The Goose Point fault between Fountainebleau State Park and Bayou Lacombe
The Goose Point fault is a part of the Baton Rouge - Tepetate fault system that extends from southeastern St. Tammany Parish in Louisiana to the Sabine River at the Texas border. The fault system generally coincides with a portion of a Cretaceous shelf-edge reef complex that rings much of the Gulf of Mexico. Shen et al, 2019 used optically stimulated luminescence dating technology to estimate rates of movement on faults in the system just east of Baton Rouge. They found vertical displacements of Pleistocene strata across the faults between 3 and 15 feet, and estimated rates of rates of fault movement between 0.03 and 0.07 mm/year (approximately 1 to 3 inches per 1,000 years). The surface escarpments of faults along this system are the result of these average rates of movement over about the last 150,000 years. Fault movement is very episodic by nature. It is very likely that these average rates of movement represent the cumulative effect of many short high-rate episodes of movement. It is possible that a single episode of movement along faults in this system may have consisted of one or two inches of instantaneous movement, and would probably been associated with a small earthquake.
The surface escarpment of the Goose Point fault is clearly visible on a LIDAR (light detection and ranging) digital elevation model. The surface escarpment of the fault coincides with the 0-depth contour of the fault plane map. The Goose Point fault extends thousands of feet below the surface and is encountered in a few oil and gas wells in the area. The relatively sharp change in elevation across the fault also affects ecological systems and patterns of human habitation.
The surface trace of the Goose Point fault can be seen in this frame of drone video taken from the Louisiana Department of Wildlife and Fisheries Scenic River Series on Cane Bayou
Three-dimensional images of the Goose Point fault show the fault plane below the surface and the surface escarpment of the fault.
One of the best places to see the Goose Point fault escarpment is near the end of the Cane Bayou Trail in Fountainebleau State Park. Where the trail turns to the south to parallel Cane Bayou, it crosses the escarpment, and there is a visible change in elevation. In walking to the south a hiker crosses from the footwall to the hanging wall of the fault. The photo in the lower left above is looking from the hanging wall toward the escarpment. There are also distinct changes in the surrounding ecosystems across the fault.
Haggar (2014) examined ecological systems along the hanging wall of the Goose Point fault near Bayou Lacombe. The change in elevation across the fault makes the marshes on the hanging wall a mixing zone affected by the salinity of freshwater runoff from the north and storm surge from the south. The red dots indicate the optimum salinity ranges for several marsh species. Haggar showed how landscape level changes in the ecological system over time indicate subsidence associated with fault movement.
Dokka, 2011 provided another good map that includes the Goose Point fault and several faults in Lake Pontchartrain. Lopez et al, 1997 published a high resolution seismic line acquired by the USGS adjacent to the Highway 11 bridge. The surface expression of the Highway 11 bridge fault is evidenced by a visible offset of the bridge. The seismic line shows that the Pleistocene strata below the lake are offset by progressively greater amounts with depth. This indicates that the fault has been continually (if episodically) active throughout the Pleistocene. Increasing offset with depth is the classic characteristic of a south Louisiana “growth fault”. Lopez et al also documented the relationship between one of these faults and the 1987 earthquake at Irish Bayou.
Hopkins et al, 2018 published elevation surveys of the bridges crossing the lake. There are clear vertical offsets on these surveys at the Highway 11 and Causeway bridge faults. A comparison of surveys for the southbound span, which was constructed in 1956 with the northbound span, which was constructed in 1969, also show evidence for continual displacement over time, as the older span shows greater offset than the younger span.
REFERENCES
Dokka, R. K., 2011, The role of deep processes in late 20th century subsidence of New Orleans and coastal areas of southern Louisiana and Mississippi, Journal of Geophysical Research, v. 116, 25 p.
Haggar, K.S., 2014 Coastal Land Loss and Landscape Level Plant Community Succession: An Expected Result of Natural Tectonic Subsidence, Fault Movement and Sea Level Rise, GCAGS Transactions, v. 64, p. 139-159
Hopkins, M., Lopez, J., Songy, A., 2018, Subsidence rates from faulting determined by real-time kinematic (RTK) elevation surveys of bridges in Lake Pontchartrain, State of the Coast Conference, New Orleans
Lopez, J., Penland, S., Williams, J., 1997, Confirmation of Active Geologic Faults in Lake Pontchartrain in Southeast Louisiana, GCAGS Transactions, v. 47, p. 299-303
Shen, D., Dawers, N. H., Tornquist, T.E., Gasparini, N.M., Hijma, M.P., Mauz, B., 2019, Mechanisms of later Quaternary fault throw-rate variability along the north central Gulf of Mexico coast: implications for coastal subsidence, Basin Research, v. 29, p 557-570