Upper Jurassic (Malm) Shallow-Water Carbonates in the Western Gorski Kotar Area: Facies and Depositional Environments (Western Croatia)
| Autor: | J. Tišljar, I. Velić |
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| Jazyk: | angličtina |
| Rok vydání: | 2010 |
| Předmět: | |
| Zdroj: | Geologia Croatica, Vol 46, Iss 2, Pp 263-279 (2010) Geologia Croatica Volume 46 Issue 2 |
| ISSN: | 1333-4875 1330-030X |
| Popis: | Shallow-water carbonates in the Upper Jurassic of the Gorski Kotar were deposited on a carbonate ramp, in beach-barrier island-lagoonal and peritidal environments. In the continuous sequence, more than 900 m thick, several facies have been found. They are: (A) low-energy shallow-water wackestones and mudstones of the Lower Oxfordian; (B) high-energy shallow-water grainstones of the Middle Oxfordian; (C) subtidal packstones of the uppermost Oxfordian and transition to the Kimmeridgian; (D) shallowing-upward/coarsening-upward units formed through progradation of beach tidal bars or barriers over the peritidal deposits of the Kimmeridgian and the beginning of the Tithonian and (E) peritidal shallowing-upward units capped by storm tidal deposits of the Tithonian and beginning of the Berriasian. Fossil assemblages adapted to the environmental changes: maximum of their abundance, in the number of the taxa, as well as in the number of individuals, corresponds to the high-energy facies B (Oxfordian), while their minimum corresponds to the peritidal shallowing-upward units of facies E (Tithonian). The "A", "B" and "C" facies of the western Gorski Kotar area (the Platak-Grobnik region) - which are Oxfordian in age - correspond, by their sedimentological characteristics, to the carbonate ramp depositional model in which the sea-level rise exceeds the sediment accumulation. In the Kimmeridgian and at the beginning of the Tithonian (the "D" facies) the sedimentary conditions changed greatly. Shoals with characteristics of the beach - barrier island - lagoonal environment system with progradation of the ooid sand bars (and/or barriers) over the subtidal (shoreface and foreshore) to intertidal-supratidal zone (backshore), have been formed. The dark colour of the Platak-Grobnik Malm facies (the "A", "B", and "C" facies units) agrees with the above mentioned sedimentary environments and conditions in the Oxfordian. These limestones are grey to dark grey, at places even almost black, rich in organic matter ("euxinic"). The dark colour in the lower part of the similar sediments of the Oxfordian - Smackover Formation in Mexican Gulf, has been explained as a consequence of deposition in carbonate ramp marine environments that were deprived of dissolved free oxygen and enriched in organic matter due to the relative sea-level rise. This rise overtook the sedimentation rate and caused the keeping up of the long-lasting carbonate ramp depositional regime. The general vertical facies succession in the Oxfordian and Kimmeridgian of the western Gorski Kotar region shows also some other similarities with the above mentioned Smackover Formation in Oxfordian carbonates of Texas, Louisiana and Arkansas, that served as a prototype for establishing the carbonate ramp sedimentation model. In western Gorski Kotar during the Malm there is an uninterrupted shallow-marine carbonate sedimentation with the formation of shallowing-upward cycles as a result of barrier ooid sand progradation over the periti-dal deposits. In the Late Tithonian, the general depositional succession show a clear difference with regard to the Kimmeridgian and early Tithonian environments and conditions. Namely, peritidal environments with long-lasting subtidal-intertidal sedimentation predominate, giving rise to shallowing-upward cycles due to progradation of tidal flats and autocyclicity. The shallowing-upward cycles are topped by coarse-grained detritus splashed by tidal waves and high storm-tides onto the intertidal and tidal flat, mainly emerged above sea-level (third member of the shallowing-upward cycles in the "E" facies unit). Sedimentary succession of the Upper Jurassic car-bonates in the western Gorski Kotar show generally-shallowing carbonate ramp environment, from low-energy shoals below fair-weather wave-base (the Late Dogger and Oxfordian age), passing into hibh-energy shoals and peri-reefal areas, to low-energy subtidal above fair-weather wave-base, and finally to the tidal flat facies during the Kimmeridgian and Tithonian age. |
| Databáze: | OpenAIRE |
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