COPPER HARBOR CONGLOMERATE ON ISLE ROYALE
LOCATION AND AREAL EXTENT
Isle Royale is the largest island in an archipelago, all of which is within the boundaries of Isle Royale National Park. The name Isle Royale is often used to refer to the entire archipelago, and this is the sense in which it is used in the title of this report. Within the body of the report, however, the term is also used to refer to the main island. It should be clear in context which meaning is intended. Isle Royale is located in the northwestern part of Lake Superior, 15 miles southeast of the Minnesota-Canadian shoreline and 40 miles northwest of the closest point on the Keweenaw Peninsula (fig. 1). Exposures of Copper Harbor Conglomerate are slightly more than 40 miles apart on opposite sides of the Lake Superior syncline. The island is 45 miles long and about 6 miles wide; the long dimension is parallel to the structural grain.
Approximately four-fifths of Isle Royale is underlain by volcanic flows and minor clastic rocks of the Portage Lake Volcanics, which dip 10°-20° to the southeast in the vicinity of their contact with the overlying Copper Harbor Conglomerate (Huber, 1973b,c). The Copper Harbor Conglomerate underlies the remaining one-fifth of Isle Royale and is confined to the southwestern part of the archipelago (pl. 1); it dips 5°-28° to the southeast. The contact between the Copper Harbor Conglomerate and the Portage Lake Volcanics appears to be conformable; the top of the Copper Harbor Conglomerate, however, is not exposed. If the Nonesuch Shale and other formations that overlie the Copper Harbor Conglomerate on the Keweenaw Peninsula are present in the Isle Royale area, they lie beneath Lake Superior to the southeast.
The thickness of the Copper Harbor Conglomerate on Isle Royale is difficult to estimate because (1) the base of the formation, concealed beneath surficial materials for most of its length, is impossible to locate accurately; (2) the top of the formation, concealed beneath Lake Superior is nowhere exposed; and (3) the attitudes of strata are uncertain in the extensive covered areas. Nevertheless, useful approximations can be made.
East of Hay Bay the base of the Copper Harbor Conglomerate can be located reasonably well. From Hay Bay west to the head of Siskiwit Bay, the contact can be projected on the basis of topographic expression of the immediately underlying lava flows; from that area west to Cumberland Point, however, projection on the basis of topography is impossible as large areas are swamp or are covered by glacial debris (Huber, 1973c). In this western area, the location of the basal contact that is shown on plate 1 was calculated by measuring stratigraphically upward from marker horizons within the uppermost 1,000 feet of the Portage Lake Volcanics. (The marker horizons are recognized in a few scattered outcrops and in a diamond drill hole logged by Lane (1898, hole no. 11).) These calculations assume a relatively uniform thickness for the upper part of the Portage Lake Volcanics and a uniform dip or rate of change of dip in covered areas between outcrops of the Portage Lake Volcanics and the Copper Harbor Conglomerate. Errors derived from non-uniform thickness are believed to be small and those derived from dip are difficult to evaluate. The total changes in dip from one formation to the other in this area are small; this fact suggests that errors from this source also may be relatively small.
Differential resistance to erosion of many horizons within the Copper Harbor Conglomerate has resulted in the development of a linear ridge and valley topography in the outcrop areas. Although this ridge and valley structure is more subdued and less continuous than that in areas underlain by the Portage Lake Volcanics, it is sufficiently well developed to permit one to trace some individual horizons for several miles or more. One such horizon, which makes up the southern coastline from Fishermans Home Cove to about 2 miles west of Attwood Beach, has been used as an upper datum plane for estimating thicknesses for at least part of the Copper Harbor Conglomerate. Other less continuous or less well controlled horizons within the formation provide additional data on the nature of lateral thickness changes.
Stratigraphic thicknesses have been graphically calculated for seven sections across the Copper Harbor Conglomerate between Rainbow Point and Francis Point. A stratigraphic diagram based on these data (fig. 4) indicates a fourfold increase in thickness (1,480-5,450 feet) from west to east along a 12-mile length of outcrop. For a section 8 miles farther east in the vicinity of Long Island and Malone Bay an estimate of approximately 6,000 feet may be obtained by assuming a uniform rate of change of dip between opposite ends of the section where known dips are quite different and there is a large gap in data. This calculation is subject to greater errors than the others, but it does suggest the possibility that the thickness of the section continues to increase in an easterly direction, but at a much lower rate than along the Rainbow Point-Houghton Ridge segment.
One other possible complication in determining stratigraphic thicknesses for the Copper Harbor Conglomerate should be considered briefly. Lane (1898, p. 54-56) postulated a major high-angle fault, more or less parallel with the strike of the rocks, through the covered interval between Rainbow Cove and Siskiwit Bay. The duplication of stratigraphic section that such an interpretation requires would have a sizable effect on thickness calculations. This interpretation was based to a large extent on his belief that apparent inconsistencies in attitudes of strata between outcrops of similar rock types on opposite sides of Rainbow Cove indicated a structural disturbance between them and on his speculation that the conglomerate on Cumberland Point on the north side of Rainbow Cove might indeed be the same horizon as that exposed on Rainbow Point to the south and on Feldtmann Ridge. As additional support, he cited the large differences in attitudes between sandstone outcrops on opposite sides of the east end of Siskiwit Bay. He also believed that except by such a fault there was no accounting for the seemingly anomalous prominence of Point Houghton, which is underlain by sedimentary rocks that are not exceptionally resistant to erosion and generally underlie lowlands. Prominent ridges underlain by rocks of the Copper Harbor Conglomerate, however, also occur on the Keweenaw Peninsula, as at Mount Lookout and Brockway Nose, where there is no evidence for fault control of their development (Cornwall, 1954a, b).
The apparent structural inconsistencies can be explained partly by the now-recognized wedge shape of the formation and partly by differential warping related to flexures on the generally monoclinal structure of the island. Such flexures in the sedimentary sequence are suggested by geophysical data from just southeast of the island (Halls and West, 1971, p. 619-621). As the postulated fault is not required to explain the geologic relations, we have not allowed for any stratigraphic duplication in making our thickness calculations.
Alternating beds of sandstone and conglomerate with varying textural characteristics are typical of the Copper Harbor Conglomerate on Isle Royale as well as on the Keweenaw Peninsula. The three major rock types are sandstone, pebble conglomerate, and boulder and cobble conglomerate. Within these major groups clast size varies widely, the material ranging from clay-sized particles to boulders larger than 2 feet in diameter. Texture and composition are closely related throughout the formation, the finer grained rocks being compositionally more mature. Conglomerate near the top of the exposed section is similar to conglomerate at the base; the sandstones show a like similarity, even though separated by large stratigraphic intervals. Because of this, general lithologic descriptions can be made for the formation as a whole prior to considering internal variations and specific localities.
Bedding thicknesses vary considerably. Most of the conglomerate beds are between 1 and 3 feet thick; the sandstone beds between 1/4 inch and 6 inches thick. Clast size descriptions follow Wentworth (1922): pebbles, between 3/16 inch (4 mm) and 2-1/2 inch (64 mm) in maximum dimension; cobbles, between 2-1/2 inch and 10 inches (256 mm); boulders, greater than 10 inches.
The conglomerate clasts are subangular to rounded with prolate, bladed, and equant shapes. The maximum dimension of most clasts is between 2 and 8 inches although many are larger, and at least one is 2 feet in diameter (fig. 5). Both the larger clasts and the matrix are poorly sorted, and a complete size gradation commonly exists from fine matrix to conglomerate clasts. The components are generally loosely packed; calcite cement fills most of the spaces between grains.
Sandstone textures are very fine to very coarse grained, and a complete gradation within these ranges may occur in a single bed. Vertical changes in grain size are both abrupt and gradational. Grains are prolate, equant, and bladelike; most grains are angular to subrounded. The degree of sorting generally increases with decrease in grain size, the very fine- and fine-grained standstones being fairly well sorted, the medium- and coarse-grained sandstones poorly sorted. Most fine-grained sandstones are tightly packed and contain less calcite cement than the coarser sandstones with their more open framework.
Siltstone beds, which are very highly jointed and have a darker red color than the coarser grained rocks, are poorly sorted, owing mainly to a bimodal size distribution with many larger quartz and feldspar grains in finer silty matrix. Very thin shale layers, commonly displaying desiccation cracks, are abundant locally.
The boulders, cobbles, and pebbles throughout the Copper Harbor Conglomerate are chiefly derived from volcanic rocks, including basalt, andesite, trachyte, latite, quartz latite, and rhyolite. Such volcanic rock fragments usually make up 50-75 percent of the conglomerate, including the finer matrix. Sedimentary rock clasts (claystone and arkosic sandstone) and metamorphic rock clasts (quartzite) make up 0-5 percent of the conglomerate, but usually less than 2-3 percent.
Three types of quartz grains (unstrained, undulatory, and polycrystalline) together make up less than 10 percent of the matrix of the conglomerate beds. Potassium feldspar and plagioclase generally amount to less than 5 percent each; in a given rock, one may be more abundant than the other. Both quartz and feldspar are less abundant in the coarser conglomerates than in the finer ones. Opaque minerals range between 0 and 9 percent.
The percentage of interstitial calcite increases with increasing grain size, and in some specimens calcite makes up as much as 25 percent of the rock. In most specimens, however, this constituent is between 5 and 15 percent of the rock. Other cementing materials present are ferric oxide, silica, and laumontite.
The average composition of five samples of pebble conglomerate is given in table 1.
TABLE 1.Composition of pebble conglomerate
In the sandstones, volcanic rock fragments make up from 0 to more than 50 percent of the rock, depending on the grain size. The abundance of these rock fragments decreases uniformly from the very coarse grained to the very fine grained sandstones. Sedimentary and metamorphic rock fragments make up less than 3 percent of most sandstones; some beds contain abundant shale chips, probably derived from the immediately underlying strata.
Quartz grains increase from about 10 percent in the coarse-grained rocks to nearly 50 percent in the fine-grained ones. Unstrained quartz is slightly more abundant than undulatory quartz, and the polycrystalline variety generally amounts to only 1-2 percent. Potassium feldspar is usually less than 10 percent and commonly 5-7 percent; the plagioclase percentages are about 5 percent greater. Magnetite is by far the dominant opaque mineral; it ranges from 2 to 30 per cent, and where most abundant, is concentrated in layers parallel to the bedding. Calcite cement constitutes as much as 20 percent of the sandstones.
TABLE 2.Composition of coarse to very coarse grained sandstone
TABLE 3.Composition of very fine to fine-g rained sandstone
The average composition of coarse- and fine-grained sandstones is indicated in tables 2 and 3.
The composition of the siltstones is more difficult to determine; 60-70 percent of these rocks consists of material so fine that it is not easily identifiable with the petrographic microscope. Staining the feldspars helped somewhat in their identification. Owing to recrystallization and alteration, it is also difficult to distinguish matrix material from cement. Much of the fine material is clay, which commonly occurs as shale fragments; some fine material is probably extremely fine quartz and feldspar. Larger quartz and feldspar grains are also present; the quartz amounts to about 15 percent of the total composition of the rock, the feldspars 10 percent or less. Volcanic clasts and opaque minerals are present in small amounts.
Even though there is a major gap in the exposed section because of the lack of outcrops in the swampy lowland between Rainbow Cove and Siskiwit Bay and, of course, in Siskiwit Bay itself, data from existing outcrops are sufficient to demonstrate a major facies change from west to east. This facies change, indicated by a decrease in clastic fragment size and an increase in textural and compositional maturitypredominantly coarse conglomerate in the west and sandstone in the east, is shown in a very general way by the geologic map (pl. 1). On plate 1 the different map units are based upon average clast size.
This lateral facies change can be clearly recognized at several horizons within the formation as well as in the overall gross change. At Cumberland Point, a minimum of several hundred feet of coarse boulder conglomerate lies at the base of the formation. Eastward toward Malone Bay, the basal rock becomes a cobble and then a pebble conglomerate (fig. 6), and at Hat Island only a few feet of fine pebble conglomerate separates a thick section of sandstone from underlying volcanic rock of the Portage Lake Volcanics. Farther east, in a small cove north of Schooner Island in sec. 33, T. 65 N., R. 35 W. (fig. 7), and on a small island at the entrance to Vodrey Harbor, the conglomerate has phased out and sandstone rests directly upon the volcanic rock. There are no outcrops of Copper Harbor Conglomerate east of Vodrey Harbor, although the basal contact must be near the shore for many miles eastward along the island coast.
One of the most dramatic and best exposed displays of the facies sequence making up the Copper Harbor Conglomerate is exhibited by the backbone of Feldtmann and Houghton Ridges and the Siskiwit Island chain. Not only can one see here the change from coarse conglomerate on Feldtmann Ridge in the west (fig. 8) to sandstone on the island chain (fig. 9), but on Houghton Ridge, between the two extremes, one can also see the vertical variations in rock type brought about by the interfingering of beds representing different clast sizes and sedimentation energy levels, as illustrated in the cross section on plate 1. A series of samples collected across Houghton Ridge shows that the only consistent compositional changes are related to changes in grain size (table 4); there is no evidence for a vertical compositional trend.
TABLE 4.Composition of very fine grained
sandstones, very coarse grained sandstones, and pebble conglomerates
taken from the measured section on Houghton Ridge
The direction of sedimentary transport of the Copper Harbor Conglomerate on Isle Royale was determined from (1) the orientation of current-formed sedimentary structures such as crossbedding, primary current lineations, and ripple markings; (2) lateral changes in coarseness and thickness of the formation; and (3) conglomerate boulder and cobble composition. Textures, bedding types, and sedimentary structures suggest that streamflow was responsible for transporting and depositing the sandstones and conglomerates of this formation.
SEDIMENTARY STRUCTURES AS DIRECTIONAL INDICATORS
Sedimentary structures that yield directional information include trough-type crossbedding (fig. 10), microcross-laminations (rib-and-furrow structure), ripple markings (figs. 11 and 12), primary current lineations, and current crescents (fig. 13) (Pettijohn and Potter, 1964; Potter and Pettijohn, 1963). Analysis of the relative abundance of these structures on Isle Royale, given in table 5, shows that trough-type cross-beds occur in both sandstone and pebble conglomerate beds and that all other sedimentary structures are confined to the sandstones. Most data on these structures are from a narrow belt parallel to the strike of the beds along the southern shoreline where sand stones are well exposed by wave action.
TABLE 5.Relative abundance of sedimentary features in sandstone
and pebble conglomerate in the Copper Harbor Conglomerate, Isle Royale
The data collected consist of 273 paleocurrent measurements. Measurements of trough crossbeds, rib-and-furrow structures, ripple marks, and current crescents indicate current directions (arrows on fig. 14), whereas measurements of primary current lineations indicate only current trend (one of two opposite directionsstraight lines on fig. 14). Half of the data are from structures with discrete directions, but it is encouraging that the measurements of current trend are parallel to these and therefore can be assigned the single directional property with reasonable confidence. Each arrow or line plotted on the sediment dispersal map represents the arithmetic mean of the azimuthal readings from the unit sample area (township-range section). The average number of readings per square-mile section is twelve.
These same data have been used to construct rose diagrams of current direction, with class intervals of 30°. They have been plotted in four groups (fig. 14) along the strike of the formation. Standard deviations of the measurements are small; the maximum is 40° for an individual group and 55° for all 273 determinations. The diagrams show that most currents flowed southeastward, but in the Houghton Ridge area and on the Siskiwit Island chain the currents flowed eastward to northeastward. These findings are in good agreement with those of Hamblin and Horner (1961), whose data consisted of 80 readings taken along the southeastern shore.
LATERAL CHANGES IN TEXTURE AND FORMATIONAL THICKNESS AS DIRECTIONAL INDICATORS
The demonstrated decrease in average clast size, increase in textural and compositional maturity, and increase in thickness of the formation from west to east suggest transport of material in a general easterly direction, consistent with the data supplied by paleocurrent analysis.
CONGLOMERATE COMPOSITION AS AN INDICATOR OF SOURCE DIRECTION
More than one hundred boulders and cobbles were sampled from the conglomerates and later identified more accurately in the laboratory. With the exception of a minor amount of sandstone, conglomerate, and quartzite clasts, all clasts are of volcanic rocks covering a wide range in composition from basalt to rhyolite. No intrusive igneous clasts were noted. Most of the matrix material of the conglomerate could have been derived from volcanic rocks.
The low degree of metamorphism and lack of schistosity of the volcanic clasts is typical of Keweenawan volcanic rocks and unlike any pre-Keweenawan volcanic rocks in the Lake Superior region. The Keweenawan volcanic sequence itself is the most likely source for the clastic debris in both the interflow conglomerates of the Portage Lake Volcanics and the overlying Copper Harbor Conglomerate.
Felsites and some of the textural varieties of mafic volcanic fragments that occur as clasts in the Copper Harbor Conglomerate are not found in flows in the exposed section of the Portage Lake Volcanics on Isle Royale, although Lane (1898, p. 94) does report a diamond drill hole with record of a felsite, which is about 6,200 feet stratigraphically below the top of the lava series. Similarly, although small bodies of intrusive and extrusive felsite are present in the Portage Lake Volcanics on the Keweenaw Peninsula, they are not sufficiently abundant to have supplied the felsic, clastic debris of the Copper Harbor Conglomerate there.
Abundant felsitic volcanic rocks do occur in the lowermost part of the Keweenawan flow sequence in the North Shore Volcanic Group of Grout, Sharp, and Schwartz (1959) and Green (1968; 1971) in Minnesota. A few of the textural types of mafic volcanic rocks found as clasts in the Copper Harbor Conglomerate on Isle Royale have not been reported from the North Shore Volcanic Group, but because the outcrops are not continuous, they could well be present. Similarly, recent work by Hubbard (1968b; 1972) indicates that while many Copper Harbor Conglomerate pebble types are not present in the Portage Lake Volcanics in westernmost Michigan, nearly all volcanic types are present in older Keweenawan flows that form the so-called South Trap Range (White and others, 1971) and predate the Portage Lake Volcanics in that area. For this and other reasons, he has proposed an unconformity within the total Keweenawan volcanic sequence (Hubbard, 1968a). Such an unconformity would permit an older Keweenawan sequence of volcanic rocks to be eroded to provide clastic debris for the intraformational conglomerates in the middle Keweenawan Portage Lake Volcanics and the overlying Copper Harbor Conglomerate.
Paleomagnetic studies by DuBois (1962), Books (1968, 1972), and Palmer (1970) have shown that the only zone of reversed magnetic polarity within the Keweenawan volcanic sequence occurs within the lower most group in western Michigan and within the lower part of the North Shore Volcanic Group in Minnesota, thereby strengthening the correlation of rocks on opposite sides of Lake Superior. By analogy, then, an unconformity can be postulated between the North Shore Volcanic Group and the Portage Lake Volcanics on Isle Royale similar to that suggested for the volcanic rocks of the South Trap Range and the Portage Lake Volcanics on the south side of the lake. The North Shore Volcanic Group would then not only provide a source terrane of appropriate composition but such an unconformity would also allow for erosion of that source terrane while deposition of the interflow sediments of the Portage Lake Volcanics and of the Copper Harbor Conglomerate present on Isle Royale took place. The North Shore Volcanic Group does not necessarily provide a unique source for the clastic material, as similar material may exist within the Keweenawan volcanic rocks of the Osler Group in Canada (fig. 1), but a likely source present in the direction indicated as the source by other criteria is helpful.
Although all evidence indicates that the North Shore Volcanic Group is stratigraphically older than the Portage Lake Volcanics on Isle Royale, interpretations other than the postulated unconformity are possible. It is possible that the North Shore Volcanic Group may have been tectonically uplifted at the western margin of the depositional basin while in the central part of the basin volcanic activity continued with the eruption of the Portage Lake Volcanics and the later deposition of the Copper Harbor Conglomerate. Alternatively, it is also possible that the North Shore Volcanic Group and the Portage Lake Volcanics may have formed in separate tectonic basins isolated from each other, rather than in a single large basin, as has been suggested for some of the Keweenawan volcanic sequences on the south side of the basin (White and others, 1971). Each of these structural settings would permit erosion of the North Shore Volcanic Group to provide clastic debris for the interflow conglomerates in the Portage Lake Volcanics and for the Copper Harbor Conglomerate.
Last Updated: 22-Jan-2009