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GEOREF RECORD

Volcaniclastic sedimentation in and around an Ordovician subaqueous caldera, Lower Rhyolitic Tuff Formation, North Wales

William J. Fritz, M. F. Howells, A. J. Reedman and S. D. G. Campbell
Volcaniclastic sedimentation in and around an Ordovician subaqueous caldera, Lower Rhyolitic Tuff Formation, North Wales
Geological Society of America Bulletin (September 1990) 102 (9): 1246-1256

Abstract

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.


ISSN: 0016-7606
EISSN: 1943-2674
Coden: BUGMAF
Serial Title: Geological Society of America Bulletin
Serial Volume: 102
Serial Issue: 9
Title: Volcaniclastic sedimentation in and around an Ordovician subaqueous caldera, Lower Rhyolitic Tuff Formation, North Wales
Affiliation: Ga. State Univ., Dep. Geol., Atlanta, GA, United States
Pages: 1246-1256
Published: 199009
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 8
Accession Number: 1990-050148
Categories: Sedimentary petrologyIgneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 2 tables, sects., geol. sketch map
N53°04'00" - N53°04'00", W04°04'60" - W04°04'60"
Secondary Affiliation: Br. Geol. Surv., GBR, United Kingdom
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute.
Update Code: 1990

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