Source-to-Sink Sand and Mud Partitioning across a 300 km Transgression of the Western Interior Seaway (Campanian)

Gomez, Carolina A. ¹, Ron Steel², Andrew Petter³ (1) University of Texas at Austin, Austin, TX (2) The University of Texas at Austin, Austin, TX (3) University of Texas at Austin, Austin

A 300 km source-to-sink transect from north-central Colorado to southwest Wyoming illustrates how sand and mud are differentially sequestered in lowstand deltas, backstepping, barrier and estuarine systems and eventually in tidal-fluvial lithosomes, during a ca.1my (ammonite zones Baculites scotti to Didymoceras stevensoni) transgression of late Campanian shorelines. The deposits of the Rusty member of the Ericson Sandstone, Wyoming, allow examination of the sourceward reaches of the transect, where fluvial, fluvial-tidal and inner-estuarine sand bodies expose the tidal and brackish-water limits of the transgression, and where clusters of unconformities are severely thinning the succession against an embryonic Laramide uplift. Individual high-frequency rises of sea level partitioned 8-25 m of sand in broad fluvio-estuarine valleys and 2-6 m of more extensive capping mudstones. The intermediate seaward reaches of this transgressive system, some 150 km basinwards, record multiple, high frequency, backstepping shorelines of the upper Iles (Iles 10-14 sand tongues) Formation in the Sand Wash Basin of north Colorado. Here individual phases of stillstand to rising sea level sequestered 20-40 m of wave and tide-influenced sand within bayhead deltas, tidal inlets and punctuated regressive deltaic shorelines, while mud is partitioned into back-barrier areas and onto the shelf. Farther seawards in this backstepping system (another 150 km basinwards), there are slightly older lowstand shoreline systems (Carter Sandstone of the Middle Park Basin, Colorado) where sand bodies up to 25m thick develop in a regime with strong tidal influence. A model is presented for the partitioning of the sand/shale sediment budget during this transgression.

AAPG Search and Discovery Article #90055©2006 AAPG Rocky Mountain Section Meeting, Billings, Montana