historical geology lecture 10. late paleozoic geology i

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HISTORICAL GEOLOGYLECTURE 10. LATE PALEOZOIC GEOLOGY I.

The Late Paleozoic (Devonian-Mississippian-Pennsylvanian-Permian) is noted for a number of geologically significant events:

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- widespread COLONIZATION of the land by large plants, reptiles and amphibians.

- a number of major OROGENIES, including the ACADIAN (Devonian), the ANTLER (Mississippian), the HERCYNIAN (Mississippian), the ALLEGHENY (Pennsylvanian), OUACHITA (Pennsylvanian) and the SONOMA (Permian).

- major non-marine deposits, including COAL beds (related to the development of LAND PLANTS), and sand dunes/evaporites (related to the widespread aridity of many parts of the northern continents.

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DEVONIAN - MISSISSIPPIAN

1. PaleogeographyThe North American craton had continued its counterclockwise rotation relative to the Equator. Most of the craton was still experiencing hot, tropical conditions. Carbonates were forming in the quieter regions of epeiric (platform) seas; clastics were being shed by the newly emergent mountainous areas along the orogenic belts; coral reefs and evaporites were forming in the more restricted shallow marine areas.

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2. Transgressions and RegressionsThe beginning of the

Devonian is marked by the TIPPECANOE REGRESSION, causing widespread early Devonian erosion and leaving a major EROSIONAL UNCONFORMITY, used to identify the boundary between the TIPPECANOE and the KASKASKIA - the next major transgressive sequence.The rest of the Devonian is characterized by THE KASKASKIA TRANSGRESSION, which continued into the Mississippian.

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The Kaskaskia transgression left coastal sands spread across New York, Pennsylvania and Virginia – the Oriskany Sandstone.

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In deeper water, shales formed (especially after the Acadian orogeny – which promoted vigorous erosion and mud production). A well-known example is the Chattanooga Shale of Tennessee. 'Black shales' are dark, as a result of being especially rich in unoxidized carbon (stagnant water, little oxygen).

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Exposure of Chattanooga Shale.

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3. OrogeniesThe Late Paleozoic is noted for 3 major orogenic events:

1. THE ACADIAN OROGENY: the continental convergence that formed the Taconic Uplands of the Early Paleozoic continued into the Late Paleozoic as THE ACADIAN OROGENY. The Acadian Mountains run from Newfoundland right down the east coast to west Virginia. They consist of folded sedimentary and metamorphic rocks containing igneous intrusions. A major consequence of this mountain building was the formation of a very large clastic wedge spreading westward across the craton - THE CATSKILL WEDGE. Coarse terrestrial sediments in the east; fine-grained clastics and carbonates in the west.

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The nonmarine sandstones and shales in this wedge are often deeply OXIDIZED (iron combines with oxygen - similar to rust), producing a red coloration - giving these beds a similar origin and appearance to the OLD RED SANDSTONES of Europe (many older buildings in eastern cities contain red sandstone blocks from the Catskill Wedge - “brown stones”).

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2. THE ANTLER OROGENY:

On the west coast, the island arc that formed back in the Ordovician had now been pushed up against the continent forming the highlands and volcanic activity of the ANTLER OROGENY, running through (what is now) Nevada, Idaho and into British Columbia. These mountains became source areas for clastics that spread across the western states in the Pennsylvanian and Permian.

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The mountainous west (Cordillera) was beginning to form.

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3. THE HERCYNIAN OROGENY: The collision between Gondwanaland and the northern continents occurred slightly earlier in Europe than in North America. The result was the Late Mississippian HERCYNIAN OROGENY (roughly equivalent to the Allegheny Orogeny of North America, which occurred slightly later - in the Pennsylvanian). A great mountain chain was pushed up along the margins of Southern Europe. Some of the clastic wedges that formed around these mountains were covered by forest and went on to become some of the great European coal basins. Much of the Hercynian Mountain chain was subsequently eroded away - only small eroded stumps remain uncovered today.

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