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TitreReappraisal of Precambrian sheet-braided rivers: Evidence for 1.9 Ga deep-channeled drainage
AuteurIelpi, A; Rainbird, R H
SourceSedimentology vol. 63, 2016 p. 1550-1581, https://doi.org/10.1111/sed.12273
Année2016
Séries alt.Secteur des sciences de la Terre, Contribution externe 20150415
ÉditeurBlackwell
Lang.anglais
DOIhttps://doi.org/10.1111/sed.12273
Mediapapier; en ligne; numérique
Formatspdf
ProvinceNunavut; Territoires du Nord-Ouest
SNRC76E; 76F; 76G; 76H; 76I; 76J; 76K; 76L; 76M; 76N; 76O; 76P; 77A; 77B; 77C; 77D; 77E; 77F; 86H/01; 86H/02; 86H/07; 86H/08; 86H/09; 86H/10; 86H/15; 86H/16; 86I/01; 86I/02; 86I/07; 86I/08; 86I/09; 86I/10; 86I/15; 86I/16; 86P/01; 86P/02; 86P/07; 86P/08; 86P/09; 86P/10; 86P/15; 86P/16; 87A/01; 87A/08; 87A/09; 87A/16; 87D/01; 87D/08; 87D/09; 87D/16; 87E/01; 87E/08; 87E/09; 87E/16
Lat/Long OENS-114.0000 -104.0000 71.0000 65.0000
Sujetsgéologie de l'arctique; systèmes fluviaux; transport fluvial; processus fluviaux; dépôts fluviaux; grès; milieu sédimentaire; antecedents de sedimentation; stratification entrecroisée; dépôts éoliens; chenaux anastomosés; niveaux des eaux souterraines; Formation de Burnside River ; sédimentologie; Précambrien; Protérozoïque
Illustrationslocation maps; geological sketch maps; cross-sections, stratigraphic; photographs; tables; graphs; satellite images
ProgrammeChantrey-Thelon de la province de Rae, GEM2 : La géocartographie de l'énergie et des minéraux
Résumé(disponible en anglais seulement)
The repetitive sedimentology of many Precambrian sheet-dominated fluvial sandstones favoured their attribution to unconfined depositional processes. This article presents outcrop evidence for deep channelled drainage in the 1.9 Ga Burnside River Formation of Kilohigok Basin, Arctic Canada. On the ground, sheet-like sandbodies with ubiquitous cross-bedding are at first consistent with classic, unconfined depositional models. However, satellite and oblique-aerial imagery of sections up to 15 km wide and 500 m thick reveals the occurrence of incised palaeovalleys hosting clustered, kilometre-scale, channel bodies with attached large foreset bars pointing to downstream-lateral accretion, sand sheets with aspect ratios (i.e. width to thickness) as high as 2500, and scattered aeolian intervals. The genetic association of these architectural elements points to aggradational fluvial piedmonts composed of low relief unit bars generated by braidplain channels several metres deep. Preservation of aeolianites was facilitated by fluctuating groundwater table and accommodation. Fluvial piedmonts were transected by weakly-sinuous channel belts up to 25 m deep and characterised by through-going or tributary planform. Aspect ratios comparable with those of late Palaeozoic to modern braided channels disprove the inference that all Precambrian streams readily widened in response to increased discharge. Previous facies models for large-scale Precambrian sheet-braided rivers failed to depict entire channel forms, possibly because they could not be resolved by ground-based observations. Based on their limited geomorphic variability and abundance of architectural elements with very high aspect ratios, this study recommends that large sheet-braided fluvial systems should still be considered separately from their post-Silurian (i.e. vegetated) braided counterparts. Parallels between sheet-braided and modern dryland rivers do not however reconcile with the deep, perennial, channelised processes described here. Yet, distal sand-bed and perennial reaches of modern sandur plains remain the closest analogue to sheet braided rivers. This conjecture contradicts the assumption that all Precambrian rivers were prone to simulate seasonal behaviours independently from their actual climate regime. Previous facies models for Precambrian sheet-braided rivers failed to depict large channelised forms, as ground methods were not aided by remote sensing on wide exposures. Based on their limited geomorphic variability and abundance of architectural elements with very high aspect ratios, we advise that sheet-braided rivers still be considered a separate entity from the braided counterparts post-dating the rise of vegetation. Parallels between sheet-braided and modern dryland rivers do not reconcile with the deep, perennial channelised processes evidenced here. Yet, distal sand-bed and perennial reaches of modern sandur plains remain the closest analogue to sheet-braided rivers. This conjecture contradicts the assumption that Precambrian rivers were prone to simulate seasonal behaviours independently from their actual climate regime.
Résumé(Résumé en langage clair et simple, non publié)
Les modèles précédents de faciès de cours d'eau anastomosés en nappes du précambrien ne réussissaient pas à représenter de grandes formes chenalisées parce que la télédétection appliquée à des champs étendus ne complétait pas les travaux sur le terrain. En tenant compte de leur variabilité géomorphologique limitée et de l'abondance de leurs éléments architecturaux présentant des rapports de forme très élevés, nous conseillons que les cours d'eau anastomosés en nappes soient considérés séparément des homologues anastomosés, qui sont généralement postérieurs à la végétalisation. Les parallèles entre les cours d'eau anastomosés en nappes et les cours d'eau en milieu aride modernes ne correspondent pas aux processus profonds et permanents de chenalisation décrits ici. Toutefois, les lits de sable distaux et les biefs pérennes des plaines d'épandage fluvioglaciaire modernes demeurent les entités analogues le plus proche des cours d'eau anastomosés en nappes. Cette conjecture contredit l'hypothèse selon laquelle les cours d'eau du Précambrien auraient tendance à simuler des comportements saisonniers indépendamment de leur régime climatique réel.
GEOSCAN ID297545