Jean-Claude Dionne

An estimate of ice-drifted sediments based on the mud content of the ice cover at Montmagny, Middle St. Lawrence Estuary

Abstract Recent measurements made at Montmagny, a locality on the south shore of the Middle St. Lawrence Estuary, 70 km northeast of Quebec City (47°N), give an idea of the volume of fine-grained sediments incorporated in the ice cover and allow an estimation of the annual load drifted by ice. At this locality, a mean thickness of 10 cm of mud was encountered in the ice cover over an area of approximately 20 km 2 . Thus, the total load of fines may be as much as 4 × 10 6 tonnes (t). It is estimated that upon melting at break-up, about 15% of this load returns to the Montmagny tidal flat while the remaining volume is carried to the offshore zone. Considering breakup characteristics, it is estimated that about 1.5−2 × 10 6 t of fines return to the turbidity zone while the remaining load is ice-drifted outside that zone. Since the shore area of the Middle St. Lawrence Estuary covered by ice during the winter is approximately 60 km 2 , it is calculated that a load of 5−6 × 10 6 t of sediment incorporated in the ice cover could escape from the turbidity zone annually. To this load should be added another 4 × 10 6 t of suspended matter which come from the freezing in situ of the turbid water in the offshore zone. An annual output by ice drifting of 10 × 10 6 t of sediment is thus likely and is in great contrast to the output during the ice-free season of approximately 1 × 10 6 t. In the Middle St. Lawrence Estuary, the annual output almost equals the input. Consequently the sediment budget is virtually in a state of equilibrium, which helps to explain why there is very little permanent mud deposition in the shore and offshore zones today. It is concluded that ice processes largely control the sedimentary budget of the turbidity zone of the Middle St. Lawrence Estuary, a particular environment within a mid-latitude inner continental shelf which is partly dominated by ice.

Formes, figures et faciès sédimentaires glaciels des estrans vaseux des régions froides

Abstract Frost and drift ice are two important processes creating distinctive features in tidal flats of cold regions. Original sedimentary forms, figures and structures, and a particular facies, characterized by two opposite grain sizes (fine and coarse together), result from ice action in muddy tidal flats. Grooves or furrows, polished and striated surfaces, tracks and trails, various drag, roll and skip marks, circular depressions, chaotic microreliefs, pushed ridges, polygonal nets, monroes, lenticular bedding and deformations are common to intertidal zones along the St. Lawrence, and the James-Hudson Bay area, Canada. They also occur elsewhere in most tidal flats and on continental shelves dominated by ice. A better knowledge of drift ice features in modern tidal flats is needed. It is a prerequisite to the discovery of similar features in consolidated rocks. The finding of such indicators should improve greatly any interpretation of the paleoenvironmental conditions of deposition.

Sedimentary structures made by shore ice in muddy tidal-flat deposits, St. Lawrence estuary, Québec

Abstract Well-preserved sedimentary structures formed by shore ice are exposed in emergent muddy tidal-flat deposits along the shoreline of the St. Lawrence estuary. The sedimentary features are exposed in high-marsh micro-cliffs 1 to 2 m high. Features from two localities where they are the most abundant (Sainte-Anne-de-Beaupre, north shore, and Montmagny, south shore) are reported here. Two categories are distinguished: first, bowl-shaped circular and elongated structures 30 to 100 cm across, which are former scoured depressions subsequently filled by stratified mud, fine sand and organic debris; and second, deformations (folds) a few decimeters to a few meters in size produced by ice-block pressure. Pebble- to boulder-size dropstones are associated with these structures. Most features were observed in mud-flat and low-marsh units (facies) vertically exposed at these localities. The age of these tidalite deposits ranges from about 200 to 600 years. These ice-made structures and deformations, well-preserved in a sequence of Recent emerged tidal sediments, suggest that similar features may be preserved in older rock formations. Consequently, they can be considered as a promising tool for interpreting the former climatic conditions in which tidalites and other shallow marine and lacustrine formations may have been deposited.

Paleoclimatic significance of late Pleistocene ice-wedge casts in Southern Quebec, Canada

Abstract Ice-wedge casts in southern Quebec are exclusive to deposits pertaining to two ice front positions during the retreat of the Laurentides ice sheet: the Highland front morainic system located at the southern margin of the St. Lawrence lowlands, and the St. Narcisse front morainic system located at the southern margin of the Laurentides highlands. Wedge casts filled with sand and gravel are relic of true ice wedges and give evidence of former permafrost conditions in the ground during the retreat of the ice sheet in that area, between 13 000 and 11 000 years B.P. Mean annual air temperature was lower than −6°C, a difference of 8–12° with the present-day temperature. Humidity was high enough to allow the growth of ice wedges in the permafrost. These rigorous climatic conditions prevailed during a few centuries only. There is no positive evidence of tundra conditions in southern Quebec young than 11 000 years ago.

Les formes et les marques de l'érosion glaciaire du plancher rocheux: Signification, terminologie, illustration

Abstract Glacial erosion is characterized by a grain by grain abrasion of the bedrock, or the removal of very small fragments which results in a polish or scratches varying in size. It may also weaken the rock by fracturation and leave a train of crescentic fractures, or shape quarried lee-forms. Differential erosion is also involved since the glacier bed is made of more or less resistant rocks and that the action of glacial flow varies in time and in space. So the kind of forms varies: hollows worked like grooves or protruding forms like veins if the rock encloses hard cores. Glacial scouring is downstream in the case of striations or concave chattermarks, and upstream in the case of quarried walls. The scale may be larger when a protuberance shaped by ice flow becomes an unsymmetrical rock so called roche moutonnee and glacial carving may affect such a form on all sides in a single primary effect. The authors aim is to present a qualitative and quantitative hierarchical classification of terms of these minor forms and marks as to better evaluate the morphological results of ice flow erosion on bedrock.

Ploughing boulders along shorelines, with particular reference to the St. Lawrence estuary

Abstract Ploughing boulders are defined as large stones moved by drift ice with a ploughing action into the substrate, leaving a linear depression on the back and building a frontal push ridge. They are commonly found on unconsolidated tidal flats and on rock shore platforms along cold-region shorelines in the marine, lacustrine and fluvial environments. It is an erosional process of great significance, particularly for consolidated substrates. A total of 176 ploughing boulders were measured in eighteen localities along the St. Lawrence estuary. The largest boulder weighed 176 tons and was moved 3 m, while a 50 t boulder was moved 40 m. On shore platforms cut in shales, one boulder 49 t, has scoured a depression 19 m long and up to 40 cm deep, and built a frontal ridge 40 cm high. Along the St. Lawrence estuary about 95 of ploughing boulders were moved seaward by ice floes entrained by ebb currents.

Structures cylindriques verticales dans du quaternaire a arthabaska, Quebec

Resume Vertical cylindric structures with circular layers, measuring from 3 to 20 cm in diameters and 45 to 225 cm high (minimum), occur at Arthabaska, in a Quaternary (Late Pleistocene) sand deposit, along the Nicolet River, at the contact between the Appalachian highlands and the St. Lawrence lowlands. Characteristics of the structures and the deposit suggest that springs pouring through stratified sands may have caused the formation of cylinders.

Typical Aspects of Cold Regions Shorelines

Cold regions shorelines (about one-third of the world’s coastline) do not differ much from those of mid-and low-latitude regions because the fundamental factors governing their development (lithology, structure, tectonics) are nearly the same. Major features (cliffs, platforms, drowned valleys, dunes and beaches) are also found in the three major morpho-climatic regions (warm, temperate, cold). Many differences, however, do exist in the details and in the processes involved in shoreline development. There are two main categories of factors to consider. Two climatic factors (ice and frost) play an important role in shaping cold regions coasts, whereas the geological background, particularly the Quaternary heritage (glaciations and marine submergence) is of major importance for an adequate knowledge of cold regions coastal features (paraglacial vs. periglacial shorelines).

Les erratiques de dolomie sur le rivage des Escoumins, Côte Nord de l’estuaire maritime du Saint-Laurent, Québec

Le rivage entre Les Escoumins et Baie-des-Bacons. sur la Cote Nord du Saint-l.aureni, est caracterise par une large batture (800 a 2000 m) argileuse et une falaise entailllant la terrasse de 30 m constituee de depots meubles quatemaires. Il est parseme de milliers de cailloux de lithologies et de tailles variees, formant des dallages sur le bas estran et des amas divers sur le haut du ravage, au pied de la falaise. n nLa presque totaliie des cailloux (plus de 90%) appartient aux diverses lithologies ignees et metamorphiques du socle precambrien a proxtmite. Mais on trouve aussi de nombreux cailloux de dolomite, une lirhologic rare dans le Bouclier Laurentidien. Sur une distance de 18 km. 3661 cailloux de dolomie ont ete observes et mesures. Il en existe plusieurs varietes; du point de vue de la couleur, Ies dolomies grises et roses dominant, totalisanl respectivement 74% et 20%. Parmi les autres caracteristiques. il y a une trentaine de blocs de dolomie a stromatolites, plus de 28% des cailloux de dolomies observes sont slries et pres de la moitie sont subarrondis. Les cailloux de petite taile (1 a 10 kg) comptent pour environ 59%, ceux de 10 a 50 kg, pour 30%, ceux de 50 a 100 kg. 7%. alors que 4% eesent plus de 100 kg. le plus gros alteignant deux tonnes. Les deux plus gros blocs de dolomie a stromatolites pesent respectivement 175 et 280 kg. n nII existe quatre grandes aires de roches dolomitiques dans Ie Bouclier precambrien: le bassin sediment aire de Mistassini. le secteur entre la Grande Ile el Inukjuak sur la cote est de la mer dHudson. Ie fosse du Labrador. ainsi que dans le Grenville, entre Hull et le lac Baslong. La source la plus plausible des erraiiques de dolomie des Escoumins demcure le bassin de Mistassini, ce qui implique un courant de glace important, au Wisconsinien. vers le SE a partir dun dome ou dune ligne de partage des glaces situe au NO du lac Mistassini. Les erratiques de dolomie du rivage des Escoumins ont dabord ete transportes par le glacier via la depression du Lac-Saint-Jean et du Saguenay avant detre relaches par les icebergs ou peut-elre meme, en partie, par une plate-forme de glace localisee en bordure de la Mer de Goldthwaii. Les deplaecments sont de lordre de 400 a 500 km. Par ailleurs plusieurs bloes ont pu etre deplaces localement par les glaces flotiantes au coursde lHolocene. n n nABSTRACTS nThe shoreline between Les Escoumins and Baie-des-Bacons on the North Shore of the Lower St. Lawrence, is characterized by a wide clayey tidal flat (800 to 2 000 m) and a cliff cut into the 30-m terrace made of (Quaternary unconsolidated deposits. The shore zone is covered by thousands of boulders of various lithologies and sizes, including mega-boulders scattered throughout the tidal flat, small boulders (25 to 40 cm) commonly forming pavements at the surface of the tidal flat, and boulders of various sizes on the beach at the base of the cliff. n nMost erraties are Precambrian igneous and metamorphic rocks from the adjacent Shield area, however, dolostone erraties are also a significant component. In an area 18 km long, 3661 dolostone erraties were observed and measured. There are various types of dolostone including 30 stromatolitic dolostone erraties; most are grey (74%) and pink (20%). Among other characteristics, 28% of the erraties are striated and about half are surrounded: small size erraties (I to 10 kg) account for about 59%. those 10 to 50 kg, for 30%, those 50 to 100 kg, 7%. while 4% weigh over 100kg, the largest boulder weighing two tons. The two largest stromatolitic dolostone erraties weigh respectively 280 and 175 kg. n nThere are four areas in the Precambrian Shield were dolostone formations occur: the sedimentary basin of Mistassini, the east coast of Hudson Bay. between Long Island and Inukjuak. the Labrador Trough, and a wide area in the Grenville between Hull and Lake Bastong. The most likely source of the dolostone erraties observed at Les Escoumins is the Mistassini sedimentary basin. This interpretation implies a major ice flow lo the SE during the Wisconsinan from an ice dome or an ice devide located to the NW of Lake Mistassini. The erraties occurring in the area of Les Escoumins were likely first transported by a glacier via the Lake St. Jean-Saguenay corridor before being released in the Goldihwait Sea most probably by icebergs or possibly by a nearby ice shelf. Distances of displacement range from 400 lo 500 km. Many erraties may also have been moved locally by shore ice during the Holoccne

Ring structures made by shore ice in a muddy tidal flat, St. Lawrence estuary, Canada

Abstract Large-size ring sedimentary structures occurring in a muddy tidal flat, on the south shore of the St. Lawrence estuary, are reported for the first time. They range from 15 to 50 m in diameter and occur as isolated features or in groups of 3 to 5. In plan they form well-defined circles scattered throughout the higher mud flat. Some structures are characterized by an inner ring and by a chaotic surface of alternating micro-ridges and depressions. The outer surrounding mud ridge is a few decimeters high and wide (25–60 cm). Ring structures occur at the emplacement of icefoot pustules and are related to vertical pressures exerted on the substrate by ice blocks moved up and down by tide motion. Icefoot pustules form every winter but not necessarily at the same location. It is likely that the related ring structures can be fossilized and preserved in the sedimentary sequence. Consequently, they can be of assistance in palaeoenvironmental interpretation.