STRUCTURAL ANALYSIS OF A LAVA-LIKE RHEOMORPHIC IGNIMBRITE: THE GREY'S LANDING IGNIMBRITE, TWIN FALLS, IDAHO, USA

Numerous voluminous rhyolitic rheomorphic ignimbrites erupted from Miocene volcanic centres along the Snake River Plain as the North American Plate passed over the Yellowstone plume. Several are extremely high-grade, and dominated by lava-like lithofacies (terms used as in Branney & Kokelaar, 1992). They exhibit abundant ductile deformation structures including flow banding, an elongation lineation, and abundant small-scale transposed (oblique) folds. We present an initial field analysis of one such deposit, the Grey’s Landing ignimbrite, which has been traced for over 450 km2, and reaches 60 m thickness where it occupies a basin formed by a contemporary half-graben. Above a basal fused ashfall layer, the ignimbrite exhibits both a basal and an upper vitrophyre. The rest is devitrified and lacks bedding and other sedimentary structures. It is inferred to represent a single massive cooling-unit. Small- and medium-scale deformation structures and fabrics were measured along 12 vertical traverses through the thick ignimbrite sheet at different locations. Results show that the styles, attitudes and orientations of the ductile deformation structures vary systematically both with height through the ignimbrite and with geographic location. The lateral and vertical variations can be used to distinguish the pyroclastic (ignimbrite) origin from the inferred effusive origin of nearby extensive silicic lavas (Bonnichsen and Kauffman, 1987). We interpret the variations with height in the Grey’s Landing ignimbrite to record temporal changes in deformation within a narrow and upwards-migrating hot shear-zone that developed rapidly, and tracked the rising aggradation surface of the ignimbrite-agglutinate while it progressively aggraded.

Bonnichsen B, Kauffman, DF, (1987) Physical features of rhyolite lava flows in the Snake River Plain volcanic province, southwestern Idaho. In: Fink JH (ed.), The emplacement of silicic domes and lava flows. Geol Soc America Spec Paper 212: 119-145.

Branney MJ, Kokelaar, P (1992) A reappraisal or ignimbrite emplacement: progressive aggradation and changes from particulate to non-particulate flow during emplacement of high-grade ignimbrite. Bulletin of Volcanology 54: 504-520.