Salt tectonics

M. K. Jenyon, Salt Tectonics, Elsevier AppHed Science PubHshers, New York, 1986. 

On passive margins, seep flows are caused by (1) the escape of petroleum and natural gas driven by salt tectonics (Figure 19.21b), (2) artesian flows, (3) catastrophic erosion of sediments, and (4) submarine slides. 

The structural style of the Heidrun area is the result of episodes of stretching and of salt tectonics. The patterns of sedimentation in the area generally reflect the simultaneous operation of these two processes. Based on seismic, well, structural and sedi-mentological data, the structural history of the Greater Heidrun study area is summarised by Bukoviks et al. (1984), Schmidt (1992), Knott et al. 

S. (eds) Salt Tectonics A Global Perspective. American Association of Petroleum Geologists Memoir, 65, 109-151. 

Rowan, M. G. 1995. Structural styles and evolution of allochthonous salt, central Louisiana outer shelf and upper slope. In Jackson, M. P. A., Roberts, D. G. Snelson, S. (eds) Salt Tectonics A Global Perspective. American Association of Petroleum Geologists Memoir, 65, 199-228. 

The present sources to the ocean are the weathering of old evaporites (75% of river flux) and CP carried by atmospherically cycled sea-salts (25% of river flux). Loss from the ocean occurs via aerosols (about 25%) and formation of new evaporites. This last process is sporadic and tectonically controlled by the closing of marginal seas where evaporation is greater than precipitation. The oceanic residence time is so long for CP ( 100Myr) that an imbalance between input and removal rates will have little influence on oceanic concentrations over periods of less than tens of millions of years. 

Shallow-water embayments provide a mechanism to isolate seawater so that evaporation can raise salt ion concentrations. Arid climates are required to ensure that the rate of water loss from evaporation exceeds the rate of water supply by rainfell, groundwater seeps, or river runoff. Seawater can be resupplied continuously via a type of antiestuar-ine circulation as illustrated in Figure 17.2 or episodically as a result of sea level change, plate tectonics, or very high tides and storm surges. 

The ancient evaporites of the Phanerozoic eon were deposited at rates as fest as 100 m per lOOOy. These rapid rates are thought to have been caused by a lowering of sea level associated with tectonic activity and glaciation. Some of the largest of the salt giants are the Messinian evaporites that formed in the Mediterranean Sea during the late Miocene epoch, 5.5 to 6.5mybp. 

The Carboniferous sediments of the Maritimes Basin were originally deposited as red-green interstratified continental to marginal siliciclastics, marine limestone, dolostone, gypsum, anhydrite, halite, and locally, potash. The salt deposits vary from stratified, with only minor structural complications, to those that have been tectonized into pillows, anticlines e.g., Penobsquis Deposit) and diapirs or domes. In the latter cases, structural complexities make the stratigraphic position of many of these deposits uncertain. 

The dyke is conserved as an isolated tectonic boudin embedded in potash salt. 

The plot of Fig. 7 is made against the maximum depth reached by the seal. For cases C-G this is the present depth for cases A, B, M, and N this was the depth at 85 myrbp. The maximum depth of the seal has been chosen as the control on the permeabilities because a body of rock salt once exposed to a pres-sure/temperature regime largely keeps the petrophysical properties acquired at that time even if subsequently discharged and cooled due to tectonic uplift (permeability hysteresis see Borgmeier and Weber,... 

Hodgson, N. A., Farnsworth, J. Fraser, A. J. 1992. Salt-related Tectonics, sedimentation and hydrocarbon palys in the Central Graben, North Sea, UKCS. In Hardman, R. F. P. (ed.) Exploration Britain. Geological Society, London, Special Publications, 67, 31-63. 

Jackson, J. S. Hastings, D. S. 1986. The role of salt movement in the tectonic history of Haltenbanken and Traenabanken and its relationship to structural style. In Spencer, A. M. (ed.) Habitat of Hydrocarbons in the Norwegian Continental Shelf. Graham Trotman, 241-257. 

We are thus left with the possibility that the oceans, including both the water and the soluble salts found therein, were present at the Earth s surface right at the beginning and subsequently have been subjected to recycling between crust and mantle via the plate tectonic cycle. 

We may assume that a certain amount of secondary quartz could be deposited from thermal and formation waters. These processes could take place in permeable layers in which formation waters and deep waters can circulate as well as a result of the lowering of the partial pressure of CO under these conditions. In any case the presence of gypsum, anhydrite and baryte in numerous sections of the rocks studied is evidence of the penetration of mineralized waters derived especially from the salt-bearing sections of the Triassic. We have also tried to find a connection between the permeability of the sandstones and the distance of these horizons from tectonic disturbances. It has been noted that the mean permeability of reservoir rocks in holes situated less than 500 m from fractures or faults differs very little from that in rocks from other holes (Fig. 3.9). 

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