Paleogeographic Setting

 

View of Lower and Upper High Falls

Picture by Tom Whiteley

 
 
   
         
 
 
 
 
 

Paleogeographic Setting:

 
 

 

Trenton Falls is located roughly between 43° 15' N and 43° 17' 30" N today, but was not at this mid-latitude position when the Trenton limestones were deposited. In fact, Trenton Falls and most of North America were not even located in the northern hemisphere. Due to relatively slow rates of continental drift over the course of nearly 460 million years, North America rotated in its orientation and has migrated from its sub-equatorial position in Ordovician time to its current temperate latitudinal position.

The following section will introduce concepts documenting the paleogeographic setting of New York State and the Trenton Falls region during the Upper Ordovician. Because of the added dimension of time, the paleogeographic discussions will also consider geographic change during the several million years of deposition encompassed in the Trenton limestones.
   
   

GLOBAL ORDOVICIAN PALEOGEOGRAPHIC RECONSTRUCTION

 

   
     

 

 

 

Paleogeographic Reconstruction for the Upper Ordovician Period (approximately 450 million years ago)

East and West Equatorial Views

 

Image Source: Dr. Ron Blakey Northern Arizona University

modified and reproduced with permission.

http://jan.ucc.nau.edu/~rcb7/paleogeographic.html

 

   

 

During the Ordovician the surface of the Earth, like the present-day, was composed of a number of continents of varying sizes that were separated by large ocean basins. The largest continents during that time were: Laurentia, Siberia, Baltica, and Gondwana, as well as a number of smaller island arcs and other terrains. The largest ocean basins of the time were the Iapetus Ocean (which separated Laurentia from Siberia, Baltica and Avalon), the Paleotethys (which separated Avalon, Baltica, and Kazhakstan from the Gondwana craton), and the Panthalassic Ocean (which dominated the northern hemisphere and separated the Gondwana craton from Laurentia). (Scotese, 1991) >>Back to Top

Note: The Paleogeographic Maps, as drawn above by Blakey (2003), are based on paleomagnetic studies of Ordovician rocks from around the world. The data, summarized by Scotese (1991), help to document the physical arrangement of continents with respect to latitudinal position and orientation. Paleomagnetic studies do not provide longitudinal constraints for the exact reconstruction of plates.

   
   

PALEOGEOGRAPHIC RECONSTRUCTION OF LAURENTIA (ANCESTRAL NORTH AMERICA)

   
    During the Ordovician, the craton of ancestral North America or Laurentia (center of image above), straddled the equator with the majority of the present day United States located in the southern hemisphere. The Laurentian craton was more or less flooded during a period of global sea-level highstand, and much of  

 

 

 

 

 

 

Image Source: Dr. Ron Blakey

Northern Arizona University

http://jan.ucc.nau.edu/~rcb7/paleogeographic.html

 

These images were modified and reproduced with permission.

 

   

 

 

 

 

the continent was covered with shallow epicontinental seas. Southeastern Laurentia (present day eastern United States) was bordered to the east by the Iapetus Ocean, which during the Ordovician began to narrow. Portions of the southeastern Laurentian craton and western Iapetan oceanic crust developed a subduction zone and volcanic island arc.

By the end of the Ordovician (as shown above), an accretionary prism (portions of scraped up seafloor sediments) and the volcanic arc were thrust onto the Laurentian margin and had depressed the margin of the craton into a foreland basin. This event is known as the Taconic Orogeny, and was named for the Taconic Mountains of present day western Connecticut and Massachusetts. These mountains are composed of the rocks from the accretionary prism and volcanic island arc complex just mentioned. Further discussion on the change in the tectonic setting from shallow epicontinental seas into deep foreland basin conditions is presented in the section on Tectonic Setting. >>Back to Top

 

   
   

PALEOGEOGRAPHIC RECONSTRUCTION OF SOUTHEASTERN LAURENTIA (EASTERN UNITED STATES)

   
   
 

 

 

 

 

 

Paleogeographic Reconstruction of the Eastern United States During the (Upper Mohawkian) Early Late Ordovician

Image modified after Cornell (2003)

   

 

Just prior to the end of the Mohawkian Stage (early Late Ordovician), the shallow shelf deposits of northeastern United States were being modified rapidly by the onset of Taconic tectonism. By approximately 460 million years ago, active thrust loading had begun to emplace large sheets of oceanic floor and cratonic margin slope and rise deposits onto the edge of the Laurentian craton. Due to the increased thickening at the cratonic margin in the vicinity of present-day central Vermont, Massachusetts and Connecticut, the southeastern most portion of ancestral North America developed a rapidly subsiding peripheral foreland basin in advance (the cratonic side) of the developing accretionary wedge. This subsidence and increased compressional forces caused older Precambrian aged Iapetan rift graben faults to be reactivated to produce a complicated antecedent topography in the region.

The diagram above demonstrates some of the key paleotopographic features associated with the Taconic Foreland Basin that were active during the deposition of the Trenton limestone. After the deposition of the Black River Group limestones (located stratigraphically just below the Trenton limestone), the pattern of deposition within Trenton aged limestones suggests that the once relatively flat Black River shelf became segmented through the subsidence of the Sebree trough (a linear intracratonic basin) and the coupled uplift of the Lexington Platform. To the north, in the vicinity of New York State and Trenton Falls, a similar pattern developed at about the same time with the activation of the Trenton Shelf and the Taconic foredeep basin. >>Back to Top

 

   
   

PALEOGEOGRAPHIC RECONSTRUCTION OF THE TRENTON FALLS REGION

   
 

 

 

Trenton Platform Reconstruction for Upper Mohawkian Series in central New York State (Early to Middle Trenton Time)

 

   

 

During the onset of deposition of the Trenton Group limestones, the cratonic edge of Laurentia had begun to be modified by tectonic processes off the southeastern seaboard associated with the Taconic Orogeny. As a result of these tectonic processes, the once stable carbonate shelf of eastern Laurentia was modified into an active foreland basin setting. The relatively flat pre-Trenton platform was modified during Trenton time into a series of smaller uplifted platforms and arches as well as smaller sub-basins, all of which have deep-seated basement affinities. For more discussion on the tectonics of this change, please refer to the sectionTectonic Setting.

In the diagram above, the "Trenton Shelf" is drawn to depict its early to mid Trenton configuration. Key physiographic features of the Trenton shelf region include: 1) the northwestern shelf margin basin (located approximately in the position of the modern Lake Ontario and St. Lawrence Rivers) referred to as the Kingston Embayment; 2) the Trenton Shelf proper (located in the northcentral portion of present-day New York State); 3) the Canajoharie arch (located in the central Mohawk River Valley near the city of Canajoharie, New York); and 4) the Champlain Trough, the main Taconic peripheral foreland basin (located in the eastern New York to western Connecticut, Massachusetts and Vermont region). The position of Trenton Falls is marked by the star. >>Back to Top

 

   
 
 
 

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