Abstract

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.

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