Rocky Mountain Section - 72nd Annual Meeting - 2020

Paper No. 8-3
Presentation Time: 2:10 PM


ZUCHUAT, Valentin1, MIDTKANDAL, Ivar1, POYATOS-MORÉ, Miquel1, DA COSTA, Sigrid1, BROOKS, Hannah L.2, HALVORSEN, Kristine1, SUNDAL, Anja1, COTE, Nathan3 and BRAATHEN, Alvar1, (1)Department of Geosciences, University of Oslo, Sem Saelandsvei 1, Oslo, 0371, Norway, (2)Department of Geological Sciences and Geological Engineering, Queen's University, Kingston, ON K7L 3N6, Canada, (3)Natural & Environmental Sciences Department, Western State Colorado University, 600 North Adams Street, Gunnison, CO 81231

Unconformities, by definition, correspond to non-depositional or erosive surfaces, separating older strata below, from younger rocks above, and encapsulating significant time gaps. However, recent studies have emphasized the composite nature of some unconformities, as well as their heterochronous and diachronous character, which restricts the use of such a definition to a near one-dimensional setting.

This study analyses the nature of the J-3 Unconformity, which separates the Middle Jurassic Entrada Sandstone from the Late Jurassic Curtis Formation (and laterally equivalent units) in east-central Utah (USA). Our detailed mapping and characterization indicates the J-3 “Unconformity” is in fact a composite surface generated by either erosion-related processes such as eolian deflation, and water-induced erosion, or by deformational processes. These multiple mechanisms interacted and overlapped in time and space, which demonstrates the composite and diachronous and non-unique nature of such boundaries. This contact has been historically interpreted as an unconformity, but our results show that this key stratigraphic surface is a time-transgressive flooding-ravinement surface that formed shortly before and during a series of transgressions that flooded the area during the Late Jurassic. Consequently, the regionally extensive, composite, heterochronous, and diachronous J-3 Unconformity does not fit the classic unconformity definition, after which an unconformity universally separates older from younger strata basin-wide.

The J3 “Unconformity” case study highlights the fact that one process can be represented by varying expressions in the stratigraphic record, and conversely many processes may result in the same stratigraphic expression. Consequently a revised definition of unconformity is discussed, focusing on processes and associated environmental changes, while abandoning its time-barrier aspect. This investigation also exemplifies some of the implications on subsurface analyses that the inaccurate characterization of the subseismic complex characters of such major stratigraphic bounding surfaces can have. These inaccuracies may lead to imprecise timing and sediment budget predictions, and ultimately have strong implications for basin evolution and reservoir models.