Inoceramid bivalve and dinoflagellate cyst integrated biostratigraphy of the topmost Campanian–Maastrichtian in a part of the Western Fore-Balkan Mountains, northwest Bulgaria

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.

Review of the ammonite genus Pseudokossmaticeras Spath, 1922 from the uppermost Campanian–lower Maastrichtian in Bulgaria

“…in its occurrence to the uppermost Campanian (see Walaszczyk et al, 1996Walaszczyk et al, , 2002. Above them were found, in superposition, the inoceramids Cataceramus subcircularis (Meek, 1876), C. barabini (Morton, 1834), and C. palliseri (Douglas, 1942), which both elsewhere and in Bulgaria have been proven to occur in the lower Maastrichtian (Walaszczyk et al, 2001(Walaszczyk et al, , 2002Pavlishina et al, 2020). From the Yankovo Formation at Tsonevo, we obtained single specimens of Pseudokossmaticeras cf.…”

“…Tzankov, 1964, p. 146), of which one, identified as "Inoceramus impressus d'Orbigny", is a typical example of Endocostea typica Whitfield, 1880. The latter taxon is the index species for the eponymous inoceramid zone of the lower Maastrichtian (Walaszczyk et al, 2001(Walaszczyk et al, , 2002, which has also been documented in Bulgaria (Dochev et al, 2018;Dochev and Metodiev, 2020;Pavlishina et al, 2020). Thus, P. brandti is again excluded as a record from Kladorub.…”

“…The total chronostratigraphic extent of these units has been determined as upper Campanian (pars. )lower Maastrichtian (e.g., Jolkičev, 1986Jolkičev, , 1989Sinnyovsky and Vangelov, 2007;Dochev, 2012;Dochev and Metodiev, 2020;Pavlishina et al, 2020). From the Darmantsi Formation, the overall record of Pseudokossmaticeras exclusively refers to P. brandti.…”

“…Thus, the only authentic late Campanian record of P. brandti, from the Darmantsi Formation, is that from Reselets, where two specimens were found in association with "Inoceramus" cf. wyomingensis Walaszczyk, Cobban and Harries 2001, immediately below the Campanian/Maastrichtian boundary (see Dochev et al, 2018;Pavlishina et al, 2020). The latter taxon is a characteristic inoceramid species for the uppermost Campanian, as evidenced in the U.S. Western Interior Basin and at Tercis, France (Walaszczyk et al, 2001(Walaszczyk et al, , 2002.…”

“…More recently, Dochev et al (2018) and Pavlishina et al (2020) published a new record of P. brandti from the uppermost Campanian near the village of Reselets.…”