Microcrystalline Calcite Research Articles

Meteoric diagenesis, marine diagenesis, and microporosity in Pleistocene and Oligocene limestones, Enewetak Atoll, Marshall Islands

Microporosity occurs in Pleistocene and Oligocene limestones cored on Enewetak Atoll. In cores of Pleistocene limestone (10–80 m deep), mi microporosity is present in aragonitic skeletal grains that underwent partial intrafabric dissolution (chalkification) leaving a porous meshwork of aragonite needles. Microporous aragonite is most abundant in 5–10 m thick intervals below zones of intense diagenesis associated with paleo-freshwater lenses. Extensive microporosity was apparently generated by slow intrafabric dissolution of aragonite in mixing zones below freshwater lenses. Microporous aragonite is an intermediate step in the calcitization or dissolution of aragonite. Most calcitized aragonite (neomorphic calcite) formed by precipitation of sparry calcite in (over) microporous aragonite. Aragonitic fossils were not converted to microporous, microcrystalline calcite. Therefore, data from the Pleistocene of Enewetak suggest that widespread, microporous, microcrystalline calcite does not form directly from pure aragonite, at least not in open meteoric systems or mixing zones. Microporous micrite occurs in Oligocene wackestones 930 m deep deposited in a slope environment. The microporous micrite is characterized by equant and prismatic crystals of low-magnesium calcite (LMC). Equant crystals are anhedral to euhedral and 2–10 μm in diameter. Prismatic calcite crystals are subhedral, 5–10 μm wide, and 20–50 μm long. Stable isotopic and trace element geochemistry suggest that microporous micrite formed by diagenetic alteration of high-magnesium calcite (HMC) micrite in deep seawater undersaturated with respect to HMC but supersaturated with respect to LMC (probably near the sediment-water interface). Mineralogy and crystal morphology of the microporous aragonite is very different from microporous shelf limestones in the Thamama Group (Cretaceous, Persian Gulf; see Moshier, 1989). Equant microcrystals of calcite in the Oligocene micrites are similar to those in microporous parts of the Thamama Group. Widespread microporosity in LMC limestones could form by alteration of HMC micrite in mixing zones or in seawater undersaturated with respect to HMC but supersaturated with respect to LMC.

Fossil cyclic calcrete in the Kuwait Group clastic deposits (Mio-Pleistocene) of Kuwait, Arabian Gulf

An investigation of the upper Kuwait Group clastic sequence (Mio-Pleistocene) exposed along the Jal Az-Zor escarpment in north Kuwait revealed several horizons of fossil calcrete. Field occurrences and textural characteristics of the studied sequence indicated that the fossil calcrete was hosted in cyclic fluviatile muddy sandstones and developed through diagenesis during inter-fluvial periods. Depositional and diagenetic features of the calcretized fluviatile sandstone units suggested that each cycle started with the deposition of a fluviatile muddy host sandstone during a humid period. This was followed by a period of semi-aridity during which calcrete profiles developed. The upper part of the calcrete profiles was weathered and terra rosa type soil was formed during wet seasons. This was succeeded by a severe arid climate and the deposition of desert aeolian sand sheets. Fossil nodular calcrete was only developed within the fluviatile muddy sandstones. The calcretized sandstones were lithified in two stages of diagenesis-an early stage that was responsible for the authigenesis of microcrystalline calcite and the development of calcrete nodules, and a later stage during which macrocrystalline calcite cement was precipitated as an intergranular cement displacing and replacing the clay matrix of the host sediments and filling fractures and cavities in the calcrete nodules. The findings and conclusions of the present study will help in understanding the lithostratigraphic setting of the Kuwait Group clastic sequence in the northern Arabian Gulf region.

Diagenesis of Devonian Reefal Carbonates in the Oscar Range, Canning Basin, Western Australia

ABSTRACT The Oscar Range, part of the extensive Devonian reef complex of the northern Canning Basin, is a large atoll (80 10 km) in which Frasnian and Famennian (Late Devonian) reefs grew around an emergent island of Precambrian metasedimentary rocks. Many sediments in the Oscar Range had high depositional porosities, but this porosity was largely occluded by diagenetic alteration. Diagenesis included marine cementation, dolomitization, pressure solution, burial cementation, and karstification. Field mapping, petrography, cathodoluminescence, and carbon and oxygen isotope analyses have been used to document six major diagenetic episodes that affected Oscar Range sediments. Earliest cementation occurred in the marine environment, where reef-margin and associated subfacies developed abundant non-luminescing radiaxial fibrous and microcrystalline calcite cements. Based on convergent covariant isotopic trends, marine cements had a characteristic Late Devonian oxygen signature of -4.5 (±0.5) (PDB) and a carbon signature of +2.0 (±0.5) (PDB). In well-cemented parts of the reef complex, synsedimentary neptunian fissures formed and were filled with cement and sediment. Cements in these fissures have isotopic ratios that are essentially identical to the values determined for marine cements, thereby suggesting that the fissures acted as conduits f r seawater flow and cementation early in the history of the reef complexes. Marine diagenesis was followed by marine-burial and possible meteoric-phreatic cementation, and dolomitization. Marine cements are overgrown by non-luminescing scalenohedral cements that grade into banded bright- and non-luminescing blocky calcite cements that are common on the platform but are missing in many reef-margin and marginal-slope deposits. Banded luminescence may represent fluctuating reducing and oxidizing conditions in pore waters across the intermittently emergent platform. The reef complexes, however, were generally drowned reefs with little chance for prolonged meteoric exposure. This sedimentologic evidence suggests that banded-luminescing cements may have formed in the marine-burial environment. The next cement generation consists of non-luminescing, non-ferroan bloc y calcites that occur throughout the reef complex. Stratigraphic relationships combined with isotopic results (18O = -6.0 to -9.0; 13C = +2.0) suggest that this spar is a burial cement resulting from Late Devonian through Early Carboniferous burial. The reef complex was exhumed and eroded in the Late Carboniferous. Dedolomite, dolo-moldic porosity, pisolitic calcrete crusts, iron oxide stained blocky cements, and deep (> 200 m) sinkholes formed at the unconformity surface. Non-ferroan, irregularly banded, moderate- to bright-luminescing blocky calcite precipitated throughout the reef complex. Unconformity-related cements, calcrete crusts, and dedolomites have distinctive isotopic signatures that show little variation in oxygen, -7.0 to -9.0, but have a broad range of carbon values, +1.5 to -8.0. Highly negative carbon values are probably related to the interaction of meteoric fluids with isotopically light soil gas at the er sion surface. In post-Carboniferous time, most of the remaining porosity was filled with non-ferroan, bright-luminescing blocky calcite that formed by pressure solution/reprecipitation during burial. Isotopic analyses (18O = -8.0 to -12.0; 13C = +1.5) support a burial origin for this generation of cements. These cements fill primary porosity, tension gashes along stylolites, and hairline fractures along which hematite is leached. A final generation of ferroan, homogeneous, dull- to non-luminescing blocky calcite cement formed in a late-burial event that, according to isotopic evidence (18O = -10.5 to -15.0; 13C = +2.0 to -9.0), was probably related to movement of basinal brines through the reef complexes.

Late Pleistocene unconformity in the Gulf of Kavalla, northern Aegean, Greece

A 147 cm core from the Gulf of Kavalla on the northern Aegean shelf displays two different lithologies separated by an erosional contact. The upper part (0–128 cm) consists of clayey silt to silty clay grading into silty sand and represents a neritic to nearshore marine environment. The lower section (128–147 cm) is a stiff calcareous mudstone with scattered microcrystalline calcite nodules, and probably represents part of a subaerial soil horizon (Bca). Seismic data indicate that at this location the unconformity between the late glacial subaerial surface and the overlying post-glacial transgressive deposits occurs at a depth of 150 cm, and has apparently been sampled by the core.

An Early Carboniferous (Visean) Lacustrine Oil Shale in Canadian Arctic Archipelago

The Lower Carboniferous (Visean) Emma Fiord Formation in the Canadian Arctic Archipelago is an oil shale of lacustrine origin composed of black carbonaceous shale, siltstone, and marlstone with interbedded sandstone, conglomerate, and oolitic and algal limestones. At Kleybolte Peninsula on Ellesmere Island, the Emma Fiord Formation was deposited on the tectonically active northwestern rim of the Sverdrup basin; it is now thermally overmature and well beyond the dry gas zone of organic maturation (vitrinite reflectance ^approx 5.0). In contrast, the Emma Fiord sequence on Grinnell Peninsula, Devon Island, near the southern edge of the Sverdrup basin, is immature to marginally mature (vitrinite reflectance = 0.26-0.44) and is composed predominantly of liptinite-rich oil sha e. These kerogen-rich rocks contain a high volume of microcrystalline calcite and compositionally are marlstones. The Emma Fiord lake system probably was a closed basin and may have been thermally and chemically stratified (meromictic). Seasonal or continuous high organic productivity by phytoplankton (and bacteria?) in the epilimnion resulted in accumulation of lipid-rich calcareous sediment below an anoxic hypolimnion. The Emma Fiord oil shales on Grinnell Peninsula contain both autochthonous and allochthonous organic material. Two types of oil shales are recognized in this sequence: (1) Alginite-rich oil shales with a clay-carbonate matrix in which liptinite content may be as high as 93% of the organic matter. The hydrogen/oxygen index indicates kerogen of intermediate composition between types I and II. These oil shales are hydrogen rich and oxygen poor. (2) Alginite-poor oil shale with a carbonate matrix, relatively hydrogen poor and oxygen rich. The hydrogen/oxygen index indicates kerogen of intermediate composition between types II and III. The Emma fiord oil shales were deposited in lakes formed immediately prior to or possibly synchronous with the initiation of rifting in the Sverdrup basin. Syntectonic red-bed conglomerates derived from uplifted horst blocks directly overlie the Emma Fiord rocks. A few beds of conglomerate and sandstone in the upper part of the Emma Fiord Formation possibly record the onset of faulting. The formation closely resembles contemporaneous sequences in northern Alaska, Yukon Territory, Greenland, and Spitsbergen. Clearly, similar tectonic and paleoclimatic factors influenced sedimentation over this area in the Early Carboniferous, with the Sverdrup basin locations lying within 10°-15° of the paleoequator.

Marine Vadose Beachrock Cementation by Cryptocrystalline Magnesian Calcite--Maui, Hawaii

ABSTRACT Active precipitation of high-magnesian cryptocrystalline calcite cement (15.5 mole % MgCO3) in the intertidal zone on the north shore of Maui, Hawaii, results in formation of an extensive beachrock exposure at Hookipa Park. Framework grains in the beachrock are coarse algal, molluscan, and intraclast-bearing sands. Meniscus fabric is dominant in the cement, indicating that most lithification occurs in the vadose zone. The cryptocrystalline cement is generally brownish gold in color, but very thin, clear isopachous fringes of microcrystalline calcite surround darker cryptocrystalline cement, especially in upper horizons of the beachrock. Peloidal texture is scarce in the cement but is present in a few samples. Silt and fine-sand-sized skeletal fragments have been trapped in he evolving cement, resulting in cement-supported fabric. Hollow rims of microcrystalline calcite occupy some pore spaces. These hollow rims may have precipitated at the air-water interface of bubbles that formed in pores, and they may therefore be additional evidence of cementation in the vadose zone. Interstitial water samples from wells that penetrate portions of the beachrock and unconsolidated beach sands reveal temporal chemical variations that suggest the importance of CO2 degassing in the formation of beachrock cement. When chemical and tidal cycles fluctuate with the same period, PCO2 values in pore water of the intertidal zone generally bear a direct relationship to tidal stand. Increasing values of PCO2 most often occur during semi-daily rising tides and decreasing values of PCO2 occur during falling tides. Saturation ratios with respect to calcite vary in direct proportion to PCO2. Saturation levels generally increase during rising tides and decrease during falling tides. T ese data suggest that CO2 degassing during falling tides results in precipitation of calcite cement from pore water of the beach.

Timing of Syntaxial Cement: ABSTRACT

Echinodermal fragments are commonly overgrown in ancient limestones, with large single crystals growing in optical continuity over their skeletal hosts (i.e., syntaxial overgrowths). Such syntaxial cements are usually considered to have precipitated from meteoric pore waters associated with a later stage of subaerial exposure. Although several examples have been reported from ancient carbonates where petrographic relationships may indicate an early submarine formation of syntaxial cement, no occurrences have been noted in Holocene submarine-cemented rocks. Syntaxial cements of submarine origin have been found in Bermuda beachrock where isopachous high-magnesian calcite cements merge with large optically continuous crystals growing on echinodermal debris. Examination of other Holocene sediments cemented by magnesian calcite indicates that echinodermal fragments are not always overgrown syntaxially, but may be rimmed by microcrystalline calcite. The reason for this difference is not clear, although it may be a function of the spacing of nucleation sites and rates of crystal growth. A review of syntaxial cements from several localities in ancient carbonate sequences reveals that many are best interpreted as having formed in the submarine setting, whereas it is more clear that others formed from meteoric precipitation. These occurrences suggest that care should be exercised in inferring meteoric diagenesis from syntaxial overgrowths and that the possibility of submarine formation should be considered. End_of_Article - Last_Page 295------------

Recristallisation des calcaires micritiques en fonction de la maturation thermique dans les Basses-Terres du Saint-Laurent du Québec

Typical sections of the Ordovician carbonate platform sequence in the St. Lawrence Lowlands (Beekmantown, Chazy, Black River, Trenton, and Utica Groups) were sampled at Neuville, Joliette, and Montreal, in the Province of Quebec. The purpose of this work was to determine the level of thermal maturation of the carbonates using organic matter and clay studies, and to correlate it with the crystal characteristics of the microcrystalline calcite.The studied formations are supermature in the Joliette and Montreal areas, whereas they are only mature at Neuville.The numerical analysis of the petrographic data shows that some microstructures observed in the rock samples and the size and morphology of the microcrystalline calcite crystals are partly related to the thermal maturation as determined by other methods.

Paleoenvironments of Middle Devonian Sandstones of Upper Mississippi Valley: ABSTRACT

The Dutch Creek, Beauvais, Lupus, and Hoing Sandstones are supermature quartzarenites associated with transgressive carbonate sequences. They generally have gradational upper and lower contacts. Fossils are mostly fragmentary crinoids, brachiopods, and trilobites. Complete fossils are mostly corals that settled during periods of temporary bottom stability and slow sedimentation. Unidirectional, planar cross-bedding dominates part of the Dutch Creek, Beauvais, and Lupus Sandstones, indicating paleocurrent movement to the northwest. Local herringbone cross-stratification with associated reactivation surfaces indicates tidal action. Lenticular bodies of Hoing and Lupus sandstone enclosed in calcilutite contain graded sequences and basal conglomerates, suggesting tidal-channeling of banks and shoals. Parallel lamination and cylindrical structures in the Hoing Sandstone indicate high intertidal environments. These sandstones all consist of bimodally distributed fine- and medium-sized quartz grains. Medium-sized grains are well rounded to rounded, contrasting with angular fine-grained quartz. Cementation is mostly by sparite and microcrystalline calcite. Locally, secondary silica replaces carbonate. Near faults, quartzitic texture probably results from pressure solution. Interlocking secondary overgrowth locally provides cementation. Bimodal grain-size distribution, seaward thickening of the sandstone, and an apparently inadequate source suggest eolian transport to the sea across a carbonate terrane prior to marine deposition. This does not require a sabkha, but depends on aridity and sparse plant cover. Subsequent tidal, wave, and current action is believed responsible for ultimate transport and deposition of the sand. End_of_Article - Last_Page 542------------

Calcareous crust (caliche) profiles and early subaerial exposure of Carboniferous carbonates, northeastern Kentucky

ABSTRACTSequences of laminated limestones found within thin Carboniferous carbonate strata of northeastern Kentucky were studied to determine their origin and palaeo‐environmental significance. These laminated zones are strikingly similar to Holocene and Pleistocene surficial calcareous crusts (caliche) profiles that occur in various parts of the world.Carboniferous laminated carbonates are associated with shallow marine carbonate units, palaeokarst, and overlying palaeosol zones. A typical laminated profile ranges in thickness from 1 to 2 m and contains brecciated, light olive‐grey to brown micrite that lacks distinctive bedding. Structures and textures common in most profiles include: (1) calcareous and silicious laminae (laminae form diffuse, alternating light and dark bands that generally parallel bedding but often fill fractures and vugs within the rocks); (2) particles (allochems, and micrite and microspar fragments) coated by brown microcrystalline calcite; (3) brecciated texture; (4) circular to elliptical fossil moulds (occur in sinuous patterns and fill fractures within the rocks); (5) large and small scale fracture patterns.Subaerial weathering and vadose diagenesis of carbonate mud banks or islands is suggested as a mechanism for the formation of these Carboniferous calcareous crust profiles. These ‘crusts’ formed by a combination of solution (karsting), brecciation, and soil development that transformed an exposed marine biomicrite (‘host’ rock) into a porous subsoil rubble. Laminated ‘crusts’ and coated particles developed as the result of dissolution and reprecipitation of CaCO3 and SiO3 from the soil and carbonate bedrock.Carboniferous laminated carbonates in northeastern Kentucky are often referred to as ‘algal limestones’ because of their superficial similarity to some modern and ancient algal structures. This study, however, reveals numerous characteristics that can only be explained by diagenesis in a subaerial environment.

Pedogenic calcite accumulation in arid and semi-arid regions of the indo-gangetic alluvial plain of erstwhile punjab (india) — Their morphology and origin

In the Indo-Gangetic plain (India), a sequence of soils, developed under different climatic conditions has been investigated micromorphologically to study the evolution of the pedogenic calcite formations. The climate of the plain grades from arid and hot to semi-arid and hot to less hot, and the soils, respectively, from Camborthids, through Calciorthids and Ustochrepts, to Haplustalfs. Nine different forms of calcite accumulations from microcrystalline calcite to coarsely recrystallized crystallaria are distinguished and described. They may form combinations which are characteristic for specific soil conditions. An evolution pattern of different forms could be determined, not only within profiles, but also among different soil types of the sequence; the highest complexity being found in the calcic horizons. Specific forms were observed in the saline and watter-logged soils. The origin of the different calcareous accumulations is discussed and the authigenic character of the calcitic nodules is propounded. In the scope of these findings, the use of the suffix Ca in horizon designation is discussed.

Stratigraphy and Petrology of the Morrison Formation (Jurassic) of the Canon City, Colorado, Area

ABSTRACT Within the Canon City area, the Morrison Formation is predominantly claystone (71%) with interbedded sandstone (20%), siltstone, (7%), and thin carbonate (2%) units. Maximum thickness is approximately 107 meters (350 ft). The sequence conformably overlies the Jurassic Ralston Creek Formation in the eastern part of the area. At places in the western part it disconformably overlies the Ralston Creek Formation, but in other places, the Morrison lies nonconformably on the Pikes Peak Granite and Idaho Springs Formation. Disconformably overlying the Morrison is the Purgatoire Formation of Early Cretaceous age. Most sandstones within the Morrison are quartzarenites or subarkoses, with grain sizes generally fine to very fine. Calcite and microcrystalline quartz are the two major cements, with a paragenetic relationship among cementing materials of (1) microcrystalline quartz, (2) calcite, and (3) hematite. Composition and texture of the sandstones indicate that source areas of sedimentary rocks contributed most of the sediments. Acid igneous and metamorphic rocks, exposed mainly in the adjacent ancestral Rockies, contributed minor amounts of detritus. Illite, the most abundant of the Morrison clay minerals, was probably derived from pre-existing illite-rich sedimentary rocks; montmorillonite, the principal clay mineral in the middle Morrison, was formed from altered volcanic ash. Sediments accumulated on floodplains formed by numerous small meandering streams. Deposition of carbonates was in small lakes formed in low areas on the floodplains.

Fossiliferous Concretions on Georges Bank

ABSTRACT Unusually shaped concretions are occasionally dredged on the north-central and northeast margin of Georges Bank. In their centers are molds and casts of molluscan fossils of Miocene age. The fauna is probably correlative with fauna of the Chesapeake Group of Maryland and suggests deposition in a shallow open sea with cool temperate water. The mineralogical and textural composition of the sandy mudstone and muddy sandstone surrounding the fossils is similar to that of some Tertiary Atlantic Coastal Plain units including Miocene deposits. The framework consists of angular quartz of fine-sand grade, feldspar, lithic grains, phosphorite, glauconite, mica, plant fibers, and dark organic grains. Ovules, probably fecal pellets, and patches of matrix reworked by organisms are common. Grains f oat in a matrix paste of microcrystalline calcite, clay minerals, phosphate, ferruginous cement, or mixtures thereof. The concretions are early diagenetic in origin and are concentrated at present as a lag pavement in areas near outcrops of Miocene deposits on the top of Georges Bank.

Petrology of the Carbonate Members of the Swope and Dennis Formations (Pennsylvanian), Missouri and Iowa

ABSTRACT Nine distinct facies, differentiated on the basis of field studies, petrographic modal analyses, and mineralogy, comprise the carbonate members of the Swope Formation (Middle Creek and Bethany Falls Limestones) and the Dennis Formation (Canville and Winterset Limestones). These facies are 1. Osagia biomicrite; 2. Cryptozoon biomicrite; 3. brachechino biomicrite; 4. fusulinid biomicrite; 5. argillaceous, pyritic biomicrite; 6. micrite; 7. algal biosparite; 8. Cryptozoon biolithite; and 9. mixed oolitic-intraclastic facies. The majority of the carbonates are calcitic, and recrystallization of microcrystalline calcite to coarser grained mosaics is widespread. Dolomite occurs as scattered grains in matrices of microcrystalline calcite, as replacements of allochems and burrow fillings, and as cavity fillings. Completely dolomitized rock units are rare. Four suites of clay minerals include various combinations of detrital illite, mixed-layered 10A expanding-non-expanding clays, chlorite, free -expanding clays, clay-size muscovite, and authigenic kaolinite that fills voided ooid molds. Overlap of facies illustrates a transgression of 70 to 110 miles during deposition of the Middle Creek Limestone. The types, temporal sequence, and geographic position of facies within the Bethany Falls Limestone are remarkably similar to those of the Winterset Limestone. The boundary between lower and upper portions of each member is marked by a shaly interval and an abrupt change in carbonate facies array. These shale seams mark the change from transgression to regression in each cycle and represent excellent time-line markers. The sea bottom on which these carbonates accumulated sloped southwestward and was locally interrupted by shoals and deeper embayments. Paleocurrents flowed north-eastward in late Bethany Falls time. Depth of water was the major control over facies arrangement, and changes in water depth were responsible for facies migration. To a lesser degree, variable amounts of terrigenous mud contributed to the differentiation of facies. Contrary to some earlier ideas, fusulinid-bearing sediment appears to have accumulated in water no deeper than that in which contemporaneous facies formed.

Alteration of host rock limestone adjacent to zinc-lead ore deposits in the Tri-State District, Missouri, Kansas, Oklahoma

The nature and lateral variation of a host rock limestone in the Tri-State zinc-lead district were petrographically studied within a single horizon, the M bed. This bed is a massive, cherty, crinoidal limestone in the Tri-State district of southwestern Missouri, northeastern Oklahoma and southeastern Kansas and is a part of the Mississippian Keokuk Formation.One hundred and twenty specimens from this bed were collected under-ground from six pull drifts in five mines: West Side, Grace B, Big John, Little Greenback, and Mudd Mines. The specimens were collected at regular intervals, 10 to 50 feet, beginning at the edge of an ore body and progressing away from the zone of mineralization and ore. Ninety-four thin sections prepared from the limestone specimens were described and classified according to Folk's classification of carbonate rocks. According to this classification M bed limestone is predominantly a sparry allochem.Distally from ore bodies the M bed limestone consists predominantly of allochems, mainly crinoids, and microcrystalline calcite, sparry calcite and microspar are much less abundant constituents. Toward ore bodies, the constituents of the M bed limestone are altered by the solutions that emplaced the ores. The allochem content progressively decreases, whereas the sparry calcite content increases. This gradual change in the limestone constituents toward the margin of ore bodies generally begins at a distance of 40 to 80 feet from the ore bodies. Microcrystalline calcite and microspar exhibit no significant lateral variations.

Petrology of a Lower Carboniferous Bryozoan Limestone and Adjacent Limestones in North Wales, Great Britain

ABSTRACT Field examination of the Carboniferous Limestone at Nant y Gamar, southeast of Llandudno on the coast of North Wales, indicates that fenestrate bryozoa are the major organic components of a shoal or knoll-like structure in limestone of S2 age. The limestone is a bryozoan calcilutite consisting of large, intact or occasionally broken fenestrate Bryozoa in a matrix of dark brown microcrystalline calcite containing fine grained crinoid and brachiopod fragments, Foraminifera and calcispheres (Biomicrudite, Folk, 1959). The bryozoan limestone is flanked by crinoidal calcirudites, slightly quartzose bioclastic calcarenites with matrices of microcrystalline calcite (respectively Biomicrudite and Biomicrite, Folk, 1959), and intraclastic calcirudites with calcite cement (Intrasparudite, Folk, 1959). In view of the structure and lateral variation of facies, it is tentatively proposed that the bryozoan rock unit represents a shoal or possibly a fore-reef deposit of a small reef-knoll. The bryozoa do not appear to be in a growth position, or to form a rigid framework, but seem to represent a loose mesh of zoaria derived by transportation or scavenging action. It is assumed that they were continually derived from a nearby reef or broken in situ, and entrapped sediment transported by currents in shallow water, thus forming a prominence. The prominence grew by accumulation, and resulted in a shoal or knoll-like structure.

Nya Fynd av Subglacialt Bildade Kalkstenar

Abstract Deposits of limestone on the lee-sides of glaciated rocks have earlier been described from Norway and Sweden (Hoeg 1929, 1931 a, 1931 b, 1932, 1934, Westergard 1934, Bohlin 1943, and Kers 1961). In 1962 and 1963 similar limestone were observed in the vicinity of Gothenburg, in front of the Austrian glacier Pasterzenkees, and at Nigardsbreen, Norway. The crusts consist of microcrystalline calcite with varying contents of other mineral grains. The limestone is laminated. Plates with a thickness of 1 cm are common but there are also ridges, furrows, and nodules. A prominent lee-side is covered with coarse, dark calcite in semispherical bodies. The contents of calcite range from 30 to 75 percent by weight. The origin is shown to be subglacial. According to the formation through precipitation, there must, at least occasionally, have been some water both beneath the inland ice and the glaciers. The new localities suggest that subglacially formed limestone can be expected where unweathered, glaciated ro...

Petrography of the Southesk-Cairn Carbonate Complex, Western Alberta, Canada: ABSTRACT

Regional and petrographic studies suggest that a specialized reef association was responsible for the build-up of the carbonate mass. Although dolomitization has preferentially affected certain formational members, some original textures and rock constituents may be recognized. At the Mount Dalhousie type section, the complex has divided into lower Cairn and overlying Southesk formations. The coarsely crystalline, light gray dolomites of the Peechee member, Southesk formation, were originally medium and coarse calcarenites with some areas of algal-stabilized sediment. The whole is suggestive of a depositional environment with strong currents and local sheltered areas. Where dolomitization is incomplete, there is a preference for replacement of integranular sparry calcite. It is suggested that this is an early diagenetic event that took place prior to complete infilling of porosity by calcite precipitation. The Arcs member, in contrast, consists dominantly of microcrystalline calcite with eyes of clear sparry calcite (dismicrite and birdseye limestone), with sporadically distributed areas of calcarenite and layered algal limestone. Comminuted skeletal remains are abundant, although locally, delicate undamaged ostracod shells occur; the latter partly infilled with microcrystalline calcite constitute geopetal fabrics and indicate quiet micro-environments where the fine calcite or aragonite particles settled out. A relatively quiet environment is postulated during deposition of most of this member, with algal-stabilized areas building up locally to further restrict circulation. The algal material was in turn frequently torn up, presumably during storms, and incorporated as intercalated calc renites in the predominantly microcrystalline rock sequence of the Arcs member. The overlying Ronde member, composed of calcarenite, microcrystalline limestone, silty limestone, silty dolomite and siltstone, probably accumulated under unsettled conditions. Globular stromatoporoids appear to have been characteristic of turbulent reef-margin areas and colonial corals of rather deeper and turbid environments. The sedimentary record terminates at an unconformity beneath the overlying Sassenach formation. Viewed as a whole, the carbonate complex has been extensively dolomitized. Various types of dolostone have been mapped and studied with a view to equating them with limestone facies, establishing a reef model and interpreting the dolomitized sequence. End_of_Article - Last_Page 536------------

Petrology and Paragenesis of the Ordovician Manitou Formation Along the Front Range of Colorado

ABSTRACT Field and laboratory studies of the Manitou Formation (Lower Ordovician) along the southern Front Range of Colorado indicate that in this area the formation originated as an allochemical limestone. Deposition occurred in an environment of moderate to low energy that was interrupted occasionally by brief periods of high energy. Biomicrite, intramicrite, and micrite comprise, in order of decreasing abundance, the major lithologies of the original limestone. This section studies show evidence of five post-depositional alterations of the sediment: (1) Grain growth recrystallization of microcrystalline calcite to sparry calcite mosaics. (2) Dolomitization of selected horizons in the northern localities and rather complete dolomitization in the more southern localities. (3) Silicification ranging in degree from partial void filling to complete silicification that produced dense chert beds. (4)Calcitization of chert and dolomite. (5) Oxidation of iron-bearing minerals, especially glauconite and ankeritic dolomite, to limonite and hematite. Glauconite replacement of calcite and dolomite and dolomite replacement of chert and glauconite can be demonstrated but are sufficiently rare that they are not considered as major diagenetic events. The occurrence of extensive void-filling quartz and recrystallized microcrystalline calcite suggest that lithification of the original sediment probably was complete after the stage of silicification. Early partial lithification is suggested by the presence of intraclasts at several horizons.

Use of Factor Analysis in Recognizing Facies Boundaries: ABSTRACT

One of the vexing problems of geology is recognizing boundaries between facies that intergrade. For example, if a shale facies grades laterally into sandstone, where do you draw the line between the two facies? No matter where the boundary is placed, it is partly subjective and depends on what you consider to be important criteria--color, lithological proportions, thickness of beds, kinds of fossils, and so on. Factor analysis, a statistical technique, can help in determining facies boundaries by objectively establishing where major changes occur, once the criteria have been decided. As an experiment, a thin, continuous limestone unit within the Lower Permian Americus limestone was sampled at 27 localities along a 250-mile-long outcrop belt in Kansas and Oklahoma. The problem was to interpret variations along the outcrop belt and to decide where facies changes occur. Specimens from each of the localities were point-counted and were also analyzed chemically. As a result, 14 different constituents (percentages of bryozoans, fusulinids, crinoids, shells, intraclasts, visibly crystalline calcite, microcrystalline calcite, silica, magnesia, the R2O3 group, manganese, titanium, strontium, and vanadium) were quantitatively determined, and permitted comparisons between each of the 27 localities in terms of each of the 14 constituents. Factor analysis brought about a great simplification by showing that two basic variables, or factors, were sufficient to mark where major changes occur and where boundaries between four different facies should be placed. Thus, 2 factors substituted for 14 constituents. End_of_Article - Last_Page 529------------