Diatremes and shock features in Precambrian rocks of the Slate Islands, northeastern Lake Superior R. P. SAGE Geological Branch, Ontario Ministry of Natural Resources, Toronto, Ontario, Canada MSS 1B3 ABSTRACT province (Goodwin and others, 1972). Along the west coast of Pat- terson Island (Fig. 2), a small area of middle and upper Precam- The Precambrian rocks of the Slate Islands exhibit shock features brian supracrustal rocks of the Southern province of the Lake that have been attributed to meteorite impact. The islands occur at Superior basin is present. This sequence consists of a thin unit (less the intersection of two major regional faults, one of which controls than 21 m) of Gunflint iron formation of middle Precambrian age, the location of late Precambrian alkalic magmatism. Alkalic intru- overlain by a 120-m-thick sequence of upper Precambrian mafic sive events north of Lake Superior occurred on the Slate Islands. volcanic flows of the Osier Formation. The upper Precambrian Port These alkalic intrusions are represented by a carbonatite dike in the Coldwell alkalic rock complex is 24 km northeast of the Slate Is- southeast corner of the island and a set of alkalic diabase dikes ex- lands. posed at several locations on the island. Diatreme breccias cut these The lower Precambrian rocks of the Superior province are older alkalic rocks and are interpreted to be late-stage phases of this than 2,480 m.y. (Stockwell and others, 1970, p. 54), whereas the volatile-rich alkalic magmatism. Shatter cones appear spatially re- Gunflint (Animikie) and Osier volcanic rocks of the Southern prov- lated to the diatremes, and quartz grains displaying deformation ince have been dated at 1,650 to 1,800 m.y. and 900 to 1,100 m.y., lamellae are present in the matrix of the breccias. Shatter-cone respectively (Stockwell and others, 1972, p. 118-119). structures and deformation lamellae are considered to be indicative Faults beneath Lake Superior, indicated in Figure 1, were taken of shock events. On the basis of field observations and data, the from Hinze and others (1966), who have, on the basis of shock and diatreme events can be correlated and related to vol- aeromagnetic data, interpreted the presence of two major faults in canism or alkalic magmatic processes associated with major re- the northeastern Lake Superior basin intersecting immediately gional fractures and not to meteorite impact. south of the Slate Islands. These faults are (1) the northeast-striking Big Bay—Ashburton Bay fault, and (2) a curvilinear fault that passes INTRODUCTION north of Michipicoten Island and then to the west, passes im- mediately south of the Slate Islands, and then strikes northwest The Slate Islands, a group of 17 islands and islets covering a sur- toward Schreiber, Ontario. The positions of these faults have been face area of approximately 39 km2 in the northeast corner of Lake slightly modified from Hinze and others (1966) by contouring of Superior (Fig. 1), were mapped in 1974 (Sage, 1975). This work bathymetric maps published by the Department of Energy, Mines showed that the islands are geologically complex, with a variety of and Resources, Ottawa (Canada Department of Energy, Mines and rock types and structures ranging in age from early to late Pre- Resources, 1971b, 1973). Contoured water depth or lake-bottom cambrian. Among these complexities are numerous diatreme brec- topographic data indicate that a linear trench, with water depths cias, shatter cones, and associated deformation lamellae in quartz. approaching 240 m trends northeast approximately 1.6 km south- These phenomena have been accredited to meteorite impact (Halls, east of the Slate Islands, and a similar linear trench trends north- 1975; Halls and Grieve, 1976), but in my opinion, they are more west through a point approximately 1.6 km southwest of the is- likely caused by endogenous processes. lands, parallel to the trend of the faults inferred by Hinze and others (1966). REGIONAL GEOLOGY The onshore extension of the Big Bay-Ashburton Bay fault of Hinze and others (1966) was interpreted on the basis of linear fea- The complexity of the Slate Islands cannot be fully appreciated tures evident on ERTS photographs (from the Canda Centre for without a general understanding of their regional geologic setting. Remote Sensing, Ottawa). Several interpretive possibilities exist for Figure 1 is a compilation of some of the pertinent geologic features joining the linear structures found north of the Port Coldwell found within the northeastern Lake Superior region. The general Alkalic Complex and the geophysically inferred faults of Hinze and geology of the region is taken from Ayres and others (1970); the others (1966), but only the most prominent trends are indicated in prominent linear passing through Chipman Lake is extended south Figure 1. to the Killala Lake alkalic rock complex and beyond to the north- The curvilinear fault south of Patterson Island does not appear ern flank of the Port Coldwell alkalic rock complex on the basis of on the mainland. Its eastern extension north of Michipicoten Island a topographic linear discernible on published topographic maps has been confirmed by Halls and West (1972). This fault appears (Canda Department of Energy, Mines and Resources, 1971a; On- similar to a number of geophysically inferred and/or geologically tario Ministry of Natural Resources, 1972). documented faults within and marginal to the Lake Superior basin Most of the islands are underlain by lower Precambrian supra- (Wold and Ostenso, 1966; Hinze and others, 1966). These cur- crustal rocks belonging to the Wawa subprovince of the Superior vilinear faults have strike lengths of many kilometres. The fault of Geological Society of America Bulletin, v. 89, p. 1529-1540, 12 figs., 1 table, October 1978, Doc. no. 81008. 1529 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/89/10/1529/3429432/i0016-7606-89-10-1529.pdf by guest on 25 September 2021 1530 R. P. SAGE Hinze and others (1966) south of Patterson Island is likely marginal topographic high or ridge that crosses Lake Superior in a to the Lake Superior basin and may be similar to the curvilinear northeast-southwest direction, from Big Bay, Michigan, to Ashbur- Keeweenaw fault in Michigan. ton Bay, Ontario, parallel to the geophysically inferred fault zone. Seismic studies in Lake Superior by Smith and others (1966) in- White (1972) divided the currently exposed Lake Superior basin dicate that the Mohorovicic discontinuity lies at a depth of approx- into five (possibly six) distinctly different volcanic basins. He inter- imately 60 km beneath Lake Superior at a position that lies approx- preted the western limits of the two most easterly basins as lying imately 80 km south of the Slate Islands, along the Big Bay- along the submarine ridge between Big Bay, Michigan, and Ashbur- Ashburton Bay fault. This 60-km depth is the most extreme depth ton Bay, Ontario, which is along the inferred fault of Hinze and in North America (Smith and others, 1966). O'Brien (1968) re- others (1966). Likewise the eastern limits of the two central basins examined the data of Hinze and others (1966) and calculated a lie along the same structural trend. This interpretation implies at depth slightly in excess of 50 km. Bottom topographic contours least minimum middle Precambrian age for the Big Bay—Ashburton compiled by Hough (1958) also indicate that the islands lie along a Bay structure, or that the ridge existed prior to the deposition of the Figure 1. Regional geology of Slate Islands, after Ayres and others (1970); inferred faulting in Lake Superior after Hinze and others (1966), modified from Canadian Hydrographie Service maps (Canada Department of Energy, Mines and Resources, 1973, 1976). Inferred faulting between Killala Lake alkalic complex and Lake Superior interpreted from ERTS imagery. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/89/10/1529/3429432/i0016-7606-89-10-1529.pdf by guest on 25 September 2021 DIATREMES AND SHOCK FEATURES, SLATE ISLANDS 1531 volcanic rocks within the basins east and west of the trend. The facies rank and consist of coarse felsic pyroclastics, felsic to mafic possibility of this ridge being pre—late Precambrian was originally tuff, feldspar porphyry flows, volcanic slump deposits, and amyg- suggested by Halls (1972), who, on the basis of paleomagnetic daloidal, pillowed, and variolitic mafic flow rocks with thin inter- studies, postulated that the upper Precambrian Osier Group vol- bedded units of argillite and siltstone (Fig. 2). Several thin units of canic rocks pinch out toward the Slate Islands. He consequently in- bedded chert-magnetite iron formation are also present. This lower terpreted the Slate Islands to lie along an axis of a prevolcanic Precambrian volcanic sequence has been intruded by lower Pre- ridge. cambrian feldspar porphyry, quartz-feldspar porphyry, and mas- The results of many investigators working independently indi- sive diorite intrusions and folded into a steeply southwesterly cate that the Slate Islands have a unique geologic setting. In addi- plunging (approximately 60°) fold (Sage, 1975). During this fold- tion, the extension of the Big Bay—Ashburton Bay fault and ing, considerable tectonic adjustment took place between Mortimer branches from this fault are the loci of several alkalic rock- and Patterson Islands, and a number of smaller fold structures were carbonatite complexes or areas of alkalic intrusive activity lying superimposed on the major structure. Relatively deep water, ap- north of Lake Superior (see Fig. 1). Of these complexes, the Prairie proaching 30 m, between Mortimer and Patterson Islands and Lake carbonatite yields a whole-rock Rb-Sr isochron age of 1,033 wide, steeply dipping breccia zones along the south side of Mor- m.y. (K. Bell and H. D. Watkinson, in prep.); the Port Coldwell timer Island suggest that faults are likely to exist here.