GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 281-4
Presentation Time: 9:00 AM-6:30 PM


REVES-SOHN, Sintra M., BALDUS, Joseph A. and ROGERS, Raymond R., Geology Department, Macalester College, Saint Paul, MN 55105

The Upper Cretaceous (Campanian) Judith River Formation in its type area in the Upper Missouri River Breaks National Monument in north central Montana represents the distal part of a major eastward-thinning clastic tongue in the Western Interior Foreland Basin. The mid-Judith discontinuity (approx. 76.3 Ma), marked by a major change in alluvial architecture in the Judith River section, essentially bisects the formation in the type area, separating the McClelland Ferry Member below from the Coal Ridge and Woodhawk members above. This discontinuity has been interpreted to coincide with the onset of the Bearpaw transgression in central Montana. The marine Woodhawk Member, marking the leading edge of this major transgression, has been interpreted to include three back-stepping fourth-order sequences (each approx. 30 m thick). General patterns of deepening and shallowing can be discerned in the Woodhawk Member on the basis of lithology, sedimentary structures, and trace fossils, but outcrop is often covered by a veneer of loose sand, making the recognition of diagnostic structures and ichnofossils difficult. This in turn has rendered the identification of parasequences and systems tracts difficult. In an effort to better resolve the internal anatomy of the Woodhawk sequences (and specifically delimit parasequences), 62 sandstone samples spanning the entirety of the two upper sequences (a total thickness of approx. 60 m) were collected at ~1 m intervals and point counted to ascertain both grain size and composition. Recurrent shifts in grain size were documented in a portion of the Woodhawk Member previously interpreted as a highstand systems tract. The thickness of stacked coarsening upward packages (presumably parasequences) ranges from 2–4 m, and preliminary data indicate that the thickness of parasequences decreases up-section in the highstand systems tract. Trends in grain size suggestive of parasequences were not resolved in the underlying transgressive systems tract (finer sampling may be required). Interestingly, a prominent sequence boundary previously identified on the basis of localized scours filled with concentrations of fossils and ferroan dolomite cement coincides with the coarsest sample in our data set and a spike in the abundance of labile grains, including amphiboles and pyroxenes.