Abstract Two small monogenetic volcanoes are exposed at Mount Early and Sheridan Bluff, in the upper reaches of Scott Glacier. In addition, the presence of abundant fresh volcanic detritus in moraines at two other localities suggests further associated volcanism, now obscured by the modern Antarctic ice sheet. One of those occurrences has been attributed to a small subglacial volcano only c. 200 km from South Pole, making it the southernmost volcano in the world. All of the volcanic outcrops in the Scott Glacier region are grouped in a newly defined Upper Scott Glacier Volcanic Field, which is part of the McMurdo Volcanic Group (Western Ross Supergroup). The volcanism is early Miocene in age ( c. 25–16 Ma), and the combination of tholeiitic and alkaline mafic compositions differs from the more voluminous alkaline volcanism in the West Antarctic Rift System. The Mount Early volcano was erupted subglacially, when the contemporary ice was considerably thicker than present. By contrast, lithologies associated with the southernmost volcano, currently covered by 1.5 km of modern ice, indicate that it was erupted when any associated ice was either much thinner or absent. The eruptive setting for Sheridan Bluff is uncertain and is still being investigated.

Abstract Salt-sediment interaction influence the stratigraphic architecture of depositional sequences during the passive margin stage of development of the Adelaide Geosyncline in South Australia. This late Precambrian to Early Cambrian succession outcrops in the Flinders ranges and is punctuated by submarine unconformities. The unconformities are interpreted as deep-water sequence boundaries that have no shallow water equivalent. However, they can be traced laterally into condensed sections or major flooding surfaces. These sequence boundaries are typically overlain by deep-water facies that reflect the high rate of subsidence coupled with high sedimentation rates typically found in halotectonic mini-basins. Large-scale slumping and canyon incision often mark the deep-water sequence boundaries. Canyon incision is associated with salt withdrawal or downbuilding in mini-basins. The architecture of submarine canyons is defined by the nature and correlation of bounding surfaces, sandstone interconnectivity and lithofacies elements. These features are often beyond the limits of seismic resolution in basins where passive margin development was influenced by salt tectonics.

Abstract Holocene volcanic deposits from Mount Baker are plentiful in the low-lying Baker River valley at the eastern foot of the volcano. Tephra set SC (8850 yr B.P.), erupted from the nearby Schreibers Meadow cinder cone, is sporadically present. Exposures of both subaerial and subaqueous facies of the associated Sulphur Creek basalt lava flow are easy to access; the lava, the most mafic product known from the entire Mount Baker volcanic field, entered Glacial Lake Baker, invaded lacustrine sediments, and formed peperites as well as subaqueous block-and-ash flows. A volcaniclastic delta was deposited in the lake above the lava. The peperite and delta can be seen in the walls of Sulphur Creek, and in the banks of Baker Lake when the reservoir is drawn down in winter and early spring. The best exposures of volcaniclastic flank assemblages from Mount Baker are found in the Baker River valley. The Boulder Creek assemblage formed a thick fan between the end of the Vashon glaciation and the deposition of the SC tephra. Now deeply trenched by Boulder Creek, lahar and block-and-ash diamicts can be seen with some effort by ascending the creek 2 km. A tiny vestige is exposed along the Baker Lake Road. Much younger deposits are also accessible. In 1843, tephra set YP, erupted from Sherman Crater, was deposited in the valley. In ca. 1845–1847, the Morovitz Creek lahar swept down Boulder, P.r., Morovitz, and Swift Creeks and inundated much of the current location of the Baker Lake reservoir. This lahar is an example of the most likely future hazard at Mount Baker as well as the most common type of lahar produced during the Holocene at the volcano—clay-rich or cohesive lahars initiated as slope failures from hydrothermally altered rock. They commonly increase in volume by entraining sediment as they flow. When thermal emissions from Sherman Crater increased in 1975–1976, the level of the reservoir was lowered to accommodate inflow of lahars such as the Morovitz Creek lahar. Renewed activity at Sherman Crater will again trigger reservoir drawdown. In 1890–1891, and again ca. 1917–1932, debris avalanches from pre–Mount Baker lavas flowed down Rainbow Creek. The largest, which flowed 10.5 km, can be visited at the Rainbow Falls overlook. Here, the peak discharge of the flow, derived from reconstructed cross sections defined by well-exposed lateral levees and from reported velocities of equivalent modern flows, is estimated to have been greater than the peak discharge of any historic flood in the Mississippi River.

Abstract Clay-poor lahars of late Holocene age from Mount Rainier change down the White River drainage into lahar-derived fluvial and deltaic deposits that filled an arm of Puget Sound between the sites of Auburn and Seattle, 110–150 km downvalley from the volcano’s summit. Lahars in the debris-flow phase left cobbly and bouldery deposits on the walls of valleys within 70 km of the summit. At distances of 80–110 km, transitional (hyperconcentrated) flows deposited pebbles and sand that coat terraces in a gorge incised into glacial drift and the mid-Holocene Osceola Mudflow. On the broad, level floor of the Kent Valley at 110–130 km, lahars in the runout or streamflow phase deposited mostly sand-sized particles that locally include the trunks of trees probably entrained by the flows. Beyond 130 km, in the Duwamish Valley of Tukwila and Seattle, laminated andestic sand derived from Mount Rainier built a delta northward across the Seattle fault. This distal facies, warped during an earthquake in A.D. 900–930, rests on estuarine mud at depths as great as 20 m. The deltaic filling occurred in episodes that appear to overlap in time with the lahars. As judged from radiocarbon ages of twigs and logs, at least three episodes of distal deposition postdate the Osceola Mudflow. One of these episodes occurred ca. 2200–2800 cal. yr B.P., and two others occurred ca. 1700–1000 cal. yr B.P. The most recent episode ended by about the time of the earthquake of A.D. 900–930. The delta’s northward march to Seattle averaged between 6 and 14 m/yr in the late Holocene.

Abstract This study discusses the petrological and geochemical features of two monogenetic Miocene volcanoes, Mount Early and Sheridan Bluff, which are the above-ice expressions of Earth's southernmost volcanic field located at c. 87° S on the East Antarctic Craton. Their geochemistry is compared to basalts from the West Antarctic Rift System to test affiliation and resolve mantle sources and cause of melting beneath East Antarctica. Basaltic lavas and dykes are olivine-phyric and comprise alkaline (hawaiite and mugearite) and subalkaline (tholeiite) types. Trace element abundances and ratios (e.g. La/Yb, Nb/Y, Zr/Y) of alkaline compositions resemble basalts from the West Antarctic rift and ocean islands (OIB), while tholeiites are relatively depleted and approach the concentrations levels of enriched mid-ocean ridge basalt (E-MORB). The magmas evolved by fractional crystallization with contamination by crust; however, neither process can adequately explain the contemporaneous eruption of hawaiite and tholeiite at Sheridan Bluff. Our preferred scenario is that primary magmas of each type were produced by different degrees of partial melting from a compositionally similar mantle source. The nearly simultaneous generation of lower degrees of melting to produce alkaline types and higher degrees of melting forming tholeiite was most likely to have been facilitated by the detachment and dehydration of metasomatized mantle lithosphere.