JURASSIC MACROSTRATIGRAPHY OF THE WESTERN UNITED STATES

Sedimentation rate, type, and preservation potential vary over time as a result of changes in eustasy, tectonics, and climate. The resulting mosaic of time gap-bound packages of sediment constitute the stratigraphic record. Macrostratigraphy utilizes these gap-bound sedimentary packages as a fundamental basis for quantifying spatiotemporal patterns in the preserved rock record. Here I present a high-resolution macrostratigraphic analysis of the Jurassic system in the western US. Data for this analysis were compiled from the literature and were synthesized into composite columns representing 40 geographic regions, from Montana southward to New Mexico and from Nevada eastward to Colorado. The sedimentary units comprising each gap-bound package were characterized on the basis of lithology, depositional environment, general lithostratigraphy, and fossil content. Hiatus durations separating packages were determined by integrating Jurassic biostratigraphy, palynology, magnetostratigraphy and available radiometric dates. Temporal resolution for this study is approximately 1 Myr. Previous work has suggested the presence of six major Jurassic unconformities in the western United States and has attributed them to regional flooding events of epeiric seaways and subsequent retreats (J-0 to J-5 sequence boundaries). The regional and temporal extent of these surfaces can be quantified in the Jurassic Macrostratigraphic Database and correlate with times of higher average rock package truncation and initiation rates. Sediment packages coded by lithology indicate that deposition during the Lower Jurassic is largely absent, eolian, or marginal marine. A sharp increase in number of packages is observed in the Middle Jurassic when the Western Interior was subjected to frequent inundation and retreat of shallow seas. An abrupt decline in package number occurs at the Middle-Upper Jurassic boundary when the Sundance Seaway retreated for the last time. This decline continues through the Oxfordian, packages increase in the Kimmeridgian, and decline once again during the Tithonian. Package turnovers along with depositional environment interpretations reflect a history of the Jurassic west that experienced uplift, erosion, and inundation to varying regional extent over 55 Myr.