Minnesota’s Evidence of an Ancient Meteorite Impact

Imagine central North America nearly 2 billion years ago, as a
meteorite 10 miles in diameter strikes the Earth near what is now
Sudbury, Ontario . The force of the collision vaporizes the meteorite and
much of the ground near the impact site, forming a crater more than 150 miles wide(1).
Shock waves race from the impact, deforming the Earth’s crust around the crater’s edge,
and causing earthquakes that shatter the ground hundreds of miles away(2). Within seconds, a cloud of ash, rock fragments, gases, and droplets of molten rock—known
collectively as ejecta—rises through the atmosphere and begins to
spread across the globe. In this turbulent cloud of ejecta, some of the
ash and vapor coalesces—much like hail stones form during thunder
storms—to create small spheres called accretionary lapilli. The lapilli
and other ejecta are propelled from the impact site at supersonic
speeds. In the shallow ocean that covered much of the region, the
impact generates huge tidal waves (tsunamis) that cross the ocean
surface, mixing together rock fragments and ejecta. Over time, this
material is buried by younger sediments, cemented together, and
fused by molten rock to form a solid layer.


In 2007, a layer of rock was discovered in Minnesota that is
thought to have formed during the Sudbury meteorite impact event.
The layer is exposed near GUNFLINT LAKE, nearly 500 miles west
of the impact site at Sudbury (Figure 2). It is sandwiched between
the Gunflint Iron Formation below, and slate of the Rove Formation
above (Figure 3). Both of these formations were deposited as muddy,
oceanic sediments. Nearly a billion years later, these rocks were
intruded by magma (Logan Intrusion) as part of a major continental
rifting event.

Most of the impact layer consists of breccia—a mixture of
fragments broken from the underlying iron-formation and cemented
together (Figure 4). These fragments represent pieces of seafloor that
were ripped loose by impact-related earthquakes and carried down
a submarine slope.

Only the uppermost part of the layer at Gunflint Lake contains
true ejecta—the most obvious of which are accretionary lapilli. Notice
in repeated layering of ash and melt droplets onto the hail-stone like
projectiles. In some of the locations near Gunflint Lake, the lapilli
are intermixed with large iron-formation fragments (Figure 6),
suggesting that the material was reworked by tsunamis.
The impact layer extends discontinuously from Thunder Bay,
Ontario(3), southward into parts of Michigan(4) (5), and westward into
Minnesota (Figure 2). Although it’s a thin layer—only about 25 feet
thick in Minnesota—it’s a very important and remarkable one. Its
importance lies in the record of global catastrophe that occurred in a
“moment” of the planet’s long geologic history and it is remarkable
that such a thin layer has survived weathering and erosion for nearly
2 billion years.

Of the 174 scientifically verified impact structures on Earth,
only one is larger, and few are older, than the Sudbury Impact(1).
For comparison, the Chicxulub Impact on the Yucatan Peninsula of
Mexico, is much younger (~65 million years old) and its crater size
is smaller. Yet, the Chicxulub event caused world-wide extinction
of many species, including dinosaurs. Clearly, the larger Sudbury
impact event would also have had global ramifications.

The internal organization of units within the impact layer at
Gunflint Lake is consistent with the sequence of events outlined
on the table. Seismic shaking from earthquakes deformed and
fragmented the underlying iron-formation, and caused submarine
debris flows that redistributed the fragments into a thick breccia
unit. This was followed by deposition of airborne ejecta that rained
down on the ocean surface and settled to the sea floor, forming the
lapilli unit. Finally, localized reworking of the ejecta and breccia units
by tsunamis produced the uppermost unit of mixed fragments and
lapilli.

Given the preceeding “context for interpretation,” it is an
interesting footnote that the entire layer of breccia and ejecta very
likely represents the catastrophic events of a single day; caught
during the 48 million years that separate the deposition of Gunflint
Iron Formation below from Rove Formation above Table

ARRIVAL TIME EFFECT MODERN ANALOG
1. ~13 seconds Fireball 3rd degree burns, trees ignite
2. ~2-3 minutes Earthquakes Richter scale 10.2 at Sudbury,
buildings collapse at Gunflint Lake
3. ~5-10 minutes Airborne ejecta a layer 1-3 meters thick, with
arrives fragments <1 cm in size
4. ~40 minutes Air Blast Maximum wind speeds ~1,400 mph
5. ~1-2 hours Tsunami None of this magnitude
F
Despite the fact that large meteorite impacts are exceedingly rare
and unlikely in our lifetime, recent geological research demonstrates
that the impact process is fundamental to the formation of terrestrial
planets. The on-going study of these ancient deposits in the Lake
Superior region (Figure 8) will enhance our understanding of the
environmental consequences of impact during the oldest time period
in Earth history.

REFERENCES AND ADDITIONAL RESOURCES
(1)www.unb.ca/passc/ImpactDatabase Website describing 174
meteorite impacts world-wide. Developed and maintained
by Planetary and Space Science Centre, University of New
Brunswick, Fredericton, New Brunswick, Canada.
(2)Dietz, R.S., 1964, Sudbury structure as an astrobleme: Journal of
Geology 72:412-434.
(3)Addison, W.D., Brumpton, G.R., Vallini, D.A., McNaughton, N.J.,
Davis, D.W., Kissin, S.A., Fralick, P.W., and Hammond, A.L.,
2005, Discovery of distal ejecta from the 1850 Ma Sudbury impact
event: Geology 33:193-196.
Figure 8. Electron microprobe image of accretionary lapilli (by McSwiggen and
Associates, PA).
8
(4)Cannon, W.F. and Addison, W.D., 2007, The Sudbury Impact layer
in the Lake Superior iron ranges: A time-line from the heavens:
Institute of Lake Superior Geology, 53rd Annual Meeting,
May 8-13, 2007, Lutsen, Minnesota, v. 53, Part 1-Proceedings
and Abstracts, p. 20-21. Available via website: (http://www.
lakesuperiorgeology.org).
(5)Pufahl, P.K., Hiatt, E.E., Stanley, C.R., Morrow, J.R., Nelson, G.J.,
and Edwards, C.T., 2007, Physical and chemical evidence of the
1850 Ma Sudbury impact event in the Baraga Group, Michigan:
Geology 35:827-830.
(6)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; Meteorite and Planetary Science 40:817-840. (www.lpl.
arizona.edu/impacteffects)
(7)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, 36:383-386.
(8)Fralick, P.W., Davis, D.W., and 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 39:1085-1091.
Or Contact:
Mark Jirsa
Minnesota Geological Survey
jirsa001@umn.edu
612-627-4780 X208
or
Paul Weiblen
University of Minnesota
Department of Geology and Geophysics
pweib@umn.edu
Prepared with editorial and technical support from Barb Lusardi and Richard
Lively-MGS