Fluid-rock interactions in the salt dome environment: An introduction and review

Chemical Geology, 74 (1988) 1-24 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

1

FLUID-ROCK INTERACTIONS IN THE SALT DOME ENVIRONMENT: AN INTRODUCTION AND REVIEW HARRY H. POSEY* and J. RICHARD KYLE Department of Geological Sciences, The University of Texas at Austin, Austin, TX 78713 (U.S.A.) (Accepted for publication July 28, 1988)

Abstract Posey, H.H. and Kyle, J.R.,1988. Fluid-rock interactionsin the saltdome environment: A n introductionand review. In: H.H. Posey and J.R. Kyle (Guest-Editors),Fluid-Rock Interactionsin the Salt D o m e Environment. Chem. Geol., 74: 1-24. The saltdome environment in the Gulf of Mexico Coast, U.S.A., provides one of the most diverse and long-lived records of fluid-rockinteractionsin an activesedimentary basin dominated by salineformation waters.The geochemicalrecord,most of which is isotopic,appears to span a temperature range between Earth surfacetemperatures and lower greenschistfacies.Most mineral-forming processes in the saltdome cap rocks appear to involve mixtures of warm salineformation fluidsfrom deep basin sources and cool dilutemeteoric waters. Papers in this issue of Chemical Geology document some of the current research on fluid-rockinteractionsin the saltdome environment. These papers highlightthe diversityof opinion about saltdome processes,offeringclues for further research.The present paper reviews some of the key and more current literatureon saltdome geochemistry, fluidconvection around saltdomes, and fluid-rockinteractionswithin saltand within the saltdome environment.

I. Introduction

The Jurassic Louann Formation in the Gulf of Mexico Coast region of southern North America represents an extensive evaporite depositional system that formed during the early stages of riftingof the Gulf Coast. This thick saltsequence has been modified by halokinesis in much of the basin and now comprises one of the world's best developed saltdome provinces. The formation of salt domes in the Gulf Coast has been investigatedby many workers, in large part prompted by the association of major petroleum reservoirswith saltstructures.The "age *Present address: 8302 Reston Drive, Austin, TX 78758, U.S.A.

of liquidfuel" was initiatedby the spectacular oil gusher at Spindletop D o m e in coastal east Texas in 1901 (Halbouty, 1979 ),and saltdome structures stillplay an important role in oiland gas production in the Gulf Coast. In general, the Gulf Coast region consists of a thick accumulation of Mesozoic and Cenozoic sedimentary rocks deposited on Precambrian and Paleozoic basement in relativelystable,but constantly subsiding extensional basins (Figs. 1 and 2; Martin, 1978; Salvador and Buffler, 1982). The northern margin of the Gulf Coast Basin is composed of Tertiary and Quaternary terrigenous siliciclasticsediments supplied by fluvialsystems along several depositionalaxes. Contemporaneous growth faultsinfluenced the geometry of individual fluvial packages, and

I J,° "'\L

EXPLANATION BL

Baling Belle Isle

BI OM

Damon Mound Gyp Hill

GH

Texas

- -']

Basin

/

~

/"

/ ~

"0

/o| :/

MISSISSIPPI

No,, ,ouisio~ Sosin (

/

I

/

GI Grand I s l e GS

/ | ,'.o~ /GS

Grand Saline Hockley

HK OK PG RA

I

/ \ ~x ''0 ; I • °

/"

//

) ~Mississippi ) ' - - . / Basin • . o~

/ / i

~/ ~/

Oakwood Poiongono Royburns

RI ST TT

VH WI WH WN •

,'

Richton Spindletop Totum Vocherie Weeks I s l a n d West H o c k b e r r y Winnfield

•

!~

TEXAS

-WN t.-

I~IK

°

~ . W H ....

• ....

e

.°

°°

,, ~-.."

"s ° o" •

¶ j / /

...

of

, . • •

"

... :

Basin

1

'~

' -...

~-,a~

I;,

I~/ .-,... L / L I

-

°

• • "" •

.m~.. "

v,j

Gulf

Coast

Basin

,

. . ".:

%

•

"-.

•....w.,.

d •

. f .eo.#

"' °

• D

m

•

.:" • _._. 26 o-

M£,YI~-~ 98 °

/

'.~

.

% ""

~-.~

( :. ',

; • ..'.:::.:..::. , , , ,

..e°e P~I."

Coast

"J..

•

/ ~I . I ' ' ' ~ I

"ML~ ......

""-.

w<:':'.

"-J

:.

..:...

.-'"

', ,-

....

,(

LOUISIANA

."/ r

",,

P~G • •

"_. /

/

k

i

2 0 0 krn

, i IOOmi

Gulf

~0

r

;

-"

/.

.

--~_ (/ •

.' ,-'"

Salt dome

, i

;'

,

,,.

Salt m a s s i f

-26 °

// /

94 °

90 °

Fig. 1. General geologic setting of the northern Gulf Coast region of North America. Salt domes mentioned in this article and/or in others in this special issue are identified.Actual size of saltdiapirs is generally exaggerated. Modified after Martin

(1978).

sedimentary formations dip and generally increase in thickness toward the present Gulf of Mexico• These contemporaneous facies tracts represent persistent depositional environments that generally prograded seaward through the Cenozoic (Martin, 1978)• This siliciclastic sequence overlies a Cretaceous, dominantly carbonate, shelf sequence. Red-bed, evaporite, carbonate and marginal marine siliciclastic deposits ranging from Upper Pennsylvanian to Jurassic underlie the Cretaceous sequence and rest unconformably on complexly deformed Paleozoic strata of the Ouachita system. Extensive Middle Jurassic evaporite deposits of the Louann Salt are of key interest to the discussion of salt domes. The Louann underlies most of the northern and western Gulf of Mex-

ico and is regarded to be equivalent to the Challenger evaporites which underlie the southern Gulf (Salvador, 1987). Bedded salts have been intersected only in the northern Gulf where they are shallow enough to have been penetrated by drilling; elsewhere, salt samples consist of salt dome materials. The Louann probably reaches thicknesses of 3000-4000 m in the central Gulf (Salvador, 1987). The thickest salt deposits lie within three zones that are defined by piercement and nonpiercement salt structures: (1) an inner belt consisting of the East Texas, North Louisiana and Mississippi Basins; (2) a middle belt containing the Louisiana-Texas coastal and inner shelf salt domes; and (3) an outer belt that includes nearly all of the continental shelf and slope (Fig. 1; Martin, 1978). Hundreds of salt tectonic structures have

N

S

East Texas Basin ..eL

co

_ - ' - ~ " ~ ~ ~ ~

/Present Coast Line

N ~

f~ ~

~

SigsbeeScarp Siqsbee

5

i 15

Oceanic Crust 0 I

I00 I

Q Quaternary UT Upper Tertiary LT Lower Tertiary

200

300

I

I

400 krn

Transitional Crust

I

UK Upper Cretaceous L K Lower Cretaceous d Middle? -Upper Jurassic

~ Middle Jurassic salt ~....... ' diapirs and pillows ~ Upper Triassic? red beds

Fig. 2. Generalized geologic cross-section of the northern Gulf Coast region. Approximate line of section is shown in Fig. 1. Modified after Salvador and Buffler (1982).

been identified in the onshore and offshore Gulf region (Martin, 1978). It is generally accepted that salt diapirs rise, isostatically, as sediments adjacent to them subside with burial. The top of the diapir maintains a more-or-less constant elevation relative to the land surface (Murray, 1966; Jackson and Talbot, 1986). The relationship between sedimentation and halokinesis makes it possible to accurately date salt movement by the response of contemporaneous sedimentation to salt withdrawal (e.g., Seni and Jackson, 1983a, b). In general, salt structures become younger toward the centers of local depositional basins. An important corollary of this relationship is that salt structures in different parts of the Gulf Coast have different ages, including the currently active salt structures in the deep Gulf (Figs. 1 and 2; Humphris, 1978). About 65% of the onshore salt diapirs in the Gulf Coast is mantled by cap rocks. These typically consist of a lower anhydrite zone that directly overlies the halite stock and an upper calcite zone commonly separated by a zone of gypsum, calcite and sulfur (Fig. 3). Murray (1966) proposed that the following general se-

quence of events results in cap rock formation. As the upper part of a rising salt diapir dissolves, the insoluble components, which are largely anhydrite, accumulate and with diapiric movement are compacted, cemented, and underplated to the base of the previously formed cap rock. Further cycles of dissolution and residue underplating result in the formation of a banded anhydrite zone which grows younger downward (Kyle et al., 1987). Calcite and sulfur are products of bacterially associated chemical reactions that, in the presence of a hydrocarbon food supply, alter the sulfate zones (Feely and Kulp, 1957). These hydrocarbons are oxidized to carbon dioxide, and aqueous sulfate derived from anhydrite or gypsum is reduced. These reactions result in precipitation of biogenic calcite and production of hydrogen sulfide which may be subsequently oxidized to elemental sulfur (Feely and Kulp, 1957). Gulf Coast salt dome calcite cap rocks are composed of several genetically distinct zones which also appear to develop from top to base (Fig. 3; Posey, 1986a; Posey et al., 1987b). The gypsum zone results from hydration of anhydrite along

EXPLANATION ~FALSE CALCITE CAP

F~--~MARINE CALCITE CAP F.~E~ VARIEGATED CALCITE CAP FI----~I7BANDED CALCITE CAP ~

GYPSUM (tronsitionel) CAP

~

ANHYDRITE CAP

~SALT ~

DISSOLUTION ZONE

SALT STOCK

Fig. 3. Schematic cross-section of a Gulf Coast salt dome cap rock showing an idealized sequence of cap rock lithotypes. See text for discussion; relative amounts of lithotype zones are highly variable among domes. Modified after Posey (1986a).

the calcite-anhydrite contact by low-temperature low-salinity waters. Renewed interest in the geochemistry of salt domes has been spawned by several recent independent research endeavours including: (1) nuclear and toxic chemical waste isolation feasibility studies; (2) exploration for commercial metal sulfide concentrations in cap rocks; (3) the discovery of what appear to be fluid convection cells around the margins of salt domes; (4) evaluations of the role of halite and bittern salts in fixing the compositions of deep formation water; and (5) traditional interests in the im-

portant economic relations between salt domes, hydrocarbons and nonmetallic resources such as cap rock-hosted sulfur concentrations. As these studies expand, traditional models explaining salt dome formation and cap rock geochemistry are being re-examined. Much of the current research focuses on the mechanisms of H2S oxidation, the sources of various fluid and mineral components including calcite carbon, native S, Sr, Ba and sulfide metals, the behavior of fluids inside diapirs, the origin of fluids that yield characteristic light carbon isotope ratios in calcite cap rocks, and the evolution of fluids in young sedimentary basins. Salt domes have a complex and long-lived diagenetic history that covers, at least, the following major chemical and mechanical features: deposition by evaporation of, presumably, marine fluids; loading and diapirism that probably accompanies chemical mixing of fluids and minerals inside the evaporites; salt solution with accompanying anhydrite cap rock accumulation; and, in many cases, reaction with hydrocarbons and other fluids leading to formation of calcite cap rocks and the precipitation of metals. Many of these reactions can be used to help reconstruct the chemical history of sedimentary basins. Because the salt domes and associated cap rocks interact with fluids from many levels of the basin over a protracted history, they appear to bear evidence of sedimentfluid interactions covering a large range of burial conditions and long periods of time; larger and longer, at least, than other sediment-hosted diagenetic minerals that are more commonly used to reconstruct burial histories. As structural disturbances in sedimentary basins, the sediments alongside salt domes often provide a port through which basin fluids escape to shallower levels through focused flow. In this special issue, we collect the results of several recent geochemical and fluid modeling studies of materials in the Gulf Coast salt dome setting. Most of these papers were presented at the 1987 mid-year meeting of the Society of

Economic Paleontologists and Mineralogists (SEPM). The focus of each study is on: cap rock formation and its relation to petroleum or metalliferous brines; fluid diagenesis or fluid evolution in young sedimentary basins; or chemical and mechanical relations between diapirs and associated fluids and solids. In this paper, the concepts of fluid-rock interactions in salt dome environments will be reviewed, highlighting the changing concepts of salt dome geochemistry. Although the contributions in this issue are limited to the Gulf Coast region, the processes discussed should be generally appropriate for fluid-rock interactions in other salt dome terranes and for sedimentary basins dominated by saline formation waters in general. I. 1. Nuclear waste isolation feasibility studies

In the early 1950's, when it was first realized that radioactive nuclear waste would be a very long-term potential environmental problem resulting from the manufacture of weapons-grade plutonium, various federal governments proposed storing nuclear waste in salt. It was assumed that salt has the natural properties essential for safe storage of nuclear waste: low water content, tectonic stability and high elasticity (NAS-NRC, 1957). Certain salt deposits, it was reasoned, must certainly be dry and stable (lest they would already have dissolved), elastic enough to absorb earthquake stresses, and plastic enough to be self-sealing. These basic assumptions have been especially studied and challenged in recent years. Detailed feasibility studies of several Gulf Coast salt domes for nuclear waste isolation were begun by the U.S. Nuclear Regulatory Commission (NRC) and the U.S. Department of Energy (DOE) in the 1970's. Rayburns and Vacherie Domes in Louisiana, Oakwood and Gyp Hill in Texas and Richton in Mississippi were evaluated by drilling, and considerable study of the underground mine at Weeks Island, Louisiana, was conducted (Fig. 1) (Nance and Wilcox, 1979; Nance et al., 1979; K n a u t h et al., 1980;

Kreitler and Muehlberger, 1981; Dix and Jackson, 1982; Giles and Wood, 1983; Kreitler and Dutton, 1983; Jackson, 1985). These studies have increased significantly the information base on salt dome structures and geochemistry. It is difficult to find documents that either accept or reject salt domes, definitively, as possible safe nuclear waste storage facilities, even though numerous geological studies were conducted for that specific purpose. Most conclusions that do address safety generally reject salt domes as safe storage media. Knauth et al. (1980) found that brine seeps in the Weeks Island dome originated as formation water and concluded that such water entered from sources external to the dome. This indicates that some salt domes are not as stable and dry as originally assumed, especially under mining conditions. Werner et al. (1988 in this special issue ) notes that formation water may have entered along the salt-anhydrite interface, causing relatively recent anhydrite hydration, and potentially affecting dome stability. Kreitler and Dutton (1983, p. 55 ), who studied the Oakwood Dome in east Texas and the Gyp Hill Dome in south Texas, were more positive about using at least some salt domes for waste storage: "The Oakwood anhydrite cap rock, which formed more than 100 million years ago, appears to be an effective hydrologic seal. During the formation of the calcite section the anhydrite/salt interface was impermeable, as it remains today. In contrast, the Gyp Hill cap rock is still forming, and the salt dome is undergoing active dissolution; this cap rock is not an effective hydrologic seal. Salt domes being considered as nuclear waste repositories should have anhydrite cap rocks with the same characteristics as Oakwood Dome cap rocks and not those of Gyp Hill."

However, Jackson (1987) dismissed the Oakwood dome from consideration for other reasons: "In terms of Nuclear Regulatory Commission licensing criteria, the presence of many boreholes in the Oakwood salt stock was considered 'adverse' and the proximity of potential Quaternary faulting, possible evidence of dissolution, and aspects of the surface hydrology were termed 'less favorable' (Office of Nuclear Waste Isolation, 1982 ). In their summary of all results

from the [East Texas Waste Isolation Feasibility] project, Jackson and Seni (1984) concluded that in terms of guidelines of the U.S. Department of Energy (1983), Oakwood Dome was disqualified as a candidate dome because 62 exploration and production boreholes are drilled through the overhang, 3 of which have not been located, and because petroleum reserves lie below the overhang and 4 km to the southeast."

1.2. Mineral occurrences

Occurrences of metal sulfides and barite in salt dome cap rocks have been known for several decades (Hanna and Wolf, 1934; Taylor, 1938), but it was not until Price et al. (1983) reported high concentrations in cap rocks at the Hockley Dome in south-central Texas that significant study of these unusual occurrences began. Since then, sulfides and barite have been documented from at least 17 domes in the Gulf Coast (Kyle and Price, 1986). These probably have genetic affinities with evaporite diapir mineralization in Northwest Africa (Rouvier et al., 1985 ) as well as classic ore deposits in sedimentary terranes including the Mississippi Valley-type and sedimentary exhalative metal deposits (Kyle and Price, 1986). The dominant minerals in the cap rocks, other than the host rocks themselves, are Fesulfides, sphalerite, galena, barite, celestite and strontianite. At least one dome (Hockley) has interesting concentrations of acanthite. These "ore" minerals occur in both calcite and anhydrite cap rocks, apparently at potentially all levels. However, economic feasibility studies have been conducted for only the Hockley Dome metal concentrations (Kyle and Price, 1986). Whether these deposits will ever become economic remains to be determined. As with any mineral deposit, market conditions will set the definition of ore, but the salt domes have other physical characteristics that will affect mining considerations. Mining in the cavernous and H2S-rich cap rocks or near the salt-anhydrite interface will present special challenges. Likewise, the extremely fine-grained character of many of the mineral concentrations may re-

quire unconventional techniques.

mineral

separation

1.3. Fluid convection around salt domes

Around and above several salt domes in the Gulf Coast there occur saline formation waters that indicate modern dissolution of halite (Fogg et al., 1983; Workman and Hanor, 1985; Hanor and Workman, 1986; Bennett and Hanor, 1987). These plumes of dense saline water overlying less dense and less saline water are gravitationally unstable, so have the potential for downward flow (Workman and Hanor, 1985). Around several salt domes (Iberia, Fordoche, Port Barre and Welsh, all in southern Louisiana), warm fluids from deep formations, below the zone of geopressure, appear to be discharging into shallower units (Workman and Hanor, 1985; Hanor and Workman, 1986; Bennett and Hanor, 1987). Coupling of these upward migrating fluids and the dense brine plumes forms flow systems akin to convection cells with diameters of ~1 km (Hanor and Workman, 1986; Bennett and Hanor, 1987). Cooler meteoric waters, above the brine plumes, probably contribute to the flow system as well as to the chemistry of the fluids. Fluid discharge from beneath the top of overpressure in the Gulf Coast has been used to explain sandstone uranium mineralization near growth faults in south Texas (Goldhaber and Reynolds, 1979; Goldhaber et al., 1978, 1983), and various diagenetic events in Mesozoic formations of the Gulf Coast (Land, 1984b; Sharp et al., 1988). Mass transport by upward-migrating fluids is an important concept as it affects many interpretations of hydrocarbon transport, ore fluid migration, secondary porosity development, porosity occlusion and salt dome dissolution. 1.4. Salt dome-formation water interactions

The relationships between salt domes and formation waters are studied, generally, from

two perspectives. One seeks to know the effects of salt dissolution on formation water composition whereas the other seeks to determine whether formation fluids have seeped into salt masses and, if so, what effects that seepage has on salt tectonics and salt chemistry. 1.4.1. Effects of salt on water salinity. With important exceptions, formation fluids grow generally more saline with depth in a sedimentary basin (see Hanor, 1979). The specific causes are not known, but have been related to mineral dissolution (especially evaporites), membrane filtration, diffusion and density stratification (see Bredehoeft et al., 1963; Hitchon and Friedman, 1969; Carpenter et al., 1974; Manheim and Horn, 1974; Land and Prezbindowski, 1981; Graf, 1982; Hanor, 1984; as reviewed in Hanor, 1987). Our focus in this paper is on just one of these mechanisms, evaporite dissolution. As noted by numerous authors, most saline formation fluids are brines (i.e. > 105 mg 1-1; see Carpenter, 1978) and are composed dominantly of Na, Ca and C1. A simple mechanism for explaining this simple composition is through a combination ofevaporite (halite) solution and albitization of Ca-plagioclase (Land and Prezbindowski, 1981). Although evidence of evaporite solution is lacking or, at best, indirect in some basins (e.g., Illinois Basin), it has been clearly documented in other basins, including the Gulf Coast. For example, Manheim and Horn (1968) found that salinities along the Atlantic Coast margin of the U.S.A. are spatially related to evaporites. Also, Hanor and Bailey (1983) found that disturbances in the normal increasing-salinity-with-depth profile in the Gulf Coast correspond with the upper surfaces of salt domes, which are being dissolved. One outcome of studies of saline formation waters is a debate over the origin of high Br/C1 ratios in such fluids. Evaporation of seawater to halite saturation or beyond produces fluids with high Br/C1 ratios because halite preferentially excludes Br from its structure (see

Holser, 1979, for a discussion). Preservation of these brines, if possible, would provide a fluid with extremely high Br/C1 ratios. Carpenter (1978) proposed such a mechanism for explaining high Br/C1 ratios in Gulf Coast formation fluids. However, Land and Prezbindowski (1981) and Land et al. (1988 in this special issue) explained similar data by concluding that high Br/C1 ratios result from incongruent dissolution of halite in the subsurface. In other words, halite recrystallization, in the presence of a fluid, regardless of its initial Br concentration, expels Br from the halite lattice, thus enriching the fluid in Br.

1.4.2. Effects of water on salt. Despite assumptions to the contrary (NAS, 1957), salt dome salt is not necessarily dry. Water occurs in halite fluid inclusions and as seeps in salt dome salt mines. The origins of salt dome fluids are probably multiple and complex, and include connate fluids as well as meteoric water and formation fluids that leak into salt mines (Kumar, 1978; Kumar and Martinez, 1978; Roedder and Belkin, 1979; Knauth et al., 1980). The time of influx of formation fluids into salt domes is not known but some data suggest that it occurs early in the history of diapirism, and that it affects the compositions of salt dome minerals. Posey (1986a), Posey et al. (1987a), and Land et al. ( 1988 in this special issue) found that 87Sr/86Sr ratios of anhydrite within salt stocks are radiogenic relative to Mid-Jurassic seawater, the presumed source of the Louann mother salt. A broad range of 87Sr/S6Sr values (0.7068-0.710) indicates that Sr came from more than one source, one being Mid-Jurassic seawater and the other being radiogenic siliciclastics (Posey et al., 1987a). Posey et al. (1987a) argued on modeling considerations for the Hockley Dome that the most likely source of radiogenic Sr is K-feldspar which is destroyed during burial (Milliken et al., 1981). Although radiogenic Sr could come from any of several K-rich source rocks, dissolved K-feld-

spar is the favored source because the volumes of fluid required are smaller than from any other source. K salts, which occur in minor abundance in salt domes, were also discarded from consideration because the abundance of Rb in the K salts and the amount of time elapsed since deposition of the Louann are collectively too small to explain the high average Sr isotope ratios. Land et al. ( 1988 in this special issue) report radiogenic Sr isotope ratios for the undeformed Louann. An implication of this observation is that radiogenic Sr invades bedded salt prior to diapirism. Whether the source of radiogenic Sr was external to the dome, as proposed by Posey et al. (1987a), or from siliciclastics within the salt section is not known. Early diagenetic reactions are likely to take place between the fluids and siliciclastics (clays, K-spar, etc.) in evaporite units owing to the extremely high activity of Na + in brine pools (S. Fisher, pers. commun., 1988). These would release Sr from the clastics. If the clastics are inhomogeneously dispersed within the basin, local pockets of brine could contain 87Sr/S6Sr ratios different from adjacent isolated pockets and these could impart different Sr isotope ratios to later-formed minerals. Except that the salt domes studied contain very limited amounts of clastics, this mechanism might explain some of the wide variation in STSr/S~Sr ratios found in diapiric anhydrite, thus negating the need for driving external fluids through the salt. The presence of water in salt and the timing of its incursion are important in cases where long-term stability of salt domes is important. Salt caverns used for product storage and salt mines could be more safely exploited if safety from fluid intrusion could be assured. In the Gulf Coast there are several cases of cavern collapse associated with active mining that can be shown to be man-induced. However, saline lakes and surface depressions over salt domes provide evidence of non-anthropogenic subsidence

as well (Mullican, 1988), and the causes of that subsidence and the sources of the fluid are not known. The observation that epigenetic fluids have interacted with the Louann salt is critical to those considering salt diapirs for product-storage because the timing and mechanism of fluidsalt interaction are not known. Furthermore, modeling studies that are used to predict rates of salt flow due to diapirism are generally scaled for dry salt. However, a small amount of water ( < 1% ) can have profound effects on salt stability because it lowers the shear strength and induces salt flow (Jackson, 1985; Jackson and Talbot, 1986). Of the seven Gulf Coast salt dome salt mines operating in 1980, at least three contain water (Martinez and Kumar, 1980). Compared with shallow formation waters found in Louisiana, these waters (brines) are very enriched in Ca and K, and slightly enriched in Mg. Based on O and H isotope studies, Knauth et al. (1980) concluded they are formation waters rather than seawater or meteoric water, and are not derived from fluid inclusions found in salt.

1.5. Economic aspects of salt domes Salt domes and associated cap rocks and sedimentary rock hosts provide one of the most diverse economic entities of any geologic feature. The importance of salt domes for the production of NaC1 and sulfur and in the formation of structural traps for oil and gas is well recognized. Salt from salt domes is produced by underground and solution mining and is used dominantly as a chemical feedstock for production of many important industrial products. Salt domes are the primary source of sulfur in the U.S.A., where sulfur is recovered by the Frasch method (Halbouty, 1979 ). Fracturing and folding along the margins of salt diapirs provide both the traps for oil and gas and the escape route for upward migrating fluids. Salt domes are of minor to insignificant im-

portance for the production of limestone, anhydrite, gypsum and uranium. Limestone, gypsum and anhydrite are quarried from shallow cap rocks, and uranium has been recovered by in situ leaching from the supercap rock sediments at Palangana Dome in south Texas. Salt caverns are of growing importance, particularly in the Gulf Coast, for storing products such as liquefied petroleum gas (LPG) (Seni, 1986b). In fact, most of the U.S. strategic petroleum reserve in the Gulf Coast is stored in salt caverns. These salt storage caverns are ordinarily excavated by injecting fresh water through a drill pipe into the halite stock. As halite is dissolved by the fresh water the brine is pumped to the surface and either re-injected into porous cap rocks, into deeper briney formations, or pumped out to sea. Greater use of this storage medium may be in the offing; for instance, the Water Commission of Texas recently approved a request for storing solidified hazardous waste in a salt cavity. The natural characteristics of salt domes and their commercial exploitation produce both negative and positive results. On the negative side, subsidence is common above salt domerelated oilfields and above Frasch sulfur mining operations (Seni and Mullican, 1986). Surface subsidence also occurs where near-surface salt plugs have been dissolved by groundwater (Mullican, 1988). Subsidence has been known to adversely affect roads, pastures, power lines, dwellings and other cultural features. Although Rn gas leakage around salt domes has not yet been identified as a problem, U, which occurs around some domes, will decay in part to Rn and could cause local concerns. On the positive side, salt domes, particularly those in swampy, coastal areas, often provide topographic mounds that are habitable relative to the surrounding lands. One such "island", the Avery Island Dome in coastal Louisiana, is the site of a game preserve, a park, and a farm that harvests Capsicum frutescens var. tabasco for a popular hot sauce.

2. Aspects of diapirism, cap rock evolution, and fluid-rock interactions 2.1. Introduction Salt domes occur principally in four basins or sub-basins in the Gulf Coast (Fig. 1 ). As far as can be determined, all of the Gulf Coast salt domes derive from a common mother salt unit, the Louann Formation, the age of which is regarded as late Middle Jurassic (Callovian) (Salvador, 1987). Like all Gulf Coast units, the Louann dips away from the continent toward the central Gulf of Mexico, except in the central Gulf where it has apparently been thrust upward over younger rocks and occurs at fairly moderate depths (Fig. 2; Humphris, 1978). The Louann does not crop out anywhere in the Gulf Coast, although carbonates of equivalent age do appear at the surface in eastern Mexico (Salvador, 1987). Diapiric salt generally contains 1-5% anhydrite and trace occurrences of doubly-terminated quartz, euhedral dolomite, Fe-bearing carbonates, clay, and a myriad of minerals occurring in ultra-trace quantities (Taylor, 1937; A.E. Smith, 1970a, b ). This assemblage of trace minerals also occurs in both anhydrite and calcite cap rocks. Erratics of several types of igneous and sedimentary rocks also occur in isolated locales. Many salt diapirs are capped and probably mantled by anhydrite, limestone, a n d / o r gypsum. Where each of these rocks occur as cap rocks above a diapir, they appear in a top-tobase sequence consisting of calcite, gypsum and anhydrite. Anhydrite rests directly above salt or above a zone of salt dissolution at the top of the diapir. Anhydrite is the most common cap rock and is almost certainly a residue of salt dissolution, reaching thicknesses in excess of 300 m (Kupfer, 1980). There appears to be no direct relationship between the relative abundances of any of these cap rocks and depth to the diapir's top (Kupfer, 1980). However, sur-

10 face subsidence is related to both depth to diapit and cap rock thickness. According to Seni and Mullican (1986), negative relief does not occur over any dome (in Texas) that is either deeper than 600 m or that has greater than 200 m of cap rock. Anhydrite apparently forms by underplating as salt is dissolved along the salt-cap rock interface ( Murray, 1966). Thus, anhydrite layers become progressively younger toward the base of the anhydrite cap. During anhydrite underplating some of the anhydrite recrystallizes and the grains are cemented by new anhydrite. During this process anhydrite loses part of its trace Sr and may experience some partial sulfate reduction or sulfate mixing because the sulfur isotope values apparently shift to slightly heavier values (Feely and Kulp, 1957; Posey et al., 1987a). Calcite apparently forms when petroleum is degraded by sulfate-reducing bacteria (Feely and Kulp, 1957; Sassen, 1980; Posey et al., 1987b; Sassen et al., 1988 in this special issue ). However, the occurrence of a variety of calcite cap rock facies (Fig. 3) indicates that pre-existing sediments, sea-floor topography and timing of calcite formation exert controls on the textures of the product calcite. The calcite zone generally comprises two major rock types (Fig. 3): an upper variegated limestone and a lower banded or "zebra-textured" zone. However, in several cases, limestones termed false calcite and marine false calcite have formed above these two zones. False calcite cap rock is composed of calcite-cemented clastics that occur above or alongside salt diapirs, and that have characteristic carbon and oxygen isotope signatures. Examples include calcite-cemented sandstones at Butler Dome, Texas (Kreitler et al., 1985 ) and calcitecemented sands at the West Hackberry Dome, Louisiana (McManus and Hanor, 1988 in this special issue). Marine calcite cap rocks are a true marine carbonate that forms on sea floor bulges uplifted by salt diapirs. An example from the ancient is

the Upper Oligocene Heterostegina reef above Damon Mound {Baker, 1979), and examples from the Recent include the West and East Flower Garden Banks offshore of Louisiana (Rezak, 1985). The upper variegated limestone commonly contains clastic detritus and generally contains variegated clasts of carbonate in a carbonate matrix. These carbonate clasts and matrix contain irregular segregations of a later, more coarsely crystalline calcite, and both carbonate phases are commonly cross-cut by calcite veins. All calcite cap rocks contain this type of variegated limestone, but only about half contain the banded calcite. The lower banded calcite zone consists generally of three stages, each similar to the three present in variegated limestone. However, the first two stages of calcite form generally horizontal bands that are commonly cross-cut by the late-stage calcite veins. Calcite cap rocks probably form progressively from the top to the base (Murray, 1966; Posey, 1986a; Posey et al., 1987b). Except for the two types of false cap, calcite cap rocks probably form in reverse stratigraphic sequence (Posey, 1986a; Posey et al., 1987b). As carbon-bearing fluids enter the salt dome environment along the anhydrite-sediment interface, calcite forms at the expense of anhydrite, sometimes replacing it to preserve original anhydrite textures. The early-formed calcite incorporates detritus from nearby sediments. As anhydrite dissolves and new calcite forms, the calcite-anhydrite interface moves downward, effectively armoring the later calcite-forming zone from overlying detritus. Brecciation caused by collapse into voids left by the dissolution of anhydrite (if Ca is conserved during the destruction of anhydrite and consequent formation of calcite, there is a molar volume change of - 1 9 . 6 cm 3 per 100 cm 3 of anhydrite ) leads to formation of open spaces in the calcite that are filled by later phases of calcite (see Prikryl et al., 1988 in this special issue).

11

2.2. Diapirism: a misconception The notion t h a t salt diapirs "intrude" the sediments or sedimentary rocks around t h e m is a misconception, recounted in numerous sources, that needs to be examined. The following passage is characteristic of the way diapirism is presented in general texts (Skinner, 1986, pp. 140-141): "Althoughmany [salt] beds are too deep to mine, nature has an interesting way of bringing salt nearer the surface. Halite has a density of 2.2 grams per cubic centimeter, whereas most of the associated sedimentary rocks have densities of at least 2.5 grams per cubic centimeter. The salt beds, being lighter and capableof plastic flow like ice in a glacier, tend to rise and 'flow' up through the overlyingrocks. If the overlyingrocks are weak enough to be ruptured by the rising salt, long thin columnsor plugs of salt float up from deeplyburied sedimentary horizons. Columns ranging from about 100 meters to more than 2 kilometers are known to have risen up through as much as 12 kilometers of overlying sediments." Although this passage is correct within the limitations presented, it leaves the impression t h a t salt is capable of intruding through m a n y kilometers of sediment, and this has not been documented. Evidence from drilling and seismic records indicate the upper surface of a salt diapir maintains a relatively constant elevation while sediments accumulate and subside like a sleeve around it. Diapirs begin forming early in the sequence of burial, passing from a phase of gentle bulging (the pillow stage) on the upper surface of layered evaporites (the m o t h e r salt) to a stage of diapirism (Trusheim, 1960). During the diapir stage of development, the top of the salt plug maintains a more or less constant elevation relative to the sediment-water interface while the enveloping sediments subside around the diapir flank and are buried. The neck of the salt is fed by intrusive salt from the m o t h e r salt below, and the diapir stem grows or is maintained through time until cut off from the m o t h e r salt. Although salt tends to rise owing to its buoyancy relative to other sedimentary rocks, its up-

ward migration may be curtailed if sediments above the diapir are deposited too quickly, interrupting the upward flow, or if the diapir encounters undersaturated fluids that dissolve the halite. Excellent explanations of the mechanics of diapirism may be found in Trusheim (1960) and Jackson and Talbot (1986).

2.3. Age of the mother salt - the Louann Owing to the absence of definitive fauna in the evaporites, the age of the Louann has been determined indirectly from fauna of the overlying and underlying units, and only slightly more directly by Sr-isotope chronostratigraphy. According to Salvador (1987), the age of the Louann is between post-Early Jurassic and pre-Late Oxfordian, and the upper surface is most likely Callovian (Late-Middle Jurassic; 165-160 Myr. B.P.). A Callovian age has been corroborated with Sr-isotope studies of salt dome cap rocks and salt-hosted anhydrite by Stueber et al. (1984), Russell (1985), Posey (1986a), Posey et al. (1987a), Land et al. (1988 in this special issue) and Russell et al. (1988 in this special issue). However, because m a n y domes in the Gulf have STSr/S6Sr ratios more radiogenic t h a n Callovian seawater, it is possible that at least part of the Louann is older t h a n Callovian (Posey et al., 1987a).

2.4. Age of diapirism During diapirism, halite is expelled from the mother salt beds mostly into the throats of diapits, and sediments adjacent to the stocks sag in response to the evacuation of halite. On seismic and stratigraphic sections, these periods of diapiric movement appear as thick stratigraphic units adjacent to the diapir compared with thinner (normal thickness) beds of equivalent age farther away from the diapir ( see Trusheim, 1960). Studied in this fashion, diapirism in the Gulf Coast appears to have been a stop-and-go process t h a t began shortly after

12 Louann deposition and continues, in some cases, to the present (Seni and Jackson, 1983b).

2.5. Age of cap rocks The age of cap rock formation is more difficult to measure than times of diapiric movement. It has been generally assumed that cap rocks form during the pre-diapiric or diapiric phase of salt movement because fractures present in anhydrite cap rocks indicate stress that is most easily attributed to upward compression from the salt mass (Murray, 1966). Although the precise timing is not generally known, there is at least one case where the age of anhydrite cap rock has been very well established. Studies by Gose et al. (1985) and Kyle et al. (1987) indicate that anhydrite cap rock at Winnfield Dome in north Louisiana formed shortly after deposition of the mother salt. Paleomagnetic studies of this dome indicate that anhydrite cap rocks accumulated along with stratiform pyrrhotite zones during the Late Jurassic and Early Cretaceous. Today, this anhydrite cap rock is juxtaposed against sedimentary rocks that are Eocene: ~ 100 Myr. younger than the anhydrite cap rock! This indicates that anhydrite cap rock at Winnfield began forming during the early stages of diapirism, and has risen isostatically from at least Late Jurassic through Eocene. A substantial part ( ~ 50 m) of this cap rock, which lies beneath the quarry floor and which is not currently accessible, may record events to the Eocene or perhaps even younger.

2.6. Review of the general model for salt dome formation In their landmark papers on the sulfur and carbon isotope geochemistry of Gulf Coast salt domes, Thode et al. (1954) and Feely and Kulp (1957) apparently solved a complex puzzle of sedimentary geochemistry by relating sulfur deposit formation to bacterially-mediated sulfate reduction in the presence of petroleum.

Stated simply, the model says that after portions of salt diapirs dissolve and a cap rock of residual anhydrite has accumulated, calcite may form in the place of anhydrite if hydrocarbons (presumably liquid petroleum) and bacteria gain access to the anhydrite. Dissolved sulfate from the anhydrite is reduced by bacteria using hydrocarbons as an energy source and then, according to the Feely and Kulp (1957), H2S is re-oxidized by SO~- to native sulfur. In the process of sulfate reduction, petroleum is oxidized and its carbon is incorporated by calcite. These papers have been cited frequently, both in salt dome studies and general studies of sulfate reduction by bacteria, and as the process of early sulfate reduction is so common in young sediments, their models have provided a significant foundation for interpreting the origin of sulfur-bearing minerals in sedimentary environments. Regarding the mechanism of sulfide oxidation, Davis et al. (1970) concluded that oxidation of H2S by SO~- is not probable at the low temperatures proposed by Feely and Kulp (1957). In view of the fact that most salt domes bearing economic concentrations of sulfur are found within major river basins, we are led to believe that H2S is oxidized, simply, by oxygenated groundwater. Feely and Kulp (1957, p. 1803 ) reported that 13C/12C ratios of calcite are: "similar to those of cap rock petroleum" and, on the basis of these "similar" ratios, concluded that the calcite cap rocks derived carbon from petroleum, and that calcite values are often lighter than petroleum because bacteria preferentially incorporate 12C during sulfate reduction. [According to Postgate (1959) and Trudinger et al. (1985), sulfate-reducing bacteria cannot use oil per se but instead depend on other microorganisms for their supplies of energy-yielding substrates.] Feely and Kulp's own data, the data of Thode et al. (1954), and subsequent data collected from salt domes and other Gulf Coast oil reservoirs indicate that cal-

13 cite cap rocks are not derived directly from liquid hydrocarbons. Feely and Kulp reported petroleum ~f13C ranging from ~ - 3 4 to ~ - 2 4 % 0 (PDB) and calcite ranging from ~ - 44 to ~ - 23%0, not including the data from Thode et al. (1954) which extended the calcite range to ~ -54%0. Although the fi~3C range of petroleum has been corroborated by more recent studies, the range for Gulf Coast oils is actually fairly narrow: - 2 8 . 0 to -23.0%0 (Kennicutt et al., 1987). The difference between the lightest oil 51~C-value ( - 28%o ) and the lightest calcite ~13C-value ( -54%0 ) is substantial, and probably unrealistically large to be explained by bacterial fractionation. As stated by Feely and Kulp (1957) and numerous researchers since, "organisms are known to extract carbon-12 preferentially from their carbon source during its utilization." However, biodegraded residues are seldom enriched more t h a n a few per mil over the original petroleum (Stahl, 1977). Facing this observation, the biogenic fractionation mechanism fails to explain the abundance of light carbon ( < - 30%0 ) found in m a n y salt domes (see Posey et al., 1987b ). The only carbon source light enough and abundant enough to provide the super light carbon in calcite is CH4. And with CH4 as the carbon source the problem focuses on a biogenic source for CH4 vs. a thermogenic CH4 source. The sequence of cap rock and associated metals formation appears similar at all domes. Ghosts of anhydrite appear in early-stage calcite; late-stage calcite fills open voids; gypsum replaces anhydrite and calcite; and sulfur fills open voids. Thus a general sequence from anhydrite to calcite to gypsum to sulfur appears to be common. However, the relative timing of the formation of metal sulfides, barite, celestite and strontianite is not well established. Metal sulfides form during at least three and possibly more episodes. One stage forms as anhydrite accumulates as a residue of salt solution into cap rock. A second sulfide stage, which is com-

posed dominantly of Fe-sulfide, precipitates apparently along with early-stage calcite. A third sulfide stage replaces late-stage calcite. Barite, celestite and strontianite occupy voids left by late-stage calcite, and so may be of the same generation as stage-3 sulfides or possibly later. The source of metals in salt domes, as in Mississippi Valley-type deposits, is best ascribed to metal-rich fluids from within the sedimentary section (Price et al., 1983). Light et al. (1987) speculated that burial of shale, organic matter and feldspar-bearing clastics leads to irreversible fluid-mineral reactions, some of which release metals and organics. Several coincident key reactions take place around 100-120 ° C and lead to the enrichment of metals and organics in pore fluids. Associated with these fluids are COe and CH4. As these fluids migrate upward through the top of the geopressured zone, carbonate may be dissolved in the presence of CO2rich fluids. If so, the carbon isotope composition will be shifted toward heavier values owing to the dissolution of carbonates, which are likely, in general, to have marine affinities. Once the carbonate has been dissolved, later portions of the hydrocarbon and fluid column will retain their isotopic character. According to Kreitler and Dutton (1983), Posey (1986a), Posey et al. (1987b), Light et al. (1987), and Prikryl et al. (1988 in this special issue), 513C-values become lighter with depth. For variegated calcite, these values range from ~ - 3 0 to ~ - 1 0 % o (PDB) whereas for banded calcite the range is ~ - 50 to ~ - 25%~. Light et al. (1987) explained this lighter-withdepth profile by surmising that the early-arriving fluids - those that produce variegated calcite - contain relatively high abundances of dissolved carbonate and so have relatively heavy values. However, the later fluids - those that produce banded calcite - are more CH4-rich and contain less dissolved carbonate and so produce relatively light 513C-values. Alternative explanations are given by Prikryl et al. ( 1988 in this special issue) and Sassen et al. (1988 in this

14 special issue). Both studies observed J13C-values no lighter than ~ - 3 0 % o , and explained them using mixtures of petroleum and other fluids. 3. S o u r c e s o f S, C, Sr a n d o t h e r m e t a l s

3.1. Sulfur Sulfur may occur in cap rocks as sulfates, sulfides and native sulfur. The most abundant sulfates are, of course, the cap rocks - - anhydrite and gypsum. Barite and celestite are the dominant trace minerals and occur generally within the calcite cap rocks. The dominant sulfides are the Fe-sulfides, pyrite, marcasite and pyrrhotite, and these often accompany sulfides of Zn, Pb, and rarely Ag and Mn (Kyle and Price, 1986 ). Native sulfur occurs in most domes and forms economically important deposits in several (Halbouty, 1979 ). Where present, it is most commonly associated with the gypsum transition zone in both the calcite above the gypsum, anhydrite below, and within the gypsum itself. This association of sulfur with gypsum argues favorably for groundwater as the source of H2S oxidation. Because of the abundance of anhydrite in the salt dome system, and particularly in the cap rock system, it is generally simplest to consider anhydrite as the sulfur source for all subsequently-formed sulfur-bearing minerals. Anhydrite is soluble in both dilute and saline solutions (Holland and Malinin, 1979). Therefore, sulfate is likely to be dissolved to some extent by any solution in the shallow salt dome environment. Furthermore, there is petrographic evidence for anhydrite dissolution in cap rocks (Feely and Kulp, 1957; Seni, 1986a; Kyle et al., 1987; Prikryl et al., 1988 in this special issue; Werner et al., 1988 in this special issue ). The range of sulfur isotope values found in salt dome minerals ( - 30 to + 78%o, CDT; Kyle and Price, 1986; Kyle and Agee, 1988 in this

special issue) is only slightly less than the range of J~4S range found in all geological materials (see Hoefs, 1973 ). The explanation for this huge range of sulfur isotope values in salt domes is that cap rock formation is considered to take place at low temperatures and to be associated with bacterial processes. The local sulfur source provided by the anhydrite of Mid-Jurassic seawater origin will have a generally heavy J34Svalue of ~ + 16%o (Posey et al., 1987b). Thus, sulfate reduction by bacteria that use petroleum as an energy source can explain the very light values (to -30%o ) found in sulfides and sulfur and the un-reduced SO~- residues can explain the very heavy ~348-values (to + 78%o ) found in late-stage barite (Kyle and Price, 1986; Posey et al., 1987b). However, this mechanism, though very reasonable on the grand scale, ignores the possible contributions from extrinsic sulfur sources such as deep basin brines and shallow-circulating meteoric water or formation water. Kyle and Agee ( 1988 in this special issue ) present a coupled study of Z n - P b - A g distribution and pyrite sulfur isotopes from a cap rock core at Hockley Dome, Texas. Because some of the sulfides apparently formed while anhydrite crystals aggregated into anhydrite cap rock after salt dissolution, and because the anhydrite cap rocks form in sequence from top to base, these stratiform sulfides probably record changes in metal and sulfur isotope compositions of the fluid over a long period of time. By analogy with the Winnfield Dome, Louisiana, where anhydrite accumulated over at least a 10-Myr. period (Ulrich et al., 1984; Gose et al., 1985; Kyle et al., 1987), these data indicate that fluids around Hockley changed significantly, but fairly regularly, probably over an equivalent time. Their results, if applicable to mineralized salt domes in general, indicate that sulfides are precipitated when hot metalliferous formation waters encounter ambient sulfate-saturated shallow cool fluids, and that each fluid contributes variable amounts of isotopically distinct sulfur.

15 3.2. Carbon

The source of carbon in calcite cap rocks is perhaps one of the more controversial topics in salt dome geochemistry. Thode et al. (1954) first noted the association between extremely 13C-depleted values in calcite cap rocks and the large difference between 32Sff4S values of sulfate and native sulfur and concluded that the reduction of sulfate is a biogenic process that occurs simultaneously with the oxidation of organic matter. Feely and Kulp (1957) followed these arguments through with experimental evidence documenting that bacteria which grow on hydrocarbon substrates are capable of reducing SO~- to H2S and thereby appealed to crude oil as the source of carbon. A key to both studies is the statement that crude oil carbon isotope values are the same as calcite, which is not correct. The lightest ~13C-value for petroleum reported in both the Thode et al. (1954) and Feely and Kulp (1957) studies is ~ - 34%o, whereas the lightest value for calcite reported in these studies is ~ -54%o. Crude oils which have been biodegraded are enriched in 13C by no more than ~3%0 (Stahl, 1977). Thus, to form calcite with a -54%0 value from petroleum with a - 34%0 value requires more carbon isotope fractionation than has been demonstrated for biodegraded crude oils. Sassen (1980) and Sassen et al. (1988 in this special issue ) also appealed to oil as a source of carbon in calcite cap rocks at Damon Mound, Texas, but the calcite ~3C-values of this dome fit the carbon isotope data more directly. At Damon Mound calcites range from ~ - 3 1 to ~ -14%o (PDB), so fit a hydrocarbon source fairly well. Kreitler and Dutton (1983) also concurred with the oil source model for calcite carbon, even though the dome they examined, Oakwood, Texas, has values as light as -49%o, much too light to be from liquid hydrocarbons. Many domes, especially those within the East Texas and North Louisiana Basins, exhibit carbon isotope signatures indicating that CH4, rather than petroleum, is the carbon source

(Posey et al., 1987b). In many domes, calcite cap rocks grow more 13C-depleted with depth (Kreitler and Dutton, 1983; Posey, 1986a; Posey et al., 1987b; Prikryl et al., 1988 in this special issue). From this information it appears that calcite cap rocks form progressively from top to base, either from a mixture of two different carbon sources or from a single carbon source that becomes depleted with time (Posey et al., 1987b; Prikryl et al., 1988 in this special issue). Upon reflection, these profiles may indicate that crude oil, C02 and CH4 arrive at the zone of anhydrite dissolution and sulfate reduction together, and that the sulfate-reducing bacteria preferentially utilize the C02 and crude oil before CH4, thus producing the lighter-withdepth carbon isotope profile that appears to characterize most cap rocks. 3.3. Strontium

Most salt dome cap rocks contain at least trace amounts of the Sr minerals celestite (SrS04) and strontianite (SrCO3). However, several domes, notably those in the Central Mississippi Basin (Fig. 1 ) have higher concentrations (up to several percent). The source of this Sr is controversial. Walker (1974) claimed that Sr minerals in the cap rocks of Mississippi domes were derived from aragonite in the carbonate units above the Louann (probably the Smackover) during early diagenesis. A problem with this interpretation is that the removal of Sr from aragonite occurs during meteoric diagenesis, and the Sr does not remain in the local environment (Kinsman, 1969). Kreitler and Dutton (1983) observed that late-stage calcite has higher Sr than adjacent earlier-stage material and concluded that late-stage calcite is a reprecipitated form of the early-stage calcite; that is, a second fluid containing high Sr dissolved the pre-existing calcite, then re-precipitated it in place. Posey ( 1986a, b) argued on the basis of Sr isotope ratios that most of the Sr in salt dome cap rocks is inherited from anhydrite because the range of Sr isotope ratios in salt-

16 anhydrite is nearly the same as all Sr-bearing minerals in the cap rocks. According to him, the relatively high Sr concentrations in late calcite are a product of the distribution behavior of Sr in calcite, that Sr builds up in the fluids that form calcite and produce late-stage calcites that are preferentially enriched in Sr relative to early stage. One outstanding fact from these studies is that late-stage calcites generally have different STSr/S6Sr ratios than adjacent early-stage calcites, so cannot be a product of sequential deposition from a single homogeneous fluid (Prikryl et al., 1988 in this special issue). Saunders et al. (1988 in this special issue) determined that Sr stripping from anhydrite cannot produce enough Sr to account for the abundance of Sr minerals in calcite cap rocks at Tatum Dome, Mississippi, and concluded that deep basin brines, which commonly contain several hundred mg 1-1 Sr (Carpenter et al., 1974; Land and Prezbindowski, 1981) are the probable source of celestite and strontianite in cap rocks.

3.4. Metal sulfides Metal sulfides occupy at least two, and possibly four paragenetic positions within salt dome cap rocks. Sulfides occur in the anhydrite cap rocks and have some textures indicating syngenetic precipitation at the cap rock-salt contact during anhydrite accretion (Ulrich et al., 1984). Pyrite occurs in early-stage calcite and appears to be either a residue from anhydrite dissolution or a coprecipitate with calcite. Sulfides that occur elsewhere in cap rocks appear to have formed both before and after calcite (Kyle and Price, 1986). The temperatures of sulfide formation are poorly constrained by existing data. Fluid-inclusion evidence for paragenetically late barite and celestite indicates precipitation between 110 ° and 140 ° C from saline solutions that range from 3 to 12 eq wt.% NaC1 (Kyle and Price, 1986). Geothermometry based on sulfur isotope pairs from sulfides within the anhydrite

portion of Hockley Dome cap rocks suggests sulfide precipitation from ~ 70 ° to ~ 200°C (Kyle and Price, 1986; Kyle and Agee, 1988 in this special issue). These high temperatures indicate that hot brines were expelled from deeper parts of the basin and transported metals, and perhaps reduced sulfur (Kyle and Agee, 1988 in this special issue). However, pyrite that appears to have coprecipitated along with calcite cap rocks could not have formed at these high temperatures because of the constraint that the biogenic calcite cap rocks must have formed at low temperatures (probably < 70°C). Singlephase fluid inclusions in cap rock calcite confirm a low temperature for calcite precipitation (Prikryl et al., 1988 in this special issue). Although local thermochemical reduction of cap rock-derived S042- is possible in the presence of hydrocarbons at temperatures of > 120 ° C (Orr, 1977 ), the "stratigraphic" trends in sulfur isotope composition of cap rock sulfides are difficult to explain by this method. The general sulfide mineralization model proposed involves the episodic mixing of metal and isotopically heavy H2S-bearing deep-sourced brines with ambient dilute formation waters of meteoric affinities in the cap rock environment (Price et al., 1983; Kyle and Price, 1986; Kyle and Agee, 1988 in this special issue). This situation provides conditions compatible with bacterial reduction of local S042- in the cool shallow cap rock environment, whereas metals are supplied episodically by pulses of relatively hot brines, which results in sulfide precipitation while temporarily halting bacterial activity. 4. F l u i d - r o c k i n t e r a c t i o n s adjacent to salt domes

The presence of hydrocarbons in cap rocks and the isotopic evidence that at least some calcite cap rocks form through biodegradation of crude oil indicate either that oil has migrated upward adjacent to salt stocks and into the cap rock area, or that cap rocks and adjacent sediments have been injected upward from signifi-

17

cantly hotter deeper formations. The presence of numerous shallow oil and gas fields adjacent to salt domes indicates that it is the oils, rather than the rocks, that have moved upward (Sharp et al., 1988). Upward migration of fluids in the Gulf Coast has been observed as "bad water" discharge at the surface, and has been speculated based on several lines of evidence (Land, 1984a; Sharp et al., 1988). Recently, evidence of vertical fluid flow has been found around several salt domes in the Gulf Coast by J.S. Hanor and colleagues. In most sedimentary sequences the salinities of associated formation fluids increase with depth, in part because the higher densities of the more saline fluids causes them to sink (Hanor, 1979). However, fluids around several salt domes have anomalous salinities (densities) owing to the local dissolution of salt from the salt dome, and are thus gravitationally unstable. Saline fluids of this type have been described by Workman and Hanor (1985), Hanor and Workman ( 1986 ), and Bennett and Hanor (1987). The high salinities reported by Hanor and colleagues appear to be affected in part by fluids from deep formations that are driven up section. These fluids apparently come from geopressured units that occur in this region between ~ 3 and ~ 4 km depth. In these cases it is the salt domes that appear to have created avenues for fluid escape, either by fracturing the rocks adjacent to the domes, or by dissolving the salt along the diapir margins and creating a conduit for flow. Elsewhere in the Gulf Coast, geopressured fluids appear to escape upward along growth faults (Goldhaber et al., 1983; Land, 1984a). Ranganathan and Hanor (1988 in this special issue) consider several hypothetical stratigraphic cases and model the isothermal behavior of saline fluid that would be produced at the salt-sediment interface. It seems, according to the models, that even in low-permeability sediments, salinity-driven convection cells form almost immediately after salt dissolves, and achieve flow velocities of 0.01 to 1

m yr. -1. These instantaneously-formed salinity plumes may explain, in part, the density inversions that have been observed around a few salt domes in southern Louisiana (Hanor et al., 1986) and may be important mechanisms for generating diagenetic minerals. In a conceptually-related study, McManus and Hanor (1988 in this special issue) report on sulfide and calcite cements that occur along the flanks of the West Hackberry Dome, Louisiana, speculating that these mineral occurrences formed by convection-related processes described by Hanor et al. (1986) and Ranganathan and Hanor (1988 in this special issue). Significant among their findings is that pyrrhotite is one of the more abundant sulfides - a situation not commonly found in sedimentary rocks, but that seems to occur in several salt domes - and that associated carbonate cements have carbon isotope compositions within the general range of salt dome cap rocks. The controversy over the origins of Gulf Coast brines is currently being evaluated from new perspectives. Land et al. (1988 in this special issue) examine the occurrences of Br in bedded and dome salts in the Gulf Coast and show that salt diapirs apparently release significant quantities of their initial Br during the structural deformation and halite recrystallization associated with diapirism. Although it is too early to know just how this phenomenon has affected the Br concentrations in Gulf Coast brines reported by Carpenter (1978) and Land and Prezbindowski (1981), and though it does not explain the regional differences in Br concentrations (see Stoessel and Carpenter, 1986), it does indicate that halokinesis, as a process, is capable of enriching fluids in Br and other incompatible elements. Sr isotopes and Sr concentrations are employed by Russell et al. (1988 in this special issue) to determine the sources of fluids in oilfields in the Mississippi Basin. They note that, on a regional scale, Sr concentrations and STSr/S6Sr ratios increase with depth and infer that carbonate minerals, detrital micas, and K-

18

feldspar all decompose with burial, and their products affect the compositions of fluids. On a local scale, they are able to correlate brines within individual oil-producing reservoirs, and show that, generally, there is no cross-formation migration of fluid. Shallow brines have Sr isotope ratios that are similar to Mid-Jurassic seawater, the mother liquor of the Louann Mother Salt, whereas deeper brines are more radiogenic.

5. Proposed research Future studies will focus on unanswered questions described, in part, in the collection of papers in this special issue of Chemical Geology. Some of these have lingered since the earliest studies of salt dome geochemistry. The source of carbon is yet an unresolved issue. It is clear that several cap rocks may have formed from oil, whereas others require a significant component of methane. For those requiring methane, the question of a biogenic vs. a thermogenic methane source must be debated. Upcoming studies will need to address problems and consequences of kinetics, a problem that has been generally avoided in the salt dome literature. Thus far, most researchers have operated under the assumption, perhaps erroneously, that equilibrium processes prevailed under cap rock-forming conditions. The problem now is to rethink existing data in light of recent advances in the kinetics of low-temperature processes. Petrographic studies indicate that at least one generation of calcite precipitates very rapidly and that particular phenomenon may affect the lattice parameters. This is the often-described late-stage pore-filling pale calcite that lines the walls of darker calcite substrates and has elongate crystals with inclined extinction. Tiezzi and Folk (1989) have indicated that this feature indicates rapid crystallization from supersaturated solutions, and as this particular calcite has both trace-element and isotopic compositions unlike the adjoining calcite phases, it should be determined whether

these geochemical features are kinetically controlled. Hanor and colleagues have found evidence of active convection probably related to the escape of fluids from overpressured zones. Just how long these fluids will continue to move out of their current host rocks, and how they affect the stability and history of salt domes has not been determined. Presumably, the fluids will not flow forever - not even throughout a significant duration of the active subsiding life of the basin - but what controls the fluid movement and what, if anything, will shut it off need to be evaluated, particularly if salt domes are to be safely used for longer-duration industrial purposes. Profiles of carbon and oxygen isotopes published to date indicate that both J13C and (~180 grow lighter with depth. These domes include Oakwood, Texas (Kreitler and Dutton, 1983), Winnfield, Louisiana (Posey et al., 1987b) and Hockley, Texas (Posey, 1986b; Posey et al., 1987b). However, the profile for Damon Mound (Prikryl et al., 1988 in this special issue) indicates that although J13C-values grow lighter with depth, oxygen grows heavier with depth. This obvious discrepancy in what was previously assumed to be a typical characteristic of calcite needs considerable evaluation. Either the oxygen reservoirs act independently of other element reservoirs, or no simple general model incorporating fluid and rock reservoirs can be made. Another avenue for research is the study of sulfate-oxygen isotopes. Werner et al. (1988 in this special issue) report some of the first such data from a salt dome noting that the values for gypsum are the same as for anhydrite. The values reported by Werner et al. are ~ 1%o lighter than predicted by the sulfate-oxygen chronostratigraphic curve of Claypool et al. (1980). However, owing to the large discrepancies and the paucity of data that compose that curve, this deviation cannot be presently evaluated. Feely and Ktflp (1957) and Posey et al. (1987a) noted

19 that sulfur isotope ratios from anhydrite cap rocks are slightly enriched, on average, compared with anhydrite hosted by salt, and speculated that the process of anhydrite cap rock accumulation may involve either partial sulfate reduction or cementation by sulfate from another sulfur reservoir with a different •348value. If SO 2- reduction occurs during anhydrite accumulation, this should show up in the analysis of oxygen isotopes as well. Trace-element studies of cap rock and salt dome minerals (Kreitler and Dutton, 1983; Posey, 1986a; Land et al., 1988 in this special issue; Prikryl et al., 1988 in this special issue; Werner et al., 1988 in this special issue) appear to offer much for the studies of fluid provenance and mineral-rock interaction phenomena. The fact that Sr concentrations are generally higher in the domes of the East Texas, North Louisiana and Mississippi Basins than in the Gulf Coast Basin (Posey, 1986) has not been investigated. This feature may have important information about processes that take place at levels far deeper than the currentlyreachable diapirs. Recent attempts to unravel fluid-rock reactions in the Gulf Coast Basin using rare-earth elements (Macpherson et al., 1988) offer considerable promise. Attempts at determining cap rock formation temperatures have had only moderate success, partly because of the scarcity of suitable materials for definitive fluid-inclusion studies and because of difficulties locating sulfide minerals that demonstrate equilibrium relationships a prerequisite of sulfur isotope geothermometry. Understanding the thermal history of salt domes would tell much of the thermal history of the Gulf Coast as cap rock minerals appear to record fluid events over, perhaps, tens of millions of years. 6. Conclusions

Theories for the generation of salt domes are being revised to accommodate recent information spawned by investigations into basin brine

evolution, fluid evolution and transport related to salt diapirism, sulfide mineralization in cap rocks and adjacent sediments, isotopic and trace-element investigations into the nature of cap rock formation, salt dome stability studies, and traditional studies of the relations between salt structures and hydrocarbon accumulations. Papers in this special issue indicate that fluid migration from deep formations is apparently common around cap rocks, that such migration may be long-lived, albeit episodic, and that significant perturbations in the chemistry of fluids may be induced by both recrystallization and dissolution of salt. Sr isotope studies indicate that fluids have interacted with salthosted anhydrite some time after primary evaporite deposition, and that these fluids may have affected the viscosity and buoyancy of salt perhaps to the point of initiating diapirism. A comparison of Br and Sr isotopes in salt diapirs and bedded salt indicates similar relationships, but indicates that the loss of Br from salt diapirs is a phenomenon that is unrelated to Sr exchange. Oxygen isotope studies indicate that calcite cap rocks require a significant component of meteoric water, if it is assumed that calcite is a product of bacterial sulfate reduction and that such activity proceeds at low temperatures ( < 100°C). For the class of calcite cap rocks that have J13C ratios heavier than ~ - 3 0 % o (PDB) an assumption that liquid hydrocarbons provided the carbon is probably most reasonable based on available information. However, those with lighter values, some of which reach values lighter than - 50%e, require a CH4 source. Whether the methane is thermogenic or biogenic is not known. There is no compelling evidence that seawater is an essential component except in the formation of marine false calcite cap rocks, although its presence could be masked by mixtures of two fluids that bracket t h e composition of seawater, both in terms of carbon and oxygen isotopes. Sulfur isotope studies of sulfates and sulfides in cap rocks and salt suggest that local SO42- of

20 Mid-Jurassic seawater parentage may not be the principal (or sole) source of sulfur for all sulfur-bearing species t h a t form following salt dissolution and cap rock accumulation. Reduced sulfur and metals may, in fact, travel in the same solutions upward along the sides of salt domes and deposit as metal sulfides in cap rocks. Salt domes have both active and passive economic utility. Active mining of salt, sulfur and cap rock minerals provides considerable income for industries in the Gulf Coast. Passive storage of L P G and other products in salt storage caverns - caverns t h a t are " m i n e d " by injecting fresh water into salt diapirs t h a t dissolves the salt - is perhaps a growth industry in this area. Continued safe use of salt domes depends on a clear understanding of the physical and chemical mechanisms t h a t combine to form them, and understanding salt dome chemistry and physics will contribute immensely to the understanding of processes throughout the Gulf Coast and to the history of other young subsiding sedimentary basins.

Acknowledgements We would like to express our appreciation to the editorial staff of Chemical Geology for providing a means to publish these collected contributions to research on the geochemistry of a major sedimentary terrane. We would like to t h a n k the contributors to this special issue for their diligence and efforts to satisfy our editorial suggestions. We are particularly grateful to Jim Prikryl and Miriam Renkin for logistical help in bringing this special issue to successful completion. We are pleased to acknowledge stimulating discussions on the geology and geochemistry of Gulf Coast Basin with our research colleagues, including W.N. Agee, M.R. Farr, R.L. Folk, W.A. Gose, L.S. Land, M.P.R. Light, W.E. Macpherson, P.E. Price, J.D. Prikryl and M.R. Ulrich, at the University of Texas at Austin, and to J.S. H a n o r and Roger Sassen (Louisiana State University), A.M. Stueber (Southern Illinois University) and L.P. K n a u t h

(Arizona State University). Research support was provided by National Science Foundation G r a n t EAR-8709319.

References Baker, H.W., 1979. General geologyof Damon Mound. In: E.M. Etter (Editor), Damon Mound, Texas. Houston Geol. Soc., Houston, Texas, Guideb.,pp. 10-25. Bennett, S.C. and Hanor, J.S., 1987. Dynamicsof subsurface salt dissolutionat the WelshDome, LouisianaGulf Coast. In: I. Lerche and J.J. O'Brien, DynamicalGeologyof Salt and RelatedStructures. AcademicPress, Orlando, Fla., pp. 653-678. Bredehoeft,J.D., Blyth,C.R.,White,W.A.and Maxey,G.B., 1963. Possible mechanism for concentration of brines in subsurface formations. Am. Assoc. Pet. Geol. Bull., 47" 257-269. Carpenter, A.B., 1978. Origin and chemical evolution of brines in sedimentary basins. Okla. Geol. Surv., Circ., 79: 60-77. Carpenter, A.B., Trout, M.L. and Pickett, E.E., 1974. Preliminary report on the origin and chemicalevolutionof lead- and zinc-richoil fieldbrines in central Mississippi. Econ. Geol.,69: 1191-1206. Claypool, G.E., Holser, W.T., Kaplan, I.R., Sakai, H. and Zak, I., 1980. The age curves of sulfur and oxygenisotopes in marine sulfateand their mutual interpretation. Chem. Geol.,28: 199-260. Davis, J.B., Stanley, J.P. and Custard, H.C., 1970. Evidence against oxidation of hydrogen sulfide by sulfate ions to produce elementalsulfur in salt domes. Am. Assoc. Pet. Geol. Bull., 54: 2444-2447. Dix, R.O. and Jackson, M.P.A., 1982. Lithology, microstructures, fluidinclusions,and geochemistryofrock salt and of the cap rock contact in the OakwoodDome,East Texas: significance for nuclear waste storage. Univ. Texas, Austin, Bur. Econ. Geol., Rep. Invest. No. 120, 59 pp. Feely,H.W. and Kulp, J.L., 1957. Origin of Gulf Coast salt dome sulfur deposits. Am. Assoc. Pet. Geol. Bull., 41: 1802-1853. Fogg, G.E., Seni, S.J. and Kreitler, C.W., 1983. Three-dimensional ground-watermodelingin depositional systems, Wilcox Group, Oakwood salt dome area, East Texas. Univ. Texas, Austin, Bur. Econ. Geol., Rep. Invest. No. 133, 55 pp. Giles,A.B.and Wood,D.H., 1983.Oakwoodsalt dome,East Texas: geologicframework,growth history, and hydrocarbon production. Univ. Texas, Austin, Bur. Econ. Geol., Geol. Circ. No. 83-1, 55 pp. Goldhaber, M.B. and Reynolds,R.L., 1979. Origin of marcasite and implicationsregardingthe genesisof roll-type uranium deposits. U.S. Geol. Surv., Open-FileRep. 791696, 40 pp.

21 Goldhaber, M.B., Reynolds, R.L. and Rye, R.O., 1978. Origin of a south Texas roll-type deposit, II. Sulfide petrography and sulfur isotope studies. Econ. Geol., 73: 1690-1705. Goldhaber, M.B., Reynolds, R.L. and Rye, R.O., 1983. Role of fluid mixing and fault-related sulfide in the origin of the Ray Point uranium district, South Texas. Econ. Geol., 78: 1043-1082. Gose, W.A., Kyle, J.R. and Ulrich, M.R., 1985. Preliminary paleomagnetic investigations of the Winnfield salt dome cap rock, Louisiana. Gulf Coast Assoc. Geol. Soo. Trans., 35: 97-106. Graf, D.L., 1982. Chemical osmosis, reverse chemical osmosis, and the origin of subsurface brines. Geochim. Cosmochim. Acta, 46: 1431-1448. Halbouty, M.F., 1979. Salt Domes, Gulf Region, United States and Mexico. Gulf Publishing Co., Houston, Texas, 2nd ed., 561 pp. Hanna, M.A. and Wolf, A.G., 1934. Texas and Louisiana salt dome caprock minerals. Am. Assoc. Pet. Geol. Bull., 18: 212-225. Hanor, J.S., 1979. The sedimentary genesis of hydrothermal fluids. In: H.L. Barnes (Editor), Geochemistry of Hydrothermal Ore Deposits. Wiley, New York, N.Y., pp. 137-168. Hanor, J.S., 1984. Salinity and geochemistry of subsurface brines in South Louisiana and their potential for reaction with injected hydrothermal waste waters. U.S. Dep. Energy, Washington, D.C., Rep. DOE/NV/10174-3, pp. 277-346. Hanor, J.S., 1987. Origin and migration of subsurface sedimentary brines. Soo. Econ. Paleontol. Mineral., Short Course No. 21,247 pp. Hanor, J.S. and Bailey, J.E., 1983. Use of hydraulic head and hydraulic gradient to characterize geopressured sediments and the direction of fluid migration in the Louisiana Gulf Coast. Trans. Gulf Coast Assoc. Geol. Soc., 33: 115-122. Hanor, J.S. and Workman, A.L., 1986. Distribution of dissolved volatile fatty acids in some Louisiana oil field brines. Appl. Geochem., 1: 37-46. Hanor, J.S., Bailey, J.E., Rogers, M.C. and Milner, R.L., 1986. Regional variations in physical and chemical properties of south Louisiana oilfield brines. Trans. Gulf Coast Assoc. Geol. Soo., 36: 143-149. Hitchon, B. and Friedman, I., 1969. Geochemistry and origin of formation waters in the western Canada sedimentarybasin, I. Stable isotopes of hydrogen and oxygen. Geochim. Cosmochim. Acta, 33: 1321-1349. Hoefs, J.D., 1973. Stable Isotope Geochemistry. Springer, Berlin, 142 pp. Holland, H.D. and Malinin, S.D., 1979. The solubility and occurrence of non-ore minerals. In: H.L. Barnes (Editor), Geochemistry of Hydrothermal Ore Deposits. Wiley, New York, N.Y., pp. 461-508. Holser, W.T., 1979. Trace elements and isotopes in evap-

orites. In: R.G. Burns (Editor), Marine Minerals. Mineral. Soc. Am., Short Course Notes, 6: 295-336. Humphris, Jr., C.C., 1978. Salt movement on continental slope, northern Gulf of Mexico. In: Framework, Facies, and Oil-trapping Characteristics of the Upper Continental Margin. Am. Assoc. Pet. Geol., Stud. Geol., 7: 6985. Jackson, M.P.A., 1985. Natural strain in diapiric and glacial rook salt, with emphasis on Oakwood dome, East Texas. Univ. Texas, Austin, Bur. Econ. Geol., Rep. Invest. No. 143, 74 pp. Jackson, M.P.A. and Seni, S.J., 1984. Suitability of salt domes in the East Texas Basin for nuclear waste isolation. Univ. Texas, Austin, Bur. Econ. Geol. Circ. No. 841,128 pp. Jackson, M.P.A. and Talbot, C.J., 1986. External shapes, strain rates, and dynamics of salt structures. Geol. Soc. Am. Bull., 97: 305-323. Kennicutt, M.C., Denoux, G.J., Brooks, J.M. and Sandberg, W.A., 1987. Hydrocarbons in Mississippifan and intraslopebasin sediments.Geochim. Cosmochim. Acta, 51: 1457-1466. Kinsman, D.J.J., 1969. Interpretationof Sr2+ concentrations in carbonate minerals and rock. J. Sediment. Petrol.,39: 486-508. Knauth, L.P.,Kumar, M.B. and Martinez, J.D., 1980. Isotope geochemistry of water in Gulf Coast saltdomes. J. Geophys. Res.,85: 4863-4871. Kreitler,C.W. and Dutton, S.P., 1983. Origin and diagenesis of cap rock, Gyp Hill and Oakwood salt domes, Texas. Univ. Texas, Austin, Bur. Econ. Geol.,Rep. Invest. No. 131, 58 pp. Kreitler,C.W. and Muehlberger, W.R., 1981. Geochemical analysisofsalt,Grand SalineDome, East Texas. In:C.W. Kreitler,E.W. Collins,E.D. Davidson, Jr. et al.Geology and Geohydrology of the East Texas Basin, A Report on the Progress of Nuclear Waste Isolation Feasibility Studies (1980). Univ. Texas, Austin, Bur. Econ. Geol., Geol. Circ.No. 81-7,pp. 188-194. Kreitler,C.W., Collins,E.W., Fogg, G.E., Jackson, M.P.A. and Seni, S.J.,1985. Hydrologic characterizationof the saline aquifers,East Texas Basin - implicationsto nuclear waste storage in East Texas salt domes. Univ. Texas, Austin, Bur. Econ. Geol. Rep. to U.S. Dep. Energy under Contract No. DE-AC97-80ET46617, 163 pp. Kumar, M.B., 1978. Characteristicsof brinesfrom the Belle Isleand Weeks Islandsaltmines. In:J.D. Martinez, R.L. Thorns, C.R. Kolb, M.B. Kumar, R.E. Wilcox and E.J. Newchurch (Editors),An Investigation of the Utility of Gulf Coast Salt Domes for the Storage or Disposal of Radioactive Wastes, Vol. 1. Inst.Environ. Stud.,Louisiana State Univ., Baton Rouge, La., pp. 235-309. Kumar, M.B. and Martinez, J.D., 1978. Sources of subsurface leaks in the Belle Isle and Weeks Island salt mines. In: J.D. Martinez, R.L. Thorns, C.R. Kolb, M.B. Kumar, R.E. Wilcox and E.J. Newchurch (Editors), An

22 Investigation of the Utility of Gulf Coast Salt Domes for the Storage or Disposal of Radioactive Wastes, Vol. 1. Inst. Environ. Stud., Louisiana State Univ., Baton Rouge, La., pp. 311-324. Kupfer, D.H., 1980. Problems associated with anomalous zones in Louisiana salt stocks. In: A.H. Coogan and L. Hauber (Editors),Proceedings of the 5th International Symposium on Salt,Vol. 1. N. Ohio Geol. Soc.,pp. 119134. Kyle, J.R. and Agee, Jr.,W.N., 1988. Evolution of metal ratios and ~34S composition of sulfide mineralization during anhydrite cap rock formation, Hockley Dome, Texas, U.S.A. In: H.H. Posey and J.R. Kyle (Guest-Editors), Fluid-Rock Interactions in the Salt Dome Environment. Chem. Geol., 74:37-55 (thisspecial issue). Kyle, J.R. and Price, P.E., 1986. Metallic sulphide mineralization in salt-dome cap rocks, Gulf Coast, U.S.A. Trans. Inst. Min. Metall.,95: 6-16. Kyle, J.R., Ulrich, M.R. and Gose, W.A., 1987. Textural and paleomagnetic evidence for the mechanism and timing of anhydrite cap rock formation, Winnfield salt dome, Louisiana. In: I.Lerche and J. O'Brien (Editors), Dynamical Geology of Salt and Related Structures. Academic Press, Orlando, Fla.,pp. 497-542. Land, L.S., 1984a. Evidence for verticalmigration of diagenetic fluids,Texas Gulf Coast. Am. Assoc. Pet. Geol. Bull.,68:498 (abstract). Land, L.S., 1984b. Frio Sandstone diagenesis,Texas Gulf Coast: a regionalisotopicstudy. In: D.A. McDonald and R.C. Surdam (Editors), ClasticDiagenesis. Am. Assoc. Pet. Geol., Mere. 37, pp. 47-62. Land, L.S. and Prezbindowski, D.R., 1981. The origin and evolution of saline formation waters, Lower Cretaceous carbonates, south-centralTexas, U.S.A. In: W. Back and R. L~tolle (Guest-Editors), Symposium on Geochemistry of Groundwater - 26th International Geological Congress. J. Hydrol., 54:51-74 (specialissue). Land, L.S.,Kupecz, J.A. and Mack, L.E., 1988. Louann salt geochemistry (Gulf of Mexico sedimentary basin, U.S.A.): A preliminary synthesis. In: H.H. Posey and J.R. Kyle (Guest-Editors), Fluid-Rock Interactions in the Salt D o m e Environment. Chem. Geol., 74:25-35 (this specialissue). Light, M.P.R., Posey, H.H., Kyle, J.R. and Price,P.E., 1987. Integrated hydrothermal model for the Texas Gulf Coast Basin; origins of geopressured brines and lead-zinc, uranium, hydrocarbon, and cap rock deposits. In: I. Lerche and J.J. O'Brien (Editors), Dynamical Geology of Salt and Related Structures. Academic Press, Orlando, Fla.,pp. 787-830. Macpherson, G.L., Farr, M.R. and Posey, H.H., 1988. Rare earth element concentrations in formation waters, Gulf Coast sedimentary basin. Soc. Econ. Paleontol. Mineral.,Mid-year Meet., Abstr. V, p. 33. Manheim, F.T. and Horn, M.K., 1968. Composition of deeper subsurface waters along the Atlantic coastal margin. Southe. Geol., 8: 215-236.

Martin, R.G., 1978. Northern and eastern Gulf of Mexico continental margin: stratigraphic and structural framework. In: Framework, facies, and oil-trapping characteristics of the upper continental margin. Am. Assoc. Pet. Geol. Stud. Geol., 7: 21-42. Martinez, J.D. and Kumar, M.B., 1980. Thermal migration of formation waters in salt domes. Eos (Trans. Am. Geophys. Union), 61: 1177-1178. Milliken, K.L., Land, L.S. and Loucks, R.G., 1981. History of burial diagenesis determined from isotopic geochemistry, Frio Formation, Brazoria County, Texas. Am. Assoc. Pet. Geol. Bull., 65: 1397-1413. Mullican III, W.F., 1988. Subsidence and collapse at Texas salt domes. Univ. Texas, Austin, Bur. Econ. Geol., Geol. Circ. No. 88-2, 36 pp. Murray, G.E., 1966. Salt structures of the Gulf of Mexico Basin - a review. Am. Assoc. Pet. Geol. Bull., 50: 439478. Nance, D. and Wilcox, R.E., 1979. Lithology of the Rayburn's dome salt core. Inst. Environ. Stud., Louisiana State Univ., Baton Rouge, La., Topical Rep. No. E51102500-3. Nance, D., Rovik, J.E. and Wilcox, R.E., 1979. Lithology of the Vacherie salt dome core. Inst. Environ. Stud., Louisiana State Univ., Baton Rouge, La., Topical Rep. No. E511-02500-5. NAS-NRC (National Academy of Sciences - National Research Council), 1957. The disposal of radioactive wastes on land. Rep. Comm. Waste Disposal, Div. Earth Sci., Publ. 519. ONWI (Office of Nuclear Waste Isolation), 1982. Evaluation of area studies of the U.S. Gulf Coast salt dome basins: location recommendation report. Battelle Memorial Institute, Columbus, Ohio, ONWI-109, 95 pp. Orr, W.L., 1977. Geologic and geochemical controls on the distribution of hydrogen sulfide in natural gas. In: J. Campos and J. Gofii (Editors), Advances in Organic Geochemistry. Enadimsa, Madrid, pp. 572-597. Posey, H.H., 1986a. Regional characteristics of strontium, carbon, and oxygen isotopes in salt domes cap rocks of the western Gulf Coast. Ph.D. Dissertation, University of North Carolina, Chapel Hill, N.C. (unpublished). Posey, H.H., 1986b. Isotope geochemistry of Hockley dome cap rock. In: S.J. Seni and J.R. Kyle (Editors), Comparison of Cap Rocks, Mineral Resources, and Surface Features of Salt Domes in the Houston Diapir Province. Guideb. Geol. Soc. Am., Natl. Meet., San Antonio, Texas, pp. 185-209. Posey, H.H., Kyle, J.R., Jackson, T.J., Hurst, S.D. and Price, P.E., 1987a. Multiple fluid components of salt diapirs and salt dome cap rocks, Gulf Coast, U.S.A. Appl. Geochem., 2: 523-534. Posey, H.H., Price, P.E. and Kyle, J.R., 1987b. Mixed carbon sources for calcite cap rocks of Gulf Coast salt domes. In: I. Lerche and J. O'Brien (Editors), Dynamical Geology of Salt and Related Structures. Academic Press, Orlando, Fla., pp. 593-630.

23 Postgate, J.R., 1959. The sulphate-reducing bacteria. Annu. Rev. Microbiol., 13: 505-520. Price, P.E., Kyle, J.R. and Wessel, G.R., 1983. Salt dome related lead-zinc deposits. In: G. Kisvarsanyi, S.K. Grant, W.P. Pratt and J.W. Koenig (Editors), International Conference on Mississippi Valley Type LeadZinc Deposits, Proc. Vol., Univ. of Missouri-Rolla, RoUa, Mo., pp. 558-577. Prikryl, J.D., Posey, H.H. and Kyle, J.R., 1988. A petrographic and geochemical model for the origin of calcite cap rock at Damon Mound salt dome, Texas, U.S.A. In: H.H. Posey and J.R. Kyle (Guest-Editors), Fluid-Rock Interactions in the Salt Dome Environment. Chem. Geol., 74:67-97 (this special issue). Ranganathan, V. and Hanor, J.S., 1988. Density-driven groundwater flow near salt domes. In: H.H. Posey and J.R. Kyle (Guest-Editors), Fluid-Rock Interactions in the Salt Dome Environment. Chem. Geol., 74:173-188 (this specialissue). Rezak, R., 1985. Local carbonate production on a terrigenous shelf.Gulf Coast Assoc. Geol. Soc. Trans., 35: 477484. Roedder, E. and Belkin, H.E., 1979. Fluid inclusions in salt from the Rayburn and Vacherie domes, Louisiana. U.S. Geol. Surv., Open-File Rep. 79-1675, 25 pp. Rouvier, H., Perthuisot, V. and Mansouri, A., 1985. Pb-Zn deposits and salt-bearing diapirs in Southern Europe and North Africa. Econ. Geol., 80: 666-687. Russell, C.W., 1985. A strontium isotope study of oil field brines and associated rocks in southeastern Mississippi. M.S. Thesis, Florida State University, Tallahassee, Fla. (unpublished). Russell, C.W., Cowart, J.B. and Russell, G.S., 1988. Strontium isotopes in brines and associated rocks from Cretaceous strata in the Mississippi Salt Dome Basin. In: H.H. Posey and J.R. Kyle (Guest-Editors), Fluid-Rock Interactions in the Salt Dome Environment. Chem. Geol., 74:153-171 (this special issue). Salvador, A., 1987. Late Triassic-Jurassic paleogeography and origin of Gulf of Mexico Basin. Am. Assoc. Pet. Geol. Bull., 71: 419-451. Salvador, A. and Buffler, R.T., 1982. The Gulf of Mexico Basin. In: A.R. Palmer (Editor), Perspectives in Regional Geological Synthesis. Geol. Soc. Am., Decade N. Am. Geol., Spec. Publ., pp. 157-162. Sassen, R., 1980. Biodegradation of crude oil and mineral deposition in a shallow Gulf Coast salt dome. Org. Geochem., 2: 153-166. Sassen, R., Chinn, E.W. and McCabe, C., 1988. Recent hydrocarbon alteration, sulfate reduction and formation of elemental sulfur and metal sulfides in salt dome cap rock. In: H.H. Posey and J.R. Kyle (Guest-Editors), Fluid-Rock Interactions in the Salt Dome Environment. Chem. Geol., 74:57-66 (this special issue). Saunders, J.A., Prikryl, J.D. and Posey, H.H., 1988. Mineralogic and isotopic constraints on the origin of stron-

tium-rich cap rock, Tatum Dome, Mississippi, U.S.A. In: H.H. Posey and J.R. Kyle (Guest-Editors), FluidRock Interactions in the Salt Dome Environment. Chem. Geol., 74:137-152 (this special issue). Seni, S.J., 1986a. Evolution of Boling Dome cap rock with emphasis on includedterrigenousclastics,Fort Bend and Wharton Counties, Texas. In: I.Lerche and J.J.O'Brien (Editors), Dynamical Geology of Salt and Related Structures.Academic Press, Orlando, Fla.,pp. 543-592. Seni, S.J.,1986b. Texas Gulf Coastal saltdomes: introduction and overview. In: S.J. Seni and J.R. Kyle (Editors),Comparison of Cap Rocks, Mineral Resources and Surface Features of Salt Domes in the Houston Diapir Province. Guideb. Geol. Am., Natl. Meet., San Antonio, Texas, pp. 3-24. Seni, S.J. and Jackson, M.P.A., 1983a. Evolution of salt structures,East Texas diapirprovince, Part I.Sedimentary record of halokinesis.Am. Assoc. Pet. Geol. Bull., 67: 1219-1244. Seni, S.J. and Jackson, M.P.A., 1983b. Evolution of salt structures,East Texas diapirprovince, Part 2. Patterns and ratesof halokinesis.Am. Assoc. Pet. Geol. Bull.,67: 1245-1274. Seni, S.J. and Mullican III,W.R., 1986. Topography over domes: Implications for dome growth and dissolution. In: S.J. Seni and J.R. Kyle (Editors), Comparison of Cap Rocks, Mineral Resources, and Surface Features of Salt Domes in the Houston Diapir Province. Guideb. Geol. Soc. Am. Natl. Meet., San Antonio, Texas, pp. 89100. Sharp, Jr.,J.M., Galloway, W.E., Land, L.S.,McBride, E.F., Blanchard, P.E.,Bodner, D.P., Dutton, S.P.,Farr, M.R., Gold, P.B., Jackson, T.J., Lundegard, P.D., Macpherson, G.L. and Milliken,K.L., 1988. Diagenetic processes in northwestern Gulf of Mexico sediments. In: G.V. Chilingarian and K.H. Wolf (Editors), Diagenesis, II, Ch. 1. Elsevier,Amsterdam, pp. 43-114. Skinner, B.J., 1986. Earth Resources. Prentice-Hall, Englewood Cliffs,N.J., 3rd ed., 184 pp. Smith, Jr., A.E., 1970a. Minerals from Gulf Coast salt domes, Part I. Rocks Miner., 45: 299-303. Smith, Jr., A.E., 1970b. Minerals from Gulf Coast salt domes, Part II. Rocks Miner., 45: 371-380. Smith, G. and Kolb, C.R., 1981. Isotopic evidence for the origin of the Rayburn's boulder zone of north Louisiana. Inst. Environ. Stud., Louisiana State Univ., Baton Rouge, La., File Rep. QR-02-4-2. Stahl, W.J., 1977. Carbon and nitrogen isotopes in hydrocarbon research and exploration. Chem. Geol., 20: 121149. Stoessel, R.K. and Moore, C.H., 1983. Chemical constraints and origins of four groups of Gulf Coast reservoir fluids. Am. Assoc. Pet. Geol. Bull., 67: 896-906. Stueber, A.M., Pushkar, P. and Hetherington, E.A., 1984. A strontium isotopic study of Smackover brines and associated solids, Southern Arkansas. Geochim. Cosmochim. Acta, 48: 1637-1649.

24 Talbot, C.J. and Jackson, M.P.A., 1987. Salt tectonics. Sci. Am., 257: 70-79. Taylor, R.E., 1938. Origin of cap rock of Louisiana salt domes. La. Geol. Surv., Bull. No. 11,191 pp. Thode, H.G., Wanless, R.K. and Wallouch, R., 1954. The origin of native sulphur deposits from isotope fractionation studies. Geochim. Cosmochim. Acta, 5: 286-298. Tiezzi, P.A. and Folk, R.L., 1989. Cloudy cements in bioherin cavities, Mississippian Lodgepole Limestone, Montana: product of bacterial activity during organic decay. Sediment. Geol. (in press). Trudinger, P.A., Chambers, L.A. and Smith, J.W., 1985. Low-temperature sulphate reduction: biological versus abiological. Can. J. Earth Sci.,22: 1910-1918. Trusheim, F., 1960. Mechanisms of salt migration in northern Germany. Am. Assoc. Pet. Geol. Bull.,44:15191540. Ulrich, M.R., Kyle, J.R. and Price, P.E., 1984. Metallic sulfide deposits in the Winnfield salt dome, Louisiana evidence for episodic introduction of metalliferousbrines

during cap rock formation. Gulf Coast Assoc. Geol. Soc. Trans., 34: 435-442. U.S. DOE (U.S. Department of Energy), 1983. Proposed general guidelines for recommendation of sites for nuclear waste repositories.Fed. Regist.,Washington, D.C., 48 (26): 5670-5682. Walker, C.W., 1974. The nature and origin of cap rock overlying Gulf Coast salt domes. N. Ohio Geol. Soc., 4th Syrup. on Salt, Vol. I, pp. 169-195. Werner, M.L., Feldman, M.D. and Knauth, L.P., 1988. Petrography and geochemistry of water-rock interactions in Richton Dome cap rock (southeastern Mississippi, U.S.A.). In: H.H. Posey and J.R. Kyle (Guest-Editors), Fluid-Rock Interactions in the Salt Dome Environment. Chem. Geol., 74: 113-135 (this special issue). Workman, A.L. and Hanor, J.S., 1985. Evidence for large scale vertical migration of dissolved fatty acids in Louisiana oil field brines: Iberia field,south-central Louisiana. Gulf Coast Assoc. Geol. Soc. Trans., 35: 293-300.