Glacial geomorphology and late Quaternary chronology of inner Nachvak Fiord, Northern Labrador

摘要

Nachvak Fiord in the central Torngat Mountains has been the subject of several detailed studies, including some which deal with the horizontal and vertical extent of the Wisconsin ice sheet. This study examines the inner fiord to the head of the fiord proper, and draws on data from the entire fiord to provide an evaluation of previous reconstructions of glacial history. -- Several stades or glaciations, involving ice of local and regional origin, are shown by the number and orientation of moraines in the inner fiord area. Examination of ice-flow indicators, including analysis of drift geochemistry, showed that regional ice was predominant during the last stade, while Kogarsok Brook valley was the main source of local ice. Glacial stades of different relative age were detected through morphologic mapping of moraines, measurement of solum development, and observation of weathering characteristics. However, absolute ages were difficult to determine. Soil pits dug on moraines of different altitude showed that time is rarely the only soil-forming factor influencing solum depth; solum depths on moraines of high altitude showed wide variability, while even at lower elevations a single moraine often showed considerable variation in solum depth. -- Analysis of the lithostratigraphy of piston cores extracted from the centre of the fiord provided additional information on the late- and postglacial environment of the region. The cores were not deep enough to penetrate into late Pleistocene sedimentary units previously identified by acoustic survey, and therefore did not provide a correlation with terrestrially-identified glacial sediments. They do, however, show ice-distal and postglacial sedimentation, the most recent facies being dominated by fluvial deposits. Radiocarbon dated shells from the cores indicate that this major facies change occurred 7300- 5200 years BP, thus providing a minimum date for the final influence of pro-glacial deposition in the area. Pollen spectra of two cores correspond well with those from other parts of Labrador, showing a postglacial tundra vegetation, followed by an early increase in shrub pollen (7300 years BP), and a subsequent return to sedge-tundra. -- Raised shorelines in the inner fiord may be indicators of individual stages of glacial retreat. Beach segments were levelled and correlated into thirteen shorelines, ranging in height from 73 to 8 metres above high tide (aht). Some of these were associated with moraines and submarine sills, suggesting that they were formed during glacial stillstands. This indicates that a stage-like retreat from the last ice maximum probably occurred. Very high, continuous shorelines with considerable gradients were levelled in Tasiuyak Arm. Their elevation and tilt may be explained through fairly rapid ice retreat, which allowed them to experience as much uplift older shorelines of the same elevation, recorded in the outer fiord. Alternative possibilities for their tilt, such as postglacial faulting, were also considered. -- At least three phases of glaciation were detected; the youngest Nachvak phase is correlated with the Late Wisconsin, and was the last regional glaciation. In the inner fiord, this phase is characterised by prominent moraines and numerous raised shorelines, suggesting that deglaciation occurred in stages, though overall retreat may have been quite rapid. A previously reported radiocarbon date has been used to show early deglaciation of the fiord, by 9000 BP. Although positive evidence of deglaciation by this time was found to extend only as far as central Tasiuyak Arm, the early date is corroborated by an 8000 BP inner fiord shoreline, and by dates from the piston cores. -- Two inner fiord moraines may be associated with the Nachvak maximum. Moraine K2 indicates that the ice-sheet reached a minimum elevation of 180m aht, while moraine K1 suggests a maximum of about 220m aht. A local ice tongue emanating from Kogarsok Brook appears to have coalesced with the regional ice-sheet at this time. These elevations concur with evidence supporting a restricted Late Wisconsin glaciation by suggesting that the Nachvak ice-sheet was not vertically extensive. Association with the Nachvak moraines dated by Evans and Rogerson (1986) and Bell (1987) implies an 'early' glacial maximum, with retreat beginning by approximately 20 ka BP. The restricted vertical extent of ice implies a restricted horizontal extent; there is no support in the inner fiord for the hypothesis that the Late Wisconsin ice sheet extended beyond the Labrador shelf. -- Older glacial phases are indicated by higher moraines, with a greater degree of weathering. An Adams Lake phase equivalent appears to be evident in moraines west of Kogarsok Brook; ice-flow indicators suggest that moraines M1 and M3 are of local origin, though M3 may be the result of coalescing local and regional ice. If these moraines are of the same age as the Adams Lake phase in the outer fiord, they suggest that local ice activity at this time was more extensive than previously thought. A much earlier glaciation is indicated by fragments of a moraine at approximately 500m aht. This is considered equivalent to the Ivitak phase observed in the Selamiut Mountains, and may be over 70 ka BP.