Stephen R. Hicock

Initiation and development of the Laurentide and Cordilleran Ice Sheets following the last interglaciation

Abstract Fossil records from sites overridden by or adjacent to the Laurentide Ice Sheet indicate that the climate of the last interglaciation (Oxygen-Isotope Substage 5e, ca. 130-116 ka) was warmer than today. Following the last interglaciation, the Laurentide Ice Sheet first developed during Stage 5 over Keewatin, Quebec and Baffin Island. Along its northern margin, the ice sheet reached its maximum extent of the last glaciation during Stage 5. The ice sheet advanced across Baffin Island onto the continental shelf early during Stage 5 (5d?), whereas the advance into the western Canadian Arctic occurred late during Stage 5 (5b?). The ice sheet also may have advanced into the St Lawrence Lowland during Substage 5b, although this event may be younger (Stage 4). The Hudson Bay lowland became ice-free during Substage 5a. Retreat of the ice sheet on Baffin Island occurred during late Stage 5, probably Substage 5a. The exact timing of retreat from the western Canadian Arctic is unknown, but it occurred before 48 ka. The southern sector, including the St Lawrence Lowland, was ice-free during late Stage 5. The Hudson Bay lowland may have remained ice free through Stage 4 and much of Stage 3. Because of conflicting chronologies, however, it is more likely that this area was glaciated throughout Stage 3 and perhaps Stage 4. Nevertheless, the data demonstrate that the lowland was ice-free during part of the last glaciation. An independent ice cap developed over the Appalachian Uplands and advanced across Nova Scotia during Stage 4, perhaps as far as the edge of the continental shelf. The ice cap remained active over Nova Scotia as a setellite to the Laurentide Ice Sheet throughout the remainder of the last glaciation. The ice sheet advanced into the St Lawrence Lowland during Stage 4 and subsequently overwhelmed the local ice cap in the Appalachian Uplands, advancing perhaps into northern New England, but not farther south. The Lowland remained covered by the ice sheet until late Stage 2. The ice sheet may also have advanced into the Lake Ontario basin during Stage 4. The position of the northern margin of the Laurentide Ice Sheet during Stage 4 is not known, but it remained an unknown distance behind its maximum position reached during Stage 5. Cores from Baffin Bay indicate a substantial decrease in high-latitude glaciation during Stage 4. Following retreat, the Keewatin sector of the ice sheet may have remained over much of northwestern Canada as a quasi-stable ice mass until it readvanced during Stage 2. Similarly, the Baffin Island sector of the ice sheet may have remained largely intact. The southern margin of the ice sheet may have advanced into the Lake Ontario basin and upper Mississippi Valley during the middle of Stage 3 (ca. 50 ka), reaching its maximum extent of the last glaciation during Stage 2 (ca. 18–21 ka). Advance of the northern margin was younger (ca. 8–13 ka) than that of the southern margin; this advance was less extensive than the penultimate advance (Stage 5). Paleoenvironmental records indicate that the last interglaciation in areas covered by and near the Cordilleran Ice Sheet was as warm as, or warmer than, present. The Cordilleran Ice Sheet appears to have developed during Stage 5 or 4. At that time, it advanced over southern British Columbia and into the northern Puget Lowland. There is no record of this event in northern areas that were later covered by the ice sheet. The ice sheet disappeared before 59 ka, at the beginning of a lengthy nonglacial interval. Paleoenvironmental records indicate that climate was similar to the present during part of this interval. The ice sheet was absent, and glaciers probably were confined to mountain areas, throughout Stage 3. Climatic deterioration marking the end of this nonglacial interval may have begun as early as 29 ka. By 14–15 ka, the ice sheet had achieved its maximum extent of the last glaciation. Because there are few suitable dating methods capable of resolving events beyond the radiocarbon limit and because sites that preserve a record of events from the last glaciation are spatially restricted, we consider this synthesis as tentative and subject to significant revision as dating methods improve. Nevertheless, this perspective of the North American ice sheets through the last glaciation demonstrates their complex and dynamic behavior and attendant rapid fluctuations in ice volume.

On the Interpretation of Subglacial Till Fabric

ABSTRACT The modal distribution of stone long-axis fabrics and their respective eigenvalues can be used to infer the genesis of subglacial till. In this paper we offer a two-axis diagram that compares fabric modality to fabric isotropy (S3/S1) and addresses the problem of eigenvectors falling between the modes of some well-developed till fabrics with low eigenvalues. Our simple five-fold scheme of modality categories includes: (1) unimodal clusters, (2) spread unimodal, (3) bimodal clusters, (4) spread bimodal, and (5) polymodal to girdle-like fabrics, and requires the analyst to study equal-area, lower-hemisphere (Schmidt) plots of the fabric data. After assigning the fabric to a morality category, isotropy is calculated and both results are plotted on the graph, whic helps to separate two main fields of subglacial till: (1) lodgement and subglacial meltout tills, and (2) deformation fill. On the basis of selected published fabrics from tills at modern glaciers, as well as our own Pleistocene till data, lodgement and subglacial meltout tills tend to have unimodal or bimodal fabrics. In contrast, deformation tills and tills that experienced multiple processes tend to have polymodal to girdle-like fabrics. Some overlap occurs between fields be. cause of the complex nature of till formation (i.e., because pure end-member till facies are rare and most tills are hybrids). We strongly recommend that Schmidt plots be visually analyzed and used in conjunction with eigenvalues when studying till. However, fabric data alone is not enough. Multiple criteria including structural, lithologic, and stone morphologic data from the till must also be considered before drawing conclusions on till genesis. Furthermore, if eigenvectors fall between fabric modes, then they cannot be used to indicate former ice movement directions. Finally, our new modality-isotropy diagram may have wider applications.

Cold glaciers erode and deposit: Evidence from Allan Hills, Antarctica

Here we report previously undescribed features of erosion and deposition by a cold (polar) glacier. A recent study challenged the assumption that cold glaciers neither slide nor abrade their beds, but no geological evidence was offered. The features we describe include abrasion marks, subglacial deposits, glaciotectonically deformed substrate, isolated blocks, ice-cored debris mounds, and boulder trains, all products of a recent cold ice advance and retreat. Mapping these features elsewhere in Antarctica will document recent shifts in the East Antarctic Ice Sheet margin, providing new insight on regional mass-balance changes.

Lobal interactions, rheologic superposition, and implications for a Pleistocene ice stream on the continental shelf of British Columbia

Abstract Data on structures, stone striae, clast pavements, fabrics, morphology, provenance, and matrix grain size of four pre-late Wisconsinan diamictons are presented from the lower accessible part of an 855 m long sea cliff at Cape Ball, east coast of Graham Island (Queen Charlotte archipelago) British Columbia. The data indicate that an ice stream crossed the continental shelf from the British Columbian mainland to the Queen Charlotte Islands. Local Queen Charlotte piedmont ice coalesced with the western edge of the Cordilleran Ice Sheet, resulting in northward flow deflection along the east coast of Graham Island. Generally consistent structural and stone data indicate that diamictons were deposited as primary subglacial tills and that subglacial dynamics and rheologic conditions were complex. Two of the tills can be traced continuously over the Cape Ball section which provides a north-south transect oblique to ice flow. Each of the two tills reveals five major changes in rheology and genesis ranging from a strong lodgement component to dominantly deformation (mainly by subglacial squeeze flow). True end-member tills are rare and in almost all places the tills are hybrids that formed within a continuum of till-forming processes commonly involving stone rotation and superposition of structures and rheologic states. This could have occurred mainly in response to fluctuating till pore water content. Along the transect, two-dimensional stretches of contrasting till types probably represent a three-dimensional patchwork of contrasting till rheologies and pore waters. If patches of ductily-deforming till were common and/or widespread, they could have accounted for much of the movement of Cordilleran ice over glaciomarine mud on the continental shelf when sea level was much lower than present. Deforming, probably saturated, muddy till (reconstituted glaciomarine) would have possessed negligible shear strength, resulting in a thin, rapidly moving glacier (most likely an ice stream) that reached Cape Ball but failed to remove the record of previous glacial events. The ice stream probably issued out of the mainlands Skeena valley which is aligned with Cape Ball and is approximately the size of valleys in Antarctica that today conduct small outlet glaciers and ice streams. Evidence for this and other ice streams on the northwestern continental shelf of North America imply that destabilization and rapid decay of the western edge of the Cordilleran Ice Sheet may have been accomplished by a combination of ice streams, sea level rise, and extensive calving.

Sunnybrook drift indicates a grounded early Wisconsin glacier in the Lake Ontario basin

Glaciotectonic analysis of Sunnybrook diamicton and underlying sediments in Scarborough Bluffs, Ontario, Canada, indicates that the diamicton investigated is lodgment and/or deformation till. The early Wisconsin overriding of deformable glaciolacustrine mud by the Ontario ice lobe was a significant glacial event in the eastern Great Lakes region. This has important implications for understanding interactions between Laurentide ice, proglacial lakes, and water-laid sediments.

Genetic till prism

A schematic diagram aids geologists and geomorphologists in visualizing the relations between modern and ancient till-forming processes. It can be modified to accommodate mechanisms of till formation not yet recognized or to include other processes involved in making glacial deposits. The prism can also be used to trace till history, from debris release from glacier ice to final emplacement, as well as to depict multiple stages of till deposition in an outcrop or geographic area. Use of the prism should improve understanding of glacier behavior and help in reconstructing the dynamics of former ice sheets.

Fragment of an ancient outlet glacier system near the top of the Transantarctic Mountains

Sirius Group tillite near the summit of Mount Feather, Transantarctic Mountains, is critically located for reconstructing past behavior of the East Antarctic Ice Sheet. New ice-direction indicators and tillite geometry show that the tillite accumulated beneath wet-based ice on a transverse paleovalley floor—a hanging remnant of a landscape largely removed by erosion. We conclude that the Mount Feather tillite formed primarily by lodgment as part of an ancient, wet-based, outlet glacier system when the Transantarctic Mountains were at least 1500 m lower than today. Till deposition took place before 20 Ma on the basis of estimated past uplift rates.

Recent Holocene changes in sedimentation in a landslide-dammed lake in the Cascade Mountains, southwestern British Columbia, Canada

Landslide-damming of Silver Lake in the Cascade Mountains took place about 1000BP. Delta growth favoured the faulted west valley side. At the commencement of logging activity in AD 1946 the course of Silverhope Creek was diverted away from access roads. As a consequence, the channel shifted from the west to the east side of the valley, forming a new delta. Six gravity lake cores and SCUBA surveys of lake sediments reveal changes in the depositional regime. In two cores, late Pleistocene-early Holocene sediments are pre served. Old delta abandonment, new delta growth, reorientation of turbidity channels and confluence zones are discussed. 137Cs data show that, following channel diversion, little or no further deposition occurred on the old delta. Assuming that the 1964 137Cs maximum has been recorded in each core, sedimentation rates appar ently decrease to the NNW, away from the new delta, and 137Cs values increase northwards. Stratigraphy, 14C and 137Cs dates indicate that there has been a recent increase in sedimentation rate of about 20 times near the deltas, to about 10 times in a mid-lake location. Erosion and the release of destabilized glaciolacustrine valley- side sediments into the creek enhances sedimentation in Silver Lake. Unstable sediments on steep valley sides prohibit forest regrowth.

Identifying kimberlite indicator mineral dispersal trains in the Pelly Bay region, Nunavut, Canada using GIS interpolation

ABSTRACT Identification of kimberlite indicator mineral dispersal trains in glaciated terrain provides important clues in determining provenance. Complex ice flow history can distribute kimberlite indicator minerals such that dispersal trains are disguised within clusters of till samples containing abundant kimberlite indicator minerals. This study uses GIS with inverse distance weighted interpolation to identify and isolate dispersal trains within areas where large clusters of till samples with abundant kimberlite indicator minerals exhibit no apparent distribution patterns. The method was tested in the Pelly Bay region of Nunavut, Canada, where many areas contain clusters of till samples with abundant kimberlite indicator minerals, the method delineated dispersal trains within these areas. The study identified trains ranging from 1.5–7 km in length, with the heads of some trains ranging in width between 225 m and 3 km. Mg-ilmenite is the most abundant kimberlite indicator mineral in the Pelly Bay area, however, several trains with distinct relative abundances of kimberlite indicator mineral species were identified that suggest the presence of kimberlites with different intrusive phases. Our study suggests that several of the identified dispersal trains likely originated from kimberlite dykes and/or sills occupying NW–SE oriented structures in the bedrock.

Aleksis Dreimanis: a legacy in Quaternary science

Abstract Aleksis Dreimanis was born and raised in Latvia. His interest in Quaternary and glacial geology began early and developed into a career that has spanned 7 decades. At age 20 he published his first paper in glacial geology and soon after began teaching at the University of Latvia. Teaching and research were interrupted by World War II but resumed at the Baltic University (Pinneberg, Germany), then at the University of Western Ontario where he has been ever since. Throughout his career, Dreimanis has successfully balanced the twin disciplines of Quaternary history and glacial geology. He was among the first to study quantitatively the relationship between till lithology and till formation and to study how glacial transport and dynamics affect till texture and deformation. With co-workers he developed the well-known stratigraphic scheme of the last glaciation in the Great Lakes region of North America. Aleksis became world-renowned through his committee work, especially as President of the INQUA Commission on Genesis and Lithology of Glacial Quaternary Deposits. His diplomacy, enthusiasm, and passion for his subject have inspired students and colleagues around the globe and resulted in remarkable international dialogue, cooperation, and consensus. Professor Aleksis Dreimanis is an honest scientist, a gentleman, and a true scholar who has left a rich legacy for future Quaternarists.