DONALD J. P. SWIFT ANIL K. LYALL

Reconnaissance of Bedrock Geology by

Sub-bottom Profiler,

Abstract: Reconnaissance with a sub-bottom profiler has revealed the structural plan of the largely submarine Acadian Triassic Basin in the Bay of Fundy. The Bay is floored mainly by Triassic clastic rocks which have been warped into a broad syncline. The basalt of the North Mountain--Cape Blomidon cuesta may be traced underwater as far as Quaco Ledge, defining the northeastern limb of the Fundy syncline. The basin's northwestern margin is a zone of normal faults, from 2 to 10 km seaward from the shore. While the basin is, therefore, technically a half-graben, the central syncline is the dominant structure.

Introduction sity, in carrying it out. f. E. Blanchard of Dalhousie University obtained financialsup - The Bay of Fundy (Fig. 1) is a funnel-shaped port from the Research Founda- body of water lying between Nova Scotia and tion for us. The profiling system was loaned the Canadian mainland. The Bay of Fundy by B. Pelletier of Bedford Institute of Ocean- proper is 144 km long, 100 km wide at the ography. We thank the officers and crew ot base, and averages 75 m deep. The northeast CNAV Sackyille for their co-operation and sup- end bifurcates into northeast-trending Chig- port. This work was conducted from the Ge- necto Bay and the east-trending . ology Department of Dalhousie University, The bay has been incised into sedimentary and Halifax, Nova Scotia, and is published with the volcanic rocks of a Triassic half-graben which assistance of the faculty of graduate studies. lies athwart the northern portion of the Ap- palachian Mountain system. Methods In March of 1966, we recorded 1110 km of The profiles were made with a Huntec- sub-bottom profiles in the Bay of Fundy (Fig. Mark-2A-Hydrosonde profiling system, a 165 1). The profiling was intended to be part of a Joule sparker. The system was programmed to study of Quaternary sedimentation, but the provide four sparks per second with a sweep profiles contained, as well, considerable infor- time of 250 milliseconds. Filters were set at mation on the bedrock geology of the bay. 152 to 2329 CPS, a band which avoided inter- Consequently, it was decided that the first ference with the ship's electrical system. The analysis of the records would be a brief assess- spark tip was towed at about 5 knots behind ment of the bedrock geology of this submerged the CNAV Sackyille, a Canadian naval auxil- and hitherto unexplored portion of the Mari- iary corvette. Fiducial lines were drawn across time Appalachians, in order to extend to the the record every 15 minutes to coincide with northeast the work of Uchupi (1966) and Tagg fixes on the ship's position. Cross sections con- and Uchupi (1966) in the Gulf of Maine. structed from these profiles were based on Further reports on Pleistocene glaciation and velocities of 1.5 kmps for seawater, and 2.0 modern sedimentation on the bay's floor are in kmps for the sediment cover. Tidal corrections preparation. for each fiducial were determined from the Atlantic Coast Tide and Current Tables, pub- Acknowledgments lished by the Canadian Hydrographic Survey. We acknowledge help from D. Stanley of the Smithsonian Institution in organizing the Marginal Stratigraphy and Structure field work, and from A. Cok, Adelphi Univer- Triassic rocks. The Bay of Fundy has been

Geological Society of America Bulletin, v. 79, p. 639-646, 6 figs., 4 pis., May 1968 639

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66° 65°

U AREA OF DETAILED SURVEY AND SAMPLING

66 Figure 1. Profile grid in the Bay of Fundy, outlined by fiducial points. Numbers and letters locate cross sections (Fig. 5) and plates.

incised into the red continental sandstones and basal Triassic contact lies largely seaward of the shales and basalt flows ol a Triassic half-graben, shoreline. Local exposures of Triassic occur at the Acadian structural basin. Triassic stratig- Waterside, Point Lepreau, and St. Martin raphy has been described in detail by Powers (Klein, 1960). Only the first exposure is large (1916) and Klein (1962). It is summarized in enough to appear in Figure 3. Figure 2. The major structure of the Acadian struc- The boundary ol the Acadian structural tural basin is the Fundy syncline, defined by basin approximates the shoreline of the bay. the great, curved cuesta of Cape Split—Cape The south coast consists of a strip of Triassic Blomidon-North Mountain, where the North rock from 15 to 20 km wide (Fig. 3). The Mountain Basalt dips from 5° to 15° toward northwestern or shoreline side of this strip is the synclinal axis. The syncline plunges gently North Mountain, a cuesta of Triassic basalt. west-southwest, down the trend of the bay. The southeastern or inland side of the strip is Second-order folds oi similar orientation occur Annapolis Valley, underlain by friable, dusky- in some fault blocks. red Triassic sandstone and shale. At the cast The major faults of the Triassic rocks are end of the bay, the basalt cuesta curves around normal faults trending parallel to the Fundy to form the hook-shaped peninsula of Cape syncline (Klein, 1960). They are especially well Blomidon (elbow of hook) and Cape Split (tip developed along the north shore of the Minas of hook). The basalt briefly reappears above Basin, where the Portapique fault (Weeks, the surface of the bay in the north limb of the 1948) and others have displacements of up to Fundy synclme on Capt1 D'or, Isle Haute, and 1700 m (Klein, I960, p. 154). Here they form Quaco ledge (Fig. 4). a series ol steps, horsts, and grabens separating On the north shore of the Bay of Fundv, the the Carboniferous rocks of the Minas Basin.

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See Figure 1 for locations. (Depth in milliseconds is not depth below surface. Datum is arbitrary.) (A) Gentle synclme in Triassic sedimentary rock. (B) Basalt outcrop. (C) Border fault. Triassic sedi mentary rock against pre-Carboniferous granitic rock.

SUB-BOTTOM PROFILES OF THE BAY OF FUNDY

SWIFT AND LYALL, PLATE 1 Geological Society of America Bulletin, volume 79

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------Sec Figure 1 for locations. (Depth in milliseconds is not depth below surface. Datura is arbitrary.) (A) Apparent non-faulted, disconformable contact between Tnassic and Carboniferous sedimentary rocks. (B) Fault in Triassic sedimentary rock. (C) Anticline in Carboniferous sedimentary rock.

SUB-BOTTOM PROFILES OF THE BAY OF FUNDY

SWIFT AND LYALL, PLATE 2 Geological Society of America Bulletin, volume 79

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Channel sandstone (arrow) and even stratification Blomidon Formation, Rossway Cliffs, St. Mary's Bay, Nova Scotia. Photo by G. de V. Klein.

SEA CLIFFS OF BAY OF FUNDY UNITS FOR COMPARISON WITH SUB-BOTTOM PROFILES

SWIFT AND LYALL, PLATE 3 Geological Society of America Bulletin, volume 79

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/79/5/639/3442752/i0016-7606-79-5-639.pdf by guest on 25 September 2021 Figure 1. North Mountain Basalt, south side of Minas Passage, Nova Scotia. Four flows crutiely outlined by vegetation.

F~gure2. Parrsboro Formdt~on,West Bay, Minas Channel, Nova Scotla.

SEz4 CLIFFS OF B.4Y OF FUNDY UNITS FOR (:OMPARTSON WITH SUB-BOTTOM PROFILES

SIVIFT AND I>YAI.L, PLA'I'I: 4 Geologlcal Socict! of Arncrica Bulletin, volume 79

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Second-order faults trend obliquely to the has stripped the Triassic sediments back to the main structures. They show displacements of Portapique fault. 30 to 70 m, and are visible mainly within the Carboniferous rocks have been thrown into Triassic system (Klein, 1960). They are par- open symmetrical folds with wave lengths up ticularly apparent where they offset the basalt to 2 km. On the north margin of the Minas of South Mountain, at Digby Gap and Gulliver sub-basin, in the vicinity of the up-thrust Hole (Fig. 3). Cobequid block, folds are asymmetrical or over- Pre-Triassic rock$- Folded and faulted Car- turned toward the basin's center (Swift, D. boniferous clastic rocks form the basement for J. P., 1966, Huntec Ltd. Rept., unpublished). the Acadian structural basin. Analyses by Bell Within the study area, pre-Carboniferous (1944) and Belt (1965) have shown that cer- rocks come to the surface in the Caledonian tain broad resemblances exist between the and Cobequid fault blocks, respectively situ- Acadian structural basin and its predecessor, ated on the north shore of the Bay of Fundy the Fundy structural basin, in which Carbon- and in the Chignecto Peninsula between iferous rocks accumulated. Both were inter- Chignecto Bay and the Minas Basin. The montane fault basins accumulating mainly rocks, slates, phyllites, and metavolcanics of clastic lacustrine, alluvial plain, and alluvial fan early Paleozoic age, are intruded by granite. deposits. Carboniferous stratigraphy is sum- marized in Figure 3. Submarine Geology Carboniferous structure in eastern Fundy is The sub-bottom profiles show that, except dominated by the Cobequid fault on the Minas for a zone on the north shore about 10 km Basin's north shore, which separates the up- wide, the Bay of Fundy is floored by gently thrown pre-Carboniferous rocks of the Co- dipping to horizontal Triassic sedimentary- bequid fault block from the Carboniferous rocks (Fig. 5; PL 1A). They are recognized by rocks to the south. This fault with its spec- their well-defined stratification, a pattern tacular scarp runs 130 km from Cape Chignecto which can be traced by sub-bottom profiler to the coalfields of Pictou in central Nova toward seacliffs of Triassic clastic rocks (PL 3). Scotia. The fault, if reactivated in Triassic The very well-bedded nature of most sub- times, may have served as the border fault ot marine Triassic clastic rocks suggests that they the Minas sub-basin. If so, post-Triassic erosion are lake deposits similar to the Blomidon and

FORMATION

nterbedded gray, purple sandstone, rlayslone, chert limestone

Klein

1962

Cumberland! a r Reddish Conglomerate

Belt I9b5

Figure 2. Summary of stratigraphy, Bay of Fundy.

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BAY OF FUNDY

GEOLOGY

Figure 3. Bedrock geology ol the Bay of Fund}'. Southeastern margin ol map is southeastern margin o! Acadian Tnassic basin.

Figure 4. East one) of the F'undy synclinc as outlined by basalt outcrops (shaded). Rectangular out- line of submarine outcrops is arbitrary except in the Minas Passagc-Minas Channel area, where pattern was obtained from unpublished data of Huntcc, Ltd., Toronto.

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CARBON- t| IFEROUS

100 . 125- CAR8ONIFEROUS m\ TRIASSIC 150- Km. IO

Figure 5. Cross sections of the Bay of Fundy. See Figure 1 for locations.

Scots Bay formations of Klein (1962). If the Formation must underlie the bay northwest oi stratigraphy of the Minas Basin area (Klein, the basalt. However, with the local exception 1962) continues under the Bay of Fundy, then of the Formation, these units cannot the Scots Bay Formation must underlie the be resolved by the profiler. southeast half of the bay between the basalt of Triassic volcanic rocks appear on several southeast shore cuesta and the basalt of Quaco profiles between Isle Haute and Quaco ledge, ledge-Isle Haute-Cape Split cuesta. The Blomi- on the north limb of the Fundy syncline (Fig. don Formation and, marginally, the Wolfvillc 5; PL IB). They are recognized on sub-bottom

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proiilcs by their relative acoustic transparency, sembles the "Fundian Fault" of Johnson lack ol stratification, and a "sharkskin" effect (1925), although it generally lies further off- due to numerous intersecting parabolic reflec- shore. These border faults appear to be equiva- tions Irom point sources on rough upper sur- lent to the first- and second-order faults of faces of the rock. This pattern may be traced Klein (1960). The larger segments run parallel by profiler to basalt seacliffs (PL 4). The con- to the structural axis of the Acadian Triassic tact ol the basalt with the adjoining sedimen- basin, as do Klein's first-order faults. The tary rock is locally discordant. However, the shorter segments, off Point Lepreau, St. profiles do not permit resolution of these con- Martins, and the mouth of Chignecto Bay, tacts into fault or intrusive contacts. trend obliquely to the regional strike. The dominant structure of the submarine Fast ol St. Martins, the Triassic system ap- Triassic rocks, as in the marginal exposures, is pears to rest disconformably on the Carbon- the Fundy synclinc. This feature may be iferous (PI. 2A). It is possible that a border traced out of Scots Bay into the Mmas Chan- fault exists here but lies within Carboniferous nel, where its axis has been repeatedly offset rocks, the Triassic rocks having been stripped by faults with a horizontal component to their back toward the southwest. movement. Beyond Isle Haute, it can be The Chignecto fault (new name), an oblique traced as far southwest as the latitude of the fault originally postulated by Cameron (1949), mouth of the Bay of Fundy. Here the axis trends along the blunt tip of Cape Chignecto, widens lo an axial zone ol flat-lying strata up and south past Isle Haute. to five km wide. Second-order (olds with wave- Both parallel and oblique faults are abundant lengths of less than a kilometer are also present within the Triassic system at the mouth of the (PI. 1A), and tend to be more common in the Minas Channel, south of Cape Chignecto. vicinity of faults. Their frequency possibly is due to draping of The profiles show a series of discontinuous the Triassic over a subsurface projection of the faults along the northwest side ol the basin, pre-Triassic rocks of the Chignecto Peninsula. which separates the Carboniferous rocks of the Key horizons in some of these faults (PI. 2B), north shore from the Triassic rocks to the show that the throw is on the order of 20 m. southwest (Fig. T; PI. 1C). The fault /.one re- Pre-Triassic rocks observed on the profiles

Figure 6. Above: Model of the Newark Triassic basin by Stose and Stose (f944). Below: model of the Connecticut Triassic basin by Barrell (1915).

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are mainly well stratified, strongly reflective and structural basins leaves Triassic rocks ex- rocks whose relations to land outcrops suggest posed on land to the southeast and has sub- that they belong to the upper Carboniferous, merged pre-Triassic rocks on the northwest, probably to the Mabou Group and "Coarse The subaerial southeast margin of the struc- Fluvial Facies" of Belt (1965). See Plates 2C tural basin consists mainly of an unconformable and 4. The only probable profiles of pre- contact between Triassic and pre-Triassic rocks, Carboniferous rocks were taken off Point while the submarine northwest margin is de- Lepreau, where massive, isotropic, strongly re- fined by a series of faults. The basin is, there- fleeting material appeared northeast of the fore, a half-graben. However, the profiles sug- border fault (PI. 1C). This border fault is as- gest that the great central syncline is a more signed to the pre-Carboniferous granitic rocks significant structure and show that the axis of of the adjacent Caledonian massif. the syncline is displaced to the southeast rather than to the northwest of the basin's topo- Condusions graphic midline. The syncline is, therefore, not closely controlled by the fault zone; and the Reconnaissance with a sub-bottom profiler Acadian Triassic basin is more truly a structural has clearly revealed the broad outlines of the basin than it is a fault trough. In this respect, structural framework of the Bay of Fundy. it more closely resembles the model for a The bay is underlain mainly by Triassic sedi- Triassic basin postulated by Stose and Stose mentary rocks. Fundy's shoreline is nearly (1944) from study of the Newark Triassic coincident with the Acadian Triassic basin. An basin, than the model of Barrell (1915) based offset of some 10 km between the topographic on the Connecticut Triassic basin. (Fig. 6).

References Cited Barrell, J., 1915, Central Connecticut in geologic past: Connecticut Geol. Survey Bull., v. 23, p. 14. Bell, W. A., 1944, The Carboniferous rocks and fossil flora of northern Nova Scotia: Geol. Sur- vey Mem. 238, 276 pp. Belt, E. S., 1965, Stratigraphy and paleography of Mabou group and Carboniferous facies, Nova Scotia: Geol. Soc. America Bull. v. 76, pp. 777-802. Cameron, H. L., 1949, Faulting in Nova Scotia: Royal Soc. Canada Trans., v. 43, pp. 13—21. Howie, R. P., and Gumming, L. M., 1963, Basement features of the Canadian Appalachians: Geol. Surv. Canada Bull. 89, 15 pp. Johnson, D. W., 1925, The New England-Acadian shoreline: New York, John Wiley and Sons, 608 pp. Klein, G. de V., 1960, Stratigraphy, sedimentary petrology, structure of Triassic sedimentary rocks, Maritime provinces of Canada: Ph.D. thesis, Yale University, New Haven, Conn., 182 pp. (Available on intcrlibrary loan.) 1962. Triassic sedimentation, Maritime provinces of Canada: Geol. Soc. America Bull., v. 73, pp. 1127-1146. Powers, Sidney, 1916, The Acadian Triassic: Jour. Geology, v. 24, pp. 1-26, 105-122, 254-268. Stose, A. J., and Stose, G. W., 1944, Geology of the Hanover-York district, Pennsylvania: U. S. Geol. Survey Prof. Paper 204, 84 pp. Tagg, A. Richard, and Uchupi, Elazar, 1966, Distribution and geologic structure of Triassic rocks in the Bay of Fundy and northeast part of the Gulf of Maine: U. S. Geol. Survey Prof. Paper 550-B, pp. B95-B98. Uchupi, Elazar, 1966, Structural framework of the Gulf of Maine: four. Gcophys. Research, v. 71. pp. 3013-3028. Weeks, L. J., 1948, Londonderry and Bass River map areas, Colchester and Hants Counties, Nova Scotia: Canada Geol. Survey Mem. 245, 86 pp.

PRESENT ADDRESS: DUKE UNIVERSITY, DURHAM, NORTH CAROLINA (SWIFT) PUERTO Rico NUCLEAR CENTER, COI.I.ECE STATION, MAYACUEZ, PUERTO Rico (SWIFT AND LYAI.I.) PRESENT ADDRESS: GEOLOGY DEPARTMENT, DAI. HOUSIE UNIVERSITY, HALIFAX, NOVA SCOTIA, CANADA (LYAI.I.) MANUSCRIPT RECEIVED BY THE SOCIETY APRIL 7, 1967

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