Abstract A critical review of American thought on salt-dome origin shows that from the discovery of American salt domes in 1862 until the establishment of their economic importance by the development of a cap-rock pool at Spindletop in 1901, little was known of the constitution of the domes except what little was expressed at the surface. The result was a wide variety of highly speculative theories of origin, chief of which, as best fitting our meager knowledge, was the theory that the domes were old Cretaceous islands in Tertiary and even Recent seas. Exploration of the known domes for cap-rock pools and sulphur deposits from 1901 to 1916–18 made us better acquainted with the salt, anhydrite, gypsum, limestone, sulphur, and various minor minerals—the salt-dome materials—and theories of deposition from solution became the vogue. The development of important oil deposits in the lateral sands flanking the salt masses, from 1912 to date, concentrated attention on the structural features of the dome. The deposition-from-solution and lifting-power-of-crystallization theories seemed to be inadequate to explain the sharp and considerable uplift caused by the formation of the salt core and cap rock, and, with a growing recognition of the similarity of American salt domes to the salt structures of Germany, Roumania, Mexico, and elsewhere, came a gradual swing to the theory of tectonic origin. This theory of tectonic origin supposes that the plastic salt was forced by pressure to flow from originally bedded deposits into its present position. The author accepts the tectonic or pressure-flowage theory for the origin of American domes and believes that the most serious objection raised by its opponents—the lack of evidence as to the existence of sedimentary salt deposits—is no longer tenable in view of the recent discovery of potash salts and fossil algae in the salt core of the Markham dome.

Abstract Salt-mining in Northwestern Europe has afforded significant data on the structure of salt domes. Salt is a relatively plastic rock under pressure, and may “erupt” in a manner analogous to igneous magmas. Highly deformed, clearly traceable key beds in the salt domes of Europe, and progressive changes from slight flowage at the salt outcrops to typical intrusive relations where the salt is deeply buried, afford basis for the conclusion that the European domes are purely tectonic in origin. The cap rock represents the residue of less soluble material in the upthrust salt as solution attacks the rising dome in the zone of active ground-water circulation. The salt domes of Europe are mostly without associated oil deposits, a feature probably due to pre-Eocene erosion which permitted escape of oil from older source beds.

Abstract Two kinds of cap rock are differentiated: the gypsum anhydrite-cap and the calcite cap. The gypsum is derived from the anhydrite by hydration and is characteristic of the upper part of the cap, where it contains much calcite and sulphur. The deeper, less altered part of the anhydrite cap shows parallel banding, which may be the result of diffusion, and breccia fragments of a parallel-banded, finer-grained anhydrite, which give evidence of sedimentary origin. Therefore, although no detrital minerals have been found in specimens of anhydrite cap, this cap is believed to be of sedimentary origin, brought up by an intrusive salt plug from depth. Possible explanations of the presence or absence of anhydrite caps on salt domes are offered. The calcite cap is the product of replacement and penetration by calcite of the sedimentary beds adjacent to the gypsum-anhydrite cap and probably of the upper part of the gypsum-anhydrite cap itself. The sulphur in cap rock is characteristically associated with this calcite. The calcite and sulphur are probably the result of reduction of the outer parts of the anhydrite cap by hydrocarbons from adjacent beds. Constituents of minor importance are sulphides, carbonates of an early generation, barite, celestite, bipyramidal quartz crystals, and inclusions of sandstone in anhydrite.

Abstract The Rumanian salt domes are confined to the axial portions of sharp anticlinal folds and fracture zones which affect the sedimentary rocks of the region. The anticlines are aligned with the Carpathian axes. Intrusion of the salt has been accompanied by intrusion of thick masses of breccia composed of sedimentary and some igneous rocks of all ages, including types of rock unknown beneath these portions of the mountains. Both salt and breccia have been thrust upward in diaper manner and even overthrust with sharp, recumbent crests. The author believes that the salt in the Rumanian domes cannot be of Miocene age, as has been previously contended, and he argues that the original salt beds from which salt domes have formed, not only in Rumania but universally, are products of the evaporation of the earliest lakes and shallow seas to form on the surface of the primitive earth. These waters were rich in chlorides and other salts taken into solution from the heated rocks of the earth's crust and condensed from the heavy blanket of the earth's atmosphere. Part of the immense quantity of salt deposited in this manner was re-dissolved in the permanent oceans, but after these had become saturated, the main part of the salt deposit remained, to be covered by the first muds laid down on primitive sea floors. They were thus protected until movement and pressure squeezed the salt upward as salt plugs. The intrusion of the Rumanian salt plugs is late Pliocene or early Pleistocene.

Abstract The distribution of the Rumanian oil fields is governed by lines of folding related to the major Carpathian overthrust faulting. Along these lines of folding, the production tends to focus around salt masses that form local “highs.” The source of the salt is unknown. Its rise is due to lateral pressure. The source of the oil is disputed, but the writer believes that it is Pliocene. A comparison of these fields with those of the American Gulf Coast region shows that, although there are some resemblances which may indicate the action of like causes to a limited extent, the dissimilarities are greater. The lateral pressure, which is proved for Rumania, cannot be accepted as the entire, or even as the major, cause of the American domes.

Abstract The Zechstein salt deposits of middle and northern Germany were laid down in a sinking basin in which sinking came to an end during the Jurassic, except in northern Germany, where it persisted into the late Tertiary. The beds deposited in this basin were involved in the Saxon mountain-building movements, which contrast with the older, Variscan movements in being intermediate in character between folding and block faulting. The Saxon movements were periodic and not continuous. That the movements were due to compressive thrust is shown by the fact that the “horsts” were uplifted. The salt bodies are found in the form of “salt beds,” “salt anticlines,” and “salt stocks.” “Salt anticlines” are normal anticlines both in the form and inner structure of the salt and in the structure of the sedimentary cover. The “salt stocks” are strongly folded, subcircular to elongated masses of salt which are upthrust into faulted rather than folded adjacent formations. The main theories proposed to explain the upthrust of the salt are three: Lachmann's “atectonic” theory, the “isostatic” theory, and the theory of upthrust by lateral thrust. Lachmann's “atectonic” theory of upthrust of the salt by an inherent autoplastic force is no longer current. The formation of the salt anticlines by the compressive thrust of the Saxon orogenic movements is very generally accepted by German geologists. But as there is every gradation in form between the characteristic salt anticline and characteristic salt stock, as the gradation from one to the other can be followed on the same anticlinal axis, and as with a rare exception the periods of movement in the salt stocks coincide with the periods of the Saxon orogenic movements, it seems reasonable to believe that all have been caused by the same force. The difference in the resulting forms is due rather to the difference in the materials acted upon than to difference in the forces acting. The salt is more plastic and therefore more mobile than the ordinary sedimentary rocks, and is therefore the more easily deformed. Under intensive deformation, it advances far ahead of the other rocks, and thus a salt stock is the extreme form of an anticlinal core. The tectonics of salt upthrust are therefore a phase of the tectonics of mobile materials and are intermediate between the normal tectonics of folding and the tectonics of magmatic intrusion. D. C. B.

Abstract The American salt-dome problems are divisible into two parallel series, the one comprising problems of description, the other problems of theory. The solution of the latter are of necessity dependent upon solution of the former. The American salt domes consist of subcircular stocklike masses of salt, capped in most cases by limestone and gypsum-anhydrite, intruded into and surrounded by Pleistocene-Eocene or Eocene-Cretaceous sediments which dip quaquaversally away from the salt core. The domes show certain tendencies to alignment. They occur in regions of geologic quiescence where there has been no compressive folding. The Roumanian salt domes are divisible into two groups, the Carpathian-Sub-Carpathian, and the Transylvanian. The Carpathian-Sub-Carpathian domes consist of narrow, elongated, vertical intrusions of the Salifere salt, clays, and marls along anticlinal axes and into a thick series of Pliocene and Miocene sediments. The domes occur on the edge of, and immediately in front of, the Carpathian sheet overthrust and are aligned along structural, mostly anticlinal, axes which reflect the effect of the Carpathian tectonics. If the difference in the structural setting is allowed for, the Roumanian domes closely resemble those in America. Their origin has usually been attributed to some phase of the tangential thrust of the Carpathian mountain-building forces. Krejci recently has raised strong objection to that theory, and has advanced a tectonicisostatic theory according to which tectonic thrust is responsible for the localization and initiation of their formation, but the weight of the overlying sediments is responsible for the upthrust of the Salifere core. The Transylvanian domes occur in the Plio-Miocene Transylvanian basin. Although only poorly known, they seem to be very similar in form to the American domes. They are aligned on anticlinal axes which are sub-parallel to the periphery of the basin. The German salt domes are a phase of the German salt deposits, which are a definitely sedimentary series with a well-defined, persistent, and characteristic section. One of the middle members carries marine fossils, and two members are potash bearing. On account of the extensive mining and exploration for this potash, and on account of the recognizable section in the salt series, the structural deformation of the salt deposits has been worked out in great detail. On the basis of form and structure of the salt deposits there is a complete gradation in type and in space from undeformed sedimentary beds through broad anticlines with slightly swollen cores of salt (Strassfurt type), sharp anticlines where the salt core is starting to pierce the cover (Asse type), to broken anticlines in which the salt core has been squeezed up between the two flanks (Leine type), or to salt stocks (Hannoverian type) in which a pluglike mass of salt has been intruded for thousands of meters vertically into the overlying sediments. There seemingly can be no dispute that the German salt domes and salt ridges are the result of the plastic deformation and flow of a sedimentary salt series. The salt domes and ridges are aligned along Rhenish and Hercynian anticlinal axes. Many of the domes seem to be at the intersection of axes. The upthrust of the salt cores is attributed to tectonic thrust by Stille, who presents substantial evidence for such a theory. Lachman, Arrhenius, Seidl, and others argue less conclusively for an isostatic upthrust. In view of the evidence of the Roumanian and German salt domes, in addition to what is known about the American domes, it would seem unreasonable to believe that the American salt domes are not the result of the plastic deformation and upthrust of a sedimentary salt series.

Abstract This paper describes the geology of the six interior salt domes in eastern Texas and discusses their relationship with the geology of the region and with the interior salt domes of Louisiana.

Abstract The eleven known interior salt domes of Louisiana, herein described, are distinct and separate from the Gulf Coast domes. They occur in an area of Claiborne Eocene exposures, on the east flank of the Sabine uplift, in the northwestern part of the state. They are distinguished from the common salines by characteristic salt-dome topography and other surface manifestations. This paper lists the more important publications on the Louisiana interior domes, dating from Forshey, in 1850, to Harris, in 1910, and adds thereto much new information secured by recent drilling and by special investigations of the author and others. All but one of these domes have been partially explored by drilling for oil and gas, but as yet no production has been found. The history, topography, and geology of each dome are described in detail.

Abstract The Bayou Bouillon salt dome is located in the swamps northeast of Martinsville, Louisiana. Cap rock appears to occur at depths of 1,500 to 2,000 feet. The salt has a rather gentle slope on the east and north, but the west and south sides of the dome are unexplored. Small quantities of heavy black oil have been found in shallow sands.

Abstract Gas and “paraffin dirt” prompted drilling of a test well northeast of St. Martinville, Louisiana. A showing of oil was found at a depth of 900 feet, and salt from 1,250 to 2,500 feet. There was no cap rock.

Abstract The Five Islands, so called, are in reality large, symmetrical hills, or rounded eminences, rising to an elevation of 100 feet or more from a flat, marshy plain in southwestern Louisiana. They are distributed at irregular intervals along a straight line which runs about northwest. They have aroused interest and published comment from scientists for more than a hundred years, and for a long time it has been generally known that these hills are underlain by immense bodies of salt and are the surface expression of recent upthrusting of salt plugs. Each of the islands is described in detail as to its geology, its record as a source of salt (several of the Islands have for years been the site of extensive salt-mining), and its possibilities for oil production. The evidence which these occurrences affords as to the origin of salt domes is analyzed. An extensive Bibliography is appended.

Abstract The Jennings oil field was one of the earliest oil fields of the first class on the Gulf Coast. It is on a dome in Tertiary sediments. Although the salt has not been drilled into, limestone which has never been drilled through and which resembles cap rock underlies the main part of the field, and is taken to indicate the presence of a salt dome. There is a surface mound that is probably the remnant of a salt dome mound and a depression that in part is probably a “central depression.” The Jennings field has been one of the most prolific of the Gulf Coast oil fields, and had many long-lived wells with enormous total production.

Abstract The Pine Prairie salt dome is a characteristic Gulf Coast salt dome, and is composed of a steep-sided, relatively flat-topped, pluglike salt core surmounted by a thimble-like cap of limestone and gypsum-anhydrite. The salt core is 1 mile in diameter and rises to within 520 feet of the surface. The limestone of the cap is exposed at the surface. The salt core and cap have been intruded into beds of Oligocene to Pliocene age. Commercial production has never been established. Four small producers were completed at a depth of about 1,300 feet on the west edge of the dome. Three later well located deep tests respectively on the northeast, southeast, and southwest sides of the dome were dry.

Abstract The Welsh oil field seems probably to be of the Goose Creek or non-salt-dome type of Gulf Coast oil field. The beds are relatively flat over the field, but dip sharply to the southwest in the southwest quadrant. Small production was developed from the Pliocene, but failed after several years, so that today there is no production at Welsh. The oil was of low gravity, 11° to 19° Baume, but had especially high fire and flash points and a viscosity that made it especially desirable for car-journal oil. The field has some possibilities for discovery of deeper productive sands.

Abstract The sulphur salt dome in Louisiana is a typical Gulf Coast salt dome, but is exceptional in its small area, about 75 acres, in the very great thickness of the cap, about 1,000 feet, and in the richness of its deposit of native sulphur. The cap is composed of anhydrite and a mantle-like mass of “lime” rock covering the top and flanks of the anhydrite. The sulphur is found as a secondary mineral in the transition zone between the “lime” rock and the anhydrite and has probably been precipitated from hydrogen sulphide or metallic sulphides in solution. After a long history of vain expensive attempts to mine the sulphur, the Frasch process was perfected, by which the sulphur is melted in place and pumped in liquid form to the surface. The total production to date has been about 9,000,000 tons of sulphur with a gross value of $150,000,000. Small amounts of heavy oil were encountered in the early sulphur wells, but not in commercial quantity. The flanks have not been well tested for oil.

Abstract The Edgerly oil field is located in the southwestern part of Louisiana in a topographically featureless part of the Gulf Coastal plain. Exploration for oil was based on gas seepages, and though a total production of more than 55,000 barrels of oil per acre has been obtained from the field, there is no indication of a salt dome other than the local steep dips away from the field. No salt, gypsum, or cap rock has been encountered in any well. There are four main producing sands, at average depths, respectively, of 2,700, 2,900, 3,000, and 3,100 feet.

Abstract The Spindletop oil field was the first and one of the most brilliant of the Gulf Coast oil fields. Spindletop is a characteristic Gulf Coast salt dome and is composed of a steep-sided, relatively flat-topped, circular salt core with a diameter of about 1 mile, and with a limestone, anhydrite, gypsum cap surmounting the salt. Most of the oil was produced from the porous cavernous limestone at the top of the cap. The early gushers have never been equaled in the United States for the size of their daily flush production. Few fields in the United States of like size, 265 acres, have had as big a production, thirty million barrels in the first three years, and a total of over fifty million barrels to date.

Abstract Although salt has not been penetrated in drilling, the Big Hill salt dome, Jefferson County, Texas, in all probability is a characteristic Gulf Coast salt dome. It has a very distinct salt-dome mound, a characteristic salt-dome cap. One well has been completed as a producer. The oil is typical Gulf Coast crude.