Description and interpretation of the Jurassic J-2 unconformity of the Western Interior (U.S.A.)

The Jurassic J-2 surface is one of the more extensive unconformities of the Western Interior (U.S.A.). Stratigraphic relationships show that this is an angular unconformity, which developed first in the north and progressively moved south. An examination of features associated with the J-2 surface reveals detrital and diagenetic chert grains, ventifacts, polygonal fracture patterns, oscillation ripples, and paleotopographic relief. Some of these features indicate that the J-2 unconformity was a hot, dry, deflationary surface, and that the level of deflation on this surface was controlled, at times, by the height of the water table. The ventifacts are a lag deposit of the overlying Gypsum Spring Formation. Three possible variables involved in the formation of the J-2 surface are fluctuations in water table, disruption of sediment supply, and tectonic activity. For the Early-Middle Jurassic, these variables can be associated with increased rifting rates in the Atlantic Ocean. The increase in rifting rate raised eustatic sea level, which resulted in a larger area for deposition of marine sediments and a rise in continental water table. A higher water table would have trapped sediment that was previously available for sand sea (erg) formation. Furthermore, a higher sea level might have changed sediment supply and transportation routes. As the increase in rifting continued, tectonic movement and tilting of the North American plate occurred. This tilting caused the uplift and erosion of some deposits, and influenced the sites available for sediment preservation.