Carbonate platform dominated by peloidal sands: Lower Ordovician La Silla Formation of the eastern Precordillera, San Juan, Argentina
Bibliographic record
Abstract
Abstract The Lower Ordovician La Silla Formation of the Precordillera of west‐central Argentina is part of the west‐facing early Palaeozoic, tropical carbonate platform succession that comprises the core of the Cuyania terrane. Up to 360 m thick, it is exposed in several thrust sheets over a distance of some 250 km along and across depositional strike over a palinspastically unrestored distance of about 35 km. La Silla Formation is a strikingly pure limestone with subordinate finely crystalline dolomite and rare chert. It accumulated on a more or less uniformly subsiding passive margin. Copious precipitation of microcrystalline calcite, probably influenced by microbial activity to varying degrees, led to the generation of peloids, ooids and aggregates of these grains, as well as small amounts of lime mud, intraclasts, stromatolites and thrombolites. Rare bioclasts are limited mostly to scattered gastropods and trilobite sclerites; bioturbation is present locally. The array of carbonate rock types is grouped into eight recurring lithofacies, in order of decreasing abundance: (i) peloidal grainstone; (ii) laminated dolostone; (iii) intraclastic rudstone; (iv) microbial laminite; (v) peloidal packstone; (vi) ooidal grainstone; (vii) thrombolite boundstone; and (viii) mudstone. These facies represent sediments that formed solely in a shallow subtidal marine environment, with no evidence of restricted conditions, hypersalinity or subaerial exposure. No events of eustatic sea‐level change are recorded. By far the dominant facies is grainstone composed of well‐sorted, fine sand‐sized peloids and peloidal aggregates in homogeneous, tabular to gently undulating, medium to thick beds; cross‐lamination is scarce. Clusters of sub‐metre‐sized microbial patch reefs developed sporadically. The shallow platform is envisaged to have been covered by extensive peloidal sand flats and low‐relief banks, and little lime mud was generated. The setting was probably microtidal and may not have been affected by strong trade winds. It was washed by frequent, relatively gentle wave action but without experiencing powerful storms. In the middle member, anomalous lenses of intraclastic rudstone and laminated dolostone occur as graded beds overlying sharply downcut scoured surfaces up to 20 cm deep; these are interpreted to indicate a phase when accretion was punctuated occasionally by tsunamis generated from rift‐faulting seaward of the platform margin. The remarkably uniform peloidal grainstone composition over a broad area shows that, given the appropriate combination of climate, environmental and ecological factors, large portions of some early Palaeozoic platforms were dominated by grainy sediment and remained under well‐agitated conditions within fair‐weather wave‐base, without distinct lateral facies differentiation or tidal‐flat aggradation.
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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) | 0.002 | 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".