ST. MARTIN IMPACT STRUCTURE
by: Charles O’Dale
- Type: Central peak
- Age: 227.8 ±0.9 Ma *
- Diameter: ~40 km (estimated minimum – see text)
- Location: Manitoba, Canada. N 51° 46′ W 98° 32′
- Shock Metamorphism: PDF in quartz grains, feldspar and maskelynite.
* 40 Ar/39 Ar-dated at a Carnian age of 227.8 ± 0.9Ma[±1.1Ma] (MSWD =0.52; P=0.59) by combin-ing two plateau ages and one isochron age obtained on differ-ent aliquots of impact-melted K-feldspar. With a relative error of ±0.4% on the isotopic age, the Lake Saint Martin impact structure has advanced to one of the most precisely dated larger impact structures on Earth (Schmieder et al 2014). Pre 2014 dating Method for the St. Martin crater: K/Ar and Rb-Sr isotopic analysis of impact melt rocks (Grieve 2006).
2014 ST. MARTIN CRATER UPDATE
* 40 Ar/39 Ar age of the Lake Saint Martin impact structure (Canada) – Unchaining the Late Triassic terrestrial impact craters
Martin Schmieder, Fred Jourdanb, Eric Tohvera, Edward A. Cloutisc
Manitoba Crater Revealed, GEOS, Vol.13, p.10-13
ABSTRACT: New 40 Ar/39 Ar dating of impact-melted K-feldspars and impact melt rock from the ∼40km Lake Saint Martin impact structure in Manitoba, Canada, yielded three plateau ages and one mini-plateau age in agreement with inverse isochron ages for the K-feldspar melt aliquots and a minimum age for a whole-rock impact melt sample. A combination of two plateau ages and one isochron age, with a weighted mean of 227.8 ±0.9Ma[±1.1Ma; including all sources of uncertainty] (MSWD =0.52; P=0.59), is considered to represent the best-estimate age for the impact. The concordant 40 Ar/39 Ar ages for the melted K-feldspars, derived from impact melt rocks in the eastern crater moat domain and the partially melted Proterozoic central uplift granite, suggest that the new dates accurately reflect the Lake Saint Martin impact event in the Carnian stage of the Late Triassic. With a relative error of ±0.4% on the 40 Ar/39 Ar age, the Lake Saint Martin impact structure counts among the most precisely dated impact structures on Earth. The new isotopic age for Lake Saint Martin significantly improves upon earlier Rb/Sr and (U–Th)/He results for this impact structure and contradicts the hypothesis that planet Earth experienced the formation of a giant ‘impact crater chain’ during a major Late Triassic multiple impact event.(Bannatyne, BB & McCabe, H. 1984,)
The St. Martin meteorite crater lies immediately north of Lake St. Martin between Lake Winnipeg to the east and Lake Manitoba to the west. Even though there is no surface expression, from drilling data, the structure was suspected as an impact crater with the discovery of shock metamorphic effects. The structure is classified as a complex meteorite crater with a central uplifted area. The uplifted area includes a surrounding annular trough. Outcrops of Precambrian granite at the crater’s outer limit may indicate an inner ring of a larger central peak basin crater. The minimum size of the crater is represented by the circle superimposed on the above Google Earth image of the area (Grieve 2006).
The impactor made contact in Ordovician to Devonian sandstones overlying Archean-aged granite of the Superior Province of the Canadian Shield. The crater is now buried by over 100 m of Jurassic red beds and glacial drift. This image taken from approximately 1000 feet over the point of impact illustrates how geology over the eons has eroded and covered any indication of an impact structure at this site. Reports have stated that the St. Martin structure is one of the best preserved craters on this planet. Drilling has revealed carbonate breccia, granitic breccia, suevitic breccias and impact melt rocks under the impact structure.
Planar Deformation Features (PDF) in shocked quartz grain from crystalline target rocks were found in the St. Martin drill samples. The passage of the shock wave through the rock changes the structure of some of the enclosed minerals. IE: change is possible in the feldspar mineral plagioclase. The shock wave can break down the structure of the mineral, changing parts of it into a diapletic glass (glass formed at high-pressure in the solid-state) which is isotropic, or uniform in all directions. This photograph of a thin slice of plagioclase, 0.03 millimetre thick, is seen here in cross-polarized light, with a ‘sensitive tint’ plate. The original plagioclase is coloured yellow and the shock-changed mineral is purple. (Courtesy Denis W. Roy & MIAC).
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 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).
Research of the Chesapeake impact structure has revealed abrupt diversions of the lower courses of the James and York Rivers (indicated by the small circles in the map at left). These diversions coincide with the Chesapeake crater rim. The cause of these diversions is the differential subsidence of the outlaying country rock compared to the breccia within the Chesapeake Bay impact crater forcing a structural sag over the subsiding breccia. The river diversions are at the “rim” of this sag (Poag, 1999). 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.
The circle, superimposed on this aeronautical chart of the area, indicates the geological extension of the St. Martin impact crater. Note on the chart that on the North East point of the crater is an almost 180° diversion of the Dauphin River. This extreme change in direction of the river coincides with the North East extension of the crater rim. This phenomenon was documented the Chesapeake impact structure. My hypothesis is that the cause of the extreme diversion of the Dauphin River at the St. Martin crater rim is a result of the differential sagging of the outlaying bedrock compared to the breccia within the impact structure. To my knowledge, there is no other report describing the cause of this river’s diversion at this specific location. The Dauphin River then follows this rim to the East and flows into Lake Winnipeg.
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).
Bannatyne, BB & McCabe, H. 1984, Manitoba Crater Revealed, GEOS, Vol.13, p.10-13
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.
Brent Dalrymple, Radiometric Dating Does Work! Reports of the National Center for Science Education
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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.
Martin Schmieder, Fred Jourdan, Eric Tohver, Edward A. Cloutis 40 Ar/39 Ar age of the Lake Saint Martin impact structure (Canada) – Unchaining the Late Triassic terrestrial impact craters.Earth and Planetary Science Letters 406(2014)37–48 2014
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