Genesis of giant, bouldery bars in a Miocene gravel-bed river: Insights from outcrop sedimentology, UAS-SfM photogrammetry, and GPR

ABSTRACT Bedsets of bouldery intraformational conglomerate partly fill a narrow, 40-m-deep paleovalley in the Arikaree Group (lower Miocene) in Nebraska, USA. Entire bar successions are exposed in 3D, offering an opportunity to improve upon sedimentological models for coarse-grained fluvial deposits. Using unmanned aircraft, structure-from-motion photogrammetry, and ground-penetrating radar, we offer the first comprehensive assessment of the lithofacies and stratigraphic architecture of this regionally unique fluvial conglomerate. We interpret three lithofacies associations: (1) beds of chaotic breccias lining the exhumed valley wall (colluvium), (2) < 5-m-thick units dominated by horizontal to gently inclined (2–3°) beds of granule to pebble conglomerates, and (3) 10–24-m-thick units of steeply inclined (∼ 30°) beds of bouldery conglomerates, which dominate the channel fill. The deposit comprises at least four stories, but its thickest part is a deep scour filled by a single story of steeply inclined bouldery deposits. The direction of accretion of the giant bars was toward the paleovalley wall, where contemporaneous valley-side colluvium and bar deposits are juxtaposed. We propose that increased discharge in a high-order trunk stream rapidly eroded the friable sandstone of the underlying Monroe Creek–Harrison formations, amassing resistant, large concretions that were subject to valley-slope mass movements and fluvial transport (subaqueous traction, grainflow, and debris flow). Giant accretion surfaces formed as fluvial bars migrated over the edges of deep scours. Thus, the funneling of increased discharges through a confined paleovalley amplified the magnitudes of valley and bed erosion, water depth, stream competence, and bar thickness, preserving an exceptionally thick, coarse-grained deposit.