Paper No. 32-4
Presentation Time: 9:05 AM
CRETACEOUS TROPIC SHALE OF SOUTHERN UTAH: AN UNDERUTILIZED NATURAL OBSERVATORY ON PUBLIC LANDS FOR RESEARCH IN SHALE DEPOSITIONAL FACIES AND PROCESSES, MID-CRETACEOUS PALEOCLIMATES AND CARBON BURIAL
Unusually good outcrops of the Cenomanian-Turonian Tropic Shale on public lands along the southern flank of the Kaiparowits Plateau in southern Utah have been called the “Book Cliffs of shale” by some of the relatively few shale sedimentologists and stratigraphers who have seen them. These rocks contain abundant, widespread event beds (bentonites, storm beds), chemostratigraphic markers (Cenomanian/Turonian boundary event, max flood chemostrat marker), widely correlative bedding couplets attributed to Milankovich-scale climate alternations, and excellent marine macrofossil and microfossil biostratigraphy. Generations of workers have correlated many of the marker beds from southern Utah to Kansas. Unusual for shale sequences where the precise level of max flood and systems tract boundaries must be inferred due to imprecise correlations with shoreline deposits at the parasequence scale, interfingering of Tropic Shale with retrogradationally-stacked shoreline sandstones of the Dakota and aggradational to progradational shoreline sandstones of the Straight Cliffs has allowed precise identification of max flood and systems tract boundaries within the mudrock stack far offshore. This allows fine details of shale sedimentology and geochemistry to be interpreted within a sequence stratigraphic context, rather than being used to infer max flood, parasequence stacking patterns and systems tracts. A single facies model does not apply to the entire transgressive-regressive cycle. There are distinctly different mudrock facies associations in transgressive, early highstand and progradational or late highstand intervals. Among many topics ripe for study are quantification of lateral changes in carbon burial across large areas within precise time slices, fine-scale temporal details within Milankovitch cycles where the limestone-equivalent beds are laminated not churned by bioturbation, fine details of storm deposits including 3D exposures of gutter casts, and lateral change in limestone concretion type from cone-in-cone in proximal, sandy prodelta to septarian in distal, clay-rich prodelta to simple in more distal, offshore depositional environments. Availability of good outcrops on public lands, public-domain core and subsurface well logs invite detailed studies of these rocks.