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On the southwest cape of the Congo craton, a subtropical carbonate bank the size of Greenland was heavily glaciated during two Cryogenian panglacial episodes spaced 10-20 Myr apart. In NW Namibia, the bank underwent crustal stretching with resultant Aegean Sea-type topography during the older and longer Sturtian glaciation (717-661 Ma). This is indicated by angular discordance between glacial and preglacial strata and diamictites sourced from all older units, including crystalline basement. In contrast, the bank was flat-topped and underwent broad thermal subsidence during Marinoan glaciation (646 [+ or -] 5-635 Ma), attested by stratal parallellism and diamictites sourced from [less than or equal to] 100 m stratigraphic depth. However, [greater than or equal to]2.0kmofreliefexistedon the Marinoan continental slope, where most glacial erosion and accumulation occurred. The average rates of Marinoan erosion (2.55-6.80 m/Myr, n = 190) and accumulation (2.65-7.07 m/Myr, n = 211) are indistinguishable, implying that the location in a continental promontory did not bias erosion over accumulation. The average accumulation rates for the Sturtian and Marinoan, scaled for different averaging times, including Marinoan uncertainty, are 3.95-4.93 m/Myr (n = 183) and 2.65-7.07 m/Myr (n = 190), respectively, suggesting that a Marinoan glacioeustatic coastal escarpment substituted for rift-related Sturtian basin-and-range topography. These slow rates, comparable to long-term pre-Quaternary accumulation rates on existing abyssal plains, reconcile glacial sedimentology with the feeble hydrologic cycle of snowball Earth. Key words: snowball Earth, glacial erosion, glacial sedimentation, Marinoan, Cryogenian, Namibia, Introduction Estimates of global average Cryogenian glacial erosion are as disparate (Partin and Sadler 2016; Keller et al. 2019) as they once were for Quaternary erosion by the Laurentide Ice [...]

Glacial sediment research using thin sections began post-1945. Microsedimentology examines sediments at the microscopic level deriving insights into the processes of glacial erosion, transport, and deposition. Two issues exist (1) the difficulty by some in recognizing microstructures in thin section and (2) the absence of quantitative data making data reproduction difficult. The latter is hard to resolve but more image capture and software methodologies are now becoming available at reasonable costs. Thin sections are two-dimensional sections of three-dimensional objects, and this must be considered when measurements, fabrics and other data are assessed. Research into the microaspects of glacial sediments followed a typical scientific trajectory: thin sections description with little uniformity or common 'language' for observed microstructures: standardization allowed comparison between different sediments; with standardization, came an open-ended classification; and with crosscomparison with multiple thin sections--a quantitative means of study needs developed. It has become apparent that the basic principles of structural geology had to be applied. Thus, micromorphology has subsumed into a microsedimentological study of glacial sediments where stress parameters, structural fabrics and the mapping of deformation structures and contextual integration allows an understanding of how these sediments have been formed. Examples of the development stages of glacial micromorphology/sedimentology are presented here as well as discussion of future avenues of study. In addition, multiple thin sections are used to illustrate the many aspects of glacial micromorphology and the parameters necessary to understand glacial sedimentology and the processes of glacial sedimentation. Key words: Micromorphology, microsedimentology, glacial geology, image analyses, Introduction The study of sediment thin sections began in the early 20th century with the publication of 'Micropedology' (Kubiena 1938). As always, the critical aspect was in making viable thin [...]

Sediment delivery rates through an entire ice age are investigated using seismic records of glacial erosion products from the Fennoscandian Ice Sheet deposited offshore of mid-Norway in the 2.7 m.y. old Naust Formation (volume 100,000 [km.sup.3]). The mean sedimentation rate over the ice age is ~0.24 m [k.y.sup.-1], with bedrock lowering of ~520 m in the ice-sheet catchment. The mean sediment delivery is 2-3 times higher for the most recent 600 k.y. than for earlier Naust sequences. The hypothesis that glacial erosion is most rapid early in an ice age, when there was presumably much weathered bedrock and preglacial sediment available, is not accepted for our study area. More important are changing ice-sheet dimensions and dynamics, varying intensity of individual glacial cycles, and complex ice-sheet dynamics in single deglaciations. The mean rate of sediment delivery from ice sheets is an order of magnitude higher than from fluvial activity in the huge Amazon and Mississippi systems, implying that ice is a very effective agent of long-term denudation during cold periods of Earth history. doi: 10.1130/G25523.1

Conodonts from U.S. Midcontinent cyclothems were studied for oxygen isotopes in order to constrain Pennsylvanian glacioeustatic sea-level fluctuations. Pennsylvanian deposits of the Midcontinent United States are composed of cyclic alternations of thin transgressive limestones, offshore gray to black phosphatic shales, and thick regressive limestones, a sequence that is underlain and overlain by nearshore to terrestrial shales with paleosols and coal beds. Glacioeustatic sea-level fluctuations are considered the primary cause for the formation of these cyclothems. Oxygen isotope analyses of conodont apatite from the black (20.1 [+ or -] 0.5 [per thousand], Vienna standard mean ocean water [VSMOW]) and gray shale units (20.5 [+ or -] 0.5 [per thousand], VSMOW) show lowest average [delta][sup.18]O values, whereas conodont elements from the regressive (21.0 [+ or -] 0.3 [per thousand], VSMOW) and transgressive limestone units (21.1 [+ or -] 0.6 [per thousand], VSMOW) are enriched in 180. The maximum change in [delta][sup.18]O of conodonts from the black shale and carbonate units from individual cyclothems is 1.7 [per thousand]. The 1.7 [per thousand] difference in [delta][sup.18]O compares relatively well to Pleistocene interglacial-glacial changes in [delta][sup.18]O of equatorial surface-dwelling foraminifers and suggests that Pennsylvanian glacioeustatic sea-level changes may have been of comparable amplitude. However, since the Pennsylvanian glacial maxima are represented by terrestrial sediments and are not documented in the conodont oxygen isotope record, Pennsylvanian glacioeustatic sea-level changes were probably larger than the 120 m fluctuations recorded for the Pleistocene glaciations. Keywords: Pennsylvanian, conodonts, oxygen isotopes, sea level.

The hydraulic behaviour of meltwater during subglacial basaltic eruptions in temperate ice is of paramount importance in understanding the eruptive processes, lithofacies and architecture of the edifices formed. Hydraulics also determines the timing, location and volume of meltwater discharge, which may be sudden and catastrophic and via subglacial and/or supraglacial routes. Increasing our knowledge of meltwater hydraulics is therefore important for understanding, predicting and mitigating the impact of meltwater release on vulnerable human communities. New observations about eruption-related meltwater hydraulics are presented for well-exposed glaciovolcanic lava-fed deltas on James Ross Island, Antarctica, and accounts of historical eruptions are also re-examined to identify the major meltwater discharge routes. The study provides the first conceptual model for how meltwater escapes supraglacially. In the absence of a crevassed layer (which will dominate any meltwater flow), overflowing may be initiated by enhanced rates of seepage, despite the intrinsically low hydraulic conductivities of snow and fun. Once overflowing is established, the rate of spillway incision is a likely overriding control on the evolution of the system and whether the discharge is unstable (fast) or stable (slower). The James Ross Island sequences demonstrate that meltwater discharge is highly dynamic and may have involved both subglacial and supraglacial escape. Subglacial discharge probably occurs throughout basaltic tuya eruptions but some periods may be dominated by concurrent overflowing. It is still unclear if overflowing systems are sufficiently stable to enable the growth of laterally extensive glaciovolcanic lava-fed deltas. Keywords: subglacial; tuya; basalt; hydraulics; glaciovolcanic; meltwater; supraglacial; James Ross Island

Field evidence in the Koryak Mountains-Lake Mainitz region of far eastern Russia supports three Pleistocene glacial advances. The early Wisconsinan and pre-Wisconsinan glaciations are represented by broad lobate moraines extending as much as 30 km north of the Koryak Mountains. Field evidence demonstrates that the terminal, lateral, and medial moraines, as well as meltwater channels, dead ice topography, kettles, and out-wash plains mark the extent of ice during the last glacial maximum (LGM), during which glaciers reached no more than 20 km beyond their present limits. Those emanating from the southern Koryak Mountains may have reached the Bering Sea. Numerical and relative dating techniques support these results and test the theoretical models of the LGM in western Beringia. Erratics on moraines and glaciofluvial terraces, common to all valleys at 13-13.8 m above river level, yield (super 36)Cl exposure ages ranging from 10.08 to 25.78 ka. The Koryak Mountains-Lake Mainitz record of glaciation is spatially and temporally consistent with the glaciation pattern across central Beringia found in other terrestrial and marine records. Glacier growth in the Koryak Mountains was sustained by possible increased summer sea surface temperatures and precipitation in the northwest Pacific region. Evidence from the northern Koryak Mountains (lat 64 degrees N, long 177 degrees E) indicates that the extent of ice in western Beringia was limited to mountain and valley glaciers during the LGM. This field-based research contradicts M.G. Grosswald's theoretical Beringian ice sheet hypothesis. Keywords: Beringia, Chukotka, cosmogenic elements, erratics, glaciation, northwest Pacific.

Mountain erosion is thought to have sped up globally over the past few million years as the climate cooled and glaciers grew. A reassessment of the data suggests that this acceleration was limited to just a few regions.Mountain erosion is thought to have sped up globally over the past few million years as the climate cooled and glaciers grew. A reassessment of the data suggests that this acceleration was limited to just a few regions., Author(s): Eric KirbyAuthor Affiliations:Doubt cast on how the pace of global glacial erosion responds to climate cooling Mountainous landscapes are the manifestation of the competition between forces that arise from [...]

We propose that hummocky terrain in south-central Alberta is the product of subglacial erosion rather than supraglacial letdown during ice disintegration (a common view of hummock formation). Extensive exposures in hummocks contain sediments marking the history of Laurentide ice advance and deglaciation in the region. Regardless of the genesis of those sediments, units are abruptly truncated at the surface that represents the hummock and trough morphology, indicating that the hummocks are the product of differential erosion. Subglacial sediments predating the erosion and subglacial eskers overlying the erosion surface suggest that the erosion was also subglacial. Hummock morphology, lithostratigraphy correlated from hummock to hummock, truncation at the land surface, and widespread coarse boulder lags support a glaciofluvial origin for hummocky terrain in this region.

The history of late Wisconsinan, pre-Valley Heads glaciation (about 18-15.5 ka) implies that major glaciological controls, as well as overall climatic warming, influenced the pattern of deglaciation in the western Mohawk Valley. The Middleville Formation in the West Canada Valley containst the most complete record of pre-Valley Heads ice recession in central New York. The lithostratigraphic succession was determined through detailed mapping and logging of measured sections, provenance study of diamicton units, and the analysis of declination of detrital remanent magnetization (DRM) in laminated lacustrine silt and clay. The initial recession of southwest-flowing ice from the Adirondacks and at least two glacial readvances of the Mohawk Lobe are recorded in these units. Glacial Lake Newport, impounded in the West Canada Valley during ice recession, attained levels that required ice-damming by coalescent Ontario and Mohawk Lobes and a subglacial outlet across the Mohawk Valley to the Susquehanna drainage basin. Initial ice recession in the Mohawk Valley was mostly by calving of active valley lobes in deep-water (up to 350 m) embayments and not by regional stagnation and simple downwasting. A shift of ice flow from the Adirondacks to ice flow from the Mohawk Lobe marks the development of low-land lobes during pre-Valley Heads time before the complete uncovering of the Adirondacks. The later dominance of the Ontario Lobe in Valley Heads time may have been the result of (1) a reduced supply of ice to the Mohawk Lobe from a diminished Adirondack ice dome, (2) changing ice-flow conditions in Quebec associated with an eastern St. Lawrence ice stream and calving bay, and (3) nonsynchronous surging of the Mohawk and Ontario Lobes as these lobes became fronted by deep lacustrine waters at different times.

Glaciers in West Antarctica, including some of the most swiftly changing glaciers in the world, are being carved away from underneath by warm water that moves through large valleys in [...]

A study of glacier-induced sackungen in the sedimentary bedrock of the Culebra range of south-central Colorado is reported. The results show that all Culebra sackungen can be postdated to the Pinedale glaciation and that seismic activity may have been responsible for their formation.

Accumulation rates of terrestrial sediment have increased in the past few million years both on and adjacent to continents, although not everywhere. Climate changes contributed to all increases in sedimentation, an obvious choice is the increase in both amplitudes and frequency of climate change in late Cenozoic time, which might have maintained a state of disequilibrium in which erosive processes continually adjusted to new conditions, as suggested for both this and other timescales.

The growth of glaciers reflects the balance between the accumulation of snow and its loss through melting. Given the close association between altitude and temperature (1), the elevation and morphology [...]