PALEOZOIC ERA
Geologists have divided the second great era of geologic time into units called Periods. The rocks deposited during a Period are called Systems, but more often than not it is convenient to discuss them in terms of easily recognized units of rock, called Formations. Formations are named after areas in which they are well exposed.
The [stratigraphic column] given in Chapter I shows the Periods and Systems in their correct order, and gives the age in years for each, as determined by radioactivity methods. As you read, refer as often as necessary to this column.
The geologic map on [page 35] will help you locate areas where the rocks discussed in the text are exposed, and will greatly facilitate your understanding of the geology of the state.
The Cambrian Sawatch Sandstone lies almost horizontally on Precambrian [granite] in Glenwood Canyon. In the foreground is the Colorado River. (Jack Rathbone photo)
Cambrian Period
(500-570 million years ago)
The first fossiliferous rocks in Colorado were deposited during the Cambrian Period, at a time when over much of the world the seas were creeping in across wide, level plains formed during the Lipalian Interval. Colorado was not covered by these seas until quite late in the Cambrian Period. Beach deposits progressively younger in age suggest that the sea invaded from the west, and spread slowly eastward, inundating most of the central part of the state but not the extreme north or south.
The beach deposits, now called the Sawatch Sandstone because they are well exposed in the Sawatch Range, are composed mostly of fine quartz sand. They are colored with glauconite, a green mineral, and [hematite], a dark red mineral, so that the rock has a variegated appearance. The post office at Manitou is built of this red and green rock, and good exposures of it exist in Williams Canyon near Manitou, along U. S. Highway 24 northwest of Manitou, near Red Cliff and Minturn, and in Glenwood Canyon.
The sea which crept over Colorado at this time contained small conical-shelled [mollusks], [brachiopods], and [trilobites]. Their shells can occasionally be found in Cambrian rocks in Williams Canyon and in the Sawatch and Mosquito Ranges. At two localities unusual [fossils] called [graptolites] have been found in thin Upper Cambrian shales overlying the Sawatch Sandstone.
These [fossils] can occasionally be found in Cambrian rocks in central Colorado.
Ordovician Period
(440-500 million years ago)
The sea deepened and widened as the Ordovician Period began, and a series of limestones and dolomites was deposited, either on top of the Sawatch Sandstone or, where the Sawatch had not been deposited, directly on the Precambrian. These rocks are now called the Manitou Formation.
The [fossils] in these rocks are much more varied than those in the Sawatch Sandstone: snails, [echinoderms], sponges, [cephalopods], [brachiopods], and [trilobites] are common. The Ordovician sea must have teemed with life, as many rocks deposited at this time are more than half composed of animal remains. In addition to hard-shelled animals which formed fossils, there were probably abundant soft-bodied animals such as jellyfish and worms, which left no record of their presence.
After deposition of the Manitou Formation, the seas receded slightly. A new series of sands was deposited above the Manitou in central Colorado. These now form the Harding Sandstone, a formation of unusual interest because it contains remains of the earth’s earliest known vertebrates, primitive jawless fish called Agnathids. In places in the Harding Sandstone there are dense accumulations of the tiny polygonal armor plates from these fish. Although no whole fish have been found, we can reconstruct their appearance by comparing individual plates or groups of plates with later, better known relatives.
Also present in great quantities in the Harding Sandstone are [conodonts], peculiar tiny brown tooth-like [fossils]. Relationships of the conodonts are unknown; they may be parts of the Agnathids, or perhaps they represent some entirely different group of animals, with no living relatives.
After deposition of the sands of the Harding Sandstone, the sea deepened locally and the Fremont Limestone, a massive gray crystalline limestone containing many marine [fossils], was deposited. [Mollusks] (some quite large), [brachiopods], and corals contributed their shells to the Fremont Limestone. The chain coral Catenipora and the horn coral Streptelasma may often be used to identify the formation.
The Fremont Limestone was deposited very late in the Ordovician Period. Probably the seas were much more extensive then than present deposits indicate; subsequent erosion has at several times erased the evidence in uplifted areas.
These Ordovician [fossils] can be found in the Manitou Formation in the Colorado Springs area.
The earliest known fish remains come from the Ordovician Harding Sandstone of central Colorado. These fragments of the protective plates have been magnified about five times.
Corals and coral-like organisms occur in the Ordovician Fremont Limestone.
Silurian Period
(400-440 million years ago)
Until very recently, no Silurian rocks or [fossils] were known in Colorado, and it was thought that seas did not extend into the state during this period. However, a few years ago good Silurian corals and [brachiopods] were discovered near the northern edge of the state. They occur in broken blocks and patches of Silurian limestone, mingled with blocks of other [sedimentary rocks] and, oddly enough, with volcanic material.
What seems to have happened here is that sedimentary layers of Silurian age were present over northern Colorado at one time. During some subsequent period of volcanism, volcanic lavas penetrated these sediments from below. Near the volcanic tubes, broken, angular fragments of the surrounding sedimentary rocks were sometimes carried upward or downward by the motion of the [lava].
Much later, both the volcanic outpourings (if the lavas ever reached the surface) and the sediments were stripped away by erosion, probably at a time when mountains were rising in the area. Only the deep portions of the tubes that fed the volcanoes were preserved. These tubes are called [diatremes], and thanks to the blocks of sedimentary rock in them we know that there were indeed seas in Colorado during Silurian time, seas containing the abundant life of a shallow marine environment very much like that existing at the same time in Illinois, Iowa, and Indiana.
Devonian Period
(350-400 million years ago)
As far as we know now, Colorado was just a little above sea level during most of Devonian time. Early and Middle Devonian deposits are lacking. Late in the period, however, Colorado was widely inundated once more. Embayments of a western sea covered most of the central part of the state and an area in southwestern Colorado around Ouray.
Deposits formed in these embayments have been given several names. Chaffee Formation is the name most commonly used in central Colorado; Ouray Formation identifies rocks of the same age in southwest Colorado. The Chaffee Formation has been subdivided into two well defined units, the Parting Sandstone or Quartzite, and the Dyer Dolomite or Limestone. Many ore deposits are associated with these rock units—notably deposits of lead and zinc. The Parting Sandstone is frequently so well cemented with silica that it is actually a quartzite; thin shale beds or “partings” make it easy to recognize. It frequently contains remains of [fossil] fish and distinctive beds of algae.
The Dyer Dolomite contains [brachiopods] and [bryozoans], [mollusks] and corals. Some of the best [fossil] hunting in Colorado is in Dyer beds around the White River [Plateau], where the fossils frequently weather out of the rock as almost perfect specimens.
These Devonian [brachiopods] come from the White River [Plateau] in western Colorado.
Mississippian Period
(310-350 million years ago)
The sea continued to cover most of Colorado after the end of the Devonian Period, well into Mississippian time. Mississippian rocks are characteristically thick, massive gray limestones collectively called the Leadville Limestone. This unit is well known as the host rock for many Colorado ore deposits, notably those around the town of Leadville.
During Mississippian time the western sea, warm and rich in organisms, covered much of North America. [Brachiopods] and corals flourished, as did many other forms of life. The seas during part of this time extended completely across Colorado to merge with seas that covered the midwestern part of the United States.
Over all this vast area, as well as southwest into Arizona, the gray, massive, fossiliferous Mississippian limestone is remarkably uniform and easily recognized, although it is called by different names in different areas.
Late in Mississippian time, the Colorado area rose slightly and the sea in which the Leadville Limestone was deposited receded. An interval of erosion followed. The surface of the limestone was dissolved and pitted, tunnels and caves formed where running water etched deep into the rock, and a reddish soil formed on the surface and in the hollows. This portion of the limestone, which in some places also contains pebbles of chert, is named the Molas Formation. Part of the Molas may be Pennsylvanian in age.
Mississippian [fossils] from western Colorado show that seas covered much of the state about 330 million years ago.
Pennsylvanian Period
(270-310 million years ago)
As the Pennsylvanian Period began, the Colorado area continued to rise. Earliest deposits of this age are fine-grained black shales and sands—the Glen Eyrie Formation along the southern Front Range and the Belden Formation in west central Colorado. Then, through millions of years, mountain-building took place. Some areas rose more than others, so that formerly flat-lying marine sediments were bent and broken, and a series of high mountain ridges and deep basins were formed. Geologists sometimes call these the Ancestral Rocky Mountains.
Although the pattern of the mountains changed repeatedly, the Ancestral Rockies consisted principally of two large ranges. One range roughly paralleled the present Front Range, but lay thirty to fifty miles further west. The other extended from the San Luis Valley northwest toward Colorado National Monument, including the area around the Black Canyon of the Gunnison and the present Uncompahgre [Plateau]. Coarse sediments washed off both sides of both ranges, and accumulated as [alluvial fans] and valley fill along the mountain margins. These exist today as the Fountain Formation of the eastern Front Range, the Minturn Formation between the ancient uplifts, and the Hermosa Formation west of the western uplift.
This paleogeographic map reflects the distribution of land and sea during the early part of the Pennsylvanian Period and shows where coarse sediments derived from the Ancestral Rockies were deposited.
FOUNTAIN FORMATION MINTURN FORMATION HERMOSA FORMATION
West of Denver, the main line of the Denver & Rio Grande Railroad tunnels beneath steeply dipping sandstones and [conglomerates] of the Fountain Formation. (Jack Rathbone photo)
Corals, [brachiopods], and [fusulinid] Foraminifurida can be found in the Pennsylvanian Minturn Formation at many places in the Mountain and [Plateau] Provinces.
In western Colorado, where vegetation is sparse, rock structures are clearly defined. This photograph shows beds of the Pennsylvanian Minturn Formation sharply folded, probably as a result of the deformation of gypsum in underlying layers. (Jack Rathbone photo)
In the Flatirons near Boulder, Red Rocks Park near Denver, and the Garden of the Gods near Colorado Springs we see well exposed examples of the Fountain Formation. The Minturn Formation is visible along the Eagle River west of Wolcott, and along Gore Creek near Vail. The Hermosa Formation forms striking red cliffs north of Durango. In the Sangre de Cristo Mountains area, exceptionally great and rapid deposition took place, and the Minturn Formation is very thick.
In west central Colorado, near the towns of Eagle and Gypsum, a large basin formed. In it, gypsum and other salts were deposited as arms of the sea were cut off from the main marine area. The unusual appearance of the hills along the Eagle River, especially north of U. S. Highway 24, is caused by the presence of gypsum in the [bedrock].
In a similar manner, the Paradox Basin was formed in southwestern Colorado. Thousands of feet of gypsum, salt, and potash were deposited here, probably also precipitated in restricted arms of the sea. These minerals, the so-called [evaporites], have since significantly controlled development of the landscape in Gypsum Valley and other parts of this region. (See [The Plateaus] in Chapter I and the section on [Gypsum] in Chapter III).
Between the mountain masses and their surrounding alluvial deposits, shallow seas repeatedly invaded the lowland areas of the state. Marine [fossils] in some parts of the Minturn Formation bear witness to as many as twenty marine cycles. Strangely, the Pennsylvanian Period appears to have been cyclical in other parts of the United States as well, for marine sediments are found alternating with nonmarine sediments in Pennsylvania, Illinois, Kansas, Nebraska, and New Mexico. In middle Pennsylvanian time, general uplift occurred in Colorado, and almost the entire state was above sea level for the rest of the period.
Permian Period
(223-270 million years ago)
By the end of the Pennsylvanian Period, the mountains of the Ancestral Rockies had been almost entirely removed by erosion, and the deep basins were filled with sediments. Colorado was once more a great plain, sloping gently to the northeast. In eastern Colorado, a shallow sea gradually dried up, leaving some thin limestone and gypsum beds along its margin. The western shore of this sea was edged with beaches and sand dunes, preserved as the Lyons Sandstone. The buildings of the University of Colorado, as well as many homes and other structures in the Boulder-Denver area, are faced with this beautiful salmon-colored sandstone.
Balanced Rock, in the Garden of the Gods northwest of Colorado Springs, is an erosional remnant of iron-rich [conglomerate] and sandstone. It remains while the rest of the surrounding layers are gone because it is harder and more completely cemented together by silica. The rock is part of the Late Paleozoic Fountain Formation. (John Chronic photo)
In the western part of the state, Permian deposits consist mostly of shales and sandstones. The red color of these rocks, and the complete absence of [fossils] in them, suggest that the environment in which they were deposited was not marine, but was a vast, level mudflat subject to alternating wet and dry periods. The shales and sandstones collectively are called the Maroon Formation, named for Maroon Bells, near Aspen, where they are dramatically exposed in the mountain cliffs.
Tracks of Permian reptiles called Laoporus coloradoensis occur in the Lyons Sandstone near Lyons. These are about life size.
During part of Permian time, a shallow sea extended from Idaho, Utah, and Wyoming into the northwest corner of Colorado. In this sea was deposited the Phosphoria Formation, a highly phosphatic limestone containing only rare, poorly preserved molluscan [fossils].
As the Paleozoic Era ended, Colorado was still flat and low-lying. By this time land plants and animals had evolved, but if vegetation grew in the Colorado area, or animals roamed it, they left few [fossil] remains. Tracks of early reptiles have been found in the Lyons Sandstone. Dune sandstones here and in adjacent areas suggest that desert conditions may have prevailed, in which case Colorado would have been very similar, scenically and climatically, to Sahara regions today.
Dark red Pennsylvanian and Permian [conglomerates] form the Flatirons that overlook the University of Colorado campus at Boulder. University buildings are faced with Permian Lyons Sandstone quarried along the foothills of the northern Front Range. (University of Colorado photo)