Anatomy of the Chesapeake Bay impact structure revealed by seismic imaging, Delmarva Peninsula, Virginia, USA

Abstract

[1] A 30-km-long, radial seismic reflection and refraction survey completed across the northern part of the late Eocene Chesapeake Bay impact structure (CBIS) on the Delmarva Peninsula, Virginia, USA, confirms that the CBIS is a complex central-peak crater. We used a tomographic P wave velocity model and low-fold reflection images, constrained by data from two deep boreholes located on the profile, to interpret the structure and composition of the upper 5 km of crust. The seismic images exhibit well-defined structural features, including (with increasing radial distance) a collapsed central uplift, a breccia-filled moat, and a collapsed transient-crater margin (which collectively constitute a $40-km-wide collapsed transient crater), and a shallowly deformed annular trough. These seismic images are the first to resolve the deep structure of the crater (>1 km) and the boundaries between the central uplift, moat, and annular trough. Several distinct seismic signatures distinguish breccia units from each other and from more coherent crystalline rocks below the central uplift, moat, and annular trough. Within the moat, breccia extends to a minimum depth of 1.5 km or a maximum of 3.5 km, depending upon the interpretation of the deepest layered materials. The images show $350 to 500 m of postimpact sediments above the impactites. The imaged structure of the CBIS indicates a complex sequence of events during the cratering process that will provide new constraints for numerical modeling.

Deep drilling in the Chesapeake Bay impact structure—An overview

“…It consists of a central crater (collapsed transient cavity) that is ~35-40 km in diameter and is surrounded by a less deformed, outer annular zone that has a radial width of ~25 km. Poag et al (2004), Horton et al (2005b), Catchings et al (2008), and most other previous studies refer to this outer zone as the annular trough ( Fig. 1).…”

“…1) led to the recognition that this structure was the result of the impact of a large extraterrestrial projectile (Poag et al, 1992(Poag et al, , 1994Koeberl et al, 1996). Since the mid-1990s, an increasing number of marine-and land-based seismic refl ection studies, and core holes, have provided the data needed to describe the basic morphology, structure, and stratigraphy of the Chesapeake Bay impact structure (e.g., Poag, 1996Poag, , 1997Poag et al, 1999Poag et al, , 2004Powars and Bruce, 1999;Powars, 2000;Horton et al, 2005aHorton et al, , 2005bHorton et al, , 2008Catchings et al, 2008).…”

Pre-impact tectonothermal evolution of the crystalline basement-derived rocks in the ICDP-USGS Eyreville B core, Chesapeake Bay impact structure

“…spe458-12 1st pgs page 5 the crystalline rocks in this section of the drill core, except in rare occurrences near impact breccia veins (Horton et al,this volume,Chapter 2), and new high-resolution seismic data (Catchings et al, 2008) have been used to suggest that these rocks represent large slumped sections from the outer parts of the transient crater wall, rather than autochthonous crater fl oor (Gohn et al, 2008). Seventy-eight samples of the basement-derived rocks collected by Gibson and Reimold were analyzed as part of this study (see Table 1 in Townsend et al, this volume, for depths).…”

“…In their present orientation, the mylonitic S-C and S-C′ structures indicate reverse slip sense, which might suggest that the D1 event involved thrusting. However, Catchings et al (2008) presented a cautionary note on the assumption that the Eyreville B core intersected in situ basement. Their seismic-refraction and refl ection study suggests that the rocks below the base of the core hole show unusually low velocities, consistent with impact-related fracturing.…”

“…1B), giving the Chesapeake Bay crater its distinctive "inverted sombrero" profi le (Collins and Wünnemann, 2005;Horton et al, 2005c;Poag et al, 2004). The crater contains a 0.9-km-high, 15-20-km-wide central peak of uplifted basement rocks (Horton et al, 2005c;Catchings et al, 2008). Modeling of the cratering event (Collins and Wünnemann, 2005) suggests that the transient crater sampled a volume of rock up to 28 km wide and 7-10 km deep.…”