Most outcrops of fluvial deposits consist of a series of cliff exposures, either natural or manmade (e.g., roadcuts). Predictions, and especially observation of plan-form geometry, such as might be made with numerical experiments or from studies of modern rivers, are challenging to test in most ancient outcrops. This study examines ancient exhumed channel belts from the Cretaceous Notom Delta of the Ferron Sandstone Member in south-central Utah. Extensive plan-view exposures with local vertical cliff exposures allowed documentation of channel plan-form, channel-belt dimension, bar migration patterns (translation versus expansion), and cross-sectional facies architecture. Channel-fill thickness and bedding structure, documented from the cliff exposures, were used in paleohydraulic reconstructions. Approximately 270 paleocurrent directions were integrated with grain-size measurements to reconstruct the 3D facies architecture. Paleocurrent measurements are consistent within specific facies architectural units (such as unit bars) and show systematic variation at the channel-belt scale that can be used to infer channel and bar migration patterns. In this example, the migration pattern of the single-thread channel bend was interpreted to change from expansion to translation with a corresponding bend sinuosity that increased from 1.01 to 1.44. Inclined large-scale foresets are interpreted to be indicative of unit bars. Empirical equations result in estimated average channel depths from 1.7 m to 3.6 m with corresponding widths of 23 m to 89 m respectively. These empirical estimates match the dimensions measured in the field. For example, channel widths of around 50 m were measured from abandoned channel fills. Bar thickness, measured from vertical outcrops, ranges from 5.4 m to 6.3 m, which yields a narrower estimate of channel width ranging from 47 to 59 m. Integration of sediment size, bedforms type, and channel depth were used to estimate the discharge of the river, which is on the order of , 400 m 3 /s. This suggests that the Ferron deltas were characterized by small, steep-gradient ''dirty'' rivers, which is consistent with the hyperpycnal nature of linked downstream deltaic systems.
Alluvial fans are usually constructed through episodic flood events. Despite the significance of these ephemeral floods on the morphodynamics of alluvial fans, depositional responses to the variations in flood conditions are still poorly documented. This greatly limits the ability to interpret ancient sedimentary successions of fans and the associated flood hydrodynamics. The Quaternary Poplar Fan from endorheic Heshituoluogai Basin provides an optimal case for addressing this issue. Based on the variations in facies associations and flood conditions, three depositional stagesnamely; lobe building stage, channel building stage and the abandonment stageare identified. During the lobe building stage the Poplar Fan is predominately constructed through incised channel flood, sheetflood and unconfined streamflood, with coeval development of distal surficial ephemeral ponds. The channel building stage is characterized by the development of gravelly braided rivers. However, only scour pool fill deposits are preferentially preserved in the Poplar Fan. During the abandonment stage, erosional lags and aeolian sands randomly occur throughout the fan, while gully deposits can only be found in the distal fan. The distinctive facies architecture of the Poplar Fan is likely to be the result of periodicity of climate fluctuations between wetter and drier conditions during the Late Pleistocene to Holocene. The ephemeral floods formed under wetter conditions usually show high discharge and sediment concentrations which facilitate the lobe building processes. During the drier periods, only gravelly braided rivers can be developed through ephemeral floods as the intensity and frequency in precipitation, discharge and sediment concentrations of the flood flows significantly decrease. The abandonment stage of the fan may occur between recurring flood episodes or during the driest periods. Furthermore, the long-term (10 5 to 10 6 year) geomorphic evolution of the Poplar Fan shows the influence of tectonic activities. The ongoing thrust uplift tectonic activities have caused destruction of the fan but can also facilitate the fan-head trench/incision of the fan, which in turn facilitate the progradation of the fan. This study proposes a new depositional model for alluvial fans constructed through episodic flood events, which shows the character of both sheet-flood dominated and stream-flow dominated end members of alluvial fans. These findings Sedimentology supplement the understanding of the variability of the alluvial fans and provide means to characterize rock record of alluvial fans and their associated flood and climate conditions.
The hydrodynamics of rivers approaching a receiving basin are influenced by the onset of backwater conditions that give rise to decelerating reach-average flow velocity and decreasing boundary shear stress. These changes occur across a spatial gradient over which decreasing sediment transport capacity triggers morphodynamic responses that include sediment deposition at the transition from uniform to nonuniform flow. As a consequence, the channel width-to-depth ratio and bed sediment grain size decrease downstream. While nonuniform flow and associated morphodynamic adjustments have been investigated in modern fluvial-deltaic systems, the impacts to fluvial-deltaic stratigraphy remain relatively unexplored. This represents an important unresolved gap: there are few contributions that link morphodynamic response to nonuniform flow, impacts on sediment deposition and influence on the rock record. This study uses a numerical model to explore how variable channel hydraulics influence long-term (1000s years) patterns of sediment deposition and development of stratigraphy. The model results indicate that: (a) nonuniform flow propagates upstream beyond the backwater transition that is traditionally estimated with a basic backwater length scale relationship. (b) Base-level fluctuations, especially rising, enhance the impact of nonuniform flow. (c) Sediment deposition shows large spatio-temporal variability, which ultimately contributes to unique stacking patterns of fluvial-deltaic stratigraphy. (d) Nonuniform flow imparts spatial variation in flow depth, channel bed slope and sediment grain size over the delta, and these signatures are potentially preserved and recognizable in the rock record.
Flow processes and sediment transport in a channel bend and associated point bar have been studied in modern rivers, theoretical models and physical experiments: however, the relationship between flow process and pointbar morphology has rarely been explained due to the complex nature of open channel flow. Plan-view exposures of an ancient point-bar complex, exposed at the top of the Cretaceous Ferron Sandstone Member of the Mancos Shale Formation, south-central Utah, allowed reconstruction of bar morphology, sediment transport and bed shear stress, which were used to extrapolate flow processes. Studies of these outcrops show that compound point bars and scroll bars were probably formed during falling and rising flood stages, respectively. A simulation model of plan-view channel form shows that channel dimensions, such as radius of curvature and sinuosity of the pointbar complex, range between 205 m and 351 m and 1Á04 and 1Á22, respectively, throughout the evolution of the channel bend. Variations in strength of the helical flow were interpreted as the main control on facies architecture and bar morphology. Strong helical flow was related to the deposition of the scroll bars, while strength of helical flow is decreased for compound bars. The use of cross-beds as a common palaeocurrent indicator was found to be inconsistent with mean flow directions and channel margin orientation.
Lowland river systems (with channel slopes of 10–5 to 10–4) inevitably shift away (retreat upstream) from the receiving basin under a sustained rate of base-level rise, even if the system can maintain a period of advance at the onset of rise. This autogenic pattern of transition from progradation to retrogradation through steady base-level rise and sediment supply is termed “autoretreat.” Using a morphodynamic model of autoretreat, this study explored the varying channel hydrodynamics of lowland fluvial systems and associated stratigraphic record under sustained base-level rise and constant sediment supply. Results from the numerical simulations show that a fluvial system will reach a state of dynamic equilibrium during autoretreat where both the backwater length and the morphodynamic adjustment of the downdip channel profile become steady. Moreover, when this dynamic equilibrium state is realized, simulated systems display a persistent twofold downstream deepening of flow depth across the backwater zone, a pattern that is also present in many natural systems. In general, backwater effects play a key role in the morphodynamics of a lowland fluvial-deltaic system during autoretreat, and this hydrodynamic condition is therefore critical for predicting river responses to sea-level change.
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