Geol 33 Environmental Geomorphology

J Bret Bennington

Major Geomorpho-tectonic Regions

Coastal Plains - these are extensive regions of flat-lying to gently seaward dipping sediments that build up as a passive continental margin subsides over millions of years. They are areas of very low relief. Generally, coastal plains are absent in tectonically active areas because active uplift of the coastline prohibits their development.

• Atlantic Coastal Plain - extending from Cape Cod south to Florida and West to Texas, the Atlantic coastal plain began forming as Pangea was rifting apart and the modern Atlantic Ocean began to widen in the Jurassic. Slow, steady subsidence of the eastern and gulf continental margin has allowed sediment to accumulate into a seaward-thickening wedge of unconsolidated strata ranging in age from Jurassic to Holocene.

Orogenic Belts - these are elongate regions that form from collisional tectonics along active continental margins. In presently active orogenic belts the metamorphic rocks may not yet be exposed on the surface - only after extensive erosion of the overlying, intensely deformed sedimentary cover are the metamorphic roots of the mountains exposed. Where they are generated by subduction of oceanic crust beneath the continent they are highly intruded by magmatic plutons.

Appalachian Orogenic Belt - Metamorphic and igneous rock representing the crustal roots of a series of orogenies developed through the Paleozoic. Mesozoic sediments and lava flows are also present in a series of elongate rift basins produced during Late Triassic / Early Jurassic breakup of Pangea.

• New England Province - Mostly Paleozoic metamorphic rock and granitic plutons, these rocks form low mountains that are glacially rounded and sculpted.
• Piedmont Province - The southern extension of the New England Province. Deeply weathered bedrock of mostly Paleozoic metamorphic rock and granitic plutons forming rolling hills.

Cordilleran Orogenic Belt - Complex assemblage of geomorphic provinces developed primarily from collision events along the western margin of North America during the Mesozoic and Cenozoic.

• Coast Ranges - Folded and thrust-faulted sedimentary strata of Late Mesozoic to Tertiary age forming coast-parallel belts of low mountains.
• Sierra Nevada - Block of Mesozoic granite and older sedimentary rock uplifted along N-S trending normal faults, glacially sculpted into high peaks and deep, wide valleys.
• Cascades - Inland belt of Tertiary and Quaternary volcanics produced by magma generated during subduction of the Juan de Fuca plate beneath western North America. Stratovolcanoes are the most impressive topographic feature.
• Rocky Mountains - Northern Rockies dominated by glacially sculpted, thrust faulted blocks of crust, which bring Proterozoic bedrock up and over Mesozoic strata. Central and Southern Rockies dominated by normal faulting that has uplifted blocks of Proterozoic meta-igneous rock. High peaks created by glacial sculpting of deep, wide valleys.

Fold and Thrust Belts - marginal to orogenic belts are regions where the sedimentary cover has been folded and thrust faulted by compressional stresses without metamorphism. Low angle thrust faults create blocks of sedimentary crust stacked one on another, suggesting large amounts of shortening of the upper crust (often refered to as "thin-skined tectonics"). Erosion of ancient fold and thrust belts near sea level produces a distinctive ridge and valley topography with surface expressions of underlying syncline and anticline folds.

• Valley and Ridge Province - Belt of folded and thrust faulted strata of Proterozoic (Blue Ridge Province) to late Paleozoic age. Blue Ridge consisting mainly of thrust sheets of Grenville-age metamorphic rock. Valley and Ridge consisting primarily of Cambrian through Mississippian sedimentary rocks. Most ridges developed above Silurian sandstones, with valleys eroded into shales and limestones.
• Ouachita Province - Re-emergence of Valley and Ridge belt west of the Mississippi Embayment.
• Northern Rocky Mountains (see above)

Plateaus - these are tectonically elevated regions of undeformed sedimentary rock (the Tibetan Plateau and the Chilean Altiplano are a somewhat different type of plateau - elevated regions within an orogenic belt created by slow erosion due to rain shadow climates). Plateaus developed in in humid regions can be very mountainous due to stream dissection. An extreme form of stream dissection is seen in the canyonlands of the Colorado Plateau, where rivers such as the Colorado have carved mile-deep canyons such as the Grand Canyon.

• Appalachian (Allegheny) Plateau - Elevated region of horizontal strata of Late Paleozoic age, stream dissected into steep, low mountains and deep valleys.
• Ozark Plateau - Similar to the Appalachian Plateau in most respects.
• Interior Plateau - Moderately elevated region exposing strata of Lower and Middle Paleozoic.
• Colorado Plateau - Large circular region in southwest where undeformed sedimentary strata have been elevated to one mile above sea level. Stream dissection has produced a series of deep canyons exposing strata from the Proterozoic to the Mesozoic. Includes the Grand Canyon, Zion Canyonlands, and Monument Valley.
• Columbia Plateau - This plateau is unusual in that its elevation is a result of the accumulation of numerous flows of basaltic lava over millions of years in the Tertiary. Layered basalt flows cover over 250,000 square miles to depths that exceed one mile. Stream dissection of the lava flows has generated the present topography of steep-sided valleys and canyons.

Stable Interior / Shield - the interior regions of continents are generally low relief areas with very mild stream dissection. Shield areas in the center of a continent are underlain by ancient, deeply eroded metamorphic and meta-igneous rocks. Surrounding the shield regions are areas of the stable interior covered with a thin veneer of nearly horizontal sedimentary strata. These strata are very subtly deformed into broad structurally upwarped regions called domes and downwarped regions called basins which generally do not have any topographic expression.

• Central Lowlands - broad regions of the midwest underlain by Paleozoic sedimentary strata. Lower Paleozoic strata are exposed where layers have been gently upwarped into domes, while Upper Paleozoic strata are exposed across broadly downwarped basins.
• Great Plains - Broad region of undisturbed strata of mostly Mesozoic and Cenozoic age.
• Shield - Southern extension of Canadian Shield exposing Late Proterozoic Grenville metamorphic rock. Most of this region is covered by glacial deposits.

Extensional Regions - regions of high mantle heat flow beneath continental crust can cause broad crustal doming and the formation of fault block mountains bounded by grabens and half grabens. Large grabens are also known as rift valleys when they are associated with continental rifting. These features form as the crust is thinned and stretched, creating tensional forces and normal or listric faults. Although fault block mountains are primarily tectonic features, they can be maintained by isostatic readjustment as the eroding upturned portion of the block rises and the sediment cover over the down-rotated portion thickens.

• Basin and Range - Extensive region of Utah and Nevada in the western US has been generated in the last 17 million years by extension of the western US by as much as 100 miles in the east-west direction. Extensional tectonics was accompanied by igneous intrusives and surface eruptions as magma worked its way upward along faults in the thinned crust. Different blocks expose different depths of crust, ranging in age from Proterozoic through Paleozoic and Mesozoic. Volcanics and sedimentary valley deposits are Tertiary through Quaternary in age.