Vitric, recrystallized components are widespread in the Onaping Formation. They occur at many different scales, from fluidal rims surrounding lithic clasts to fluidal fragments, green shards, bombs, and aphanitic intrusions, and are clearly recognized by their devitrified, often banded appearance, their igneous textures (when preserved), and the presence of vesicles. The abundance of vitric clasts is consistent with the occurrence of significant magmatic activity during the deposition of the Onaping Formation, including volcanism related to dissemination of the impact melt sheet. The angular to cuspate shape of fluidal fragments, green shards, and bomb margins (Figures 6a and 6b, 10, and 11) indicate hydroclastic fragmentation of magma due to contact with water. In general, the rocks making up the Onaping Formation resemble hyaloclastite breccias from more recent submarine volcanic deposits (e.g., Yagi et al., 2009; Cas and Giordano, 2014; Soriano et al., 2014). Specifically, morphological features of green shards such as cuspate margins and angular vesiculated fragments are reminiscent of mafic pyroclasts fragmented in subaqueous conditions (Murtagh and White, 2013). Fluidal margins on fragments are most easily explained by eruption of low-viscosity magma in water (cf., Simpson and McPhie, 2001). The wide range of sizes of vitric fragments also supports hydrodynamically enhanced magma fragmentation in a subaqueous environment (Cashman and Fiske, 1991; Rotella et al., 2013). In this context, the sharp change from coarse grained Sandcherry Member to fine-grained, green shard-rich Dowling Member suggests that the fragmentation mechanism changed with time, and that volcanism became more explosive at the Dowling Member.
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The presence of vitric bombs with fluidal wings in the upper Sandcherry Member and Contact Unit in the Dowling Member (Figure 7) suggests that the depositional environment might have been intermittently subaerial. At the smaller Ries impact structure in Germany, “glass bombs” were historically described as having “aerodynamic” shapes and, therefore, interpreted as having an airborne mode of origin (Hörz, 1982). More recently, however, Osinski et al. (2011) showed that such shapes can form in any confining flow, such as in dykes injected into the crater floor that have never been airborne. At the Sudbury structure, eutaxitic textures reported locally in the Contact Unit (Ames et al., 2002; Grieve et al., 2010) lend further support to subaerial conditions. It follows that the Sudbury basin was not completely or at least not immediately submerged after the impact. This situation could be explained by water intermittently or periodically breaching the basin rim, so that volcanism could be subaqueous and subaerial in different areas at low stratigraphic levels in the Onaping
Formation before the basin was completely flooded. In addition, syndepositional faults could have created palaeotopography and channelled the ascent of magma up to subaerial
environments as suggested by Grieve et al. (2010). Alternatively, where the water column was relatively shallow, an erupting explosive plume could have breached the head of water (Allen and McPhie, 2009; Cas and Giordano, 2014, and references therein) providing local subaerial conditions.
Aphanitic intrusions in the Onaping Formation can be considered coeval with the deposition of the breccias and tuffs given their wavy, billowed margins that often transition into peperite (Figure 8) and globules of vitric material intermixing with the Onaping
Formation. These relations are normally found where magma has come in contact with water- saturated, unconsolidated sediments (Kokelaar, 1982; White et al., 2000; Hooten and Ort, 2002; Skilling et al., 2002; Befus et al., 2009; Lago et al., 2012; Cas and Giordano, 2014). Hence, field relationships between aphanitic intrusions and host rocks provide evidence of syndepositional intrusion of magma into wet and not fully consolidated breccias and tuffs throughout the Onaping Formation.
Field and petrographic evidence shows that explosive volcanism contributed to deposit the Onaping Formation as water flooded the basin after the impact. There is growing evidence linking volcanic processes with impact structures on Earth (e.g., Branney and Brown (2011) in a study of the Stac Fada outflow deposit of Scotland). Explosive reactions due to magma-water interaction were identified in earlier studies at the Sudbury basin (e.g., Grieve et al., 2010) and have also been suggested at the smaller Ries crater in Germany (e.g., Meyer, 2013; Stöffler et al., 2013). However, deposits filling the Sudbury and Ries structures show significant
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been related to impact into mixed sedimentary-crystalline target rocks (Osinski et al., 2016, and references therein). In the light of our new data, it is worth noting that the range and prominence of hydroclastically fractured igneous materials preserved throughout 1.5 km worth of stratigraphy is exceptional in the context of other known impact sites
State-of-the-art surface analysis of inner Solar System planetary bodies has identified postimpact volcanism as a common process (e.g., Mercury (Marchi et al., 2013), Venus
Figure 14. Major and trace element composition of green shards and fluidal fragments in the Dowling Member Contact Unit. (a) Scanned thin section of a Contact Unit sample with green shards and fluidal fragments; color-coded arrows indicate location of analyses in Figures 14b and 14c. (b) Major element compositional plot where compositions in the sample are plotted on top of the general data set for the
Onaping Formation (see Figure 13 for details). (c) Normalized rare earth element (REE) diagram showing the median composition of green shards and fluidal fragments in the sample, as well as the composition of N- MORB (McDonough and Sun, 1995) for comparison; data are normalized against the average SIC
composition (Lightfoot et al., 1997) and then normalized to the total REEs so that each trend averages to 1. Note the contrasting compositional trends of green shards and fluidal fragments within a single sample: green shards are more mafic and have light REE depletions similar to N-MORB, whereas fluidal fragments have trace element compositions equivalent to the SIC. Data set available as supporting information Data Set S4.
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(Herrick and Rumpf, 2011), Mars (Edwards et al., 2014), and the Moon (Elkins-Tanton et al., 2004)). At the Rembrandt basin on Mercury, Whitten and Head (2015) found melt deposits related to the impact, as well as volcanically produced smooth plains younger than the impact. On the Moon, impact craters are typically flooded by younger mare basalts. Zhu et al. (2015a) proposed multiple stages of mare volcanism within the Orientale basin, for a protracted period of up to 2 Ga. Timing of volcanism relative to basin formation is difficult to ascertain in extraterrestrial bodies, but volcanism typically postdates the impact by hundreds of millions of years (e.g., Hiesinger et al., 2011). A recent study on Mercury’s Caloris basin (Ernst et al., 2015), however, found no clear evidence of ghost craters in volcanically flooded plains, suggesting that volcanism may have occurred quite shortly after basin formation. The
implications of these findings to the Sudbury structure are not straightforward, given the much larger size of the Caloris basin (1420 km). In addition, the key role of water at the Sudbury structure, producing explosive volcanism and fragmented volcanoclastic deposits, complicates the comparison with postimpact scenarios on planetary bodies such as Mercury or the Moon, where volcanism in the absence of water produces primarily basaltic smooth planes. Presently, the type of volcanism found in the Sudbury basin, where the entire Onaping Formation was deposited within the 1.5 Ma error of U/Pb dates of zircon and titanite, is unique. It may have more importance as a petrologic analogue model for long-destroyed Hadean and Eoarchean terrestrial impact structures than for basins on planetary bodies lacking a liquid hydrosphere.