In the late Ordovician period of geologic time, subduction pulled a volcanic island arc and ancestral North America (Laurentia) together. The result was an extensive mountain belt that produced a large amount of eroded sediments. Both the resulting sedimentary rocks and the metamorphic and igneous rocks that formed at the roots of the mountain belt are used by geoscientists today to examine this episode of mountain-building, known as the Taconian (“Taconic”*) Orogeny.
Earth as it appeared in the late Ordovician period of geologic time. Find ancestral North America (“Laurentia”) and note the mountain belt along its southern edge. This is the Taconian Mountain Belt. This dynamic visualization was created by Ian Webster using tectonic and paleogeographic maps by C.R. Scotese’s PALEOMAP Project, and is embedded here with permission. Grab it and spin it around! Zoom in and out! Explore!
The cause of the Taconian Orogeny was a collision between two tectonic plates: the ancestral North American plate’s continental leading edge, and another plate of oceanic affinity, now deceased. The oceanic plate was one of the plates that floored the Iapetus Ocean, and as it moved toward the ancestral North American plate, the oceanic lithosphere that was part of the North American plate subducted, down and under the overriding plate of oceanic lithosphere. This resulted in a volcanic island arc, out in the middle of the Iapetus Ocean.
Part of the context of the orogeny is therefore on the ancestral North American continent, and part is out in the volcanic island arc. Rocks that formed in that island arc journeyed toward ancestral North America and accreted to the continent during the orogeny. Isotopic ages reflect this two part history: an initial crystallization from magma in the arc, and a later metamorphic age from the orogeny. The Port Deposit Tonalite, a metamorphosed granitoid, provides a nice example. Prior to the Taconian Orogeny, it was not yet metamorphosed: just a granitoid, under a volcano, moving along at a few cm per year, getting closer and closer to the Laurentian continental slope.
Prior to the orogeny, the edge of ancestral North America was a passive margin setting: it was the edge of the continent, but not the edge of the plate. Through the Cambrian and well into the Ordovician, there was no tectonic activity anywhere nearby, and there had not been for a very long time. Submerged under an epeiric sea, it was the site of limestone and dolostone deposition in a Bahama-like carbonate bank setting. Primary sedimentary structures such as ooids and stromatolites testify to shallow water depths. The proportion of clastic detritus such as clay and silt was quite low. There are abundant fossiliferous limestones from this time full of brachiopods, bryzoans and other common Paleozoic filter feeders, indicating clean water: a lack of excess runoff and sedimentation.
But not for long…
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The roots of the mountains
Since the Taconian mountain range itself is long gone, we can consider the orogeny from two different perspectives: (1) that of the eroded roots of the mountains, and (2) that of the sedimentary basins “next door,” which received sloughed-off sediment eroded from the mountains.
Let’s first examine the roots of the mountains, which can be found in the Piedmont geologic province of Virginia, Maryland, Washington, D.C., Pennsylvania, New Jersey, and New York, as well as assorted provinces in New England TKTKTKTK. The rocks in question are the crushed, cooked remains of the Iapetus Ocean and the Taconian volcanic island arc.
The rocks of the Piedmont have been metamorphosed to various degrees, from greenschist facies all the way up to partial melting. Their protoliths range from basalt and gabbro (oceanic crust) to mudstone, graywacke, and limestone (oceanic sediments), as well as the volcanic rocks of the volcanic island arc (both intrusive and extrusive, both mafic and felsic). In some cases, the subsequent metamorphic recrystallization had a light enough touch that primary structures are still preserved, both volcanic and sedimentary. Graded beds in meta-graywacke of the Mather Gorge Formation are a nice example of a primary sedimentary structure that speaks specifically of oceanic processes. These graded beds formed from deep submarine deposition of clastic sediment by turbidity currents in the Iapetus Ocean.
We can estimate the timing of the Taconian Orogeny by looking at metamorphic ages for these rocks (K/Ar, Ar/Ar, and Rb/Sr methods), as well as crystallization ages for the migmatites produced by partial melting (U/Pb). In both cases, the answer returned is ~460 Ma, a late Ordovician age. The Piedmont is also home to many plutons of felsic igneous rock like the Occoquan Granite, the Georgetown Intrusive Suite, and the Kensington Tonalite, and these all also yield isotopic ages in the 474 to 450 Ma range.
Explore this gigapixel panorama of a sample of migmatite from Orange County, Virginia, and search for pockets of speckled granite. These “leucosomes” represent the formerly-molten portion of this rock, which is otherwise a schist. Similar partial melting is occurring today beneath active modern mountain belts such as the Himalayas.
Deformation was another major signature of mountain-building in the Piedmont region. Primary structures were distorted by folds and disrupted by faults as the Taconian volcanic island arc docked with ancestral North America, compressing the Iapetan sediments caught in between.
In New England, Iapetus seafloor rocks including both oceanic lithosphere and overlying deepwater sedimentary deposits moved upward onto continental rocks, and westward for close to 5o km of distance. A major thrust fault allowed this relative motion. Today the trace of this fault is called “Cameron’s line,” after the geologist who first described it. Isolated klippen of the overthrust rocks remain in the eponymous region of the Taconic Mountains, but the trace of the fault also runs through New England and even through downtown New York City.
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The sedimentary signature
When mountains are raised, they erode. The erosion of the mountains produces clastic sediment, and a lot of it. While this does not accumulate on-site (i.e., atop the mountain belt), neighboring sedimentary basins may be sufficiently low-lying that they can receive and preserve this sediment through geologic time. Long before geologists understood the thermal or tectonic origins of metamorphic rocks and granites, orogenies were known from their clastic sedimentary signature. Gravel, sand, and mud don’t just magically spring into existence, after all – they require a source. A large amount of clastic sediment showing up in a stratigraphic sequence implies that there must have been a lot of nearby mountainous rock to be eroded.
The resulting clastic sediment (my students like to call it “mountain dandruff”) comes in two essential varieties: a deep marine turbidite package that Alpine geologists call “flysch,” and a terrestrial red-bed package dubbed “molasse.” Though these European terms are a bit out of vogue in modern America, they very nicely summarize the sedimentary signature of the Taconian Orogeny. We find both Taconian flysch and Taconian molasse in the stratigraphic sequence of sedimentary rocks in the Valley & Ridge province.
The pre-orogeny limestones get dirtier and dirtier as the end of the Ordovician approaches. Their increased clay and silt content is seen as the first indication of the clastic onslaught to come, like a whiff of smoke before a forest fire. Over time, going up through the stratigraphic sequence, these passive margin carbonates give way to limy shales and then clastic shales with no calcite, and finally to graywacke turbidites interbedded with shale. The interpretation for this “dirtying upward” pattern is the increasing proximity and prominence of the Taconian mountain range, shedding more and more sediment the larger it grew. This flysch in the marine sedimentary record of Taconian mountain-building (and erosion).
The record of these turbidity currents is a series of graded beds in graywacke, separated by layers of shale. These Bouma sequences are distinctive deep sea sedimentary sequences that speak about submarine avalanche after submarine avalanche, delivering huge quantities of sand and mud into the oceanic deep:
Here is a hand sample of rock showing a Bouma sequence:
The transition from pre-Taconian shallow-water carbonates to during-the-Taconian deepwater turbidites suggests that the water got deeper. There may have been a role for crustal flexure here: where the tectonic loading of the Taconian arc and its accretionary wedge onto the edge of ancestral North America caused the crust to sag downward under this extra weight, deepening the sedimentary basin next door.
In the Mid-Atlantic region of the Valley & Ridge province, the major geologic unit showing full-on flysch is the Martinsburg Formation. Fossils in the Martinsburg Formation allow us to constrain the timing of the mountain building from both biostratigraphic and paleoecological points of view. As the Ordovician limestone platform sediments get dirtier and more clay rich, shallow water filter feeders are replaced with species that are better suited for muddier and deeper conditions. Here are two examples showing deeper water faunas, one showing graptolites and one showing brachiopods, crinoids, and a nautiloid; both shown as gigapixel panoramas:
Layers of ash are preserved too, presumably sourced to the approaching volcanic island arc. These ash layers weather today to a yellowish, crumbly clay material called bentonite, but they include zircons that can be dated, and that helps constrain the age of the sedimentary strata above and below the bentonites. Two widespread bentonite beds, named the Deicke bentonite (457 Ma) and the Millbrig bentonite (454 Ma), are found in a vast swath of Appalachia and the Midwest. They can be correlated all the way from southern Minnesota and Texas to Alabama and Georgia to upstate New York.
Once the flysch basin filled up, rivers draining the Taconian mountain belt stretched out across the flysch, reaching westward toward the Tippecanoe epeiric sea. As they flowed, they transported sediment. The sediment built up in river channels and flood plain deposits. In the Mid-Atlantic region, these occur mainly in the Juniata Formation.
Here’s a Google Maps Street View of one such exposure:
Note the sandstone-filled channel edge poking up from the grass at the right side of the screen, like half a smiley face. There are half a dozen red sandstone/shale layers to its left. A bit further left still, you can see just red shale (no sandstone). This is a small snapshot of the relationship between a river and its floodplain. The river is the channel sandstone with the smiley-face shape, and the red shale represents its floodplain. The transitional zone with the many small sandstone/shale couplets are interpreted as crevasse splay deposits, places where the river overflowed its banks in a flood, and spilled over its own natural levee.
The Juniata Formation is part of a more massive arcuate deposit of terrestrial deposits, called the Queenston Clastic Wedge. Some geologists dub it the “Queenston Delta,” though that’s probably not literally accurate. It was probably more like an alluvial plain fed by many rivers draining the Taconian mountain belt. In map view, it has a big fan-like shape, but in cross section, the name “wedge” makes more sense: it’s thickest (and coarsest) in the east, and then thins systematically to the west, pinching out to a feather edge in Michigan.
The Queenston Clastic Wedge is reckoned to be about half the sediment that was shed off the Taconian Mountains (with the other half having gone off east of the mountain belt, into the Iapetus). If this is correct, an estimate of the volume of the mountains can be made: 600,000 cubic km of rock. Since we know the width of the metamorphic belt (the “roots” of the mountains, as outlined in the previous section), this allows up to convert our volume estimate into an interpretation of height. As with the estimates from metamorphic pressures, this calculation suggests Taconian peaks on the order of 4000 m high.
After the Taconian Mountains had been worn down, in the Silurian, conditions returned to passive margin sedimentation, and a new layer of carbonate was laid down in the Silurian and into the Devonian. This was a temporary reprieve from active margin conditions, which would resume with the Acadian Orogeny in the middle to late Devonian.
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For a detailed look at the sediments shed off the Taconian mountain belt, see the Massanutten Synclinorium VFE.
Multiple lines of evidence point to the accretion of a volcanic island arc with (what is today) eastern ancestral North America during the late Ordovician period of geologic time (about 460 Ma in the Mid-Atlantic region). This tectonic collision, called the Taconian Orogeny, resulted in an extensive metamorphic belt marking the roots of a mountain chain thousands of km long. As they were worn down by erosion, these ancient mountains shed copious quantities of sediment, which accumulated in neighboring low-lying basins (both marine and terrestrial).
* What’s in a name?
The Taconian Orogeny is also called “the Taconic Orogeny” by enough geologists that it’s probably worth exploring the different names here. The authors of this text believe that “Taconian” is the better term, and that “Taconic” is misleading. Let us briefly explain why…
The Taconic Mountains are a small, modern mountain range in upstate New York are located east of Albany, on the border with Massachusetts, close to the southwestern corner of Vermont:
Foundational work on understanding Ordovician mountain-building was first completed in these (modern) mountains, and thus the local landmarks provided the name for the orogenic episode. However — and this is the key point — the entire mountain chain from the Ordovician was not limited to the area of the modern Taconic Mountains. Instead, the ancient mountains extended from Newfoundland in eastern Canada all the way down to Alabama.
Not only were the ancient Taconian Mountains much longer as a range than the modern Taconic Mountains, they were taller, too. The highest peak of the Taconic Range today is only about 600 meters tall. In contrast, estimates from metamorphic minerals formed during the Taconian Orogeny suggest that the peaks of the Ordovician-aged Taconian Range must have been much taller. Peak metamorphic pressures of 1.5 GPa imply something on the order of 20 km of overlying crustal material. The Taconian Mountains, in other words, would have been an Alpine-scale range. In the modern Alps, the highest peak is more than 4000 meters tall.
Those ancient mountains are gone now, eroded away over geologic time. We can observe their eroded roots, and we can observe the sediment that resulted from that erosion, but the mountains themselves as topographic features are long gone. That ancient range deserves its own name, and that name should be distinct from the name that’s applied to the modern range. If the modern range is the Taconic Mountains, then the Ordovician mountains need a different name: Taconian.
Let’s summarize with a quick comparison and contrast: The Taconian Mountains were an ancient landscape feature, thousands of kilometers long, with peaks that likely once exceeded 4000 m elevation, and are now completely eroded away. The Taconic mountains are a modern landscape feature, about 20 kilometers long, with maximum elevation of merely 400 m, and the mountains are not yet completely eroded away.