The sedimentary dynamics of fine-grained rivers: a novel application of marine geophysics to develop new fluvial facies models

Lead Research Organisation: University of Birmingham
Department Name: Sch of Geography, Earth & Env Sciences

Abstract

A lot is known about how rivers with sandy or gravelly beds operate and how these processes lead to characteristic depositional features such as point bars. By comparison, those dominated by finer-grained sediments are much less well understood. This is especially the case for their deposits, for example, as a result of this basic lack of knowledge it has been suggested that within channel fine-grained river sediments may have been incorrectly interpreted to be environments of overbank floodplain sedimentation. Thus very little is known about how the different building blocks of a fine-grained river are put together and what factors may control these building blocks. In short, the science of fine-grained rivers lags that of sandy/gravelly ones. This can be attributed in large part to the fact that the geophysical techniques that have revolutionised the study of sand and gravel bed rivers simply do not work in fine-grained ones. For example, for one of the most popular techniques, ground penetrating radar, the nature of the signal is so severely affected in fine grained environments that no data can be collected. This proposal seeks to address this shortcoming through the application of a geophysical technique more normally used in marine and deltaic settings. The parametric echosounder has proven capability to generate high-resolution data from fine-grained sediments. By application of a novel cross-over of technology the aim of this proposal is to use this technique in a river environment to generate the first large-scale, 3-D quantification of fine-grained river sedimentology. This is important because fines in ancient rocks will severely impact the ability of water and oil/gas to move. There are thus significant social (drinking water supply from aquifers) and economic (successful recovery of hydrocarbons) benefits from being able to understand fine-grained sediments fully so that they can be properly taken in to account in reservoir models.
 
Description The results demonstrate that the PES has the ability to provide high resolution (decimeter) datasets from fine-grained rivers that are equivalent to the more commonly used GPR, which works in coarser grained environments. This allows the larger unit bar sets that are seen lower down in the profile to be imaged in addition to the individual bounding surfaces of dune sets from which set thickness can be measured. Additionally, the internal architecture of both active dunes at the surface and preserved dune sets can also be determined. Analysis of the data reveals that the deposits of the Rio Bermejo are characterized by a lower unit comprising long, low-angle surfaces associated with active point bar evolution, and large-scale scour surfaces resulting from channel migration. These sediments are truncated and overlain by vertical accretion deposits, with sets associated with small bars, dunes and climbing ripples, and cut and fill structures resulting from cross-bar channels. This overall style of alluvial architecture is very different from other modern silt-bed-rivers in the literature that tend to emphasise the presence of oblique accretion. The Rio Bermejo differs from these other rivers because it is much more active, with very high rates of bank erosion and channel migration. Modern examples of this type of highly active fine-grained river have been rarely reported in the literature, although ancient examples are more prevalent. For example, the Lower Permian (Wolfcampian) Abo Formation of south-central New Mexico displays very similar alluvial architectures as the Rio Bermejo, which thus represents a useful analogue for the identification and interpretation of these ancient deposits. More broadly, it is concluded herein that the PES has the potential to transform our understanding of fine-grained rivers in as significant a way as GPR has for coarser-grained rivers. The PES is capable of imaging the detailed large scale alluvial architecture, which is simply not possible with cores or shallow trenches. In such a way, the full complexity of the sedimentology of this under-reported channel type has been revealed and all project objectives have been realised.
Exploitation Route improved surveys of river bed for utility detection
Sectors Environment
 
Description collaboration with hydrocarbon industry
First Year Of Impact 2006
Sector Energy
Impact Types Economic