St. Rose Fault — McLindon Geosciences, LLC

The St. Rose fault offers one of the best examples of surface expression not associated with a significant elevation change in southeast Louisiana. The surface traces of the Baton Rouge fault system are easily imaged with lidar digital elevation models. The surface escarpments, which may exhibit 10 to 15 feet of elevation change show up as sharp lineations on lidar (Shen et al, 2017). The St. Rose fault does not create an elevation escarpment, but it does create a sharp lineation that is visible on satellite imagery. The tree line in the swamps of St. Charles Parish clearly shows the fault. More investigation is needed to determine exactly why that is.

The St. Rose fault is one of network of surface fault traces that have been mapped with oil and gas industry seismic and subsurface data in preparation for the construction of a fault atlas. The fault traces to the north have been published in peer-reviewed technical journals. The fault traces to the south have been published in university research projects and assembled on the LA Dept of Transportation website. The publication of the results of these research projects in peer-reviewed journals is anticipated in the coming months.

A profile across southeast Louisiana shows the complex interrelationships between faults, salt domes and the sedimentary layers that have accumulated to thickness of up to 40,000 feet over the last 50 million years. The St. Rose fault is at the northern boundary of the thick Miocene accumulations that dominate the subsurface of southeast Louisiana. Movement on the faults has continually interacted with ductile, low density salt, which has been squeezed up from a layer originally deposited during the Jurassic. The ancestral deltas of the Mississippi River continually delivered their sedimentary load to the geological basins formed by the faults and salt. Loading of delta sediments in these basins certainly triggered episodes of fault movement. The faults also acted as conduits for fluid migration as the sediments compacted and expelled pore waters. The organic-rich shales of the Cretaceous also expelled oil and gas, which migrated up the faults and into reservoirs in the deltaic sands of the Miocene.

Evidence for recent movement can be found on other faults in the vicinity of St. Rose. The Vacherie fault ruptured the surface of the Earth in an episode of fault slip in 1943. A high-resolution seismic profile shot by the USGS in Lake Pontchartrain shows the recent offset of Pleistocene sediments on the Frenier fault. This type of high-resolution seismic data is essential to accurately mapping faults and determining patterns of recent movement, but it is extremely rare.

The smooth planar surface of the St. Rose fault is exhibited by a map of subsurface depth contours. The color code of the fault plane grades from cool to warm based on depth. The contour lines indicate the depth of the fault below the surface in 1,000-foot increments. The 0-depth contour is coincident with the surface trace of the fault plane, and the tree line in the swamp.

The fault roughly parallels a reach of the Mississippi River west of New Orleans and appears to run under the New Orleans International Airport. The surface trace is just west of the interchange of Interstate Highway 310 and US Highway 61 (Airline Highway).

Subsurface geological structure maps are the essential tools for oil and gas exploration. A subsurface depth contour map on a Middle Miocene sand layer shows the geological structure. The subsurface trace of the fault is mapped where the fault plane intersects the mapped horizon – in this case at about 9,000 feet below the surface. The St. Rose fault offsets the Middle Miocene sand layer by about 500 feet at this depth. It is the offsetting of the sand layer below the surface that creates the trap where natural gas accumulates. The gas accumulations in the Middle Miocene sand on the downthrown side of the St. Rose fault make up the St. Rose gas field. Wells drilled into the gas reservoirs in this field have produced significant volumes of natural gas. The concentric contours of the Good Hope oil field to the north of St. Rose indicate that it is rooted by a salt dome.

A profile constructed from wireline logs taken in wells drilled in the vicinity of St. Rose field shows the St. Rose fault offsetting the sedimentary layers. Sand and shale layers can be correlated between the well logs used to construct the profile. The depths of each layer can also be used to construct a subsurface depth map on that horizon. The profile shows that offset of the layers extends up through the Upper Miocene and Pliocene intervals and into the Quaternary. Direct determination of fault offset in the Quaternary cannot be made with oil and gas industry data. In order to make this determination it would be necessary to construct a profile of shallower borings or cone penetrometer readings across the fault. This type of profile could be combined with the acquisition of high resolution seismic for the most definitive determination.

The importance of attempting to determine recent fault movement, or more importantly to estimate associated rates of fault slip, is based on the proximity of the surface trace of the St. Rose fault to critical infrastructure. The visible surface trace of the fault appears to cross LA State Highway 626, the Kansas City – Southern Railroad, Airline Highway and the St. Rose levee and drainage structure. Extrapolations of the possible surface fault trace to the east and west of the visible trace could cross I-310 and may even reach the airport at a possible eastern extent of the fault trace and the Mississippi River levees at the possible western extent. It is also possible that movement on the fault may affect rates of settling around the I-310 – Airline Interchange and the floodwall and levee reach to the south of the fault. An elevation survey across the Highway 11 Bridge fault by the Lake Pontchartrain Basin Survey (Hopkins et al, 2018) showed that the drop in elevation across the fault affected the entire southern span of the bridge, not just the section on the immediate downthrown side of the fault.

Potential indications of the effects of fault movement can be seen at two locations along the visible trace of the fault. An elevation survey on Highway 626 shows a distinct offset at the point where the fault crosses the highway. An offset crack in the T-Wall structure at the St. Rose drainage structure also appears to be almost exactly coincident with the surface trace of the fault. These potential impacts of fault movement are consistent with the type of slow-slip movement that is believed to typify faults in southeastern Louisiana most of the time. The potential for a more significant fault slip event, such as that which occurred on the Vacherie fault is the real source of concern.

While the sharp tree line in the swamp clearly delineates the surface trace of the St. Rose fault, it is not clear why it does. The tree line exists because trees on the downthrown or hanging wall side of the fault have died. This may be because fault slip has lowered the elevation of the base of the swamp increasing the water depth on the downthrown side, effectively causing the trees to drown. Cypress trees are generally tolerant of a range of water depths, however, and it seems unlikely that a change in water depth significant enough to have killed them exists along this fault. A more likely explanation may be offered by the role that faults have played throughout geological history as conduits for fluid migration. The migration of saltwater up faults and into freshwater drinking aquifers has been documented along the Baton Rouge fault trend. Kuecher et al, 2001 interpreted salinity anomalies along the surface trace of the Montegut fault in Terrebonne Parish, and Gagliano et al, 2003 related saltwater migration along this fault to the death of cypress trees in the area. This may be the case along the St. Rose fault, but more investigation is needed.

Much more study is needed to understand the St. Rose fault and its potential impacts on the natural environment and manmade infrastructure in the area. A complete investigation of the fault should include detailed mapping of the fault at depth with 3-D seismic data, determination of recent fault movement with borings, cone penetrometers and high-resolution seismic, and determination of possible saltwater migration along the fault. It would not be wise to wait for a catastrophic event associated with fault slip to begin this type of investigation. The area around the visible trace of the fault would also be an excellent place to attempt to measure subsidence rates. GPS and/or InSAR velocity measurement techniques could be employed. The integration of the subsurface geological interpretation with the measurement of land surface motion should be employed across the coastal plain.

References

Gagliano, S.M., et.al., 2003. Active Geological Faults and Land Change in Southeastern Louisiana. Prepared for U.S. Army Corps of Engineers, New Orleans District, Contract No. DACW 29-00-C-0034.

Hopkins, M., Lopez, J., Songy, A. 2018, Subsidence rates from faulting determined by real-time kinematic (RTK) elevation surveys of bridges in Lake Pontchartrain, presentation, State of the Coast Conference 2018, New Orleans, Louisiana.

Kuecher, G.J., Roberts, H.H., Thompson, M.D. and Matthews, L., 2001, Evidence for Active Growth Faulting in the Terrebonne Delta Plain, South Louisiana: Implications for Wetland Loss and the Vertical Migration of Petroleum., Environmental Geosciences, v. 8, p. 77‐94

Shen, Z., Dawers, N. H., Tornquist, T. E., Gasparini, N. M., Hijma, M. P., and Mauz, B., 2017, Mechanisms of late Quaternary fault throw-rate variability along the north central Gulf of Mexico coast: implications for coastal subsidence, Basin Research, v. 29, p. 557-570