The Unmetamorphosed Sedimentary Fill of the Brent Meteorite Crater, Southeastern Ontario

Abstract

A 10 000-ft (-3050-m) diameter circular structure that exhibits rimmed crater form, shockmetamorphic features, and underlying unbrecciated basement rocks occurs near Brent, Ontario, close to the southern margin of the Canadian Shield. In its center 863 ft (263 m) of Middle Ordovician sediments are preserved. Shortly after impact a nearly level crater floor was established and the subsequent sequence appears to have been deposited close to mean sea level. Repeated sediment laminae probably reflected wind-tidal marine incursions onto the low relief margin of the contemporary Ordovician epicontinental sea. Throughout much of its period of sediment accumulation, the crater floor appears to have been nearly flat. A 380-ft (115.8 m) sequence of dolostones, arkosic siltstones, and evaporite layers and veins formed within a breccia-rimmed depression. Initially sea water canying fine silt invaded the crater and refluxed through the porous and permeable crater rim. Subsequently some 100 ft (30.5 m) of silty arkose blanketed the area, probably resulting from further transgression of the Middle Ordovician seas and breaching of the crater walls. An upper 380 ft (115.8 m) of predominantly thin-bedded lagoonal and shallow shelf sea limestones are divided into upper and lower sequences by a middle regressive set of red beds.The implied near-flat crater floor, coupled with preservation of over 800 ft (>244 m) of crater sediments, suggests continued slow subsidence. Earlier on, this subsidence affected only the crater area, but later episodes of subsidence were regional, involving Ordovician, Silurian, and Devonian sedimentation. The superincumbent load further compacted the total crater sequence. Preservation of the rocks described here is due to final depression of the sequence into a position below the general level of the surrounding Precambrian terrain. If Brent can be considered to be a typical pen-marine meteoritic impact crater, all such craters should have in common an inwarddipping succession of open-circulation sediments overlying a crater-rim restricted sedimentary sequence, which in turn overlies a shock-metamorphosed series of breccias.Compaction of the impact-generated breccias and subsequent unmetamorphosed crater-filling sediments influenced both sediment accumulation and the ultimate crater structure. Can. J. Earth Sci., 12,606-628 (1975) Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by McMaster University on 11/25/14 For personal use only. LOZEJ AND BEALES: BRENT METEORITE CRATER un cratere de meteorite de choc, tous ces crateres devront avoir en commun une succession a circulation ouverte de sediments a pendage vers I'interieur et se trouvant au-dessus d'une sequence sedimentaire restreinte en bordure du cratere qui B son tour est au-dessus de series de breches metamorphosees par mttamorphisme de choc. La compaction des breches mktamorphosees par metamorphisme de choc. La compaction des brtches fonnees par choc et des sediments subsequents et non-metamorphoses remplissant le cratere influen~a l'ac...

When a cosmic impact strikes the sea bed

“…comm. 1998); water depth: Lindström et al 1996b and this paper), Flynn Creek (Roddy, 1977), Brent (Lozej & Beales, 1975;age: Grahn & Ormö, 1995; water depth: this paper based on Lozej & Beales, 1975). …”

“…The nearshore Brent crater (Lozej & Beales, 1975;Grahn & Ormö, 1995) has features of a land-target crater, such as melt sheets and fall-back ejecta. The crater was transgressed shortly after its formation, which explains the marine influences in the infill.…”

Suevite of the ries crater, Germany: Source rocks and implications for cratering mechanics

“…Hydrothermal formation of montmorillonite have been observed in impact breccias and impact melt sheets at many craters, e.g. at Manicouagan (FLoP, AN et al 1978), East Clearwater Lake (PALME et al 1979) and Brent (LOTZEJ andBEALS 1975, GRIEVE 1978).…”

Intra-crater sedimentary deposits at the Haughton impact structure, Devon Island, Canadian High Arctic

“…This is a problem not unique to Haughton, as the post-impact sedimentary evolution within impact craters has not generally been well-studied and is not considered to be an important process associated with impact events. As a result, the few previous studies of intra-crater sedimentary deposits have typically focused on understanding the paleoenvironmental and paleobiological record they contain (e.g., Lozej and Beals 1975;Hall 1978;Gronland et al 1990;Arp 1995).…”