Strato-tectonic analysis of the Niobrara Formation and its equivalent though out the mid-Cretaceous seaway and their contained hydrocarbon endowment (mainly as shale oil) indicate that the Niobrara Formation occupies a unique position in Laramide-Sevier tectonic evolution. On a time space strato-tectonic diagram, the Niobrara appears as the first Laramide formation which rests on a continent wide (and indeed globally distributed) unconformity known as the Turonian unconformity (see Figure 1).
The period of non-deposition represented by the unconformity lasted for about 500,000 years, and may be associated with a global plate reorganization event (Coney, 1972) that featured opening of major new ocean basins in the south Pacific half of the planet. The appearance of new oceanic spreading by a global ‘chiropractic adjustment’ that featured increased ocean floor generation, gravitational pull at trenches, increased global spreading rates, flattening of subduction zones around the entire circum-Pacific Rim, and retreat of mid-Cretaceous epeiric seas off the continents. The circum-Pacific subduction zone flattening, of course, included southwestern North America. In southwestern North America Sevier Orogeny can and has been viewed as developed in the context of constant dip subduction whereas Laramide orogeny dynamics were driven by variable dip flattening (Keith, 1978, 1982). Progressive regional epeiric uplift associated with the flat subduction ultimately lead to the development of another regional unconformity referred to as the Eocene unconformity/surface by many workers (e.g, Chapin and Cather, 1981, Keith and Wilt, 1986).
The approximately 500,000 year depositional hiatus represented by the Turonian unconformity though out the Rocky Mountain region records a profound shift in the far field regional strain regime which is driven by underthrust subduction tectonics. Sevier orogenics beneath the unconformity were driven by WNW directed absolute motion of the North America Plate which attempted to ‘A’ subduct at a rate of about 3.5 cm/yr beneath the Sevier belt between the western interior seaway and the Cretaceous coastal batholiths to the west (Figure 2). The Laramide tectonic regime appears above the unconformity during Niobrara time when NE-SW directed shortening strain related to the underriding Farallon plate dramatically ‘jumped’ northeastward during the global ‘chiropractic adjustment’ (Figures 3, 4, and 5) and was transferred to the crust beneath the Cretaceous seaway which was withdrawing to the northeast in an oscillatory fashion producing extensive coal swamps during the withdrawals (Figure 6). The strain rates in the crust also dramatically increase due to the greatly increased descent rates of the ‘B’ subducting Farallon beneath the region at about 10 cm/year.
URTeC 1582709
