Winnfield Salt Dome Research Articles

Winnfield Salt Dome Cap Rock, North Louisiana Basin: Evidence for Late Mesozoic to Early Tertiary Cold (Warm?) Seeps

Winnfield Salt Dome Cap Rock, North Louisiana Basin: Evidence for Late Mesozoic to Early Tertiary Cold (Warm?) Seeps

Paleomagnetic Dating of Sulfide Mineralization and Cap Rock Formation in Gulf Coast Salt Domes

Remnant magnetization of stratiform sulfides within salt dome cap rocks provides a means of dating the timing of mineralization and its relationship with tectonic, sedimentation, and fluid evolution events within the local basin. The Winnfield salt dome in northern Louisiana hosts stratiform laminae of pyrrhotite and other sulfides in anhydrite cap rock. Detailed paleomagnetic analyses indicate the presence of numerous magnetic reversals that can be correlated with the sea-floor magnetic anomaly sequence, thus providing the first direct determination of a salt dome cap rock formation age. The sampled section that represents about two thirds of the total anhydrite thickness formed between 157 and 145 Ma (latest Jurassic). Accumulation rates calculated from these data indicate that the average rate for the oldest cap rock sampled is 5.7 m/m.y. and decreases to 2.8 m/m.y. for the younger strata. The Winnfield diapiric salt contains about 3% anhydrite, indicating the diapir growth rate was at least 30 times faster, a figure that is compatible with independent geologic estimates. The best documented cap rock-hosted sulfide concentrations occur at the Hockley salt dome in south-central Texas. Hockley is a relatively young diapir with the cap rock believed to have developed within the last 45 ma. Sulfidesmore » occur throughout the 285-m thick cap rock, but major metal concentrations occur within a 20-m zone within the central cap rock stratigraphy. If this zone formed at comparable rates to the Winnfield cap rock, then the main pulse of Hockley mineralization lasted only a few million years. Paleomagnetic studies are in progress to constrain timing of mineralization of Hockley.« less

Evidence for episodic basin dewatering in salt-dome cap rocks

A detailed record of episodic basin-dewatering events is preserved in the anhydrite cap rocks of two Gulf Coast salt domes. Metalrich brines were intermittently expelled from geopressured zones deep in the stratigraphic section and were channeled upward along escape structures bounding the salt diapirs. Overhanging anhydrite cap rock helped to focus some escaping fluid into the zone of dissolution between the top of salt and overlying residual anhydrite cap rock. Iron, lead, and zinc sulfide solubilities were exceeded in this zone, possibly in response to dissolution and reduction of cap-rock sulfates. Because the metalliferous brines entered the dissolution zone intermittently, they were recorded as relatively thin horizontal bands of sulfide sandwiched between thicker accumulations of anhydrite. Continued dissolution of salt and underplating of residual anhydrite caused the sulfide bands to be displaced upward relative to the base of the cap, leading to an inverted stratigraphic record of basin-dewatering events. Paleomagnetic data from the Winnfield salt dome suggest that sulfide-producing basin-dewatering events and anhydrite cap-rock accumulation occurred between 157 and 145 Ma.

Preliminary Paleomagnetic Investigations of Winnfield Salt Dome Cap Rock, Louisiana: ABSTRACT

Quarrying operations at Winnfield salt dome in the North Louisiana basin provide access to calcite, gypsum, and anhydrite cap-rock zones. Sulfide laminae in the anhydrite zone are comprised dominantly of pyrhotite with lesser amounts of sphalerite, galena, pyrite, and marcasite. Sulfides cement euhedral anhydrite grains and represent the products of the episodic introduction of metalliferous waters along the salt-anhydrite interface during halite dissolution and residual anhydrite accumulation. Thus, sulfide laminae provide a chronological record of anhydrite cap-rock accretion. Two hundred oriented samples were collected in stratigraphic sequence, covering much of the exposed anhydrite section. Alternating field demagnetization readily revealed the magnetic polarity of most samples. Using only reversely magnetized samples with a well-defined stable magnetization (N = 50, ^agr95 = 6.2°) yields a pole position at 71.4°N, 125.7°E, which implies that the sampled cap-rock sequence formed in the Late Jurassic. This age is consistent with geologic evidence indicating that cap-rock formation began in the late Jurassic and was most intense during the Early Cretaceous. A densely sampled 45-ft stratigraphic interval contains a sequence of normal and reverse polarity zones. Assuming a constant formation rate, these zones can be compared with the M-anomaly sequence. A growth rate of about 67 ft/m.y. (20 m/m.y.) is indicated. This value is about 30 times less than estimates of salt dome growth rates. Because a 10 to 50-fold decrease in volume is associated with halite dissolution and anhydrite accretion, the paleomagnetically determined value for cap-rock formation rate is reasonable. This investigation is the first such study undertaken. The results are encouraging and offer a unique approach for investigating the timing of various geologic processes related to salt dome formation. End_of_Article - Last_Page 1422------------

Metallic Sulfide Deposits in Winnfield Salt Dome, Louisiana: Evidence for Episodic Introduction of Metalliferous Brines During Cap Rock Formation: ABSTRACT

ABSTRACT Winnfield Dome is a shallow piercement salt structure which penetrates late Jurassic through early Tertiary siliciclastic and carbonate strata of the North Louisiana Basin. Quarrying operations in the calcite and anhydrite portions of the cap rock have exposed zones of metallic sulfides and barite. Within the anhydrite portion of the cap rock, sulfides form laminar zones parallel to the tightly intergrown, layered anhydrite; sulfides occur either as cement between anhydrite grains or as coarse euhedral crystals in open space. The sulfide layers are up to several centimeters thick and extend for tens of meters. These layers locally thicken into laminated, flat-bottomed concave downward massive sulfide mounds. Monoclinic pyrrhotite is the dominant sulfide; sphalerite, galena, or barite may also be present. In weathered samples hydrating to gypsum, marcasite is common and appears to be an alteration product of pyrrhotite. A roughly laminated massive sulfide lens is exposed at the calcite/anhydrite transition zone. This sulfide body has a maximum thickness of about 5 m (16 ft) and extends for about 45 m (150 ft). The edges of the lens taper to a layer about 30 cm (1 ft) thick which separates the overlying calcite from the anhydrite. The massive sulfide lens is comprised of marcasite and pyrite complexly intergrown with calcite, gypsum, and anhydrite. Colloform sphalerite, galena, marcasite, pyrite, and barite line vugs and fractures; pyrrhotite is found locally within the sulfide lens. Examination of rotary drill samples suggests that massive sulfide concentrations, similar to the exposed example, are common along the calcite/anhydrite transition zone. These sulfide concentrations are believed to have originated from the interaction of metalliferous basinal brines with reduced sulfur trapped within the cap rock. Textural relationships and variations in chemical compositions between the sulfide layers in the anhydrite portion of the cap rock suggest that distinct pulses of metalliferous brines were responsible for the sulfide concentrations. Anhydrite grains that are completely surrounded by sulfides are euhedral and undeformed, similar to the anhydrite disseminated throughout the salt mass. Anhydrite grains outside the mineralized areas are deformed and tightly intergrown. These textures suggest that mineralizing fluids were introduced episodically along the salt/anhydrite interface at the zone of salt dissolution, before that portion of the anhydrite zone was compressed and accreted to overlying anhydrite cap rock. Therefore, the earliest-formed sulfides originating by this mechanism occur at the top of the anhydrite cap rock zone while the last sulfides to form are found at the base. Extensive sulfide concentrations along the anhydrite/calcite contact suggest that this contact also acted as a permeable zone allowing metalliferous brines into the cap rock. Textural and compositional relationships suggest that sulfides that formed along the anhydrite/calcite contact are locally superimposed on sulfides that formed at the salt/anhydrite contact.

Metallic Sulfide Deposits in Winnfield Salt Dome Louisiana ...: ERRATUM

AAPG Bulletin (September 1984), v. 68, no. 9, p. 1221, first column. Abstract, Metallic Sulfide Deposits in Winnfield Salt Dome Louisiana. . ., by Mark R. Ulrich, should include the additional authors, J. Richard Kyle and Peter E. Price. End_of_Article - Last_Page 1931------------

Excess radiogenic argon in fluid inclusions

Most minerals, with the possible exception of micas, commonly contain fluid inclusions, Argon contained in such inclusions may be considered a possible source of error in dating very young or low potassium minerals by the K-Ar method. Wahler [1956] first demonstrated spectrographically the presence of Ar in fluid inclusions in quartz. Elinson and Polykovskii [1961, 1963] used a vacuum ball mill to extract the gases present in fluid inclusions in various pegmatite minerals. They obtained Ar from quartz in quantities up to 8.9×10−3 cc/g. Preisinger and Huber [1964] state that they determined the Ar content of the gas emitted by heating very small samples (10−6 g) of feldspar crystals from a granite in a cycloidal mass spectrometer. The total gas emitted (water-free basis) varied from 0.8 to 3.3×10−3 cc/g. Nesmelova [1959] found up to 1.7% (Ar + Kr + Xe) in the gas from gas inclusions in sylvites from the Bereznikovsk mine in the USSR. Hoy et al. [1962] found 0.4 volume % Ar in the gases of highpressure gas inclusions in ‘popping-salt’ from the Winnfield salt dome in Louisiana. Sample weight and total gas volume were not specified. Damon and Kulp [1958] extracted excess radiogenic Ar from beryl, cordierite, and tourmaline in quantities ranging from 10−5 cc/g to 10−2 cc/g, some of which was assumed to come from fluid inclusions (p. 449). Lippolt and Gentner [1963] reported the finding of excess radiogenic Ar of the order of 10−7 cc/g in several fluorite samples and indicated that much of the excess was probably contained in fluid inclusions.

Internal Structure and Growth of Salt Domes: ABSTRACT

End_Page 352------------------------------Grand Saline salt dome, Texas, and Winnfield salt dome, Louisiana, have well-developed cap rock on top of them and topographic lows at the surface. In contrast to this, the Five Islands domes of South Louisiana form topographic highs at the surface and give evidence of recent and perhaps fairly rapid movement. Differences in types of fold structure observed in these domes also suggest more recent and rapid movement of the South Louisiana domes. Salt petrofabric patterns indicate that there is a more distinct preferred orientation of salt crystals in the central parts of domes than in their periphery. The relatively stable domes have a more distinct orientation than those that have been subject to Recent differential movement. The best preferred orientation patterns of salt from Grand Saline are derived from samples taken farthest from the dome margin, and can be related to dodecahedral or cubic gliding of halite if the axial planes of the folds are considered to be the planes of motion. Samples from Winnfield dome show less distinct preferred orientation patterns than those from Grand Saline, but are interpreted as combinations of cubic and dodecahedral gliding or superimposed patterns resulting from successive movements of the salt in different directions. Recent surveying of a water-etch line formed about 27 years ago inside the Winnfield dome when the mine was temporarily flooded show irregularities that may indicate slight differential movement during that interval. Irregular uplift probably tends to b ur or destroy the patterns of preferred orientation that formed during slow unidirectional movement, and distinct petrofabric patterns therefore probably indicate a condition of relative stability. End_of_Article - Last_Page 353------------

Structure of Winnfield Salt Dome, Winnfield Parish, Louisiana: DISCUSSION

Structure of Winnfield Salt Dome, Winnfield Parish, Louisiana: DISCUSSION

Structure of Winnfield Salt Dome, Winn Parish, Louisiana

Megascopic structural features were mapped of that part of the Winnfield, Louisiana, salt dome exposed in the Carey Salt Company's mine, located 811 feet below the surface and about 400 feet below the flat top of the dome. Layering of the salt is visible throughout the mine. White to dark gray beds of halite and anhydrite, 1 inch to 1 foot in thickness, are intricately folded. Both the bedding layers and fold axes dip and plunge steeply toward the center of the dome. The increased intensity of layering and the concordancy near the boundary of the dome show the close relation between the structural attitude of the plastically deformed original salt layers and the edge of the salt dome. Pencil-shape aggregates of anhydrite, as long as 18 inches, parallel the fold axes; most of them plunge steeper than 70° inward toward the dome and support the interpretation of upward and outward movement of the salt during its diapiric emplacement in the surrounding rocks. The structural data also suggest the possibility that the dome overhangs the enclosing sedimentary rocks on the south and west. Structurally the Winnfield dome is similar to the Jefferson Island, Louisiana, and Grand Saline, Texas, domes as mapped by Balk. Special features in the Winnfield dome include the emanation of interstitial brine and CO2 into the mine. Blowouts which result in cavities up to 100 feet in length have occurred in parts of the mine near the salt-dome boundary. Microscopic examination and chemical analyses of the salt indicate the blowouts probably are due to instability caused by high CO2 pressures in numerous small gas-filled cavities close to a free-air surface.

THE EFFECT OF SHOTPOINT MEDIUM ON SEISMIC COUPLING

Two experiments on the coupling of seismic waves to the surrounding medium have been conducted. One was performed at two different overburden depths in a tunnel complex at the Nevada Test Site in the Oak Spring Tuff. The other was done at a depth of 800 ft in the Carey Salt Mine at the Winnfield Salt Dome near Winnfield, Louisiana. Free‐field measurements within 400 ft of the chemical charges indicate that three to four times more energy was propagated into the elastic zone in the tuff than in the salt. There is some indication that in tuff the amount of energy transmitted into the elastic zone was dependent upon the lithostatic overburden pressure. Increasing the overburden pressure by a factor of about five almost quadrupled the energy propagated into the elastic zone.