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SUDBURY DISTAL EJECTA (fallout from the Sudbury impact)

The Sudbury impact produced a red hot rain of glowing glass and shattered melted rock, that fell on the Thunder Bay area 1.85 billion years ago. It formed a layer over a metre thick of this hot debris. The last debris to fall was fine dust, and it took as long as a year to fall, scattering a dusty film right around the world. Some evidence of this has been discovered in the Thunder Bay area. The breccia is sandwiched between Gunflint Iron Formation and sedimentary strata of the Rove Formation. (Addison et al 2005 & 2010).

The Gunflint chert (1.88 Ga[1]): is a sequence of banded iron formation rocks that are exposed in the Gunflint Range of northern Minnesota and northwestern Ontario along the north shore of Lake Superior. The black layers in the sequence contain microfossils that are 1.9 to 2.3 billion years in age.

The Rove Formation: is located in the upper northeastern part of Cook County, Minnesota, United States, and extends into Ontario, Canada. It is the youngest of the many Animikie layers, a layer of sedimentary rocks. Before the Rove sediments were laid down, during the Archean Eon, the Algoman orogeny added landmass along a border from South Dakota to the Lake Huron region; this boundary is the Great Lakes tectonic zone. Several million years later a thin layer of hypervelocity impact ejecta from the Sudbury impact event was deposited on the older, underlying, Gunflint Iron Formation, and the Rove was then deposited on top of the ejecta; it is estimated that at ground zero the earthquake generated by the meteor impact would have registered 10.2 on the Richter scale. During the Middle Precambrian a shallow inland sea covered much of the Lake Superior region and formed the Animikie Group, layers of sedimentary rocks overlying 2700-million-year-old Archean rocks. The Rove Formation is the youngest of the many Animikie layers.

Addison, W.D., Brumpton, G.R., Davis, D.W., Fralick, Philip W., and Kissin, S.A., 2010, Debrisites from the Sudbury impact event in Ontario, north of Lake Superior, and a new age constraint: Are they base-surge deposits or tsunami deposits? Geol. Soc. Am., Special Paper 465. 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. Paul Weiblen, Sudbury impact breccia - Forest Fire on the Gunflint Trail leads to discovery of further evidence for an ancient, giant meteorite impact at Sudbury, Ontario, Canada. May 16, 2007

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. At Hillcrest Park in Thunder Bay is a ten to twenty foot-thick layer over 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 (Weiblen 2007).

These images of the Sudbury Impact Distal Ejecta were taken by the author at Hillcrest Park, Thunder Bay Ontario - 2013.

Comment by Roland Dechesne, geologist and fellow RASC member about the Hillcrest deposits: "The lapilli were poorly preserved and could have, potentially, been any number of things. However, features were present, and given the regional context, it's probable that what (we) saw was them (Sudbury Distal Ejecta). Interestingly, they were in a coarse sandstone that had discontinuous thin blebs of cherty quartz that gave me the impression of being fiamme. If Earth impacts can create frothy pumice-like clasts, then all would be very consistent".

Lapilli: a size classification term for tephra, which is material that falls out of the air during a volcanic eruption or during some meteorite impacts.

Fiamme: lens-shapes, usually millimetres to centimetres in size, seen on surfaces of some volcanic rocks.

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