Geomorphological facies reconstruction of Late Quaternary alluvia by the application of fluvial architecture concepts

This paper investigates the methodical implications, the benefits, and the constraints of applying the fluvial architecture concept to Late Quaternary shallow-subsurface alluvial deposits. The focus is upon a typical small- to mesoscale valley floor of the temperate zone. The studied reach is part of the river Wetter catchment (517 km2) in central Germany. A large number of studies on Late Quaternary terrestrial paleoenvironmental change of the temperate zone refer to such small- to mesoscale catchments, which are mostly occupied by mixed- to suspended-load rivers. As in many cases mostly coring-based evidence is available, the observation and measurement is limited to lithology and lithofacies boundaries; facies geometry must be inferred. Moreover, in those environments differences in lithofacies are obscured by a significantly reduced range of grain size distributions.An example of an ancient channel belt of Late Glacial to Holocene age serves to delineate the methodical practice and utility of the fluvial architecture approach. Field evidence is obtained from detailed cross-sectional surveys and comprises descriptions of lithofacies, structural, pedogenetical, biotic features, Munsell color, and total organic carbon magnetic volume susceptibility. Cross-sectional lithofacies information is represented by spatially attributed, scaled borehole logs. The example also accentuates the need for applying additional stratigraphical methods such as physical age determination, macrofossil analysis, and tephrological stratigraphy.These methods form the basis to discern stacked channel facies and derive a diachrony of channel forms. Thus, the adapted architecture approach provides a significant surplus of information on channel dimensions, ages, and channel–floodplain interconnectedness. Distinct fluvial landforms such as channels, levees, abandoned channels, swamps, and floodplain flats can be highlighted. A number of methodical constraints are discussed in detail, e.g., the sharpness of channel element geometry and the underlying ordering of bounding surfaces. Moreover, it is shown that the analytical process resembles an iterative looping process that is led by deduction.Many geomorphologists and multi-disciplinary floodplain researchers collect sedimentary data but still neglect utilizing the potential of architectural analysis. The study makes clear that sedimentological approaches used in 'big river' floodplains also apply to small valleys; in fact, the procedures for facies reconstruction need to be adjusted to each individual case study. Alluvial architecture analysis provides the tools to reveal interconnectedness (or disconnectedness) of channel, near-channel, and overbank fluvial landforms, which is essential for a geomorphological understanding of floodplain evolution.