Forest Fire on the Gunflint Trail leads to discovery of
further evidence for an ancient, giant meteorite impact at Sudbury, Ontario,
Canada
By Paul Weiblen
May 16,
2007
It will be small
comfort for those whose property was consumed by the Ham Lake fire to learn that
in the matter of a few minutes some 1, 850 million years ago the entire Gunflint
region suffered a far greater catastrophe.
But, bad things happen on the good planet Earth and the Ham Lake forest fire that left an ugly,
scorched footprint in the beautiful Gunflint region is certainly one of
them. Still, it is nothing compared
to the devastation wrought by a meteorite, probably over five miles in diameter
that struck the earth at Sudbury, Ontario.
The impact scattered a blanket of debris from a crater, presumed to be
160 miles in diameter, over nearly a million square miles - much as the eruption
of Mount Vesuvius buried Pompeii.
Just as there are benefits from fires in forests, the Sudbury Impact Event is just one of the many geologic
processes that produced the scenic wonders of North
America.
Minnesota Geological Survey geologist, Mark Jirsa and
University of
Minnesota Geology Professor Emeritus Paul
Weiblen were scheduled to lead a geology field trip
at the end of the Gunflint Trail on Saturday, May 12th. The trip was part of the annual meeting
of the Institute of Lake Superior Geology (ILSG), held during the week at the
Lutsen Resort on Lake Superior. As the fire grew from 17,000 to nearly 75,000 acres (117 square
miles), the trip was of course cancelled. However, on May 8th, Jirsa
was still able to visit one of the planned field trip stops in the vicinity of
the Gunflint Lodge. During his
examination of rock exposures at the stop, he discovered some unusual features
in the rocks at the top of the Gunflint Iron Formation. Jirsa was able to show samples of the
rocks to William Addison, a colleague attending the ILSG meeting. Addison
recognized immediately that the samples exhibit the typical textures of material
that is ejected from a meteorite crater and deposited over a large area around
the crater (somewhat like volcanic ash around an erupting volcano).
Over the past two decades, geologists have reached a
consensus that a large meteorite impact occurred at Sudbury Ontario, 1,850 million years ago. This is also
considered to be the time when the formation of a succession of iron-rich
sedimentary rocks in a shallow ocean basin in northeastern Minnesota and southern Ontario was coming to an end. The impact
created a large, 162 mile-diameter crater,
like those still visible on the Moon and other terrestrial planets. From studies of lunar craters and model
calculations, impacts of this magnitude produce "blankets" of ejected material
over an area as much as five times the radius of the crater. On the Moon, the ejecta blankets have
been relatively easy to identify because the only active geologic processes have
been successive cratering, deposition of ejecta blankets, and in-filling of the
cratered terrane on the front side of the Moon with lunar lava. Thus, the geology of the Moon is
relatively simple, consisting only of an ancient cratered terrane (the bright
reflective highlands), the lava infilling (the dark maria forming the Man in the Moon), and the succession of
lunar ejecta blankets that have remained undisturbed over geologic time. By contrast, impact ejecta deposits on Earth
are only a minor component in the very complex succession of igneous,
sedimentary and metamorphic rocks that make
up our continents. Hence solely
from a consideration of abundance, it is not surprising that impact ejecta have
not been readily recognized by geologists. In addition, it is not clear what the
effects of Earth's atmosphere and hydrosphere were on the alteration and
preservation of impact ejecta.
The complex products of impact range from angular fragments of
preexisting rocks and partially melted, recrystallized, or glassy fragments, to
spherules that condense from vapor in the ejecta cloud (much like hail stones
form in rain clouds). The shock
wave produced by impact transports ejecta away from the site of impact at
velocities of miles per second. On Earth the shock wave would produce giant
tsunamis. The force of the currents
on the bottom of shallow ocean basins would disrupt the layering and other
features of sediments accumulating on the sea floor and probably even some of
the sea floor itself. The layer of
sediment that would accumulate after the tsunami had passed would be a very
complex mixture of disrupted sediments and the ejecta material. Oxidation and
hydration would further alter impact ejecta. Only a handful of geologists have
tackled the difficult problem of recognizing and positively identifying the
presumably meager trace of impact ejecta in rock exposures around the
world. Addison and a fellow high school science
teacher, Gregory Brumpton, driven by curiosity and the belief that ejecta from
the Sudbury impact must exist, began a search
about 15 years ago in the Thunder
Bay area.
They were aided and encouraged by the recognition in Australia of unusual rocks with some
of the characteristics of ejecta materials described above. Addison and Brumpton focused their search on
exposures of Gunflint Iron Formation
in the vicinity of
Thunder
Bay. Ironically, at Hillcrest Park in Thunder Bay,
where Addison played as a child, they found a
ten to twenty foot-thick layer at the top of the Gunflint Iron Formation that
fits the now accepted criteria for impact ejecta transported and deposited in a
tsunami surge. This exposure had
been described and dismissed by earlier geologists as a "chaotic mess" at the
top of the Gunflint Iron Formation.
Building on and expanding Addison's and Brumpton's search for
remnants of the elusive Sudbury Impact ejecta blanket, William Cannon of the
United States Geological Survey has found and documented exposures of the ejecta
blanket in or near five of the iron ranges in the Lake Superior region in
Ontario, Michigan, Wisconsin, and Minnesota (see map below).
Jirsa's discovery adds an occurrence in the Gunflint Iron
Formation just off the Gunflint Trail.
The significance of Jirsa's find and the other occurrences of
the Sudbury Impact ejecta blanket is that a unique layer of rock formed over an
area more than 400 miles from Sudbury in the matter of a few days or less,
1,850 million years ago. As Cannon
points out, the layer is a "time line" that defines a precise moment in time for
the rocks with which it is interbedded. In addition, some of the interbedded
rocks contain fossil remains of cyanobacteria, the earliest preserved forms of
life on earth. Excellent examples
of these fossil remains, called stromatolites are well exposed in the upper
layers of the Gunflint Iron Formation at the field trip stop off the Gunflint
Trail where Jirsa found the ejecta layer.
There is a major world-wide concentration of iron-formations
of the same age as the Gunflint Iron Formation. The reason for this is generally thought
to be related to the build up of oxygen in the atmosphere by photosynthesis. At
low levels of oxygen, iron would have been soluble in the early oceans, but at
high levels the dissolved iron was oxidized and highly insoluble. This would
result in the precipitation of iron and its incorporation into sedimentary rocks
forming at the time. Cyanobacteria
played a crucial role in the oxygen build up and the origin of
iron-formations.
William Canon of the USGS has noted that the occurrence of
iron-formations world-wide decreases precipitously around 1, 850 million years
ago. Since this is also the date of the Sudbury Impact, Cannon has raised the
question of whether the impact had world-wide consequences, the principal one
being the extinction of cyanobacteria, which would have abruptly ended the
favorable conditions for formation of iron-rich sedimentary rocks.
Further documentation of the Sudbury Impact and its effects
on geologic processes at the time will be an increasingly exciting area of
research. But it is really only the
beginning of exciting research for geologists interested in the early history of
the Earth. The record of meteorite
impact on the Moon shows that the size and intensity of impact decreased over
time. The craters that define the "Man on the Moon" were created by giant
impacts around 4,000 million years ago.
After that time the impact rate and size decreased to only sporadic
impacts after about 2,500 million years.
Thus the effects of impact on terrestrial geologic processes before 2,500
million years ago should be even more dramatic than those associated with the
Sudbury Impact. Only one older and larger impact than Sudbury has been documented
on Earth. It is the Vredefort
structure in South
Africa, dated at 2,500 million years. Products of the impact like the
Sudbury ejecta
blanket remain to be discovered and documented. It is unfortunate that as the
impact rate increases with age on Earth, other geologic processes have had a
longer time to obscure the record of impact. The oldest rocks in North America,
dating back to at least 3,800 million years are found in the Minnesota River Valley in southwestern Minnesota. Unfortunately it has been demonstrated
that these rocks have undergone four periods of burial, deformation,
metamorphism, and uplift. Professor
Emeritus Paul Weiblen has puzzled
over the question of what was the fate of the impact products during the
processes that formed the ancient rocks now exposed in the Minnesota River
Valley. Weiblen concludes that a sure way of
"proving" that the impact record has been totally obliterated is not to look for
it. It is interesting to note that
it took two high school science teachers not professional geologists to discover
the rocks that record the effects of the Sudbury Impact.
Approximate
locations of Sudbury Impact Layer Sites and the Sudbury
Impact
Map from 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. Locality
of impact ejecta in Minnesota shown by
star.
NEXT TO
DO
Paul analyzes
samples
Get TS of carbonate breccia
from stop 8b
Map out tsunami
deposits