TECTONICS BLOG Rev. 2025-03-02
Gregory Charles Herman, PhD   Flemington, New Jersey, USA

Structural and geophysical aspects of some known and suspected astroblemes of the Great Lakes region, North America

A geology report with maps, cross sections, field photographs and structural records from the Sudbury, Slate Island, and Wanapitei astroblemes in Ontario Canada leading to a tectonic interpretation of the Great Lakes region invoking impact tectogenesis, including a suspected large-bolide impact event in the Ohio River basin of early Grenville age.

Click on a picture for more detail.  Figure 1. Google Earth (GE) displays of Ontario known and suspected multi-ring astroblemes using 4 geospatial themes including (B) the bedrock geology of Canada and the USA, (C) Global magnetics, (D) Global Bouguer gravity anomalies and (E) the NASA ETOPO1 global physiography. The locations of the known Sudbury, Slate Island, and Wanapitei craters and the suspected Lake Nipigon strewn field circled at varying radii for mapping far-field strain effects. The down-range headings of each impact are mapped using the thick yellow lines. Note the location of the Lake Nipigon igneous suite where the leading crater of a suspected impact strewn field is located.   Figure 2. Time line and stratigraphic column summarizing the relative ages of the Ontario astroblemes in relation to the bedrock geology and some other large impacts and igneous provinces. Crater sizes are scaled at 1 in. = 300 km. Figure 3. A summary of outcrop locations, photographs, and structural readings taken during the excursion. Figure 4. Nineteen photographic contact sheets for the set of Ontario photos. Click on an image to see the file names. Access a photo by typing in a file name using the following filename prefix: www.impacttectonics.org/2024/Ontario/. For example: www.impacttectonics.org/2024/Ontario/sulfidelacing.jpg Table 1. Location, size, and age parameters for the known and suspected impact craters addressed in this work. Figure 5. The Sudbury and Wanapitei Lake astroblemes in GE using three spatial themes including the (A) bedrock geology (ref), (B) global magnetics (ref) and (C) a Space Shuttle Radar Tomography (SRTM) digital terrain model. Figure 6. SRTM image of the Sudbury and Wanapitei Lake astroblemes. Interpreted own-range headings traced with yellow lines. Field stops and roads taken highlighted along with the axial traces of anticlines (red polylines) and syncline (blue polylines) . Figure 7.  GE bedrock geology map of the Sudbury basin. The roads traveled, field stops, bedrock units and faults, copper mines, and mafic dikes associated with the basal, mafic melt layer and superjacent granophyre.   Figure 8. GE maps using a 100-km scale to summarize the geological expression of the Slate Islands astrobleme. A. Features mapped on a standard GE satellite image. B. Bedrock geology. The central peak of Slate Islands lies close to the Terrace Bay shoreline along the head of Lake Superior. The roads traveled, field stops, mapped bedrock faults, copper mines, and seismic-reflection surveys in Lake Superior are labeled. Concentric rings around Slate Islands and the suspected impact crater at Lake Nipigon to the North are labeled using radii noted in kilometers (km).  Interpreted down-range heads are mapped for each impact crater. Please note that the Lake Nipigon one appears to be one of many in a strewn field extending southward (fig. 1). Figure 9. GE base imagery (left) and a registered SRTM image covering the Slate Islands impact area along the north shore Of Lake Superior. Down-range headings for the Slate Island's and suspected Lake Nipigon impact craters are accompanied by line tracings of magnetic anomies and seismic-reflection lines in the Lake Superior. White lines trace interpreted faults intersecting ground surface and the circular histogram summarizes their strike orientations using 10o bins. Figure  10. Dave Buthman points to one of the many faults cutting rocks along the shoreline of Terrace Bay. We collected a combined set of 28 fault readings along a shoreline traverse. A dominant fault set strikes sub parallel to the interpreted down0range heading (SW) and dip away from the Slate Islands central uplift. The assortment of faults having variable slip directions cutting the Mesoproterozoic basalt was highly varied and complex. Fault orientations and dip azimuths are summarized with the lower-hemisphere equal-angle stereographic projection (above) and a circular histogram (below, using 10o bins).  The rocks have been multiply deformed with presumed Paleozoic, brittle-ductile faults offsetting earlier, copper-mineralized veins with autoclastic breccia of probable Keweenewan age (lower right). Figure 11. A SketchUp Pro model of the Slate Islands impact crater was made using the GLIMPCE seismic-reflection line A-A'. The line runs due south from the crater and is mirrored on the other half to complete the structural model. The assumption that the structure is symmetric as portrayed is likely improbable but nevertheless is built to see if the structures imaged beneath Lake Superior can be extended onto land.   Figure 12. A suspected large-meteorite strewn field may be headed up by a crater lying beneath the Lake Nipigon basin in Ontario. A set of aligned geophysical anomalies stretch from lake Nipigon  to the south end of Lake Michigan. This event may have stemmed from a fragmented, larger body or entered the atmosphere as a clustered set of projectiles. It's alignment and association with Keweenewan age igneous activity suggests that the enigmatic mid-continental rift may have originated from impact tectogenesis   Figure 13. STOP 30 folds part of the upturned rim of the transient crater at 28 km (~17 mi.) distance from the Slate Islands central uplift. The outcrop holds sub vertical layering including sills of Keweenewan age that are sheared with a sense of western displacement. Figure 14. Geological map centered on the Great Lakes region of North America summarizing the distribution and form of high-intensity magnetic and gravity anomalies thought to reflect mid-continental rifting. Cross sections A and B included below in figure 15. Figure 15. Interpretive cross sections constructed across the southernmost part of the Canadian shield (top) and the mid-continental rift system (bottom), Figure 16. A series of maps centered on the suspected Columbus-Grenville 1 astrobleme. Figure 17. Structural framework of the suspected Columbus-Grenville 1 astrobleme. Figure 18. The high-intensity band of the EMAG2.0 vers. 3.0 Earth magnetics theme was extracted and augmented with labeled tectonic elements, large impact craters, and cross section trace A-A' (fig. 15).

Data and methods * Sudbury and Wanapitei Lake astroblemes  * The Slate Islands Astrobleme and suspected Lake Nipigon - Superior - Michigan strewn field * The suspected Columbus-Grenville 1 astrobleme * Summary * References * Google Earth KMZ file (4.9 MB)

Introduction

More recently in 2021, David Buthman contacted me after visiting impacttectonics.org to share his similar views on impact tectogenesis that he has realized from decades of hydrocarbon exploration and his recognition of far-field structural effects around large impact craters that come to bear on petroleum entrapment and commodity production (Buthman, 2022).  After vetting one another virtually over a year's time, and with continued interaction, we arranged to meet to explore the Nevada desert in 2023 to see the grounded strains of the elusive Alamo impact crater who's near-field effects have been confirmed despite having a firm solution to where the crater lies! This initial effort of studying large impact structures together in the field was an attempt to better understand their nature, reach, and significance. In retrospect, launching a combined effort starting with the Alamo impact was somewhat frustrating but exhilarating at the same time as we failed to find the crater but touched upon some amazing geology. The frustration stemmed from realizing that the crater central peak probably lies within military grounds with restricted access, so we had to focus solely on ITFF strains occurring just beyond the transient crater, of which there were many. Exhilaration came from gaining a new perspective over a week's time on the regional disruption within a central Nevada astrobleme.

Our second, collaborative field venture was formulated soon afterwards when we took aim at the Sudbury and Slate Island astroblemes of Ontario (fig. 1). The Sudbury lies at the northern head of the Georgian Bay to the east of Lake Huron and Slate Islands lies at the northern head of Lake Superior (fig. 1). Sudbury is the third largest and fifth oldest (~1085 Ma) known impact crater on Earth with an estimated transient crater of 139 km diameter and radiometric ages of about 1850 million years (Ma; Earth impact database, 2024).  The Slate Islands crater is ranked 27th in size having an estimated transient crater of 30 km diameter and is the 50th oldest known crater with radiometric ages constraining a 436-450 Ma age range (upper Ordovician to lower Silurian).  The Sudbury basin has been mapped in detail because of its high-grade nickel-copper ore bodies including platinum, silver, tin and gold that stoked Sudbury-area mining beginning in the late 19th century. The Slate Islands structure is comparatively less defined beyond the central uplift that forms the set of islands for which its named. It too has spatial association with iron, copper and gold mines in the region. We planned a trip for May 31 to June 7 and met in Toronto with the goals of exploring both structures over the course of a long week.  We met on the morning of May 31 near the Toronto airport and immediately drove North to Sudbury before hitting outcrops starting about noon. Following is an illustrated account of the preparation, field excursion, and findings leading to some unexpected conclusions and hypotheses about the physical nature of the Great Lakes basin.

Geological and geophysical data and analysis methods

The Sudbury and Wanapitei Lake astroblemes

The first three field days were spent visiting outcrops around the Sudbury and Wanapitei Lake craters that I had never seen before including shatter cones and a variety of thick-bedded impact-generated breccia that struck me with a sense of enormity and widespread disruption that was awe inspiring. During that time we logged 19 field stops along traverses across the Sudbury basin's western and eastern ends, the latter of which touched upon the smaller Wanapitei impact crater (figs. 1, 4, 5, and 6).  A local billboard there welcomes visitors to the Wahnapitae First Nation. But the name Wanapitei is used here in conformance with that listed in the EID and in GE Pro.

Dave had prepared the first day's itinerary based on some field guidebooks covering the area including those by Lightfoot and others (1997) and Rousell and Card (2009). The first outcrop that we visited within the Sudbury City limits has pervasive, nested sets of shatter cones (photos 0585, 586, 587, 588, 589) within Paleoproterozoic metasedimentary bedrock of the Huronian Supergroup. These bedrock units are elsewhere extensively brecciated and cemented with impact-generated melt (photos 0592, 593, 594, 595).  We also touched upon the the impact-generated melt sheet (figs. 4A, 4B, and 6) and fall-back breccia (photos 0606, 0608), both of which also carry post-impact, hydrothermal mineral veins and tectonic fractures. Nearby outcrops of the subjacent impact-injection breccia (photos 0610, 0611, 0612, 0613) preceded stops in the superjacent basin fill of sedimentary origin (photos 0597, 0598, 0599, 0600) that were once deeply buried and then tectonically compressed and compacted, uplifted and exhumed to reveal the astrobleme core structure today. There are probably similar, fractionated melt sheets associated with other large craters around the world, but they remain deeply buried beneath thick sediment blankets until regional tectonism may one day reveal their roots too. 

The elliptical geometry of the Sudbury basin partly reflects post-impact tectonic overprinting of the basin from compression directed subnormal to the current basin axis during the Grenville Orogenic cycle (fig. 2). The Grenville post dates the the Sudbury impact by about 700 Ma with the deformation front skirting the southern side of the Sudbury basin (fig. 4C). The line drawn along the Grenville front corresponds to a pronounced, linear magnetic low where crustal rocks have been structurally compounded and thickened from tectonic accretion along the southern margin of the Canadian shield (fig. 1C). The Grenville front angles acutely away from the Sudbury basin to the south and merges with the straight-line projection of the down-range heading line to the northeast. The front appears structurally complicated immediately south of the Sudbury basin from crustal compounding of previously impact-faulted bedrock lying within the southern, lateral blast sector of the Sudbury astrobleme where large crustal faults flare outward from the basin into adjacent bedrock to the north and south along with probable dike emplacement signaled by high-intensify magnetic anomalies (fig. 4). The southern blast sector of the Sudbury astrobleme was therefore first subject to mixed-mode faulting striking about normal to the basin axis resulting from the low-angle impact of less than 30^o obliquity based on the elliptical crater, and impact experimentation (Gault, D.E., and Wedekind, 1978). Crustal compounding of heterogeneous bedrock with shock-induced faults striking about normal to the subsequent shortening direction therefore results in a complex block-fault slip history  with complicated geological history and geophysical expressions. An example of complex structure exhibiting these traits occurs on Route 400 that was our last stop of the trip during the day 7 drive back to Toronto (photos 0627, 0628, 0629). It occurs about 150 km SSE from the Sudbury basin and exemplifies the structural complexity of multiply deformed Mesoproterozoic basement subject to at least two tectonic episodes. It is reasonable to assume that one was Sudbury and another Grenville; those tectonic events of which we have radiogenic ages for.  STOP 21 on day 4 located about 170 km to the west of Sudbury also held probable ITFF structures including massive stock-work veining with secondary mineralization including Bornite (photos 0649, 0650, 0651, 0652, 0653, 0654, 0655). The impact-generated crustal faults and dikes swarming laterally outward and away from the Sudbury basin (figs. 4, 5 and 6) extend for over 500 km and link with highly complex crustal structures elsewhere in the shield like the Kapaskasing fault zone and prior dike swarms. ITFF strains stemming from the Sudbury impact event therefore cover a map diameter exceeding 1000 km.

The Sudbury impact is thought to have succeeded the Penokean orogeny, a proposed orogenic event reported in the Lake Superior region where a volcanogenic arc was docked onto the Archean Superior craton just prior to impact and during a time when the region experienced major contraction (fig. 2; Schulz and Cannon, 2007). But sedimentation in the subsiding foreland began at 1850 Ma in the south and slightly later to the north correspond in age with the Sudbury impact, so the need for a Penokean orogeny is unnecessary if the recognized tectonism resulted from the Sudbury impact. Traditionally, the Grenville Orogeny is also thought to stem from arc collision accompanying tectonic plate drift, plate subduction and suturing of accreted Mesoproterozoic terrane onto the Superior craton (Stein and others, 2017). But as exemplified below, there are also alternative explanations for the Grenville tectonic cycle that hypothetically arise from impact tectogenesis. The Grenville front in this region was influenced by crustal heterogeneity imparted by the Sudbury impact and then structurally overprinted by the Wanapitei Lake crater. The manner in which structural interference is manifest is portrayed in figures 5 to 7, but more work is needed to better understand how the Eocene-aged Lake Wanapitei impact structurally modified the Sudbury basin and the bedrock shield down range where the SRTM data show areas of anomalous crustal reflectance within 100-km distance of the crater (fig. 5D).

The serendipitous alignment of the Wanapitei Lake impact crater immediately downrange of the Sudbury one has created overlapping strain fields that complicates the area's geology (figs. 4 and 5). The Wanapitei Lake astrobleme is younger (table 1) and one-order-of-magnitude smaller than Sudbury with a transient crater mapped at ~20 km diameter (table 1). It too holds ample shock evidence including localized suevites, shatter cones, and breccia (Dressler, 1982). We only stopped at a few places on the western side of the Lake where the two transient craters overlap. A recent road excavation exposed some shatter cones within a finely crystalline diorite (photos 0632, 0633, 0634, 0635) that may have resulted from the Wanapitei event, but assignment to a specific event is hypothetical as it falls within the area covered by both transient craters. Another stop in the overlap area held younger quartz veins overprinting an offsetting older ones within a low-grade metamorphic, subarkosic wasckestone near the base of the Huronian Supergroup (Mississagi Fm. photos 0637, 0638). The complex vein interplay demonstrates the need for detailed field mapping in order to understand how inherited structure here are systematically modified by younger episodes of impact tectogenesis. Over the course of those three days, the tapestry of structures that we saw in the Sudbury astrobleme are of such variety and magnitude that it left us somewhat stunned, but more familiar with the geological havoc arising within these energized structures.

On day 4 we arose earlier, grabbed breakfast and hit the road west and north to Wawa, ON, bringing us into the realm of the Slate Island astrobleme. Day 4 was thus spent driving over half of the ~700 km between Sudbury and Terrace Bay on the northern tip of Lake Superior (fig. 7). We planned to stop at a motel in Wawa by mid afternoon so that we could head out to find outcrops of the Firesand carbonatite located nearby (close to STOP 23 on figure 7). This intrusion is of Keweenewan age (1090 + 10 Ma; Symons, 1989) and forms a low hill within a forested area accessible by an unpaved and rutted dirt trail. Prior to this we had been conducting urban and suburban geology but this was rural; big-animal country. The trial was dry so we drove in as far as we could in our sedan, got netted and deated up to counter the black flies and mosquitos, but by the time we hiked to the hill, it was dusk and an imposing swamp situated between the trail and hill thwarted our attempt.  Sometimes plans just don't work out, so we headed back to the motel, nevertheless excited for what lay ahead.

The Slate Islands Astrobleme and suspected Lakes Nipigon-Superior-Michigan strewn field

Upon descending from the shield uplands into Terrace Bay, the sky was vivid blue with puffy white clouds being wind swept northward. It was a brilliant vista with the dark green evergreen forests rolling unbroken beneath the drifting clouds that altogether struck me with a sense of majesty. We didn't know at the time that this would be the last fair weather that we would see over the next few days as a fog bank set in shortly afterwards that led to periods of intermittent showers that hampered our field work somewhat over the next three days. Field work during days 5 through 8 left us damp but exhilarated by providing evidence of the transient crater reaching at least 120 km diameter based on the distributed pattern of crustal faulting and shatter structures. As mentioned above, the Slate Islands transient crater is officially listed at 30 km, but we found evidence making it at least four times larger than previously thought. This impact likely helped sculpt the Lake Superior physiography, overprints and partly remobilized older copper deposits of likely early Keweenewan age (~ 1100 Ma; fig. 2) and emplaced new polymetals in the area; especially down range to the west. We saw proof of this at many places around Terrace Bay with some road cuts providing amazing glimpses into the structural and hydrothermal processes operating near the edge of the transient crater. For example, a series of three road cuts (STOPS 30-32) just to the west of Terrace Bay on Route 17 hold a spectacular assemblage of impact-generated structures including very thick (>3), sulfide-laced pseudotachylyte showing dynamic metal emplacement accompanying shock-generated frictional melting (STOP 32 photos annotated below). These road cuts are about 28 km distance from the central uplift and testify to the manner in which crustal rocks are melted, plasticized, and broken within the transient crater from the impact's grounded energy. Structural details and photographs collected at these three stops are outlined and summarized below. Please follow the instructions provided in the caption of figure 4 to view the photographic details.

STOP 30 Archean granodiorite cut by younger granite and even younger (~1.10 Ga?) sills cut by pseudotachylyte w/ sulfides (photos 0691-0721).  This outcrop includes Archean intermediate rocks that are intruded with younger granite, both of which are cut by basic sills and dikes of presumed Keweenewan age (~110 Ma; fig. 2). The rocks are turned up on end with layering dipping near vertical (fig. 12). Relatively young, low-angle shear planes and faults with slip lineation cut all older structures and show western-directed reverse shearing, compounding and transport.

STOP 31 Complex quartz-calcite vein in sheared, sulfide-infused diorite. The lesser of the three series of outcrops nevertheless holds some intriguing features, among which is a thick calcite vein holding euhedral, milky quartz crystals in the medial area. Sulfide-infused, quartz-mineralized shear planes are also seen here that were a prelude to what we were about to see in splendor in the next set of outcrops likely corresponding to the rim of the transient crater.

STOP 32 Tectonic mélange with low-angle thrusting, massive pseudotachylyte and polymetal infusion. I was motivated to see the Ontario astroblemes in order to experience the suite of impact-generated structures including pseudotachylyte that I had only read about but hadn't never see before. We didn't recognize any in Nevada, and we saw some at Sudbury, but we were both somewhat startled to see it developed so profoundly here and holding evidence of polymetal infusion accompanying the sudden, frictional melting of the shield basement down range at 28 km distance from the central peak. This was the next to last outcrop sequence seen on day 6. It was so impressive that we decided to revisit in as part of our day 7 itinerary which was first aimed at seeing ITFF structures further west before backtracking and heading north to explore structures at around the 28 km radius there. Below is an annotated list of features photographed at STOP 32 by IMG number.

1. Hydrothermal blistering and polymetal infusion  0729, 0732, 0734, 0735, 0736, 0737, 0738

2. Sulfur-infused meso-scale shear zone and tectonic mélange 0730, 0732, 0733, 0739, 0742

3. Polymetal infused, massive pseudotachylyte showing ductile shearing   0769, 0770

4. Sulfur-infused meso-scale injection breccia 0740, 0741,

5. White, milky effluent seeping from the shear zone 0743

The furthest western reach of our excursion brought us near to where the a relatively young fault system striking parallel to the interpreted heading helps bracket the Slate Island central peak at 66 km distance when combined with its mirrored equivalent to the south (figs. 8 and 9). Together they exemplify axial splitting as part of the Slate Islands astrobleme surface expression. Rocks approaching the trace of the bracketing fault system are complexly sheared with thin pseudotachylyte planes cutting and offsetting both granite and younger trap rock of probable Keweenewan age (photos 0765, 0766). On a return trip to see STOP 32 again, we pulled over to photograph a new road cut that exposed a thicker pseudotachylyte shear zone and other uncovered structures that remain to be studied.

The Slate Islands impact is indeed profound, but visually and geologically subordinate to the Keweenewan-age structures in the region currently ascribed to mid-continental rifting (MCR; figs. 3, 4, and 11). But one problem portraying the MCR as a failed continental rift zone is that as such it would be the only known not to develop into an oceanic spreading center. The MCR is currently interpreted as continental rift system developed above a rising mantle plume resulting in the Nipigon large-igneous province with basic sill and dike emplacement that just ceased along with continued rifting at some point in time. Alternatively, if one looks at the physiographic expression of Lake Nipigon, and recognizes the probability of this ovoid basin being a melt-filled impact basin, then the MCR interpretation can be relaxed in favor of one involving many aligned impacts spread south across Lakes Superior and Michigan for which thee is ample geophysical evidence for to entertain this as a reasonable tectonic alternative to the current reigning hypotheses.

The prominent physiographic, geological, and geophysical expression of Lake Nipigon lines up with other suspected, buried impact craters stretching south over the Great Lakes region (figs. 1, 11, and 14). I suspect that mid-continental rifting stems from crustal strains imparted by many large impacts in the region including one abbreviated as LSNM for the suspected Lakes Nipigon-Superior-Michigan strewn field of early Keweenewan age (~ 1100 Ma). Rather than the enigmatic MCR resulting from an upwelling mantle plume randomly situated beneath the Canadian shield, it's possible that the continent interior was partially split by a clustered set of km-scale bolides impacting the shield obliquely along a southerly heading and spurring an associated large-igneous province (LIP). The fault geometry is compatible with an oblique event with a southern bearing headed up by the Lake Nipigon structure and stretching southward to Lake Michigan (figs. 1, 11, 14, and 15). A set of low-intensity magnetic anomies are aligned but slightly offset with the Lake Nipigon structure that together help define its pronounced geophysical expression that led to the popular MCR structural interpretation .  The suspected LSNM strewn field gives cause to the effect of interior  continental rifting that during the Mesoproterozoic from a bolide-impact episode involving multiple, or fragmented projectiles hitting and splitting the shield with resulting structures symmetrically disposed about its oblique, down-range heading (figs. 11, 14, and 15). However, if this is the case, then logically, it may not be the only impact event contributing to such an expansive, complex structure.

The suspected Columbus-Grenville 1 impact crater and astrobleme

Some cultural aspects and trip summary

Dave and I traveled to Ontario in order to gain a better understanding of the physical nature of astroblemes by visiting one of the biggest and oldest ones on Earth. Having  two sets of eyes trained on the same geological features enabled us to cover more ground and compare our characterizations. What we saw provided insights into the nature of some very old, very complex geological structures. But geological inquiry usually is expansive, that is, more often than not, when inspecting something closely, unexpected results oftentimes lead to more questions and ensuing mysteries.  Such is the case for this study which led to a reassessment of the Grenville tectonic setting. This was completely unexpected and quite serendipitous as I have been concurrently working on a detailed 3D model of the New Jersey Zinc Company ore deposit within the Highlands of Sussex County (location noted on fig. 18). That work details the geometry and nature of Grenville isoclinal folding in Mesoproterozoic marble hosting the zinc ore, and is the subject of my next blog. To imagine that a large-bolide impact in Ohio could have led to granulite ductility of the metamorphosed limestone previously formed during the rise of oceanic life on Earth is a fascinating prospect. But could that impact directly contributed to such complex structures lying 700 km to the east within a marginal blast sector? Could that happen? This hypothesis needs further testing. 

In addition to the splendid geology, I found Canada's culture rich and their people friendly. It's a societal blend similar to that in the USA with deep, indigenous roots overprinted by a colonial culture in a sparely populated region with many recent immigrants. I was particularly impressed by the extensive array of cairns perched atop bedrock outcrops that lined the trans-Canadian highways (photos 0805, 0806, 0824). A  waitress at dinner in Terrace Bay one evening explained that most of the cairns are  Inuksuks; anthropomorphic cairns erected by indigenous peoples for whatever reason. I suspect they're representative sentries erected to remind those travelling through the land of the original dwellers, or perhaps that we're being watched.  Below is sketch of a typical structure that she drew for us.

A second noteworthy cultural event occurred at breakfast on Day 4 when I experienced my first robotic waitress in a restaurant.  It wasn't as advanced as Rosie from The Jetsons, but nevertheless was programmed to autonomously trail our waitress around through the aisles wheeling plates of food stacked onto 'her' tray tiers.  She was named 'Kiwi' the Bellabot and we couldn't help but take note. I found it somewhat remarkable to first see such a technological adaption in this mining town.

To conclude, this Ontario excursion was the most fulfilling, exciting geological venture that I have participated in to date.  But Dave and I are planning a trip to South Africa this fall to evaluate tectonic aspects of the world's largest astrobleme, the Vredefort complex near Johannesburg. We expect that the splendor of that one will likely surpass this one. Please stay tuned.

References

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Sandwell, D. T., R. D. Muller, W. H. F. Smith, E. Garcia, R. Francis, 2014, New global marine gravity model from CryoSat-2 and Jason-1 reveals buried tectonic structure, Science, vol. 346, no. 6205, pp. 65-67, www.doi.10.1126/science.1258213.

Schruben, P. G., Arndt, R. E., and J. Bawiec, W. J., 1974, Geology of the Conterminous United States at 1:2,500,000 Scale — A Digital Representation of the 1974 P.B. King and H.M. Beikman Map: By  GIV display software by Russell A. Ambroziak; Abacas display software by Innovative Technologies Inc. (ITA); DIGITAL DATA SERIES 11, Release 2 

Schulz, K. J. and Cannon, W. F., 2007, The Penokean orogeny in the Lake Superior region: Precambrian research, vol. 157, iss. 1-4, p. 4-25. doi.org/10.1016/j.precamres.2007.02.022

Stein, C. A., Stein, S., Elling, R., Keller, G. R., and Kley, J., 2017, Is the "Grenville Front" in the central United States really the Midcontinent Rift?: GSA Today, v. 28, 7 p., doi.org/10.1130/GSATG357A.1.

Swedan, N. H. 2013: Energy of plate tectonics calculation and projection, Solid Earth Discussions,
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Symons, D. T. A., 1989, Age of the Firesand River carbonatite complex from paleomagnetism: Canadian Journal of Earth Sciences, vol 26., no. 11., 2401-2405, doi.org/10.1139/e89-205

Therriault, A. M., Fowler, A. D., and Grieve, R. A. F., 2002, The Sudbury Igneous Complex: A Differentiated Impact Melt Sheet: Economic Geology, vol. 97, no. 7, p. 1521–1540. doi.or/10.2113/gsecongeo.97.7.1521.

Wheeler, J.O., Hoffman, P.F., Card, K.D., Davidson, A., Sanford, B.V., Okulitch, A.V., and Roest, W.R. (comp.), 1997, Geological Map of Canada, Geological Survey of Canada, Map D1860A: By 1997: 

Zhou, H., Hu, J., Dal Zilio, L. et al. India–Eurasia convergence speed-up by passive-margin sediment subduction. Nature 635, 114–120 (2024). www.doi.org/10.1038/s41586-024-08069-6