Paleoecology and biostratigraphy in the Mossano section: effects of the paleoecological bias on the biostratigraphical resolution

The larger foraminiferal assemblages from the Mossano section (Berici Mts., Vicenza province, Northern Italy) were re-examined, mainly based on the data reported by Papazzoni & Sirotti (1995).An attempt was made to assign to each species (or species-group) a paleoenvironment (inner, middle, or outer platform) by comparing the percentages of co-occurrence with eight selected species (or species-groups). They are, respectively: Orbitolites sp., Calcarina sp., Alveolinidae, and Miliolidae for the inner platform (IP), Orbitoclypeus spp. for the middle platform (MP), Asterocyclina spp., Discocyclina spp., and Nemkovella spp. for the outer platform (OP) (Tab. 1). Attributing to each paleoenvironment a score (IP=100; MP=200; OP=300) lead to recognize six sets of species (or species-groups) belonging to different parts of the platform, plus a set of ubiquitous (UBIQ) species (or species-groups). Making the arithmetical mean of the individual scores of each species recognized in a sample (excluding the ubiquitous ones) allowed assigning a score (and therefore a paleoenvironment) to every sample considered (Tab. 2).The numeric results were plotted into a diagram showing a quite detailed "signal" of the shifting paleoenvironments. We can use this diagram to test the biostratigraphical data, i.e. to determine if the appearance and disappearance of the species could be or not linked to the shifting paleoenvironments.The succession of the paleoenvironments shows, into the limestones, a more or less gradual transition from an outer platform environment, roughly corresponding to the Nummulites lyelli Zone, to a middle platform (N. biedai Zone), and an inner platform (N. variolarius/incrassatus Zone). The lithological change from the limestones to the marls corresponds to a sudden change in the assemblages (with a "jump" from the score 158 of the sample MOSS 18 to the 253 of MOSS 19), marking a shift from the inner to the outer platform. This is consistent with the interpretation made by Papazzoni (1994).It does appear clearly that some of the key species to establish the biozonation are facies-dependant. This is the case of Nummulites lyelli and N. cf. dufrenoyi (=N. maximus of Serra-Kiel et al., 1998) both attributed here to the middle/outer platform (M/OP), and of Spiroclypeus carpaticus (=S. granulosus auct.), indicating the outer platform. It seems that the species of the middle and inner platform are less tightly linked to the facies (see e.g. N. beaumonti/discorbinus, attributed to the inner platform, but widespread in nearly all the limestones).The new proposal of biozonation of Serra-Kiel et al. (1998) raises some problems if applied to the Mossano section. The main incongruity is the simultaneous presence of N. lyelli and N. biedai. According to Serra-Kiel et al. (1998, fig. 3) these species belong to different SB Zones (17 and 18 respectively) and their range never overlap. This could be the usual situation, but the species belong to different paleoenvironment, so it is difficult to ascertain if the disappearance of N. lyelli is real or due to a facies change. At least in Mossano (and in Pradipaldo; see Papazzoni & Sirotti, 1995, tab. 2) the stratigraphical range of the two species do overlap.A similar problem is the disappearance of N. biedai. In Mossano this event corresponds to a facies change from the middle to the inner platform, but N. biedai results characteristic of the middle platform, therefore the considerations exposed above could be repeated. The uncertainties in recognizing the disappearance of N. biedai complicate also the definition of the N. variolarius/incrassatus Zone (=N. aff. fabianii Zone).Nevertheless, I think the effects of the widespread sea level lowering near the end of the Middle Eocene forced most of the carbonate platforms to regression. Therefore, the succession of the paleoenvironment is nearly the same in several different localities, leading to about the same succession of events. This should be the reason why, even in presence of some paleoenvironmental bias, the biozonation still works.ReferencesPapazzoni C.A. (1994) - Macroforaminifera and paleoenvironments near the Middle-Upper Eocene boundary in the Mossano section (Berici Mts., Vicenza, northern Italy). In Matteucci, R., Carbone, M.G. & Pignatti, J.S. (eds), Studies on Ecology and Paleoecology of Benthic Communities. Boll. Soc. Paleont. Ital., Spec. Vol. 2: 203-212.Papazzoni C.A. & Sirotti A. (1995) - Nummulite biostratigraphy at the Middle/Upper Eocene boundary in the northern Mediterranean area. Riv. Ital. Paleont. Strat., v. 101, n. 1: 63-80.Serra-Kiel J., Hottinger L., Caus E., Drobne K., Ferràndez C., Jauhri A.K., Less G., Pavlovec R., Pignatti J., Samsó, J.M., Schaub H., Sirel E., Strougo A., Tambareau Y., Tosquella J. & Zakrevskaya E. (1998) - Larger foraminiferal biostratigraphy of the Tethyan Paleocene and Eocene. Bull. Soc. géol. France, 169(2): 281-299.