28 Geologic Mapping in the West (Posters)

This poster session highlights geologic mapping in the western Cordillera and Rocky Mountains. We encourage submissions that explore bedrock and surficial geology, geophysics, hydrology, hazards, and more. Both professional and student submissions are welcome.

Tuesday, 19 May 2026
8:00 AM - 5:30 PM
Alvarado D/E

Geologic mapping of the Fort Collins, Colorado 7.5’ quadrangle and new geochronology of MIS 5 deposits

Paper number: 28-4
Program: Technical Sessions
Time: 8:15 AM – 8:15 AM

Authors

Abstract

The Colorado Geological Survey (CGS) has been mapping in the northern Colorado Front Range for more than a decade as a part of the STATEMAP Program, which receives equal funding from the state of Colorado and the U.S. Geological Survey.

Marine Isotope Stage (MIS) 3, 4, and 5 deposits have not been widely recognized along the Front Range in Colorado in the South Platte River basin; however, in the last several years, in the Gowanda and Kersey quadrangles, CGS mappers have used luminescence analysis to identify MIS 3 to 5 correlative deposits that aggraded from the underlying MIS 6-aged (Qa4, previously Louviers Alluvium) sediments to the overlying MIS 2 (Qa3, previously Broadway Alluvium) sediments. New mapping in the Fort Collins quadrangle revises the age of a large alluvial terrace, adjacent to the Cache la Poudre River, from MIS 2 (Qa3) to MIS 5a (Qa4). This change is based on a thermally transferred optically stimulated luminescence sample age-estimate of ~77 ka and soil carbonate development up to Stage II. This deposit in the Fort Collins quadrangle is the first documented occurrence of a late MIS 5-aged deposit with a finite age estimate assigned by geochronological analysis that constrains the age of the deposits upper geomorphic surface in the South Platte valley. This recent work by CGS mappers sheds new light on the complex Pleistocene fluvial history of the South Platte River basin and its tributaries.

Pierre Shale members comprise most of the bedrock underlying the quadrangle. Old alluvial fan deposits (unit Qafo), deposited by drainages issuing from the west side of the mapped area during the Pleistocene, cover much of the western and center parts of the quadrangle. Fluvial gravels deposited by the ancestral Cache la Poudre River, from the upper Middle Pleistocene through the Holocene, cover much of the remaining area. Eolian sediments form weakly developed dunes in the northeastern corner of the quadrangle and mantle much of the old alluvial fan deposit in the quadrangle, though it is only mapped overlying unit Qafo in the southeast corner.

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

doi: 10.1130/abs/2026RM-410552

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