EVOLUTION OF A PLIO-PLEISTOCENE EMERGENT TUFF CONE USING DETAILED STRATIGRAPHIC SECTIONS, WESTERN SNAKE RIVER PLAIN, IDAHO

The products of basaltic hydrovolcanism (maars, tuff rings, tuff cones) are second only to scoria cones as the most abundant volcanic landforms on Earth (Cas and Wright, 1988, p.376). However, the origin of these deposits remains poorly understood. This project integrates both volcanology and sedimentology to study Sinker Butte, an erosional remnant of a very large basaltic tuff cone located at the southern edge of the western Snake River Plain. It is emergent in that it erupted beneath Plio-Pleistocene Lake Idaho and grew to more than 250 m above the lake surface. Subaerial and subaqueous deposits from this center are well exposed on both sides of the Snake River Canyon, creating an unusual opportunity to study the entire eruptive sequence. Over 25 detailed stratigraphic sections from 10 to 100 m thick were measured and described on three sides of the eroded edifice. The lowest units are generally massive, poorly sorted deposits, many of which have Bouma sequence turbidity features. They grade upward into better sorted sediments with large scale cross stratification. The transition to subaerial deposits is marked by an abrupt change to well bedded palagonite and accretionary lapilli tuffs. Sections of subaerial tuffs close to the old crater rim are well bedded throughout, with bedding typically being planer and uniform in thickness. Sections intermediate and distal from the crater rim contain well-bedded tuffs in their lower parts, but cross stratification is much more common than in the proximal sections. Moreover, the well bedded tuffs in these sections grade upward into layers consisting of 75-95% juvenile clasts, which in turn are overlain by massive units of tuff breccia. Welded spatter and an extensive crater filling lava flow cap the clastic sequences. Based on the field data, the lowest Sinker Butte units are interpreted as being deposited mainly by subaqueous density flows. As deposits built close to the lake surface, they acquired wave induced cross beds. The well bedded subaerial units are mixed fall and wet surge deposits. Beds of juvenile lapilli indicate a temporary decrease in the magma/water ratio, but the massive tuff breccias near the top of the sequence suggest downward coring may have intersected a deeper aquifer. Finally, the volcano effectively sealed itself off from external water, producing a magmatic cap.