Lutetian magnetostratigraphy in the Santa Marina

Lutetian magnetostratigraphy in the Santa Marina section (Balzes anticline, Southwestern Pyrenees) Magnetoestratografía Luteciense en la sección de Santa Marina (anticlinal del Balzes, Pirineo Meridional) A. Rodríguez-Pintó1,2, Pueyo, E. L.1, Barnolas, A.3, Samsó, J. M.4, Pocoví, A.2, Gil-Peña, I.3, Mochales, T.5 and Serra-Kiel, J.6 1 Unidad del Instituto Geológico y Minero de España (IGME) de Zaragoza. C/ Manuel Lasala, 44, 9B, 50006 Zaragoza, Spain [email protected] 2 Departamento de Ciencias de la Tierra, Universidad de Zaragoza. c/ Pedro Cerbuna 12, 50009 Zaragoza (Spain). [email protected] 3 Instituto Geológico y Minero de España, c/ La Calera s/n, 28003 Tres Cantos, Madrid (Spain). [email protected] 4 Consultant Geologist: [email protected] 5 University of Queensland, Australia, [email protected] 6 Dpt. d’Estratigrafia, Paleontologia i Geociències Marines. Univ. de Barcelona. C/ Martí i Franquès, s/n, 08028 Barcelona. [email protected]

Abstract: Santa Marina magnetostratigraphic profile (189 samples, 630 m) has been performed at the western limb of Balzes Anticline (Southern Pyrenees). We sampled limestones of Guara Formation with the aim of dating the Lutetian sequence and the age of folding (recorded in a progressive unconformity). Thermal and alterning field demagnetizations (194 and 209 respectively) were done and revealed characteristic unblocking temperatures up to 435ºC being magnetite and iron sulphides the main magnetic carriers. We could calculated directions in 81% data. However many of them are biased by anomalous declinations and shallow inclinations suggesting an important overlapping. Trying to avoid this problem, restrictive and rigorous filters have been applied on the dataset. All in all, 27% of the measured samples prevail to build the Local Polarity Sequence (109 VGPs), displaying three pairs of magnetozones that can be tentatively correlated with Chrons 21 to 19. Higher uncertainty is observed towards the top of the profile. Key words: Guara Fm., External Sierras, Overlapping.

Resumen: El perfil magnetoestratigráfico de Santa Marina (189 muestras, 650 m) se realizó en el flanco W del Anticlinal del Balzes (Pirineo Meridional). Las calizas de la Formación Guara se muestrearon para datar la secuencia Luteciense y la deformación registrada en una discordancia progresiva. Desmagnetizamos muestras térmicamente y por campos alternos (194 y 209 análisis respectivamente). Los análisis revelan temperaturas de desbloqueo medias de 435º y magnetita y sulfuros como principales portadores del registro. Se calcularon direcciones del 82% de los análisis. Sin embargo, declinaciones anómalas y bajas inclinaciones son el patrón común, sugiriendo un solapamiento de componentes. Para evitar el solapamiento se aplicaron filtros restrictivos. Sólo un 27% de los análisis prevalece para construir una Secuencia Local de Polaridades (109 VGPs) con tres pares de magnetozonas correlacionables con los crones 21, 20 y 19 y con una incertidumbre creciente hacia el techo del perfil. Palabras clave: Fm. Guara, Sierras Exteriores, Solapamiento.

(the Western structure of External Sierras), present important features to take into consideration for potential studies: firstly, excellent outcrops of continuous marine series of 450 and 630 m of Ypresian and Lutetian times respectively. And secondly and also very important, the possibility of dating the folding chronology of the structure, since there is very well exposed progressive unconformity at the western limb in the Santa Marina section.

INTRODUCTION The External Sierras are characterized for the abundant and well-exposed outcrops of Cenozoic series. The Ypresian-Lutetian sequences display the larger thickness at the Eastern part of the External Sierras (Balzes and Boltaña anticlines). In the last decade, the basin chronology has been considerably improved, with numerous paleomagnetic, biostratigraphic and structural data. Studies as Mochales et al., in 2012b, Rodriguez-Pintó et al., in press (a), have addressed the Ypresian to Priabonian times in marine facies at Ainsa Basin and External Sierras respectively. Moreover, the Balzes anticline

GEOLOGICAL SETTING The Balzes anticline (BA) is the N-S easternmost fold of the External Sierras. It extends for 17 km long

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and has a curved axis trending N014E in the northern sector and N150E in the southern part (RodriguezPintó 2010). The structure formed during the main tectonic pulse in this part of the Pyrenees (middle Eocene) where an imbricate system was developed. A second stage of thrusting during Oligocene-Miocene times affects the structure (Millán et al., 2000). The outcropping lithology involves three main marine platform sequences developed at the distal foreland margin of the Jaca basin (Barnolas and Teixell, 1994). Ypresian (locally Illerdian) Alveoline limestones of the first platform outcrop in the core of the structure. The Boltaña Formation (late Ypresian, locally Cuisian, 2nd platform) is represented by ≈300 m of shallow limestones and siliciclastic input. The third platform (Guara Formation) is made of 630 m of Lutetian limestones and some marls and it has been widely sampled in this study. The sedimentation of Guara formation was controlled by the growing of the Balzes anticline. Upper Guara Member lays on an unconformity (Barnolas and Gil-Peña, 2001) over older members of Guara Fm (Fig. 2) and even the Boltaña Formation.

SAMPLING, RESULTS AND METHODOLOGY We drilled 189 cores along the Santa Marina profile (SM). SM was performed at the western limb of Balzes anticline, near of Bagüeste and Rodellar Villages, along 650 m of limestones and marls of Guara Formation. Cores were drilled and oriented with a magnetic compass. There were drilled cores of 4-7 cm long every 3 m of profile. Subsequently, they were cut in paleomagnetic standard samples before measurement procedures. Laboratory measurements were performed at the Universities of Burgos (Spain) and Utrecht (The Netherlands) using superconducting 2G cryogenic magnetometers for stepwise thermal demagnetizations (TH) and additionally a robot ARM (Utrecht) for alternating field demagnetizations (AF). We thermally demagnetized 194 samples and 209 AF demagnetizations of sister samples. Individual directions were calculated by means of end-point and principal component analyses (PCA). In general, the dataset display low Natural Remanent Magnetizations of about 0.3 mA/m and they are usually related to diamagnetic susceptibilities. Rock magnetism diagrams suggest that magnetite and undifferentiated ferromagnetic sulphides are the main carriers of the paleomagnetic record. Maximum unblocking temperatures averaged out 435ºC (Rodriguez-Pintó et al., in press (b)). We calculated directions from an 82% of the performed analysis. Calculated directions are dominantly of reverse polarity (TH: 79% and AF: 73%). Values of declination are very heterogeneous and there are shallow inclinations in most of the calculated directions. Additionally, unexpected counterclockwise rotations (up to -18º) contrast with numerous evidences of clockwise rotation in the area (Mochales et al., 2012a and Pueyo 2000 among others; +45º in the lower Lutetian and +40º in the Bartonian of Boltaña anticline respectively). Initially, a conical geometry of the structure was claimed as a possible cause of this anomalous dataset in the western limb of the BA (Rodríguez-Pintó et al., 2010), but new data displayed in intra-Lutetian marls near Santa Marina agree with the general CW rotation in the region. Therefore, a model of overlapping of components is suggested to explain and control this source of error (RodriguezPintó et al., in press (a and b)). Taking into account the hypothesis of the overlapping, we applied very restrictive filters to the raw magnetostratigraphic log to overcome the less reliable data. The restrictions were applied to all calculated directions: they must be fitted with more than 3 steps of demagnetization, with a Maximum Angular Deviation (MAD) less than 20º, Virtual Geomagnetic Poles (VGPs) larger than |40º|, inclinations higher than |20°| and NRM < 3mA/m. All calculated directions were classified in quality types (T1, T2 and T3) and only T1 and T2 were used.

FIGURE 1. Location of the study zone and ortophoto displaying the profile sampled (UTM 30T-ED50).

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After the application of the restrictions, only 27% of the samples (TH: 59 and AF:50) prevail to build the Local Polarity Sequence (LPS based on 109 VGP). The resulting LPS display three pairs of magnetozones. Each one has been defined at least with two consecutive sites with the same polarity. The LPS along the 630 m of profile shows three pairs of magnetozones. Reversed polarities are dominant wellcharacterized but the normal ones are shorter and of worst quality. R1 spreads from the base of SM up to meter 36 defined with 6 filtered VGPs. N1 is defined from meter 36 to meter 65 and it is defined with 3 filtered VGPs. The second pair of magnetozones is well defined in both polarities (R2: 65-230 m, n: 6 and N2: 230-318 m, n: 10). The reversed R3 is the most extended 608 m and it is defined with numerous VGPs.

Around meter 500 there are two filtered normal VGPs but we have discarded them because of for its short extension (less than 10 m). Finally there is high uncertainty on the top of the section and it seems to be defined by a poor N3 from 608 to 630 m with only two filtered VGPs. The three pairs of magnetozones can be correlated with Chrons C21, C20 and C19 from the base to the top of the section. The quality of the reverse magnetozones is much bigger than the normal ones. The calibration of the LPS with the Geomagnetic Polarity Time Scale (GPTS) is also founded on six biostratigraphic samples with abundant Nummulitids; which lead us the identification of associations of benthic foraminifers (SBZ). We could identify the SBZ15 and SBZ16 zones in the middle and upper part of the section.

FIGURE 2. The Santa Marina Lutetian progressive unconformity picture.

FIGURE 3. Stratigraphy column and Local Polarity Sequence of Santa Marina section.From left to right panels: Declinations, Inclinations (after bedding correction), Virtual Geomagnetic Poles(VGPs),filtered VGPs and the Local Polarity Sequence.

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Utrecht. Paleomagnetic data processing has been made using "Paldir" and “Palfit” programs from Utrecht Paleomagnetic Laboratory. REFERENCES Barnolas, A. and Teixell, A. (1994): Platform sedimentation and collapse in a carbonatedominated margin of a foreland basin (Jaca basin, Eocene, southern Pyrenees). Geology, 22: 11071110. Barnolas, A. and Gil Peña, I. (2001): Ejemplos de relleno sedimentario multiepisódico en una cuenca de antepaís fragmentada: La Cuenca Surpirenaica. Boletín Geológico y Minero, 112 (3): 17-38 Gradstein, F.M. and Ogg, J.G. (2004): A geologic Time Scale. In: Lethaia, 37: 175-181 Millán, H., Pueyo, E. L., Aurell, M., Luzón, A., Oliva, B., Martínez-Peña, M. B. and Pocoví, A. (2000): Actividad tectónica registrada en los depósitos terciarios del frente meridional del Pirineo central. In: Revista de la. Sociedad Geológica de España, 13(2): 279-300. Mochales, T., Casas, A.M., Pueyo, E.L. and Barnolas, A. (2012a): Rotational velocity for oblique structures (Boltaña anticline, southern Pyrenees). In: Journal of Structural Geology, 35: 2-16. Mochales, T., Barnolas, A., Pueyo, E.L., Casas, A.M., Serra-Kiel, J., Samsó, J.M. and Ramajo. J. (2012b): Chronostratigraphy of the Boltaña anticline and the Ainsa Basin (Southern Pyrenees). In: Geological Society of American Bulletin. doi: 10.1130/B30418.1 Pueyo, E. L. (2000): Rotaciones paleomagnéticas en sistemas de pliegues y cabalgamientos. Tipos, causas, significado y aplicaciones. PhD thesis, Universidad de Zaragoza. 296 p. Rodríguez-Pintó, A., Pueyo, E. L., Pocoví , A. and Barnolas, A. (2010): Paleomagnetic analysis in the Balzes anticline (Southern Pyrenees); Vertical-axis rotations and kinematic implications. In: Trabajos de Geologia, 30 (1): 169- 175. Rodríguez-Pintó, A., Pueyo, E.L., Serra-Kiel, J., Samsó, J. M., Barnolas, A. and Pocoví, A.: Lutetian magnetostratigraphic calibration of larger foraminifera zonation (SBZ) in the Southern Pyrenees. In: Palaeogeography, Palaeoclimatology, Palaeoecology: In press (a). Rodriguez-Pintó, A., Pueyo, E. L., Barnolas, A., Pocoví, A., Oliva-Urcia, B. and Ramón, M. J. (2012) Overlapped paleomagnetic vectors and fold geometry: A case study in the Balzes anticline (Southern Pyrenees). In: Physics of the Earth and Planetary Interiors: In press (b).

FIGURE 4. Magnetostratigraphic data deduced from Santa Marina section and the average “per chron” accumulation rates. At the y axis the local polarity sequence of SM compared to the Geomagnetic Polarity Time Scale from Gradstein et al., 2004 (Horizontal). CONCLUSIONS A tentative local polarity sequence of magnetozones can be built in the Lutetian Santa Marina section. This can be possible after rigorous filtering of the dataset focused on avoiding the effects of a strong overlapping of paleomagnetic components. The LPS is built with only 27% of the initial measured data (109 VGP). This sequence displays three pairs of magnetozones properly calibrated with biostratigraphic data that enables the correlation with Chrons C21, C20 and C19. The uncertainty rises and the quality decreases in the upper part of the profile. The resolution on the top of the profile is not enough to make a precise dating of the end of the folding stage of the structure with this paleomagnetic dataset. Finally, an increment in the accumulation rate can be observed in the profile. While C21n displays 1.5 cm/Kyr, then accelerates in C20 (n & r: 6.4 and 7.5 cm/Kyr) and reaches 31.5 cm/Kyr during C19r. This accumulation cycle is comparable to values found in the other locations of the Guara platform such as the Isuela section (Rodriguez-Pintó, et al, in press (a). ACKNOWLEDGEMENTS Research financial support comes from the projects CGL2009-14214 (MICINN) and 3DR3 & (PI165/09 Gob. Aragón). Thank you very much to people who help us in the development of this work: Paleomagnetic work was performed in the Universities of Burgos and

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