25.4: 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.

Taconian Flysch

Figure \(\PageIndex{1}\): Over the course of the late Ordovician, light-colored shallow limestones gave way to increasingly dark deep-water limestones and shales.

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).

Figure \(\PageIndex{2}\): 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.”

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:

Figure \(\PageIndex{3}\): Late Ordovician bentonite layer between limestone layers in the Valley & Ridge province of northern Virginia.

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.

Taconian Molasse

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.

Figure \(\PageIndex{4}\): 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.

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.

Did I Get It? - Quiz

Exercise \(\PageIndex{1}\)

Which package of sediments is a general term for deposits of deep marine clastic sediment such as black shale and graywacke from turbidity currents?

a. evaporite

b. flysch

c. molasse

d. migmatite

e. carbonate

Answer

b. flysch

Exercise \(\PageIndex{2}\)

With their fluvial sandstones and redbed shales, the strata of the Queenston clastic wedge are _________.

a. Acadian molasse

b. Acadian flysch

c. Taconian molasse

d. Taconian flysch

Answer

c. Taconian molasse

Exercise \(\PageIndex{3}\)

Crustal flexure of the ancestral North American margin resulted in which of these?

a. Uplift of the sedimentary basin immediately adjacent to the Taconian mountain belt.

b. Deepening of the sedimentary basin immediately adjacent to the Taconian mountain belt.

Answer

b. Deepening of the sedimentary basin immediately adjacent to the Taconian mountain belt.

For a detailed look at the sediments shed off the Taconian mountain belt, see the Massanutten Synclinorium VFE.