LATERAL VARIABILITY OF SPLAY DEPOSITS IN A DISTRIBUTIVE FLUVIAL SYSTEM: UPPER TRIASSIC CHUGWATER GROUP, FREMONT COUNTY, WYOMING

Modern distributive fluvial systems (DFS) are known from many tectonic settings, but relating modern geomorphology to ancient sedimentary architecture has been limited. Crevasse splays in modern DFS display lateral variability in clast size and sedimentary structures, but comparable variability in ancient deposits is poorly documented.

Of the two uppermost formations of the Upper Triassic Chugwater Group, the Crow Mountain Formation has been described as a marine to tidal flat environment and the Popo Agie Formation as a tributary fluvial system. However, laterally extensive sand beds, few channels, and common upper flow regime sedimentary structures all suggest a distributive fluvial environment. The Crow Mountain Formation contains largely medial- to proximal-splay deposits, consisting of planar-laminated sands, upper flow regime structures (antidunes and chutes-and-pools), and interbedded silty sands. Proximal splay deposits grade laterally from massive and trough cross-stratified pebble conglomerates into planar and cross-stratified sands. Medial splay deposits are laterally extensive over mile to sub-mile scale and consist mainly of very fine sands and silty sands. These splays commonly contain ripple lamination, dune cross stratification, and planar bedding, although some are massive and lack sedimentary structures. Paleosols are laterally extensive over miles, but some pedogenic features, such as root mottling, are commonly more localized to 10’s of meters. These paleosols are interpreted as oxisols, owing to their lack of mineral horizonation, but many paleosols also display vertic features, such as slickensides. The Popo Agie Formation contains mainly distal DFS facies, specifically, distal crevasse splays and ponded deposits. These facies are dominated by laminated and massive mudstone, vortex-rippled silty mudstone, and analcime-rich beds. Massive and laminated mudstones are the most common facies in the Popo Agie Formation, and correlating specific mudstone beds is difficult beyond 100 meters. Pedogenic alteration of the mudstone beds is seen over at least 100’s of meters laterally. Analcime-rich and vortex-rippled silty muds cannot be traced beyond distances of 100 meters and were presumably localized to small, ponded areas similar to modern distal DFS.