IMPACT CRATER EXPLORATIONS
by: Charles O'Dale
ST. MARTIN IMPACT STRUCTURE
A gravity low, possibly due to underlying rock fracturing and brecciation, was documented near the centre of the structure. The absence of a "gravity high" from a "central peak" is possibly explained by the lack of density contrast between the Precambrian basement granites and the Ordovician-Lower Silurian carbonates.
A central magnetic anomaly is due to the formation of hematite from the alteration of mafic silicates in the floor of the central uplift (Grieve 2006).
A study (2007) detailed in the journal Geology suggests meteor impacts with the Earth can produce effects of a more subtle and insidious kind than just catastrophic extinction. The scientists said a good example was found at the Canadian town of Gypsumville, Manitoba, located near the Lake St. Martin meteor impact crater. Domestic wells in the town have elevated salinity, sulfate and fluoride concentrations. The groundwater with elevated fluoride is shown to occur exclusively within the impact structure, and the study is thought to be the first to document enhanced groundwater fluoride concentrations associated with impact structures (Boyle et al, 2007).
With respect to the St. Martin structure, this phenomenon "may" be the cause of the "diversion" in the Dauphin River on the crater's North East rim.
Summary of “Lake St. Martin Structure” presented to Manitoba Mineral Society on March 7, 2007
by: Jim Bamburak Industrial Minerals Geologist Manitoba Geological Survey
Definitions (American Geological Institute “Glossary of Geology”, 1974)
Crypto-explosion Crater – non-genetic, descriptive term designating a roughly circular structure formed by the sudden, explosive release of energy and exhibiting intense, often localized deformation with no obvious relation to volcanic or tectonic activity.
Astrobleme – an ancient erosional scar on the Earth’s surface, produced by the impact of a cosmic body, and usually characterized by a circular outline and highly disturbed rocks showing evidence of intense shock.
Near the eastern edge of the Western Canada Sedimentary Basin.
Centrally located in Manitoba’s Interlake area.
In the immediate vicinity of Gypsumville, Manitoba.
Straddling the northern shoreline of Lake St. Martin.
Along the Fairford and Dauphin rivers.
N 51°47‘, W 98°32‘.
Numerous coreholes have been drilled into the Lake St. Martin Structure, indicating:
Complex crater; 40 km in diameter, Age: 219 ± 32 Ma (Kohn et al., 1995).
Geological Setting; Within the outcrop belt of Silurian Interlake Group dolomite (S). Rimmed by structurally disturbed Ordovician Red River (ORR), Stony Mountain (OSM) and Stonewall formation (OS) and Silurian Interlake Group. Core of possible Jurassic Amaranth gypsum (J), partially surrounded and underlain by remobilized Permian Lake St. Martin Complex (P) and Precambrian granite (PC).
Geophysical Characteristics - Geological Components; Disruption limit – possibly the original crater limit in Silurian Interlake Group dolomite, prior to erosion. Crater rim – uplifted, undeformed Precambrian basement granite, granitic gneiss and amphibolite. Exposed in outcrop on east side of Lake St. Martin Structure, east of PR513. Central uplift – shocked granite Exposed in outcrop in centre of Lake St. Martin Structure, north of PR513.
Present in corehole LSM-4: 53’ depth = shock-metamorphosed granitic gneiss intruded by thin pseudotachylyte veinlets. 122’ depth = pegmatite cut by pseudotachylyte vein. Crater fill – St. Martin Series, which includes trachyandesite meltrock, granitic breccia, polymict breccia and Paleozoic carbonate breccia.
Present in corehole LSM-3: 44’ and 285’ depth = massive trachyandesite meltrock containing abundant fine to coarse granitic inclusions. 189’ depth = fallback breccia underlying Jurassic Amaranth red shale. 223’ depth = fallback polymict breccia with blobs of reddish, vesicular aphanitic meltrock and buff carbonate.
Present in Bralorne Gypsumville 8-20-32-8W well: 433’ depth = polymict breccia = granitic, argillaceous and igneous fragments in finely fragmental matrix.
Present in corehole LSM-1: 260’ depth = complexly brecciated carbonate rock.
Thin sections of crater fill meltrock show the following: Feldspar (clear) and quartz fragments with several sets of planar features. Fragments of melted rock in fallback breccia; note planar features. Shock-metamorphosed, partially melted inclusions of granite. Glassy to partly devitrified fragments with skeletal crystals. Post-crater red beds (conglomerate, sandstone and siltstone) and evaporites (gypsum and anhydrite). Gypsum exposed in former quarry on west side of Lake St. Martin Structure, north of PR513. Jurassic Amaranth red shale present in LSM-3, 189’ depth. Glacial till, and possibly Cretaceous sediment.
Gypsum and gypsum wallboard production. Gypsumville (1901 to 1990). Wallace and Greer (1927) reviewed the early development of the Gypsumville deposits. And Bannatyne and Watson (1982) described the more recent history of the Lake St. Martin gypsum and anhydrite deposits.
Aggregate production. Limited amounts of aggregate have been produced from gravel pits in the Lake St. Martin area. Groom (2006) produced a map that shows the location of gravel pits in the Rural Municipality of Grahamdale.
Base metal potential. According to McCabe and Bannatyne (1970), native copper has been reported from the Lake St. Martin Crater structure over the years. Trace element analysis of Lake St. Martin Series carbonate breccia core samples from LSM-1 have indicated that copper is anomalous (up to 710 ppm, according to Gale and Conley, 2000).
Boyle, D.R. et al, Geochemistry, geology, and isotopic (Sr, S, and B) composition of evaporites in the Lake St. Martin impact structure: New constraints on the age of melt rock formation, GEOCHEMISTRY GEOPHYSICS GEOSYSTEMS, VOL. 8, 2007.
M.H.L. Deenen, M. Ruhl, N.R. Bonis,W. Krijgsman, W.M. Kuerschner, M. Reitsma, M.J. van Bergen, A new chronology for the end-Triassic mass extinction. Earth and Planetary Science Letters 2009.
Grieve, R.A.F., Impact structures in Canada, Geological Association of Canada, no. 5, 2006.
Robertson, P.B., Grieve, R.A.F., Impact Structures in Canada: their recognition and characteristics. The Journal of the Royal Astronomical Society, February 1975.
Smith, R. Dark days of the Triassic: Lost world - Did a giant impact 200 million years ago trigger a mass extinction and pave the way for the dinosaurs? NATURE 17 Nov. Vol#479 2011.
Tetsuji Onouea, et al; Deep-sea record of impact apparently unrelated to mass extinction in the Late Triassic. Rutgers University/Lamont-Doherty Earth Observatory, Palisades, NY, October 3, 2012