Date of Thesis

Spring 2022

Description

Topographic complexity on floodplains can route flow, control sediment dispersal, and influence channel behavior, but studying floodplain-channel interactions in modern rivers is challenging because of human modifications and the short timescales of observable data. This project assesses the link between different types of floodplain microtopography and avulsion style in the Devonian Catskill Formation, north-central Pennsylvania, where thick stacks of fluvial strata provide a lengthy record of channel-floodplain interaction. Using a combination of field observations and computer modeling, this study identifies sedimentary features indicative of floodplain complexity and analyzes their impact on avulsion style at fourteen Catskill Formation outcrops.

Based on outcrop analysis, we identify three types of floodplain microtopography indicators in the Catskill Formation: paleogilgai topography, floodplain channel scours, and reactivated mud plugs. Paleogilgai are preserved as complexes of 0.8-7.0-meter wide pedogenic slickensides with 0.2-0.9 meters of vertical relief in distal floodplain facies with a regular bowl- and-chimney arrangement. Floodplain channel scours are physical erosion surfaces with 0.5-to- 3.1 meters of vertical relief within floodplain facies. Mud plugs with incisional scour surfaces of a similar scale within their fill are interpreted as ancient oxbow lakes that became reoccupied by channelized floodwater after abandonment. For our analysis of avulsion style, we identify five stratigraphic succession in the Catskill Formation that we interpret as progradational avulsion deposits, and ten sequences that we interpret as incisional avulsion deposits. All ten of these incisional sequences have paleogilgai deposits underneath the main channel sand body, leading us to hypothesize that gilgai topography influenced avulsion style in the Catskill Formation.

To test this hypothesis, we use the morphodynamic modeling software Delft3D-FLOW to explore the connection between gilgai topography and avulsion style. Model results show that regularly spaced mound-and-depression topography typical of gilgai limits sedimentation and increases incision of the floodplain, suggesting that gilgai topography increases the likelihood of an incisional avulsion. Overall, field and modeling results show that Catskill Formation floodplains had complex microtopographies that complicated overbank flow and influenced channel-floodplain interaction, adding to our understanding of the control of floodplain hydrology on avulsion style in alluvial systems.

Keywords

Sedimentology, Fluvial, Catskill, Late Devonian, Geology, Avulsion

Access Type

Honors Thesis

Degree Type

Bachelor of Science

Major

Geology & Environmental Geosciences

Minor, Emphasis, or Concentration

German Studies

First Advisor

Ellen Chamberlin

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