Abstract
A 4-km thick, vertically dipping, south-facing, homoclinal, intermediate to felsic, volcaniclastic sequence in central Lake of the Woods, is underlain and overlain by subaqueous, tholeiitic basalt flows. Volcaniclastic lithologies comprise a lower tuff-breccia and lapilli-tuff assemblage, an upper turbiditic greywacke formation, and three lenticular, upward-fining, conglomerate formations interspersed within other units. Tuff-breccia and lapilli-tuff are characterized by very thick, poorly defined beds, pronounced heterogeneity, mostly angular blocks and lapilli, and a plagioclase+quartz crystal component that, in places, is incompatible with phenocryst populations in fragments. These appear to be subaqueous debris flow deposits that resulted from multistage mixing, during downslope mass transport, of fragments produced by subaerial phreatomagmatic vulcanian-type eruptions. Anomalous crystal contents represent intermixing of subaerial subplinian or plinian ash. Conglomerate formations comprise very thick, poorly defined beds composed almost entirely of well rounded, fine- to medium-grained trondhjemite. They are also subaqueous debris flow deposits but were apparently derived from wave erosion of large, flank dome complexes that periodically blocked normal transport of volcanic debris from higher on the volcano. The upper greywacke represents a shift in eruptive processes to largely plinian or subplinian eruptions.
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have