Structures, architecture, vertical profiles, palaeohydrology and taphonomy of an upper‐flow‐regime‐dominated fluvial system, the Triassic Dockum Group of the Palo Duro Canyon, Texas

Abstract

Triassic strata of the Tecovas and Trujillo formations in the Palo Duro Canyon of West Texas record deposition dominated throughout by upper‐flow‐regime channel flow. These strata provide an opportunity for new insights into the architecture, floodplain/channel processes, vertical profiles, palaeohydrology and taphonomy typical of broader upper‐flow‐regime‐dominated fluvial systems. Thicker channel‐fills (ca 2 m, aspect ratio ca 24) host upper‐plane‐bed, symmetrical antidune, breaking‐antidune, chute‐and‐pool, and cyclic‐step structures. The thinner channel‐fill population (<1 m, aspect ratio ca 35) comprises upper‐plane‐bed and some antidune structures. Some of the larger channel fills developed low‐angle downstream‐accreting bars, with rare examples of laterally accreting side‐attached bars. This appears to be the first documentation of an ancient upper‐flow‐regime side‐attached bar. Lamina orientation of structures parallel accretion surfaces because these followed the original bed surface, and structures drop in flow regime down the bar surfaces as formative Froude numbers also declined. A generalized vertical sequence for large upper‐flow‐regime channel storeys has upper‐plane‐bed structures above the basal scour, transitioning upward into higher flow regime structures (for example, antidunes, cyclic steps, etc.), then back to a mix of upper‐plane‐bed and antidunes or lower‐flow‐regime structures towards the top. The Tecovas Formation and overlying Trujillo Sandstone both bear layers of cross‐cutting laterally amalgamated valley fills, multivalleys, and are separated by a regional sequence boundary. Calculated palaeodischarges for Tecovas (ca 90 m3/s) and Trujillo (ca 76 m3/s) rivers record flow at the stage where symmetrical antidunes were deposited. These discharges occurred at some point on the falling limb of a flood hydrograph of currently unknown shape. Slopes for the rivers were on the order of 10−4 to 10−5, consistent with typical continental‐interior rivers. Large and flashy discharge was, thus, the likely cause for supercritical‐flow conditions instead of excessive slopes. Rivers in both the Tecovas and Trujillo formations record a fluvial system developed in a tropical to subtropical megamonsoonal environment. The tendency for preservation of metre‐scale channel‐fills and catastrophic inundations of floodplains and lakes in single floods has also made the Dockum Group a prolific preserver of large Triassic vertebrates.

Garzapelta muelleri gen. et sp. nov., a new aetosaur (Archosauria: Pseudosuchia) from the Late Triassic (middle Norian) middle Cooper Canyon Formation, Dockum Group, Texas, USA, and its implications on our understanding of the morphological disparity of the aetosaurian dorsal carapace

“…The Dockum Group was deposited primarily by braided and meandering river systems originating from the Ouachita‐Marathon Orogenic Belt to the South and Amarillo‐Wichita Uplift to the North, with some strata dominated by lacustrine deposits (Lehman & Chatterjee, 2005; Walker & Holbrook, 2023). In northeastern New Mexico and the Texas Panhandle the stratigraphy of the Dockum Group is composed of five or six formations; from oldest to youngest they are the Santa Rosa and/or Camp Springs, Tecovas and/or Garita Creek, Trujillo, Bull Canyon, and Redonda formations (Lehman & Chatterjee, 2005; Lucas, 1993; Martz & Parker, 2017).…”