The Bastian Bay fault
The best inputs to a scientific investigation always come from the people in the field. Woody Gagliano spent many years in the marshes of coastal Louisiana studying the patterns of change. He collected scientific data, but also spoke to the local oyster fishermen and camp owners. It was Pete Hebert that told Gagliano about his camp built in the 1960s in Plaquemines Parish west of Buras. The camp was located on the banks of Bayou Ferrand, but by the mid-1970s it was standing in open water. Hebert reported that the land surface under the camp sank by 3 to 3 ½ feet over that time span. Gagliano realized that the submergence of Hebert’s camp was due to movement on the Bastian Bay fault, which he had found by constructing profiles of the shallow subsurface from auger borings. Gagliano later referred to the Bastian Bay fault as the “Rosetta Stone” for deciphering relationships among fault movement, subsidence and land loss in southeast Louisiana. Subsequent scientific investigation of the Bastian Bay fault by geologists at Tulane and the University of Texas at Austin confirmed the location of the fault using seismic data (Armstrong et al, 2013; Dawers and Martin, 2006)
The Bastian Bay fault is near the eastern end of the Terrebonne Trough. The fault is one of a series of down-to-the-south faults (highlighted in blue) that bound the northern rim of the trough. They appear to have a conjugate relationship with a set of northern-dipping faults (highlighted in red) and salt domes that bound the southern rim. Miocene, Pliocene and Quaternary sedimentary layers are all thicker within the trough, and the weight differential created by these thick piles of sediment appears to have driven subsidence and fault movement for at least the past 15 million years.
Most of the major faults bounding the Terrebonne Trough appear to extend to the surface where recent fault movement has played a critical role in causing wetlands loss. The figure below shows the subsurface depth contours of the Bastian Bay fault plane in yellow and their connection to the Lake Washington salt dome shown in green. The zero-depth contour coincides with the surface trace of the Bastian Bay fault. Similar fault plane contour maps for each of the other faults that reach the surface could be constructed using seismic data and well logs.
Like most of the other faults along the Terrebonne Trough, the Bastian Bay fault controlled the formation of geological structures at depth. Most of these structures control the accumulation of oil and gas in the Miocene, Pliocene and Pleistocene sands. Bastian Bay Field is an example of a “rollover fault structure” in which an anticlinal structure is created by the thickening of sedimentary layers into the fault at the time of deposition. At Bastian Bay Field the producing gas reservoirs are found in sand layers that were deposited by ancestral deltas of the Mississippi River during the upper Miocene. The subsurface structure map on the “O” Sand published by the New Orleans Geological Society shows the concentric contours of the anticlinal structure, which has a crest at depth of 12,400 feet below the surface. The red area indicates the extent of gas accumulation at the top of the rollover structure. The Bastian Bay fault runs along the northern edge of the anticline and forms a boundary of the gas reservoir. A small synthetic fault running parallel to the Bastian Bay fault forms the southern boundary of this gas reservoir. Wells drilled to a depth of 12,400 feet or greater within the red area encountered gas in the “O” Sand.
A cross section of well logs from the field shows the arrangement of the fault and the producing gas reservoirs. The “throw” of the fault at any horizon is the difference in elevation across the fault. At the upper blue Miocene horizon the throw is about 1,500 feet. At the “X” Sand the throw is over 3,000 feet. The interval thickness between each of the labeled sand layers is greater on the downthrown (or hanging wall) side of the fault than it is on the upthrown (or footwall) side. This indicates continuous fault movement during the deposition of the deltaic sands, and it is why faults like this are referred to as “growth faults”.
A simplified model for the history of the Bastian Bay fault can be constructed by isolating the fault in a depositional sequence in which all other faults and salt domes have been removed and sedimentary layers are considered to be flat and continuous away from the fault. The reality is that the complex arrangement of structural features in the subsurface have all been interacting with each other throughout the deposition of the sedimentary layers, but this is very difficult to model.
The Bastian Bay fault probably began as slide structure on the lower continental slope during the lower Miocene. A study of the mechanics of faults by geologists W. Crans, G. Mandl and J. Harembore at Shell Research Laboratory found that when loose sediment is deposited on a gently inclined slope (about 3 degrees), its weight has a surface-parallel component that tends to pull it down the slope. As long as this force is balanced by the reactive shear stress along the slope, the sediment will stay on the slope in stable equilibrium. As sedimentation continues and the height of the sediment layer increases, both the “pulling” weight component and the reactive shear stress increase proportionally. The equilibrium will remain stable as long as the slope-parallel shear stress remains below the limit determined by the shear strength of the sediment.
The shear strength of the sediment may be reduced when pore fluids become “geopressured” meaning that they support a portion of the overburden. The greater the magnitude of geopressure, the lower the shear strength of the sediments. At a critical point when the driving weight component exceeds the friction along the base of a sedimentary layer it may begin to slide along the basal slip plane. In this model the initial lateral slip of the fault results in a negative vertical component of slip in the active plastic region at the head of the fault and a positive vertical component in the passive plastic region at the toe of the fault. The accommodation capacity created at the head of the fault allows for a differential accumulation of sediment on the hanging wall of the fault, which will continue to drive fault movement in a feedback loop.
The influx of terrigenous sediments into the Terrebonne Trough during the middle Miocene triggered episodes of dramatic fault movement on all of the bounding faults including the Bastian Bay fault. It is significant to note that these faults tended to act as an interconnected system of faults throughout this time. It is probable that a triggering event on one fault could result movement that rippled throughout the system. The simplified model on the Bastian Bay fault during this period shows that the original lateral slip plane of the fault begins to arc upward to a steeper angle. This is the result of more competent normally pressured sediments. The curving arc of the fault plane defines the nature of a listric fault. The model also shows that fluids migrate along the fault plane, including hydrocarbons. It is almost certain that the gas accumulated in the Miocene reservoirs at Bastian Bay Field migrated up the Bastian Bay fault and into the sand layers at some time after their deposition.
Movement on the Bastian Bay fault continued to propagate throughout the Pliocene and the Quaternary. The movement of fluids up the fault plane continued throughout this time period and into the present. It is probable that saline fluids reach the surface along these major faults. This may be responsible for some of the salinity anomalies that have been measured in the coastal wetlands (Keucher et al, 2001).
A subsurface structure map on the top of the Pliocene stratigraphic interval shows the trace of the Bastian Bay fault. The fault trace is constructed on this map by marking the intersection of the depth of the mapped horizon and the depth of the fault plane. In the vicinity of the Bastian Bay Field the top of the Pliocene horizon is at a depth of about 2350 feet on the hanging wall and about 2250 feet on the footwall. The two sides of the fault trace on the map are coincident with the equivalent 2350-foot and 2250-foot contours on the fault plane map. This means that the throw of the fault in this area at the top of the Pliocene is about 100 feet. Because the elevation of the surface in south Louisiana is effectively zero, a map on the depth of the top of the Pliocene is also a map of the thickness of the Quaternary (the Pleistocene and the Holocene). The Quaternary is therefore 100 feet thicker on the downthrown side of the Bastian Bay fault than it is on the upthrown sides. This indicates that the fault continued to be active in the Quaternary, which generally qualifies it as an “active fault”.
A detailed well log cross section of the Quaternary across the Bastian Bay fault shows that the fault remained continually active throughout the interval. Individual stratigraphic intervals in the Pleistocene are vertically offset by the fault and they are generally thicker on the hanging wall side than they are on the footwall side. This pattern extends into a tentative interpretation of the base of the Holocene interval from well log correlation. It is apparent from this interpretation that there is stratigraphic evidence that the Bastian Bay fault was active during the Holocene. It is probable that late Holocene activity on the fault would have been associated with the deposition of sediments in the Bayou Robinson delta lobe between 950 and 300 years ago (Kulp et al, 2005). Bayou Ferrand was originally formed as a distributary channel in this delta lobe.
Gagliano realized that the submergence of the marshes around Pete Hebert’s camp was a result of the latest episode of movement on the Bastian Bay fault. It is the nature of all faults to move episodically in (geologically) short bursts of activity. In the tectonic regions of California these episodes are generally very brief, and fault slip happens dramatically in a matter of seconds. In a delta system where the forces are all driven by the weight of the deltaic sediments fault slip can happen much more slowly and will generally not create a detectable seismic signal. The most recent episode of fault slip on the Bastian Bay fault appears to have happened over a period of about a decade between the early 1970s and the early 1980s. Imagery of the area taken from the ProPublica publication “Losing Ground” is used in this short video sequence to show the submergence of the marsh on the hanging wall side of the Bastian Bay fault during that time period. The interconnected nature of the system of faults bounding the Terrebonne Trough throughout their geological history strongly suggests that similar patterns of movement happened on most of these faults during the same time period. This is likely to be the principal cause of the episode of wetlands loss that occurred between the 1970s and 1980s in southeastern Louisiana.
REFERENCES
Armstrong, C., Mohrig, D., Hess, T., George, T., Straub, K.M., 2014, Influence of growth faults on coastal fluvial systems: Examples from the late Miocene to Recent Mississippi River Delta, Sedimentary Geology, v. 301, p. 120-132
Crans, W., Mandl, G., Haremboure, J., 1980, On the Theory of Growth Faulting: A Geomechanical Delta Model Based on Gravity Sliding, Journal of Petroleum Geology, v. 2, p. 265-307
Dawers, N. H. and Martin, E., 2006, Fault Related Changes in Louisiana Coastal Geometry, Louisiana Governor’s Applied Coastal Research and Development Program, GACRDP Technical Report Series 05-000, 21 p.
Gagliano, S.M., Kemp III, E. B., Wicker, K. M., Wiltenmuth, K. S., Sabate, R. W., 2003 Neo-Tectonic Framework of Southeast Louisiana and Applications to Coastal Restoration, Transactions G.C.A.G.S., v. 53, p. 262-272
Gagliano, S.M., Kemp III, E. B., Wicker, K. M., Wiltenmuth, K. S., 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
Kulp, M. A., Fitzgerald, D., Penland, S., 2005, Sand-Rich Lithosomes of the Holocene Mississippi River Delta Plain, SEPM Special Publication No. 83, SEPM (Society for Sedimentary Geology), ISBN 1-56576-113-8, p. 279–293.