ABSTRACT Unconformities and related karst systems are studied worldwide. A unique red calcite gen... more ABSTRACT Unconformities and related karst systems are studied worldwide. A unique red calcite generation, which fills fractures/cavities, is hosted by Mesozoic carbonates in the Transdanubian Range, Hungary. Solid inclusions are located along growth zones of calcite. Hematite, the most abundant solid inclusion, gives the red colour of it. Outcrop-scale geometry, mineralogical features and detrital mineral assemblage (hematite, gibbsite, goethite, kaolinite, smectite, illite, Cr-spinel, monazite, xenotime, zircon, apatite, and Ti-oxide) of calcite precipitates suggest strong correlation between the calcite and nearby karst bauxite deposits. Fluid inclusion petrography and microthermometry (T<50 °C; salinity from 0 to 0.17 NaCl eq. w%) of primary fluid inclusions, and the stable isotope trend of the calcite, following the meteoric water line, clearly indicate vadose and phreatic meteoric origin in a near-surface karst system.The late Cretaceous to mid-Eocene unconformity-related cavity-filling deposits occur close to the surface; indicating that the most recent Quaternary exhumation re-exposed those surfaces that existed at the time of calcite mineralization. Thus red calcite precipitates are interpreted as being speleothems, vestiges of the subterranean part of the pre-Middle Eocene karst. The infiltrated, fine bauxite particles enclosed by the calcite are the witnesses of the once areally extensive pre-Middle Eocene bauxitic blanket that became partially eroded by the time of the deposition of the cover beds.Red calcite when found in core samples may provide good evidence on bauxite formation associated with the overlying unconformity, even if it was later removed by erosion. Therefore, presence or absence of red calcite may be used as distinguishing criteria between karst episodes with or without bauxite formation.This article is protected by copyright. All rights reserved.
Calcite veins and related sulphate–sulphide mineralisation are common in the Buda Hills. Also, ab... more Calcite veins and related sulphate–sulphide mineralisation are common in the Buda Hills. Also, abundant hypogenic caves are found along fractures filled with these minerals pointing to the fact that young cave-forming fluids migrated along the same fractures as the older mineralising fluids did. The studied vein-filling paragenesis consists of calcite, barite, fluorite and sulphides. The strike of fractures is consistent—NNW–SSE—concluding a latest Early Miocene maximum age for the formation of fracture-filling minerals. Calcite crystals contain coeval primary, hydrocarbon-bearing- and aqueous inclusions indicating that also hydrocarbons have migrated together with the mineralising fluids. Hydrocarbon inclusions are described here for the first time from the Buda Hills. Mixed inclusions, i.e., petroleum with ‘water-tail’, were also detected, indicating that transcrystalline water migration took place. The coexistence of aqueous and petroleum inclusions permitted to establish the entrapment temperature (80°C) and pressure (85 bar) of the fluid and thus also the thickness of sediments, having been eroded since latest Early Miocene times, was calculated (800 m). Low salinity of the fluids (2 and CH4 are associated with hydrocarbons. Groundwater also contains small amounts of HC and related gases on the basin side even today. Based on the location of the paleo- and recent hydrocarbon indications, identical migration pathways were reconstructed for both systems. Hydrocarbon-bearing fluids are supposed to have migrated north-westward from the basin east to the Buda Hills from the Miocene on.
governed by climate. Burial diagenesis usually resulted in only moderate dolomitization, either i... more governed by climate. Burial diagenesis usually resulted in only moderate dolomitization, either in connection with compactional fluid flow or via thermal convection. The Tri-assic fault zones provided conduits for fluid flow that led to both replacive dolomitization and dolomite cement precipitation. In the Late Triassic extensional basins, synsedimen-tary fault-controlled dolomitization of basinal deposits was reconstructed.
Dolomite most commonly forms via replacement of precursor carbonate minerals. For this reason, di... more Dolomite most commonly forms via replacement of precursor carbonate minerals. For this reason, diagnosing primarily precipitated organogenic dolomite in microbial mat deposits from the rock record is not straightforward, even though the deposits exhibit microbial fabric. Single and multiple dolomite crusts exhibiting microbial fabric occur in a pervasively dolomitized Middle Triassic platform succession. Two sections were studied in the Transdanubian Range. In both sections, two fabric types occur in the upper part of the metre-scale cycles. One of that is microbial boundstone (fabric type 1)—characterised by clusters of dolomite microcrystalswhich display diagnostic microbial features, such as calcimicrobes, clotted–spherular aggregates and globules. The other one is different in the two sections. In Section 1, it is micritic dolomite (fabric type 2) that is characterised by predominantly fine crystals and contains obscured microbial components. In Section 2, it is bioclastic dolomite (fabric type 3) that is rich in reworked dasycladalean alga fragments and consists of dolomite crystals of wide sizerange from fine to coarse. The precipitation of the microcrystalline dolomite phase is interpreted as being facilitated by mats and biofilms favouring/tolerating an increasing frequency of subaerial conditions in the upper intertidal setting. Petrographic analyses revealed that organogenic calcite was also precipitated, especially in mat deposits rich in bioclasts. Synsedimentary dolomitization, resulting in fine crystals, was coupled with aragonite dissolution and it postdated the organogenic precipitation. It took place only in the peritidal caps of the shallowing-upward depositional units. Petrographic analyses provide circumstantial evidence constraining that microcrystalline dolomite did not formvia mimetic replacement. Accordingly the microcrystalline dolomite, which shows microbial microfabrics in the studied samples, is interpreted as an organogenic primary precipitate. Both peritidal processes, dolomite precipitation and replacement, were likely controlled by the environmental factors in a semi-arid climate. Those components of the platform succession that were not dolomitized in the peritidal environment were replaced and cemented by mediumand coarsely crystalline dolomite during further burial at elevated temperature, as shown by fluid inclusion homogenisation temperature (62 to 83 °C) and negative stable oxygen isotope values. Thus, the majority of the studied formation consists of fabric-destructive dolomite (fabric type 4).
ABSTRACT Dolomite most commonly forms via replacement of precursor carbonate minerals. For this r... more ABSTRACT Dolomite most commonly forms via replacement of precursor carbonate minerals. For this reason, diagnosing primarily precipitated organogenic dolomite in microbial mat deposits from the rock record is not straightforward, even though the deposits exhibit microbial fabric. Single and multiple dolomite crusts exhibiting microbial fabric occur in a pervasively dolomitized Middle Triassic platform succession. Two sections were studied in detail in the north-eastern part of the Transdanubian Range (north-central Hungary). In both sections, thin layer-couplets of two fabric types occur in the upper part of the metre-scale cycles. Microbial boundstone (fabric type 1)––characterised by clusters of dolomite microcrystals which display diagnostic microbial features, such as calcimicrobes, clotted–spherular aggregates and globules––composes the upper layer of couplets. The lower layer of the couplets is different in the two studied sections. In Section 1, it is micritic dolomite (fabric type 2) that is characterised by predominantly fine crystals and contains obscured microbial components. In Section 2, it is bioclastic dolomite (fabric type 3) that is rich in reworked dasycladalean fragments and consists of dolomite crystals of wide size-range from fine to coarse. The precipitation of the microcrystalline dolomite phase, which process is the focus of this study, is interpreted as being facilitated by mats and biofilms favouring/tolerating increasing frequency of subaerial conditions in the upper intertidal setting. Petrographic analyses revealed that organogenic calcite was also precipitated, especially in mat deposits rich in bioclasts. Synsedimentary dolomitization, resulting in fine crystals, was coupled with aragonite dissolution. Moreover, it postdated the organogenic precipitation and fibrous calcite cementation. Synsedimentary replacive dolomitization took place only in the peritidal caps of the shallowing-upward depositional units. Petrographic analyses provide circumstantial evidence constraining that microcrystalline dolomite did not form via mimetic replacement. Accordingly the microcrystalline dolomite, which shows microbial microfabrics in the studied samples, is interpreted as organogenic primary precipitate. Both peritidal processes, dolomite precipitation and replacement, were likely controlled by the environmental factors in a semi-arid climate. Those components of the platform succession that were not dolomitized in the peritidal environment were replaced and cemented by medium and coarsely crystalline dolomite during further burial at elevated temperature, as shown by fluid inclusion homogenisation temperature (62 to 83 °C) and negative stable oxygen isotope values. Thus, the majority of the studied formation consists of fabric-destructive dolomite (fabric type 4).
Dolomite most commonly forms via replacement of precursor carbonate minerals. For this reason, di... more Dolomite most commonly forms via replacement of precursor carbonate minerals. For this reason, diagnosing primarily precipitated organogenic dolomite in microbial mat deposits from the rock record is not straightforward, even though the deposits exhibit microbial fabric. Single and multiple dolomite crusts exhibiting microbial fabric occur in a pervasively dolomitized Middle Triassic platform succession. Two sections were studied in the Transdanubian Range. In both sections, two fabric types occur in the upper part of the metre-scale cycles. One of that is microbial boundstone (fabric type 1)‒‒characterised by clusters of dolomite microcrystals which display diagnostic microbial features, such as calcimicrobes, clotted–spherular aggregates and globules. The other one is different in the two studied sections. In Section 1, it is micritic dolomite (fabric type 2) that is characterised by predominantly fine crystals and contains obscured microbial components. In Section 2, it is bioclastic dolomite (fabric type 3) that is rich in reworked dasycladalean alga fragments and consists of dolomite crystals of wide size-range from fine to coarse. The precipitation of the microcrystalline dolomite phase is interpreted as being facilitated by mats and biofilms favouring/tolerating an increasing frequency of subaerial conditions in the upper intertidal setting. Petrographic analyses revealed that organogenic calcite was also precipitated, especially in mat deposits rich in bioclasts. Synsedimentary dolomitization, resulting in fine crystals, was coupled with aragonite dissolution and it postdated the organogenic precipitation. It took place only in the peritidal caps of the shallowing-upward depositional units. Petrographic analyses provide circumstantial evidence constraining that microcrystalline dolomite did not form via mimetic replacement. Accordingly the microcrystalline dolomite, which shows microbial microfabrics in the studied samples, is interpreted as an organogenic primary precipitate. Both peritidal processes, dolomite precipitation and replacement, were likely controlled by the environmental factors in a semi-arid climate. Those components of the platform succession that were not dolomitized in the peritidal environment were replaced and cemented by medium and coarsely crystalline dolomite during further burial at elevated temperature, as shown by fluid inclusion homogenisation temperature (62 to 83 °C) and negative stable oxygen isotope values. Thus, the majority of the studied formation consists of fabric-destructive dolomite (fabric type 4).
ABSTRACT Disintegration of dolostones to dolomite powder (powderization) was a widespread phenome... more ABSTRACT Disintegration of dolostones to dolomite powder (powderization) was a widespread phenomenon in Triassic dolostones of the Buda Hills, where the areal extent of powdered dolostones is large compared to similar occurrences elsewhere in the world. In the Buda Hills, dolostone disintegration proceeded in four stages that correspond to a gradual decrease in particle size, that is, from the parent dolostone to (1) crackle breccia; via (2) mosaic breccia (diameter\2 cm); via (3) mosaic breccia blocks ‘floating’ in dolomite powder; to (4) dolomite powder (diameter 100–300 lm). Stable isotope ratios and trace element compositions of dolomite remained constant throughout these stages, and there are no indications of dissolution in most locations, suggesting that disintegration was predominantly a mechanical process. Combining these findings with the geological history of the region, and supported by a simple freezing/thawing experiment and pertinent experimental studies on weathering of building stones, it appears that powderization in the Buda Hills was caused by repeated freeze–thaw cycles during and/or after the Pleistocene glaciations. Subaerial exposure under cold climate conditions involves multiple freeze–thaw cycles that create mechanical stresses in the rock framework related to the opposing thermal expansion of rock and water that freezes and of ice that liquefies. This process is herewith called ‘cryogenic powderization’. Our data further suggest that the synergy of four factors promoted dolostone powderization in the Buda Hills: (1) tectonics, which created a pervasive fracture network; (2) intercrystalline porosity of the dolostone; (3) relatively high water saturation; and (4) subaerial exposure under cold climate conditions.
ABSTRACT Unconformities and related karst systems are studied worldwide. A unique red calcite gen... more ABSTRACT Unconformities and related karst systems are studied worldwide. A unique red calcite generation, which fills fractures/cavities, is hosted by Mesozoic carbonates in the Transdanubian Range, Hungary. Solid inclusions are located along growth zones of calcite. Hematite, the most abundant solid inclusion, gives the red colour of it. Outcrop-scale geometry, mineralogical features and detrital mineral assemblage (hematite, gibbsite, goethite, kaolinite, smectite, illite, Cr-spinel, monazite, xenotime, zircon, apatite, and Ti-oxide) of calcite precipitates suggest strong correlation between the calcite and nearby karst bauxite deposits. Fluid inclusion petrography and microthermometry (T<50 °C; salinity from 0 to 0.17 NaCl eq. w%) of primary fluid inclusions, and the stable isotope trend of the calcite, following the meteoric water line, clearly indicate vadose and phreatic meteoric origin in a near-surface karst system.The late Cretaceous to mid-Eocene unconformity-related cavity-filling deposits occur close to the surface; indicating that the most recent Quaternary exhumation re-exposed those surfaces that existed at the time of calcite mineralization. Thus red calcite precipitates are interpreted as being speleothems, vestiges of the subterranean part of the pre-Middle Eocene karst. The infiltrated, fine bauxite particles enclosed by the calcite are the witnesses of the once areally extensive pre-Middle Eocene bauxitic blanket that became partially eroded by the time of the deposition of the cover beds.Red calcite when found in core samples may provide good evidence on bauxite formation associated with the overlying unconformity, even if it was later removed by erosion. Therefore, presence or absence of red calcite may be used as distinguishing criteria between karst episodes with or without bauxite formation.This article is protected by copyright. All rights reserved.
ABSTRACT Unconformities and related karst systems are studied worldwide. A unique red calcite gen... more ABSTRACT Unconformities and related karst systems are studied worldwide. A unique red calcite generation, which fills fractures/cavities, is hosted by Mesozoic carbonates in the Transdanubian Range, Hungary. Solid inclusions are located along growth zones of calcite. Hematite, the most abundant solid inclusion, gives the red colour of it. Outcrop-scale geometry, mineralogical features and detrital mineral assemblage (hematite, gibbsite, goethite, kaolinite, smectite, illite, Cr-spinel, monazite, xenotime, zircon, apatite, and Ti-oxide) of calcite precipitates suggest strong correlation between the calcite and nearby karst bauxite deposits. Fluid inclusion petrography and microthermometry (T<50 °C; salinity from 0 to 0.17 NaCl eq. w%) of primary fluid inclusions, and the stable isotope trend of the calcite, following the meteoric water line, clearly indicate vadose and phreatic meteoric origin in a near-surface karst system.The late Cretaceous to mid-Eocene unconformity-related cavity-filling deposits occur close to the surface; indicating that the most recent Quaternary exhumation re-exposed those surfaces that existed at the time of calcite mineralization. Thus red calcite precipitates are interpreted as being speleothems, vestiges of the subterranean part of the pre-Middle Eocene karst. The infiltrated, fine bauxite particles enclosed by the calcite are the witnesses of the once areally extensive pre-Middle Eocene bauxitic blanket that became partially eroded by the time of the deposition of the cover beds.Red calcite when found in core samples may provide good evidence on bauxite formation associated with the overlying unconformity, even if it was later removed by erosion. Therefore, presence or absence of red calcite may be used as distinguishing criteria between karst episodes with or without bauxite formation.This article is protected by copyright. All rights reserved.
Calcite veins and related sulphate–sulphide mineralisation are common in the Buda Hills. Also, ab... more Calcite veins and related sulphate–sulphide mineralisation are common in the Buda Hills. Also, abundant hypogenic caves are found along fractures filled with these minerals pointing to the fact that young cave-forming fluids migrated along the same fractures as the older mineralising fluids did. The studied vein-filling paragenesis consists of calcite, barite, fluorite and sulphides. The strike of fractures is consistent—NNW–SSE—concluding a latest Early Miocene maximum age for the formation of fracture-filling minerals. Calcite crystals contain coeval primary, hydrocarbon-bearing- and aqueous inclusions indicating that also hydrocarbons have migrated together with the mineralising fluids. Hydrocarbon inclusions are described here for the first time from the Buda Hills. Mixed inclusions, i.e., petroleum with ‘water-tail’, were also detected, indicating that transcrystalline water migration took place. The coexistence of aqueous and petroleum inclusions permitted to establish the entrapment temperature (80°C) and pressure (85 bar) of the fluid and thus also the thickness of sediments, having been eroded since latest Early Miocene times, was calculated (800 m). Low salinity of the fluids (2 and CH4 are associated with hydrocarbons. Groundwater also contains small amounts of HC and related gases on the basin side even today. Based on the location of the paleo- and recent hydrocarbon indications, identical migration pathways were reconstructed for both systems. Hydrocarbon-bearing fluids are supposed to have migrated north-westward from the basin east to the Buda Hills from the Miocene on.
governed by climate. Burial diagenesis usually resulted in only moderate dolomitization, either i... more governed by climate. Burial diagenesis usually resulted in only moderate dolomitization, either in connection with compactional fluid flow or via thermal convection. The Tri-assic fault zones provided conduits for fluid flow that led to both replacive dolomitization and dolomite cement precipitation. In the Late Triassic extensional basins, synsedimen-tary fault-controlled dolomitization of basinal deposits was reconstructed.
Dolomite most commonly forms via replacement of precursor carbonate minerals. For this reason, di... more Dolomite most commonly forms via replacement of precursor carbonate minerals. For this reason, diagnosing primarily precipitated organogenic dolomite in microbial mat deposits from the rock record is not straightforward, even though the deposits exhibit microbial fabric. Single and multiple dolomite crusts exhibiting microbial fabric occur in a pervasively dolomitized Middle Triassic platform succession. Two sections were studied in the Transdanubian Range. In both sections, two fabric types occur in the upper part of the metre-scale cycles. One of that is microbial boundstone (fabric type 1)—characterised by clusters of dolomite microcrystalswhich display diagnostic microbial features, such as calcimicrobes, clotted–spherular aggregates and globules. The other one is different in the two sections. In Section 1, it is micritic dolomite (fabric type 2) that is characterised by predominantly fine crystals and contains obscured microbial components. In Section 2, it is bioclastic dolomite (fabric type 3) that is rich in reworked dasycladalean alga fragments and consists of dolomite crystals of wide sizerange from fine to coarse. The precipitation of the microcrystalline dolomite phase is interpreted as being facilitated by mats and biofilms favouring/tolerating an increasing frequency of subaerial conditions in the upper intertidal setting. Petrographic analyses revealed that organogenic calcite was also precipitated, especially in mat deposits rich in bioclasts. Synsedimentary dolomitization, resulting in fine crystals, was coupled with aragonite dissolution and it postdated the organogenic precipitation. It took place only in the peritidal caps of the shallowing-upward depositional units. Petrographic analyses provide circumstantial evidence constraining that microcrystalline dolomite did not formvia mimetic replacement. Accordingly the microcrystalline dolomite, which shows microbial microfabrics in the studied samples, is interpreted as an organogenic primary precipitate. Both peritidal processes, dolomite precipitation and replacement, were likely controlled by the environmental factors in a semi-arid climate. Those components of the platform succession that were not dolomitized in the peritidal environment were replaced and cemented by mediumand coarsely crystalline dolomite during further burial at elevated temperature, as shown by fluid inclusion homogenisation temperature (62 to 83 °C) and negative stable oxygen isotope values. Thus, the majority of the studied formation consists of fabric-destructive dolomite (fabric type 4).
ABSTRACT Dolomite most commonly forms via replacement of precursor carbonate minerals. For this r... more ABSTRACT Dolomite most commonly forms via replacement of precursor carbonate minerals. For this reason, diagnosing primarily precipitated organogenic dolomite in microbial mat deposits from the rock record is not straightforward, even though the deposits exhibit microbial fabric. Single and multiple dolomite crusts exhibiting microbial fabric occur in a pervasively dolomitized Middle Triassic platform succession. Two sections were studied in detail in the north-eastern part of the Transdanubian Range (north-central Hungary). In both sections, thin layer-couplets of two fabric types occur in the upper part of the metre-scale cycles. Microbial boundstone (fabric type 1)––characterised by clusters of dolomite microcrystals which display diagnostic microbial features, such as calcimicrobes, clotted–spherular aggregates and globules––composes the upper layer of couplets. The lower layer of the couplets is different in the two studied sections. In Section 1, it is micritic dolomite (fabric type 2) that is characterised by predominantly fine crystals and contains obscured microbial components. In Section 2, it is bioclastic dolomite (fabric type 3) that is rich in reworked dasycladalean fragments and consists of dolomite crystals of wide size-range from fine to coarse. The precipitation of the microcrystalline dolomite phase, which process is the focus of this study, is interpreted as being facilitated by mats and biofilms favouring/tolerating increasing frequency of subaerial conditions in the upper intertidal setting. Petrographic analyses revealed that organogenic calcite was also precipitated, especially in mat deposits rich in bioclasts. Synsedimentary dolomitization, resulting in fine crystals, was coupled with aragonite dissolution. Moreover, it postdated the organogenic precipitation and fibrous calcite cementation. Synsedimentary replacive dolomitization took place only in the peritidal caps of the shallowing-upward depositional units. Petrographic analyses provide circumstantial evidence constraining that microcrystalline dolomite did not form via mimetic replacement. Accordingly the microcrystalline dolomite, which shows microbial microfabrics in the studied samples, is interpreted as organogenic primary precipitate. Both peritidal processes, dolomite precipitation and replacement, were likely controlled by the environmental factors in a semi-arid climate. Those components of the platform succession that were not dolomitized in the peritidal environment were replaced and cemented by medium and coarsely crystalline dolomite during further burial at elevated temperature, as shown by fluid inclusion homogenisation temperature (62 to 83 °C) and negative stable oxygen isotope values. Thus, the majority of the studied formation consists of fabric-destructive dolomite (fabric type 4).
Dolomite most commonly forms via replacement of precursor carbonate minerals. For this reason, di... more Dolomite most commonly forms via replacement of precursor carbonate minerals. For this reason, diagnosing primarily precipitated organogenic dolomite in microbial mat deposits from the rock record is not straightforward, even though the deposits exhibit microbial fabric. Single and multiple dolomite crusts exhibiting microbial fabric occur in a pervasively dolomitized Middle Triassic platform succession. Two sections were studied in the Transdanubian Range. In both sections, two fabric types occur in the upper part of the metre-scale cycles. One of that is microbial boundstone (fabric type 1)‒‒characterised by clusters of dolomite microcrystals which display diagnostic microbial features, such as calcimicrobes, clotted–spherular aggregates and globules. The other one is different in the two studied sections. In Section 1, it is micritic dolomite (fabric type 2) that is characterised by predominantly fine crystals and contains obscured microbial components. In Section 2, it is bioclastic dolomite (fabric type 3) that is rich in reworked dasycladalean alga fragments and consists of dolomite crystals of wide size-range from fine to coarse. The precipitation of the microcrystalline dolomite phase is interpreted as being facilitated by mats and biofilms favouring/tolerating an increasing frequency of subaerial conditions in the upper intertidal setting. Petrographic analyses revealed that organogenic calcite was also precipitated, especially in mat deposits rich in bioclasts. Synsedimentary dolomitization, resulting in fine crystals, was coupled with aragonite dissolution and it postdated the organogenic precipitation. It took place only in the peritidal caps of the shallowing-upward depositional units. Petrographic analyses provide circumstantial evidence constraining that microcrystalline dolomite did not form via mimetic replacement. Accordingly the microcrystalline dolomite, which shows microbial microfabrics in the studied samples, is interpreted as an organogenic primary precipitate. Both peritidal processes, dolomite precipitation and replacement, were likely controlled by the environmental factors in a semi-arid climate. Those components of the platform succession that were not dolomitized in the peritidal environment were replaced and cemented by medium and coarsely crystalline dolomite during further burial at elevated temperature, as shown by fluid inclusion homogenisation temperature (62 to 83 °C) and negative stable oxygen isotope values. Thus, the majority of the studied formation consists of fabric-destructive dolomite (fabric type 4).
ABSTRACT Disintegration of dolostones to dolomite powder (powderization) was a widespread phenome... more ABSTRACT Disintegration of dolostones to dolomite powder (powderization) was a widespread phenomenon in Triassic dolostones of the Buda Hills, where the areal extent of powdered dolostones is large compared to similar occurrences elsewhere in the world. In the Buda Hills, dolostone disintegration proceeded in four stages that correspond to a gradual decrease in particle size, that is, from the parent dolostone to (1) crackle breccia; via (2) mosaic breccia (diameter\2 cm); via (3) mosaic breccia blocks ‘floating’ in dolomite powder; to (4) dolomite powder (diameter 100–300 lm). Stable isotope ratios and trace element compositions of dolomite remained constant throughout these stages, and there are no indications of dissolution in most locations, suggesting that disintegration was predominantly a mechanical process. Combining these findings with the geological history of the region, and supported by a simple freezing/thawing experiment and pertinent experimental studies on weathering of building stones, it appears that powderization in the Buda Hills was caused by repeated freeze–thaw cycles during and/or after the Pleistocene glaciations. Subaerial exposure under cold climate conditions involves multiple freeze–thaw cycles that create mechanical stresses in the rock framework related to the opposing thermal expansion of rock and water that freezes and of ice that liquefies. This process is herewith called ‘cryogenic powderization’. Our data further suggest that the synergy of four factors promoted dolostone powderization in the Buda Hills: (1) tectonics, which created a pervasive fracture network; (2) intercrystalline porosity of the dolostone; (3) relatively high water saturation; and (4) subaerial exposure under cold climate conditions.
ABSTRACT Unconformities and related karst systems are studied worldwide. A unique red calcite gen... more ABSTRACT Unconformities and related karst systems are studied worldwide. A unique red calcite generation, which fills fractures/cavities, is hosted by Mesozoic carbonates in the Transdanubian Range, Hungary. Solid inclusions are located along growth zones of calcite. Hematite, the most abundant solid inclusion, gives the red colour of it. Outcrop-scale geometry, mineralogical features and detrital mineral assemblage (hematite, gibbsite, goethite, kaolinite, smectite, illite, Cr-spinel, monazite, xenotime, zircon, apatite, and Ti-oxide) of calcite precipitates suggest strong correlation between the calcite and nearby karst bauxite deposits. Fluid inclusion petrography and microthermometry (T<50 °C; salinity from 0 to 0.17 NaCl eq. w%) of primary fluid inclusions, and the stable isotope trend of the calcite, following the meteoric water line, clearly indicate vadose and phreatic meteoric origin in a near-surface karst system.The late Cretaceous to mid-Eocene unconformity-related cavity-filling deposits occur close to the surface; indicating that the most recent Quaternary exhumation re-exposed those surfaces that existed at the time of calcite mineralization. Thus red calcite precipitates are interpreted as being speleothems, vestiges of the subterranean part of the pre-Middle Eocene karst. The infiltrated, fine bauxite particles enclosed by the calcite are the witnesses of the once areally extensive pre-Middle Eocene bauxitic blanket that became partially eroded by the time of the deposition of the cover beds.Red calcite when found in core samples may provide good evidence on bauxite formation associated with the overlying unconformity, even if it was later removed by erosion. Therefore, presence or absence of red calcite may be used as distinguishing criteria between karst episodes with or without bauxite formation.This article is protected by copyright. All rights reserved.
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primarily precipitated organogenic dolomite in microbial mat deposits from the rock record is not straightforward,
even though the deposits exhibit microbial fabric. Single and multiple dolomite crusts exhibiting microbial
fabric occur in a pervasively dolomitized Middle Triassic platform succession. Two sections were studied in the
Transdanubian Range. In both sections, two fabric types occur in the upper part of the metre-scale cycles. One
of that is microbial boundstone (fabric type 1)—characterised by clusters of dolomite microcrystalswhich display
diagnostic microbial features, such as calcimicrobes, clotted–spherular aggregates and globules. The other one is
different in the two sections. In Section 1, it is micritic dolomite (fabric type 2) that is characterised by predominantly
fine crystals and contains obscured microbial components. In Section 2, it is bioclastic dolomite (fabric
type 3) that is rich in reworked dasycladalean alga fragments and consists of dolomite crystals of wide sizerange
from fine to coarse. The precipitation of the microcrystalline dolomite phase is interpreted as being facilitated
by mats and biofilms favouring/tolerating an increasing frequency of subaerial conditions in the upper intertidal
setting. Petrographic analyses revealed that organogenic calcite was also precipitated, especially in mat
deposits rich in bioclasts. Synsedimentary dolomitization, resulting in fine crystals, was coupled with aragonite
dissolution and it postdated the organogenic precipitation. It took place only in the peritidal caps of the
shallowing-upward depositional units. Petrographic analyses provide circumstantial evidence constraining
that microcrystalline dolomite did not formvia mimetic replacement. Accordingly the microcrystalline dolomite,
which shows microbial microfabrics in the studied samples, is interpreted as an organogenic primary precipitate.
Both peritidal processes, dolomite precipitation and replacement, were likely controlled by the environmental
factors in a semi-arid climate. Those components of the platform succession that were not dolomitized in the
peritidal environment were replaced and cemented by mediumand coarsely crystalline dolomite during further
burial at elevated temperature, as shown by fluid inclusion homogenisation temperature (62 to 83 °C) and negative
stable oxygen isotope values. Thus, the majority of the studied formation consists of fabric-destructive dolomite
(fabric type 4).
primarily precipitated organogenic dolomite in microbial mat deposits from the rock record is not straightforward,
even though the deposits exhibit microbial fabric. Single and multiple dolomite crusts exhibiting microbial
fabric occur in a pervasively dolomitized Middle Triassic platform succession. Two sections were studied in the
Transdanubian Range. In both sections, two fabric types occur in the upper part of the metre-scale cycles. One
of that is microbial boundstone (fabric type 1)—characterised by clusters of dolomite microcrystalswhich display
diagnostic microbial features, such as calcimicrobes, clotted–spherular aggregates and globules. The other one is
different in the two sections. In Section 1, it is micritic dolomite (fabric type 2) that is characterised by predominantly
fine crystals and contains obscured microbial components. In Section 2, it is bioclastic dolomite (fabric
type 3) that is rich in reworked dasycladalean alga fragments and consists of dolomite crystals of wide sizerange
from fine to coarse. The precipitation of the microcrystalline dolomite phase is interpreted as being facilitated
by mats and biofilms favouring/tolerating an increasing frequency of subaerial conditions in the upper intertidal
setting. Petrographic analyses revealed that organogenic calcite was also precipitated, especially in mat
deposits rich in bioclasts. Synsedimentary dolomitization, resulting in fine crystals, was coupled with aragonite
dissolution and it postdated the organogenic precipitation. It took place only in the peritidal caps of the
shallowing-upward depositional units. Petrographic analyses provide circumstantial evidence constraining
that microcrystalline dolomite did not formvia mimetic replacement. Accordingly the microcrystalline dolomite,
which shows microbial microfabrics in the studied samples, is interpreted as an organogenic primary precipitate.
Both peritidal processes, dolomite precipitation and replacement, were likely controlled by the environmental
factors in a semi-arid climate. Those components of the platform succession that were not dolomitized in the
peritidal environment were replaced and cemented by mediumand coarsely crystalline dolomite during further
burial at elevated temperature, as shown by fluid inclusion homogenisation temperature (62 to 83 °C) and negative
stable oxygen isotope values. Thus, the majority of the studied formation consists of fabric-destructive dolomite
(fabric type 4).