DEPOSITION OF FLUVIAL GRAVELS IN LATE ALBIAN KIOWA MARINE CYCLE FLUVIAL-ESTUARINE SETTING, NISHNABOTNA MEMBER, DAKOTA FORMATION ALONG THE CRATONIC MARGIN, CRETACEOUS WESTERN INTERIOR BASIN

Prior to the sea-level rise of the Late Albian Kiowa marine cycle (~ 100 Ma), an episode of fluvial incision spanning from ~ 160-100 Ma cut westward-draining paleovalleys in western Iowa and eastern Nebraska, with relief of about 80 m. During Albian flooding, the extremely low gradients in these cratonic paleovalleys (stratigraphic estimates range from 0.3-0.6m/km) resulted in formation of estuarine depositional systems considerable distances inland. Deposition of estuarine mudrocks was coeval with the aggradation of more proximal fluvial conglomeratic sandstones of the basal Nishnabotna Member of the Dakota Formation, and these two contrasting lithofacies intertongue in and around the Ash Grove Quarry in Cass County, NE, and points farther eastward. In Guthrie County, IA, ~ 200 km east of the Ash Grove quarry, Ordovician to Pennsylvanian fossils in chert pebbles, jasper chert, quartzite and bull quartz, are in clasts that range up to 5 cm in maximum diameter. Compositions of these deposits and regional paleogeology, indicate that parts of the fluvial sediment load must have traveled distances of 400 to 1,200 km. Conglomerate clasts of locally-derived cemented siltstone having maximum diameters that range up to 40 cm, and quartzite, bull quartz, chert, and ironstone clasts ranging up to 18 cm, occur in local pockets within the lower parts of the Nishnabotna Member of the Dakota Formation at Ash Grove Quarry.

An overall rising sea level during the Late Albian created accommodation space for gravelly lithofacies in eastern Nebraska and western Iowa to accumulate. As Western Interior sea level rose, regional stream gradients decreased, resulting in loss of competency of stream systems, progressively stranding the coarse-grained bed load in areas that were more proximal, stratigraphically recording regional fluvial aggradation. Mud-draped cross-bedded sandstone bodies, laminated mudstone intervals, and burrows exposed in lower Nishnabotna Member in Guthrie County, IA, indicate that estuarine conditions likely existed several hundred kilometers inland from the interpreted Late Albian coast. These observations support the concept that estuarine conditions stepped up the incised valleys as fluvial sediments aggraded in response to regional transgression that continued through the Albian.