Paleohydrological features and detrital compositions of the Late Triassic Chinle Formation in southwestern Utah, U.S.A

Abstract

The present study reconstructed the early Norian drainage basin in equatorial western Pangaea based on paleohydrological and petrographical analyses of the Chinle Formation in southwestern Utah, U.S.A. The formation in this region is subdivided into the Shinarump and Cameron members, and the “purple pedogenic beds” in ascending order, and is characterized mainly by fluvial channel and channel complex deposits and pedogenically modified muddy floodplain deposits. Paleohydrological features, which were calculated using the thicknesses of bar deposits and cross-sets, show an upward decrease in bankfull discharges, and revealed the length of estimated upstream drainage basins of up to 260–640 km to the south of the study area. The clast compositions of the conglomerate demonstrate an upward decrease in quartzite and quartz, and an upward increase in chert and acidic volcanic rock fragments. The framework compositions of the 53 sandstone samples indicate that the Shinarump Member has a higher quartz grain content than the Cameron Member. X-ray diffraction analyses of 74 mudstone samples indicate that the clay mineral composition of the Shinarump Member consists of nearly equal amounts of illite and kaolinite, and less chlorite. The Cameron Member and the “purple pedogenic beds” are characterized by a dominance of kaolinite and smectite. No authigenic structures of the clay minerals were observed in scanning electron microscope images, and so most clay minerals are of clastic origin. Stratigraphic variations in the clast composition of conglomerate, and the detrital compositions of sandstone and mudstone, are interpreted to have been controlled by a combination of multiple factors: a temporal decrease in the intensity of chemical weathering of the source rocks due to climatic drying with seasonality, enhancement of volcanic activity in the provenance terranes, and erosion of the basement rocks in the source areas. These compositions differ slightly from those of the Chinle Formation in the Four Corners region, which is located 300 to 400 km east of the study area. These differences suggest the development of two distinct drainage basins. In addition, the reconstructed short length of the drainage basins indicates that the source areas of the Chinle Formation in southwestern Utah were mainly the Mogollon Highlands and Cordilleran volcanic arc, and the headwaters were interpreted as not extending to the East Mexico arc.