ABSTRACT: Fluvial Response to Subsidence Determined from Remote Sensing

Mary J. Kraus

Well-exposed rocks of the fluvial Willwood Formation, covering approximately 5000 km² of the central part of the Big Horn basin, Wyoming, were analyzed with Thematic Mapper (TM) data. False-color images and field analysis were used to characterize and map large-scale lithologic packages in this lower Eocene unit. Field criteria used to distinguish among the packages include mudstone coloration and type and abundance of nodules, both of which reflect the type of alluvial paleosol that developed; abundance, geometry, and paleotransport direction of sand bodies; and abundance and geometry of carbonaceous shales. The lithologic packages reflect both spatial and temporal variability; biostratigraphic data were used to establish which packages are time correlative.

Differences among time-correlative fluvial packages (facies) reflect variability in local moisture regimes and sediment accumulation rates, factors that influenced the location of major stream channels and the types of palesols that formed on overbank deposits. Facies distribution demonstrates that east-west-trending lineaments, which segment the Bighorn Mountains, extend into the basin and were active faults during the early Eocene. The lithologic heterogeneity is attributed to differential crustal subsidence on either side of the lineaments.

Vertical changes in lithology record temporal variability in basin subsidence rates. Subsidence rates slowed over time producing brighter mudstones (more mature paleosols) higher in the section and changes in carbonaceous shale type and abundance. The location of major channel sand bodies also appears to have shifted westward over time. Fluvial Willwood rocks are capped by the lacustrine Tatman Formation in certain parts of the basin. TM images suggest that the north-south extent of the lake deposits was determined by the location of several of the lineaments.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990