Quaternary stratigraphy and events in southeastern Quebec.

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Geological Society of America Bulletin Quaternary Stratigraphy and Events in Southeastern Quebec B. C McDONALD and W. W SHILTS Geological Society of America Bulletin 1971;82;683-698 doi: 10.1130/0016-7606(1971)82[683:QSAEIS]2.0.CO;2

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B. c. MCDONALD W. W. SHILTS

r Geological Survey of Canada, Ottawa, Canada

Quaternary Stratigraphy and Events in Southeastern Quebec ABSTRACT Quaternary stratigraphic units have been mapped in the Appalachian region of southeastern Quebec, and formal stratigraphic names for these units are proposed. Evidence exists for four separate glacial phases, the last three of which are represented by tills. The three tills, from oldest to youngest, have been named Johnville, Chaudiere, and Lennoxville, respectively. Stratified sediments interbedded with the tills record significant nonglacial intervals between each of the glacial phases. It is suggested that the last three glacial phases are of Wisconsin age and that the Lennoxville Till represents the entire late Wisconsin. Ice-flow directions were determined using dispersal shadows (indicator trains), till fabrics, and striations. Johnville ice flowed from the northwest; Chaudiere ice flowed initially from the northeast, but later from the northwest; Lennoxville ice flowed from the northwest. Late Wisconsin glaciers did not flow northward or northwestward into Quebec from New England. Pre-Johnville stratified sediments probably record preWisconsin deposition. The Massawippi Formation, recording the nonglacial interval between the Johnville and Chaudiere glacial phases, may correlate with the St. Pierre peat beds of the St. Lawrence Lowlands. The Gayhurst Formation, recording the nonglacial interval between the Chaudiere and Lennoxville glacial phases, may correlate with some of the Port Talbot interstadial sediments of southern Ontario. The Quebec Appalachians were finally deglaciated by about 12,500 C 14 -yrs B.P.

INTRODUCTION Quaternary stratigraphy in an area from the Sutton Mountains east to the mountains on the Quebec-Maine border (Fig. l) has been studied in detail since 1964. The pur-

pose of this paper is to provide a stratigraphic framework for a portion of the relatively mountainous Appalachian region of Quebec, to summarize the Quaternary record there, and to assign formal names to the stratigraphic units. The authors believe that the events detailed herein are sufficiently widespread and documented to promise closer correlation between Quaternary histories of the midwestern United States and northern New England. The region examined lies in Quebec just north of Vermont and New Hampshire, and it abuts the northwest border of Maine. Present drainage is toward the north or northwest. Most streams are ultimately tributary to either the St. Francis or Chaudiere rivers, which flow northwest across the strike of the Appalachians to the St. Lawrence River and thence to the Gulf of St. Lawrence. Lakes were ponded in southeastern Quebec whenever the drainage was blocked by glaciers occupying the St. Lawrence Lowlands. Thus, the distribution of glacial-lake sediments in the region has implications for the regional extent of associated glaciers. Data will be presented from the St. Francis River drainage basin (McDonald), and also from the Chaudiere River drainage basin (Shilts). Details are given elsewhere by McDonald (1966a, 1966b; 1967a, 1967b, 1967c; 1968, 1969) and by Shilts (I969a, 1969b). BEDROCK GEOLOGY AND INDICATORS A complex sequence of deformed rock units strikes northeastward across the study area (Cady, I960; Cooke, 1937, 1950, 1957; Doyle, 1967; Lord, 1938; Marleau, 1968; and Reid, I960). Information pertinent to the present paper has been extracted from these sources and is shown on Figure 2. Major units are, from northeast to southwest: (1)

Geological Society of America Bulletin, v. 82, p. 683-698, 3 figs., March 1971 683


684 MCDONALD AND SHILTS-QUATERNARY STRATIGRAPHY AND EVENTS, QUEBEC

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Figure 1. Locations of sections and radiocarbon dates. Green Mountain lithologic complex—ultra- QUATERNARY STRATIGRAPHY basic, volcanic, metasedimentary, and minor The stratigraphy presented here is sumacidic rocks. (2) Connecticut Valley-Gaspe lithologic complex—a sequence of slate, gray- marized in Table 1. Units will be described wacke, and minor limestone is penetrated by from oldest to youngest. The Ascot River granodioritic stocks of the New Hampshire section (Fig. 1, No. 1) is the composite type Plutonic Series and by nordmarkite, granite, section for all of the units except for the and gabbro at Mount Megantic. (3) Bound- Gayhurst Formation, the type section of ary Mountain lithologic complex—gneissic which (Fig. 1, No. 2) is on the Chaudiere andschistosemetasandstone,metagraywacke, River. Type sections and a reference section are described briefly in the appendix; these basic intrusive, and extrusive rocks. Dispersal shadows (indicator trains) on and other reference sections are numbered Figure 2 show the distribution of boulders and located on Figure 1. Dreimanis (i960) referred the interstade and cobbles on the surface and in the surface 14 till. They were selected for presentation on between about 50,000 and 23,000 C -yrs the basis of their prominence and the avail- B.P. to the "Mid-Wisconsin"; earlier Wisability of sufficient data. For example, consin events were referred to "Eatly Wisalthough an ultrabasic shadow occurs south- consin." This usage is followed here, and 14 east of the entire ultrabasic outcrop belt, only events postdating about 23,000 C —yrs B.P. the particularly prominent part of it trailing are referred to "late Wisconsin." from the large outcrop area near Thetford Mines has been outlined on Figure 2. Char- Pre-Johnville Sediments The oldest known Quaternary sediments acteristics of these shadows are presented in more detail elsewhere (McDonald, 1966a, in southeastern Quebec are exposed only at the Ascot section (No. 1) and at the Grande 1967b; Shilts, 1969a, 1969b). The dispersal shadows are used to support Coulee section (No. 3). At the Ascot section, pre-Johnville sediideas developed in the paper and will be more fully discussed along with the strati- ments are represented by noncalcareous fluvial pebble gravel which is tightly cemented graphic data.

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Figure 2. Generalized bedrock geology and boulder dispersal data.

with bright red-orange iron oxide. Some silt and clay, contain current structures indipebbles in overlying gray Johnville Till have cating that stream flow was in the same direcsimilar thick coatings of iron oxide, an ob- tion as is the present drainage. This implies servation which supports the view that the northward-flowing drainage in the Chaudiere gravel was cemented and thoroughly oxidized valley, at least above this altitude, before before being partially eroded and redeposited deposition of Johnville Till. Pebbles in the by the Johnville glacier. upper gravel are heavily coated with iron At an altitude of 300 m at the base of the oxide, and the upper 1.3 m of underlying Grande Coulee section (No. 3), two fluvial laminated silty clay is oxidized, suggesting gravel units, each underlain by interlaminated prolonged weathering prior to deposition of TABLE 1. QUATERNARY STRATIGRAPHIC COLUMN, SOUTHEASTERN QUEBEC Time-Stratigraphic Unit

post-Lennoxville sediments

Lennoxville Till Gayhurst Formation Chaudiere Till Massawippi Formation

pre-Wisconsin stage(s)

Chronologic Control

Rock-Stratigraphic Unit

Johnville Till pre-Johnville sediments

12,640 12,570 12,000 11,500

+ + + ±

190 (GSC-312; peat) 220 (GSC-419; peat) 230 (GSC-936; marine shells) 160 (GSC-475-2; marine shells)

>20,000 B.P. (GSC-1137) ca. 4000 varves > 54,000 B.P. (Y-1683) >41,500 B.P. (GSC-507) > 40,000 B.P. (GSC-1084)


686 MCDONALD AND SHILTS-QUATERNARY STRATIGRAPHY AND EVENTS, QUEBEC Johnville Till. The gravel contains granite 1.54, a value locally typical of tills with gneiss pebbles derived from the Canadian northwest provenance (Table 2). Shield, 120 km away on the north side of the The lowest of three tills at site Number 4 St. Lawrence River. Because stream gradients has a strong southeast two-dimensional pebare northward, these pebbles probably were ble fabric and is regarded as Johnville Till. transported initially to the Chaudiere valley Johnville Till has also been identified tentaby a glacier and subsequently incorporated tively in a bore-hole (No. 5) on the basis of in a stream gravel. stratigraphic position beneath Chaudiere Till, The stratigraphic units just described and in a detached lens dragged up along a directly underlie Johnville Till which is over- shear plane at the base of section Number 6. lain in turn by the Massawippi Formation Johnville Till is the oldest till recognized (dated at >54,000 CI4-yrs B.P., see below). in southeastern Quebec. It is considered to Pre-Johnville sediments are thought to be of be of early- or pre-Wisconsin age because of pre-Wisconsin age because of: (1) deep "greater-than" dates on sediments higher in weathering, (2) evidence of northward-flow- section. Johnville ice apparently flowed from ing drainage, and (3) occurrence beneath a the northwest; it was preceded by and followed till that is more than 54,000 C14—yrs old. by significant nonglacial intervals. Evidence for at least one glaciation and a possible interglacial stage has been recog- Massawippi Formation nized in pre-Johnville sediments at Grande Coulee, and interglacial weathering is inferred This unit is named after the Massawippi from oxidation of correlative sediments at River that flows within 6.0 km of the type the Ascot section. section (No. l) on the Ascot River. At its type section, the Massawippi Formation comJohnville Till prises noncalcareous, compact, interstratified The oldest known till in the region is silt and sand, containing finely divided plant exposed at the type section on the Ascot debris and rare wood fragments as much as River (No. l) and is named after the nearby 2 cm long. It is directly overlain by Chaudiere village of Johnville. At the Ascot River sec- Till, and it overlies Johnville Till in a low tion, Johnville Till is 0.3 m thick and non- river bank 175 m downstream. Organic debris calcareous; it directly overlies oxidized gravel from the Massawippi Formation at the type and is in turn overlain by pollen-bearing section gave an age of > 54,000 C14—yrs sediments of the Massawippi Formation. A B.P. (Y-1683). Palynological data and infernorthwest provenance is inferred from the ences based on them are presented below in high (13.1 percent) content of magnetite in the section entitled "Discussion." heavy minerals of fine-sand size and from The Massawippi Formation has been identithe high (12 percent) content of volcanic fied at three other widely separated sites in pebbles, both indicating derivation from bed- the study area. Along the Magog River (No. rock units to the northwest (Fig. 2; also see 7), a 20-m-thick sequence of noncalcareous later discussion of provenance of theLennox- lake sediments contains finely disseminated plant material. No till was observed at this ville and Chaudiere tills). At the Grande Coulee site (No. 3), a lens section. The organic material was dated at of Johnville Till is covered by a silty, pebble- >41,500 C14-yrs B.P. (GSC-507). On the rich diamicton. The till is locally absent, and Eaton River (No. 8), laminated silt and sand, its stratigraphic position is indicated there physically identical to sediments on the Ascot only by the diamicton. The till is gray and River except that no organic matter was noncalcareous and is stained by irregular noted, underlie Chaudiere Till. patches of iron oxide to a meter below its On the Grande Coulee River (No. 3), a upper surface. Its two-dimensional pebble lens of medium- to coarse-grained crudely fabric has a strong maximum at 130°. The fissile sand containing disseminated organic till is much richer in feldspar than younger fragments, dated at >40,000 C14-yrs B.P. tills, except for the portion of Lennoxville (GSC-1084), overlies the same diamicton Till that was deposited in a granodiorite dis- which rests on Johnville Till in an exposure persal shadow. No alteration of clay minerals 15m downstream. The sand lens is traceable was noted and the 10 A/7 A ratio for clay is for only 30 m laterally and apparently was


TABLE 2. Property I Color

II Texture 1. Grain size

2. Fabric

Ill Composition 1. Pebbles and boulders (In all cases, local

PROPERTIES OF LENNOXVILLE AND CHAUDIERE TILLS

Location

Lennoxville Till

SF

Oxidized: moderate yellowishbrown Unoxidized: olive-gray and medium-gray Oxidized: moderate yellowishbrown Unoxidized: olive-black, olivegray, and medium-dark gray; usually not distinguishable from Chaudiere Till on basis of color

Olive-gray to medium-gray; not distinguishable from Lennoxville Till on basis of color

Generally compact, cobbly, sandy, silt-clay till; locally represented by loose, sandy, ablation till with little silt or clay. Based on 14 samples: sand = 13% to 65%, silt = 25% to 69%, 0.5 m-diameter boulders; locally loose, sandy, ablation facies present. Based on 40 samples: sand = 13% to 50%, silt = 32% to 53%,