Detrital zircon geochronology of the Roberts Mountains allochthon, Nevada
Bibliographic record
Abstract
U-Pb geochronologic analyses have been conducted on 205 individual detrital zircon grains from Cambrian through Devonian sandstones of the Roberts Mountains allochthon in central Nevada. These strata were tectonically emplaced onto the Cordilleran margin during the mid-Paleozoic Antler orogeny, but their original depositional settings and provenance have been controversial. Our data, combined with previous detrital zircon studies, define four different age signatures for the eugeoclinal strata: (1) 690-715 and 1065-1350 Ma grains in a minor group of sandstones in the Upper Cambrian(?) Harmony Formation, (2) 1745-1790, 1820-1860, and 2595-2700 Ma grains for most of the Harmony Formation, (3) 1410-1445, 1665-1690, and 1705-1740 Ma grains for lower Middle Ordovician sandstones of the Vinini Formation, and (4) 1020-1045, 1815-1860, 1905-1940, and 2645-2740 Ma ages for lower Upper Ordovician sandstones in the Vinini, Valmy, Snow Canyon, and McAfee Formations, for the Silurian Elder Sandstone, and for the Devonian Slaven Chert. Comparison of these data with the detrital zircon reference for the Cordilleran miogeocline and with ages of basement provinces in cratonal North America indicates that sandstones in the lower Vinini and parts of the Harmony Formations were derived from 1.0-1.3, -1.43, and 1.6-1.8 Ga provinces of southwestern North America. In contrast, most older and younger units contain few grains of the appropriate ages to have come from the southwestern part of North America, and instead have strong similarities with the Peace River arch region of western Canada. We propose that detritus in most of the Harmony Formation was shed from off-shelf basement rocks exposed along the Canadian continental margin, perhaps as a western continuation of the Peace River arch or as extensional fault blocks. In contrast, detritus in the lower Upper Ordovician through Devonian strata is interpreted to have been recycled from platformal strata exposed along the flanks of the Peace River arch. Transport of the detritus is interpreted to have been largely via turbidity currents flowing in off-shelf basins or trenches, rather than by longshore currents on the shelf. These provenance links provide new insights into the paleodispersal history along the Cordilleran margin, and indicate that sandstones of the Roberts Mountains allochthon received detritus from, and therefore accumulated near, western North America.
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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 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 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".