MESOZOIC ERA
The Mesozoic Era, popularly known as the Age of Reptiles or Age of Dinosaurs, is divided into three periods. The climate of the entire earth appears to have been warmer then than it is at present, perhaps because of a different distribution of land and sea areas, or because continental areas were not as high and mountainous as they are just now. Colorado was a rather low land area for most of the first two Mesozoic periods; then a vast sea covered the entire state for the remainder of the era.
The pink cliffs of Colorado National Monument are made of Wingate and Entrada Sandstones. Underlying them, in the valley bottom, Chinle shales form steep red slopes. (William C. Bradley photo)
Triassic Period
(180-225 million years ago)
Saharan conditions continued to prevail in western North America during the early part of the Mesozoic Era. In central Colorado, the lowest Mesozoic deposits are the Triassic Lykins Formation, a series of soft, bright red sandstones and shales. Where the Lykins is exposed along the Front Range, its bright red color identifies it. Because of its softness, it is often less prominent than adjacent rock layers in the mountain foothills. The Lykins Formation includes some [evaporites], apparently derived from Permian evaporites washed into the Triassic ponds and lakes which existed occasionally in this region.
Over almost the entire state, the rocks deposited at this time are very similar. Formation names may differ—Lykins, Moenkopi, Chinle, Ankareh, Wingate—but the rocks are almost universally fine-grained sandstones and shales with a red or pink color. They represent ancient coastal plain, dune, or delta deposits. Toward the western edge of the state they coarsen, and contain layers of [conglomerates] similar to the Triassic conglomerates of northern Arizona and Utah. These suggest that mountain-building was taking place west of here at that time.
There are virtually no [fossils] known from Triassic rocks in Colorado, although some fossil palm fronds have been found west of the San Juan Mountains, in the southwestern corner of the state.
Jurassic Period
(135-180 million years ago)
During the Jurassic Period, Colorado was still a low, flat desert area with intermittent streams flowing eastward over the surface of older sediments. The Navajo Sandstone, formed from dune sands, was deposited in the western part of the state. Streams flowing eastward from Utah brought fine sediments—silts and muds—to western Colorado, forming what is now the Carmel Formation. Near Canon City, coarse gravels bear witness to local uplift in Jurassic time. Both these gravels and the Carmel Formation were overlain by more dune sands, now hardened into the Entrada Sandstone.
In Late Jurassic time the Colorado area, which had been predominantly desert since Permian time, appears finally to have been submerged once more. Fine calcareous muds of the Curtis Formation, containing [ammonites], [belemnites], and other marine shellfish, show us that a shallow sea transgressed from the west over the wind-blown sands. This sea was, geologically speaking, of short duration—only a few million years. Bounded on almost all sides by desert, it seems to have dried up, depositing the gypsum that is now present in a thin layer along the Front Range between Denver and Canon City in the Ralston Formation.
At about this time, however, the climate underwent a major change. Deposits above the Ralston indicate an increasingly moist environment, the environment in which the Morrison Formation was deposited over most of Colorado and parts of the adjacent states of Kansas, Arizona, Utah, and Wyoming. The Morrison Formation is exposed in many places, and is characteristically composed of layers of fine, limy mud, brightly colored in streaks of red, brown, green, and blue. In most areas it is so soft that it becomes soil-covered; it is well exposed only in roadcuts or where it is protected from erosion by a “caprock” of harder sediments or [lava]. Spectacular outcrops can be seen in new roadcuts along U. S. Interstate highway 70 just west of Denver.
In this roadcut along U. S. Interstate 70 west of Denver, Jurassic and Cretaceous rocks are unusually well exposed in the Dakota [hogback]. Green and purple shales represent the dinosaur-bearing Morrison Formation. The Cretaceous Dakota Group forms the eastern, higher half of the cut. Black layers are carbon-rich clays of the South Platte Formation, frequently quarried locally for ceramic uses. (John Chronic photo)
[Fossil] dinosaur bones occur in great numbers in the Morrison Formation near the towns of Morrison and Canon City and at several other places in Colorado. Those at Canon City have been quarried extensively, and are now mounted in a number of museums in the United States. At Dinosaur National Monument, in eastern Utah and northwestern Colorado, many excellent remains have been found; those in Utah can be seen in place in the rock in a striking exhibit at the National Monument.
In an old painting, a paleontologist contemplates [fossil] bones found near Morrison. The date is 1877. The bones are those of the 70-foot dinosaur Apatosaurus, more commonly known as Brontosaurus, shown below in reconstruction.
Some of the dinosaurs known from the Morrison Formation reached 80 feet in length. Both plant-eating and meat-eating types are known. In addition to the bones themselves, gastroliths or gizzard stones can frequently be found; these highly polished stones were as essential to dinosaur digestion as gravel is to a chicken or a caged canary.
Along with the dinosaur [fossils] are found abundant remains of water plants called charophytes. These plants formed tiny spiralled balls of calcite as part of their reproductive activities; both the little balls and the stalks of the plants themselves occur in many parts of the state. In western Colorado, near Grand Junction, silicified shells of freshwater snails can also be found in the Morrison.
Early in the 1900s vanadium, radium, and uranium were discovered in Jurassic sandstones and mudstones of western Colorado. Extensive mining in this area has revealed that these elements often become concentrated by groundwater in organic material such as [fossil] plant stems or dinosaur bones. The search for radioactive minerals has thus brought to light many ancient fossil accumulations.
Cretaceous Period
(70-135 million years ago)
Early in Cretaceous time, marine conditions once more prevailed in Colorado. This is indicated by a marked change in rock types from beach and near-shore deposits to true marine sediments.
Between the Front Range and the Plains the Cretaceous Dakota Formation forms a [hogback] ridge which can be traced for 200 miles or more. The well-cemented sandstone resists erosion, and so remains as a ridge when softer layers are stripped away. (Jack Rathbone photo)
The sandstones derived from beach sands sometimes include coarse pebbles of chert which can be traced to sources in Permian rocks of Utah and Nevada. Occasionally the beach and near-shore deposits include marine shells like oysters, indicating that there were brackish and salt water lagoons and marshes along the shore. The Dakota Formation represents the beach of the transgressive or advancing sea. This formation contains oil in eastern Colorado, Nebraska, and Wyoming; the oil itself may have been derived from decay of organic materials in swamps behind the beaches and bars.
As the sea deepened in eastern Colorado, finer sediments were deposited. These included the black muds of the Benton Shale, and the Niobrara Limestone, a shallow-water deposit containing abundant shells of clams (Inoceramus and Ostrea) and [ammonites] and tiny one-celled animals called [Foraminiferida]. Above the Benton and Niobrara Formations lie the fine gray muds of the Pierre Shale. Several thousand feet thick, the Pierre contains occasional beautifully preserved ammonite shells as well as bones from [fossil] fish and swimming reptiles.
Cretaceous rocks in Colorado are rich in [fossil] pelecypods. Each of the fossils illustrated above may grow to a much larger size than shown.
Shales of the Laramie Formation contain many recognizable plant [fossils].
The rocks deposited in western Colorado at this time are markedly different from those deposited in eastern Colorado. In the east, deposits are fine and very limy, containing abundant shells and little in the way of coarse debris. In the west, sandstones of the [Mesa] Verde Formation dominate, and coal beds suggest marshy or swampy conditions inshore from the ancient ocean. This is just the pattern we would expect from a low-lying region bordering a shallow sea, a region similar perhaps to the southeastern Atlantic and Gulf coasts of the United States today.
Toward the end of the Cretaceous Period, the sea receded from Colorado. Beaches and bars of the retreating sea left a sandstone layer which now outcrops prominently east of the Front Range as the Fox Hills Sandstone. Above lie interbedded sands and coals, the Laramie Formation. The presence of coal above beach sands shows that the coal swamps moved eastward as the sea retreated.
The exact age of the shoreline deposits and coal beds varies from place to place in such a way as to indicate that the sea withdrew slowly and irregularly. In general the shore moved eastward, but there are localities such as North Park where deposition lasted much longer than elsewhere. In some places no real beach was formed at the ancient strand line.
In western Colorado, the end of Cretaceous time is marked by coarser beds, indicating an increased rate of uplift in Utah. [Conglomerates] were deposited in the beds of the McDermott Formation, now visible along the Animas River south of Durango.