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

Jesse T. Korus, R. Matthew Joeckel and Shane T. Tucker
Genesis of giant, bouldery bars in a Miocene gravel-bed river; insights from outcrop sedimentology, UAS-SfM photogrammetry, and GPR
Journal of Sedimentary Research (January 2020) 90 (1): 27-47

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 degrees ) beds of granule to pebble conglomerates, and (3) 10-24-m-thick units of steeply inclined ( approximately 30 degrees ) 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.


ISSN: 1527-1404
EISSN: 1938-3681
Serial Title: Journal of Sedimentary Research
Serial Volume: 90
Serial Issue: 1
Title: Genesis of giant, bouldery bars in a Miocene gravel-bed river; insights from outcrop sedimentology, UAS-SfM photogrammetry, and GPR
Affiliation: University of Nebraska-Lincoln, School of Natural Resources, Lincoln, NE, United States
Pages: 27-47
Published: 202001
Text Language: English
Publisher: Society for Sedimentary Geology, Tulsa, OK, United States
References: 140
Accession Number: 2020-046953
Categories: Applied geophysicsSedimentary petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. sects., 1 table, sketch map
N41°43'08" - N41°43'08", W103°44'07" - W103°44'07"
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2020, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by SEPM (Society for Sedimentary Geology), Tulsa, OK, United States
Update Code: 202013
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