12.2: Evidence for the Taconian Orogeny

Introduction

During the late Ordovician, subduction pulled a volcanic island arc into Laurentia (ancestral North America), causing mountain-building in the region that is today New England. The Taconian, or Taconic Orogeny, produced an extensive mountain belt, with peaks rivaling those found in the Swiss Alps and vast volumes of sediments from their erosion. Geoscientists today use both the resulting sedimentary rocks and the metamorphic and igneous rocks that formed at the roots of the mountain belt as evidence for this important event.

Figure \(\PageIndex{1}\): The sequence of events that led to the Taconic (Taconian) Orogeny. Image from Lyn Topinka, United States Geological Survey. Modified from Plank and Schenck, 1998, Delaware Piedmont Geology, Delaware Geological Society., Public domain, via Wikimedia Commons.L

Before the orogeny, the edge of Laurentia was a passive margin: through the Cambrian and well into the Ordovician, there was no tectonic activity anywhere nearby. Submerged under a shallow 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, and abundant fossils of filter feeders like bryozoans and brachiopods, and low quantities of clastic sediments, indicate clear water and no highlands.

Figure \(\PageIndex{2}\): Ooids from the pre-Taconian Cambrian-aged Conococheague Formation, Shenandoah County, Virginia.

Evidence from Igneous and Metamorphic Rocks

The cause of the Taconian Orogeny was a collision between two tectonic plates: the oceanic segment of the ancestral North American plate and the plate including the floor of the Iapetus Ocean. Subduction resulted in a volcanic island arc forming in the middle of the Iapetus Ocean.

Figure \(\PageIndex{3}\): The tectonic situation that would lead to the Taconian Orogeny: subduction of the oceanic margin of the ancestral North American plate resulted in a volcanic island arc that drew ever closer, building up an accretionary wedge of Iapetan ocean floor and deepwater sediments.

Isotopic ages of rocks from this time tell us two things: that subduction was happening and, later, the volcanic arc accreted onto Laurentia. The Port Deposit Tonalite, a metamorphosed granitoid, is a nice example. Prior to the Taconian Orogeny, it was an unmetamorphosed body of intrusive igneous rock, the product of subduction-generated magma. Zircons from the tonalite reveal a magmatic crystallization age of 515 Ma. At the same time, biotites indicate the igneous rock was metamorphosed 490-480 Ma when the Taconian volcanic island arc collided with the ancient continent.

Figure \(\PageIndex{4}\): The Port Deposit Tonalite of Cecil County, Maryland.

Additional evidence for the accretion of the Taconic Island Arc is contained in the remains of the ancient mountains of this region, the Piedmont geologic province of Virginia, Maryland, Washington, D.C., Pennsylvania, New Jersey, and New York, and parts of New England. The "story-tellers" are the crushed and cooked remains of the Iapetus Ocean and the Taconian volcanic island arc. These rocks have been metamorphosed to various degrees, from greenschist facies 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 (intrusive and extrusive, mafic and felsic). In some cases, the subsequent metamorphic recrystallization was slight enough to preserve primary volcanic and sedimentary structures, such as the graded beds in the meta-graywacke of the Mather Gorge Formation, which reflect oceanic processes. Specifically, these graded beds formed from deep submarine deposition of clastic sediment by turbidity currents in the Iapetus Ocean.

Figure \(\PageIndex{5}\): Relict graded bed in Mather Gorge Formation metagraywacke, near Potomac, Maryland.

These rocks also help tell a broader story of the timing of the Taconian Orogeny. Radiometric dating shows metamorphism happening around 460 Ma, as reflected by the crystallization ages for migmatites. The Piedmont is also comprised of felsic igneous rocks that crystallized out of the subduction-made magma. These include formations like the Occoquan Granite, the Georgetown Intrusive Suite, and the Kensington Tonalite, all of which have isotopic ages in the 474 to 450 Ma range, which are the product of later-stage subduction. The gigapixel image below is of a sample of migmatite from Orange County, Virginia. It contains “leucosomes” that represent the formerly molten portion of this rock, which is otherwise a schist. Similar partial melting occurs today beneath active modern mountain belts such as the Himalayas.

Figure \(\PageIndex{6}\): Migmatite from Orange County, Virginia representing partial melting that occurs beneath active mountain belts.

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 smashed into Laurentia, compressing and metamorphosing the Iapetan ocean sediments caught in between. Today, traces of ancient Taconian Orogeny faults are found throughout New England and New York City.

Figure \(\PageIndex{7}\): Folded metamorphosed turbidites: alternating schist & metagraywacke layers (former shale & graywacke) were folded by Taconian mountain-building. Outcrop in Chesapeake & Ohio Canal National Historical Park, near Potomac, Maryland.

Evidence from Sedimentary Rocks

As mountains grow, they produce large amounts of sediment through erosion. This debris accumulates in the adjacent basins, sometimes terrestrial and sometimes marine. Before plate tectonics was understood to be the driving force of mountain building, geologists used accumulations of clastic sediment as evidence of orogenies because gravel, sand, and mud must come from somewhere. Sequences of clastic sediment can be organized into two groups: deep marine turbidity current deposits called “flysch,” and a terrestrial “red-bed” sequence, termed “molasse.” These European terms are useful for summarizing the sedimentary signature of the Taconian Orogeny.

Taconian Flysch

As the Ordovician approaches, the stratigraphic record shows limestones get “dirtier” as clay and silt deposits mix with the carbonate deposits. The first indication of tectonic uplift. Over time, going up and younger in the stratigraphic sequence, the passive margin carbonates give way to shales with limestone, 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 as it grew taller. This flysch is the marine sedimentary record of Taconian mountain-building and its erosion.

Figure \(\PageIndex{8}\): The deepening of the sedimentary basin adjacent to the young Taconian mountain belt was accomplished as the edge of ancestral North American continent flexed downward. Turbidity currents flowed into this deepened basin, depositing shale and graywacke: the Taconian “flysch.”

A record of these turbidity currents is preserved as graded beds in graywacke, separated by shale layers. These distinctive deep-sea sedimentary sequences resulted from repeated submarine avalanches that delivered vast quantities of sand and mud into the deep ocean.

Figure \(\PageIndex{9}\): Evidence of turbidity currents in the Devonian Trimmers Rock in Newport, Pennsylvania.

The transition from pre-Taconian shallow-water carbonates to Taconian Orogeny deepwater turbidites suggests that the water got deeper. There are likely two causes of this: (1) compression along the convergent boundary, which caused the leading edge of the ancestral North America plate to fold downward, and (2) the increasing weight of the accretionary wedge of sediment caused the oceanic crust of the passive margin to sag downward under this extra weight. Fossils further confirm this scenario, as deeper water faunas like brachiopods, crinoids, and a nautiloid are preserved in some of these deposits. Layers of ash are preserved, too, presumably sourced from the approaching volcanic island arc. These ash layers weather today to a yellowish, crumbly clay material called bentonite, including zircons that can be dated, which helps constrain the age of the sedimentary strata above and below the bentonites.

Figure \(\PageIndex{10}\): Gigapixel image of a rock containing brachiopods and crinoids.

Taconian Molasse

Eventually, the basin collecting the flysch deposits filled. Rivers flowing out of the Taconian mountains deposited their load of sediment in alluvial fans, stream channels, flood plains, and deltas. Using the volume of flysch and molasse deposits, one can estimate the total volume of the mountains to be 600,000 cubic km of rock. Considering the width of the metamorphic belt that was previously discussed and the metamorphic grade of the rock within the belt (which is a reflection of the overlying pressure acting on those rocks), we can use the volume estimate to estimate the height of the Taconian peaks to be around 4000 meters or 13,000 feet.

Figure \(\PageIndex{11}\): The Queenston Clastic Wedge was deposited west of the Taconian mountain belt. The molasse is thickest and coarsest close to the mountain belt to the east, and thins and fines to the west.

After the Taconian Mountains had been worn down, conditions returned to passive margin sedimentation in the Silurian, and new carbonates were deposited into the Devonian. This reprieve from active margin conditions would resume with the Acadian Orogeny in the middle to late Devonian.

Key Terms

• migmatites - a rock having both metamorphic parts and igneous parts resulting from partial melting
• protolith - the original, unmetamorphosed rock
• Taconic Orogeny - an extensive mountain building event 440-470 million years ago caused when subduction pulled a volcanic island arc into Laurentia