Historical Geology of the PeriodThe Lower Cretaceous Period
At the beginning of the Lower Cretaceous in North America, the Mexican Sea of the late Jurassic period spread over Texas, Oklahoma, New Mexico, and parts of Arizona, Kansas, and Colorado. Deposits from this inland sea, known as the Comanchean Sea, were chiefly limestone (up to 1,500 ft/457 m thick in Texas) but some continental sediments (i.e., sandstone, shale, and conglomerate) mark the reemergence of land, which brought the Lower Cretaceous to a close. The Comanchean Sea was probably separated by a land barrier from contemporaneous seas in the California areas, where 26,000 ft (7,925 m) of Shastan shales, with sandstone and thin limestone, were laid down. The sediments were derived by rapid erosion from the recently elevated Sierra Nevada and Klamath mts. In Montana, Alberta, and British Columbia the Kootenai deposits of sandstone and sandy shale, which contain workable deposits of good coal, were formed; along the Atlantic coast the unconsolidated sandy clay, gravel, and sand of the Potomac series were deposited.
The Lower Cretaceous opened in NW Europe with the deposition of a continental and freshwater formation, the Wealden sand and clay, best displayed in England. The sea, meanwhile, expanded from the Mediterranean, finally overlaying successive Wealden strata with limestone. There was at the same time an extensive sea in N Europe. At the close of the Lower Cretaceous, there was some recession of the seas; by the Upper Cretaceous, the great transgression of seas submerged lands that had been open since the Paleozoic.
The Upper Cretaceous opened in W North America with the deposition of continental sands (now the Dakota sandstone), which, however, were covered by the ensuing rise of the Colorado Sea. The Colorado Sea was the greatest of the North American Mesozoic seas and extended all the way from Mexico up into the Arctic, covering most of central North America. The Colorado deposits were composed chiefly of shales, limestone, and some chalk in Kansas and South Dakota. Slight shifting of the sea was followed by the deposition of the Montana shale and sandstone and then by withdrawal of the sea. Near the end of the Upper Cretaceous, conditions in the west were similar to those of the Carboniferous period in other regions; swamps and bogs were formed that later became valuable deposits of coal.
At the close of the Cretaceous the Laramide revolution occurred—at least two different epochs of mountain building and one of relative quiet. In this disturbance the Rockies and the E Andes were first elevated, and there were extensive flows of lava. The Appalachians, which had been reduced almost to base level by erosion, were rejuvenated, and the seas retreated from all parts of the continent. The intermittent character of the Laramide disturbance makes difficult the demarcation of the Mesozoic and the succeeding Cenozoic era.
The striking feature of the European Upper Cretaceous are great chalk deposits from small carbonate-bearing marine algae and calcareous fauna, now exposed in the cliffs of the English Channel. In India the late Upper Cretaceous was marked by an overflow of lava in the Deccan plateau. The area covered by igneous rocks dating from this period now comprises over 200,000 sq mi (518,000 sq km) and was formerly much larger, having been reduced by erosion. Near Mumbai the formation is 10,000 ft (3,000 m) thick.
During the Cretaceous period the massive continents of Gondwanaland and Laurasia continued to separate. South America and Africa had separated, with the consequent widening of the S Atlantic. The N Atlantic continued to expand, although it appears that Europe, Greenland, and North America were still connected. Madagascar had separated from Africa, while India was still drifting northward toward Asia. The Tethys Sea was disappearing as Africa moved north toward Eurasia. Antarctica and Australia had yet to separate.
Sections in this article:
See more Encyclopedia articles on: Geology and Oceanography