DISCOVERY OF FLUVIAL RIDGES ON THE GREAT PLAINS FILLS BIG DATA GAP IN ROCKY MOUNTAIN-GULF OF MEXICO SOURCE-TO-SINK SYSTEMS

Cenozoic fluvial deposits on the Great Plains (GP) are products of the post-orogenic unroofing of the Rocky Mountains. They record long-term sediment routing to the Gulf of Mexico, but sparse outcrop and subsurface data have prevented researchers from adequately unraveling their histories. Consequently, we use high-resolution imagery and LiDAR data to map ~3100 fluvial ridges—erosionally inverted alluvial-fan, channel-fill, channel-belt, and valley-fill deposits—extending from the Rocky Mountain front (RMf) to the margin of the Central Lowland.

Ridge-forming deposits exhibit distinct spatial and temporal patterns relatable to ancient streams. Late Eocene–Oligocene (White River and basal Arikaree groups) streams were morphologically diverse. Small (<10 km in radial distance) alluvial fans adjacent the RMf changed eastward to parallel or downstream-divergent, fixed, single-thread, straight to slightly sinuous (S =1.0–1.5) streams <50 m in width. At ~100 km from the RMf, streams became sinuous and migrated laterally, forming amalgamated channel bodies as much as 3 km in width. Streamflow in all these systems was highly dispersed (SE to NE). Channels of different ages commonly intersected at high angles. These geographic patterns reveal a nascent GP alluvial apron hosting small, poorly integrated drainages undergoing abrupt changes. By the Miocene, more uniform streamflow generally trended E-NE. Channel deposits are identifiable 500 km from the RMf. Middle Miocene valley fills (lower Ogallala Group) gave way to fixed, multithread channels a few km in width by the late Miocene (upper Ogallala Group). These patterns evince a mature alluvial apron hosting bigger rivers draining a more integrated drainage basin.

We interpret the systematic changes between fixed and mobile channel styles in both late Eocene–Oligocene and Miocene strata to record spatially and temporally variable aggradation rates. The widening of channels in the late Miocene likely reflects increased discharge relating to wetter climates upstream or to the integration of once-isolated Rocky Mountain drainage basins into a continental-scale drainage system. Our study of ancient GP streams is the most areally comprehensive to date, and it will doubtless reveal yet more about late Paleogene and Neogene landscape evolution.