Facies analysis of an ancient, dismembered, large caldera complex and implications for intra-arc subsidence: Middle Jurassic strata of Cobre Ridge, southern Arizona, USA

Abstract

The Middle Jurassic (∼ 170 Ma) Cobre Ridge caldera is an elongate caldera complex that formed during the eruption of the tuff of Pajarito, a crystal-rich rhyodacite ignimbrite with an estimated volume > 1000 km 3. The caldera subsided in two subequal blocks: to the southeast, caldera-forming ignimbrite is as much as 3000 m thick, whereas the northwest half of the caldera subsided to lesser depths during the initial eruption, but was the locus of subsidence during subsequent eruptions, providing a depocenter for a > 1.5 km-thick section of secondary- and post-collapse volcanic and sedimentary strata. These strata record moat formation and filling and waning volcanism, including: (1) areally restricted ignimbrites up to several tens of meters thick that may have been ponded between caldera margins and/or fault scarps; (2) a ⩾ 600 m-thick ignimbrite whose eruption probably caused further collapse of the caldera or enlargement of the caldera margins; (3) a localized debris apron deposit up to 500 m thick that represents material reworked from intra-caldera pyroclastic deposits and possibly caldera walls; and (4) eolian and fluvial sandstones and water-lain tuffs. Megabreccia blocks as large as 0.5 km along strike probably demarcate approximate caldera margins, suggesting that the composite caldera was approximately 50 km long by 25 km wide. The preserved thickness of the strata of Cobre Ridge ( > 4500 m) is greater than many ancient continental volcanic sequences, suggesting external (i.e. tectonic) controls on subsidence, but preservation of this great thickness of strata is apparently due entirely to volcanic subsidence.