Deep Red Sandstone

DEEP RED SANDSTONE Beginning in the late Silurian and continuing into the Devonian, from about 400 million to 350 million years ago, collisions between present-day eastern North America and northwestern Europe lifted the Acadian Mountains (Figure 103). The red layer of the terrestrial Catskills in the Appalachian Mountains southwest of New York State to Virginia consists of sandstone and shale cemented by red iron oxide and is the primary expression of Acadian orogeny in North America. Extensive frozen activity and metamorphism accompany mountain building at its climax. The Devonian Antler orogeny is another mountain-building episode, resulting from the collision of an island arc with the western margins of North America.The island arc appears to have formed about 470 million years ago off the west coast of North America.The orogeny is intensely deformed rock in the Great Basin region from the California-Nevada border to Idaho. Innuitian orogeny, from the Devonian through the Carboniferous, damaged the northern margin of the present-day North American continent. The mountain-building episode raised the Innuitian Mountains on Ellesmere Island in the Canadian Arctic. It resulted from a collision with another crustal plate, possibly the mass of the eastern Siberian continent. Block faults and basin fillings replaced mountain buildings in the region. The middle Devonian Crimson sandstone, a thick sequence of mainly nonmarine sediments in Great Britain and northwestern Europe (Fig.104), is a major expression of an episode of mountain building called the Caledonian orogeny. The formation consists of large masses of sand and silt that accumulate in basins between the Caledonian Mountain ranges from Great Britain to Scandinavia.The sediments are poorly sorted (differing in size) and consist of red, green, and gray sandstones and gray shales that often contain fish fossils. Erosion leveled the continent and shallow seas flowed inland, flooding more than half of the land. Inland seas and wide continental margins along with a stable environment provide favorable conditions for marine life to thrive and breed around the world. The seas that flooded North America during the Devonian produced abundant coral reefs that were lithified (into stone) into widespread limestone (Fig. 105). The rising Acadian Mountains on the northeast side of the interior are eroded. Their sediments produced flat fossil shale deposits in western New York State, perhaps the finest part of the Devonian in the world. The vast Chattanooga Shale Formation, which covers almost the entire interior of the continent, was laid down during the Devonian and Carboniferous.The sea also enveloped much of Eurasia in the late Devonian. Terrestrial clastic containing eroded rock fragments from the Caledonian Mountains cover the western part of the continent. The second half of the Paleozoic followed the Silurian ice age, when Gondwana wandered into the south polar region about 400 million years ago and acquired a thick ice sheet. Gondwana, up until this point located in Antarctica, is now shifting its position. Its location can be demonstrated by paleomagnetic data, which show the location of continents relative to magnetic poles by analyzing the magnetic orientation of ancient iron-rich lava. The south magnetic pole drifts from present-day South Africa in the Devonian, crosses Antarctica in the Carboniferous, and ends in southern Australia in the Permian. The location of the south pole is also indicated by widespread glacial deposits and erosional features on the continent comprising Gondwana during the late Paleozoic. The mass extinctions of the late Ordovician 440 million years ago and the central Devonian 365 million years ago coincided with glacial periods that followed long intervals of ice-free conditions. Gondwana in the southern hemisphere and Laurasia in the northern hemisphere are separated by the Tethys Sea (Fig. 106). Into this sea flow thick sedimentary deposits washed from the surrounding continent. Their accumulated weight forms a long, deep depression in the ocean's crust, called the geosyncline, which then lifts into a folded mountain belt when Gondwana and Laurasia collide. The warm climate and desert conditions over a large area are indicated by the wide distribution of evaporite deposits in the Northern Hemisphere, coal deposits in the Canadian Arctic, and carbonate reefs. Warm temperatures in the past were commonly recognized by abundant marine limestone, dolomite, and calcareous shale. The coal belt, stretching from northeastern Alaska across the Canadian archipelago to northernmost Russia, suggests that vast swamps are prevalent in this region. Evaporite deposits generally form in dry conditions between 30 degrees north and south of the equator. However, extensive evaporite deposits are not currently forming, suggesting a relatively cooler global climate. The existence of ancient evaporite deposits as far north as the Arctic region implies that these regions were once closer to the equator or the global climate was much warmer in the geological past. The Devonian year is 400 days and the lunar cycle is about 30.5 days as determined by the daily growth rings of fossil corals. Paleomagnetic studies show that the equator moved from California to Labrador and from Scotland to the Black Sea during the Devonian and Carboniferous (Fig. 107). Ideal climate settings helped spur the emergence of amphibians inhabiting large Carboniferous swamps. After discussing Devonian marine and terrestrial life forms, the next chapter follows the evolution of the first amphibians of the Carboniferous period.