Dynamic interaction between impact melt and fragmented basement at Manicouagan: The suevite connection

Abstract

The interface between impact melt rocks and underlying footwall lithologies within the Manicouagan impact structure is defined by a zone of dynamic mixing (<20 m thick). This zone transitions as a continuum from clast‐free to clast‐bearing impact melt rocks, through melt‐bearing breccias to melt‐free breccias. Field observations; microscopy; and major, trace, and rare earth element analysis indicate that the breccias are derived by blending two endmembers during the impact process: impact melt and brecciated footwall. The product is a basal breccia sequence, which locally includes the rock type referred to as suevite. In this occurrence, the suevite is a submelt sheet variety, in contrast to similar lithologies that are developed atop impact melt sheets, or beyond crater rims. Dynamic mixing between impact melt and basal clastic material at Manicouagan is attributed to the initial high‐speed centrifugal outflow of superheated, low viscosity impact melt over underlying fractured and fragmented footwall, and its centripetal return during the earlier stages of the crater modification process. The interaction of two fluids (melt with a mobilized granular medium) possessing contrasting densities, and moving at different velocities, can facilitate shear instabilities and turbulent mixing that may be characteristic of Kelvin–Helmholtz behavior.

Density current origin of a melt-bearing impact ejecta blanket (Ries suevite, Germany)

“…We propose that initial ejection of the suevitic material rapidly formed a gas-particle density current during the early excavation stage of the crater. This ejection may have been enhanced by degassing of volatile-bearing target lithologies (Thompson and Spray, 2017) and substantially enhanced by the very rapid post-impact pressure release (Collins et al, 2012). The ejected dispersion of gas and particles was too dense to loft through the atmosphere so it fountained, collapsing and dispersing outward as a radial density current (t 1 in Fig.…”

Heterogeneity of melts in impact deposits and implications for their origin (Ries suevite, Germany)

“…; Coney et al. ; Thompson and Spray ). Furthermore, this is the site where high‐pressure minerals, e.g., coesite and stishovite, and diagnostic features of impact processes were first discovered (Shoemaker and Chao ), and where the earliest hypotheses were developed about how impact melt forms and is incorporated from its place of origin into suevite, involving comminution, melt‐fragmentation, mixing, transport, and deposition (e.g., Stöffler ; Engelhardt and Graup ; Engelhardt ).…”

“…Some are suitable for dating impact events (e.g., Jourdan et al 2012) and some preserve traces of the meteorite (Koeberl et al [2012] and references therein). The Ries impact structure is where suevite was first defined (type locality), and is often used as a key location for the description of other suevite occurrences (e.g., Claeys et al 2003;Coney et al 2010; Thompson and Spray 2017). Furthermore, this is the site where high-pressure minerals, e.g., coesite and stishovite, and diagnostic features of impact processes were first discovered (Shoemaker and Chao 1961), and where the earliest hypotheses were developed about how impact melt forms and is incorporated from its place of origin into suevite, involving comminution, melt-fragmentation, mixing, transport, and deposition (e.g., St€ offler 1977;Engelhardt and Graup 1984;Engelhardt 1997).…”

Thermo‐mechanical interaction of a large impact melt sheet with adjacent target rock, Sudbury impact structure, Canada

“…Our study of the thermal aureole of the SIC may also pertain to other impact structures hosting thick impact melt sheets, notably the 213.2 AE 5.4 Ma Manicouagan impact structure, Quebec (Van Soest et al 2011). Although on different length scales, the lithological and structural characteristics of the lower portion of the >200 m thick impact melt rock and adjacent target rock (Spray and Thompson 2008) at Manicouagan are (1) fragmented to brecciated target rock grading into (2) allochthonous clastic or meltbearing breccia, overlain by (3) clast-laden impact melt rock and topped by (4) clast-free, igneous-textured impact melt rocks (Thompson and Spray 2017). The similarity of these lithological characteristics to the Footwall Breccia realm at Sudbury may indicate mass redistribution processes akin to those inferred for the basal SIC and adjacent target rock.…”