SUDBURY IMPACT STRUCTURE – GROUND EXPLORATION (DISTAL EJECTA)
Discovery of distal ejecta from the 1850 Ma Sudbury impact event
Evidence of this impact layer has been located in the Thunder Bay Area by Lakehead University, and the Ontario Ministry of Northern Development and Mines. The “impactite” rock appears as shattered fragments of the Gunflint Iron Formation and chert (quartz rich cemented in a rusty matrix of volcanic ash). One dramatic example of this rock occurs at Hillcrest Park in the City of Thunder Bay .
These images of the Sudbury Impact Distal Ejecta were taken by the author at Hillcrest Park, Thunder Bay Ontario – 2013.
ABSTRACT A 25–70-cm-thick, laterally correlative layer near the contact between the Paleoproterozoic sedimentary Gunflint Iron Formation and overlying Rove Formation and between the Biwabik Iron Formation and overlying Virginia Formation, western Lake Superior region, contains shocked quartz and feldspar grains found within accretionary lapilli, accreted grain clusters, and spherule masses, demonstrating that the layer contains hypervelocity impact ejecta. Zircon geochronologic data from tuffaceous horizons bracketing the layer reveal that it formed between ca. 1878 Ma and 1836 Ma. The Sudbury impact event, which occurred 650–875 km to the east at 1850 ± 1 Ma, is therefore the likely ejecta source, making these the oldest ejecta linked to a specific impact. Shock features, particularly planar deformation features, are remarkably well preserved in localized zones within the ejecta, whereas in other zones, mineral replacement, primarily carbonate, has significantly altered or destroyed ejecta features. (Addison 2005).
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.
Accretionary lapilli from the Sudbury impact event
Matthew S. Huber, Christian Koeberl
Abstract
Meteorite impact-generated accretionary lapilli are not well studied. The recently discovered distal ejecta from the 1850 Ma Sudbury impact event contain abundant accretionary lapilli generated during the impact and deposited at great distances from the crater. We petrographically and geochemically examined lapilli from five sites (McClure, Connors Creek, Hwy 588, Pine River, and Grand Trunk Pacific, approximately 480–750 km from the center of the Sudbury structure). The compositions of quartz, K-feldspar, calcite, biotite, and chlorite minerals are similar to each other in all of the samples, although the relative proportions of the minerals vary from site to site. The lapilli occur in a matrix of coarse-grained quartz, carbonate, and feldspar grains. Zonation within lapilli appears to be due to grain size distribution rather than compositional variation. The inner zones are coarser grained than outer zones. The relative abundances of calcite, phyllosilicates, and feldspars are similar in each zone within individual lapilli. A meteoritic component is indicated by up to 1.8 ppb Ir in one lapillus from the Pine River site, and Ni and Cr ratios are on a chondritic trend line for many of the lapilli. Mechanisms previously proposed for accretionary lapilli formation seem inadequate to explain deposition of distal accretionary lapilli resulting from impact events. A new mechanism for upper atmospheric accretion is proposed, whereby ash ejected from impact events concentrates at altitudes of neutral buoyancy, where it then accretes and is deposited later than ballistically emplaced particles. Likely, multiple processes are taking place in the chaotic postimpact environment
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