Glenn D. Thackray

Latest Pleistocene alpine glacier advances in the Sawtooth Mountains, Idaho, USA: Reflections of midlatitude moisture transport at the close of the last glaciation

Mountain glaciers in the Sawtooth Mountains responded strongly to reinvigorated atmospheric moisture transport following the last ice-sheet maximum. The glaciers constructed an extensive moraine belt in the southeastern Sawtooth Mountains, allowing determination of a detailed radiocarbon chronology from lake and marsh cores. Moraine morphometry and soil data indicate construction of seven to nine moraines in each of four valleys during two main late Pleistocene ice advances. A basal radiocarbon date from marsh cores in the Alturas valley documents that the maximum advance during marine oxygen isotope stage 2 occurred shortly before ca. 16,900 calibrated (cal.) yr B.P. Minimum limiting dates on glacial-lacustrine sediment in lake and marsh cores from three valleys, clustered around 13,950 cal. yr B.P., document the maintenance or reestablishment of extensive ice volume during early late-glacial time. The ca. 16,900 cal. yr B.P. advance postdates the last ice-sheet maximum by ∼4000 yr and is broadly correlative with maximum advances of the North Yellowstone outlet glacier and the Puget Lobe of the Cordilleran Ice Sheet and a near-maximum advance of Wallowa Mountain glaciers. These synchronous advances indicate response of a variety of ice systems to reinvigorated moisture transport following the ice-sheet maximum, as does the subsequent Sawtooth advance during early late-glacial time. Together, these responses indicate strong sensitivity of certain ice systems to moisture-delivery fluctuations.

EVIDENCE FOR EXPANDED MIDDLE AND LATE PLEISTOCENE GLACIER EXTENT IN NORTHWEST NELSON, NEW ZEALAND

Abstract. The extent of Late Quaternary glaciation in the northwest Nelson region of New Zealand has traditionally been regarded as minor, with small‐scale valley glaciation in confined upland reaches. New geomorphological evidence, including moraines, kame terraces, till‐mantled bedrock and outwash terraces, indicate that greatly expanded valley glaciers flowed into the lowland valley system at the mouths of the Cobb‐Takaka and Anatoki drainages. The timing for this ice advance into lowland valleys is constrained by lowland landform characteristics and a single cosmogenic exposure age, suggesting Late and Middle Pleistocene ice expansion, respectively. Evidence for expanded upland ice on the Mount Arthur Tableland and adjacent areas includes trimlines, boulder trains and roche moutonées. Two cosmogenic exposure ages on upland bedrock surfaces suggest that major ice expansion occurred during MIS 3 and/or 4, while previously published exposure dating from Cobb Valley suggests large MIS 2 ice expansion as well. The inferred, markedly expanded ice left little or no clear geomorphic imprint on the Cobb–Takaka Gorge, and required temperature depression of 4–6°C with near‐modern precipitation levels.

Landslide surveillance: New tools for an old problem

Landslides are one of the most widespread geological hazards on Earth, responsible for hundreds of deaths and billions of dollars in property damage per year. Landslides commonly occur with other natural disasters (e.g., earthquakes, floods) and leave the landscape prone to sedimentation, erosion, and further mass wasting. Remote sensing, the Global Positioning System (GPS),and geographic information systems (GIS) are now mature technologies that can be used to monitor landslides and landslideprone areas with greater accuracy than could be accomplished previously with field reconnaissance alone.

Cycles of Doming and Eruption of the Miocene Kisingiri Volcano, Southwest Kenya

Volcanic cycles of doming and eruption of the Miocene Kisingiri volcano produced three sedimentary cycles recorded in the volcaniclastic strata of Rusinga and Mfangano Islands, Lake Victoria, Kenya. Each of the three cycles began with the deposition of cobble and boulder conglomerates shed from the volcanically domed Precambrian basement, followed by deposition of pyroclastic and volcaniclastic strata, representing nephelinite-carbonatite eruption of the Kisingiri volcano. Volcanogenic strata produced by the first two cycles (lower two-thirds of the Rusinga Group) are predominantly fine-grained tuffs and medium-grained volcaniclastic deposits, indicating alluvial deposition from a low-relief volcanic edifice. The third cycle is dominated by boulder debris flows, lava flows, and minor tuffaceous beds (upper part of the Rusinga Group and overlying Kisingiri Group). This last cycle records the formation of the high-relief Kisingiri stratocone, much of which is preserved in the dissected flanks of the volcano. The first two cycles are recorded in distal apron deposits but are not well preserved in the core of the volcano. Second-order sedimentary cycles, consisting of fining-upward sequences (5-10 m thick) of granular tuffaceous sandstones and conglomerates that fine into siltstones, stacked floodplain paleosols, and airfall tuff beds, dominate the strata of the first two cycles. These fining-upward sequences represent alluvial aggradation that accompanied and followed eruptive episodes that were much shorter in duration than the main cycles of doming and eruption.

Holocene scarp on the Sawtooth fault, central Idaho, USA, documented through lidar topographic analysis

High-resolution lidar data reveal a prominent latest Pleistocene–Holocene scarp on the Sawtooth fault (central Idaho, United States). The fault scarp marks 55–65 km of the range front, and may comprise two segments. The scarp is 4–9 m high in latest Pleistocene glacial landforms (11–14 ka) and 2–3 m high in Holocene alluvial landforms, implying 2–3 postglacial rupture events. Patterns of fault scarp continuity, coupled with existing gravity data, suggest that active faulting may have migrated northward during Pleistocene time. Detailed comparisons of raw lidar digital elevation models (DEMs), bare-earth lidar DEMs, and field surveys indicate that the bare-earth lidar data document the fault scarp morphology accurately and allow for detailed fault analysis where field evaluation is difficult. The documentation of Holocene motion on the Sawtooth fault demonstrates that ENE-directed extension extends across central Idaho, and that the fault contributes to seismic hazards.

Differential Synthetic Aperture Radar Interferometry to Investigate Surface Deformation of the Eastern Snake River Plain, Idaho, U.S.A.

The Eastern Snake River Plain (ESRP) is a northeast-trending volcanic basin that marks the track of the Yellowstone hotspot. Subsidence has characterized the ESRP since at least 4 Ma. To test whether subsidence of the ESRP surface is an active process, synthetic aperture radar interferometry has been applied to detect deformation during 1993–2000. Results show that no regionally consistent deformation occurred across the ESRP during the periods of observations. However, local displacements of 1–3-cm magnitude have been detected in the adjacent Basin-Range province. This deformation is not attributed to long-term movement of the ESRP but instead to local tectonic and/or groundwater processes.

MIS 3 glaciation in the middle Rakaia valley, New Zealand, documented through stratigraphy and luminescence geochronology

ABSTRACT Stratigraphy, geomorphology, and luminescence dating of ice-proximal sediments document extensive MIS 3 Rakaia valley glaciation. Basal outwash contains ice-meltout depressions filled with upward-coarsening and progressively deformed outwash, overlain by undeformed ice-proximal diamicton and outwash. The most reliable luminescence date from basal outwash is 35.2 ± 0.7 ka, consistent with site stratigraphic evolution and overlying exposure ages. The site thus indicates MIS 3 ice of similar extent as the main MIS 3/2 advance and demonstrates that ice retreated at least a short distance up-valley between the two advances. The results correlate with similar ages on ice-proximal lake sediments 10 km up-valley, on distal outwash 65 km down-valley, and with cosmogenic radionuclide dates elsewhere in the Southern Alps. This study thus confirms independently that MIS 3 glaciation was nearly as extensive as the main LGM advances in central South Island, New Zealand.

Systematic variation of Late Pleistocene fault scarp height in the Teton Range, Wyoming, USA: Variable fault slip rates or variable landform ages?

Fault scarps of strongly varying height cut glacial and alluvial sequences mantling the faulted front of the Teton Range (western USA). Scarp heights vary from 11.2 to 37.6 m and are systematically higher on geomorphically older landforms. Fault scarps cutting a deglacial surface, known from cosmogenic radionuclide exposure dating to immediately postdate 14.7 ± 1.1 ka, average 12.0 m in height, and yield an average postglacial offset rate of 0.82 ± 0.13 m/k.y. using simple scarp height (average 11.2 m, offset rate 0.76 ± 0.11 m/k.y. using vertical separation). We apply the offset rate to higher fault scarps to develop preliminary age estimates for the geomorphically older landforms, with an initial assumption of constant offset rate through time. The landform age estimates of 16.2 ± 3.9 ka to 45.9 ± 11.0 ka imply that glaciation and alluviation influenced the range front during marine isotope stages 2 and 3. However, fault offset rate variability, suggested by previous work to be attributable to Yellowstone ice cap deglacial processes, suggests that the fault scarp height pattern might also be interpreted as a reflection of strongly variable offset rates in landforms of only slightly contrasting age. These results demonstrate the need for detailed geochronology of isochronous landforms and sediments of multiple ages, in order to understand both faulting and glaciation on faulted range fronts.