"Rock of the Month #159, posted for September 2014" ---

Coarse lapilli and related impact-derived ash-fall ejecta, northwest Ontario

1. A Bad Day, Long Ago

Dated at 1,850 million years ago, the impact occurred long after the majority of major collisions in the solar system. Most of these, as recorded on the surface of our Moon and other airless worlds, occurred in the first 15% of solar system history. The Sudbury and Chicxulub (KT) events were later, some 60% and 98.5% into the current age of the Sun and its retinue of planets and smaller bodies, such as asteroids and comets.

The impact's effects were simultaneously local and worldwide, transient and lasting in nature. Because of subsequent erosion, the fallout from the impact is not preserved in the strata near Sudbury, but have in recent years been discovered hundreds of km away, in northwest Ontario and in northern reaches of the three Great Lakes states adjacent to Lake Superior.

The discovery of the ejecta layer was a major accomplishment for two amateur geologists, Bill Addison and Greg Brumpton, and professional colleagues who joined them on their quest (e.g., Kissin et al., 2000). Further aspects of the discovery of Sudbury ejecta west and south from the impact point, across northern Michigan, Minnesota and Ontario have been published widely in the past decade, especially in the annals of the Institute on Lake Superior Geology (ILSG). It is fitting that, in 2010, Addison and Brumpton were jointly awarded the "Goldich Medal", the highest recognition bestowed by ILSG for service in advancing the understanding of the geology of the region.

The Thunder Bay area is some 660 km from the impact site. The local Gunflint chert is noted for a range of microfossil remains, most notably the colonial algal structures known as stromatolites (Addison and Brumpton, 2012). Eight localities displaying the Sudbury impact layer have now been recognized in and near the city of Thunder Bay. Stromatolites occur at most localities, unconformably overlain by the detritus of the `debrisites'. The stromatolites would have formed near-shore, and there is some evidence for a paleosol, indicative of subaerial conditions over part of the area (Addison et al., 2009, 2012). An earthquake, estimated to have arrived some 2 minutes after the impact began, fractured the local Gunflint strata. Accretionary lapilli formed, and a fluidized base surge may have allowed fining upwards within the debris pile, which included some (now-devitrified) vesicular glass clasts, which formed from molten rock. There is however no clear evidence for previously-postulated tsunami deposits. In northern Michigan the Sudbury impact, some 550 km to the east, produced a modelled magnitude 10.5 earthquake, about 30 times stronger than any recently recorded earthquake. The first seismic waves would have arrived about 1.5 minutes after the impact, and caused slumping in the sedimentary sequence. About four minutes later the first airborne ejecta arrived. Massive tsunami waves should have arrived about an hour later, but this is not obvious in the deep water sediments, which do not readily preserve lapilli (Cannon et al., 2013).

A layer 25 to 70 cm thick is identified near the contact of the Gunflint iron formation and overlying Rove Formation, and between the Biwabik iron formation and overlying Virginia Formation in nearby Minnesota. The layer contains accretionary lapilli, shocked quartz and feldspar. Zircon crystals from nearby tuffaceous horizons bracket the deposits between 1878 and 1836 Ma in age (the Sudbury event occurred 1850±1 Ma, 650 to 875 km to the east (Addison et al., 2005). Impact models suggest that ejecta, largely target rock, would follow a ballistic trajectory to about 5 times the estimated 130 km crater diameter, i.e., to 650 km distance from the impact centre. Most known occurrences in Ontario and the northern US states of Michigan, Wisconsin and Minnesota lie within this distance (Cannon and Addison, 2007). Based on the Thunder Bay area, clues to the likely presence of the ejecta deposits include 1) unmetamorphosed rock circa 1850 Ma; 2) black chert breccia; 3) cm- to m-scale clasts; 4) bedded lapilli above accretionary lapilli and 5) devitrified glass (Addison et al., 2007; Fralick et al., 2010; Karman and Fralick, 2013).

2. Further Finds in the Great Lakes States

Accretionary lapilli were found in a layer 1-2 m thick in the lower 10 m of the Michigamme Formation in northern Michigan (Kring et al., 2006; Cannon et al., 2006). At the McClure locality, west of the town of Marquette in the Upper Peninsula of Michigan, there is a thick sequence of sandstones and breccia, the latter with chert clasts in a groundmass containing glass shards, all deposited on older banded iron formation and overlain in turn by black shales, all preserved in the Proterozoic outlier of the Dead River basin. This is the best-exposed site of the Sudbury ejecta layer in Michigan, and at some 500 km the closest known to the impact site (Cannon, 2008). The history of the Penokean foreland basin is recorded in the early Proterozoic rocks of the Animikie Group and Marquette Range Supergroup. The Sudbury impact layer provides a unique stratigraphic marker for reconstructing the architecture of the basin. At the time of the impact, the northern margin of the basin, near Thunder Bay, was very shallow to subaerial, with algal stromatolites. In Minnesota conditions were different, with a deepening basin recorded at Gunflint Lake and along the Mesabi Range. Banded iron formations (BIF) underlie the impact layer, which was succeeded by black shale of the Virginia and Rove Formations. Curiously, the western Marquette Range contains the only BIF above the Sudbury ejecta layer (Cannon and Schulz, 2009). The Hiawatha greywacke is a coarse clastic unit, including breccias with clasts as large as 1 m in diameter. Long considered a submarine slump and possible marker of an ancient earthquake, it is now viewed as the southernmost and deepest-water known occurrence of the Sudbury ejecta layer. It is derived in large part from the underlying Riverton BIF. The Hiawatha is the deep-water equivalent of the Gunflint Trail impactite in Minnesota (Cannon et al., 2013).

3. Aftermath

The immediate effects of the impact are easy to enumerate: shock wave, earthquake, fallout, tsunami, etc. A rocky body roughly the size of Mount Everest, travelling at a velocity of many kilometres per second, would have caused the superheated melting of tens of thousands of cubic kilometres of rock, at a temperature approaching 2000°C. The original crater, a multi-ring basin excavated by the impact, may have been about 300-330 km in diameter. But, what happened after the dust settled?

A. The Sudbury igneous complex and mineral deposits. Some have speculated that the impactor contained metals that are mined today, but there is a more plausible explanation for the phenomenal mines of the Sudbury camp. The region already contained a belt of gabbroic igneous bodies enriched in nickel, copper and platinum-group elements (PGE), as well as associated volcanic rocks with appreciable background levels of Cu and Ni. The East Bull Lake intrusive suite, trending E.N.E. for about 250 km in the vicinity of the Sudbury basin, is dated at 2480 Ma, and includes mineralized bodies such as the East Bull Lake and River Valley intrusives (Easton, 2003; Easton et al., 2010). It is likely that such rocks contained sulphide-bearing metal-enriched rock that was sampled and concentrated by the awesome energy released by the impact. The orebodies at Sudbury include a variety of disseminated to massive sulphide orebodies, including super-rich veins in the footwall of the igneous complex. A number of ore minerals of platinum and palladium were first described in samples from Sudbury, including sperrylite, froodite, michenerite and sudburyite.

B. The Sudbury basin. Sudbury, Vredefort and Chicxulub have been recognized as three terrestrial examples of multi-ring basins. After the impact strucure cooled, it gradually became filled with three sedimentary formations, extending west under what are now the western suburbs of Sudbury. The original basin structure was modified by later mountain-building episodes, including the Grenville orogeny.

C. Banded iron formations. In the Archean and early Proterozoic periods, huge deposits of iron were deposited in chemical sediments with variable content of silicate, oxide, carbonate and sulphide minerals. Perhaps coincidentally, these deposits, still mined on a huge scale today in Western Australia and elsewhere, disappear from the geological record at about 1800 Ma, soon after the Sudbury event. There is a body of evidence on the gradual chemical evolution of the Earth's atmosphere, which in the earlier eons would have been quite unsuited to higher lifeforms. The atmosphere may have been essentially oxygen-free until 2350 Ma, but in the next 250 million years the O content shot up with the inception of photosynthesis (Krupp et al., 1994).

D. The mining legacy. The Sudbury mining camp, working deposits first discovered in the late 19th century, is a world-class source of Ni, Cu, PGE and other elements The scale of operations is huge. Across more than 130 years, Sudbury continues to yield new surprises, new discoveries. Past production and resources in the camp were estimated (Keays and Lightfoot, 1999) to exceed 1,548 million tonnes of ore grading on average 1.2% Ni, 1.1% Cu, 0.4 ppm Pt and 0.4 ppm Pd. Today, Sudbury is the largest city in central and northern Ontario, and has been a major industrial centre and focus of mining and metal refining for well over 100 years. The initial workings, in the style of the Industrial Revolution, did not consider the environmental costs of the operations, but since the 1960s the city of Sudbury has seen great progress in cutting harmful emissions, "re-greening" blackened, deforested hillsides, and bringing life back to local lakes (Roussell and Jansons, 2002).

E. Science and Technology. The unique conjunction of this massive impact structure with complex regional geology, and the mines that sprang up long after, have spawned a vast amount of research and further discoveries in both pure and applied science. More than 1.400 records in the MINLIB bibliographic database refer to Sudbury. Geology, mineralogy, planetary science, mining and metallurgy, environmental science and urban planning have all made great strides, in a relative blink of an eye, 1,850 million years after one very bad day in the Proterozoic.

References

Addison,WD and Brumpton,GR (2012) Sudbury impactoclastic debrisites at Thunder Bay. Institute on Lake Superior Geology, volume 58 part 2, 219pp., trip 1&13, 2-26, Thunder Bay, ON.

Addison,WD, Brumpton,GR, Fralick,PW and Kissin,SA (2009) The complex Gunflint-Rove formations boundary at Thunder Bay, Ontario: two disconformities and a base surge debrisite. Abs. 55th Annual Meeting, Institute on Lake Superior Geology, vol.55 part 1, 83pp., 1-2, Ely, MN.

Addison,WD, Brumpton,GR, Vallini,DA, McNaughton,NJ, Davis,DW, Kissin,SA, Fralick,PW and Hammond,AL (2005) Discovery of distal ejecta from the 1850 Ma Sudbury impact event. Geology 33 no.3, 193-196.

Addison,WD, Cannon,WF and Brumpton,GR (2007) How to identify Sudbury impact ejecta in the Lake Superior region. Abs. 53rd Annual Meeting, Institute on Lake Superior Geology, vol.53 part 1, 89pp., 1-2, Lutsen, MN.

Bleeker,W and Kamo,S (2022) The Sudbury structure, Earth's largest (partially) preserved impact crater - a review. Abs. 68th Annual Meeting, Institute on Lake Superior Geology, vol.68 part 1, 55pp., 5-6, Sudbury, Ontario.

Cannon,WF (2008) The Sudbury impact layer at the McClure site. Institute on Lake Superior Geology, volume 54 part 2, 199pp., trip 5, 115-126, Marquette, MI.

Cannon,WF and Addison,WD (2007) The Sudbury impact layer in the Lake Superior iron ranges: a time-line from the heavens. Abs. 53rd Annual Meeting, Institute on Lake Superior Geology, vol.53 part 1, 89pp., 20-21, Lutsen, MN.

Cannon,WF and Schulz,KJ (2009) Reconstructing the Penokean foreland basin using the timeline of the 1850 Ma Sudbury impact layer. Abs. 55th Annual Meeting, Institute on Lake Superior Geology, vol.55 part 1, 83pp., 9-10, Ely, MN.

Cannon,WF, Horton,JW and Kring,DA (2006) The Sudbury impact layer in the Marquette Range Supergroup of Michigan. Abs. 52nd Annual Meeting, Institute on Lake Superior Geology, vol.52 part 1, 72pp., 10-11, Sault Ste. Marie, ON.

Cannon,WF, Woodruff,LG and Schulz,KJ (2013) The Hiawatha graywacke of the Iron River-Crystal Falls district, Michigan: a megaturbidite triggered by seismicity related to the 1850 Ma Sudbury impact. Abs. 59th Annual Meeting, Institute on Lake Superior Geology, vol.59 part 1, 83pp., 14-15, Houghton, MI.

Easton,RM (2003) Geology and mineral potential of the Paleoproterozoic River Valley intrusion and related rocks, Grenville province. OGS OFR 6123, 172pp.

Easton,RM, James,RS and Jobin-Bevans,LS (2010) Geological Guidebook to the Paleoproterozoic East Bull Lake Intrusive Suite Plutons at East Bull Lake, Agnew Lake and River Valley, Ontario: a field trip for the 11th International Platinum Symposium. 11th International Platinum Symposium, Sudbury, field trip guide B3. OGS OFR 6253, 108pp.

Fralick,P, Brumpton,GR, Jirsa,MA, Kissin,SA and Severson,MJ (2010) Sedimentology of the Paleoproterozoic ejecta layer from the Sudbury impact event. Abs. 56th Annual Meeting, Institute on Lake Superior Geology, vol.56 part 1, 76pp., 18-19, International Falls, MN.

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Karman,MM and Fralick,PW (2013) Sedimentology and paleographic reconstruction of the strata adjacent to the Sudbury impact layer in a cored drillhole. Abs. 59th Annual Meeting, Institute on Lake Superior Geology, vol.59 part 1, 83pp., 43-44, Houghton, MI.

Keays,RR and Lightfoot,PC (1999) The role of meteorite impact, source rocks, protores and mafic magmas in the genesis of the Sudbury Ni-Cu-PGE sulphide ore deposits. In `Dynamic Processes in Magmatic Ore Deposits and their Application to Mineral Exploration' (Keays,RR, Lesher,CM, Lightfoot,PC and Farrow,CEG editors), GAC Short Course Notes 13, 477pp., 329-366.

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Pope,KO, Ocampo,AC, Fischer,AG, Alvarez,W, Fouke,BW, Webster,CL, Vega,FJ, Smit,J, Fritsche,AE and Claeys,P (1999) Chicxulub impact ejecta from Albion Island, Belize. Earth Planet.Sci.Letts. 170, 351-364.

Pye,EG, Naldrett,AJ and Giblin,PE (editors) (1984) The Geology and Ore Deposits of the Sudbury Structure. OGS Spec.Vol.1, 603pp.

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Tucker,ME (2011) Sedimentary Rocks in the Field: a Practical Guide. Wiley-Blackwell, 4th edition, 276pp.