Abstract A recent resurgence in the study of slope deposits is strongly related to the complexity inherent with the depositional setting, and the continued prospectivity for hydrocarbons in associated units. Intraslope basins are particularly intriguing to explorationists, because they provide accommodation space for coarse-grained facies in areas often characterized by otherwise muddy deposits. An outstanding outcrop of a growth-fault-controlled slope minibasin and its sandy fill are exposed in Cretaceous strata of southern Chile. A turbiditic sandstone package (TSP) 60 m thick sharply overlies fine-grained deposits of the Cerro Toro Formation, and defines the base of the Tres Pasos Formation in the study area, 65 km north of Puerto Natales, Chile. Individual sedimentation units are rarely amalgamated, and are tabular for at least hundreds of meters, suggesting deposition in a relatively unconfined setting. Trace-fossil assemblages are grouped into the Zoophycos or Cruziana ichnofacies, and are consistent with slope deposition rather than deposition on the basin floor. Beds at the base of the TSP are truncated by a normal fault at the southern end of the outcrop, and lap out towards the north onto the tilted hanging wall. Stratal thickening and thinning across the fault, and unfaulted overlying deposits, suggest that the fault was active during sand deposition (a growth fault), and that it created accommodation space for the partially ponded TSP. This, and other growth faults in the underlying strata, suggest deposition in an intraslope minibasin setting. The TSP is overlain by ~700 m of amalgamated slump, slide, and debris-flow units, suggesting that deposition immediately preceded rapid slope progradation.
Abstract Although it has long been recognized that deposition along meandering rivers is not restricted to convex banks (i.e., point bars), the consensus is that sediment deposition on concave banks of channel bends mostly occurs when meander bends translate downstream because erosion-resistant barriers inhibit their lateral migration. Using a kinematic model of channel meandering and time lapse satellite imagery from the Mamoré River in Bolivia, we show that downstream translation and associated concave bank deposition are essential, autogenic parts of the meandering process, and resulting counter point bars are expected to be present whenever perturbations such as bend cutoffs and channel reoccupations create short bends with high curvatures. The implication is that zones of concave bank deposition with lower topography, finer-grained sediment, slack water, and riparian vegetation that differs from point bars are more common than previously considered.
An ancient (Upper Cretaceous, 77-76.5 Ma, Oldman Formation) river meander deposit, exposed in the Steveville Badlands of Dinosaur Provincial Park, AB., exhibits extensive deformed and chaotically bedded strata. The most impressive features are large scale rotations of inclined heterolithic stratified (sandstone and shale) blocks, up to 6 m high and 50 m long, dipping in the opposite direction to that of the lateral accretion trend (Fig. 2). We observe three separate sets of reversely dipping beds along one badland gully, oriented parallel with the direction of lateral accretion. The large reverse cross-stratified structures rest on shale failure planes, suggesting the structures formed as back-rotational slumps of inclined heterolithic strata that slid down an active point bar slope into the channel before it was buried by subsequent lateral accretion sediments. Chaotic and disturbed sandstone and shale blocks, soft sediment deformation, and evidence for sediment foundering in the upper 3 m of point bar stratigraphy are common throughout the ancient meander bend, atypical of meandering river deposits (Fig. 1). Some of the broken and blocky sandstone strata displays a domino-like effect, with all blocks leaning in the same direction. Overturned sandstone beds resting on interpreted failure planes, attributed to slumping, are suggestive of down-slope failures (Fig. 3). Faulting represents the final form of deformation of stratigraphy with displacements of up to 2 m, and the hangingwall always on the channel side of the meander lobe (Fig. 4). We interpret all of these structures as having been caused by large magnitude earthquakes and tremors associated with Laramide thrusting. The three sets of reversed inclined heterolithic strata encased within normal lateral accretion bedding, are interpreted to record three major seismic events, separated by periods of relative
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Deciphering depositional age from deposits that accumulate in deep-water slope settings can enhance understanding of shelf-margin evolutionary timing, as well as controlling mechanisms in ancient systems worldwide. Basin analysis has long employed biostratigraphy and/or tephrochronology to temporally constrain ancient environments. However, due to poor preservation of index fossils and volcanic ash beds in many deep-water systems, deducing the timing of slope evolution has proven challenging. Here, we present >6600 new U-Pb zircon ages with stratigraphic information from an ∼100-km-long by ∼2.5-km-thick outcrop belt to elucidate evolutionary timing for a Campanian–Maastrichtian slope succession in the Magallanes Basin, Chile. Results show that the succession consists of four stratigraphic intervals, which characterize four evolutionary phases of the slope system. Overall, the succession records 9.9 ± 1.4 m.y. (80.5 ± 0.3 Ma to 70.6 ± 1.5 Ma) of graded clinoform development punctuated by out-of-grade periods distinguished by enhanced coarse-grained sediment bypass downslope. Synthesis of our results with geochronologic, structural, and stratigraphic data from the basin suggests that slope evolution was largely controlled by an overall decline in basin subsidence from 82 to 74 Ma. In addition to providing insight into slope evolution, our results show that the reliability of zircon-derived depositional duration estimates for ancient sedimentary systems is controlled by: (1) the proportion of syndepositionally formed zircon in a stratigraphic interval; (2) the magnitude of the uncertainty on interval-bounding depositional ages relative to the length of time evaluated; and (3) the geologic time (i.e., period/era) over which the system was active.
Over the last couple of decades, fluvial geomorphology and fluvial sedimentary geology have been developing in parallel, rather than in conjunction as might be desired. This volume is the result of the editors' attempt to bridge this gap in order to understand better how sediments in modern rivers become preserved in the rock record, and to improve interpretation from that record of the history of past environmental conditions. The catalyst for the volume was a conference with the same that was hosted at the University of Aberdeen School of Geosciences, in Aberdeen, Scotland, on 12-14 January 2009. The conferences brought together a broad spectrum of geomorphology and sedimentology researchers, from academia and industry. This interdisciplinary mix of experts considered and discussed ideas and examples ranging through timescales from the annual movement of individual river bars to sequence stratigraphic analysis of major sedimentary basins spanning millions of years. The articles in this volume are a mixture of novel concepts, new evaluations of the perceived wisdom about rivers and their sediments, and improved understanding derived from recent experience in interpreting the rock record. This volume usefully illustrates the current state of knowledge and will provide a stimulus for further research, particularly work that integrates geomorphological and sedimentological approaches and emphasizes crossdisciplinary communication.
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