PENNSYLVANIAN - PERMIAN. 1. Paleogeography

Harry Williams, Historical Geology 1

PENNSYLVANIAN - PERMIAN.1. PaleogeographyThe Pennsylvanian opened with the Kaskaskia Regression, which left a widespread erosional unconformity - the basis for the division of the CARBONIFEROUS into the MISSISSIPPIAN and PENNSYLVANIAN in North America. The regression, combined with continuing orogenic uplift in the east, and the continuing deposition of clastics, resulted in a large land area forming down the eastern half of the North American Craton. Africa was now joined to the north east of the North American Craton, the SUTURE ZONE marked by a vast mountain chain (the Appalachians).

HISTORICAL GEOLOGYLECTURE 11. LATE PALEOZOIC GEOLOGY II.


The Craton continued its counterclockwise rotation and was now moving towards the north pole - bringing the Equator into a WSW - ENE alignment across the Craton. The climate was still, however, Tropical. The Antler Mountains were continuing to grow in the west and some other small upland areas resulting from plate convergence were developing further north along the west coast.



2. Transgressions and RegressionsThe KASKASKIA REGRESSION continued until mid-Pennsylvanian time; the rest of the Pennsylvanian is characterized by a marine transgression - THE ABSAROKA TRANSGRESSION.

An important part of Pennsylvanian deposits were the COAL BEDS that formed in low-lying areas to the west of the eastern mountains. These deposits probably originated as thickly vegetated swampy areas near the coast. Frequent marine inundation would have killed off the vegetation and ensured rapid burial under swampland muds. This would have prevented decomposition of the organic material by OXIDATION; resulting in the formation of carbon-rich peat-like deposits. Continuing burial and compaction, eventually transformed this to COAL.


Features often associated with the coal beds are CYCLOTHERMS, which record frequent and rapid sea-level changes in the form of a cycle of land and sea deposits. Typically, the cyclotherm consists of a basal layer of terrestrial swampy sands, shales and clays, usually capped by coal; aburptly overlain by shallow marine shales and limestones, which gradually graded up into terrestrial deposits once again. Each cyclotherm is typically about 30 - 50 feet thick; in Missouri and Kansas, stacked cyclotherms are up to about 2500 feet in thickness. The cause of the repeated sea-level fluctuations isn't certain, but it may have been periodic regional uplift or subsidence related to the continuing orogeny in the east; or eustatic (global) sea-level changes related, perhaps, to glaciations in Gondwanaland.




By the Early Permian the sea was regressing, resulting in restricted shallow inland seas and, conequently, the development of evaporite deposits; especially notable are the large Permian salt beds in Kansas.



The Permian Basin of Texas and the Guadalupe Mountains - a Permian reef.


El Capitan - part of the Permian reef.



3. OrogeniesThe major orogenic episode occurred during Early Pennsylvanian time - THE ALLEGHENY OROGENY. This was essentially the continuation of the (slightly) earlier HERCYNIAN OROGENY in Europe. Northwest Africa collided with eastern North America, producing mountain building, mainly in the Southern Appalachians. It was this compression that caused widespread thrust-faulting throughout the Appalachians, resulting in the "valley and ridge" province.


Satellite image of severe folding recording the Allegheny Orogeny.


At about the same time, South America collided with North America crumpling up the Ouachita Trough and forming a great mountain chain along the southern margin of North America - THE OUACHITA OROGENY.


View of balcones escarpment.


Erosion has since removed much of these mountains leaving only the Ouachita Mountains of Oklahoma/Arkansas, the Arbuckles of Oklahoma and the Marathon Mountains of Texas.This collision between Gondwanaland and Luarussia not only threw up mountain belts around the edge of the Craton, but also transmitted stress into the interior of the Craton, resulting in INTRACRATONIC OROGENESIS - the results were the COLORADO MOUNTAINS (the Ancestral Rockies).


Arbuckles pix


Turner Falls


In the west, the ANTLER OROGENY continued into the Pennsylvanian. Subduction also continued, building an island arc down the entire west coast. These uplands provided large amounts of clastic and volcanic deposits to this region. Many of these volcanic uplands continued moving eastward, pushing up onto the North American Craton and becoming part of it in the late Permian SONOMA OROGENY. The mountainous CORDILLERA region was clearly beginning to take form.


Permian sand dunes in Arizona


Pennsylvanian in north Texas: variety of coastal environments – black shelf muds, patch reefs, deltas, limestones (we see these, and more, on the field trip)


PANGAEAWith the suturing of Baltica and Laurentia by the Caledonians " Laurussia and Gondwana " Hercynians, Appalachians, Ouachitas" Laurussia and Siberia " Urals.....Pangaea was now assembled.

Of course, it promptly breaks up again…

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PENNSYLVANIAN - PERMIAN. 1. Paleogeography