Flow-depth scaling in alluvial architecture and nonmarine sequence stratigraphy: Example from the castlegate sandstone, Central Utah, U.S.A.

Changes in the proximity and arrangement of channel-belt sandstone bodies in the rock record and associated unconformities in alluvial successions (i.e., alluvial architecture and nonmarine sequence stratigraphy) are often interpreted to reflect changes in basin boundary conditions, including subsidence and sea level. Such interpretations do not often consider how the size of a depositional system influences alluvial architecture and sequence stratigraphy. We investigate how paleoflow depth scaling affects fluvial stratigraphic interpretations using the Castlegate Sandstone (Campanian) at its type section in central Utah as an example. Using LIDAR imaging, channel preservation and paleoflow depths are estimated from bar clinoforms throughout the unit, and these data are compared to scour spacing, scour continuity, and mudstone content within the study interval. Within the Castlegate Sandstone bar clinoforms are routinely close to fully preserved (i.e., rollover of the upper part of the bar is present), and an overall increase in paleoflow depth up-section is coincident with changes in other alluvial-architecture characteristics including increases in scour-surface spacing and mudstone abundance up-section. This suggests that alluvial architecture commonly ascribed to changes in accommodation may result primarily from changes in the scale and style of fluvial deposition rather than accommodation-limited reworking and amalgamation. Additionally, although sequence boundaries have previously been identified in the section, no scour surfaces deeper or more extensive than what could be produced autogenically by large rivers typical of the Castlegate system were found in the study interval. This study demonstrates that estimates of paleoriver depth can enhance studies of alluvial architecture and provide a more robust basis for correlating and interpreting fluvial deposits.