Serge Occhietti

Sakami moraine, Quebec: A 500-km-long moraine without climatic control

The Laurentide ice sheet in eastern Canada disintegrated step by step, as evidenced by several morainic complexes. Although commonly interpreted as reflecting climatic events, it seems probable that the disintegration simply related to changes in the dynamics of the ice margin, without climatic control. From the example of the Late Glacial Sakami moraine in Quebec, formed during the drainage of Lake Ojibway into the Tyrrell Sea, the concept of a “re-equilibration moraine,” already briefly proposed, is developed. The construction of such a moraine implies a stabilization of the ice front when the glacier margin, previously floating or calving in a lacustrine or marine basin, is suddenly grounded because of topographic blocking or abrupt drop of water level due to drainage. The glacial retreat then pauses until the equilibrium profile of the ice is restored, and a morainic accumulation can form during the halt.

Palynostratigraphy and Th/U ages of upper Pleistocene interglacial and interstadial deposits on Cape Breton Island, eastern Canada

On Cape Breton Island (Nova Scotia), near the margin of the Wisconsinan ice sheet, karst depressions in Mississippian gypsum-bearing rocks contain interglacial and interstadial organic-rich deposits. Three palynostratigraphic units have been observed and tentatively dated by Th/U measurements on embedded fossil wood. The first, dated at ca. 125 ka, may be assigned to the oceanic /sup 18/O substage 5e; thermophilous forests (Quercus, Ostrya, pinus strobus) developed in response to a climate warmer than the present. The second unit, rich in Abies balsamea pollen and dated at ca. 87 ka, may relate to the /sup 18/O substage 5a; it reflects a cool and wet climate not unlike that of today. The third unit probably spans part of the mid-Wisconsinan (/sup 18/O stage 3); it shows alternating boreal forest-tundra forest assemblages indicative of climatic oscillations during a generally cold interval.

Glacial and nonglacial sediments of Matuyama paleomagnetic age on Banks Island, Canadian Arctic Archipelago

Preliminary paleomagnetic investigations of unconsolidated sediments from Duck Hawk Bluffs on Banks Island, in the Canadian Arctic Archipelago, have defined magnetite-residing magnetizations that exhibit systematic polarity correlations between temporally equivalent units from spatially separate stratigraphic sections. The preglacial Worth Point Formation sediments and overlying Duck Hawk Bluff Formation sediments (including marine and glacial deposits laid down during the Banks Glaciation) have magnetically reversed directions and therefore are probably of Matuyama age (<730 Ka). Deposits of the younger Morgan Bluffs and Cape Collinson interglacials and Thomsen and Amundsen glaciations are normally magnetized and therefore of Brunhes age (<730 Ka). These sesults provide the first minimum-age estimate for the Worth Point Formation organic deposits and for the Banks Glaciation, the oldest and strongest glaciation recorded in the western Arctic. The new time framework will facilitate correlations with terrestrial sequences of Beringia and early Pleistocene Arctic Ocean sediments.

Fréquence des datations Au 14C de faunes marines post-glaciaires de l'est du Canada et variations paléoclimatiques

Abstract A statistical analysis of radiocarbon dates for 365 samples associated with postglacial marine deposits of Eastern Canada and New England, U.S.A., yield a non-random statistical distribution. This distribution pattern is explained by two principal controls (that are partly interdependent): (1) secondary eustatic fluctuations, creating relative stabilizations of the sea level with an uplifting land; and (2) more generally the climatic variations. Other factors involved may include: (1) limitations of 14C dating method; (2) non-statistical sampling methodology adopted by the Quaternary geologists; (3) lack of intensive study of some of the marine basins; and (4) incidence of rapidly changing paleogeographic events (opening and shoaling of marine basins, glacial re-advances and changes in the ice flow). The statistical appraisal of errors in published 14C dates and the standard deviation, calculated with sliding means, enable us to screen out the influence of most of the non-climatic factors. With this technique, we are also able to draw a curve based on a general synthesis of all variations in 14C dates distribution. In this way, the irregularities observed in the synthesis curve are now correlated with paleoclimatic and paleogeographic events that took place in Eastern Canada between 15,000 years B.P. to present. That curve shows a close correlation during the Holocene with other climatic indicators and gives some original informations concerning the period 15,000–10,000 B.P. No attempt has been made to adjust radiocarbon years to calendar years for two reasons: (1) lack of agreement between comparative curves prior to 7,000 B.P.; and (2) lack of information concerning the effect of “old waters” on shell dates.

Late Winconsinan glacial dynamics, deglaciation, and marine invasion in southern Quebec

Deglaciation patterns of the Laurentide ice sheet in southern Québec were related to climatic and nonclimatic factors. Thinning of the ice sheet and thermolatitudinal ice retreat are directly linked to the global warming at the end of late Wisconsinan time, between 17 ka and 11 ka. However, correlations between regional deglacial events and global climatic oscillations during that period have yet to be established, except for the St. Narcisse Moraine event, which has been assigned to the Dryas III, and perhaps for the reactivation of Laurentide ice in the middle Chaudiere Valley area during an older cold event. Nonclimatic factors also played a major role on the deglaciation of the region. After the last glacial maximum, the Laurentide ice sheet began to decrease, and the St. Lawrence corridor channelized a major ice stream, the St. Lawrence ice stream, which became a major feature of the southeast sector of the ice sheet. The St. Lawrence ice stream is a flow convergence zone caused by a combination of ice dynamics and topographic factors and rapid ablation at its terminus. The head of the flow convergence migrated deeply into the Laurentide ice sheet and caused thinning of adjacent ice masses. As a consequence of this accelerated ablation, an Appalachian sector became differentiated from the main ice sheet. Regionally, the terminus of the ice stream was a calving bay that retreated along the Laurentian channel to the mouth of the Saguenay fjord. The ice stream and the deglaciated estuary generated the well-known flow reversal along much of the northern margin of the Appalachian sector. In addition to these generalized deglaciation processes, local and regional topographic features influenced the ice dynamics and the final deglaciation patterns. Occhietti, S., Parent, M., Shilts, W.W., Dionne, J.-C., Govare, É., and Harmand, D., 2001, Late Wisconsinan glacial dynamics, deglaciation, and marine invasion in southern Québec, in Weddle, T.K., and Retelle, M.J., eds., Deglacial History and Relative Sea-Level Changes, Northern New England and Adjacent Canada: Boulder, Colorado, Geological Society of America Special Paper 351, p. 243–270. E-mails: Occhietti, ochietti.serge@uqam.ca; Parent, parent@gsc.nrcan.gc.ca; Shilts, shilts@geoserv.isgs.uiuc.edu; Dionne, ggr@ggr.ulaval.ca; Govare, e.govare@sympatico.ca; Harmand, harmand@clsh.univ-nancy2.fr on February 28, 2015 specialpapers.gsapubs.org Downloaded from

Late Wisconsinan—Early holocene deglaciation of Québec-Labrador

Abstract During the Wisconsinan, the area of Quebec-Labrador was completely covered by the Laurentide Ice Sheet (LIS), except for nunataks in the Torngat Mountains. For this reason, pre-Upper Pleistocene events are only documented in scattered stratigraphic sections, and by erosional glacial marks. After the Last Glacial Maximum (LGM), the Labradorean Sector of the LIS evolved from a single, predominant dispersal centre with subsidiary ice divides, into peripheral ice domes and masses which remained connected or not to the central dome. The central dome was not a simple and stable dome-shaped ice mass, but an evolving ice mass. Thinning of the LIS through ablation, and mechanical drawdown along its margins as the result of diachronic ice streams in the St. Lawrence Corridor and Hudson Strait are the main features of Late-Glacial ice flow dynamics. In the areas south of the St. Lawrence Corridor, ice masses over the Appalachian uplands evolved from a glacier complex confluent with the LIS into separate local ice caps. During a part of the warm Bolling-Allerod phase, a series of ice front positions mark a fast retreat of the ice front in the Appalachians of southern Quebec, between about 11,900 (or as recent as 11,600) and 11,600 (or 11,300) conventional 14 C years B.P. (equivalent of ages of terrestrial fossils). The ice mass over the Canadian Shield, north of the St. Lawrence Corridor, dissipated slowly, between about 11,000 (or several centuries later) and 6,500 B.P. The deglaciation pattern includes the differentiation of an ice mass over the Hudson Bay, early deglaciation of the Labrador Highlands, a major change of ice flow in the Ungava Bay, and a very roughly concentric ice retreat pattern in the south-west, south and south-east margins of the remnant main ice mass. In the northern Ungava and Labrador peninsulas, major glacial lakes in low-lying areas were dammed between the ice front and the tilted deglaciated land. Lowlands depressed by glacioisostasy were momentarily invaded by marine waters, mostly between 13 ka and 7 ka. The last glacial ice masses were located in the Labrador Trough and Nunavik and finally disappeared c. 6.5 ka.

Laurentide ice sheet: oceanic and climatic implications

Abstract An expanded and improved Wisconsinan stratigraphy of southeastern Canada, with revised correlations, has led to a new hypothesis on ice-mass shape and movement, since the last glacial maximum, which involves: (1) a refinement of the interpretations and correlations of continental climatostratigraphy; (2) the formulation of a model for the dynamics of the margins of the ice sheet which is not necessarily related to climate; and (3) the determination of the contribution of the ice mass to (a) variations in ocean volume and (b) to variations in oceanic 18 O. In contradiction to oceanic sequences which are both continuous and sensitive to climatic changes, continental sequences are characteristically discontinuous, reflect local or regional paleoenvironments (grounded margins, ice-shelf, etc.), are subject to the limitations of 14 C dating and tend to locally exaggerate fluctuations of global climate (glacial/interglacial, stadial/interstadial). During the last glacial maximum of the Wisconsinan State, the ice sheet was a multidome complex with several centres of outflow and satellite ice caps. The eastern margin of grounded ice corresponded roughly to the present coastline. The volume of ice was less than in the model of the single-dome mega-ice sheet which is commonly used. This problem must be considered when calculating the oceanic volume and 18 O variations. Ice retreat was characterized by large fluctuations of the ice margin, that were unrelated to climatic variations. These fluctuations reflected phases of restoration of ice-profile equilibrium, as evidenced by reequilibration moraines (e.g. Sakami moraine) and by huge ice surges (e.g. Cochrane episodes). Temporary retention of large water volumes in glacial lakes and inland seas induced a time-lag between the climatic amelioration and subsequent oceanic response (e.g. sea levels). The irregular ice retreat and the retention of water masses on the continent combined to produce a step-by-step increase in ocean volume ( eustatic pulses ). Complex geoidal readjustments during Late Glacial and Postglacial times might therefore be expected.

Fluvial sedimentation in a semi-arid region: the fan and interfan system of the Middle Souss Valley, Morocco

The Souss Valley is a structural basin located between the High Atlas Range and the Anti-Atlas Range in southern Morocco. Quaternary deposits in this valley are stacked as torrential piedmont fans, in which coarse deposits are dominant, and as flat gently sloping interfans, in which silty deposits are dominant. The visible part of the deposits, referred to as the Taroudannt Group, is composed of a lower silty unit (S0), three sequences (SI, S2 and S3), and a discontinuous cover of Rharbian (Late Weichselian—Holocene) deposits. Each sequence begins with an erosional disconformity and contains three superimposed members: a torrential cobble unit, lenses of highly calcareous sand, and a silt unit related to piedmont silt fans composed of aeolian deposits reworked by diffuse rain-wash and sheet-wash. A climatic origin is ascribed to this repeated superposition of facies. Calcareous epigenesis has modified all these deposits. Percentages of kaolinite, palygorskite and hornblende decrease from the base to the top of the sections, neogenic palygorskite and silica being observed by scanning electron microscope on quartz grains from the two lower silt units. The upper sequence (Maader Formation) is correlated with the upper sequence of the Atlantic coast of Morocco and represents the interglacial—glacial Ouljian—Soltanian (Eemian—Weichselian) climatic cycle. The older units may range from the Early to the Middle Pleistocene.

Pleistocene palynostratigraphy in the St. Lawrence Valley and middle Estuary

The litho-biostratigraphic framework of the upper middle and upper Quaternary units of the St. Lawrence basin results from the interplay of ice advance/retreat, isostatic depression/rebound, fluvial processes and climatic changes. Each main climatic cycle is characterized by a sedimentary sequence representative of the associated changing depositional environments. The climatic significance of the units is assessed with reference to the pollen content, and the evolving trend of the inferred vegetation. Five assemblages are defined: (1) the mixed woods assemblage related to temperate deciduous forests, (2) the boreal forest subdivided into a balsam fir (A. balsamea) dominated assemblage, a pine (P. banksiana or P. divaricata) dominated assemblage, and a spruce dominated assemblage (Picea mariana), (3) the forest tundra (subarctic zone), (4) the shrub tundra with subarctic plants, and (5) the herb tundra (arctic zone). Three marine invasions, older than Late Wisconsinan, record a progressive vegetal recolonization: a pre-Illinoian marine episode, the post-Illinoian marine invasion of Guettard Sea (6–5 transition), and the Cartier Sea invasion (substage 5b?). Three types of lake inundated the lowlands: (1) lake systems associated with interstadial or interglacial fluvial drainage, (2) deglacial lakes, with a gradual vegetal recolonization, (3) pre-glacial lakes, whose pollen indicates boreal forest replacement by forest tundra and herb tundra.

The last interglacial/glacial group of sediments in the St Lawrence Valley, Quebec, Canada

Abstract During the last interglacial-glacial (Sangamonian-Wisconsinian) cycle, the St Lawrence Valley was invaded by allochthonous ice flowing from the Nouveau-Quebec-Labrador ice dome. The valley was inundated by lakes during the englaciation phases and by post-glacial lakes and glacio-isostatic seas during at least two deglaciation episodes. During the climatic optimum of the last interglacial stage (Isotopic Substage 5e) and during at least two interstades, a fluvial system was inset into the previous units. The base level of the drainage was related to contemporaneous mean sea level and glacio-isostatic state of equilibrium. According to published and original data, the Quaternary group of sediments of the valley can be divided into five parts: (1) Illinoian glacial deposits; (2) fluvial deposits of the climatic optimum of the Sangamonian; (3) a set of intermediate sedimentary sequences which may comprise fluvial, glaciolacustrine, glacial and marine deposits; (4) the upper main sequence; and (5) Holocene fluvial and lacustrine deposits. The age and correlation of the intermediate sequences are debated. They indicate at least one glacial event during Isotopic Stage 5b or 4. The upper sequence is a complete succession of fluvial, glacial and post-glacial marine facies (Les Becquets Interstade and Trois-Rivieres Stade). According to contradictory 14 C and TL dates, the valley was covered by ice after 75 ka (Stage 4) or after 61 ka (during Stage 3) until about 11 ka.