Abstract

Sequence stratigraphic models emphasize the role of external, allogenic controls such as relative sea level, tectonic subsidence, and climate on alluvial to coastal plain stratigraphic architecture, at the expense of internal, autogenic behaviors such as avulsion. These models are tested via sedimentologic analysis of fine-grained floodplain deposits and their relationship to channelized fluvial sandbodies in a well exposed alluvial to coastal plain succession developed under progressively increasing distance from the coeval shoreline and an accompanying decrease in the rate of accommodation creation (Late Cretaceous Blackhawk Formation, Wasatch Plateau, central Utah, U.S.A).

The studied alluvial to coastal plain succession contains fourteen lithofacies, which are grouped into four associations and eight architectural elements. Facies associations consist of: 1) channelized fluvial sandstone bodies; 2) nonchannelized fluvial sandstone and siltstone bodies; 3) fine-grained floodplain deposits; and 4) brackish floodplain deposits. Paleosols are immature, and are classified as entisols, inceptisols, and histosols. Integrated analysis of paleosol character, facies analysis, and architectural-element analysis indicates deposition on a rapidly aggrading floodplain that was subject to frequent crevassing and channel-belt switching.

The character, relative abundance, and vertical stacking of the paleosol types, architectural elements, and the first three facies associations exhibit little variation stratigraphically, from base to top of the Blackhawk Formation (c. 250 m), or paleogeographically between the northern and southern limits of the outcrop belt (c. 100 km). Local stratigraphic architectures record three styles of avulsion, representing avulsion by reoccupation, avulsion by progradation, and avulsion by incision. Avulsion style displays no consistent pattern with stratigraphic position or paleogeographic location. In combination, these results suggest that floodplain sedimentation and local stratigraphic architecture are dominated by autogenic avulsion. The three observed avulsion mechanisms operated in apparent uniformity over a range of tectonic subsidence rates (c. 80–700 m/Ma) and distances from the coeval shoreline (c. 0–100 km).