Ichnology and Sedimentology Reveal Depositional Characteristics of Bay-Margin Parasequences in the Miocene Amazonian Foreland Basin
Bibliographic record
Abstract
Abstract Proposed depositional models for Miocene Amazon foreland basin strata (Pebas Formation, Peru) are controversial. Recent depositional models include lacustrine and tidally influenced, brackish-water embayment. This paper presents data that support tidally influenced, brackish-water deposition for at least part of Pebas time (10-14 Ma). Two parasequences are presented (Santa Julia and Tamshiyacu). Both crop out along the Amazon River in Upper Amazonia near Iquitos, Peru. At these locations, abundant evidence of brackish-water, tidally influenced deposition is documented, including marginal marine bioturbation, sedimentary couplets, semidiurnal couplets (preserved in burrows), and pinstripe lamination. The deposits are locally highly bioturbated. At both locations ichnogenera normally associated with marine to brackish-water depositional environments are common. Three normally marginal-marine ichnofabrics are reported: (1) a Chondrites-reburrowed, Planolites ichnofabric resident only in massive-appearing muds; (2) a Scolicia (Laminites), Thalassinoides, Ophiomorpha ichnofabric that is manifested as intensely bioturbated silty sands; and (3) a Thalassinoides-generated ichnofabric that is interpreted to have descended into consolidated substrates and is thus representative of the Glossifungites Ichnofacies. Several trace fossils contain laminated infills organized into distinct sedimentary couplets that are best interpreted as resulting from semidiurnal processes. Six conclusions are arrived at: (1) sedimentological and ichnological data consistently indicate that sediment accumulation dominantly occurred in sporadically dysaerobic, marine to brackish water, under a tidal influence; (2) sediment accumulation occurred in bay-margin environments that prograded into a shallow, quiescent bay; (3) a stratified water column is indicated by the ichnofauna; (4) low accommodation space, repetitive and rapid adjustments of relative sea level, shallow wave base, and a stratified water column combined to generate an atypical parasequence architecture; (5) previously published isotopic data are consistent with sediment accumulation in brackish to marine water; and (6) marine incursion into Amazonia occurred during the Middle Miocene.
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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) | 0.002 | 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".