Mountain Building Part II: the Taconic Mountains

Figure 1.5: The Grenville Mountains gradually eroded over millions of years, depositing sediments on either side of the range, becoming layered with carbonate rocks that were forming in the proto-Atlantic Ocean along the margin of the continent. Figure by J. Houghton.

The continental margin of North America was broad and flat as the Grenville Mountains were wearing down. Sea level at this time, the Cambrian period, was very high worldwide, and warm, shallow seas covered most of the Northeast. The Iapetus Ocean continued to widen during this time. Sediments from the eroding Grenville Mountains were still being deposited on the shoreline to the east, though in far lesser amounts (Figure 1.5). In the Northeast, near the end of the Cambrian, sand deposits were gradually replaced by carbonates in the Iapetus Ocean. Carbonates were widely deposited on a broad flat shelf along the margin of North America.  

Figure 1.6: Ordovician: 458 million years ago. Shaded areas represent land that was above water.

Sometime during the middle of the Ordovician period, about 470 million years ago, the Iapetus Ocean began to close as two plates came together. The plate carrying Baltica (proto-Europe) approached the North American plate from the southeast (Figure 1.6). Though Baltica did not collide with North America until several million years later, the convergence of the two plates created a whole new look for the eastern margin of North America. As the continents approached one another, the oceanic crust in the middle was forced under the Baltica plate. The friction and melting of the crust form the intense pressure of the colliding plates created a string of volcanic islands along the area where the plates converged (know as the subduction zone). 

Figure 1.7: Volcanic islands formed where the plates were forced together as the Iapetus Ocean closed. The compression crumpled the crust to form the Taconic Mountains and a shallow inland sea. Figure by J. Houghton.

The carbonates that had been deposited in the Iapetus Ocean were squeezed and pushed ahead of the volcanic islands up onto the margin of the continent along with deeper water silts, sands and clays (Figure 1.7). These volcanic remnants may be found in a thin band of rocks in northernmost Vermont that extend through northern New Hampshire, and up the western and northern-most section of Maine. 

As the Iapetus Ocean closed, folding, thrust-faulting, uplift, and intrusion occurred along the margin of the continent from the intense pressure of the colliding plates, causing another mountain chain to from in the place of the worn-away Grenville Mountains. This mountain-building event is called the Taconic Orogeny. 

Figure 1.8: The Queenston Delta formed as sediments eroded from the Taconic Highlands and were transported downward by streams, forming the characteristic wedge-shaped delta deposits. Figure by J. Houghton.

The compression induced by the collision of the two plates, caused a downwarp in the crust to the west of the Taconic Mountains. This sagging crust became a basin filled with a broad, shallow inland ocean and sediments from the eroding Taconic Mountains. As sediment was eroded from the western side of the Taconic Mountains, the Queenston Delta deposits formed a wedge of sediments spreading away from the Taconics through New York and Pennsylvania (Figure 1.8). Some of the delta sediments settled in the shallow inland sea, gradually filling the basin. Sediments were also being eroded and deposited east of the Taconics into the trench formed where the plates converged. Eventually, the Taconic Mountains eroded away to only the inner core, as had the Grenville Mountains previously.

The Taconic Mountains that we see today in eastern New York are not the ancestral Taconic Mountains. Further compression of the crust during the Taconic and Acadian mountain-building events thrust huge slabs of the ancestral Taconic Mountains westward from Vermont and Massachusetts. Thus older, more resistant rocks from the Taconics ended up on top of younger sedimentary rocks from the inland ocean. This is unusual in geology; usually the oldest sediments are on the bottom. The resistant blocks form the ancestral Taconic Mountains weathered much more slowly than surrounding rocks, eventually forming the Taconic Mountains of today.  

Figure 1.9: Taconic Mountain Building:

• Baltica approaches North America after breaking away earlier.
• Volcanic islands form over subduction trench.
• Volcanic islands collide with North America, form Taconics.
• Inland sea forms to the west of Taconics.
• Taconic Mountains erode.
• Queenston Delta deposited west of Taconics.