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Oxfordian to Valanginian palaeoenviron- mental evolution on the western Moesian Carbonate Platform: a case study from SW Bulgaria

Ivanova D., Kołodziej B., Koleva-Rekalova E., Roniewicz E.

Annales Societatis Geologorum Poloniae

2008

Vol. 78, No 2

65-90

Three sections (Rebro, Lyalintsi and Velinovo) of the Upper Jurassic-Lower Cretaceous carbonate sequences from the Lyubash unit (Srednogorie, Balkanides, SW Bulgaria) have been studied for elucidation of biostratigraphy and palaeoenvironmental evolution. Palaeontological studies of foraminifera, supplemented by studies of calcareous dinoflagellate cysts and corals, enabled the determination of the Oxfordian-Valanginian age of the analysed sequences. They were deposited on the Dragoman Block (western part of the Moesian Platform), and during Mid-Late Cretaceous included to the Srednogorie. A possible Middle to Late Callovian age of the lowermost part (overlying the Bajocian-Lower Bathonian Polaten Formation) of the studied sections assumed till now has not been confirmed by the present studies. Eleven facies have been distinguished and attributed to depositional environments. Marine sedimentation on a homoclinal ramp started in the Oxfordian and till the Early Kimmeridgian - in all three sections - was dominated by fine-grained peloidal-bioclastic wackestones to grainstones. Since the Late Kimmeridgian, when a rimmed platform established, facies pattern underwent differentiation into (i) the inner platform (lagoon and tidal flat facies) - only in Velinovo, (ii) reef and peri-reef facies/bioclastic shoals - mainly in Lyalintsi, and (iii) platform slope - mainly in Rebro. Sedimentation generally displays a shallowing-upward trend. Two stages in evolution of the rimmed platform are postulated. The mobile stage lasting till the Tithonian/Berriasian boundary was followed by a more stable stage in the Berriasian to Valanginian time. Reefs are developed mainly as coral-microbial biostromes, lower coral bioherms or coral thickets, in the environment of moderate energy and sedimentation. They contain highly diversified corals (72 species). Micro- bialites contributed to the reef framework, but they never dominated. Locally, microencrusters and cement crusts formed important part of reefal framework. During the mobile stage of the platform evolution a relative sea-level rise interrupted reef development, as evidenced by intercalations of limestones with Saccocoma. During the second stage high carbonate production and/or regressive eustatic events, not balanced by subsidence, decreased accommodation space, limiting reef growth and enhancing carbonate export to distal parts of the platform.

Biostratigraphy and palaeoenvironmental development of the Callovian-Valanginian carbonate platform from the Lyubash unit (SW Bulgaria)

Ivanova D., Koleva-Rekalova El, Kołodziej B., Metodiev L., Roniewicz E.

Volumina Jurassica

2006

Vol. 4, no. 1

174-175

This study deals with the successions located in the Lyubash unit (SW Bulgaria), which represents a part of the Moesian Carbonate Platform in Middle Jurassic – Early Cretaceous time interval. According to the recent tectonic scheme of Bulgaria (Dabovski et al. 2002) the Lyubash unit belongs to Srednogorie tectonic zone of the Balkan orogenic system. Three sections have been sampled and studied – Rebro, Lyalintsi and Velinovo, which are built up by thick-bedded to massive light grey to whitish organogenic and micritic limestones of the Javorets, Gintsi and Slivnitsa formations. Detailed studies on foraminifera and calcareous dinocyst, provided new biostratigraphic data, and confirmed the Middle Callovian-Valanginian age of the examined sediments as assumed previously by Sapunov et al. (1985). Foraminifera are particularly abundant in the lagoon facies, and represented by genera: Globuligerina, Ophthalmidium, Cornuspira, Ammobaculites, Mesoendothyra, Labyrinthina, Kurnubia, Pseudocyclammina, Dobrogelina, Rumanoloculina, Hechtina, Meandrospira, Valvulina, Trocholina, and Neotrocholina. The sedimentological analysis allowed recognizing microfacies groups assigned to three facies zones: platform slope facies, reef and peri-reef facies and lagoon-tidal flat facies. The development of the platform is characterized by complex vertical and lateral alternation of the studied microfacies. Platform slope facies are prevalent at the section Rebro; the Lyalintsi section is dominated by reef and peri-reef facies, and the section Velinovo is mainly dominated by lagoon facies. Sea-level fluctuations are observed, however generally, a shallowing-upward trend in the platform evolution is observed in all three sections. Coral-microbial reefs are developed as biostromes, and contain numerous and highly differentiated scleractinian corals: 70 species (6 new) from 50 genera (4 new). Among them following corals are the most common: phacelloid Latomeandra, Stylosmilia, Thecosmilia, Cladophyllia, ramose Solenocoenia, Meandrophyllia, lameller Synastraea and Microsolena. Reef development has been disturbed by sea level rise, evidenced by limestones with Saccocoma, intercalated with coral biostromes.