CAPÍTULO 7 - ESTADO LÍMITE ÚLTIMO DE INESTABILIDAD
1. CONCEPTOS PREVIOS SOBRE EL FENÓMENO DE INESTABILIDAD
Beginning in the late Silurian and continuing into the Devonian, from about 400 million to 350 million years ago, a collision between present eastern North America and northwestern Europe raised the Acadian Mountains (Fig.
103).The terrestrial red beds of the Catskills in the Appalachian Mountains of southwestern New York State to Virginia are composed of sandstones and shales cemented by red iron oxide and are the main expression of the Acadian orogeny in North America. Extensive igneous activity and metamorphism accompanied the mountain building at its climax.
The middle Devonian Old Red Sandstone, a thick sequence of chiefly nonmarine sediments in Great Britain and northwestern Europe (Fig. 104), is the main expression of a mountain building episode called the Caledonian orogeny.
The formation comprises great masses of sand and mud that accumulated in the basins between the ranges of the Caledonian Mountains from Great Britain to Scandinavia. The sediments are poorly sorted (dissimilar in size) and consist of red, green, and gray sandstones and gray shales that often contain fish fossils.
Erosion leveled the continents and shallow seas flowed inland, flooding more than half the landmass. The inland seas and wide continental margins along with a stable environment provided favorable conditions for marine life to flourish and proliferate throughout the world. Seas flooding North Amer-ica during the Devonian produced abundant coral reefs that lithified (became rock) into widespread limestones (Fig. 105).
The rising Acadian Mountains on the east side of the inland sea eroded away. Their sediments produced flat-lying, fossiliferous deposits of shale in western New York State, possibly the best Devonian section in the world.The vast Chattanooga Shale Formation, which covers virtually the entire conti-nental interior, was laid down during the Devonian and Carboniferous. The seas also blanketed much of Eurasia in the late Devonian. Terrestrial clastics containing rock fragments eroded from the Caledonian Mountains overlay the western part of the continent.
Figure 103 Location of
Figure 104 Location of the Old Red Sandstone in Northern Europe.
Figure 105 A limestone formation of the Bend Group of the Sierra Diablo Escarpment, Culberson County,Texas.
(Photo by P. B. King, courtesy of USGS) Old red sandstone
Mixed marine and non-marine Marine
EUROPE
The second half of the Paleozoic followed a Silurian ice age, when Gond-wana wandered into the southern polar region around 400 million years ago and acquired a thick sheet of ice. Gondwana, to this point located in the Antarc-tic, now shifted its position. Its location can be shown by paleomagnetic data, which indicate the locations of continents relative to the magnetic poles by ana-lyzing the magnetic orientations of ancient iron-rich lavas.The south magnetic pole drifted from present South Africa in the Devonian, ran across Antarctica in the Carboniferous, and ended up in southern Australia in the Permian.
The location of the southern pole is also indicated by widespread glacial deposits and erosional features on the continents that comprised Gondwana during the late Paleozoic. The mass extinctions of the late Ordovician 440 million years ago and the middle 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 were separated by the Tethys Sea (Fig. 106). Into this seaway flowed thick deposits of sediments washed off the surrounding continents.
Their accumulated weight formed a long, deep depression in the ocean crust, called a geosyncline, which later uplifted into folded mountain belts when Gondwana and Laurasia collided.
A warm climate and desert conditions over large areas are indicated by the widespread distribution of evaporite deposits in the Northern Hemi-sphere, coal deposits in the Canadian Arctic, and carbonate reefs. Warm tem-peratures of the past are generally recognized by abundant marine limestones, dolomite, and calcareous shales. A coal belt, extending from northeastern Alaska across the Canadian archipelago to northernmost Russia, suggests that vast swamps were prevalent in these regions.
Figure 106 Around 400
Evaporite deposits generally form under arid conditions between 30 degrees north and south of the equator. However, extensive evaporite deposits are not currently being formed, suggesting a comparatively cooler global cli-mate. The existence of ancient evaporite deposits as far north as the Arctic regions implies that either these areas were once closer to the equator or the
Figure 107 The approximate positions of the continents relative to the equator during the Devonian and Carboniferous periods.
NORTH AMERICA
SOUTH AMERICA
AFRICA
EURASIA
Equator
T
his chapter examines the evolution of the amphibians in the great coal swamps of the Carboniferous period and follows the building of the great supercontinent Pangaea. The Carboniferous, from 345 to 280 million years ago, was named for the coal-bearing rocks of Wales, Great Britain. It is further divided into the Mississippian and Pennsylvanian peri-ods in North America. Flora that appeared in the Devonian was plentiful and varied during the Carboniferous. Great coal forests of seed ferns and true trees with seeds and woody trunks spread across Gondwana and Laurasia in the lower Carboniferous.All forms of marine fauna that existed in the lower Paleozoic flourished in the Carboniferous except the brachiopods, which declined in number and types. The fusulinids appeared for the first time in the Carboniferous. They were large, complex protozoans that resembled grains of wheat and ranged from microscopic size up to 3 inches in length. Primitive amphibians inhab-ited the swampy forests, which were abuzz with hundreds of different types of insects, including large cockroaches and giant dragonflies. When the climate grew colder and widespread glaciation enveloped the southern continents at