Hummocky Cross-Stratification Vs. Swaley Cross-Stratification Sequences in South-Central Wyoming, Mesa Verde Sandstone and Haystack Mountains Formation Outcrops: ABSTRACT

Abstract

Progradational shoreline sandstones, showing evidence of combined flow (waves and currents), and distributary-channel sandstones were deposited in Cretaceous (Campanian) time as part of a wave-dominated delta of the upper part of the Haystack Mountains Formation in the Rawlins, Wyoming, area. Depositional processes, water depths, and flow directions were interpreted utilizing physical structures and trace-fossil assemblages in two progradational inner-shelf to upper shoreface sequences. The uppermost formal member of the Haystack Mountains, the Hatfield Sandstone Member, where it crops out northeast of Rawlins, is a shoreline deposit. A typical suite of well-developed shoreface trace fossils is recognized, the upper units including Ophiomorpha, Asterosoma, Rosselia, and Teichichnus. Hummocky Cross-stratification (HCS) and swaley cross-stratification (SCS) are abundant in two of the upper members of the Haystack Mountains Formation. HCS and SCS, where they occur in the same sequence, differ in form (synform, antiform); grain size ( =/ 125 /mu/); bed thickness (0.1-1 m, 0.5-4 m); amalgamation (uncommon to common, very common); stratigraphic position (below SCS, above HCS and below foreshore); and environments of deposition (inner shelf and lower shoreface, middle and upper shoreface). In SCS, antiforms are essentially absent (<5%) and horizontal laminae are rare (<10%), In HCS, antiforms usually occur inmore » amounts greater than 20% by volume. Within a single bed of SCS, synforms usually occur almost to the exclusion of antiforms. The thickness of individual lamina sets is highly variable and is strongly dependent on the lateral dimensions (up to 3 m) of individual HCS or SCS.« less