Paper No. 11
Presentation Time: 8:00 AM-12:00 PM
ALLUVIAL ARCHITECTURE OF A DISTAL ANCESTRAL ROCKY MOUNTAINS FLUVIAL SYSTEM, PERMIAN HERMIT FORMATION, SEDONA, ARIZONA
The Lower Permian Hermit Formation represents the evolution of an arid, ephemeral, fluvial system that sourced highlands of the southern Ancestral Rocky Mountains. Hermit streams flowed SW across the Sedona arch and onto arid coastal plains that bordered epicontinental seas to the south and west of Sedona. Five facies assemblages have been recognized: 1) Fine-grained sandstone comprises laterally extensive, convex-up cross sets with upper plane bed stratification in complex beds 1-4 m thick. Ephemeral, probably catastrophic floods spread sand-choked river deposits as lateral- and downstream-accreting alluvial ridges; sand was derived from extensive eolian dune and loess deposits that accumulated on the coastal plain. The ephemeral deposition coupled with extensive lateral reworking prevented formation of larger bedforms; even ripples were relatively rare on the dynamic sandy ridges and channels. 2) Bioturbated sandstone and intercalated mudstone beds 5-30 cm thick grade laterally and vertically from facies 1 and are interpreted as splay deposits that broke through alluvial ridges onto floodplains. 3) Intraformational conglomerate formed in arroyo channels and as thalweg deposits of larger bedload streams. 4) Mudstone consists of 1-5 m thick tabular bodies that contain carbonate rhyzoliths and caliche nodules. This facies is the source of clasts for the conglomerate and formed on broad, arid floodplains. 5) Siltstone consists of beds 0.5-4 m thick. Lack of stratification and other sedimentary structures suggests deposition as loessite.
Overall alluvial architecture comprises tabular ledges and slopes in sequences 10-20 m thick. Successive floods of unconfined river systems constructed coalesced, sheet-like alluvial ridge deposits. Floodplain complexes vertically sandwiched between sandstone bodies include locally abundant loessite and eolianite deposits as well as incised arroyo-formed conglomerate. Arroyos may have occurred as tributary streams on the arid floodplains or may represent incision caused by climate, tectonic, or eustatic controls between accreting fluvial sandstone intervals. The second hypothesis suggests that the overall tabular ledge-slope geometry probably reflects changing external controls rather than intrinsic shifting of floodplain-channel processes.