The evolution of the Onaping Formation at the Sudbury impact structure

Abstract

Abstract-The 1.4-1.6 km thick Onaping Formation consists of a complex series of breccias and ''melt bodies'' lying above the Sudbury Igneous Complex (SIC) at the Sudbury impact structure. Based on the presence of shocked lithic clasts and various ''glassy'' phases, the Onaping has been described as a ''suevitic'' breccia, with an origin, at least in part, as fallback material. Recent mapping and a redefined stratigraphy have emphasized similarities and differences in its various vitric phases, both as clast types and discrete intrusive bodies. The nature of the Onaping and that of other ''suevitic'' breccias overlying impact melt sheets is reviewed. The relative thickness, internal stratigraphic and lithological character, and the relative chronology of depositional units indicate multiple processes were involved over some time in the formation of the Onaping. The Sudbury structure formed in a foreland basin and water played an essential role in the evolution of the Onaping, as indicated by a major hydrothermal system generated during its formation. Taken together, observations and interpretations of the Onaping suggest a working hypothesis for the origin of the Onaping that includes not only impact but also the interaction of sea water with the impact melt, resulting in repeated explosive interactions involving proto-SIC materials and mixing with pre-existing lithologies. This is complicated by additional brecciation events due to the intrusion of proto-SIC materials into the evolving and thickening Onaping. Fragmentation mechanisms changed as the system evolved and involved vesiculation in the formation of the upper two-thirds of the Onaping.

The petrogenesis of impact basin melt rocks in lunar meteorite Shi r 161

“…Comparison with the stratigraphic succession that is to be expected for an impactite deposit in such a 300-500 km diameter lunar impact basin reveals that the fractionally differentiated cumulate rocks are overlain by a decameter-thick succession of crystallized and partly assimilated melt rocks that represent a quenched layer. Fallback breccia on this layer was likely much thinner than at Sudbury (Settle 1980), where the equivalent Onaping breccia is interpreted as excessively thick due to impact melt interacting with water (Kieffer and Simonds 1980;Grieve et al 2010). Nonetheless, a breccia deposit of quenched fallback debris, possibly on the order of ~100 m thick, appears probable.…”

Evidence for an explosive origin of central pit craters on Mars

“…First, the ejecta deposit is expected to contain abundant fractured and fragmented glassy impact melt, similar to the Onaping Formation at Sudbury (Grieve et al, 2010). This layer of glassy deposits should overlay more coherent impact melt deposits.…”

Ries crater and suevite revisited—Observations and modeling Part II: Modeling

“…The apparent excessive volume of the Onaping Formation and other geological evidence allowed for suggesting a new mechanism of its formation (Grieve et al. ): seawater could have entered the inner crater and reacted violently with the underlying impact melt, leading to the initiation of the FCI process. It may be a coincidence, but the total thickness of the final Onaping sequence (1.4–1.6 km) is such that it would create a lithostatic load on the underlying impact melt sheet of approximately 30 MPa that could suppress further water vaporization and, hence, would have terminated the FCI activity.…”