Abstract

In many parts of the Colorado Plateau continuous conglomerate and sandstone lenses within the Burro Canyon formation (Early Cretaceous age) present a contrast to the dominantly mudstone upper part of the Morrison formation (Late Jurassic age). In the Four Corners area of southwestern Colorado, however, the lithologic character of the Burro Canyon is extremely variable, and in places the formation consists entirely of mudstone. The mudstone of the Burro Canyon formation and mudstone of the Brushy Basin member of the Morrison formation characteristically weather differently, the mudstone of the Burro Canyon to hacky fragments and mudstone of the Brushy Basin member to a hard frothy crust. This difference is believed due to original differences in composition rather than diagenetic processes, and it was found to persist throughout south-western Colorado. The compensatory thickening of the Burro Canyon formation in relation to the Morrison formation, lateral change within short distances from mudstone of the Burro Canyon to mudstone of the Brushy Basin member, lack of evidence of a persistent unconformity between the Morrison and Burro Canyon, and the partial interbedding and intertonguing of the lowest conglomerate and sandstone of the Burro Canyon with mudstone of the Brushy Basin indicate that the Burro Canyon formation and Brushy Basin member of the Morrison formation are conformable and intertongue. The lack of fossil evidence prohibits precise placement of the Jurassic-Cretaceous time boundary. The lowest conglomerate in the Burro Canyon formation is part of a system of channel-filling conglomerate and conglomeratic sandstone lenses. The name Karla Kay conglomerate member is proposed for this system of lenses. A new sedimentary cycle accompanied by reduction in volcanic ash deposition is suggested by the Karla Kay conglomerate arid adjacent mudstones of the Burro Canyon formation. The basal conglomerate of the Burro Canyon and Stokes' Buckhorn conglomerate member of the Cedar Mountain formation of central Utah may be lithogenic equivalents. The general decrease in the number of conglomerate and sandstone lenses southward in the Four Corners area of Colorado suggests that this area may be near the southern edge of deposition of the coarse clastic material of the Burro Canyon.

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