Gypsum Pseudomorphs Research Articles

Dolomitization by penesaline sea water in Early Jurassic peritidal platform carbonates, Gibraltar, western Mediterranean

Peritidal carbonates of the Lower Jurassic (Liassic) Gibraltar Limestone Formation, which form the main mass of the Rock of Gibraltar, are replaced by fine and medium crystalline dolomites. Replacement occurs as massive bedded or laminated dolomites in the lower 100 m of an ≈460‐m‐thick platform succession. The fine crystalline dolomite has δ18Ο values either similar to, or slightly higher than, those expected from Early Jurassic marine dolomite, and δ13C values together with 87Sr/86Sr ratios that overlap with sea‐water values for that time, indicating that the dolomitizing fluid was Early Jurassic sea water. Absence of massive evaporitic minerals and/or evaporite solution‐collapse breccias in these carbonate rocks indicates that the salinity of sea water during dolomitization was below that of gypsum precipitation. The occurrence of peritidal facies, a restricted microbiota and rare gypsum pseudomorphs are also consistent with penesaline conditions (salinity 72–199‰). The medium crystalline dolomite has some δ18Ο and δ13C values and 87Sr/86Sr ratios similar to those of Early Jurassic marine dolomites, which indicates that ambient sea water was again a likely dolomitizing fluid. However, the spread of δ18Ο, δ13C and 87Sr/86Sr values indicates that dolomitization occurred at slightly increased temperatures as a result of shallow (≈500 m) burial or that dolomitization was multistage. These data support the hypothesis that penesaline sea water can produce massive dolomitization in thick peritidal carbonates in the absence of evaporite precipitation. Taking earlier models into consideration, it appears that replacement dolomites can be produced by sea water or modified sea water with a wide range of salinities (normal, penesaline to hypersaline), provided that there is a driving mechanism for fluid migration. The Gibraltar dolomites confirm other reports of significant Early Jurassic dolomitization in the western Tethys carbonate platforms.

Pedosedimentary and climatic reconstruction of the last interglacial and early glacial loess–paleosol sequence in South Tadzhikistan

The thick loess–paleosol-sequences in Tadzhikistan, which indirectly record the environmental changes from the Upper Pliocene to the present-day, have been extensively described from a chronostratigraphical and archaeological point of view. The genesis of the soil-sedimentary complexes and their implications for palaeoenvironmental reconstruction in Tadzhikistan, however, have been considered only relatively recently [Bronger, A., Winter, R., Derevjanko, O., Aldag, S., 1993. Loess–paleosol sequences in Tadjikistan as a palaeoclimatic record of the Quaternary in Central Asia. Quaternary Proceedings 4, 69–81; Lomov, S.P., Tursina, T.V., Zhou, L.P., 1996. Magnetic susceptibility of soils: Micromorphology and possible reconstructions of climate. In: Soil micromorphology. Studies in Soil Diversity, Diagnostics, Dynamics, 10th International Working Meeting on Soil Micromorphology. Moscow, Russia, July 8–13, 1996, p. 145]. The pedocomplex of Last Interglacial and Early Glacial age (Oxygen Isotope Stage 5) in the Tagidjar and Darai Kalon sections near the town of Khovaling is represented by three major stages of soil formation. The soil characteristics have been micromorphologically analysed. The distinction between primary and secondary carbonates is of major importance to understand the soil forming processes. The relative distribution of needle fibre calcite (NFC), micritic infillings (M), hypocoatings (CHC), coarse calcitic grains (CCG) and calcified root cells (CRC) provides detailed insights into the environmental conditions prior, during and following pedogenesis. The abundance of coprolitic aggregates (CA) and pupal chambers gives a clear indication of the rate of biological activity which prevailed during the last two soil forming stages. Void pseudomorphs after gypsum crystals ( gypsum pseudomorphs) are frequently observed beneath the Cca horizons of the paleosols. Using the present-day climate and soilscape as a reference base, the obtained pedosedimentary events detected in the Tadzhikistan sections provide a detailed palaeoenvironmental record. Indeed, glacial and interglacial climates and soils respond to contemporaneous climatic schemes and soil forming processes, which are still active in the region today. The amount and the seasonal variability of precipitation, mostly, are at the origin of the variable soils formed during glacial and interglacial ages.

Small scale secondary CaCO 3 accumulations in selected sections of the European loess belt. Morphological forms and potential for paleoenvironmental reconstruction

Eighteen important Quaternary loess paleosoil sites have been studied across the European loess belt. They included approximately 50 buried soils, the over and/or underlying loess and the present-day surface soils (where possible). From the numerous types of secondary CaCO 3 accumulations recognized, only the small scale accumulations are discussed in this paper, including calcified root cells, CaCO 3 hypocoatings, needle-fiber calcite, powdery calcite coatings, earthworm biospheroliths, and calcite pseudomorphs on gypsum. Most of the features studied are found in the rhizosphere microecosystem. Each of these features showed a unique relationship with organic matter, micro-organism and plant distribution, moisture and leaching processes. For example, the exceptional abundance of calcified root cells is an indicator of a long stability of soil surface during favorable climatic conditions. Needle-fiber calcite is related to biomineralization of saprophytes in soils with sufficient moisture and decomposing organic matter. The presence of powdery calcite coatings covering clay and Mn coatings in non-calcareous soils indicates a change from leaching to non-leaching soil moisture regimes. Earthworm biospheroids provide arguments for a relative stability of the soil surface, without rapid burial processes or drastic, prolonged water stress. Finally, gypsum pseudomorphs are indicators of changes in local climatic parameters. In addition to a discussion of the genesis and the dynamics of the various accumulations, several examples are given to show how these accumulations can be used as indicators of a changing environment, such as renewed loess deposition, the extent of drought or varying intensity of leaching during the year. To conclude, various forms of small scale secondary CaCO 3 accumulations are common in the loess paleosoils and each type has the potential to contribute to a better understanding of the evolution of the paleoenvironment. Therefore, it is proposed that their individual presence or absence throughout the pedostratigraphical levels and throughout the (paleo)topography should be recorded at all scales of observation.

Latest Cretaceous (Maastrichtian) aridity indicated by paleosols in the McRae Formation, south-central New Mexico

The McRae Formation in south-central New Mexico contains paleosols that indicate a significant increase in aridity occurred during the latest Cretaceous (late Maastrichtian). The McRae Formation is approximately 420 m thick and consists of interbedded fluvial channel sandstone and conglomerate, floodplain mudstone and fine sandstone, and siliceous ash-fall tuff. It has been divided into a lower Jose Creek Member and an upper Hall Lake Member. A Lancian dinosaur fauna is present in both members, indicating a late Maastrichtian age for the McRae Formation. Jose Creek paleosols are classified as argillisols and probably formed under humid to subhumid conditions. Paleosols in the Jose Creek member consist of A, ±E, and Bt horizons and are associated with numerous in situ petrified stumps, the largest of which is 1.7 m in diameter. The argillic B horizons (Bt) contain vertical clay-filled root traces, rhizoliths, blocky peds, embedded-grain argillans, and ped argillans. In contrast, paleosols in the Hall Lake Member are calcisols and vertic calcisols. These paleosols are characterized by Bw, Bt, Btk, Bk, and K horizons. The calcic horizons contain vertical calcite-filled tubules and carbonate nodules, in addition to calcans, spar coats, brecciated zones, alveolar-septal fabric and gypsum pseudomorphs. The vertic calcisols contain wedge-shaped peds and slickensides. The presence of calcic horizons in the Hall Lake Member suggest that the paleoclimate was significantly drier than during deposition of the Jose Creek Member. In addition, the vertic features indicate periods of wetting and desiccation (seasonality of precipitation?) during Hall Lake Time. Late Maastrichtian aridity, based on paleosols, has been interpreted for the Big Bend region of Texas, central Utah, and for Alberta, Canada, suggesting that the increase in aridity found in south-central New Mexico occurred throughout the Western Interior of North America.

The East Kirkton Limestone, Viséan, of West Lothian, Scotland: introduction and stratigraphy

ABSTRACTA brief history of research into the unusual laminated and spherulitic East Kirkton Limestone (Brigantian, Viséan) is given. It documents the paucity of fossil finds until 1984, when S. P. Wood discovered a previously unsuspected terrestrial biota. Some early finds, such as stromatolites, have since been overlooked, while others were misinterpreted, e.g. a ‘fish-bone’ collected before 1870 proves to be amphibian. A formal lithostratigraphic description is given of the three beds that comprise the c. 15 m sequence in East Kirkton Quarry: the East Kirkton Limestone, overlain by Little Cliff Shale and Geikie Tuff, the latter two being here formally named. Lithological descriptions and a unit-by-unit distribution of major elements of the biota are provided for a measured section comprising 88 units. This is based on bed-by-bed excavations carried out 1985-92, supplemented by nine boreholes. Two 3 m-thick massive limestones lenses are recorded from the quarry and a probable third was proved in a borehole. Brief reports are given on the ‘stromatolites’ (by M. R. Walter), bivalve molluscs (R. M. C. Eagar and R. B. Wilson), ostracods (J. Pollard) and gypsum pseudomorphs.

Tourmaline mineralization in the Barberton greenstone belt, South Africa: early Archean metasomatism by evaporite-derived boron.

Tourmaline-rich rocks are common in the low-grade, interior portions of the Barberton greenstone belt of South Africa, where shallow-marine sediments and underlying altered basaltic and komatiitic lavas contain up to 50% tourmaline. The presence of tourmaline-bearing rip-up clasts, intraformational tourmaline pebbles and tourmaline-coated grains indicate that boron mineralization was a low-temperature, surficial process. The association of these lithologies with stromatolites, evaporites, and shallow-water sedimentary structures and the virtual absence of tourmaline in correlative deep-water facies rocks in the greenstone belt strengthens this model. Five tourmaline-bearing lithologic groups (basalts, komatiites, evaporite-bearing sediments, stromatolitic sediments, and quartz veins) are distinguished based on field, petrographic, and geochemical criteria. Individual tourmaline crystals within these lithologies show internal chemical and textural variations that reflect continued growth through intervals of change in bulk-rock and fluid composition accompanying one or more metasomatic events. Large single-crystal variations exist in Fe/Mg, Al/Fe, and alkali-site vacancies. A wide range in tourmaline composition exists in rocks altered from similar protoliths, but tourmalines in sediments and lavas have similar compositional variations. Boron-isotope analyses of the tourmalines suggest that the boron enrichment in these rocks has a major marine evaporitic component. Sediments with gypsum pseudomorphs and lavas altered at low temperatures by shallow-level brines have the highest delta 11B values (+2.2 to -1.9%); lower delta 11B values of late quartz veins (-3.7 to -5.7%) reflect intermediate temperature, hydrothermal remobilization of evaporitic boron. The delta 11B values of tourmaline-rich stromatolitic sediments (-9.8 and -10.5%) are consistent with two-stage boron enrichment, in which earlier marine evaporitic boron was hydrothermally remobilized and vented in shallow-marine or subaerial sites, mineralizing algal stromatolites. The stromatolite-forming algae preferentially may have lived near the sites of hydrothermal discharge in Archean times.

Depositional and Diagenetic Controls on Porosity Permeability and Oil Production in McFarland/Magutex (Queen) Reservoirs, Andrews County, West Texas

The McFarland/Magutex Queen reservoir complex lies along the northeastern edge of the Central basin platform in the west Texas Permian basin and produces oil from the Permian Queen Formation. Current production from this complex totals 42 million stock-tank barrels (MMSTB) of an estimated 219 MMSTB of original oil in place, with an estimated 90 MMSTB of remaining mobile oil (RMO). The gross pay interval contains two parasequences consisting of progradational, 30-ft-thick, upward-shoaling facies packages. Facies include shoreface, mixed tidal channel and intertidal flat, and supratidal. Elongate shoreface facies are characterized by poorly consolidated, massive to thinly laminated sandstones. The supratidal facies, which act as permeability barriers, are characterized by algal-laminated dolostone and nodular, laminated, and massive anhydrite containing halite and gypsum pseudomorphs. Highest production and the largest amount of the 90 MMSTB of RMO is associated with the shoreface and tidal-channel facies. Bulk pore volume storage capacity and permeability are also highest within these two facies. Sandstones are arkosic, containing anhydrite and dolomite cements. Accessory minerals are clays, authigenic feldspar, and dolomite. Three main pore types are recognized: interparticle, moldic and intraconstituent, and micropores. Moldic and intraconstituent porosity is associated with leached feldspars and anhydrite cement dissolution. Microporosity is associatedmore » with syndepositional, grain-coating corrensite, dissolution-enhanced feldspar cleavage planes, and authigenic multifaceted dolomite. Microporosity derived from clays and dolomite is formed preferentially in tidal-channel and intertidal flat facies.« less

Sulphide-mineralised algal breccias in a Devonian evaporitic lake system, Orkney, Scotland

Algal breccias in the Middle Devonian of Orkney, Scotland, are mineralised by sphalerite, galena, and minor celestite, and barite. The algal sediment is dolomicritic, having an internal thrombolitic texture and micrite-walled tubules referable to Girvanella. In some samples, the thrombolitic texture is replaced by sulphide minerals and sulphides are attached to organic tissues. Circumstantial evidence suggests that algal sediment helped to localise sulphide mineralisation, possibly by the reductive nature of its organic content. The mineralisation was later remobilised into vein deposits. The mineralised algal breccias are concentrated within erosive runnels that appear to be aligned normal to the palaeoshoreline of a Devonian lacustrine system. Widespread gypsum pseudomorphs in the lake sediments show that sulphate was readily available for reduction to sulphides.

Saline lake carbonates within an Upper Jurassic‐Lower Cretaceous continental red bed sequence in the Atacama region of northern Chile

ABSTRACTThe Codocedo Limestone Member is a thin but laterally persistent lacustrine sequence within the red beds of the Upper Jurassic‐Lower Cretaceous Quebrada Monardes Formation, in the Atacama region of northern Chile. The thick succession of clastic terrigenous sediments of the Quebrada Monardes Formation was deposited in an arid to semi‐arid environment. Sedimentary facies are indicative of deposition of aeolian dunes, alluvial fans and braided streams, playa‐lake mudflats, and saline lakes and coastal lagoons. The strata accumulated in a N‐S elongated extensional back‐arc basin on the landward side of an active volcanic arc.The 3 m thick Codocedo Limestone Member marks striking facies changes within the Quebrada Monardes Formation. It is underlain by a thick sequence of conglomerates and sandstones, deposited on alluvial fans. The limestone itself is characterized by evaporite minerals and laterally continuous laminations, indicative of deposition by vertical accretion in a perennial saline lake. The overlying siltstones and fine sandstones contain geodes and gypsum pseudomorphs and were deposited on playa‐lake mudflats. The limestone therefore represents a relatively short period of lacustrine deposition within an essentially terrigenous succession. The lake was possibly formed quite suddenly, for example by damming of the basin by a lava flow.Sedimentation in the perennial lake was predominantly cyclical. Seasonal planktonic algal blooms produced millimetre‐scale laminations. Interbedded with these laminites are centimetre‐scale beds of evaporitic gypsum, anhydrite and minor halite. The evaporite minerals have been largely replaced by calcite, chalcedony and quartz. The centimetre‐scale cycles may have resulted from periodic freshwater input into the lake. After a period of about 3000 yr the lake dried up, to be replaced by extensive playa‐lake mudflats.The Codocedo Limestone Member possibly formed a plane of detachment during an early Tertiary phase of E‐W directed regional compression. The limestones and evaporites were folded and extensively brecciated. This deformation probably resulted from simple shear along the bedding plane of the relatively weak evaporite minerals prior to their replacement by calcite and quartz.

Sedimentology of part of the Lower Oil-Shale Group (Dinantian) sequence at Granton, Edinburgh, including the Granton “shrimp-bed”

ABSTRACTThe Dinantian Lower Oil-Shale Group of the Edinburgh area records the evolution of a SW–NE trending basin within an area of fluvial-dominated deltaic sedimentation, with rare marine incursions from the E. The sequence at Granton records the abandonment of a delta lobe, with deltaic sandstones overlain by lagoonal mud-shales. These thinly-laminated non-bioturbated mud-shales record low-energy, stagnant conditions. Fish, ostracodes and conchostracans inhabited the lagoon at this time.Within the mud-shale sequence is the dolomitic Granton “shrimp-bed” which contains fenestrae, possible gypsum pseudomorphs andin situshrinkage breccias suggesting periods of emergence. The “shrimp-bed” contains a number of laminae packed with soft-bodied fossils including marine forms, which resulted from brief marine incursions into the stagnant lagoon. The fluctuating salinity conditions seem to have favoured the development of an unusual fauna of shrimp-like crustaceans, fish, conchostracans, ostracodes and bellerophontid gastropods. This fauna appears to have suffered periodic mass mortalities during marine incursions which brought orthocone cephalopods, polychaete worms and conodonts into the lagoon, according to our current knowledge of the distribution of these marine taxa in the sequence.The Granton sequence displays a variety of ductile and brittle deformation features most of which can be attributed to local SW-directed extension related to regional dextral strike-slip across the Midland Valley of Scotland.

Sedimentación lacustre-palustre, formación de ópalos y otras silicificaciones en el Mioceno medio al sur de Villaluenga

The petrological and sedimentological analysis of Middle Miocene carbonate sediments and siliceous rocks , outcropping North Toledo, provides new ideas on the interpretation of these lacustrine and paludine sedimentation. The main lithofacies found are: 1) micrite with crytalgal (?) laminite, caleite gypsum pseudomorphs, and trace of diatoms; 2) biomicrite with varying amount of lacustrine mollusks, ostracodes and charophytes, which form laminar structure; 3) biomicrite with the same skeletal components (charopytes, ostracodes, etc .) and sometimes diatoms but disorganized, 4) pseudospar and microspar calcite with lenticular gypsum pseudomorphs; 5) sepiolitic marls. The sedimentation environment was very shallow, with low salinity, calcareous, highly productive with abundant vegetation (sometimes diatoms) and periodic subaerial exposures. All type of sediments (paludine or lacustrine) have silicifications although they are different. An early discontinuos diagenetic silicification process, which forms only silica minerals (mainly opal C-T) and where the silica probably comes from the diatoms dissolution, has been observed. Also early in the diagenesis, but later than the former, a silicification process takes place, originating the continuous opal C-T and clay minerals (mainly sepiolite) horizonts, which can be homogeneous or present Liesegang bands (rings and laminations). This second silicification seems to happen in the interface water-sediment, during periods of evaporation, when the sha1low lake is near disecation. The silica cements in the different porosities of the rock (fenestral karst dissolution,…etc.) indicate that after the second silification, there is a remobilization of the primary silica, which sometimes produce other silicifications of quartz and silica cementations.

Dedolomitization and Calcitization of Gypsum in Mississippian Arroyo Penasco Group, North-Central New Mexico: ABSTRACT

The Espiritu Santo Formation of the Mississippian Arroyo Penasco Group represents the oldest Paleozoic stratigraphic unit preserved in north-central New Mexico. The Espiritu Santo Formation is a diagenetically complex carbonate unit that exhibits a well-developed cement stratigraphy reflecting changes from meteoric fresh to marine-phreatic environments. Recrystallization of the algal-laminated sediments occurred during subaerial exposure of the overlying Macho Member of the Tererro Formation, a collapse breccia produced by the dissolution and removal of gypsum. The breccia and recrystallized limestone are indicative of broad, low-relief topography and shallow water table. Cathodolumninescent petrography reveals the presence of pseudomorphs of dolomite and gypsum throughout the Espiritu Santo carbonates. Typical dedolomite morphologies are: inclusion-rich cores surrounded by limpid rims; corroded Mn-rich rhombs within calcite pseudospar; highly zoned rhombs; and uniformly luminescent rhombs enclosed in gypsum pseudomorphs. Calcitized gypsum, occurring as bladed to hexagonal crystals and nodules, varies from highly zoned to uniformly luminescent crystals. The varying luminosity is a possible relict of the original trace-element distribution and/or the diagenetic environment. Meteoric waters migrating from the Macho Member were enriched, but undersaturated, in dissolved CaSO4 and have low Mg/Ca ratio. Thus these pore fluids within the Espiritu Santo carbonates dissolved gypsum and dolomite. The solution, supersaturated with respect to CaCO3, precipitated calcite. Therefore, the dissolution of gypsum and dolomite and the precipitation of calcite occurred simultaneously during diagenesis. The reaction terminated once the supply of gypsum was exhausted. End_of_Article - Last_Page 536------------

Marginal Marine Evaporites, Lower Cretaceous of Arkansas: ABSTRACT

The mixed evaporite/carbonate/terrigenous clastic sediments of the DeQueen formation, in southwestern Arkansas, were deposited at the End_Page 1467------------------------------ landward margin of a broad shallow lagoon formed behind the Glen Rose reef. About 60% of the sedimentary volume consists of mudstone, silt, and sand, with brackish-water to hypersaline ostracod faunas believed to result from influx of flood waters from the Ouachita highlands a few miles to the north. The lower part of the formation contains discontinuous beds of gypsum, ranging in thickness from a few centimeters to composite beds > 3 m (10 ft), and displaying mosaic structure with vertically oriented, elongate nodules. These beds, which are lenticular, are interpreted to result from subaqueous precipitation of vertical selenite crystals (subsequently recrystallized) in discrete ponds and pools on microtidal-range mud flats. Intrastratal growth of gypsum nodules and displacive hali e occurred at the margins of the pools. The upper part of the formation contains no gypsum beds, but halite pseudomorphs at the base of and within some of the thin limestones suggest the presence of supratidal brine pools. Several minor unconformities exist, of which one has a regional extent and is underlain by red-brown mudstones. Algal-mat lamination, lenticular gypsum pseudomorphs (an intrastratal growth form), and syneresis cracks occur in the limestones, and a supratidal environment is envisaged for a significant proportion of the time of deposition. The limestones generally have a restricted fauna of ostracods, bivalves, cerithid gastropods, serpulid worms, and miliolid Foraminifera, and range in texture from lime mudstones to grainstones. The most abundant grain types are pellets, superficial ooliths, and terrigenous quartz. A paucity of dolomite is a striking feature. Some of the thinner bedded units are rippled, and some ripples were truncated during periods of emergence. The limestones are believed to represent periods of shallow water, slightly hypersaline to slightly hyposaline conditions of variable energy. The regressive trend displayed by these two divisions continued with deposition of the overlying formation. The uppermost 3 m (10 ft) of the DeQueen consists of mudstone with a thin marl at the top. A conformable contac exists with the overlying Antlers formation, which has basal mudstones becoming more silt- and sand-rich upwards and finally giving way to the typical Antlers (Paluxy) sands. End_of_Article - Last_Page 1468------------

Alluvial‐fan‐carbonate facies with evaporites, Eocene Guarga Formation, Southern Pyrenees, Spain

ABSTRACTContinental carbonates of the basal part of the Guarga Formation, Upper Eocene, have been studied in the field and in the laboratory. Both geographic pattern and vertical sequences served as a basis for sedimentary models. The pattern reveals that carbonate deposition was greater in the western part of the Tremp‐Graus Basin than in the eastern part. Studies of slabs and thin sections suggest high energy deposition in the eastern part of the basin and low energy deposition in the western part. Gross lithology, petrography, and faunal/floral associations indicate sedimentary environments which include calcimorphic palaeosoils, caliche nodule zones, solid caliche nodule zones, caliche crusts, fluvial carbonates, paludal carbonates, and pond/lake carbonates. Distribution pattern of the carbonate bodies and lithologies seem to be controlled by alluvial‐fan processes. They are typical fining‐upward sequences in the sense of Collinson (1978) which furthermore can be assigned to a channel‐bound group and a succession independent of fluvial channels. Referring to fan‐physiography the sequences can be subdivided into fluvial proximal interchannel, fluvial distal interchannel, lacustrine distal interchannel, nearshore lacustrine‐paludine, and lacustrine offshore carbonate facies. The two last categories are distal alluvial plain and/or fan‐edge associations. The presence of especially charophyte oogonia adjacent to pseudomorphs of lenticular gypsum in the lacustrine carbonates of the western Tremp‐Graus Basin indicates that their depositional environment was that of shallow lakes with alternating salinities.Diagenetic alterations consist of three major phenomena: (a) general cementation of the carbonates by low‐Mg calcite, (b) pseudomorphs of lenticular gypsum, and (c) silification. The relationship of gypsum and silica diagenesis to the regional distribution of lacustrine carbonates suggests early diagenetic processes for their formation. Gypsum pseudomorphs are small and are found to be replaced by Fe‐calcite throughout the whole area, or are large and replaced preferentially by silica in the west only. There is experimental evidence (Cody, 1979) that distribution of gypsum morphology and crystal sizes can be related to increased salinities and increased organic production in the west.Chert‐gypsum fabrics from that area resemble the ‘silex nectique’ of Cayeux (1929). The occurrence of this noteworthy type of chert together with length‐slow chalcedony is an indication of relatively early silification of the gypsum. It is suggested that the sediments containing the silicified lenticular gypsum hosted brines of partly marine origin which alternately became brackish or fresh.An attempt was made to check the influence of evaporation during carbonate deposition by determining the stable isotope composition of the carbonates and of the chert. The data show no distinct variations with respect to the presumed palaeoenvironments. The ranges in the carbonates (δ13C − 5,0 to −7,9; δ18 O −4,5 to −7,7) fit among the data given in the literature. The values derived from chert (δ18O 26,9 to 31 SMOW) support a concept of alternating salinities.Distribution of carbonate lithofacies indicate a systematic change in sedimentation from fluvial/ephemeral to permanent lacustrine from east to west. Diagenetic alterations suggest an increase of salinities of pore waters in the same direction. In addition the lacustrine limestones in the west were due to frequent changes in pore‐water salinity. Similar sequences lacking subtidal and/or lagoonal fossil associations but including pseudomorphs of carbonate and chert after lenticular gypsum were described from the Lower Purbeckian of Dorset (West, 1964, 1975, 1979), and from the Tertiary of the Paris Basin (Fontes, 1968). Guarga limestone environments of especially the western Tremp‐Graus Basin probably resembled the modern semi‐arid ephemeral saline lakes in Australia (Burne, Bauld & De Dekker, 1980). Gypsum precipitation in these environments can be compared to the one in modern sabkhas from the Mediterranean coast of Egypt (West, Ali & Hilmy, 1979; Ali West & Hilmy, 1980).

Tidal flat and shallow marine sediments from the Craignish Phyllites, Middle Dalradian, Argyll, Scotland

SummarySediments from 6 localities showing negligible deformation are described from a generally strongly deformed formation of metasediments. The tidal flat facies include decimetre-thick tabular sands, interpreted as the flood tide storm deposits of subtidal to intertidal channels, as well as centimetre to millimetre thick, rippled and laminated sands and silts with gypsum pseudomorphs. The gypsum crystals grew at interfaces where more permeable, overlaid less permeable, sediment and then underwent partial solution before being replaced by ferroan calcite and quartz. The shallow marine facies are centimetre-thick rippled sands interbedded with muds, interpreted as storm sediments.

Ancient Anhydrite Facies and Environments, Middle Devonian Elk Point Basin, Alberta

ABSTRACT The association of evaporites with a large percentage of the world's oil and gas reservoirs makes these rocks economically significant. This study was intended to increase our knowledge of environments of deposition and diagenesis of these rocks in a basin in which good core control was available -- the Middle Devonian Elk Point basin, Alberta. We have recognized four major environments of anhydrite deposition -- supratidal, shallow-water, deep-water, and replacement. Supratidal anhydrite includes gypsum pseudomorphs and most of the nodular, nodular-mosaic, and mosaic types. These are identified by comparison with the anhydrite in the sabkha sequence of the Trucial Coast. Bedded-massive anhydrite is interpreted to have been deposited in shallow water under conditions comparable to those in the Pekelmeer of Bonaire Island. Tiny nodules and thin laminae of anhydrite which are usually within or interbredded with laminated limestone are interpreted as deep-water in origin, from their stratigraphic position in the deeper parts of the Elk Point basin. No modern analogues of these types are known. Some massive, nodular, nodular-mosaic, and mosaic anhydrites are considered to be postlithification replacement types. They are commonly located on the edges of shoals, banks, and reefs and are characterized by their relatively greater thickness and by the inclusion of carbonate fragments of the host rock within the anhydrite. By detailed core studies of the anhydrite environments and their order of occurrence in three subbasins, we have been able to recognize eleven major sedimentary events. By correlating these events between subbasins, with the aid of additional core and density-log control, the history of the Middle Devonian basin has been reconstructed.

Ancient Anhydrite Facies and Environments--Their Role in Reconstructing Geologic History of Middle Devonian Elk Point Basin, Alberta: ABSTRACT

The association of evaporites with a large percentage of the world's oil and gas reservoirs makes these rocks economically significant. Good core control for study of environments of deposition and diagenesis of these rocks is available in the Middle Devonian Elk Point basin, Alberta. We have recognized 4 major types of anhydrite formation--supratidal, shallow-water, deep-water, and replacement. Supratidal anhydrite includes gypsum pseudomorphs and most of the nodular, nodular-mosaic, and mosaic types. These are identified by comparison with the anhydrite in the sabkha sequence of the Trucial Coast. Bedded anhydrite is interpreted to have been deposited in shallow water under conditions comparable with those in the Pekelmee of Bonaire Island. Tiny nodules and thin laminae of anhydrite, which are usually within or interbedded with laminated limestone, are interpreted as deep-water in origin, from their stratigraphic position in the deeper parts of the Elk Point basin. No modern analogues of these types are known. Some massive, nodular, nodular-mosaic, and mosaic anh drites, considered to be postlithification replacement types, are located on the edges of shoals, banks, and reefs, and are characterized by their relatively greater thickness and by the inclusion of carbonate fragments of the host rock within the anhydrite. By detailed core studies of the anhydrite environments and their order of occurrence in the subbasins, we have recognized 11 major sedimentary events. By correlation of these events End_Page 769------------------------------ between subbasins, with the aid of additional core and density-log control, the history of the Middle Devonian basin has been reconstructed. End_of_Article - Last_Page 770------------