investigation are not conclusive.
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