The Whitewater Group preserved within the Sudbury Basin structure conforms to a logical succession of beds to be expected for sediments accumulated in a large impact-generated crater. The lowest beds of the Group, the lower Onaping Formation, represent impact-generated breccia and fall-back material, the upper Onaping Formation and the overlying Onwatin Slates represent the restricted crater series, while the succeeding Chelmsford Sandstone represents an open marine turbidite sequence. The turbidites swept across the area at a time when the crater rim was no longer influencing sea bottom current circulation. The apparent anomaly of the occurrence of sediments deposited in an extensive marine environment and presently only found within the Sudbury Basin is attributed to preferential preservation, due to compaction-generated subsidence over the crater and impact-fractured area.

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...

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It has been suggested that the Sudbury Basin could be compared with the Brent meteorite impact crater and that Sudbury may also have developed a relatively flat crater floor at a very early stage. More recent paleomagnetic interpretations appear to fit an original, subhorizontal, sill-like form for the Nickel Irruptive at Sudbury. These suggestions in turn permit formulation of a more satisfying model for the early stages of Sudbury basin evolution. I1 a Cte suggeri que le bassin de Sudbury pourrait ktre compare au cratere form6 par le meteorite a Brent et qu'aussi Sudbury aurait pu developper, tres t6t, un fond de cratere relativement plat. Des interpretations palComagnetiques plus recentes suggerent que le 'Nickel Irruptive' de Sudbury etait a I'origine un sill sub-horizontal. Ces suggestions permettent la formulation d'un modele plus satisfaisant pour les etapes primaires de I'evolution du bassin de Sudbury.