On uncrevassed regions of polythermal glaciers, englacial conduits can form by incision of supraglacial stream channels followed by roof closure. The origin and evolution of examples in Longyearbreen, Svalbard, and Khumbu Glacier, Nepal, were determined by speleological survey. The development of perennial incised channels requires that incision is significantly faster than glacier surface ablation, and thus will be favoured by high meltwater discharges in combination with cool climatic conditions or thick debris cover. Incised canyons can become blocked by drifted winter snow, refrozen meltwater, ice rafting from non-local sources (allochthonous breccias) and roof collapses (autochthonous breccias). Conduit closure can also occur in response to ice creep, particularly at depth. Following isolation from the surface, englacial conduits continue to evolve by vadose incision down to local base level. In the case of Longyearbreen, incision allowed the channel to reach the glacier bed, but on Khumbu Glacier deep incision is prevented because an effectively impermeable terminal moraine provides a high base level for the glacier drainage system. During our period of observations, deeper parts of the Longyearbreen conduit became blocked by a combination of ice accumulation and creep, causing the stream course to be re-routed to higher levels. On that glacier, incision, blockage and upward re-routing are cyclic. We conclude that 'cut and closure' is the dominant mechanism of englacial conduit formation on uncrevassed regions of polythermal glaciers.

Detailed strati graphic and structural studies, performed in connection with the Nevada Nuclear Waste Storage Investigations program, have been in progress since 1978. The purpose of these studies is to characterize volcanic rocks underlying Yucca Mountain a volcanic highland situated along the western boundary of the Nevada Test Site in southern Nye county, Nevada. Core hole USW G-4 was cored from 41 ft (13 m) to a depth of 3,001 ft (915 m) at a location approximately 300 ft (91 m) southwest of the proposed site of an exploratory shaft that will be used for in situ studies of the geotechnical, geologic, and hydrologic characteristics of rock in the unsaturated zone to aid in evaluating the suitability of Yucca Mountain for storage of high-level nuclear wa'ste. The primary objectives of this study were to (1) verify that geologic conditions are similiar to those identified in nearby boreholes and (2) determine geologic and geophysical characteristics for use in the design and construction phases of the exploratory shaft. Strati graphic section in core hole USW G-4 is composed entirely of thick sequences of ash-flow tuff that are separated by fine-to coarse-grained ashfall tuff and tuffaceous sediments. All rocks are of Tertiary age and vary in composition from rhyolitic to quartz latitic. Major stratigraphic units include the Paintbrush Tuff, tuffaceous beds of Calico Hills, and Crater Flat Tuff. All four members of the Paintbrush Tuff were identified in USW G-4. In descending order, the members are the: Tiva Canyon, Yucca Mountain, Pah Canyon, and Topopah Spring. The Tiva Canyon and Topopah Spring Members are dominantly densely welded and devitrified, except for the basal part of the Tiva Canyon and upper and lower parts of the Topopah Spring where the rock is non-to partially welded and vitric. In contrast, the Yucca Mountain and Pah Canyon Members are entirely non-to partially welded and vitric. They represent the distal edges of ash-flow tuffs that thicken to the north and northwest. Plots of poles of fractures indicate that conspicuous fracture sets have orientations of (1) N. 22° E., 65° N.W. in the Tiva Canyon Member, (2) N. 12° W., 89-90° N.E. and S.W. in the Topopah Spring Member, and (3) N. 23° E., 45° N.W. and N. 50° E., 55° S.E. in the Crater Flat Tuff. Oriented fracture data obtained from television camera observations show no preferential strike of fractures in the Tiva Canyon Member and a wide range in the strike of fractures in the Topopah Spring Member, most of which strike between N. 30° W. and N. 60° E. Rock quality characteristics, as defined by the core index, indicate that greater amounts of broken core are associated with densely welded zones in the Tiva Canyon and Topopah Spring Members, and in zones that contain concentrations of shear fractures. Greater amounts of lost core and excessive hole enlargement are associated with vitric, non-to partially welded tuff. Geophysical logs from USW G-4 correlate well with logs from other drill holes at Yucca Mountain. Anomalous differences between t...

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This report is preliminary and has not heen reviewed for conformity with U.S. Geological Survey editorial standards and stratigraphic nomenclature. Any use of trade names is for descriptive purposes only and does not imply endorsement by the USGS.

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