NEW CONSTRAINTS FOR CRATERING PROCESSES INTO MARINE ENVIRONMENTS FROM THE ICDP-USGS EYREVILLE CORE B, CHESAPEAKE BAY IMPACT CRATER

In contrast to most terrestrial impact craters where suevites represent the uppermost impactite unit, the Eyreville core B revealed a different sequence (from top to bottom: 444 m post-impact sediments, 652 m Exmore breccia, 297 m lithic blocks, 157 m suevites and melt breccias, 216 m apparent basement lithologies). Generally suevites are interpreted as the fallout product of the collapsing ejecta plume. The period for their deposition is unconstrained but may take hours [1]. In the Eyreville core B the suevites are overlain by 950 m of impact-related deposits. This may allow for the first time a more precise estimate of the formation and deposition period of suevites. Collins and Wünnemann [2] found by hydrocode modeling of the Chesapeake Bay impact that the time span between the formation of the transient cavity and rushing back of a weak (water-saturated) upper sediment layer (resulting in deposition of the Exmore breccia) does not exceed 10 minutes. This suggests that the suevites never left the transient cavity. The suevites exposed at 1393-1550 m could represent base-surge material that covered the crater floor. A fraction of the ejecta plume distributed in the atmosphere may have been simultaneously deposited with the Exmore material. The granite block is virtually undeformed and does not show shock features. Therefore, its original position might have been the rim area around the transient cavity that, according to [2], had a diameter of about 28 km. Hence the transport distance is ~5-10 km. Although the size of the block is only known in one dimension, a crude estimate of the force required to move the block as part of the Exmore Formation provides constraints on the dynamics and viscous properties of the Exmore breccia suspension during resurge. The basement beneath the suevites displays a more complex deformation history but likewise shows weak to absent shock metamorphic overprinting, also indicating inward displacement of the basement sequence. At least two pre-impact deformation events can be seen in these rocks: the first one led to ductile folding and local mylonitization, the second occurred under brittle-to-ductile conditions. The impact itself caused the formation of dike breccias and fault networks. [1] Wittmann, A. et al. 2007 GSA Bull. 119 (in press); [2] Collins, G. & Wünnemann, K. 2005, Geology, 33, 925-928.