Ch 5 The Siluro-Devonian Interorogenic Event PDF

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Ch. 5: The Siluro-Devonian Interorogenic Event (435-400 Ma) Silurian-Devonian Events in Georgia  

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The Siluro-Devonian calm lasted for 35 million years o Half the time span of the Cenozoic Taconic highlands are eroded and leveled during this time period o Much of SE Georgia was above sea level as this was the location of the eroding Taconic highlands o Central and northwest Georgia formed the edge of a large inland sea Inland sea forms in the central Appalachians o This sea may not have been as extensive in Georgia as elsewhere because SE Georgia was a highland area during this time (the Taconic highlands) o Both clastic and carbonate deposition occurred to the northwest/west o Periods of closed circulation within the inland sea resulted in evaporate deposition to the north Land plants appear o Recent fossil finds now extend land plants into the Silurian, but they probably evolved in the Ordovician (Silurian ones are the oldest found to date) o Land plants became abundant in the Devonian

The Laurentian margin returned to a passive continental margin conditions during this time

Sedimentary History 

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Silurian o Clastics were sourced from the Taconic highlands to the east (Lower to Middle Silurian) o Followed by carbonates and evaporites deposition (Upper Silurian) o These units interfingered (carbonates to the west, clastics to the east) Devonian o Cherts and sandstones o Cherts are often altered carbonates Unconformities o Many units are bounded above and below by unconformities o Pre-Lay Dam Unconformity, Pre-Mineral Bluff Unconformity Silurian regression followed by Devonian transgression Iron-rich sediments (Silurian Red Mountain Formation) From Valley and Ridge Stratigraphy: From Mountain Sandstone and Armuchee Chert (younest: Devonian) and Red Mountain Formation (oldest: Silurian) Chert most likely a silicified limestone o Some volcanic ash or something volcanic evolved to supersaturate groundwater with silica o That silica goes in and replaces and silicify the limestone with chert o Something tectonic going one because there are volcanoes o Chert (silicified limestone) indicates some tectonic activity o Documenting the beginning of the Acadian orogeny

The Famous Silurian Ironstones       

Known as the Red Mountain Formation These were primarily carbonate rocks that underwent later diagenesis that included significant iron replacement/ enrichment Red Mountain Formation iron ore was used to produce steel in Birmingham, AL Rocks that are sedimentary originally and somehow (usually carbonate (limestone)) gets replaced by iron The limestone gets replaced by iron Mineral hematite Ooids: sand in carbonate environment

Georgia During the Siluro-Devonian 

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Highlands existed to the SE o Remnants of the Taconic Mountains o Probably showed very modest topography o Coarsest sediment these produced were fine sands  Well worn sands  Mature both mineralogically and texturally  You don’t get this from a big mountain range  You get this from gently rolling hills o Highlands were barely above sea level o Recycling sediment that had been deposited there during the Cambrian  All that was left of the Taconic mountains A large shallow inland sea existed to the NW (these rocks are found in the Valley and Ridge) o The ironstones and cherts and everything were all deposited in the Central Appalachian basin (inland sea) A vast tidal flat existed across NW Georgia o Some argue that this may have been the largest tidal flat ever o Rocks at Ringold Gap (N. Georgia) show excellent tidal flat features o Mudcracks, raindrop imprints Georgia probably had a hot, arid ?, tropical climate o Georgia was inside the tropics Georgia may or may not have had land plants o Earliest land plants were Silurian...