The basal Great Valley sequence in Napa and southern Lake Counties, California, is a mappable chaotic unit composed largely of ophiolitic debris. Serpentinite flows and breccias, mafic breccias and associated finer-grained clastic rocks, and blocks of extrusive greenstone, mafic breccia, chert, bedded and unbedded clastic sedimentary rocks, phyllites, actinolitic greenschists, and hornblende amphibolites are mixed with Great Valley sequence mudstone and serpentinous mudstone. The chaotic unit extends along strike for at least 50 km. Cross-sections indicate that it extends for at least 20 km across strike and is up to 1 km thick. It is involved in complex folds caused by imbricate thrust faulting. The unit lies directly above the serpentinite that represents the Coast Range Ophiolite within the study area and below the well-bedded Great Valley sequence of Upper Jurassic and Cretaceous age. Its lower contact is enigmatic but is probably depositional; the upper contact is sheared and gradational. Locally the unit represents the entire Tithonian Stage. Ophiolitic detritus in the lower Great Valley sequence is also found elsewhere in the Northern California Coast Ranges—near the Geysers, in Rice Valley, near Wilbur Springs, along the Bartlett Springs Road near Walker Ridge, near Cooks Springs, and at Crowfoot Point west of Paskenta. Other accumulations of ophiolitic debris are inter-layered in the Great Valley sequence at various stratigraphic levels in and near the study area. This detritus takes four forms, which may be mixed together: (1) sedimentary serpentinite debris flows; (2) mafic breccias; (3) basaltic sandstones; and (4) polymict, polymorphous chaotic units with blocks-in-matrix texture, like the rocks in Napa County described here. Widespread detrital textures and the common occurrence of spaced, rather than penetrative, shear foliation in its matrix demonstrate that the chaotic unit in Napa County is not a tectonic melange, and I interpret it to be an amalgam of olistostromes. These ophiolitic olistostromes are a facies distinct from the overlying turbidites. Thus, the basal Great Valley sequence in this area is composed of two different rock types: very proximal ophiolitic debris flows, and substantially more distal subsea-fan rocks derived from a volcanic arc. Ophiolites may form at mid-ocean ridges, in back-arc or forearc basins, or in island arcs. Ophiolitic detritus may be eroded and deposited on ophiolitic basement in any environment in which the oceanic crust is deformed. The geology of surrounding terranes and the petrologic features of the ophiolitic basement below the Great Valley sequence suggest that the basement was formed in a back-arc basin. The stratigraphy of the chaotic rocks that overlie the basement in Napa County suggests that they were deposited on deeply eroded basement in a technically active forearc basin. Large volumes of rock stuffed under the hanging-wall slab after the onset of subduction may have uplifted the forearc basin and subjected its basement to erosion. A wave of uplift may have passed across the basin, so that debris shed from eroding oceanic basement was deposited directly on freshly exposed harzburgite tectonite. Some blocks may have been carried completely across the forearc basin and into the trench, and incorporated into the Franciscan melange wedge, which is also rich in ophiolitic blocks. The change from back-arc to forearc basin was probably caused by collisional tectonics and the establishment of a new subduction zone off the western coast of California during the Late Jurassic Nevadan orogeny. Stratigraphic relationships in and above the Coast Range Ophiolite are unusual through much of the Northern Coast Ranges. Nearly complete ophiolites are the exception rather than the rule, and in many areas only serpentinite is present. In some areas, ophiolitic debris different from that described here overlies the serpentinite. In other areas, arc-derived submarine fan rocks of the Great Valley sequence directly overlie serpentinized harzburgite tectonite. The relationships described here suggest that many of these contacts are not tectonic, and that the Coast Range Ophiolite does not owe its fragmentary nature to tectonic dismemberment. Rather, it is likely that the ophiolitic basement below the Great Valley sequence was deeply eroded during Mesozoic time. Many of the contacts throughout the Coast Ranges along which sedimentary rocks overlie serpentinite—which must represent deep layers of the oceanic crust or the upper mantle—are in the main nonconformities.

Abstract This field-trip guide outlines the glacial history of the upper Arkansas River valley, Colorado, and builds on a previous GSA field trip to the area in 2010. The following will be presented: (1) new cosmogenic 10 Be exposure ages of moraine boulders from the Pinedale and Bull Lake glaciations (Marine Isotope Stages 2 and 6, respectively) located adjacent to the Twin Lakes Reservoir, (2) numerical modeling of glaciers during the Pinedale glaciation in major tributaries draining into the upper Arkansas River, (3) discharge estimates for glacial-lake outburst floods in the upper Arkansas River valley, and (4) 10 Be ages on flood boulders deposited downvalley from the moraine sequences. This research was stimulated by a new geologic map of the Granite 7.5′ quadrangle, in which the mapping of surficial deposits was revised based in part on the interpretation of newly acquired LiDAR data and field investigations. The new 10 Be ages of the Pinedale terminal moraine at Twin Lakes average 21.8 ± 0.7 ka ( n = 14), which adds to nearby Pinedale terminal moraine ages of 23.6 ± 1.4 ka ( n = 5), 20.5 ± 0.2 ka ( n = 3), and 16.6 ± 1.0 ka ( n = 7), and downvalley outburst flood terraces that date to 20.9 ± 0.9 ka ( n = 4) and 19.0 ± 0.6 ka ( n = 4). This growing chronology leads to improved understanding of the controls and timing of glaciation in the western United States, the modeling of glacial-lake outburst flooding, and the reconstruction of paleotemperature through glacier modeling.

Abstract The Workshop Lecture held in association with The Clay Mineral Society annual meeting in Billings, MT included a two-day field trip to Yellowstone National Park. The scheduled stops (Figure 1 ) focus on the variety of hydrothermal features, although the route takes us through an area that all too few visitors to the Park ever experience, a drive over the Beartooth Mountains of Wyoming and Montana. This is one of the most spectacular drives in the continental United States. Here you will see (Stop 1) a glaciated, alpine tundra terrain with some of the oldest rocks exposed in North America. From there we will descend to the small community of Cook City, MT, adjacent to the northeast entrance of the Park. After driving in the Park, the trip will head along the eastern loop road, south towards Canyon Village with a stop at the new Canyon Educational Center (Stop 2). This center, which opened in the summer of 2006, has a variety of interactive exhibits that emphasizes the geology of the national park. After leaving Canyon Village, we make our last field stop of the day at Mud Volcano (Stop 3). Here we see a variety of acid geothermal springs, fumaroles, and mud pots. All are the product of a steam-dominated hydrothermal system. With a little luck, we will also see some bison. They frequent this area, migrating from Hayden Valley to the south. The remainder of our journey will take us in part along Yellowstone Lake to the

Abstract White Island lies at the north tip of the TVZ and is among the most active and accessible andesite-dacite volcanoes in the world. It is also perhaps one of the best understood, having been intensively studied in terms of both its physical and chemical characteristics (e.g. Hamiliton and Baumgart, 1959; Giggenbach, 1987; Houghton and Nairn, 1989; Houghton and Nairn, 1991). According to legend, White Island is the source of volcanic-hydrothermal activity in the North Island. One story describes the arrival of Chief Ngatoro-i-rangi who brought fire from Hawaiiki. He left his sisters at Whakaari and travelled south to Tongariro. He found it very cold there and he called his sisters to send fire. The subterranean passage taken by the spirits carrying fire marked a trail of thermal activity, which is the subject of this tour. The first European to record sighting White Island is Captain James Cook, who named it as it appeared to him during his frrst visit to New Zealand in 1769. The island is made up of two overlapping cones, an older western cone and a younger central cone. The main crater occupies the eastern and central portions and is breached to the southeast (Fig. 25). The western part of the main crater includes several deep coalescing subcraters, formed about the most active vents. Their deepest point is about 150m below sea level. Fumarolic activity is concentrated in this area, but acid brines periodically discharge from springs further to the east. Baseline studies are in progress for

Abstract Northward Ho! In the spring of 1936, Carter Oil Co. created a new crew to work in Canada for Imperial Oil Co., an affiliate of Standard Oil N.J. (Exxon). I had graduated from “trainee” status and was chosen assistant interpreter on this crew. I was also now allowed an expense account, fixed at a whopping $75/month. In actuality, I was also the assistant operator and assistant shooter, as we took only five trained positions to Canada and filled out the personnel with local citizens. We considered ourselves to be the first professional seismograph crew to operate in Canada. Previous seismic work had been done in Alberta, but that was by a crew from a US college which had been composed of upper classmen and graduate students. We based our claim of “professionalism” on our many years of collective experience. We proved our point when we shot the six-month assignment in six weeks. This was considered a foreign assignment, more or less. Although the language was English (almost), most everyday products (cigarettes, candy, magazines, clothing) were quite different from those to which we were accustomed. Our party chief, the only married man on the crew, lived in a furnished apartment. The four singles were provided a house and cook, known as “Cooksie” (what else?). He was a transplanted Cockney and had lost none of his accent. Our crew came from Louisiana, Texas, Oklahoma, and Kansas so we spent the first month trying to interpret his “English” and he ours. After that,

Limulus polyphermis is an estuarine species ranging from Maine to Yucatan. Within the Gulf of Mexico large populations are found only along the coasts of Florida, Yucatan, and Campeche, i.e., only along the “entrances” to the gulf. It is a bottom feeder, burrowing into the substrate for clams and other invertebrates or scavenging over the bottom. Specimens have been dredged during all seasons of the year, throughout most of the range of the species, from “mud” and sand bottoms at depths up to 12.5 fathoms in water ranging from 12.5° C to 28.5° C and from 11 to 30.6‰ in salinity. In the northern part of its range Limulus apparently remains buried in the bottom most of the winter. Large concentrations of adult limuli and the immature stages and their exuviae are common in many estuaries of the United States. The adults migrate shoreward during the spring and summer, spawning at high tide along the beaches. The large numbers of mating animals reported by Cook (1857) have been much reduced by the extensive fishing of these animals for fertilizer. Concentrations of larvae are found in the nests on the beaches (Battey, 1883); young limuli on intertidal flats (Townes, 1938). The exuviae sometimes collect by the thousands, even millions, in the beach wrack during the summer. Supposing that similar large concentrations of the adults, young, and exuviae occurred in past ages, it is interesting to note that the extinct Limulidae are apparently only well represented in the lithographic limestone of the...