Breton Island
The Breton National Wildlife Refuge was formed in 1904 by executive order of President Theodore Roosevelt. It is the second oldest refuge in the National Wildlife Refuge system. Looking at this video of Roosevelt’s visit to Breton Island in 1915, it’s not hard to understand why. The visit was sponsored by the Audubon Society and they played an instrumental role in preserving ecosystems that support wildlife.
Breton Island is at the southern end of a chain of barrier islands that form an arcuate perimeter to Breton and Chandeleur Sounds. The sand that makes up these islands was deposited in bars at the mouths of historical lobes of the Mississippi River over 2,000 years ago. Geologists Bryan Rogers, Mike Miner, and Mark Kulp at the University of New Orleans reconstructed the patterns of the historical delta lobes. Their research suggests that Breton Island may have gotten its start as early as 4,000 years ago from delta lobe 5 at about the time the Pyramids of Giza were being constructed. The ancestral Mississippi River revisited the area several times, and the deposits of delta lobes 5, 8, 9 and 13 are all preserved in the shallow subsurface. While the swamps and marshes of those deltas have subsided below the surface, the sand remains near the surface as barrier islands. Breton Island is different than the rest of the chain. The long linear pattern of the islands from Hewes Point to Curlew Island gives way to a distinctly rounded shape at Breton Island.
The Louisiana Barrier Island Comprehensive Monitoring (BICM) program was established by Louisiana Coastal Protection Restoration Authority (CPRA). The goal of the program is to provide long-term data on the barrier islands of Louisiana that could be used to plan, design, evaluate, and maintain current and future barrier-island restoration projects. CPRA partnered with the the U.S. Geological Survey (USGS) and the University of New Orleans to help achieve these goals. The BICM program used both historical and newly acquired data to assess and monitor changes in the aerial and subaqueous extent of islands, habitat types, sediment texture and geotechnical properties, environmental processes, and vegetation composition. This program is an excellent example of the value of data collection as an essential component of coastal sustainability planning.
The detailed bathymetric maps around Breton Island produced by the BICM program show the mound-shape accumulation of sand that supports the island. The island itself appears to wrap around the mound, suggesting that the sediment supply is from the northeast. A comparison of the 1870 and 2007 maps shows that while the island has suffered significant degradation, the supporting mound has remained fairly stable. If the sediment supply to the island was primarily littoral drift along the island chain from the northeast, it may be possible that the dredging of the Mississippi River Gulf Outlet channel caused an interruption of the natural sediment supply, which contributed to the degradation of the island. The hook-shaped configuration of Breton Island that appears to be wrapping around a point of resistance is interesting because the island is underlain by a salt diapir.
Subsurface geological interpretation reveals the dome-shaped top of a salt diapir at depth of about 18,000 feet directly beneath the dome-shaped bathymetric feature that supports the island. The diapir has a distinct convex upper surface typical of the conceptual “salt dome”, but it has a concave lower surface with the suggestion of a “tail”. This indicates that the diapir was squeezed up from deeper source of salt to the northwest.
The southern face of the salt diapir is cross by a fault. The depth contours of the fault plane map indicate that the fault is slightly deflected by the diapir, and it is likely that there is a genetic relationship between the emplacement of the salt feature and the formation of the fault. The fault appears to extend to the surface, and the projected surface trace of the fault appears to arc around the bathymetric mound.
Natural gas accumulations were found in Miocene sand layers beneath Breton Island in the 1940s. The well logs from the drilling that followed has allowed for detailed mapping of the Miocene sands. Subsurface geological structure maps show that the gas accumulations are trapped in anticlinal structures the directly overlie the salt diapir. The concentric depth contours on the Miocene 1 and Miocene 2 sands, at depths of about 3,800 and 5,800 feet respectively, define the anticline feature. The relationship between the anticline and the fault is that of a classic “growth fault rollover”. The subsurface structure maps on the Miocene sands and the well profile across the area show that the anticline nests within the arcuate shape of the fault plane. The thickness of the sedimentary layers is greater on the downthrown side of the fault, indicating that the fault was actively slipping during the deposition of the Miocene sediments. The sedimentary layers also get thinner onto the crest of the anticline and then thicken into the fault on the downthrown side. This is indicative of active fault slip during deposition, but it also indicates a relative upward movement of the crest of the anticline. The fact that the area of relative upward movement on the rollover anticline directly coincides with the underlying salt diapir suggests the salt feature has continued to exert a structural influence on the overlying sedimentary structure long after the upward movement of active diapirism had ceased.
An analog for this type of influence from below may be found in the art installation “The Impact of a Book” by Jorge Mendez Blake. This work uses the physical expression of a wall of bricks stacked across a book to create a powerful metaphor for the intellectual and sociological impacts of a book. The physical expression itself also provides a useful image for the geology of Breton Island. To make the analogy complete imagine that instead of placing the book on the floor and stacking bricks on top of it, a moveable foundation is set at the level of the top of the wall. The book is placed on this foundation, and as each row of bricks is laid down, the foundation is lowered. In this configuration the progressive downward movement of the foundation is equivalent to subsidence, the book is equivalent to salt diapir, and the addition of each new row of bricks is equivalent to sediment deposition. The impact of the diapir is continually expressed at the surface as each new layer is added on, and the structural expression of the diapir is propagated all the way to the surface. This appears to be the relationship between the Breton Island salt dome and the Breton Island bathymetric feature.
A panel of four geological reconstructions through time shows the progression of structural development and the continual influence of the salt diapir. At the end of the Paleogene Period (the Paleocene Epoch through the Oligocene Epoch) the salt diapir was near the end of its period of active diaprism. The salt appears to have been squeezed up from the northwest under the weight of a gradually advancing sedimentary load. The area was in a deep water depositional environment which the huge influx of terrigenous sediments from the Rocky Mountains had yet to reach. The initial upward movement of the salt diapir and the formation of the fault may have been related to the compressional “toe” feature of a fault system further to the north, but this is not entirely clear. The influx of course terrigenous sediments into this area began in the lower Miocene. The rate of sediment accumulation was so great that the continental margin prograded southward, and by the middle Miocene a structural graben had formed across the crest of the diapir in the upper continental slope environment. The antithetic fault component of the graben ceased its motion at the end of the middle Miocene, and the primary fault developed into a normal growth fault by the upper Miocene. This coincided with the progradation of the continental shelf across the area. The outer shelf deltas of the ancestral Mississippi River during the upper Miocene deposited the sand layers that would become the reservoirs for the natural gas accumulations. The transition of the structure across the top of the diapir from a graben to a fault rollover happened during this time. The red box at the right of the panel shows the interval covered by the well profile.
It appears that the rollover anticline continued to be expressed throughout the Pliocene and Pleistocene Epochs. The shallowest horizon that can be mapped with well log data is a Pleistocene sand at a depth of about 800 feet below the surface. This map shows the rollover anticline at this horizon indicating that the fault was active and that the impact of the diapir was being expressed. The indication of fault slip during the Pleistocene generally classifies the fault as being an “active fault”.
The evolution of the Holocene delta across this area was primarily driven by the interplay of deposition and subsidence. When delta lobe 9 was active emergent wetlands covered most of what is now Breton and Chandeleur Sounds. The mouth bars of this delta formed the arcuate fringe of the delta lobe that would eventually evolve into the barrier island chain. That evolution began with the abandonment of the delta lobe when the river changed course and began building delta lobe 10 in Barataria Bay. Radiocarbon dating on an in situ cypress stump encountered in a sediment boring near Breton Island indicates it is a remnant of this delta environment from about 2100 years ago. The stump was found at a depth of about 30 feet below the surface. Given that it was at the surface 2100 years ago, this indicates an average subsidence rate of about 5 millimeters per year over that time span. This is same rate of current subsidence that was measured in a recent study of the Breton Basin by CPRA.
Breton Island appears to have maintained a surface expression at nearly the same place over the same 2100 year time span. The influence of a salt diapir over 18,000 feet below the surface may have played a role in maintaining the surface expression of the island.
REFERENCES
Frazier, D.E., 1967, Recent deltaic deposits of the Mississippi River: their development and chronology, Trans. G.C.A.G.S., v. 17, p. 287‐315
Rogers, B.E., Kulp, M.A., Miner, M.M., 2009, Late Holocene chronology, origin, and evolution of the St. Bernard Shoals, Northern Gulf of Mexico, USA, Geo-Mar Lett, v. 29, p.379-394