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Description of proposed impact melts on the coast of Encounter Bay South Australia consistent with an oblique impact

Presentation #405.06 in the session Asteroids: Planetary Defense (Poster + Lightning Talk)

Published onOct 23, 2023
Description of proposed impact melts on the coast of Encounter Bay South Australia consistent with an oblique impact

Apparent impact-melt seams were documented in-situ in exposed outcrops on the South Australian mainland at Middleton Head and Port Elliot. A site [1] in Encounter Bay just south east of Port Elliot is the prime candidate for the origin of widely dispersed putative ejecta found in the Adelaide Hills [2-4]. The quartz seams lie within Cambrian sedimentary rocks at Middleton and Ordovician-Cambrian granite at Port Elliot respectively. Of these quartz seams, those in granite at Port Elliot contain obvious black glass material, while those at Middleton contain a grayer glass and more iron than the Port Elliot examples. It has been proposed [1] that an oblique (shallow angle) impact, centered on what is now continental shelf at coordinates -35.56, 138.72, came from the S-SW horizon, consistent with a proposal by Haines [3] that the Fleurieu Peninsula and Adelaide Hills experienced an impact at ~5 degrees from horizontal hailing from that direction more than 35 mya. Considering all evidence from the Middleton-Port Elliot section of the Encounter Bay coast, including iron-melt that is putatively from projectile material, a sequence of impact related- and then hydrothermal- events accounts well for all the observed features. Initially target-rock melt would have been forced down to depth before rising again, or else, given the expected depth of a several-kilometer-long crater, melt may have risen from the crater floor and flowed in seams in all directions preferentially forward. Fractures which the impact would have caused in near-target strata proximal to the crater, appear to have been initially both filled by- and partially widened by the melt glass, then subsequently widened further by hydrothermal quartz. The dark glassy material and the quartz, in many cases have not mixed, lending support to the two-stage upwelling scenario. Local indigenous people’s oral traditions can be interpreted as estimating the time of the event as Paleocene, and imply that their knowledge of geological strata and fossil records may have been far more advanced than was previously known, as this timing is consistent with that proposed by Haines [3] and subsequently by this author [1]. If the event pans out then the impact melts described here are the first strong evidence of a meteor trajectory not lying in the plane of the ecliptic, which has repercussions for planetary defense scenarios from left of field, presumably rare. [1] Moore RB (2022) MetSoc LXXXVI #6148. [2] Haines PW et al. (1999) MetSoc LXII Meteoritics & Planet. Sci., 34, A49 [3] Haines P. (2000). Catastr Events: Geozentrum, Vienna #3093. [4] Moore RB (2021) AGU Fall Meeting EP55A-1102.

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