PETROLOGIC ANALYSIS OF A REGOLITH/BEDROCK INTERFACE IN THE JURASSIC MORRISON FORMATION, NORTHEASTERN ARIZONA

The Blue Gap study area, in northeastern Arizona, is characterized by a 500 ft. high escarpment formed in gently dipping sedimentary beds of the Jurassic Morrison Formation, particularly the Salt Wash Member, a fluvial, friable, silty, sandstone. Small (~0.5 km²) drainage basins adjacent to the escarpment exhibit evidence for extreme aggradation and erosion events occurring in century and decadal time scales. The availability of loose sediment on the cliffs of the Morrison sandstone, in the form of a weathered regolith, may, in part, explain this extreme erosion behavior. The regolith, sometimes up to 25 cm thick, contains primary sedimentary structures suggesting that it weathers in situ. The transition from regolith to bedrock is abrupt, usually less than 1 cm. The regolith is distinct because of its relatively lower bulk density, higher porosity, diminished shear strength and the presence of roots and organic matter. Characterizing the processes behind the weathering of bedrock into regolith will contribute to an understanding of the geomorphic history of the basins in the study area.

In this study we examine the mineralogy and texture of the regolith and underlying bedrock in order to assess the processes involved in the transformation of bedrock to regolith. Samples of the Salt Wash Member were collected at depth intervals in the regolith from the surface to the bedrock interface, and were examined in thin section. Preliminary examination indicates that Morrison bedrock is largely clay cemented with kaolinite, chlorite and mixed layer clays and exhibits little evidence of calcite or silica cementation. We hypothesize that this clay cementation plays a significant role in the weathering of bedrock to regolith. To further test this hypothesis, we are conducting additional petrographic and XRD analysis.