RUMORS OF THE J5’S DEMISE HAVE BEEN GREATLY EXAGGERATED: ICHNOLOGY AND SEDIMENTOLOGY REVEAL DETAILS OF AN OXFORDIAN FORCED REGRESSION (Invited Presentation)

Outcrops of the Bathonian-Oxfordian Sundance Formation (SF) at Wyoming’s Glendo (GR), Alcova (AR), and Seminoe (SR) Reservoirs offer the opportunity to test the hypotheses that: 1) the J5 unconformity does not exist, 2) the Windy Hill Member (WH) accumulated as a large tidal delta analogous to the Han River delta, and 3) the SF is divisible into a lower, carbonate and an upper, silicic section.

At GR, <2 m thick, sandy tidal mouth bars of the WH, hosting Conichnus, Palaeophycus, Siphonichnus internally, and dinosaur tracks on their surfaces, accumulated unconformably on offshore deposits of the Redwater Shale (RS) containing Rhizocorallium, Thalassinoides, Neonereites, and Chondrites. The basal WH at SR records sandy intertidal flat deposits with Pteraichnus and dinosaur tracks, deposited on a sharp surface truncating the RS's shell coquinas and glauconitic, HCS beds containing robust offshore trace assemblages. At AR, the WH incises shell coquinas and offshore deposits of the RS and again, hosts Pteraichnus and dinosaur tracks demonstrating intertidal environments. Accumulation of intertidal deposits directly on offshore sediments, deposited below fair-weather wave base requires a significant unconformity (the J5) to account for 10’s of m of missing strata.

The WH at AR is 10 m thick, composed of heterolithic successions of delta fronts, intertidal flats, and tidal dunes with minimum flow depths of 6 m during high water, while none of the bars preserved at GR exceed 2 m height. The Han River has ≤9 m tidal range, 10-40 m-thick tidal bars, and builds into the open ocean, making it a poor analog for the WH. Instead, given the storm-dominated setting of the underlying SF, it is likely that barrier islands and spits protected broad, shallow, tide-dominated lagoons with tidal creeks and distributary channels, analogous to the Laguna Madre on the lower Texas coast.

The change from carbonate- to clastic-dominated deposition reported in the Bighorn Basin is not present in the study area, reflecting proximity to clastic sources from the east and west and greater turbidity in the water column. The variability of the SF across a relatively small range stresses the importance of detailed ichnological and sedimentological analysis and the risks of extrapolating regional, or basin-scale depositional trends from local studies.