Ribbed moraines in northern Manitoba, Canada: characteristics and preservation as part of a subglacial bed mosaic near the core regions of ice sheets

摘要

Ribbed moraines are enigmatic glacial landforms for which different models, with contrasting paleoglaciologic implications, have been proposed to explain their formation. Despite the great deal of attention this type of landform has received over the last several decades, ribbed moraine fields in northern Manitoba, Canada are among the largest in the world but have been seldom studied. Ribbed moraines in this part of the world overlie the low-relief Canadian Shield, are not constrained by topography, and are part of a spatial subglacial-landform assemblage associated with drumlinoid ridges within palimpsest and relict-type Glacial Terrain Zones. Field observations herein provide new insights into the characteristics of these transverse-to ice-flow ridges at landscape (mapping and spatial analysis) and landform (internal structure using high-resolution shear wave (S-wave) seismic reflection surveys, sedimentological characteristics, clast-fabric analyses) scales. Two main types of ribbed moraine are recognized: 'pristine', high amplitude straight-crested ridges and secondarily-modified subdued 'drumlinized' ridges. Ribbed moraine in northeast Manitoba consist of massive, matrix-supported till at surface, which is similar in matrix texture and composition to the regional till sheet, though pristine moraines show a higher concentration of boulders. A seismic profile reveals subparallel-to surface layered stratigraphy with only minor folding and no major unconformities (stacking or faulting). The demonstrated fragmentary nature of ribbed moraine fields, the inherited signature of the till within these fields, the secondary patchy drumlinization of the ridges, and the more abundant granitoid outcrops in areas of intense modification all suggest that subglacial ribbed moraine formed from preexisting sediments and were later preserved and/or partially reworked through a regional mosaic of shifting subglacial bed conditions.We therefore make the case that pristine (unmodified) moraines were preserved beneath stable sticky spots. We support a link between ribbed moraine and widespread coldbased and/or dewatered subglacial conditions in inner-core regions of ice sheets, but for reasons of preservation, and not necessarily formation.