5: Overview of the Eocene Castle Rock Conglomerate, east-central Colorado: Remapping the fluvial system, and implications for the history of the Colorado Piedmont and Front Range
Stephen M. Keller, Matthew L. Morgan, 2016. "Overview of the Eocene Castle Rock Conglomerate, east-central Colorado: Remapping the fluvial system, and implications for the history of the Colorado Piedmont and Front Range", Unfolding the Geology of the West, Stephen M. Keller, Matthew L. Morgan
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The Castle Rock Conglomerate is a late Eocene fluvial deposit flanking the east side of the Colorado Front Range and lying within the Colorado Piedmont. It occurs as a northwest-southeast–trending swath ~63 km in length and between 3 and 10 km in width, and is ~70 m in thickness. The conglomerate consists of a matrix of arkosic coarse sand and granules along with pebble- to boulder-sized clasts that vary in abundance. Locally, the upper portion of the conglomerate is well exposed in cliffs and ledges and also in flat outcrops along drainages. Large to very large-scale cross- bedding, of both the planar and trough types, is characteristic of the unit. Clasts are dominantly Front Range granitic rocks and Wall Mountain Tuff, and boulders of the latter can exceed 0.5 m in diameter. Minor quantities of quartzite and vein quartz, and rare sedimentary clasts, also are present. The large sizes of the bedforms and clasts indicate deep water and high velocity during deposition. A recent, extensive paleocurrent study of the upper part of the conglomerate has produced a new map of the fluvial system, indicating a main, southeast-trending paleochannel with two major northeast-trending tributaries. This field trip will present an overview of the Castle Rock Conglomerate lithology, bedforms, and associated geologic units; ideas about its deposition; and its bearing on uplift along the Front Range and in the southwest Denver Basin. Planned stops will be in Douglas and Elbert Counties, south of Denver, and include Castlewood Canyon State Park, Prairie Canyon Ranch open space, private farm and ranch properties, and an inactive quarry. Several of the stops provide excellent vistas of the Front Range, majestic Pikes Peak, and also of the Colorado Piedmont.
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Prepared in conjunction with the 2016 GSA Annual Meeting in Denver, Colorado, this volume contains sixteen guides to field trips in this rich geologic region. The four “Great Surveys” of the late 1800s ventured west to explore and document the region’s unknown natural resources and collect valuable geologic information. Many of the field guides in this volume, aptly titled Unfolding the Geology of the West, will cover the same hallowed ground as the early geologic expeditions. Organized into four sections, this volume spans some of the major subdisciplines of geology: (1) stratigraphy, sedimentology, and paleontology; (2) structure and metamorphism; (3) Quaternary landscape evolution; and (4) engineering and environmental geology.