by: Charles O’Dale

  • Type: Complex
  • Age Ma: 435 ±10 (Geological dating)aSILURIAN
  • Diameter: ~5 km
  • Location: N 51° 27.5’ W 097° 41.2’

a The age granite from the southwestern rim was annealed. An alternate impact age estimate: Raw fission track ages for the High Rock Lake impact are 389 ± 26 and 396 ± 33 Ma (Kohn et al 1995).

Manitoba – St. Martin, High Rock and Hartney structures. The High Rock Lake structure is indicated by the RED dot below the bore hole # M-3-00 between Lake St. Martin impact structure to the west and Fisher Bay which is off Lake Winnipeg to the east.
The approximate 5 Km diameter High Rock Lake structure is indicated by the red circle. (Courtesy Google).
Location of coreholes in the High Rock Lake structure magnetic map and of cross-section A-A’ (after McCabe, 1982).
The High Rock Lake structure from the north. The “point of impact” is in the immediate foreground right with High Rock Lake in the background.
High Rock Lake structure “ground zero”.

ABSTRACT: The High Rock Lake structure, on the northeastern flank of the Williston Basin in Manitoba, is a circular crater commonly thought to be an astrobleme. Apatite fission-track (FT) ages from basement rocks in the structure is markedly younger than those previously derived in the region. Constraints from regional geohistory combined with forward modelling of apatite FT data indicate that at High Rock Lake, apatites in a weakly foliated granite and a brecciated and metasomatised granite from the uplifted southwestern crater rim, were totally, or nearly totally, annealed in the range of ~435 ± 10 Ma. This range is interpreted as dating the time of cratering and is in excellent agreement with stratigraphic evidence which constrains the event as Late Ordovician to Mid-Silurian. The resetting of apatite FT clocks within the basement rocks by two discrete Phanerozoic cratering events (High Rock Lake and St. Martin structures) provides a unique opportunity to study the post cratering thermal history of the region. At High Rock Lake and Lake St. Martin samples achieved maximum paleotemperatures (~60-70° C) during the Eocene. This temperature range is in good agreement with data independently attained from organic maturity indicators elsewhere in the northeastern Williston Basin area (Bezys 2000).

The High Rock Lake structure approximately over the north west rim of the structure. The “point of impact” is in the background under the wing, with High Rock Lake to the left.
A study conducted by H.R. McCabe indicated the presence of a highly disturbed area, possibly representing a crypto-explosion meteorite-impact structure (McCabe, 1981).

Two Precambrian granite inliers near High Rock Lake, south of Lake St. Martin, suggest a circular feature of very complex origin (McCabe, 1981; 1982). The two Precambrian granite inliers are approximately 180 m above their expected position in the regional structure. As well, disturbed Paleozoic outcrops exist north and west of the granite outcrops. The structurally disturbed area is shown to be coincident with a well defined aeromagnetic low (McCabe, 1981).

Cross-section A-A’ (after McCabe, 1982).
Corehole M-3-00 was drilled to provide an undisturbed reference hole in the High Rock Lake area. It was hoped that the hole would reach the Precambrian, but it encountered unconsolidated sand in the Winnipeg Formation and drilling had to be terminated prematurely. The hole is undisturbed and does not appear to be affected by the High Rock Lake structure.



R.K. Bezys STRATIGRAPHIC INVESTIGATIONS AND COREHOLE DRILLING PROGRAM, 2000 in Report of Activities 2000, Manitoba Industry, Trade and Mines, Manitoba Geological Survey, p. 196-201.

Barry P. Kohn, Kirk G. Osadetz, Ruth K. Bezys – Apatite Fission-Track Dating of Two Crater Structures in the Canadian Williston Basin Bulletin of Canadian Petroleum Geology Vol. 43 (1995), No. 1. (March), Pages 54-64

Brent Dalrymple, Radiometric Dating Does Work! Reports of the National Center for Science Education

McCabe, H.R. High Rock Lake structure. Manitoba Mineral Resources Division, Report of Field Activities, p. 78-82 1981.

McCabe, H.R. High Rock Lake Crater structure. Manitoba Mineral Resources Division, Report of Field Activities, p. 69-74 1982.