Abstract

West Hawk Lake is a near-circular, 11,700-ft -wide, deep lake located 110 mi east of Winnipeg, Canada. The lake now completely submerges an ancient meteorite impact crater. A central drillhole passes through nearly 1080ft of slump and fallback breccia into 655 ft of rupture zone bedrock below a well-defined crater base. Petrographic analysis of core from this hole, a hole near the original crater rim, and an off-center hole reveals numerous shock features which, supplementing earlier geophysical studies, demonstrate an impact origin for this structure. These results are significant:

(1) The original crater was approximately 8000 ft wide; the rim has recessed by erosion; the target rock consists of steeply dipping units of metasediments and metavolcanics.

(2) Microscope criteria for shock include: abundant planar features in quartz; diagnostic parallel extension fractures; extensive isotropization of tectosilicates in quartz-mica schists (analcite is a common product of recrystallization of the isotropized phases); variable decomposition of biotite and actinolite; abundant glassy fragments and matrix-binding “melt,” now generally devitrified; shock-induced twinning and granulation in pre-impact carbonates.

(3) Distribution of shock features is strongly dependent on mineral composition and particle size. Unless intensely shocked, amphibole-rich rocks are usually devoid of shock evidence, even where fragments are intermixed with shocked schistose metasediments.

(4) The vertical distribution within the central hole core of intensity of shock in the breccias, as plotted on a shock log, is notably erratic.

(5) Autobrecciation, cataclastic fragmentation, and injection of shocked breccia into open fractures account for observed effects in the rupture zone.

(6) Zoisite and secondary calcite develop extensively within the breccia lens by solution and neo-mineralization; serpentine var. antigorite forms with the breccia matrix, and in a network of veinlets in fragments and in the rupture zone; sodium-calcium zeolites form in possibly replaced melt material at the crater base.

(7) Analysis of strongly shocked rock fragments from the breccias by infrared absorption, X-ray fluorescence, electron microprobe, and argon loss methods demonstrates significant changes in the crystal structure and chemistry of constituent minerals in these materials as compared with unshocked equivalents collected off-structure.

West Hawk Lake is compared with the Brent structure, the best-studied model of a simple impact crater in crystalline rocks.

First Page Preview

First page PDF preview
You do not currently have access to this article.