Origin of the polymict, allochthonous breccias of the Onaping Formation, Sudbury Structure, Ontario, Canada

Abstract

Favoring an impact model for the Sudbury Structure, the polymict, allochthonous breccias of the Onaping Formation were investigated petrographically and geochemically and interpreted as various impact breccias. The Basal Member at the base of the Onaping Formation is a clast-rich melt breccia on top of the impact melt (Sudbury Igneous Complex). The overlying Gray Member consists of suevitic meltrich breccias and represents ground-surge deposits grading into fall-back breccias. Going upward from the Gray to the Green Member, the latter is interpreted as an agglomerate of fine-grained and gradational final fall-back material, which was affected by high temperatures. Movements during the final phase of crater formation initiated lateral transport of suevitelike material into the central depression of the crater. The Lower Black Member is composed of these redeposited suevitic breccias and a carbonaceous matrix. The Upper Black Member at the top of the Onaping Formation represents reworked impact breccia material, with obvious sedimentary features, and forms a transition to the overlying slates of the Onwatin Formation.

The suevite of drill hole Yucatàn 6 in the Chicxulub impact crater

“…The more than 1500 m thick suevite breccia unit, known as the Onaping Formation, is also clearly stratified regarding texture but not with respect to chemical composition. It is divided, from top to bottom, into aquatic "suevitic" sediments with carbonaceous matrix (Upper Black member), a reworked suevite breccia with carbonaceous matrix (Lower Black member), a melt-rich suevitic fall-back layer (Green member), a suevitic clastic matrix breccia (Gray member), and a clast-rich impact melt agglomerate with suevitic parts (Avermann 1994;. The geologic setting of this sequence of clastic impact breccias and melt rocks is assumed to represent the central impact basin inside the (now eroded) peak ring of the Sudbury structure, in contrast to the Y6 well of Chicxulub, which is located outside of the peak ring.…”

“…For the derivation of an emplacement model for these breccias, sources of information besides the petrographic and compositional data reported in the previous sections need to be taken into account: 1) the structural and geophysical setting of the Y6 drill core (Morgan et al 1997); 2) the comparison with suevites from other terrestrial craters (e.g., Engelhardt and Graup 1984;Avermann 1994;Engelhardt 1997;Graup 1999); and 3) the results of numerical simulations of the Chicxulub impact (Alvarez et al 1995;Pierazzo et al 1998;Pierazzo and Melosh 1999).…”

The importance of being cratered: The new role of meteorite impact as a normal geological process

“…Most of the Sudbury carbon occurs as poorly-characterized carbonaceous organic material ("kerogen") in a distinctive unit of crater-filling impact breccia, the Onaping formation. In the black, carbon-rich upper part of the unit (Muir and Peredery 1984;Avermann 1994;Bunch et al 1999;Heymann et al 1999), carbon contents of 0.1-1 wt% are typical. This "Black Onaping" is a large and significant unit, even in the wider context of Precambrian organic geochemistry, with a thickness of nearly 1 km and a preserved volume of ~2000 km 3 .…”

Terrestrial and Extraterrestrial Fullerenes

“…At Shunga and Sudbury there are no fossil remains in the fullerene-bearing rocks. [176] In the case of Shunga the opinions on an algal parentage of the carbonaceous matter are divided [108,177] . For Sudbury, the biologic origin of the carbon in the BMOF was firmly proven.…”