18 Surficial geology and neotectonics in the West: Celebrating and advancing the legacy of Kirk Bryan through field-based geomorphology studies

This session invites and examines recent field-based geomorphology studies in the West. Contributions regarding soils geomorphology, neotectonics, pediments, fluvial terraces, and landscape development are invited.

Tuesday, 19 May 2026
8:00 AM - 12:00 PM
Alvarado A

New cosmogenic age constraints for classic Colorado Rocky Mountain Pleistocene deposits and the Central Colorado Icefield reveal slow neotectonic processes punctuated by <1.5 Ma episodic, rapid, glaciofluvial erosion

Paper number: 18-2
Program: Technical Sessions
Time: 8:30 AM – 8:50 AM

Authors

Abstract

       Kirk Bryan provided foundational conceptual models, primarily using field observation, that established relationships between geologic time and geomorphology.  Recent decades have seen the rise of innovative models and novel terminology that often overlook the foundational field observations of earlier geologists. We integrate modern geochronologic techniques at four classic locations where pre-glacial/glacial stratigraphy and associated geomorphology were originally defined: (1) The Central City Gravel (1.44 ± 0.26 Ma), equivalent to the Prairie Divide Gravels of Bryan and Ray (1939) and Ives (1953), is the oldest glacial deposit in the Southern Rocky Mountains (SRM) and the foundation for Plio-Pleistocene geomorphic studies of the South Platte River system. Paired with (2) the Rocky Flats Alluvium (>1.5 Ma-400 ka) and Great Plains glacial record, together define the geochronologic framework of SRM-Great Plains Pleistocene history relative to the maximum glacial extent of the Laurentide Ice Sheet ~1.3 Ma. (3) The early Pleistocene pre-glacial Malta Gravel (1.97 ± 0.04 Ma) paired with the (4) Nussbaum Gravel (1.30 ± 0.08 Ma) and other recently documented ages downstream, establishes the coeval onset of regional Laurentide-Rocky Mountain glaciation and the evolution of the Arkansas River system, inducing incision of the Royal Gorge, and all canyons downstream to the Mississippi River.

       These sites demonstrate that the SRM-Great Plains landscape is a product of long-term neotectonic rates of 0.05-0.3 mm/yr exceeded by episodic glacial and periglacial erosion rates >0.5 mm/yr beginning <1.4 Ma. Following major glacial-periglacial processes ~1.4-1.0 Ma, deposition of the Lava Creek B ash from Yellowstone at ~630 ka, and marine oxygen isotope stages (MIS) 12-11 (~478-374 ka), the strongest glacial-interglacial signal in the Cenozoic MIS record, major incision and topographic development in western North America on the order of ~200-300 meters occurred as follows.  MIS 9-7 (~337-191 ka) is generally an erosional interval, with very weak glacial and strong interglacial frequencies.   MIS 6-2 (~200-12 ka) reintroduced strong glacial-interglacial frequencies, producing abundant glacial and glaciofluvial deposits at much lower base levels, or inset within canyons.  These fluvial systems emanate from transient Ice Age-generated regions above 2,200 m asl, an elevation constrained by geologic mapping and by rock glacier and glacial morphometric studies.

Geological Society of America Abstracts with Programs. Vol. 58, No. 4

doi: 10.1130/abs/2026RM-410521

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