Evaporite Basin Research Articles

Structure, chemistry and origins of gypsum crusts in southern Tunisia and the central Namib Desert

ABSTRACTGypsum crusts are broadly defined as accumulations at or within about 10 m of the land surface from 0.10m to 5.0 m thick containing more than 15% by weight gypsum (CaSO4·2H2O) and at least 5.0% by weight more gypsum than the underlying bedrock. The deposits are often, but not invariably, consolidated owing to cementation by gypsum. The crusts are found in many of the world's deserts where mean monthly potential evaporation exceeds mean monthly precipitation throughout the year.Using structural, fabric and textural criteria, three main types of crust may be distinguished:(1) bedded crusts, found either at or beneath the land surface, which are made up of discrete horizontal strata up to 0.10 m thick, each showing a gradation in gypsum crystal size from less than 50 μm at the top to more than 0.50 mm at the base; (2) subsurface crusts, of which there are two forms, one made up of large, lenticular crystals (up to 0.50 m in diameter)—the desert rose crusts—and the other, a mesocrystalline form, with gypsum crystals up to about 1.0 mm in diameter; and (3) surface crusts, which are subdivided into columnar, powdery and cobble forms, all of which are made up of predominantly alabastrine gypsum (crystallites less than 50 μm in diameter).In southern Tunisia and the central Namib Desert, bedded crusts are found around ephemeral lakes and lagoons. They are characterized by size‐graded beds, gypsum contents of 50–80% by weight and comparatively high concentrations of sodium, potassium, magnesium and iron. They are interpreted as shallow‐water evaporites which accumulate when saline pools evaporate to dryness. Desert rose crusts or croûtes de nappe generally contain 50–70% by weight gypsum, and have higher sodium concentrations than the second subsurface form. Texturally they are characterized by poikilitic inclusion of clastic material within large lenticular crystals. They are interpreted as hydromorphic accretions, which precipitate in host sediments at near‐surface water tables through the evaporation of groundwater. The second form of subsurface crust—the mesocrystalline—often occurs in close association with the various surface forms. Unlike the hydromorphic crusts, they are not restricted to low‐lying terrain. They are characterized by gypsum contents reaching 90% by weight, and have a close chemical and textural similarity to columnar surface crusts. This mesocrystalline form represents an illuvial accumulation; the surface forms—excluding the bedded crusts—are exhumed examples at various stages of solutional degradation. Subsurface precipitation of gypsum from meteoric waters containing salts leached from the surface, results in displacive gypsum accumulation in the soil zone. In southern Tunisia, the gypsum is derived from sand and dust deflated from evaporitic basins; in the central Namib, salts dissolved in fog water are the most likely source. Where other salts are present, differential leaching may form two‐tiered crusts, calcrete—gypsum or gypsum—halite, if rainfall is sufficient to mobilize the less soluble salt yet insufficient to flush the more soluble. Gypsum crust genesis is restricted to arid environments, and if their susceptibility to post‐depositional alteration is acknowledged, they can provide valuable palaeoclimatic indicators.

Organic geochemical studies of a Messinian evaporitic basin, northern Apennines (Italy) I: Hydrocarbon biological markers for a hypersaline environment

This paper describes the occurrence and significance of hydrocarbons present in two bituminous marl layers and one distinct gypsum layer from a Messinian sedimentary basin, where hypersaline conditions prevailed. Several new compounds were detected and tentatively identified: of these 20R and 20S 4α, 24-dimethyl-5α(H),14β(H),17β(H) and 20R and 20S 4β,24-dimethyI-5α(H),14β(H),17β(H) cholestanes; 4-methylspirosterenes; 4,4-dimethyl-5α(H),14β(H),17β(H) pregnanes and homopregnanes are discussed in this paper. Several of these compounds might be considered as biological markers for a (hyper)saline environment. The short side chain 4-desmethylsteranes, 5α(H),14β(H),17β(H), 5α(H),14β(H),17α(H) and 5α(H),14α(H), 17α(H) pregnanes and homopregnanes, are the most abundant compounds in the extract from the gypsum sample. It is suggested that in this case these compounds do not reflect the stage of diagenesis but are related to certain organisms exclusively occurring in hypersaline environments. In addition the very low pristane/phytane ratio, often considered as an indicator for anoxicity, could also be interpreted as a useful indicator for hypersalinity.

Geología de tos diapiros triásicos en el noreste de la provincia de Murcia.

Main stratigraphic characteristics of triassic diapires: Jumilla, La Celia, Yecla and La Rosa, their tectonics and their environements are studied; all of them are situated in the Northeast Prebetic of Murcia province. Stratigraphically, three stretches are estimated: a detrital-argillaceous-dolomitic one with ocher-yellowish colours in its base, another gypseous-argillaceous intermediate one with vivid red colours and a third one, in the top, with well stratiphied grey scarfed gypsum and some interpolations of dolomites and black clays. Salt ls under the basal stretch and it's developed in most of the up to here studied diapires. As a whole, it's an evaporitic basin in a central position where saline and penesaline litotopes, with tendency to euxinic ones, have been identified. Moreover, we emphasize the existence of volcanic rocks of a lamproitic type which are inserted in the just described materials. From a structural point of view, all these diapires are injected with faults of a regional character; inside them a radial and occasionally concentric fracturation is observed. The mis en place of these autcroppings is connected with the main alpine folding stage, Nevertheless, the alocinetic effects work almost to this day providing a neotectonic connected to diapirism which has been displayed on folds , faults and peripheric grooves of materials belonging to Pliocene and Quaternary; in these last ones, old lakes formed where turbidites associated to evaporites settled.

EVAPORITES AS OIL AND GAS SOURCE ROCKS

Shale‐ and micritic limestone‐bound organics have long been considered as hydrocarbon source rocks. However, a conceptual model of hydrocarbon generation in salt basins does not require the entrapment of hydrocarbons in impervious strata, shales or micritic limestones as source rocks.In evaporite basins, organic matter derived from surface waters is preserved beneath the pycnocline of a density‐stratification in anaerobic hypersaline bottom brines. Not being recycled, it is decomposed by bacteria, goes into solution or settles on the bottom. Montmorillonites, calcites, gypsum and potash deposits can alter organic matter catalytically. Impervious clays and micritic limestones trap large hydrocarbon molecules permanently; smaller molecules are flushed through a permeable substrate into aquifers and delivered to reservoirs. After burial, maturation equal to other hydrocarbons is achieved at a lower vitrinite index.

Oligocene deposits of the Zara-Hafik region (Sivas, Central Turkey): evolution from storm-influenced shelf to evaporitic basin

New stratigraphical and micropalaeontological data, supplemented by detailed sedimentological and petrographical observations, have been obtained from an almost completely exposed Oligocene-Miocene succession near Celalli in the Zara-Hafik basin, which forms part of a long-lived zone of subsidence associated with the North Anatolian Fault system. Four laterally extensive lithostratigraphical units have been distinguished and ostracod microfaunas indicate that the lower two units are of Middle Oligocene age, while bivalve/brachiopod faunas from the third unit are also Oligocene. The status of the uppermost unit of redbeds remains equivocal, but microfaunas from the capping marls are of lowermost Miocene age.

Depositional Environment of Lower Permian Stone Corral Formation: ABSTRACT

The Lower Permian Stone Corral Formation is a 5-8 ft thick unit of dolomitic limestone and shale in a sequence of red beds that crops out in south-central Kansas. It forms a good stratigraphic marker bed in the subsurface, where it consists mainly of anhydrite (the Cimmaron anhydrite) up to 100 ft thick. At outcrop, the formation is weathered and consists of 2 interfingering facies--a crinoidal-echinoidal grainstone (in the north) and a laminated mudstone (in the south). Both the grainstone and mudstone exhibit diagenetic alteration. The grainstone is composed of well-rounded allochems of crinoid and echinoid fragments with scattered ooids, sponge spicules, peloids, and possible foraminifers and ostracods. Layers of sponge spicules alternate with peloid or ooid layers. The mudstone facies has a clotted texture and has laminations defined by the occurrence of fenestral fabric and peloids. Dolomite occurring in the Stone Corral Formation is of secondary origin. The grainstone is interpreted as being lagoonal in origin, whereas the mudstone facies is interpreted as supratidal. The grumous (clotted) texture may indicate phreatic diagenesis. The absence of anhydrite on outcrop, where it was presumably leached by surface waters leaving the less soluble dolomite, is additional evidence for a regional disconformity above the unit on the eastern side of a large, shallow, evaporitic basin. End_of_Article - Last_Page 269------------

Stable-isotope mapping in Messinian evaporative carbonates of central Sicily

Mapping stable-isotope isolines in both modern and ancient evaporative carbonate formations demonstrates that regional variations of δ 18 O values exist. In the case of the Messinian Central Sicilian Basin, an ancient evaporite basin, the distribution pattern of the δ 18 O isolines can be related to the paleogeometry and paleohydrology of the basin. Isolines of equal δ 13 C values also show large regional variations possibly reflecting the domination of different bacterial processes (oxidation, sulfate reduction, or methanogenesis) in different regions. The potential application for stable-isotope mapping in evaporative carbonate formations deposited in both marine and lacustrine environments lies in its ability to predict the location of depositional centers in ancient evaporite basins.

Controls on Porosity and Dolomite Distribution in Upper Smackover Formation (Upper Jurassic), Southwestern Alabama and Western Florida: ABSTRACT

Upper Smackover carbonates of southwestern Alabama and western Florida exhibit arcuate trends of porous dolomitized grainstones separated by areas of impermeable muddy limestones. The origin of these porous trends is related to the depositional and diagenetic history of upper Smackover carbonates and overlying Buckner evaporites. Shoal-water oolitic and peloidal grainstones of the upper Smackover were deposited across basement and salt-related topographic highs. Subsequent aggradation and stepwise progradation of oolitic shoals over low-energy packstones and wackestones produced a complex sea-floor topography of arcuate oolite ridges (highs) and elliptical muddy lagoons (lows). Marine regression during Buckner deposition led to the formation of saline ponds and sabkhas th t were initially located over Smackover lagoonal lows and rimmed by Smackover oolite ridges. Precipitation of evaporites within these depressions created magnesium-rich brines that selectively dolomitized adjacent Smackover carbonates. Both the outflow of brine and the pattern of dolomitization were controlled by the fluid transmissibility of Smackover sediments. Consequently, permeable oolitic and peloidal grainstones were preferentially dolomitized over less-permeable muddy carbonates. A possible hydrologic analog for the study area exists within the MacLeod Evaporite basin, western Australia. Following burial, dolomites maintained greater effective porosity on the flanks of basement and salt-related topographic highs than on the crests. End_of_Article - Last_Page 274------------

Historical Geomorphology and Geoarchaeology in the Southwestern Makgadikgadi Basin, Botswana

Abstract Arid landscapes of the southwestern Makgadikgadi, Botswana, are dominated by sandy lacustrine plains, which are veneered by grassy stabilized dunes and small evaporite basins (pans). In addition, the exotic Boteti River delivers a nearly permanent supply of water to the region from the Angolan highlands via the Okavango marshes. The history of this unusually intricate landscape was investigated through regional geomorphological and geoarchaeological studies focused at two archaeological sites. The first site, Toromoja I, preserves residues of Later Stone Age settlement along the shore of a now-dry lake; a mid-Holocene age is suggested. At the second site, Gwi Pan West, a Later Stone Age burial was excavated. This burial appears to be only several hundred years old, and can be referred to a landscape much like the present. The spatial and environmental contexts of these archaeological residues can best be understood in terms of a dynamic historical geomorphology shaped by past climatic, tectonic, ...

Possible Salt Movement in Northern Wasatch Plateau: ABSTRACT

The Jump Creek 7 ½-minute quadrangle in the northeastern Wasatch Plateau is an area where Late Cretaceous to early Tertiary formations are fractured by at least two complex systems of faults. Southern and eastern sections of the area are dominated by north-south-trending faults, such as the Gordon Creek fault zone, whereas the northern section is dominated by northwest-southeast-trending faults. Strong evidence, including horst and graben structures, rim synclines, and other collapse-related features, suggests that north-south-trending faults may be related to salt movement within the underlying Jurassic sediments. Indeed, the Gordon Creek fault zone could represent part of the eastern boundary of the Arapian evaporite basin. The northwest-southeast-trending faults appear to parallel major lineaments that cut across the Wasatch Plateau, Book Cliffs, and San Rafael swell areas and suggest origins not related to salt tectonics. This fault system, however, may be part of these lineaments that have been accentuated by the salt movement. The possibility of the structures in the Jump Creek 7 ½-minute quadrangle producing excellent hydrocarbon traps cannot be overlooked. Structures similar to those found in this area are proven producers in Joes Valley and Pleasant Valley fields, both located within several miles of the Jump Creek quadrangle. End_of_Article - Last_Page 938------------

Aragonite Crusts and Pisolites Beneath Dolomitic Tepees, Lake MacLeod Evaporite Basin, Western Australia: ABSTRACT

Research currently being conducted by the Sedimentology and Marine Geology Group, under Brian W. Logan at the University of Western Australia, has recently concentrated on Lake MacLeod, a 2,000 km2 (770 mi2) coastal salina on the western coast of Australia. This work has shown that this evaporite basin, which is 3 to 4 m (10 to 13 ft) below sea level, is separated from the Indian Ocean by a topographic barrier, but seawater under hydrostatic head, seeps freely through the barrier and discharges from several vents and springs in a carbonate mud flat at the north end of the basin. From there, seawater flows slowly across the basin, evaporating and depositing carbonate, gypsum, and ephemeral halite. About 10 to 12 m (33 to 39 ft) of evaporites have been deposited in the past 5,300 years. In July 1982, the authors visited the carbonate mud flats and discovered abundant aragonite pisolites and botryoidal-mammillary crusts of fibrous aragonite cement beneath lily-pad tepee slabs of cemented protodolomite. This protodolomite host-rock is well-lithified, intraclast, peloid packstone with abundant coarse fenestrae. Thick aragonite crusts cover both the undersides of lily-pad slabs and the lithified floors of tepees. Crusts covering the floors are more botryoidal and consist of both aragonite nubs and mounds (0.2 to 2.5 cm, 0.08 to 1 in., in diameter), and a few scattered, loose pisolites, several millimeters in diameter. Pisolites are composed of multi-generation fibrous layers of square-tipped aragonite rays surrounding peloid-intraclast nuclei. Thus, it seems that per odic deposition of a fine layer of carbonate mud, peloids, and intraclasts across the floor of a tepee is a prerequisite to pisolite growth. Stable isotope analysis of the host rock and aragonite cements gave expected marine values (^dgr18O = +0.25 to +1.14 PDB and ^dgr13C = -0.18 to +0.16 PDB) and reflect precipitation from ground water (marine composition) discharging from seeps in the carbonate mud flats. The manner in which crusts, pisolites, and tepees occur at Lake MacLeod raises the possibility that they and their ancient counterparts from the Permian basin share a common origin. Perhaps Permian pisolites and aragonite crusts formed beneath cemented slabs of peritidal sediments in tepees bathed by marine water which seeped across exposed portions of the shelf crest. End_of_Article - Last_Page 478------------

Paleotectonic Control of Pennsylvanian Sedimentation in Paradox Basin: ABSTRACT

The Paradox basin of the east-central Colorado Plateau province is an elongate rhombic evaporite basin of Middle Pennsylvanian age. It is bounded on the northeast by the Uncompahgre and San Luis segments of the Ancestral Rockies. The northwest-trending basin sagged along preexisting basement rifts by strong east-west extension during Desmoinesian time. The dominant zone of weakness was the northwesterly Olympic-Wichita basement lineament that lies along the eastern margin of the salt basin and the southwestern front of the Uncompahgre and San Luis uplifts. Less prominent northwest and northeast shear zones are ubiquitous, but are especially well developed in basement and Paleozoic rocks underlying the San Juan basin and the southwest shelf of the Paradox basin. It is a cl ssic pull-apart basin developed along anastomosing wrench faults which first developed at about 1,700 m.y.B.P., and were rejuvenated repeatedly throughout the Paleozoic. Initial subsidence and associated sedimentation occurred in the southeastern divergent termination of the basin in Atokan and early Desmoinesian time, when the Grenadier and Sneffels horst blocks were elevated to form the San Luis uplift. The quartzite-dominated horsts shed large volumes of relatively nonfeldspathic quartzose sediments into shallow seaways of northern New Mexico and southwestern Colorado. By mid-Desmoinesian time, the true Uncompahgre uplift was elevated to the north, and shed vast accumulations of arkose into the eastern margins of the basin. Meanwhile, cyclic evaporites, dominated by salt, accumulated in the structurally restricted basin. The southwestern shallow shelf of the basin developed along a broad zone of basement rifting, extending across the northern San J an basin, the Four Corners platform, and northwest to beyond the Henry Mountains. The Four Corners lineament served to terminate salt deposition along the structural shelf and create local shoaling conditions to host myriad algal bioherms that would become prolific petroleum reservoirs. These fields include the giant Aneth field and the recently developed Bug field complex. Unlike the dramatic events along End_Page 1329------------------------------ the eastern Paradox basin, tectonic involvement of carbonate sedimentation on the southwest shelf was very subtle. Sea-floor topography of a few feet over deep-seated faults would have been sufficient to localize biohermal development. Subtle tectonically induced topography on the present Monument upwarp and San Rafael swell limited sedimentation along the western basin margin. The northwesterly rift fabric of the basin is transected by the conjugate northeast-trending basement fractures of the Colorado lineament. The most important of these underlies the length of the Colorado River, and terminates or offsets the major salt diapirs of the eastern basin. Northwest of the structure, the Paradox basin becomes compressional rather than extensional as to the south, due to convergent termination of the pull-apart basin; and marine sedimentation becomes rapidly limited to the narrow Oquirrh sag between the Emery and Uncompahgre uplifts. Also, the large influx of arkose from the Uncompahgre becomes much younger (Early Permian). Every aspect of sedimentation (clastic, evaporite, and carbonate) in the Paradox basin was greatly influenced by contemporaneous rejuvenati n of the basement tectonic fabric. End_of_Article - Last_Page 1330------------

A Synthesis of Marine Geological/Geophysical Data for Ocean Margin Drilling on Continental Margin of Morocco: ABSTRACT

The continental margin of Morocco represents the conjugate segment to the North American margin south of Nova Scotia. The total sedimentary cover in the continental margin province is considerably thinner off Morocco than it is off the east coast of North America. Further, selected portions of the Morocco continental margin have undergone major tectonics, either exposing or bringing near the surface older elements of the stratigraphic section. Thus, scientific objectives requiring sampling of the earliest drift phase sediments, the oldest oceanic crust, and the basement rocks in the ocean-continental transition zone can be more easily addressed off Morocco than off North America. We have recently compiled a synthesis of marine geological/geophysical data for the offshore regions bordering the continental margin of Morocco. The synthesis was undertaken in order to evaluate future scientific deep-sea drill sites. The final product of the synthesis is a set of maps on Mercator projection for the area 29° to 35°N, 5° to 22°W. The maps include: bathymetry contours, free-air gravity anomaly contours, total intensity magnetic anomalies plotted normal to the ship's tracks, seismic velocity crustal structure, reflection time to basement, blue reflector (upper Jurassic) and red reflector (upper Cretaceous), isopach (in seconds of two-way reflection time) of sea floor to basement, sea floor to blue reflector, sea floor to red reflector, and blue refle tor to red reflector, locations of surface sediment samples, hazards, and tectonics. In addition, seismic crustal sections from the nearshore across the disturbed piercement structure province to the deep ocean floor have been compiled. Our synthesis reveals that the continental margin of Morocco consists of several unique segments. A dominant structural unit is the NNE-SSW-trending zone of salt diapirs and piercement structures observed on the continental shelf and slope. The salt is presumed to be of Late Triassic to Early Jurassic age. A prominent magnetic anomaly which may represent a magnetic edge effect anomaly can be traced along most of the western boundary of the Moroccan diapiric zone. On the conjugate continental margin south of Nova Scotia, a diapir zone is also observed. The seaward edges of the Moroccan and eastern North American diapiric zones match very well in pre-drift reconstructions, thus suggesting that before the separation of the African and North American plates a single evaporite basin existe . The magnetic and seismic data suggest that the seaward boundaries of the piercement fields mark the likely boundary between oceanic and continental basement. The westernmost portion of the synthesis area is characterized by the well-defined Mesozoic sequence of magnetic anomalies from M0 (^cong 108 m.y.B.P.) to M25 (^cong 152 m.y.B.P.). These anomalies are situated on seismically observed oceanic crust formed by sea-floor-spreading processes. Between the seaward edge of the diapiric zone and magnetic anomaly M25 we observe a broad structural arch beneath which our single-channel seismic records in general fail to resolve the underlying basement. This is the region of the Jurassic magnetic quiet zone. End_of_Article - Last_Page 537------------

Former Evaporites Within English Zechstein First Cycle Carbonates: ABSTRACT

The Upper Permian Cadeby Formation was deposited on the western margin of the Zechstein evaporite basin. The formation comprises two carbonate members which were nearly entirely dolomitized during early diagenesis. Recent meteoric dissolution in the shallow subsurface precludes the presence of evaporites within the carbonates at outcrop, but both direct and indirect evidence indicates the existence of former displacive and replacive evaporites. Direct evidence of former evaporites is present in the form of both pseudomorphs and casts. In the lower member, textural preservation of internal fabrics within some anhydrite nodules was produced by calcitization of both anhydrite cleavage flakes and possible small chevron halite crystals . Where no replacement preceded dissolution, cauliflower-shaped molds 2 to 3 cm (.78 to 1.18 in.) in diameter evidence the former presence of anhydrite nodules in lower member lagoon wackestones and mudstones. Anhydrite nodule development distorted bedding laminae, indicating displacive growth within soft sediments. Halite hopper casts surround some former nodules. Lack of evidence of subaerial exposure or intertidal sedimentation suggests both sediment deposition and anhydrite nodule growth were s baqueous; evaporites forming when lagoon waters became restricted and progressively hypersaline. Further direct evidence of interbedded evaporites is shown by fabrics of vertically oriented calcite crystals with scattered pseudomorphs of gypsum crystal margins. Ghosts of foraminifera, only visible under cathodoluminescence, and numerous dolomite inclusions are present within the calcite. The vertical orientation suggests relict gypsum precipitates on the sediment surface. These calcitized gypsum horizons are underlain by skeletal lime wackestones and packstones and overlain by irregular dolomite mudstones with casts of displacive gypsum rosettes, indicating subaqueous evaporite growth within sediment during periodic hypersaline conditions. In the upper member, indirect evidence of former bedded evaporites overlying the Cadeby Formation is provided by an irregular bed of dedolomite (calcitized dolomite). The dedolomite transects facies boundaries, being found in supratidal carbonates with incipient tepee structures, intertidal and subtidal cryptalgal laminate boundstones, and ooid grainstone shoals. Dedolomite thickness varies from 0.5 to 2.5 m (1.5 to 8 ft) within individual outcrops. Dedolomitization was due to the action of fluids with high Ca2+/Mg2+ ratios, created by dissolution of the overlying Hayton Evaporite Formation, with dedolomite thickness dependent on local differences in the permeability of the preexisting dolomites. Petrographically, these dedolomites contrast with ferroan dedolomit s elsewhere in the Cadeby Formation and not spatially associated with evaporites. The presence of former replacive evaporites is shown by irregular vugs which crosscut sedimentary laminations. Such vugs are more numerous in the upper member. Petrography indicates this replacement occurred before final compaction of the carbonates and was possibly related to diagenetic changes within the overlying anhydrite formation. End_of_Article - Last_Page 480------------

Mineralogy of clay marker seams in some Saskatchewan potash mines

Water-insoluble material is present mainly as thin stratigraphic layers throughout the potash ore zone(s) of the Prairie Evaporite. These clay seams constitute about 6% of the ore as mined. After clay minerals, which make up about one third of the total, the main water-insoluble constituents, in approximate order of decreasing abundance, are anhydrite, dolomite, hematite, quartz, potassium feldspar, hydrocarbon, and sporadic non-diagnostic palynomorphs.Clay mineralogy in the following mines has been studied: Cominco (Vanscoy); Central Canada Potash Co. Ltd. (Colonsay); and Potash Corporation of Saskatchewan (Allan and Lanigan). A total of 49 samples has been examined. In each sample, following concentration of clay minerals by centrifuging, X-ray diffractograms were obtained for untreated, glycolated, and heat-treated material (300, 450, 580, 650, and 725 °C). Additional runs were made on several samples under conditions of controlled humidity.The main clay minerals are Fe–Mg chlorite (14 Å (1.4 nm)), illite, and Mg-septechlorite (7 Å (0.7 nm)). Of the two chlorites, septechlorite is the more thermally stable. One or more of sepiolite, smectite, mixed layer (chlorite–smectite), and possibly traces of vermiculite are also present in some samples. The septechlorite, sepiolite, and vermiculite very likely originated as direct products of evaporation under hypersaline conditions, or are the result of diagenesis. Absence of otherwise ubiquitous septechlorite in a sample from Second Red Beds west of the 0-edge of the evaporite basin supports this concept. The proportions and kinds of clay minerals present in the ore zone(s) seem to reflect the extent to which hypersaline conditions were developed. The illite and 14 Å (1.4 nm) chlorite are of regional (detrital) origin.

A long geoclimatic record from the Permian

A 260,000‐year, continuous, annual geoclimatic time series has been compiled from the varved Permian Castile and Bell Canyon Formations of the Delaware Basin, southeastern New Mexico, and southwestern Texas. The last 200,000 years of the record was obtained from calcite‐laminated anhydrite and anhydrite‐laminated halite, which formed as a result of seasonal and annual deposition in the evaporite basin. The time series is based on measured thickness of individual varves. Analyses of calcium sulfate, calcium carbonate, and organic matter were made on 50‐year intervals in the evaporite part of the sequence. Time series of the major components were examined by using smoothed graphic plots, variance spectra, and moving correlation coefficients. The longest oscillation recorded in the series that may be of climatic origin has a period of about 100,000 years. The calcium sulfate record contains 9–11 distinct oscillations with an average period of about 20,000 years. The strongest oscillation has a preferred spectral period of 2700 years and is recorded throughout the series as thickness changes in calcium sulfate, as events controlling halite deposition, and as episodes of basin freshening. A broad spectral response near a period of about 200 years reflects changes in sulfate, carbonate, and halite thickness. Shorter periods do not have consistent spectra. The time series contains no preferred periods that have not already been identified in previously described records. The climatic changes appear to be the result of a deterministic response to orbital effects at the longer periods and stochastic processes at the shorter periods. An unusually strong response near 2700 years is associated with episodic freshening of the basin. This period has been well established in the Holocene paleoclimatic record, and its presence in the Permian suggests a deterministic origin.

Gypsum Precipitation by Mixing Seawater Brines

Experiments indicate that gypsum can be precipitated by the mixing of seawater brines of different stages of evaporation, in addition to the previously recognized mechanisms of direct evaporative crystallization and crystallization due to temperature changes. A proposed geologic model shows how gypsum precipitation by brine mixing might occur in an evaporite basin. Conclusions based on the experiments and their relations to the geologic model are as follows. 1. Precipitation of gypsum can occur in a marine evaporite basin by mixing brines of different composition and specific gravity. 2. Precipitation occurs without further water loss by evaporation. 3. Precipitation can occur from a brine that was undersaturated before mixing. 4. The only form of calcium sulfate to precipitate in these experiments was gypsum. 5. Brine mixing could cause different salts to be deposited simultaneously in different parts of a basin depending on the stage of the evaporite cycle.

Inferred Paleosalinity and Phosphate Content of Carbonate Rocks from a Cyclic Evaporite-Carbonate Rock Sequence

ABSTRACT Covariance of phosphate content of carbonate rocks and inferred paleosalinity was studied from a core interval characterized by cyclic carbonate rock alternating with anhydrite. Phosphate content of limestone was found to have an inverse relationship to inferred salinity. Inferred paleosalinity was based upon the following: fossil sequences within the carbonates, containing Texagryphaea, established normal marine salinities; anhydrite (originally gypsum) and thin-bedded laminated limestone and dolomite indicated a saline environment. A total of thirteen cyclic carbonate rock alternations with anhydrite were studied. The core, from the Lower McKnight Formation, Comanchean (Cretaceous), South Texas, records periodic influxes of seawater into the marginal evaporitic McKnight basin where dilution of the brine induced an increase in pH. This enhanced the dissociation of HPO4-2 to PO4-3 resulting in elevation of phosphate uptake by calcium carbonate and an increase in total phosphate measured in limestone.

Geometry of Tertiary Coal Seams as a Response to Changes in Structural Framework: ABSTRACT

Diminished subsidence of the Williston basin at the end of the Cretaceous was followed by accumulation of thick lignite seams in the Ravenscrag Formation (Paleocene). The formation of seven coal fields in southern Saskatchewan was also influenced by contemporaneous subsidence structures that originated as collapse of overlying strata above areas of salt leaching from the buried Devonian Elk Point evaporite basin. Lignite seams, up to 10 m thick, accumulated in successively younger coal fields that prograded to the southeast across southern Saskatchewan toward the cratonic depocenter in western North Dakota. The shape of the older coal basins conformed to the areas of salt solution activity and both the coal fields and component beds accumulated as circular forms. Salt-sol tion collapse effects diminished upsection with more pronounced cratonic influence that originated with intermittent movements between Precambrian crustal blocks forming the Williston basin. The thicker areas of coal seams in the middle of the 300-m coal-bearing section accumulated parallel with the cratonic lineaments but oblique to the coal-basin forms that retained a geometric relation to the salt solution. The uppermost coal basins were almost completely dominated by cratonic influence with thick coal bed areas parallel with cratonic lineaments and dispersed in a reticulate pattern. Younger coal fields distant from the cratonic depocenter were elongated parallel with lineaments oriented toward the cratonic depocenter, whereas coal fields next to the center of the Williston basin accu ulated as arcuate forms along the strike of the basin. The activity of salt-solution subsidence structures during the formation of the uppermost coal fields was sufficiently weak to favor greater regional bed thickness but not affect lateral basin form. End_of_Article - Last_Page 553------------

Subduction in the Hellenic Trench: probable role of a thick evaporitic layer based on Seabeam and submersible studies

Summary A Seabeam survey of the Hellenic Trench in 1978 was followed by a submersible survey in 1980. Fifteen dives were completed between 1500 and 3000 m covering 58 km on the bottom and obtaining 48 samples. Thus, the Hellenic Trench is probably now the most intensively studied trench as previous work included seismic reflection, dredging, coring, bottom photography, side-scan sonar and drilling. Because of the high sedimentation rate and the relatively shallow depth, we did not expect outcrops except along faults and in canyons. Active tectonics with numerous faults and folds have been observed. But we have also discovered vertical cliffs with hard rock exposure over a depth range of 1000–1500 m. In this paper, we discuss the nature of the hard rock outcrops observed both on the inner wall and on two hills that we interpret as diapirs. We conclude that at least part of these hard rocks are probably Messinian evaporites and concentrate on the tectonic role of this evaporitic layer in the Hellenic subduction. It is likely that the evaporites do not subduct but tend to pile up in the trench, to form a floating evaporitic basin.