Vertical and lateral facies transitions

3 . 5 . 1 . Interpretation Hyaloclastite generally develops as a response to thermal contraction at flow tops and fronts Dimroth et al., 1978; McPhie et al., 1993 or may form as a result of subaqueous lava fountaining Smith and Batiza, 1989. The fluidal texture re- sulting from the combination of devitrified sideromelane containing microlites and granules and sedimentary material in the Peltier Formation suggests that the hyaloclastite formed by auto- brecciation when hot lava came into contact with cool, wet sediment, akin to the formation of peperite Schminke, 1967. The fluidal texture of the hyaloclastite is attributed to en- trainment of very fine-grained wet sediment in a vapor film at the magma-sediment interface Busby-Spera and White, 1987. Stratified hyalo- clastite in the Beaulieu River volcanic belt is attributed to resedimentation of reworked pillow breccia and autoclastic hyaloclastite McPhie et al., 1993. 3 . 6 . Bedded tuffs Bedded, fine- to medium-grained volcaniclastic deposits are the on-strike equivalents of massive flows at locality B Fig. 3. The 10 – 35 m-thick, andesitic-dacitic volcaniclastic rocks are locally massive, but are generally characterized by 10 – 50 cm-thick planar beds. The rocks are poorly sorted and contain 0.1 – 1.2 mm, euhedral, subangular, and broken plagioclase crystals, B 1.6 mm subangular to subrounded quartz crystals, 0.3 – 0.8 mm relic, euhedral hornblende crystals, and 0.2 – 2 mm subangular volcanic lithic frag- ments. 3 . 6 . 1 . Interpretation The andesitic-dacitic volcaniclastic rocks at lo- cality B are referred to as tuffs, based on the grain size classification of Fisher 1961, 1966. The tuffs are interpreted as Bouma Ta divisions Bouma, 1962 or S 3 beds Lowe, 1982, the results of turbidity current deposition McPhie, 1995. Vol- caniclastic rocks are typically the direct or redeposited products of subaerial andor sub- aqueous eruptions, or are deposited following erosion and remobilization reworking of erup- tion products. Distinguishing between primary, redeposited, and reworked deposits is often problematic, but the abundance of angular and broken crystals in addition to lithic fragments in the tuffs argues for a primary or redeposited pyroclastic origin. Subaerial eruptions that settle through the water column are typically well-sorted and are distributed over an extensive area McPhie et al., 1993. In contrast, the poor sorting, generally unmodified to slightly modified crystal and lithic fragment shapes, and the limited extent of the bedded tuffs at locality B are consistent with deposition or redeposition from a nearby subaqueous eruption McPhie, 1995.

4. Vertical and lateral facies transitions

Volcanic facies in the Peltier Formation and Beaulieu River volcanic belt generally conform to the ‘standard’ sequence of Dimroth et al. 1978, where massive parts of flows laterally and verti- cally become pillowed, overlain by pillow breccia, and capped by hyaloclastite or hyalotuff. Most flows display only one or two facies transitions, rarely recording the entire sequence of divisions. Vertical changes from pillowed to pillow breccia were identified at locality B Fig. 6; flows I, V, in addition to lateral transitions from pillowed to pillow breccia, locally grading into hyalo- clastite Fig. 6; flows IV, V. Discrete shale units, 1 – 4 m thick, separate pillow breccia and hyalo- clastite Fig. 6; flows III, IV, V. Similar lateral transitions from lobate to pillowed isolated pil- lows, to pillow breccia Fig. 8; flow III, and vertical changes from pillowed to pillow breccia Fig. 11, were identified in the Beaulieu River volcanic belt. All visible contacts between the massive facies and overlying pillows and pil- low breccia are sharp Figs. 6 and 8. Bedded, fine- to medium-grained tuffs are interstratified with the summital massive flow unit at locality B Fig. 3. Stratified hyaloclastite in the Beaulieu River volcanic belt, located immediately below the unconformity with the overlying sedimentary se- quence, sharply overlies a massive flow Fig. 4C. Vertical transitions from pillowed to pillow breccia are explained by changing discharge rates during single eruptive events, whereas lateral changes represent slope differences andor dimin- ishing magma supply at the distal parts of flows as a result of increasing surface area Cas, 1992. The lobe to pillow transition in the Beaulieu River volcanic belt indicates decreasing flow rate Griffiths and Fink, 1992. Autoclastic pillow breccia and disorganized hyaloclastite in the Peltier Formation is interpreted to have developed at flow tops and fronts from quench fragmenta- tion during the late stages of an eruption in a remote part of a volcanic edifice where lava sup- ply was sufficiently decreased Dimroth et al., 1978; Busby-Spera, 1987. Stratified hyaloclastite, or hyalotuff, in the Beaulieu River volcanic belt represents the reworked deposits of pillowed flows and autoclastic pillow breccia and hyaloclastite Dimroth et al., 1978. Bedded tuffs interstratified with massive flows indicate syn-volcanic deposi- tion of localized subaqueous eruption material. Shale units intercalated with pillow breccia and hyaloclastite mark the boundaries between sepa- rate flow events, indicating periods of volcanic quiescence.

5. Discussion