IMPLICATIONS OF SANDSTONE “CIGAR” STRUCTURES FOR FLUVIAL SYSTEM GEOMETRY ACROSS THE CRETACEOUS-PALEOGENE BOUNDARY IN EASTERN MONTANA

Systematically oriented, elongate bodies (termed “cigars”) of well-cemented sandstone are common in the dominantly fluvial latest Cretaceous Hell Creek Formation and the early Paleocene Fort Union Formation. in Montana and North Dakota. The cigars commonly contain trough crossbedding whose dip directions generally coincide with the long axes of the cigars, thus indicating paleocurrent direction. They are much more strongly cemented (dominantly by calcite) and more well sorted than the sandstone matrix in which they occur, although cross bedding in the matrix sandstone is continuous with that in the cigars. Cigars are probably not primary sedimentary structures, but are revealed by differential weathering and erosion, details of which are the subject of ongoing study. Nonetheless, the value of cigars as paleocurrent indicators appears valid and their spatial and temporal distributions provide insights into local fluvial system evolution. The dimensions of cigars (0.5-2 m x 0.5-2 m x 2-100 m) render them easily detectable in satellite imagery. We have used Google Earth to systematically measure the orientations and geographic coordinates of >1800 clusters of cigars over an area of ~3,300 km² south and east of Fort Peck reservoir, Montana. Individual cigars are commonly curved in plan view, and clusters of them commonly vary systematically in orientation, apparently defining meanders. Results indicate that the cigars formed in moderately to highly sinuous meandering streams whose overall flow was from west to east, consistent with sediment transport from ancestral Rocky Mountain sources to the waning inland seaway. The radii of curvature of meanders delineated by cigars indicate channel widths of 50-300 m (mean = 229 ±100 m). Preliminary results suggest that the latest Cretaceous streams had lower sinuosity and narrower channels than those of the early Paleocene. This work improves our understanding of fluvial hydrodynamics through the Hell Creek Formation and lower Fort Union Formations, potentially shedding light on changing landscapes across the Cretaceous-Paleogene boundary.