Episode One: Development of an Atlantic-Type Continental Margin;
Deposition of Sediment
(about 650 to 450 million years ago)
As this episode began, the supercontinent Rodinia was breaking up and separate plates were diverging. As they did, they carried smaller continental pieces, including ancestral North America (sometimes labeled Laurentia), with them.
For outstanding maps showing the position of the continents during the Precambrian Era and Cambrian Period, please visit Christopher Scotese's website: http://www.scotese.com
Through most of this epsode, North America straddled the equator and was rotated somewhat clockwise relative to its present orientation. What is now Pennsylvania was situated at the southeastern margin of North America, about 20 degrees south of the equator, and at the edge of a widening ocean basin, the proto-Atlantic (Iapetus) Ocean. As part of an intraplate, tectonically passive, Atlantic-type continental margin, Pennsylvania was a low, featureless coastal shelf sloping gently southeastward toward the sea.
North America during the Late Cambrian, ~500 million years ago. Image courtesy Ron Blakey, Colorado Plateau Geosystems, Inc.
The geologic record of this episode in Pennsylvania is a thick sequence of sedimentary rocks that record the accumulation primarily of carbonate sediment (now limestones and dolostones. Even though the average rate of sediment accumulation was only millimeters per year, with deposition taking place over something like 200 million years, the total thickness of strata reached several kilometers (below).
Cross section of strata of latest Precambrian through Middle Ordovician age in Pennsylvania (from Rankin and others, 1989). Later folding has been removed; limestones colored blue.
The weight of the accumulating sediment caused the crust to subside continually so that depositional conditions were persistently shallow-marine. At those periodic times when global sea level became relatively lower, Pennsylvania became an emergent coastal plain and some deposition took place on sandy beaches.
Because the geographic location of this shallow marginal sea was in warm, tropical waters of the southern hemisphere (see paleogeographic map above), much of the sediment was in the form of calcium carbonate (now limestone), either precipitated directly out of sea water or the skeletal remains of organisms.
Upper Ordovician carbonate strata (limestone) along U.S. 322 near Reedsville, PA.
These organisms included familiar early Paleozoic forms, such as brachiopods, bryozoans, and corals, but in addition the delicately laminated cyanobacterial growths known as stromatolites (below) were present.
Algal stromatolites in limestone strata near Birmingham, PA
Much evidence, including those stromatolites, indicates that water depths were quite shallow, so shallow in fact that storm waves made characteristic sedimentary structures in the loose sediment (below). If you could have been here 450 million years ago, you might have thought these tropical sea conditions rather idyllic. Little would you suspect that the quiet calm that had persisted for some 200 million years was about to end...
Wavy laminae and burrows in the Trenton Limestone, near Oak Hall, PA.
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