Abstract The frontispiece of the symposium on Gulf Coast Oil Fields is a mosaic of parts of 40 photographs which were taken at elevations of approximately 12,000 feet. The successful reading of an aerial photograph, such as this one of the Barbers Hill salt dome requires special training and skill. The significance of various elements of aerial pictures is pointed out so that the reader may recognize the cultural and physiographic features and determine the nature of the geologic structure underlying the surface.

Abstract The salt of the salt domes of some areas is known to be sedimentary. Geological observation of those domes and laboratory experimentation show that salt flows plastically under differential pressure. The German salt domes are known to have been formed by the plastic flowage of the Zechstein salt series. Any type of differential pressure should tend to produce plastic flowage of sedimentary salt whenever certain critical conditions such as those of pressure, temperature, and time have been exceeded. The plausible sources of pressure are two: (1) the static pressure of the overlying sediments; and (2) the dynamic pressure of tangential compression or thrust. Under (1), growth of the dome by upthrust can take place only if the available energy is sufficient both to overcome friction and to uplift the salt core and some sediments against gravity; growth of the dome by downbuilding can take place if the mother salt bed is sinking in earth space; the position of maximum uplift is below that of isostatic equilibrium of the salt core; and the form of the salt dome should evolve progressively through a characteristic series of forms. Under (2), the horizontal dynamic pressure will act indirectly upward through anticlines and downward through synclines in competent beds; and directly through horizontal squeezing of the salt in relatively upthrust cores. The static thrust of (1) will be active and may be more important than the dynamic thrust of (2); the position of maximum upthrust of the salt core may be far above its position of isostatic equilibrium; the form of the domes should be varied. The Gulf Coast domes have been formed by the plastic flowage of sedimentary salt intrusively into the overlying sediments. The evidence for that origin of the domes comes from the structure which is revealed by oil-field drilling, from algal remains in the salt, and from the close similarity of the American salt domes to the German salt domes. The age of the salt is greater than most of the Lower Cretaceous. The motive force of the formation of the domes has been the static weight of the sediments. Growth of the domes has taken place throughout the Tertiary and has taken place on a few domes in the most recent past. There was no dynamic tangential compression in the Gulf Coastal Plain area during the Tertiary and Quaternary; therefore, the motive force presumably must have been the static thrust of the sediments. Subsidence of the mother salt bed took place almost continuously through the Tertiary and into the Quaternary. The difference between the specific gravity of the salt and of the sediments is small; and the calculated force of upthrust is small, rather too small to overcome friction and to uplift the salt core and some sediments against gravity. Growth of the domes, therefore, must have been largely by downbuilding. Partially corroboratory evidence is given by the concomitant cessation of growth of the Clay Creek dome and cessation of the regional subsidence of the general surrounding area. But some actual upthrust has taken place on the Gulf Coast domes; and, as theoretically expectable, it tends to be greater on the domes of larger diameter. Growth has not continued into recent time on all the domes and has ceased finally at different times on different domes. The law holds crudely that the deeper the dome, the older the time of cessation of growth. The final cessation of growth in general may have been caused by exhaustion of the salt in the mother salt bed, attainment of isostatic equilibrium, frictional freezing of the salt core to the sediments, and, in the case of downbuilding, by cessation of the subsidence. The succession of retrograde movement of the salt core after the cessation of growth is suggested, inconclusively, by the Clay Creek dome. The presence of rim synclines has been suggested by the results of torsion-balance surveys, but has not been identified from geologic data. Rim synclines could be formed: (r) by solution of the flank of the salt, and (2) by the pushing-in of the deep flank of a flaring salt core in the growth of the dome. Overhang of the salt and cap is present in two types: (1) tilting of the vertical axis of the dome, and (2) mushrooming (Barbers Hill). Type (r) is produced perhaps by the seaward flowage of the deeper sediments. The explanations oftype (2) are not satisfactory.

Abstract Barbers Hill is a moundlike elevation in the extreme northwestern portion of Chambers County, Texas, 26 miles northeast of Houston. It is oval in shape, covers an area of 1,718 acres, and rises 45 feet above the surrounding prairie. This elevation is the result of an intrusive salt plug which has raised the surface above its normal position. The salt plug is composed of almost pure rock salt and is capped by deposits of gypsum anhydrite and limestone. Formations surrounding the salt plug are inclined at steep angles dipping away from the salt plug. Numerous indications of geologic disturbance are present on and about the dome. The most prominent of these are mineralized water-and-gas seepage in shallow water wells, paraffin dirt, and gas seepage on the surface. Prior to 1924, 120 wells were drilled, of which 26 produced oil and 91 were failures. The total production for this period is 784,520 barrels, all of which was obtained from an area of 100 acres on the southwestern side of the dome. The average production of producing wells has been 30,174 barrels, and the average depth 2,251 feet. The oil has an asphaltic base with gravities ranging from 17° to 39°7 Baumé; the oil produced from cap rock averages about 20° Baumé. Production around the periphery of the dome is obtained from sands and limestones of the Fleming formation.