SEQUENCE STRATIGRAPHY AND INCISED-VALLEY FILL ARCHITECTURE OF THE LOWER PART OF THE UPPER TRIASSIC CHINLE FORMATION IN THE DINOSAUR NATIONAL MONUMENT REGION, UTAH AND COLORADO

The Gartra Member of the lower part of the Upper Triassic Chinle Formation in the Dinosaur National Monument region represents a terrestrially deposited incised-valley fill succession. Previous lithostratigraphic work on the Gartra in this region has defined a single fining-upward conglomeratic sandstone unit that that was incised into the Lower Triassic Moenkopi Formation and graded into the overlying "mottled member". Detailed field work and sequence stratigraphic correlations have identified 3 unique packages of fluvial deposits (G1-G3) within the Gartra and related "mottled member". The basal G1 unit is a coarse conglomeratic sandstone that is laterally discontinuous and found at the base of only the deepest incised valleys. At one site, the G1 cuts into lake beds that had previously filled the incised valley indicating that the there were other deposits in the valley previous to Gartra deposition. The G2 is the thickest Gartra deposit and cuts down into the G1 and the Moekopi. The G2 is generally finer grained than the G1 and occasionally has silty interbeds at channel margins. G2 channels are found in both major fluvial systems that extend for several kilometers and in single channel deposits that laterally pinch out into thick floodplain and paleosol deposits. Upper G2 beds become increasingly mottled as the Gartra grades into the pedogenically modified "mottled member". The "mottled member" consists of sandstones and siltstones that have been bioturbated and altered by mineral translocation during groundwater fluctuations. Within the "mottled member" are sporadic, laterally discontinuous white sandstone channels which are the G3 deposits. The "mottled member", with its G3 beds, is the most regionally extensive lower Chinle unit in the area and it is interpreted to represent the overtopping of the previously cut paleovalley. As the fluvial system aggraded, it overlapped the valley margins, depositing high-sinuosity alluvial facies across an unconfined landscape, in contrast to the underlying, confined fluvial deposits of the lower Gartra units.