STEEN RIVER IMPACT STRUCTURE (SRIS)
by: Charles O’Dale
- Type: Complex
- Age (ma): 91 ± 7a – CRETACEOUS
- Diameter: 25 km
- Location: N 59° 31′ W 117° 39′
- Shock Metamorphism: magnesioferrite MgFe2O4 (Walton et al 2017), shock deformation and transformation features in quartz and feldspar (Carrigy and Short, 1968; Winzer, 1972)
a Based on a single K‐Ar whole rock age obtained from a ‘pyroclastic vesicular rock’ (Carrigy and Short, 1968), recalculated using more recent decay constants of Steiger and Jager (1977) to be 91 ± 7 Ma, is roughly consistent with stratigraphic constraints. The SRIS remains a candidate for further isotopic analysis to better constrain this estimate on impact timing.
The Steen River impact structure (SRIS) is a buried complex crater in NW Alberta, Canada, first detected as an anomaly magnetic and seismic surveys. It is ascribed to hypervelocity impact based on the presence of shock deformation and transformation features in quartz and feldspar (Carrigy and Short, 1968; Winzer, 1972). The target rocks include 70 m of Mississippian calcareous shale underlain by ~1530 m of Devonian marine sedimentary rocks including evaporites, carbonates and shales. This ~1.6 km thick package of sedimentary rock overlies Lower Proterozoic crystalline rocks of the Hottah Terrane and Great Bear Magmatic Arc, thought to be joined along a faulted contact (Burwash et al., 1994). With a roughly elliptical shape and ~25 km diameter length, the SRIS is the largest known impact structure in the Western Canada Sedimentary Basin. The eroded crater lies buried under ~200 m of cover with no surface expression necessitating geophysical and drilling projects for its exploration.
Fractured basement has been raised 800–1000 m above the regional level (Winzer, 1972). This central uplift is surrounded by a rim syncline, which locally downthrows the basement by ~180 m in the southeast but >500 m in the northwest. Further from the center, a raised rim forms a positive 20–50 m feature. Post-formation, the crater was emergent and eroded, with the uppermost units truncated by a regional unconformity. Deposition of Lower Cretaceous marine shale and sandstone, now ~200 m thick, preserved the crater fill deposits (Molak et al., 2001). Target rocks at the time of impact consist of predominantly Devonian carbonates, evaporites, and shales.
A New Occurrence of Magnesioferrite, MgFe2O4
A rare spinel mineral, magnesioferrite, has been documented in the recrystallized impact breccias of the SRIS. Textural relations indicate that the magnesioferrite occurs in equilibrium with clinopyroxene, calcite, and quartz. The formation of these assemblages could be due to reactions involving precursor phases such as anhydritic carbonates, dolostones, and shales, which occur in target materials and as xenoliths in the upper, less thermally affected portions of the core. Magnesioferrite at the impact structure possesses characteristics such as grain size, shape, and composition akin to those that define the K-Pg boundary layer within the Pacific Basin Kyte and Bostwick, 1995) and occur as a small component of the boundary clay in Spain (Bohor et al., 1986) and Italy (Smit and Kyte, 1984). The discovery of magnesioferrite at the SRIS provides context lacking in the Chicxulub distal ejecta, suggesting that vaporized sedimentary rocks were an important component of the impact-generated vapor cloud from which the MgFe2O4 grains condensed. Thus, magnesioferrite may serve as a novel indicator mineral for impact into carbonate-bearing target rocks at other craters.
Investigating the response of biotite to impact metamorphism: Examples from the Steen River impact structure, Canada
Impact metamorphic effects from quartz and feldspar and to a lesser extent olivine and pyroxene have been studied in detail. Comparatively, studies documenting shock effects in other minerals, such as double chain inosilicates, phyllosilicates, carbonates, and sulfates, are lacking. In this study, we investigate impact metamorphism recorded in crystalline basement rocks from the Steen River impact structure (SRIS), a 25 km diameter complex crater in NW Alberta, Canada. An array of advanced analytical techniques was used to characterize the breakdown of biotite in two distinct settings: along the margins of localized regions of shock melting and within granitic target rocks entrained as clasts in a breccia. In response to elevated temperature gradients along shock vein margins, biotite transformed at high pressure to an almandine-Ca/Fe majorite-rich garnet with a density of 4.2 g cm−3. The shock-produced garnets are poikilitic, with oxide and silicate glass inclusions. Areas interstitial to garnets are vesiculated, in support of models for the formation of shock veins via oscillatory slip, with deformation continuing during pressure release. Biotite within granitic clasts entrained within the hot breccia matrix thermally decomposed at ambient pressure to produce a fine-grained mineral assemblage of orthopyroxene + sanidine + titanomagnetite. These minerals are aligned to the (001) cleavage plane of the original crystal. In this and previous work, the transformation of an inosilicate (pargasite) and a phyllosilicate (biotite) to form garnet, an easily identifiable, robust mineral, has been documented. We contend that in deeply eroded astroblemes, high-pressure minerals that form within or in the environs of shock veins may serve as one of the possibly few surviving indicators of impact metamorphism (Walton, et al 2017).
Frictional melting processes and the generation of shock veins in terrestrial impact structures: Evidence from the Steen River impact structure, Alberta, Canada
Shock-produced melt within crystalline basement rocks of the Steen River impact structure (SRIS) are observed as thin (1–510 μm wide), interlocking networks of dark veins which cut across and displace host rock minerals. Solid-state phase transformations, such as ferro-pargasite to an almandine–andradite–majorite garnet and amorphization of quartz and feldspar, are observed in zones adjacent to comparatively wider (50–500 μm) sections of the shock veins. Shock pressure estimates based on the coupled substitution of Na+, Ti4+ and Si4+ for divalent cations, Al3+ and Cr3+ in garnet (14–19 GPa) and the pressure required for plagioclase (Ab62–83) amorphization at elevated temperature (14–20 GPa) are not appreciably different from those recorded by deformation effects observed in non-veined regions of the bulk rock (14–20 GPa). This spatial distribution is the result of an elevated temperature gradient experienced by host rock minerals in contact with larger volumes of impact-generated melt and large deviatoric stresses experienced by minerals along vein margins.
Micrometer-size equant crystals of almandine–pyrope–majorite garnet define the shock vein matrix, consistent with rapid quench (100–200 ms) at 7.5–10 GPa. Crystallization of the vein occurred during a 0.1–0.15 s shock pressure pulse. Majoritic garnet, formed during shock compression by solid state transformation of pargasite along shock vein margins, is observed in TEM bright field images as nanometer-size gouge particles produced at strain rates in the supersonic field (106–108). These crystals are embedded in vesiculated glass, and this texture is interpreted as continued movement and heating along slip planes during pressure release. The deformation of high-pressure minerals formed during shock compression may be the first evidence of oscillatory slip in natural shock veins, which accounts for the production of friction melt via shear when little or no appreciable displacement is observed. Our observations of the mineralogy, chemistry and microtextures of shock veins within crystalline rocks of the SRIS allow us to propose a model for shock vein formation by shear-induced friction melting during shock compression (Walton, et al 2016).
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Department of Physical Sciences, MacEwan University, Edmonton, Alberta T5J 4S2, Canada Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada
32602–38 Metropole Pvt., Ottawa, Ontario K1Z 1E9, Canada
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