Download Geologic History of Central Pennsylvania

Geologic History of Central Pennsylvania
Introduction
by Edward Cotter, Bucknell University
If you were flying over central
Pennsylvania you would see
long, parallel, linear ridges
curving across the region
(Figure 1) and between them
fertile pastoral valleys with a
quilt-like array of small farms
(Figure 2). What could have
caused this unique valley and
ridge landscape pattern?
A look into the rocks
underlying the landscape
reveals a thick sequence of
sedimentary layers that had
been deformed so that the
strata are no longer horizontal
but are warped and folded.
Ridges are made of hard,
difficult-to-erode sandstones,
while the valleys are sites of
easier-to-erode limestones
and shales.
Figure 1. High-Altitude image of folded strata in the
Valley and Ridge Province of central Pennsylvania.
Figure 2. A view of the Kishacoquillas Valley, looking north from Jack's Mountain
So, to understand why central Pennsylvania has these unique features, we have to explore three chapters in
its geologic history:
•
the origin of the layered sedimentary sequence,
•
the timing and cause of the folding and warping of these strata, and
•
the millions of years of weathering and erosion that produced the present landscape.
The geology of central Pennsylvania can be understood in a context of moving tectonic plates, different types
of continental margin, and changing directions of plate movement. This involves the powerful concepts known
as Plate Tectonics. Before continuing, you might want to review an introduction to Plate Tectonics. Remember
our objective is to explain and understand these three chapters in the geologic history. The explanation will be
organized in terms of episodes:
The accumulation of the layered sedimentary sequence:
Episode 1: The sedimentary sequence began to form toward the end of the Precambrian as Pennsylvania was
part of the passive trailing southeastern edge of ancestral North America, an intraplate continental margin of
Atlantic-type.
Episode 2: Changing plate movement made eastern North America a tectonically active, Pacific-type margin,
and central Pennsylvania received great quantities of additional sedimentary deposits largely from the erosion
of the mountain ranges that were located along the continent's edge to the east.
Last Stage of Episode 2: Nearly final closure of the proto-Atlantic Ocean.
The folding and warping of that sequence:
Episode 3: As the plate convergence continued, finally the proto-Atlantic Ocean was closing completely as the
continental mass of Africa collided with North America; this collision compressed, thrusted, and folded the
previously deposited sedimentary sequence back toward the continent interior.
The long-term weathering and erosion:
Episode 4: Weathering and erosion of the folded and thrusted sedimentary rocks of Pennsylvania began when
North America was part of the super-continent Pangaea, and continued as Pangaea rifted apart and the
continents moved away from each other. Over time, difficult to erode strata, such as cemented sandstones,
stood higher as ridges, while easier to erode strata, such as limestones and shales, became the sites of
valleys.
Plate Tectonics
Before we begin...
An Introduction to Plate Tectonics
Earth history is easier to understand once you have two important forms of large-scale thinking. The first of
these is to think in terms of geologic time, measured in millions and billions of years.
While time of this magnitude is difficult for humans to comprehend, without it one cannot grasp how our globe
has come to have its present characteristics. With the availability of immense amounts of time, the history of
the Earth can be seen to result from the operation of normal forces and processes seen to be operating today.
Even small, incremental changes, such as the few feet of movement that accompanied the recent earthquake
in Turkey, can have major consequences if there is sufficient time.
THE GEOLOGIC TIME SCALE
Eon
Era
Period
Epoch
Lower Age
Boundary
(millions of
years ago)
Quaternary
Recent
Pleistocene
1.6
Pliocene
Miocene
23.7
Oligocene
Eocene
Paleocene
66
Cenozoic
Tertiary
Mesozoic
Phanerozoic
Cretaceous
144
Jurassic
208
Triassic
245
Permian
286
Carboniferous
Paleozoic
Pennsylvanian
320
Mississippian
360
Devonian
408
Silurian
438
Ordovician
505
Cambrian
545
Late
Precambrian
Middle
Early
+4500
It also helps to be able to think about Earth forces operating on a global scale; this is summed up by the theory
called Plate Tectonics. An informative review of this theory can be found at the United States Geological
Survey site called This Dynamic Earth. The essence of this theory is that heat from radioactivity causes the
Earth's interior to flow slowly, resulting in the lateral (sideways) movement of thin rock slabs (called plates)
over the Earth's surface. While once considered revolutionary, plate tectonics theory is now so solidly
supported by evidence of many kinds that it is second nature for geologists to explain local histories in terms of
such things as the collision of two plates or the movement of a continent from one climate zone to another.
How fast do plates move? About 2 to 5 centimeters per year (1 to 2 inches per year), about the same speed
that your fingernails grow.
We know, then, that the outermost part of Earth consists of a series of large slabs
(tectonic plates; lithospheric plates) that move slowly over the globe, powered by
flow in the interior mantle.
At some locations tectonic plates (lithosphere) move away from each other
(diverge), and in the rift thus opened molten magma wells up from the mantle below
to make new plate material. At opposite locations around the Earth, those same
moving tectonic plates crash into each other (converge), raising up mountain
systems, such as the Himalayas or the Alps. Continents are those parts of the plates
that consist of lighter, older rocks that ride passively, like rafts, on the moving plates.
The forward, leading edge of a continent is the site of volcanoes, earthquakes, and
developing mountains; because these are the conditions presently around the
margin of the Pacific, such continental margins are said to be of Pacific-type.
The trailing edge of a continent lies within a plate, so it has no volcanoes,
earthquakes, or young mountains and typically looks like a low-relief, gentle coastal
plain where thick sequences of sedimentary rock accumulate over millions of years.
As these are the conditions on those parts of the present continents flanking the
Atlantic Ocean, they are said to be Atlantic-type continental margins. At the present
time, the basic pattern of plate movement is away from the center of the Atlantic
Ocean, and toward the Pacific Ocean.
You can see these two types of continental margin on the global relief map of Earth
(below). Around the margin of the Pacific Ocean, where plates are converging, the
continents are mountainous, and the region has many volcanoes and earthquakes.
Those parts of continents bordering the Atlantic Ocean, on the other hand, are much
flatter, with little happening other than the deposition of sediment. The Atlantic
Ocean basin is progressively widening, while the Pacific Ocean Basin is becoming
smaller as the tectonic plates move over it.
But every half billion years or so, the flow directions in the underlying mantle
change, and the pattern of tectonic plate motion reverses. The super-continent that
formed under the old regime of movement now begins to be split apart as smaller
continent segments move away from each other. New ocean basins form in the
widening rifts between the separating plates, and the former ocean basins that
surrounded the super-continent begin to close. That is the situation today (see
diagram below), as broken-apart continental fragments of the former super-continent
Pangaea are moving away from each other.
Notice that since the continents at the leading edges of moving plates are converging
together, they will gradually join together in the form of a large super-continent, a composite
continent.
When plate movement directions change, margins of continents can undergo drastic
transformations. A continental margin that had been within a plate and passive
(Atlantic-type) when the ocean basin was widening will be converted to an active
(Pacific-type) continental margin when motion directions reverse and the adjacent
plates now converge (and vise versa). The history of plate movement shows that
those parts of continents that border ocean basins alternate repeatedly between two
states: (1) a trailing edge margin of Atlantic-type that is tectonically quiet and
where large quantities of sediment accumulate to great thicknesses, and (2) a
leading edge margin of Pacific-type that is tectonically very active, with large
volcanoes, many earthquakes, and high mountains. Another way of looking at this
history is that it shows the cyclically repeated opening and closing of ocean basins,
with a full cycle lasting something like one half billion years. This cycle is commonly
called the Wilson Cycle, in honor of J. T. Wilson, who first got the idea.
The features we refer to as mountain systems (orogens) tend to have a consistent
pattern of development. First, kilometers-thick sequences of sedimentary deposits
accumulate over a couple of hundred million years onto passive Atlantic-type
continental margins. Then, when mantle flow patterns change and plate movement
directions reverse, the continental margin changes to Pacific-type, and those thick
sedimentary deposits are compressed, deformed and intruded by molten magma
from below.
Episode One
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 episode, 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.
Episode Two
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.
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
Two layers of volcanic ash (recessed
zones) in Ordovician limestones near
Reedsville, PA.
Limestones of Ordovician age (light gray)
interbedded with dark shales.
complex of rivers flowed westward.
The record of this alluvial plain is a
very thick sequences of riverdeposited 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.
Part of thick sequence of Upper Ordovician
sandstones (Bald Eagle Formation). Note
truck for scale.
Cross stratification in Ordovician
sandstones show sediment transport
toward the northwest.
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 protoAtlantic 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 finingupward 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.
Calcium carbonate nodules in floodplain soils
of the Catskill Formation indicate semi-arid
climate in Late Devonian time.
Plant root marks in floodplain deposits
of the Catskill Formation.
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.
End of Episode Two
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.
Episode Three
Episode Three - Final closure of the proto-Atlantic Ocean; Thrusting and Folding
(about 270 million years ago)
Paleogeographic map showing the supercontinent Pangea during the Permian. Note the
large mountain range that was built from the collision of North America (Laurasia) and Africa
(Gondwana). Diagram courtesy of Dr. Ron Blakey's Global Earth History page, used with
permission.
In early Permian time, not long after the marginal mountains had been elevated
again in the Late Carboniferous, a more extreme collision took place as the
continental mass of Africa collided with the continental mass of Laurentia.(see
diagram at right). Compressive stress accompanying this collision was so intense
that great portions of the Laurentian crust and overlying sedimentary sequence were
thrust westward toward the continent interior. [Figure] Above the thrust planes, the
sequence of sedimentary strata many kilometers thick, which had been deposited
over hundreds of millions of years (see Episode 1 and Episode 2 above), was
warped and folded as the strata were forced toward the west.
A NW - SE hypothetical cross section through central Pennsylvania, illustrating the folding
and thrusting of sedimentary layers during the Permian. Modified from R. T. Faill in The
Geology of Pennsylvania, 1999.
That compression did not happen overnight. As ten or more million years passed,
strata in the sedimentary sequence underwent a series of deformational stages: first
a squeezing up of the sedimentary materials, then the beginning of small internal
movements, and finally significant bending and warping of the strata as the thrust
planes glided northwestward. (see photos below). In central Pennsylvania, the
evidence of this compressive deformation can be seen in the major landscape
features of the region, as well as in the bent and folded strata shown in many
outcrops.
Folded and thrusted Tuscarora formation
near Milroy, Pennsylvania
Kink folds in Trimmers Rock Formation,
Watts Exit, Routes 22/322, central
Pennsylvania.
Episode Four
Episode Four - Back to an Atlantic-type continental margin;
erosion of the deformed stratigraphic sequence
(about 270 million years ago to present)
Paleogeographic reconstruction of the continents during Late Jurassic time. North
America has begun to rift apart from South America and Africa, creating the present-day
Atlantic Ocean. From Dr. Ron Blakey's Global Earth History page, used with permission.
The result of this major period of tectonism was an Alpine-size mountain system
with elevations up to 15,000 feet or so, and nearly 200 miles across, with its axis
somewhere near the eastern border of Pennsylvania. Since there is no mountain
system there at present, the next stage of geologic history is essentially nothing
more than the gradual lowering of the mountainous terrain through operation of
the forces of weathering and erosion. This erosion took place in the passive
tectonic conditions characteristic of an Atlantic-type continental margin. Pangaea
did not stay as an assembled supercontinent for very long. Beginning in Late
Triassic time, the tectonic plates once again began to diverge to rift Pangaea
apart (see Figure at right). As the Atlantic Ocean basin widened again, eastern
North America resumed its passive ride on a westward-moving plate. Another
Wilson Cycle was beginning.
While an enormous amount of erosion has taken place in the 270 million years
since mountain-making, the rate at which that erosion took place has not been
constant. For the first 50 million years or so erosion rates were accelerated in the
near-equatorial conditions, and much of the elevation of the initial mountain
system was lost. Erosion rates probably lessened as Pennsylvania drifted
northward through arid subtropical latitudes, but in the Cretaceous Period, rates
were again high as both global temperature and world-wide sea level were quite
high. Through all this, isostatic rebound raised the regional elevation continually
to make possible still more erosion.
Central Pennsylvania's characteristic linear valley and ridge landscape (above) is the
consequence of this long-continued erosion. A close correlation exists between the
elevation of particular rock units and their relative resistance to weathering and
erosion (denudation). Well-cemented quartz sandstone is very resistant to
denudation, so it is not surprising that the higher ridges consist of quartz-rich
sandstones. On the other hand, limestones and shales can be eroded much faster,
so once again it is not surprising that the valleys are underlain by limestones and
shales. You will remember that the sedimentary sequence in central Pennsylvania
consists of "packages" of strata that alternate between shallow-marine carbonate
rocks (limestones and shales) and quartz-rich sandstones that were deposited in
river systems.
Because those "packages" have been thrusted and folded into a particular
configuration (see Figure immediately below) by the collision with Africa at the end of
Episode 2, the elongate ridges are produced on top of several different sandstonedominated rock "packages", while more than one "package" of limestones and
shales underlie the intervening valleys (see photo below).
A NW - SE hypothetical cross section through central Pennsylvania, illustrating the folding and thrusting
of sedimentary layers during the Permian. Modified from R. T. Faill in The Geology of Pennsylvania,
1999.
Kishacoquillas Valley as Viewed from Jack's Mountain. This topography is typical of the
Valley and Ridge Province of central Pennsylvania.Valleys are typically underlain by easilyeroded underlain by shales and limestones, surrounded by ridges of highly-resistant
sandstone.
One challenging question concerning the
central Pennsylvania landscape has vexed
geologists for more than a century. How did it
come about that the major rivers, such as the
Susquehanna, cut transversely across the
ridges instead of flowing down the valleys?
This issue is still being debated, but some
elements of the answer appear to have a
consensus. For most of the Paleozoic Era (see
Episode 2), rivers in central Pennsylvania
flowed westward away from easterly mountains
toward the continent interior. In the early
Mesozoic Era, the more intense denudation
associated with equatorial climates significantly
lowered those mountains. Then in the Late
Triassic Period, the stretching and rifting apart
of the crust associated with opening of the
Atlantic made it possible for rivers in some of
central Pennsylvania to carry sediment toward
the southeast. Because their shorter routes to
the sea were more efficient, gradually, over
time the southeastward-directed rivers
captured more and more of central
Pennsylvania's drainage.
EROS-Landsat image of the
Susquehanna River as it crosses
the Valley and Ridge Province of
central Pennsylvania. Confluence
of the North and West Branch is
visible at the top of the photo.