HISTORICAL STUDIES OF THE PRECAMBRIAN GEOLOGY OF THE CENTRAL COLORADO ROCKY MOUNTAINS COLORADO

1858 – 1910s - The first geological studies of the central Front Range were prompted by the 1858 discovery of gold in Clear Creek and Gilpin Counties. The Hayden Survey included the first detailed study of Precambrian rocks in the area, which identified the rocks as Precambrian metasedimentary units intruded by granites.

Different interpretations were offered during this time as to the nature, origin, and age relations of the various units present in the area. There was general agreement that the quartzites and mica schists are of sedimentary origin, but the origin of the hornblende and microcline gneisses had been argued and variously interpreted as original sedimentary, intrusive, or extrusive units. There was even less agreement on age and age relations of the units. Further studies continued to address relationships between the metasedimentary rocks, quartzites, and intruded granites. Variations in metamorphic grade in the rocks were also noted, from rocks demonstrating little to no metamorphic texture to those showing schistose and gneissic foliation.

1930s-1950s - Studies of Precambrian rocks over a greater area of the Colorado Front Range detailed the relationship between the different rock units as well as the origin of those units. Names were assigned to various rock units, and relative ages of the rock units were examined. The Coal Creek Granite Gneiss (Boulder Creek Batholith) was identified as a unit intruding the Coal Creek quartzite. A metasedimentary origin for hornblende and felsic gneisses was favored by many, but a volcanic origin was now gaining acceptance. A more modern approach was taken to interpretation of units, and serious mapping was initiated by the USGS in the 1950s.

1960 – 1970s - Additional studies mapped and defined major lithologies and stratigraphic relationships of the Precambrian rocks. Formation names were proposed and discussed, but “indiscriminate use” of the names Idaho Springs Formation, Swandyke Hornblende Gneiss, Coal Creek Quartzite, and others negated any stratigraphic meaning. However, the names Idaho Springs Formation and Coal Creek Quartzite remain in casual, if incorrect, use today.

Late 1970s – Ushered in ‘modern’ studies of the Colorado Precambrian terrane, with interpretations of protolith and tectonic associations. Discussion follows in subsequent presentations.