Mel A. Kuntz

Contrasting magma types and steady-state, volume-predictable, basaltic volcanism along the Great Rift, Idaho

The Great Rift is an 85-km-long, 2- to 8-km-wide volcanic rift zone in the Snake River Plain, Idaho. Three latest Pleistocene to Holocene basaltic lava fields, Craters of the Moon, Kings Bowl, and Wapi, are located along the Great Rift. The Craters of the Moon lava field is a composite of more than 60 lava flows, 25 cinder cones, and at least 8 eruptive fissure systems. It covers 1,600 km2 and contains ∼30 km3 of lava flows and associated pyroclastic deposits. Field, radiocarbon, and paleomagnetic data show that the Craters of the Moon lava field formed in eight eruptive periods, each of which was typically several hundred years or less in duration and was separated from others before and after by intervals of several hundred to several thousand years. The first eruptive period began ∼15,000 yr B.P., and the latest eruptive period ended ∼2100 yr B.P. The small Kings Bowl lava field (3.3 km2, 0.01 km3) and the larger Wapi lava field (330 km2, 6 km3) both formed ∼2250 yr B.P. Three magma types have fed flows along the Great Rift. The types are (1) a contaminated type that has a SiO2 range of ∼49%–64% and commonly shows petrographic evidence of contamination, (2) a fractionated type that has a SiO2 range of ∼44%–54% and shows no evidence of contamination and whose chemical and mineralogical variation can be accounted for mainly by crystal fractionation, and (3) a Snake River Plain type that has a SiO2 range of ∼45%–48%, shows little evidence of fractionation, and is represented by Kings Bowl–Wapi flows and olivine basalts of the Snake River Plain. The contaminated and fractionated magma types were erupted at the Craters of the Moon lava field, and the Snake River Plain magma type was erupted at the Kings Bowl and Wapi lava fields. These relations imply that the magma reservoirs are spatially isolated. The magma output rate for the Craters of the Moon segment of the Great Rift was constant at ∼1.5 km3/1,000 yr for the period from 15,000 to 7000 yr B.P. The rate increased to ∼2.8 km3/1,000 yr from 7000 to 2000 yr B.P., mainly as a result of the addition of contaminated magma to the nearly constant output rate of fractionated magma. The Craters of the Moon segment of the Great Rift has experienced quasi-steady-state, volume-predictable volcanism for the last 15,000 yr. The recurrence interval of eruptive activity for the Craters of the Moon lava field ranges from several hundred to ∼3,000 yr. Because the present interval has lasted ∼2,100 yr, another eruptive period seems likely to occur within the next 1,000 yr. The steady-state, volume-predictable relationship suggests that 5–6 km3 of lava will be erupted in the next eruptive period.

Emplacement and deformation history of the western margin of the Idaho batholith near McCall, Idaho: Influence of a major terrane boundary

Cretaceous plutons of the western margin of the Idaho batholith were emplaced along and to the west of the major terrane boundary separating middle Proterozoic and Paleozoic continental rocks from mostly Mesozoic accreted oceanic-arc terranes of the Blue Mountain Province. This boundary is marked by a change in the lithology of pendants and inclusions within the batholith. Plutons form two newly named complexes of igneous and metamorphosed igneous rocks. The Hazard Creek Complex, emplaced west of the boundary between the oceanic arc and the continental margin, consists primarily of a series of variably deformed and metamorphosed quartz diorite to trondhjemite plutons. The Little Goose Creek Complex, which intruded the boundary between the oceanic arc and the continental margin, is primarily porphyritic granodiorite to granite orthogneiss. A preliminary U-Pb age of 111 Ma for this porphyritic orthogneiss is a minimum age for the formation of the oceanic-arc-continent boundary. The plutonic rocks were deformed both during and after emplacement in response to east-west compressive stresses. Cretaceous deformation was localized along the boundary between the accreted terranes and the continental margin and is interpreted to have occurred after the formation of this boundary. The major deformation of the Hazard Creek Complex occurred during its emplacement. The dominant fabric in the Little Goose Creek Complex is due to subsolidus ductile deformation. The localization of two deformation events along the pre-existing boundary between the accreted terranes and the continental margin suggests that a terrane boundary may form a long-lasting, crustal flaw.

Radiocarbon studies of latest Pleistocene and Holocene lava flows of the Snake River Plain, Idaho: Data, lessons, interpretations

Abstract Latest Pleistocene-Holocene basaltic lava fields of the Snake River Plain, Idaho, have been dated by the radiocarbon method. Backhoe excavations beneath lava flows typically yielded carbon-bearing, charred eolian sediment. This material provided most of the samples for this study; the sediment typically contains less than 0.2% carbon. Charcoal fragments were obtained from tree molds but only from a few backhoe excavations. Contamination of the charred sediments and charcoal by younger carbon components is extensive; the effects of contamination were mitigated but appropriate pretreatment of samples using acid and alkali leaches. Twenty of the more than 60 lava flows of the Craters of the Moon lava field have been dated; their ages range from about 15,000 to about 2000 yr B.P. The ages permit assignment of the flows to eight distinct eruptive periods with an average recurrence interval of about 2000 yr. The seven other latest Pleistocene-Holocene lava fields were all emplaced in short eruptive bursts. Their 14 C ages (yr B.P.) are: Kings Bowl (2222± 100), Wapi (2270 ± 50), Hells Half Acre (5200 ± 150), Shoshone (10,130 ± 350), North Robbers and South Robbers (11.980 ± 300), and Cerro Grande (13,380 ± 350).

Proximal bedded deposits related to pyroclastic flows of May 18, 1980, Mount St. Helens, Washington.

Thin-bedded, dacitic, pumiceous pyroclastic-flow deposits partly cover the steep northern flank of Mount St. Helens volcano, Washington. They are termed proximal bedded pyroclastic-flow (PBPF) deposits and were formed during the eruption of May 18, 1980. These unconsolidated deposits, as much as 20 m thick, are characterized by well-defined, chiefly plane-parallel bedding sets separated by erosion surfaces. Most beds dip generally parallel to the slope of the underlying volcano flank, which averages 15° to the north but locally is as much as 30°. Individual beds range in thickness from 2 mm to >1 m. Cross-bedding, in which bedding sets dip gently to the north or locally to the south, is abundant. Cross-bedding is generally associated with large longitudinal dunes, apparently antidunes. Some cross-bedding sets, however, constitute small longitudinal anti-dunes (chute-and-pool structures) containing stoss-side beds that migrated south and dip steeply south toward the crater source. We suggest that PBPF deposits, base-surge deposits, and ignimbrite-veneer deposits are products of a broad spectrum of high-flow–regime pyroclastic processes. PBPF deposits at Mount St. Helens formed during deposition of high-velocity, partly turbulent, dry, fluidized pyroclastic flows that poured profusely down the volcano flank on May 18. Some beds of the PBPF deposits probably were deposited from typical high-concentration laminar pyroclastic flows. Other beds, especially those that display evidence of high-angle stoss-side accumulation, were probably deposited from low-concentration turbulent flow under high velocity as pyroclastic surges. Most beds in the PBPF deposits, however, may have formed during transitional flow conditions. Pyroclastic-surge and transitional flow-surge conditions may have resulted when the flows exceeded the threshold velocity or threshold minimum particle concentration for pyroclastic flows while lofting over obstacles or depressions or after collapse from Plinian ash columns.

Petrography, age, and paleomagnetism of basaltic lava flows in coreholes at Test Area North (TAN), Idaho National Engineering Laboratory

The petrography, age, and paleomagnetism were determined on basalt from 21 lava flows comprising about 1,700 feet of core from two coreholes (TAN CH No. 1 and TAN CH No. 2) in the Test Area North (TAN) area of the Idaho National Engineering Laboratory (INEL). Paleomagnetic studies were made on two additional cores from shallow coreholes in the TAN area. K-Ar ages and paleomagnetism also were determined on nearby surface outcrops of Circular Butte. Paleomagnetic measurements were made on 416 samples from four coreholes and on a single site in surface lava flows of Circular Butte. K-Ar ages were measured on 9 basalt samples from TAN CH No. 1 and TAN CH No. 2 and one sample from Circular Butte. K-Ar ages ranged from 1.044 Ma to 2.56 Ma. All of the samples have reversed magnetic polarity and were erupted during the Matuyama Reversed Polarity Epoch. The purpose of investigations was to develop a three-dimensional stratigraphic framework for geologic and hydrologic studies including potential volcanic hazards to facilities at the INEL and movement of radionuclides in the Snake River Plain aquifer.