W. P. Nash

Fallout tuffs of Trapper Creek, Idaho—A record of Miocene explosive volcanism in the Snake River Plain volcanic province

A 900-m-thick section of tuffaceous sedimentary rock, vitric fallout tuff, and ash-flow tuff is well exposed along Trapper Creek in south-central Idaho. This section provides nearly continuous exposure through the fill of the Goose Creek basin, a major north-trending Miocene extensional basin located along the southern margin of the Snake River Plain volcanic province (SRPVP). Some 51 separate units of vitric fallout tuff are recognized in the Trapper Creek section. Petrographic and chemical characteristics of these vitric tuffs indicate that most are from SRPVP sources. New 40Ar/39Ar laser-fusion dating, along with prior isotopic age determinations, show that the Trapper Creek tuffs span the period ca. 13.9 – 8.6 Ma. Chemical correlation indicates that fallout tuffs in the central part of the Trapper Creek section (12.5 – 10.0 Ma) are from sources in the Bruneau-Jarbidge volcanic field of the SRPVP centered ≈100 km west of Trapper Creek. Underlying fallout tuffs may have had sources in the Owyhee-Humboldt field of the SRPVP centered ≈200 km west of Trapper Creek, while overlying fallout tuffs, interlayered with several ash-flow tuffs, had a relatively proximal source, possibly in the proposed Twin Falls volcanic field centered ≈60 km north of Trapper Creek. The Trapper Creek tuffs provide insight into the characteristics of explosive silicic volcanism within the SRPVP during middle – late Miocene time. From ca. 13.9 to ca. 9.5 Ma, major eruptions (those depositing ≥1.5 m of fallout tuff) were frequent (about one event per 200 k.y.); their products display a trend toward the eruption of progressively less evolved, higher temperature silicic magma after 12.5 Ma. This trend to higher temperature eruptions, termed the Cougar Point “flare-up,” culminated in the eruption of high-temperature (≈1000°C), plagioclase-rich magma during the period 10.5 – 9.5 Ma. In contrast to these eruptions, later (<7.0 Ma) major silicic eruptions within the SRPVP were characterized by the lower temperature (≈850°C) of the erupted magma and by the longer intervals (about one event per ≈500 – 600 k.y.) between eruptions. Variations in the character of SRPVP explosive silicic eruptions may reflect changes in the structure, composition, or state of stress in the crust beneath the eastward propagating SRPVP, or, perhaps, changes in the Yellowstone hot-spot plume that may drive the SRPVP volcanism.

Sequence, age, and source of silicic fallout tuffs in middle to late Miocene basins of the northern Basin and Range province

The latest Cenozoic (<6 Ma) ash beds in the western United States have been intensively studied for several decades. The more widespread of these ash beds are well-documented event horizons that are of great value in studies of the timing and pace of geological, climatological, and biological events throughout the region. Because explosive volcanism was not restricted to latest Neogene time in this region, many older ash beds are likely to prove as useful as younger beds as event horizons, once they are located, characterized, and dated. As a first step in developing a useful chronology of older Cenozoic ash beds in the western United States, we have sampled and analyzed silicic fallout tuffs in middle to late Miocene sedimentary basins across the northern Basin and Range province. The northern Basin and Range basins, ideally situated in the vicinity of major coeval silicic volcanic centers, contain numerous relatively unaltered, silicic fallout tuffs. We have correlated tuffs between all sampled sections on the basis of glass shard composition. The composite stratigraphic sequence established by the correlations contains more than 200 individual tuffs, including 59 widely distributed tuffs termed correlative tuffs. The tuffs vary widely in composition, but most are in one of two compositional groups: gray metaluminous vitric tuffs (Gm tuffs) or white metaluminous vitric tuffs (Wm tuffs). Distribution patterns, compositional characteristics, and correlation with ash-flow tuffs show that the source for most Gm tuffs was the Snake River Plain volcanic province along the northern edge of the northern Basin and Range, and the source for most Wm tuffs was the southwestern Nevada volcanic field in the southern part of the northern Basin and Range. The northern Basin and Range tuffs range in age from ca. 16–6 Ma. The ages of individual tuffs are determined variously by direct isotopic dating, by correlation to previously dated fallout and ash-flow tuffs, or by interpolation age estimation. Ages for most tuffs are known to within 0.25 m.y. (1σ) or less and for many tuffs to within 0.1 m.y. or less. The sequence and ages of tuffs established in this study provide insights into the evolution of the northern Basin and Range basins and patterns of explosive volcanism in coeval volcanic centers, and contribute to the development of a high-resolution stratigraphy and chronology of coeval sedimentary deposits throughout the western United States.

Phosphate Minerals in Terrestrial Igneous and Metamorphic Rocks

Phosphorus is a ubiquitous although minor component of most igneous and metamorphic rocks. Under the normal oxidative conditions of the Earth’s crust and upper mantle, phosphorus is present as phosphates, and Koritnig (1978) lists over 200 phosphate minerals. Nonetheless, in common igneous and metamorphic rocks phosphate mineral diversity is extremely limited and is dominated by the occurrence of apatite. This chapter is devoted to phosphates in terrestrial igneous and metamorphic rocks in the strictest sense. Metasomatic, hydrothermal, ore-forming and pegmatite associations are not treated; the latter has been described in particular detail by Moore (1973). Accordingly, considerable attention is paid to apatite; other phosphates discussed include monazite, xenotime and whitlockite. Also omitted is any discussion of crystallography which is provided by Moore (this Vol.).

Lake Cycles in the Bonneville Basin, Utah

A paleomagnetically dated core through the basin floor near Great Salt Lake reveals a remarkably complete Pleistocene and late Pliocene continental record. The Blake event, the Brunhes-Matuyama boundary, and the Bishop and Pearlette “O” ashes provide time lines in the upper third of the core. The Bishop ash is found to be about 15,000 yrs younger than the Brunhes-Matuyama boundary. During the last 800,000 yrs, about 28 episodes of subaerial exposure interrupted lacustrine deposition at the core site. Comparison with a composite central European loess section and several deep-sea cores shows good correlation of inferred climatic fluctuations at the Brunhes-Matuyama boundary and in the late Brunhes, but not in the early Brunhes.

An Appalachian isochron: a kaolinized Carboniferous air-fall volcanic-ash deposit (tonstein)

The Fire Clay tonstein is a kaolinized, airfall volcanic ash bed that was deposited in a widespread late Carboniferous peat-forming mire. Eleven samples from Kentucky and West Virginia, spanning a distance of 200 km, and two samples from Tennessee and Virginia indicate a characteristic mineralogical signature, as compared with other Appalachian tonsteins, consisting of well-crystallized kaolinite, beta-quartz crystal paramorphs, sanidine, ilmenite, zircon, and brookite. Detrital illite and quartz are rarely present or are in very small amounts, which indicates rapid deposition in a mire. Several normal graded cycles in this tonstein suggest repeated episodes of pyroclastic activity that produced a composite ash layer. A high-silica alkalic rhyolitic source is suggested by the geochemistry of immobile elements and by electron-probe analyses of glass inclusions in volcanic quartz from the Fire Clay tonstein. The rare-earth-element plots (chondrite normalized) of the tonstein show a pronounced negative Eu anomaly and relatively high concentrations of Zr and Th, which are both indicative of a rhyolitic source. Probe analyses of the Fire Clay glass inclusions from four states indicate a chemically identical high-silica rhyolite with peraluminous affinities. 40 Ar/ 39 Ar sanidine plateau dating indicates an age of 312 ± 1 Ma for the Fire Clay tonstein, which is consistent with previous 40 Ar/ 39 Ar dates for this tonstein. This age is in agreement with a late Westphalian B age in the European Carboniferous chronostratigraphy on the basis of an age of 311 Ma for the Westphalian B/C boundary. A new isopachous map of the Fire Clay ash-fall deposit indicates an area of 37,000 km 2 and a probable source to the present-day southwest. The deposit has a minimum preserved compacted volume of 2.8 km 3 , which corresponds to an original uncompacted volume of about 20 km 3 . This preserved volume indicates an ultraplinian volcanic explosion. Pindell and Dewey (1982) proposed an Andean-type arc in this block during the late Carboniferous, prior to South American-North American plate collision. We hypothesize an associated back-arc caldera system in the Yucatan block to explain the high-silica, potassic rhyolitic ash that gave rise to the Fire Clay tonstein.

Sequence stratigraphy of lacustrine deposits: A Quaternary example from the Bonneville basin, Utah

The late Quaternary lacustrine sedimentary record in the Bonneville lake basin in the eastern Great Basin provides an excellent example of sequence stratigraphy. Two sequences, referred to as the Little Valley and Bonneville Alloformations, are exposed in the bluffs of the Sevier River where it has entrenched its Pleistocene delta between Leamington and Delta, Utah. Both alloformations contain offshore marl units and fine-grained deltaic or underflow-fan deposits. They can be identified and mapped by tracing the unconformity separating them and employing a number of geochronometric tools, including amino acid epimerization in fossil gastropods, radiocarbon and thorium-230 ages, and basaltic tephrochronology. Thin transgressive sand of the Little Valley Alloformation is overlain by deeper-water marl and down-lapping regressive-phase deltaic silt. The Bonneville Alloformation lies unconformably above the Little Valley deposits. Fine-grained deltaic sediments deposited during the transgressive phase of Lake Bonneville fill the entrenched Sevier River valley that was eroded subsequent to the Little Valley lake cycle. Marl deposited during the deep-water phase is overlain by down-lapping deposits of the regressive phase below the Provo shoreline but is the uppermost unit in the altitudinal range where the lake was lowered catastrophically during the Bonneville Flood. The sequence stratigraphic interpretation leads to the conclusion that the Sevier River delta as a whole is probably made up of a number of sediment sequences, each composed of several facies. Recognition of this complexity could be important in potential applications of the stratigraphic model.

Mineralogy and Petrology of the Iron Hill Carbonatite Complex, Colorado

Bulk rock and mineral chemistry of the rock types occurring in the Iron Hill carbonatite complex have been examined in order to determine the genetic link between carbonatites and the alkaline igneous rocks with which they are ubiquitously associated. A variety of silicate rock types, including pyroxenite, uncom-pahgrite, ijolite, and nepheline syenite is developed at Iron Hill. The carbonatites are composed dominantly of calcite, ankeritic dolomite, or siderite. Only a rare variety of calcite carbonatite contains significant amounts of silicate minerals. Silicate minerals in the calcite carbonatite are generally intermediate in composition to those occurring in ijolite and nepheline syenite. These data, together with the scarcity of rock types gradational between silicate and carbonate varieties, suggest the possible separation of a discrete carbonate fluid phase during the latter part of the crystallization of the silicate series postdating formation of the ijolite but prior to the crystallization of the nepheline syenites. Calcite-dolomite and pyrite-pyrrhotite pairs indicate temperatures of crystallization of late-stage carbonatites of the order of 435° C to 290° C at sulfur fugacities of 10 −7.8 b and oxygen fugacities of the order of 10 −25 to 10 −26 b.

High-fluorine rhyolite: An eruptive pegmatite magma at the Honeycomb Hills, Utah

The Honeycomb Hills rhyolite dome in western Utah displays chemical and mineralogical features characteristic of a rare-element pegmatite magma. The lavas show extreme enrichments in such trace elements as Rb (≤1960 ppm), Cs (≤78), Li (≤344), Sn (≤33), Be (≤270), and Y (≤156). Phenocrysts (10%-50% by volume) include sanidine (Or 66-70 ), plagioclase (Ab 83-92 ), quartz, biotite approaching fluorsiderophyllite, and fluortopaz, as well as accessory phases common to highly differentiated granites and pegmatites, including zircon, thorite, fluocerite, columbite, fergusonite, and samarskite. Low temperatures (600 to 640 °C), coupled with high phenocryst and silica content, might normally preclude eruption due to the extremely high viscosity of the melt. However, high concentrations of fluorine (2%-3%) could domal lavas significantly reduce viscosity and allow eruption of domal lavas even after dewatering of the mama during the initial pyroclastic phase of the eruptive cycle. Fractionation of phenocrysts and accessory phases, for which partition coefficients have been measured, is sufficient to account for most compositional gradients inferred in the preeruptive magma body, although transport by a fluid phase formed a may have caused upward enrichments in Li, Be, and Cs. If the Honeycomb Hills magma had crystallized at depth, it would have formed a rare-element pegmatite.

Petrology of Tertiary and Quaternary volcanic rocks, Washington County, southwestern Utah

In Washington County, southwestern Utah, a thick sequence of volcanic rocks ranges in age from latest Oligocene to Holocene. The Tertiary units consist predominantly of ash flows of intermediate calc-alkalic composition, whereas the youngest volcanic rocks are mostly basaltic andesites. Thermochemical calculations indicate that some of the mafic lavas could be derived from melting of spinel peridotite within the upper part of the low-velocity zone at depths of 50 to 65 km. Some basaltic andesites contain megacrysts of quartz; if the quartz is cognate, the magmas may have formed by partial fusion of quartz eclogite at depths of approximately 120 to 150 km. We suggest that this quartz eclogite is a remnant of the Farallon plate, which underwent subduction at the time of calc-alkalic Tertiary volcanism.