Franklin F. Foit

The age of glacier peak tephra in west-central Montana

At Sheep Mountain Bog, near Missoula, Montana, a late-glacial tephra, that probably fell in late summer, is preserved as an 8-mm-thick graded bed overlain by another 8 mm of redeposited ash mixed with lake deposits. Sediment surrounding the ash was 14C dated to about 11,200 yr B.P. Electron-microprobe analyses of the volcanic glass and hornblende phenocrysts from this ash layer indicate that they are similar in major-element chemistry to those of Glacier Peak layer G previously considered to be about 12,000 yr old or older.

Paleomagnetic and tephra evidence for tens of Missoula floods in southern Washington

Paleomagnetic secular variation and a hiatus defined by two tephra layers confirm that tens of floods from Glacial Lake Missoula, Montana, entered Washington’s Yakima and Walla Walla Valleys during the last glaciation. In these valleys, the field evidence for hiatuses between floods is commonly subtle. However, paleomagnetic remanence directions from waterlaid silt beds in three sections of rhythmically bedded flood deposits at Zillah, Touchet, and Burlingame Canyon display consistent secular variation that correlates serially both within and between sections. The secular variation may further correlate with paleomagnetic data from Fish Lake, Oregon, and Mono Lake, California, for the interval 12,000‐17,000 14 C yr B.P. Deposits of two successive floods are separated by two tephras derived from Mount St. Helens, Washington. The tephras differ in age by decades, indicating that a period at least this long separated two successive floods. The beds produced by these two floods are similar to all of the 40 beds in the slack-water sediment sequence, suggesting that the sequence is a product of tens of floods spanning a period of perhaps a few thousand years.

Fe2+ -F avoidance in silicates

Abstract Fe 2+ -F avoidance, reported in the literature in micas and amphiboles, can be accounted for by crystal field theory. The crystal field splitting parameter, Δ O , of Fe 2+ octahedrally coordinated to F − is significantly smaller than its value when (OH) − is the coordinating anion. Thus, the presence of Fe 2+ is not favored at sites where F − substitutes for (OH) − due to smaller crystal field stabilization energy.

Composition, Fe3+/∑Fe, and crystal structure of non-asbestiform and asbestiform amphiboles from Libby, Montana, U.S.A.

Abstract Compositional data and Fe3+/ΣFe ratios obtained by electron microprobe and Mössbauer analyses are given for a suite of three amphibole and amphibole-asbestos samples collected from the former vermiculite mine near Libby, Montana. A crystal structure analysis, compositional data, and Fe3+/ΣFe values for two samples from a previous study are also reported. The results confirm the conclusion drawn in the previous study that these amphiboles are dominantly compositions ranging from winchite to richterite. Mössbauer spectroscopy yielded Fe3+/ΣFe ratios from 58% to 72% for the five samples. The crystal structure was determined for a single crystal selected from a bulk sample. Its formula (as determined by electron microprobe analysis and Mössbauer spectroscopy) is (K0.19 Na0.32)A(Na0.85 Ca1.12Mn0.03)B(Mn0.01Mg4.43Fe3+ 0.34Fe2+0.19Ti0.01Al0.02)(Al0.03Si7.97O22)(OH1.63F0.37). The refinement was carried out based on space group C2/m, with a = 9.879(2), b = 18.024(3), c = 5.288(1) Å, b = 104.377(3)° and using data collected at room temperature. Mg is partitioned among the M1, M2, and M3 sites. All of the Fe3+ occupies M2, while Fe2+ is split between M2 and M3; Ca and Na fill the M4 site, while Na and K occupy the partially filled A site. The A-site occupancy is calculated as 0.51 based on chemical data, but only 0.48 based on X-ray diffraction results. Minerals with the former values would be classified as richterite and those with the latter as winchite.

A paleoclimate record for the past 250,000 years from Summer Lake, Oregon, USA: I. Chronology and magnetic proxies for lake level

This study presents the age control and environmental magnetism components of a new, late Pleistocene paleoclimate record for the Great Basin of western North America. Two new cores from the Summer Lake sub-basin of pluvial Lake Chewaucan, Oregon, USA are correlated to basin margin outcrops on the basis of tephrochronology, lithostratigraphy, sediment magnetism and paleomagnetic secular variation. Eleven tephra layers were found in the cores that correlate to tephra identified previously in the outcrop. The Olema ash was also found in one of the cores; its stratigraphic position, relative to 3 dated tephra layers, indicates that its age is 50-55 ka, somewhat younger than has been previously reported. The Summer Lake sediments are divided into deep and shallow lake lithosomes based on sedimentary features. The stratigraphic position of these lithosomes support the tephra-based correlations between the outcrop and the cores. These sediments contain a well resolved record of the Mono Lake Excursion (MLE) and an earlier paleomagnetic excursion as well as a high quality replication of the paleosecular variation immediately above the MLE.Relative sedimentation rates increased dramatically toward the depocenter during intervals of low-lake level. In contrast, during intervals of high-lake level, relative sedimentation rates were comparable along the basin axis from the basin margin to the depocenter. The magnetic mineralogy of the Summer Lake sediments is dominated by pseudo-single domain (titano)magnetite and intervals of high/low magnetite concentration coincide with lithosomes that indicate high/low lake levels. Magnetic grain size also varies in accord with bulk sediment grain size as indicated by the silt/clay ratio. To a first order, variations in magnetic parameters, especially those attributable to the concentration of magnetic minerals, correlate well with global glacial/interglacial oscillations as indicated by marine oxygen isotope stages. This relationship can be explained by increased dissolution of (titano)magnetite minerals as lake level dropped and the lake became more productive biologically. This inference is supported by a correspondence between lower concentrations of magnetite with higher levels of total organic carbon and vice-versa.

Lithostratigraphy, Tephrochronology, and Rare Earth Element Geochemistry of Fossils at the Classical Pleistocene Fossil Lake Area, South Central Oregon

One of the most famous fossiliferous Pleistocene sites in the Pacific Northwest is Fossil Lake, Oregon. Until recently, fossil collections from the area were not stratigraphically controlled, owing to the lack of a detailed stratigraphic and chronologic framework. Our field studies reveal at least nine exposed thin rhythmic fining‐upward depositional packages, most separated by disconformities. Analysis of interbedded tephras reveals that the Rye Patch Dam (∼646 ka), Dibekulewe (∼610 ka), Tulelake T64 (∼95 ka), Marble Bluff (47 ka), and Trego Hot Springs (23.2 ka) tephra layers are present in the section, indicating deposition from more than ∼646 ka to less than 23 ka, which includes both the late Irvingtonian and Rancholabrean North American land mammal ages, a much longer time span than previously believed. Bones analyzed from eight of the defined units have distinctly different rare earth element (REE) signatures. Fossils obtain REE during early diagenesis, and signatures are probably closely related to lake water compositions. REE signatures in fossils from lower packages suggest uptake from neutral pH waters. In contrast, REE signatures become increasingly heavy REE–enriched up‐section, with positive Ce anomalies in the upper units. REE signatures in fossils from the upper units are very similar to waters from modern alkaline lakes, such as Lake Abert, Oregon, suggesting diagenetic uptake in increasingly alkaline and saline waters. These REE changes suggest increasing aridity up‐section, a contention reinforced by the habitat preferences of the terrestrial vertebrates preserved.

The stability of transition metal dolomites in carbonate systems: a discussion

Abstract The instability of transition metal dolomites [CaR 2+ (CO 3 ) 2 where R 2+ is Fe, Co, Ni, Cu, or Zn] and the limited substitution of transition metal cations for Mg in the dolomite structure can be accounted for by the effect of octahedral distortion. For example, trigonal elongation of the Fe octahedron, due to the Jahn-Teller effect, observed in siderite and ankerite, results in elongation of the Ca octahedron which is sensitive to distortion because the radius of Ca 2+ is close to the upper limit for octahedral coordination. Co, Ni, Cu, Zn octahedra are also thought to be deformed, relative to Mg octahedra, in carbonates. The free energy of formation (ΔG o f ) of R 2+ CO 3 becomes more positive with increasing octahedral distortion. Estimated ΔG o f(dolomite) as well as stabilities and solubility limits of R 2+ in natural and synthetic dolomites suggest a series in order of decreasing stability: Mg > Mn > Zn > Fe > Co > Ni > Cu . ΔG o f(est.) for the terminal Fe-dolomite solid solution [72 mol% CaFe(CO 3 ) 2 ] in the system CaCO 3 -MgCO 3 -FeCO 3 may represent an empirical threshold value for dolomite stability which lies between ΔG o f for Mn- and Zn-dolomites. While Zn-dolomite is probably not a stable phase, very extensive solid solution toward CaZn(CO 3 ) 2 is to be expected in the system CaCO 3 -MgCO 3 -ZnCO 3 . The tendency for transition metal dolomites to contain excess CaCO 3 can also be accounted for in terms of octahedral distortion and AG o f .

The crystal structure of gillulyite, Tl 2 (As,Sb) 8 S 13 , from the Mercur gold deposit, Tooele County, Utah, U.S.A.

The crystal structure of manandonite-2H2, an AI-, Li-, B-rich analogue of amesite, has been refined in space group C1 to R = 5.7%. Tetrahedral Si, AI, and B are partly ordered to give two mean T-O bond lengths near 1.603(1) A and two near 1.667(1) A. B has a greater tendency to order than do Si and Al. There are one relatively Li-rich octahedron and two relatively Al-rich octahedra in each layer with mean M-O,OH bond lengths of 1.997(1), 1.960(1), and 1.955(1) A, respectively, in layer 1 and 2.014(1), 1.941(1), and 1.956(1) A, respectively, in layer 2. It is the partial ordering of the octahedral cations that decreases the ideal P63 symmetry to PI (or C1, as used for refinement). Tetrahedral rotations of 18.3° are required to match the larger lateral dimensions of the tetrahedral sheet with those of the octahedral sheet, coupled with basal 0 corrugations because ofthe different sizes of the octahedra. The H+ protons of the six surface OH molecules point directly toward their acceptor basal 0 atoms to give H bond contacts between 2.647 and 2.773 A.

Holocene tectonics and fault reactivation in the foothills of the north Cascade Mountains, Washington

We use LiDAR imagery to identify two fault scarps on latest Pleistocene glacial outwash deposits along the North Fork Nooksack River in Whatcom County, Washington (United States). Mapping and paleoseismic investigation of these previously unknown scarps provide constraints on the earthquake history and seismic hazard in the northern Puget Lowland. The Kendall scarp lies along the mapped trace of the Boulder Creek fault, a south-dipping Tertiary normal fault, and the Canyon Creek scarp lies in close proximity to the south-dipping Canyon Creek fault and the south-dipping Glacier Extensional fault. Both scarps are south-side-up, opposite the sense of displacement observed on the nearby bedrock faults. Trenches excavated across these scarps exposed folded and faulted late Quaternary glacial outwash, locally dated between ca. 12 and 13 ka, and Holocene buried soils and scarp colluvium. Reverse and oblique faulting of the soils and colluvial deposits indicates at least two late Holocene earthquakes, while folding of the glacial outwash prior to formation of the post-glacial soil suggests an earlier Holocene earthquake. Abrupt changes in bed thickness across faults in the Canyon Creek excavation suggest a lateral component of slip. Sediments in a wetland adjacent to the Kendall scarp record three pond-forming episodes during the Holocene—we infer that surface ruptures on the Boulder Creek fault during past earthquakes temporarily blocked the stream channel and created an ephemeral lake. The Boulder Creek and Canyon Creek faults formed in the early to mid-Tertiary as normal faults and likely lay dormant until reactivated as reverse faults in a new stress regime. The most recent earthquakes—each likely M w > 6.3 and dating to ca. 8050–7250 calendar years B.P. (cal yr B.P.), 3190–2980 cal. yr B.P., and 910–740 cal. yr B.P.—demonstrate that reverse faulting in the northern Puget Lowland poses a hazard to urban areas between Seattle (Washington) and Vancouver, British Columbia (Canada).

Active faulting on the Wallula fault zone within the Olympic-Wallowa lineament, Washington State, USA

The Wallula fault zone is an integral feature of the Olympic-Wallowa lineament, an ∼500-km-long topographic lineament oblique to the Cascadia plate boundary, extending from Vancouver Island, British Columbia, to Walla Walla, Washington. The structure and past earthquake activity of the Wallula fault zone are important because of nearby infrastructure, and also because the fault zone defines part of the Olympic-Wallowa lineament in south-central Washington and suggests that the Olympic-Wallowa lineament may have a structural origin. We used aeromagnetic and ground magnetic data to locate the trace of the Wallula fault zone in the subsurface and map a quarry exposure of the Wallula fault zone near Finley, Washington, to investigate past earthquakes along the fault. We mapped three main packages of rocks and unconsolidated sediments in an ∼10-m-high quarry exposure. Our mapping suggests at least three late Pleistocene earthquakes with surface rupture, and an episode of liquefaction in the Holocene along the Wallula fault zone. Faint striae on the master fault surface are subhorizontal and suggest reverse dextral oblique motion for these earthquakes, consistent with dextral offset on the Wallula fault zone inferred from offset aeromagnetic anomalies associated with ca. 8.5 Ma basalt dikes. Magnetic surveys show that the Wallula fault actually lies 350 m to the southwest of the trace shown on published maps, passes directly through deformed late Pleistocene or younger deposits exposed at Finley quarry, and extends uninterrupted over 120 km.