Sunday, November 25, 2012

Examples of Devonian deposits around the globe

Earth during Devonian
According to current views Earth was during the Devonian period largely covered by oceans and water was high. The ocean is called Panthalassa (gr. "all ocean"). From the water was separe three major land massives and when the water level lowered towards the end of Devonian the shallow coastal areas grew in size (light blue in map) and had major reefs and corals. There were no ice caps.
  1. Gondwana in the South
  2. Euramerica  in the middle
  3. Siberia in the North
The reconstruction of Devonian continents belongs to the field of paleogeography. Because much of the evidence is thoroughly mixed by continental drift and the study of the bottom of the sea is very difficult and expensive, it is wise to be careful and critical with the proposed mappings of Devonian. Do we really know or are we just using computer models that have a statistical level of probability? 

Ocean bottom?
Since Devonian is commonly called "The Age of Fish" so do we know it mainly by drilling the bottom of sea? 


The geological study of ocean bottom is its own discipline that requires special equipment and is very expensive. 19th century geologists could only dream of such a possibility.

But Nature has helped scholars massively by folding and pushing up ancient deposits from the bottom of the seas, some even high up to mountains! Thus it is possible to study exposed deposits in dry land without diving underwater.

Great Britain and Scandinavia - "Old Red"
Devonian sediments in Great Britain contain characteristic reddish sandstone, that was recognized as a specific rock type already at the end of the 18th century (Old Red Sandstone).

The Old Red consists of sandstone sediments that have formed on dry land and shallow waters and lacks therefore fossils of deep sea life. The rock contains alluvial deposits from low-lands, and also conglomarates packed tight in flowing water (now found by Curiosity also on Mars), mudstone and other finer stones from riverbeds, lagoones and lakes. The reddish color is due to iron-oxide (haematite). 

The Old Red sandstone is related to the massive Caledonian orogeny that has been studied in British Isles, Scotland, Scandinavia and Baltic regions.  Unfortunately this much studied orogeny must be among the most complicated and difficult to analyse births and deaths of mountains and presents Geology at its most difficult. (See more in wikipedia)

North American Devonian
Devonian tectonics tell about the rise of Caledonian mountains and the associated birth of Old Red sandstone.  Similarly continental plate movements caused in Euramerica the rise of northern Appalachian mountains. The western part of the continent was more quiet and consisted of lowlands, river deltas and reefs. Further to the west there was volcanic activity in Panthalassa. 

The map shows known devonian deposits in North America today. Note that in Devonian period the continent was Euramerica and looked quite different - the modern continent has passed through many phases after that. 

Devonian deposits in North America
North American Tapestry

What about Africa, India, Siberia?
Do you really want to know?

Baltic evidence of devonian period?

For example, William C. Mahaney, Volli Kalm: Comparative scanning electron microscopy study of oriented till blocks, glacial grains and Devonian sands in Estonia and Latvia. Boreas 29.1 2000, pp. 35-51 (abstract)

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