3D seismic geomorphology and stratigraphy of the late Miocene to Pliocene Mississippi River Delta : fluvial systems and dynamics

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Armstrong, Christopher Paul

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This study uses a 1375 km2 3D seismic dataset located in the late Miocene to Pliocene Mississippi River Delta in order to investigate the external characteristics, lithology, and evolution of channelized deposits within the seismic survey. Fluvial thicknesses range from about 11 m to 90 m and widths range from about 100 m to 31 km. Channel fill can be generalized as sandy with low impedance and high porosity (~ 35%), though heterogeneity can be high. Three distinct fluvial styles were recognized: incised valleys, channel-belts, and distributive channel networks. Fluvial styles were interpreted as a result of changes in sea-level and a speculative late Miocene to Pliocene Mississippi River Delta sea-level curve constructed using these relationships. Additionally, a characteristic interval between the major changes in fluvial style was found. These fluvial systems interact with and are affected by other elements in the landscape. Growth faults in particular are common within the survey area; however, the dynamic between fluvial systems and growth fault related subsidence has been poorly understood and so was also a focus of this project. Previous work as well as this study found little evidence that growth faults are able to affect the course or geometry of the majority of small (with most < 500 m in width and < 20 m in depth) channels. However, the relationship between growth faults and larger scale channel-belt systems (between 1 km and 5 km in width and > 25 m in depth) has not been previously evaluated in this area. In contrast to the majority of small distributary channels found within the survey, channel-belts appear to be steered by growth faults. Fluvial response or insensitivity to fault induced subsidence is related to the relative timescales of avulsion and faulting. Channel-belts are longer lived features than more ephemeral small distributary channels. Channel-belts, due to their relatively low mobility compared to small channels, are more likely to experience punctuated faulting events which results in greater apparent sensitivity to faulting than seen in small channels.



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