Abstract
El Grupo de Asha de los Urales meridionales, inicialmente definido por Albert Olli como complejo sedimentario paleozoico, hace más de medio siglo, fue considerado como una sección clave para el Véndico (Ediacárico) de la vertiente occidental de los Urales meridionales. Sin embargo, secuencia sedimentaria terrígena de unos 1.700 m de espesor se caracteriza por una biodiversidad inusualmente baja de macrofósiles ediacaranos, lo que podría explicarse por la edad relativamente temprana del grupo y el posible evento de extinción masiva que marca el final del período Ediacárico. La edad relativamente temprana fue confirmada por la datación de 547.6±3.8 Ma obtenida de una toba en la parte inferior de la Formación de Basa, así como por el hallazgo de la iconospecie Didymaulichnus tirasensis en la parte superior de la Formación de Zigan. Sin embargo, en 2018 se ha documentó la aparición masiva de D. tirasensis en la parte más baja de la Formación de Basa. Por lo tanto, la edad Véndico (Ediacárico) del Grupo de Asha de los Urales meridionales puede ser reconsiderada en favor del Ediacárico terminal-Cámbrico o incluso del Cambrian temprano.
Highlights
It is widely accepted by the Russian geologists that there were two stages in evolution of sedimentary cover of the East European Platform: (1) aulacogen stage, which is characterized by the sporadic occurrences of thick sedimentary complexes in rift zones of ancient crystalline basement during Riphean time (~ 1.6–0.6 Ga); (2) plate complex development stage, which is featured by a trend to subsidence of basement coincident with its sedimentary covering, with the exception of the Baltic and Ukrainian shields and some areas of the Timan Ridge (Milanovsky, 1981; Khain & Lomize, 2005)
The fossil assemblage is presented by abundant complex of ichnofossils Didymaulichnus tirasensis and skeletal macrofossils Palaeopascichnus linearis
A new occurrence of the Didymaulichnus tirasensis together with Palaeopascichnus linearis near the Shubino village can be coincident with a Figure 2.—Specimens from the fossil locality in the new quarry near the Ust-Katav city, lower part of the Basa formation: A-C
Summary
It is widely accepted by the Russian geologists that there were two stages in evolution of sedimentary cover of the East European Platform: (1) aulacogen stage, which is characterized by the sporadic occurrences of thick sedimentary complexes in rift zones of ancient crystalline basement during Riphean time (~ 1.6–0.6 Ga); (2) plate complex development stage, which is featured by a trend to subsidence of basement coincident with its sedimentary covering, with the exception of the Baltic and Ukrainian shields and some areas of the Timan Ridge (Milanovsky, 1981; Khain & Lomize, 2005). The relatively recent studies show that there is some evidence of younger age (close to Ediacaran-Cambrian boundary) of the Asha Group (Grazhdankin et al, 2011; Levashova et al, 2013; Kolesnikov et al, 2015). 1700-m-thick Asha Group (Fig. 1) into four formations: Uryuk, Basa, Kuk-Karauk and Zigan (Kolesnikov et al, 2015).
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