Vadose Silt Research Articles

Multistage selective diagenesis increasing poro-perm properties of Anisian sponge-microbial-coral patch reefs and adjacent facies in the Muschelkalk of Upper Silesia, Southern Poland

Carbonate rocks, particularly reefs and circum-reef facies, are significant reservoir rocks whose reservoir properties depend on their primary composition and diagenetic history. This paper reconstructs the influence of multistage selective diagenesis on the pore system evolution and porosity-permeability properties of the Anisian tropical reef system (Karchowice and Diplopora Beds) in the Muschelkalk of Upper Silesia, southern Poland, using field and slab observations, standard and cathodoluminescence petrography, bulk and SIMS carbon and oxygen stable isotope analyses, porosity-permeability plug study, and computer image analysis of macroporosity (>5 mm across). The reef system comprises two distinct levels of sponge-microbial(-coral) reef mounds surrounded by diverse bioclastic, peloidal, coated-grain, and micritic facies rich in Balanoglossites burrows. The strata experienced complex diagenetic alterations related to various fluids (marine, evaporitic, meteoric) and diagenetic environments (shallow substratal, burial, telogenetic), including (in chronological order): 1) early-marine precipitation of isopachous fibrous rims; 2) early dissolution during emersion; 3) brine-reflux dolomitization; 4) patchy silicification; 5) recrystallization; 6) burial calcite cementation by heated marine fluid; 7) deposition of vadose silt; 8) meteoric calcite cementation; 9) chemical compaction; 10) fracturing; and 11) final karstification, dedolomitization, FeO precipitation, and Liesegang ring formation by meteoric water during Cenozoic telogenesis. Many processes were selective and affected the same (more permeable) parts of the rocks. The strata exhibit porosities reaching 50% (combined plug porosity of 6–35% and macroporosity of up to 20%) and plug permeabilities of 0.07–70 mD, with the highest values occurring in the reefs and burrowed facies. The dominant late-stage telogenic dissolution pores originated from selective removal of dolomitized micrite within burrows and between sponge-microbial automicrite as well as dolomite crystals. The study illustrates the transition of the reef system from tight to porous due to multistage selective diagenesis and fluid flow, which enhanced subtle permeability differences between various types of micrite. This is the first case where four different fluids (dolomitizing, early dissolving, calcite-forming, and late dissolving) selectively migrated through the same rock parts at different times and modified them one after another.

Alteration effects of karstification and hydrothermalism on middle Permian Qixia formation at the Wulong section, South China

Middle Permian Qixia Formation in the southwestern region of Sichuan (SW China) has experienced multiphase fluidisation, resulting in an unclear understanding of the reservoir reconstruction effect. In this study, a systematic analysis of the Qi2 member in Wulong Town was carried out by combining field outcrop petrology and geochemistry. The results demonstrated that multiple sets of crystalline dolomite-bioclastic limestone cycles were stacked vertically in the Qi2 member, accompanied by the development of fractures and karst channels. The dolomite was mainly composed of silty-fine dolomite (D1) and recrystallised dolomite (D2). Furthermore, obvious multiphase dolomitic cements (Cd1-Cd2) were present in the fractures and pores. Early karst is known to have lithologic mutation surface development and karst channel development at the top of several secondary cycles. The vadose silt dolomites (Cd1) having karst channels developed dull luminescence under cathode luminescence (CL). Both the geochemical indicators of elements and rare earth element (REE) content indicated dysoxic-oxic environmental conditions. The hydrothermal solution displayed tectonic carniole characteristics in the strata burial stage. Fractures and pores were filled with hydrothermal minerals such as coarse dolomites-saddle dolomites (Cd2, with some caused by recrystallisation of the Cd1 hydrothermal solution) and fluorites. Coarse dolomites-saddle dolomites developed dull-red luminescence with a bright-red rim under CL and their δ18OVPDB values were more negative than those of middle Permian limestone samples. Both the geochemical indicators of elements and REE content indicated the suboxic-anoxic environmental conditions. Karstification had minor constructive impact on the reservoir of the Qi2 member in Baoxing in southwestern Sichuan. Most products of karstification were distributed as fillings in channels. Aside from creating certain networked fractures, the hydrothermal solution was mainly filled with hydrothermal minerals along the fractures, pores and early karst channels. Karst and the hydrothermal solution mainly damaged the middle and upper parts of the middle Permian Qixia Formation in Southwest Sichuan. The impact of episodic fluid on the restoration of the carbonate reservoir was mainly restricted by channels for fluid migration and thickness differences among the reservoir. However, certain thick-layered and massive crystalline dolomite may hold promise for exploration.

Bryozoan‐rich stromatolites (bryostromatolites) from the Silurian of Gotland and their relation to climate‐related perturbations of the global carbon cycle

ABSTRACTBryozoan–stromatolite associations (bryostromatolites) formed conspicuous reef structures throughout the Sheinwoodian (Wenlock) to Ludfordian (Ludlow) stratigraphy on Gotland but have not been described so far. They are mainly composed of encrusting bryozoans forming a complex intergrowth with porostromate and spongiostromate microbes and are different from the abundant stromatoporoid–coral–algal reefs with respect to their composition. In the bryostromatolite different growth stages can be identified. The observed succession can be taken as evidence for cyclic environmental changes during reef formation. Stenohaline reef‐dwelling organisms, such as echinoderms, sponges, corals and trilobites, indicate fully marine salinities. Ten localities exposing bryostromatolites were discovered. Individual bryostromatolites are small with few decimetres up to one metre in size, and occur solely in shallow marine areas. Common features of these reefs on Gotland are cauliflower‐like growth, a high bryozoan diversity, a high abundance of phosphatic fossils and components such as bryozoan pearls and inarticulate phosphatic brachiopods, enhanced bioerosion, Palaeomicrocodium crusts, vadose silt and gypsum pseudomorphs. The high abundance of Palaeomicrocodium, as well as the alternation with other crust‐forming contributors, suggest that it could have been formed directly at the palaeo‐sea surface, probably in times of minor but high‐frequency sea‐level fluctuations. Vadose silt and pseudomorphs after gypsum in reef cavities indicate subaerial exposure shortly after reef growth. The high amount of phosphatic components indicates a high nutrient input, probably by dust. All bryostromatolites were formed in times of strongly elevated δ13C values. The unusual combination of sedimentological and palaeoecological features, as well as their occurrence exclusively during strong positive δ13C excursions, are evidence that the bryostromatolite development responded to climatic/oceanographic changes, which may have played an important role in reef control.

Origin of the penecontemporaneous sucrosic dolomite in the Permian Qixia Formation, northwestern Sichuan Basin, SW China

Abstract The characteristics, formation time, and origin of the sucrosic dolomite reservoirs in the Permian Qixia Formation of northwestern Sichuan Basin are analyzed. Core and outcrop description and microscopic analysis of the sucrosic dolomite samples are carried out. It is found that the dolomite has typical features different from other kinds of dolomites: (1) This dolomite is generally medium-coarse in crystal size, and often associated with very finely to finely crystalline dolomite and cave-filling dolomite. (2) Typical identification marks of eogenetic karstification are developed at the top of the upward-shallowing sequence. (3) The medium-coarse crystalline sucrosic dolomite is cut by the early diagenetic karst fabric, and is characterized by dolomite with dissolution edge, dolomite vadose silt in pores, and transgression clay filling between the medium-coarse dolomite crystals. The medium-coarse crystalline sucrosic dolomite was formed earlier than the eogenetic karstification. The sucrosic dolomite with occasional cloudy core and clear rim has bright cathodoluminescence, high inclusions temperature, significant negative skewness carbon and oxygen isotopic compositions, and rare-earth element (REE) pattern similar to seawater, indicating it experienced two periods of dolomitization, evaporative concentration reflux-infiltration and penecontemporaneous seawater circulation hydrothermal fluid dolomitization. The study results not only update the understanding on the dolomitization time of Qixia Formation, demonstrate that the sucrosic dolomite can be formed in the penecontemporaneous stage when seawater reflux superimposed with hydrothermal fluid effects, but also show that the taphorogenesis in the Dongwu period began in the Early Permian. Moreover, the dolomite controlled by the grain bank migration and terrain in the slope break appears in bands of large scale, this knowledge provides basis for expanding the exploration field of this type of reservoirs.

Diagenesis and reservoir quality of carbonates rocks and mixed siliciclastic as response of the Late Carboniferous glacio-eustatic fluctuation: A case study of Xiaohaizi Formation in western Tarim Basin

Abstract Late Carboniferous-Permian was a longest-lived and most widespread ice age. The depositional environment, diagenesis and reservoir quality of carbonate rocks as response to the glacio-eustatic changes was analyzed in this study. The Upper Carboniferous Xiaohaizi Formation in the western Tarim Basin is an important oil reservoir. The dolostones from the Xiaohaizi Formation have very good physical properties, with maximum porosity and permeability of 16.6% and 214mD, respectively. The sedimentary facies of the Xiaohaizi Formation consist of carbonate shoals and lagoon on a mixed siliciclastic-carbonate platform. There was a strong heterogeneity of carbonate rocks deposited on shoals. Porous dolostones always alternate with tight sparry allochemical limestones. Based on the petrographic observation and stable isotope measurement, the controlling factors for the heterogeneity and reservoir quality were revealed. The palaeosol, vadose silt, karst breccia, detrital kaolinite, darkening of dolostones and enrichment of 18O indicate the short-lasting subaerial emergence during the glacial period. During the glacial sea level fall, the mixed siliciclastic and carbonate allochems deposited on bioclastic and intraclastic shoals, while dolomitization of calcic grains occurred under a relatively low-temperature. Through the correlation analysis of physical property, lithology, rock components, it's suggested that the reservoir quality of the Xiaohaizi Formation was controlled by mixed sedimentation and dolomitization associated with glacio-eustatic fluctuation. At the burial diagenesis, the effect of hydrothermal fluid on physical properties of dolostones with relative high primary porosity was positive, whereas resulted in the poikilotopic calcite cementation. The assembly of fluorite, dickite and authigenic pyrite were indicators for hydrothermal fluid accumulation. The control of glacio-eustatic fluctuations on sedimentation and diagenesis in carbonates could be used to forecast the reservoir quality.

Characteristics and significance of the penecontemporaneous karst in lacustrine carbonate, Da'anzhai Member, Lower Jurassic, Beibei area, eastern Sichuan Basin

Abstract There are few reports about the penecontemporaneous karst of the lacustrine carbonate of Da'anzhai Member which developed on the top of Lower Jurassic Ziliujing Formation, eastern Sichuan Basin. Based on the analysis of the macroscopic and microscopic characteristics of Heping Reservoir section in Beibei area, It is discovered that the penecontemporaneous karst in the lacustrine carbonate, Da'anzhai Member, and its characters as follows: (1) The dissolved fractures and karrens are common, which have been filled with vadose silts, bioclasts, terrigenous quartzes, crystal and intergrown calcites; (2) The penecontemporaneous karstification mainly occurs in the micrites and shell limestones of the middle and upper part of the shallowing-upward sequence, and the desiccation cracks occur in some shallowing-upward sequences; (3) Three meteoric-water diagenetic lenses are identified in this section. By further study we also find the factures, stylolite and burial dissolution pores occur along the karst system (tubes, conduits and caves of the karst). And the asphalt not only fills the stylolites and fractures, but also is mixed with the vadose silt in the karst system, although the pore space formed by penecontemporaneous karstification is filled with vadose material and cements. Therefore it is concluded that the development and modification of the tight lacustrine carbonate reservoir of Da'anzhai Member, to some extent, are relevant with the penecontemporaneous karstification, and the penecontemporaneous karst reservoir may develop in the buried Da'anzhai carbonate of the basin.

Subaerial exposure and drowning processes in a carbonate platform during the Mesozoic Tethyan rifting: The case of the Jurassic succession of Western Sicily (central Mediterranean)

The Liassic carbonate platform succession outcropping at Monte Maranfusa (central Western Sicily) consists of a shallowing-upward sequence of peritidal carbonates, with Jurassic to Paleogene pelagic limestone and siliciclastic Tertiary covers above. The cyclic sequences of subtidal wackestones/packstones, intertidal microcrystalline carbonates with bird's-eye pores, and supratidal bioclastic grainstones are interbedded with dark layers of the following composition: 1) dark-gray, compact, and well-cemented limestone with blackish clasts, interpreted as calcretes (a type of carbonate soil) and 2) reddish calcite laminae, deformed by elongated cavities, filled with vadose silt, interpreted as paleokarst. This succession is crossed by almost vertical faults, of the Late Liassic to Miocene, which often coincides with neptunian dykes, filled by several generations of Toarcian–Early Miocene pelagic sediments. Another system of dykes, known as neptunian sills, filled by injected Upper Lias–Dogger pelagic sediments, lies parallel to the stratification. The parallel dykes were caused by the flexure of the platform during the Jurassic and presumably by a planar slip in the carbonate rocks, whereas neptunian dykes are caused by faulting episodes. Here, we present evidence that the dark layers in the Liassic succession of Monte Maranfusa, previously described by many authors only as parallel dykes, can actually be interpreted as a) neptunian sills, b) pedogenic calcretes, and c) paleospeleothems. Therefore, we found evidence of exposure/flooding intervals in the evolution of the carbonate platform during the Liassic, linked to different pulses in both the subsidence/tectonic activity and the sea-level oscillations. At the top, Fe–Mn crusts (hardgrounds) seal the carbonate platform succession, which is in turn overlain by condensed pelagic deposits, confirming its drowning during rifting processes.

ENVIRONMENTAL CONTROLS ON THE GENESIS OF MARINE MICROBIALITES AND DISSOLUTION SURFACE ASSOCIATED WITH THE END-PERMIAN MASS EXTINCTION: NEW SECTIONS AND OBSERVATIONS FROM THE NANPANJIANG BASIN, SOUTH CHINA

Abstract A widespread marine microbialite and underlying truncation surface occur in Permian–Triassic sections of South China. We interpret the microbialite to have formed as a shallow, open-marine benthic framework stimulated by high seawater CaCO3 saturation. The widespread distribution across platform interiors and lack of asymmetry or thickening toward platform margins is incompatible with an alternative hypothesis, that microbialite deposition was stimulated by upwelling anoxic, alkaline waters. The truncation surface beneath the microbialite is irregular with overhangs and small caverns extending up to 30 cm beneath the surface indicating a dissolutional origin. Petrographic observations refute the interpretation that strata immediately beneath the surface contain pendant cements, meniscus cements, and vadose silt. Measurements of the anisopachous fibrous cements show that thickened areas have random, not downward orientations. Pores retain the pointed geometry consistent with isopachous cement. Car...

Formation mechanism of reservoir oolitic dolomite in Lower Triassic Feixianguan formation, northeastern Sichuan Basin, southwest China

A series of marine natural gas fields were recently discovered in oolitic dolomites of the Lower Triassic Feixianguan formation, northeastern Sichuan Basin, southwest China. The mechanism forming these reservoir dolomites is debatable, limiting the ability to characterize these reservoir successfully. Based on the investigation of the representative Dukouhe, Luojiazhai, and Puguang areas, this issue was addressed by examining the distribution, petrology, and geochemistry of the dolomites, the most comprehensive study to date was provided. Dolomitization occurred at a very early stage of diagenesis, as shown by the petrological features of the rock fabric. Vadose silt, which is composed primarily of dolomitic clasts, is found in the primary and secondary pores of the oolitic dolomite. This indicates that the overlying strata were subjected to dolomitization when the Feixianguan formation was located in the vadose zone. Therefore, it may be inferred that the dolomitization which occurred before the formation was exposed to meteoric conditions. The spatial distribution and geochemical characteristics of the dolomite indicate that dolomitization occurred as a result of seepage reflux. The degree of dolomitization decreases with increasing distance from the evaporative lagoon. Furthermore, the type and porosity of the dolomite vary in different zones of the upward-shoaling sequence, with the porosity gradually decreasing from the highest layer to the lowest layer. This reflects a close relationship between dolomitization and seawater evaporation during the formation of the dolomite. Geochemical analysis provided further evidence for the relationship between the dolomitization fluid and the coeval seawater. The 87Sr/86Sr and δ13C isotopes, as well as the abundances of trace elements, Fe and Mn, indicate that seawater concentrated by evaporation acted as the dolomitization fluid. These results also show that dolomitization most likely occurred in a semi-closed diagenetic environment. Therefore, the main mechanism of oolitic dolomite formation is seepage reflux, which occurred at an early stage of diagenesis.

Pliocene–Pleistocene sequences bounded by subaerial unconformities within foramol ramp calcarenites and mixed deposits (Salento, SE Italy)

Nine discrete, metre-scale sequences, of Early Pliocene to Middle Pleistocene age, were deposited in the small Novoli graben (Salento peninsula, Puglia, S-Italy). They consist of carbonate and mixed carbonate-siliciclastic sediments, deposited on a slowly subsiding foreland ramp. Skeletal concentrations and intervening less fossiliferous intervals have been examined to provide information on major environmental parameters and infer the dynamics of their changes. Taphonomic and palaeoecological analyses indicate that storm-induced waves and currents, reduced sediment input, and settling behaviour of components were the main factors controlling the features of the various shellbed types. The concentrations were formed below fair-weather wave base in low-stress inner-to-outer shelf environments and are often associated with surfaces or intervals that are characterized by sedimentary condensation. Vertical change in the fossil content within individual cycles indicates water depth changes that were parallel to climatic fluctuations, hence may result from glacio-eustatic sea-level changes. Most sequences are bounded by subaerial, karstic unconformities. Because of the regional setting of low subsidence rate, the record of the relative sea-level fluctuations is incomplete. Episodes of subaerial exposure and concomitant effects of vadose diagenesis are documented by: (i) diagenetic changes leading to hardening of unconformity horizons; (ii) local subvertical solution pits developed in the vadose zone below unconformity surfaces; (iii) networks of polygonal cracks below unconformities; and (iv) infilling of solution pits and polygonal cracks with vadose silt and marine sediment inwashed during transgressions following the subaerial stages. Lower sequences are characterized by tighter cementation and significant increase in moldic and vuggy porosity, due to superimposition of the diagenetic effects of repeated high-amplitude sea-level fluctuations.

Development of a Lithocodium (syn. Bacinella irregularis)-reef-mound- A patch reef within Middle Aptian lagoonal limestone sequence near Nova Gorica (Sabotin Mountain, W-Slovenia)

A Middle-Aptian (zone of Palorbitolina lenticularis) „patch reef“ of about 40 m maximum thickness with marked morphology was analyzed at the Sabotin Mountain near Nova Gorica. It is developed above an basal unit of superficial oolites within lagoonal sediments and is overlain by shallow subtidal to intertidal sediments which reveal short time periodically subaerial exposure and early diagenetic freshwater influxes (birds eyes, vadose silt, characeans). Above these sediments within the zone of Salpingoporella dinarica finebedded to platy, laminated bituminous limestones occur. The central part of the reef structure reveals an alternation of individual lenses of Lithocodium-boundstones and of rudist-beds (up to 4 m thickness) which are separated by coarse- to medium-grained, moderately to poorly sorted bioclastic sands. These facies types show also lateral interfingering and are concentrated in the central part of the buildup where the greatest thickness can be observed and where packstones, grainstones and boundstones prevail. The neighbouring lagoonal sediments which consist of mudstones and wackestones predominantly were analyzed in the so-called Sabotin-standardprofile which is located north of the patch reef at a lateral distance of about 300 m. Three vertical profiles (A = 90 m, B = 100 m, C = 64 m thickness) were analyzed. The middle profile B of greatest thickness is taken as reference profile and documents best the vertical facies development within the patch reef directly overlying a basal unit of peloidal packstones with superficial ooids. The patch reef itself is characterized by the faunal associations within the unit rich in Lithocodium and rudists. It is overlain by a subtidal unit of peloidal mudstones with very minor biogenic allochems. An intertidal unit above rich in birds-eyes and vadose silt is followed by fine-laminated black shales which are covering the general seqeunce of interfingering patch-reef - lagoonal sediments. Using different time lines (marker horizons) for correlation it can be shown that already very early differential compaction of fine-grained uncemented sediments in comparison to the core area of the patch reef is of great importance. The greater thickness of the patch reef itself is caused by an intensive early diagenetic marine phreatic cementation within the core zone and by the early fixing of sediment by Lithocodium aggregatum (syn. Bacinella irregularis) resulting in a greater resistivity against compaction. Furthermore a lagoonal side and an more open marine side of the patch reef can be determined. Lithocodium aggregatum is the main constructing organism within the buildup investigated, beginning with the incrustation of varying substrates and biogenic particles. Finally, a dense network of encrustation-sequences is formed interfingering with the general sedimentary textures intensively and resulting in larger „biogenic-cemented“ patches within the sediment. The correlation of the three profiles A, B, and C allows to reveal the history of differential compaction of the associated lagoonal sediments in comparison to the more massive patch reef limestones. It becomes obvious that the main compaction must have occurred within the time span of about 50 m sediment-deposition overlying the patch reef. Differential compaction resulted in differences in thickness of about 10 m from the patch reef (profile B) to the more lagoonal influenced sediments (profile A) within a lateral distance of about 50 m.

Lacustrine and palustrine carbonate petrography: an overview

Lacustrine limestones were formerly identified by their faunistic (limnea, planorbis) and floristic (Charophytes) content. For 30 years, indications of pedogenesis have been found in many lacustrine deposits, and consequently the concept of palustrine limestone was defined. Lacustrine fabrics are not that numerous: varved, laminated, homogeneous, peloidal, brecciated, gravelly, bioturbated (burrows), bioclastic, algal, and stromatolitic. Detrital beds are sometimes present and are interpreted as bottomset deposits. Palustrine fabrics result from exposure and pedogenesis of lacustrine mud. The main processes involved in this evolution are: cracking, with planar, curved, craze and skew planes, colonization by plants resulting in root traces, marmorization (redistribution of iron due to water table fluctuation), and redistribution of carbonates (needles, subspherical or cylindrical vertical nodules, carbonate coatings, early and late diagenetic crystals, Microcodium). Carbonate palustrine features can be associated with other minerals: palygorskite, gypsum, or silica. Alternation of lacustrine sedimentation and exposure/pedogenesis leads to the pseudo-microkarst facies resulting from enlargement of the complex network of root traces and horizontal cracks. The voids in the pseudo-microkarst facies are infilled with a polyphased internal sediment composed of carbonate and vadose silt and phreatic and vadose cements. Traces of exposure and pedogenesis are less in evidence in lacustrine bioclastic sands and algal-stromatolitic limestones. Finally, under certain conditions, the surficial laminar horizon and its associated perlitic crust (ooids) develops on palustrine muds and form a desert stromatolite.

Transgressive oolites onlapping a Silurian rocky shoreline unconformity, Gotland, Sweden

Stratigraphical relationships, facies and petrographical evidences define a rocky shoreline in the Late Wenlock of Gotland. The major facies-elements forming the rocky shoreline are 1) several subaerially exposed reefs, which were eroded during falling base-level and which form the topography, and 2), onlap of oolite grainstones which were deposited during the subsequent rise of the base-level. Five localities along the rocky shoreline are discussed in detail here, viz., Svalings 1 where the unconformable boundary is accessible, and Bara 1–4 where the petrography of the onlapping oolite has been studied. A regression at the end of the latest Cyrtograptus lundgreni Biozone resulted in emersion of parts of the Gotland carbonate platform. Erosion during the short time interval of maximum lowstand (top C. lundgreni Biozone) caused the topography of this rocky shoreline and resulted in truncations, corrosion and vadose silt in the exposed reefs. The following parvus-nassa Biozone starts with the transgressive Bara Oolite Member (lowermost Halla Formation) which onlaps the southern flanks of the youngest reef generation of the Slite Group. The rocky shoreline can be traced for 11 km striking SW-NE. Calculated base-level for the erosion is situated more than 16 metres below the top of the highest preserved part of one Slite reef, indicating the probable minimum magnitude of the sea-level drop. Detailed petrographic work on the onlapping oolite is consistent with the interpretation that it was formed in the tidal zone of the shores bordering the palaeocoast. The Slite Rocky Shoreline Unconformity (SRSU) is herein considered as the platform-proximal equivalent to the sequence boundary separating the regressive Fröjel Formation and the transgressive Bara Oolite Member (lowermost Halla Formation) in the Klintehamn area on western Gotland. The rocky shoreline development is taken as the most reliable evidence of a rapid regressive-transgressive cycle in the mid-Homerian of Gotland.

Lower Cisco Formation (Pennsylvanian- Virgilian) Paleokarst, Wolf Flat Field, Palo Duro Basin, Texas: ABSTRACT

Wolf Flat field produces from karstified Lower Cisco Formation at the rimmed northeast shelf margin. Mounds, peripheral skeletal debris, and oolitic facies form the vertical/lateral succession. Mounds are composed of crinoids, bryozoans, corals and algae. Skeletal packstone/grainstones surround mounds, With back-mound wackestone. Ooid grainstone formed in high-energy passages onto the shelf and cap shallowing-upward cycles. Karstification resulted from two 100+ ft. (30+ m) sea level drops, that dropped in a step-by-step manner to form [approximately]20 porous intervals that extend through all facies and lithologies in the field. Aragonitic mound and skeletal grainstone/wackestones experienced intense dissolution. Calcitic ooid grainstones dissolved less and form cave roofs and floors. Dissolution was in two phases: first, small vertically oriented pipes formed, resembling a [open quotes]swiss cheese[close quotes] texture; second, was intense lateral dissolution that created a [open quotes]sponge[close quotes] texture, with porosities up to 60-70%. Eventually, collapse breccias formed, with some clasts transported in the karst system. Porosity was reduced by vadose silt, breccia clasts, speleothem cements, transgressive shale infilling, and burial saddle dolomite. Portions of the field nearest the rimmed margin were dolomitized. [delta][sup 18]O is within the range of early meteoric diagenesis and [delta][sup 13]C is negative and becomes more negative towardmore » the exposure surface.« less

Recognizing exposure, drowning, and "missed beats"; platform-interior to platform-margin sequence stratigraphy of Middle Ordovician limestones, East Tennessee

Stratigraphic sequences in a Middle Ordovician platform-interior carbonate succession (Steinhauff and Walker, in press) are correlated to the platform margin. Surfaces of subaerial exposure are correlated among seven stratigraphic sections, four from the platform succession and three from platform-margin rocks. Surfaces of subaerial exposure define stratigraphic sequences within a carbonate succession that shows little evidence of cyclicity in the field. Interpretation of carbonate facies geometry is based on outcrop observation, petrographic and geochemical analysis of samples, and consideration of paleobathymetric curves. In the platform succession, most sequences are bounded by exposure surfaces, but in more continuously subtidal areas periods of shallowing and drowning are discerned in bathymetric curves on the basis of multiple lines of evidence. Exposure surfaces are evidenced by subjacent meteoric cements or other features indicating exposure such as truncated marine cements, vuggy porosity with vadose silt and pendant cements, or mud cracks. At some localities, a marked shallowing event correlates to exposure elsewhere. In subtidal sediments, drowning is indicated by the lack of shallow-water physical sedimentary features and water-depth curves based on multiple lines of evidence including the presence of red algae, thin-shelled (eyeless) trilobites, delicate arborescent bryozoa, and deposit-feeding organisms. Evidence for drowning unconformity is in some cases provided by cephalopod-rich, black to blood-red limestone beds rich in iron and manganese oxides, and occasionally by corroded surfaces encrusted with these minerals. We have referred to the succession as apparently noncyclic because evidence for exposure and shallowing is subtle and is not always observable in the field. Nonetheless, these strata show fourth-order subsequences that can be grouped into third-order sequences by the existence of pronounced and less pronounced exposure surfaces. Pronounced surfaces of exposure are those that overlie fenestral mudstones that contain exposure features, such as crystal silt and calcite cement in vugs lined by reddish crusts, and are truncated by deeper-water lithologies. For example, seven pronounced surfaces of subaerial exposure define six third-order sequences, 10-50 m thick, on the platform near Thorn Hill, Tennessee. Within the lower four third-order subsequences, 19 less pronounced surfaces showing fewer exposure features are present in approximately 400 m of section. These surfaces define fourth-order subsequences that are correlative from place to place in the platform succession. Many, but not all, of these platform fourth-order subsequences can be correlated to the platform margin. Also, some fourth-order subsequences are present only at the platform edge. Fifth-order, meter-scale packages, or parasequences, are present within these fourth-order sequences, but are not correlative over wide areas as are the other sequences. Most are probably the result of migration of lateral environments. Geometries unlike those of siliciclastic sequences result where condensed sections or deepening events are followed by exposure in this carbonate succession. The sequences defined here constitute shallowing-upward packages that are, then, a type of cycle. These are used to construct modified Fischer plots for both the platform-interior and platform-margin stratigraphic successions. Surfaces of subaerial exposure are correlated from the platform to the platform margin by matching pronounced exposure surfaces and deepening events shown in water-depth curves for the platform with pronounced exposure surfaces and deepening or shallowing events at the platform margin. This analysis provides several possible solutions, the most conservative of which suggests that some sequences or cycle beats are missing. Beats are missed in three ways: (1) sea level remains near the platform margin, causing deposition to be restricted to that area; (2) tectonic uplift exposes the platform interior but not platform-margin areas until subsidence allows resubmergence; and (3) drowning carries platform margin areas to depths precluding carbonate deposition there, though sediment does accumulate on the platform. In Cases 1 and 2 the missed beats would be on the platform, but in Case 3 they would be at the platform margin.

Calcretes from a palaeosinkhole in jurassic palaeokarst (subbetic, southern Spain)

Calcretes have been recognized in the bottom of a palaeokarstic sinkhole etched into Early Jurassic carbonates from the Subbetic (Southern Spain). These are 3 to 10 cm thick and they show an internal structure in which three units can be distinguished. The lower unit is composed of alternate irregular bands of unfossiliferous micrite and sparry calcite. In the micrite, several features point out the activity of endolithic microorganisms as fungi and bacteria. The middle unit, which has a laminated structure with palisade calcite spar and some vadose silt is interpreted as a speleothem and, finally, the upper unit shows calcrete pisoids and rhizocretions. The isotope ratio analysis gives results in which δ 13C ranges from −10.1‰ to −10.9‰ and δ 18O ranges from −5.7‰ to −7.0‰. These values are very similar to those from palaeosols of other ages and localities, which is an important argument favouring emersion stages, with continental deposition in an island with an arid or semi-arid climate. Nevertheless, these calcretes outcrop in association with a palaeokarstic relief reflecting wet climate before the calcrete formation.

Anatomy of a phylloid algal buildup, Raytown Limestone, Iola Formation, Pennsylvanian, Southeast Kansas, U.S.A.

Quantitative petrography and temporal-spatial relationships of microfacies were used to interpret the depositional history of Iola carbonates. The Paola Limestone, consisting of four microfacies, was deposited on a carbonate ramp and appears to record the maximum transgression within the Iola cycle of deposition. The overlying Raytown Limestone was composed of nineteen microfacies arranged in three vertically successive depositional models of similar general configuration. These models revealed an initial bioclastic bar evolving upward into a complex bioaccumulated-bioconstructed phylloid algal buildup. The latter was replaced landward by a siliciclastic distal deltaic environment and graded basinward into upper ramp and lower ramp carbonates. Locally, restricted circulation caused the development of hypersaline conditions. The shallowing-upward depositional evolution of the phylloid algal buildup consisted of four distinct stages: bioclastic (hydrodynamic buildup), bioaccumulation, bioconstruction, and bioclastic (destruction in storm-dominated conditions and intermittent emergence). Diagenesis of Iola microfacies was complex and included: marine phreatic 1 (bioturbation, syneresis, micritization, micrite lithification, and hardground genesis; freshwater vadose—phreatic undersaturated 1 (dissolution of evaporites, moldic dissolution, collapse brecciation, and vadose silt infiltration); marine phreatic 2 (isopachous rim and botryoidal cementation); freshwater phreatic saturated 2 (sparite cementation, and extensive neomorphism); deep burial (anhydritization, dolomitization, stylolitization, and silicification); and late uplift (fracturation and cementation). Most of the diagenesis was microfacies specific.

Subaerial exposure and cement stratigraphy of a Silurian bioherm in the Klinteberg Beds, Gotland, Sweden

Abstract The cementation of Upper Silurian biohermal rudstones and bedded crinoidal grainstones on Gotland, has been investigated with eathodoluminescence microscopy. The cement stratigraphy includes early marine cement succeeded by clear calcite cements indicating transition from oxidizing to increasingly reducing pore water conditions. The distribution of cement generations shows a systematic variation which indicates preferential penetration of the coarse-grained biohermal rocks by oxidizing meteoric water. This cement stratigraphy together with internal sediment, interpreted as vadose silt, indicates late Silurian emergence and subaerial exposure. The final stages of cement growth and pore occlusion took place after reburial of the locality.

Petroleum Reservoirs in Silurian Dolomite of Western Illinois: ABSTRACT

The basal Silurian Alexandrian carbonates, sandwiched between the Ordovician-Silurian and Silurian-Devonian unconformities, have produced more than 3 million bbl of oil from the Buckhorn East and Brooklyn fields on the western margin of Illinois. These fields are shallow (450-670 ft), remote from the most-productive portion of the Illinois basin, and potential exists for additional production from this unconformity play. Three lithofacies were recognized in Alexandrian carbonates in detailed core and wireline studies that included thin-section, x-ray, and SEM analyses. These carbonates are regressive sequences composed of shallow-shelf, restricted-subtidal, and intertidal facies. The shallow-shelf facies is primarily biosparite composed of crinoid and brachiopod debris; t is facies is dolomitic in places. The restricted-subtidal facies consists of both bioturbated and undisturbed gray micrites containing stylolites, green chloritic clay, and glauconite; sparry clacite fills vertical cracks. The intertidal facies is tan, mottled, vuggy dolomite composed of algal mats, anhydrite-filled birdseye structures, burrows, and scattered marine fossils. Petroleum reservoirs apparently developed as a result of dolomitization in intertidal and shallow-shelf carbonates. Vuggy porosity, a possible result of karst leaching, is found in pay zones within the dolomitic intertidal facies. Evidence of emergence and subaerial weathering include the presence of vadose silt, anhydrite, and nontectonic breccia. End_of_Article - Last_Page 306------------

Recognition of Paleosilcretes: Example from North Texas: ABSTRACT

Recognition of fossil silcretes may be hampered by weathering, erosion, or subsequent burial. Silcretes develop during periods of exposure and nondeposition and can represent cumulative sedimentation for geologically important periods of time. The basal Cretaceous conglomerate in north Texas (Antlers and Twin Mountains formations) was deposited and lithified on the Wichita paleoplain. The silcrete formed in a tropical climate without seasonal variation. Outcrops are sporadic yet widespread and topographically prominant. The rock is a quartz and chert-pebble conglomerate with a variety of silica cements. It is recognized as a silcrete by the following criteria. (1) Both the detrital and authigenic components are composed primarily of silica. (2) Vadose quartz overgrowths are found in association with phreatic chalcedonic cements. Vadose silt occurs with both cement types. Zonation within the cements suggests intermittant cementation. (3) Reworked cemented grains indicate syndepositional cementation, necessarily a surface phe omenon. (4) Petrified wood is abundant. Abraded petrified wood further suggests recurring silicification and transportation. End_of_Article - Last_Page 291------------