Explosion‐breccia in the Wrangell district, southeastern Alaska

Abstract

Unusual breccias were noted at several places in the vicinity of Groundhog and Glacier Basins, about 13 miles east of Wrangell on the mainland of southeastern Alaska, in 1942 and 1943. They are similar in some respects to the clastic dikes in Colorado described by Burbank [see 1 of “References” at end of paper] and Haff [2], and to some explosion breccias in the Samoan Islands described by Stearns [3].

Combined description of map units and correlation of map units for the Petersburg-Wrangell area 1:63,360-scale geologic maps, southeastern Alaska

“…Occurs in broad swarm of dikes from 0.5 to a few m wide with little included country rock that is more or less centered on the granite stock (Tag) at Groundhog Basin (see below); a few small isolated plugs occur along strike. Several exposures within the unit of vent breccia similar to the Vent Breccia unit (QTv) in the Kuiu-Etolin belt to the west of this quadrangle; the breccias consist of dominant 5-20 cm angular rhyolite and some granitic and metamorphic rock fragments with essentially no matrix of any kind; see Gault (1954) and Gault and others (1953) for some further information. Age of the rhyolites is inferred from that of the previously mentioned closely associated granite stock at Groundhog Basin and from a K-Ar age of 15.0±0.6 Ma on muscovite (Douglass and others, 1989, p.65).…”

Reconnaissance geologic map of the Petersburg C-1 quadrangle, southeastern Alaska

“…Several exposures within the unit are of vent breccia similar to the "Vent Breccia" (QTv) in the Kuiu-Etolin belt to the west of this quadrangle. The breccias consist of dominant angular 5-20 cm maximum dimension rhyolite, granitic, and metamorphic rock fragments with essentially no matrix; see Gault (1954) and Gault and others (1953) for details. Age of the rhyolites is inferred from the closely associated granite stock at Groundhog Basin in the southeastern corner of this quadrangle ("Tag", described below).…”