Original Aragonite Mineralogy Research Articles

Ediacaran Corumbella has a cataphract calcareous skeleton with controlled biomineralization.

Corumbella is a terminal Ediacaran tubular, benthic fossil of debated morphology, composition, and biological affinity. Here, we show that Corumbella had a biomineralized skeleton, with a bilayered construction of imbricated calcareous plates and rings (sclerites) yielding a cataphract organization, that enhanced flexibility. Each sclerite likely possessed a laminar microfabric with consistent crystallographic orientation, within an organic matrix. Original aragonitic mineralogy is supported by relict aragonite and elevated Sr (mean= ca. 11,800 ppm in central parts of sclerites). In sum, the presence of a polarisation axis, sclerites with a laminar microfabric, and a cataphract skeletal organization reminiscent of early Cambrian taxa, are all consistent with, but not necessarily indicative of, a bilaterian affinity. A cataphract skeleton with an inferred complex microstructure confirms the presence of controlled biomineralization in metazoans by the terminal Ediacaran, and offers insights into the evolution of development and ecology at the root of the 'Cambrian radiation'.

Characteristics and primary mineralogy of fibrous marine dolomite cements in the end-Ediacaran Dengying Formation, South China: Implications for aragonite–dolomite seas

Fibrous marine cements are widely distributed in the terminal Ediacaran Dengying Formation of South China and are an important record for reconstructing paleoseawater chemistry. However, the morphological characteristics and filling sequence of these cements, and their formation mechanism and paleoceanographic significance, are poorly understood. Our investigation of five outcrop sections in the northern upper Yangtze region allows four types of fibrous dolomite cement to be identified: fascicular fast dolomite (FFD), radial fast dolomite (RFD), radial slow dolomite (RSD), and fascicular slow dolomite (FSD), where “fast” and “slow” refer to the optical character of cement crystals. FFD cements possess original aragonitic mineralogy, whereas RFD cements had high-Mg calcitic precursors that subsequently underwent mimetic dolomitization via syntaxial replacement. RSDs and FSDs are primary marine precipitates, whereas fibrous dolomite cements with length-fast optical characteristics (FFDs and RFDs) are of secondary origin. These fibrous cements are widely distributed laterally in the Dengying Formation, although the RSDs and FSDs occur only in the second member of this formation. Therefore, the seawater chemistry of the terminal Ediacaran period in the upper Yangtze area was dominated by an aragonite sea, with a regional short-lived “dolomite sea” during the late depositional stage of the second member of the Dengying Formation, from which dolomite may have directly precipitated.

Primary mineralogy of Dalichai carbonate formation and separation of upper boundary with Lar formation based on geochemical studies in northeast of Semnan

The Dalichai Formation is the first rock unit of the Middle Jurassic in the Alborz sedimentary basin that has occurred in a marine environment. The boundary of the Dalichai formation with Shemshak Formation is erosional unconformable while it is concordant with Lar formation. Due to lithological similarity, the boundary of the Dalichai formation with the Lar formation is unclear. It is also very difficult to identify the primary carbonates mineralogy, due to the effects of diagenetic processes solely on the basis of petrographic studies. We studied a section of Sharaf mountain with a thickness of 1420m and 149 samples was studied. The study of the major and minor elements, the Oxygen and Carbon isotopes, and plotting of these values ​​against each other represents that the Dalichai formation have had original aragonite mineralogy, and the nonmarine diagenesis have had the highest impact on them. Plotting the Sr/Ca ratio versus Mn value, shows that these carbonates are affected in a semi-closed to open diagenetic system. Using Sr, Sr/Na, Sr/Ca, Oxygen and Carbon isotopes, the boundary of this formation with the Lar Formation is recognized in thickness of 1200m. The diagenetic environment temperature for this section limestones is 32.9oC.

Lower Jurassic corals from Morocco with skeletal structures convergent with those of Paleozoic rugosan corals

Neorylstonia nom. nov. pseudocolumellata, a replacement name for Mesophyllum pseudocolumellatum Beauvais, 1986, is only known from uppermost Sinemurian to uppermost Pliensbachian (Lower Jurassic) strata of Morocco. This solitary coral species went extinct during the Pliensbachian-Toarcian transition (~183.5 My), which is currently considered to be a second order biodiversity crisis linked to a period of rapid and global paleoenvironmental disturbances. Neorylstonia pseudocolumellata has a distinctive skeletal organization. The axial structure of the corallite, which has no equivalent in Mesozoic taxa, is reminiscent of the skeletal organization of some Paleozoic rugose corals such as Amygdalophyllum Dun and Benson, 1920, or Rylstonia Hudson and Platt, 1927. This similarity is based on the occurrence of a calicular boss also named pseudocolumella. As Rugosa and Scleractinia do not appear closely related, we consider that this feature cannot be due to evolutionary inheritance from a Rugosan ancestor. The morphological aspects of the skeleton have been assessed in order to understand the function and growth of this axial “calicular boss” as well as to more precisely describe the genus and species characters. The organization of the septal apparatus points to a typical Scleractinian pattern, which is also supported by the original aragonitic mineralogy of the skeleton. This implies a convergence phenomenon, leaving open the question of the functional significance of such a calicular organization. We posit that this feature was used for sexual reproduction and so did not always develop in a population.

Guadalupian cool versus warm water deposits in central Iran: a record of the Capitanian Kamura event

AbstractAn integration of geochemical and grain association studies were carried out on Middle Permian deposits in central Iran where both cool and warm water carbonates are found. The recrystallization of most bioclasts, lime-mud matrix and ooids along with high Sr contents suggests a probable original aragonite mineralogy for carbonates of the Middle Permian Jamal Formation at the Shotori section. Low bulk carbonate δ18O values imply pervasive diagenetic alteration in this section. Conversely, Middle Permian deposits at the correlative Bagh-e Vang section have a probable calcite precursor supported by low Sr contents and no evidence of recrystallization. This mineralogical variation in these coeval carbonates is considered to be due to the change in depth and temperature of the depositional palaeoenvironment. δ13C values started to rise over 2 ‰ PDB and reached a maximum of 4.3 ‰ PDB at the Wordian–Capitanian boundary at the Bagh-e Vang section. This δ13C rise is attributed to high primary productivity as previously reported in the Capitanian Abadeh Formation in central Iran. The positive δ13C excursion in these sections is correlated with the Capitanian ‘Kamura event’ identified from the mid-Panthalassian sections in Japan. No noticeable positive excursion occurs in the δ13C plot at the Shotori section making the interpretation of palaeo-productivity difficult. It is suggested that an active oceanic upwelling was the probable driver of the Middle Permian oceanic productivity in central Iran. Remarkable negative δ13C excursions around 3.7 and 4.2 ‰ PDB in Capitanian carbonates close to the Guadalupian–Lopingian boundary at the Bagh-e Vang and Abadeh sections, respectively are recorded, which are a proxy for low palaeo-productivity and a transition from a cool to warm climate, consistent with an early Lopingian sea level rise.

Borings and etchings in the Upper Bathonian-Lower Callovian oolite of the Paris Basin (France)

The oolite of the Dalle Nacree Formation in the Paris Basin is made of marine calcareous ooids with, from base to top, radial (and therefore likely to have been calcite), concentric and micritic fabrics, each corresponding to a discrete stratigraphic unit. Several hardgrounds and oolitic pebble-cobble layers in the succession are encrusted and bored. Three main types of boring have been identified ranging in sizes from some tens of µm (sponge borings) to centimeters (bivalve borings), with an intermediate category (worm borings). Some worm borings have rough walls, where early marine fibrous cement is less corroded than the cortices of cemented ooids. The key to understanding this differential dissolution could be related to organic matter, present within the ooid cortices but lacking in the fibrous cement. Polychaete worms that use chemical means (enzymes or acids) to bore are probably responsible for these peculiar borings. A secondary conclusion is that partly or fully leached ooid cortices do not necessarily indicate an original aragonitic mineralogy of the dissolved parts.

Inorganic control on original carbonate mineralogy and creation of gas reservoir of the Upper Jurassic carbonates in the Kopet-Dagh Basin, NE, Iran

The Upper Jurassic Mozduran Formation (Oxfordian–Tithonian) is the main petroleum reservoir in the Kopet-Dagh Basin, northeast Iran, which consists predominantly of carbonate rocks with subordinate evaporites and siliciclastics deposited in platform to deep marine settings of a subtropical sea. Detailed field surveys, petrographic investigations, facies and wire line log analyses were carried out at eight surface sections and four wells in the Kopet-Dagh Basin. Integration of petrographic and isotopic data suggests primary low-Mg calcite mineralogy of Oxfordian and Tithonian ooids. On the other hand, in the wells, Kimmeridgian ooids and cements are dominantly aragonitic and high-Mg calcite mineralogy (HMC). Marine cements with isopachous, fibrous and isopachous bladed fabrics indicate original aragonite and HMC mineralogy, respectively. The domination of aragonite mineralogy could be related to increased hypersalinity, evaporite precipitation and consequently an increase in Mg/Ca ratio, which resulted in formation of aragonite in preference to calcite. Preserved ooids with radial and concentric cortices in shallow-water settings that are nearby siliciclastic source, together with aragonitic and HMC ooids accompanied by evaporites in the drilled fields, suggest original mineralogy was probably controlled inorganically following local salinity variations. This study suggests that Kimmeridgian pay zones are mainly controlled by depositional facies, aragonitic and HMC mineralogy, and diagenetic processes such as dolomitization and dissolution.

Geochemistry, microfacies and diagenetic evidences for original aragonite mineralogy and open diagenetic system of Lower Cretaceous carbonates Fahliyan Formation (Kuh-e Siah area, Zagros Basin, South Iran)

The Fahliyan Formation is an important reservoir unit of Cretaceous strata in the south of Iran (Zagros Mountains). This formation with the age of Lower Cretaceous overlies unconformably the Hith Formation and is conformably covered by the Gadvan Formation in the studied area. To evaluate the original carbonate mineralogy, paleotemperature, and diagenetic environment in Kuh-e Siah anticline in Boushehr Province, the Fahliyan Formation was investigated by petrographic and geochemical analyses. Petrographic studies led to the recognition of 25 microfacies that were deposited in four facies belts: tidal flat, lagoon, and shoal in inner ramp and shallow open marine in mid-ramp environment. Of these microfacies, twenty-one belong to inner ramp and four are located in the mid ramp. Calcareous algae and benthic foraminifera are abundant in the shallow-marine carbonates of the Fahliyan Formation. Evidences such as absence of calciturbidite deposits and reefal facies, the occurrence of widespread tidal flat deposits, gradual facies changes, and abundant micrites indicate that the Fahliyan Formation was deposited in a homoclinal carbonate ramp environment. Abundant aragonite skeletal and non-skeletal components and presence of dolomites in Fahliyan carbonates indicate original aragonite mineralogy. Radial ooids generated in low-energy environments similar to modern aragonitic forms. Deformed and spalled ooids as well as shattered micritic envelopes indicate aragonite dissolution during meteoric diagenesis. Isopachous and fibrous sparry calcite cements resemble modern aragonite morphologies. Micritization, geopetal fabric, bioturbation, neomorphism, compaction, cementation, dissolution, dolomitization, dedolomitization, and fracturing are diagenetic processes in the Fahliyan Formation, occurring in marine to meteoric and burial diagenetic environments. Cementation, dissolution, and dolomitization are the main diagenetic processes that affected the original texture. Values of major (Ca and Mg) and minor (Sr, Na, Fe and Mn) elements (such as high Sr value) and δ18O and δ13C designate that the Fahliyan carbonates were deposited in a shallow warm-water sub-tropical environment and aragonite was the original carbonate mineralogy. Variations of Sr/Ca and δ18O values versus Mn suggest that diagenetic alteration must have occurred in an open diagenetic system, with high water–rock interaction. Dolomitization, dissolution and cementation are the main diagenetic features observed in the Fahliyan Formation of Kuh-e Siah and confirm high water–rock interaction in open diagenetic system. Dissolution as leaching is the most important diagenetic event in the evolution of porosity, particularly where green algae and other benthic foraminifera are abundant. This may indicate that the original carbonate mineralogy of the mentioned bioclasts might be aragonite, rather than low-Mg calcite. Temperature calculation based on the oxygen isotope value of the least-altered sample, using Anderson and Arthur (Stable isotopes in sedimentary geology, vol 10. Society of Economic Paleontologists and Mineralogists. Short Course, pp 1–151, 1983) equation, indicates paleotemperature to be around 29.1 °C.

Microfacies and geochemical evidence for original aragonite mineralogy of a foraminifera-dominated carbonate ramp system in the late Paleocene to Middle Eocene, Alborz basin, Iran

The Ziyarat Formation is a Late Paleocene to Middle Eocene carbonate sequence, located in the north of Tochal Village (south-east of Tehran). The Ziyarat Formation with a total thickness of 212.5 m conformably overlies the Fajan conglomerate and is overlain by greenish tuffaceous siltstone of the Karj Formation. Petrographic studies have led to the recognition of 11 microfacies. Of these microfacies, five belong to inner ramp, four belong to middle ramp and two are located in the outer ramp. Due to the great diversity and abundance of large benthic foraminifera (e.g., Nummulites, Alveolina, Discocyclina), the Ziyarat Formation represents an example of “foraminifera-dominated carbonate ramp settings”. Paleobathymetry, light level and water energy were determined during deposition of Ziyarat carbonates based on the test shape and size variation of different types of benthic foraminifera and other components in different facies. Micritization, cementation, compaction, dissolution, dolomitization and fracturing are important diagenetic processes in the Ziyarat Formation, occurring in marine to meteoric and burial diagenetic environments. Geochemical studies indicate that Ziyarat carbonates were deposited in a shallow warm-water sub-tropical environment and aragonite was the original carbonate mineralogy with mean Sr values of 3,470 ppm. It should be noted that the maximum Sr content in abiotic calcite is 1,000 ppm. In Ziyarat limestone, dissolution as leaching is the most important diagenetic event in the evolution of porosity, particularly where Nummulite and other benthic foraminifera are abundant. This may indicate that the original carbonate mineralogy of Nummulites might be aragonite, rather than low-Mg calcite. In the Jahrum Formation of Iran and in the El Garia Formation in north-east of Libya, the widespread moldic porosity in Nummulite beds enhanced the reservoir potential of the carbonate sequence. Thus, it is proposed that the original carbonate mineralogy of nummulitid shells may be varied in different environments, and those with metastable mineralogy are excellent for forming hydrocarbon reservoirs. Bivariate plots of Mn versus Sr/Ca and δ18O values illustrated that Ziyarat limestones were affected by open diagenetic system with high water/rock interaction. Early burial diagenetic temperature calculation based on heaviest oxygen isotope values of micrite and δw of Eocene seawater of −0.85 SMOW showed that temperature was around 39 °C. Cathodluminescence studies of carbonate cements illustrated dull luminescence, because these carbonates were affected mainly by burial diagenesis. This statement is confirmed by δ18O and δ13C isotope evidences. Based on different facies and evidences such as absence of reefal facies, calciturbidite deposits, the occurrence of widespread tidal flat deposits and abundant micrites in all microfacies indicate that the Ziyarat Formation was deposited in a homoclinal carbonate ramp environment. In addition, many researchers suggested that due to extreme global warming during the Paleocene–Eocene time, platform margin coral reefs declined in low latitude and heterozoan calcitic foraminifers became abundant and, thus, widespread carbonate ramp systems developed.

Uplifted Pleistocene marine sediments of the Central Adriatic Island of Brusnik

The Island of Brusnik, located in the Central Adriatic Sea, is mainly known for its Triassic igneous rocks. However, it also contains Pleistocene conglomerates, limestones and Neptunian dikes, as well as Holocene rock debris, soil, and beach gravels. Quaternary sediments unconformably overlie tectonically disturbed Triassic basement. The majority of Pleistocene limestones, as well as the matrix of the conglomerates, are predominantly bioclastic grainstones and rudstones. Gastropod shells in these sediments retain their original aragonite mineralogy, and may also display their original colours. The majority of the conglomerates and limestones originated in lower beachface and shoreface environments. Similar sediments have not been identifi ed in the surrounding area and the Dinarides in general. Some of the Pleistocene sediments originated during MIS 5e of the Last Interglacial based on radiometric evidence. The island experienced uplift of about 30m during the Middle and Late Pleistocene and this process probably continued in the postglacial period. The combination of uplift and cyclic sea-level changes is envisaged to have resulted in an overall downstepping pattern of the Pleistocene deposits.

Incremental growth and mineralogy of Pannonian (Late Miocene) sciaenid otoliths: paleoecological implications

Incremental growth and mineralogy of Pannonian (Late Miocene) sciaenid otoliths: paleoecological implicationsOntogenetic age and body dimensions were studied on three extremely well-preserved sciaenid fish otoliths from sublittoral marls of Lake Pannon from Doba, Bakony Mts, Hungary. Macroscopic and microscopic observations offered clear evidence for the preservation of the genuine structural characteristics, for instance the bipartite incremental features. Ontogenetic ages were assigned for the three specimens as 16, 7 and 6 years by counting the annuli of the sagittae. Analytical results prove that the original aragonitic mineralogy has been preserved making them, and probably other Late Miocene teleost fossils, suitable for future microchemical analysis to reconstruct the past physicochemical environment.

Depositional environment, sequence stratigraphy and geochemistry of Lower Cretaceous carbonates (Fahliyan Formation), south-west Iran

The Fahliyan Formation is a carbonate sequence of Lower Cretaceous (Berriasian–Hauterivian) age and was deposited in the Zagros sedimentary basin, Iran. In this investigation, the Fahliyan Formation at the type section and in the subsurface has been studied. Facies analysis and petrographic studies led to the recognition of 10 microfacies that were deposited in three facies belts: lagoon, shoal and open marine. The observed facies patterns indicate a carbonate rimmed-shelf depositional environment. Based on field observations, microfacies analysis and sequence stratigraphic concepts, two-third-order sequences in the type section and three-third-order sequences in the subsurface section were identified. The transgressive deposits display a predominance of deep subtidal facies, while highstand deposits show shallow subtidal facies. Some petrographic evidence such as an abundance of aragonite skeletal and non-skeletal components shows that this formation was deposited in a sub-tropical environment with original aragonite mineralogy. Geochemical evidence such as high Sr/Na ratios also support original aragonite mineralogy. δ 18O and δ 13C values suggests that alteration occurred during burial diagenesis, in a closed system, with low water/rock interaction. Palaeotemperature calculation, based on the heaviest oxygen isotope value in micritic samples of the Fahliyan limestone, shows that ambient water temperature was around 24 °C during the deposition of this formation.

A sedimentary prelude to Marinoan glaciation, Cryogenian (Middle Neoproterozoic) Keele Formation, Mackenzie Mountains, northwestern Canada

Continental margin deposition between the Sturtian- and Marinoan-aged global glacial periods is represented in NW Canada by a siliciclastic-to-carbonate grand cycle. The Keele Formation, a 300–600 m thick carbonate and siliciclastic deposit, comprises the upper part of this interglacial cycle. Sedimentary facies in the lower, carbonate-dominated member of the Keele span a broad spectrum of depositional environments, from below storm-wave-base to peritidal to continental, and are disposed across a relatively narrow paleogeographic belt. Lateral arrangement of facies and the gradual stratigraphic transitions between them, suggest deposition in a ramp setting; local slump structures and occasional debrites in deep-water facies imply that the ramp was distally steepened. Fluvial facies are concentrated at a single locality, indicating that terrigenous-clastic sediment was derived from a point source or sources. Most strata of the carbonates are “Bahamian” in character and similar to Phanerozoic warm-water carbonates in that they contain abundant calcite ooids of interpreted original aragonitic mineralogy and have well-developed muddy peritidal facies. Nevertheless, close juxtaposition of deep- and shallow-water facies in Keele rocks indicates high-amplitude sea-level change during deposition, on the order of ∼100 m, suggesting accumulation in a climatic icehouse period. This interpretation implies that although the globe possessed polar and high-altitude glaciers at this time, sea-surface temperature, at least at tropical latitudes, warmed dramatically between Neoproterozoic glacial episodes. The onset of Marinoan glaciation here is heralded by an abrupt shift to nearshore marine siliciclastic sedimentary rocks. These deposits form the upper member of the Keele Formation (informally called the Keele clastic wedge) and directly underlie Ice Brook Formation diamictites. Carbonates are rare and are restricted to mudstones and flat-pebble conglomerates that lack any indication of tropical conditions.

Petrology and diagenetic evolution of Neoproterozoic ooids (Libby Formation, western Montana, U.S.A.)

Radial/concentric and twinned/sparry ooids from the upper Belt Supergroup originally formed as mixed Mgcalcite and aragonite, and aragonite. Textural evidence suggests that Mgcalcite ooids may have formed along with the ooids having an original aragonite mineralogy. Strontium concentrations of 2400 to 2900 ppm support an aragonitic precursor to the twinned/sparry ooids, and Sr values of 2500 ppm strongly suggest that the radial/concentric ooids had an aragonitic component in addition to the Mgcalcite suggested by the preserved fabric. Trace element (Mn, Mg, Fe, and Sr) values of ooids and early carbonate cements suggest that diagenetic trends for these Proterozoic carbonates are similar to those established for their Phanerozoic and Archean counterparts. Mg, Fe, and Mn increase and Sr decreases with successively later carbonate phases.

Petrographic and geochemical evidence for original aragonite mineralogy of upper Jurassic carbonates (Mozduran formation), Sarakhs area, Iran

Diverse skeletal and non-skeletal grains with abundant red and blue-green calcareous algae, evaporites and early diagenetic dolomites in the Mozduran carbonates are similar to those of modern warm-water shallow-marine carbonates. Isopachous and fibrous intragranular sparry calcite cements resemble modern aragonite morphologies. Concentric and radial ooids are like modern aragonitic forms which are formed in turbulent and in low-energy environments, respectively. Deformed ooids, shattered micritic envelopes and spalled ooids indicate aragonite dissolution during meteoric diagenesis. Meteoric cements are equant, mosaic to drusy calcites. Burial cements only occur as vein fillings.Sr and Na covariance with Mn supports evidence of inversion of aragonite to calcite, with subsequent open phreatic diagenesis. Fe concentrations are a magnitude higher than Mn due to a reducing phreatic environment. High SrNa ratios suggest original aragonite mineralogy.The δ18O and δ13C field of Mozduran limestone lies at the edge of the modern shallow-marine field indicating a range from marine to meteoric cements. Differences in δ18O values between marine calcites and meteoric-derived calcites is similar to that in the modern tropics. The surface marine diagenetic trend line that passes through shallow-marine modern whole carbonates cuts the Mozduran limestone isotope field due to their formation in warm, shallow-marine waters, given that atmospheric CO2 levels were similar to those at present.Increasing Mn and Fe values, decreasing Na and uniform Sr values with increasingly lighter values of both δ18O and δ13C are exlained by extensive meteoric diagenesis. The least altered marine calcite δ18O value of −2.6‰ PDB corresponds to 29 or 24°C, depending on whether δω = 0‰ (of present seawater) and δω = − 1.2‰ (of ancient seawater with no ice caps) are taken as a base. An original calcite mineralogy postulated for some Jurassic limestones is related to a non-tropical origin.

Probable calcified metaphytes in the latest Proterozoic Nama Group, Namibia: origin, diagenesis, and implications.

Samples from the Huns Limestone Member, Urusis Formation, Nama Group, at two adjacent localities in southern Namibia contain thin foliose to arched, sheet-like carbonate crusts that are 100-500 micrometers thick and up to 5 cm in lateral dimension. Morphologic, petrographic, and geochemical evidence supports the interpretation of these delicate crusts as biogenic, most likely the remains of calcified encrusting metaphytes. The original sediments of the fossiliferous samples contained aragonitic encrusting algae, botryoidal aragonite cements, and an aragonite mud groundmass. Spherulites within the precursor mud could represent bacterially induced mineral growths or the concretions of marine rivularian cyanobacteria. Original textures were severely disrupted during the diagenetic transition of aragonite to low-magnesian calcite, but some primary structures remain discernible as ghosts in the neomorphic mosaic. Gross morphology, original aragonite mineralogy, and hypobasal calcification indicate that the crusts are similar to late Paleozoic phylloid algae and extant peyssonnelid red algae. Structures interpreted as possible conceptacles also suggest possible affinities with the Corallinaceae. Two species of Cloudina, interpreted as the remains of a shelly metazoan, are also known from limestones in the Nama Group. It is possible, therefore, that skeletalization in metaphytes and animals arose nearly simultaneously near the end of the Proterozoic Eon.

Comments on a paper supposedly giving the first evidence of aragonitic mineralogy in tetradiid tabulate corals

WENDT (1989) has claimed recently to be the first to recognize that a tabulate coral group, the family Tetradiidae, had an original aragonitic mineralogy. He has added that the "unusual mineralogic composition" of the family "(whose original rnicrostructure remains unknown) has been completely ignored" by workers since BASSLER (1950). These statements are incorrect, for nearly twenty years ago, SEMENIUK (1971) reached much the same conclusions in his studies of the Ordovician carbonates and associated faunas in central New South Wales. SEM~NItSK (1971:945-49) recognized that there were two markedly different types of fossil preservation in the same samples. First there are the fossils like Tetradium, the inarticulate brachiopod Eodinobolus, the cylindrical stromatoporoid Alleynodictyon and the gastropods which have been much modified by diagenetic leaching processes and which "may have been composed originally of aragonite". Secondly there are most of the other fossils (such as the various articulate brachiopods, the heliolitine and rugose corals, the bryozoans and echinoderm remains) which exhibit little alteration to their internal structures, and which he regarded as having an original calcitic composition. These observations are little different from those espoused recently by WEND'r (1989), namely, that the skeletons of Tetradium DANA 1846 and PhytopsisJ. HALL 1847, p. 39 (a name which is strictly preoccupied by trace fossil PhytopsisJ. HALL 1847, p. 38 see HANTZSCHEL, 1975) are "completely transformed into a coarse mosaic of neomorphic spar which has left no trace of the original microstructures... The only possible interpretation for such a diagenetic overprint is that the original mineralogy of the skeleton was aragonitic. This interpretation is supported by the observation that other skeletal remains, the primary calcitic composition of which is well established and which occur in the same samples (e. g. brachiopods, ostracods), have retained their original wall structures".

Late Proterozoic aragonitic cement crusts, Bambuí Group, Minas Gerais, Brazil

ABSTRACTThe Late Proterozoic Pedro Leopoldo facies (Bambuí Group) in the vicinity of Belo Horizonte, Brazil, comprises alternating laminated microsparitic limestones (10–35 mm thick beds) and fibrous limestones (10–55 mm thick). The latter are composed of a mosaic of sparry calcite crystals. These irregularly crosscut rays and fans are composed of feathery precursor crystal bundles with squared‐off growth zones. Ghosts of an original fibrous mineral, hexagonal in cross‐section, are visible. The petrographic characteristics, very high strontium content and low magnesium content of the fibrous beds, as well as microspar beds, strongly argue for an original aragonitic mineralogy. The rays are interpreted as having formed by precipitation at the sediment‐water interface, whereas the micrite was precipitated from the water column prior to deposition on the sea floor. The lack of emergence features suggests widespread aragonite precipitation under persistently subtidal conditions.

Geochemistry of the Gordon Limestone (Ordovician), Mole Creek, Tasmania

The Gordon Limestone is characterized by low Mn, moderate Na and high Sr concentrations, similar to those in modern tropical aragonitic carbonates. Mn reflects predominantly original aragonite mineralogy, oxidizing peritidal conditions and the influence of meteoric water. Sr and Na concentrations generally decrease with increasing Mn. Na concentrations are similar in limestones and dolomitic limestones because of the lack of appreciable freshwater influence during dolomitization. Sr concentrations decrease with increasing Mg concentrations, indicating dolomitization in predominantly marine waters. Similar amounts of Mn in both limestones and dolomitic limestones indicate prevalence of oxidizing conditions during their formation. A Ba content of mostly <60 parts/106 indicates mainly marine origin for limestones and dolomitic limestones. Ti reflects the clay mineral content in each sample. Positive correlation between Rb and K2O supports marine origin of clays within the carbonates. Positive correlation between insoluble residue percentage and K2O suggests that illite is the predominant non-carbonate mineral. The dolomites are enriched relative to calcite by 2–3% PDB in δ18O and about 1% PDB in δ13C, indicating formation in mainly marine and mixed-marine waters at temperatures averaging 23°C.

Geochemistry of the Gordon Limestone (Ordovician), Mole Creek, Tasmania

The Gordon Limestone is characterized by low Mn, moderate Na and high Sr concentrations, similar to those in modern tropical aragonitic carbonates. Mn reflects predominantly original aragonite mineralogy, oxidizing peritidal conditions and the influence of meteoric water. Sr and Na concentrations generally decrease with increasing Mn. Na concentrations are similar in limestones and dolomitic limestones because of the lack of appreciable freshwater influence during dolomitization. Sr concentrations decrease with increasing Mg concentrations, indicating dolomitization in predominantly marine waters. Similar amounts of Mn in both limestones and dolomitic limestones indicate prevalence of oxidizing conditions during their formation. A Ba content of mostly <60 parts/106 indicates mainly marine origin for limestones and dolomitic limestones. Ti reflects the clay mineral content in each sample. Positive correlation between Rb and K2O supports marine origin of clays within the carbonates. Positive correlation between insoluble residue percentage and K2O suggests that illite is the predominant non-carbonate mineral. The dolomites are enriched relative to calcite by 2–3% PDB in δ18O and about 1% PDB in δ13C, indicating formation in mainly marine and mixed-marine waters at temperatures averaging 23°C.