GIANT, BOULDERY FLUVIAL BARS IN AN EXHUMED MIOCENE PALEOVALLEY, NEBRASKA PANHANDLE

A deep (~ 40 m), narrow (< 2 km) incised paleovalley in the lower Miocene Arikaree Group atop the Wildcat Ridge in the Nebraska Panhandle contains giant (~ 24 m), steeply inclined (~ 30°) beds of bouldery conglomerates composed almost entirely of intraformational carbonate-cemented sandstone. We assess the lithofacies and sedimentary architecture of the deposit aided by structure-from-motion photogrammetry and ground-penetrating radar. Eleven lithofacies include both clast- and matrix-supported fluvial conglomerates, as well as colluvial to fluvial breccias. Conglomerate facies compose two associations: (1) thin (< 5 m) units dominated by gently inclined (2 – 3°) granule to pebble conglomerates (unit bar accretion deposits), and (2) thick (< 24 m) units dominated by steeply inclined (~ 30°) cobble to boulder conglomerates. The latter association is interpreted as compound-bar-margin accretion deposits; it forms the majority of the paleovalley fill. Breccias compose one facies association defined by thin (1 – 2 m), chaotic beds lining the exhumed valley wall. 3D outcrop models show that the thickest accretion surfaces are continuous from the top to the base of the channel fill, that they drape unit bar deposits and fill deep scours, and that the direction of bar accretion was toward the paleovalley wall.

The conglomerates represent particular conditions of sediment source, valley dimensions, and hydrology. The potentially rapid erosion of loose sand or very friable sandstone in the local Monroe Creek and Harrsion formations, perhaps resulting from a marked increase in fluvial discharge, left behind abundant, large concretions and nodules that were recruited as sedimentary clasts. The juxtaposition of ancient colluvium (breccia) adjacent to bar deposits indicates that coarse clasts could have been emplaced directly onto bar tops by mass movements. Furthermore, the giant accretion surfaces indicate that fluvial bars migrated over the edge of a scour into water at least 24 m in depth, and probably even deeper. We propose that the funneling of high discharges through the confined paleovalley promoted the development of giant, bouldery accretion surfaces by amplifying the magnitudes of bed erosion, water depth, stream competence, and bar thickness.