25 Paleo-climate and -ecosystems of the Southwest and High Plains: From the Paleozoic to the Quaternary (Posters)

Many Phanerozoic climates and ecosystems serve as potential analogues for the future; others highlight the Earth system response to tectonic and climatic forcings. We invite abstracts that probe the paleoclimate/ecosystems of the US Southwest

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

New Observations of Sand Wedges in Northeastern Colorado and their Paleoclimatic Significance

Paper number: 25-3
Program: Technical Sessions
Time: 8:30 AM – 8:30 AM

Authors

Abstract

In 2024, Colorado Geological Survey (CGS) geologists observed approximately 16 Pleistocene sand wedges at Black Hollow Reservoir in the Severance 7.5’ quadrangle, located 97 km north of Denver. Sand wedges act as significant paleoclimate indicators for periglacial conditions. While documented throughout various Wyoming basins, they are rare in the Colorado Plains.

At Black Hollow Reservoir, a resistant promontory extends north and west from the lakeshore, with ~3-4 meters tall bluffs. The bluffs consist of Late Cretaceous Pierre Shale overlain by Upper Pleistocene eolian silty sand; the sand wedges occur in the Pierre Shale at the contact of these units. North of the promontory, the overlying sand thins out, and the Pierre Shale is capped by a Cenozoic conglomeratic sandstone and Middle Pleistocene sand and gravel deposits.

The sand wedges consist of silty sand with rare 2 cm pebbles. They range from 0.6–2 m in height and 3–80 cm in width. Processes that formed the wedges mechanically deformed the underlying Pierre Shale. Eolian sediment later filled in the wedges and developed soil with prismatic structures. Two samples were collected and analyzed with optically stimulated luminescence techniques: one from a sand wedge and one from the base of the eolian sand near the Pierre Shale, yielding ages of 13,755 ± 685 and 13,860 ± 645 years, respectively. CGS geologists first documented and dated sand wedges in the Colorado Plains on the Anton Escarpment in 2004. These samples range in age from ~130 to 18 ka. In 2025, although not documented in detail, CGS geologists found a sand wedge-like feature in the Milton Reservoir 7.5’ quadrangle with a thickness of about 3 cm and a height of 45 cm.

Previous research indicates that the Colorado Plains could have been cold, windy, and dry during the Pinedale glaciation. Insect fossils from Lamb Springs near Denver showed the plains were a dry grassland. The presence of sand wedges provides further supporting evidence that the Colorado Plains were cold enough to undergo periglacial conditions. The arid environment could have resulted in low snow cover, exposing the surface to the extreme cold. Subsequently, the surface thermally expands and contracts, creating cracks which are later filled in by eolian sediment.

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

doi: 10.1130/abs/2026RM-410567

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