Abstract Rhyolites in the vicinity of Snowdon (North Wales) are intimately associated with the evolution of the Lower Rhyolitic Tuff Formation (LRTF) caldera of Ordovician (Caradoc) age. They occur as deep-seated dykes, sills and small stocks, shallow-level intrusive domes, and domes extruded within a predominantly shallow-marine environment. Extrusion occurred during three main phases, indicating the episodic availability of rhyolite magma. The rhyolites can be divided on their trace element ratios (e.g. Nb/Zr) into five main groups. Extrusive representatives indicate that each group correlates strongly with a single phase of rhyolite extrusion. Within each group, the distribution and variation of intrusive form with stratigraphic level suggests that geochemically similar rocks were emplaced at approximately the same time. Consequently, the groups represent discrete magma compositions tapped from the evolving Snowdon subvolcanic magma system. Differences in distribution of the groups reflect changes in structural controls of emplacement before and after development of the LRTF caldera.
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Abstract Detailed mapping and biostratigraphic data provide new evidence of a major break below the base of the Caradoc succession along the northern margin of the Harlech Dome in south central Snowdonia. Within this outcrop the sequence is locally complicated by subsequent tectonic and volcanic events, but undisturbed sections indicate a break of at least 10 Ma between upper Arenig and middle, or upper, Llandeilo strata. The break is greatest between two N‐S trending fracture systems, the Cwm Pennant Fault Zone in the west and Trawsfynydd Fault Zone in the east, which both have a persistent history of reactivation. Between these two fractures neither Llanvirn nor lower Llandeilo strata occur. This contrasts with the thick, and perhaps complete, sequence preserved in the Cadair Idris district on the southern margin of the dome and suggests that, during Llanvirn times, the Harlech Dome formed a major uplifted and tilted block, with a tectonically active northern margin. Subsequently, uplift and tectonism either ceased before, or was overwhelmed by, the ensuing sea‐level rise associated with the gracilis (early Caradoc) transgression. The preservation of ooidal ironstones around the dome suggests that it may have became a large shoal or platformal area at this time. Renewed uplift and erosion along the northern margin of the Harlech Dome during the early Caradoc ( gracilis to multidens ) led to large‐scale disruption of the stratigraphic succession by mass gravity flow and slumping, overprinting and locally accentuating the effects of the earlier hiatus.
Research Article| September 01, 1990 Volcaniclastic sedimentation in and around an Ordovician subaqueous caldera, Lower Rhyolitic Tuff Formation, North Wales WILLIAM J. FRITZ; WILLIAM J. FRITZ 1Department of Geology, Georgia State University, Atlanta, Georgia 30303 Search for other works by this author on: GSW Google Scholar M. F. HOWELLS; M. F. HOWELLS 2British Geological Survey, Bryn Eithyn Hall, Aberystwyth, Dyfed SY23 4BY Wales, United Kingdom Search for other works by this author on: GSW Google Scholar A. J. REEDMAN; A. J. REEDMAN 2British Geological Survey, Bryn Eithyn Hall, Aberystwyth, Dyfed SY23 4BY Wales, United Kingdom Search for other works by this author on: GSW Google Scholar S.D.G. CAMPBELL S.D.G. CAMPBELL 2British Geological Survey, Bryn Eithyn Hall, Aberystwyth, Dyfed SY23 4BY Wales, United Kingdom Search for other works by this author on: GSW Google Scholar Author and Article Information WILLIAM J. FRITZ 1Department of Geology, Georgia State University, Atlanta, Georgia 30303 M. F. HOWELLS 2British Geological Survey, Bryn Eithyn Hall, Aberystwyth, Dyfed SY23 4BY Wales, United Kingdom A. J. REEDMAN 2British Geological Survey, Bryn Eithyn Hall, Aberystwyth, Dyfed SY23 4BY Wales, United Kingdom S.D.G. CAMPBELL 2British Geological Survey, Bryn Eithyn Hall, Aberystwyth, Dyfed SY23 4BY Wales, United Kingdom Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1990) 102 (9): 1246–1256. https://doi.org/10.1130/0016-7606(1990)102<1246:VSIAAA>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation WILLIAM J. FRITZ, M. F. HOWELLS, A. J. REEDMAN, S.D.G. CAMPBELL; Volcaniclastic sedimentation in and around an Ordovician subaqueous caldera, Lower Rhyolitic Tuff Formation, North Wales. GSA Bulletin 1990;; 102 (9): 1246–1256. doi: https://doi.org/10.1130/0016-7606(1990)102<1246:VSIAAA>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search 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. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
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