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ABSTRACT

This field trip examines a sequence of ejecta and deformed substrate resulting from the 1850 Ma meteorite impact. An impact origin for the Sudbury structure in Ontario has long been accepted, but knowledge of the corresponding ejecta was limited to fall-back breccia in the relict crater at Sudbury. The more distant ejecta blanket was discovered only recently near Thunder Bay, Ontario, and later in other parts of the Lake Superior region. Known informally as the Sudbury impact layer (SIL), it occurs at and near the stratigraphic top of Paleoproterozoic iron-formation. The impact-related deposits in the western Lake Superior region include (1) autochthonous material interpreted to be seismically folded and shattered iron-formation and carbonate rocks (breccia), overlain by (2) strata composed largely of allochtho-nous material (ejecta) derived in part from target rocks, and (3) irregular layers that appear to be mixtures of locally and distally derived material. Definitive microscopic evidence of an impact origin includes the occurrence of accretionary lapilli, ash pellets, spherules, devitrified glass, and quartz fragments marked by planar deformation features. The SIL exhibits extreme lithologic variability from place to place within each exposure area and between exposure areas. Nevertheless, the stratigraphic relationships that are presented by these exposures can be used to devise a sequence of deformation and depositional events that is largely consistent with experimental and empirical evidence of impact processes. This field trip will demonstrate that the stratigraphic arrangement of facies in the SIL has important temporal implications for understanding mechanisms of ejecta delivery and deposition.

Introduction

This field trip examines a sequence of ejecta and deformed substrate resulting from the 1850 Ma (Krogh, et al., 1984; Davis, 2008) Sudbury meteorite impact. Of the 178 known and scientifically verified terrestrial impacts, the Sudbury event was the second largest (based on crater size) and fourth oldest (www.unb.ca/passc/ImpactDatabase). Estimates of crater diameter range from 160 km to as large as 250 km. Only the Vredefort structure in South Africa is larger, and the Chicxulub impact in Mexico is equivalent or only slightly smaller (Grieve et al., 2008). An impact origin for the Sudbury structure in Ontario has long been accepted (Dietz, 1964; French, 1967). Although fall-back breccia occurs at Sudbury, the more distant ejecta blanket was discovered only recently near Thunder Bay, Ontario (Addison et al., 2005). Known informally as the Sud-bury impact layer (SIL), it occurs at and near the stratigraphic top of the Paleoproterozoic Gunflint Iron Formation (or Gunflint Formation, as it is known in Canada). The SIL was identified subsequently in equivalent iron ranges of the Lake Superior region (Fig. 1), including those in Michigan (Pufahl et al., 2007; Cannon et al., 2010), in Minnesota near Gunflint Lake (Jirsa, 2010), and along the Mesabi Iron Range (Addison et al., 2005). Collectively, these deposits occur at distances of 500 to 950 km from the target, thus spanning the inferred boundary between proximal and distal ejecta at ∼5x crater radii. As such, the Sudbury ejecta field holds promise for enhancing the growing body of knowledge about impact processes.

The impact-related deposits in the western Lake Superior region include (1) autochthonous material interpreted to be seismically folded and shattered iron-formation and carbonate (breccia), overlain by (2) strata composed largely of allochthonous material (ejecta) derived in part from target rocks, and (3) irregular layers that appear to be mixtures of locally and distally derived material. Definitive microscopic evidence of an impact origin includes the occurrence of accretionary lapilli, ash pellets, spherules, relict glass, and quartz fragments marked by planar deformation features. The SIL exhibits extreme lithologic variability from place to place within each exposure area and between exposure areas. Nevertheless, the stratigraphic relationships that are presented by these exposures can be used to devise a sequence of deformation and depositional events that is consistent with experimental and empirical evidence of impact processes. The creation of such models will prove useful not only for understanding processes operative during Earth impacts, but also may be applicable to defining processes and products of impacts on extraterrestrial bodies (Branney and Brown, 2011; Fralick et al., 2011). Highlighting this is the imaging of impact breccias on Mars (Grant et al., 2008), and the analogy that Addison et al. (2010) drew between the thick Sudbury impact deposits in the Gunflint Lake area and rampart deposits produced by impact base surges on Mars (Barnouin-Jha et al., 2005; Kenkmann and Schonian, 2006; Osinski, 2006; Mouginis-Mark and Garbeil, 2007).

Figure 1.

Map of the Lake Superior region, showing Sudbury and the iron ranges where the Sudbury impact layer has been identified, including the subject locations of this field trip at Gunflint Lake, Minnesota, and Thunder Bay, Ontario (black boxes). Gray bold lines represent the strike of iron-formations. Gradient circle surrounding Sudbury approximates estimated size of final crater (250 km diameter).

Figure 1.

Map of the Lake Superior region, showing Sudbury and the iron ranges where the Sudbury impact layer has been identified, including the subject locations of this field trip at Gunflint Lake, Minnesota, and Thunder Bay, Ontario (black boxes). Gray bold lines represent the strike of iron-formations. Gradient circle surrounding Sudbury approximates estimated size of final crater (250 km diameter).

Ejecta emplacement is a complicated process, especially in the distal regions of the ejecta blanket, as we will observe in the various SIL deposits. As described in Gault et al. (1968), at the moment of impact the energy and momentum carried by the projectile act to melt and vaporize both projectile and target material, as well as generate a shock wave through the target material. Target material is pushed both downward and outward by the expanding shock wave. As the target decompresses after the passage of the shock wave, the flow direction of material is bent up toward the surface, causing some of the target material to leave the surface on ballistic trajectories. Highly shocked, very fast moving material leaves the target surface first, nearest the impact point. As the crater continues to grow, the ejection velocity of the material slows until, at the very rim of the crater, minimally shocked target material is simply overturned. Therefore, the more distal ejecta falls faster, eroding and entraining the local surface material in a process known as ballistic sedimentation (Oberbeck, 1975).

In the case of the Moon, the lack of volatiles and an atmosphere make the deposition of ejecta a fairly straightforward and predictable process. However, impacts on the Earth and Mars are complicated by the presence of an atmosphere and volatiles. (Much debate remains about which effect dominates ejecta emplacement. See Barnouin-Jha et al., 2005, for more discussion.) Rather than a smooth progression of ballistic impacts that increase in impact velocity with distance from the impact point, ejecta emplacement on Earth or Mars more likely mimics a density-driven debris flow, similar to what is seen in turbidites or base surges (e.g., Barnouin-Jha et al., 2005). The larger ejecta particles are less affected by the atmosphere, and make up the standard advancing curtain, but the smaller particles would be entrained within the atmosphere and create a vortex behind the advancing curtain (Fig. 2). This vortex generates winds behind the curtain that scour the local surface, resulting in the erosion and deposition of a mixture of material from the projectile, original target surface (Sudbury, in our case), and final deposition location (i.e., the Gunflint Iron Formation). Indeed, through ballistic sedimentation, large amounts of surface material can be incorporated into the advancing ejecta curtain (e.g., Melosh, 1989).

Beyond this, other complications exist in the deposition of distal ejecta. At these distances (650 and 750 km at Thunder Bay and Gunflint Lake, respectively), the ejecta deposits become discontinuous, and their morphometry can vary widely from location to location. If the impact is oblique, which most are, the distal ejecta deposits will record this initial asymmetry because these are the materials ejected from nearest the impact point. Finally, local topography and the presence of a shallow sea—a likelihood for the deposits at Gunflint Lake—would also affect how the ejecta are emplaced and what is observed in the field. This field trip will demonstrate that the stratigraphic arrangement of facies in the SIL has important temporal implications for depositional mechanisms, which are not clearly understood at this time. Thus, ongoing research of the SIL will greatly aid our understanding of these highly complex processes, leading to more accurate models for the emplacement of distal ejecta from large impacts on Earth (and Mars).

Geologic Setting

The SIL in the western Lake Superior region is sandwiched between the underlying Gunflint Iron Formation and the overlying slate and siltstone of the Rove Formation (Fig. 3). The underlying Gunflint Iron Formation has an age of 1878 ± 1 Ma, acquired from zircons in a tuff zone ∼60 m below the SIL (Fralick et al., 2002). In detail, the SIL is overlain by a thin succession of ferroan dolomite (ankerite), which was erosively transgressed by the Rove Sea. A basal flat pebble conglomerate was deposited on the erosive surface, followed by deposition of black shale, silt-stone, and tuff—the latter having a U-Pb zircon date of 1835 Ma (Addison et al., 2005). All three units are gently dipping and, for the most part, planar bedded. The Gunflint is the eastern extension of the Biwabik Iron Formation exposed along the Mesabi Iron Range, which was bisected by a large area of Mesoproterozoic intrusions of the Duluth Complex (ca. 1100 Ma). Both iron-formations lie unconformably on deformed Archean greenstone-granite terrane of the Wawa subprovince of the Superior Province, and Paleoproterozoic diabasic dikes that cut the Archean rocks locally. The iron-formation, SIL, and overlying Rove Formation were intruded by Mesoproterozoic hypabyssal dikes and sills of the ca. 1115 Ma Logan Intrusions (Heaman et al., 2007), which were omitted from Figure 3 for brevity. Metamorphism adjacent to the Logan Intrusions was minor in both the Gunflint Lake and Thunder Bay areas; however, the deposits at Gunflint Lake lie within the contact metamorphic aureole of the Duluth Complex, evidenced by the presence of amphibole- and pyroxene-rich hornfels (Floran and Papike, 1978). Despite this metamorphism and pervasive carbonate alteration that have obscured some of the original mineralogic features, macroscopic textures and geochemical content that convey information about protolith and depositional processes are preserved.

Figure 2.

Qualitative models for the emplacement of ejecta, showing the effect of an atmosphere that entrains smaller ejecta particles into a vortex following the advancing curtain, scouring the surface and incorporating local material into the ejecta. This material is eventually deposited through dynamic debris flow, similar to a turbidity current. Modified from Barnouin-Jha et al. (2005).

Figure 2.

Qualitative models for the emplacement of ejecta, showing the effect of an atmosphere that entrains smaller ejecta particles into a vortex following the advancing curtain, scouring the surface and incorporating local material into the ejecta. This material is eventually deposited through dynamic debris flow, similar to a turbidity current. Modified from Barnouin-Jha et al. (2005).

Figure 3.

Generalized map of the western Lake Superior region (modified from Pye, 1963; Jirsa et al., 2011), showing geologic setting of field trip areas at Gunflint Lake (Stops 1–6) and Thunder Bay (Stops 7–12). A complex array of Mesoproterozoic sills (Logan and related intrusions) are common in the Rove Formation, but are not shown here for simplicity.

Figure 3.

Generalized map of the western Lake Superior region (modified from Pye, 1963; Jirsa et al., 2011), showing geologic setting of field trip areas at Gunflint Lake (Stops 1–6) and Thunder Bay (Stops 7–12). A complex array of Mesoproterozoic sills (Logan and related intrusions) are common in the Rove Formation, but are not shown here for simplicity.

As on the Mesabi Iron Range, the Gunflint Iron Formation in Minnesota has historically been subdivided into so-called cherty (granular) and slaty (argillaceous) informal subunits or members (Fig. 4). In actuality, granular and argillaceous strata are interbedded on all scales; however, the members have been designated by an abundance of one lithotype over the other. The members are denoted lower cherty, lower slaty, upper cherty, and upper slaty (Wolff, 1917; Broderick, 1920). Although this terminology has some descriptive utility in the field, the subdivision employed here is based instead on a sedimentological model (after Pufahl and Fralick, 2000). In this model the lower cherty, lower slaty, and part of the upper cherty members represent deposition during a marine transgressive-regressive cycle. This cycle defines the lower sequence of the Gunflint Iron Formation (or lower member of the Gunflint Formation of Goodwin, 1956). It grades from conglomerate and sandstone at its base (Kakabeka Conglomerate), deposited in paleo-depressions in the underlying Neoarchean bedrock, to locally stromatolitic chert and microbialite affixed to conglomerate boulders and the Archean bedrock paleo-surface. This is overlain by siliceous grainstone, or ankeritic grainstone in more shore-proximal areas. A thinning and fining-upward sequence grades to dominantly iron-rich mudstone with thin grainstone layers. The sequence then coarsens and thickens upward to the regressive surface, where it is capped by strata deposited sub-aerially (Fralick and Barrett, 1995). During the initial stages of the next transgression, microbial mounds (now stromatolites) developed on the silicified surface and are overlain by coarse siliceous grainstone. At this point in the stratigraphy an influx of fine-grained volcanic ash occurs, possibly related to basaltic volcanism ∼10 km north of the Canada-U.S. border. The ash layers consist of iron-rich chlorite with some beds of volcanic accretionary lapilli. In the north near Thunder Bay, grainstone layers interbedded with black shale become progressively more abundant up the stratigraphic section, until the grainstone dominates. In more southerly localities (i.e., Gunflint Lake), volcanic ash is mixed with chemical sediment, creating the upper slaty member. The siliceous grainstone units are composed of ∼15-m-thick coarsening- and thickening-upward cycles, representing parasequences in an overall transgressive system.

Figure 4.

Schematic diagram of stratigraphy at Gunflint Lake and near Thunder Bay, incorporating older (4-5 members) and newer (2 sequences) nomenclatural subdivision of units. Section is hung from the 1850 Ma Sudbury impact layer. The diastem representing maximum regression and likely subaerial exposure defines the contact between sequences.

Figure 4.

Schematic diagram of stratigraphy at Gunflint Lake and near Thunder Bay, incorporating older (4-5 members) and newer (2 sequences) nomenclatural subdivision of units. Section is hung from the 1850 Ma Sudbury impact layer. The diastem representing maximum regression and likely subaerial exposure defines the contact between sequences.

The upper parts of the formation in the two areas differ significantly. Near the top of the Gunflint Formation at Thunder Bay, additional volcanism results in chloritic ash mixed with the chemical sediments. The top of this unit is marked by a regressive surface with intense silicification (Fralick and Burton, 2008) that is overlain by breccia and ejecta of the SIL. In the area near Gunflint Lake the uppermost several meters of iron-formation strata are variably brecciated and/or chaotically folded, carbonate-bearing, and capped by granular ejecta of the SIL. Field evidence near Gunflint Lake indicates that much of the deformation in the uppermost layers of iron-formation occurred during and after silicification, but prior to complete lithification. Assuming that brecciation and folding are the products of seismic waves generated by the Sudbury bolide impact (see discussion below), it logically follows that impact occurred while the Gunflint Lake section was at or only slightly below sea level. By contrast, the strata farther north toward Thunder Bay contain a calcite-bearing unit 1–2-m thick that separates the silicified regression surface from the overlying impact layer. The intervening unit is composed of sand-size chloritic ash pellets and chloritic layers in stromatolites, with pervasive meteoric (groundwater) calcite cement formed prior to deposition of the impact layer (Fralick and Burton, 2008). This indicates that when the impact occurred the Thunder Bay section was lithified and above sea level. The contrast in apparent sea level and lithification at the time of impact between these two stratigraphic sections is enigmatic and requires further study to clarify.

The overlying Rove Formation consists of gray silt-stone to carbonaceous slate, and graded, medium- to finegrained, clay-rich sandstone beds. The basal several tens of centimeters of the formation is irregularly bedded with scour truncations, is carbonate-rich, and contains locally derived conglomerate. Zircons taken from tuffaceous layers 1 m above the base of the formation in Ontario yielded ages of 1827 ± 8 and 1836 ± 5 Ma (Addison et al., 2005). This indicates a considerable hiatus separating the deposition of the SIL (1850 Ma) from that of the overlying Rove, which is similar in duration to the hiatus that must have separated deposition of the basal Gunflint Formation (1878 Ma) from that of the overlying SIL. In many ways the transgression of Rove seas produced sediments that were lithologically similar to pre-impact deposits, with one important difference: Sediments deposited prior to impact were iron-rich, and the overlying Rove strata are not. This may reflect a fundamental shift in the oxygenation of seawater, as suggested by Slack and Cannon (2009). Most research supports the concept that preimpact oceanic oxygen levels were stratified, with higher levels in relatively shallow water. In Slack and Cannon's model, impact-generated mixing created a suboxic ocean, which reduced the flux of anoxic iron-rich waters onto more oxygenated shelf environments. This resulted in what they refer to as the "demise" of iron-formation deposition, which is consistent with a shift in the metallogeny of other types of submarine mineral deposits.

Sudbury Impact Layer

The Sudbury impact layer in the western Lake Superior region records earthquake-induced deformation of the substrate, followed by the base surge and/or ejecta curtain sweeping across the area. These processes produced two main recognizable lithologic components: chaotically folded and brecciated iron-formation composed of locally derived fragments, and exotic material derived to varying degrees from the fragmented, liquefied, and vaporized target and bolide. We have subdivided the SIL into generalized facies on this basis as either ejecta-absent or ejecta-bearing. The ejecta layers and lenses typically are 1 m or so in thickness. The great majority of the several meter- to 10-m-thick deposits mapped as SIL is breccia composed of thoroughly disheveled fragments that appear to have been derived from subjacent loose sediment and disaggregated bedrock. We believe this disrupted iron-formation horizon is a product of seismic deformation related to the Sudbury impact event for the following reasons:

  1. 1.

    In every outcrop containing a recognizable ejecta layer the layer is underlain by either brecciated or folded iron-formation.

  2. 2.

    This style of deformation is not observed elsewhere in the thick and well-exposed stratigraphic section of the iron-formation.

  3. 3.

    The folding and brecciation affect only the uppermost layers of the formation. Presumably, lithostatic load prevented deeper penetration of surface waves. In some localities, straight-bedded iron-formation grades both vertically and laterally into chaotically deformed strata.

  4. 4.

    At Gunflint Lake the rheologic response to deformation varied irregularly over relatively short intrastratal distances from brittle brecciation to semi-ductile and ductile folding.

  5. 5.

    Deformation is nonsystematic; i.e., folds have random axial trends and plunges, and detachments of fold limbs are common. This argues against deformation by a regional tectonic event.

  6. 6.

    Breccia fragments are angular and completely unsorted, locally with blocks as large as 5 m long. At many localities, large, elongate fragments stand vertically and are surrounded by a mass of smaller blocks.

  7. 7.

    Breccias consist primarily of disheveled fragments of subjacent or adjacent strata, and no evidence exists for significant transport of detritus.

  8. 8.

    Although karstification might produce similar brecciation, no evidence for dissolution collapse exists, and the association of breccia with soft-sediment deformation features precludes such an interpretation.

  9. 9.

    The contact between breccia and the overlying material that we infer is ejecta is very sharp and lacks evidence for chemical weathering and paleosol development.

  10. 10.

    The deformation style is consistent with that associated with younger impacts, such as Alamo (Pinto and Warme, 2008) and Chicxulub (see discussion in Jannett and Terry, 2008).

A number of factors raise questions about this interpretation of the observed deformation, however. For example, an apparent 28 m.y. hiatus separates deposition of iron-formation (ca. 1878 Ma) and the Sudbury impact event (1850 Ma). The age date for ironformation was taken from a sample 60 m below the top of formation, so in that time period at least 60 m of sediment presumably was deposited. Nevertheless, the duration is long and opens the door to many other depositonal and erosional events. Furthermore, the Gunflint Lake exposures lie within a broad anticline-syncline pair and adjacent to a fault zone (Fig. 6), making regional deformation a possibility—despite observations to the contrary. An intraformational mass flow origin for breccia also cannot be totally ruled out with existing observations.

Overlying the disrupted iron-formation in many locations are lenses and layers of ejecta, typically only a few meters in total thickness. The macroscopically most apparent feature of the ejecta in all localities is the presence of 0.1–1.5 cm, concentrically zoned spheres inferred to be accretionary lapilli (Fig. 5). These occur as scattered grains within finer grained gritstone and as tightly packed (grain-supported) lapillistone units. In most outcrops the lapilli display delicate internal zonation, and they are rarely abraded or deformed by compaction. The latter observation indicates that lapilli were rigid during deposition. Microscopic evidence that this material has an impact origin includes the presence of quartz fragments in lapilli that are marked by planar deformational features and by various particles of devitrified and altered glass. Material having a glassy protolith and other diagnostic petrographic attributes (e.g., French and Koeberl, 2010) are present in the Thunder Bay area but are largely obscured by metamorphism near Gunflint Lake. Stratigraphic and sedimentological characteristics of the ejecta are extremely varied from outcrop to outcrop; this is not surprising, given the chaos of impact processes and the likelihood that the ejecta surge interacted with an uneven topographic surface. Nevertheless, a very generalized stratigraphic description can be made. Typically, breccia (with or without ejecta content) is overlain by layered gritstone and mesobreccia composed of a mixture of sand- to pebble-size ejecta and locally derived fragments. This is overlain in turn by lapilli-rich lenses and layers that commonly grade or pass more abruptly up-section into finer grained zoned lapilli, unzoned accreted particles (pellets), and relict spherules that may have been glass. Logically, this stratigraphic arrangement of lithologies should reflect a sequence of depositional events, which will be discussed below.

Gunflint Lake Area

The general geologic framework at Gunflint Lake is shown in Figure 6. Despite the complex appearance in map view, the cross section shows that Paleoproterozoic strata and Mesoproterozoic sills generally are gently south dipping. Much of the apparent complexity is a product of shallowly dipping formations, local faults and folds, and moderate to high topographic relief resulting from contrasting resistance to erosion. The Sudbury impact layer lies at the stratigraphic top of the Gunflint Iron Formation, typically along bluffs shielded by diabase sills of the Logan Intrusions. The SIL consists of brecciated and complexly deformed iron-formation as thick as 10 m, overlain locally by <1 m of mesobreccia and granular ejecta. Both deformed (seismically shattered and chaotically folded) iron-formation and ejecta are inferred to be related to the Sudbury meteorite impact event (Jirsa, 2010), and thus both are considered to be part of the SIL.

Figure 5.

Scanning electron microscope (SEM) X-ray element map of lapillistone from Gunflint Lake. This mosaic backscatter electron image shows elemental variations based on mass (higher mass produces lighter colors). Zonation primarily reflects proportions of silicate versus carbonate minerals; bright spots are oxides. (Analyses courtesy of McSwiggen and Associates, www.mcswiggen.com.)

Figure 5.

Scanning electron microscope (SEM) X-ray element map of lapillistone from Gunflint Lake. This mosaic backscatter electron image shows elemental variations based on mass (higher mass produces lighter colors). Zonation primarily reflects proportions of silicate versus carbonate minerals; bright spots are oxides. (Analyses courtesy of McSwiggen and Associates, www.mcswiggen.com.)

Figure 6.

Geologic map and schematic cross section of the Gunflint Lake area, showing approximate location and stratigraphic position of some field trip stops. Geology modified from Jirsa (2011). Note that horizontal scale of section is much greater than map, and that its vertical exaggeration is ∼3.5x, so that apparent dips of contacts are much steeper than true.

Figure 6.

Geologic map and schematic cross section of the Gunflint Lake area, showing approximate location and stratigraphic position of some field trip stops. Geology modified from Jirsa (2011). Note that horizontal scale of section is much greater than map, and that its vertical exaggeration is ∼3.5x, so that apparent dips of contacts are much steeper than true.

Facies of the Sudbury Impact Layer at Gunflint Lake

Figure 7 depicts the apparent stratigraphic relationships in eight sections of the unit exposed over a 2-mi strike length. In no single outcrop are all facies present; however, an approximation of temporal relationships can be inferred from the juxtaposition of two or more facies in individual outcrops.

In the following section, facies are described in apparent stratigraphic order from oldest to youngest.

Ejecta-Absent

Contorted iron-formation facies: Exposed at Stops 4 and 5. The uppermost layers of iron-formation are chaotically folded and exhibit both ductile and brittle behavior in close proximity at the scale of individual outcrops. The rheologic response of the strata appears to have depended on the rigidity of material at the time of deformation. Silica-rich layers display brittle, shattered to semi-ductile, boudinage-like textures. By contrast, much of the iron-silicate mudstone layers behaved in a ductile fashion, locally showing evidence of fluidization and injection into superjacent strata. Folds are nonsystematic in trend and style, and multiple hinge detachments occur. These attributes counter-indicate a regional tectonic origin, and instead are best viewed in the context of impact-generated seismicity imposed on locally silicified, but generally un-lithified, substrate.

Parautochthonous breccia facies: Stop 5. At several localities, straight-bedded iron-formation passes laterally along strike into irregular zones in which the silica-rich layers have been broken and disheveled while still retaining some semblance of a jigsaw-puzzle fit.

Megabreccia facies: Stops 2–6. This term is used for breccia composed of unsorted slabs (as large as 5 m), blocks, and smaller fragments of iron-formation. The fragments are angular and in most places have random orientations. Fragments of green, iron-silicate mudstone typically show some evidence of semi-ductile behavior, and locally this material was fluidized to form irregular matrix and clastic (muddy) dikes. Although a debris flow mechanism cannot be totally ruled out for some of the megabreccia, there is no evidence that this material was transported significant distances to depositional sites. For example, the lithologies of all breccia fragments are identical with those present in directly adjacent and subjacent iron-formation layers. The possibility that breccias are the product of solution collapse (karst) is similarly unlikely, as no dissolution features have been recognized in strata underlying the breccia deposits.

Ejecta-Bearing

Mesobreccia facies:Stops 2–6. This is fragmental rock containing angular, subrounded, and amoeboid clasts (up to 5 cm long) of dark-green material and scattered accretionary lapilli. Petrography shows that much of the original structure of the clasts has been metamorphically recrystallized and annealed; however, relicts of amygdaloidal and fluid-looking textures remain, implying some component of a glassy protolith.

Lapillistonegritstone facies:Stops 2–6. Accretionary lapilli as large as 1.5 cm occur as lenticular masses and layers inter-bedded with and overlying sandy to silty gritstone. In areas least affected by metamorphism, several grains of shocked quartz with planar deformation features have been identified in thin section.

Spherule, pellet, small lapilli facies:Stops 3, 6. The upper parts of ejecta zones locally contain layers, lenses, and interbeds of accretionary or relict glass grains that are smaller than typical lapilli and generally lack concentric zonation. These apparently accreted particles may represent waning ejecta plume deposition (cf. Branney and Brown, 2011).

Ejecta-bearing conglomerate facies:Stop 2. In a few localities the uppermost part of the deposits consists of conglomerate containing subrounded fragments of iron-formation (in contrast to angular fragments typical in breccias described above), and matrices containing variably abraded lapilli.

Interpretation

The arrangement of facies described above and depicted in Figure 7 can be interpreted in the context of experimental evidence and observations from Lunar and smaller terrestrial impacts. Using calculations from Collins et al. (2005), one can predict arrival times for various effects of the impact here, some 750 km (480 mi) from the impact site, as follows:

EventApproximate arrival time
(1) Fireball∼13 s (modern equivalent of 3rd degree burns)
(2) Earthquake∼2–3 min (10.9–13 at epicenter)
(3) Ejecta ground surge∼5–10 min (predicts ejecta 1–3 m thick, grain sizes ∼1 cm)
(4) Air blast∼40 min (sonic boom)
(5) Tsunami∼1–3 h (speculation—arrival and effects dependent on basin bathymetry and pre-impact position relative to strand line, which are difficult to establish)
EventApproximate arrival time
(1) Fireball∼13 s (modern equivalent of 3rd degree burns)
(2) Earthquake∼2–3 min (10.9–13 at epicenter)
(3) Ejecta ground surge∼5–10 min (predicts ejecta 1–3 m thick, grain sizes ∼1 cm)
(4) Air blast∼40 min (sonic boom)
(5) Tsunami∼1–3 h (speculation—arrival and effects dependent on basin bathymetry and pre-impact position relative to strand line, which are difficult to establish)

Nearly all contacts between individual facies are gradational, with one very important exception—in all exposures, the boundary between ejecta-absent and ejecta-bearing facies is extremely sharp. This contact lacks evidence for weathering and paleosol development. Instead, it is inferred to reflect an abrupt and fundamental shift in geologic process from intense seismic perturbation of uppermost iron-formation represented by the ejecta-absent facies, to deposition by the passing ejecta plume. The uppermost conglomerate facies represents mixing of local and exotic detritus, presumably by tsunamis or post-impact fluvial or marine processes.

Figure 7.

Stratigraphic framework derived from eight exposures along a 2-mi strike length near Gunflint Lake; hung from the contact between ejecta-bearing and ejecta-absent facies of the Sudbury Impact Layer (bold dashed line). SIL—Sudbury impact layer.

Figure 7.

Stratigraphic framework derived from eight exposures along a 2-mi strike length near Gunflint Lake; hung from the contact between ejecta-bearing and ejecta-absent facies of the Sudbury Impact Layer (bold dashed line). SIL—Sudbury impact layer.

Deposits near Gunflint Lake appear to be consistently thicker than in other areas, even though these sites are more distal than those in Michigan and Ontario. The impact deposits at sites farther away from the crater than Gunflint Lake are much thinner, and lapilli are only rarely present. This has led Addison et al. (2010) to hypothesize that the Gunflint Lake deposits may represent thick ramparts, as described for end-of-flow Martian base-surge deposits (Barnouin-Jha et al., 2005; Kenkmann and Schonian, 2006; Osinski, 2006; Mouginis-Mark and Garbeil, 2007). An alternative explanation is that the thick breccia deposits at Gunflint Lake reflect the passage of seismic waves through variably silicified, but un-lithified, iron-formation substrate, which was rheologically more susceptible to deformation than other localities. For example, the more proximal deposits at Thunder Bay appear to have been fully lithified, and the more distal sites on the Mesabi range may have been in a deeper submarine environment at the time of impact.

Petrology and Geochemistry

The SIL and underlying iron-formation at Gunflint Lake were variably replaced by carbonate and metamorphosed by the superjacent Duluth Complex to amphibole and pyroxene hornfels. Although macroscopic features are well preserved in outcrop, these features are obscured microscopically by pervasive recrystallization, primarily to amphibole and carbonate minerals. Nevertheless, scanning electron microscopy (Fig. 8A) and geochemistry (Figs. 8B and 8C) can be used to characterize the rocks, differentiate between ejecta-bearing and ejecta-absent materials, and speculate about possible source-rock contributions to the ejecta. In the following discussion we use the analyses and images from a single outcrop, Stop 5, because it contains a nearly complete stratigraphic record from disrupted iron-formation (ejecta-absent) to various ejecta-bearing facies, including mesobreccia, gritstone, and lapillistone. These appear in stratigraphic order in Figures 8A–8C. Figure 8A shows scanning electron microscope images and semiquantitative energy dispersive X-ray analyses of amphibole crystals liberated from the fine-grained matrix of samples by electric pulse disaggregation (Saini-Eidukat and Weiblen, 1996). Figure 8A5 shows relative proportions of CaO, MgO, and FeO. Notice that the amphibole compositions do not differ significantly from bulk oxide compositions shown in Figure 8B, reflecting the observation that amphibole minerals dominate the metamorphic assemblage in all Gunflint Lake samples. The diagram of the Mg number (Fig. 8A6) shows comparatively larger Mg contents for all facies of ejecta than for the underlying iron-formation, likely reflecting contribution of distal source rocks.

The geochemical make-up of the ejecta-bearing strata should represent some mixture of the vaporized and liquefied impactor; vaporized, liquefied, and fragmented target rocks at Sudbury; and disaggregated surface materials including the underlying iron-formation. Using this assumption, we compare in Figures 8B and 8C analyses from the finegrained fraction of outcrop samples, with proxy compositions for impactor and target rocks. The geochemical analyses and analytical methods utilized in this section are available by contacting the authors, and they are also available on the Minnesota Geological Survey Web site (http://www.mngs.umn.edu).

Figure 8B demonstrates that the major oxide content of ejecta-bearing layers from Stop 5 differs significantly from that of the subjacent iron-formation. It allows us to speculate about the possible sources of ejecta by comparing these analyses with the compositions of average continental crust (Rud-nick and Gao, 2009) as a proxy for target rocks, and the C1 chondrite meteorite Orgueil (Henderson, 1982) as a proxy for the impactor. The figure can be interpreted as a mixing diagram. For example, it shows that SiO2, Al2O3, and MgO in the ejecta can be derived from a combination of average continental crust (target) and Orgueil (impactor). The Al2O3 content of the basal mesobreccia cannot have been derived from the underlying iron-formation, and instead may reflect arrival of earliest ejecta exhumed from shallowest crustal levels at the target. The FeOt content (total iron as FeO) is greater in the ejecta than in the crust or impactor, implying contribution from an impactor more iron-rich than our chosen chondrite proxy, the presence of an unknown iron-formation in the target, or more likely from the underlying Gunflint Iron Formation. The diagram implies that CaO content of ejecta is not a simple mixture of possible source rocks, highlighting the probable role of carbonate replacement.

Like our conclusion from Figure 8B, the rare earth element (REE) data shown in Figure 8C demonstrates that the composition of ejecta can be modeled as a mixture of an impactor, represented by C1 chondrite ("IMPACTOR" composition from Koratev, 2009), and target rocks ("CRUST") represented by Archean migmatite (Southwick, 1991). Because the analyses of Southwick are incomplete, we also show light rare earth element (LREE) analyses taken from a dioritic gneiss of the North Range at Sudbury (Lafrance, et al., 2008), and heavy rare earth element (HREE) analyses of an amphibolite (Vetrin, et al., 2002). Notice that the REE content of mesobreccia is closer to crustal abundances than other ejecta rock types. This is similar to the relationship shown for oxide compositions in Figure 8B, which may reflect earliest arrival of ballistic ejecta from the upper target zone.

More geochemical and modal data are needed to test and expand these preliminary interpretations. With further evaluation, it may be possible to establish the actual proportions of source-rock contributions—terrestrial versus extraterrestrial, proximal versus distal—in a stratigraphic context to explore temporal variations during deposition of the Sudbury ejecta blanket. This would refine our understanding of ejecta delivery mechanisms (e.g., base surge, ejecta curtain, plume collapse, fallout, etc.).

Figure 8

(continued on following page). (A) Scanning electron microscope images (1–4) and compositions (5, 6) of amphibole crystals extracted from the fine-grained fraction of samples of ejecta (mesobreccia, gritstone, lapillistone) and substrate iron-formation at Stop 5. (B) Geochemical comparison of the finegrained fraction of ejecta-bearing facies and underlying iron-formation layers at Stop 5, average continental crust ("CRUST"), and the Orgueil carbonaceous chondrite (IMPACTOR) using five major oxides. Oxide contents represent >90% of sample mass for all samples except that used for impactor. Analyses of Gunflint Lake samples are shown in stratigraphic order. (C) Rare earth element plot, showing the relationship of ejecta, iron-formation substrate, and postulated source-rocks, normalized to "total crust" (Rudnick and Gao, 2009), which refers to estimated proportions of upper, middle, and lower continental crust. See discussion in text for further details.

Figure 8

(continued on following page). (A) Scanning electron microscope images (1–4) and compositions (5, 6) of amphibole crystals extracted from the fine-grained fraction of samples of ejecta (mesobreccia, gritstone, lapillistone) and substrate iron-formation at Stop 5. (B) Geochemical comparison of the finegrained fraction of ejecta-bearing facies and underlying iron-formation layers at Stop 5, average continental crust ("CRUST"), and the Orgueil carbonaceous chondrite (IMPACTOR) using five major oxides. Oxide contents represent >90% of sample mass for all samples except that used for impactor. Analyses of Gunflint Lake samples are shown in stratigraphic order. (C) Rare earth element plot, showing the relationship of ejecta, iron-formation substrate, and postulated source-rocks, normalized to "total crust" (Rudnick and Gao, 2009), which refers to estimated proportions of upper, middle, and lower continental crust. See discussion in text for further details.

Figure 8

(continued).

Figure 8

(continued).

STOP DESCRIPTIONS

These outcrops are scientifically important, the subjects of ongoing research, and many lie on private or National forest lands. For this reason we ask that you please refrain from hammering and sampling without checking with the leaders. All UTM coordinates are given in NAD 83; Zone 15.

Stop 1. Gunflint Iron Formation—Largely Argillaceous and Gently Dipping (Inferred to Lie Several Meters Stratigraphically beneath Sudbury Impact Layer)

Location: UTM: 663,754E/5,328,2I2N, gravel pit north of Gun-flint Trail on U.S. Forest Service Road 1347.

Description:This dip-slope exposure consists of interbedded granular (cherty) and laminated (slaty) strata of the uppermost Gunflint Iron Formation. The stop is included to provide an overview of regional geology and a look at typical iron-formation that underlies the Sudbury impact layer. The slope defines the southern limb of a large, shallowly east-plunging anticline. The gentle dip of this limb illustrates the observation that open folding and moderate-relief topography are responsible for the complex map pattern (Fig. 6). This exposure of relatively planar-bedded and gently dipping strata is typical of the nearly 100-m-thick iron-formation in the area. It contrasts sharply with chaotically deformed strata directly beneath the Sudbury impact layer that subsequent stops will highlight. Small exposures of breccia occur a short distance to the south; however, metamorphism at this location nearest the Duluth Complex (the ridge just to the south) precludes their utility in demonstrating an impact origin.

Figure 9.

Quartz veining on eroded bedding surface of thinly bedded Gunflint Iron Formation.

Figure 9.

Quartz veining on eroded bedding surface of thinly bedded Gunflint Iron Formation.

Figure 10.

Outcrop photographs of Stop 2. (A) Megabreccia of the ejecta-absent facies. (B) Same photograph, showing outlines of iron-formation fragments (white solid lines) and internal foliation of fragments (white dashed lines); hammer for scale. (C) Mesobreccia and gritstone containing scattered accretionary lapilli. (D) Lapilli-bearing conglomerate; note that iron-formation fragments (darker colored) are rounded in contrast to those in breccia of photos A and B.

Figure 10.

Outcrop photographs of Stop 2. (A) Megabreccia of the ejecta-absent facies. (B) Same photograph, showing outlines of iron-formation fragments (white solid lines) and internal foliation of fragments (white dashed lines); hammer for scale. (C) Mesobreccia and gritstone containing scattered accretionary lapilli. (D) Lapilli-bearing conglomerate; note that iron-formation fragments (darker colored) are rounded in contrast to those in breccia of photos A and B.

Bedding surfaces here are marked by what have been referred to in earlier literature as syneresis cracks. The cracks, now filled with quartz, occur both concentrically and radially around a central, apparently raised core within a single granular layer of siliceous iron-formation (Fig. 9). Syneresis cracks form by sediment shrinkage during dewatering in a gel or colloidal suspension. In most sediments they form by subaqueous dewatering owing to higher salinity in the overlying water column than in sediment pore water. Surprisingly diverse interpretations can be found in the literature about syneresis cracks (summarized in Pratt, 1998). There is, however, general agreement that they represent localized tensional failure during sediment dewatering. The explanation for localized semi-brittle response to what likely were formation-wide stresses—caused by compaction or vibration from syn-sedimentary earthquakes—is more contentious. It has been ascribed variously to the localization of cements, locally increased pore pressure, or zones of granular sediment made coherent by "microbial glue." Recent mapping by Jirsa (2011) indicates that these quartz-filled cracks occur only in granular siliceous layers that lie near the stratigraphic top of the iron-formation (but beneath the SIL). This stratigraphic position, and their enigmatic structural attributes, imply that the trigger mechanism may have been the passage of impact-induced seismic waves through cohesive, semi-rigid, siliceous sediment during the Sudbury impact event.

Stop 2. Sudbury Impact Layer—Breccia, Ejecta, and Conglomerate; Intruded by Sill and Dike of the Mesoproterozoic Logan Intrusions

Location: UTM: 664,785E/5,329,200N, north of Magnetic Rock Hiking Trail.

Description:This traverse provides a cross section through diabase of the Logan Intrusions and underlying deposits of iron-formation, breccia, and ejecta. The diabase is medium to coarse grained in its core to the south, and grades to finer grained and more porphyritic near its base to the north. The northernmost outcrops lie along a steep cliff that exposes basal Gunflint Iron Formation, a thick sequence of breccia (Figs. 10A, 10B), localized pods of bedded lapillistone and mesobreccia (Fig. 10C), and what is inferred to be reworked breccia containing rounded fragments of iron-formation in a matrix composed largely of accretionary lapilli (Fig. 10D). The precise stratigraphic positions of the latter two facies are not entirely clear, though the strata containing accretionary lapilli (true ejecta) lie near the top of the deposit.

Stop 3. Breccia and Lapillistone of Sudbury Impact Layer; Basal Rove Formation and Logan Intrusion

Location: UTM: 665,200E/5,329,300N, adjacent to U.S. Forest Service Road I347.

Description:This outcrop affords a great number and variety of views of the ejecta and breccia (Fig. 11) because the exposed surface is nearly parallel with the strike and dip of the formations. The stratigraphic sequence is similar to that at Stop 2; however, this site lies along the top of the SIL, showing the relationship between ejecta and breccia more clearly. Just to the south is a bluff held up by the same Logan sill that was traversed at Stop 2. The sill is underlain by ∼3 m of slate, clayrich sandstones, and conglomerate inferred to be the basal section of the Rove Formation.

Figure 11.

(A) Concentrically zoned accretionary lapilli. (B) Breccia containing ductily deformed (pre-lithification) siliceous fragments (light gray).

Figure 11.

(A) Concentrically zoned accretionary lapilli. (B) Breccia containing ductily deformed (pre-lithification) siliceous fragments (light gray).

Stop 4. Sudbury Impact Layer—Folded Iron-Formation Overlain by Ejecta

Location: UTM: 663700E/5328967N, offMagnetic Rock Hiking Trail.

Description:Folded siliceous and argillaceous iron-formation overlain by a thin, discontinuous layer of mesobreccia containing scant accretionary lapilli. Note the structural detachment at the base of the outcrop that separates gently dipping, planar-bedded iron-formation layers from the overlying meter or so of folded strata. The chaotic fold style (Fig. 12A) indicates soft-sediment deformation prior to deposition of ejecta, which lends credence to the inference that iron-formation was not yet fully lithified at the time of impact.

Figure 12.

Outcrop photographs of soft-sediment deformation in the ejecta-absent facies of SIL, locally overlain by ejecta, and demonstrating that deformation occurred during and after silicification of mudstones but prior to complete lithification. (A) Folded siliceous (light-colored) and iron-silicate (darker) iron-formation overlain by a thin skin of ejecta containing accretionary lapilli (Bill Addison and Bevan French for scale). (B) Irregularly layered siliceous and iron-silicate mudstone cut by a mudstone clastic dike (darkest narrow feature running up-down in center of photo). (C) Folded and brecciated iron-formation in which the siliceous layer (light gray) is attenuated and shattered in contrast to the enclosing iron-silicate mudstone that is ductily folded.

Figure 12.

Outcrop photographs of soft-sediment deformation in the ejecta-absent facies of SIL, locally overlain by ejecta, and demonstrating that deformation occurred during and after silicification of mudstones but prior to complete lithification. (A) Folded siliceous (light-colored) and iron-silicate (darker) iron-formation overlain by a thin skin of ejecta containing accretionary lapilli (Bill Addison and Bevan French for scale). (B) Irregularly layered siliceous and iron-silicate mudstone cut by a mudstone clastic dike (darkest narrow feature running up-down in center of photo). (C) Folded and brecciated iron-formation in which the siliceous layer (light gray) is attenuated and shattered in contrast to the enclosing iron-silicate mudstone that is ductily folded.

The walk from here to Stop 5 crosses several exposures of variably deformed iron-formation, all considered part of the ejecta-absent facies of SIL. These outcrops demonstrate the rheologic contrasts of substrate during deformation and highlight the interpretation that at least some components of iron-formation were unlithified at the time of impact deformation (Figs. 12B and 12C).

Stop 5. Sudbury Impact Layer—Deformed Substrate, Mesobreccia, Gritstone, and Lapillistone

Location: UTM: 663,535E/5,329,100N, off Magnetic Rock Hiking Trail.

Description:This small outcrop provides a complete cross section of the SIL, and some unique sedimentological features not seen elsewhere. The stratigraphic sequence is shown in Figure 13A. Of particular importance are the scoured (channelized) appearance at the base of the lapillistone and the presence of larger fragments of gritstone in lapillistone (Fig. 13B). Both indicate moderately high energy delivery of detritus—presumably by the passing ejecta plume or ground surge. Inferences about the geochemical make-up of strata at this outcrop are discussed above in the "Petrology and Geochemistry" section (Fig. 8).

In detail, the basal part of this deposit consists of disorganized-bedded boulder "megabreccia," with clasts composed of rock types characteristic of the underlying Gunflint Iron Formation. The megabreccia is overlain by a decimeters-thick, matrix-supported, pebble "mesobreccia" and massive, pebbly sandstone—here termed gritstone owing to its content of moderately sorted but primarily angular grains. Scattered accre-tionary lapilli occur in this unit locally, implying that it may be a mixture of ejecta and locally derived detritus. The meso-breccia and gritstone are overlain by lapillistone, composed of tightly packed accretionary lapilli. These fill shallow scours in the top of the mesobreccia and gritstone, or deeper scours that removed strata all the way down to megabreccia locally. The bases of the scours are commonly overlain by a 1-cm-thick wisp of coarse-grained gritstone, followed vertically by the accretionary lapilli. The scours give a paleocurrent direction of 260°; the bearing from Sudbury to Gunflint Lake is 280°. At other localities where individual smaller scours at the base of the lapillistone are not present, the basal, clast-supported lapillistone bed drapes shallow erosive scours. The lowermost accretionary lapillistone is massive textured, as are overlying accretionary lapilli-rich beds, except where rare, small-scale, low-angle cross-stratification dipping toward 060° is visible. The diameters of accretionary lapilli in the bed at the base of the lapilli-rich interval average 0.7–0.8 cm, and those higher in the section and interbedded with sandstone range from 0.2 to 0.4 cm. Gritstone beds become more dominant in the upper few decimeters. Here they are medium to fine grained with stringers and patches of small accretionary lapilli. Some beds are massive with abundant isolated lapilli. Parallel lamination to undulating parallel lamination is common in the non-massive beds. Approximately 10 cm of thinly laminated siltstone caps the impact deposit.

Figure 13.

(A) Photo and graphic sedimentological analysis of Stop 5. Black angular polygons represent fragments of iron-formation. White box shows approximate location of photo B. (B) Close-up view of lapillistone containing entrained fragment of layered gritstone.

Figure 13.

(A) Photo and graphic sedimentological analysis of Stop 5. Black angular polygons represent fragments of iron-formation. White box shows approximate location of photo B. (B) Close-up view of lapillistone containing entrained fragment of layered gritstone.

Stop 6. Sudbury Impact Layer—Deformed Gunflint Iron Formation Overlain by Thin Ejecta Layer That Includes Small Spherules

Location: UTM: 663,628E/5,329,186N, off Magnetic Rock Hiking Trail.

Description:This cliff and ridge-top exposure includes a 7-m-thick breccia, abruptly overlain by mesobreccia (Fig. 14A), and capped by strata composed of small (2–5 mm) accretionary pellets and slightly larger, concentrically zoned lapilli (Fig. 14B). Some of these small particles may be relict glass spherules; however, metamorphism precludes definitive identification.

THUNDER BAY AREA STOP DESCRIPTIONS

Stop 7. Silicified Ankerite Grainstones of the Gunflint Formation, Ejecta Layer, and Rove Formation Basal Conglomerate and Overlying Siltstone

Location: UTM: 340,097E/5,372,489N, cliff faces near entrance to Terry Fox Lookout, Highway 11/17, east edge of the City of Thunder Bay, Ontario.

Description:Ankeritic grainstone forms the base of this outcrop. It is highly silicified in places and overlain by the basal ejecta layer along an irregular surface (Fig. 15). Silicified ankerite clasts occur in this basal unit, along with crushed spherule clusters. Abundant, subhorizontal slickenside surfaces have striae aligned at 140° (Addison et al., 2010). Subaerial deposition from a base surge cloud is indicated by these features. This 40-cm-thick bed is overlain by the main 3-m-thick body of the flow. This unit is organized into lenses up to 2 m by 30 m, which decrease in size upward. Pebble-size devitrified glass forms most of the clasts, although they are difficult to see because a pervasive ankeritic alteration has affected these large outcrops. What appears to be faint cross-stratification is present in some lenses. They are overlain by a 5-20-cm-thick, spherule-rich, recessively weathered layer, which in turn is overlain by agate, showing open space fills of miniature stalagmites and stalactites, indicating a period of subaerial exposure after deposition of the ejecta layer. An iron oxide and sulfide layer lies above the agate and is overlain by a thin pebble conglomerate composed of fragments of mudstone and pieces of the iron alteration layer (Addison et al., 2010). This is overlain by beds of ankerite, the uppermost of which were ripped up to form the conglomerate at the base of the Rove Formation. Rove siltstone with thin black shale layers, and a thick diabase sill, cap the large cliff exposures in the area.

Figure 14.

(A) Megabreccia, sharply overlain by mesobreccia and other ejecta. (B) Layers composed of accretionary pellets, small lapilli, and inferred relict spherules.

Figure 14.

(A) Megabreccia, sharply overlain by mesobreccia and other ejecta. (B) Layers composed of accretionary pellets, small lapilli, and inferred relict spherules.

Stop 8. Limestone of the Gunflint Formation and Overlying Basal Ejecta

Location: Private yard, City of Thunder Bay (location omitted for owner's privacy). PLEASE: no hammers.

Description:This exposure contains 1 m of partially silici-fied ankerite grainstone, with a heavily silicified zone at its top, sharply overlain by more pristine limestone. This 40-cm-thick

"limestone" is an odd rock unit in that the clastic component is composed of iron-rich chlorite sand grains. Chlorite also forms layers in the cabbage-shaped stromatolites (Fig. 16A), whereas the carbonate simply forms pervasive, meteoric, blocky, calcite cement. The geochemistry of this unit is also unusual, as the chlorite contains 1.5% vanadium and the calcite is enriched in U, Y, and REE, which have a curve shape similar to modern fresh water (Fralick and Burton, 2008). This denotes subaerial exposure and may indicate that cements formed where somewhat oxidized groundwater encountered a redox boundary. The ejecta layer sits sharply on the "limestone," and in places the tops of stromatolites were removed and deposited as fragments in the overlying ejecta layer (Fig. 16B). The breccia matrix is composed of amoeboid, vesicular devitrified glass fragments (Fig. 16C). The ejecta layer here was an open framework that developed blocky calcite cement.

Stop 9. Ejecta Layer with Accretionary Lapilli Overlying Truncated Stromatolites

Location: UTM: 334,728E/5,366,952N, Hillcrest Park, City of Thunder Bay.

Description:The "limestone" here overlies a silicified bed and has been partly replaced by ankerite. Cabbage-shaped stromatolites on its upper surface have been planed off by the base surge, creating a sharp contact with the overlying ejecta layer. The ejecta is organized into large overlapping lenses composed of pebble-sized devitrified glass and local lithologies. Extensive ankerite replacement and agate veining make the original clasts difficult to distinguish in places. The ejecta unit is at least 4 m thick, without an exposed upper contact. The lenses become smaller upward, and the size of the clasts decreases. Small lenses of accretionary lapilli in clast to matrix support occur 2.3 m above the base of the ejecta unit.

Stop 10. Chaotic Boulders of Lithified Ejecta Separated by Thin Layers of Black Shale

Location: UTM: 395,129E/5,367,236N, hill down from Waverley Park, City of Thunder Bay.

Description:This is a perplexing outcrop. It appears to be composed of large boulder-size blocks of ejecta-bearing sediment arranged in chaotic layers separated by carbonaceous and pyritic, black shale that resembles parts of the Rove Formation. The sides of some blocks show the development of large, centimeter-size, dolomite-ankerite rhombohedral crystals, similar to crystals directly below the carbonate-Rove contact in core. This appears to be a series of boulder slide deposits of the lithified substrate initiated after flooding by the Rove Sea.

Stop 11. Seven Meters of Boulder Conglomerate Overlain by Ejecta with Devitrified Glass

Location: UTM: 326,399E/5,363,836N, Old Grand Trunk Pacific Railway cut; off Mapleward Avenue.

Description:Here, thrust faults cut grainstone in the underlying basement, causing an increase in stratal dip. These beds, dipping at a high angle, form one side of a depression into which a thick sequence of blocks from the adjacent basement accumulated. The contact between the moved blocks and those in situ, but fractured, is difficult to locate in places. Large slabs of "limestone" up to 5 m long are present with ankerite and chert cobbles to boulders (Fig. 17). The platy slabs appear to define a scree slope against the tilted strata. Pebbles of devitrified glass are present approximately halfway up the cliff face. Their appearance defines the separation between the underlying boulder unit that was probably created by the earthquake activity and the capping unit deposited from the base surge. On top of the hill the ejecta unit is dominated by pebble-sized devitrified glass fragments with a calcite cement. It is also apparent that the basement is at a much higher elevation here than along the railway cut, and the boulder unit is missing.

Figure 15.

Stratigraphie section of units present at Stop 7. From Addison et al. (2010).

Figure 15.

Stratigraphie section of units present at Stop 7. From Addison et al. (2010).

Figure 16.

Outcrop features at Stop 8. (A) Stromatolites composed of chlorite and calcite cement in plan view, surrounded by fine- and coarse-grained grainstone. (B) Ripped-up boulders of "limestone" in ejecta layer. (C) Matrix composed of devitrified, vesicular glass with calcite cement.

Figure 16.

Outcrop features at Stop 8. (A) Stromatolites composed of chlorite and calcite cement in plan view, surrounded by fine- and coarse-grained grainstone. (B) Ripped-up boulders of "limestone" in ejecta layer. (C) Matrix composed of devitrified, vesicular glass with calcite cement.

Stop 12. Ankerite-Replaced Stromatolites Overlain by Accretionary Lapilli

Location: UTM: 307,539E/5,357,977N, Highway 588.

Description:The drainage ditch by the side of the road contains an outcrop of the cabbage-shaped stromatolites partially replaced by ankerite. In places their tops were sheared off by the base surge, and a layer of coarse sand–size devitrified glass was deposited. This was eroded, leaving only remnants in depressions and a thin layer in other places. Accretionary lapilli accumulated on top of this erosive surface (Fig. 18). Cobbles and boulders of the underlying bedrock are sporadically present. Devitrified glass larger than sand size is not present. It is interesting that generally in outcrops where there are pebbles of devitrified glass, there are no accretionary lapilli; and where there are abundant accretionary lapilli, there are no pebbles of devitrified glass. Upward through the sequence, sand-size material becomes more common and only stringers of accretionary lapilli are present. Most of this outcrop was blown-up during road improvement, so samples are present—please take only a small, fist-size piece, leaving material for future visitors. Drill core from two holes drilled ∼200 m north of this outcrop will be examined (Fig. 19).

Figure 17.

Scale card on a slab of limestone with an orange-stained, upside-down stromatolite to the right and slightly below the card. A large chert boulder is above the stromatolite.

Figure 17.

Scale card on a slab of limestone with an orange-stained, upside-down stromatolite to the right and slightly below the card. A large chert boulder is above the stromatolite.

Figure 18.

Photographs of cut slabs from Stop 12. (A) Erosively scoured bedrock composed of Gunflint carbonate and chert overlain by the impact deposit. Very coarse grained sandstone with granules fills the scour. Coin is 19 mm. (B) Another example of a scoured depression filled by a remnant of granule-rich sandstone. Note that the very coarse grained, granule-rich sandstone in the scour has a different grain size than the matrix of the overlying lapilli-rich bed. (C) Truncated top of a hemispherical stromatolite, overlain by a layer of very coarse grained sandstone, which in turn is overlain by accretionary lapilli in clast-support. Slab courtesy of W. Addison. (D) Lapilli directly overlying Gunflint carbonate bedrock. (E) Large block of underlying bedrock encased in accretionary lapilli-rich ejecta. (F) Coarse-grained sandstone, which lies above the more massive, accretionary lapilli-rich bed (not present in this slab). Note that the stringers of lapilli are associated with and overlain by coarser sand-size material than that constituting the underlying layer.

Figure 18.

Photographs of cut slabs from Stop 12. (A) Erosively scoured bedrock composed of Gunflint carbonate and chert overlain by the impact deposit. Very coarse grained sandstone with granules fills the scour. Coin is 19 mm. (B) Another example of a scoured depression filled by a remnant of granule-rich sandstone. Note that the very coarse grained, granule-rich sandstone in the scour has a different grain size than the matrix of the overlying lapilli-rich bed. (C) Truncated top of a hemispherical stromatolite, overlain by a layer of very coarse grained sandstone, which in turn is overlain by accretionary lapilli in clast-support. Slab courtesy of W. Addison. (D) Lapilli directly overlying Gunflint carbonate bedrock. (E) Large block of underlying bedrock encased in accretionary lapilli-rich ejecta. (F) Coarse-grained sandstone, which lies above the more massive, accretionary lapilli-rich bed (not present in this slab). Note that the stringers of lapilli are associated with and overlain by coarser sand-size material than that constituting the underlying layer.

Figure 19.

Core drilled through the ejecta layer ∼200 m north of Stop 12.

Figure 19.

Core drilled through the ejecta layer ∼200 m north of Stop 12.

References Cited

Addison
,
W.D.
Brumpton
,
G.R.
Vallini
,
D.A.
McNaughton
,
N.J.
Davis
,
D.W.
Kissin
,
S.A.
Fralick
,
P.W.
Hammond
,
A.L.
,
2005
,
Discovery of distal ejecta from the 1850 Ma Sudbury impact event
:
Geology
 , v.
33
, p.
193
196
,
Addison
,
W.D.
Brumpton
,
G.R.
Davis
,
D.W.
Fralick
,
P.W.
Kissin
,
S.A.
,
2010
,
Debrisites from the Sudbury impact event in Ontario, north of Lake Superior, and a new age constraint: Are they base-surge deposits or tsunami deposits?
, in
Gibson
,
R.L.
Reimold
,
W.U.
, eds.,
Large Meteorite Impacts and Planetary Evolution IV: Geological Society of America Special Paper 465
 , p.
245
268
.
Barnouin-Jha
,
O.S.
Baloga
,
S.
Glaze
,
L.
,
2005
,
Comparing landslides to fluidized crater ejecta on Mars
:
Journal of Geophysical Research
 , v.
E04010
,
Branney
,
M.J.
Brown
,
R.J.
,
2011
,
Impactoclastic density current emplacement of terrestrial meteorite-impact ejecta and the formation of dust pellets and accretionary lapilli: Evidence from Stac Fada, Scotland
:
Journal of Geology
 , v.
119
, p.
275
292
,
Broderick
,
T.M.
,
1920
,
Economic geology and stratigraphy in the Gun-flint iron district, Minnesota
:
Economic Geology and the Bulletin of the Society of Economic Geologists
 , v.
15
, p.
422
452
,
Cannon
,
W.F.
Schulz
,
K.J.
Horton
,
J.W.
, Jr.
Kring
,
D.A.
,
2010
,
The Sudbury impact layer in the Paleoproterozoic iron ranges of northern Michigan, USA: Lake Superior iron ranges: A time-line from the heavens
:
Institute of Lake Superior Geology
,
Annual Meeting, 53rd, Proceedings
 , v.
53
,
pt. 1
, p.
20
21
.
Collins
,
G.S.
Melosh
,
J.H.
Marcus
,
R.A.
,
2005
,
Earth impact effects program: A web-based computer program for calculating the regional environmental consequences of a meteoroid impact on Earth
:
Meteoritics and Planetary Science
 , v.
40
, p.
817
840
().
Davis
,
D.W.
,
2008
,
Sub-million-year age resolution of Precambrian igneous events by thermal extraction–thermal ionization mass spectrometer Pb dating of zircon: Application to crystallization of the Sudbury impact melt sheet
:
Geology
 , v.
36
, p.
383
396
,
Dietz
,
R.S.
,
1964
,
The Sudbury structure as an astrobleme
:
Journal of Geology
 , v.
72
, p.
412
434
,
Floran
,
R.J.
Papike
,
J.J.
,
1978
,
Mineralogy and petrology of the Gunflint Iron Formation, Minnesota-Ontario
:
Journal of Petrology
 , v.
19
, p.
215
288
.
Fralick
,
P.W.
Barrett
,
T.J.
,
1995
,
Depositional controls on iron formation associations in Canada
, in
Plint
,
A.G.
, ed.,
Sedimentary Facies Analysis
 :
International Association of Sedimentologists
Special Publication 22
, p.
137
156
.
Fralick
,
P.W.
Burton
,
J.
,
2008
,
Geochemistry of the Paleoproterozoic Gunflint Formation carbonate: Implications for early hydrosphere-atmosphere evolution
:
Geochimica et Cosmochimica Acta
 , v.
72
, p.
A280
.
Fralick
,
P.W.
Davis
,
D.W.
Kissin
,
S.A.
,
2002
,
The age of the Gunflint Formation, Ontario, Canada: Single zircon U-Pb age determinations from reworked volcanic ash
:
Canadian Journal of Earth Sciences
 , v.
39
, p.
1085
1091
,
Fralick
,
P.W.
Grotzinger
,
J.
Edgar
,
L.
,
2011
,
Possible recognition of accre-tionary lapilli in distal impact deposits on Mars: A facies analog provided by the 1.85 Ga Sudbury impact deposit
, in
Grotzinger
,
J.
Milliken
,
R.
, eds.,
Martian Sedimentology: Journal of Sedimentary Research Special Publication
  (
in press
).
French
,
B.M.
,
1967
,
Sudbury structure, Ontario: Some petrographic evidence for origin by meteor impact
:
Science
 , v.
156
, p.
1094
1098
,
French
,
B.M.
Koeberl
,
C.
,
2010
,
The convincing identification of terrestrial meteorite impact structures
:
What works, what doesn't, and why: Earth-Science Reviews
 , v.
98
, p.
123
170
,
Gault
,
D.E.
Quaide
,
W.L.
Oberbeck
,
V.R.
,
1968
,
Impact cratering mechanics and structures
, in
French
,
B.M.
Short
,
N.M.
, eds.,
Shock Meta-morphism of Natural Materials
 :
Baltimore, Mono
, p.
87
99
.
Goodwin
,
A.M.
,
1956
,
Facies relationships in the Gunflint iron-formation
:
Economic Geology and the Bulletin of the Society of Economic Geologists
 , v.
51
, p.
565
595
,
Grant
,
J.A.
Rossman
,
P.I.
Grotzinger
,
J.P.
Milliken
,
R.E.
Tornabene
,
L.L.
McEwen
,
A.S.
Weitz
,
C.M.
Squyres
,
S.W.
Glotch
,
T.D.
Tomson
,
B.J.
,
2008
,
Hirise imaging of impact megabreccia and sub-meter aqueous strata in Holden Crater
:
Marine Geology
 , v.
36
, p.
195
198
.
Grieve
,
R.A.F.
Riemold
,
W.U.
Morgan
,
J.
Riller
,
U.
Pilkington
,
M.
,
2008
,
Observations and interpretations at Vredefort, Sudbury, and Chicxulub: Towards an empirical model of terrestrial impact basin formation
:
Meteoritics & Planetary Science
 , v.
43
, p.
855
882
,
Heaman
,
L.M.
Easton
,
R.M.
Hart
,
T.R.
Hollings
,
C.A.
MacDonald
,
C.A.
Smyk
,
M.
,
2007
,
Further refinement to the timing of Mesoproterozoic magmatism, Lake Nipigon region, Ontario
:
Canadian Journal of Earth Sciences
 , v.
44
, p.
1055
1086
,
Henderson
,
P.
,
1982
,
Inorganic Geochemistry
 :
Elmsford, New York
,
Pergamon Press
,
353
p. [
analysis of sample from Orgueil meteorite from Table 1.3
].
Jannett
,
P.A.
Terry
,
D.O.
, Jr.
,
2008
,
Stratigraphic expression of a regionally extensive impactite within the Upper Cretaceous Fox Hills Formation of southwestern South Dakota
, in
Evans
,
K.R.
Horton
,
J.W.
, Jr.
King
,
D.T.
, Jr.
Morrow
,
J.R.
, eds.,
The Sedimentary Record of Meteorite Impacts: Geological Society of America Special Paper 437
 , p.
199
213
.
Jirsa
,
M.A.
,
2010
,
Stratigraphy of Sudbury "impactite" near Gunflint Lake, NE Minnesota
:
Institute of Lake Superior Geology
:
Annual Meeting, 56th, Proceedings and Abstracts
 , v.
56
,
pt. 1
, p.
31
32
.
Jirsa
,
M.A.
,
2011
,
Bedrock Geology of the Western Gunflint Trail Area, Northeastern Minnesota
:
Minnesota Geological Survey Miscellaneous Map M-191
 ,
scale 1:24,000, 1 sheet.
Jirsa
,
M.A.
Boerboom
,
T.J.
Chandler
,
V.
Mossler
,
J.M.
Runkel
,
A.C.
Setterholm
,
D.R.
,
2011
,
Geologic map of Minnesota, Bedrock Geology
:
Minnesota Geological Survey State Map Series S-21
 ,
scale 1:500,000, 1 sheet.
Kenkmann
,
T.
Schonian
,
F.
,
2006
,
Ries and Chicxulub: Impact craters on Earth provide insights for Martian ejecta blankets
:
Meteoritics & Planetary Science
 , v.
41
, p.
1587
1603
,
Koratev
,
R.L.
,
2009
,
http://meteorites.wustl.edu/goodstuff/ree-chon.html [recommended normalization #11, representing C1 carbonaceous chondrite]
.
Krogh
,
T.E.
Davis
,
T.W.
Corfu
,
F.
,
1984
,
Precise U-Pb zircon and bad-deleyite ages for the Sudbury area
, in
Pye
,
E.G.
Naldrett
,
A.J.
Giblin
,
P.E.
, eds.,
The Geology and Ore Deposits of the Sudbury Structure
 :
Ontario Geological Survey
Special Volume
1
, p.
431
446
.
Lafrance
,
B.
Legault
,
D.
Ames
,
D.E.
,
2008
,
The formation of the Sudbury breccia in the North Range of the Sudbury impact structure
:
Precambrian Research
 , v.
165
, p.
101
119
[
analysis of dioritic gneiss, Table 3, sample #02AV20
].
Melosh
,
H.J.
,
1989
,
Impact Cratering
:
A Geologic Process
 :
New York
,
Oxford University Press
,
245
p.
Mouginis-Mark
,
P.J.
Garbeil
,
H.
,
2007
,
Crater geometry and ejecta thickness of the Martian impact crater Tooting
:
Meteoritics & Planetary Science
 , v.
42
, p.
1615
1625
,
Oberbeck
,
V.R.
,
1975
,
The role of ballistic erosion and sedimentation in lunar stratigraphy
:
Reviews of Geophysics and Space Physics
 , v.
13
, p.
337
362
,
Osinski
,
G.
,
2006
,
Effect of volatiles and target lithology on the generation and emplacement of impact crater fill and ejecta deposits on Mars
:
Meteoritics & Planetary Science
 , v.
41
, p.
1571
1586
,
Pinto
,
J.A.
Warme
,
J.E.
,
2008
,
Alamo Event, Nevada: Crater stratigraphy and impact breccia realms
, in
Evans
,
K.R.
Horton
,
J.W.
, Jr.
King
,
D.T.
, Jr.
Morrow
,
J.R.
, eds.,
The Sedimentary Record of Meteorite Impacts: Geological Society of America Special Paper 437
 , p.
99
138
.
Pratt
,
B.R.
,
1998
,
Syneresis cracks: Subaqueous shrinkage in argillaceous sediments caused by earthquake-induced dewatering
:
Sedimentary Geology
 , v.
117
, p.
1
10
,
Pufahl
,
P.K.
Fralick
,
P.W.
,
2000
,
Depositional environments of the Paleo-proterozoic Gunflint Formation
:
Thunder Bay, Ontario
,
Institute of Lake Superior Geology
,
Annual Meeting, 46th, Proceedings, Field Trip Guidebook
 , v.
51
,
Field Trip 4, no pagination.
Pufahl
,
P.K.
Hiatt
,
E.E.
Stanley
,
C.R.
Morrow
,
J.R.
Nelson
,
G.J.
Edwards
,
C.T.
,
2007
,
Physical and chemical evidence of the 1850 Ma Sudbury impact event in the Baraga Group, Michigan
:
Geology
 , v.
35
, p.
827
830
.
Pye
,
E.G.
,
1963
,
Atikokan-Lakehead Sheet; Kenora, Rainy River, and Thunder Bay Districts
:
Ontario Geological Survey Geological Compilation Series Map 2065
 ,
scale 1:253,440, 1 sheet.
Rudnick
,
R.L.
Gao
,
S.M.
,
2009
, , p.
52
53
.
Saini-Eidukat
,
B.
Weiblen
,
P.W.
,
1996
,
A new method of fossil preparation, using high-voltage electric pulses
:
Curator
 , v.
39
, p.
139
144
,
Slack
,
J.F.
Cannon
,
W.F.
,
2009
,
Extraterrestrial demise of banded iron formations 1.85 billion years ago
:
Geology
 , v.
37
, p.
1011
1014
,
Southwick
,
D.L.
,
1991
,
On the genesis of Archean granite through two-stage melting of the Quetico accretionary prism to a transpressional plate boundary
:
Geological Society of America Bulletin
 , v.
103
[
analysis #2 on Table 2
, p.
1390
].
Vetrin
,
V.R.
Turkina
,
O.M.
Ludden
,
J.
,
2002
,
Petrology and geochemistry of rocks from the basement of the Pechenga paleorift
:
Russian Journal of Earth Science
 , v.
4
[analysis from Table 3, sample #4], http://rjes.wdcb.ru/v04/tje02085/tje02085.htm#flon20.
Wolff
,
J.F.
,
1917
,
Recent geologic developments on the Mesabi Iron Range, Minnesota
:
American Institute of Mining and Metallurgical Engineers Transactions
 , v.
56
, p.
229
257
.
www.unb.ca/passc/ImpactDatabase (and References therein)
Planetary and Space Science Centre
 ,
University of New Brunswick
,
Fredericton, New Brunswick, Canada
.

Figures & Tables

Figure 1.

Map of the Lake Superior region, showing Sudbury and the iron ranges where the Sudbury impact layer has been identified, including the subject locations of this field trip at Gunflint Lake, Minnesota, and Thunder Bay, Ontario (black boxes). Gray bold lines represent the strike of iron-formations. Gradient circle surrounding Sudbury approximates estimated size of final crater (250 km diameter).

Figure 1.

Map of the Lake Superior region, showing Sudbury and the iron ranges where the Sudbury impact layer has been identified, including the subject locations of this field trip at Gunflint Lake, Minnesota, and Thunder Bay, Ontario (black boxes). Gray bold lines represent the strike of iron-formations. Gradient circle surrounding Sudbury approximates estimated size of final crater (250 km diameter).

Figure 2.

Qualitative models for the emplacement of ejecta, showing the effect of an atmosphere that entrains smaller ejecta particles into a vortex following the advancing curtain, scouring the surface and incorporating local material into the ejecta. This material is eventually deposited through dynamic debris flow, similar to a turbidity current. Modified from Barnouin-Jha et al. (2005).

Figure 2.

Qualitative models for the emplacement of ejecta, showing the effect of an atmosphere that entrains smaller ejecta particles into a vortex following the advancing curtain, scouring the surface and incorporating local material into the ejecta. This material is eventually deposited through dynamic debris flow, similar to a turbidity current. Modified from Barnouin-Jha et al. (2005).

Figure 3.

Generalized map of the western Lake Superior region (modified from Pye, 1963; Jirsa et al., 2011), showing geologic setting of field trip areas at Gunflint Lake (Stops 1–6) and Thunder Bay (Stops 7–12). A complex array of Mesoproterozoic sills (Logan and related intrusions) are common in the Rove Formation, but are not shown here for simplicity.

Figure 3.

Generalized map of the western Lake Superior region (modified from Pye, 1963; Jirsa et al., 2011), showing geologic setting of field trip areas at Gunflint Lake (Stops 1–6) and Thunder Bay (Stops 7–12). A complex array of Mesoproterozoic sills (Logan and related intrusions) are common in the Rove Formation, but are not shown here for simplicity.

Figure 4.

Schematic diagram of stratigraphy at Gunflint Lake and near Thunder Bay, incorporating older (4-5 members) and newer (2 sequences) nomenclatural subdivision of units. Section is hung from the 1850 Ma Sudbury impact layer. The diastem representing maximum regression and likely subaerial exposure defines the contact between sequences.

Figure 4.

Schematic diagram of stratigraphy at Gunflint Lake and near Thunder Bay, incorporating older (4-5 members) and newer (2 sequences) nomenclatural subdivision of units. Section is hung from the 1850 Ma Sudbury impact layer. The diastem representing maximum regression and likely subaerial exposure defines the contact between sequences.

Figure 5.

Scanning electron microscope (SEM) X-ray element map of lapillistone from Gunflint Lake. This mosaic backscatter electron image shows elemental variations based on mass (higher mass produces lighter colors). Zonation primarily reflects proportions of silicate versus carbonate minerals; bright spots are oxides. (Analyses courtesy of McSwiggen and Associates, www.mcswiggen.com.)

Figure 5.

Scanning electron microscope (SEM) X-ray element map of lapillistone from Gunflint Lake. This mosaic backscatter electron image shows elemental variations based on mass (higher mass produces lighter colors). Zonation primarily reflects proportions of silicate versus carbonate minerals; bright spots are oxides. (Analyses courtesy of McSwiggen and Associates, www.mcswiggen.com.)

Figure 6.

Geologic map and schematic cross section of the Gunflint Lake area, showing approximate location and stratigraphic position of some field trip stops. Geology modified from Jirsa (2011). Note that horizontal scale of section is much greater than map, and that its vertical exaggeration is ∼3.5x, so that apparent dips of contacts are much steeper than true.

Figure 6.

Geologic map and schematic cross section of the Gunflint Lake area, showing approximate location and stratigraphic position of some field trip stops. Geology modified from Jirsa (2011). Note that horizontal scale of section is much greater than map, and that its vertical exaggeration is ∼3.5x, so that apparent dips of contacts are much steeper than true.

Figure 7.

Stratigraphic framework derived from eight exposures along a 2-mi strike length near Gunflint Lake; hung from the contact between ejecta-bearing and ejecta-absent facies of the Sudbury Impact Layer (bold dashed line). SIL—Sudbury impact layer.

Figure 7.

Stratigraphic framework derived from eight exposures along a 2-mi strike length near Gunflint Lake; hung from the contact between ejecta-bearing and ejecta-absent facies of the Sudbury Impact Layer (bold dashed line). SIL—Sudbury impact layer.

Figure 8

(continued on following page). (A) Scanning electron microscope images (1–4) and compositions (5, 6) of amphibole crystals extracted from the fine-grained fraction of samples of ejecta (mesobreccia, gritstone, lapillistone) and substrate iron-formation at Stop 5. (B) Geochemical comparison of the finegrained fraction of ejecta-bearing facies and underlying iron-formation layers at Stop 5, average continental crust ("CRUST"), and the Orgueil carbonaceous chondrite (IMPACTOR) using five major oxides. Oxide contents represent >90% of sample mass for all samples except that used for impactor. Analyses of Gunflint Lake samples are shown in stratigraphic order. (C) Rare earth element plot, showing the relationship of ejecta, iron-formation substrate, and postulated source-rocks, normalized to "total crust" (Rudnick and Gao, 2009), which refers to estimated proportions of upper, middle, and lower continental crust. See discussion in text for further details.

Figure 8

(continued on following page). (A) Scanning electron microscope images (1–4) and compositions (5, 6) of amphibole crystals extracted from the fine-grained fraction of samples of ejecta (mesobreccia, gritstone, lapillistone) and substrate iron-formation at Stop 5. (B) Geochemical comparison of the finegrained fraction of ejecta-bearing facies and underlying iron-formation layers at Stop 5, average continental crust ("CRUST"), and the Orgueil carbonaceous chondrite (IMPACTOR) using five major oxides. Oxide contents represent >90% of sample mass for all samples except that used for impactor. Analyses of Gunflint Lake samples are shown in stratigraphic order. (C) Rare earth element plot, showing the relationship of ejecta, iron-formation substrate, and postulated source-rocks, normalized to "total crust" (Rudnick and Gao, 2009), which refers to estimated proportions of upper, middle, and lower continental crust. See discussion in text for further details.

Figure 8

(continued).

Figure 8

(continued).

Figure 9.

Quartz veining on eroded bedding surface of thinly bedded Gunflint Iron Formation.

Figure 9.

Quartz veining on eroded bedding surface of thinly bedded Gunflint Iron Formation.

Figure 10.

Outcrop photographs of Stop 2. (A) Megabreccia of the ejecta-absent facies. (B) Same photograph, showing outlines of iron-formation fragments (white solid lines) and internal foliation of fragments (white dashed lines); hammer for scale. (C) Mesobreccia and gritstone containing scattered accretionary lapilli. (D) Lapilli-bearing conglomerate; note that iron-formation fragments (darker colored) are rounded in contrast to those in breccia of photos A and B.

Figure 10.

Outcrop photographs of Stop 2. (A) Megabreccia of the ejecta-absent facies. (B) Same photograph, showing outlines of iron-formation fragments (white solid lines) and internal foliation of fragments (white dashed lines); hammer for scale. (C) Mesobreccia and gritstone containing scattered accretionary lapilli. (D) Lapilli-bearing conglomerate; note that iron-formation fragments (darker colored) are rounded in contrast to those in breccia of photos A and B.

Figure 11.

(A) Concentrically zoned accretionary lapilli. (B) Breccia containing ductily deformed (pre-lithification) siliceous fragments (light gray).

Figure 11.

(A) Concentrically zoned accretionary lapilli. (B) Breccia containing ductily deformed (pre-lithification) siliceous fragments (light gray).

Figure 12.

Outcrop photographs of soft-sediment deformation in the ejecta-absent facies of SIL, locally overlain by ejecta, and demonstrating that deformation occurred during and after silicification of mudstones but prior to complete lithification. (A) Folded siliceous (light-colored) and iron-silicate (darker) iron-formation overlain by a thin skin of ejecta containing accretionary lapilli (Bill Addison and Bevan French for scale). (B) Irregularly layered siliceous and iron-silicate mudstone cut by a mudstone clastic dike (darkest narrow feature running up-down in center of photo). (C) Folded and brecciated iron-formation in which the siliceous layer (light gray) is attenuated and shattered in contrast to the enclosing iron-silicate mudstone that is ductily folded.

Figure 12.

Outcrop photographs of soft-sediment deformation in the ejecta-absent facies of SIL, locally overlain by ejecta, and demonstrating that deformation occurred during and after silicification of mudstones but prior to complete lithification. (A) Folded siliceous (light-colored) and iron-silicate (darker) iron-formation overlain by a thin skin of ejecta containing accretionary lapilli (Bill Addison and Bevan French for scale). (B) Irregularly layered siliceous and iron-silicate mudstone cut by a mudstone clastic dike (darkest narrow feature running up-down in center of photo). (C) Folded and brecciated iron-formation in which the siliceous layer (light gray) is attenuated and shattered in contrast to the enclosing iron-silicate mudstone that is ductily folded.

Figure 13.

(A) Photo and graphic sedimentological analysis of Stop 5. Black angular polygons represent fragments of iron-formation. White box shows approximate location of photo B. (B) Close-up view of lapillistone containing entrained fragment of layered gritstone.

Figure 13.

(A) Photo and graphic sedimentological analysis of Stop 5. Black angular polygons represent fragments of iron-formation. White box shows approximate location of photo B. (B) Close-up view of lapillistone containing entrained fragment of layered gritstone.

Figure 14.

(A) Megabreccia, sharply overlain by mesobreccia and other ejecta. (B) Layers composed of accretionary pellets, small lapilli, and inferred relict spherules.

Figure 14.

(A) Megabreccia, sharply overlain by mesobreccia and other ejecta. (B) Layers composed of accretionary pellets, small lapilli, and inferred relict spherules.

Figure 15.

Stratigraphie section of units present at Stop 7. From Addison et al. (2010).

Figure 15.

Stratigraphie section of units present at Stop 7. From Addison et al. (2010).

Figure 16.

Outcrop features at Stop 8. (A) Stromatolites composed of chlorite and calcite cement in plan view, surrounded by fine- and coarse-grained grainstone. (B) Ripped-up boulders of "limestone" in ejecta layer. (C) Matrix composed of devitrified, vesicular glass with calcite cement.

Figure 16.

Outcrop features at Stop 8. (A) Stromatolites composed of chlorite and calcite cement in plan view, surrounded by fine- and coarse-grained grainstone. (B) Ripped-up boulders of "limestone" in ejecta layer. (C) Matrix composed of devitrified, vesicular glass with calcite cement.

Figure 17.

Scale card on a slab of limestone with an orange-stained, upside-down stromatolite to the right and slightly below the card. A large chert boulder is above the stromatolite.

Figure 17.

Scale card on a slab of limestone with an orange-stained, upside-down stromatolite to the right and slightly below the card. A large chert boulder is above the stromatolite.

Figure 18.

Photographs of cut slabs from Stop 12. (A) Erosively scoured bedrock composed of Gunflint carbonate and chert overlain by the impact deposit. Very coarse grained sandstone with granules fills the scour. Coin is 19 mm. (B) Another example of a scoured depression filled by a remnant of granule-rich sandstone. Note that the very coarse grained, granule-rich sandstone in the scour has a different grain size than the matrix of the overlying lapilli-rich bed. (C) Truncated top of a hemispherical stromatolite, overlain by a layer of very coarse grained sandstone, which in turn is overlain by accretionary lapilli in clast-support. Slab courtesy of W. Addison. (D) Lapilli directly overlying Gunflint carbonate bedrock. (E) Large block of underlying bedrock encased in accretionary lapilli-rich ejecta. (F) Coarse-grained sandstone, which lies above the more massive, accretionary lapilli-rich bed (not present in this slab). Note that the stringers of lapilli are associated with and overlain by coarser sand-size material than that constituting the underlying layer.

Figure 18.

Photographs of cut slabs from Stop 12. (A) Erosively scoured bedrock composed of Gunflint carbonate and chert overlain by the impact deposit. Very coarse grained sandstone with granules fills the scour. Coin is 19 mm. (B) Another example of a scoured depression filled by a remnant of granule-rich sandstone. Note that the very coarse grained, granule-rich sandstone in the scour has a different grain size than the matrix of the overlying lapilli-rich bed. (C) Truncated top of a hemispherical stromatolite, overlain by a layer of very coarse grained sandstone, which in turn is overlain by accretionary lapilli in clast-support. Slab courtesy of W. Addison. (D) Lapilli directly overlying Gunflint carbonate bedrock. (E) Large block of underlying bedrock encased in accretionary lapilli-rich ejecta. (F) Coarse-grained sandstone, which lies above the more massive, accretionary lapilli-rich bed (not present in this slab). Note that the stringers of lapilli are associated with and overlain by coarser sand-size material than that constituting the underlying layer.

Figure 19.

Core drilled through the ejecta layer ∼200 m north of Stop 12.

Figure 19.

Core drilled through the ejecta layer ∼200 m north of Stop 12.

EventApproximate arrival time
(1) Fireball∼13 s (modern equivalent of 3rd degree burns)
(2) Earthquake∼2–3 min (10.9–13 at epicenter)
(3) Ejecta ground surge∼5–10 min (predicts ejecta 1–3 m thick, grain sizes ∼1 cm)
(4) Air blast∼40 min (sonic boom)
(5) Tsunami∼1–3 h (speculation—arrival and effects dependent on basin bathymetry and pre-impact position relative to strand line, which are difficult to establish)
EventApproximate arrival time
(1) Fireball∼13 s (modern equivalent of 3rd degree burns)
(2) Earthquake∼2–3 min (10.9–13 at epicenter)
(3) Ejecta ground surge∼5–10 min (predicts ejecta 1–3 m thick, grain sizes ∼1 cm)
(4) Air blast∼40 min (sonic boom)
(5) Tsunami∼1–3 h (speculation—arrival and effects dependent on basin bathymetry and pre-impact position relative to strand line, which are difficult to establish)

Contents

References

References Cited

Addison
,
W.D.
Brumpton
,
G.R.
Vallini
,
D.A.
McNaughton
,
N.J.
Davis
,
D.W.
Kissin
,
S.A.
Fralick
,
P.W.
Hammond
,
A.L.
,
2005
,
Discovery of distal ejecta from the 1850 Ma Sudbury impact event
:
Geology
 , v.
33
, p.
193
196
,
Addison
,
W.D.
Brumpton
,
G.R.
Davis
,
D.W.
Fralick
,
P.W.
Kissin
,
S.A.
,
2010
,
Debrisites from the Sudbury impact event in Ontario, north of Lake Superior, and a new age constraint: Are they base-surge deposits or tsunami deposits?
, in
Gibson
,
R.L.
Reimold
,
W.U.
, eds.,
Large Meteorite Impacts and Planetary Evolution IV: Geological Society of America Special Paper 465
 , p.
245
268
.
Barnouin-Jha
,
O.S.
Baloga
,
S.
Glaze
,
L.
,
2005
,
Comparing landslides to fluidized crater ejecta on Mars
:
Journal of Geophysical Research
 , v.
E04010
,
Branney
,
M.J.
Brown
,
R.J.
,
2011
,
Impactoclastic density current emplacement of terrestrial meteorite-impact ejecta and the formation of dust pellets and accretionary lapilli: Evidence from Stac Fada, Scotland
:
Journal of Geology
 , v.
119
, p.
275
292
,
Broderick
,
T.M.
,
1920
,
Economic geology and stratigraphy in the Gun-flint iron district, Minnesota
:
Economic Geology and the Bulletin of the Society of Economic Geologists
 , v.
15
, p.
422
452
,
Cannon
,
W.F.
Schulz
,
K.J.
Horton
,
J.W.
, Jr.
Kring
,
D.A.
,
2010
,
The Sudbury impact layer in the Paleoproterozoic iron ranges of northern Michigan, USA: Lake Superior iron ranges: A time-line from the heavens
:
Institute of Lake Superior Geology
,
Annual Meeting, 53rd, Proceedings
 , v.
53
,
pt. 1
, p.
20
21
.
Collins
,
G.S.
Melosh
,
J.H.
Marcus
,
R.A.
,
2005
,
Earth impact effects program: A web-based computer program for calculating the regional environmental consequences of a meteoroid impact on Earth
:
Meteoritics and Planetary Science
 , v.
40
, p.
817
840
().
Davis
,
D.W.
,
2008
,
Sub-million-year age resolution of Precambrian igneous events by thermal extraction–thermal ionization mass spectrometer Pb dating of zircon: Application to crystallization of the Sudbury impact melt sheet
:
Geology
 , v.
36
, p.
383
396
,
Dietz
,
R.S.
,
1964
,
The Sudbury structure as an astrobleme
:
Journal of Geology
 , v.
72
, p.
412
434
,
Floran
,
R.J.
Papike
,
J.J.
,
1978
,
Mineralogy and petrology of the Gunflint Iron Formation, Minnesota-Ontario
:
Journal of Petrology
 , v.
19
, p.
215
288
.
Fralick
,
P.W.
Barrett
,
T.J.
,
1995
,
Depositional controls on iron formation associations in Canada
, in
Plint
,
A.G.
, ed.,
Sedimentary Facies Analysis
 :
International Association of Sedimentologists
Special Publication 22
, p.
137
156
.
Fralick
,
P.W.
Burton
,
J.
,
2008
,
Geochemistry of the Paleoproterozoic Gunflint Formation carbonate: Implications for early hydrosphere-atmosphere evolution
:
Geochimica et Cosmochimica Acta
 , v.
72
, p.
A280
.
Fralick
,
P.W.
Davis
,
D.W.
Kissin
,
S.A.
,
2002
,
The age of the Gunflint Formation, Ontario, Canada: Single zircon U-Pb age determinations from reworked volcanic ash
:
Canadian Journal of Earth Sciences
 , v.
39
, p.
1085
1091
,
Fralick
,
P.W.
Grotzinger
,
J.
Edgar
,
L.
,
2011
,
Possible recognition of accre-tionary lapilli in distal impact deposits on Mars: A facies analog provided by the 1.85 Ga Sudbury impact deposit
, in
Grotzinger
,
J.
Milliken
,
R.
, eds.,
Martian Sedimentology: Journal of Sedimentary Research Special Publication
  (
in press
).
French
,
B.M.
,
1967
,
Sudbury structure, Ontario: Some petrographic evidence for origin by meteor impact
:
Science
 , v.
156
, p.
1094
1098
,
French
,
B.M.
Koeberl
,
C.
,
2010
,
The convincing identification of terrestrial meteorite impact structures
:
What works, what doesn't, and why: Earth-Science Reviews
 , v.
98
, p.
123
170
,
Gault
,
D.E.
Quaide
,
W.L.
Oberbeck
,
V.R.
,
1968
,
Impact cratering mechanics and structures
, in
French
,
B.M.
Short
,
N.M.
, eds.,
Shock Meta-morphism of Natural Materials
 :
Baltimore, Mono
, p.
87
99
.
Goodwin
,
A.M.
,
1956
,
Facies relationships in the Gunflint iron-formation
:
Economic Geology and the Bulletin of the Society of Economic Geologists
 , v.
51
, p.
565
595
,
Grant
,
J.A.
Rossman
,
P.I.
Grotzinger
,
J.P.
Milliken
,
R.E.
Tornabene
,
L.L.
McEwen
,
A.S.
Weitz
,
C.M.
Squyres
,
S.W.
Glotch
,
T.D.
Tomson
,
B.J.
,
2008
,
Hirise imaging of impact megabreccia and sub-meter aqueous strata in Holden Crater
:
Marine Geology
 , v.
36
, p.
195
198
.
Grieve
,
R.A.F.
Riemold
,
W.U.
Morgan
,
J.
Riller
,
U.
Pilkington
,
M.
,
2008
,
Observations and interpretations at Vredefort, Sudbury, and Chicxulub: Towards an empirical model of terrestrial impact basin formation
:
Meteoritics & Planetary Science
 , v.
43
, p.
855
882
,
Heaman
,
L.M.
Easton
,
R.M.
Hart
,
T.R.
Hollings
,
C.A.
MacDonald
,
C.A.
Smyk
,
M.
,
2007
,
Further refinement to the timing of Mesoproterozoic magmatism, Lake Nipigon region, Ontario
:
Canadian Journal of Earth Sciences
 , v.
44
, p.
1055
1086
,
Henderson
,
P.
,
1982
,
Inorganic Geochemistry
 :
Elmsford, New York
,
Pergamon Press
,
353
p. [
analysis of sample from Orgueil meteorite from Table 1.3
].
Jannett
,
P.A.
Terry
,
D.O.
, Jr.
,
2008
,
Stratigraphic expression of a regionally extensive impactite within the Upper Cretaceous Fox Hills Formation of southwestern South Dakota
, in
Evans
,
K.R.
Horton
,
J.W.
, Jr.
King
,
D.T.
, Jr.
Morrow
,
J.R.
, eds.,
The Sedimentary Record of Meteorite Impacts: Geological Society of America Special Paper 437
 , p.
199
213
.
Jirsa
,
M.A.
,
2010
,
Stratigraphy of Sudbury "impactite" near Gunflint Lake, NE Minnesota
:
Institute of Lake Superior Geology
:
Annual Meeting, 56th, Proceedings and Abstracts
 , v.
56
,
pt. 1
, p.
31
32
.
Jirsa
,
M.A.
,
2011
,
Bedrock Geology of the Western Gunflint Trail Area, Northeastern Minnesota
:
Minnesota Geological Survey Miscellaneous Map M-191
 ,
scale 1:24,000, 1 sheet.
Jirsa
,
M.A.
Boerboom
,
T.J.
Chandler
,
V.
Mossler
,
J.M.
Runkel
,
A.C.
Setterholm
,
D.R.
,
2011
,
Geologic map of Minnesota, Bedrock Geology
:
Minnesota Geological Survey State Map Series S-21
 ,
scale 1:500,000, 1 sheet.
Kenkmann
,
T.
Schonian
,
F.
,
2006
,
Ries and Chicxulub: Impact craters on Earth provide insights for Martian ejecta blankets
:
Meteoritics & Planetary Science
 , v.
41
, p.
1587
1603
,
Koratev
,
R.L.
,
2009
,
http://meteorites.wustl.edu/goodstuff/ree-chon.html [recommended normalization #11, representing C1 carbonaceous chondrite]
.
Krogh
,
T.E.
Davis
,
T.W.
Corfu
,
F.
,
1984
,
Precise U-Pb zircon and bad-deleyite ages for the Sudbury area
, in
Pye
,
E.G.
Naldrett
,
A.J.
Giblin
,
P.E.
, eds.,
The Geology and Ore Deposits of the Sudbury Structure
 :
Ontario Geological Survey
Special Volume
1
, p.
431
446
.
Lafrance
,
B.
Legault
,
D.
Ames
,
D.E.
,
2008
,
The formation of the Sudbury breccia in the North Range of the Sudbury impact structure
:
Precambrian Research
 , v.
165
, p.
101
119
[
analysis of dioritic gneiss, Table 3, sample #02AV20
].
Melosh
,
H.J.
,
1989
,
Impact Cratering
:
A Geologic Process
 :
New York
,
Oxford University Press
,
245
p.
Mouginis-Mark
,
P.J.
Garbeil
,
H.
,
2007
,
Crater geometry and ejecta thickness of the Martian impact crater Tooting
:
Meteoritics & Planetary Science
 , v.
42
, p.
1615
1625
,
Oberbeck
,
V.R.
,
1975
,
The role of ballistic erosion and sedimentation in lunar stratigraphy
:
Reviews of Geophysics and Space Physics
 , v.
13
, p.
337
362
,
Osinski
,
G.
,
2006
,
Effect of volatiles and target lithology on the generation and emplacement of impact crater fill and ejecta deposits on Mars
:
Meteoritics & Planetary Science
 , v.
41
, p.
1571
1586
,
Pinto
,
J.A.
Warme
,
J.E.
,
2008
,
Alamo Event, Nevada: Crater stratigraphy and impact breccia realms
, in
Evans
,
K.R.
Horton
,
J.W.
, Jr.
King
,
D.T.
, Jr.
Morrow
,
J.R.
, eds.,
The Sedimentary Record of Meteorite Impacts: Geological Society of America Special Paper 437
 , p.
99
138
.
Pratt
,
B.R.
,
1998
,
Syneresis cracks: Subaqueous shrinkage in argillaceous sediments caused by earthquake-induced dewatering
:
Sedimentary Geology
 , v.
117
, p.
1
10
,
Pufahl
,
P.K.
Fralick
,
P.W.
,
2000
,
Depositional environments of the Paleo-proterozoic Gunflint Formation
:
Thunder Bay, Ontario
,
Institute of Lake Superior Geology
,
Annual Meeting, 46th, Proceedings, Field Trip Guidebook
 , v.
51
,
Field Trip 4, no pagination.
Pufahl
,
P.K.
Hiatt
,
E.E.
Stanley
,
C.R.
Morrow
,
J.R.
Nelson
,
G.J.
Edwards
,
C.T.
,
2007
,
Physical and chemical evidence of the 1850 Ma Sudbury impact event in the Baraga Group, Michigan
:
Geology
 , v.
35
, p.
827
830
.
Pye
,
E.G.
,
1963
,
Atikokan-Lakehead Sheet; Kenora, Rainy River, and Thunder Bay Districts
:
Ontario Geological Survey Geological Compilation Series Map 2065
 ,
scale 1:253,440, 1 sheet.
Rudnick
,
R.L.
Gao
,
S.M.
,
2009
, , p.
52
53
.
Saini-Eidukat
,
B.
Weiblen
,
P.W.
,
1996
,
A new method of fossil preparation, using high-voltage electric pulses
:
Curator
 , v.
39
, p.
139
144
,
Slack
,
J.F.
Cannon
,
W.F.
,
2009
,
Extraterrestrial demise of banded iron formations 1.85 billion years ago
:
Geology
 , v.
37
, p.
1011
1014
,
Southwick
,
D.L.
,
1991
,
On the genesis of Archean granite through two-stage melting of the Quetico accretionary prism to a transpressional plate boundary
:
Geological Society of America Bulletin
 , v.
103
[
analysis #2 on Table 2
, p.
1390
].
Vetrin
,
V.R.
Turkina
,
O.M.
Ludden
,
J.
,
2002
,
Petrology and geochemistry of rocks from the basement of the Pechenga paleorift
:
Russian Journal of Earth Science
 , v.
4
[analysis from Table 3, sample #4], http://rjes.wdcb.ru/v04/tje02085/tje02085.htm#flon20.
Wolff
,
J.F.
,
1917
,
Recent geologic developments on the Mesabi Iron Range, Minnesota
:
American Institute of Mining and Metallurgical Engineers Transactions
 , v.
56
, p.
229
257
.
www.unb.ca/passc/ImpactDatabase (and References therein)
Planetary and Space Science Centre
 ,
University of New Brunswick
,
Fredericton, New Brunswick, Canada
.

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