Abstract

The upper Jurassic Morrison Formation in the southern part of the San Juan Basin contains uniquely etched detrital garnets characterized by smooth, crystallographically controlled faceted surfaces. The etched garnets occur in relatively homogeneous Morrison sandstones in discrete stratigraphic zones that are bounded above and below by sandstones bearing unetched garnets. Diagenetic alterations associated with etched garnets include etched staurolite, skeletal plagioclase, regularly interstratified illite-smectite, iron-rich chlorite, ankerite, albite, diagenetically altered (organic-poor) uranium ore, and extensive secondary porosity. Etching of garnets and partial to complete dissolution of other aluminosilicate minerals were caused by high concentrations of organic acids generated during the maturation of epigenetic organic matter (predominantly type-III kerogen) in the Morrison Formation. The presence of authigenic phases that form near 100 degrees C indicates that temperatures were high enough during diagenesis to cause the thermal degradation of kerogen. The subsequent release of reactive organic acids promoted a variety of water-rock reactions. This hypothesis was corroborated by experiments in which dicarboxylic acid solutions created facets on garnets identical to those developed on naturally etched Morrison garnets. The excellent complexing ability of organic acid anions accounts for the characteristic lack of alteration products on etched surfaces.

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