PALEOSHORELINE EVIDENCE FOR POSTGLACIAL TILTING IN SOUTHERN MANITOBA

Detailed air photo interpretation and four seasons of field mapping an d surveying in southern Manitoba have revealed that the once-level pal eoshorelines of Lake Winnipegosis and Dauphin Lake and the Burnside sh oreline of former Lake Agassiz have been tilted up to the northeast by postglacial differential rebound. Our investigation has also revealed that Lake Winnipegosis has the best preserved paleoshoreline record o f any of the large lakes in southern Manitoba, including lakes Winnipe g and Manitoba. This is because northeasterly uptilting shifts the reg ion's lakes to the southwest. Lakes with southern outlets, like Lake W innipegosis, undergo general regression as the outlet is lowered relat ive to the rest of the basin. Lakes with northern outlets, like lakes Winnipeg and Manitoba, undergo general transgression as northeasterly uptilting raises the outlet relative to the rest of the basin. Along t he northeastern shore of Lake Winnipegosis a staircase of at least 32 abandoned Winnipegosis shorelines exists that is consistent with north easterly tilting. The Dawson level represents the major mid-Holocene h ighstand on Lake Winnipegosis. It persisted for about 500 years, peaki ng at 5290 C-14 yr B.P. (early Dawson) and then falling about 3 m by 4 740 C-14 yr B.P. (late Dawson). The early Dawson shoreline is tilted a t 13.5 cm km(-1) in a direction N24.3 degrees E. Three other shoreline s informally named shoreline 4, shoreline 3, and shoreline 2 are also tilted up to the northeast. Their radiocarbon ages (and slopes in cm k m(-1)) are 3330 yr B.P. (2.2), 1510 yr B.P. (1.3), and 1080 yr B.P. (0 .7), respectively. On Dauphin Lake shoreline IV is the oldest level ma pped for this study. It has a C-14 age of 7910 yr B.P. and is tilted a t 21.7 cm km(-1) in a direction N44.4 degrees E. The Id shoreline mark s the major mid-Holocene highstand for Dauphin Lake. It peaked at 4640 C-14 yr B.P. followed by a rapid decline of about 1 m to the Ib shore line, which is dated at 4320 C-14 yr B.P. Id is tilted up at 8.8 cm km (-1) in a direction N53.4 degrees E. The next major shoreline is Ia3 w hich has a C-14 age of 3020 yr B.P. and is tilted up at 5.3 cm km(-1) in a direction N62.3 degrees E. Tilt directions are significantly more easterly for the Dauphin Lake shorelines than those from Lake Winnipe gosis or any of the much older Lake Agassiz shorelines. Taken together , the Winnipegosis and Dauphin isobases indicate that the direction of tilt in southern Manitoba is more complex than a simple uni-direction al pattern. The observed pattern of tilting for paleoshorelines in sou thern Manitoba agrees better with predictions derived from the recentl y revised loading history model ICE-4G than with those from its predec essor ICE-3G. In general, the calculated tilt based on the ICE-3G load tends to exceed the observed tilt, while ICE-4G tends to underestimat e it. Both ice load models appear to disagree most with our observed t ilts in this region during the interval before about 9000 cal yr B.P., when deglaciation was proceeding rapidly and the large water load ass ociated with Lake Agassiz covered the region. Because both of these ic e load models have been estimated mainly from a global data set of rel ative sea level curves from marine coast sites, it is not unexpected t hat model tilts derived from them do not agree well with observations in the North American continental interior. The pattern of postglacial crustal deformation for southern Manitoba described in this paper cou ld be used to further refine ice load models for the North American co ntinental interior.