|Titre||Pre-Late Wisconsinan shells in Rae Isthmus Ice Stream tills: implications for LIS dynamics and deglaciation of northwestern Hudson Bay|
|Auteur||McMartin, I; Campbell, J E; Dredge, L A|
|Source||CANQUA-CGRG biannual meeting, abstracts; 2013 p. 167|
|Séries alt.||Secteur des sciences de la Terre, Contribution externe 20130053|
|Réunion||CANQUA-CGRG biannual meeting; Edmonton; CA; août 18-21, 2013|
|Lat/Long OENS||-88.0000 -80.0000 70.0000 66.0000|
|Sujets||dépôts glaciaires; tills; déglaciation; topographie glaciaire; géologie des dépôts meubles/géomorphologie; Cénozoïque; Quaternaire|
|Programme||Gisements polymétalliques - Presqu'île Melville (Nunavut), GEM : La géocartographie de l'énergie et des minéraux|
|Résumé||(disponible en anglais seulement)|
A previously unknown, exotic calcareous till was observed in north-northwest trending streamlined terrain over Archean gneisses on the mainland south of Repulse
Bay, Nunavut. The carbonate-rich till contains small but robust marine shell fragments with corrected finite ages varying between 31,660 ± 210 BP 14C yr BP and 39,190 ± 460 14C yr BP at 5 separate sites. Radiocarbon ages for field duplicate samples
collected at one site and for laboratory duplicates analyzed at two different laboratories confirm the mid-Wisconsinan age for the shells. All shell sample sites are located on a high plateau above the Holocene marine limit defined by clear trimlines
at 145 m a.s.l. Radiocarbon ages on marine shells collected below the maximum limit of post-glacial marine inundation support a minimum age for deglaciation between about 6.1 ka 14C BP and 6.9 14C BP in the same area.
The majority of striations
measured in the field are parallel to the streamlined forms, indicating the area was influenced primarily by ice flowing north-northwestward that converged into the low-lying Rae Isthmus north of Repulse Bay as part of an ice stream. However, as far
inland as 80 km from the coast of Repulse Bay, late southeastward and eastward ice-flow indicators overprint this dominant northward flow reflecting a late ice-flow convergence into Repulse Bay associated with opening of the bay during deglaciation.
The relatively high northward-streamlined terrain with carbonate fossiliferous till appears to have largely escaped the southeastward and eastward ice flows as there is little imprint of these late deglaciation phases on the high plateau.
Surface tills in the north-trending ice stream landforms have a calcareous sandy silty matrix (6 to 32% CaCO3 in <63 µm size fraction) and contain 11 to 38 % Paleozoic carbonate clasts in the pebble fraction (8-30 mm). The closest carbonate
up-ice sources are distant Paleozoic outcrops lying to the south-southeast in Roes Welcome Sound and/or Southampton Island, over 25 km from the most southern site where the old shells were collected. The pre-Late Wisconsinan shells were likely
incorporated in the calcarerous till directly from the floor of Roes Welcome Sound or with pre-Holocene raised coastal marine sediments, and redeposited in the streamlined landforms during the ice streaming event.
Implications for these findings
are numerous. 1) In contrast to most paleogeographic models of the LIS that suggest full ice conditions in Hudson Bay during marine isotope stage 3, the pre-Late Wisconsinan ages of the marine shells in the streamlined till suggest the northwestern
part of Hudson Bay through Roes Welcome Sound was ice-free for a minimum of 7500 radiocarbon years. Alternatively, the opening of sea waters may have occurred as a result of a topographically controlled calving bay developed along the deep Foxe
Channel and further to the northwest into the even deeper Frozen Strait that connects to Repulse Bay. 2) The presence of carbonate-rich till in streamlined landforms concordant with streamlined landforms north of Repulse Bay extends the southern
limit of the Rae Isthmus Ice Stream south of the bay, indicating that the onset zone for the ice stream lay in the north Southampton Island - White Island region, not Foxe Basin as previously suggested. This discovery changes the interpretation of
source location for the carbonate-rich till, hence the ice-flow dynamics in the Repulse Bay ¿ Southampton Island ¿ southern Melville Peninsula area. The age of the Rae Isthmus Ice Stream is unknown but likely LGM and was maintained until early
deglaciation. 3) The discovery of distally-derived till in ice stream landforms south of Repulse Bay, as well as late oblique to opposite ice flows around Repulse Bay, may have significant implications for mineral exploration; specifically for
following up indicator mineral dispersal trains that are from more distal sources, or in discordant directions with the regional northward landform orientation.
|Résumé||(Résumé en langage clair et simple, non publié)|
Jusqu'alors inconnus, des sédiments glaciaires provenant d'une source lointaine et contenant des coquillages marins anciens qui remontent à une
époque d'avant la dernière glaciation continentale ont été découverts près du village de Repulse Bay, au Nunavut central continental. Cette découverte a de grandes implications pour la paléo-histoire des derniers glaciers et fournit de nouvelles
connaissances géologiques sur la composition des matériaux de surface. Ces matériaux sont utilisés pour retracer des débris minéralisés à leurs sources dans le substratum rocheux et ainsi développer des méthodes efficaces d'exploration minière dans
des terrains anciennement couverts par les glaciers. Ce travail fait partie du projet de la péninsule de Melville à la Commission géologique du Canada, dans le cadre du Programme en géocartographie de l'énergie et des minéraux (GEM) de Ressources