A Sequence Of Glacial Deformation, Erosion, And Deposition At The Ice–Rock Interface During The Last Glaciation: Cranbrook, British Columbia, Canada

Abstract

ABSTRACT. A comparison of bedrock fra ctures with orientation of structures and fabric in the overlying late Pleistocene basal till indicates a 'sequence of common glacial origin. The deforma ti ons in bedrock and till, a nd the d eposition of the till layer, were accomplished by two glac ia l flows that were part of a mountain ice sheet: the r egional (moving south-south-east) and the local (moving south-south-west ) a long Peavine Valley.The following sequence of events is postulated from this investigation: the initial glacial advance from the north-north-west that produced conjugate low-angle shear fractures in the b edrock was followed by competition of both the regional and the Peavine Valley glacial flows which also produced low-a ngle fra cture systems. As the over-riding ice becam e thi cker, high-angl e conjugate fra ctures were form ed by both glacial flows. Subseq uently, south-east-dipping injection wedges were form ed by the regional ice, which was concurrently erodi ng the down-glacier pa rts of the wedged blocks, depositing local till, and produci ng sigmoidal deformations in this till. The main deposition of the overlying lodgement till was accomplished by an interaction of both the r egio nal a nd the Peavine Valley glacia l flows, and the lodgement was supplemented by shearing . of the till, producing fissility. Bedrock wedging, as a significant process of glacial erosion, is believed to be discussed here for the first time.R ESUME . Une sequence de diformation, d'erosion et depots glaciaires cl l'inter.face glace-roche pendant la derniere glaciation ; Cranbrook, Colombie Britannique, Canada . Un e comparaison des fa ill es du lit rocheux avec les orientations des st ruct ures et des grains dans la moraine d e fond qui recouvre to ut le Pleistocene recent, indique un e sequence d 'origine glaciaire ordinai re. Les deforma tions dans le lit roch eux et les moraines et le depot du niveau morainique ont ete accomplis par deux ecoulements glaciaires qui appartenaie nt a un e calotte glaciaire d e montagne: l'un regional (se mouvant Sud Sud-Est) et l'un local (se depla<;ant Sud Sud-Ouest) le long de la P eavine Valley.De ces investigations on deduit que la succession des evenements a ete la suivante : l'avance glaciaire initiale depuis le Nord Nord-Ouest qui produisit des cassures conjuguees a u cisaillement sou s un angle faibl e dans le lit rocheux fut suivie de la combinaison d es courants de glace regionaux et du Peavine Valley qui produisit aussi d es systemes de cassures a angles fa ibles. Comme la glace surmontante devint plus epaisse, des cassures conjuguees a angles forts furent form ees par les deux coura nts d e glace. Pa r la suite, des coins d'inj ection s'enfon<;ant vers le Sud Est furent formes p a r la glace regiona le qui erodait e n m e me temps la partie aval d es blocs en coin, d eposa it une morain e locale et produisait d es deformat ions sigmoidales dans cette moraine. La plus grande partie des depots de la moraine de cou verture se fit par une interaction des ecoulements d e...

Clastic injection dynamics during ice front oscillations: A case example from Sólheimajökull (Iceland)

“…The multimodal grain-size distribution and the facetted and striated clasts indicate that the diamicton was initially formed at the glacier bed. The fissility observed in the diamicton deposits suggests shear-induced deformation or/and dewatering during or after deposition (Broster et al, 1979). Deformation induced by shear is typical of subglacial deposits, and triggered by the shear stress transmitted to the bed by overriding ice during ice-bed coupling periods.…”

Glaciotectonic structures as useful ice-movement indicators in glacial deposits: four Canadian case studies

“…Tension fractures (extension cracks) dip downglacier as a result of drag under the glacier sole, at which time the fractures can be opened and even infilled with sediment (Dreimanis 1969;Broster et al 1979;Derbyshire and Jones 1980). At Ocean Park, tension fractures dip southward in the massive clayey silt (Fig.…”

“…These fracture orientations indicate that the local glacial movement over the till was toward the northwest. Till wedges are believed to develop by the opening of tension fractures in the downglacier direction during continued glacial drag and subsequent infilling with till (Dreimanis 1969;Broster et al 1979). A till wedge occurs within the lower till at this site and dips northwest (Fig.…”

Laurentide and Cordilleran fast ice flow: some sedimentological evidence from Wisconsinan subglacial till and its substrate

“…Also of note are the studies of Broster et al (1979), (Hicock & Fuller 1995); 2. Puget Lobe (Brown et al 1987); 3.…”