Prairie, Peak, and Plateau: A Guide to the Geology of Colorado

by John Chronic; Halka Chronic

PRECAMBRIAN ERA

Only part of the earth’s very early history is represented in Colorado, where the oldest known rocks are the [gneisses] and schists of the Idaho Springs Formation, at least 1,800,000,000 years old. These rocks appear to be the remains of ancient sediments, folded and metamorphosed into vast mountain areas long before recognizable life inhabited the earth.

Precambrian rocks in Colorado are on the whole very poorly known. They have, however, been studied in detail in the Front Range west of Denver and Boulder, where they have been intensively explored for valuable minerals. The lack of [fossils] in the oldest rocks makes their close correlation difficult, but from studies of radioactive minerals contained in these rocks, and of the relationships of the rock units themselves, we can list them in order of their relative ages.

Note that the rock sequence given below reads chronologically from bottom to top—a logical pattern in geology since younger rocks, especially those of sedimentary origin, normally lie above older ones. Recent studies indicate that the sequence may be much more complex than shown here.

(youngest) Silver Plume [Granite]:	light pinkish gray, fine-grained granite.
Pikes Peak Granite:	pink, coarse-grained granite.
Boulder Creek Granite:	dark gray, faintly banded granodiorite.
Coal Creek Quartzite:	light gray quartzite with grains ranging in size from fine sand to boulders, with some interbedded [schist].
Swandyke Hornblende [Gneiss]:	dark gray to black, strongly banded gneiss.
(oldest) Idaho Springs Formation:	gray to black schist and gneiss.

From a sequence such as this, it is possible to reconstruct some features of Colorado’s early history. The first chapter of which we have a record is the deposition of the Idaho Springs Formation, probably as an accumulation of mud, sand, and limy mud in an ancient sea. Swandyke deposition followed—the sediments were iron-rich, perhaps derived from ancient volcanic materials. The original Coal Creek sediments were sands and gravels, some of them quite coarse and therefore indicative of near-shore deposition. The [schist] layers suggest that muds must have been deposited also.

South of Ouray, Cambrian sandstones of the Sawatch Formation lie almost horizontally across the vertical Precambrian metamorphic rocks. (Jack Rathbone photo)

Together these three formations represent some 40,000 feet of sedimentary layers. Deposition of such a great thickness of mud, sand, and lime must have taken a very long period of time. Details of the geography of the continent during that period have of course been obscured by later events, when these rocks were subjected to repeated uplift, crumpling, folding, various degrees of remelting and recrystallization, and erosion. But the ancient sediments must have been derived from even more ancient highlands, either folded and faulted mountains or volcanoes, and probably were deposited under water in broad expanses of sea that covered portions of the continent.

Geologic map of Colorado. Geologic maps show the age of rocks appearing at the surface, disregarding soil cover. A more detailed geologic map of Colorado may be obtained from the U.S. Geological Survey at the Federal Center in Denver.

PRECAMBRIAN PALEOZOIC MESOZOIC CENOZOIC SEDIMENTS CENOZOIC VOLCANICS Yampa River White River Fort Collins South Platte River Glenwood Springs Denver Colorado River Grand Junction Aspen Gunnison River Colorado Springs Gunnison Salida Dolores River Rio Grande Arkansas River La Junta Walsenburg Alamosa Durango

The Boulder Creek, Pikes Peak, and Silver Plume [Granites] cut through the metamorphic rocks, and are therefore younger. They represent pulses of molten rock forced upward from deep within the crust, probably during three separate episodes of mountain building. As each set of mountains was formed, it was worn down, perhaps to low rolling hills, perhaps to flat plains almost at sea level, and partially or entirely covered with thick layers of sediment. Each time, another mountain building episode followed, with new intrusions of granite and new metamorphism of the pre-existing rocks.

Each succeeding period of metamorphism and mountain building further changed the nature of the rocks involved, complicating the patterns of folding and faulting, adding recrystallization to recrystallization, until the oldest of the rocks bore little trace of their original sedimentary nature. In general, the rocks that are oldest were most altered by the repeated metamorphism, while the younger rocks were less altered.

The Black Canyon of the Gunnison River is one of the state’s deep and spectacular chasms. Canyon walls are of Precambrian [gneiss] intruded by many [dikes] and highly fractured by later uplifts. The flat upper surface of the Precambrian rocks represents an ancient plain on which, during Jurassic time, the dinosaur-bearing Morrison Formation was deposited. (John Chronic photo)

The Precambrian Era ended with a long period of erosion, a period known to geologists as the Lipalian Interval. During this time, over almost the entire world there was no mountain building. The land lay sleeping, subject only to the forces of erosion. The last mountains were flattened nearly to sea level. Slow, sluggish streams and rivers carried sand and mud toward the oceans—oceans in which perhaps primitive, soft-bodied organisms, with no hard parts to be preserved as [fossils], were beginning to evolve.

On the continents, the time of intense metamorphism was over; most rocks of later eras are preserved today in pretty much their original state. The boundary between the Precambrian and later rocks is normally well defined. It is visible at many places in Colorado: in Williams Canyon near Colorado Springs, in Glenwood Canyon, near Red Rocks west of Denver, just west of La Veta Pass, at the top of Royal Gorge and the Black Canyon of the Gunnison. At most of these localities it is a smoothly beveled surface, with highly contorted Precambrian rocks below it and flat-lying Paleozoic sediments above it. Near Red Rocks and La Veta Pass, the same relationship prevails, but the entire contact, and the rocks above and below it, have been steeply tilted by the uplift of the present mountains.

In portions of western North America, deposition late in Precambrian time has left a series of flat-lying rocks between the contorted Precambrian and later Paleozoic sediments. These rocks can be seen in northwestern Colorado, where they form the dark red sedimentary core of the Uinta Mountains.