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...
