Episode Two: Development of a Pacific-type Continental Margin;
Further Deposition of Sediment

(about 450 - 270 million years ago)

About 450 million years ago, in the middle of the Ordovician Period, the direction of mantle flow inside Earth changed. The proto-Atlantic Ocean (Iapetus) stopped widening and began to close. That means that conditions along the southeastern margin of North America ceased being tectonically passive (Atlantic-type margin) and instead became tectonically active (Pacific-type margin). From the above background comments, you know that this means that two tectonic plates were colliding together (converging), producing volcanic mountains similar to those around the present-day Pacific Ocean.

In central Pennsylvania, initial signs that the margin was under compression included the warping or flexing of the gently sloping continental margin, causing some parts of the continental margin to deepen while other parts were lifted above sea level. At about the same time, offshore volcanoes began to erupt enormous ash clouds, now seen as thick beds of volcanic ash interbedded with the limestones. For a while, the configuration of Laurentia's southeastern margin resembled the volcanic island complexes around the Pacific, such as Japan or the Aleutians, but after a few tens of millions of years, plate convergence had crushed the volcanic islands onto the continent. The edge of North America then probably looked more like the Andes Mountains of South America. This enormous Late Ordovician mountain complex is generally known as the Taconic Mountains.


Two layers of volcanic ash (recessed zones) in Ordovician limestones near Reedsville, PA.

These Taconic Mountains are now completely eroded away. But then how do we know where they were and that they were so large? We know principally from the sediment that came from them and was deposited in central and eastern Pennsylvania. For millions of years, the Taconic Mountains were weathered and eroded, producing enormous quantities of mud, sand, and gravel that were transported down river systems back toward the interior of North America. First to reach central Pennsylvania was the fine-grained mud that was flushed out to sea to pile up on the shallow limey shelves (see Figure at right). The great quantities of incoming sediment exceeded the capacity of the interior basin to accommodate it, so central Pennsylvania was built up above sea level. The scene then resembled a broad alluvial plain across which a complex of rivers flowed westward. The record of this alluvial plain is a very thick sequences of river-deposited sandstones (see Figure at lower right). We know the rivers flowed westward because of the pattern of distribution of the different types of rock and because of preserved sedimentary structures, such as cross stratification (see Figure at lower left), that indicate the flow direction of ancient currents.


Cross stratification in Ordovician sandstones show sediment transport toward the northwest.


Limestones of Ordovician age (light gray) interbedded with dark shales.




Part of thick sequence of Upper Ordovician sandstones (Bald Eagle Formation). Note truck for scale.

Through subsequent parts of the Paleozoic Era, the proto-Atlantic (Iapetus) Ocean continued to close, even though the initial phase of more intense compression had ended. By the time of the Early Silurian Period, the Taconic Mountains were lowered so much that less sediment arrived to the interior basin, and with continued subsidence, shallow-marine conditions returned. The sediment deposited in this interior seaway was primarily shale and limestone, but in the middle of the Silurian Period, very unusual chemical conditions in the ocean led to formation of a very interesting type of rock known as the Clinton Ironstone.


Bed of ooidal ironstone, Keefer Formation, Route 22/322 near Millerstown, PA


Ooids of hematite, some with nuclei of quartz sand.


Brachiopods and crinoids coated with hematite.


Tonoloway Limestone; Iddings Quarry west of Mifflinburg, PA.

As the Silurian Period came to a close, central Pennsylvania was again the site of limestone accumulation in a tropical sea (see photo at right). Some parts of this limestone preserve desiccation cracks as evidence of temporary exposure above the high tide line (see photo at lower right). At other times, environmental conditions were subtidal and sustained the development of small reefs constructed by stromatoporoids (see photo below).


Curved, laminated patterns are stromatoporid fossils seen on quarry wall.

Desiccation cracks in Tonoloway Limestone.

That shallow interior seaway was disrupted during the Devonian Period by another major tectonic disturbance along the margin of Laurentia. As the proto-Atlantic continued to close and plates converged, the ancestral part of Europe (Baltica) collided with the northern part of Laurentia (see Figure below).


Paleogeographic reconstruction of the earth for the Mid Silurian. Note that much of North America (Laurentia) is below sea level, except for the mountain belt caused by the collision between North America and Avalonia/Baltica. Diagram from the Paleomap Project by Christopher Scotese (www.scotese.com). Used by permission.

This compression raised up a high mountain range (the Acadian Mountains) that stretched from eastern Canada down to Virginia (see Figure below).


Paleogeographic reconstruction of the North America region during the Middle Devonian. Note the Acadian Mountains, and location/absence of southern portions of the North American continent. Diagram from Dr. Ron Blakey's Global Earth History Home Page. Used by permission.

With another high mountain system on its eastern border, another long period began when Pennsylvania received enormous quantities of sediment. These now-vanished Acadian Mountains must have been impressively large, judging from the great thicknesses (kilometers thick) of river and delta sediments that flowed westward from them into central Pennsylvania.

In central Pennsylvania, this river complex is preserved in the kilometers-thick redbed sequence known as the Catskill Formation.


The principal organizational pattern of the Catskill Formation is the fining-upward sequence, which is characteristic of the deposits of meandering rivers.


Meander bend in the Mississippi River; analogue of rivers that deposited much of the Catskill Formation.


Plant root marks in floodplain deposits of the Catskill Formation.

Calcium carbonate nodules in floodplain soils of the Catskill Formation indicate semi-arid climate in Late Devonian time.

But, just as happened earlier following the Taconic Orogeny, this Acadian Mountain range was also reduced by erosion over millions of years. As a result, gradually less and less siliciclastic sediment came into central Pennsylvania, and just as before, shallow-marine conditions began to return.

continue to: End of Episode 2...

 

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