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.
Frictional melting processes and the generation of shock veins in terrestrial impact structures: Evidence from the Steen River impact structure, Alberta, Canada (Walton, et al 2016)