Edward B. Evenson

How glaciers entrain and transport basal sediment: Physical constraints

Abstract Simple insights from the physics of ice, water and sediment place constraints on the possible sediment-transport behavior of glaciers and ice sheets. Because glaciers concentrate runoff, streams generated by glaciers transport much sediment and may erode bedrock rapidly. Deforming glacier beds also can transport much sediment, particularly in marginal regions. Rapid sediment entrainment producing thick debris-rich basal zones may occur by regelation into subglacial materials, and by freeze-on from rising supercooled waters. Numerous other mechanisms may be important but primarily near ice margins, especially those of advancing or fluctuating glaciers. Several sediment-entrainment mechanisms may be active beneath a single glacier, but one process is likely to be dominant at any place and time.

Beryllium-10 Dating of the Duration and Retreat of the Last Pinedale Glacial Sequence

Accurate terrestrial glacial chronologies are needed for comparison with the marine record to establish the dynamics of global climate change during transitions from glacial to interglacial regimes. Cosmogenic beryllium-10 measurements in the Wind River Range indicate that the last glacial maximum (marine oxygen isotope stage 2) was achieved there by 21,700 � 700 beryllium-10 years and lasted 5900 years. Ages of a sequence of recessional moraines and striated bedrock surfaces show that the initial deglaciation was rapid and that the entire glacial system retreated 33 kilometers to the cirque basin by 12,100 � 500 beryllium-10 years.

Glaciohydraulic supercooling : a freeze-on mechanism to create stratified, debris-rich basal ice : I. Field evidence

Debris-laden ice accretes to the base of Matanuska Glacier, Alaska, U.S.A., from water that supercools while flowing in a distributed drainage system up the adverse slope of an overdeepening. Frazil ice grows in the water column and forms aggregates, while other ice grows on the glacier sole or on substrate materials. Sediment is trapped by this growing ice, forming stratified debris-laden basal ice. Growth rates of >0.1 m a -1 of debris-rich basal ice are possible. The large sediment fluxes that this mechanism allows may have implications for interpretation of the widespread deposits from ice that flowed through other overdeepenings, including Heinrich events and the till sheets south of the Laurentian Great Lakes.

Cosmogenic 36Cl and 10Be ages of Quaternary glacial and fluvial deposits of the Wind River Range, Wyoming

We measured cosmogenic 36 Cl in 56 samples from boulders on moraines and fluvial terraces in the vicinity of the Wind River Range, Wyoming. We also measured 10 Be in 10 of the same samples. Most of the 10 Be ages were in good agreement with the 36 Cl ages, indicating that rock-surface erosion rates were very low. The oldest moraine investigated, the type Sacagewea Ridge site, yielded only a limiting minimum age of >232 ka. The oldest moraines in the type Bull Lake complex also could be constrained only to >130 ka. The main sequence of type Bull Lake moraines yielded age distributions indicating deposition within the intervals 130 to 100 ka and 120 to 100 ka; the best estimates are closer to the upper limits of these ranges, and associated uncertainties are in the range of 10% to 15%. These uncertainties could permit deposition in either marine isotope stage 6 or stage 5d. We found no evidence of glacial deposits dating to marine isotope stage 4. Both Bull Lake–age moraines from Fremont Lake, on the opposite side of the Wind River Range, and boulders on a fluvial terrace above the Wind River, gave age distributions very similar to that of the second oldest Bull Lake advance (ca. 130 to 100 ka). The distribution of boulder ages for Pinedale moraines at Bull Lake indicated deposition between 23 and 16 ka, nearly identical to the distribution of 10 Be ages previously reported for the type Pinedale moraines at Fremont Lake.

Stabilizing feedbacks in glacier-bed erosion.

Glaciers often erode, transport and deposit sediment much more rapidly than nonglacial environments, with implications for the evolution of glaciated mountain belts and their associated sedimentary basins. But modelling such glacial processes is difficult, partly because stabilizing feedbacks similar to those operating in rivers have not been identified for glacial landscapes. Here we combine new and existing data of glacier morphology and the processes governing glacier evolution from diverse settings to reveal such stabilizing feedbacks. We find that the long profiles of beds of highly erosive glaciers tend towards steady-state angles opposed to and slightly more than 50 per cent steeper than the overlying ice–air surface slopes, and that additional subglacial deepening must be enabled by non-glacial processes. Climatic or glaciological perturbations of the ice–air surface slope can have large transient effects on glaciofluvial sediment flux and apparent glacial erosion rate.

Glaciohydraulic supercooling : a freeze-on mechanism to create stratified, debris-rich basal ice : II. Theory

Simple theory supports field observations (Lawson and others, 1998) that subglacial water flow out of overdeepenings can cause accretion of layered, debris-bearing ice to the bases of glaciers. The large meltwater flux into a temperate glacier at the onset of summer melting can cause rapid water flow through expanded basal cavities or other flow paths. If that flow ascends a sufficiently steep slope out of an overdeepening, the water will supercool as the pressure-melting point rises, and basal-ice accretion will occur. Diurnal, occasional or annual fluctuations in water discharge will cause variations in accretion rate, debris content of accreted ice or subsequent diagenesis, producing layers. Under appropriate conditions, net accretion of debris-bearing basal ice will allow debris fluxes that are significant in the glacier sediment budget.

Knickzone propagation in the Black Hills and northern High Plains: A different perspective on the late Cenozoic exhumation of the Laramide Rocky Mountains

Geomorphic research in the Black Hills and northern High Plains poses an intriguing hypothesis for the Cenozoic evolution of this salient of the Laramide Rockies. Most recently, geologists have appealed to late Cenozoic epeirogenic uplift or climate change to explain the post-Laramide unroofing of the Rockies. On the basis of field mapping and the interpretation of long-valley profiles, we conclude that the propagation of knickzones is the primary mechanism for exhumation in the Black Hills. Long profiles of major drainages show discrete breaks in the slope of the channel gradient that are not coincident with changes in rock type. We use the term knickzones to describe these features because their profiles are broadly convex over tens of kilometers. At and below the knickzone, the channel is incising into bedrock, abandoning a flood plain, and forming a terrace. Above the knickzone, the channel is much less incised, resulting in a broad valley bottom. Numerous examples of stream piracy are documented, and in each case, the capture is recorded in the same terrace level. These observations are consistent with migrating knickzones that have swept through Black Hills streams, rearranging drainages in their wake. We demonstrate there are two knickzone fronts associated with mapped terraces. Preliminary field evidence of soil development shows that these terraces are time transgressive in nature. Our data strongly suggest that knickzone propagation must be considered a viable mechanism driving late Cenozoic fluvial incision and exhumation of the northern High Plains and adjacent northern Rocky Mountains.

Greatlakean Substage: A replacement for Valderan Substage in the Lake Michigan basin

Abstract New evidence from recent field and seismic investigations in the Lake Michigan basin and in the type areas of the Valders, Two Creeks and Two Rivers deposits necessitates revision of late-glacial ice-front positions, rock- and time-stratigraphic nomenclature and climatic interpretations and deglaciation patterns for the period ca. 14,000–7,000 radiocarbon years B.P. The previously reported and long accepted pattern of deglaciation for the Lake Michigan basin started with a regular retreat from the Lake Border Morainic System, with a minor oscillation marked by the Port Huron moraine(s) and then an extensive Twocreekan deglaciation followed by a major (320 km) post-Twocreekan advance (Valders). However, we now record a major retreat between the times of the Lake Border and Port Huron moraines, followed by a gradual retreat from the Port Huron limit and interrupted by a minor standstill (deposition of Manitowoc Till), a retreat (Twocreekan) and a readvance (Two Rivers Till). No Woodfordian or younger readvance was as extensive as had been the preceding one. This sequence argues for a normal, climatically controlled, progressive deglaciation rather than one interrupted by a major post-Twocreekan (formerly Valderan) surge. This revision appears finally to harmonize the geologic evidence and the palynological record for the Great Lakes region. Our investigations show that Valders Till from which the Valderan Substage was named is late-Woodfordian in age. We propose the term “Greatlakean” as a replacement for the now misleading time-stratigraphic term “Valderan”. The type section and the definition of the upper and lower boundaries of the Greatlakean Substage remain the same as those originally proposed for the Valderan Substage but the name is changed.

Basal-crevasse-fill origin of laminated debris bands at Matanuska Glacier, Alaska, U.S.A.

The numerous debris bands in the terminus region of Matanuska Glacier, Alaska, U.S.A., were formed by injection of turbid meltwaters into basal crevasses. The debris bands are millimeter(s)-thick layers of silt-rich ice cross-cutting older, debris-poor englacial ice. The sediment grain-size distribution of the debris bands closely resembles the suspended load of basal waters, and of basal and proglacial ice grown from basal waters, but does not resemble supraglacial debris, till or the bedload of subglacial streams. Most debris bands contain anthropogenic tritium (3H) in concentrations similar to those of basal meltwater and ice formed from that meltwater, but cross-cut englacial ice lacking tritium. Stable-isotopic ratios (δ 18 O and δD) of debris-band ice are consistent with freezing from basal waters, but are distinct from those in englacial ice. Ice petrofabric data along one debris band lack evidence of active shearing. High basal water pressures and locally extensional ice flow associated with overdeepened subglacial basins favor basal crevasse formation.

Microstructures of glacigenic sediment-flow deposits, Matanuska Glacier, Alaska

Microstructures of glacigenic sediment gravity-flow deposits formed at the terminus of the Matanuska Glacier, Alaska, were analyzed to characterize flow type. These sediment flows have been classified into four types based primarily on water content and sedimentological characteristics (Lawson, 1979a, 1982). Thin sections of flow deposits show a variety of microand mesoscale characteristics that vary according to water content of the source flow. Wet-type flow deposits are characterized in thin section by a well-defined parallel and imbricated microclast fabric and thin laminations resulting from laminar to plastic flow regimes. Dry-type flow deposits are characterized in thin section by bior polymodal or random microclast fabrics, greater textural heterogeneity, and deformational microstructures associated with plastic to brittle flow regimes. Thin laminations and a “laminar flow fabric” in wet-type flow deposits may be characteristic of sediment gravity flow in a glacial environment. Characterization of these microstructures supports the contention that micromorphological analyses can be used to elucidate sediment flow genesis and the conditions of the flow just prior to deposition. Thus, micromorphology may also be useful for differentiating sediment-flow type in Pleistocene diamictons in other locations. Lachniet, M. S., Larson, G. J., Strasser, J. C., Lawson, D. E., Evenson, E. B., and Alley, R. B.,1999, Microstructures of glacigenic sediment-flow deposits, Matanuska Glacier, Alaska, in Mickelson, D. M., and Attig, J. W., eds., Glacial Processes Past and Present: Boulder, Colorado, Geological Society of America Special Paper 337. 45 *Current address: Department of Earth Sciences, Syracuse University, Syracuse, New York 13244 type sediment-flow deposits correspond approximately to Lawson type III and IV flow deposits (high water content; Lawson, 1979a, 1982; see below for sediment flow type characteristics). Here we evaluate the use of micromorphological analysis to differentiate contemporary dry-type from wet-type sediment-flow deposits formed at the terminus of the Matanuska Glacier. This study deals exclusively with the micromorphology of sediment-flow deposits; the study of the micromorphology of tills is beyond the scope of this study and has not been undertaken at the Matanuska Glacier. Future investigation on the micromorphology of known glacial sediments will allow the further distinction between sediment-flow deposits and true tills.