The sedimentary architecture of meandering rivers: The influence of bend migration style

Sanchez, Alexandra

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https://hdl.handle.net/2142/117834 Copy

Description

Title

The sedimentary architecture of meandering rivers: The influence of bend migration style

Author(s)

Sanchez, Alexandra

Issue Date

2022-12-06

Director of Research (if dissertation) or Advisor (if thesis)

Best, James

Department of Study

Geology

Discipline

Geology

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

M.S.

Degree Level

Thesis

Keyword(s)

• geomorphology
• Wabash River
• sedimentology
• facies
• geology
• geophysics
• meandering rivers
• bend migration
• ground penetrating radar
• multibeam echo sounder survey
• GPR
• MBES
• sedimentary facies

Abstract

Meandering rivers are ubiquitous in nature and play a major role in shaping the landscape and creating ecological diversity. Their deposits can also form a significant component of ancient alluvium. Past research has shown that meander bends and their point bars can display different modes of planform migration, such as expanding across the floodplain, translating down valley, and expanding/rotating. Numerical models and interpretations from outcrops have suggested that these different migration styles influence what is preserved in the sedimentary facies. However, few studies have sought to assess the validity of these assertions in modern rivers: the present thesis studies the classic Wabash River in Illinois as a site to test these ideas. In this study, ground penetrating radar (GPR) surveys, multibeam echo sounder surveys (MBES), and remote sensing have been used to examine the Wabash River and test if migration style influences the subsurface sedimentary features. Two bends with different migration styles – an expanding (Maier Bend) and a translating (TB3) bend - were chosen to study the nature and rate of bend movement and links to facies characteristics and type. GPR surveys were conducted on the exposed meander point bars at low flow and established a series of flow normal and flow parallel GPR lines. When interpreting GPR data, the GPR facies are first described as the types of reflectors seen in the GPR profiles, allowing interpretation of the GPR facies in terms of sedimentary facies and subsurface architecture. In addition, the facies interpretations of the present thesis are compared to other models and studies conducted on meandering rivers. Six GPR facies were identified. Facies 1 is divided into two categories. Facies 1a consists of low-angle (~5-13˚) reflectors with high-angle reflectors (20-30˚) or small-scaled trough-shaped (concave-upwards) reflectors packaged in between, which were interpreted as the point bar (accretion surfaces) migrating downstream with smaller scale superimposed bedforms. Facies 1b has the same low-angle reflectors but lacks the small-scale features (bedforms) in between. Facies 2 consists of high angle reflectors (~15-32˚) and is 0.2-2m thick and interpreted as the deposits of unit bars. Facies 3 comprises small, concave-upwards, trough-shaped reflectors (1-2 m wide) interpreted as the deposits of dunes. Facies 4 is represented as larger-scale (10’s of meters wide) trough-shaped reflectors that were interpreted as bar top hollows. Facies 5 consists of planar, parallel, reflectors, interpreted as deposition on low-angle planar surfaces, possibly by bedforms that were smaller than the resolution of the GPR. Facies 6 consists of hyperbolae, which are interpreted as either sections of the point bar with an abundance of gravel or buried tree trunks or branches. All six of these facies were found in both bends with differing migration style, but in different abundance as assessed by quantification of facies types in every GPR section. Facies 1 was the dominant facies overall within Maier and TB3 bends. However, the distinguishing characteristic between the two bends is the abundance of Facies 1a (lateral accretion surfaces with small scale surface features) versus Facies 1b (lateral accretion surfaces with no small-scale features). Facies 1b is interpreted more frequently in Maier Bend than in TB3, and Facies 1a occurs more frequently in TB3 compared to Maier Bend. This difference is less likely due to differing bend migration styles and more likely due to grain size differences and the angle at which the GPR surveys were taken. Facies 2 (unit bars) were found more frequently in TB3 in comparison to Maier Bend, possibly due to the dominance of unit bars in a downstream translating bend, potentially making it a distinctive characteristic of a translating bend. Facies 3 (dunes) increases in abundance from upstream to downstream in both bends but is found more frequently in TB3. The abundance of Facies 3 is also likely due, similar to Facies 1, to the difference in grain size with respect to the resolution of the GPR, making it difficult to identify small dunes in the GPR profiles. Facies 4 (bar top hollows) is only found at the tail end of the point bars. TB3 has a much larger bar top hollow compared to the Maier Bend point bar that is likely due to the migration pattern of TB3 as it wraps around the bend during translation. Facies 5 (planar deposition) is only found at either thalweg depth within the point bars or filling the bar top hollow and is the least abundant facies within the point bars. Lastly, Facies 6 (hyperbolae) is highly abundant but dependent on the amount of material (e.g., gravel or trees ) present to cause the point reflectors within the GPR profile. The results of this thesis were compared to three other depositional models for meandering rivers. The first model, the depositional model of Jackson, was based on the premise that the depositional facies are dependent on the nature of flow development within bends of different curvature. The sedimentary facies determined by Jackson were compared with the facies in the present thesis and similarities are found notably in the presence of unit bars (transverse bars in Jackson’s model), dunes, and accretion surfaces. Willis’s theoretical model predicts variation in the three-dimensional geometry of meander bend large-scale stratigraphy with differing bend geometry and hydraulics. The slopes and depositional patterns of Maier bend and TB3 point bars were compared to the Willis’s predicted bedding geometry, showing similarities in the angle and shape of the bedding geometry slopes. However, the grain size distribution was a significant discrepancy between Willis’s model and the Wabash River and thus may impart some discrepancies between the two. Lastly, the model of Hagstrom et. al was based on high-quality 3-D seismic data and information from hundreds of wellbores, testing if the accretion-package geometry affects the lithological variability of point bars. When comparing this model to the results of the present thesis, it was found that they were not entirely comparable due to the width of accretion surfaces not being able to be measured in the GPR profiles. However, the overall depositional patterns found in the specific bend migration styles found in both studies were similar.

Graduation Semester

2022-12

Type of Resource

Thesis

Copyright and License Information

Copyright 2022 Alexandra Sanchez

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