INTEGRATED LITHOSTRATIGRAPHIC, BIOSTRATIGRAPHIC, AND GAMMA RAY CORRELATION TECHNIQUES FOR IMPROVED SEQUENCE STRATIGRAPHIC ANALYSIS OF MIDDLE JURASSIC STRATA, WYOMING AND MONTANA

Incorporation of lithologic descriptions, fossil zonations, and outcrop to subsurface gamma-ray correlations improves the relative chronostratigraphic framework for reconstruction of the depositional history of the Middle Jurassic units of northern Wyoming and southern Montana. Difficulty in defining lithostratigraphic and chronostratigraphic relationships between the Bajocian to Callovian Gypsum Spring, Piper, Sawtooth, and Sundance formations has resulted in inconsistent and often contradictory stratigraphic correlations. Lithostratigraphic analyses provide the regional framework for examining depositional facies over time. Biozones provide a temporal framework, provided that they are placed within their lithologic/depositional context. Finally, outcrop to subsurface gamma-ray correlations afford direct comparisons between lithologies and well-log patterns, thereby improving regional correlations.

Seven regionally significant surfaces are recognized through these integrated techniques. The stratigraphic surfaces divide the Middle Jurassic section into four major transgressive-regressive (T-R) cycles. The lower portion of the Gypsum Spring Formation of Wyoming and the equivalent Piper Formation of Montana records the first T-R cycle. The upper section of these same formations record the second T-R cycle. A regionally significant horizon marked by the coral Coenastraea hyatti Wells, microbial (thrombolite) buildups, and the Pleuromya compressa bivalve assemblage marks the surface of maximum sediment starvation within this second cycle. The “lower” Sundance and “upper” Sundance formations represent the third and fourth T-R cycles respectively.