Chalcedony vein horizons and clastic dikes in the White River Group as products of diagenetically driven deformation

Abstract

Chalcedony veins occur as local stratabound arrays at multiple levels within the finer-grained sediments of the White River Group, making up to 2%–3% of the outcrop volume. The veins are commonly deformed by small folds, faults with well-developed striae, and various fold-fault combinations, and they also exhibit striae and slickenslides on vein walls. These indicate significant vertical shortening of the veins. The combination of a stratabound distribution and vertical shortening is consistent with an origin by diagenetically driven deformation, where changes in clay and/or silica phases drive syneresis and associated dewatering and compaction. In this way, the chalcedony veins bear similarities in origin to stratabound polygonal normal fault systems seen in fine-grained marine strata. Smectite clays, silica phases, and clinoptolite in the White River Group are associated with diagenetic reactions that could produce syneresis. At different localities, vein strike distributions vary from being statistically random to highly organized. These distributions are also consistent with a syneresis origin, with local stress fields organizing the distribution into multiple coeval directions in some cases. Chalcedony veins locally occur inside and parallel to clastic dikes, clearly indicating that the veins were emplaced at the same time as or after the dikes. Thin-section textures from dike-vein composites indicate that vein formation occurred while the clastic fill was unlithified and still mobile. These relationships, along with common orientations when in proximity, link clastic dike and chalcedony vein formation. Dikes also show complex strike orientation distributions that differ by locality. Internal dike features indicate multiple fill events with intervening lithification. Evidence for vertical dike shortening suggests synchronous or later compaction. The clastic dikes are also postulated to result from syneresis. We suggest that chalcedony vein formation, silica mobilization, local uranium mineralization, and clastic dike formation are part of diagenetically driven fracture development that produced a fluid flow network, initiating feedback relationships among diagenesis, dewatering, fluid migration, and associated compaction. Given that the clastic dikes occur within the Sharps Formation, the event was Miocene or later.