An Evaluation of Faulting in Holocene Mississippi River Delta Strata Through the Merger of Deep 3-D and 2-D Seismic Data with Near Surface Imaging and Measurements of Vertical Motion at Three Study Areas

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Description

This study is broken down into two different phases, with the end goal of determining how near-surface faults in Lake Pontchartrain and Lake Borgne affect the geomorphology in the surrounding area. The first phase will involve the interpretation of three data sets, including two 3-D seismic surveys in Lake Borgne, and 18 2-D seismic lines that span Lake Pontchartrain. As part of this initial phase, wells containing foraminifera-aged sands were correlated from Lake St. Catherine to Lake Borgne to age constrain the 3-D seismic surveys. The 3-D seismic surveys of Lake Borgne revealed four major, east-west trending faults, three of which were grouped together spatially. It is these three, grouped faults that are the focus of this study. These particular faults are considered major because of both their lateral and depth extents, with trace lengths of 6.4, 11.2, and 12.4 km, and depth offsets to approximately 3,700 m (or Lower Miocene age). The greatest displacement of strata (~300 m) caused by these faults occurs at Lower Miocene depths. The fault planes have a uniform dip angle of approximately 68°. Faults found in Lake Pontchartrain using the 18 2-D seismic lines are similar in trend to the faults in Lake Borgne. The Lake Pontchartrain faults also show similar offsets (274 m), and fault plane dip angles (65°) to those in Lake Borgne. It is these similarities that allow faults between Lake Borgne and Lake Pontchartrain to be correlated to one another with high confidence. This would mean that faults cross the New Orleans “Land Bridge”, a strip of land which plays an integral part in the city’s flood protection. The second phase of this study will determine if the faults discovered in Lake Borgne were still active recently (Holocene). The collection of high resolution seismic data took place over the summer of 2016 to answer this question. A total of 30 2-D high resolution lines were collected, covering a distance of 50 km. After examining these data, there was no clear evidence of brittle or ductile deformation caused by faulting in the Holocene sediment. However there was other evidence for fault movement in the strata. A 150 m wide, paleo-river channel is preserved in the strata, and parallels one of the faults within Lake Borgne for approximately 6.4 km before it is lost off of the data collection area. The size and position of the paleo-channel deposits suggest that this is an ancient distributary of the Mississippi River, most likely active around 2,000-4,000 YBP, when the St. Bernard delta lobe was active. It is known that river channels will align themselves with areas of the greatest subsidence, because of this it seems that this fault in Lake Borgne was active during the time that the paleo-river channel was occupied. Subsurface sampling of the Lake Borgne sites near to the three major faults will include sediment coring, with multiple goals, including (1) developing a detailed stratigraphic section to merge with geophysical data; (2) developing chronostratigraphic control over important offset facies using radiocarbon (last ~50,000 YBP); and (3) developing sediment accretion and age models using fallout radionuclides (7Be, 137Cs, 210Pb) to investigate possible responses to fault motion over the historical period (last ~100 YBP).
StatusFinished
Effective start/end date8/1/177/15/20

Funding

  • University of New Orleans: $21,381.00

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