METHOD FOR RECOGNIZING LOCAL BEDROCK INFLUENCE ON DRIFT PLAINS∗
1
Citation
17
Reference
10
Related Paper
Citation Trend
Abstract:
Analysis of topographic and bedrock surface data from 41 sites 23.3 km2 (9 m2) within Midwestern areas glaciated during the Late Wisconsinan identifies an average drift thickness-maximum bedrock relief transition equation for the presence or absence of bedrock surface influences on topographic detail. For sites with average drift thicknesses between 15 and 35 m, the transition occurs when average drift thicknesses exceed 0.5 maximum bedrock relief + 10 m. This equation may provide a practical tool in searching for buried bedrock valleys.Keywords:
Bedrock
Bedrock
Trough (economics)
Cite
Citations (25)
Bedrock
Forearc
Lithology
Tectonic uplift
Cite
Citations (87)
Meltwater
Bedrock
Lineament
Cite
Citations (0)
Airphoto analyses, geologic field mapping, and study of borehole logs and surficial topographic maps in ice-thrust terrains in central Alberta suggest that all these techniques are needed to identify ice-thrust terrains that may or may not have topographic expressions.Three geomorphological settings susceptible to glaciotectonic deformation are described as escarpment, valley, and plains settings. The bedrock slopes in the escarpment setting ranged from 19 to 51 m/km. The bedrock slopes in the valley setting ranged from 23 to 32 m/km, and the valley was 8.5 km wide between crests and 4 km at the bottom. The bedrock slopes of the plains setting ranged from 2 to 5 m/km.Ice-thrust features are found in topographic troughs in front of an ice sheet where water bodies were impounded. These proglacial water bodies thawed the permafrost in front of the glacier. The disintegration of proglacial permafrost decreased the resistance of subglacial strata to ice thrusting.Ice-thrust features can be expected in areas where local slopes are inclined upglacier toward the former glacier margin, where proglacial water bodies could be impounded, rather than in areas where slopes inclined downglacier away from the glacier margin, where the existence of a proglacial water body is unlikely.
Escarpment
Bedrock
Cite
Citations (27)
Although there is an established relationship between geological structure and the morphology of certain glacial erosional landforms, the role of lithology is less clear. This is particularly true of the surface wear characteristics of glaciated bedrock. In order to examine this relationship, the surface wear characteristics of eight recently deglaciated metamorphic bedrock slabs in the Kongsfjorden area of Svalbard were mapped and recorded using detailed "micro‐maps." Features recorded included lee‐side fracture surfaces, lee‐side cavities, and the location and depth of open joints and quartz veins. On schist, glacial erosion is favored by situations where ice movement is parallel to the trend of the bedrock foliation. In these situations, cavities may be elongated in the direction of ice flow. On more homogeneous lithologies such as marble, cavity formation is suppressed and more uniform glacially abraded rock surfaces develop. On all the metamorphic rocks examined, glacial abrasion is favored in situations where bedrock foliation is normal to ice flow. The structure of the parent bedrock, especially the orientation of foliation, exerts a strong influence on the surface wear characteristics of glaciated bedrock slabs and on the location of subglacial cavities. Geological structure therefore has the potential to influence rates of ice flow across bedrock surfaces.
Bedrock
Lithology
Landform
Foliation (geology)
Cite
Citations (31)
Field and archival data have been used in compiling the Glaciotectonic Map of Estonia. Two principal types of glaciotectonic deformations shown on this are discussed here: dislocations of rigid bedrock, and soft bed deformations associated with unconsolidated drift masses. Most bedrock disturbances occur in the narrow zone south of the Baltic Klint and in the tectonically crushed zone where the fractured bedrock was easy to break, displace and deform by the moving glacier. Some of the bedrock dislocations are related to ice-marginal deposits of the Late Weichselian Glaciation (Palivere and Pandivere Phases). Most subglacial deformations of soft sediments are simple in style, namely: shear and ductile deformations within a thin layer. The spatial organisation and efficiency of drainage beneath the local ice streams determined the deformational behaviour of sediments at the ice/bed interface. Ice-marginal deposits of the Late Weichselian deglaciation have not been subjected to large-scale compressive deformation. This suggests that most marginal deposits were formed as the result of brief standstills of the ice margin which caused sediment deformation either at the ice margin or beneath the ice sole.
Bedrock
Deglaciation
Cite
Citations (26)
Tunnel valleys are large elongated depressions eroded into unconsolidated sediments and bedrock. Tunnel valleys are believed to have been efficient drainage pathways for large volumes of subglacial meltwater, and reflect the interplay between groundwater flow and variations in the hydraulic conductivity of the substrate, and basal meltwater production and associated water pressure variations at the ice-bed interface. Tunnel valleys are therefore an important component of the subglacial hydrological system. Three-dimensional modelling of geophysical and lithological data has revealed numerous buried valleys eroded into the bedrock unconformity in northeast Alberta, many of which are interpreted to be tunnel valleys. Due to the very high data density used in this modelling, the morphology, orientation and internal architecture of several of these tunnel valleys have been determined. The northeast Alberta buried tunnel valleys are similar to the open tunnel valleys described along the former margins of the southern Laurentide Ice Sheet. They have high depth to width ratios, with undulating, low gradient longitudinal profiles. Many valleys start and end abruptly, and occur as solitary, straight to slightly sinuous incisions, or form widespread anastomosing networks. Typically, these valleys are between 0.5 and 3 km wide and 10 and 30 m deep, although the depth of incision along some thalwegs exceeds 100 m. Several valleys extend for up to 60 km, but most are between 10 and 30 km long. Valley fills comprise a range of lithofacies, including stacked sequences of diamict, glaciofluvial sands and gravels and glaciolacustrine silts and clays. Displaced bedrock, presumably of glaciotectonic origin, also occurs within several anastomosing valleys. Several channel bodies are exposed along a number of valley sections suggesting progressive valley development through repeated cycles of sediment discharge. Cut-and-fill structures that are capped by fine-grained sequences of rippled sand and mud-rich drapes within these channel bodies suggest unstable flow regimes within the valley and the discharge of sediment-laden basal meltwater under flood-like conditions followed by wane flow events or periods of lower meltwater discharge, likely concomitant with localized modification by glacial ice. Basal meltwater is inferred to have been released as episodic jökulhlaups beneath the western Laurentide Ice Sheet, which at times re-used existing valley systems, which were spatially and temporally stable features, and at other times incised new valleys.
Meltwater
Bedrock
Cite
Citations (27)