Architecture and sedimentology of an active braid bar in the Wisconsin River based on 3-D ground penetrating radar

Abstract

The internal architecture of sandbars in modern braided streams has not been adequately documented, especially in medium-scale braided rivers. Identification of the architecture and development of an understanding of the formative processes for these macroforms is important for (1) understanding sedimentation in braided streams, (2) understanding reservoir and aquifer compartmentalization in ancient deposits, and (3) predicting the controls on deposition in similar settings. A 225 MHz GPR survey was conducted within a braided reach of the Wisconsin River near Spring Green, Wisconsin, USA, to characterize the subsurface architecture of a midchannel bar. A 20 20 m survey grid consisting of sixteen GPR transects oriented approximately in flow-parallel and flow-transverse directions was established on the bar. Three-dimensional analysis of the GPR profiles resulted in the interpretation of five major radar facies that represent depositional mechanisms that controlled bar growth and modification. Vertical accretion (aggradation) was the primary depositional mechanism for bar growth and was augmented by much smaller amounts of downstream accretion, lateral accretion, and upstream accretion. A channel fill pattern was also recognized and correlated between multiple profiles, and it provided evidence for two preexisting, independent macroforms that converged to form the studied bar. The work provides insight into bar morphology within sandy braided reaches that closely resembles that of similar GPR studies performed in both smaller and larger rivers and supports a scale-independent model for some aspects of bar growth and modification in sandy, braided rivers.