Abstract

The Siegas Formation occurs in a conformable sequence of marine sedimentary rocks from Middle Ordovician to Upper Silurian in age. Application of transition matrices to sequences of the lithotypes of the lithic wacke facies defines two lithologic associations. The persistent assemblage which is composed of limestone and slate is a continuation of the sedimentary sequence of the underlying Carys Mills Formation. This assemblage is a product of relatively steady, long-term depositional processes. A second exogenic assemblage consists of sandstone, limestone-conglomerate and chert and was produced during three distinct and relatively short-lived depositional periods that were superposed on the long-term processes that produced the persistent assemblage. The middle exogenic period was most important and is represented by a turbidite sequence beginning and ending in distal facies and passing through a highly proximal phase. Limestone-conglomerate beds were deposited in all three exogenic periods by turbidity currents and by at least two slumping mechanisms. Within the lithic wacke facies there is a proximal to distal lateral variation across the area studied. All erosional surfaces and outcrops of the limestone-conglomerate lithotype in the Siegas Formation are associated with the highly proximal region. In the most proximal region submarine erosion probably due to the development of a submarine canyon cut at least 100 feet into the underlying Carys Mills Formation. Collapse of undercut canyon walls probably provided source material for the limestone-conglomerate lithotype and an initiating mechanism for the high-competence turbulent flows of the conglomerate debris.

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