Precambrian fluvial systems, lacking the influence of rooted vegetation, probably were characterised by flashy surface runoff, low bank stability, broad channels with abundant bedload, and faster rates of channel migration; consequently, a braided fluvial style is generally accepted. Pre-vegetational braided river systems, active under highly variable palaeoclimatic conditions, may have been more widespread than are modern, ephemeral dry-land braided systems. Aeolian deflation of fine fluvial detritus does not appear to have been prevalent. With the onset of large cratons by the Neoarchaean–Palaeoproterozoic, very large, perennial braided river systems became typical. The c. 2.06–1.88 Ga Waterberg Group, preserved within a Main and a smaller Middelburg basin on the Kaapvaal craton, was deposited largely by alluvial/braided-fluvial and subordinate palaeo-desert environments, within fault-bounded, possibly pull-apart type depositories.
Palaeohydrological data obtained from earlier work in the Middelburg basin (Wilgerivier Formation) are compared to such data derived from the correlated Blouberg Formation, situated along the NE margin of the Main basin. Within the preserved Blouberg depository, palaeohydrological parameters estimated from clast size and cross-bed set thickness data, exhibit rational changes in their values, either in a down-palaeocurrent direction, or from inferred basin margin to palaeo-basin centre. In both the Wilgerivier and Blouberg Formations, calculated palaeoslope values (derived from two separate formulae) plot within the gap separating typical alluvial fan gradients from those which characterise rivers (cf. [Blair, T.C., McPherson, J.G., 1994. Alluvial fans and their natural distinction from rivers based on morphology, hydraulic processes, sedimentary processes, and facies assemblages. J. Sediment. Res. A64, 450–489.]). Although it may be argued that such data support possibly unique fluvial styles within the Precambrian, perhaps related to a combination of major global-scale tectono-thermal and atmospheric–palaeoclimatic events, a simpler explanation of these apparently enigmatic palaeoslope values may be pertinent. Of the two possible palaeohydrological formulae for calculating palaeoslope, one provides results close to typical fluvial gradients; the other formula relies on preserved channel-width data. We suggest that the latter will not be reliable due to problematic preservation of original channel-widths within an active braided fluvial system. We thus find no unequivocal support for a unique fluvial style for the Precambrian, beyond that generally accepted for that period and discussed briefly in the first paragraph. 相似文献
The evolution, migration and deposits of a gravelly braid bar in the Sagavanirktok River, northern Alaska, are described in unprecedented detail using annual aerial photographs, ground‐penetrating radar (GPR) profiles, trenches and cores. Compound braid bars in the Sagavanirktok River form by chute cut‐off of point bars and by growth of mid‐channel unit bars. Subsequent growth is primarily by accretion of unit bars onto their lateral and downstream margins. The upstream ends of braid bars may be sites of erosion or unit bar deposition. Compound braid bar deposits vary in thickness laterally and are thickest in medial sections and near cut banks. Compound bar deposits are typically composed of three to seven sets of simple large‐scale inclined strata, each simple set formed by a unit bar. The simple large‐scale strata contain medium‐scale cross‐strata (from dune migration) and planar strata (from migration of bedload sheets). The upstream and medial parts of compound braid bar deposits show very little vertical variation in grain size, but downstream and lateral margins tend to fine upwards. The deposits are mostly poorly sorted sands and gravels, although sands tend to be deposited at the top of the braid bar, and open‐framework gravels preferentially occur near the top and base of the braid bar. The patterns of braid bar growth and migration, and the nature of the deposits, described from the Sagavanirktok River are generally similar to other sandy and gravelly braided rivers, and consistent with the theoretical braid bar model of Bridge (1993). 相似文献
On the southwest-facing slopes of a bedrock ridge lying between Cardigan Bay to the north and the Afon Teifi to the south stands a group of hills in which 30-35 m of cross-laminated and parallel-laminated sands with lenticular upward-fining gravel sequences are overlain by 10-12 m of gravel in a single foreset bed. The sediments mantle a surface of till sloping gently toward the southwest, were transported toward the southwest (across one margin of the Afon Teifi valley), and were cut by a system of densely arranged conjugate normal faults striking northwest-southeast. The lenticular gravels and fault system suggest that the deposits accumulated as a glaciofluvial outwash spread, and on top of an ice-lobe that became isolated in the Teifi valley during the downwasting of a glacier which had occupied Cardigan Bay and much of the country to the south. The large gravel foresets capping the succession are the only indication at Banc-y-Warren of the former existence of a lake, but neither a large nor deep body of water need be envisaged. 相似文献
The relation between morphological change and patterns of variation in bedload transport rate in braided streams was observed by repeated, daily topographic surveys over a 25 day study period in a 60 m reach of the proglacial Sunwapta River, Alberta, Canada. There are two major periods of morphological change, each lasting several days and each involving the complete destruction and reconstruction of bar complexes. Bar complex destruction was caused by redirection of the flow and by downstream extension of the confluence scour zone upstream. Reconstruction involved accretion of unit bars on bar head, flank and tail and in one case was initiated by disection of a large, lobate unit bar. High rates of sediment movement, measured from net scour and fill of the cross-sections, coincided with these morphological changes. Sediment was supplied from both bed and bank erosion, and patterns and distances of transfer were highly variable. Rates of transport estimated by matching upstream erosional volumes with downstream deposition were much greater than those estimated from either a step-length approach or a sediment budget. Measurements of scour and fill and observations of morphological change indicate that step lengths (virtual transport distances) were typically 40–100m during a diurnal discharge cycle. Shorter step lengths occurred when transfer was confined to a single anabranch and longer steps involved channel changes at the scale of the entire reach. Sediment budgeting was used to describe the spatial patterns of sediment transport associated with the morphological changes and to estimate minimum daily reach-averaged transport rates. Mean bedload transport rates correlate with discharge, but with considerable scatter. The largest deviations from the mean relation can be tied to phases of channel incision, bank erosion, scour hole migration, bar deposition and channel filling apparently controlled by changes and fluctuations in sediment supply from upstream, independent of discharge. These are interpreted as field evidence of ‘autopulses’ or ‘macropulses’ in bedload transport, previously observed only in laboratory models of braided streams. 相似文献
Braided and anastomosing channels make up two major coexistent networks in the mud-dominated fluvial system of Cooper Creek, Southwest Queensland. The floodplain is characterized by a system of mud braids operative when floods inundate the whole alluvial surface. Anastomosing channels are inset deeper into the floodplain, operate at modern flows, and transport a traction load of sand. Shallow stratigraphic data show that an underlying sand sheet is unrelated to surface channel patterns and was formed by a system of meandering streams. According to preliminary dates based on thermoluminescence, the change from a sand- to mud-dominated fluvial regime took place between 50 000 and 200 000 years B.P., and probably reflects increasing aridity. 相似文献
In braided fluvial deposits, consisting of monotonous successions of sandstone or conglomerate, it may be difficult to distinguish regionally significant bounding surfaces (sequence boundaries) from autogenic channel-scour surfaces. Major surfaces may be characterized by erosional relief and draped by lag deposits, but not all sequence boundaries show these characteristics. Other clues to the presence of a major surface are sharp changes in detrital composition, shifts in regional palaeocurrent trends and evidence of early diagenesis of the sandstones immediately below the sequence boundary. Examples of these attributes of cryptic sequence boundaries are illustrated from three Mesozoic units in the Colorado Plateau area of the United States. In the Chinle Formation (Triassic), near Moab, Utah, angular intraformational unconformities overlie sandstones showing evidence of early diagenesis. In the Castlegate Sandstone (Upper Cretaceous) of east-central Utah, a cryptic sequence boundary can be discriminated from other erosion surfaces by the evidence of detrital petrography and early diagenesis. Palaeocurrent data indicate changes in regional palaeoslope at two sequence boundaries within this unit. Evidence of early diagenesis is also present at a sequence boundary in the Kayenta Formation (Jurassic) of westernmost Colorado. 相似文献
This paper presents flume and field observations of a bank-confined braided river. Morphological features, including plan form configuration, channel width, and main channel migration, were examined by a series of experiments. Repeated measurements of channel morphology, provided a basis to estimate the relationship between noncumulative frequency of bars and bar area. Additional results from the Dajia River, located in Central Western Taiwan, were presented to provide a reference data set for comparing the laboratory and field data. The results indicate that the relationship between bar length and width can be predicted by a simple best-fit power function relating to self-similarity characteristics. The Hurst index by Walsh and Hicks (2002) provides acceptable predictions of the bar length and width observed in the experiments and confirmed by the field investigations. Eexperimental and field results both show that large river width yields a uniform distribution of bar areas with the similar discharge, leading to a large value of exponent (β) in the model. The river width is confirmed to be a critical parameter in the main channel shift. A small increase in channel width likely increased rapidly the shift cycle. 相似文献