The Lower Rhyolitic Tuff Formation of Ordovician age in North Wales records the collapse, infilling, and subsequent resurgence of a volcanic caldera with an original diameter of about 15 km. This volcanic center controlled patterns of volcaniclastic sedimentation, providing enough topographic relief for both a shallow-lagoon depositional basin within the caldera and a source of sediment derived from the rim. Within the caldera, sediment consists of tuffaceous laminated siltstone and immature, coarse-grained, volcaniclastic sandstone containing plane beds, ripple cross-laminations, symmetrical wave ripples, and hummocky cross-stratification. Coarse-grained, matrix-supported, conglomerate layers and layers of ash-flow tuffs are also present. These sediments accumulated in shallow water above fair-weather wave base. Conglomerate units represent debris flows from the caldera rim, a nearby shoreline, and elevated areas associated with resurgent domes.

Sedimentation outside of the caldera consisted of deposition of background suspension and volcanic-ash suspension, and turbidite deposition on a pyroclastic apron. The outer margin of the apron was dominated by fine-grained suspension and turbidite deposition, whereas the inner margin of the apron contains hummocky cross-stratification and other evidence of reworking by episodic storm waves. Local highs with associated shallow-water sedimentation existed outside the caldera.

Even though deposited in high-energy marine environments, all sedimentary rocks are both texturally and mineralogically very immature. This textural immaturity differs from the typical very mature marine sediments and was caused by rapid depositional rates and a local volcanic sediment source.

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