Continental Sequence Stratigraphy of the Upper Triassic (Norian Rhaetian) Chinle Strata, Northern New Mexico, U.S.A.: Allocyclic and Autocyclic Origins of Paleosol-Bearing Alluvial Successions

Abstract

Abstract Two age-equivalent Upper Triassic fluvial successions deposited on the continental interior of the southwestern United States were evaluated using an adapted marine stacking-pattern analysis methodology. A three-tier cyclic hierarchy is present in the strata at both study areas. Meter-scale fining-upward fluival aggradation cycles (FACs) comprise fluvial aggradational cycle sets (FACSETs) 4–15 m thick (avg. 8.4 m). FACSETs in turn stack into four fluvial sequences 26–48 m thick (avg. 41 m). Within these sequences, transgressive-systems-tract equivalents (TE) are characterized by channel sands and associated minor overbank deposits and relatively immature paleosols (i.e., high rates of deposition), whereas highstand- to falling-stage-systems-tract equivalents (HFE) are dominated by overbank muds and relatively well-developed paleosols (i.e., lower rates of deposition). These two fluvial successions, which are 200 km apart, contain age-equivalent fluvial sequences that record similar histories of deposition and pedogenesis: Sequence 1 contains only an incomplete HFE; Sequence 2 includes both the TE and HFE; Sequence 3 is an HFE; and Sequence 4 contains only a TE. Fluvial sequences likely accumulated in response to pulses of source area uplift and/or basin subsidence, which resulted in changes in accommodation. Conversely, higher-frequency FACs and FACSETs that occur within sequences do not correlate between study areas and are likely the products of autocyclic processes, such as channel avulsion, floodplain aggradation, and channel migration. These results suggest that regionally significant tectonic episodes may be discernible in suspended-load fluvial deposits that accumulated over a broad area.