The Sequence Stratigraphy of the Chinle Formation in the Dinosaur National Monument Region, Utah and Colorado, USA

A Thesis submitted to the faculty of the Graduate School of the University of Minnesota by Ryan Eric Erickson in partial fulfillment of the requirements for the degree of Master of Science, October 2007.Deposition of Late Triassic Chinle Formation fluvial and lacustrine sediments in the Dinosaur National Monument (DNM) region of Utah and Colorado is bounded by two major periods of landscape degradation (stratal truncation) and punctuated by a period of non-deposition (well-developed paleosol pedogenesis). Correlation of these unconformable regional features in outcrop and geophysical well-logs enable the identification of two genetically related Chinle depositional sequences. The first depositional sequence includes the Gartra Member and the mottled member. These units are located within regional paleovalley networks that are attributed to Mid-Triassic Tr-3 sequence boundary erosion. The basal Gartra Member conglomeratic sandstone was deposited in topographically-constrained, low-sinuosity fluvial systems. As aggradation continued, the lateral constraint of the paleovalley on the fluvial system decreased and higher sinuosity fluvial deposits and minor floodplain deposits are preserved. Overlapping the paleovalley margins and covering the entire landscape are fine grained mottled member floodplain deposits. Capping the mottled member is a well-developed paleosol that is a non-depositional sequence boundary and the end of the first depositional sequence. The second depositional sequence includes the ocher member and the upper member. Ocher member siltstone deposition occurred in large, shallow, evaporative, nonmarine influenced, lacustrine environments. Initial upper member sandstone and siltstone units were deposited in fluvial-deltaic environments. Later upper member deposits are weak-red lacustrine siltstones with few interbedded fine-grained sandstones. The J-0 erosional unconformity marks the top of preserved Chinle deposition and the bottom of the subsequent Jurassic eolian sandstones. After identification and analysis of Chinle depositional environments and known allogenic influences (a gradually drying climate), an autogenic mechanism is proposed to be the primary influence on the observed facies changes. In a basin with pre-existing erosional topography, the annealing of this landscape could initiate autogenic retreat of the depositional systems leading to distinct facies changes in the distal basin. In the first Chinle depositional sequence (Gartra Member-mottled member), the annealing of the Tr-3 paleovalley network caused a decrease in fluvial energy and the autogenic retreat of deposition into the proximal Eagle Basin. In the distal basin, continued subsidence and a period non-deposition resulted in a well-developed paleosol horizon (mottled member paleosol) and the transgression of lacustrine facies (ocher member). As the sediment-charged fluvial mega-fan prograded out of the Eagle Basin, sedimentation in the distal basin gradually increased until thick fluvial-deltaic deposits (upper member sandstones) developed filling excess basin accommodation. Subsequent allogenic drying climatic conditions may explain a decrease in later distal basin sedimentation and the second observed lacustrine transgression (upper member siltstones)