| Title | Reappraisal of Precambrian sheet-braided rivers: Evidence for 1.9 Ga deep-channeled drainage |
| |
| Author | Ielpi, A; Rainbird, R H |
| Source | Sedimentology vol. 63, 2016 p. 1550-1581, https://doi.org/10.1111/sed.12273 |
| Image |  |
| Year | 2016 |
| Alt Series | Earth Sciences Sector, Contribution Series 20150415 |
| Publisher | Wiley-Blackwell |
| Document | serial |
| Lang. | English |
| Media | paper; on-line; digital |
| File format | pdf |
| Province | Nunavut; Northwest Territories |
| NTS | 76E; 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 WENS | -114.0000 -104.0000 71.0000 65.0000 |
| Subjects | sedimentology; arctic geology; fluvial systems; fluvial transport; fluvial processes; fluvial deposits; sandstones; depositional environment; depositional history; crossbedding; eolian deposits; braided
channels; groundwater levels; Burnside River Formation; Kilohigok; Precambrian; Proterozoic |
| Illustrations | location maps; geological sketch maps; cross-sections, stratigraphic; photographs; tables; graphs; satellite images |
| Program | GEM2: Geo-mapping for Energy and Minerals Rae Province, Chantrey-Thelon |
| Released | 2016 05 25 |
| Abstract | 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. |
| Summary | (Plain Language Summary, not published)
Previous facies models for Precambrian sheet-braided rivers failed to depict large channelized 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 be considered separately from the braided counterparts, which generally
post-date the rise of vegetation. Parallels between sheet-braided and modern dryland rivers do not reconcile with the deep, perennial channelized 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 Precambrian rivers were prone to simulate seasonal behaviours independently from their actual climate regime. |
| GEOSCAN ID | 297545 |
|
|