Burial Dolomitization Research Articles

Événements diagénétiques carbonatés, marqueurs de l'évolution géodynamique du Moyen Atlas (Maroc)

Carbonate diagenetic events, markers of the geodynamical evolution in the Middle Atlas (Morocco). The carbonate episodes dated from Lias to Paleogene in the Middle Atlas and the High Moulouya areas, have recorded many diagenetic events of which the main markers are grouped in two categories that are significant to the geodynamical evolution of these regions. Then, the sedimento-diagenectic markers represented by earlies cementation and dolomitiza-tion, characterize the eogenectic domain, reflect the sedimentological and paleogeographical dynamic environnements and correspond to the period of stability dated from Lower and Middle Lias. The dynamo-diagenetic markers of mesogenetic and telogenetic domains, materialized by a compaction, a later dolomitization, a tectonic fissuration which is associated with a later cementation and a meteoric dissolution, reflect the geodynamic of these sectors : during the Toarcian and Dogger, the period of filling (burial of liasic carbonate deposits), is expressed by the lithostatic compaction and the burial dolomite ; the intracontinental stage of the atlasic chain is the result of the atlasic compression since Cretaceous, it is materialized by tectonic fissuration and later dolomitization ; finally, the formation of the continental chain with the exhumation of former buried deposits, lead to their partial dissolution by meteoric waters.

Stratigraphic Control on Facies and Diagenesis of Dolomitized Oolitic Siliciclastic Ramp Sequences (Pinda Group, Albian, Offshore Angola)

Pervasively dolomitized, oolitic to siliciclastic ramp sequences are prolific oil reservoirs in the Albian Pinda Group of northern Angola. During the Late Cretaceous, this reservoir series was segmented by salt tectonic gravitational gliding into halokinetic turtle-back and raft structures that provide hydrocarbon traps today. The Albian ramp sequences contain a large variety of reservoir facies, reaching from littoral sandstones with interparticle porosity to various types of dolomitized marine shelf to shoreline carbonates with moldic, vuggy, and intercrystalline porosity. Reservoir prediction is substantially improved by an integrated approach combining high-resolution sequence stratigraphy with carbonate geochemistry. As high-resolution stratigraphy documents, most of the important sandstone reservoirs formed at the base of stratigraphic sequences during shoreface transgression. Pisolitic dolostones with moldic porosity are the best carbonate reservoirs. They developed as transgressive shoreface sand sheets above the basal sandstone reservoirs. Regressive facies tracts contain oolitic and pisolitic dolomites with intercrystalline and moldic porosity that originated in progradational barrier and sand-belt settings. The combined stratigraphic and geochemical approach allowed us to determine the Pinda Group dolomites as early diagenetic in origin, whereas fluid inclusion and oxygen isotope analysis alone suggested a late burial dolomite formation. Dolomite neomorphism at higher burial temperatures modified most of the original geochemical signatures of early, near-surface dolomitization. Cathodoluminescence data and trace element analysis suggest that dolomitization was associated with gradually decreasing pore water salinity. Accordingly, barrier oolites were completely dolomitized and developed intercrystalline porosity during the earliest stages of diagenesis. At a later phase of early diagenesis, as waters became less saline and undersaturated with respect to calcium carbonate, fabric-selective dissolution of the remaining calcareous components occurred. The decrease in pore water salinity and associated porosity generation are genetically linked with high-frequency events of relative sea level lowering. This possible relationship implies that high-resolution sequence stratigraphy can be a valid reservoir prediction tool even in pervasively dolomitized series.

Type, origin, and reservoir characteristics of dolostones of the Ordovician Majiagou Group, Ordos, North China Platform

Dolostones are well developed in the Ordovician Majiagou Group in the Ordos area, North China Platform. These dolostones can be divided into four types: mud-sized to silt-sized crystalline dolostones not associated with gypsum and halite beds (type I), mud-sized to silt-sized crystalline dolostones associated with gypsum and halite beds (type II), mottled silt-sized to very fine sand-sized crystalline dolostones (fine saccharoidal dolostones) (type III), and mottled coarse silt-sized to fine sand-sized crystalline dolostones (coarse saccharoidal dolostones) (type IV). Type I dolostones consist of mud-sized to silt-sized dolomite crystals. Laminar stromatolites, ripple marks, mud cracks and birdseyes are common. Such dolostones are not associated with gypsum and halite beds, but lath-shaped pseudomorphs after gypsum are common. The ordering of dolomites averages 0.59, and molar concentration of CaCO 3 averages 51.44%. δ 13C averages −0.8‰ (PDB Standard), δ 18O averages −2.9‰, δCe averages 0.83. The above characteristics suggest that type I dolostones result from penecontemporaneous dolomitization of lime mud on supratidal flat environments by hypersaline sea water. Type II dolostones mainly consist of mud-sized to silt-sized dolomite crystals. They are commonly well laminated but show no desiccation structures. Such dolostones are intercalated within laminated gypsum and halite beds or are intermixed with them. Such dolostones resulted from dolomitization of lime mud by hypersaline sea water in gypsum and halite precipitating lagoons. Type III dolostones consist of coarse silt-sized to very fine sand-sized dolomite crystals. They commonly underlie type I dolostones and grade downwards to dolomite-mottled limestones and pure limestones. The ordering of dolomites averages 0.63, and molar concentration of CaCO 3 averages 55.64%. δ 13C averages −0.2‰, δ 18O averages −3.3‰, δCe averages 1.24. Such dolostones resulted from reflux dolomitization by hypersaline sea water. Type IV dolostones consist of coarse-silt-sized to fine-sand-sized dolomite crystals. In such dolostones, stylolites are cut by dolomite crystals. Fluid inclusions are present, and the homogenization temperature commonly ranges from 104°C to 203°C. The ordering of dolomites averages 0.85, and molar concentration of CaCO 3 averages 50.65%. δ 13C averages 0.6‰, δ 18O averages −7.4‰, and δCe averages 1.16. Such dolostones resulted from deep burial dolomitization. In the Ordos area, type I and II dolostones modified by palaeokarstification are the major gas reservoir rocks of the Ordos Gas Field at present. Type IV dolostones show good reservoir characteristics and may also be potential reservoir rocks.

Regional dolomitization of the Waulsortian limestone in southeastern Ireland: Evidence of large-scale fluid flow driven by the Hercynian orogeny

The Lower Carboniferous Waulsortian limestone has been affected by late diagenetic, regional burial dolomitization over an area of ∼7000 km 2 extending 170 km northeastward from the Hercynian front. The dolostone is composed of two components: (1) a very fine crystalline replacive component, and (2) a coarse-crystalline, baroque component that fills vuggy porosity developed within the replacive dolostone. Fluid inclusions within the coarse-crystalline component of the regional dolostone indicate that the dolomitizing solutions were warm (∼100 °C) and moderately saline (10–13 wt% NaCl equivalent). The replacive dolomite displays systematic decreases in crystal size and degree of xenotopic texture northward. Both components of the regional dolostone display a regular northward increase in mean δ 18 O values; the replacive dolomite has 87 Sr/ 86 Sr values that decrease northward. These laterally variable characteristics suggest that regional dolomitization resulted from large-scale, north-directed fluid flow of heated brines developed in response to topographic uplift associated with collapse of the continental margin south of Ireland during the Hercynian (Variscan) orogeny.

Geometry of Dolomite Bodies within Deepwater Resedimented Oolite of the Middle Jurassic Vajont Limestone, Italy: Analogs for Hydrocarbon Reservoirs Created Through Fault-Related Burial Dolomitization: ABSTRACT

Geometry of Dolomite Bodies within Deepwater Resedimented Oolite of the Middle Jurassic Vajont Limestone, Italy: Analogs for Hydrocarbon Reservoirs Created Through Fault-Related Burial Dolomitization: ABSTRACT

Epigenetic dolomitization of the Přaídolí formation (Upper Silurian), the Barrandian basin, Czech Republic: implications for burial history of Lower Paleozoic strata

Stratabound epigenetic dolomite occurs in carbonate facies of the Barrandian basin (Silurian and Devonian), Czech Republic. The most intense dolomitization is developed in bioclastic calcarenites within the transition between micritic limestone and shaledominated Přidoli and Lochkov formations deposited on a carbonate slope. Medium-crystalline (100–400 µm), inclusion-rich, xenotopic matrix dolomite (δ18O=−4.64 to −3.40‰ PDB;δ13C=+1.05 to +1.85‰ PDB) which selectively replaced most of the bioclastic precursor is volumetrically the most important dolomite type. Coarse crystalline saddle dolomite (δ18O=−8.04 to −5.14‰ PDB;δ18C=+0.49 to +1.49 PDB) which precipitated in fractures and vugs within the matrix dolomite represents a later diagenetic dolomitization event. In some vugs, saddle dolomite coprecipitated with petroleum inclusion-rich authigenic quartz crystals and minor sulfides which, in turn, were post-dated by semisolid asphaltic bitumen. The interpretation of the dolomitization remains equivocal. Massive xenotopic dolomite, although generally characteristic of a deeper burial setting, may have been formed by a recrystallization of an earlier, possibly shallow burial dolomite. Deeper burial recrystallization by reactive basinal pore fluids that presumably migrated through the more permeable upper portion of the Přidoli sequence appears as a viable explanation for this dolomitization overprint. Saddle dolomite cement of the matrix dolomite is interpreted as the last dolomitization event that occurred during deep burial at the depth of the oil window zone. The presence of saddle dolomite, the fluid inclusion composition of associated quartz crystals, and vitrinite paleogeothermometry of adjacent sediments imply diagenetic burial temperatures as high as 160°C. Although high geothermal gradients in the past or the involvement of hydrothermally influenced basinal fluids can account for these elevated temperatures, burial heating beneath approximately 3-km-thick sedimentary overburden of presumably post-Givetian strata, no longer preserved in the basin, appears to be the most likely interpretation. This interpretaion may imply that the magnitude of post-Variscan erosion in the Barrandian area was substantially greater than previously thought.

Burial dolomitization and dedolomitization of the late Cambrian Wagok Formation, Yeongweol, Korea

The Cambrian Wagok Formation in Yongweol, Korea, involved two stages of dolomitization, separated by an intermediate phase of dissolution, dolomite cementation, and dedolomitization. Textural, isotopic, and chemical investigations of the dolomites and calcite indicate that they all formed or recrystallized in a burial diagenetic environment. Cathodoluminescence petrography shows two stages of dolomitization (Types I & II). Type I dolomite is non- to dully luminescent with a heavily corroded outre rim, comprising more than 90% of the whole sequence, whereas Type II dolomite is brightly luminescent and overgrew mostly on the corroded Type I dolomite. Replacement of Type I dolomite by calcite before the Type II dolomitization is supported by the following textural evidences: 1) the tip of Type I dolomite is heavily corroded; 2) relic crystal boundaries as well as ghosts of Type I dolomite can be observed within calcite; 3) calcite contains relic crystals of Type I dolomite; and 4) Type II dolomites always show sharp crystal surfaces, and cross-cut into calcite. Carbon isotopic compositions of Type I dolomite are in the range of −0.6 to +0.8‰ (PDB), which suggests that carbon isotopes were buffered by preexisting marine carbonates. Lighter oxygen isotopic values (−9.3 to −8.2‰, PDB) as well as xenotopic texture indicate that Type I dolomites formed in a burial diagenetic environment. Type I & II dolomites and dedolomite are Fe-, Mn-, Na-, and Sr-poor, all less than 300 ppm. Thus, similar trace and minor elemental contents of the three phases suggest that the diagenetic fluids responsible for all the phases were chemically similar in terms of trace and minor elements. The higher initial87Sr/86Sr ratio of Type I dolomite (0.7102–0.7105) than that of Cambrian seawater (0.7088–0.7092) suggests that the dolomitizing fluids were basin-derived.

Pre-ore burial dolomitization adjacent to the carbonate-hosted Gays River Zn–Pb deposit, Nova Scotia

The Gays River Formation of the Lower Windsor Group in the vicinity of the Gays River Zn–Pb deposit was completely dolomitized via a volume for volume transformation prior to mineralization. Premineralization porosity was the precursor limestone porosity, comprising fenestrae, dissolution molds, and intercrystalline, intraskeletal, and growth pores, which remained essentially unchanged through dolomitization. Regionally, the dolomites (one replacement and one cement) overlapped with sediment compaction and dissolution of aragonite. The dolomites are characterized by low δ18OPDB values (−14 to −4[Formula: see text]; average −8[Formula: see text]), slightly radiogenic 87Sr/86Sr ratios (0.70778–0.70900), and locally high percentages of FeCO3 and MnCO3 (up to 13.5 and 9.5 mol%, respectively). The presented petrographic and geochemical data compiled with existing fluid-inclusion microthermometry data suggest dolomitization by hot brines, in a burial setting. High Fe and Mn concentrations south of the studied area, with progressive depletion towards the northeast and northwest, suggest either a northward flow of an Fe- and Mn-rich fluid or interaction with local sources of Fe and Mn, during single or multiple flows. By preserving the host-rock permeability, dolomitization played a role in ground preparation but was not genetically associated with the Zn– and Pb–rich fluid that mineralized the Gays River deposit.

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

Paleozoic Fluid History of the Michigan Basin: Evidence from Dolomite Geochemistry in the Middle Ordovician St. Peter Sandstone

ABSTRACT The isotope (Sr and O) and elemental (Mg, Ca, Mn, Fe, and Sr) compositions of the various dolomites in the Middle Ordovician St. Peter Sandstone in the Michigan Basin are determined and the variations are modeled in terms of fluid-rock interaction or as mixing relations. These geochemical models, combined with the paragenetic sequence of the dolomites and late anhydrite cement, suggest the existence of at least four distinct diagenetic fluids in the St. Peter Sandstone during the Paleozoic. Fluid 1 (Sr = 30 ± 10 ppm, Sr/Ca = 0.019, 87Sr/86Sr = 0.7093 ± 1) has a composition consistent with a modified older (pre-Middle Ordovician) seawater origin, which indicates that the flow path for this fluid had a major upward component. This fluid resulted in th first and volumetrically most important burial dolomitization event, producing dolomite in both carbonate and quartz sandstone lithofacies in the St. Peter Sandstone. Fluid 2 (Sr = 30 ± 10 ppm, Sr/Ca = 0.019, 87Sr/86Sr = 0.7084 ± 1, low Fe) has a composition consistent with a modified Middle to early Late Ordovician seawater origin, suggesting a major downward component for fluid flow. Fluid 2 produced dolomite cement in the carbonate lithofacies that postdates Fluid 1 dolomite. The composition of Fluid 3 (Sr > 50 ppm, Sr/Ca = 0.044-0.110, 87Sr/86Sr = 0.710-0.711, high Fe) is best interpreted to reflect a heated, deep basinal brine that had previously interacted with the K-feldspar-rich rocks near the Cambrian-Precambrian unco formity in the Michigan Basin, indicating a major upward component for fluid flow. Fluid 3 produced dolomite cement in quartz sandstone lithofacies that postdates Fluid 1 dolomite. Fluid 4 (87Sr/86Sr = 0.7085 ± 1) resulted in precipitation of late anhydrite in fractures. The 87Sr/86Sr ratio of the anhydrite is consistent with Fluid 4 originating as a dilute fluid that interacted extensively with Silurian gypsum in the Michigan Basin; this indicates that the flow path of Fluid 4 had a major downward component. The St. Peter Sandstone was an important migration pathway for many different types of fluids in the Michigan Basin, and the proposed fluid origins indicate that both upward and downward cross-formational fluid flow occurred during the Paleozoic. Older and younger modified seawater produced most of the burial dolomite in the St. Peter Sandstone.

Determining fluid source and possible pathways during burial dolomitization of Maryville Limestone (Cambrian), Southern Appalachians, USA

ABSTRACTDetailed petrographic analyses along a depositional transect from a carbonate platform to shale basin reveals that dolomite is the principal burial diagenctic mineral in the Maryville Limestone. This study examines the role of burial dolomitization of subtidal carbonates. Dolomite occurs as a replacement of precursor carbonate and as inter‐ and intraparticle cements. Four different types of dolomite are identified based on detailed petrographic and gcochemical analyses.Type I dolomite occurs as small, irregular disseminations typically within mud‐rich facies.Type II dolomite typically occurs as inclusions of planar euhedral rhombs (ferroan), 5–300 μm in size, in blocky clear ferroan calcite (meteoric) spar. Type II dolomite is non‐luminescent. Type I and II dolomite formed during shallow to intermediate burial diagenesis. Type III dolomite consists of subhedral to anhedral crystals 10–150 μm in size occurring as thin seams along stylolites and as thick bands a few millimetres in width. This dolomite consists of dominantly non‐luminescent rhombs and, less commonly, orange luminescent and zoned rhombs. Type IV dolomite consists of baroque or saddle‐shaped, 100–1500 μm crystals, and is non‐luminescent. Type IV dolomite formed during the period of maximum burial.Types III and IV dolomite increase in abundance downslope. Type III dolomite contains 1.2–2.6 wt% Fe and a maximum of 1000 ppm Mn. The distribution of these elements displays no distinct vertical or lateral trends. In contrast, Fe and Mn distributions in Type IV dolomite exhibit distinct spatial trends, decreasing from 3.5–4.5 wl% Fe and 0.1–0.3 wt% Mn in the west (slope/basin) to 1.5–2.5 wt% Fe and less than 600 ppm Mn in the east (shelf margin), a distance of approximately 60 km. Spatial trends in Fe and Mn distributions in Type IV saddle dolomite, suggest a west‐east fluid flow during late burial diagenesis. Types III and IV dolomite have a mean δ18O value of ‐ 7.8%00 and a mean δ13C value of + 1.1%00 (relative to the PDB standard). Based on a range of assumed basinal water composition of 2.8%00 SMOW, temperatures calculated from δ18O values of Types III and IV dolomite range between 75 and 160°C. 87Sr/86Sr data for Types III and IV dolomite range from 0.7111 to 0.7139. These values are radiogenic when compared to Cambrian marine values and are consistent with the presence of a diagenetic fluid that interacted with siliciclastic sediments.The distribution of Palaeozoic facies in the southern Appalachians indicates a Cambrian shale source for the fluids, whilst burial curves suggest a Middle Ordovician age for burial fluid movement.

Controls on Reservoir Development in a Shelf Carbonate: Upper Jurassic Smackover Formation of Alabama

Hydrocarbon reservoirs of the Upper Jurassic Smackover Formation in Alabama are predominately oolitic and pelletal dolostone. Pore systems are dominated by moldic and secondary intraparticle pores, intercrystalline pores, or mixtures of these pore types. All Smackover reservoirs in Alabama have been strongly affected by early cementation, dissolution of calcium-carbonate allochems, and dolomitization. Marine-phreatic cement occluded primary interparticle porosity in much of the Smackover reservoirs in Alabama. Dolomitization of the Smackover in Alabama included penecontemporaneous, early burial, and late (deep) burial episodes. Early burial dolomite predominates. Fabric-selective dolomitization yielded reservoirs strongly influenced by both depositional fabric and diagene is. Nonselective dolomitization yielded reservoirs with intercrystalline pore systems shaped primarily by diagenesis. Porosity evolution was controlled regionally by level of thermal exposure, mode of dolomitization, and proximity to the Wiggins arch. Thermal exposure is inversely related to porosity, but the relationship is weak (r2 < 0.5). Fabric-selective dolostone is slightly more porous than nonselective dolostone when averaged over the entire study area (averages of 18.1% and 15.1%, respectively; p = 0.0001), but nonselective dolostone is more porous at a given level of equivalent vitrinite reflectance. Smackover fields on the north flank of the Wiggins arch are unusually porous given their level of thermal maturity, and are unusual in other ways as well. Local porosity variation was controlled by depositional fabric, early cementation, dissolution, and burial compaction and cemen ation. Regional permeability variation cannot be explained using existing data. Permeability is locally controlled by pore-throat size, the effects of dolomite crystal-size distribution, early cementation, fracturing, and burial compaction and cementation. Pore-throat size exhibits the strongest overall correlation with permeability (r2 = 0.54). Permeability and porosity are strongly correlated locally, but the regional correlation is weak.

Early-replacement dolomitization and deep-burial modification and stabilization: A case study from the late precambrian of the Zawar area, Rajasthan (India)

The dolomites of Zawar area belong to the late Precambrian upper Aravalli Supergroup. The initial dolomitization process was initiated by seawater through tidal pumping in the tidal flat environment. It is evidenced by low REE content and δ13C values (+1.12 to +2.85 o/oo PDB) and subsequent burial dolomitization is shown by xenotopic texture, depletion in Na and Sr content and lighter oxygen isotopic composition. The early seawater and later burial dolomitization processes played the major roles in the formation of carbonates of the Precambrian upper Aravalli Supergroup.

Morphology and significance of super-mature calcretes from the Upper Old Red Sandstone of Scotland

Thick calcrete profiles occur in the Late Devonian–early Carboniferous Kinnesswood Formation of the Midland Valley of Scotland. These profiles consist of highly brecciated peloidal and coated grain fabrics with laminar calcrete horizons and linings to fractures. The profiles resemble supermature calcretes associated with late Cenozoic surfaces in the southwestern United States and reflect pedogenesis over periods of several hundred thousand years or more. The possible origins of the highly complex fabrics seen are reviewed and the polygenetic nature of such profiles is stressed. It is suggested that brecciation takes place co-incident with subaerial exposure of the calcrete. Mature profiles may be predisposed to brecciation and surficial weathering because of their susceptibility to soil erosion as a result of changes in vegetation cover. The calcretes underwent phreatic cementation, burial dolomitization and silicification.

Shallow burial dolomitization and dedolomitization of Mid-Cenozoic, cool-water, calcitic, deep-shelf limestones, southern Australia

ABSTRACT Oligocene to mid-Miocene, deep-shelf, bryozoan-rich limestones across southern Australia are variably altered to gray to orange, Ca-rich, medium-crystalline, sucrosic dolomite. Degree of replacement ranges from scattered rhombs to complete dolostone units several tens of meters thick and many kilometers in areal extent. The locale and timing of dolomitization are tightly constrained to shallow burial and mid- to late Miocene. Dolostone varies from friable to dense and well lithified. Dolomitization is fabric specific; muddy sediments are preferentially replaced; calcite bryozoans and brachiopods form biomolds. Geochemistry suggests that dolomitization was predominantly by seawater-limestone interaction, but admixing of continental possibly meteoric groundwater is required by data on stable and radiogenic isotopes and trace elements. Sr isotopes confirm a mid-Miocene age for the dolomite, if precipitated from seawater with no inherited limestone values. Dolomite crystals have undergone variable degrees of dissolution, and meteoric calcite cement has locally filled the resultant rhombic voids. Dissolution began in the crystal cores, implying some sort of metastability, and expanded outward until, in some ins ances, the whole dolomite crystal was dissolved, leaving dolomolds in otherwise unaffected limestone. This dolomite occurrence, well constrained by regional geology, shows that widespread, porous and permeable, fabric-destructive, medium-crystalline, sucrosic dolostone can form in the shallow subsurface soon after sediment deposition. Such dolomite, however, may be metastable in the presence of variably mixed continental-marine waters, and newly formed crystals can undergo dedolomitization soon after precipitation.

Eustatic control on early dolomitization of cyclic peritidal carbonates: Evidence from the Early Ordovician Upper Knox Group, Appalachians

Research Article| July 01, 1992 Eustatic control on early dolomitization of cyclic peritidal carbonates: Evidence from the Early Ordovician Upper Knox Group, Appalachians ISABEL P. MONTAÑEZ; ISABEL P. MONTAÑEZ 1Department of Earth Sciences, University of California, Riverside, California 92521 Search for other works by this author on: GSW Google Scholar J. FRED READ J. FRED READ 2Department of Geological Sciences, Virginia Polytechnic Institute, Blacksburg, Virginia 24061 Search for other works by this author on: GSW Google Scholar Author and Article Information ISABEL P. MONTAÑEZ 1Department of Earth Sciences, University of California, Riverside, California 92521 J. FRED READ 2Department of Geological Sciences, Virginia Polytechnic Institute, Blacksburg, Virginia 24061 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1992) 104 (7): 872–886. https://doi.org/10.1130/0016-7606(1992)104<0872:ECOEDO>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation ISABEL P. MONTAÑEZ, J. FRED READ; Eustatic control on early dolomitization of cyclic peritidal carbonates: Evidence from the Early Ordovician Upper Knox Group, Appalachians. GSA Bulletin 1992;; 104 (7): 872–886. doi: https://doi.org/10.1130/0016-7606(1992)104<0872:ECOEDO>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The Early Ordovician Upper Knox Group is characterized by stacked meter-scale peritidal cycles that repeat at high frequencies (104 to 105 yr). Stacking patterns (stratigraphic trends in lithofacies and cycle thickness) of meter-scale cycles define five depositional sequences that, in conjunction with Fischer plots, delineate five long-term relative sea-level fluctuations during Upper Knox deposition. Intrabasinal and interbasinal correlation of Upper Knox Fischer plots suggests that the third-order sea-level events were eustatic. Meter-scale peritidal cycles likely formed in response to high-frequency, fourth-and fifth-order, eustatic sea-level fluctuations superimposed on these third-order sea-level events.Upper Knox cyclic carbonates are extensively dolomitized; as much as 85% of all dolomite is stratiform and consists of early dolomite exhibiting minor to extensive modification by burial dolomite. Synsedimentary dolomitization likely occurred in modified sea water during tidal-flat progradation governed by high-frequency sea-level events. This is suggested by the common association of dolomite with mud-cracked laminites and silicified evaporite nodules, the systematic decrease in dolomite abundance below laminite cycle caps, and the presence of dolomite clasts in regoliths veneering high-frequency cycle tops or in transgressive limestones of the overlying cycle. Dolomite distribution within depositional sequences shows a strong relationship to third- and fourth-order eustatic sea-level events, indicating that long-term eustasy also strongly controlled early dolomitization of Upper Knox carbonates.Mass-balance calculations show that the proposed sabkha model of dolomitization in concert with composite eustasy could generate stratiform dolomite of considerable vertical and lateral extent in peritidal cyclic carbonates. This reflects the duration of progradation and supratidal exposure (104 to 105 yr) available for dolomitization to proceed, and the broad zone of active dolomitization that would develop during continued progradation throughout each cycle period. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Dolomitization of a Carbonate Platform During Late Burial: Lower to Middle Cambrian Shady Dolomite, Virginia Appalachians

ABSTRACT Outer platform carbonates of the Lower to Middle Cambrian Shady Dolomite, Virginia Appalachians, record a lengthy and complex diagenetic history, ranging from marine cementation to burial diagenesis during late Paleozoic overthrusting. Analysis of petrographically well-preserved marine cements has provided an estimate of the isotopic composition of Lower Cambrian marine carbonate (18O = -7.5 to -6.1 PDB; 13C = +0.2 to +0.8 PDB; 87Sr/86Sr = 0.70869 to 0.70975), establishing a base line with which to assess later diagenetic carbonate . Meteoric dissolution and early calcite cement postdate marine cementation and formed during subaerial exposure. The platform margin carbonates were extensively dolomitized; dolomitization postdates calcite cementation and the initial development of stylolites and tectonic fractures. Several episodes of dolomite replacement and cementation are evident. Zone 1 dolomite consists of rare relict dolomite cores which were replaced and overgrown by zone 2A dolomite, the dominant replacement phase. Zone 2A dolomite is depleted in 18O (18O = -10.2 to -7.0) and enriched in 13C (13C = +1.0 to +1.6) relative to marine carbonate, reflecting precipitation from warm formation fluids buffered by the host carbonate. Uniformly low Sr contents (25 to 65 ppm) and nonradiogenic 87Sr/86Sr (0.70900 to 0.70971) also attest to pore water equilibration with the host rock. Replacement dolomitization was followed by widespread carbonate dissolution, which formed vuggy porosity and local solution breccias. The host dolomite locally is encrusted by Mississippi Valley-type (MVT) occurrences of ore mineralization consisting of sphalerite, galena, and pyrite. This major generation of ore minerals is overgrown by three generations of dolomite cement. The first dolomite cement, zone 2B, has similar 18O composition (13O = -10.3 to -7.4) but slightly depleted 13C values (13C = +0.4 to +1.1) relative to zone 2A dolomite and has similar Sr contents (30 to 45 ppm) and 87Sr/86Sr ratios (0.70933 to 0.70966). Zone 2B dolomite cement exhibits petrographic, isotopic, and geochemical affinities with zone 2A replacement dolomite, implying that the two dolomite types formed from fluids of similar composition and temperature. Primary fluid inclusions in zone 2B dolomite indicate precipitation from warm (minimum 120 to 150°C) brines (23-26 wt. % NaCl equiv.). These temperatures imply that the dolomitizing fluid was enriched in 18O due to extensive water-rock interaction with host carbonate, which also buffered the fluid 13C, Sr, and 87Sr/86Sr content. Zones 3 and 4 dolomite cement are depleted in 18O (13O = -13.8 to -11.3) relative to the previous dolomites, implying a temperature increase to 150 to 210°C. The late dolomite cements have 13C compositions (13C = -0.7 to +0.9) ranging from those similar to the earlier dolomites to more depleted values. Zones 3 and 4 dolomite cement are enriched in Sr (60 to 135 ppm) which is radiogenic (87Sr/86Sr = 0.71025 to 0.71445). These later cements thus record fluids that were no longer buffered by the host carbonate. Dolomite cements locally were overgrown by sphalerite, followed by authigenic quartz and calcite cement. Secondary three phase fluid inclusions in quartz and calcite indicate hot (175 to 225°C) saline (30 to 33 wt. % NaCl equiv.) fluids. Temperatures and pressures interpreted from the fluid inclusions are compatible with estimated burial depths in excess of 5 km. The dolomites formed during deep burial, coeval with late Paleozoic, Alleghanian-age thrusting. The Mg required for burial dolomitization was derived from pressure solution of structurally lower, platform interior dolomites that were overridden by the Shady Dolomite thrust sheet. Tectonic uplift and deformation resulted in regional fluid flow, which transported this Mg to the Shady Dolomite outer platform during burial dolomitization.

Sedimentology and Diagenesis of the St. Peter Sandstone, Central Michigan Basin, United States (1)

The Middle Ordovician St. Peter Sandstone occurs at depths between 1600 and 3600 m in the central Michigan basin. Integration of conventional core and wireline log studies indicates that the formation consists of, from base to top, (1) up to 250 m of sandstone deposited in intertidal and supratidal sand flats with associated dolomitic lagoonal deposits, and shallow subtidal shoreface environments, (2) sandstones deposited in subtidal shoreface to upper offshore environments, and, (3) dolomitic and argillaceous sands deposited in a storm-dominated epeiric sea. Sandstone composition is closely related to depositional environment: quartz arenites occur in higher energy littoral facies, whereas feldspathic and carbonate-rich sands occur in predominantly lower energy shelfal f cies. Important modification of primary mineralogy and porosity occurred during diagenesis. A generalized, basinwide model for paragenesis includes (1) early calcite marine cement, (2) syndepositional dolomitization, (3) widespread precipitation of quartz and K-feldspar overgrowths, (4) pervasive replacement of early carbonate by burial dolomite, (5) local dissolution of unstable framework grains (e.g., K-feldspar) and carbonate cement, (6) growth of authigenic clay, and (7) pressure solution and additional precipitation of quartz overgrowths. Authigenic clays apparently formed along with or subsequent to economically significant secondary porosity. The different pathways of sandstone diagenesis observed in the St. Peter Sandstone are largely dependent on primary textures and composition inherited from the environment of deposition rather than other factors, such as depth of burial or position in the basin. The relationships among primary textures and composition, diagenesis, and reservoir sandstone properties is useful for prediction of reservoir quality in the St. Peter Sandstone in the Michigan subsurface. Weakly cemented sandstone reservoirs characteristic of high-energy shoreface and shallow offshore depositional environments have the best reservoir quality in the basin. Clay-cemented sandstone reservoirs deposited on offshore shelves have the poorest reservoir quality. Incompletely quartz-cemented sandstone reservoirs, comm n in intertidal/supratidal facies, have intermediate reservoir quality.

Fracture and Fault-Controlled Burial Dolomitization, Late Devonian Wabamun Group, Alberta

Fracture and Fault-Controlled Burial Dolomitization, Late Devonian Wabamun Group, Alberta

Large-scale fluid flow in the Middle Devonian Presqu'ile barrier, Western Canada Sedimentary Basin

Saddle dolomite cements are regionally extensive in the Middle Devonian Presqu9ile (or Keg River) barrier and are spatially associated with Mississippi Valley-type mineralization at Pine Point (Northwest Territories). From northeastern British Columbia to Pine Point over a lateral distance of 400 km, these dolomite cements show general trends of decreasing 87 Sr/ 86 Sr ratios (0.7106 to 0.7081) and homogenization temperatures (178 to 92 °C), with some increase in δ 18 O values (-16‰ to -7‰ PDB). These regional trends suggest that hotter and more radiogenic basinal fluids moved eastward updip along the Presqu9ile barrier and mixed with cooler ambient formation waters. These movements of basinal fluids were probably related to tectonic thrusting and compression, sedimentary loading, and tectonic uplift on the western margin of the Western Canada Sedimentary Basin, which began to form during Late Jurassic to Early Cretaceous time and climaxed during the Late Cretaceous to Paleocene. The Middle Devonian Presqu9ile barrier appears to have acted as a deeply buried regional conduit system that played an important role in focusing and channeling these basinal fluids. These regional fluid flows appear to have been responsible for extensive burial dolomitization, secondary migration of hydrocarbons, and local Mississippi Valley-type mineralization in the Western Canada Sedimentary Basin.