The Upper Triassic carbonate rocks near Bosnek village ... Upper Triassic carbonate rocks near...
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87 БЪЛГАРСКО ГЕОЛОГИЧЕСКО ДРУЖЕСТВО, Национална конференция с международно участие „ГЕОНАУКИ 2017“ BULGARIAN GEOLOGICAL SOCIETY, National Conference with international participation “GEOSCIENCES 2017” The Upper Triassic carbonate rocks near Bosnek village (Western Srednogorie Unit): transition from peritidal to shallow subtidal sedimentation Горнотриаските карбонатни скали при село Боснек (Западносредногорска зона): преход от приливно-отливна към плитка подотливна седиментация Athanas Chatalov, Kristina Metodieva Атанас Чаталов, Кристина Методиева Sofia University “St. Kliment Ohridski”, 15 Tsar Osvoboditel Blvd., 1504 Sofia; E-mails: [email protected]; [email protected] Keywords: carbonate, peritidal, shallow subtidal, Upper Triassic, Western Srednogorie Unit. The only exposure of Triassic carbonate rocks in Bulgaria, in which dolostones of the Rusinovdel Fm (Tronkov, 1968) are covered by limestones of the Tran Fm (Tronkov, 1983), is located in the vicinity of Bosnek village (Pernik district). The chronostrati- graphic range of the two units is defined as Carnian– Norian on the basis of their stratigraphic position and benthic foraminifera taxa in the Tran Fm (Budurov et al., 1995). The Rusinovdel Fm has total thickness of >200 m and consists of medium- to thick-bedded, grey and yellowish-grey dolostones. Laminated rocks are predominant in the upper part of the section as they altеrnate in a non-cyclic pattern with massive dolos- tones and discrete layers of intraclastic dolostones (showing imbrication of flat-pebble clasts). In thin- sections, the massive rocks have obliterated original textures as a result of burial dolomitization. However, diverse depositional and early diagenetic fabrics are recognizable in the laminated dolostones including dark- and light-colored laminae with fine crystal size, desiccation cracks (Fig. 1a), tabular intraclasts (Fig. 1b), tepee structures (Fig. 1c), and dolomite-filled fenestrae (laminoid and birdseye types). These char- acteristics are indicative of deposition in a peritidal environment with low terrigenous input where the primary lime sediments were affected by penecon- temporaneous dolomitization. The absence of skeletal remains and trace fossils suggests arid climate condi- tions and elevated salinity of seawater. The laminated and intraclastic deposits are products of the intertidal and supratidal zones, while the bulk of the massive do- lostones are probably shallow subtidal deposits. This general interpretation is similar to published results for the Rusinovdel Fm from exposures in the West Balkan Zone (e.g., Chatalov, 2014). The Tran Fm has thickness of about 30 m and con- sists of thick-bedded to crudely bedded, light grey and whitish limestones. Thin dolostone layers occur in the lowermost part of the unit. The carbonate rocks typi- cally have massive appearance and only few beds show fenestral structure. The predominant (>90%) micro- facies type (MFT) is bioclastic-peloidal-intraclastic grainstone (Fig. 1d), which locally grades into pack- stone. The commonly rounded and moderately sorted intraclasts were derived from mudstones, fossiliferous wackestones, peloidal and intraclastic wackestones/ packstones, while others represent aggregate grains that comprise the major type of intraclasts in some rocks. The associated peloids originated through re- working of weakly lithified carbonate mud (mud pe- loids) and microbial micritization of small bioclasts and ooids (bahamite peloids). Subordinate MFT in the Tran Fm include intraclastic wackestone, foramini- feral-peloidal packstone with scarce intraclasts and bioclastic mudstone with stromatactis-like cavities. Benthic foraminifera and bivalves are predominant skeletal components in the limestones while gastro- pods and ostracods show lesser occurrence. The lack- ing evidence for peritidal deposition (e.g., lamina- tions, mud- or prism-cracks, microbial mats, tepees, dolomicrites) indicate shallow subtidal origin of the carbonate sediments. This conclusion is supported by the presence of abundant intraclasts, grapestones, bio- clasts with micritic envelopes (cortoids), winnowed textures, and rare micritic ooids. Deposition occurred above fair-weather wave base on an open marine plat- form interior under variable, but dominantly moderate hydrodynamic conditions. The very shallow depth of the sea floor is emphasized by unequivocal evidence for episodic subaerial exposure and vadose diagen- esis, e.g., crystal silt in the stromatactis-like cavities
Transcript of The Upper Triassic carbonate rocks near Bosnek village ... Upper Triassic carbonate rocks near...
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БЪЛГАРСКО ГЕОЛОГИЧЕСКО ДРУЖЕСТВО, Национална конференция с международно участие „ГЕОНАУКИ 2017“BULGARIAN GEOLOGICAL SOCIETY, National Conference with international participation “GEOSCIENCES 2017”
The Upper Triassic carbonate rocks near Bosnek village (Western Srednogorie Unit): transition from peritidal to shallow subtidal sedimentationГорнотриаските карбонатни скали при село Боснек (Западносредногорска зона): преход от приливно-отливна към плитка подотливна седиментацияAthanas Chatalov, Kristina MetodievaАтанас Чаталов, Кристина Методиева
Sofia University “St. Kliment Ohridski”, 15 Tsar Osvoboditel Blvd., 1504 Sofia; E-mails: [email protected]; [email protected]
Keywords: carbonate, peritidal, shallow subtidal, Upper Triassic, Western Srednogorie Unit.
The only exposure of Triassic carbonate rocks in Bulgaria, in which dolostones of the Rusinovdel Fm (Tronkov, 1968) are covered by limestones of the Tran Fm (Tronkov, 1983), is located in the vicinity of Bosnek village (Pernik district). The chronostrati-graphic range of the two units is defined as Carnian–Norian on the basis of their stratigraphic position and benthic foraminifera taxa in the Tran Fm (Budurov et al., 1995).
The Rusinovdel Fm has total thickness of >200 m and consists of medium- to thick-bedded, grey and yellowish-grey dolostones. Laminated rocks are predominant in the upper part of the section as they altеrnate in a non-cyclic pattern with massive dolos-tones and discrete layers of intraclastic dolostones (showing imbrication of flat-pebble clasts). In thin-sections, the massive rocks have obliterated original textures as a result of burial dolomitization. However, diverse depositional and early diagenetic fabrics are recognizable in the laminated dolostones including dark- and light-colored laminae with fine crystal size, desiccation cracks (Fig. 1a), tabular intraclasts (Fig. 1b), tepee structures (Fig. 1c), and dolomite-filled fenestrae (laminoid and birdseye types). These char-acteristics are indicative of deposition in a peritidal environment with low terrigenous input where the primary lime sediments were affected by penecon-temporaneous dolomitization. The absence of skeletal remains and trace fossils suggests arid climate condi-tions and elevated salinity of seawater. The laminated and intraclastic deposits are products of the intertidal and supratidal zones, while the bulk of the massive do-lostones are probably shallow subtidal deposits. This general interpretation is similar to published results for the Rusinovdel Fm from exposures in the West Balkan Zone (e.g., Chatalov, 2014).
The Tran Fm has thickness of about 30 m and con-sists of thick-bedded to crudely bedded, light grey and whitish limestones. Thin dolostone layers occur in the lowermost part of the unit. The carbonate rocks typi-cally have massive appearance and only few beds show fenestral structure. The predominant (>90%) micro-facies type (MFT) is bioclastic-peloidal-intraclastic grainstone (Fig. 1d), which locally grades into pack-stone. The commonly rounded and moderately sorted intraclasts were derived from mudstones, fossiliferous wackestones, peloidal and intraclastic wackestones/packstones, while others represent aggregate grains that comprise the major type of intraclasts in some rocks. The associated peloids originated through re-working of weakly lithified carbonate mud (mud pe-loids) and microbial micritization of small bioclasts and ooids (bahamite peloids). Subordinate MFT in the Tran Fm include intraclastic wackestone, foramini-feral-peloidal packstone with scarce intraclasts and bioclastic mudstone with stromatactis-like cavities. Benthic foraminifera and bivalves are predominant skeletal components in the limestones while gastro-pods and ostracods show lesser occurrence. The lack-ing evidence for peritidal deposition (e.g., lamina-tions, mud- or prism-cracks, microbial mats, tepees, dolomicrites) indicate shallow subtidal origin of the carbonate sediments. This conclusion is supported by the presence of abundant intraclasts, grapestones, bio-clasts with micritic envelopes (cortoids), winnowed textures, and rare micritic ooids. Deposition occurred above fair-weather wave base on an open marine plat-form interior under variable, but dominantly moderate hydrodynamic conditions. The very shallow depth of the sea floor is emphasized by unequivocal evidence for episodic subaerial exposure and vadose diagen-esis, e.g., crystal silt in the stromatactis-like cavities
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(Fig. 1e) as well as sheet cracks and laminoid fenestrae (Fig. 1f) in some grainstones. The interpretation pro-posed herein differs from other sedimentological re-sults for the Tran Fm based on studied exposures in the Kraishte Zone (or Strouma Superunit). Thus, Vaptsarova (1986) suggested deposition in a peritidal setting with local development of Tubiphytes bio-herms, while Belivanova (1996) interpreted a middle shelf environment with open circulation dominated by diverse skeletal deposits and formation of algal-Tubi-phytes patch reefs.
The carbonate succession cropping out near Bosnek village shows evidence for temporal transition from peritidal to shallow subtidal sedimentation during the Late Triassic. Published literature data indicate that such facies transition has not been reported from other regions of the country with exposed or drilled Upper Triassic rocks. A possible analogue occurs in the area between the towns of Gabrovo and Sliven where do-lostones of the Troyan Fm (Carnian) are overlain by limestones of the Tvarditsa Fm (Norian).
ReferencesBelivanova, V. 1996. Sedimentology of the Trun Formation
north of the town of Trun. – Rev. Bulg. Geol. Soc., 57, 2, 19–24 (in Bulgarian with an English abstract).
Budurov, K., E. Trifonova, I. Zagorchev. 1995. The Triassic in Southwest Bulgaria. Stratigraphic correlation of key sections in the Iskar Carbonate Group. – Geologica Balc., 25, 1, 27–59.
Chatalov, А. 2014. Contribution to the lithostratigraphy and sedimentology of Iskar Carbonate Group (Lower–Upper Triassic) in the Elovitsa (Melyane) anticline, Northwestern Bulgaria. II. Sedimentology of the dolostones. – Ann. Univ. Sofia “St. Kliment Ohridski”, Fac. Geol. and Geogr., 104, 1–Geol., 5–42 (in Bulgarian with an English abstract).
Tronkov, D. 1968. Die Grenze Untere Trias – Mittlere Trias in Bulgarien. – Bull. Geol. Inst., Ser. Palaeont., 17, 113–131 (in Bulgarian with German and Russian abstracts).
Tronkov, D. 1983. Stratigraphic problems of Iskar Carbonate Group (Triassic) in South-west Bulgaria. – Geologica Balc., 13, 3, 91–100 (in Russian with an English abstract).
Vaptsarova, A. 1986. Lithological and facies characteristics of the Triassic rocks in Vlahina Mountains (Southwest Bulgaria). – Geologica Balc., 16, 4, 55–71 (in Russian with an English abstract).
Fig. 1. Laminated peritidal dolostones of the Rusinovdel Formation (a–c) and limestone microfacies type (MFT) of the Tran Formation (d–f): a, desiccation cracks (arrows); b, tabular intraclasts; c, tepee structure; d, bioclastic-peloidal-intraclastic grainstone with single ooids (arrows); e, stromatactis-like cavity with geopetal fill of crystal silt (arrow) and blocky cement in bioclastic mudstone; f, laminoid fenestra in bioclastic-peloidal-intraclastic grainstone
