Ichnology of the Upper Triassic Chinle Formation, Colorado Plateau: Interpreting Ancient Environment, Hydrology and Climate

Abstract

The Upper Triassic Chinle Formation (CF) contains diverse and abundant trace fossil associations (TFA) in clastic and carbonate deposits from numerous areas on the Colorado Plateau and Rocky Mountain region. The sedimentologic and stratigraphic succession of TFA serve as proxies to reconstruct the spatial and temporal changes in environments, landscapes, local and regional hydrology, and climate during CF deposition. At least one study to date has used multiple proxy data to propose that sedimentary facies, pedofacies, and TFA record latitudinal climatic conditions that CF deposition experienced from 5o to 30oN latitude. These relations can be used to interpret sedimentation rate, accommodation, paleosol formation, and landscape stability to construct a sequence stratigraphic framework for the continental deposits that have no marine influence on stratal architecture and stacking patterns. Deposits interpreted as active meandering and braided river channels contained little to no bioturbation; only short U-shaped (Arenicolites) and horizontal surface burrows (Planolites), trails (Cochlichnus, Gordia, Mermia, Haplotichnus), and rare footprints, reflecting periods of low flow or short-term subaerial exposure with wet surfaces. Deposits interpreted as subaerially exposed bars, levee, and crevasse splays show a range of ichnodiversity, bioturbation intensity, tiering, and pedogenesis depending on degree and frequency of sediment accumulation, length of time between depositional events, and water table position. Such traces as Steinichnus, Planolites, Palaeophycus, and shallow roots comprise shallow tiers in sand- to mudstone-dominated deposits, mostly under high soil moisture conditions. As time between events and water-table depth increases, paleosol development also increases in concert with the presence of such traces as Camborygma, Scoyenia, Celliforma, insect cocoons, Naktodemasis, Archeoentomichnus, and more complex and deeper rooting patterns; more common particularly in proximal and distal floodplain deposits. Finer grained deposits also occupy more distal environments when accommodation and the grain sizes were available. Well-developed paleosols represent relatively stable landscapes that could serve as sequence boundaries with lateral extent. Patterns in floodplain deposits from weakly to better developed paleosols infer changes in accommodation thru time; weaker equates to higher accommodation, whereas better equates to lower accommodation.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015