SEQUENCE STRATIGRAPHY OF THE CHINLE FORMATION IN DINOSAUR NATIONAL MONUMENT, UT AND CO

Deposition of the fluvial and lacustrine sediments comprising Late Triassic Chinle Formation in the Dinosaur National Monument (DNM) region of Utah and Colorado was punctuated by two major periods of overall landscape degradation as evidenced by truncated strata and well-developed paleosols. The correlation of these degradational features both in outcrop and geophysical well logs permit the division of the Chinle into two genetically related depositional sequences.

The first depositional sequence includes the Gartra Member and the mottled member. These units are located above the Early-Late Triassic sequence boundary and are within regional paleovalleys that were incised into the Early Triassic Moenkopi Formation. At the base of these paleovalleys is the Gartra, which is a conglomeratic sandstone that formed in low sinuosity fluvial systems. Within the Gartra, periods of landscape degradation during the deposition of the paleovalley fill create small scale sequence boundaries. As paleovalley aggradation continued, the river systems became less constrained and high sinuosity fluvial deposits and paleosols were preserved. This fining upward trend continues into the mottled member where small sandy alluvial channels are surrounded by extensively pedogenically modified overbank deposits. This unit marks the overlap of the basal paleovalley and the boundary between the two Chinle depositional sequences.

The second depositional sequence includes the red siltstone member, the ocher siltstone member, the upper sandstone unit, and the upper carbonate unit. These units are generally lithologically consistent on a regional scale. The red siltstone member and the ocher member are found above the mottled member sequence boundary and represent a large, shallow, non-marine influenced, lacustrine environments. The overlying upper sandstone unit is a massive sheet delta deposit that varies regionally in thickness and character. The upper carbonate unit is another shallow lacustrine deposit that alternates between red mudstones and carbonate rich green beds. Jurassic eolian sands unconformably overly the upper portion of the Chinle and truncate the Triassic as deeply as the upper sandstone unit.