New Magnetostratigraphy from the Punta Grohmann Section

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General Assembly New magnetostratigraphy from the Punta Grohmann section Online, 4-8 May 2020 (Dolomites, NE Italy): an improvement of the Geomagnetic Polarity Università degli Studi di Milano Time Scale around the Ladinian/Carnian boundary University of Milan

PG 21 PG 38 40 Matteo70 Maron1*, Giovanni Muttoni1, Marica Ghezzi1, Manuel Rigo2 and Piero Gianolla3 Università degli Studi di Padova 35 60

30 University of Padua 1)50 Department of Earth Sciences “A. Desio”, University of Milan, via Mangiagalli 34, 20133 – Milan, Italy 25 2)40 Department of Geosciences, University of Padova, via Gradenigo 6, 35131 – Padova, Italy 20 30 Università degli Studi di Ferrara 15 3) Department of Physics and Earth Sciences, University of Ferrara, via Saragat 1, 44122 – Ferrara, Italy

0 0 0 100 200 300 400 500 600 700 0 100 200 300 400 500 600 700 Temperature (°C) THE LADINIAN/CARNIANTemperature (°C) BOUNDARY PG 49 PG 57 16 The40 Ladinian/Carnian boundary (LCB) is officially defined at Prati di Stuores (GSSP of the Carnian Stage; Mietto et al., 2012) with the first appearance datum of ammonoidDaxatina canadensis in bed 14 SW435 (Broglio Loriga et al., 1999; Mietto et al., 2012). The age of the LCB has been recently estimated at ca. 236.8 Ma (Maron et al., 2019). In the attempt to refine the chronology of the Carnian Stage, 12 0.12 T we 30investigated the Ladinian-Carnian marine section of Punta Grohmann, which is time-calibrated by two U-Pb datings from the same ash-bed (237.58±0.04 Ma, 237.68±0.05 Ma; Storck et al., 2019). 0.4 T 10 25 2.5 T 8 20 11°30’E 11°45’E 12°00’E Punta Grohmann

Sass de Putia 46°40’N 6 15 THE PUNTA GROHMANN SECTION Dolomites Magnetization (A/m) Magnetization (A/m) Lithostratigraphy Ammonoids Biostratigraphy J NRM (×10-3 A/m) Susceptibility (×10-6 SI) VGP Latitude (°N) 4 10 Chiusa 46°40’N The Punta Grohmann section (e.g. Gianolla et al., 1998, 2010; Mietto N Odle Group Badia 2 5 ITALY GR4r Puez Group Fanes N Ortisei et al., 2012) is located in the Dolomites (Southern Alps, NE Italy), close GR4n 0 0 Group - GRH 3 Selva di Val Gardena 0 100 200 300 400 500 600 700 0 100 200 300 400 500 600 700 0 250 Km CARNIAN Corvara to the Sassopiatto massif. It is a basinal sequence of ~270 m of mainly 500 GR3r Temperature (°C) TemperatureRio Frommer (°C) - GRH 80 Sella - GRH 8 Rio Nigra Sassopiatto Group volcaniclastic sediments (Wengen Fm.) followed by ~300 m of mainly - GRH 7 Prati di GR3n BOLZANO Sciliar Stuores 46°30’N Punta Grohmann GR2r carbonatic deposits belonging to the San Cassiano Fm. The Punta Celtites sp. PG 61 PG 73 400

16 Zestoceras sp.

LADINIAN

46°30’N 25 Canazei Catinaccio Orestites sp. Celtites epolensis

Grohmann section lies above the Marmolada Conglomerates (Wengen Fm.) SAN CASSIANO FM. 14 GR2n Group - GRH 6 Asklepioceras sp.

Zestoceras cf. lorigae 12 20 Marmolada and is topped by the caotic breccias of the Cassian Dolomite Fm., marking

10 0 10 km 300 Latemar Moena 15 the progradation of the Cassian platforms on the basinal sequences. In the - GRH 5 8 11°30’E 11°45’E 12°00’E lower part of the section, the ammonoid biostratigraphy is characterized by 6 10 237.579 ± 0.042 Ma Magnetization (A/m) Magnetization (A/m)

Lecanites glaucus GR1r 200 237.680 ± 0.047 Ma 4 assemblages typical of the Regoledanus Subzone, while in the upper part (~490 m) the first appearance ofZestoceras cf. - GRH 2 5 - GRH 9 2 lorigae (Canadensis Sbz.; Mietto et al., 2012) approximates the LCB. The conodont assemblages recovered from Punta - GRH 100 0 0 0 100 200 300 400 500 600 700Grohmann0 100 (e.g.200 Russo300 400 et500 al., 6001997)700 are typical of the upper Ladinian - lower Carnian interval (Gianolla et al., 2010). The 100 - GRH 9 cf. Joannites sp. WENGEN FM. TemperaturePG (°C) 21 TemperaturePG 38 (°C) Frankites regoledanus - GRH 4 Frankites apertus 70 Frankites sp. 40 U-Pb ages of 237.58±0.04 Ma and 237.68±0.05 Ma (Storck et al., 2019) are located at ~217 m. Maclearnoceras spp. - GRH 1 Lobites ellipticus 35 PG 21 PG 38 GR1n 60 40 70 PG 81 30 35 60 0 m 12 400050 Marmolada Conglomerates 0 500 1000 1500 2000 0 5000 10000 15000 -90 -60 -30 0 30 60 90 30 25 50 3000 10 40 25 20 40 20 PALEOMAGNETISM OF PUNTA GROHMANN 200030 8 30 15 15 vertical

Magnetization (A/m) Magnetization (A/m) 20 Magnetization (A/m) 1000Magnetization (A/m) 20 10 A total of 93 samples for paleomagnetism have been horizontal 6 10 PG1 PG50 10 Units: A/m ×10-2 IN SITU TILT CORRECTED 100 5 Units: A/m ×10-3 5 collected from the Punta Grohmann section, The 5000 1000 1500 2000 25000 W,U W,U

Magnetization (A/m) 0 0 4 10 8 0 -10000 0 100 200Field (mT)300 400 500 600 700 0 100 200 300 400 500 600 700 PG34 Temperature (°C) Temperature (°C) highest values of magnetic susceptibility and NRM -2

0 100 200 300 400 500 600 700Magnetization (A/m) 0 100 200 300 400 500 600 700 Units: A/m ×10 2 W,U Temperature (°C) -2000 PG20 PG50TemperaturePG71 (°C) PG74 PG82 PG84 PG 49 PG 57 3 16 40 magnetization are generally located in the lower part of S N 0 -3000 5 20 PG 49 14 PG 57 35 0 16 100 200 300 400 500 600 700 40 the section (within Wengen Fm.), except for a peak in 12 0.12 T 30 Temperature (°C) PG 21 PG 38 PG 21 0.4 T 70 PG 38 14 3540 4010 2570 2.5 T both curves located in a level of the San Cassiano Fm. S N 270 90 270 90 8 20 5 1 12 0.12 T 3035 35 6060 306 15 0.4 T 30 50 and probably related to locally weathered rocks. IRM

10 Magnetization (A/m) 25 50Magnetization (A/m) 2.5 T 254 10 40 S N 25 4 2 8 20 2 acquisition and thermal demagnetization of three-axis 20 405 30 3 6 1520 150 0 E,D

Magnetization (A/m) 0 100 200 300 400 500 600 700 30Magnetization (A/m) 20 0 100IRM200 on300 representative400 500 600 700 samples indicate magnetite as the Magnetization (A/m)

Magnetization (A/m) 10 4 1015 Temperature (°C) Temperature (°C) 10 5 180 180 Magnetization (A/m) Magnetization (A/m) 20 main carrier of the remanence in the Punta Grohmann 2 105 0 0 35 4 E,D E,D 0 100 200 300PG 61400 500 600 700 10 0 100 200 300PG 73400 500 600 700 5 16 0 0 Temperature (°C) 25 section.Temperature (°C) 0 100 200 300 400 500 600 700 0 100 14 200 300 400 500 600 700 MEAN DIRECTIONS 0 0 Temperature (°C) 12 Temperature (°C)PG 49 20 PG 57 PG87 0 100 16 200 300 400 500 600 700 40 0 100 200 300 400 500 600 700 PG59 In Situ Tilt-Corrected -4 10 Temperature (°C) Temperature (°C) Units: A/m ×10-2 Units: A/m ×10 14 3515 8 W,U W,U Site N k 95 Dec. Inc. k 95 Dec. Inc. After thermal demagnetization, 56 of 93 samples revealed a ChRM component oriented north-down15 12 0.12 T 30 5 PG 61 6 PG 73 10 Magnetization (A/m) PG 49 PG 57 0.4 T Magnetization (A/m) PG70   16 16 10 4025 Punta Grohmann 56 5.4 17.3°E 61.6° 5.5 6.4°E 40.5° 25 4 2.5 T and south-up in in situ coordinates. The mean directions obtained from the ChRMUnits: A/m ×10components-3 (Dec.: 5 8 14 2 20 W,U 14 35 6 PALEOMAGNETIC POLE AND PALEOLATITUDE 6 20 0 150 6.4°E, Inc.: 40.5°, a = 9°) in tilt-corrected coordinates yield to a paleopole position having Lat.: 66.3°N 12 S N Magnetization (A/m) 95 12 0 100 200 300 400 500 600 700 30Magnetization (A/m) 0 100 200 300 400 500 600 700 95 4 0.12 T 10 20 5 Site Lat. Long. k A Paleolatitude 10 Temperature (°C)0.4 T Temperature (°C) 1510 2 255 and Long.: 176.7°E (A = 6.6°). The stratigraphic sequence of the Virtual Geomagnetic Poles (VGP), Punta Grohmann 66.3°N  5.5 6.6° 23.1°N +7.2°/-6.1° 2.5 T 95 8 0 8 PG 81 200 12 0 100 200 300 400 500 600 700 4000 0 100 200 300 400 500 600 700 S N 6 10 calculated for each ChRM component, was used for intepreting the polarity2 stratigraphy.10 The latitude Temperature (°C) Temperature (°C) Magnetization (A/m) S N Magnetization (A/m) 6 300015 2 1 4 10 Magnetization (A/m)

Magnetization (A/m) of the samples VGP defined 8 magnetozones named GR. 5 2000 4 8 10 2 PG 61 PG 73 16 1000 2 525 0 6 0 14 0 Wayao 0 100 200 300 400 500 600 700 0 100 200 300 400 500 600 700 500 1000 1500 2000 2500 0 Magnetization (A/m) 4 020 Prati di Stuores 12 -1000 Option C (China) Field (mT) Temperature (°C) 0 100 200 Temperature300 400 (°C) 500 600 700 0 100 200 300 400 500 600 700 102 Magnetization (A/m) (Italy) Temperature (°C) -200015 PG20 PG50 PG71TemperaturePG74 (°C)PG82 PG84 8 4 10 40 0 -3000 E,D s E,D E,D Age 6 0 100 200 300 400 500 600 700 10 Wa5 GAP

PG 81 Magnetization (A/m)

Magnetization (A/m) (Ma) 12 4000 Temperature (°C) 4 0.4 1.0 Wa4 Wa5 5 PG 61 PG 73 0.5 2.0 Wa4 300016 2 0.0 10 25 0.0 Mayerling 234 0-0.4 0 -0.5 Punta Grohmann

14 aon 2000 t-value 0 100 200 300 400 500 600 700 0 100 200 300 400 500 600 700 1.5 -0.8 t-value -1.0 (Austria) 8 200 lygnathiform i Temperature (°C) 20 Temperature (°C) (Italy) 12 -1.2 -1.5 Wa3 Wa3 1000 -2.0 180 6 10 S4n 1.0

0 PG 81 15 s 160 12 4000 235 8 500 1000 1500 2000 2500 140 S3r Magnetization (A/m) 4 Mayerling Mayerling -1000 10 Field (mT) 3000 MA5r 0.5 6 10 120 S3n Wa2 Magnetization (A/m)

Magnetization (A/m) 2000 S2r 60 Magnetization (A/m) 2 -2000 PG20 8 PG50 PG71 PG74 PG82 PG84 100 CARNIAN 4 MA5r MA5r 0 Myr 1000 ragondolella p o Wa1r 5 canadens i 80 FAD 0 -3000 60 6 60 S2n

Punta Grohmann P a 2 0 60 LCB MA5n Daxatina 236 0 100 200 300 400 500 600 700 500 1000 1500 2000 2500 MA5 Magnetization (A/m) 4 GR4r TRIASSIC LATE E -10000 Field (mT) E (thick beds filtered) 40 canadensis Temperature (°C) 0 MA5n MA5n S1r 50

PuntaMagnetization (A/m) Grohmann GR4n Rio Nigra 0 100 2 200 300 400 500 600 700 -2000 0 PG20100 PG50200 PG71300 PG74400 PG82 500PG84 600 700 20 Zestoceras cf. lorigae 50 D 50 D 500 S1n 500 GR3r MA4r Temperature (°C) GR4n Temperature (°C) GR4n (Italy) 0 t = 0.173 500-3000 t = 0.333 0 m MA4r GR3r MA4r GR3r GR3n 0 100 200 300 400 500 600 700 400 40 400 GR2r ??? Temperature (°C) 400 237 t = -0.476 GR2n-GR3n t = -0.558 GR2n-GR3n MA4 40 PG 81 40 300 GR2n MA4n

12 C 4000300 C regoledanu s

300 edanus edanus MA4n MA4n 200 237.58 ± 0.04 Ma 3000200 30 MA3r 237.68 ± 0.05 Ma 10 GR1r GR1r 200 rego l

30 30 rego l ??? MA3r 100 ??? MA3r 100 Seceda ??? RN2n 2000 GR1r olygnathiformis 30 238

8 t = 0.311 GR1n t = 0.477 GR1n 7.58 ± 0.04 Ma 7.68 ± 0.05 Ma B B p 0 m 0 m (Italy) 100 20 RN1r MA3 1000 2 3 2 3 20 MA3n 10 6 20 MA3n 20 MA3n GR1n Pg . 0 m RN1n 0 SC6n 0 m

500 1000 1500 2000A 2500 neumayri

Magnetization (A/m) A 4 65 MA2r MA2 LADINIAN -1000 Field (mT) MA2r MA2r 239.04 ± 0.10 Ma 239 Magnetization (A/m) 10 neumayri 10 10 60 SC5r MA2n 2 MA2n -2000 PG20 PG50 PG71 PG74MA2n PG82 PG84 t = -0.225 t = -0.144 MA1r MA1r 55 MA1r 0 -3000 7.77 ± 0.05 Ma

50 MA1n MIDDLE TRIASSIC 0 100MA1n 200 t =300 -1.248 400 500 600 700 MA1n t = -1.464 0 m SC4 0 m 0 m SC5n 2 3 Temperature (°C) 45 239.04 ± 0.10 Ma 240 40

SC4r Budurovignathus mostleri 35 m SC4n MAGNETOSTRATIGRAPHIC CORRELATIONS (Ma) Age vs. Height Mayerling The magnetostratigraphy of the Punta Grohmann section has been correlated to the Prati di Stuores section (Mietto et al., 2012) using 235.0 (Option C) the ammonoid biostratigraphy to calibrate the magnetostratigraphic correlation, obtaining only one option that is in agreement with both MA5r 235.5

236.0 magnetostratigraphic and biostratigraphic constraints. The correlation between Punta Grohmann and Mayerling section (Gallet et al., 1998) MA5n 236.5 is based on the statistic evaluation of all the possible correaltion options obtained from sliding the Punta Grohmann magnetostratigraphy MA4r 237.0 alongside Mayerling, each time comparing the thickness of the magnetozones involved (similar to Muttoni et al., 2004; Maron et al., 2015, MA4n 237.5 2019; Marini et al., 2020). The short (single and two-points) reversals in Punta Grohmann are excluded from the correlation. Among five MA3r 238.0 MA3n y=-0.0625x + 239.53 possible options, only option C appears to be the more reliable, although with a low t-value. Inspired by the approach used on turbiditic 238.5 MA2r MA2n 239.0 basins by Marini et al. (2020), we excluded from the stratigraphy the thick conglomerate and slump beds, which are considered to erratically MA1r increase the accumulation rates within the section. Also in this case, option C appears to be most reliable correlation option. MA1n 239.5 240.0 Option C correlates the magnetostratigraphy of Punta Grohmann to the MA3n-MA5n.1r interval in Mayerling. Using the U-Pb ages from 0 10 20 30 40 50 60 70 (m) MA1n MA1r MA2n MA2r MA3n MA3r MA4n MA4r MA5n MA5r Punta Grohmann (Storck et al., 2019) and Seceda (239.04±0.10 Ma; Wotzlaw et al., 2018) as tie-points, we obtained an age model for the Mayerling section. The age of the LCB can be interpolated through this age model at ~236.3 Ma, about 0.5 Myr younger than previously 237.58 ± 0.04 Ma 239.04 ± 0.10 Ma estimated (236.8 Ma; Maron et al., 2019). 237.68 ± 0.05 Ma Carnian Base (236.3 Ma)

CONCLUSIONS The magnetostratigraphy of the Punta Grohmann section highly improved the magnetostratigraphic record around the Ladinian/Carnian boundary (LCB), currently represented by few sections (e.g. Mayerling and Prati di Stuores). The U-Pb age from Punta Grohmann provides a further chronological reference for the Middle Triassic geomagnetic polarity time scale, leading to a Ladinian/Carnian boundary age of ~236.3 Ma.

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