Abstract
AbstractLandforms and sediments on the palaeo–ice stream beds of central Alberta record glacitectonic raft production and subsequent progressive disaggregation and moulding, associated substrate ploughing, and grooving. We identify a subglacial temporal or developmental hierarchy that begins with incipient rafts, including en échelon hill-hole complexes, hill-hole pairs, and strike-slip raft complexes, all of which display patterns typical of transcurrent fault activation and pull apart. Many display jigsaw puzzle–style fragmentation, indicative of substrate displacement along shallow décollement zones and potentially related to patchy ice stream freeze-on. Their gradual fragmentation and smoothing produces ice flow-transverse ridges (ribbed moraine), hill-groove pairs, and paraxial ridge and groove associations. Initiator scarp and megafluting associations are indicative of raft dislodgement and groove ploughing, leading to the formation of murdlins, crag-and-tails, stoss-and-lee type flutings and drumlins, and Type 1 hogsback flutings. Downflow modification of rafts creates linear block trains (rubble stripes), stoss-and-lee type megaflutings, horned crag-and-tails, rubble drumlinoids, and murdlins, diagnostic of an immature palaeo–ice stream footprint. Lateral ice stream margin migration ingests disaggregated thrust masses to form ridged spindles, ladder-type morphologies, and narrow zones of ribbed terrain and Type 2 hogsback flutings, an assemblage diagnostic of ice stream shear margin moraine formation.
Highlights
IntroductionAND RATIONALEThe occurrence of large slabs of bedrock and coherent sediment intraclasts in glacigenic stratigraphic sequences has long been widely recognised and linked to the glacitectonic process of raft detachment, facilitated by displacement along décollement zones in horizontally bedded strata (e.g., Christiansen, 1971; Moran, 1971; Ruszczynska-Szenajch, 1976, 1987; Stalker, 1973, 1976; Ringberg et al, 1984; Christiansen and Sauer, 1988; Aber et al, 1989; Hopson, 1995; Aber and Ber, 2007; Burke et al, 2009; Evans et al, 2012; Sigfusdottir et al, 2018; Fig. 1)
The proposal by Moran et al (1980) that such rafts were displaced and transported subglacially, especially in the submarginal, frozen bed zone implies that some mechanism of incorporation into the basal transport zone was in operation (e.g., Vaughan-Hirsch et al, 2013) and that upwards thrusting of the detached raft was driven by compressive ice flow (Clayton and Moran, 1974; Bluemle and Clayton, 1984)
Incipient rafts include en échelon hill-hole complexes (EHHCs), hill-hole pairs, strike-slip raft complexes (SSRCs), and pull-apart zones developed in composite thrust moraines
Summary
AND RATIONALEThe occurrence of large slabs of bedrock and coherent sediment intraclasts in glacigenic stratigraphic sequences has long been widely recognised and linked to the glacitectonic process of raft detachment, facilitated by displacement along décollement zones in horizontally bedded strata (e.g., Christiansen, 1971; Moran, 1971; Ruszczynska-Szenajch, 1976, 1987; Stalker, 1973, 1976; Ringberg et al, 1984; Christiansen and Sauer, 1988; Aber et al, 1989; Hopson, 1995; Aber and Ber, 2007; Burke et al, 2009; Evans et al, 2012; Sigfusdottir et al, 2018; Fig. 1). Deep-seated deformation and liberation of bedrock rafts in association with pressurised aquifers has been widely proposed as the likely mechanism of such subglacial raft origins (cf Moran et al, 1980; Cite this article: Evans DJA, Phillips ER, Atkinson N (2021). Glacitectonic rafts and their role in the generation of Quaternary subglacial bedforms and deposits.
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