Abstract

Numerous volcanic ash beds (K-bentonite) are recognized globally in Ordovician to Early Devonian sedimentary successions, and are important for regional stratigraphic correlations. Relatively little, however, is understood of the origin and transport of these ash deposits. Here, we examine ash beds preserved in the Upper Ordovician and Silurian dark shales and mudstones of the Holy Cross Mountains, Poland, and discuss their origin in the context of contemporaneous tectonic configurations. The Ordovician ash beds occur in the upper Sandbian and Katian mudrock succession, whereas the Silurian K-bentonites form prominent beds in the upper Wenlock and lower Ludlow graptolite shales. Their various thickness and irregular distribution reflect fluctuation of volcanic activity and contribution of physical and biogenic factors in their preservation. Deposition under dysoxic or anoxic conditions and relatively rapid burial by muddy sediment seem to be dominant factors preventing physical and biological reworking and mixing of the volcanic ash. The Upper Ordovician ash beds in the Holy Cross Mountains, as well as the Baltoscandian K-bentonites, appear to be accumulated from pyroclastic material delivered by westerlies of the Southern Hemisphere from the Avalonian volcanoes. In the Silurian, the distribution of pyroclastic material was controlled by SE trade winds, and therefore the source area for the Wenlock and Ludlow K-bentonites in the Holy Cross Mountains appears to have been an active subduction zone developed close to the southeastern side of Baltica. However, the delivery of volcanic ash to the central part of Baltica might have been strongly affected by climatically driven latitudinal migration of the Intertropical Convergence Zone.

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