The Cretaceous / Paleogene ( K / Pg ) boundary in the Mezdra and Lyutidol syncline , Vratza District ( West-Fore Balkan , Bulgaria )

This paper discusses the unjustified assignment (based on calcareous nannofossils) of a large portion of the Maastrichtian strata in the Mezdra and Lyutidol synclines (West Fore Balkan, Bulgaria) to the Paleogene. The co-occurrence of Paleocene nannofossils, reported by some authors, and Maastrichtian macrofossil taxa in these sections indicates diachronism in the appearance of macroand nannofossils across the K/Pg boundary. Thus, this boundary cannot be precisely localised except if the Maastrichtian fossils are assumed to have been redeposited, but there is no evidence of resedimentation. Maastrichtian macrofossils are found not only within the range of the Paleogene nannofossil zones, but also in sections overlying them in the Kajlâka Formation where new Maastrichtian macrofossil taxa, such as the echinoid Hemipneustes striatoradiatus (LESKE), appear and some inoceramid and cephalopod taxa range into this unit. These facts shed doubt over the applicability of nannofossils in determining the K/Pg boundary where this has already been firmly documented by macrofauna.

extended the studies of this stage to the north of the Iskar River -between Mezdra and Roman, and later they (BON^EV & KAMENOV, 1934) continued these to the west -between the rivers of Iskar and Ogosta.Based on inoceramids, cephalopods, echinoids and other macrofossil taxa, they documented in detail the biostratigraphy of the Maastrichtian Stage in the western Fore Balkan.The Maastrichtian age of the Upper Cretaceous sediments in this area was confirmed by all subsequent investigators, based on macrofossil fauna (COHEN, 1946;TZANKOV, 1968;JOLKI^EV, 1982, 1986, 1989, and others).
During recent years, calcareous nannofossils have been assumed to be of extreme importance for the subdivision of Upper Cretaceous and Cenozoic sediments -an importance that, seemingly, cannot be put in question.However, NAIDIN (2002, p. 46) has recently pointed out that "nevertheless we should have some doubts" of the applicability of nannofossils.
Under the influence of nannoplankton euphoria, a number of publications have recently appeared in which the Cretaceous/Paleogene boundary in the study area was traced without taking into account the presence of characteristic macrofauna in the same sections that were subdivided by means of nannoplankton.The Cretaceous/Paleogene boundary as determined by macrofauna was disregarded in these papers.
The macrofaunal data presented below raise questions about the applicability of nannofossils in defining the Cretaceous/Paleogene boundary in the study area.

Facts and discussion
This paper discusses the Maastrichtian strata in the southern limb of the Mezdra syncline and the same deposits in the northern and southern limb of the Lyutidol syncline in the southern parts of the West Fore Balkan (Fig. 1).
The Mezdra Formation in the two structures comprises three lithological units of variable thickness: the lower unit -microgranular limestones with flint concretions; the middle unit -argillaceous limestones without flint concretions with interbeds or in alternation with marls and the upper unit -microgranular limestones with flint concretions (Figs.2-4).SINNYOVSKY & CHRISTOVA-SINNYOVSKA (1993, p. 32) referred to the middle unit in the Lyutidol syncline as the "Limestone Formation".In this unit EK.DIMITROVA (Geological Institute, Bulgarian Academy of Sciences (BAS), unpublished data) identified a foraminiferal assemblage (see Fig. 3).From the same strata at the southerly limb of the Mezdra syncline (at the village of Chelopeck), Y. MALIAKOV (Geological Institute, BAS) collected eighteen echinoid tests (now housed at the museum of the Geological Institute, BAS No F.002525 to 002542).Among these, the following taxa have recently been identified (Fig. 2): Echinocorys conoidea GOLDFUSS as well as several Echinocorys sp. which belong to a group of species morphologically close to E. gr.marginata/subglobosa (of early to late Campanian age; compare ERNST, 1972ERNST, , 1975;;JAGT et al., 2004); this may represent a continuation into, or recurrence(?)during the Maastrichtian of such test morphologies (compare JAGT, 2000).In the Maastrichtian type area, these forms occur as well, and are nearly always associated with typical Fig. 1.Sketch map (inset) of Bulgaria with location and geological map of the study area (after TZANKOV et al., 1991, modified)  Echinocorys gr.conoidea, hence a Maastrichtian age is not in doubt (J.W.M. JAGT, pers. comm., April 2006).
In outcrops south of the village of Perchovtzi (Section I in Fig. 1; Fig. 3) and at the Malata reka River (Section II in Fig. 1; Fig. 4), SINNYOVSKY & CHRISTOVA-SIN-NYOVSKA (1993, p. 38, fig. 8) andSINNYOVSKY (2001, p. 15, fig.3) did not include the normal and complete magnitude of the Mezdra Formation.Furthermore, in the "Paleocene nannofossil zone" in these sections, single specimens of Cataceramus sp. and Cataceramus cf.regularis (Figs.5A, B), and a juvenile ammonite undeterminable to the species level cf.Pachydiscidae (Figs. 5C,  D) have been found.Ammonites are common in the "limestone formation" along the southern limb of the Lyutidol syncline, SW of the village of Lipnitza (Fig. 1).
The transitional limestones are followed upwards by whitish (with beige), indistinctly bedded micro-to medium-grained quarry limestones -the so-called "Vratza which is figured here (Fig. 5E).
In the area of the Lyutidol syncline, the Kajlâka Formation is preserved only in the southern limb of the structure -along the left bank of the Malata River, at the southern end of the village of Lyutidol.There, different horizons of Maastrichtian strata are transgressively overlain by terrigenous Middle Eocene deposits (TZANKOV et al., 1991), which SINNYOVSKY & CHRISTOVA-SINNYOVSKA (1993) and SINNYOVSKY (1993SINNYOVSKY ( , 2001) ) assumed to be in allochthonous position and of Campanian-Maastrichtian age, as defined by nannofossils (Figs. 3, 4).I subscribe to the transgressive, but not allochthonous, position of the terrigenous sediments upon the Maastrichtian ones.The nannofossil samples have presumably been collected from Upper Cretaceous blocks, included as a common component within Middle Eocene terrigenous sediments.
Disregarding the presence of inoceramids, cephalopods and characteristic Maastrichtian echinoid fauna in the whole section of the Upper Cretaceous series in these structures, SINNYOVSKY & CHRISTOVA-SINNYOV-SKA (1993), SINNYOVSKY (1991SINNYOVSKY ( , 1993SINNYOVSKY ( , 1998SINNYOVSKY ( , 2001) ) and STOYKOVA et al. (2000), on the basis of nannofossils, defined the Paleocene age for most of this section (Figs.2-4).They assumed (pers. comm., 2004) the Maastrichtian inoceramid, cephalopod and echinoid fauna, which occurs in the range of their "nannofossil zones", as well as the macrofauna from the Kajlâka Formation, to have been redeposited.I assert that this does not correspond to the fossil sequences in the section and there is no physical evidence of resedimentation of Maastrichtian macrofossils.
The outcrops of the Mezdra Formation continue into the Fore Balkan and to the west of the Mezdra syncline as far west as the valley of the Ogosta River.There, in a quarry at the village of Lyuta (now Vladimirovo), Vratza District, BON^EV & KAMENOV (1934, p. 80) found Pachydiscus neubergicus (VON HAUER) together with numerous echinoids, analogous in specific content to those from the Mezdra Formation in the area of Mezdra (determined also by the present author).SINNYOVSKY (2003, p. 152) analysed the limestones in this quarry for nannoplankton and "proved" that they are of Paleocene and not of Maastrichtian age.SINNYOV-SKY is well aware of the presence of Maastrichtian macrofossil taxa at this locality, cited by him in this paper (p.149), but fails to comment on this fact.

Conclusion
The normal superposition of lithostratigraphic units, which form the limbs of the Mezdra and Lyutidol synclines, as well as their macro-and microfossil content unambiguously confirm their Maastrichtian age.
The co-occurrence of Paleocene nannofossils and Maastrichtian macrofossil taxa in the sections of these  1).For dating the "Danian" strata SINNYOVSKY refers to the nannofosil taxa mentioned in the same paper (SINNYOVSKY, 2001, p. 12).For legend see Fig. 2 structures indicates the diachronic appearance of macroand nannofossils at the Cretaceous/Paleogene boundary.From this viewpoint, the respective boundary cannot be fixed by nannofossils except if it is assume the Maastrichtian macrofossils to have been re-deposited, but this is not the case.Furthermore, the Maastrichtian macrofauna is found not only within the ranges of the "nannofossil zones" but also in the sections overlying them -in the Kajlâka Formation, where a number of new Maastrichtian taxa, such as Hemipneustes stria- toradiatus, appear.Accompaning to this taxon, inoceramids and cephalopods continue to occur (Fig. 2).These facts call into question the applicability of nannofossils for defining the Cretaceous/Paleogene boundary.
This recalls the situation in the type area of the Maastrichtian Stage, where all nannofossil taxa except one (Biantholithus sparsus), including the ones held to be indicative of the lower Paleocene, already occur in the underlying Maastricht Formation [(MAI et al, 1994;MAI et al, 1997a;MAI et al, 1997b;MAI, 1999;MAI et al, 2003), yet in a different size category], which is well dated by macrofossil taxa as late Maastrichtian (J.W.M. JAGT, pers. comm., 2005).
Diachronism in the occurrences of macro-and nannofossils is observed not only at the boundary Cretaceous/Paleogene, but also at other boundaries, e. g. the Campanian/Maastrichtian boundary in some European outcrops (JAGT & FELDER, 2003;KÜCHLER & WAGRE-ICH, 1999;WAGREICH et al., 2003).ROBASZYNSKI et al. (1985) also expressed some doubts on the applicability of nannofossils in determining the Campanian-Maastrichtian boundary and pointed out that "the Campanian-Maastrichtian boundary is somewhat difficult to recognize with nannoplankton because of problems in determining the index species and possible diachronism of their appearances and extinctions from the Tethyan to the Boreal realms".WAGREICH (1987, p. 85) stated that "no exact correlation of nannoplankton and macrofossil zonation at the Campanian/Maastrichtian boundary for low and high latitudes exists".According to BURNETT (1998, p. 137) "stages have been historically defined onshore using macrofossils.In the absence of macrofossil data from oceanic cores, stages boundaries started to be "defined" using microfossil events".Finally, BURNETT (1998, p. 137) concluded: "Nannofossils do not define the bases of any Upper Cretaceous stages."This evidence, as well as the data presented above, shows that nannofossils should be used in biostratigraphy with more care in the case of chronostratigraphic boundaries already fixed by macrofauna.
Fig. 4. Schematic stratigraphic section of the Maastrichtian Stage near to the village of Lyutidol -on the left bank of the Malata River (Section II in Fig.1).For dating the "Danian" strata SINNYOVSKY refers to the nannofosil taxa mentioned in the same paper(SINNYOVSKY, 2001, p. 12).For legend see Fig.2