Preliminary report on mudlumps in lacustrine deltas of the Monitor Butte Member of the Upper Triassic Chinle Formation, southeastern Utah

Abstract

Mudlumps in the Monitor Butte Member of the Upper Triassic Chinle Formation are the product of syndepositional deformation in lacustrine, delta-front, and prodelta deposits. Diapirism in lacustrine and delta deposits caused intraformational folds and faults that were penecontemporaneous with sedimentation. Deposition of the Shinarump, Monitor Butte, and Moss Back Members of the Chinle Formation in southeastern Utah was in a complex fluvial-lacustrine system, in which fluvial channel systems with abundant wetland floodplain environments flowed generally west into a large lake. Intraformational deformation occurred in lacustrine and deltaic units near the lake margin. Lacustrine deposits in and near the upper part of Glen Canyon are locally well exposed and are composed of red and purple bentonitic mudstone, which, in places, contain ostracodes, and burrowed, sandy limestone. Prodelta deposits are composed of gray and purple bentonitic mudstone. These lacustrine and prodelta deposits contain high-angle reverse faults, locally vertical beds, and diapiric strata interpreted as mudlumps. Delta-front deposits comprise green, micaceous and carbonaceous, coarsening upward sequences of foresetbedded siltstone and sandstone that dip as much as 28° and are as much as 24 m thick. Rapid progradation of the delta front over prodelta and lacustrine sediments produced overloading and subsequent deformation by diapirism. The mechanism of formation of the Monitor Butte mudlumps appears to have been similar to that proposed for modern Mississippi River delta mudlumps. The processes that formed the mudlumps in the vicinity of Glen Canyon may be the same processes that produced the vertical and contorted bedding that is more poorly exposed in the Monitor Butte Member elsewhere on the Colorado Plateau.