Abstract

This study combines ground-penetrating radar (GPR) and capacitively coupled resistivity (CCR) for geophysical architecture-analysis of a bar platform and channel bend on the floodplain of a poorly organized wandering gravel-bed river. An important objective of fluvial architectural analysis is linking fluvial style with preserved subsurface sedimentology. However, architectural analysis relies on opportunistic outcrops with locations or orientations that may not provide appropriate data. GPR is a well-established geophysical method that images reflections interpreted to represent bed geometry and bounding surfaces and is therefore ideal for imaging fluvial architecture. Unfortunately, grain-size information, which is integral to architectural classification, is more elusive using GPR. Resistivity data can be used as a proxy for sediment grain size. When GPR and CCR are combined they offer an effective tool for geophysical fluvial architectural analysis.

Five trenches provided direct observation of the subsurface sedimentology and are used to calibrate the two geophysical methods. Eight radar facies and one radar element are classified from the GPR survey and grouped into four categories: horizontal and subhorizontal, laterally continuous reflections (Group 1), clinoform reflections (Group 2), discontinuous reflections (Group 3), and concave-up elements (Group 4). The 2-D resistivity data are combined in a GIS to establish a 3-D resistivity model for the upper 5 m of the floodplain. Resistivity values correlate well with grain size and are categorized into fine-grained (< 400 ohm-m), sand-size (400 to 800 ohm-m) and gravel-sized (> 800 ohm-m) sediments. In general, resistivity values indicate coarse sediment in the bar platform and sand to fine-grained material in the main channel and subordinate depressions. Resistivity profiles were extracted from the 3-D model along the GPR lines so that resistivity values could be directly compared to radar facies. The shape and spread of the resistivity distributions provide dominant grain size as well as an indication of sediment sorting for individual radar facies. In some cases, the same radar facies is associated with markedly different grain sizes, indicating different architectural elements (i.e., horizontally bedded gravel verses horizontally bedded sand or fine-grained sediment). This demonstrates the utility of combining GPR and CCR, insomuch as radar architecture alone is not diagnostic of fluvial architectural elements.

In this case study, the bar platform and channel have a planform that might suggest a meandering fluvial style. However, GPR-CCR results indicate that the dominant depositional process across the bar platform was vertical accretion of gravel sheets, an architecture more consistent with a wandering gravel-bed fluvial style. Lateral migration was limited to the outer downstream margin of the platform, a location dominated by a mix of sand and gravel. Coarse gravel likely occupies the base of the main channel, with fine-grained sediment contributing to the remainder of the channel fill. A four-phase history is presented where the initial phase of development involves vertical accretion and migration of stacked gravel-sheets during floods, forming the core of the bar platform. After flooding subsides and the interior bar emerges, the channel becomes established and lateral migration becomes the dominant depositional process. The evolution concludes with avulsion and eventual abandonment of the channel.