Proximal ejecta of the putative parent impact crater for Australasian tektites at the Bolaven Plateau

Sieh et al. (1) 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.

Fig. 1.

Map showing distribution of the diamicton (circles with thickness in the legend) and lava flows on and around BP. Triangles indicate lavas with ⁴⁰Ar-³⁹Ar ages older than or roughly contemporaneous with the AAT impact; their color indicates alkaline (red) or tholeiitic (white) basalts. The map has been constructed by merging maps in figures 7 and S7 from ref. 1, and complemented with age and composition data from supplements in ref. 2.

Fig. 2 demonstrates that a basalt contribution to AAT source materials must have been quite minor, if any. There is no trend among the Muong Nong, splash-form, and ablated splash-form AAT. For microtektites, the observed trend may permit addition of preimpact alkaline basalt, but not the tholeiite predominant on BP and in the diamicton. The trend observed for microtektites can be explained by their origin from the finest clay fraction of the target, enriched in light rare earth elements and depleted in heavy, Zr-rich minerals (7), in microtektites indicated by Mg and ¹⁰Be enrichment (5, 8), similarly to trends observed in the CL (9, 10) possibly related to AAT source materials (6).

Fig. 2.

Distinction of the BP primary lithologies using the Nb/Zr vs. Yb/La dependence, with AAT (MN, Muong Nong, SF, splash-form, ASF, ablated splash-form, and MT, microtektites) and Chinese loess (CL) data added for comparison. Basalt and sandstone fields were adopted from figure S5 in ref. 1, individual basalt data represented by numbers indicating age (Ma) from supplements in ref. 2, and AAT and CL data from supplements in refs. 5 and 6, respectively.

In conclusion, the description of the diamicton in ref. 1 disqualifies rather than confirms its impact origin and the hypothesis of the parent impact crater for AAT located at BP.