Investigations in the Late Eocene Castle Rock Conglomerate of Colorado from 1869 to the present - Part 1: Description, Nomenclature, and Mapping

Abstract

The late Eocene Castle Rock Conglomerate occurs mostly in Douglas and Elbert counties, Colorado. It is the uppermost and youngest Cenozoic unit in the southern Denver Basin and its outcrops occur in a swath trending from Sedalia southeast to Calhan. The unit is well exposed and topographically prominent, forming flat mesas, steep cliffs, and narrow canyons. The conglomerate is a fluvial unit deposited by a 3- to 10-km-wide braided stream system. Large-scale crossbedding, massive bedding, angular blocks of welded tuff, a variety of other clast lithologies, cut-and-fill structures, fining-upward sequences, fossil logs, and occasional fossilized bones are readily observable. Because the conglomerate is both geologically and scenically striking, it has interested geologists since the late 1860s. Because of improved access to the unit over the last 60 years (in Castlewood Canyon State Park and in county and municipal open spaces) it has increasingly attracted educators, students, and the public. The purpose of this two-part article is to be a source document for future investigators of the formation. Part 1 of the article (the present publication) is a chronology of the description, nomenclature, and mapping of the formation as presented by various investigators over the decades. Part 2 (for future publication) will cover the formation’s geologic history, depositional environment, age, and diagenesis and will present several suggestions for future research.