A. Nicora

D.V. Kent

G. Muttoni

H. Rieber

P. Brack

English

D. V. Kent

AIR Universita degli studi di Milano

MAGNETO-BIOSTRATIGRAPHY OF THE 'BUCHENSTEIN BEDS' AT FROTSCHBACH (WESTERN DOLOMITES, ITALY) G.Muttoni, D.V. Kent, A. Nicora, H. Rieber and P. Brack Introduction 51 In this short contribution we present preliminary results on the magneto-biostratigraphy of the Anisian/Ladinian boundary interval at Frotschbach in the Dolomites of northern Italy. The interdisciplinary research carried out at this and on other coeval sections such as Aghia Triada and Vlichos on Hydra island, Greece (Muttoni et al., in prep.) is aimed at correlating in detail the well-known Tethyan biozonation to a geomagnetic polarity sequence of reversals for the construction of a standard Triassic time scale. Location of Frotschbach section and lithologies The Frotschbach section near Seisin the western Dolomites (Fig. 1 l was recorded in outcrops close to trail n.1 a between Bad Ratzes and Prossliner Schwaige on the way to Seiser Aim. Its trace follows the exposure along the trail (lower part, 0-23 m) and a nearby southern tributary of the Frotschbach creek (upper part, 24 m ff.); both portions are separated by a small fault with a vertical displacement of less than eight metres. As a result of a recent heavy storm water the basal section ('Lower Plattenkalke') is now fully exposed along the Frotschbach creek in the bottom of the main valley. 46"30' ~~;~F~,;~~h-4::.-:--. 44"N ~}~ ~ "Carnian Platform" =:= basinal 'Buchenstein Beds' ~ "Anisian Platform" .-._ .. _. . :.-.: Frotschbach 0 km 5 (Contrin Fm.) . ::·._. ..........  .-.  · Fig. 1. Simplified geological sketch of Middle Triassic units in the St. Ulrich (Ortisei} area (Dolomites, northern Italy). The Frotschbach section is located to the southeast of the village Seis am Schlern (Siusi allo Sciliar). Other well known and coeval sections in the area are found at Seceda and in the Pufels gorge between these two localities. Albertiana 17, May 1996 52 The section comprises the 'Buchenstein Beds' ( = Livinallongo Fm. in the Italian terminology; for an alternative illustration of the section see Abb. 2 of Brandner, 1982) consisting of two stratigraphically superposed units (Fig. 2): (i) the 'Lower Platten kalke', which is a 8 to 9 m-thick succession of evenly bedded black siliceous and calcareous mudstones, and (ii) the 'Knollenkalke', consisting of 30 to 35 m of calcareous mudstones arranged in dm-thick wavy to nodular beds with abundant chert. Higher up (i.e., above the 42 m-level in Fig. 2), the 'Knollenkalke' are replaced by more evenly-bedded light dolomitic limestones and dolomites which are in part of a turbiditic origin. Acidic volcaniclastic layers ("Pietra verde") occur throughout the 'Buchenstein Beds' as silty to sandy horizons (e.g., Tc, Td, Te in Fig. 2) between a few centimetres and several decimetres thick. Correlation with other sections and biostratigraphic constraints On the basis of the vertical distribution of volcaniclastic layers, distinct lithological marker beds and few macrofossils, the Frotschbach section (Fig. 2) is closely tied to the nearby succession of 'Buchenstein Beds' at Seceda, around 13 km to the northeast (Fig. 1 ). Correlation of the lower portions of both sections on a bed-scale is based on the unambiguous recognition of the volcaniclastic markers T c-e and of distinct nodular limestone beds n. 1-6 (corresponding to the 14.7-16.5 m-interval at Seceda) . This has been illustrated in Fig. 6 of Brack and Rieber (1993). Probably as a result of closer proximity to coeval carbonate platforms, intervals of 'Knollenkalke' are on the average between 10-20% thicker at Frotschbach than at Seceda. The tuffite-bearing interval above the 34 m-level at Frotschbach most likely corresponds to the lower part of the interval rich in volcaniclastic layers between the 30 m and 40 m-levels at Seceda (see Brack and Rieber, 1993, Fig. 7). The Seceda section has been correlated in detail with other well-known Anisian/Ladinian boundary sections further afield in the Southern Alps (e .g., Bagolino, Monte San Giorgio; see Brack and Rieber, 1993, 1994). At Frotschbach the 'Lower Plattenkalke' yielded palynomorphs of the "vicentinense-scheuringii Phase" (Brugman pers . comm., in Brack and Rieber, 1993, p.435; see also p . 63 in Gaetani [Ed.], 1993), as well as few ammonoids (Aplococeras sp., Parakellnerites sp.) and thin-shelled 'pelagic' bivalves of the Daonella e/ongata group (Daonella angulata, Fig. 2). The conodont species Gondolella trammeri, G. fueloepi fueloepi and Gladigondolel/a tethydis occur from the upper part of the 'Lower Plattenkalke' upwards. Gondolella trammeri is considered as a good proxy for the base of the (Nevadites) secedensis Zone (e.g ., Krystyn, 1983; Kovacs, 1994; Nicora and Brack, 1995) although at Frotschbach it does occur already somewhat earlier. Other conodont species in this section show an as yet discontinuous distribution and do not provide further biostratigraphic detail. Several specimens of the ammonoid Chieseiceras chiesense were collected from a rock slab found at the base of a cliff of the lowermost 15 metres of 'Knollenkalke' (Fig. 2). This non in-situ finding is in agreement with data from Seceda where Chieseiceras chiesense occurs around three metres above the volcaniclastic Te-level (Brack and Rieber, 1993). According to data from eastern Lombardy and Giudicarie, this ammonoid species marks the very top of the (Nevadites) secedensis Zone (Brack and Rieber, 1993; Bracket al., 1995). Slightly higher up at Seceda Eoprotrachyceras cf. recubariense was found around 7.5 metres above the volcaniclastic Te-level; this ammonoid species is attributed to the curionii Zone (Brack and Rieber, 1993). Finally, at Frotschbach a specimen of Arpadites was collected close to the top of the recorded section. In the Bagolino section ammonoids of the genus Arpadites occur in an interval with a few conodonts of the genus Budurovignathus (8. truempyi slightly below, B. hungaricus somewhat higher up; Nicora and Brack, 1995) . This association may be referred to the gredleri Zone. Albertiana 17, May 1996 53 Based on these fossil data and correlations, the bottom of the Frotschbach section (i.e., the 'Lower Plattenkalke') is ascribed to the reitzi Zone, whereas its top, although less well constrained, presumably represents part of the gredleri Zone (Fig. 2). The Frotschbach section therefore straddles two out of the three chief candidates for the ultimate collocation of the Anisian/Ladinian boundary, namely the base of the secedensis Zone and the base of the curionii Zone. In analogy with Bagolino (Brack et al., 1995), the base of the secedensis Zone is drawn immediately below volcaniclastic layer Tc. The base of the curionii Zone is constrained by the occurrence of Chieseiceras chiesense and Eoprotrachyceras at Seceda. At Frotschbach this boundary most probably falls in an interval between four to eight metres above the volcaniclastic Te-layer. Radiometric age data High resolution U-Pb ages were obtained by single grain analysis of zircons from selected volcaniclastic layers at Seceda, Bagolino and Monte San Giorgio (Mundi! et al., in press; Bracket al., in press). Zircons from a thin crystal-tuff at the base of the volcaniclastic Tc-layer at Seceda have an average radiometric age of 241.2 + 0 .8/-0.6 Ma (errors given at 95% confidence level) . The same layer is also identified at Frotschbach. Zircons from a tuff layer at the 72.5 m-level of the Bagolino section have yielded an age of 238.0 + 0.4/-0.8 Ma. This level can be approximately correlated via Seceda to the upper part of the Frotschbach section (35-40 m interval in Fig. 2) . These data allow an estimate of the duration of the Frotschbach interval shown in Fig. 2 at about three millions of years. Magnetostratigraphy Samples for palaeomagnetic analysis were taken over a stratigraphic interval of 31.6 metres equally subdivided in two complementary sections (Fig. 2). A sampling interval of 2-4 samples per metre was adopted, yielding a total of 101 samples for thermal demagnetization. Samples typically show an initial steeply inclined component which in geographic (i.e., in situ) coordinates is consistent with the present-day field. A bipolar northwest-and-down (southeast-and-up) characteristic component was successively unblocked in 92 samples between about 200°C and 500°C, with a maximum upper limit of 575°C. Magnetic susceptibility is typically low and stable over the heating procedure. We interpret the characteristic directions as carried by a magnetite phase, and assign normal (reversed) polarity to the positive (negative) inclinations after correction for bedding tilt. For each of the characteristic component directions a virtual geomagnetic pole (VGP) was calculated. The latitudes of the VGPs plotted with respect to stratigraphic thickness define a pattern of four main polarity intervals, i.e., Fr1 n to Fr2r, with two short polarity intervals within Fr1 nand Fr1 r (Fig. 2). Final remarks The magnetostratigraphic record established so far at Frotschbach indicates the existence of at least one main reversal in the Anisian/Ladinian boundary interval (i.e., between Frl nand Frl r in Fig. 2). This event falls in the upper part of the (Nevadites) secedensis Zone. Based on its magnetostratigraphic pattern, the Frotschbach section can be successfully correlated to a similar record from the Aghia Triada and Vlichos sections on Hydra island in Greece. The sampling rate and, therefore, the resolution at Aghia Triada has been improved compared to what was reported in Muttoni et al. (1994). These new data and correlations shall be presented in a forthcoming publication by the authors. The main result will be a composite magneto-chrono-biostratigraphic record across a boundary interval of around five million years duration. Albertiana 17, May 1996 54 CD ~ ~·· ""· 21 20-19-16-14-, .. 11 1v· 8-6 5 • 3 2 "" ·!r 1---1fr18.10 11r15.89 11r1S.45 11r1S.07 tfrt4.42 1fr14.03 1fT13.62 1tr13~4 1fr12.88 1fr12.0S 11.1s ~ Gl. mafayensis 1fr11.63 11, 11 .27 tr ... o -G. trammerijuvenile, Gl. malayensis 1tr10.0.C Hr10.66 1fr1020 1fr10.00 Hr9.55 Ur9.21 1h8.85 11<e.53 11<8.32 ttr7.89 ttr7.SO ttr7.13 11<e.S6 11r6.33 1~.28 "'l~:gg • (105.23) • 1tr4.~ 1fr.C.M 1fr4.38 11r3.65 1fr3.24 ---------Chieseiceras chiesense Chieseiceras chiesense at Seceda (in block) a.oo ~ Gl. malayensis Te- Tuff Td- Tuff Tc- Tuffs Nevadites interval at Seceda 3.oo - Gl. tethydis ttr2.82 2.1a -G. trammeri 1fr2.43 111'2.10 1fr11.27 11r10JW 1frt0.66 1fr10.2V 1fr8.8S 11rtl.S3 11r4.60 Ur4.38 1fr3.SS 1ft3.24 mn mn 11' 1·40 Ticinites horizon at Seceda 11' 1·40 11rt.20 ~ 1fr1.20 1tr1.oo -G.trammeri, G. f. fueloepi, Gl. tethydis 1fr0.73 tfrO.SO 1h0.27 t ftO.OO Daonella angulata Parakellnerites sp. ro ~ c CD - Aplococeras sp. -ro a_ .... CD ;:: 0 _J .,. 1~ CONTAIN FM. 7 'Lower Plattenkalke' at Frotschbach: palynomporphs of "vicentinense scheuringii" phase Albertiana 17, May 1996 Fr1 r.2r Fr1r.1n Fr1r.1r CD c 0 N -~ (f) c CD Fr1 n.2n "0 CD 0 CD C/) Fr1n.1r Fr1n.1n ?: a: :5 -'0 0(ij01 a..e!!? 0'-01 O> -cOl 1- .... W.!!llll z-l<:·-(!)<.>01 <co= CD ::2 e.:~ c 0 N (f) .N w -z ·a; 0 N a: 0 6 z 0 ::2 ::2 < Q) .:::t:. ctS .:::t:. c Q) 0 c :::X:: minor fautt ~ t --2tr13.75 2tr13.<48 2trt3.2S 2tr11 .33 2tr11.t6 2fr10.&1 2tr10.55 2tr10.53 2fr10.30 2fr10.05 2fr8.n 2f/Q.40 Section continues upwards 0' ..__ Arpadites sp. 21'r15 .-46 2!r15.25 2tr14.88 2tr1-4 .55 2tr14 .15 2fr8.g7 2fr8.7S 2fr8.S5 2fr8.15 ~ Proarcestes-layer in main stream • 2tr7.78 • (21'r7.58) 21re.sa 2tr6.48 2tr6.1 8 2!rii.03 Gt. tethydis 2trS.7t s .1o-G. transita 2!15.30 2tr4.gjjl 2''4·62 4.s.-G. f. fue/oepi, Gl. tethydis, G. transita ~:::~~ -G. f. fueloepi. Gl. tethydis 2tr3.~ 2'r3.66 2'r3.30 --G. aff. f. fueloepi 2tr3.04 2fr2.Q.4 ~~:~ vo-G. trammeri, G. transita 2tr2.27 2111 .112 --Gt. matayensis 2111 "67 1.4s -G. bakatovi, G. tonga 2tr1 .33 2h13.2S 2fr12..1l0 2fr12.6S 2tr10.55 24~= ~::,-~ '-06-G/. tethydis. G. bakalovi, G. transita ~1010. 8863 062 -G 1 • G 1 -1 Eoprotrachyceras cf. recubariense •· · · · rammen. · rans' a at Seceda 2110.211 o.2s -G. tonga, G. trans1ta 2110.00 -G. f. fueloepi 55 Q) c 0 N Q) c 0 N c 0 ·;:: :::1 0 Fig. 2. Section of 'Buchenstein Beds' at Frotschbach: lower part (0-23 m}, upper part {24 m ff.) with the position of samples for paleomagnetism (black dots to the right of stratigraphic column labelled 1fr0.00 to 2fr15.46) and the derived magnetic polarity stratigraphy. Volcaniclastic layers are indicated by black bars. Zones are based on ammonoid distribution at Seceda, Bagolino and few fossils from Frotschbach (see text). Conodonts indicated with sample positions are from Frotschbach. See Brack and Rieber {1993} for further information on macrofossils and correlation with other Anisian/Ladinian boundary sections. Albertiana 17, May 1996 56 References BRACK, P. and RIEBER, H., 1993. Towards a better definition of the Anisian/Ladinian boundary: New biostratigraphic data and correlations of boundary sections from the Southern Alps. Eclogae geol. Helv.: 86/2: 415-527. BRACK, P. and RIEBER, H., 1994. The Anisian/Ladinian boundary: retrospective and new constraints. Albertiana, 13: 25-36. BRACK, P., RIEBER, H. and MUNDIL, R., 1995. The Anisian/Ladinian boundary interval at Bagolino (Southern Alps, Italy): I. Summary and new results on ammonoid horizons and radiometric dating. Albertiana, 15: 45-56. BRACK, P., MUNDIL, R. , OBEALI, F., MEIER, M . and RIEBER, H., 1996. Biost ratigraphic and radiometric age data question the Milankovitch characteristics of the Latemar cycles (Southern Alps, Italy). Geology, in press. BRANDNER, R., 1982. Mittel- und Obertrias in Frotschbach und Seiser Aim; Exkursion 2. In: Mostler, H., (Ed.), Exkursionsfuhrer zur 4. Jahrestagung der osterreichischen geologischen Gesellschaft, Seis am Schlern, Sudtirol 1982. 80-97. GAETANI, M. [Ed.). 1993. Anisian/Ladinian boundary field workshop Southern Alps - Balaton Highlands, 27 June - 4 July; Field-guide book. I.U.G.S. Subcommission of Triassic Stratigraphy; 118 pp. KovAcs, S., 1994. Conodonts of stratigraphic importance from the Anisian/Ladinian boundary interval of the Balaton Highland, Hungary. Rivista ltal. Paleont. Strat., 99/4: 473-514. KRYSTYN, L., 1983. Das Epidaurus-Profil (Griechenland) - Ein Beitrag zur Conodonten-Standardzonierung des tethyalen Ladin und Unterkarn. Schriftenreihe der Erdwissenschaftlichen Kommissionen, 5: 231-258. MUNDIL, R., BRACK, P., MEIER, M., RIEBER, H. and 0BERLI, F. 1996. High resolution U-Pb dating of Middle Triassic volcaniclastics: time-scale calibration and verification of tuning parameters for carbonate sedimentation. Earth and Planetary Science Letters, in press. MUTTONI, G., CHANNELL, J.E.T., NICORA, A . and RETTORI, R. , 1994. Magnetostratigraphy and biostratigraphy of an Anisian-Ladinian (Middle Triassic) boundary section from Hydra (Greece). Palaeogeogr. Palaeocl., Palaeoecol. , 111: 249-262. MUTTONI, G., KENT D. V. , BRACK, P., NICORA, A. and BALINI, M ., in prep. Middle Triassic Magneto-Biostratigraphy from the Dolomites and Greece. NICORA, A. and BRACK, P., 1995. The Anisian/Ladinian boundary at Bagolino (Southern Alps, Italy): II. The distribution of conodonts, Albertiana, 15: 57-71. A L 8 Ernl1ul A N A Comments, reactions or contributions to ALBERTJANA? Send it to the editor! Hans Kerp, WWU - Abt. Palaobotanik Hindenburgplatz 57-59, W-48143 Munster, Germany Tel : + 49-251 -833966; Fax: +49-251 -834831 E-mail: kerp@uni-muenster.de Albertiana 17, May 1996 

Magneto-biostratigraphy of the 'Buchenstein Beds' at Frötschbach (western Dolomites, Italy)

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Magneto-biostratigraphy of the 'Buchenstein Beds' at Fr\uf6tschbach (western Dolomites, Italy)

Magneto-biostratigraphy of the 'Buchenstein Beds' at Fr\uf6tschbach (western Dolomites, Italy)

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