The Campanian-Maastrichtian boundary interval at the Oslen-Krivodol reference section, Bulgaria: nannofossils, dinoflagellate cysts, inoceramids and strontium isotope stratigraphy

The Campanian-Maastrichtian boundary (CMB) as defined by the GSSP at Tercis-de-Landes, France, is currently in investigation by the newly installed Maastrichtian Working Group. Although the base of the Maastrichtian was set at a mean of marker bioevents, it was actually defined near the guide event of the first occurrence (FO) of the ammonite Pachydiscus neubergicus, ca. 35 ka younger than the GSSP level. The main calcareous nannofossil marker is the last occurrence (LO) of Uniplanarius trifidus (Quadrum trifidum), estimated at ca. 750 ka younger. Paleogeographically, Tercis was situated in the Tethys-Atlantic transition to the cooler northern temperate realm.

A largely similar paleogeographic position at the northern Tethyan-European margin is provided by the Upper Cretaceous sedimentary successions cropping out in the western Fore-Balkan Mountains area in Bulgaria. One of the most representative section comprising the Upper Campanian-Lower Maastrichtian interval is the Oslen-Krivodol section, situated in the western part of the Central Balkan–Fore-Balkan Tectonic Zone, that forming the northern (marginal) segment of the Balkan Orogen in Bulgaria.

At the section of Oslen-Krivodol, a several meter-thick succession of glauconitic limestones, clayey to nodular limestones and chalky sediments is exposed that includes inoceramids and belemnites. The lower part of the over 7 m thick Oslen-Krivodol succession is composed of green to greenish glauconitic limestones of the Darmantsi Formation. The succeeding Kunino Formation is represented by thin- to medium bedded limestones, clayey and nodular limestones. Belemnite rostra occur in the entire section, whereas inoceramid bivalves were collected only from two levels in the upper part of the Kunino Formation. A complete nannofossil zonal succession could be established, from the uppermost Campanian to the lowermost Maastrichtian. The following subzones could be distinguished: UC15c^TP, UC15de^TP, UC16a^TP (uppermost Campanian), and UC16b^TP (basal Maastrichtian). The nannofossil bioevent for the base of the Maastrichtian, the LO of U. trifidus, is at 3.25m in the section. Strontium isotope stratigraphy indicates the CMB interval around values of 0.707728 ±0.000004. Additional nannofossil bioevents in the CMB interval include the FO and LO of Microrhabdulinus ambiguus below those two CMB levels, and the sudden decrease of warm-water nannofossil indicators like Watznaueria spp. which seems to indicate a threshold crossing for global cooling along the Campanian-Maastrichtian boundary isotope event. Higher in the section, Arkhangelskiella maastrichtiensis s.str. occurs.

Two dinocyst zones have been recognized in the section: the upper Campanian Areoligera coronata and the lower Maastrichtian Cerodinium diebelii Zone. Their boundary is marked by the last occurrence of typical Campanian taxa (Odontochitina, C. robusta, C.horridum, A. fenestra) and the appearance of characteristic Maastrichtian forms (C. diebelii, M. carpentierae, Glaphyrocysta).

Only one level from the upper part of the Kunino Formation have yielded abundant, but badly preserved (as internal moulds) inoceramid bivalves. The inoceramids are represented mainly by the genus Cataceramus, but Endocostea is also present: Endocostea typica Whitfield, 1880; Cataceramus pallisieri (Douglas, 1942); Cataceramus subcircularis (Meek, 1876) and Cataceramus barabini (Morton, 1834). Based on the presence of E. typica we can indicate the eponymous inoceramid zone at the base of the Maastrichtian.