Normal Waters Research Articles

Bacteriological aspects of Florida red tides: A revisit and newer obervations

A brief review is presented of the literature involving the occurrence of bactera in red tides caused by the dinoflagellate Ptychodiscus brevis. Both quantitative and qualitative studies were made on outbreaks along the Florida Gulf coast in 1982 and 1987 and a bloom along the North Carolina coast in 1987. Non-bloom waters in both areas were sampled in 1988. Bacteria were also recovered from batch cultures of P. brevis. Bacterial isolates from red tide and normal waters and dinoflagellate cultures were tested for their ability to kill fish under laboratory conditions. Numbers of bacteria were always higher than reported for a 1971 outbreak but did not routinely correlate directly with P. brevis numbers. Storage of red tide water produced increased bacteria counts in some cases and decreased levels in another. Members of the genera Aeromonas, Alteromonas/Pseudomonas, and Vibrio were frequently isolated from the three blooms studied as well as from non-bloom waters; MPN of> 1100 100 ml of Vibrio alginolyticus were recorded from the latter. Dominant chromogenic bacteria, reported during previous studies, were not observed in our samples. Several bacteria killed fish experimentally including isolates from red tide and normal waters and P. brevis cultures. It was concluded that a given dinoflagellate bloom is an individual event from a bacteriological standpoint, based on quantitative and qualitative comparison of historical observations and the present study. Recent reports of tetrodotoxin production by the bacterial genera isolated above suggests a further consideration of bacteria in toxic dinoflagellate blooms.

An hydrothermal origin for copper-impregnated prehnite and analcime from Boylestone Quarry, Barrhead, Scotland

Field, petrographic, mineralogical, fluid inclusion and stable isotope studies of prehnite, analcime and calcite in basalt lava of Visean age (about 360 Ma) from Boylestone Quarry, Barrhead, Scotland are all consistent with an hydrothermal origin. In the hydrothermal system, water interacted with hot alkaline basalt soon after eruption. Feldspars were digested in an alkaline solution and alumina and silica were reprecipitated in fractures and veins as the hydrous alumino-silicates analcime and prehnite. The prehnite generally occurs below the analcime in more oxidized host basalt. Minor calcite crystallized late, usually on earlier analcime or prehnite. Fluid inclusion measurements give homogenization temperature ( Th ) of 83–184 °C (mean 145 °C) and salinities c . 1.5 equiv. wt% NaCl for analcime; and c . 46-234°C and c . 26 equiv. wt% NaCl for calcite. Fluid inclusions in analcime appear to contain some SO 2– 4 or CO 2– 3 . Inclusions in calcite associated with prehnite have a lower Ca 2+ :Na + ratio than those in calcite associated with analcime. Using a fluid inclusion temperature of 145 °C for analcime of δ 18 O = 15.6% (V-SMOW) implies δ l8 O for the fluid was 1.4% and this leads to 184 °C for the temperature of precipitation of prehnite of δ 18 O = 12.5%. The δD values of –35% for prehnite are consistent with surface water (seawater or low latitude meteoric water) heated to 180 °C. Two groups of calcite are recognised on fluid inclusion and isotopic evidence: both groups have δD values ranging from —61 to — 117% but one group is characterized by Th > 100 °C and δ 18 O = 20 to 22% (δ 18 O water = 10.3%) and is believed to have formed from surface waters which had exchanged 18 O with igneous rocks at a high temperature whereas the second group with Th = 85 °C and δ l8 0i = 17.2% crystallized from normal surface waters of δ 18 O water = –0.4%. The carbonate δ 13 C data (range –8.3 to –42.4% PDB) attest to the presence of organic-derived carbon dioxide in the hydrothermal fluid, as expected since organic material is observed in some fluid inclusions. The low δ I3 C value, —42.4%, indicates oxidation of methane whereas the higher values point to decarboxyla-tion of organic matter Copper was oxidized and remobilized from disseminated chalcopyrite in basalt and probably from disseminated copper minerals which had resulted from Carboniferous weathering of underlying basalts. Dissolved copper was reprecipitated on cooling and reduction, generally within and above greenish prehnite. The fluid inclusion and isotope studies indicate that the water could well have been mainly meteoric with in addition, some warm Lower Carboniferous seawater and/or saline groundwaters; the water interacted with organic-rich sediments to provide CH 4 and CO 2 then to various degrees with hot igneous rock. The hydrocarbons may have been mobilized into groundwaters by baking of organic sediments in the Cementstone Group. The model is consistent with active volcanogenic geothermal activity during Lower Carboniferous times in the Midland Valley of Scotland.

Multistage Dolomitization in Rainbow Buildups, Middle Devonian Keg River Formation, Alberta, Canada

ABSTRACT More than 80 partially to completely dolomitized buildups of Middle Devonian Keg River Formation occur in the Rainbow sub-basin of northwestern Alberta. In six buildups (A, B, E, F, G, and Tehze), four petrographic types of dolomites are identified: gray, fine crystalline dolomite; floating dolomite rhombs/patches; matrix dolomite; and saddle dolomite. The petrographic and geochemical data indicate that these dolomites probably formed in three principle stages of dolomitization during progressive burial. Gray, fine crystalline dolomite (less than 1% of total dolomite by volume) is interpreted as penecontemporaneously dolomitized lime muds by either normal marine waters or evaporitic brines during early exposure. The following evidence suggests that this dolomite probably formed early: 1) it usually occurs in fractures and breccias presumably related to the early exposure of the buildups; 2) it is absent in rugs and molds that formed in the subsurface environment; and 3) locally, clasts of gray, fine crystalline dolomite are embedded in marine limestones. Rhomb/patch dolomite is the major type of dolomite in dolomitic limestones and is widespread throughout the limestone buildups. Matrix dolomite is fabric destructive and is most abundant in the dolostone buildups. Both these dolomites probably formed during shallow to intermediate burial because they 1) postdate stylolites, 2) are widespread throughout the buildups and crosscut various facies, and 3) have light oxygen isotope compositions and low Sr but high Fe and Mn concentrations. Dolomitizing fluids for dolomite rhombs/patches and matrix dolomites were possibly derived from 1) mechanical compaction, 2) dehydration of the adjacent Muskeg gypsum at relatively shallow burial depths, and 3) intermediate-depth basinal fluids conducted updip along porous and permeable conduits. During D vonian and carly Carboniferous times, compaction fluids were probably responsible for the early dolomite rhombs and patches. However, the main phase of matrix dolomitization took place later, probably during Late Mississippian to Jurassic time when the east side of the basin was intermittently tilted and uplifted, causing updip flow of intermediate-depth basinal brines. Saddle dolomite occurs as late-stage vug and fracture fillings. It is most depleted in 18O but most enriched in Mn and Fe, suggesting precipitation during deeper burial from warm basinal fluids. These fluids were expelled during Upper Cretaceous sedimentary and tectonic loading of the western side of the basin and then moved up the east side of the basin along existing platform, fault, and fracture conduits. The source of the Mg is uncertain. Some or most of the Mg may have resulted from chemical compaction of the earlier matrix dolomite.

A macrofaunal analysis of the Eocene of southeastern Nigeria

One hundred and nine macroinvertebrat species, seventy eight genera and fourty six families have been recorded from the Eocene Ameki Formation. The fauna is dominated by gastropods, followed by bivalves and echinoids. Turridae, Carditidae and Veneridae are the most diverse molluscan families while the gastropod species Turritella amekiensis and the bivalves Glans (A) costaenodulosis display the highest number of individuals. The ecological requirements of the Ameki macrofauna are shallow to moderately deep, normal to near normal marine tropical waters in a humid climatic zone.

Late Pleistocene mixing zone dolomitization, southeastern Barbados, West Indies

ABSTRACT Field, geochemical, and petrographic data for late Pleistocene dolomites from southeastern Barbados suggest that the dolomite precipitated in the zone of mixing between a coastal meteoric phreatic lens and normal marine waters. The dolomite is localized in packstones and wackestones from the algalAmphistegina fore‐reef calcarenite facies.Stable isotopic evidence suggests that meteoric water dominated the diagenetic fluids responsible for dolomitization. Carbon isotopes in pure dolomite phases average about ‐15%0 PDB. This light carbon is attributed to the influence of soil gas CO2, and precludes substantial mixing with seawater. A narrow range of oxygen isotopic compositions coupled with a wide range of carbon compositions attest to the meteoric diagenetic overprint. Dolomitization likely occurred with as little as a five per cent admixture of seawater.Strontium compositions of the dolomites indicate probable replacement dolomitization of original unstable mineralogy. The dolomite is characterized by low sodium values. Low concentrations of divalent manganese and iron suggest oxidizing conditions at the time of dolomitization.A sequence of petrographic features suggests a progression of diagenetic fluids from more marine to more meteoric. Early marine diagenesis was followed by replacement dolomitization of skeletal grains and matrix. Limpid, euhedral dolomite cements precipitated in primary intra‐ and interparticle porosity subsequent to replacement dolomitization. As waters became progressively less saline, dolomite cements alternated with thin bands of syntaxial calcite cement. The final diagenetic phase precipitated was a blocky calcite spar cement, representing diagenesis in a fresh‐water lens. This sequence of diagenetic features arose as the result of a single fall in eustatic sea‐level following deposition.A stratigraphic‐eustatic‐diagenetic model constrains both the timing and rate of dolomitization in southeastern Barbados. Dolomitization initiated as sea‐level began to fall immediately following the oxygen isotope stage 7–3 high stand, some 216 000 yr bp. Due to the rapidity of late Pleistocene glacio‐eustasy, dolomitization (locally complete) is constrained to have occurred within about 5000 yr.

Paleoecology of the eocone ameki formation of southeastern Nigeria

A well preserved fauna dominated by molluscs, foraminifera and ostracodes has been studied from five lithofacies recognised in the Ameki Formation (Eocene) in southeastern Nigeria. The ecological requirements of the fauna are inferred from the ecology of their recent representatives. It is suggested that the Ameki fauna lived under warm (tropical) conditions in a humid climatic zone at littoral to sublittoral depths. The maxium depth probably did not exceed 80m. Normal to near normal marine waters dominated the site of deposition.

Distribution of heavy metals in soft bodies and shell cavity fluids of Crassostrea gigas.

A number of oysters were collected at the intertidal zone and kept in ice boxes, from which the specimens were serially picked up over 17 times at the interval of 6 hours for 96 hours. Copper, Zn Fe, Mn and Cd contents in soft bodies and shell cavity fluids were determined. At the same time, nitrogen contents in shell cavity fluids were also determined. Regression and correlation coefficients between the metal and nitrogen contents and the serial times were not significant. Namely, the contents fluctuated independently without regard to the elapsed time. This proved that the preserving method was adequate to confine the metabolic activity of the specimen oysters. Of the total absolute amounts of Cu and Zn in soft bodies and shell cavity fluids, more than 94% distributed in soft bodies, and consequently, less than 1% of the two metals existed in the fluids. Cadmium more than 98%, Mn more than 97% and Fe more than 96% distributed in soft bodies. Although the levels of metal contents in shell cavity fluid were clearly less than those in soft bodies, the levels in the fluids were surprisingly higher than those in normal sea waters. Taking into account the fluctuations of the heavy metal and the nitrogen contents in the shell cavity fluids mentioned above: it was suggested that the pre-concentration in shell cavity fluid through the uptake process of heavy metals is the main factor for the high heavy metal contents in the fluids. Since, for specimens frozen deeply with shell valves, heavy metal contents in shell cavity fluid have been determined for a long time together with those in soft bodies, heavy metal contents of lamellibranchs are estimated somewhat higher than those obtained from fresh specimens.

Diagenesis and pyritization of crinoid ossicles

Diagenetic alteration of original high-Mg calcite crinoid skeletal material was achieved by reaction of the calcium carbonate with isotopically light meteoric water. The light δ18O values (−10.57 ± 0.74‰, PDB) are a reflection of the water source, whereas the δ13C values (−3.49 ± 2.63‰, PDB) are a consequence of the high reactive-organic-matter content in the sediments of the Brush Creek at Sewickley, Pennsylvania. High Fe (22 970 ± 30 440 ppm) and Mn (3760 ± 1450 ppm) contents suggest further that the diagenetic fluids were reducing waters that reacted with the crinoid ossicles in the shallow-burial marine and meteoric environments.Four different types of pyrite are found within crinoid ossicles. Framboidal pyrite, aggregate pyrite, and nanopyrite are generally found in the pores of the stereom. Micropyrite, which is present in the form of octahedral, dodecahedral, and pyritohedral crystals, and nanopyrite and aggregate pyrite replace the calcitic stereom. The degree of pyritization and the pore filling increase toward the outer periphery of the crinoid ossicles. Pyrite formation of the crinoid ossicles took place in two stages. In stage I, upon death, the crinoids disarticulated and were quickly buried. In this shallow sediment layer (1–10 cm) the pore water was both undersaturated with respect to calcite and oxidizing. This brought about oxidation of the organic tissue in the stroma of the crinoids and selective dissolution of the high-Mg calcite skeletons. With further burial in stage II, in the presence of reactive organic matter local iron was solubilized, marine sulphate was reduced to sulphide, and isotopically light carbon was produced by bacterial action. Reactive iron combined with sulphur to form framboidal and nanopyrite in the pores of the stereoms. With further burial the micropyrite formed in the crinoid ossicles. Termination of the pyritization process came about with depletion of the iron and (or) sulphur, and this process proceeded very rapidly under shallow-burial conditions while the crinoids resided in the microbial sulphate-reduction zone of the marine–phreatic environment.Fossils are preserved in different stages, with brachiopod valves preserved in their original low-Mg calcite mineralogy, whereas molluscs and crinoids show the complete trend from preserved aragonite and high-Mg calcite, respectively, to diagenetic low-Mg calcite. The preservation potential of fossils is closely linked to the thermodynamic stability of the skeletal carbonate in the presence of diagenetic fluids. Carbon/sulphur ratios support the assertion that Brush Creek sediments were deposited in normal marine waters and favour generally oxic redox conditions for Pennsylvanian seawater.

Stratigraphy, paleogeography and depositional environments of the Upper Middle Carboniferous (Desmoinesian) cyclothemic Excello Black Shale of the Midcontinent Region, USA

The Excello Shale is one of the best exposed examples of Pennsylvanian cyclothemic organic-rich shales in the midcontinent region. This study aimed to describe detailed stratigraphy of the study interval along the outcrop belt and to relate the environments of depositon, and paleogeography of the study area during the glacial-related paleoclimatic episodes. Certain present-day shallow silled basins were used as a possible analogue of the Pennsylvanian cyclic epeiric seas. Eustatic changes in sea-level were closely related to cyclic glaciation, global tectonics, basin subsidence and sedimentation pattern. Because of paleobathymetric relief in the cyclic epeiric seas, some units disappear and new ones appear, commonly with a change of facies. In many cases, cyclic coal beds underlie black shales. This suggests that swamp environments were present intermittently due to minor regression preceding episodes of cyclic maximum transgression. The occurrence of the cyclic seas and swamps between 5° and 8° N paleo-latitude all suggest a tropical-wet climate with possible seasonally controlled rainfall. Thin laminae, fine particle size and high TOC content (up to 17 wt%) indicate stagnant conditions with bottom-water anoxia in the Excello Sea. Anaerobic sediments were deposited in more than 100 m depth of water, poor circulation, shallow mixing with atmosphere, and high organic productivity and/or better preservation was present during maximum transgressive episode. The establishment of a density-stratified water column or pycnocline was the most important factor in development of the anaerobic sediments. Perhaps a halocline between bottom normal marine salinnity waters and a surface fresh-water layer probably caused the water stratification and anoxia leading to better preservation of organic matter in the Excello Sea.

Lower Cretaceous Carbonate Megabank, South Florida, Depositional Setting and Petroleum Potential: ABSTRACT

Reinterpretation of the South Florida basin reveals that it was a keep-up carbonate shelf during the Lower Cretaceous. Until the Cenomanian, it was connected to the Bahamas and the Cay Sal Bank north of Cuba. The Cenomanian sea level rise was at least partially responsible for the formation of the Florida Straits, isolating the platform. The combination of an abrupt worldwide rise in sea level and subsidence caused the South Florida platform to founder, ending shallow-water and evaporitic sedimentation over the majority of the shelf. The Cay Sal and Bahamian platforms remained as active shallow shelves separated by the Old Bahamas Channel. The Lower Cretaceous sediments are characterized by shallow shelf limestones, dolomites, and evaporites (mostly anhydrite). The evaporite-rich sections, originally thought to represent basinal facies, are reinterpreted as supratidal to shallow subtidal evaporites, based on examination of core and cuttings. Influx of normal marine waters over rudist fringing reefs controls both the carbonate/evaporite cycles and biota. A major salinity barrier within the platform can be defined. Production to date from the South Florida platform is from rudist patch reefs within the Sunniland formation onshore Florida. The petroleum potential of the Lower Cretaceous Sunniland formation increases offshore where more normalmore » marine waters would have been encountered and might have allowed larger, coalescing, or even stacked rudist patch reefs to develop. The most porous reservoir in the Lower Cretaceous is the Brown dolomite member of the Lehigh Acres formation. This unit also would have much greater potential offshore as the underlying section of carbonate/evaporite source rocks thickens.« less

Dolomites formed under conditions of deep burial: Hunton Group carbonate rocks (Upper Ordovician to Lower Devonian) in the deep Anadarko Basin of Oklahoma and Texas

Progressive burial diagenesis of Hunton Group (Upper Ordovician to Lower Devonian) rocks of the deep Anadarko Basin of Oklahoma and the Texas Panhandle is evident from petrographic and geochemical study of cores and cuttings from more than 25 boreholes up to 30,000 ft deep. Limestone of the Hunton Group, which originated as shallow shelf carbonates, has been replaced, chiefly below present depths of about 10,000 ft, by dolomite that is commonly ferroan and is associated with shale. This diagenetic dolomite is inferred to have formed under deep-burial conditions. The dolomite occurs as finely disseminated, 10μm and larger rhombic crystals, and is most abundant near the base of the Hunton Group, particularly where an oolite unit (the Keel Formation) overlies the thick marine Sylvan Shale that is inferred to be the chief source of Fe2+ and Mg2+ ions. Ferroan dolomite also occurs where clay minerals are abundant in the Middle Hunton Group. In shallow wells, dolomite crystals are euhedral. Below 10,000 ft(3.0 km), where dolomitization of the oolite has been more complete, hypidiotopic and xenotopic textures result. Hydrocarbon-associated fluids are inferred to have dissolved the calcite that was not replaced, and to have created intercrystalline and moldic porosity. X-ray diffraction verifies a trend of higher dolomite concentrations with increasing depth in the same oolite horizon. For example, oolite samples from outcrop lack dolomite and are 100% CaCO3; cores from 9,200 ft (2.8 km) are about 25% dolomite; and cores from 15,000 ft (4.6 km) and below are more than 85% dolomite. Radioisotope-induced x-ray fluorescence shows that dolomites below 10,000 ft(3.0 km) are iron-enriched relative to both non-dolomitized oolite and dolomites of surface origin. Where the Hunton Group and the Sylvan Shale are buried below 10,000 ft(3.0 km), well logs show high densities in the lowermost Hunton (above the Sylvan Shale) which can be interpreted as the occurrence of Fe2+- rich dolomite. Stable isotope ratios suggest a higher temperature of origin for burial dolomites than for dolomites of surface origin. Formation waters recovered from within the shale and carbonate are greatly depleted in Mg2+ ions compared to normal marine waters. General restriction of this shale-associated ferroan dolomite to strate that are currently buried below 10,000 ft (3.0 km) in the Anadarko basin supports other lines of evidence for the belief that previously deeply buried strate can be recognized even if the strata have been subsequently uplifted. The transformation of smectite to illite in the Sylvan Shale (conformably underlying the oolite) is suggested as a possible source of Mg2+ and Fe2+. Smectite/illite ratio decreases with increasing depth in the Sylvan Shale. Furthermore, Sylvan Shale below 10,000 ft(3.0 km) is depleted in iron by 70% relative to Sylvan Shale less than 10,000 ft(3.0 km) deep, suggesting that under deep conditions shales have given up their iron (and magnesium), presumably to the overlying carbonates.

Meltwater influences and palaeocirculation changes in the North Atlantic during the last glacial termination

Summary Major meltwater plumes during the last glacial termination 13 000 years bp are recorded in the δ 18 O anomalies of planktonic foraminifera from North Atlantic and Gulf of Mexico marine sediments. These distinct meltwater plumes represent the discharge of isotopically light freshwater from specific portions of the Northern Hemisphere ice sheets. Plume geometries show that northward incursions of warm, normal saline waters occurred during the last glacial termination, thereby delivering latent and sensible heat to high latitudes of the North Atlantic and enhancing the rate of ice sheet melting.

Depositional and Diagenetic History of Rocks in Petoskey Formation Type Section, Petoskey, Michigan: ABSTRACT

The Petoskey Formation type section is exposed near the shore of Little Traverse Bay, in the city of Petoskey, Michigan. The rocks are Middle Devonian and are the uppermost unit of the Traverse Group in western Michigan. The outcrop consists of limestone and dolomite, containing abundant stromatoporoids. At the top of the outcrop is a biostrome, consisting of a massive bed of stromatoporoids. Thin-section staining with Alizarin Red-S and potassium ferricyanide indicates that dolomite selectively replaced the calcite matrix, but has not altered the stromatoporoids greatly. Other faunas show varying degrees of dolomitization. Gypsum molds are found within some of the stromatoporoids, indicating hypersalinity nearby. The depositional environment for the Petoskey Formation type section was nearshore shallow marine. Carbonate buildups were present in the form of biostromes and bioherms. Evaporites were formed in nearby areas, which were restricted from influx of normal marine waters. Dolomitization occurred in the Petoskey Formation rocks soon after deposition, as hypersaline waters moved through the sediments or poorly cemented rocks. Well-cemented beds were apparently unaffected by the dolomitizing waters.

Conodont palaeobiogeography and thermal maturation in the Caledonides

Conodont elements are important in biostratigraphy, in palaeoenvironmental and palaeobiogeographical analyses, and in assessing the thermal history of Lower Palaeozoic strata throughout the Caledonian belt. The differentiation of Ordovician conodont faunas into the North Atlantic and North American Midcontinent provinces is not closely comparable with the provincial patterns shown by other faunal groups and does not relate clearly to criteria for recognizing continental separation across the Iapetus Ocean. Some species associations adapted to cool, normal marine waters were able to cross the ocean at the time of its presumed maximum extent. The colours of conodont elements reflect the thermal history of strata and can be used in studies of low-grade metamorphism. In S. Wales, for example, metamorphic temperatures indicated by conodont colour are too high to he accounted for by simple burial beneath the known stratigraphical overburden under normal heat flow conditions. Elsewhere, conodont colour trends also serve to constrain and test models of metamorphism that involve sedimentary burial in subsiding basins, tectonic burial under nappes, or the thermal effects of intrusions.

Separation of seawater by Soret diffusion

Samples of artificial seawater were separated in a Clusius-Dickel (thermogravitational) column at temperatures from 5 to 40°C at atmospheric pressure. The ions represented were sodium, magnesium, potassium, calcium, chloride and sulfate. The samples were analyzed by atomic absorption for the positive ions, and by titration for the negative ions. From the separations, ionic Soret coefficients were calculated. A multi-component effect was found, that is, the ionic coefficients were not the same as those we measured previously for the same ions in binary salts, but, except for the sulfate ions, the Soret coefficients were similar to those in binary salts. Because the difference between binary salts and this complex solution affects the coefficients only moderately, the results should be applicable to a wide range of natural waters. In the interfaces above the Red Sea brines, Soret diffusion opposes Fickian diffusion, tending to keep the sulfate in the normal Red Sea waters from penetrating to the sulfate-depleted brines.

Role of organophosphates in adaptations of an obligate water-breathing teleost (Tilapia nilotica L.) to hypoxia

The red cell concentrations of ATP and GTP were investigated in Tilapia nilotica in normal and hypoxic waters. Reduction of water oxygen content diminished the cellular concentrations of both organophosphates. The concentration of ATP fell by about 70% of its normoxic values (2.4 ± 2.6 µ mol Pi 100 ml) at an O2 concentration of 4.5 mg l−1 in the water; further reductions of aquatic oxygen levels were not associated with significant changes in cellular ATP. Normal cellular levels of GTP (25.8 ± 2.7 µ mol Pi: 100 ml RBC) decreased progressively with advanced hypoxia especially at critical aquatic O2 concentrations. The ratio of the concentrations of ATP:GTP (2.4:1) in normoxic water decreased with hypoxia till it approached the equivalence in water of less than 5.5 mg l−1 O2. It is suggested that ATP is more important in modulation of blood oxygen affinity during mild or transient hypoxia and that GTP exerts its regulatory action over a wider range of aquatic O2 concentrations.

Ciclos pelíticos de bahía Interdistributaria del nivel intramareal inferior del grupo Roda-Oroel (sector occidental del paleodelta de Ager, Lérida)

The deposits of the ancient Ager deltaic system (Lerida, Spain) are characterized in some way by the existence of cycles constituted, in a large scale, by alternances of pelites and sandstone bodies. According to geological literature these deposits represent bay marls and stream-mouth-bars, in the eastern-most counterpart. In this area sorne pelitic sections show a pronounced cyclical pattern devoid to repetitions of two terms: A marly bioturbated one, and the other one, constituted by undisturbed delicately laminated clays. Part of the deposits of the Intramarea1 Inferior level (''Tramo Superior of the Roda-Oroel depositional system) in the area comprised between La Regola cementery and Ribellera gorge, have been studied taken specially into account the faunistic content, sedimentary structures sequences and lateral relationships of these pelitic cycles in order to characterize them. In that area, three cycles constituted by alternances of marls and clays have been distinguished. As a whole these three cycles are bounded at: the top and the bottom by sanstone bodies that have been interpreted as minor-mouth-bars. The mar1y terms of these cycles are light brown in colour and show a faunistic content that may be ascribed to a lagoon with normal salinity or slight1y hipersaline waters. On the other hand, fauna in the laminated clays is scarce and there is not evidence of bioturbation. The high content of plant remains ad evaporites in these clays suggest that they were developed in a reducing environment (probably anoxic). Boundaries between marls and laminated clays are always net and not erosive. They migth be originated by drastic changes in the physical-chemical characteristics of the sedimentary environment where the were deposited. Taking into account the sedimentary context of these clays, this cyclicity migth be interpreted as the response to the alternance of succesive periods of marine flood and periods of starved waters in a deltaic interdistributary bay when it was disconnected from the open sea.

Ground Water Redox Reactions: An Analysis of Equilibrium State Applied to Eh Measurements and Geochemical Modeling

Computer modeling of 611 high-quality analyses of normal ground waters from diverse geographic areas reveals that aqueous oxidation-reduction reactions are generally not at equilibrium. Multiple redox couples present in individual samples yield computed Nernstian Eh (redox potential) values spanning as much as 1000 millivolts. The computed Eh values do not agree with each other, nor do they agree with the single "master" value measured in the field with a platinum electrode. Because of internal disequilibrium, the use of any measured Eh value as input to equilibrium hydrogeochemical computer models will generally yield misleading results for normal ground waters.

Intertidal Cementation: Some Geochemical, Mineralogical, and Petrographic Considerations: ABSTRACT

The chemistry, mineralogy, and petrography of intertidal cements are dependent on the chemical and physical characteristics of the waters from which they were precipitated--as well as on the biota this water supports. The study of Holocene cementation in Boiler Bay, St. Croix, Virgin Islands, is well suited to illustrate the basic constraints that the chemistry, mineralogy, and petrography of the cements place on any model of beach rock origin. Boiler Bay contains a well-developed beach rock pavement cemented on its western extremity by rhombohedral-bladed, circumgranular magnesian calcite cement, and on its eastern end by acicular, circumgranular aragonite cement. These facts indicate that, although the precipitational fluid was dominantly marine and cementation occurred under phreatic conditions, western beach rock cements were precipitated under a different geochemical regime than were those cements in the east. Pelleted magnesian calcite micrite cements that are characteristic of the seawardmost parts of the beach rock pavements (as well as submarine cements occurring in offshore reefs) were found by scanning electron microscopy to be biologic in origin. Backbeach cemented zones are characterized by low to high magnesian calcite, rhombohedral, equant-to-bladed circumgranular crust cements. This combination of cement mineralogy-petrography indicates fresh to marine, phreatic precipitational conditions in the backbeach area. Cement chemistry, particularly magnesian carbonate content in the calcites and strontium contents of aragonite and calcite, indica es that the beach rocks of western Boiler Bay were precipitated from mixed marine-fresh meteoric waters, and that the eastern Boiler Bay beach rocks were precipitated from normal marine waters with no freshwater influence. Backbeach cements of western Boiler Bay show a complete gradation of cement fabric, mineralogy, and chemistry that would indicate a classic fresh-marine water mixing zone. Hydrogeochemical studies of beach and near-beach interstitial waters confirm that western Boiler Bay is a locus of freshwater influx into the marine system. It would seem, then, that the mineralogy of Boiler Bay beach rock is controlled by fresh-marine water mixing. Artificial substrate experiments, using the western Boiler Bay observation wells, pinpoint the locus of cementation in this area as being within the fresh-marine water mixing zone near the upper surface of the water table. In addition, these experiments indicate that cement growth can be independent of substrate mineralogy and that organic coatings on cement substrates may ultimately be one of the major controls over initial cementation patterns. Finally, the experimental approach in the intertidal environment may well enhance our understanding of controls over carbonate cementation. End_of_Article - Last_Page 518------------

Bahamian Subtidal Stromatolites (Oolitic!): ABSTRACT

Subtidal oolitic stromatolites are forming in normal marine waters (1-5 m) in the high energy oolitic sand environment on Eleuthera Bank, Bahamas. Penecontemporaneous marine cementation transforms these stromatolites into hardgrounds, some of which may localize subsequent reef development. Accretion of oolitic stromatolites results from trapping and binding of ooids by various algae. Direct precipitation from seawater of aragonite and/or high magnesium calcite, calcification of algal filaments by high magnesium calcite, or commonly a combination of both processes lithify these stromatolites to create hardground substrates. Degree of marine cementation increases downward from stromatolite surfaces. Stromatolites themselves are localized on other low-relief oolitic hardgrounds. Morphologies of oolitic stromatolites are strikingly similar to some Shark Bay algal stromatolites. Bahamian stromatolites originate as small pinnacles which can evolve into mounds over a meter in height. Individual pinnacled stromatolites also coalesce laterally into continuous elongated ridges which develop preferred orientations in response to local hydrographic conditions. Internal crude algal laminations often are destroyed by macroborers. Oolitic stromatolite growth is ephemeral, apparently controlled by the rate of burial by shifting oolitic sand. This physical stress, therefore, sufficiently excludes grazers and encrusters, permits algal binding of ooids and explains stromatolite development in normal marine waters. Buried stromatolites that become exposed are recolonized by algae and begin accreting upward. Where physical stresses are removed for longer periods of time, oolitic stromatolites become susceptible to colonization by coralgal organisms and represent an early stage of reef development. End_of_Article - Last_Page 565------------