Evidence for proximal ejecta from the 2.02 Ga Vredefort impact event
Bibliographic record
Abstract
• Previously unrecognized ejecta from world’s largest impact crater discovered. • Discovery of shocked minerals in iron mining district. • World’s largest impact crater was probably on land, not sea. • South African iron mines are partially excavating impact ejecta. The Vredefort impact structure in South Africa is among the largest and oldest impact structures currently known on Earth. Due to extensive erosion, the proximal ejecta has been thought to have been removed completely. However, we find that sediments of the Doornfontein Member of the Gamagara Formation in the Griqualand West Basin resemble impact ejecta. They are located ca. 500 km west of the center of the Vredefort structure, which is within the modelled distance for the proximal ejecta, and are chronostratigraphically constrained to be between 1928 ± 4 Ma and 2222 ± 13 Ma, overlapping with the Vredefort event age (2019 ± 2 Ma). The stratigraphy of the Doornfontein Member includes an up to 100 m thick polymict breccia sequence that includes fragments of banded iron formation, chert, and carbonates, capped by accretionary lapilli. Shock metamorphic evidence in the form of quartz grains with planar deformation features and planar fractures, as well as granular zircons, are entrained within the breccia, confirming an impact origin. Primary compositions are obscured due to hematite alteration. We suggest that the economically significant Doornfontein Member represent proximal ejecta generated by the Vredefort event. Our discovery could represent the oldest described crater-ejecta pair and shows the potential for the preservation of massive Paleoproterozoic ejecta layers.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.001 | 0.000 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.004 | 0.001 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".