Rivista Italiana di Paleontologia e Stratigrafia volume luz numero 3 pagine 363-366 Dicembre 1996 PALEOENVIRONMENTAL CONTROL ON THE MORPHOLOGY OF NUMMULITES FABIANII (PREVER) IN THE LATE PRIABONIAN PARASEQUENCES OF THE MORTISA SANDSTONE (VENETIAN ALPS, NORTHERN ITALY) ENRICO TREVISANI1 E. CE,SARE ANDREA PAPAZZONI2 Key-utords: Facies analysis, Numtnulites, Paleoenvironments, Late Priabonian, Venetian Alps, Nonhern Italy. Riassunto. Viene descritta la distribuzione dt Nummalites fa- biani.i. fabianii e Nurnmulites fabianii retiatus nelle arenarie di Moni- sa, un'unità prevalentemente silicoclastica del Priaboniano superiore affiorante a sud dell'Altipiano di Asiago (Prealpi Venete). Lanalisi di facies ha permesso di riconoscere una organizzazione ciclica in para- sequenze tbickening-coarsening upward prodotte dalla progradazione di sistemi costieri. Nelle facies di base e di tetto di ogni ciclo sono presenti abbondanti macroforaminiferi, fra i quali numerosi esemplari della specie Nummulites fabia.nii. La distribuzione dei due norfotipi corrispondenti alle "sottospecie" N. fabianii fabianii (Prever) e N. fa- bianii retiatus Roveda è risultata strettàmente legata alle condizioni paleoambientalì,. N, fabianii reti.dtus, forma relativamente appiattita, si trova nella facies marnosa di offsbore a base ciclo, carattertzzata da bassa energia idrodinamica e bassa intensità luminosa. N. fabi.anii fa- bianii, forma più bombata, è invece tipica della facies biocalcarenitica di tetto ciclo, caratteristica di un ambiente meno profondo con ener- gia idrodinamica ed intensità luminosa relativamente elevate. Dalle osservazioni riponate non si evidenzia alcun legame di- retto tra la morfologia e la posizione stratigrafica. Lutilità strarigrafi- ca comunemente riconosciuta alle due "sottospecie" (utilizzate per di- stinguere Priaboniano inferiore e superiore) deve pertanto essere at- tentamente verificata. Abstract. The distribution of Nurnmulites fabianii fabianii and Nummulites fabi.anii reti.dtas in the Mortisa sandstone is discussed. The Monisa sandstone is a terrigenous unit of late Priabonian age occurring south of the Asiago Plateau (Venetian Alps, northern Italy). The facies analysis allowed to recognize thickening-coarsening upward cycles interpreted as shallowing upward parasequences corre- sponding to depositional regressions. In each cycle five different fa- cies were recognized. The first and the last facies of every cycle con- tain larger foraminiferal assemblages with numerous specimens of Nummulites fabianii. Two different morphotypes of this species were attributed to the "subspecies" N. fabianii fabianii (Prever) and N. /a- bianii retiatus Roveda. Their distribution was influenced by the pa- leoenvironmental conditions. The flat "subspecies" N. fabianii retia- lrs occurs in the base-cycle marly facies, deposited under low-energy, lowlight conditions. The more inflated N. fabianii fabianii charac- terises the shallower top-cycle limestone, deposited under high-energn highlight conditions. Our observations suggest there is no obvious link between the morphology and the stratigraphic position, so the biostratigraphic si- gnificance of the two "subspecies" (currently used to divide the Pria- bonian in a lower and an upper part) remains uncertain. Introduction. This note deals with the problem of the biostrati- graphic significance of the so-called subspecies N. fabia- nii fabianii and N. fabianii retiatus. They have been used since now to divide the Priabonian in a lower (with -M fabianii fabiani) and an upper (with N. fabia- nii retiatus) part, but some authors (Herb & Hekel, 1973; Barbin, 1988) pointed out the possible paleoenvi- ronmentai control on their distribution. In the frame of the IGCP 393 ("Neritic events at the Middle-Upper Eo- cene boundary") the problem of the biostratigraphic usefulness of these "subspecies" has to be carefully con- sidered. In this paper we cannot discuss the problem exhaustively: the data presented here are only the first step. Additional studies on more morphological features could help to better understand ìf N, fabianii fabianii and N fabíanii retidtus are real subspecies or simply morphogroups ecologically controlled. Our study was made on N, fabianii fabianii and N. fabianii retiatus fabianii populations from the Morti- sa sandstone unit. This is a terrigenous unit of late Pria- bonian age deposited on the eastern margin of the Lessi- ni Shelf. It crops out south of the Asiago Piateau, along the piedmont zoîe between the Astico river and the Brenta river (Fig. 1). Its thickness decreases from 'Sfest to East: about 60 m near Calvene and about 10 m in the Bassano del Grappa area. The Mortisa sandstone unit has been studied since the past century and dated as Lower Oligocene (Oppen- heim, 1900, 1901). More recently the age'was corrected ' M,r."o di Storia Naturale di Ferrara, Via De' Pisis 24, I-44100 Ferrara, Italy. 2 Dipartimerto di Scienze della Terra (Paleontologia), Università di Modena, Via Università 4, I-41100 Modena, Italy. LES SINI Mortisa la Calvene@ "1 Lusiana o ovalle di Sopra NIAROSTICA a I ì PADOVA BASSA BASSANO DEL GRAPP km -. =- 05 364 E. Treaisani Ct C. A. Papazzoni as Upper Eocene-I-ower Oligocene (Frascari Ritondale Spano, 1967). The regressive features of the unit and its variable thickness 'were pointed out by Frascari Ritonda- le Spano (1969). Frascari Ritondale Spano & Bassani (1973) considered the Mortisa sandstone as sublittoral or deltaic deposits forming the basal member of the Calve- ne Formation. Trevisani (1993) remarked the cyclic orga- nizatron of the Mortisa sandstone and interpreted the cycles as shallowing upward parasequences. Facies analysis. The Mortisa sandstone consists of thickening-coar- sening upward cycles (T-C-U), where five different facies have been recognized (Fig. z). The A facies is repre- sented by bioturbated marls, less frequently marly lime- stones, with nummulites; the B facies by bioturbated coarse siltstones to fine sandstones; the C facies by sands and sandstones vrith storm layers; the D facies by con- glomeratic sandstones to sandy conglomerates with storm layers; the D facies by biocalcarenites with num- mulites. The thickness of each cycle increases westward, from about 10 m in the Bassano area to over 40 m near Calvene. On the basis of facies analysis these T-C-U cy- cles are interpreted as shallowing upward Parasequences which are the result of depositional regressions produ- ced by rhe progradation of shallow water siliciclastic sy- stems. These progradational phases were interrupted by abrupt relative sea level rise, resulting in a backshift of the various facies belts toward more internal positions. These parasequences ^ppear to be part of a depositional sequence bounded in their lower part by an erosional limit underlying the Pradelgiglio Formation. Therefore, in terms of sequence stratigraphy, the Mortisa sandstone Fig. 1 - l-ocation map of the study area and sites of the exami- ned srretigraphic sections. l) Mare section; 2) Crosara sec- tion; 3) Pradipaldo section (from Trevisani, 1993) represents the lowstand phase of this depositional se- quence. For further details see Trevisani (1993). Paleontologic analysis. Although the Mortisa sandstone is prevailing sili ciclastic, in some parts 'we found assemblages of larger foraminifera. In particular, they are abundant in the A facies at the base of the cycles, which corresponds to an offshore environment below wave base level, and in the E facies at the top of the cycles, corresponding to bio- clastic sandy shoals. Also in the uppermost part of the D facies the larger foraminifera are often abundant. Among the larger foraminifera the specimens of Nummulites fabianii (Prever) were recognized to show different morphotypes in different facies: in the A facies the tests are distinctly flatter than in the E facies. These morphotypes correspond to the so-called subspecies N. fabianii fabianii (the more inflated ones) and N. fabianii retiatws (the flatter ones). The distinction between the two "subspecies" relies mainly on external features such as the ornamentation and the more or less inflated pro- file. Roveda (1959, l97O) reported that N. fabianii retia- tus has an ornamentation with larger mesh than N. in- termedius (:N. fichtel) and a transverse lamina weaker than that of N. fabianil. Moreover, N. fabianii retidtus is flatter than N. fabianii fabianii. The profile is well ap- proximated by the diameter/thickness (d/t) ratio. From the data reported by Roveda (1970) this ratio resuits about 2.0 for N. fabianii fabianii and about 2.7 for N. fabianii retiatws. In the Upper Eocene section of Possagno, some fifteen km north-east of Bassano del Grappa, Flerb & :ts : D C B -+2 A - {rt HD TD I Fig. 2 - Modal cycle (thickness variable from 10 to 40 m) of the Monisa sandstone. A-E) different facies recognized; TD) transgressive deposits; HD) highstand deposits; 1 and 3) marine-fl ooding surface (parasequence boundary); 2) surfa- ce of maximum flooding (modified from Trevisani, 1993). Hekel (1973) found a lithologically controlled distribu- tion of the two "subspecies": the limestones contain À{ fabianii fabianii and the silty-argillaceous sediments N. fabianii retíatul Barbin (1988) made a comparison be- tween the test flattening of -M fabianii fabianii from Buso deila Rana, near Priabona, and the N. fabianii re- tiatus descrrbed by Roveda (1959), concluding that N Axial sections of Nummulites fabianii (x 25). a) N. fabia' nii rctiatus; sample MV 9, Mare section, lower part of the A {acies; b) N. fabianii fabianii; sample MV 21, Mare sec- tion, E facies. N. fabianii motpbology in the Mortisa sandstone 365 fabianii retiatus is an ecologically controlled morphoty- pe of N. fabianiifabianii. Our samples from the Mortisa sandstone were examined to test the possible environmental control on the morphology. The d/t ratio of the specimens of N. fabianii from different facies was measured. In the E fa- cies, represented only by hard limestones, diameter and thickness were measured on thin sections only where the proloculus was visible and the centrai pillar well re- cognizable. Nevertheiess, a correction was made for the possible obliquity of the section, reducing all the mea- sured values by about 10%. The correctness of this as- sumption was tested on two samples where both free specimens and thin sections were available. The mean d/t ratio of the N. fabianii from the A facies amounts to about 2.8, whereas it is about 2.7 for the samples colleoed in the E facies. In particular, the sample MV 9 (Fig. 3a), from the base of the A facies in the Mare section, shows a d/t ratio of 2.89 (twenty-four isolated specimens measured); the sample MV 21 (Fig. 3b), from the E facies of the same section, about 45 m above MV t has a d/t ratío o[ 2.05 (six axial sections measured on two thin sections; thickness vaiues correc- ted). Therefore, the E facies is characterizedby N. fabia- nii fabianii, whereas N. fabianii retiatus occurs in the A facies. Discussion and conclusions. In living larger foraminifera the test shape depends mainly on light intensity (influencing algal symbiosis) and water energy (Reiss & Flottinger, 1984; Hallock Ec Glenn, 1986; Hallock et al., 1986, I99l). The Recent nummulitid Operculina ammonoides shows flatter tests on soft substrates than on hard ones at the same depth (Pecheux, 1995). In the Mortisa sandstone the A facies contains flat species of nummulitids: together with N. fabianii retiatus there are N. chaoannesi de la Harpe and Operculina spp. This agrees well with the sedimentoiogi- cal evidences of a low-energy, lowJight, relatively deep paleoenvironment with soft substrate. On the other hand, in the E facies N. fabianii fabianii is associated with the inflated species N. incrassatus de la Harpe and l'.1. variolariu.s (Lamarck). F{ere a high-energy, highJight, shaliower paleoenvironment (probably with hard sub- strate) is inferred. In conclusion, the late Priabonian N fabianii shows two different morphotypes, probably controlled by the paleoenvironmental conditions (light intensity, water energy, substrate) . Nummulites fabianii fabianii prevails on hard substrates, with high light intensity and high water energy, whereas Numrnulites fabianii retiatus occurs in opposite environmental conditions. F;. 1 366 E. Trevisani & C. A. Papazzoni The data here reported are not conclusive to ascer- tain if we are facing two morphogroups of the same ta- xon, produced by different paleoenvironmental condi- tions (like Recent Operculina arnmonoi.de) or really two distinct subspecies with stratigraphic significance. Ne- vertheless, we underline that the sample MV 9 (with N fabianii retiatu) is well below MV 21 (with N. fabianii fabiani). Therefore, the two "subspecies" do not appe r one after the other but are present at the same time, at least in the upper part of the Priabonian. Thus, our observations suggest that the ,'subspe- cies" of N. fabianii could be ecological morphotypes, but a more comprehensive study is needed to solve the problem of their biostratigraphic value. Acknotaledgements. '!le thank Prof. A. Sirotti (Modena University) for his help in determining the larger foraminifera. Many thanks to prof. M. Gnaccolini (Milan University) and Prof. L. Hottinger (Basel University) for their useful suggestions to improve the manuscrrpt. REFERENCES Barbin V. (1988) - Réflexions srr Nurnmulites fabianii et Nummulites fabianii retiatus, leur signification strati- graphique et leur origine phylétique. Rezt. Paléobiol., v. spéc. 2, n. 2, pp. 609-614, Genève. Frascari Ritondale Spano F. ( 1967) - Osservazioni stratigrafi- che e rertoniche sui dintorni di Calvene nel Vicentino. Giorn. Geol., v. 3a (966), pp. 307-340, Bologna. Frascari Ritondale Spano F. (1969) - Serie paleogeniche nell'area pedemontana a sud dell'Altipiano di Asiago (Vicenza). Mém. B.R.G.M., v. 69, pp. 773-182,Parìs. Frascari Ritondale Spano F. Ec Bassani P. (1973) - Ricerche geologiche nei dintorni di Bassano del Grappa (Vicen- za). Mus. Tiident. Sc. Nat., v. 19, pp. 65-106, Trento. Hailock P., Forward L.B. E{ Flansen H.J. (1986) - Influence of environment on the test shape oÍ Amphistegina. Journ. Foram. Res., v. 16, n. 3, pp.224-231, Washington. Hallock P. & Glenn E.C. (1986) - Larger Foraminifera: A Tool for Paleoenvironmental Analysis of Cenozoic Car- bonate Depositional Facies. Palaios, v. 1, n. l, pp.55-64, Tulsa. Hallock P., Róttger R. 8c 'Wetmore K. (1991) - Hypotheses on form and function in foraminifera. In Lee J.J. Er Ander- son O.R. (Eds) - Biology of Foraminifera. Academic Press, pp. 47-72, London. Herb R. & Hekel H. (1973) - Biostratigraphy, variability and facies reiations of some upper Eocene Nummulites from Northern Italy. Ecl. Geol. Helo., v. 66, n. 2, pp. 419-445, Basel. Oppenheim P. (1900) - Ueber eine riesige Perna (paclryperna n. sbg. laoerdana n. sp.) aus dem Alrtertiàr Venetiens und die ùbrigen Perniden des Gebietes. Zeitschr. Deutsch Geol. Ges., v. 52, pp. 232-243, Stuttgart. Oppenheim P. (1901) - Die Priabonaschichten und ihre Fau- na. Palaeontograpbica, v. 47, pp. 1-348, Stuttgarc. Pecheux M.J.-F. (1995) - Ecomorphology of a recenr large fo- raminifer, Opercalina ammonoides. Géobios, v. 28, n. 5, pp. 529-566, Lyon. Reiss Z. 8c Hottinger L. (1984) - The Gulf of Aqaba. Ecologi- cal Micropaleontology. y. oÍ 354 pp., Springer-Verlag, Beriin. Roveda V. (1959) - Nummulites retiatus, nouvelle espèce de nummulite réticulée des Abruzzes (Italie). Rea, Mioo- paLéontol., v. 1., n. 4, pp. 207-207, Paris. Roveda V. (1970) - Revision of the Nummulites (Foraminife- rida) of the N. fabianii.fichteli group. Rio. It. paleont. Strat., v. 76, n, 2, pp. 235-324, Milano. Trevisani E. (1993) - Analisi di facies e ciclicità delle Arenarie di Mortisa (Priaboniano superiore, Prealpi Venete). Mem. Sc. Geol., v. 45, pp. 57-65, Padova. ReceitLed Marcb 18, 1996; accepted, October 3, 1996