The Somers ignimbrite and related volcanics, Mt Somers, mid-Canterbury, New Zealand (1994)
AuthorsSmith, Timothy R.show all
Mount Somers, mid-Canterbury, New Zealand, consists of rocks of the Mt Somers Volcanic Group, a suite of mid-Cretaceous (89+/-2.0Ma) calcalkaline intermediate and silicic volcanics. Revision of the stratigraphic nomenclature is recommended, with the addition of two new formations previously incorporated within the Somers Rhyolite Formation: the Somers Pitchstone and Somers Ignimbrite. The Somers Ignimbrite is a multiple flow-unit/compound cooling-unit, high-grade pyroclastic flow deposit. Dense welding and pervasive devitrification in the Somers Ignimbrite resulted in a lack of preservation of glass shards and pumice. A relatively low abundance of lithics, widespread rheomorphism, and the presence of layering similar in appearance to lava flow-layering, combine to give the Somers Ignimbrite a lava-like appearance. It is concluded that the Somers Ignimbrite is of pyroclastic origin on the basis of: (1) it's multiple sheet-like form; (2) the presence of multiple sub-units within most sheets; (3) co-ignimbrite ash deposits at the top of some sub-units (4) a moderate abundance of broken phenocrysts; (5) layering produced by extreme attenuation of pumice fiamme; (6) abundance of lithics at the base of sheets; and (7) vertical geochemical zonation. The Somers Ignimbrite was probably produced by a low-explosivity 'boil-over' eruption from a source to the north of Mt Somers. Twelve members are recognised, each recording a major explosive phase in the eruption. Contacts between sheets are gradational indicating the sheets were emplaced rapidly, probably over a period of days to weeks. Most sheets contain multiple temporally discreet sub-units. Rare co-ignimbrite ash deposits at the top of some sub-units indicate they are flow-units; other sub-units may have been produced by step-wise aggradation at the base of a continuous over-riding pyroclastic flow. Welding occurred during primary pyroclastic flow as 'within-flow agglutination'. Millimetre-scale laminar layering comprises intercalated (devitrified) fragmental and pumiceous material. Pumiceous layers are interpreted to be extremely attenuated pumice fiamme produced by laminar shearing of the agglutinating pyroclasts during primary pyroclastic flow. Lineations on the top surfaces of some sub-units reflect prolate shearing. Rheomorphism and rapid thickness variations reflect deposition on a paleotopographic surface of significant relief. Thick accumulation of ignimbrite on the north side of Mt Somers reflects ponding in a paleotopographic depression. Vertical geochemical zonation reflects extraction from a compositionally zoned magma chamber comprising more differentiated magma overlying less differentiated magma.