Volcanic Lithofacies Research Articles

Late Miocene valley-confined subglacial volcanism in northern Alexander Island, Antarctic Peninsula

Isolated, Late Miocene volcanogenic sequences in northern Alexander Island, Antarctic Peninsula, form an unusual, cogenetic association of volcaniclastic, sandy-gravelly lithofacies (including tillites) and volcanic (lava/hyalocalstite) lithofacies. Using simple lithofacies analysis and theoretical considerations of hydrodynamic effects of subglacial eruptions, valley-confined volcanic activity beneth thin, wet-based ice is suggested. The Alexander Island successions are complete enough to be regarded as model sequences for this uncommonly recorded type of eruptive/depositional activity. The sedimentary lithofacies represent resedimented tuffs and meltout or flow tills, which were probably deposited in subglacial ice tunnels eroded or enlarged by volcanically heated meltwater. The volcanic lithofacies formed by the interaction of hot magma with the ice tunnel walls (generating abundant meltwater) and water-saturatedsediments, resulting in the formation of heterogeneous masses of lava and hyaloclastite. There is no obvious sequence organisation in the sedimentary sections. This is probably due to a complex interplay of eruption-related and environmental hydrodynamic factors affecting the relative proportions of water and entrained sediment.

Evolution of a multi-vent volcanic complex within a subsiding arc graben depression: Mount Wrightson Formation, Arizona

The Mount Wrightson Formation in the Santa Rita Mountains south of Tucson, Arizona, is a homoclinally dipping section of Lower Jurassic volcanic, subvolcanic hypabyssal, and lesser sedimentary rocks that is 4.5 km thick and crops out over a strike length of 25 km. The nature and distribution of volcanic lithofacies in the Mount Wrightson Formation are interpreted to record the evolution of a multi-vent volcanic complex that formed within a subsiding cratonal intra-arc graben. These lithofacies are grouped into the following facies assemblages: (1) the near-vent facies assemblage, consisting of silicic to intermediate hypabyssal intrusive bodies and peperites and dacite lava flows; (2) the proximal facies assemblage, containing intermediate lava flows, ignimbrites, and debris-flow deposits; (3) the medial facies assemblage, comprising ignimbrites, debris-flow deposits, and reworked pyroclastic deposits; and (4) the distal facies assemblage, including nonwelded ignimbrites and fluvial deposits. These facies assemblages are characteristic of stratovolcano complexes; in the Mount Wrightson Formation, however, the paucity of debris-flow deposits and the burial of near-vent facies assemblages by ignimbrites suggest that the formation was deposited in a low-relief multi-vent complex with no high-standing central edifice. Some of the thickest, most densely welded ignimbrites in the Mount Wrightson Formation probably represent outflow from a more distant calders or calderas within the intro-arc graben, rather than being products of volcanism within the multi-vent complex. Intro-arc subsidence resulted in a complex juxtaposition of vent, proximal, medial, and distal facies as vent areas were repeatedly buried by debris derived from adjacent vents in the multi-vent complex and, to a lesser extent, by debris from calderas outside the complex. This subsidence resulted in accumulation and preservation of a thick section of terrestrial volcanic and sedimentary rocks. The Mount Wrightson complex evolved from predominantly effusive (lower member) to predominantly explosive (middle member), followed by waning volcanism (upper member). Rapid intra-arc subsidence resulted in burial of vent areas by craton-derived eolian quartz arenites correlative with either the Wingate or Navajo Sandstones on the Colorado Plateau; this was particularly important as volcanism waned.

Evolution of a Middle Jurassic back-arc basin, Cedros Island, Baja California: Evidence from a marine volcaniclastic apron

A rifted arc-ophiolite assemblage and over-lying volcaniclastic rocks, all of late Middle Jurassic age, represent a fragment of the frontal-arc margin of a back-arc basin, now exposed on Cedros Island in Baja California (Mexico). Ophiolite generation was immediately followed by progradation of a deep- marine volcaniclastic apron (Gran Cannon Formation) contemporaneous with the growth of island-arc volcanoes. This back-arc apron shows relatively simple, uniform sedimentation patterns that may be recognizable in other back-arc basins isolated from terrigenous sediment influx. The progradational back-arc apron sequence includes, from base to top, (1) the tuff lithofacies—thin-bedded, well-sorted, laterally continuous tuffs that thicken into basement paleo-lows. These were deposited from dilute sediment gravity flows generated by eruptions from a deeply submerged, nascent arc. (2) the lapilli tuff-tuff breccia lithofacies—medium- to very thick-bedded lapilli tuffs and tuff breccias. These were deposited from debris flows on proximal parts of the apron, and high-density turbidites were deposited on more distal parts. This lithofacies records increasing generation and resedimentation of scoriaceous and hyaloclastic debris as the summits of the arc volcanoes grew nearer to sea level. (3) the primary volcanic lithofacies—monolithologic dacite pyroclastic flows were produced during the eruption of differentiated magmas, which climaxed the growth of the island arc. Basalt lava flows were fed from fissures that extended down the apron within the back-arc basin. Pyroclastic rocks of the back-arc apron were blanketed with silty basinal turbidites (epiclastic facies) that record abrupt cessation of volcanism and erosion of the arc within about 10 m.y. of ophiolite generation, reflecting the temporal episodicity of back-arc basins. It is predicted from this study that the frontal-arc side of back-arc spreading centers is preferentially preserved in the geologic record relative to the center of the basin. It is suggested that the hot frontal-arc side of the basin is prone to intra-plate volcanism but is unaffected by faulting subsequent to basin initiation. Several factors lead to slumps occurring more frequently on the arc flank of back-arc basins than in any other tectonic setting, except perhaps intra-arc marine basins.

Redefinition of Cenozoic Stratigraphic Column in Split Mountain Gorge, Imperial Valley, California: GEOLOGIC NOTES

This paper proposes some modifications of the stratigraphic terminology as it presently is applied throughout the southwestern Imperial Valley. The Split Mountain Formation is redefined and its stratigraphic limits restricted to include beds critical to its recognition in the original typological sense. Lithologically distinctive lower strata are defined formally as a new stratigraphic unit, the Anza Formation. The Alverson Andesite has been assigned anomalous positions within the stratigraphic column, either above or below the Split Mountain Formation. The formation predates the Split Mountain Formation and is in part a volcanic lithofacies of the Anza Formation. Andesite boulders are a minor component of conglomerate lenses within the upper Anza Formation. No change in efinition of the younger Neogene formations is necessary. Biostratigraphic evidence indicates that the Imperial Formation is of Pliocene age, and that the overlying Palm Spring Formation accumulated throughout late Pliocene to middle Pleistocene time. Marine connection between the Imperial Valley and the Gulf of California was maintained intermittently until the middle Pleistocene.