The Don formation, Toronto, Canada: a record of the sangamonian interglacial and early wisconsinan (warm part of MIS 5e to a MIS 5 cold substage)
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Quaternaire Revue de l'Association française pour l'étude du Quaternaire vol. 27/4 | 2016 Volume 27 Numéro 4 The Don formation, Toronto, Canada: a record of the sangamonian interglacial and early wisconsinan (warm part of MIS 5e to a MIS 5 cold substage) La formation de Don, Toronto, Canada, de l’interglaciaire sangamonien au début du wisconsinien (de l’optimum du MIS 5e à un stade froid du MIS 5) Serge Occhietti, Martine Clet and Pierre J.H. Richard Electronic version URL: http://journals.openedition.org/quaternaire/7746 DOI: 10.4000/quaternaire.7746 ISSN: 1965-0795 Publisher Association française pour l’étude du quaternaire Printed version Date of publication: 1 December 2016 Number of pages: 275-299 ISSN: 1142-2904 Electronic reference Serge Occhietti, Martine Clet and Pierre J.H. Richard, « The Don formation, Toronto, Canada: a record of the sangamonian interglacial and early wisconsinan (warm part of MIS 5e to a MIS 5 cold substage) », Quaternaire [Online], vol. 27/4 | 2016, Online since 01 December 2016, connection on 01 May 2019. URL : http://journals.openedition.org/quaternaire/7746 ; DOI : 10.4000/quaternaire.7746 © Tous droits réservés
Quaternaire, 27, (4), 2016, p. 275-299 THE DON FORMATION, TORONTO, CANADA: A RECORD OF THE SANGAMONIAN INTERGLACIAL AND EARLY WISCONSINAN (WARM PART OF MIS 5e TO A MIS 5 COLD SUBSTAGE) n Serge OCCHIETTI1, Martine CLET2 & Pierre J.H. RICHARD3 ABSTRACT Regarded as a reference unit of the Sangamonian Interglacial in North America, the Don Formation was deposited however during a longer interval of time, including the interglacial climatic optimum of MIS 5e followed by an erosive and cool phase (intra- MIS 5e or MIS 5d), a boreal phase with a fir forest (late MIS 5e transition or MIS 5c), erosive episodes and the early part of a cold stadial (MIS 5d or MIS 5b). From the planar geometry of the beds exposed at the former Don Valley Brickyard in Toronto, the forma- tion comprises five continuous allozones with lateral variations of thickness. Each allozone corresponds to dominant conditions of sedimentation in a shoreface of Lake Coleman within a limited accommodation space. Allozone D2 is correlated to other interglacial units of Eastern Canada, with ages between 128 and 115 ka. The interglacial climatic optimum is interrupted in the area by two cool events, the first one similar to the Tunturi event of Finland, the second one probably related to a Greenland Stadial. The heterogeneous upper Allozone D5 was deposited in shallow waters when a Northern Boreal Forest prevailed in the watershed. This zone is corre- lated to the Deschaillons Varves of the central St. Lawrence River Valley, deposited prior to the first regional Wisconsinan glacial advance in the central St. Lawrence River Valley and the resulting glacial Lake Scarborough Formation. From the 80 ka minimal U/Th age of the Deschaillons Varves and the 98 ± 8 ka TL age of the later marine La Pérade Clay, Allozone D5 was deposited during the beginning of one of the cold episodes of MIS 5. Keywords: Interglacial, MIS 5, Sangamonian, pollen analysis, Lake Ontario basin RÉSUMÉ LA FORMATION DE DON, TORONTO, CANADA, DE L’INTERGLACIAIRE SANGAMONIEN AU DÉBUT DU WISCONSI- NIEN (DE L’OPTIMUM DU MIS 5e À UN STADE FROID DU MIS 5) Unité de référence de l’Interglaciaire Sangamonien en Amérique du Nord, la Formation de Don a été en fait déposée pendant un plus long laps de temps, incluant l’optimum climatique interglaciaire du MIS 5e suivi d’une phase fraîche avec érosion, une phase boréale à sapinière (transition à la fin de MIS 5e ou MIS 5c), suivie de phases d’érosion et du début de l’un des stades subarctiques MIS 5d ou MIS 5b. À partir de la géométrie planaire des affleurements de l’ancienne carrière Don Valley à Toronto, la Formation de Don est subdivisée en cinq allozones d’épaisseur variable latéralement. Chaque allozone correspond à des conditions de sédi- mentation dominantes sur une avant-plage du lac Coleman, dans un espace d’accommodation restreint. L’Allozone D2 fossilifère est corrélée aux autres unités interglaciaires de l’est du Canada, d’âge compris entre 128 et 115 ka (MIS 5e). L’optimum climatique interglaciaire est interrompu dans la région par deux refroidissements, le premier similaire à l’événement Tunturi de Finlande, le second probablement en relation avec un Stade Glaciaire du Groenland. L’Allozone supérieure D5, hétérogène et déposée dans des conditions subarctiques et de bas niveau lacustre, succède à une phase érosive. Elle est corrélée avec les Varves de Deschaillons de la vallée centrale du Saint-Laurent, et précède la première invasion glaciaire wisconsinienne de la vallée et la Formation glaciolacustre de Scarborough. D’après l’âge U/Th minimal de 80 ka de ces varves et l’âge TL de 98 ± 8 ka de l’Argile marine de La Pérade posté- rieure à l’invasion glaciaire, l’Allozone D5 a été déposée au début de l’un des épisodes froids du MIS 5. Mots-clés: Interglaciaire, MIS 5, Sangamonien, analyse pollinique, bassin du lac Ontario 1 - INTRODUCTION In Europe, this time scale is applied to several terrestrial sequences, being aware that the duration of continental The time scale of the worldwide climatic fluctuations climatic episodes is variable and depends on the geographic during the last interglacial Marine Isotope Stage 5e (MIS 5e) settings of latitude, elevation and continentality. and following episodes of MIS 5d to 5a, is now preci- In North America, MIS 5 continuous terrestrial sely delimited by the marine and glacial isotopic records. sequences are rare and with variable climatic accuracy 1 Département de géographie, Université du Québec à Montréal, C.P. 8888 succursale Centre-ville, MONTREAL, QC, H3C 3P8, Canada. Email: email@example.com 2 Retraitée du Laboratoire Morphodynamique Continentale et Côtière M2C CNRS-UMR 6143, 24 rue des Tilleuls, FR-14000 CAEN. Email : firstname.lastname@example.org 3 Département de géographie, Université de Montréal, C.P. 6128, succursale Centre-ville, MONTREAL, QC, H3C 3J7, Canada. Email: email@example.com Manuscrit reçu le 27/05/2016, accepté le 05/09/2016
276 (see section 5.10). However, tens of units are related to the climatic optimum of MIS 5e, the Sangamonian Inter- glacial sensu stricto, equivalent to the Eemian in conti- nental Europe. The lower beds of the Don Formation, in the Toronto area (fig. 1 and 2), described as early as 1894 by Coleman and in 1895 by Chamberlin, are the most studied continental sediments of this interglacial in North America (review by Kelly et al., 1987). The entire Don Formation rests on the York Till, related to the Illinoian (Saalian) Glacial Stage and is overlain by the Scarborough Formation deposited in a glacial lake. Some valleys are carved into these two formations, partly filled in by the Pottery Road Formation (fig. 3 and 4). This succession is then overlain by several stadial and interstadial units of Wisconsin age (Karrow & Occhietti, 1989; Karrow, 2004). The Don Formation was deposited as lower shoreface Fig. 1: Location of Toronto and USA sites with Sangamonian soil, beds or speleothems referred to in Section 5.10. The limits of inter- beds (Eyles & Clark, 1988), near the mouth of a paleo- glacial Lake Coleman changed during the interglacial and transi- river named the Laurentian River (White & Karrow, tional time; the limit on the figure is tentative. 1971), in the non-glacial Lake Coleman (Terasmae, 1960; Fig. 1 : Localisation de Toronto et des sites aux USA avec un sol, des lits ou des spéléothèmes du Sangamonien cités dans la section 5.10. Les Karrow, 1990). Most of the studies conducted before limites du lac interglaciaire Coleman ont changé au cours de l’inter- 1950 on the exposures of the Don Valley Brickyard glaciaire et de la transition postérieure. La limite indiquée est très approximative. in Metropolitan Toronto (fig. 5) focused on the lower fossiliferous beds, with a fauna including Bison, Ursus, Castoroides and two types of Cervidae (Coleman, 1933, the Non Arboreal Pollen (NAP) and the spores were not 1941). These lower beds correspond to the interglacial counted, and the number of samples was limited. Eyles & climatic optimum. Gray (1950) was the first to describe Clark (1988, 1989) made a detailed facies analysis before all the lithologic parts of the Don Formation. His unpu- the closure of the Don Valley Brickyard in 1984, without blished thesis, difficult to access and with obsolete corre- stratigraphy. Some of their conclusions were debated by lations, contains original lithologic and lithostratigraphic Karrow (1989). Thus, up to now, the studies after 1950 data that have not been subsequently taken up. Later, did not use nor define a lithostratigraphic framework for the climatic sequence constructed by Terasmae (1960) the Don Formation. from the pollen content and other fossils of the forma- Based mostly on the sequence of Terasmae (1960), tion became the reference frame for numerous further climatostratigraphic charts have been used for several studies and remained so. However, the pollen content of decades. A long hiatus between the Don Formation and the the lowermost and uppermost beds was not identified, overlying glaciolacustrine Scarborough Formation was James Bay 75°W 70°W 65°W 60°W Moose R. Nottaway R. 50°N Sangamonian sites 50°N y ua r Est Ontario Québec Sag er uen ay R low Gulf of Newfoundland- . St. Lawrence Labrador e Magdalen I. L. Ile aux Coudres dl Le id Bassin Woody Superior m Orléans I. Cove Ott . Rivière Plante Green Point Cape Breton I. aw Deschaillons Georgian aR Montréal . Chaudière R. East bay Pointe-Fortune . L. Bay R Addington Forks Michigan en ce USA Miller t ia S co 45°N L. wr Creek Milford 45°N Laurentian R. L a Huron St. L. Champlain va Salmon No Atlantic Ocean Toronto L. Ontario Moh River 0 300 awk R kilometers 80°W Fernbank . 70°W 65°W 60°W L. Erie Hudson R. Fig. 2: Simplified path of the ancestral Laurentian River and location of Toronto and other sites with Interglacial Sangamonian beds in eastern Canada (from Richard et al., 1999) including the Moose River site (Mott & Dilabio, 1990), and in New York State (Karrow et al., 2009). Fig. 2 : Tracé simplifié de l’ancienne Rivière Laurentienne et localisation de Toronto et des sites de l’Interglaciaire Sangamonien de l’est du Canada (d’après Richard et al., 1999) incluant le site de Moose River (Mott & Dilabio, 1990), et de l’État de New York (Karrow et al., 2009).
277 VIII Sunnybrook Drift VII Pottery R.Fm VI Scarborough Fm IV V Don Fm III York Till II I Fig. 3: Upper part of the Don Valley Brickyard working face, Toronto (enlargement of a color photo of Paul Karrow, 1957): planar geometry of the Don Formation with continuous allozones and a gentle slope from the East (right) to the West (left). Pale bands (even numbers) indicate more sandy and drier groups of beds. Darker bands correspond to moister groups of beds. The continuous and thin pale band IV is related to an erosional discontinuity and indicates the upper limit of the climatic optimum of the Sangamonian Interglacial. Fig. 3 : Partie supérieure de la carrière de Don Valley, Toronto (agrandissement d’une photo couleur de Paul Karrow, 1957): géométrie planaire de la Formation de Don, avec des allozones continues et une pente douce d’Est (à droite) en Ouest (à gauche). Les bandes pâles (chiffres pairs) indiquent des groupes de lits plus sableux et plus secs. Les bandes foncées correspondent à des groupes de lits plus humides. La bande claire continue et peu épaisse IV marque une discontinuité d’érosion et la limite supérieure de l’optimum climatique de l’interglaciaire Sangamonien. TORONTO area Hu mb er 401 R. Y E. W. HW 2 Do Y D HW on nR * R. Woodbridge Brimley . Road Leaside * Sun Brickyard * Scarborough Don Valley * Don R Brickyard * Bluffs HW . Y 427 N N Lake Ontario 0 20 km Fig. 4: Lower units of the Toronto Quaternary sequence and Allo- 0 20 km zones D1 to D5 of the Don Formation, at the Don Valley Brickyard (enlargement of a color photo of Paul Karrow, 1984). Fig. 5: Location of the local sites in the Toronto area, referred to in Fig. 4 : Unités inférieures de la séquence quaternaire de Toronto et the text. Allozones D1 à D5 de la Formation de Don, carrière de Don Valley Fig. 5 : Localisation des sites de Toronto et des environs cités dans le (agrandissement d’une photo couleur de Paul Karrow, 1984). texte. inferred, which corresponds to an interval between concern the climatic variations and the inception and respectively the Sangamonian Interglacial and related fluctuations of the Laurentide Ice Sheet during MIS 5. transitional episode equivalent to MIS 5e, and an early Regarding the major influence of the continental glacier Wisconsinan cold stage attributed either to MIS 5b or of North America on global change, the significance of to MIS 4 (Prest, 1970; Terasmae et al., 1972; Karrow & the Don Formation is of international importance, and Occhietti, 1989). Karrow et al. (2000) propose an event clear litho- and biostratigraphy of this formation are chart with the same long hiatus for the Eastern and prerequisites for any additional dating and finer corre- Northern Great Lakes area (see chapter 5.8). Conversely, lations. based on facies analysis, Eyles & Clark (1988) conclude The objective of this paper is to provide a renewed look to a continuous sedimentation in a deepening Coleman at the published data and present new field observations Lake. Without reference to other regions, the Don and and biological data on the Don Formation. The first part Scarborough Formations are then related to the entire applies an allostratigraphic approach to the formation, MIS 5 (Eyles & Williams, 1992). Later, based on the followed by a detailed pollen analysis including NAP same facies analysis, a contradictory chart is proposed countings. After the reassessment of the biotic content by Berger & Eyles (1994) from thermoluminescence established by previous studies and of the sedimentation (TL) ages: the Don Formation, with calculated ages processes involved, the different allozones of the forma- circa 80 ka is related to substage MIS 5a, and the Scar- tion are correlated to dated units of the St. Lawrence borough Formation with calculated ages circa 68 ± 9 ka River Valley and Estuary, and alternative chronologies corresponds to MIS 4. All those divergent interpretations are proposed. Variations of the level of lakes Coleman
278 and Scarborough through time are then tentatively sampling had to be applied to beds with silt and clay when reconstructed. Finally, the interglacial and transitional possible. Therefore, the sampling intervals in the open sequence of units of the Toronto area and St. Lawrence part of the Don Formation are not regular, with most of Valley is compared to other sequences in North America. them close to 10 cm. Gravel beds and several sandy beds were not sampled (fig. 8). The sandy upper part, poorly studied so far, was closely sampled, every 5 cm or less. 2 - METHODOLOGY The lower part of the overlying Scarborough Formation was sampled every 10 or 20 cm, for a total of 18 samples. 2.1 - LITHOSTRATIGRAPHY The samples (20 g) were sieved between 100 µm and 10 µm after removal of carbonates with hydrochloric acid This study takes into account the various approaches and removal of silica with hydrofluoric acid. They were developed in previous work (Occhietti, 1990; Richard, prepared without acetolysis. The concentrations (number 1994; Clet-Pellerin & Occhietti, 2000) and in the studies of pollen grains per gram of sediment) were calculated of other authors (Gray, 1950; Karrow, 1967, 1990; using the weighted aliquot method of Jørgensen (1967). Eyles & Clark, 1988; Kelly & Martini, 1986; Lamothe, The pollen percentages were calculated using a standard 1989). Given that the Don Formation accumulated sum of all the pollen of terrestrial vascular plants counted at the margin of a fluvial system, we propose a strati- (300-400 grains per sample). The representation of spores graphy of the formation based on allostratigraphic and (Pteridophyta, Sphagnum) was expressed over this Pollen sequence analyses (Bhattacharya, 2001) coupled with Sum. Identification was made according to the criteria biostratigraphy. From the sequence stratigraphy prin- of Richard (1970); however, because of morphological ciples (see Catuneanu et al., 2011) and their application uncertainties, Pinus banksiana/divaricata and Pinus resi- to sequences of the Toronto area by Martini & Brookfield nosa pollen grains were grouped within the Diploxylon (1995), attention is given to discontinuities (fig. 3, 4, 6 pines and Pinus strobus was specified as an Haploxylon and 7) and sedimentary system tracks. The paleoenvi- pine. Besides, identification of spruce pollen to the species ronmental interpretations and paleoclimatic reconstruc- level is tentative: Picea glauca type is nevertheless distin- tions are based on converging facts at local, regional and guished from Picea mariana/rubens type. Regarding alder global scales. identification, Alnus incana ssp. rugosa type is clearly distinguished from Alnus viridis ssp. crispa type. The pollen taxa are grouped into the following categories: ther- mophilous trees, other trees, shrubs, herbs, spores (fig. 8). The diagram was prepared with Tilia, version 1.7.16 (2011), and plotted with TGView, version 2.0.2 (2004), developed by Dr Eric C. Grimm, from the Illinois State Research and Collections Center at Springfield, Illinois. The climatic significance of the Late Pleistocene units is assessed according to the pollen and spore content, the inferred past vegetation and its evolving trend. The pollen and spore assemblages are compared to the Holo- cene assemblages and their associated climatic signifi- cance, as expressed in modern vegetation (Ritchie, 1987; Anderson et al., 1991; Richard, 1994). From our previous work mostly in the St. Lawrence River Valley and Basin (Clet & Occhietti, 1994; 1995; Occhietti & Clet, 1989; Richard et al., 1999), distinct pollen and spore assemblages occurred during Late Pleis- Fig. 6 : Detailed view of the Don Formation at the Don Valley Brickyard (sample log by S. Occhietti, 1998). The lower Allozones tocene and characterized the regional paleoenvironmental D1 and D2a were covered by debris. dynamics (Clet-Pellerin & Occhietti, 2000). These are as Fig. 6: Détails de la Formation de Don, carrière de Don Valley (coupe follows, from warm temperate to subarctic assemblages échantillonnée par S. Occhietti, 1998). Les Allozones inférieures D1 et D2a étaient couvertes d’éboulis. (tab. 1): 1) the Mixed Hardwoods (MW) assemblage (MWa, MWb, MWc) which contains trees and shrubs found in temperate deciduous forests; 2) the Southern Boreal Forest (SBF) which is subdivided into SBFa: a 2.2 - POLLEN ANALYSIS balsam fir (Abies balsamea) dominated assemblage, and SBFb: a pine (Pinus diploxylon e.g. P. banksiana and/ Former pollen analyses by members of our team, or P. resinosa) dominated assemblage; 3) the Northern mainly on the Scarborough Formation, revealed the Boreal Forest (NBF): a spruce (Picea cf. mariana) necessity to tally the NAP grains and to analyse more dominated assemblage; 4) the Forest Tundra (FT) with samples within a given unit (Richard et al., 1999). With evidences of an open tree canopy with a subarctic sandy and gravelly beds of the Don Formation usually character: Alnus cf. crispa, Ericaceae, Sphagnum. These sterile in pollen and spore grains, a close and selective pollen and spore assemblages are related to vegetation
279 Visual silt gravel Allozones Pollen Other fossils Units bands clay sand cobble assemblages Gray 1950 Erosional discontinuities rhythmites Forest Tundra R Scarb. Northern Boreal Forest T, R brown sand g sand shale cobbles f VIII white sand and fine gravel e no Unit C 6 D5 green sand d Northern Boreal Forest fauna pavement shale wood green sand bc Don 5 green shale clasts a VII d fine sand VI 5 and c poor fauna D4 Southern Boreal Forest T, R Unit B V silt beds b Don 4 4 fine sand a cold event Don 3 g ravel “pebble IV sand D3 no pollen sterile conglomerate” fine sand e diatoms 3 ostracods Mixed Hardwood Forest T, R insects III silt and sand d Don 2.5 molluscs no pollen fine sand cool event Don 2.4 2 Unit A and c abundant silt beds II D2 Deciduous Forest T, R molluscs with with Liquidambar insects shells Don 2.3 1 cool event Don 2.2 “pebble gravel b diatoms conglomerate” a Mixed Hardwood Forest T ostracods sand I Don 2.1 insects molluscs ? Lower Clay lower clay and stones D1 (Don 1) ? (HK) cladocera 0 m York Till Fig. 7: Log of the section of the Don Formation sampled in 1998: allozones, pollen zones and fossil assemblages from other studies referred to in tab. 4. The covered basal part of the section (D1 and D2a) is simplified from Karrow (1969). HK: Hann & Karrow, 1993; R: Richard et al., 1999; T: Terasmae, 1960. Fig. 7 : Log de la Formation de Don échantillonnée en 1998: allozones, palynozones et assemblages de fossiles d’études citées dans le tab. 4. La partie inférieure masquée de la formation est redessinée d’après Karrow (1969). HK: Hann & Karrow, 1993; R: Richard et al., 1999; T: Terasmae, 1960. types which correspond in most cases to the canadian 2.3 - CORRELATION biomes (Strong et al., 1989; Dyke, 2005). Pollen or spores from species living along the banks of Using a system track approach, the correlations between lakes and rivers were grouped in an azonal group labelled the Don Formation and units of the St. Lawrence River RB (River Banks). It comprises mainly Alnus rugosa, and Valley are based on the stratigraphic relative position, the part of Cyperaceae, Poaceae, other herbs and Pterido- nature of the units and their elevation, the climatic condi- phyta. This group is present throughout the sedimentary tions deduced from the biotic content, and the inferred sequence but Sphagnum is however best represented in sea level at the time of deposition established from the the overlying Scarborough Formation. global climatic conditions.
Thermophilous trees Other trees Shrubs Herbs Out of Sum a lia a a a ex r ry fo ea ns uc ian on n e /Il e e s st di e m yl ylo os pa al us ea ea re s y ) ba n a s a r u be gla ar m l ox lox µm µm µm rug cris g t h m y a m a s /O ra c e l . . . p p 0 5 0 . . a e f. n a e rae ae a a diac llac um e m spo um ore a lla ium um rsity rsit tion e a (c us s ne u g us sa ba cf cf cf ha d i 3 2 2 cf cf a e s ce c ra pa um e ce lo lor isi si o hy ae n go ea tru r te di sp d e d gn th og id ya rc an a a a s s l p o es un gin po a dive div entr ep ol sa qu r ta in a s us in es s a la la s s x ce ylu ica ica ice o rn ea e ra lif lif m ro nop op ce go ta iac lic ha ha ole D i th ys Li Ca ue u gl ilia as arp sug agu cer lm rax upr bie arix ice ice ice i nu inu tu tu tu nu nu li ica or yr yr n em bu ac p bu gu te b e ry ia ly an m a p up on ly ilet sm la co Sph ree erb onc L N Q J T C C T F A U F C A L P P P P P Be Be Be Al Al Sa Er C M M Lo N Vi Po Cy Tu Li Ar Am Ch Ca Ap Po Pl La Th Ty N M Po Tr O Se Ly T H C Zones 0 50 100 150 200 Scar 1b 250 300 350 Scar 1a st_2 Don 5.3 400 st_5 st_3 Don 5.2 450 Don 5.1 280 Don 4.3 500 n.s. Don 4.2 550 Don 4.1 600 n.s. Don 3 650 n.s. st_1 700 Don 2.5 750 n.s. Don 2.4 800 n.s. 850 st_3 Don 2.3 900 Don 2.2 Don 2.1 20 40 20 40 20 20 20 40 60 20 40 60 80 20 20 20 400 20 20 40 80 N N 1000 grains/g Blank levels are either not sampled (n.s.) or proved sterile (st); the number of sterile samples is indicated (st_3). Analysis: Martine Clet, 1995-2001 Silt Sand Gravel/Cobble Except for Carya, Quercus, Picea cf. mariana and Pinus diploxylon, low percentages are amplified 10 times (hollow curves). Fig. 8: Pollen and spore diagram of the sampled parts of the Don and Scarborough Formations at the Don Valley Brickyard, Toronto. Fig. 8 : Diagramme sporopollinique de la partie échantillonnée des Formation de Don et de Scarborough exposées à la carrière de Don Valley, Toronto.
281 Vegetation Type Pollen assemblages Milder Zonal Harsher MW MW-a = Deciduous MW-b MW-c Forest Acer, Ulmus, Tsuga, Pinus cf. Mixed Hardwood Liquidambar, Nyssa, Fraxinus, Fagus, strobus, Betula, Forest Celtis, Quercus, Pinus cf. strobus, Picea cf. rubens, Carya, Juglans, Tilia, Corylus, Viburnum Pinus cf. resinosa Castanea, … SBF SBF-a SBF-b Southern Boreal Tsuga, Pinus strobus, Abies balsamea, Pinus cf. resinosa Forest Betula lutea, Picea cf. Betula papyrifera rubens NBF NBF Northern Boreal Picea cf. mariana Forest Pinus cf. banksiana FT FT-a Forest Tundra FT-b Forest Tundra Pinus cf. banksiana Picea mariana, Picea Sphagnum Betula papyrifera glauca, Betula, Alnus crispa HT HT-a Shrub Tundra HT-b Herb Tundra HT-c Tundra Alnus crispa, Betula Salix herbacea, Sphagnum glandulosa, Salix spp. Oxyria, Dryas…, Lycopodium and other Herbs (Poaceae, Cyperaceae, Ericaceae) Present at Scarborough Bluffs RB Larix, Alnus rugosa, Myrica, Pteridophyta, various herbaceous and River Banks aquatic plants. Tab. 1: Upper Pleistocene pollen assemblages identified in Eastern Canada, from Clet-Pellerin & Occhietti (2000). Tab.1: Types d’assemblages polliniques du Pléistocène supérieur identifiés dans l’est du Canada, de Clet-Pellerin & Occhietti (2000). 2.4 - PARADIGMS applied for example to the late-glacial Younger Dryas “event” on both sides of the Atlantic Ocean, even if this Changes in orbital forcing is the primary mechanism of cold episode is not directly linked to orbital forcing. The climate changes, at the scale of thousands of years. Due reassessed continental record of the Don Formation can to changes of the insolation through time and latitudes, therefore be compared to the European, Greenland and to complex retroactions of the global climatic system marine series, using an unambiguous terminology. and to variable inertia, the climatic responses in oceans, continents and glaciers are not perfectly synchronous 2.5 - STRATIGRAPHIC DEFINITIONS: WHAT IS (see the report of Wohlfarth, 2013). Nevertheless, signifi- THE SANGAMONIAN? cant climatic changes are recorded within a defined time span. For example, continental beds with markers of the Marine Isotope Stage 5 (MIS 5) occurred between climatic optimum of the last interglacial can be correlated 135-130 and 72-70 ka (Shackleton et al., 2003; Sánchez with marine beds with benthic foraminifers and pollen Goñi et al., 2012; Capron et al., 2014; Simon et al., which indicate a marine high stand and elevated surface 2016). From 18O and other markers, in concordance with temperature. Despite some differences in ages given to the orbital forcing, this stage comprises five main climatic the limits of MIS 5e and clear differences in the dura- substages: the interglacial climatic optimum (MIS 5e) tion and representation of the Eemian - Sangamonian and subsequent colder or mild substages (MIS 5d to 5a). Interglacial according to the regional setting, a careful In Europe, the pollen content of several continuous conti- correlation between the continental and oceanic strati- nental series, mainly from peat bogs (La Grande Pile, graphic systems gives a global significance to local data. Woillard, 1978; Les Echets, de Beaulieu & Reille, 1984) The Clear Lake pollen record in California (Adam et al., and lake fillings (Reille & de Beaulieu, 1990), and from 1981) and the reassessed chronology of the Devils Hole discontinuous fluvio-deltaic accumulations (Rhine delta, samples of calcite (Moseley et al., 2016) are examples Zagwijn, 1996), indicates the same climatic pattern. The of the global response within North America. Shorter climatic optimum is related to the Eemian, considered as climatic events (in broad sense), identified on conti- the interglacial sensu stricto, and the subsequent substages nents and in Greenland ice cores may be related, or at are referred to the Early Weichselian. Nevertheless, as least compared as similar types of events. This is usually mentioned, the boundaries of the continental and oceanic
282 records are not exactly synchronous. From Shackleton et to 2) was debated mostly in Canada (Dreimanis, 1977; al. (2003), the base of the Eemian is younger than the Karrow & Occhietti, 1989; Grant, 1989; Occhietti et al., base of MIS 5e and falls within the isotopic “plateau” of 1995), and the extent of the glaciers in North America the marine isotope stage (tab. 2). during substages 5d and 5b is still to be assessed from In continental North America, the direct equivalent to direct continental evidence. Sangamonian Stage sensu MIS 5, both in time span and climate, was the Sanga- stricto was also used by some authors in order to distin- monian Stage in its former definition. This period was guish the warmer interglacial phase equivalent to the defined, in Illinois, from a paleosol developed in Illi- Eemian. Richmond & Fullerton (1984) proposed to use noian drifts and related colluvial deposits, and overlain Sangamonian Stage as the equivalent to the Eemian, and by Wisconsinan till or loess (Willman & Frye, 1970). Eowisconsinan Stage as the equivalent to MIS 5d to 5a According to this definition, which was in use for a long substages. The Sangamonian sensu stricto is now used as time (Fulton, 1989; Zhu & Baker, 1995), the Sangamo- the interglacial stage (see 6. Delineation of the Last Inter- nian Stage (sensu lato) was equivalent to the European glacial - Last Glacial stage boundary, in Otvos, 2015). Eemian and Early Weichselian altogether. The usage of Using a diachronic nomenclature, Karrow et al. (2000) Sangamonian Stage in its broad sense came also from distinguishes the Sangamon Episode related to MIS subs- a tacit assessment that the development of glaciers in tage 5e, and the Ontario Subepisode of the Wisconsin North America was limited during substages MIS 5d Episode as the equivalent of MIS substages 5d to 5a and and 5b, and very limited or even close to interglacial MIS 4, in the eastern and northern Great Lakes area. conditions during respectively substages MIS 5c and 5a. In this paper on continental deposits, the climatic Nevertheless, this model with a short glacial time (MIS 4 optimum of the last interglacial is related to the beds Toronto area Estimated Central and Southern Time Greenland Marine Isotope duration Europe scale Phases Stages (names of units by Karrow, 1990) Guiot et al.., 1993 NGRIP, 2004 Shackleton Sunnybrook ka ka et al..,. 2003 Drift 50 Pottery Rd Fm MIS 3 erosion Scar. Sunnybrook Drift MIS 4 Don 5 proglacial beds 71-70 2 Interstadial GI 19 ? 72 1.5 Stadial II 74 GS 20 ** erosion fluvial lower beds of Pottery 3 Ognon 77 GI 20 1 Stadial I Road Formation 78 GS 21 MIS 5a Don 4 5-7 St. Germain II erosional valleys and gullies 85-82.5 GI 21 Don 3 Scar. Scar. 3-4.5 Melisey II MIS 5b erosion Don 5 D5defg 88-87 GS 22 D2/D3 D5c ? GI 22 GS 23 Don 4 erosion 12-16 St. Germain Ic Don 2.5 MIS 5c ? GI 23 100 2.5 102 Don 3 Scar. Montaigu St. G. Ib *** Don 3 erosion D2/D3 D5b 105 GS 24 Don 2.4 erosion Don 2.5 3 St. Germain Ia GI 24 D2/D3 ? Don 5 D5a 2-3 109 MIS 5d Don 2.4 Melisey I *** 110 GS 25 ? erosion erosion 112 Sanchez Goñi cooler GI 25 et al., 2012 Don 2.3 Don 4 Don 3 115 GS 26 Don 2.3 Don 2.5 erosion D2/D3 12-18 Eemian Don 2.4 Don 2.5 120 Don 2.4 GS event ? Don 2.3 Don 2.3 climatic optimum MIS 5e Don 2.2 Tunturi event? Don 2.1 128 Don 1? D1 ? ? 130 A B C D E (135 ka) * Sanchez Goñi et al ., 2012 Tab. 2: MIS 5 chronology in Europe and Greenland, and age hypotheses of the allozones of the Don Formation. * 135 ka: MIS 5 lower limit by Sánchez Goñi et al. (2012). ** Lower limit of MIS 4 above Ognon proposed by Sánchez Goñi et al. (2013).***Limits of cold events Melisey I and Montaigu from Drysdale et al. (2007). Tab. 2 : Chronologie du MIS 5 en Europe et au Groenland. Hypothèses de l’âge des allozones de la Formation de Don. * 135 ka: base du MIS 5 d’après Sánchez Goñi et al. (2012). ** Base du MIS 4, après la phase Ognon, proposée par Sánchez Goñi et al. (2013).***Âges des épisodes froids Melisey I et Montaigu d’après Drysdale et al. (2007).
283 with indicators of climatic conditions warmer than today thickness reaches 7.6 m, but the thickness of each unit and to the beds which directly precede and follow those is variable. Gray (1950) stated (p. 12) that “sections fail beds with indicators of climatic conditions as warm as to correlate in detail; no on (one?) stratum can be said today. Some beds or palynozones which record very short to extend completely around the quarry in a consistent cooler events can be included in the series of beds or paly- fashion, except for the basal clay”. According to Coleman nozones of the climatic optimum. The climatic optimum (1941), the “basal blue clay with logs of wood and unios beds are attributed to the main part of the Sangamonian (clams)... are of warm climate”. From Karrow (1969), the Interglacial and to the “plateau” of MIS 5e (Shackleton top of the Don Formation slopes southeasterly from 102 m et al., 2003). Without a complete series such as the one a.s.l. in central Toronto to 93 m at the Don Brickyard at La Grande Pile (Woillard, 1978) and because of the and 70 m a.s.l. below the Scarborough Bluffs along the homotaxy of beds with interstadial or stadial markers present Lake Ontario shore. The sedimentary structures (Karrow, 1989; Occhietti, 1990), the overlying units or and changes of facies were related to a river entering beds with transitional and colder indicators of climate the shallow interglacial Lake Coleman and the position have an equivocal stratigraphic position, as observed by of the river channel (Terasmae, 1960). Eyles & Clark Mott & Dilabio (1990) in the James Bay Lowlands. In this (1988) described the “Don Beds” as sands interbedded study, all the transitional beds of the Don Formation are with bioturbated peaty muds. They stated that “Most sand revealed to have an equivocal position. For this reason, beds are composite in origin,… each in erosional contact they are temporarily not related to the Sangamonian with the underlying unit.” These beds were deposited in a (sensu stricto), until univocal evidence can be obtained. lacustrine lower shoreface environment subject to episodic In many papers, Last Interglacial (LIG), Eemian, storms and to increasing depth, from 2 to about 18 m, Sangamonian and MIS 5e refer to the same event. Never- that is 12 to 28 m above present level of Lake Ontario. theless, the given limits of the last interglacial and transi- They did not recognize any major erosional discontinuity tional phases which precede the major glaciation related within the Don Formation or between the top of it and the to MIS 4 are somehow changing from one reference to overlying Scarborough Formation. the other. For example, the age given to the lower limit of Numerous studies of the biotic content were done Stage 5e varies from 135 ka (Sánchez Goñi et al., 2012) (reviewed in Kerr-Lawson et al., 1992). Terasmae (1960) to 132 ka (Shackleton et al., 2003), and to the widely established the pollen sequence of the Don Formation used value of 130 ka (Simon et al., 2016). Usually, from 33 pollen assemblages. According to his sequence MIS 5d is related to the Melisey I continental cold phase, and reassessing previous works (Coleman, 1933; Gray, from 115-112 ka to 105 ka, but covers also the end of the 1950; Watt, 1953), the lower part of the Don Formation Eemian in Sánchez Goñi et al. (2012), from 120 ka to was deposited during the climatic optimum of the Sanga- 105 ka. From a wide compilation including Antarctic ice monian Interglacial, and the upper part of the formation cores, Capron et al. (2014) define the LIG period between is related to a change towards a boreal climate. This is 129 and 116 ka, and Veres et al. (2013) develop a time confirmed by other biota (see chapter 5.3), by a stratigra- scale for the last 120 ka. Establishing formal limits to phic diagram of prominent tree pollen by McAndrews (in MIS 5 and to the ice core and continental phases/episodes Westgate et al., 1999), and by a preliminary study of the is out of the scope of this study. In tab. 2, the data from pollen content of a part of the Don Formation by Richard the Toronto lower sequence of units are compared to a et al. (1999). perfectible climatostratigraphic and chronostratigraphic The stratified sand beds with some pebbles and cobbles, framework based on several sources, keeping in mind located at the top of the unit (Gray, 1950; Terasmae, 1960) that the limits given to the different phases/events can were interpreted as evidence for a further shallowing vary by some thousands of years (see Tables 6-2 and 8-1 of the lake accompanied by a cooling of the climate in Wohlfarth, 2013). Toronto is located at the same lati- (Terasmae, 1960). Their significance has been controver- tude than Nice, France; therefore, despite some continen- sial. Gray (1950) concluded to a low lake level and even tality buffered by the proximity of the Great Lakes, the to the exposure of the formation, while Terasmae (1960) Toronto units are compared to the climatostratigraphic proposed a considerable hiatus between the Don and chart of central and southern Europe (tab. 2). the Scarborough Formations. Karrow (1969) concluded there were deposition in shallow waters and a disconfor- mity. During the study of fossil caddishflies, Williams & 3 - REVIEW OF THE DON FORMATION Morgan (1977) identified the following sequence of IN THE TORONTO AREA sediments: lowermost beds deposited on the substrate, lower quiet-water deposits with current-bedding struc- 3.1 - DON FORMATION tures and some large logs, a buff-grey coarse cross- bedded sand unit which suggests a more littoral facies, In the Don Valley Brickyard, Gray (1950) recognized clay laminae and ripple-marked structures with detrital a decimetric Basal Clay and three other units in the Don organics related to deeper water conditions, and at the Formation, a lower deltaic sand (Unit A) up to 5.4 m thick, top of the sequence, limonite stained sands. One of two an intermediate lacustrine unit up to 1.4 m thick, with groups of caddishflies is similar to assemblages found some stones, deposited in deeper waters (Unit B), and an on exposed shores of large lakes, the second group is upper weathered sand (Unit C) up to 1.8 m thick. The total related to large rivers. Morgan (1979) evoked a signifi-
284 cant interval of weathering. On the west side of the Don Valley Brickyard, Poplawski & Karrow (1981) observed Laurentian River: coarse, rust-coloured sand and gravel below 1 m of iron- L. Superior interglacial outlet stained deposits. According to Hann & Karrow (1984), sill > 120 m the organic matter content of about 4 % in the lower fossi- 183 ? m St. Lawrence liferous part decreases abruptly to 1 and 2 % upwards. Georgian R. The percentages of carbonate content decrease slowly, Bay L. Simcoe 176 - ? m from 12 to 8 %, in the lower beds, and rapidly to 2 % in L. the overlying part. Conversely, for Eyles & Clark (1988) Huron 75 + 18 m L. the rusty upper sand bed is related to post-sedimentation Niagara Coleman L. underground waters and the sedimentation is continuous between the Don and Scarborough units. Michigan 174 ? m N L. Erie 500 km 3.2 - EXTENT OF THE DON FORMATION AND FLUVIAL NETWORK OF THE LAURENTIAN RIVER Fig. 9: Inferred drainage pattern between the Great Lakes during the Last Interglacial and transitional phase, from Spencer (1890), Scattered findings indicate that a large sedimentary White & Karrow 1971) and Westgate et al. (1999). Fig. 9 : Mode présumé de drainage entre les Grands Lacs pendant body of the order of one hundred to several hundred km2 le Dernier Interglaciaire et la phase de transition suivante, d’après was deposited in the Toronto area, in the lower reaches of Spencer (1890), White & Karrow (1971) et Westgate et al. (1999). the Laurentian River valley at the edge of Lake Coleman. The Don Formation was exposed naturally in the banks ment on offshore seismic profiles (Eyles & Clark, 1989; of the Don River and widely accessible in the former Anderson & Lewis, 2012). Upstream from the Don Valley Sun Brickyard (Coleman, 1933) and, since 1899, in the Brickyard, shallower shoreface deposits are observed on now abandoned Don Valley Brickyard (fig. 5). From the the Leaside site (Hann & Karrow, 1993) (fig. 5), and presence of abundant mollusc shells, and sometimes further northward in the Toronto vicinity, sandy facies large pieces of wood, the part of the formation deposited correspond to fluvial deposits. If not erased by the during the interglacial climatic optimum was recognized Wisconsinan glaciers, concealed fluvial sediments of the in many temporary excavations downtown Toronto interglacial Laurentian River are potentially present in (Karrow, 1990). The largest excavations were done during the middle and upper reaches toward the Georgian Bay, the construction of subways in the 1950 and 1960’s in as suggested by the interglacial beds of the Woodbridge Metropolitan Toronto. These data and subsequent excava- Cut, 21 km northwest of the Don Brickyard (Sharpe, tions were compiled by Sharpe (1980) and Eyles (1987) 1987; Karrow et al., 2001) (fig. 5). as a series of cross sections with a total length of nearly The status of the Don beds has changed since the early 105 km. In the Scarborough area, downstream the Metro- works of Coleman (1894, 1933) and Chamberlin (1895), politan Toronto on the North shore of Lake Ontario, the regarding the interpretation given to the Don-Scarbo- Don Formation was reached by coring (Terasmae, 1960) rough succession of formations (Eyles & Clark, 1989; and by drilling (Hann & Karrow, 1993) under the Scar- Karrow, 1990). Either, the two units are in continuity borough Formation. (Coleman, 1933, who maintained the name of Toronto A major interglacial and early stadial river, with no Formation; as reused by Eyles & Clark, 1988), or they modern equivalent, is inferred from this large sedimen- represent distinct units (Don and Scarborough Forma- tary body (White & Karrow, 1971; Eyles, 1987; Martini & tions, Karrow, 1967, 1990). Brookfield, 1995) and from the biotic studies. The river flowed along a topographic low in the bedrock identified 3.3 - CHRONOLOGY as the Laurentian River channel by Spencer (1890). The In the Toronto area, a set of thermoluminescence dates bedrock contours of the buried valley are established by Berger & Eyles (1994) gives an age of 80 ± 19 ka at the from drilling and water wells (White & Karrow, 1971; base of the Don Formation, 78 ± 17 ka in the middle part, Westgate et al., 1999). From quartzite cobbles in the Don and 67.6 ± 9 ka near the top of the formation. The same Formation, the interglacial drainage is thought to have set provides also TL ages of 59.8 ± 8.5 and 54.1 ± 8.2 ka connected Lake Huron to the Lake Ontario basin (Gray, at the base of the Scarborough Formation. 1950), over an approximate distance of 110 km (fig. 9; see Fig. 1 in Eyles, 1987). 3.4 - CORRELATIONS Most of the knowledge of the Don Formation is based on the exposures followed through time at the Don Valley Several early correlation charts between the units of the Brickyard. The distal part of the sedimentary body was Lake Ontario Basin and the St. Lawrence River Valley, deposited in deeper waters, as suggested by cladocerans referred to in the introduction, were based on the 14C in logs of the Brimley Road site (Hann & Karrow, 1993) age (see discussion in Dreimanis, 1977) of the St. Pierre (fig. 5). This part, equivalent to prodelta and bottom Sediments found in the central St. Lawrence River Valley deposits, was eroded during subsequent glacial phases (Gadd, 1971; Ferland & Occhietti, 1990a). From the age (Martini & Brookfield, 1995) and the post-glacial Lake of 74.7 ka + 2.7/-2.1 (Stuiver et al., 1978), these sedi- Iroquois low phase, as indicated by a prominent escarp- ments are related to MIS 5a. The contradictory charts
285 proposed by Eyles & Clark (1988); Eyles & Williams the York Till at the bottom of the lowermost beds D1 or (1992), and Berger & Eyles (1994) do not take notice D2a (fig. 7). of stratigraphic successions of other areas. Meanwhile, the stratigraphy of the central St. Lawrence River Valley 4.2.2 - Allozone D2 was revised, with the identification of an early Wiscon- The lower Allozone D2a is mainly sandy and corres- sinan glacial and non glacial succession and the reassess- ponds approximately to the lower grey band I (fig. 4), ment of several units (Lamothe, 1985, 1989; Occhietti & with an irregular erosional upper limit with ripples and Clet, 1989; Occhietti, 1990; Besré & Occhietti, 1990; lenses of gravel (Allozone D2b; the unconsolidated Ferland & Occhietti, 1990b; Bernier & Occhietti, 1991). “pebble conglomerate”of Gray, 1950; see also the lower Preliminary correlations between the lower units of the Gr beds in Fig. 5 of Eyles & Clark, 1988). Just above, two areas were proposed by Clet & Occhietti (1994) and Allozone D2c (fig. 4 and 6) comprises the sequence by Richard et al. (1999). of sand beds related to the pale band II. Allozone D2d (fig. 6) corresponds to the conformable grey band III (fig. 3 and 4) composed of planar silt and fine sand beds, 4 - RESULTS and is overlain by the pale grey sand beds of Allozone D2e (fig. 6). The erosional upper limit is irregular. 4.1 - TABULAR GEOMETRY OF THE DON FORMATION: METRIC TO SUBMETRIC VISUAL 4.2.3 - Allozone D3 BANDS Allozone D3 is limited to coarser beds with gravel related to the end of a major erosional hiatus in the pale On the working face of the former Don Valley Bric- band IV. This allozone represents a continuous marker kyard, the formation presented eight distinct metric to unit on the faces of the brickyard (fig. 3 and 4). On the submetric continuous bands, either pale or dark grey sample log (fig. 6), the thickness of these beds varies (fig. 3, 4 and 6). Light bands are composed of drier sandy between 20 and 40 cm. This unit corresponds to the beds (bands with even numbers II to VIII ; fig. 3 and 4). horizon with unconsolidated “pebble conglomerate” and The darker bands (bands with odd numbers) are more gravel described on several sections by Gray (1950) at clayey and moister, the basal water-saturated sandy band the top of his Unit A. Allozone D3 is overlain by distinct I being the exception. In the Don Formation, the mois- finer deposits. ture varies within a range of 22 to 6 % of water content (Hann & Karrow, 1984). The bands show a progressive thickness variation at the scale of a single working face 4.2.4 - Allozone D4 (fig. 3) and stronger variations at the scale of the past Allozone D4 comprises the sequence of silt and fine working faces and between working faces with different sand beds of the grey bands V and VII (fig. 4), and the orientations (Gray, 1950). The visual bands are approxi- intermediate sandy beds of pale band VI which is thin- mate and informal units, as the moisture limits are only ning from the West to the East on the brickyard working indicative of the dominant grain size and may vary with face (fig. 3). This unit corresponds to the ‘’lacustrine’’ the seasons. Thus, this visual approach provides only a Unit B of Gray (1950). schematic view of the tabular geometry and gentle slope of the Don Formation. From a detailed analysis, each 4.2.5 - Allozone D5 continuous band corresponds to a sequence of disconti- At the top of the formation, Allozone D5 (fig. 4 and nuous beds with roughly the same dominant grain size or 6) is over 1.2 m thick, heterogeneous and corresponds same dominant lithofacies. roughly to the upper visual band VIII (fig. 4 and 6). During sampling, we observed a basal bed (10 cm) with 4.2 - DON FORMATION: ALLOSTRATIGRAPHY green shale fragments (D5a) (uppermost part of grey band VII, fig. 4 and 6), covered by a thin bed of green Based on this general geometry, together with sampling sand (D5b). Above it (fig. 7), a pavement of green shale logs and published detailed studies, the Don Formation flat cobbles and wood fragments (D5c) underlies beds can be subdivided into five allozones (D1 to D5), sepa- of greenish sand (D5d) and white sand and fine gravel rated mostly by erosional limits (fig. 3,4 and 6). beds (D5e). The uppermost beds (D5f and D5g) consist respectively of grey sand with frequent flat cobbles of 4.2.1 - Allozone D1 green shale and of strongly weathered coarse brown The lowermost Allozone D1 corresponds to the discon- sand. tinuous “Basal Clay” described by Coleman (1933) and The seasonal, annual or multi-year beds of the forma- Gray (1950), and mentioned by Terasmae (1960). The tion are discontinuous, as stated by Gray (1950) and Eyles unit is composed of layered clay, interbedded with coarse & Clark (1988). These inner discontinuities are the rule dark sand. The thickness is decreasing (40 to 15 cm) in every log of the formation and every continuous allo- from the early working faces of the brickyard to those zone. For this type of sediments, the thickness of the five observed by Gray (1950). On the late exposures, the clay allozones and their subdivisions varies within the entire beds were apparently discontinuous. This zone belongs sedimentary body. Nevertheless, despite these lateral thic- to the lower grey band I. Some cobbles and blocks lay on kness variations, the relative position of each allozone is
286 constrained within the formation, as it can be pointed out changing taphonomy throughout the record. An excep- in all the photos of the former Don Valley Brickyard (see tion may be found in the behavior of the Monolete spores photos by Coleman, 1941, p. 74; Coleman in Eyles & (most probably derived from erosion of river banks occu- Clark, 1988; Eyles, 1987; Karrow, 1990; this study). pied by ferns) that parallels the total pollen concentration. However this does not affect the sequence of changes in 4.3 - POLLEN STRATIGRAPHY: VEGETATIONAL the mainland vegetation, as translated by the successive AND CLIMATIC INTERPRETATION pollen assemblages. The exposed middle and upper parts of the Don Forma- 4.3.1 - Virtual Palynozone Don 1 (from Hann & tion (Allozones D2b to D5) were resampled (1994 and Karrow, 1984) 1998), resulting in 16 sterile samples and 42 pollen Terasmae (1960) did not sample the basal clay, and assemblages. From this analysis, five palynozones can these beds were buried in 1994 and 1998. Therefore, be distinguished in the formation, with a close but not the pollen content of Allozone D1 is unknown, but perfect relationship between allostatigraphy and biostra- Hann & Karrow (1984) identified in the lowermost tigraphy (tab. 3; fig. 7 and 8). beds of the Don Formation a cladoceran fauna that The pollen assemblages are essentially interpreted can be found today in glacial lakes and boreal to as indicating climatic changes. Given the fact that the subarctic habitats. This cladoceran biozone and the sedimentological sequence represents shoreface depo- equivalent virtual pollen zone Don 1 would corres- sits and is not deposited in an enclosed lake basin, the pond to a transition after the Illinoian glaciation. pollen and spore content of the formation should be Nevertheless, trunks and branches of red cedar and affected by changes of catchment size in the river system Unio shells with both valves united observed in the feeding the shoreface, or by changes in discharge resul- lowermost blue clay by Coleman (1941) correspond ting in sorting. In such a situation, it may appear unwise to warm interglacial conditions. This contradiction is to attribute change in palynological assemblages directly not resolved. to climatic change when variations arising from changing taphonomy could potentially explain much of the identi- 4.3.2 - Palynozone Don 2 fied variability. The interpretations of climate instability Palynozone Don 2 corresponds to a deciduous and from our record, presented below, are nevertheless valid mixed hardwood vegetation, with harsher episodes. because there is no evidence of a reaction to obvious Units Palynozones Vegetation cover Inferred climate Estimated water depth Dominant fluvial Continental on the watershed Coleman, 1933; Gray, 1950 input processes Eyles & Clark, 1988 Karrow, 1990; this study Scarborough Scar 1b remote ice margin Formation Tundra Forest: open hemiarctic distal input sporadic permafrost Scar1b forest with peat deep water non glacial and & ( Richard et al ., 1999) proglacial seasonal ice Scar 1b debris spring floods Scar1a flooding phase g Don 5.3 f taiga to open taiga shallow clasts D5 d,e Don 5.2 wood fragments seasonal ice taiga: Northern subarctic local exposures? spring floods a, b, c Don 5.1 Boreal Forest littoral channels? bedrock clasts seasonal ice on shore shallow ? bottom erosion d Don 4.3 harscher fir forest, deciduous 18 m fine silty sand decrease D4 c Don 4.2 fir forest:Picea forest: cf. mariana increase climatic change fine sand from cooler than seasonal frost b Don 4.1 fir forest: Southern Boreal Forest today to boreal deepening fine silty sand a Don 3 harsher Southern Borea lForest fine sand D3 hiatus ? bottom erosion sand and gravel ? e Don 2.5 Mixed Harwood Forest today and cooler shallow water fine sand d Don 2.5 mild Mixed Harwood Forest warm deepening water silty sand c Don 2.4 Southern Boreal Forest cool event cross-bedded sand D2 c Don 2.3 Deciduous Forest warmest shallow water sand weathering and fluvial b Don 2.2 Southern Boreal Forest cool event minor bottom erosion sandy gravel a Don 2.1 Mixed Harwood Forest (Terasmae, 1960) warm very shallow 2 m sand hiatus bottom erosion ? D1 (Don 1) ? late glacial ? ? seasonal silt input glacial lake ? Tab. 3: Processes and conditions related to the deposition of the Don and Scarborough Formations, from the Illinoian/Sangamonian (MIS 6/5) transition to an early Wisconsinan stadial (MIS 5d or b). Tab. 3 : Processus et contextes de la sédimentation des formations de Don et de Scarborough, de la transition Illinoien/Sangamonien (MIS 6/5) à un stade précoce du Wisconsinien (MIS 5d ou b).
287 – Palynozone Don 2.1 (from Terasmae, 1960) and the Picea rubens type (Richard, 1970) is identified Allozone D2a was buried in the 1990’s and only one almost exclusively in this zone. The total concentration sample could be obtained from the top of the unit. The lowers progressively upwards the section (13,000 to analyses of Terasmae (1960: fig. 19) are used to define 7000 grains/g.). The uppermost assemblage with low the type of assemblage related to these beds. The pollen concentration (3000 grains/g.) is composed of Quercus, content of 10 samples from Terasmae and one from this Carya, Pinus diploxylon (cf. Pinus resinosa) and Picea study (fig. 8; Palynozone Don 2.1) can be related to a cf. mariana; the diversity of trees and herbaceous plants zonal Mixed Hardwood Forest (MWb) assemblage with is significantly poorer than in the rest of Palynozone Quercus, Carya, Tilia, Acer and Ulmus, and without Don 2 with the exception of the colder episodes Don 2.2 Liquidambar. and 2.4. – Palynozone Don 2.2: short colder episode 4.3.3 - Palynozone Don 3 The new pollen spectrum contains Southern Boreal The gravelly Allozone D 3 is sterile. The assem- Forest (SBF) element with diploxylon pine pollen blage immediately above the gravel corresponds to (probably Pinus cf. resinosa) and Picea cf. mariana. This a harsher Southern Boreal Forest (SBFb) with Pinus is related to a short cooler episode. diploxylon (P. cf. resinosa/banksiana), Picea cf. mariana and Quercus. The low concentration could – Palynozone Don 2.3: maximum warmth of the indicate leaching of the sediment, but the abundance of interglacial stage with a Deciduous Forest Picea cf. mariana (34 %) would confirm real harsher Palynozone Don 2.3 is characterized by the occurrence conditions for the vegetation than the overlying zonal of Liquidambar and Nyssa along with most of the other Southern Boreal Forest of Palynozone Don 4. Paly- Mixed Hardwood Forest (MWa) species. The basal part nozone Don 3 follows a gap and represents a drastic comprises dominant Carya (37 %) and Quercus, then change in comparison to the preceding zonal Mixed dominant Quercus, Pinus diploxylon (cf. P. resinosa) Hardwood Forest; the pollen grains were deposited and Carya. In the upper part, the forest contains domi- at the end of an unknown vegetation episode, but the nant Quercus, Ulmus, Pinus diploxylon (cf. P. resinosa), harsher assemblage is representative of an interme- Tilia and Tsuga. The River Bank (RB) species with Larix diate colder episode. The observed discontinuity in and mostly fern monolete spores and herbaceous plants the studied log may be different laterally: some beds, (Poaceae, Cyperaceae) are present (between 7 and 10 %). truncated at the sampled section, may be present and Pollen and spore concentration is low (2000 grains/g.), contain pollen assemblages from a part of the interme- with increasing values in the upper part. The diversity is diate vegetation between the two zonal forests. high (fig. 8). 4.3.4 - Palynozone Don 4 – Palynozone Don 2.4 Palynozone Don 4 is characterized by Southern Boreal Increased representation of small size Betula pollen Forest assemblages with Abies balsamea, Pinus diploxylon grains (cf. Betula glandulosa), Alnus cf. crispa and Erica- (cf. Pinus resinosa/banksiana), Picea cf. mariana, and a ceae pollen characterize Palynozone Don 2.4, indicating sharp decrease of deciduous trees (Quercus, Carya). The an important cooling. The Palynozone is related to the Palynozone is subdivided in three parts. upper half of sandy Allozone D2c (fig. 8). In the lower samples, elevated pollen representation of Quercus is – Palynozome Don 4.1 maintained along with that of Juglans, but increasing of Palynozone Don 4.1 is characterized by a rapid increase Picea cf. glauca, albeit low amounts, is in harmony with of Abies (from 8 to 14 %), with Pinus diploxylon and the presence of the mentioned shrubs. The corresponding Picea cf. mariana, and represents the zonal Southern vegetation may still be a deciduous forest, but experien- Boreal Forest (SBFa) with some Tsuga. This vegetation cing a harsher climate. Some thermophilous tree pollen developed under wetter conditions than during Palyno- may be reworked from underlying beds of the shoreface. zones Don 2 and Don 3. The highest concentrations in The upper sample is characterized by a Southern Boreal the pollen diagram of the formation are observed in this Forest assemblage with a low pollen representation of palynozone (up to 70,000 grains/g.). thermophilous trees and increasing amounts of Abies, Pinus diploxylon (cf. Pinus resinosa/banksiana) and – Palynozome Don 4.2 Picea cf. mariana. A peak of Picea cf. mariana pollen, at the base of this Palynozone, corresponds to a more sandy band (D4c, – Palynozone Don 2.5 fig. 6 and 8), translating a vegetational response to a Diploxylon pine (cf. Pinus resinosa) and Quercus are climate change promoting erosion. Opening of the forest dominant with Carya and Ulmus of the Mixed Hard- cover in a colder environment is inferred. wood Forest. Then the vegetation evolves to Quercus, Pinus cf. resinosa, Carya, Ulmus, Fagus and Acer. – Palynozome Don 4.3 Palynozone Don 2.5 corresponds to a zonal Hardwood Abies balsamea decreases slowly (7 to 2 %), indicating Forest (MWb). Picea cf. mariana is present (7 to 10 %) the extent of a harsher Southern Boreal Forest (SBFb)
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