SEDIMENTOLOGICAL ANALYSIS OF THE BASAL MEMBER OF THE CHINLE FORMATION AND ITS PALEOGEOGRAPHIC AND PALEOHYDROLOGIC IMPLICATIONS

The Late Triassic Chinle formation was deposited over a broad portion of the Colorado Plateau, outcropping in regions of Utah, Colorado, New Mexico, and Arizona. Its depositional environment consisted of broad flood plains, which we see today as the characteristic colorfully banded badland topography, and meandering streambeds, which can be observed as conglomerates or sandstone cliffs. The primary area of study presented in this research is located in northeastern New Mexico near the town of Abiquiu, with additional data obtained from Springdale, Utah, just outside of the southern border of Zion National Park. In these regions, the basal Chinle, sometimes referred to as the Shinarump member, forms cliff-like features sitting unconformably on top of the Permian system. While the Chinle has many members that vary in appearance depending on their location, the basal Chinle consistently appears as a coarse-grained sandstone and pebble conglomerate throughout its outcrop area. This conglomerate is stratified into distinct units which vary in grain size, shape, orientation, and lithology. From bottom to top, these units consist of a poorly sorted sandstone unit up to 30 centimeters in thickness with occasional crossbedding, a thin clast-supported unit up to 8 centimeters in thickness with 3-8 centimeter sized rounded quartzite clasts, a 1.5-3 meter thick sandstone unit with crossbedding, and another 8 centimeter thick unit with round coarse grained chert and quartzite clasts. While these units are part of the same member, each unit represents a different environment within the streambed during its deposition. By performing a detailed analysis of the grains within each unit of an outcrop through a pebble count, it is possible to determine the source rock and its proximity to the place of its final deposition, the general strength of the flow of the river, as well as the flow direction of the ancient stream. These analyses include measuring grain size, determining lithology, describing the nature of the basal contact, describing graded bedding, and measuring flow directions using flow-direction indicators such as cross bedding and pebble imbrication.