SEDIMENTOLOGICAL CONSTRAINTS ON THE EVOLUTION OF THE LATE TRIASSIC CORDILLERAN ARC: NEW EVIDENCE FROM THE SONSELA MEMBER OF THE CHINLE FORMATION, PETRIFIED FOREST NATIONAL PARK, AZ

The Sonsela Member of the Upper Triassic Chinle Formation in Petrified Forest National Park (PEFO) consists of heterolithic bodies of sandstone, conglomerate, and mudstone that represent an anomalous coarse-grained interval within a dominantly mudstone formation. The coarse-grained nature of these deposits, dominated by extrabasinal siliceous clasts, suggests that the Sonsela fluvial system was governed by starkly contrasting depositional controls compared to those of other units in the Chinle Formation. This study is a first attempt to evaluate both the scale and nature of these controls.

The Sonsela within PEFO comprises 3-5 fining-upward sandstone-mudstone sequences; sandstone units consist of well-exposed arenites and wackes, locally volcanic-rich, with grains that range from coarse and very coarse sand to pebbles. Each of these horizons, characterized by sharp, scoured bases and dominated by trough- to tabular-cross-stratified sandstone and horizontally stratified sandstone and conglomerate, mark the incursion and evolution of major north- to northeast-flowing fluvial channel systems. The mudstone units consist of poorly exposed bentonitic claystone to sandy mudstone interpreted as overbank deposits. The diverse sand body geometry ranges from single- and multi-story sheets to ribbons. Preliminary analysis of the overall sedimentary architecture and of the sandstone fill suggests a broad range of stream types from bedload-dominated braided to mixed-load meandering systems. An abundance of felsic volcanic clasts, subhedral to euhedral volcanic glass and zircon grains, biotite grains, and bentonite document strong volcanic influence on sedimentation.

The Chinle Formation has been interpreted as a mud-dominated fluvial system deposited within a backarc basin controlled by dynamic subsidence. We believe that Sonsela deposition reflects sharp uplift of the incipient Cordilleran arc and subsequent rapid sediment influx to the backarc basin. As such, the Sonsela marks the initial incursion of Cordilleran-derived, coarse-grained sediment onto the Colorado Plateau.