Figure 1. Three views of coarse ejecta samples, records of an ancient, unimaginably large catastrophe. The impact of a km-scale asteroid, 1,850 million years ago, on what is now the Sudbury basin of central Ontario, is the largest terrestrial impact for which evidence has been identified, with an original diameter (before deformation and erosion) of at least 300 km and probably slightly more (Bleeker and Kamo, 2022). That is, larger than the remnants of the Vredefort structure in South Africa, and larger than the Cretaceous-Tertiary (KT) impact in the region of the Yucatan peninsula of Mexico that ended the Reign of Reptiles and brought the dawn of the Age of Mammals. In this tale we will look at the origins of these samples, the immediate effect on the world, and the lasting legacy of the impact. Samples from the collection of Bill Addison. For photos of prepared, sawn surfaces, see also: "Rock of the Month 213, 2019".
These rocks appear to be formed largely of a granular ashy deposit. The third photo shows, on its left side, irregular fragments of chert, a black silica-rich sediment. The round bodies in the samples are called accretionary lapilli, typically 2 to 10 or 20 mm in diameter, deposited as layers of ash accreted about a solid nucleus, and most often formed in a wet volcanic eruption column (McPhie et al., 1993, pp.29-30; Jerram and Petford, 2011, pp.97-98,103; Tucker, 2011, pp.73-75). They are thus found in volcanic ashfall deposits, and also in impact-related deposits, such as the fallout from the Chicxulub (KT) impact (Pope et al, 1999). Lapilli occur in rocks of ages from Archean to Recent, in common volcanic types as well as more exotic forms, such as kimberlites associated with tuff rings. Well-documented examples occur in the Ordovician volcanics of the English Lake District (Moseley, 1983).
"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?
Structure Country Dia (km)
Age (Ma) Sudbury Canada 300 1850
Vredefort South Africa 300 2020
Chicxulub Mexico 180 __65 Nordlinger Ries Germany _25 __15
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.
Jerram,D and Petford,N (2011) The Field Description of Igneous Rocks. Wiley-Blackwell, 2nd edition, 238pp.
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.
Kissin,SA, Okamoto,M, Addison,WD and Brumpton,GR (2000) A possible Sudbury ejecta layer in the Gunflint Formation, northwestern Ontario. Abs. 46th Annual Meeting, Institute on Lake Superior Geology, vol. 46 part 1, 73pp., 31-32, Thunder Bay, ON.
Kring,DA, Horton,JW and Cannon,WF (2006) Discovery of the Sudbury impact layer in Michigan, USA. Meteoritics & Planetary Science 41, A100).
Krupp,R, Oberthur,T and Hirdes,W (1994) The early Precambrian atmosphere and hydrosphere: thermodynamic constraints from mineral deposits. Econ.Geol. 89, 1581-1598.
McPhie,J, Doyle,M and Allen,R (1993) Volcanic Textures: a Guide to the Interpretation of Textures in Volcanic Rocks. Centre for Ore Deposit and Exploration Studies, University of Tasmania, 198pp.
Moseley,F (1983) The volcanic rocks of the Lake District, a geological guide to the Central Fells. Macmillan, 111pp.
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.
Roussell,DH and Jansons,KJ (editors) (2002) The Physical Environment of the City of Greater Sudbury. OGS Spec.Vol. 6, 228pp. plus 3 maps.
Tucker,ME (2011) Sedimentary Rocks in the Field: a Practical Guide. Wiley-Blackwell, 4th edition, 276pp.
Visit the Turnstone "Rock of the Month" Archives!
Related Rocks of the Month include numbers 5 (violarite, Ni ore), 81 (sperrylite, Pt arsenide), 97 (massive sulphides), 115 and 190 (chalcopyrite-rich ores) from Sudbury, 75 (shatter cones, evidence of impact, preserved in the Sudbury host rocks and at other related structures), and 213 (lapilli tuff, Kakabeka Falls, from the Sudbury event).