Proximal ejecta of the putative parent impact crater for Australasian tektites at the Bolaven Plateau / Reply to Mizera and Strunga: Evidence for the Bolaven impact crater and its ejectaOPEN ACCESS
Jiří Mizera and Vladimír Strunga
PNAS, May 13, 2024
LETTER
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“Sieh et al. (Proximal ejecta of the Bolaven extraterrestrial impact, southern Laos ) support their hypothesis locating the parent impact crater for the Australasian tektites (AAT) at the Bolaven Plateau (BP) in Southern Laos (2) with description of the putative proximal ejecta—pebbly to bouldery breccia (“diamicton”) overlying the sandstone/mudstone bedrock, covered with silt, and AAT found between the diamicton and silt. The hypothesis has been debunked by Mizera (3) who demonstrated failings of all presented lines of evidence, particularly the geochemical unsuitability of a sandstone–basalt mixture as AAT source materials. The criticism also included doubts on the putative proximal ejecta, arguing that it must have originated from the preimpact volcanism followed by fast weathering and short transport, and in the wider area of the Khorat Plateau and the Mekong River Basin from weathering and alluvial transport of the Mesozoic sandstones–mudstones–conglomerates spiked with basalts from scattered minor Cenozoic volcanic outcrops. The silt layer may have originated from the aeolian transport enhanced due to postimpact deforestation or later during glacials. Fig. 1 shows that the diamicton has accumulated mainly at the W-SW-S foothills of BP, at the edge of preimpact lava flows, in south overlaid with postimpact flows. Both the diamicton (autobreccia?) and silt bear basaltic signatures. Another accumulation with two thickest sites lies directly on BP and stretches SE of the putative crater. The local geomorphology and hydrology rather than the impact may explain basaltic contamination of the bauxitized sedimentary laterite found there. The bauxite deposit is situated within a lake area probably fed by draining the area with the oldest (>12 Ma) flows; see figure S2 in ref. 2. Also, the sporadic presence of large sandstone slabs near fresh volcanic vents, on a >20° slant, does not need the impact explanation; see Fig. 1 here, figure S2 in ref. 1, and figure 2 in ref. 2. Instead, their survival unshattered upon landing and through 780 ky of intense weathering is doubtful. So is the presence of preimpact basalts and sandstones without any ejecta in a wide area between the putative crater and the diamicton. Finally, the diamicton thickness is quite insufficient for proximal ejecta, and typical impact cratering products (megablocks, suevite, and polymict breccia) are missing; cf. ref. 4.”
Reply to Mizera and Strunga: Evidence for the Bolaven impact crater and its ejecta
Kerry Sieh, , Jason Herrin, and Dayana Schonwalder Angel
PNAS, May 13, 2024
REPLY
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“Five independent, multidisciplinary, field-based lines of evidence indicate that the source crater of the Australasian strewn field lies buried within a basaltic field in southern Laos (1, 2). In this short-format rebuttal to two recent critics (3, 4), we address just two of their several contentions.
They argue that trace-element systematics preclude a significant basaltic component in Australasian tektites. To the contrary, our Fig. 1 overlay of binary mixing models onto their figure 2 demonstrates that tektite compositions are consistent with a 30 to 40% basaltic component, in agreement with estimates from our principal component analysis of major elements [(1) p. 3 and figure S5]. Abundant stratigraphic and sedimentological evidence favors formation and emplacement of the regionally extensive Bolaven diamicton by impact-related processes. It is normally graded, contains unabraded tektites in its upper portions, and thickens and coarsens toward the plateau, all of which are consistent with theory and experiment [(5) and (2), figure 12]. Its fine component bears chemical residues of basalt even at some sites in drainages without basaltic outcrops. At some sites very near the impact site, diamicton clasts have basaltic affinities even though the bed rests upon sandstone or mudstone bedrock [(2) p. 5, figures 6B and S12)]. The argument for postdepositional basaltic contamination of these deposits, which calls upon unspecified effects of “local geomorphology and hydrology” “within a lake area” (4), is untenable. It ignores the character of the sediments, the position of some of the deposits on high, flat, erosional surfaces without upslope basaltic sources, and that the lakes are postimpact modern reservoirs for hydroelectric power generation (6) that lie in canyons cut into Mesozoic fluvial sandstones (7). Their arguments that we have misinterpreted the bouldery diamicton elsewhere confuse and conflate two distinctly different types of outcrop. One type is the complex diamiction depicted in the composite sketch of our figure 2 and illustrated with many examples. These we explain as deposits of the ejecta curtain. The other type is represented by three fields of dislodged but not ejected sandstone boulders just outside the perimeter of the buried crater [(1) p. 7, figure S16, and (2) p. 9, figures S2 and S7]. We propose the latter to have been lifted from subjacent bedrock by shock-related processes earlier in the impact process [(2) p. 9]. Incidentally, other recent field-based studies reinforce the Bolaven-impact hypothesis (8–10). Although some argue that definitive proof of the Bolaven impact crater will depend on drilling into it (11), we contend that discovery of the Bolaven diamicton essentially puts the case to rest and should serve to encourage further studies of the environmental effects of the impact and the detailed nature of its crater.”
