Montagnais: a Submarine Impact Structure on the Scotian Shelf, Eastern Canada

Montagnais: A submarine impact structure on the Scotian Shelf, eastern Canada

LUBOMIR F. JANSA Geological Survey of Canada, Atlantic Geoscience Centre, Bedford Institute of Oceanography, P.O. Box 1006, Dartmouth, Nova Scotia, B2Y4A2, Canada GEORGIA PE-PIPER Department of Geology, St Mary's University, Halifax, Nova Scotia, B3H 3C3, Canada P. BLYTH ROBERTSON Geological Survey of Canada, 1 Observatory Crescent, Ottawa, Ontario, K1A 0Y3, Canada OTTO FRIEDENREICH Shell Canada Limited, P.O. Box 100, Station M, Calgary, Alberta, T2P 2H5, Canada

ABSTRACT Dating by 39Ar/40Ar indicates that the im- bytownite, whereas the rock is highly acidic

pact occurred at about 51 Ma, late early Eo- with as much as 75% Si02. The petrochemical The first impact crater in the ocean has cene, and this is confirmed by biostratigraph- disparity was the first indication that the crystal- been identified on the outer continental shelf ic data from immediately overlying sediments. line rocks encountered in the Montagnais well off Nova Scotia, Canada. The crater is well The compositional and structural imprint left were not of normal igneous origin. Subse- preserved and buried by 510 m of Tertiary on the Montagnais structure by the colliding quently, petrographic and geochemical study of and Quaternary marine sediments. The crater body points to the impactor being most prob- the breccia revealed evidence of a wide spec- is a circular structure at least 45 km wide and ably the nucleus of a comet. trum of shock metamorphism features, including 2.7 km deep, with a central structural uplift. planar deformation features in quartz, diaplectic The uplift is at least 1,250 m and possibly INTRODUCTION mineral glasses, and inhomogeneous, flow- 2,900 m in height. Its upper surface is 11.5 km textured, vesicular melt rocks. Such features are across, and it has a central pit 3.5 km wide in A nearly circular, structurally deformed area, produced in nature by hypervelocity meteorite the center. An exploratory oil well located at least 45 km in diameter, has been recognized impact (see papers in French and Short, 1968; near the center of the structure encountered on the Scotian Shelf from multichannel reflec- Stoffler, 1972, 1974). The similar chemistry of Cambro-Ordovician metamorphic rocks of tion seismic data obtained during oil explora- the Meguma metasedimentary rocks and the the "basement," which are fractured with tion. The structure, named after exploratory oil "basalts," and the presence of shocked mineral planar deformation features in quartz repre- well Union et aL Montagnais 1-94, drilled near clasts in the latter, as discussed below in this sentative of pressures in the 8- to 12-GPa its center (42°53'N, 64°13'W) in 1974, is lo- paper, indicate that these crystalline rocks are range. The uplifted basement is covered by cated -200 km south of Halifax, Nova Scotia, impact melts. 552 m of breccia. Shock-induced features are near the edge of the Scotian Shelf, in water The serendipitous recognition of Montagnais common; they include planar lamellae in depths of 112.7 m (Fig. 1). Although hydrocar- marks the first discovery of a submarine impact quartz and feldspars; partial to total isotropi- bon potential was not realized, the structure was structure on Earth (Jansa and Pe-Piper, 1987). zation of silicates; and the occurrence of of sufficient interest to foster several hypotheses The virtually continuous, mid-Jurassic to Holo- mixed tectosilicate glasses, some of which ex- for its origin, including igneous activity, a dia- cene sediment sequence in the adjacent areas of hibit flow textures with total dissociation of treme, intersecting faults, or meteorite impact the Scotian Shelf has apparently preserved a the mafic components to ill-defined aggre- (Internal Report by R. Blum, 1972, for Shell largely uneroded record of a major, marine gates. These features are diagnostic of a 35- Canada; Union Oil Company, 1975; Jansa and impact event. The opportunities that are pre- to 50-GPa shock level for parts of the breccia. Wade, 1975; Wade and MacLean, in press). sented to compare the morphologic character of The top of the breccia is covered by a 40.5-m- The Montagnais 1-94 well, drilled to 1,646 m marine and dry-land structures, to assess the ef- thick suevite zone. Two crystalline melt-rock depth, penetrated successively Holocene to Eo- fects of sea water on impact melt sheets, and to horizons 72 and 34 m thick are enclosed in cene marine sediments, a thick breccia, and up- test some aspects of the iridium anomaly/ the breccia. Even though the feldspars in the lifted and disturbed metasedimentary rocks of impact/mass-extinction hypothesis have the po- melt are labradorite to bytownite in composi- the Cambro-Ordovician Meguma Group, which tential to make the discovery of Montagnais of tion, the chemical composition of the rock is forms the regional basement. Three relatively much greater significance than the simple recog- rhyolitic, with 70%-77% of Si0 , similar to 2 thin lithological horizons in the breccia are mac- nition of one more addition to the world's crater the composition of the basement. There is no roscopically similar to fine-grained basalts and population. obvious enrichment in siderophile elements, were described as such in the Well History Re- except an increase in iridium (0.1-0.3 ppb). port (Union Oil Company, 1975). These "ba- The occurrence of a thick section of impact Laboratory Methods salts" were subsequently analyzed, as part of an melt rocks and breccia capping the central on-going study of Cretaceous igneous activity on uplift is a feature not recognized in other Major elements were determined by electron the Scotian Shelf (Jansa and Pe-Piper, 1985). complex impact craters and may be a func- microprobe analysis of fused samples; rare- The analyses showed, unexpectedly, that plagio- tion of impact into a marine environment. earth-element (REE) and selected other trace- clase compositions range from labradorite to element values were obtained through instru-

Geological Society of America Bulletin, v. 101, p. 450-463, 7 figs., 3 tables, April 1989.

450

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/101/4/450/3380425/i0016-7606-101-4-450.pdf by guest on 26 September 2021 Figure 1. General location of Montagnais impact structure, showing the location of the Montagnais 1-94 well centered over the impact structure, other exploratory wells on the Scotian Shelf, and the reflection seismic line shown in detail on Figure 2 (solid line). The dashed line shows the continuation of the seismic line not presented on Figure 2 but used in the data interpretation.

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" ' '' ' -3.0 - ii/ I /« ' - — SEISMIC REFLECTORS

S _ OLIGOCENE/EOCENE '/L^' ' » I -> '• POLYMICTIC R BARREMIAN (ROSEWAY UNIT) IWÏ EVAPORITES u UNCONFORMITY - ' MOHAWK GR> p:Ç>:t> BRECCIA

METAMORPHIC BASEMENT c EOCENE M CALLOVIAN (MISAINE SHALE) 0 Km 5 SUEVITE UNCONFORMITY 1 i (MEGUMA)

B HORIZONTAL SCALE B BASEMENT (MEGUMA GROUP) MELT ROCK HORIZON C CONIACIAN MARKER

INTERPRETED SEISMIC SECTION THROUGH MONTAGNAIS 1-94 WELL.

Figure 2. A. Uninterpreted, 60-fold, time-variant, scaled-migration, reflection seismic section across the Montagnais structure and the Montagnais 1-94 well. Seismic line 3203-82 has been provided for the study by Petro-Canada and partners. B. Interpreted seismic line 3203-82 as above; shown is the crater, breccia fill, central uplift, and location of melt rocks. For the orientation, see Figure 1.

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mental neutron activation analysis; transition structure. Such a feature was interpreted by MONTAGNAIS 1-94 WELL: elements were analyzed by atomic absorption Jansa and Wade (1975, their Fig. 28) to be the PETROGRAPHY spectrometry. Analysis of iridium and selected result of a megaslump. It is intriguing that this trace elements was performed by C. Orth, Los feature is centered on the Montagnais structure, The Montagnais 1-94 well (Figs. 2 and 3) Alamos National Laboratory, New Mexico, but whether these two events are related is penetrated 112.7 m of water to sea bed, 510 m using instrumental neutron activation tech- unknown. of post-structure Cenozoic marine sediments, niques. Because of the small amount of sample A fault-bounded, central uplift of Meguma and a 993-m impact section, to a total depth of available, in some cases we were unable to pro- basement, which is 11.5 km across at the top 1,646 m (all depths given from rotary table vide a complete geochemical analysis on a single and irregularly circular in plan view, occurs in standing 30 m above sea level). The impact sec- sample, and thus stratigraphically higher or the center of the structural depression (Figs. 2 tion comprises two types of breccia (653-1,205 lower samples were used to complete the and 3). The generally flat surface of the central m) enclosing two zones of crystalline melt rocks analyses. uplift is interrupted by a central, shallow depres- and one zone of predominantly glass fragments sion, 3.5 km in diameter and about 200 m deep (suevite; Fig. 4). The breccia overlies shocked GEOLOGY OF THE MONTAGNAIS (Fig. 2). The top of the basement in the uplift is and uplifted, metasedimentary basement rocks STRUCTURE -1,295 m above its regional level on the La- (1,205-1,646 m). Petrographic studies were car- Have Platform. The central uplift lies near the ried out on a 3-m section of drill core obtained Multichannel reflection seismic data collected eastern edge of the Mohawk graben, where the from the 1,608- to 1,618-m interval in the by oil companies over the southern portion of Meguma basement, although not adequately re- basement, and on drill cuttings from the re- the Scotian Shelf, in combination with results of solved by reflection seismic profiles, could lie mainder of the well. Drill cuttings recovered exploratory drilling, confirm the presence of a -1,000 m below the regional level on the La- from a given drilled interval include material Paleozoic basement overlain by generally flat- Have Platform. If this is the case, then the cen- from that interval, intermixed with material col- lying, Middle Jurassic to Holocene sedimentary tral uplift would have been generated from lapsed from higher elevations in the well, so that rocks (Fig. 2). Locally, grabens in the basement, within the graben, implying uplift of as much as structural and compositional interpretations are several tens of kilometers in width, are filled by 2,300 m or more, depending upon how much reliable only on a scale of -10 m. Given these presumed Upper Triassic or Carboniferous clas- Meguma was removed from the uplift by the constraints, the contacts are sharp between the tics and evaporites (Jansa and Wade, 1975; impact. Reflection profiling data indicate that breccia and the younger sediments but are not Given, 1977). One of these grabens, the Mo- the uplift is capped by a seismically transparent well defined within the breccia and with the hawk graben, southwest of Montagnais, can be zone, which is identified as a breccia from the shocked basement below. The position of con- seen on the Petro-Canada multichannel reflec- Montagnais 1-94 well samples. A seismically tacts is more precisely defined by geophysical tion seismic profile (Fig. 2) that connects the similar transparent lens as much as 800 m thick logs (Fig. 4). Petrography of the younger Ceno- Montagnais 1-94 and Mohawk B-93 wells. Strat- extends as far as 8,500 m from the sides of the zoic sediments is not presented in this paper, igraphic ages have been obtained from the central uplift into the annular depression, abut- as they are later deposits which buried the Mohawk well for a number of seismic reflectors ting sharply on the central uplift, and is likewise structure. which can be traced eastward over and beyond interpreted as breccia. In the breccia overlying the central uplift, two horizontal reflectors can the Montagnais structure. The pre-Eocene re- Basement in the Central Uplift flectors are progressively eroded, are discontinu- be seen restricted to the bowl-shaped area of the central uplift; these correspond to the so-called ous, and show evidence of faulting and down- The basement is relatively uniform in litho- "basalts" of the early interpretations and indi- warping as far as 22 km from the center of the logic composition and consists primarily of light cate that they are of restricted extent. Montagnais structure. The resulting shallow, green, fine- to medium-grained, chloritic meta- synclinal-type structure, some 45 km in width, is Reflections from Eocene to Holocene sedi- subgraywackes and phyllites. Albite is the dom- almost symmetrically disposed about a central ments are continuous along seismic reflection inant feldspar of the poorly sorted metasub- region of uplifted basement (Figs. 2 and 3). profiles across the Montagnais structure (Fig. 2) graywackes. The presence of epidote in these Downfaulting, disruption, and lack of coherent and show evidence of draping on the flanks of rocks and in locally intercalated metaquartzites reflectors in the post-basement section are most the Montagnais uplift. Seismic profiles show ev- attests to a low metamorphic grade. Rock com- pronounced within 10 km of the edge of the idence for several periods of current erosion of position, color, texture, and grade of metamor- central uplift, forming an annular structural de- these post-impact ocean-floor sediments during phism closely resemble those of the Cambro- pression. This depression is well defined on the Tertiary time at distances as far as 22 km Ordovician Meguma Group described from seismic structure map constructed through in- northwest from the uplift. Such erosion is par- outcrops on the Nova Scotia mainland (Schenk, terpretation of more than 1,000 km of multi- ticularly recognizable by the deep channel cuts 1970; Liew, 1979). The metasubgraywackes channel reflection seismic data acquired in a in the synclinal deposits southeast of the Mon- and metaquartzites of the basement in the Mon- 3-km by 3-km grid over the area. The map, tagnais central uplift (reflector "O", Fig. 2). The tagnais well occur in 3- to 10-m-thick beds al- which was constructed by mapping the Eocene depth of erosional channeling diminishes north- ternating with thinner phyllite beds; the entire seismic horizon at the top of the structure, shows westward from the center of the structure, indi- section dips -50° southeast, as indicated by the faults, particularly at the north-northwest side of cating that the uplift was a positive relief feature dipmeter log. An 8-m-thick metafelsite encoun- the structure (Fig. 3). The southeastern side of above the sea floor that presented an obstacle to tered near the top of the Meguma section, at the structure was modified later by marine ero- post-early Eocene ocean-bottom currents. The 1,308 m, is composed of iron-stained sericitized sion on the upper continental slope and outer microfaunal assemblage of foraminifera, nanno- feldspar, quartz, and minor epidote and biotite. shelf, particularly during the Oligocene, as estab- fossils, diatoms, radiolarians, and sponge spic- Felsic volcanic rocks are known to occur in the lished by dating of seismic reflectors by oil ex- ules, along with glauconite grains in the Ordovician-Silurian White Rock Formation in ploratory drilling. An extensive collapse of the calcareous, zeolitic mudstone that overlies the western Nova Scotia (Sarkar, 1978). outer continental shelf (Jansa and Wade, 1975) breccia on the Montagnais uplift, are indications was noted to extend for about 320 km along the Dating of the Meguma basement rocks in the that the central uplift was not subaerially ex- 40 39 shelf edge and is centered just at the Montagnais Montagnais well by Ar/ Ar provided an iso- posed for any period. topic age on whole rock of 359 ± 1 m.y.

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Figure 3. Seismic time structure map on top of Eocene unconformity (top of Montagnais structure). Contour interval is 0.1 sec. The map shows central uplift, plane view of the crater shape, and location of some major boundary faults. The solid line is the location of seismic section shown on Figure 2.

(R. Bottomley, 1988, personal commun.). This oped in quartz grains throughout the 440-m 4). Detailed analysis of the lithic composition compares favorably with dates of 374 to 361 vertical section of the basement. Universal-stage and size distribution of the breccia clasts is diffi- Ma for the South Mountain Batholith (Reynolds measurements on samples from 1,611 m de- cult to reconstruct from the drill cuttings, which and others, 1981), 329 ± 14 m.y. for the granite tected the development of basal features (001) in average only a few millimeters in size. In places, at the bottom of the Mohawk B-93 well (Given, only 30% to 70% of quartz grains (Type A de- however, the breccia is monomictic over several 1977), and 359 ± 12 m.y. for the granite in the formation), indicative of shock pressures in the tens of meters, which may indicate the dimen- Ojibwa E-7 well (Table 1). The date of our 6.5- to 7.8-GPa range (Robertson and Grieve, sion of the largest blocks. Montagnais basement sample thus is concluded 1977). These weak shock levels are typical of The lower 345 m of the breccia is composed to represent thermal resetting associated with the those encountered at depth in the central uplift almost exclusively of Meguma-type rocks and emplacement of the granitic plutons, as sug- of complex impact structures, or beneath the could be appropriately described as "para- gested by Dallmeyer and Keppie (1987). floor of simple craters (Robertson and Grieve, autochthonous breccia." The trace amounts of The most pervasive evidence of deformation 1977). sand and clay-sized particles at the base of the in the uplifted basement is the development of breccia zone, however, might represent admixed hairline fractures within the metasubgraywacke Breccias Mesozoic-Cenozoic sedimentary-rock particles that result in a highly fractured mosaic fabric in within the breccia, even though such particles which the fragments are not displaced (Fig. 5A). The Meguma basement of the central uplift is could also be generated by friction of the clasts Planar deformation features are weakly devel- overlain by a 552-m-thick breccia deposit (Fig. during brecciation.

BIOSTRATI- LITH. sentation of other rock types, such as glauconitic FORMATION G.R. SONIC METRES LITHOLOGY GRAPHY COL. mudstones, chalks, and quartz sand grains. Paly- nological analysis shows mixed Eocene and Late LAURENTION Cretaceous microflora near the base of the al- FORMATION 331 m base of casing lochthonous breccia. Higher up in this zone, 378m only late early Eocene microflora were Oligocene? 400 identified. i « J Evidence of shock metamorphism is found lilO c undisturbed, throughout the breccia, but the over-all impres- BCf= .È marine glauconitic LU C "a < a> 500 siltstone, rare sand and sion is of a relatively low level of dynamic de- conglomerate beds. formation. Less than 10% of mineral grains or 2 CE p Middle <2 i Eocene lithic fragments display distinctive shock effects, i 600 and most clasts appear unshocked or only ^top of impact deposits 653m weakly disturbed. Planar deformation-feature suevite zone development in quartz of Meguma clasts is typi- late Early 693.5m Eocene 700 cally at the type A or B levels (Robertson and clastsof metamorphics, o n others, 1968), representative of shock pressures Jurassic limestone, granite S'8 Cretaceous and Cenozoic in the 8- to 12-GPa range (Robertson and .c œ sediments. Grieve, 1977); type C or D (shock levels 12-20 800- 814.0m GPa) are rarely observed. Although grains doc- ral breccia of metamorphic basement umenting the latter shock levels are nowhere mixed shocked and variably abundant, they are more common in the brec- 900- melted clasts with a horizon cias immediately above the uplifted basement of crystalline melt rock \ 953.5m and in the interval between the upper and lower upper melt zone *987.5m crystalline melt-rock zones (Fig. 4). 1000 Shock metamorphism indicative of pressures mixed shocked and variably in the 20- to 35-GPa range, such as planar de- melted clasts \ 1069m lower 1100m formation features in feldspars, is virtually ab- 1100 melt zone crystalline melt rock sent, but incipient to total isotropization of 1141m quartz and feldspars (diapletic glass) is common t>08 (Fig. 5B). Examples of mixed tectosilicate ^bottom of excavated cavity M 1200 glasses are also common, some of which exhibit , top of shocked, 1205m basement flow textures (Fig. 5C), with total dissociation of 2 mafics to an ill-defined aggregate. They are di- O 1300 agnostic of 35- to 45-GPa shock levels (Stoffler, 1971). The glass is rarely fresh, mostly devitri- cc low grade metamorphics fied or recrystallized, and usually altered into O (metasubgraywacke.phyllite, microcrystalline aggregates, which have various 1400 metaquartzite); fractured, (Paleozoic) low pressure shock metamorphism textures (and compositions?) reflecting the in- features > fc homogeneity of the target rock. These mixed 2 !" glasses are rare in the lower breccia, below the UJ S 1500- Q _1 — basal crystalline melt-rock zone, and are also O en infrequent in the breccia that overlies the lower melt-rock zone. Their abundance increases up- 1600 ward in the upper breccia (polymictic breccia 3 1646m T.D o zone) where they account for up to ~ 15% of the LU material immediately underlying the suevite zone capping the breccia (Fig. 4).

Figure 4. Vertical stratigraphic column of the Union et al Montagnais 1-94 exploratory well. Impact Melt Zones The well encountered three main lithologic units: (1) slightly shocked Paleozoic basement, (2) zone of shocked breccia and impact melt rocks, and (3) undisturbed cover of Cenozoic sedi- The three crystalline melt-rock-rich zones in- ments. G.R. = gamma-ray log, Lith. col. = generalized lithological column. Note: scale change tersected in the Montagnais well are apparently on sonic log at 908.6 m. Depths are measured from rotary table, 30 m above sea level. The in a structurally horizontal position. The lower water depth is 112.7 m. two zones are represented by fine crystalline melt rocks; the uppermost zone, by vesicular glass fragments, similar to a suevite (Fig. 4). The The upper 161 m of breccia is clearly al- stones similar to Middle Jurassic limestones two lower zones, 72 and 34 m thick, respec- lochthonous and polymictic. Clasts of variably occurring in the Mohawk B-93, and rare granite tively, have a lateral extent of at least 2.5-3.5 epidotized and chloritized Meguma lithologies fragments resembling Devonian granite pluton km as interpreted from the reflection seismic are mixed with a coarse-grained quartz sand, encountered in the Mohawk well. The polymic- profile (Fig. 2). The uppermost layer is 40.5 m with fragments of oolitic and bioclastic lime- tic breccia further contains a significant repre- thick, and caps the breccia.

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Figure 5. Photomicrographs from Mon- TABLE 1. ISOTOPE CHRONOLOGY tagnais 1-94 samples. A. Hair-line microfrac- Geological Survey of Canada Krueger Geochron. laboratory York, Bottomley tures and rare undecorated shock lamellae (1) Geochron. laboratory University of Toronto in quartz in metaquartzite of the Meguma 9 4 basement, 1611.8 m, crossed polars, scale bar Method K/Ar K/Ar 3 Ar/ °Ar

= 0.25 mm. B. Isotropization of the minerals Results (K %) age (K %) age Age in metagraywacke clast of the breccia (gray colors; i), and extensive microfracturing, 698 Montagnais 1-94 m, crossed polars, scale bar = 0.2 mm. 661.4 m (suevite) 52.2 ± 0.6 m.y. C. Flow texture in a glass fragment in the allochthonous breccia. Glass is fresh, enclosing 987.5 m (middle melt rock) 51.9 ± .2 m.y., 57 ± 0.2 m.y. light brownish, vesicular, devitrified, glass 1,115 m (lower melt rock) (2.6%) 55.8 ± 0.9 m.y. (2.55%) 49.9 ± 2.1 m.y. 48.9 ± 0.5 m.y., 50.5 ± 0.8 m.y. fragments (f), 738 m, plane-polarized light,

scale bar = 0.5 mm. D. Crystalline melt rock 1,610 m (Meguma basement, 359.26 ±1.1 m.y. from the lower melt zone composed of feld- whole rock)

spar microliths arranged in radial fibrous nests Ojibwa E-07 (n), minute opaque minerals (o) (magnetite), and rare chloritized biotite crystals in dusty 2,319 m (granite, biotite (6.85%) 359 ± 12 m.y. concentrate) devitrified glass matrix, 1127.7 m, crossed polars, scale bar = 0.4 mm. E. Highly shocked, Mohawk B-93* vesiculated clast of Meguma rock from suevite zone. Unabsorbed quartz grains (q), (granite, biotite concentrate) 329 ± 14 m,y.

some with shock lamellae, are surrounded by •Given, 1977. a devitrified, vesicular glass matrix sprinkled by dust-sized opaque minerals. Some of the vesicles (v) have brown microcrystalline rim, 652 m, crossed polars, scale bar = 1 mm. hedral magnetite grains; and (4) localized The clasts exhibit flow textures, are highly ve- F. Melt glass in the suevite zone. Thin section cryptocrystalline devitrified glassy matrix. Aver- sicular, and are generally strongly pigmented by shows a patch of fresh glass with rare, age length of the plagioclase needles decreases minute iron oxide particles. The vesicles in the needle-shaped, highly elongated microliths from 0.2 mm near the center of the subunit to 86 glass and in the frothy textured melt rock near (m), two types of vesicles, one filled by green- microns at its top, where penetrating nests of the the top of the subunit are infilled by chlorite ish glass, others with brownish rim of proba- feldspar crystallites (Fig. 5D) define a xenomor- and/or by chalcedony. In general, the mixed bly chlorite or zeolite (z). The clear, fresh phic texture. Rounded aggregates of fine-grained glasses are less abundant in the upper part of the glass (g) is surrounded by incompletely quartz with mosaic texture, representing rem- melt-rich breccia subunit. Lower grades of shock melted glass (darker colored), enclosing a few nant, recrystallized, shocked quartz clasts, are metamorphism in the upper level, characterized quartz grains, 689 m, plane-polarized light, commonly enclosed near the base of the melt by planar deformation features and reduced scale bar = 0.25 mm. zone. birefringence in tectosilicates, are an indication The crystalline melt-rock subunit is overlain that the residual heat of the impact melt at this by a melt-rich breccia subunit composed of level was insufficient to equilibrate these rela- highly shocked, partially melted, recrystallized tively cold clasts. clasts in a poorly defined groundmass with a Middle Crystalline Melt-Rock Zone (953.5- variable melt component. The groundmass var- 987.5 m). Macroscopically, this zone is com- Basal Crystalline Melt-Rock Zone (1,070- ies from very fine-grained crystalline melt rock, posed of black, dark green, and gray glassy 1,141 m). The seismic profile (Fig. 2) indicates through an inhomogeneous mix of melt rock clasts, many of which are highly vesicular. A that the basal melt-rock zone has a minimum and recrystallized comminuted material, to a thin recrystallized melt-rock zone ~6 m thick lateral extent of 3,500 m and is restricted to a generally recrystallized comminuted matrix. The occurs at the bottom of this zone. The ground- 200-m-deep, bowl-like depression in the center percentage of recrystallized matrix decreases mass of melt rock includes pockets of fresh glass of the uplifted Meguma basement. This melt- upward in the unit. Quartzose clasts in the lower but generally comprises a mixture of nielted and rock zone can be divided into two subunits on portion of the subunit are entirely recrystallized, recrystallized comminuted material, dominated the basis of texture: a lower, fine-grained crystal- and any evidence of shock deformation has been by quartz and feldspar. Feldspars have a ten- line melt rock (1,100-1,141 m) overlain with a thermally annealed. Recognizable metasub- dency to form aggregates with radial texture. reasonably sharp contact by a melt-rich breccia graywacke clasts show recrystallization of feld- Chlorite or chloritized mica are also common subunit (1,100-1,069 m). The crystalline melt spars to spheroidal aggregates. Other clasts, matrix components. Myriads of tiny, granular rock of the lower subunit has a gray color, is fine showing little evidence of thermal effects, com- particles, possibly glass or spinel, are distributed crystalline, and is comprised of (1) narrow, prise agglomerates of fragmented crystals. Many unevenly in the crystalline melt and recrystal- polysynthetically twinned plagioclase needles, clasts, however, have been shocked to a mixture lized groundmass. The overall crystallinity of the sometimes arranged into radial, fibrous aggre- of diaplectic and melt glasses so that their pre- melt rock is finer grained than of the lower crys- gates; (2) chlorite and mica laths; (3) tiny, sub- impact lithologies cannot be readily determined. talline melt-rock zone (see above). Clasts and

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TABLE 2. CHEMICAL ANALYSES OF THE MELT ROCKS AND MEGUMA METASEDIMENTS IN THE MONTAGNAIS 1-94 WELL

Suevite Middle melt Lower melt Meguma basement

661- 670- 679- «670- 960- 978- 981- 1,075- 1,082- 1,097- 1,109- 1,118- 1,136- 1,600- 1,627- 670m 679 m 689 m 679m 969 m 987 m 990m 1,085m 1,091m 1,106 m 1,118m 1,127 m 1,146 m 1,609m 1,636m

Si02 76.75 76.53 76.40 71.48 71.45 74.66 73.41 77.59 75.78 72.21 75.13 73.37 73.40 71.08 70.58

Ti02 0.61 0.70 0.58 0.55 0.68 0.50 0.61 0.50 0.57 0.64 0.72 0.61 0.60 0.59 0.68

AI2O3 13.02 12.77 12.59 11.93 14.76 12.82 13.48 11.00 12.04 13.57 12.54 12.90 13.41 15.20 15.10 FeO, 3.32 3.40 3.50 1.77 4.47 3.70 4.06 3.03 3.14 3.82 3.45 3.35 3.50 4.99 5.22 MnO 0.00 0.01 0.00 0.04 0.01 0.00 0.02 0.00 0.00 0.00 0.00 0.00 0.00 0.06 0.01 MgO 0.90 1.02 0.93 0.51 1.68 1.32 1.41 1.24 1.38 1.47 1.26 1.43 1.73 1.62 1.87 CaO 1.60 1.88 2.44 1.09 1.24 1.63 1.35 1.78 2.08 2.38 2.06 2.59 1.81 0.78 1.04 Na20 2.14 2.13 1.94 1.67 2.97 2.32 2.39 1.88 2.22 2.45 2.11 2.41 2.51 2.60 2.56 K2O 1.60 1.46 1.54 3.36 3.65 2.97 3.18 2.90 2.78 3.41 2.75 3.34 3.02 2.41 2.86 p2°5 0.00 0.03 0.00 0.05 0.03 0.03 0.03 0.00 0.00 0.00 0.03 0.00 0.00 0.04 0.00 Total 99.94 99.93 99.92 92.45 99.92 99.95 99.94 99.92 99.99 99.95 100.05 100.00 99.98 99.37 99.92

'Average chemical composition of colorless glass droplets (26 electron-probe analyses). ^From Sarkar (1978) rhyolite from the White Rock Formation. FeO, total iron expressed as FeO.

grains enclosed in the crystalline melt are recrys- able. The glass occurs as groundmass and entirely recrystallized grain mosaics. Quartz tallized in the lower part of the melt-rock zone. distorted droplets in the clasts, as a narrow par- mosaics commonly display crescentic fractures. The proportion of melted clasts and clastic tial rim around some of the clasts, or as flow- Other clasts in the breccia that do not show grains decreases upward, with weakly shocked textured zones that weld together fragments of shock metamorphism were not raised to high recognizable Meguma metasediments dominat- different composition. Whereas all of these glass thermal states by the shock process and retain an ing near the top of the zone. The lower occurrences in the upper portions of the unit are angular and nonrecrystallized form. Least com- homogenization of the melt in comparison to the devitrified and/or altered to a brownish, micro- mon in the upper melt subunit are fragments of lower melt-rock zone suggests that there is a crystalline aggregate, a few fresh patches are diaplectic glasses and inhomogeneous, shock- lower shock metamorphism in this zone and a found in the groundmass of the clasts at deeper produced glasses in which the glass is generally more rapid crystallization, as documented by the stratigraphie level. Some of the clasts in the fresh (Fig. 5F). Vesicles in the mixed glasses are smaller size of crystals in the melt rock. breccia have rounded margins and are generally slightly distorted and are rimmed or filled by the Upper Melt-Rock Zone (Suevite Blanket) brownish altered glass. Clasts of crystalline im- (653-693.5 m). The part of the zone below 671 pact melt rock, which are not abundant, are dis- m comprises three, 3- to 5-m-thick breccia horizons with a glass matrix or a prominent glass component, intercalated with thinner, allochthonous breccia layers. Highly shocked, vesicu- MELT ROCKS 200 lated clasts of Meguna metaquartzite and MEGUMA METASEDIMENTARY ROCKS metagraywacke are prominent in the intercalated breccia horizons (Fig. 5E). Fractures in ENVELOPE FOR ONSHORE MEGUMA diaplectic quartz grains are filled by glass with METASEDIMENTARY ROCKS (Liew,1979) 100 very fine-grained, highly refractive aggregates arranged along planes of different orientation.

The grain size within the aggregates is less than 1 0C ¡JL. The color of the aggregates appears brownish, o Z 504 which distinguishes them from the colorless o X glass. Such aggregates, from petrographic evi- o dence, are similar to coesite, as described by o Stoffler (1971). Rare, Jurassic oolitic limestones o with planar features in the quartz nucleus of the tr ooids indicate that the limestone fragments are derived from the site of impact and not retrans- ported from the edges of the basin. Sandy, glauconitic mudstone, typical of Late Cretaceous and Tertiary deposits on the Scotian Shelf, is 10- also identifiable among the clasts. The part of the zone above 671 m is dominated by vesicular glass fragments (Fig. 5F). La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tn Yb Lu The white lining of many vesicles has been identified as ferroan sparry calcite, zeolites, and sil- Figure 6. REE spectra for melt rocks (solid lines) and Meguma basement rocks (dashed ica. On a microscopic scale, three varieties of lines) in the Montagnais well. Stippled envelope denotes field of Meguma Group from onshore crystalline- or glass-rich material are recogniz- studies (Liew, 1979).

TABLE 3. CHEMICAL ANALYSIS OF REE AND OTHER TRACE ELEMENTS (values in ppm) FOR REPRESENTATIVE SAMPLES OF MELTS AND MEGUMA BASEMENT

Meguma basement

t652- 670- Í679- 1,082- 1,097- 1,109- ti.itf- 1,118- 1,136- 1,600- ti,6l0m +1,610.5 1,627- 661m 679 m 689 m 1,091m 1,106 m 1,118 m 1,124 m 1,127 m 1,146 m 1,609 m m 1,636 m

La 21 35.29 21 30.78 34.43 30.71 22 34.70 32.92 29.52 35 42 33.49 Ce 60.82 54.17 59.35 57.00 60.36 58.66 55.67 59.24 Nd 27.64 23.14 26.63 25.03 26.85 25.81 26.43 29.26 Sm 5.43 4.94 5.34 4.61 5.55 5.09 4.65 5.35 Eu 1.24 1.11 1.26 1.11 1.19 1.13 0.98 1.04 Tb 0.27 0.66 0.79 0.59 0.75 0.66 0.61 0.68 Yb 2.33 2.40 2.40 2.12 2.66 2.52 2.06 2.21 Lu 0.34 0.34 0.36 0.31 0.37 0.35 0.33 0.33 Ba 538 602 951 611 658 564 478 583 Co 7.7 7.9 9.91 10.2 8.4 10.3 10.6 10.4 17 n.d. Cr 39 44 47 62.2 64.8 47 40 68.6 65 48 58 106 58 Cs 5.16 1.59 3.19 3.30 3.01 2.13 3.04 3.35 Hf 5.93 5.25 5.20 4.90 5.72 5.39 4.34 4.35 Sb 1.15 0.25 0.29 1.64 0.21 0.23 0.94 1.13 Sc 6.7 9. 7.1 8.79 9.96 8 7.3 9.67 9.78 10 7.7 19 11 Ta 2.85 0.74 0.77 2.92 0.86 0.74 2.06 1.96 Th 7.5 7.94 7.5 7.27 7.88 7.22 7.3 8.24 7.91 7.60 15 12 8.48 U 2.15 1.64 1.69 1.50 1.90 1.60 2.36 2.50 •Au 0.71 0.78 0.34 0.49 n.d. Ir 0.24 0.016 Ni 27 31 32 29 22 35 Li 13 74 27 30 Zn 70 53 64 58 Cu 11 12 18 21 Cr 49 56 50 61 V 57 58 64 76

•Values of Au, Ir are ppb. Analysis by Ch. Orth, Los Alamos National Laboratory. Elements from La to Au analyzed by INAA (Dr. P. Jagam, Guelph University); Ni to V analyzed by Atomic Absorption (Dalhousie University, Halifax, Nova Scotia).

tinguishable from the other glass-rich materials Meguma Basement AI2O3, FeO, and MgO than the Meguma anal- through their general lack of alteration. Such yses from the Montagnais well (Table 2). clasts are characterized by very fine-grained The chemical composition of Meguma base- Trace-element and REE contents of the two feldspar(?) microlaths and granular oxides(?) in ment subgraywacke from the Montagnais 1-94 lithologies are closely comparable within the a colorless glass mesostasis. The circular to oval well (Table 2) is similar to that reported for limits of analytical error, and there is no clear vesicles are not plentiful and are rimmed or interbedded phyllites and metasubgraywackes evidence for any meteoritic contribution to the filled by brownish, altered glass. Occurrence of occurring in the Meguma Group elsewhere in REE patterns of the melt rocks (Table 3; Fig. 6). vesiculated glass is, according to Stoffler (1971), Nova Scotia (Liew, 1979). The composition is REE spectra from the melt rocks are very sim- indicative of shock peak pressure of 55-60 GPa -70% silica, 15% alumina, 5% iron oxide, and ilar and have only slight differences in some and post-shock temperatures of approximately 5% alkali oxides. The rare-earth elements deter- rare-earth elements in individual samples; there 1300-1500 °C at the impact site. mined for two samples of the basement Meguma is a slight enrichment in La and heavy REE rocks show a highly fractionated pattern (Fig. from Eu to Lu compared with the Meguma Overlying Sediments 6). This pattern lies within the field of Meguma basement at Montagnais. It is difficult to evalu- analyses of Muecke and Clarke (1981) but is ate how representative the two Meguma anal- The upper vesicular glass subunit is overlain slightly more fractionated than typical individ- yses are of the basement throughout the area of in sharp contact by brownish-gray, glauconitic, ual samples reported by Liew (1979). Liew the crater. Although the chemical composition slightly calcareous, silty mudstone. Nannofossils showed that there are no systematic differences falls within the range presented by Liew (1979), and planktonic foraminifera account for the cal- in REE content between the metapelites and the it is possible that stratigraphic variations in careous component, and siliceous diatoms, radi- metawackes that he analyzed; he suggested, basement chemistry could account for many of olarian debris, and sponge spicules represent a therefore, that the REE's occur principally in the differences observed between melt rocks and minor, siliceous biogenic component. There is accessory heavy minerals. It might therefore be the analyzed basement. For example, relatively no evidence of shock metamorphism in the expected that there would be minor variations in small increases in zircon content in the basement mudstone, which is a post-impact deposit. These REE content between the Montagnais site and rocks could account for the apparent enrichment sediments were deposited in a low-energy, deep the localities analyzed by Liew, which lie on- in heavy REE. outer neritic to upper bathyal marine environ- shore about 200 km to the west of Montagnais. Siderophile elements, with the exception of ment (-200-600 m) as indicated by the iridium, show almost identical abundances in microfauna. Impact Melt Rocks the basement and impact melt rocks. The analyses of samples from the Montagnais structure GEOCHEMISTRY The three crystalline melt-rock horizons have provided iridium concentrations for Meguma similar major-element compositions which are basement rocks of 0.016 and 0.047 ppb, and Results of geochemical analyses of the melt equivalent to that of rhyolite (Zannetin, 1984, concentrations of 0.21 and 0.24 ppb for the rocks and Meguma basement are summarized in Table 2). On the average, the melt rocks are basal melt rock and the upper suevite zone. Iri- Tables 2 and 3. slightly higher in Si02 and CaO and lower in dium content of up to 0.32 ppb was recently

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determined in the middle melt by Ch. Orth Eocene) and correspond to about 52-53 Ma on 39Ar age of 35.3 ± 0.2 m.y. (Bottomley and (1988, personal commun.). The melts are thus the Kent and Gradstein (1986) stratigraphic York, 1988), in good agreement with earlier enriched in iridium by an order of magnitude chart. A slightly older age was obtained by stud- measurements (Keller and others, 1987). Varia- when compared to the Meguma basement rocks. ies of nannofossils which indicate the presence tions in Rb-Sr and Sm-Nd isotopic composition The level of enrichment is also higher than is of Zone NP 12-NP 13 (52-54 m.y, Kent and show that the DSDP Site 612 tectites are pro- found in the overlying shelf deposits (0.05 ppb). Gradstein, 1986; Jansa and others, 1988). Paly- duced from an impact site other than the North Electron microprobe analyses of feldspar mi- nology also indicates an Eocene age of the sedi- American tectites (Stecher and others, 1988). crolites in crystalline melt rocks and in the ments overlying the suevite zone, and early Until analytical results similar to those for the colorless glass droplets in the suevite zone indi- Eocene dinoflagellates are thermally altered in glass of the Montagnais structure and more reli- cate variable plagioclase compositions from lab- the beds intercalated in the lower part of the able dating for sediments in Site 612, particu-

radorite (An50) to bytownite (An74), and suevite zone (E. H. Davies, 1984, Internal Re- larly by magnetostratigraphy, are available, we port EPGS-PAL.8-84EHD). Thus, very com- prefer to leave the impact source for DSDP Site sanidine (Or80) (Fig. 7). The high Si02 content (70%-77%, Table 2) of the glasses within normal parable results were obtained by the geochrono- 612 tectites open. igneous conditions would result in crystalliza- logical and biostratigraphic methods, demon- tion of a sanidine but would have more albitic strating that the bolide impact occurred during DISCUSSION plagioclases than present. The very fine-grained late early Eocene time, at -50.5 Ma. This age microlites are an indication that melting and determination does not support the suggestion of Cratering Processes subsequent crystallization did not take place Glass (1988) and Bohor and others (1988) that the Montagnais impact crater is the source of under equilibrium conditions, resulting in pres- Several authors have discussed meteorite im- ejecta for the North American tectite strewnfield ervation of local chemical inhomogeneities. It is pacts into the ocean (for example, Strelitz, 1979; and for tectites found off New Jersey in Deep postulated, therefore, that one of the possible McKinnon, 1982; Melosh, 1982; O'Keefe and Sea Drilling Project (DSDP) Site 612. Both lat- consequences of impact into a shallow ocean is Ahrens, 1982; Roddy and others, 1987). Gault ter occurrences are of late Eocene age (Keller rapid quenching and heat dissipation by the sea and Sonett (1982) commented that an impact and others, 1987). A sample of North American water, preventing the attainment of homoge- into 1 to 2 km of water would leave an easily tectite was analyzed simultaneously with the neity and equilibrium in the impact melt and identified structure, in which, according to Stre- Montagnais melt rocks and provided an 40At/ crystallization processes. litz (1979), crater shape should correspond to

AGE An

The basal melt rock was dated by whole-rock K/Ar and 39Ar/40Ar radiometric methods (Table 1). The analysis of melt rock from the interval 1,115-1,124 m provided a K/Ar isotopic age of 55.8 ± 0.9 m.y. (Geochronological Laboratory of the Geological Survey of Canada) and 49.9 ± 2.1 m.y. (Geochron Laboratory, Krueger Enterprises). The 39Ar/40Ar isotopic study (R. Bottomley and D. York, 1988) provided additional ages for the Meguma basement, melt rocks, and a glass of the suevite blanket. The radiometric age for the lowermost homo- geneous recrystallized melt rock is 50.5 ± 0.8 m.y. and 48.9 ± 0.5 m.y. The middle melt shows two age plateaus giving an older age of 57 ± 0.2 m.y. and a younger age of 51.9 ± 0.2 m.y. The glass of suevite zone provided an age of 52.2 ± 0.6 m.y. On the basis of petrographic studies, we explain the older age to be the result of unequili- brated clasts of target rock enclosed in the melt. The 39Ar/40Ar dating of the lowest melt rock probably best represents the cooling age of the melt and the age of the impact. As the impact occurred onto a shallow- marine environment, the age of the sediments directly overlying the suevite blanket provides another independent time constraint of the im- Figure 7. Normative feldspar composition diagram of whole-rock analyses of melt rocks, pact. The foraminiferal microfauna from the Meguma metasediments, colorless glass droplets, and feldspar microlites in the melt rocks. The sediments directly overlying the impact structure leucite-feldspar field boundary after Franco and Schairer (1951) and the limit of ternary solid belong to the foraminiferal zone P-9 (late early solution in natural feldspars after Smith and Mackenzie (1958) are also shown.

that on land. In terms of the cratering processes, uplift (Fig. 2) as another criterion to estimate and others, 1978). It may be argued that these the Scotian Shelf would have presented a transient cavity depth level, the base of the tran- allochthonous materials have been eroded from layered target with the 200- to 600-m-thick up- sient cavity and of the excavation could have the summits of the uplifts in craters on land and permost layer having a density near 1.0 g cm-3. reached depths of at least 2,900 m. Uplift of thus they appear different from the Montagnais The effect of the sea water as a heat-sink remains 2,900 m in view of seismic data is therefore structure. to be evaluated. realistic, but more difficult to prove because of A number of structures have been discovered The gross morphological aspects of the Mon- lack of seismic reflectors in the Meguma meta- that were protected from major erosion by a tagnais structure, an uplifted central peak sur- morphic rocks. Such a magnitude of uplift is in thick blanket of post-crater sediments that com- rounded by a depressed and apparently down- close agreement with SU calculations using the pletely buries or partially obscures the complex faulted annulus, in turn surrounded by a Grieve and others (1981) equation. crater. In the Ries, a 200-m-thick suevite breccia disturbed zone in which inward downfaulting at Therefore, an impact into marine environ- might have formed a continuous layer covering a megablock scale diminishes in intensity out- ment (less than 1 km deep) does not seem to the central region of the structure (Pohl and oth- ward, are comparable with the characteristics of result in any major structural or morphological ers, 1977). As this central region has the form of complex impact structures on land (Dence and differences between large complex impact cra- a shallow basin and was subject to river erosion, others, 1977). Some of the morphological pa- ters formed in this environment and those however, the preservation of the suevite is only rameters obtained, however, are different from created on land. The one structural difference patchy. Further, as the Ries lacks any prominent those predicted for land impacts. observed is the width of the central uplift, which central uplift, a strict analogy cannot be drawn Grieve and others (1981) derived the rela- is 11.5 km at the top and -13.75 km at the base with the Montagnais situation. On the other u hand, the Red Wing Creek structure, buried by tionship SU = 0.06 Da for the amount of (Figs. 2 and 3). The latter width is significantly structural uplift (SU) of the central peak in ter- broader than expected for an impact crater of 45 younger sediments in the Williston Basin of restrial complex craters as a function of final km in diameter (Hale and Head, 1979). A sim- North Dakota, is morphologically comparable (Brenan and others, 1975), but the central uplift crater diameter (Da). Neither feature at Mon- ilar conclusion is reached by using Melosh's (in tagnais is unequivocally defined from the seis- press) crater rim-crest diameter/central-peak di- is surmounted by a megabreccia of carbonate mic data as yet available. The crater diameter ameter for planetary craters, which shows that blocks. The buried Steen River structure, Al- berta, may provide the closest analogy with (Da) lies beyond the annular depression and is corresponding crater size to the magnitude of the logically marked by the outermost evidence of Montagnais central uplift should be in range of Montagnais. The flat-topped central uplift of faulting. From the available seismic records, it 60-65 km. The reason for such difference is Precambrian basement gneisses is apparently the may be difficult to distinguish faulting that oc- presently not clear. Three possible causes can be highest morphological feature, rising 1,700 m curred as part of the cratering event from later considered: (1) the crater diameter is larger than above the peripheral trough (Winzer, 1972). faulting that resulted from tectonic disturbance interpreted. Even though there is some weak The peak is covered by 184 m of post-crater or regional subsidence. It may also be difficult to evidence that this might be the case, the current sediments, which thicken to an average 1,100 m distinguish the crater margin from channel ero- geophysical data do not allow us to change the on the flanks. Winzer (1972) described shocked sion of bottom sediments. Nevertheless, in the interpretation; (2) the difference is the result of materials that resemble suevite breccia forming a light of current evidence, we interpret the disrup- an impact into the ocean environment; (3) the 121-m-thick deposit atop the central uplift. Al- tions symmetrically displaced at -22-23 km uplift is broader because the impactor body was though this situation resembles that at Montag- from the structure's center as the outer limit of of a lower density and bigger in size than aver- nais, these observations were based on a single drill hole into the uplifted basement, and the crater disturbance, yielding Da = 45 km, for age Apollo asteroid. morphology of Steen River and the nature and which SU calculated from Grieve and others The considerable thickness (552 m) of brec- (1981) would be 2,970 m. distribution of the crater deposits must be con- cias and impact melt rock overlying the central sidered poorly constrained. The amount of structural uplift directly ob- uplift has not been encountered at other com- servable is the difference between positions of plex impact craters. Similarly, the occurrence of Evidence for vertical zonation in crater depos- (1) Meguma basement in the central uplift and multiple impact melt-rock horizons and the ex- its, particularly on the basis of shock level, in- (2) the regional level of these metasedimentary tensive lateral continuity they display have not cluding impact melt rock, has been hinted at but rocks. Extending the Meguma basement to link been identified elsewhere conclusively, even in not firmly established at Brent and Lonar Lake its interpreted, undisturbed reflections 20 km the allochthonous deposits filling simple craters. craters (Fudali and others, 1980; Dence and east and west of the central uplift (Fig. 2) would The majority of complex impact structures rec- Guy-Bray, 1972). At neither Lonar Lake nor place the top of the basement below the central ognized so far have been discovered because of Brent are there seismic data to support extensive peak as -2,500 m below present sea level. A their considerable surface exposure. This in- lateral continuity of any possible zoning, and number of Triassic grabens occur in the area, cludes sites where central uplift materials are certainly not on the scale of several kilometers however, and there is an indication from seismic exposed and can be sampled at first hand and indicated for the Montagnais impact melt-rock data that the flank of the Montagnais structure those where central uplift is covered by water or horizons. may straddle one of them. If that was the case, post-impact sediments, where sampling has been In summary, the occurrence of the thick sec- then the top of the Meguma basement west of carried out through drilling (Dence and others, tion of impact melt rocks and breccia capping the central uplift could have been -1,000 m 1965). In several of these examples, breccia and the central uplift at Montagnais is a feature not deeper than was its regional level. Then the min- impact melt-rock deposits of some thickness recognized at other complex impact craters, and imum uplift of the central peak would be at least occur on the flanks of the central uplift or in so may be a function of impact into a marine 2,300 m. If we use the downward bending of depressed areas in the central peak complex but environment. The stages in the cratering process, seismic reflectors immediately overlying the do not overly the entire uplift (Grieve and oth- including central uplift formation, were proba- basement off the faulted flanks of the central ers, 1981; Simonds and McGee, 1979; Simonds bly the same at Montagnais as those outlined by

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Grieve and others (1981) for complex crater de- known from Ganymede, Callisto, Mars, and a mafics to an ill-defined aggregate, and diagnostic velopment. Such a scenario envisages allochtho- few from the Moon, but none of such structures of a 35- to 50-GPa shock level. nous breccia and impact melt near the floor of has been known from impact craters on Earth. Two zones of crystalline melt rocks (71 and the transient cavity, slumping outward into the A number of explanations have been offered for 35 m thick) are enclosed in the breccia. These peripheral trough region with the upwelling of the formation of central pits. One of them, fa- melt rocks have the same basic geochemistry as the crater floor to produce the central uplift. The vored by us, is that the central pit is the result of the underlying Meguma basement. The melt process and the final configuration of these de- unusually low velocity impact, and that the pit is rocks lack enrichment in siderophile elements posits become more complete in large structures the trace of the projectile that produced the except for an increase in iridium (0.3 ppb). where collapse of the central peak leads to the crater (Melosh, in press). The presence of the The basic cratering processes in the shallow development of one or more concentric rings. central pit in the Montagnais structure thus ocean (200- to 600-m water depth at the time of This scenario is supported at other complex argues against high-velocity impact and, to- the impact) are believed to be similar to those craters where allochthonous breccia and impact- gether with the broader width of the central up- known from the land impacts. The occurrence melt deposits are found concentrated in the pe- lift compared to other craters, may indicate of a thick section of impact melt rocks and brec- ripheral trough. At Montagnais, the zone of impact by a lower density, larger in size body. cia capping the central uplift, and thick para- incoherent reflectors on the flanks of the central Such information, together with low iridium en- autochthonous breccia are features not recog- peak, interpreted as allochthonous breccia, is no richment and lack of enrichment in other sider- nized at other complex impact craters on land doubt the result of this process. The central uplift ophile elements, suggests that the impactor may and as such may be the function of the impact breccias and the impact melt must have resulted have been a low-density, lower-velocity body— into a shallow sea. The 3.5-km-wide shallow the nucleus of a comet. from failure of the central uplift to fully shed central pit at the central uplift is a unique these materials as it rose. Further meaningful feature—observed for the first time in a complex discussion of this aspect will benefit from addi- SUMMARY impact crater on Earth. The central pit, which is tional drilling and geophysical surveys that are thought to be a result from an impact by an currently in the planning stage. Geophysical, geochemical, and petrographic unusually low velocity impactor (Melosh, in studies of a circular structure located near the press), has been preserved due to lack of strong Impactor shelf edge off Nova Scotia, Canada, demonstrate erosion of the structure in the marine environ- that the structure is the result of an impact by an ment, which otherwise rapidly modifies most of Enrichment of melts in siderophile elements extraterrestrial object. The crater is buried under the land-located impact craters. The larger width such as Ni, Co, Os, Ir, and Cr, were proven to be 510 m of Tertiary and Quaternary sediments of the central uplift, presence of the central pit, sensitive indicators of meteoritic contamination and 112.7 m of water. The crater, identified low-iridium enrichment, and lack of enrichment on the Earth and Moon (Lambert, 1982). With from multichannel reflection seismic data, is at in other more volatile siderophile elements may the exception of Ir, these elements are not ap- least 45 km in diameter, and evidence for severe collectively indicate that the Montagnais com- preciably enriched in the Montagnais melt disturbance can be observed to a depth of 2.9 plex impact crater has been formed by a lower- rocks. Low Ir enrichment, with Ir below 0.3 km at its center. A central uplift, 1.2 km or density, larger-diameter body, perhaps the nu- ppb, is characteristic of most achondrites, some possibly 2.9 km high and 11.5 km in diameter, cleus of a comet. classes of iron meteorites, and particularly for has been observed. The basement rock in the comets, according to Bazilevskiy and others central uplift displays hairline fractures and ACKNOWLEDGMENTS (1984). When the siderophile elements listed planar features in quartz, indicative of pressure above are arranged in order of increasing volatil- of 8-12 GPa. The crater is partially infilled by The progress on the study of the Montagnais ity, iridium is the least volatile, followed by Os, seismically isotropic masses which we interpret structure would have been impossible without Re, Ni, Co, and Cr. Thus, the relatively low Ir as fallback breccia. This breccia is up to 850 m substantial assistance from oil companies and enrichment in Montagnais melt rocks and lack thick and extends to a distance of 8.5 km many colleagues. We are indebted to Shell Can- of enrichment in other more volatile siderophile beyond the central uplift. The uplifted basement ada Ltd. for providing data for construction of elements may not indicate bolide composition of the central uplift is covered by 552 m of seismic structure maps; to Petro-Canada and but may reflect a high impact velocity. Compu- breccia. The basal 345 m is composed almost partners for making available the reflection ter modeling of large asteroid impacts by Roddy entirely of fractured basement rocks (para- seismic line used in this study; to Ch. Orth for and others (1987) indicates temperatures reach- autochthonous breccia). The upper 161 m is his kind agreement to provide the analysis of ing up to 20,000 °K above the rim of the crater polymict, and clearly allochthonous, containing iridium; to R. Bottomley and D. York (Univer- and total evaporization of the asteroid, with the a mixture of fragments of Meguma basement, sity of Toronto) for 39Ar/40Ar absolute age dat- vapor ejected above the tropopause during the Devonian granite, Jurassic limestone, and Late ing of the melt rocks; and to M-P. Aubry and first 60 seconds after the impact. By such a Cretaceous and Eocene clastics and chalks. Evi- F. M. Gradstein for micropaleontological dating mechanism, most of the geochemical signature dence of shock metamorphism is found of sediments overlying the structure. The authors of the impacting body could escape from the throughout the vertical section of the breccia, benefitted in formulating their ideas from discus- Earth. A unique piece of evidence recognized at but the over-all characteristic is a relatively low sions with T. Tankard, H. J. Welsink, J. A. the Montagnais complex crater, however, argues level of deformation. Shock metamorphism in- Wade, and B. C. MacLean. D.J.W. Piper, M. E. against the high-velocity impact. The evidence is dicative of pressures in the 20- to 35-GPa range, Best, D. J. Milton, K. A. Howard, and an un- the presence of a central pit at the center of the such as planar deformation features in feldspars, known reviewer provided critical reviews and uplift. The pit, as described above, is a shallow is virtually absent. Prominent are examples of helpful suggestions. L. F. Jansa and P. B. depression about 3.5 km in diameter located in mixed tectosilicate glasses, some of which ex- Robertson acknowledge support of the Geolo- the center of the central uplift. Such pits are hibit flow textures, with total dissociation of gical Survey of Canada in this study.

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N., 1987, Computer simulations of large asteroid im- MANUSCRIPT RECEIVED BY THE SOCIETY MARCH 4,1988 and morphometry: Proceedings of the 10th Lunar and Planetary pacts into oceanic and continental sites—Preliminary results on atmo- REVISED MANUSCRIPT RECEIVED NOVEMBER 22,1988 Science Conference, p. 2623-2633. spheric, cratering and ejecta dynamics: International Journal of Impact MANUSCRIPT ACCEPTED NOVEMBER 30,1988 Jansa, L. F., and Pe-Piper, G., 1985, Early Cretaceous voicanism on the north- Engineering, v. 5, p. 525-541. GEOLOGICAL SURVEY OF CANADA CONTRIBUTION NO. 2186

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