Geology and paleoecology of a Middle Wisconsin fossil occurrence in Zorra Township, southwestern Ontario, Canada

Abstract

Nonglacial deposits of Middle Wisconsin age are being discovered with increased frequency across a broad region of southern Ontario, Canada, and provide strong evidence for a time of significant ice withdrawal from the lower Great Lakes region. With each new discovery, a refined understanding of regional climatic and paleoecological environments is emerging. In this paper, we present the results of a sedimentological and paleoecological study of a subtill organic deposit in Zorra Township, southwestern Ontario. The organic deposit, which lies beneath Nissouri Phase Catfish Creek Till (Late Wisconsin), has been dated by accelerator mass spectrometry at between 50.5 and 42.9 14 C ka BP. The organic remains are contained within slack water pond deposits infilling a channel incised into till either of Early Wisconsin or Illinoian age. The fossil assemblage appears to be strongly influenced by taphonomic processes, including degradation due to oxidation, bacterial and fungal decay, and glacial overriding. Reworking and (or) recycling and selective sorting as well as long-distance transport has also influenced the composition of the fossil assemblage preserved. Nonetheless, meaningful paleoecological information is still obtained from this record. Collectively, the pollen and plant macrofossils indicate a boreal-type pine-spruce forest with temperatures cooler than present. The absence of arctic tundra plants, as are found in many other deposits of similar age in the lower Great Lakes basin, is notable. A pond or wetland inhabited by shoreline herbs, shrubs, and trees was present at or proximal to the site. The freshwater mollusc and ostracode assemblages are consistent with a shallow water habitat with dense submerged vegetation. The terrestrial mollusc assemblage suggests a taiga or transitional taiga-tundra fauna. Together, these fossil groups provide one of the most comprehensive environmental reconstructions of Middle Wisconsin time (oxygen isotope stage 3 or OIS3) in southern Ontario and serve to build on the ever-increasing database of paleoecological information accumulating for this episode of the late Quaternary.Résumé : Des dépôts non glaciaires d'âge wisconsinien moyen sont découverts de plus en plus fréquemment dans une vaste région du sud de l'Ontario (Canada) et fournissent des indices probants d'une période de retrait important des glaces de la région du bassin inférieur des Grands Lacs. Chaque nouvelle découverte permet une compréhension plus pointue des milieux climatiques et paléoécologiques régionaux. Nous présentons les résultats d'une étude sédimentologique et paléoé-cologique d'un dépôt organique qui sous-tend un till dans le canton de Zorra, dans le sud-ouest de l'Ontario. La datation par spectrométrie de masse par accélérateur du dépôt organique, qui sous-tend le till de Catfish Creek de la phase de Nissouri (Wisconsinien tardif), a donné des âges entre 50,5 et 42,9 14 C ka BP. Les restes organiques sont contenus dans des dépôts d'étang d'eau stagnante qui remplissent un chenal creusé dans du t...

The pre-Late Wisconsin stratigraphy of southern Simcoe County, Ontario: implications for ice sheet buildup, decay, and Great Lakes drainage evolution

“…The consistent coarser-textured and more poorly sorted lower till (SU1) facies compared with the finer-grained upper till (SU3) facies (Figs. 4, 5, 7) is similar to trends observed within Late Wisconsin till successions in southwestern Ontario (e.g., Catfish Creek and overlying Port Stanley or Tavistock tills), which have been interpreted to record shifts from regional ice flow during the main phases of the Late Wisconsin to more lobate flow during deglaciation (Barnett 1992;Bajc et al 2015a). The fine-grained nature of SU3 results from the incorporation of silty and clayey glaciolacustrine sediments deposited up-ice within the Georgian Bay basin.…”

“…10) complicates direct correlations between Simcoe County and regions to the south. Similar problems have existed for decades at various other pre-Late Wisconsin organic sites across southern Ontario (Dreimanis and Karrow 1972;Berti 1975; Karrow and Warner 1984;Bajc et al 2015a). We emphasize the implications of the southern Simcoe County stratigraphy on interpretations based on classic "type sections" as well as additional characterization of processes and the nature and timing of environmental change that is possible through detailed analysis of cored boreholes.…”

“…The interstadial organic remains commonly yield finite radiocarbon ages ranging from 38 to 50 14 C ka BP (Fig. 10), indicating nonglacial conditions persisted in the region until the Middle Wisconsin (Berti 1975;Warner et al 1988;Bajc et al 2015a). Eyles et al (1985) used coarsening-upward successions interpreted from geophysical profiles throughout the region in an attempt to map the northward extension of the Scarborough delta.…”

Three-dimensional hydrostratigraphy of the Orangeville Moraine area, southwestern Ontario, Canada

“…Precise dating of these older deposits is not yet available; however, there is a growing database of radiocarbon dates and paleoecological information from overlying organic-rich sediments in Ontario (Bajc et al 2009(Bajc et al , 2015. Dates range from >50 to 23.5 ka BP, suggesting widespread nonglacial conditions that may have spanned the Early and Middle Wisconsin (Bajc et al 2009).…”

“…Ice lobes effectively blocked drainage during the initial breakup so that meltwater and sediments flowed into the interlobate zone, forming a complex sequence of interbedded diamictons and stratified deposits that constitute the Dorchester, Waterloo, and Orangeville moraines ( Fig. 1A; Karrow and Paloschi 1996;Bajc and Shirota 2007;Burt 2011;Bajc et al 2015;Burt and Dodge 2016).…”