Palaeogeography and fluvial to estuarine architecture of the Dakota Formation (Cretaceous, Albian), eastern Nebraska, USA

摘要

Regional mapping in easternmost Nebraska indicates that the Dakota Formation (Cretaceous) fills broad palaeovalleys incised into a Pennsylvanian bedrock surface with at least 115 m of regional relief. These palaeovalleys contain successions of cross-stratified sandstones, conglomerates and mudrocks, with evidence for multiple episodes of cut-and-fill. Palaeocurrent measurements from large sandstone bodies indicate dominant westward to northward flow paralleling the orientations of the mapped sub-Dakota palaeovalleys. Coarse clasts from both local and distant sources appear in coarse sandstones and conglomerates at the base of the Dakota at multiple sites. These observations are compatible with the interpretation of a dominant fluvial component in Dakota deposition. There are also features, however, that are compatible with marine tidal influence, such as laminites, flaser and linsen bedding, mud drapes, reactivation surfaces in cross-stratified sandstones and marine to brackish trace fossils. Well-logs through complete Dakota sections west of the study area show smaller scale coarsening upward and fining upward trends, as well as medium-scale stratigraphical trends. In these wells, the Dakota typically shows 7-10 discrete, medium-scale packages, each a few to several tens of metres thick. Thick valley-filling channel sandstones, meandering fluvial deposits and possible deltaic or shoreface deposits are interpreted from electronic logs of these wells. At least one well, represented by a thorough lithological description, electronic logs and partial core, also appears to contain tidal facies in the upper Dakota. Thus, the deposition of the Dakota in eastern Nebraska, particularly the filling of palaeovalleys in the outcrop belt, involves a complex history of fluvial and estuarine deposition.