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Titel |
Supercritical strata in Lower Paleozoic fluvial rocks: a super critical link to upper flow regime processes and preservation in nature |
VerfasserIn |
David Lowe, Bill Arnott |
Konferenz |
EGU General Assembly 2015
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Medientyp |
Artikel
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Sprache |
Englisch
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Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 17 (2015) |
Datensatznummer |
250107573
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Publikation (Nr.) |
EGU/EGU2015-7279.pdf |
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Zusammenfassung |
Recent experimental work has much improved our understanding of the lithological
attributes of open-channel supercritical flow deposits, namely those formed by
antidunes, chutes-and-pools and cyclic steps. However their limited documentation in
the ancient sedimentary record brings into question details about their geological
preservation.
Antidune, chute-and-pool and cyclic step deposits are well developed in sandy ephemeral
fluvial deposits of the Upper Cambrian – Lower Ordovician Potsdam Group in the Ottawa
Embayment of eastern North America. These high energy fluvial strata form dm- to a few
m-thick units intercalated within thick, areally expansive successions of sheet sandstones
consisting mostly of wind ripple and adhesion stratification with common deflation lags.
Collectively these strata record deposition in a semi-arid environment in which rare,
episodic high-energy fluvial events accounted for most of the influx of sediment
from upland sources. Following deposition, however, extensive aeolian processes
reworked the sediment pile, and hence modified profoundly the preserved stratigraphic
record.
Antidune deposits occur as 0.2 – 1.6 m thick cosets made up of 2 – 15 cm thick lenticular
sets of low angle (≈¤ 20o) cross-stratified, medium- to coarse-grained sandstone bounded by
low-angle (5 – 15o) concave-upward scours and, in many cases, capped by low angle (10 –
15o) convex-upwards symmetrical formsets. Chute-and-pool deposits form single sets, 5 – 55
cm thick and 0.6 – 6 m wide, with scoured bases and low to high angle (5 – 25o)
sigmoidal cross-strata consisting of medium- to coarse-grained sandstone. Cyclic step
deposits consist of trough cross-stratified sets, 20 cm – 1.6 m thick, 2.5 – 12 m long
and 7 – 35 m wide, typically forming trains that laterally are erosively juxtaposed
at regularly-spaced intervals. They are composed of medium- to coarse-grained
sandstone with concave-up, moderate to high angle (15 – 35o) cross-strata with
tangential bases that conform to the shape of the basal bounding surface of the set.
Antidune and cyclic step deposits are common and fill 0.4 – 1.8 m deep channels,
which then are generally overlain by extensive (>1 km) aeolian deflation surfaces.
Chute-and-pool strata, however, are rare and only occur as isolated scour-filling sets within
unconfined floodplain deposits. Nowhere in outcrop do different kinds of supercritical
bedform deposits interfinger or appear related to the same flow event, suggesting that
individual packages of supercritical strata were deposited by discreet, rapidly waning
flows with little time for incremental growth or deposition under changing flow
conditions.
The stratal characteristics and geometries of channel-filling antidune and cyclic step
cosets in the Potsdam are similar to those produced in steady experimental flows with high
rates of aggradation. Similar conditions in the Potsdam were probably attained because
the flows were channelized, which also caused the freshly deposited sediment to
lie beneath the water table, and hence beneath the effects of extensive post-flood
aeolian deflation. Conversely, scour-filling chute-and-pool deposits formed on the
floodplain where highly unsteady, erosive and rapidly waning unconfined flows
formed isolated, partly-filled, erosively-based, ephemeral structures. Moreover,
being formed on the surface of the floodplain subjected these deposits to extensive
post-depositional reworking, and as a consequence caused them to be poorly preserved. |
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