No abstract

We present the inoceramid and dinoflagellate cyst record from the topmost Campanian–Maastrichtian strata of three key sections of the Western Fore-Balkan Mountains, northwest Bulgaria. The following inoceramid zones were recognized: “Inoceramus” redbirdensis Zone, Endocostea typica Zone, Trochoceramus radiosus Zone and “Inoceramus” ianjonaensis Zone. The Campanian/Maastrichtian boundary was tentatively drawn slightly below the first occurrence of Endocostea typica. The uppermost inoceramid assemblage was confined to the lower part of the upper Maastrichtian. Dinoflagellate cyst ranges, as well as the first and the last occurrence events of dinocysts, provided valuable markers for the stratigraphic subdivision of the Maastrichtian. The first occurrence of Microdinium carpentierae was documented in proximity to the Campanian/Maastrichtian boundary. The Cerodinium diebelii Zone was recognized with a stratigraphical range from the lower Maastrichtian to the lower upper Maastrichtian. The last occurrence of Alterbidinium acutulum was recorded in the lower Maastrichtian and used as aid for tracing the lower/upper Maastrichtian substage boundary. The marked domination of delicate chorate dinocysts in all sections and the encountered low P/G ratio values are indicative of stable low-energy depositional environment and oligotrophic conditions, with normal marine productivity and nutrient availability in the basin during the Maastrichtian.

<p>The Central Srednogorie Zone of Bulgaria represents a chain of strike-slip and pull-apart basins, part of the of the peri-Tethyan arc/back-arc basin system. The Upper Cretaceous volcano-sedimentary sequence in the western part of the Central Srednogorie Zone, forms two strips, spanning the Turonian-Maastrichtian interval. This succession is represented by basal siliciclastic sediments, an interval with magmatic rocks followed by volcanoclastic and epiclastic deposits, covered by white, red, grey limestones, with fast transition to sandy low-density turbidites. One of the most representative and continuous sedimentary record in the Panagyurishte strip is exposed east of the Petrich village (Petrich section).  </p><p>The base of the Petrich section is composed of the rocks from the so-called lower epiclastic unit (Coniacian-Santonian), followed by grey, pink to variegated limestones of the Mirkovo Formation (Santonian-Campanian). The middle and upper part of the section consists of muddy-sandy turbidites of the Chugovitsa Formation (Campanian-Maastrichtian). The lower part of this formation, the Voden Member, composed of grey thin bedded marls with rare sandstones beds, has yielded a comparatively rich macro- and microfossil record. Upwards, thin to medium bedded sandstones and marls are in alternation, with documented mudstone dominated intervals.  </p><p>The present study of the Petrich section is focused on integrated biostratigraphical analysis, based on three important fossil groups for the Campanian – inoceramid bivalves, nannofossils and dinoflagellate cysts. The study in progress creates a comprehensive calibrated dataset, in which the nannofossil and dinoflagellate cyst ranges and inoceramid occurrences, provide valuable markers for age assessment and stratigraphic subdivision of the Campanian. The presence of the nannofossil <em>Ceratolithus aculeus</em> marks a middle to late Campanian age, followed by a typical late Campanian assemblage including <em>Broinsonia parca parca</em>, <em>Reinhardtites levis</em> and rare <em>Eiffellithus eximius</em>. A high diversity dinocyst association is identified and ranges of key Campanian species as <em>Corradinisphaeridium horridum</em>, <em>Raetiaedinium truncigerum</em>, <em>Palaeohystrichophora infusorioides</em> and <em>Cannosphaeropsis utinensis</em> provided valuable markers for the stratigraphic subdivision of the Campanian. Typical middle Campanian “<em>Inoceramus</em>” <em>ellipticus</em> and “<em>Inoceramus</em>” <em>agdjakendsis</em> were documented from the Voden Member. The paleoenvironmental analysis, based on dinocyst assemblages and palynofacies data, suggests stable open-marine depositional environment and oligotrophic conditions, with normal marine productivity and nutrient availability during the Campanian in the basin.</p><p>Acknowledgements. The study is part of the Bilateral Bulgarian-Austrian collaboration within project KP-06-Austria/9.</p>

(Tröger, 1967) and M. carpathicus (Simionescu, 1899). Mytiloides incertus and Mytiloides scupini are index species for the eponymous Upper Turonian inoceramid biozones.

Taxonomic descriptions of newly recorded inoceramid species from the Maastrichtian of the Western Fore-Balkan Mountains (Bulgaria) are presented. The following taxa have been determined: Cataceramus subcircularis (Meek, 1876); Cataceramus palliseri (Douglas, 1942); Cataceramus barabini (Morton, 1834); Cataceramus? glendivensis Walaszczyk, Cobban and Harries, 2001; Cataceramus? cf. bebahoensis (Sornay, 1973); Cataceramus aff. goldfussianus (d’Orbigny, 1847); Trochoceramus tenuiplicatus (Tzankov, 1981); Endocostea typica Whitfield, 1880; Endocostea jolkicevi Walaszczyk, Odin and Dhondt, 2002; Endocostea sp. aff. E. coxi (Reyment, 1955); and “Inoceramus” howletti Walaszczyk, Kennedy and Klinger, 2009. The present record allowed two lower Maastrichtian inoceramid zones, namely the Endocostea typica and Trochoceramus radiosus zones, to be indicated. The material allows for correlation with the Northwestern and Boreal areas of Europe and elsewhere.