ABSTRACT: Mid-Holocene dune and sand-sheet environment, Lauder Sandhills, Glacial Lake Hind Basin, Southwestern Manitoba, Canada.

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Abstract

A buried mid-Holocene eolian sand deposit is observed in three cut-bank exposures along the Souris River in the Lauder Sand Hills, glacial Lake Hind Basin, southwestern Manitoba. This deposit, formed from 6760-5350 RCYBP, has no known surface expression. Here, we present a partial reconstruction of the Lauder Sandhills landscape ~6000 RCYBP, supporting SCAPE (Study of Cultural Adaptations within the Prairie Ecozone), a multi-disciplinary project focused on Holocene human-environment interaction in the Canadian Prairies. The mid-Holocene eolian deposit overlies a lacustrine/wetland unit formed between 10,420 and 6,760 RCYBP. Where the sand overlies silt/clay the contact is distorted by loading. Where it overlies peat, wood fragments are incorporated in the basal eolian sand and in-situ tree stumps extend up into the sand. Sand-sheet/fluvial deposits formed between 5350 and 3250 RCYBP overlie the eolian unit. A late Holocene parabolic dune unit caps the sequence. The eolian unit comprises two subangular to subrounded, fine-medium sand facies. 1) Packages up to 1 m thick of low-angle cross-stratified-horizontally laminated strata consist of thin, laterally continuous-lenticular beds with interbedded lenses and layers of coarser (more medium-grained fraction) sand. Planar-irregular sub-horizontal to gently dipping strata (<100) contain small (<10 cm) packages of more steeply dipping strata (12-150) locally. 2) A set of high-angle cross-strata up to 1.5 m thick contains packages up to 3 cm thick of millimeter-scale laminations alternating with predominantly medium-grained strata up to 3 cm thick. Locally, laminated packages are lacking. Strata dip NE to ESE (42-1070) at angles up to 320. One or both facies are observed in any one profile. The high angle facies typically overlies the low angle facies where both are present. The high-angle facies represents slipface deposition with grainfall/grainflow strata alternating with packages of wind ripple strata. Parabolic or crescentic dunes >1.5 m high migrated northeast, or eastward in a bimodal wind regime. This contrasts with late Holocene dune migration to the southeast. The low-angle facies represents an irregular sand sheet surface with vegetation disrupting flow, producing irregular and discontinuous strata and packages of steeper strata formed on small shadow dunes. Together, these facies suggest a landscape where dunes interspersed with and migrated over vegetated sand sheets. Similar dune activity is reported prior to about 4550 RCYBP in the Brandon Sand Hills, about 100 km to the northeast. More precise ages are needed to determine whether dune activity in these two areas are coeval.

Bibliography of Canadian Geomorphology