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The Accotink Schist, Lake Barcroft Metasandstone, and Popes Head Formation: Keys to an Understanding of the Tectonic Evolution of the Northern Virgini (en Inglés)
Avery Ala Drake
(Autor)
·
Peter T. Lyttle
(Autor)
·
Bibliogov
· Tapa Blanda
The Accotink Schist, Lake Barcroft Metasandstone, and Popes Head Formation: Keys to an Understanding of the Tectonic Evolution of the Northern Virgini (en Inglés) - Drake, Avery Ala ; Lyttle, Peter T.
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Reseña del libro "The Accotink Schist, Lake Barcroft Metasandstone, and Popes Head Formation: Keys to an Understanding of the Tectonic Evolution of the Northern Virgini (en Inglés)"
The newly named Accotink Schist and Lake Barcroft Metasandstone of the Eastern Fairfax sequence are the structurally lowest metamorphic rocks in the northernmost Piedmont of Virginia. The Accotink consists of beds of pelitic schist that have thin basal intervals containing graded, very fine grained metasiltstone, as well as interbeds of metasandstone like that in the overlying Lake Barcroft Metasandstone. The unit is characterized by the Bouma turbidite sequences Te and Tde and can be assigned to turbidite facies D and E. The thickness of the Accotink is not known because its base is not exposed. The Accotink Schist grades up into the Lake Barcroft Metasandstone, which consists of two types of metasandstone. Type I metaarenite is quartzofeldspathic granofels which forms thick sequences of amalgamated beds that can best be described as belonging to the Bouma turbidite sequence Ta and to turbidite facies B 2 . Type II metagraywacke of the Lake Barcroft Metasandstone consists of micaceous metagraywacke in thin to medium beds, which can be described as belonging to the Buoma turbidite sequences Tabe and (or) Tae and to turbidite facies C. The Lake Barcroft Metasandstone appears to be about 400 m thick. It and the Accotink Schist are thought to represent a coarsening-upward sequence of an outer submarine-fan association of rocks. The Eastern Fairfax sequence is overlain by the Sykesville Formation. We believe that this contact is a movement surface upon which the Sykesville was emplaced by subaqueous sliding. The Sykesville contains isoclinally folded fragments, thought to be rip-ups, of Accotink and Lake Barcroft rocks. The Eastern Fairfax sequence is intruded by rocks of the Occoquan Granite batholith, which contains pendants of isoclinally folded schist and metagraywacke. Mter intrusion, the metasedimentary and plutonic rocks were folded together. Gamet and chlorite porphyroblasts within the Eastern Fairfax sequence appear to be related to the emplacement of the batholith. The minimum age of the Eastern Fairfax sequence is that of the Occoquan Granite batholith, currently thought to be about 560 m.y. The sequence, then, is considered to be of Early Cambrian age or older. The Accotink Schist and Lake Barcroft Metasandstone have some lithic similarity to the Loch Raven Schist and Oella Formation of Crowley (1976) of the Baltimore area, but a correlation is very uncertain at this time. The newly named Popes Head Formation overlies all other metasedimentary and transported meta-igneous rocks in northernmost Virginia west ofthe Occoquan Granite batholith and is intruded by the batholith. The Popes Head consists of a lower Old Mill Branch Metasiltstone Member and an upper Station Hills Phyllite Member.
