This report is based on the study of a diamond-drill core through the entire gypsum-anhydrite cap rock. The anhydrite cap rock forms on top of a salt stock by cementation of anhydrite freed from the salt as a result of solution of the salt by ground water. Intermittent accumulation or compaction results in distinct layers called katatectic layers or beds, separated by katatectic surfaces. The anhydrite cap rock as thus formed is designated primary anhydrite cap rock. All changes in it or additions to it are epigenetic. Gypsification, under present conditions, probably does not take place, in the cap rock represented in the core on which this report is based, below a depth of about 1183 feet, except perhaps adjacent to faults. Gypsifying waters penetrate the cap rock either along a few widely separated fissures from which they advance on a fairly definite front, or, where deformation has been more intense, along an intimate mesh of closely spaced fissures, causing what is here called “diffuse gypsification.” Gypsification does not seem generally to have been accompanied by an increase of volume. The excess calcium sulfate must therefore have been removed. Gypsum first forms as sharply defined crystals mostly smaller than the anhydrite crystals they replace, but not necessarily falling within the limits of a single anhydrite crystal. Integration into larger crystals takes place very irregularly and complexly, but the end product is large, clear, homogeneous crystals. To explain the complexity in integration it is suggested that the water of crystallization of the gypsum acts as a medium in which the lattice of crystals is unstable so that some growing crystals can influence and reorient the lattice of other crystals at a distance. The influence of stress on integration and factors determining the boundaries of integrated crystals are briefly discussed. Along better-defined narrow fissures gypsum, instead of forming originally as equant crystals, is elongated transversely to the fissure. But it formed this way by replacement of the anhydrite adjacent to the fissure, not by growth into an open fissure. It usually forms along steep fissures that are probably due to tension, not along low-angle fissures generally associated with shear. As a result of the upward “flowage” of the salt of the salt stock, the cap rock is pushed against and through the overlying beds. “Flow” is faster in the central than in the peripheral part of the salt stock so that the cap is bent. The net results are shear and tension fissures, faults, and mylonization of the cap rock to various extents in different parts. Faulting may also result from the lowering of the salt table—the flat top of the salt stock. The effects of stress on the individual minerals of the cap are discussed. Certain minerals, especially the coarse rhombic carbonates, seem to retain evidence of deformation when recrystallization has made it difficult or impossible to recognize such evidence in the enclosing anhydrite or gypsum rock. They can therefore record past deformation. Evidence is brought out in support of the hypothesis that deformation of the anhydrite cap rock resulting in mylonization and “flowage” takes place by movement along a mesh of fissures forming a more or less rhomboidal to lenticular pattern. As deformation proceeds, the rhomboids become more and more elongate, and the fissures consequently more nearly parallel, resulting in the lenticular and braided pattern that gives the appearance of “flowage.” However, the evidence indicates that except along faults displacement of any two adjacent parts of the cap rock is slight. But deformation of the cap rock as a whole by the summation of many such small movements may be considerable. Apparently even very slight movement can result in intense mylonization. In some specimens of anhydrite rock the cleavages and the spaces between crystals have been opened up, but there is no mylonite. This is considered the first effect of stress on the rock. It gives the polished face a saccharoidal appearance. It is here called “emphasized cleavage.” Mylonization is apparently more intense below than above faults, and more intense below than above katatectic surfaces along which there has been movement. It is suggested that this is due to deformation as a result of the upward thrust of the salt, partly relieved by shearing along fault planes or katatectic surfaces. Although the forces that produce fissures cannot be sharply differentiated, there are three general types of fissures in the cap rock: shear fissures generally having low dip roughly parallel to katatectic surfaces; tension fissures, generally steep; and echelon fissures, of intermediate dip, attributed to a combination of shear and tension. Where the steep tension type of fissure formed it generally preceded the low-angle fissure, but in an association of the three types the order seemed to be echelon fissure, shear fissure, tension fissure. Two phases of recrystallization in the anhydrite cap rock are discussed. One phase reintegrates the fragments of mylonized anhydrite into larger crystals, the other results in elongation of anhydrite crystals. Criteria for the recognition of reintegrated anhydrite rock both in hand specimen and in thin section are discussed. Elongation by recrystallization is believed to take place adjacent and parallel to fissures. Random elongation is ascribed to growth parallel to the rhomboidal mesh of fissures along which “flowage” is postulated. The common, more advanced recrystallization immediately above rather than immediately below katatectic surfaces is ascribed to circulation of water upward from these surfaces. Some specimens of anhydrite rock from the deeper part of the cap show an irregular, intimate interpenetration of grains; some of these grains are crushed though there is no general mylonization. This is attributed to solution as a result of pressure between grains. Many phenomena attributed to pressure solution resemble those attributed to recrystallization, and criteria are presented for differentiating pressure solution from recrystallization. In some specimens of anhydrite rock gypsum, instead of forming a matrix for the irregularly corroded anhydrite grains from which it was derived, forms a film between seemingly unmodified, straight-sided anhydrite crystals. This is here called “rim gypsum.” It is concluded that it was introduced in solution and that it was formed, partly in place, partly in adjacent rocks, by the action of sulfuric acid waters, derived from sulfides, on calcium carbonate. It is associated with sulfides, with a mineral believed to be kaolinite, with corroded-looking carbonates, and with an ocherous stain. The source of the sulfides and of the sulfuric acid waters may have been in sulfide-bearing beds adjacent to the parts of the cap rock in which the rim gypsum occurs. However, much of the rim-gypsified rock seems to have been exceptionally porous, and possibly the rim gypsum and its associated phenomena are due to this porosity rather than to localized external factors. In many parts of the cap rock gypsum has reverted to anhydrite, here called “regenerated anhydrite.” Some of these regenerated parts occur above what is regarded as the present lower limit of gypsification at about 1183 feet. In them regeneration probably took place when they were more deeply buried. The outstanding feature of regenerated crystals is that, where scattered through a gypsum matrix, they have smooth boundaries in contrast with the irregular corroded boundaries of anhydrite crystals residual from gypsification. Most regenerated anhydrite is rather impure. Where, as most commonly, it has formed around a residual nucleus of primary anhydrite, these impurities may differentiate it from the nucleus. As a result of deformation of the original anhydrite before gypsification, and of the separation of parts of a single grain by intervening gypsum, regeneration results in complexly forked and intergrown grains. Undeformed regenerated crystals, where free to grow in gypsum, tend to develop what appear to be dome faces instead of the pinacoidal and basal faces that characterize primary anhydrite in the cap rock. Spindle-shaped regenerated grains are common and are attributed to imperfect development of these domed forms. The gypsum in which regenerated anhydrite forms is generally completely integrated. These and related criteria for the recognition of regenerated anhydrite are presented and illustrated, and differentiation of regenerated and recrystallized anhydrite is discussed. Criteria for the recognition of regypsified regenerated anhydrite, which is not uncommon, are presented. The following minerals are discussed: dolomite, calcite, aragonite, sulfur, sulfides, quartz, chert, celestite, barite, kaolinite (?), fluorite, and an unidentified mineral, perhaps strontianite. Of these only dolomite gives definite evidence of being in large part syngenetic with the cap rock. As quartz is common in all salt deposits much of the quartz must also be syngenetic. But both of these in many instances, and all the others in almost every instance, give definite evidence of introduction into the cap rock after its formation. Study of the carbonates was facilitated by the fact that gypsum and anhydrite are soluble in sodium thiosulfate (“hypo”). Dolomite occurs as well-formed rhombohedra, and all well-formed rhombohedra were assumed to be dolomite. Calcite envelopes on dolomite rhombohedra are common. Calcite occurs as coarse, spindle-shaped crystals and in two types of finer-grained crystals—a more or less equant one, and a spindle-shaped one. The relations and genesis of these are discussed. It is suggested that the fine-grained spindle-shaped type originates in the deeper parts of the cap. Some of the coarse crystals may be syngenetic with the cap rock, but the associations of most of them indicate they are epigenetic. All fine-grained calcite appears to be epigenetic. Aragonite is common and has at least two habits. Complex intergrowths of crystals are common, and much aragonite occurs as skeletal crystals in gypsum. The skeletons apparently are the residue of aragonite crystals partially replaced by gypsum. Various habits of sulfur and their associations are discussed. There is some indication that coarse sulfur crystals form in anhydrite rock, finer-grained sulfur in gypsum, but both seem to be related to gypsification. Sulfur is generally associated with calcium carbonate. In the upper part of the cap rock some sulfur appears to have been decomposed into an extremely fine-grained mass by the superheated water used in extracting sulfur. The origin of the sulfur is attributed, on theoretical grounds, to the action of hydrocarbons on calcium sulfate, probably in peripheral parts of the cap. Disseminated sulfides are common. They consist of opaque sulfides, believed to be mainly pyrite, and translucent brown sphalerite. Greater concentrations occur in places, especially in some rim-gypsified rock, and in some of these sphalerite is common. Possible relation of the sphalerite to organic remains in adjacent sedimentary beds is suggested. Quartz in well-formed but very small doubly terminated crystals was found in all solution residues of the gypsum-anhydrite cap rock, but the crystals are not readily seen in thin section. Some more permeable zones contain great concentrations of quartz, obviously secondary. A fibrous or platy mineral, believed to be kaolinite, is common, especially in association with rim gypsification. Almost all of it seemed obviously epigenetic in the cap. Only one grain of chert, a minute circular body, was found. Celestite is common, almost all of it evidently secondary. One unidentified grain, possibly barite, was seen. A mineral believed to be fluorite, apparently epigenetic, was noted in only one specimen, although the literature indicates that fluorite is common in calcium sulfate rocks. All the minerals found in this cap rock have also been found in the salt of salt stocks, but several that are common in the cap rock, especially sulfur, have not been found in bedded salt deposits. It is therefore suggested that those not found in bedded salt deposits, which include those that appear generally to be epigenetic in this cap rock, are epigenetic also in the salt stock after its intrusion.

Abstract The Beagle Platform forms a structurally complex outboard boundary to the Mesozoic Beagle Sub-basin in the Northern Carnarvon Basin in 50–1000 m of water c. 250 km offshore on Australia's North West Shelf. North- and NE-trending conjugate fault systems link to form c. 5–10 km-wide rhomboidal horsts bound by c. 5 km-wide graben in complex orthorhombic symmetry. Interpretation of the Canning TQ3D three-dimensional (3D) seismic survey identified four populations of faults comprising: (1) latest Triassic–Early Cretaceous north–south-striking normal faults (Fault Population I); (2) latest Triassic–Early Cretaceous NE–SW-striking normal faults (Fault Population II); (3) Cretaceous polygonal faults (Fault Population III); and (4) Neogene–present-day north–south- and NE–SW-trending en echelon conjugate fault arrays (Fault Population IV). Structural interpretation, comprising fault seismic interpretation, displacement analyses and fault orientation analysis, illustrates that orthorhombic extensional faulting occurred penecontemporaneously. Three-dimensional non-plane strain with non-zero intermediate ( σ 2 ) extension magnitude controls the near-synchronous displacement of fault population I and II conjugate fault sets in orthorhombic symmetry to create the characteristic rhomboidal fault geometry. Neogene–present-day north–south- and NE–SW-striking en echelon conjugate fault arrays (Fault Population IV) form in response to oblique reactivation of these subjacent latest Triassic–Early Cretaceous polymodal faults (Fault Population I and Fault Population II). Fault Population I, II and IV together form a vertically decoupled (soft-linked) pseudo-conjugate fault system partitioned by a Cretaceous interval characterized by polygonal fault systems (Fault Population III) and monoclinic draping. The structural interpretation of the Beagle Platform illustrates a seismic-scale orthorhombic fault symmetry accommodating 3D strain and the insufficiency of plane-strain ‘Andersonian’ conjugate fault theory to resolve complex polymodal faulting evolved penecontemporaneously under a single stress regime. Subsequent oblique extension reactivation of these polymodal fault systems also demonstrates that complex soft-linked en echelon extensional conjugate fault arrays form in response to a single stress regime. This research supports the evolution of complex polymodal faulting under simple states of stress, and has implications for understanding the distribution, linkage and age of extensional faults within the Beagle Sub-basin and other extensional basins.