Ice‐marginal forced regressive deltas in glacial lake basins: geomorphology, facies variability and large‐scale depositional architecture

Abstract

Brandes, C. 2018 (October): Ice-marginal forced regressive deltas in glacial lake basins: geomorphology, facies variability and large-scale depositional architecture.This study presents a synthesis of the geomorphology, facies variability and depositional architecture of ice-marginal deltas affected by rapid lake-level change. The integration of digital elevation models, outcrop, borehole, groundpenetrating radar and high-resolution shear-wave seismic data allows for a comprehensive analysis of these delta systems and provides information about the distinct types of deltaic facies and geometries generated under different lake-level trends. The exposed delta sediments record mainly the phase of maximum lake level and subsequent lake drainage. The stair-stepped profiles of the delta systems reflect the progressive basinward lobe deposition during forced regression when the lakes successively drained. Depending on the rate and magnitude of lake-level fall, fanshaped, lobate or more digitate tongue-like delta morphologies developed. Deposits of the stair-stepped transgressive delta bodies are buried, downlapped and onlapped by the younger forced regressive deposits. The delta styles comprise both Gilbert-type deltas and shoal-water deltas. The sedimentary facies of the steep Gilberttype delta foresets include a wide range of gravity-flow deposits. Delta deposits of the forced-regressive phase are commonly dominated by coarse-grained debrisflow deposits, indicating strong upslope erosion and cannibalization of older delta deposits. Deposits of supercritical turbidity currents are particularly common in sand-rich Gilbert-type deltas that formed during slow rises in lake level and during highstands. Foreset beds consist typically of laterally and vertically stacked deposits of antidunes and cyclic steps. The trigger mechanisms for these supercritical turbidity currents were both hyperpycnal meltwater flows and slope-failure events. Shoal-water deltas formed at low water depths during both low rates of lake-level rise and forced regression. Deposition occurred from tractional flows. Transgressive mouthbars form laterally extensive sand-rich delta bodies with a digitate, multi-tongue morphology. In contrast, forced regressive gravelly shoal-water deltas show a high dispersion of flow directions and form laterally overlapping delta lobes. Deformation structures in the forced-regressive ice-marginal deltas are mainly extensional features, including normal faults, small graben or half-graben structures and shear-deformation bands, which are related to gravitational delta tectonics, postglacial faulting during glacial-isostatic adjustment, and crestal collapse above salt domes. A neotectonic component cannot be ruled out in some cases. facies and the larger-scale depositional architecture. The results are compared with other marine and lacustrine delta examples from the literature, and provide information about the distinct types

Sedimentary architecture of a glaciolacustrine braidplain delta: proxy evidence of a pre‐Middle Wisconsinan glaciation (Grimshaw gravels, Interior Plains, Canada)

“…; Postma ; Winsemann et al . ). The geographical position of the foreset beds of FA4 is significant because it is situated directly adjacent to the part of the Peace River lowlands in which a lake would have most likely been impounded by the LIS (cf.…”

“…High braiding intensity is associated with areas of cyclic steps (hydraulic jumps), which have been previously recorded in the proximal zone of ice‐contact subaqueous fans (Winsemann et al . ) or at channel confluences in the fluvial plain of a braidplain delta (Fig. ; Nemec ).…”

“…; Winsemann et al . ). The predominance of normally graded gravel composing foreset beds suggests that sediment was transported on the foreset slope by concentrated turbulent flows (perhaps initiated by avalanching of debris) and deposited by density sorting and frictional freezing on the bed (Lowe ; Thomas ; Nemec et al .…”

“…Alternatively, lenses of cross‐bedded gravel may record backset or antidune deposits formed by supercritical turbidity currents at a hydraulic jump on the delta slope (Winsemann et al . ). Individual foreset beds with a sigmoidal relationship to overlying beds of FA5 (L10; Fig.…”

“…Winsemann et al . ) that indicate previous impoundment of a river valley. If evidence of other mechanisms of valley blockage (e.g.…”