Bladed Spar Research Articles

Torosites: A New Microbially Mediated Speleothem from Sistema los Toros, Nuevo Léon, Mexico

Although several kinds of speleothems are known to grow with pendant morphology (i.e., helictites, pool fingers, and stalactites), we describe here a new form found in Sistema Los Toros, Nuevo Léon, Mexico, which we propose to call torosites. A laminated, bladed, spar crust grew beneath remnants of a stromatolitic micritic crust and extended mostly downward as irregular finger-like protrusions (torosites) that are 2–3 mm wide and 2–3 cm long. Although mainly pendants, the individual torosites are quite irregular with growth sideways and even upward in places. The torosites thicken and thin, like knuckles on a finger, but maintain a relatively uniform diameter. Laminations in the bladed spar and protrusions are defined by micritic intervals with minor intermixed detrital grains (clay, quartz, and carbonate silt) and rare, ropy biofilm. Bladed-spar crystals nucleate off thicker detrital layers, but they extend through thinner lamina. The torosites lack the central canal of helictites and most stalactites. Unlike pool fingers and stalactites, torosites only grow on the end of the speleothem, not on the sides. We hypothesize that the torosites grew from dripping water at their ends. Microbial biofilm communities colonized the growth surface, probably during drier conditions, forming the micritic layers. We speculate that the microbial mat likely caused the irregular sideways growth, but we cannot identify how on these fossil forms. Torosites are unique and add to the growing list of known microbially mediated speleothems.

Early Diagenesis of Plitvice Lakes Waterfall and Barrier Treavertine Deposits

Travertine forms spectacular waterfalls, barriers, and subaqueous finegrained lake-fill accumulations throughout the Plitvice National Park, Croatia, northwestern Yugoslavia. Barrier deposits form dams, behind which, the lakes of the Plitvice complex are situated. Three generations of low-magnesian calcite spar comprise the waterfall and barrier forming travertines. The initial precipitates generally are composed of cloudy, very finely to medium crystalline equant to bladed spar. A later generation is composed of clear, isopachous layers of medium to coarsely crystalline bladed crystals. Additionally, centimeter-thick laminated speleothem-like crusts, composed of clear, bladed to columnar spar, are the common precipitates around micritic accumulations within the older travertine. In comparison, the lake-fill deposits are primarily composed of moderately (recent lake-fill deposits) to well-developed (relict lake-fill deposits) 3-8 ^m calcite rhombohedrons. Petrographie analyses clearly show that cyanobacteria, fungi, and/or other microbial organisms bore into the spar and micritize it. This sparmicritization is pervasive throughout the waterfall and barrier deposits. Bladed spar crystals range from those which are pristine to those whose original bladed morphology can only be interpreted by comparison with laterally adjacent crystals. Individual samples display multiple generations of spar which have undergone various degrees of sparmicritization. Sparmicritization results in a thoroughly micritized accumulation in which evidence of the original spar composition has been completely obliterated.

Marine cements in mid‐Tertiary cool‐water shelf limestones of New Zealand and southern Australia

Large areas of southern Australia and New Zealand are covered by mid‐Tertiary limestones formed in cool‐water, shelf environments. The generally destructive character of sea‐floor diagenesis in such settings precludes ubiquitous inorganic precipitation of carbonates, yet these limestones include occasional units with marine cements: (1) within rare in situ biomounds; (2) within some stacked, cross‐bedded sand bodies; (3) at the top of metre‐scale, subtidal, carbonate cycles; and (4) most commonly, associated with certain unconformities. The marine cements are dominated by isopachous rinds of fibrous to bladed spar, interstitial homogeneous micrite and interstitial micropeloidal micrite, often precipitated sequentially in that order. Internal sedimentation of microbioclastic micrite may occur at any stage. The paradox of marine‐cemented limestone units in an overall destructive cool‐water diagenetic regime may be explained by the precipitation of cement as intermediate Mg‐calcite from marine waters undersaturated with respect to aragonite. In some of the marine‐cemented limestones, aragonite biomoulds may include marine cement/sediment internally, suggesting that dissolution of aragonite can at times be wholly marine and not always involve meteoric influences. We suggest that marine cementation occurred preferentially, but not exclusively, during periods of relatively lowered sea level, probably glacio‐eustatically driven in the mid‐Tertiary. At times of reduced sea level, there was a relative increase in both the temperature and the carbonate saturation state of the shelf waters, and the locus of carbonate sedimentation shifted towards formerly deeper shelf sites, which now experienced increased swell wave and/or tidal energy levels, fostering sediment abrasion and reworking, reduced sedimentation rates and freer exchange of sediment pore‐waters. Energy levels were probably also enhanced by increased upwelling of cold, deep waters onto the Southern Ocean margins of the Australasian carbonate platforms, where water‐mass mixing, warming and loss of CO2 locally maintained critical levels of carbonate saturation for sea‐floor cement precipitation and promoted the phosphate‐glauconite mineralization associated with some of the marine‐cemented limestone units.

Cementation scenarios for New Zealand Cenozoic nontropical limestones

Abstract Cenozoic limestones are widely distributed in New Zealand, especially in the Oligocene‐earliest Miocene in both islands, and the Pliocene‐Pleistocene in North Island. A spectrum of limestone types exists, but all are skeletal‐dominated (>70%), with usually <20% interparticle cement‐matrix and <10% siliciclasts, and they have facies attributes typical of nontropical carbonates. The range of diagenetic features identified within the limestones is the basis for assigning them to a small number of “end‐member” cementation classes that are inferred to be associated with four, broad, diagenetic settings. Class I limestones have very open fabrics dominated by abraded bryomol (bryozoan + bivalve mollusc) facies skeletons coated with isopachous rinds of typically nonferroan, dull to blotchy luminescent, fibrous to bladed spar, often with porosity occlusion by detrital and/or precipitated micrite. The limestones are usually thin and rare, of Oligocene — early Miocene age, and are interpreted to have formed...

Quaternary dune carbonates from the Mediterranean Coast of Egypt: Petrography and diagenesis

The Quaternary carbonates of the Mediterranean coast of Egypt between Alexandria and Salum appear as parallel limestone ridges rising up to 100 m above sea level. These ridges are dominated by dunal carbonates which differ not only in their primary composition but also by distinct grades of meteoric water diagenesis. Oolitic facies dominates the younger aeolianites of the first and second ridges. Bioclastic facies with abundant coralline algae, benthonic foraminifers, molluscs, echinoderms and intraclasts represents the major rock type in the older aeolianites. Features of meteoric water diagenesis include precipitation of increasing amounts of avoid-filling low Mg-calcite spar, dissolution of aragonite and stabilization of aragonite and high Mg-calcite to low Mg-calcite. Aeolianites below paleosol horizons contain abundant calcrete cements, micritized fossils and detrital grains which are commonly corroded and replaced by calcite. Three stages of progressive meteoric diagenesis are recognised in the Egyptian Quaternary aeolianites. In stage 1 minor precipitation of low Mg-calcite occurs at the grain boundaries. Stage 2 is marked by partial dissolution of aragonite, partial loss of high Mg-calcite and precipitation of low Mg-calcite in some pore spaces. In stage 3, most of the remaining pores are occluded by cementation. At the end of stage 3, Mg is removed from high Mg-calcite but some aragonite still persists. The early vadose cements are represented by miniscus, bridge and pendant cements. The phreatic cements were precipitated as bladed spar in the isopachous rims and equant spar in the intergranular and mouldic porosity. The late vadose cements are represented by micritic cements that were related to calcrete formation. Elemental behaviour during meteoric water diagenesis indicates that it leads to a gradual decrease in bulk Sr along with Na in progressively altered aeolianites. Mn distribution is controlled by the carbonate mineralogy (aragonite versus calcite) as well as the proximity of the aeolianites to the overlying paleosol horizons.

Crystal habit, geochemistry, and cathodoluminescence of magnesian calcite marine cements from the lower slope of Little Bahama Bank

Research Article| April 01, 1991 Crystal habit, geochemistry, and cathodoluminescence of magnesian calcite marine cements from the lower slope of Little Bahama Bank R. P. MAJOR; R. P. MAJOR 1The University of Texas at Austin, Bureau of Economic Geology, Austin, Texas 78713-7508 Search for other works by this author on: GSW Google Scholar R. JUDE WILBER R. JUDE WILBER 2Sea Education Association, P.O. Box 6, Woods Hole, Massachusetts 02543 Search for other works by this author on: GSW Google Scholar Author and Article Information R. P. MAJOR 1The University of Texas at Austin, Bureau of Economic Geology, Austin, Texas 78713-7508 R. JUDE WILBER 2Sea Education Association, P.O. Box 6, Woods Hole, Massachusetts 02543 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1991) 103 (4): 461–471. https://doi.org/10.1130/0016-7606(1991)103<0461:CHGACO>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 R. P. MAJOR, R. JUDE WILBER; Crystal habit, geochemistry, and cathodoluminescence of magnesian calcite marine cements from the lower slope of Little Bahama Bank. GSA Bulletin 1991;; 103 (4): 461–471. doi: https://doi.org/10.1130/0016-7606(1991)103<0461:CHGACO>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 petrography and geochemistry of marine magnesian calcite cements recovered from the sea floor on the lower-thermocline lower slope of Little Bahama Bank address three issues concerning inorganic carbonates: (1) crystal habit of marine magnesian calcite cements, (2) cation composition of marine precipitates, and (3) origins of cathodoluminescence in calcite.Bladed and blocky limpid spar cements formed contemporaneously within the same intraparticle pore, as shown by banded cathodoluminescence traced without interruption through both cement habits. Bladed and blocky cements exhibit a positively covariant linear relationship for Mg and Sr, having low proximal (early formed) compositions and high distal (late formed) compositions. Linear covariance of Mg and Sr is interpreted to indicate that the concentrations of these cations were controlled by kinetic effects as they formed, with early precipitation occurring more rapidly than later precipitation. Precipitation rates apparently did not affect cement habit.Limpid, isopachous, bladed spar cements, the distal third of which are brightly cathodoluminescent, have a wide range of Mg and Sr compositions, which is almost completely spanned by both distal cathodoluminescent and proximal noncathodoluminescent parts of cements occurring within the same pore. Mg and Sr do not have a linear relationship. These cements are interpreted to have recrystallized in deep, cool sea water. During recrystallization, magnesium was lost, and in some cases, manganese, the principal cathodoluminescence excitor, was presumably gained. 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.

Diagenesis and Sea Level Change in a Pleistocene Coral Reef, San Salvador, Bahamas: ABSTRACT

Near the Cockburn Town reef (dated 125,000 yr B.P.), precisely surveyed bench marks are related to accurately measured mean sea level, and they provide a convenient datum plane. This coral reef developed during a sea level highstand of no more than 10,000 yr, which was insufficient time for significant subsidence; however, subsidence of approximately 3 m may have occurred since the formation of the reef. Sea level changes were caused by fluctuations in glacial-ice volume. The upper beach to dune transition, which is in the calcarenites overlying the reef, is at +4 m. A minimum highstand of +7 m is indicated when corrected for subsidence. Below +2.5 m, marine aragonite cement occurs within the intragranular pore space of the following: Halimeda plates, benthic foraminifera, Favreina (Callianassid fecal pellets), gastropods, and corals. Marine aragonite cement also occurs as intergranular isopachous rims on matrix grains of coral rubblestone. Remaining pore space was partly to completely occluded by freshwater vadose calcite cements, which occur without marine cements in the overlying shallow subtidal, beach, and dune calcarenites. No unequivocal freshwater phreatic cements have been found, although syntaxial overgrowths and irregular calcite rims about grains do occur in finer grained sediments where local patches of freshwater saturation occurred within the vadose zone. Later calichification, which affected all facies is characterized by alveolar texture, whisker calcite, microsparite, rare bladed calcite spar, Microcodium, and rhizocretions. End_of_Article - Last_Page 316------------

Carbonate cements in the algal nodules from the Lawson's Bay, east coast of India

Modern algal nodules and encrustations on rock fragments occur in the beach and littoral environment of the Lawson's Bay, east coast of India. Mg-calcite is the predominant mineral form of the carbonates, with minor amounts of aragonite. Scanning electron microscopic observations have shown several polymorphs of Mg-calcite: micrite, peloids, columnar crystals with truncated terminations and bladed spar with rhombic terminations. Aragonite is confined to voids and occurs as acicular and fibrous needles arranged in a random fashion. The different crystallites of Mg-calcite are considered represent an evolutionary sequence of carbonate cements. Some of the Mg-calcite rhombs show the diagenetic effect of fresh water periodically entering the bay. The δ 13C of the algal carbonates ranges from 0.19 to −0.63‰, the δ 18O from −2.76 to −4.92‰ PDB. The lighter oxygen values result from the dilution effect of 18O-depleted fresh water influx into the bay.

Submarine Lithification on Windward Reef Slopes: Capricorn-Bunker Group, Southern Great Barrier Reef

ABSTRACT Windward reef slopes of the Capricorn-Bunker Group, southern Great Barrier reef, are the sites of extensive submarine lithification. Both crystalline (aragonite and bladed Mg-calcite spar) and microcrystalline (micrite and peloids) cements occur within skeletal pores and chambers, whereas interskeletal cements are mainly microcrystalline. These cements are considered to be contemporaneous with reef growth and have developed in an environment characterized by slow accretion rates. Continuous precipitation of aragonite cement has, in some places, been inhibited by the formation of Mg-calcite micrite. Apparent dissolution of some of the Mg-calcite cements was observed, but the processes involved are unknown.

Petrology of Carbonate Nodules from a Cambrian Tidal Inlet Accumulation, Central Texas

ABSTRACT Carbonate nodules which occur within the trough cross-stratified highly glauconitic Cambrian Lion Mountain Sandstone of central Texas are primarily an expression of depositional fabric. Trilobite and brachiopod skeletal material accumulated as shell lag deposits within troughs in front of migrating, subaqueous megaripples in a tidal inlet and form the framework of the nodules. High depositional porosity, due to the morphology of the skeletal material, was filled, at least partly, by passively precipitated acicular to bladed spar while the sediment was still at or near the sediment-water interface. Clastic constituents rest on the upper surfaces of large skeletal grains in geopetal fashion; consequently, the calcite cement is best developed as downward-elongated spar crystals on the ndersides of the skeletal debris. Large areas within the nodules are now filled with radiaxial, fascicular-optic, and planar cross-twin lamellae calcite cement as a result of neomorphism of the original acicular spar. All three types of spar occur as length-fast and length-slow calcite.

Facies and Fabric Specificity of Early Subsea Cements in Shallow Belize (British Honduras) Reefs

ABSTRACT Early subsea cement and cemented sediment are restricted to marginal facies of shallow Belize barrier and atoll reefs. In these facies cementation is not ubiquitous, but is sporadic within seaward-facing spurs and within the upper meter of the reef pavement. The presence of internal sediment, composed of well-sorted lime silt, is critical to the precipitation of rock-making cement. This lime silt is made up of Mg-calcite peloids of unknown origin and angular chips of aragonite excavated from coral and mollusk skeletons by boring sponges. In Belize the reef flat pavement, a subtidal bed of lithified coral conglomerate 1 m thick and 10 to 100 m wide, occurs in the lee of all marginal reefs. Acropora palmata fronds and logs (radiocarbon age ca. 450 years) are cemented together by a mortar of lime packstone to wackestone. In seaward-facing spurs, cavities between in situ coral and Millepora are locally filled with cemented skeletal packstone. Cemented packstone and wackestone also partially fill intraskeletal pores and borings in coral fronds lying loose on the sea floor. The predominant cement of internal sediments is Mg-calcite (14.5-18.6 mole % Mg) as micrite and bladed spar. Aragonite, although common, is found only in coral pores. The crystal form of the cement and the degree of lithification can be related to variations in the texture of the internal sediments. Mg-calcite micrite, the first cement, develops preferentially in the small pores, less than 15 µ, between lime silt grains. Mixed lime sand and lime silts, with a wider range of pore sizes, are cemented after the lime silts by thin rinds of micrite that grade outward into bladed spar. In the larger pores of grainstones, bladed spar follows thorough cementation of lime silts in the same sample.