Phreatic Conditions Research Articles

Invasion of a karst aquifer by hydrothermal fluids: evidence from stable isotopic compositions of cave mineralization

AbstractMineral deposits in the Cupp‐Coutunn/Promeszutochnaya cave system (Turkmenia, central Asia) record a phase of hydrothermal activity within a pre‐existing karstic groundwater conduit system. Hydrothermal fluids entered the caves through fault zones and deposited sulphate, sulphide and carbonate minerals under phreatic conditions. Locally, intense alteration of limestone wall rocks also occurred at this stage. Elsewhere in the region, similar faults contain economic quantities of galena and elemental sulphur mineralization. Comparisons between the Pb and S isotope compositions of minerals found in cave and ore deposits confirm the link between economic mineralization and hydrothermal activity at Cupp‐Coutunn. The predominance of sulphate mineralization in Cupp‐Coutunn implies that the fluids were more oxidized in the higher permeability zone associated with the karst aquifer. A slight increase in the δ34S of sulphate minerals and a corresponding δ34S decrease in sulphides suggest that partial isotopic equilibration occurred during oxidation. Carbonate minerals indicate that the hydrothermal fluid was enriched in 18O (δ18OSMOW ∼ + 10‰) relative to meteoric groundwater and seawater. Estimated values for δ13CDIC (δ13CPDB ∼ − 13‰) are consistent with compositions expected for dissolved inorganic carbon (DIC) derived from the products of thermal decomposition of organic matter and dissolution of marine carbonate. Values derived for δ13CDIC and δ18Owater indicate that the hydrothermal fluid was of basinal brine origin, generated by extensive water–rock interaction.Following the hydrothermal phase, speleothemic minerals were precipitated under vadose conditions. Speleothemic sulphates show a bimodal sulphur isotope distribution. One group has compositions similar to the hydrothermal sulphates, whilst the second group is characterized by higher δ34S values. This latter group may either record the effects of microbial sulphate reduction, or reflect the introduction of sulphate‐rich groundwater generated by the dissolution of overlying evaporites. Oxygen isotope compositions show that calcite speleothems were precipitated from nonthermal groundwater of meteoric origin. Carbonate speleothems are relatively enriched in 13C compared to most cave deposits, but can be explained by normal speleothem‐forming processes under thin, arid‐zone soils dominated by C4 vegetation. However, the presence of sulphate speleothems, with isotopic compositions indicative of the oxidation of hydrothermal sulphide, implies that CO2 derived by reaction of limestone with sulphuric acid (‘condensation corrosion’) contributed to the formation of 13C‐enriched speleothem deposits.

Diagenesis in the coastal quaternary carbonates in Qatar, Arabian Gulf

Combined field studies, thin-section examination and scanning electron microscopy were utilized to study the diagenesis of the Quaternary carbonates in twelve stratigraphic sections along the coastline of Qatar. The sequence is dominated by calcarenites. It comprises three lithostratigraphic units representing two transgressive phases of the Pleistocene sea separated by a regressive phase. Diagenesis has modified the original textural and compositional characteristics of the Quaternary rocks considerably. Dissolution and cementation played major roles, whereas neomorphism and compaction were much less effective. Dissolution resulted in the development of fabric-and nonfabric-selective types of porosity. Cementation was mainly by calcite and, rarely, aragonite, gypsum and silica. Calcite crystals are nonferroan, mainly low-magnesian and have wide ranges of size, habit and distribution patterns. Aragonite cements are commonly crystals are nonferroan, mainly low-magnesian and have wide ranges of size, habit and distribution patterns. Aragonite cements are commonly isopachous and consist of fibrous and needle-like crystals. Massive and fibrous gypsum and cryptocrystalline and microcrystalline silica fill intergranular pores and microchannels. Aggrading and degrading neomorphism were selective. Compaction effects were more profound in calcarenites which were not subjected to early cementation. The diagenesis of the Quaternary carbonates occurred in a wide spectrum of settings which prevailed during the eogenetic, mesogenetic and telogenetic phases. The marine phreatic environment witnessed the commence of cementation with aragonite and high-magnesian calcite and the formation of micritic envelopes of fossil shells. The early phases of dissolution, micritization of other allochems and aggrading neomorphism occurred under mixed phreatic conditions. Most of the recorded diagenetic effects were developed in the meteoric vadose and phreatic environments. These include the completion of the dissolution events, the cementation with sparitic low-magnesian calcite, silica and gypsum, and the conversion of aragonite into low-magnesian calcite. There are remarkable variations in the nature and intensity of freshwater cementation throughout the sequence which indicate marked fluctuations in the groundwater levels.

Micromorphological and chemical complexities of a lateritic profile from basalt (Jos Plateau, Central Nigeria)

A ferricrete-capped lateritic profile developed on basaltic saprolite is described. Within both ferricrete and the immediately underlying soft laterite, two types of sesquioxidic cement are distinguished: the isopachous goethitic intergranular cement which is predominant and is interpreted as an indication of past phreatic conditions; and the Fe-stained meniscus cement which is interpreted as a sign of vadose conditions. The observed features are suggested to reflect changing drainage conditions related to morphological rejuvenation of the profile, in full accordance with previous hypotheses developed on the basis of geomorphological consideration.

Meteoric phreatic speleothems and the development of cave stratigraphy: An example from Tounj Cave, Dinarides, Croatia

Speleothems occurring in some caves of the carbonate Dinarides line all channel surfaces, and have been deposited from meteoric waters under phreatic conditions. Such phreatic speleothemic deposition modifies common experience (l) that meteoric phreatic conditions cause dissolutional widening of cave voids, and (2) that speleothems imply vadose conditions. The phreatic speleothems described here postdate an early polygenetic evolution of the cave voids, and predate the last, vadose stage. They were likely produced during the late/postglacial warming period, when dissolved carbonate was amply supplied, and when there was much water available for saturation of underground voids. Phreatic speleothems may be used as a tool for time correlation of internal deposits, both within one cave and within a karst region. They indicate an important stage in the history of the ground-water regime of an area. In general, phreatic speleothems help in better understanding of the development of subterranean voids and related karst/palaeokarst.

Use of cement types in the palaeoenvironmental interpretation of coastal aeolianite sequences

Abstract The interpretation of palaeodune sequences is an important component in palaeoenvironmental analyses. Studies of the diagenetic alteration of palaeodunes is one of the main ways of determining post-depositional environmental conditions. This paper questions the long-held and generally accepted ideas that cement types/fabrics are a ‘major method of recognising the original mineralogy of the cements and by inference their origin under marine, vadose or phreatic conditions’ (Arthur et al. 1982). A detailed study of last interglacial aeolianites from Mallorca, Tunisia and the Bahamas has shown that although many cement types may be characteristic of the meteoric vadose zone, none are unique. No single cement type is diagnostic of the vadose zone.

Tourism development and desalination systems: Comparative analysis of systems' suitability for coastal areas of the Red Sea and Gulf of Aqaba, Egypt

Several areas along the Egyptian coasts of the Red Sea and Gulf of Aqaba have been identified as priority zones for tourism development, as they possess exceptionally beautiful beaches, coral reefs and marine life. These zones however, suffer deficiencies in infrastructure, particularly the lack of adequate quantities of potable water. Therefore, water desalination plays a major role in infrastructure planning. The Red Sea coastal aquifer extends into Sinai and comprises two main components. The Quaternary Aquifer has developed at the deltaic areas of the main Wadis which cross the coastal plains. The water exists under phreatic conditions nearly at sea level, and its salinity is about 700–2000 ppm. The Tertiary Aquifer has developed at great depths, and its salinity reaches up to 20,000 ppm. The available quantity of ground water, however, is not adequate to supply the promising touristic areas. Sea water desalination, with salinity up to 47,000 ppm, cannot be avoided at least as a temporary solution. Choosing from available technology options should be based not only on comparisons of benefits and costs, but also on due consideration of the environmental impact of the chosen technology on the highly sensitive and fragile ecological system of the area. Viability would also naturally be based on studying the cost of producing water including consideration of required energy, capital investment, chemical consumption, operation, maintenance and labour. This paper will present an overview of available technology options relative to the needs and conditions of the areas under consideration, and detailed analyses of their suitability. Analysis of various factors peculiar to these areas have lead to the conclusion that thermal systems are viable to produce quantities up to 10,000 cubic meters of potable water per day. For smaller quantities of up to 500 cubic meters per day, the Reverse Osmosis system is the viable alternative. In all cases, environmental considerations must be strictly observed.

Evolution of Gypsum Karst in the Kirschberg Evaporite Member Near Fredericksburg, Texas

ABSTRACT At the Fredericksburg Gypsum Quarry in Gillespie County near Fredericksburg, Texas the quarry operations expose modern karst features in a 10-m (30 ft) thick gypsum bed that is part of the Cretaceous Kirschberg Evaporite Member. The following sequence of events is documented: 1) Formation of nodular daisy-head gypsum. Replacement by gypsum was at first slow but pervasive, with phreatic fluids oozing through the calcium sulfate bed. The main permeability pathways were provided by fractures and the intercrystalline porosity in dolomite films surrounding the calcium sulfate nodules and along thin dolomite beds. Large aligned gypsum crystals nucleated off these flow paths and grew by replacement of the earlier calcium sulfate phase. A change to more active hydrologic conditions modified the rate of secondary gypsum growth, so that the remaining gypsum grew rapidly as a microcrystalline alabastrine form. This adjustment in gypsum type was due to the relative uplift of the bed into more active phreatic conditions closer to the water table. There the bed was subjected to more pronounced temperature changes and increased groundwater flow. 2) Karst Structures and Speleothems. The gypsum bed was introduced into the vadose zone and the process of karstification of the bed began. Dissolution took place forming vertical pipes or caves in the Fredericksburg gypsum bed. Gypsum and calcite speleothems, mainly in the form of popcorn and flowstone, were deposited in the pipes and caves. Extreme dissolution of the gypsum bed produced zones of collapse breccia. This collapse occurred gradually. Locally, the overlying beds were disrupted and folded; regionally, the overlying beds were minimally disrupted by widespread lowering due to complete dissolution and gentle collapse folding. Coarse, botryoidal calcite grew in void spaces in the breccia as it was collapsing. The effects of the vadose regime on the Fredericksburg gypsum bed suggest that the end result of such an unroofed evaporite bed, even if it is never subaerially exposed, is a stratiform layer that has associated with it a breccia consisting of calcite popcorn, boxwork and botryoidal calcite, mixed with fragments of the overlying strata. Little if any evidence of the gypsum remains in the breccia. Textures and structures described herein are used to interpret similar features in other ancient evaporite beds.

Soil moisture and groundwater responses to snowmelt on a drumlin sideslope

Hillslope soil moisture and groundwater fluxes at a site in south-central Ontario were examined during the 1986 spring snowmelt. Near-surface soil moisture was found to respond rapidly to the daily snowmelt cycle and a perched aquifer was formed in the latter stages of melt due to a progressive decrease in soil permeability with depth. The delay between the initiation of snowmelt and the production of an extensive perched aquifer appears to have resulted from microscale variations in wetting behaviour induced by heterogeneity in soil texture and porosity. Subsequent additions of rainwater to the aquifer's capillary fringe produced an upward extension of phreatic conditions into more permeable near-surface layers, resulting in increased groundwater contributions from the hillslope. Previous studies indicate that saturation overland flow dominates the runoff response of catchments in this region during snowmelt; the present results suggest that much of the return flow component of this runoff mechanism is produced by these transient perched aquifers. This has implications for both hydrochemical studies and the use of environmental isotopes in investigations of snowmelt runoff.

Effect of Fractures on Reserve Calculations as Determined by Petrology: Birthright Field, Hopkins County, Texas: ABSTRACT

ABSTRACT Birthright field in Hopkins County, Texas produces oil and gas from Jurassic carbonate grainstone deposits of the upper Smackover Formation. The field was discovered in 1965 by the drilling of the well Schneider and Corey #1 E. M. Strode. This well initially tested 491 BOPD from a 34 ft interval at 9519 ft. The field has an oil-water contact at subsea 9090 ft and a gas-oil contact at subsea 8980 ft. Ten wells were originally drilled in the 800 acre field but only 6 oil and 1 gas wells were put on production. The estimated inplace reserves in 1969 were 6.4 MMBO, 1.3 MMBC, and 14.97 BCF, with an estimated primary recovery of 22%. The total cumulative oil production to 1986 was 3.19 MMBO. The grainstones of the producing interval are composed of ooids, pisoids, pelecypod fragments, grapestones, and fecal pellets. This shallow marine bar deposit has been extensively leached and cemented. Fracturing and minor brecciation has taken place causing many grains to be broken and sheared. Many grains have been coated by early diagenetic fibrous rim-cement indicative of leaching in vadose and phreatic conditions. These conditions have caused significant variations in the structural conditions and sizes of the coated grains. Anhydrite and sparry calcite has subsequently filled many of the pores, especially below the oil-water contact. It is thought that these factors may have affected actual recoverable reserves, and that petrology can be an effective tool in reservoir evaluation especially where recovered reserves are greater than had been initially calculated.

The speleogenetic role of air flow caused by convection. 1st contribution

In the past some authors described the speleogenetic role of convection in phreatic conditions. Similar effects exist also in the air-filled part of vadose passages of caves as a consequence of an air circulation due to a relevant temperature gradient; the effects can be enhanced by the presence of some acids as, e.g., H2S, H2SO4, etc. In this paper the conditions matching convection and condensation which produce typical forms, very similar to those found under phreatic conditions, are discussed both for limestone and gypsum caves.

Origin of Dolomite Cement in Chesapeake Group (Miocene) Siliciclastic Sediments: An Alternative Model to Burial Dolomitization

ABSTRACT Ferroan dolomite and calcite cement fossiliferous sandstone unite in the Choptank and Calvert Formations (Miocene) of western Maryland and northern Virginia. The average composition of the dolomite is Ca.55Mg.43Fe.02CO3, though nonferroan dolomite is also present. The dolomite is generally euhedral and zoned, and it may have fluid-inclusion-rich cores. Calcite cements are dominantly composed of equant spar, though a fine, acicular, isopachous rim cement may be present. Calvert Formation dolomite is chemically similar to Choptank Formation dolomite, though the former contains both calcite inclusions and hollow centers. A relatively simple burial history for ferroan dolomite and calcite cement, coupled with petrographic and isotopic evidence, indicates that some of the dolomite has a marine origin, possibly under the influence of sulfate-reducing organisms. Later recrystallization may have occurred under the influence of alternating marine and meteoric-phreatic conditions. Zoned dolomite rhombs, dolomite with calcite cores, and clear dolomite rims indicate episodic crystallization, possibly due to multiple periods of phreatic conditions. Most of the calcite cement is demonstrably meteoric in origin. Dolomite cement is generally confined to permeable sandy intervals bounded on the top and bottom by clays of low permeability. When buried, this sequence would be similar to dolomite sequences inferred to be of deep burial in origin. The timing of dolomite formation in Chesapeake Bay sediments is significant in that most of the dolomite has derived its magnesium from sources other than the clays, as has been suggested in deep-burial dolomitization models.

Early-Diagenetic Sheet-Crack Cements of Guadalupian Shelf, Yates and Tansill Formations, New Mexico--a Field and Chemical Study: ABSTRACT

Tepee-associated sheet-crack structures and carbonate cements that largely fill them are developed in the carbonate facies of the back-reef Capitan Reef complex, Guadalupe Mountains, New Mexico. Sheet cracks, their cements, and associated teepee structures were studied to better understand the timing, nature of precipitating waters, and environment of deposition of the sheet-crack cements. Sheet-crack cements were field classified into 8 morphologic fabrics. Contrary to previous work, this study found that sheet-crack fillings developed symmetrically from roofs and floors. Sheet-crack cements, the sheet cracks, and associated teepee structures developed repeatedly and episodically. Most are erosionally truncated and cross-cut by overlying surfaces. Most cements show isopachous growth indicative of precipitation under phreatic conditions. Some pendant cements suggest precipitation in a vadose environment, indicating occasional exposure of the outer-shelf and shelf-crest facies. Calcite-cement fabrics (39 samples) were analyzed for Sr, Mg, Fe, ^dgr13C, and ^dgr18O. Chemical compositions range from: Sr = 116 to 5,244 ppm; Mg = 215 to 9,188 ppm; Fe = 7 to 758 ppm; ^dgr18O = -5.6 to + 0.4^pmil; and ^dgr13C = +4.9 to 7.5^pmil (PDB). Sr exhibits distinct groupings of more than 2,000 ppm and fewer than 500 ppm. Samples with high Sr are associated with lower ^dgr13C, ^dgr18O, and Mg compositions. The converse is true for the low Sr values. Isotopically heavier samples are interpreted as precipitated from restricted marine waters. Isotopically lighter samples are interpreted as precipitated from mixed meteoric and marine waters. The meteoric water is inferred to have originated from shelfward, terrestrial source areas. It is hypothesized that these waters traveled through an aquifer system and percolated upward as springs to the sheet-crack area. End_of_Article - Last_Page 302------------

LAGOA FEIA FORMATION (LOWER CRETACEOUS), CAMPOS BASIN, OFFSHORE BRAZIL: RIFT VALLEY STAGELACUSTRINE CARBONATERESERVOIRS ‐ I

The Campos Basin is one of the coastal basins of eastern Brazil formed during the separation of the African and South American continental blocks. The Lagoa Feia Formation consists of the sediments deposited during the rift valley stage of the Campos Basin, and varies from 200 m to more than I, 500m in thickness. 17 microfacies were recognized, and are grouped into four main sequences, dominated respectively by terrigenous supplies, ostracods, pelecypods and basic volcaniclastics.The vertical sequence of microfacies and the associations of syndepositional diagenetic minerals were used to reconstruct the general environment of deposition. It consisted of saline alkaline lakes, oscillating between a playa lake stage with ostracod microfacies and a pluvial lake stage with pelecypod microfacies.Five diagenetic stages were recognized: syndepositional alteration of lithics to trioctahedral smectites, early dolomitization, early silicification, early cementation by bladed rim calcite and zeolites; fresh water vadose, consisting mainly of dissolution of bioclasts and lithics; fresh water phreatic, with sparite cementation and neomorphism; mixed saline‐fresh water, represented mainly by silicification; and burial with compaction, late dolomitization and partial conversion of smectites to illite.Pelecypod‐rich limestones with primary interparticle, secondary intraparticle, moldic and moldic‐enlarged porosities constitute the main potential reservoirs. The ideal conditions for the generation and preservation of such reservoirs consisted of periods of exposure followed by rapid lake expansion and burial. In this manner, the secondary porosity formed in the vadose fresh water environment was preserved or only partially occluded by calcite cementation during fresh water phreatic conditions.

Diagenesis of Coeymans (Lower Devonian) Patch Reefs, Northern Appalachian Basin: ABSTRACT

Fourteen Coeymans-age patch reefs and bioherms have been identified along the Silurian-Devonian outcrop belt in northeastern Pennsylvania, northwestern New Jersey, and central New York. Detailed petrographic analysis of samples from five reefs has led to development of a regional diagenetic model. Diagenetic fabrics show evidence of near-surface conditions that developed in five successive stages. The reconstruction of the diagenetic history of the reefs is as follows: (1) lithologic development of the reef facies, (2) syndepositional infilling by detrital micrite in pores, fractures, and cavities to form geopetal structures, (3) early subaerial emergence in which minor amounts of calcite cement were precipitated in a freshwater vadose environment, (4) a regional transgression that briefly resubmerged the reefs, followed by subaerial exposure, which led to a mixing of fresh and marine pore waters resulting in localized development of silica cements and dolomite precipitation, and (5) continued subaerial exposure of the reefs where large-scale freshwater phreatic conditions resu ted in total porosity occlusion by stable low-Mg calcite. Stage 4 shows the most regional variation in fabrics and conditions. An abundance of various forms of silica has been described in the Coeymans reefs of central New York. These varied forms resulted from fluctuations in pH and salinity. Stage 5 is the most pervasive within the reefs studied. Common phreatic fabrics are syntaxial overgrowths on crinoidal grainstones, drusy calcite rims, and neomorphic spar with crystals coarsening and completely filling the central parts of all remaining pores. The model developed in this study leads us to infer that Coeymans reefs found in the shallow subsurface would not be favorable hydrocarbon reservoirs. The possibility does exist that localized porosity development occurs in untested reefs within the deeper subsurface portions of the basin. End_of_Article - Last_Page 1927------------

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------------

Porosity Preservation in the Upper Smackover (Jurassic) Carbonate Grainstone, Walker Creek Field, Arkansas: Response of Paleophreatic Lenses to Burial Processes

A section of the ARCO Bodcaw No. 1 core from the Upper Smackover (Jurassic) of southern Arkansas was studied in detail in an effort to understand the control of porosity. Petrographic, isotopic, atomic absorption, and cathodoluminescence data on whole rocks and components were generated on 24 samples from a 115-foot length of core. The core is composed almost entirely of accretionary and cryptocrystalline grains and cement. Porosity is unrelated to grain type, size. or sorting on a sample-to-sample basis. The porous intervals are interpreted to have been stablized to calcite under fresh-water phreatic conditions. The porous intervals approach values of -5.0 per thousand delta 18 O, 1.5 mol percent Mg, and 300 ppm Sr and are further characterized by luminescent oolites and relatively slight solution compaction effects. The nonporous intervals represent relict vadose and subphreatic environments which stabilized after burial. The nonporous intervals approach values of -2.8 per thousand delta 18 O, 2.5 mol percent Mg, and 600 ppm Sr and are further characterized by nonluminescent grains and intense solution compaction. Data indicate that oolites were initially composed of a metastable mineralogy, cryptocrystalline grains were composed of a more stable mineralogy, and algal grains were composed of a mixture of mineralogies. Oolites in the vadose and subphreatic environments were not fully stabilized under fresh water conditions, but vadose grains carried with them a light delta 13 C soil gas signal after burial. These grains were fully stabilized under post-fresh-water, burial conditions. Observed porosity distribution is readily explained in terms of solution compaction and cementation. Grains with metastable mineralogy in relict vadose and subphreatic intervals were heavily solution-compacted at relatively shallow depths before being converted to stable calcite. Intense solution compaction destroyed pore space in these intervals. Dolomite cementation seems linked with the compaction process. The stablized mineralogy of the phreatic lens, together with associated meteoric cements, resisted solution compaction and thereby preserved pore space.

The Behavior of Zn2+ and Mn2+ During Carbonate Diagenesis: Theory and Applications

ABSTRACT The partitioning behaviors of zinc and manganese can serve as important tracers of diageneticprocesses in limestones. A review of the literature indicates kcalciteZn2+, the trace element partition coefficient of zinc into calcite, equals 5.5 and kcalciteMn2+e 15, at the probable temperatures and solution compositions under which most limestone lithification is achieved. Since k is greater than unity for both these elements, the trace element/calcium ratio in a calcite being precipitated is always greater than the ratio in the solution. Thus in aragonite/calcite transformation by dissolution-reprecipitation in a closed system, the initial calcite is greatly enriched in e ther zinc or manganese relative to the parent aragonite. As closed system, homogeneous dissolution-reprecipitation progresses, the enrichment of the calcite falls exponentially until the ratio in the calcite being precipitated equals the ratio in the aragonite being dissolved. In an open system, dissolution-reprecipitation usually results in a calcite that is enriched in zinc or manganese. The more pure water that enters and leaves the diagenetic site, the greater excess loss of calcium occurs, since calcium preferentially remains in the liquid phase during calcite precipitation. In order to integrate the information provided by the distributions of manganese and zinc in calcites with that of strontium and magnesium, a general model of trace element behavior is proposed. Using the parameters of distribution coefficient, water flow relative to reaction rate (open or closed), and chemistry of the water before it enters the diagenetic site, it is possible to determine the enrichment or depletion of a calcite relative to its parent aragonite. Four distinct situations are noted. Autodepletion refers to the preferential loss of trace element from the diagenetic site due to a high water flow rate relative to reaction rate, where the trace element has a partition coefficient less than unity. A calcite depleted in trace element relative to the parent arag nite (or calcite) results. Autoenrichment is the preferential removal of calcium from the diagenetic site, with consequent enrichment of the calcite in trace element. This may occur for trace elements with k greater than unity. Alloenrichment is the result of calcite precipitation from a liquid that was enriched in trace element (a trace element/calcium ratio greater than that of the parent aragonite or calcite) before it entered the diagenetic site. It may occur for any value of k, but the trace element/calcium ratio of the liquid required for such enrichment increases with decreasing k. Allodepletion results when a liquid rich in calcium and poor in trace element flushes through the diagenetic site. For values of k close to one, each cycle of liquid may introduce s much calcium that after the liquid dissolves aragonite, its trace element/calcium ratio is still less than that of the aragonite. A depleted calcite is precipitated. Examination of the Zn2+, Mn2+, Mg2+, and Sr2+ content of Pleistocene corals from Barbados, West Indies, reveals an increase in Mn2+ and Mg2+ as a result of aragonite/calcite transformation. Sr2+ was lost in this process and Zn2+ remained approximately the same. Note that unlike kcalciteZn2+ and kcalciteMn2+, kcalciteSr2+ and kcalciteMg2+ are both < 1. Such changes denote an open diagenetic water system. Strong intercorrelations among these elements were found in corals altered under late vadose conditions. These suggest that liquid flow rate controlled the trace ele ent chemistry of these corals. Corals altered under phreatic conditions did not exhibit such systematic behavior; their chemistry was likely influenced by neighboring diagenetic events. The general trace element model gives a quantitative indication of the openness of these diagenetic systems. On average each liter of diagenetic liquid dissolved and reprecipitated between 10 and 20 millimoles of calcium carbonate. The trace element model can be applied to calcite/calcite recrystallization and cement precipitation later in the evolution of a limestone. Where there is no significant external source of these trace cations, later calcites should be depleted in strontium and magnesium, and enriched in zinc and manganese.

Vadose and Phreatic Diagenesis: Processes, Products and their Recognition in Corals

ABSTRACT Pleistocene reef corals from Barbados, West Indies, have provided petrographic and chemical criteria for distinguishing the products of mineralogic equilibration (aragonite to calcite transformation) under vadose and phreatic conditions. Petrographically, corals that have equilibrated to subaerial conditions in a phreatic zone contain statistically more calcite cement and secondary porosity than their vadose counterparts. Chemically the phreatic-altered corals contain more magnesium and less strontium than vadose-altered samples. Discriminant function analysis of combined point count and chemical data allows clear distinction of vadose and phreatic populations. Texturally the phreatic calcite mosaic often consists of coarse crystals of calcite (up to several millimeters in maximum dimension) which extend out from the coral skeleton into void filling cement. These mosaics are termed cross-cutting for the manner in which the former walls of filled primary voids do not serve as crystal boundaries. No vadose-altered corals were observed with such cross-cutting mosaics. In contrast the vadose mosaics are termed fabric selective since the boundaries of filled primary voids usually serve as crystal boundaries in the calcite mosaic. These two mosaics result from different modes of aragonite/calcite transformation in the vadose and phreatic zones. Vadose transformation takes place across micron (or less) width films of water, aragonite being dissolved on one side and calcite precipitated on the other. Textural detail is preserved to the level of viewing at low magnification in a light microscope. Phreatic diagenesis often involves the development of zones of chalky aragonite (several millimeters across) between fresh coral skeleton and the growing calcite mosaic. The chalk is a region where extensive dissolution of the coral leaves a very white, friable framework. As the chalk front moves through the coral a calcite product with areas of secondary porosity and calcite cement results. Since water is always present, single calcite crystals may grow to large size and extend beyond the coral skeleton into primary void, yielding the characteristic cross-cutting mosaic. A two water model is proposed in both zones. Dissolution-reprecipitation takes place in a static water held immobile in thin films (vadose) or wider chalk zone (phreatic). In the vadose this water intermittently comes in contact with water percolating through the larger connected pores of the reef sediments. Thus chemical exchange is limited and episodic; strontium values in the calcite build up a la Kinsman (1969) and little magnesium is externally introduced. The immobile water of the chalk areas is constantly in contact with a large reservoir of water passing through the reef; the site of transformation acts as a relatively open chemical system. The resultant calcites have low strontium values due to diffusion from the site and high magnesium values as this ion is constantly introduced from the surrounding high-magnesium calcite sediments. The relative chemical homogeneity of the phreatic zone in time and space is borne out in microprobe analysis of phreatic corals; far more variability is encountered in vadose probe runs.

Mg/Ca Ratio and Salinity: Two Controls over Crystallization of Dolomite

Carbonate precipitation is governed by both thermodynamic and kinetic considerations. The concentration and ionic composition of natural solutions influence not only the mineralogy of the precipitate, but also the crystal size and habit of both metastable and stable phases. Under marine conditions sparry calcite does not precipitate because of poisoning of sideward crystal growth by Mg, which forces Mg-calcite and aragonite to assume a micritic, steep-rhombic, or fibrous habit. Blocky calcite can form only in low Mg/Ca waters, either from primary meteoric solutions, or where Mg has been removed from marine water by dolomite or clays. In hypersaline environments precipitation is commonly rapid and, together with the high concentrations of foreign ions, it is difficult for dolomite to form because of the precise Ca-Mg ordering required. Instead, aragonite or Mg-calcite crystallizes. Dolomite can form only if the Mg/Ca ratio exceeds 5-10:1, and even then it is aphanitic and poorly ordered. At progressively reduced salinities dolomite is able to nucleate at still lower Mg/Ca ratios, approaching 1:1 in meteoric waters. The absence of foreign ions and slow crystallization, commonly under phreatic conditions, cause both dolomite and calcite to form exquisitely limpid, euhedral rhombs. Thus, dolomite probably forms most readily by a reduction in salinity, particularly in a schizohaline environment (alternating between hypersaline and near-fresh conditions, as in a floodable sabkha or a phreatic mixing zone). Flushing marine saline waters with fresh water lowers salinity but maintains a high Mg/Ca ratio; crystallization is slower and the interfering effect of foreign ions is reduced.

Phreatic and Vadose Diagenetic Modification of Pleistocene Limestone: Petrographic Observations from Subsurface of Barbados, West Indies

Data from a shallow cored borehole through 105,000-year-old reef-tract sediments on the south coast of Barbados indicate that the carbonate sediments have been altered more rapidly and more extensively in freshwater phreatic diagenetic environments than in the vadose or marine phreatic diagenetic environments. The upper part of the borehole section has been exposed only to subaerial and shallow vadose processes since emergence from the marine depositional environment about 105,000 years ago. These sediments are recrystallized only partly. Total amounts of cement, mainly needle-fiber cement and dense micrite coatings, are high. Porosity is low. Dissolution, a minor feature in this part of the section, is associated with localized development of needle-fiber fabrics. Those parts of the borehole section which have been occupied by a freshwater phreatic lens have been cemented by calcite microspar. Aragonite grains have been dissolved on a massive scale, resulting in a very well-developed moldic porosity. Neomorphic grain alteration has been volumetrically important. Packstones and grainstones in the lowest part of the borehole are largely unaltered; metastable carbonate mineralogy dominates. This part of the sedimentary column has been subjected alternately to vadose conditions during low stands of sea level (> 70,000 years total) and to marine phreatic conditions during high stands of the sea (< 35,000 years total). Despite prolonged exposure to vadose conditions, diagenetic modification is slight. If the recrystallized lime muds contain greater than 25 percent cement (micrite), then mass-balance calculations suggest large amounts of calcium carbonate have been transported to that part of the sedimentary section exposed to freshwater phreatic diagenetic processes. The source of the additional carbonate may be updip in the aquifer, from mixing of underlying marine pore fluids with the freshwater pore fluids of the coastal water lens, from the overlying shallow vadose zone, or from marine water soon after deposition. Processes which affect early diagenetic modification of carbonate sediments on the south coast of Barbados operate more rapidly and more effectively in the freshwater phreatic environment than in vadose or marine phreatic environments.