Cyclicity in sediment signals from combined aeolian and fluvial systems
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Abstract Alluvial fans on Mars, which are primarily sourced from erosional alcoves incised into crater rims, record a period of increased surface runoff which ended >1 Ga. However, we lack quantitative constraints on the frequency and duration of river‐forming processes and the climatic conditions that accompanied these long‐term habitable episodes. Here we use bedrock erosion and sediment transport models to show that the cumulative time span of wet activity (i.e., nonzero erosion and deposition) was between 100 years to 1 Myr excluding dry years. We use Context Camera (CTX) digital elevation models to compile a data set of >200 channels upstream of depositional fans and determine key fluvial geometry metrics. Results from calculating Mars stream power parameters are compared to great escarpment channels and globally distributed bedrock rivers on Earth. Although Martian channel profile morphologies fall within the range of those on Earth, they are slightly less concave‐up (concavity index, ) and steeper for a given drainage area (reference steepness index, , for reference drainage area, A r = 1 ×10 7 m 2 ). Timescales depend strongly on poorly constrained variables such as erodability and grain size. Channel morphologies, intermittencies, spatial distributions, and orientations collectively suggest an arid climate and a source from snowmelt on steep crater rims, possibly from obliquity‐paced insolation variations or orographic accumulation. Derived timescales are consistent with erosion rates and intermittencies observed in arid environments on Earth and do not support short‐lived or catastrophic triggers for the warm climate conditions (such as impacts or individual volcanic eruptions).
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Abstract Ancient dryland terminal fluvial systems are often recognized within the rock record for having a progressive downstream decrease in the size and amalgamation of channel elements and systematic downstream increase in sheet and overbank elements, alongside the downstream decrease in grain size that is displayed by most fluvial systems. The spatial distribution and downstream trends displayed by the fluvial sediments of the Lower Jurassic Kayenta Formation of south‐western USA, have been examined quantitatively. The results indicate many trends that are similar to those of a dryland terminal fluvial system, including; a lack of confinement of the fluvial system, a downstream decrease in channel and sheet element amalgamation and width‐to‐thickness ratios, a downstream decrease in grain size, albeit very small, and an increase in the percentage of overbank elements downstream. However, the study highlights several downstream relationships that are atypical. While some of these relationships may be the result of external factors inherent in this study, others, including the thicknesses of channel‐fill and sheet elements that display no significant relationships to distance downstream, and channel‐fill elements that display no significant variation in average grain size with distance downstream, may be a consequence of fluvial interaction with a competing and coeval aeolian system. This work demonstrates the inherent complexity in arid dryland fluvial systems and the downstream architectural and compositional relationships that they depict. Consequently, models for fluvial style may provide only a first‐order approximation for downstream trends in dryland systems, because the controlling factors upon these systems are inherently difficult to unravel, and the sedimentary detail is strongly dependent upon external setting and internal complexity. Consequently, a generalized model may not always be applicable to these systems.
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ABSTRACT Ephemeral fluvial systems are commonly associated with arid to semi‐arid climates. Although their complex sedimentology and depositional settings have been described in much detail, depositional models depicting detailed lateral and vertical relationships, and interactions with coeval depositional environments, are lacking compared to well‐recognized meandering and braided fluvial systems. This study critically evaluates the applicability of current models for ephemeral fluvial systems to an ancient arid fluvial example of the Lower Jurassic Kayenta Formation of the Colorado Plateau, USA . The study employs detailed sedimentary logging, palaeocurrent analysis and photogrammetric panels across the regional extent of the Kayenta. A generic model that accounts for the detailed sedimentology of a sandy arid ephemeral fluvial system (drawing upon both ancient and geomorphological studies) is developed, along with analysis of the spatial and temporal interactions with the aeolian setting. Results show that the ephemeral system is dominated by laterally and vertically amalgamated, poorly channelized to sheet‐like elements, with abundant upper flow regime flat beds and high sediment load structures formed between periods of lower flow regime conditions. Through interaction with a coeval aeolian system, most of the fluvial deposits are dominated by sand‐grade sediment, unlike many modern ephemeral fluvial systems that contain a high proportion of conglomeratic and/or finer grained mudstone and siltstone deposits. During dominantly fluvial deposition, high width to thickness ratios are observed for channelized and sheet‐like elements. However, with increasing aridity, the aeolian environment becomes dominant and fluvial deposition is restricted to interdune corridors, resulting in lower width to thickness ratio channels dominated by flash‐flood and debris‐flow facies. The data presented here, coupled with modern examples of ephemeral systems and flood regimes, suggest that ephemeral flow produces and preserves distinctive sedimentological traits that can not only be recognized in outcrops, but also within core.
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