THE SEQUENCE STRATIGRAPHIC ARCHITECTURE OF THE MIDDLE JURASSIC GYPSUM SPRING AND PIPER FORMATIONS IN THE EASTERN BIGHORN BASIN OF WYOMING

The Jurassic of Western North America is divided by a series of regional unconformities, J-0 through J-5. In northern Wyoming and southern Montana, strata between the J-1 and J-2 unconformities, the Gypsum Spring and Piper formations, represent the earliest deposition in the Sundance Seaway, a retro-arc foreland basin produced by the Nevadan Orogeny. In this study, the sequence stratigraphy of these units was examined along the eastern flank of the Bighorn Basin. In addition to the J-1 and J-2 unconformities, two additional sequence boundaries are recognized here. The J-1a separates the lower and upper portions of the Gypsum Spring Formation, and the J-1b separates the Gypsum Spring from the overlying Piper Formation. The first sequence is characterized by alternating thick gypsum beds and red siliciclastic mud, and represents a mosaic of costal salinas and supratidal flats during the TST. Overlying gypsiferous red to chocolate siliciclastic mud represents supratidal mudflat deposition during the HST. The J-1a sequence boundary is marked by a purple mudstone, interpreted as a paleosol, which caps the first sequence, and an abrupt deepening into carbonates of the second sequence. This second sequence contains a thick carbonate TST overlain by a thicker carbonate HST. Carbonate facies in this sequence include open shallow-subtidal peloidal to skeletal packstone and wackestone and lime mudstone, restricted shallow-subtidal lime mudstone to skeletal wackestone, poorly developed ooid grainstone, and peritidal dolomite with stromatolitic and thrombolitic texture. The J-1b sequence boundary is marked in updip areas by a chert horizon and in downdip areas by abrupt shallowing above open shallow-subtidal ooid grainstone. The J-1b surface is overlain by maroon mudstone of the Piper Formation, and records deposition on a muddy tidal to supratidal flat. The lack of recognizable flooding surfaces in this third sequence suggests aggradational stacking. All three sequences record the same evaporite-carbonate ramp facies association. This refined sequence stratigraphy aids in regional correlation, establishes three early Middle Jurassic marine incursions in the Sundance Seaway, and describes the sequence stratigraphic context of a widespread ancient evaporite deposit.