Thursday, July 26, 2018

Triassic Conglomerates - Deep River Triassic Basin - Morrisville, NC

Rock Type: Conglomerate

Geologic terrane or major geologic element:  Deep River Triassic basin - Durham sub-basin

Age: Triassic - approximately 220 million years old

Location: Google Maps Link

Site Access: This is an active railroad line.  Great care must be taken while visiting.

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Technical Information:  Carolina Geological Society Field Trip guide 1994 - Stop 2

http://carolinageologicalsociety.org/CGS/1990s_files/gb%201994.pdf

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Simplified Information:

The rocks exposed in the railroad cut help tell part of the geologic story of North Carolina. 

When the Supercontinent Pangea (Figure 1) began to split apart approximately 245 million years ago, a system of rift basins (similar to the modern day East African Rift system) were formed all along the east coast of North America (Figure 2).  Called the Newark Rift System, the splitting apart of Pangea formed the Atlantic Ocean and several inland fault bounded rift valleys. 




Figure 1: Sketch of the supercontinent Pangea.



Figure 2: Sketch of rift basins along the Atlantic Ocean.

Land to either side of the rift basin began to erode rapidly filling the fault bounded lowlands with boulders, cobbles, sand, silt and clay.  The deposits later turned into the red to maroon colored conglomerates, sandstones, siltstones and mudstones common in the basin (Figures 3 and 4). Excellent examples of these deposits are exposed in the railroad cut.



Figure 3: Photograph of outcrop at railroad cut showing Triassic-aged sedimentary rocks.



Figure 4: Photograph of details of outcrop at railroad cut showing alternating beds of Triassic-aged conglomerates.

The Morrisville area during the late Triassic was full of life.  North Carolina was located near the equator and had a semi-arid tropical climate. Situated between rugged mountains, the ancient area was dominated by lakes, swamps and meandering rivers and streams that would periodically dry-up.  Crocodile-like animals called phytosaurs, early dinosaur ancestors, and primitive mammals roamed the land of the Triassic basin; fish, clams and various crustaceans lived in the lakes and rivers; insects crawled on the ground and flew through the air; and abundant conifer trees and cycads grew.  Evidence of this abundant life is seen by the common occurrence of petrified wood in Triassic sediments, the occasional discovery of fossilized bones of reptiles and the footprints of early dinosaurs (Figure 5) in Triassic basin sediments throughout North Carolina.



Figure 5: Artist rendition of flora and fauna of the Triassic basins – From News and Observer.