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Setting the Scene.

The Triassic-Jurassic boundary is now thought to be around 200 million years ago according to the latest dating techniques.  It is difficult to obtain an accurate date, as there are few igneous rocks from this period of time, which would have been used to date the boundary by geochronology.

The boundary has been identified in many localities biostratigraphically using ammonoids. 

On land

Covering a quarter of the Earth’s surface and stretching about 60° in latitude from pole to pole, the huge supercontinent, Pangea comprised most of the world’s terrestrial land. 

The Picture shows a rough layout of the worlds continents at the end of the Traissic Period.

 

For a more detailed map CLICK HERE.

Paleomagnetic evidence has located Britain (which was part of Pangaea at the time) to being about 20° north of the equator.  There is much evidence to say that at this latitude during the Triassic, conditions were arid and extensive desert basins (similar to what can be seen in Death Valley today) covered much of inland Pangaea.

Desert sandstones, such as the New Red Sandstone in Britain (poorly fossilised cross bedded red beds showing well oxidised and well sorted millet seed grains, typical of wind deposited sediments) and evaporites (e.g. Salt plains in northern Britain) are the main rock types found in the continental Triassic system.          

MarlsCalcareous siltstones and fossils of freshwater organisms are found in many localities such as the Newark Basin (North America) and the Keuper Marl (Germany). They tell us,  lakes covered some of the basin floors.  Desiccation cracks and halite pseudomorphs formed in places suggesting, some of these lakes may have been temporal due to the high rate of evaporation.

Land fossils such as scorpions have been identified supporting evidence for a warm and dry environment.

In Britain during this time, mountains formed by the Variscan orogeny were eroding and material was transported and deposited in huge alluvial fans along the edges of basins. 

Coal deposits, which are late Triassic in age, found in many countries such as Australia, Antarctica and Russia suggest some regions were humid, temperate, and tropical.

At Sea

Surrounding Pangaea and covering the rest of the globe was the huge ocean, Panthalassa. 

Pangaea the supercontinent during the late Triassic began separating and formed the two continents named Gondwana (in the south) and Laurasia (in the north). The two continents lay separated by the shallower Tethys Sea, which grew larger as Pangaea separated. The concept of the supercontinent of Pangaea was part of the theory of Continental Drift proposed by Alfred Wegener in 1912, a German climatologist. 

In the higher latitudes and quiet deep waters, clastics such as sandstones and shales dominated the depositional facies in Panthalassa.  

Toward the lower latitudes, on the shallow marine platforms and in the Tethys Sea, limestones were deposited and coral reefs formed within the tropics.  The Dolomites of Northern Italy formed during this time in this tropical shallow water environment. 

There was a lack of shoreline during this time as all of the continents were juxtaposed.  Consequently, there were less shallow shelf sediments in the Triassic compared with some other periods.

 

 

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