Abstract Cox and Perkins and DDD Energy were partners in an exploration project in the Wilcox trend, onshore Texas, and had difficulty deciding where to locate an exploratory well. The play was in a heavily faulted zone below a high-velocity layer associated with the Wilcox, where two high-amplitude events truncated into a major fault, as evident on the 3D data set. The events were prospects, since no corresponding high-amplitude events were visible on the other side of the fault. Underneath the major fault was what appeared to be another fault, although a minor one. It seemed to break the two events representing the prospective layers. While the amplitudes were equally strong on either side of the apparent minor fault, it was possibly a permeability barrier creating two reservoir compartments not in hydraulic communication. If so, the partners wanted the first well to be placed in the larger compartment, to drain the greater amount of oil, before a second or sidetrack well was drilled. On the other hand, if the minor fault was not real, the initial well should be placed in another location. If the minor fault is not real, why the apparent break of the horizon on the seismic section? The minor fault was nearly parallel to the major fault and could be mapped over a fairly extensive region. However, in cross section it was almost directly under the break of the high-velocity layer from the major fault. The partners suspected a potential fault shadow distortion and elected to examine

Abstract Opening of the central Atlantic Ocean basin during the past 200 Ma separated North America from Africa and created a classic example of plate tectonic divergent motion and associated geologic features (LePichon, 1968; Morgan, 1968). The entire history of relative motion of these two plates is preserved in the fabric of sea-floor spreading (SFS) recorded by magnetic lineation and fracture zone (FZ) patterns (Fig. 1) (Vine and Matthews, 1963; Heezen and Tharp, 1965) on both flanks of the Mid-Atlantic Ridge. Age calibration of the SFS magnetic anomaly pattern (Cox, 1973; Harland and others, 1982; Kent and Gradstein, this volume) enables us to treat SFS lineations as isochrons of sea-floor crustal ages. FZs mark the path of spreading center offsets (transform faults) through time, providing an approximate flowline trace of the motions that separated the North American and African plates. Reconstruction poles of rotation and stage poles of motion can be determined from the SFS lineation and FZ data sets (Bullard and others, 1965; McKenzie and Parker, 1967; McKenzie and Sclater, 1971; Harrison, 1972). The kinematic history described by these poles provides a framework for examining major tectonic events, anomalous plate behavior, geologic phenomena, paleooceanographic events, etc. (e.g., Vogt and others, 1969; Tarling and Runcorn, 1973; Dewey and others, 1973; Vail and others, 1977; Sclater and others, 1977; Pitman, 1978; Rona and Richardson, 1978; Schwan, 1980; Kerr and Fergusson, 1981;

Abstract Laterally extensive beds of acicular, radiating carbonate fans, locally known as “Coxco needles”, are particularly common within a distinct stratigraphic interval (—1640 Ma) in the Proterozoic of northern Australia. In the southern McArthur Basin, they are the distinctive feature of the Coxco Dolomite Member and occur throughout a number of lithofacies across the platform. Mounding and onlapping of sediment laminae, their upwardly divergent aspect, brecciated Coxco needle clasts infilling synsedimentary fractures, and their intimate association with stromatolites supports the precipitation of Coxco fans from ambient seawater directly onto the seafloor. Individually, they consist of acicular crystal casts up to 10 cm long, which form radiating, bottom-nucleated fans. Needle terminations are commonly blocky or square and in cross section appear pseudohexagonal with crystal casts generally having six-sided forms. Needles consist internally of an irregular mosaic of dolospar cement easily distinguished from the more finely crystalline dolomicrite matrix. These features are entirely consistent with criteria for the recognition of aragonite in ancient carbonate sequences and imply an original aragonitic mineralogy for the Coxco fans. The sequence through the middle McArthur Group (Emmerugga Dolomite, Teena Dolomite, Coxco Dolomite Member, and Barney Creek Formation) is broadly transgressive, and precipitation of Coxco needles occurred during the onset of a period of tectonically induced subsidence. The mechanism for the widespread chronostratigraphic precipitation of CaCO 3 is thought to be upwelling of highly alkaline, HCO 3 − -rich anoxic bottom water onto the carbonate platform coeval with changes in the bathymetry of the basin. Mixing with relatively Ca 2+ -rich surface waters resulted in widespread precipitation of carbonate seafloor cement (i.e., Coxco fans) across the platform in several distinct lithofacies. The reason that macroscopic carbonate cement formed in preference to widespread precipitation of finely crystalline micrite remains unclear, although it is suggested that elevated concentrations of Fe 2+ and Mn 2+ in the basin waters may have inhibited micrite precipitation and thus favored development of macroscopic seafloor Coxco fans.