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The salt-dome location map (plate 1) was compiled from eight recently published sources: Anderson and others, 1973; New Orleans and Lafayette Geological Societies, 1973; Halbouty, 1979; Martin, 1980; Louisiana Geological Survey, 198la and 1981b; U.S. Department of the Interior, 1983; and Jackson and Seni, 1984. Four of these references display both size and location of the salt domes by giving their outlines at various depths: New Orleans and Lafayette Geological Societies (1973) at 10,000 feet; Martin (1980) from 5,000 to 8,500 feet (the depth range representative of one second of two-way travel time on a seismic survey, where the depth depends on the nature of the sediment overlying the dome); U.S. Department of the Interior (1983) at an unspecified depth; and Jackson and Seni (1984) at 10,000 feet. Four sources aided in locating the salt domes although they did not show their shape. Anderson and others (1973) shows only dome locations. The other three sources (Halbouty, 1979; and Louisiana Geological Survey, 198la and 1981b) show the location of salt domes not related to hydrocarbon production and the location and size of oil and gas fields that have a spatial association with possible salt domes.

Halbouty (1979) lists only those fields where the presence of salt has been confirmed by drilling. Oil and gas fields displayed on the Louisiana Geological Survey (198la and 1981b) references were used only to augment the other six sources. Fields located above and around the perimeter of structures that were presumed domes adds validity to the other sources. The importance of maintaining consistency between references while plotting dome locations on plate 1 required that the salt domes be split into two groups, offshore and onshore domes. Offshore domes were located by aligning latitude and longitude lines because the references disagree on the exact location of offshore area boundaries.

The offshore area boundaries for this map were taken from the U.S. Department of the Interior (1983). Onshore domes were located by aligning county and parish boundaries because all sources did not have latitude and longitude lines, and those that did have lines did not agree on their exact location in reference to county and parish boundaries. Although map projections and scales varied among and within reference maps, a good fit was achieved through repeated registration across the map and the use of a variable-scale copying machine. The locations of salt domes that penetrate the base of individual layers (aquifers and permeable zones) are posted at the approximate center of the dome structure in figures 2-11.

The number of salt domes that penetrate the base of each layer ranges from a high of 154 for layer 9 (lower Pliocene-upper Miocene deposits) to a low of 29 for layer 6, the upper Claiborne aquifer (fig. 12). Younger layers generally extend farther downdip (gulfward), which explains how a younger layer can be penetrated by more domes than an older layer. The distribution of salt domes among the salt-dome basins is quite variable (fig. 13). The percentage of domes within each basin with known depths and penetrating into the gulf coast aquifers (above the top of the Midway confining unit) also varies widely. About 44 percent of the domes have unknown depth and 44 percent penetrate the top of the Midway confining unit, whereas only in 12 percent of the domes is the top of salt deeper than the top of the Midway. The Midway confining unit was chosen to differentiate depths because it is an areally extensive, massive marine clay. Above it are the gulf coast aquifers (Grubb, 1984) and below it are Cretaceous sediments. The depths of most of the offshore salt domes are unknown (fig. 13).

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