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Abstract

Widespread Cambrian deposits in Northern Siberia are notable for the absence of stratigraphie breaks within the sequences and for the abundance of fossils. Middle Cambrian rocks conformably overlie those of the Lenian Stage. The boundary is determined by the disappearance of Protolenidae and the appearance of Oryctocephalidae, Agnostidae, and Paradoxididae. The Middle Cambrian is divided into the Amginian and Mayian Stages. Amginian rocks range from limestone to black shale, but they contain similar trilobite assemblages and are divisible into three zones. The Amginian-Mayian boundary is fixed by the disappearance of typical Amginian species and the appearance of Ano-mocariodes, Metanomocare, Anopolenus, Dorypyge, and Linguagnostus. Mayian deposits show little facies variation, grading upward from red argillaceous limestone to green and gray limestones. Distinct changes in trilobite assemblages allow division into three zones. The Middle-Upper Cambrian boundary is located in a continuous section on the basis of the disappearance of Lejopyge, Anomocarina, and Bonneferrina, and the appearance of species of Agnostus, Homagnostus, Damesella, Buttsia, Proceratopyge, and other genera. Although lithologically varied. Upper Cambrian rocks are generally rhythmic clastic-carbonate strata. Trilobite assemblages allow division of the Upper Cambrian into six zones.

A trend toward uniformity of environments prevailed from the end of Early Cambrian time through the first half of the Mayian Age. By the end of Middle Cambrian time, shallowing and partial separation of the marine basin caused formation of different facies sequences and local development of the Upper Cambrian faunal assemblages.

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