The formation of the Sudbury Structure, Canada: Toward a unified impact model

Abstract

Recent field studies and petrographic, chemical, and isotopic analyses of rocks from the 1.85-Ga Sudbury Structure lead to two major conclusions: (1) the Sudbury Structure is the erosional remnant of a tectonically deformed and metamorphosed peak ring (or possibly multiring) impact basin with an original rim diameter of approximately 220 km, and (2) the Sudbury Igneous Complex, >2,500 m thick, and the related ore deposits were produced by impact melting of the upper and lower crust and subsequent differentiation of the melt, whose original volume was on the order of 12,500 km 3 . There is clear textural, chemical, and isotopic evidence for the absence of any significant magmatic or volcanic process involved in the formation of the melt complex and the melt-bearing breccias (Onaping Formation). All formations related to the Sudbury Structure, as defined by their structural position or by typical breccia textures and shock metamorphic effects, are compatible with an origin by a single impact event. The impact produced a transient cavity of about 110 km diameter that collapsed to a 220-km-wide shallow peak ring or multiring basin on a time scale of less than 30 min and, subsequently, became filled by pelitic sediments. The depth of the transient cavity, the maximum depth of excavation, and the maximum depth of melting are in the estimated ranges of 28 to 37 km, 15 to 21 km, and 25 to 35 km, respectively. The total impact energy was on the order of 8.6 x 10 30 Present address: Stöffler, Museum für Naturkunde, Invalidenstr. 43, 303 304 D. Stoffler and Others ergs and the size of the projectile near 14 km (for an impact velocity of 20 km/s and a projectile density of 3 g/cm 3 ). On the basis of this impact model, which is supported by the most recent models of impact mechanics, we have reinterpreted the stratigraphy and origin of the individual formation observed at the Sudbury Structure as follows (from bottom to top): (1) Sudbury Breccia, pseudotachylite and clastic matrix breccia dikes of the crater basement; (2) Shocked Footwall rockswith shatter cones and shocked quartz, the upper section of the crater basement below the melt complex; (3) Footwall Breccia, polymict megabreccia of the parautochthonous crater floor, thermally metamorphosed by the overlaying melt complex; (4) Contact Sublayer and Offset Dikes with Ni-Cu ore, clast-bearing basal layer of the coherent impact melt sheet lining the crater floor and forming apophyses of melt into the basement, respectively; (5) Main Mass of the Sudbury Igneous Complex (>2,500 m of norites, quartz gabbro, and granophyre), bulk of the clast-free, internally differentiated melt sheet of the central depression of the peak ring structure; (6) Basal Member of the Onaping Formation (OF), clast-rich impact melt breccia at the top of the melt complex; (7) Gray Member of the OF, polymict allochthonous clastic matrix breccia (suevite) formed by ground surging within the transient cavity; (8) Green Member of the OF, polymict suevitic breccia representing fallback material from the co...

Differentiation and emplacement of the Worthington Offset Dike of the Sudbury impact structure, Ontario

“…1), the dip of lower contacts at surface (Rousell 1984) and the asymmetric deep structure (Milkereit et al 1992) of the layered SIC is a function of its post-impact orogenic deformation (Shanks and Schwerdtner 1991;Cowan and Schwerdtner 1994;Riller et al 1998;Riller 2005). Along with the overlying Onaping Formation, a heterolithic impact melt breccia (Muir and Peredery 1984;Stöffler et al 1994), and post-impact sedimentary rocks, the synformal Main Mass of the SIC forms the Sudbury Basin (Brocoum and Dalziel 1974). At surface, the Basin is divided into the North Range, the South Range and the East Range (Fig.…”

“…Around the Main Mass, there are radially and concentrically arranged quartz-diorite dikes, which due to their segmentation are known as Offset Dikes (Fig. 1b), and are interpreted as part of the impact melt system (Stöffler et al 1994;Deutsch et al 1995).…”

Geochemistry of drill core samples from Yaxcopoil‐1, Chicxulub impact crater, Mexico

“…The target rocks of the Sudbury structure consist of Proterozoic metasediments and volcanic rocks on top of an Archean crystalline basement (e.g., Stöffler et al 1994). Within the (now eroded) inner ring of the crater, a ~1.5 km thick suevite sequence, the Onaping formation, occurs on top of the ~3 km thick Sudbury Igneous Complex that represents impact melt rock (Stöffler et al 1994).…”

“…The target rocks of the Sudbury structure consist of Proterozoic metasediments and volcanic rocks on top of an Archean crystalline basement (e.g., Stöffler et al 1994). Within the (now eroded) inner ring of the crater, a ~1.5 km thick suevite sequence, the Onaping formation, occurs on top of the ~3 km thick Sudbury Igneous Complex that represents impact melt rock (Stöffler et al 1994). The Onaping formation consists of the following subunits from top to bottom: black member (reworked, suevitic sediments with carbonaceous matrix), green member (melt-rich fallback suevite), grey member (suevitic clastic matrix breccia), and basal member (clast-rich impact melt agglomerate with suevitic parts) (Brockmeyer 1990;Avermann 1994;Stöffler et al 1994).…”

Terrestrial and Extraterrestrial Fullerenes

“…[129] It has also been argued that the rock fragments and glasses of the Black Member were actually stream-carried from outside the central depression into the central depression where redeposition occurred under euxinic aquatic conditions. [127,130] When did the first rains come after the impact? Did they cool the surface sufficiently to allow preservation of fullerenes?…”