The Venice Dome — McLindon Geosciences, LLC

Many have wondered about the network of canals and slips southwest of Venice that boaters and fishermen call “the wagon wheel”. The outer canal forms an almost perfect circle. These canals and slips were dredged in the 1950s and 60s as drilling operators explored their way around the edges of a circular salt stock. This pillar-like body of salt is called the Venice dome. It is not only nearly perfectly circular, its sides are nearly perfectly vertical along much of its length. This certainly appeared to be the case to drillers who penetrated the flanks of the dome to depths of about 12,000 feet. A subsurface structure map on the C6 Sand at Venice Field shows the outline of the salt stock. The concentric contours, which are crossed by a network of faults, indicate a structural anticline in the sedimentary layers around the dome. Sand layers which dip upward toward the salt stock are terminated by the faults or the salt creating perfect traps for the accumulation of oil and gas. Unlike some salt domes where the oil is found across the crest of the dome, at Venice the reservoirs are all trapped around the edges of the circular salt stock – hence the circular network of canals.

Venice dome is a part of a linked tectonic system of faults and salt domes that underlies the Mississippi River delta. A previous blog post explores the relationships between the tectonic system and the development of the delta:

www.mcgeo.me/blog/geological-underpinnings-of-the-modern-mississippi-delta

The major faults in the tectonic system all extend to the surface. The network of blue surface fault traces connect the domes and outline the eastern edge of the Terrebonne Trough. The Venice Dome is intersected by one of the major faults which dips northwestward toward the center of the trough.

It is commonly accepted that the salt in the domes of south Louisiana originated from a sedimentary layer called the Louann Salt that accumulated in a restricted Gulf of Mexico Basin about 160 million years ago during the Jurassic Period. A lack of open ocean circulation caused seawater in the restricted basin to precipitate salt resulting in a thick layer on top of the faulted basement of granite and Paleozoic sediments. The Louann predominantly consists of the mineral Halite, which has the distinct physical characteristics of being low density, ductile and incompressible. As the Gulf of Mexico opened to ocean circulation and the influx of sediments being shed from the North American continent, the Louann was buried by layers of terrigenous sediment that were higher density and less ductile than the salt. The result was a disequilibrium that caused the salt to become mobile.

The history of the Venice Dome includes several common elements of salt movement over time, and paints a more complex picture than the simple vertical salt stock conceived by the early explorers. The vertical stock of the Venice Dome represents that last stage in development. It appears that the vertical stock pierced the plane of the large fault that wraps around it. This happened fairly recently in history of salt movement, perhaps within the last 10 million years. Prior to the piercement of the vertical stock the salt moved on a inclined path dipping to the northwest. This inclined salt body was coincident with the footwall of the fault, and there appears to be a genetic relationship between the salt and fault. In simplest terms the upward movement of the salt may have triggered fault slip in which the fault effectively slid off the inclined salt body down to the northwest.

The movement of the inclined salt body prior to vertical piercement is evidenced by the remnant pathway of salt movement called a salt weld. The Venice Weld dips upward from a deep tabular salt body to the base of the Venice Dome. The dome is completely detached from the original salt layer. It may be possible that a very thin layer of salt remains within the weld helping to make it visible on seismic data, but strictly speaking the Venice Dome is no longer connected to the tabular salt body from which it came.

The tabular salt body appears to evidence the first stage of salt movement sometime in the Mesozoic Era after the accumulation of terrigenous sediment on top of the original salt layer. The shape of the salt body is consistent with the lateral flow of salt, as can be seen beneath the floor of the deep Gulf of Mexico today. Deep welds beneath the tabular salt body indicate the pathways of salt movement up from the original salt layer into a lateral flow of salt beneath the Gulf of Mexico when it was thousands of feet deep at the site of the modern delta.

This complex of welds and salt bodies indicates a four-stage history for the emplacement of the Venice Dome beneath the wagon wheel. Disequilibrium caused by the deposition of terrigenous sediment on top of the salt roughly 150 million years ago caused the salt to move upward by the process of halokenisis. As the upwardly mobile salt reached the shallow sedimentary layers on the bottom of the Gulf of Mexico the properties of the uncompacted sediments were much closer to that of salt than the deeper sedimentary layers. The salt was relatively unconstrained by differences in density and ductility, and it flowed laterally just beneath the ocean floor. The shape and orientation of the tabular salt body suggest a relationship with lineations of basement faulting suggested by Stephens, 2009. It is not possible to image the subsurface with current technology at the depths necessary to fully comprehend these relationships.

Sometime in the late Mesozoic or early Cenozoic Era the salt began to move again. This may have been associated with first incursion of a major delta system into this part of the basin during deposition of the Cretaceous Tuscaloosa formation. Some manner of disproportional loading probably caused salt to move upward from the tabular body along the inclined path of the Venice Weld. This inclined salt feature probably stayed on the footwall side of the fault for about 10 million years before breaking through to form the vertical salt stock. The dome became detached from the tabular salt body during this time.

The latter stages of the history of the Venice Dome are relatively easy to document thanks to the wealth of subsurface data generated by oil and gas exploration. Sabate, 1968 examined the sedimentary layers of the Quaternary Period around Venice Field. Subsurface structure maps on the base of the Quaternary horizon show a distinct anticlinal structure across the crest of the Venice Dome. The salt reaches a depth of about 2,000 feet below the surface, just beneath the base of Quaternary. Sabate’s profile across the dome shows the relationships between the salt stock and the anticline above it. The profile also shows that the sedimentary layers of the Quaternary are thinner across the top of the dome. These layers were deposited during the cyclical ice ages of the Pleistocene over the last two and a half million years. Thinning across the top of the dome indicates a relative interaction between the salt stock and the surrounding sedimentary layers. The salt has been moving upward relative to the sediments throughout this time period. It may be that the differences in density simply mean that the dome is subsiding more slowly than the surrounding area. Whatever the mechanism, the implied lower rates of subsidence over the dome during the Pleistocene are significant, and they likely apply to the Holocene as well. It should be possible to measure variations in subsidence rates around the dome with the installation of the proper measurement infrastructure. Understanding the impact of salt domes on subsidence could have significant implications for sustainability planning.

This 3-D visualization shows the relationships between the complex of salt bodies and welds associated with the Venice Dome

REFERENCES

Hudec, M.R., Norton, I.O., Jackson, M.P.A., Peel, F., 2013, Jurassic evolution of the Gulf of Mexico salt basin, AAPG Bulletin, v. 97, pgs. 1683-1710

PaleoData, Inc. Biostratigraphic chart https://www.paleodata.com/chart/

Rowan, M. G., 1995, Structural styles and evolution of allochthonous salt, central Louisiana outer shelf and upper slope, in M.P.A. Jackson, D.G. Roberts, and S. Snelson, eds., Salt tectonics: a global perspective: AAPG Memoir 65, pgs. 199-228

Sabate, R.W., 1968, Pleistocene Oil and Gas in Coastal Louisiana, GCAGS Transactions, v. 18., pgs 373-386