University of Tasmania
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Characteristics and origin of subaqueous pumice-rich pyroclastic facies : Ohanapecosh formation (USA) and Dogashima formation (Japan)

posted on 2023-05-26, 00:15 authored by Jutzeler, M
This thesis discusses the processes that generate subaqueous, pumice-rich pyroclastic facies in below wave-base environments. The principal method is field-based facies analysis. I also present grain size distributions and hydraulic sorting ratios of selected samples using a new technique that combines image analysis and functional stereology. Two Tertiary volcaniclastic successions have been studied. The Ohanapecosh Formation (Washington State, United States) is dominated by subaqueous pumice-rich facies derived from subaerial explosive eruptions and subaqueous sediment remobilisation in a continental basin environment. The Dogashima Formation (Izu Peninsula, Japan) contains well-preserved examples of subaqueous pumice-rich facies produced by subaqueous explosive eruptions and below wave-base sediment remobilisation in an oceanic arc setting. The common processes of lithification and welding prevent quantification of grain size by conventional sieving for most clastic rocks in the geological record. In addition, the true grain size distribution of clastic rocks is finer grained than its representation in a random 2D section. I show that image analysis combined with functional stereology can be used to infer 3D volume fractions and weight percent of clast populations >0.25-2 mm from 2D cross-sectional images. Data from synthetic rocks correlate well with results from sieving of the same samples while still unconsolidated. The method can be applied to any type of coarse grained clastic rock, regardless of age, and therefore has a wide application in volcanology and clastic sedimentology. The >800-m-thick Ohanapecosh Formation records voluminous sedimentation of volcanic clasts during the Eocene-Oligocene in the Central Cascades. Most volcaniclastic beds are dominated by angular pumice clasts and fiamme of intermediate composition, now entirely devitrified and altered. Very thick to extremely thick (1‚Äö-50 m) and very thin to thick (0.001‚Äö-1 m) beds are laterally continuous and have even thickness; erosion surfaces, cross-beds and other traction structures are almost entirely absent, which strongly suggests a below wave-base environment of deposition for most of the succession. The Chinook Pass Member is mostly composed of extremely thick, graded, matrixsupported, pumice-and-fiamme-rich beds that commonly include a coarse basal breccia comprising sub-rounded dense clasts. The abundance of angular pumice clasts and extreme thickness suggest that this facies was generated by magmatic volatile-driven explosive eruptions, and the sub-rounded dense clasts were probably rounded above wave-base. Thus, these beds are interpreted to have been deposited in a below wave-base setting by subaerial pyroclastic flows that crossed the shoreline, and transformed into eruption-fed, water-supported subaqueous volcaniclastic density currents. Reversely to normally graded pumice breccia facies that contains sub-rounded pumice clasts, wood and accretionary lapilli is interpreted to have formed by settling from pumice rafts, also related to subaerial explosive eruptions. The White Pass Member chiefly contains massive to normally graded volcanic breccia and coarse volcanic breccia that suggest deposition from subaqueous high-concentration density currents and subaqueous debris flows. The abundance of angular pumice clasts suggests minor reworking above wave-base. Very thin to thick interbeds of fine sandstone to mudstone are interpreted to be derived from subaqueous and subaerial sources, and to have mostly been deposited from low density turbidity currents and suspension. The presence of shallow basaltic intrusions and mafic volcanic breccia composed of scoria lapilli and that contains rare beds of accretionary lapilli indicate the presence of intra-basinal scoria cones that may have been partly subaerial. The lateral transition to thinner and finer-grained facies in the Johnson Creek Member and western part of the White Pass Member suggests that the principal sources were to the east of the preserved exposures of the Ohanapecosh Formation. The Pliocene Dogashima Formation (Izu Peninsula, Japan) is composed of three volcaniclastic sequences erupted under water. Dogashima 1 is mostly composed of pumice breccia, shard-rich siltstone, and cross-bedded and planar bedded pumice breccia/sandstone. The base of Dogashima 2 is dominated by very thick, clast-supported, massive grey andesite breccia composed of very coarse andesite clasts with quenched margins. It is gradationally overlain by very thick, clast-supported white pumice breccia. The massive grey andesite breccia is confined to a pal‚àö¬¿o-valley eroded into beds of Dogashima 1. The white pumice breccia is hydraulically sorted and stratified in proximity to the wall of this pal‚àö¬¿o-valley, and is stratified and finer grained in the adjacent overbank setting. The top of Dogashima 2 is dominated by very thick cross-bedded pumice breccia-conglomerate and planar bedded pumice breccia that contains coarse pumice clasts. Dogashima 2 has an erosive contact with overall monomictic andesite breccia of Dogashima 3. The similar mineralogy and composition of white andesite pumice and grey andesite clasts in Dogashima 1 and 2 suggests they were co-magmatic and erupted from the same vent. Dogashima 2 is interpreted to record explosive destruction of a subaqueous hot lava dome by a subaqueous, magmatic volatile-driven explosive eruption. Most of the products of this eruption were deposited in two gradational units from cohesionless, water-supported volcaniclastic density currents. Coarse pumice clasts and ash present in overlying planar beds were settled from suspension. This sequence demonstrates that lava or dome effusion on the sea floor can switch to an open-vent, pumice-forming, magmatic volatile-driven explosive activity, as in subaerial analogues. Pumice breccia of Dogashima 1 is interpreted to be the product of precursory explosive activity, whereas Dogashima 3 records a late, dome-building episode. Pumice breccia and dome-related clasts indicate cyclic effusive and explosive activity throughout the Dogashima Formation. Cross-bedded pumice breccia/breccia conglomerate facies in the Dogashima Formation are most likely to be products of resedimentation of pumiceous aggregates, and development and destruction of subaqueous dune fields in a below wave-base, canyon setting. I apply the image analysis and functional stereology method to pumiceous volcaniclastic rocks of the Ohanapecosh and Dogashima formations and the Manukau Sub-Group (New Zealand) and the Sierra La Primavera caldera (Mexico). Samples from these waterlain successions were grouped into three broad facies, on the basis of bed thickness, abundance of matrix and clast size sorting. The volume of pumice clasts, dense clasts and matrix (<2 mm), modal grain size distribution and hydraulic sorting ratio between pumice and dense clasts were used to characterise these three facies. On plots of sorting versus median diameter, the three facies overlap, which suggests that all three facies have overall good hydraulic sorting in their coarse clasts (>2 mm), and that the pumice clasts were fully waterlogged during transport and deposition. In addition, the studied subaqueous volcaniclastic samples overlap with the fields defined by matrix-free subaerial pyroclastic flow deposits, which confirms that outputs from functional stereology and conventional sieving give comparable results. The volcaniclastic successions in the Ohanapecosh and the Dogashima formations include good examples of subaqueous, pumice-rich pyroclastic facies that were erupted onland or under water, and deposited in below wave-base settings. Eruption-fed facies generated by subaqueous pumice-forming explosive eruptions are exemplified by the extremely thick and graded sequence in Dogashima 2 (Dogashima Formation), which contains dense clasts that were deposited hot. Numerous beds of graded, extremely thick, pumice-rich facies in the Ohanapecosh Formation are interpreted to be deposited from eruption-fed, water-supported high concentration density currents that were fed by subaerial pyroclastic flows, primarily because rounded dense clasts, accretionary lapilli and wood in these and associated facies imply that the source vents were subaerial. Resedimentation events can occur during eruptions and after, especially after eruptions that produce large volumes of pyroclasts in unstable environments. However, resedimented facies can be difficult to distinguish from eruption-fed facies in below wave-base successions, essentially because clast reworking below wave-base is minimal during resedimentation. In addition, eruption-fed facies can contain clasts that were previously abraded in above wave-base settings, for example during long-distance transport in subaerial pyroclastic flows, in pumice rafts (e.g. Chinook Pass Member, Ohanapecosh Formation) or during transport in the dense-clast-dominated bedload of subaqueous volcaniclastic density currents (e.g. Dogashima 2). Short-distance transport in pyroclastic flows might not be able to abrade pumice clasts (e.g. the eruption-fed facies in the Chinook Pass Member, Ohanapecosh Formation).


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