ALLO- AND AUTOCYCLIC SIGNATURES IN PALEOSOL-BEARING ALLUVIAL SUCCESSIONS: THE TRIASSIC (NORIAN-RHAETIAN) CHINLE GROUP, NORTHERN NEW MEXICO

Two age-equivalent Upper Triassic fluvial successions deposited on the continental interior of the southwestern U.S. have been evaluated using an adapted marine stacking pattern analysis methodology. Location 1 near Montoya, NM contains the Bull Canyon (Norian) and Redonda (Rhaetian) Formations, and location 2 near Abiquiu, NM contains the upper portion of the Petrified Forest Formation (Norian) and the Rock Point Formation (Rhaetian). Strata in both areas exhibit a three-tier cyclic stratal hierarchy. Meter-scale fining upward fluvial aggradation cycles (FACs) comprise 4-15m thick (avg. 8.4m) fluvial aggradational cycle sets (FACSETs). FACSETs in turn stack into four 26-48m thick (avg. 41m) fluvial sequences. Four sequences (two Norian and two Rhaetian) have been identified at each location. Within these sequences, transgressive system 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). The two study areas, which are over 200 km apart, contain age-equivalent fluvial sequences that record similar histories of deposition and pedogenesis: the earlier Norian contains only an incomplete HFE, the latter Norian includes both the TE and HFE, the earlier Rhaetian is dominantly a HFE, and the latter Rhaetian contains only a TE. These four third-order fluvial sequences likely accumulated in response to tectonic and/or climatic mechanisms. Conversely, higher frequency FACs and FACSETs that occur within sequences do not correlate between study areas and are likely the product of autocyclic processes. These results suggest that regionally significant climatic shifts and/or tectonic episodes may be discernable in suspended-load fluvial deposits that accumulated over a broad area.