The Last Stage of Episode 2 - Closure of the proto-Atlantic Ocean;
Last Sediment Deposited

(about 270 million years ago)

Toward the close of the Paleozoic Era, all continents of Earth became coalesced together to produce the supercontinent known as Pangaea. (see Figure at below) Remember that the proto-Atlantic had begun to open more than 650 million years ago with the breakup of the supercontinent Rodinia; now the proto-Atlantic was undergoing the final closing, as Africa (part of Gondwanaland) collided with North America. This would conclude the Wilson Cycle by marking the culmination of one complete opening and subsequent closing of an ocean basin.

Paleogeographic map of the Late Permian, showing the formation of the supercontinent, Pangea. Diagram from Dr. Ron Blakey's Global Earth History page, used with permission.

The first phase of this culminating compression took place during the Late Carboniferous (Pennsylvanian) Period, probably as the result of a collision between a volcanic island arc complex and the continent of Laurentia. Once more, to the east of central Pennsylvania high mountains were elevated, and increasing quantities of coarse, gravelly sediment flowed westward to be deposited in complex braided river systems central Pennsylvania. (Figure at lower left) These rivers continued to flow westward from the mountains, but over time, as gradients lessened, the rivers became less gravelly and were bordered by extensive swamps. In those swamps great amounts of plant matter accumulated to become the coal seams (Figure at lower right) mined in Pennsylvania in the present day.

To sum up Episodes 1 and 2, here at the end of the Late Carboniferous (Pennsylvanian) Period, close to 270 million years ago, a kilometers-thick sequence of horizontal sedimentary strata had been piled up under where Pennsylvania is today. The lower part of this thick sequence (the older part, deposited from about 600 to 450 million years ago) consists largely of carbonate rocks (limestones, etc.) that accumulated on a continental margin that sloped gently eastward toward an open ocean (Episode 1). In contrast, the upper part of the sequence (the younger part, deposited from about 450 to 270 million years ago) is largely siliciclastic (sandstones, etc.) that accumulated on a surface that sloped westward away from marginal mountains toward the interior of the continent (Episode 2). But that upper part is not uniformly siliciclastic, for at those times between the major elevations of the mountains, shallow-marine conditions returned to central Pennsylvania, and limestones and other marine rocks were deposited. You will see later how the alternation of "packages" of strata that have different dominant compositions (some carbonate; some siliciclastic) is to a large degree responsible for the characteristic valley and ridge landscape of central Pennsylvania.

Bucknell Geology field trip to Bear Valley near Shamokin, PA, to study the 300 million year-old coal-bearing strata of the Llewellyn Formation.

Well, the strata are now assembled, it is now time to see about their deformation.

continue to Episode 3...


Introduction Plate Tectonics Episode 1 Episode 2 End of Episode 2 Episode 3 Episode 4