An Eocene post-kimberlite maar lake: lacustrine oil-shale crater-fill deposits, Lac de Gras area, Northwest Territories, Canada
Bibliographic record
Abstract
Sedimentology and organic petrology-geochemistry from a 160-metre core have been integrated to study the characteristics of an isolated pocket of fine-grained, siliciclastic Eocene sediments deposited within a small kimberlite crater basin, Lac de Gras, Northwest Territories, Canada. These sediments overlie the "Giraffe Pipe", a kimberlite occurrence located at about 65°N/110°W, 25 km northeast of the Ekati Diamond Mine. The strata recovered in the studied core represent an overall shallowing-upward succession of lacustrine-peat mire basin-fill, interpreted to portray the deposits of a maar lake, one of the first identified in Canada (and one of the few maars anywhere to be associated with a kimberlitic pipe). Previously-published palynological data suggest a late Early Eocene to early Middle Eocene age, approximately congruent with a previous radiometric date of 47.4 +/- 0>5 Ma. These deposits include three stratal units: a) a Lower Lacustrine Zone dominated by dark, freshwater, organic-rich mudstone and thick occurrences of oil shale (% total organic carbon: TOC = 15-50 %), interpreted as recording low energy, shallow sub-lacustrine deposition within the crater basin; b) a thin Middle Transitional Zone with evidence of very shallow subaqueous deposition and subaerial exposure (TOC = 2-12 %); and c) an Upper Mire Zone characterized by thick subaerial peat deposits (TOC = 39-55 %), with minor sublacustrine mudstones, interpreted to represent accumulation in a primarily continental setting filling the crater basin. Both lower and upper portions of the succession are characterized by shallowing-upward, higher order sequences whose boundaries correspond with marked changes in TOC, and Rock Eval hydrogen and oxygen indices. These are interpreted to represent either cyclic climatic wet/dry phases or episodic kimberlite/diatreme collapse (downward stoping) subsidence/fill phases. The distribution and type of microscopic organic matter reflects the sedimentological observations: the lower, lacustrinedominated zone includes abundant freshwater diatoms, chrysophytes and liptinites (e.g. sporinite, alginite), whereas the upper, peat mire zone consists predominantly of woody/peat macerals with wellpreserved plant macrofossils. Huminites through the section have reflectance values averaging 0.23 %Ro, indicating that the thermal maturity of these ~47 M.Y.-old sediments, remarkably, is only slightly greater than modern peats, and that post-kimberlite burial and thermal alteration have been insignificant, with temperatures no greater than ~ 30°C. An abrupt cooling and the presence of bentonites during the rapid transitional conversion from lacustrine to mire facies, may indicate a previously-unrecognized regional, post-eruptive, uplift phase, which is recorded in both geochemical and sedimentological indicators.
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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 | 0.002 |
| 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) | 0.003 | 0.000 |
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".