Mid-Cretaceous alluvial-plain incision related to eustasy, southeastern Colorado Plateau

Abstract

Eustatic effects on the deposition of ancient coastal and marine rocks are well known, but eustasy also can affect depositional patterns and processes well inland from the sea and play an important role in the development of nonmarine unconformities. In the southeastern part of the Colorado Plateau, fluvial rocks of the lowermost Cenomanian (lowermost Upper Cretaceous) Encinal Canyon Member at the base of the Dakota Sandstone fill paleovalleys incised into underlying formations. In most of region, the Encinal Canyon disconformably overlies fluvial rocks of the Aptian to middle Albian (upper Lower Cretaceous) Burro Canyon Formation. Along the southern margin of the plateau, the Encinal Canyon overlies progressively older Mesozoic and Paleozoic formations toward the south. In the latter part of the Early Cretaceous, an epicontinental sea lay about 240 km east of the southeastern Colorado Plateau and was base level for streams in the plateau region. Near the end of the Early Cretaceous, sea level fell, base level was lowered, and streams incised valleys into alluvial deposits of the Burro Canyon Formation and into older formations. The resulting incised paleodrainage surface was preserved as the sub-Dakota unconformity when the succeeding sea-level rise, in earliest Late Cretaceous time, caused Dakota streams to aggrade and backfill the paleovalleys with alluvial sediments of the Encinal Canyon Member. Unconformities in continental rocks have rarely been attributed to sea-level changes. The example of sub-Dakota unconformity, however, suggests that eustatic lowering of sea level can result in erosion far inland from the sea and play an important role in the formation of unconformities in continental strata.