GeoResJ 13 (2017) 66–82 Contents lists available at ScienceDirect GeoResJ journal homepage: www.elsevier.com/locate/georesj Magnetostratigraphy and magnetic parameters in Quaternary sequences of Balcarce, Argentina. a contribution to understand the magnetic behaviour in cenozoic sediments of South America Mauro L. Gómez Samus a , b , c , ∗, Yamile Rico a , c , d , Juan Carlos Bidegain a , d a Laboratorio de Entrenamiento Multidisciplinario para la Investigación Tecnológica (LEMIT). Calle 52 S/N. La Plata b Consejo Nacional de Investigación Científica y Técnica (CONICET) c Facultad de Ciencias Naturales y Museo (FCNyM-UNLP) d Comisión de Investigaciones Científicas (CIC) a r t i c l e i n f o Article history: Received 27 October 2016 Revised 23 February 2017 Accepted 28 February 2017 Available online 3 March 2017 Keywords: Paleomagnetism Environmental magnetism Brunhes Matuyama Pampean sediments a b s t r a c t The South America Loess plateau covers a large part of Argentina. In this country, the Balcarce Hills rep- resent at the moment a poorly studied area from a stratigraphic and paleoenvironmental point of view. This area is located in an intermediate place between other zones with a greater data density; for this reason, the geochronological and environmental knowledge of this area is key to a better regional un- derstanding of Argentine loess formation mechanisms. For these purposes, Paleomagnetism, Rock Mag- netism and chemical analyses were used as the main supports for this study. Two stratigraphic sections in the Balcarce area were studied. Both sections have lithostratigraphic units in common which allow to correlate them. Three lithostratigraphic and six pedogenetic cycles were recognized. The two younger lithostratigraphic units were deposited during the Brunhes Chron ( < 0.781 Ma) while the oldest unit dur- ing Matuyama Polarity Chron (0.781 - 2.588 Ma). Towards the base of the oldest unit, normal polarity levels were recorded and they referred to Olduvai (1.778 - 1.945 Ma). The youngest reversal, assigned to the Brunhes-Matuyama Boundary (BMB), was found in the paleosoil of the top of oldest lithostrati- graphic unit. This is very important because it allows to correlate it with other paleosols in the BMB of Argentina. With regards to magnetic signature, this is relatively homogeneous along the profiles and cor- responds mainly to ferrimagnetic minerals, as titanomagnetites and probably maghemite in the paleosols. The magnetic grain size would dominantly be SD and the low coercivity fine to ultrafine particles increase in the pedogenetic horizons (A, Bw and Bt). The source rocks giving rise to the observed magnetic con- tribution seem to have been relatively constant in the last 1.9 Ma. In a first approach, the susceptibility values recorded in the sediments of Balcarce are higher than in areas located further north from the pampean loess plateau, but lower than those in the south. The mentioned differences would be linked to both the distance and the mineral composition of the source rocks. © 2017 Elsevier Ltd. All rights reserved. c i q o b p w t h t 1. Introduction The loess deposits cover a wide region of South America, mainly in northern central Argentina, although they are also found in Uruguay and south of Brazil and Paraguay ( Fig. 1 ). Zárate [76] indicated that the sedimentation of loess in South Amer- ica would have started around 12–11 million years ago following the Paranaense Sea regression. According to Teruggi [70] , these loess deposits consist mainly of silts with subordinated amounts of sands and clays. They have a characteristic volcaniclastic ∗ Corresponding author. E-mail addresses: gomez_samus@yahoo.com.ar (M.L.G. Samus), jcbidega@yahoo.com.ar (J.C. Bidegain). C t r w http://dx.doi.org/10.1016/j.grj.2017.02.005 2214-2428/© 2017 Elsevier Ltd. All rights reserved. omposition, which differentiates them from other loess deposits n the world, being mainly integrated by plagioclase, orthoclase, uartz, volcanic glass, feldspar, lithoclasts of volcanic rocks, organic pal, ores (magnetite, ilmenite, limonite), pyroxenes and amphi- oles, that were originally deposited in the Andes piedmont (ap- roximately 10 0 0 km west from study area). According to several orks, for example [35,36,60,61,66,78,79] and [76] , the transporta- ion of sediments occurred in different stages. The materials would ave been transported from the Andean region by the river sys- ems (e.g. Negro, Colorado and Desaguadero-Salado-Chadileuvú or uracó fluvial systems) ( Fig. 1 ); once deposited on the floodplains hey were re-transported by the winds during cold and dry pe- iods (glaciations). Finally, an important portion of the materials ere re-worked at a more local level by means of rivers, winds http://dx.doi.org/10.1016/j.grj.2017.02.005 http://www.ScienceDirect.com http://www.elsevier.com/locate/georesj http://crossmark.crossref.org/dialog/?doi=10.1016/j.grj.2017.02.005&domain=pdf mailto:gomez_samus@yahoo.com.ar mailto:jcbidega@yahoo.com.ar http://dx.doi.org/10.1016/j.grj.2017.02.005 M.L.G. Samus et al. / GeoResJ 13 (2017) 66–82 67 Fig. 1. Buenos Aires province (Argentina) and surrounding areas. Location of main areas subjected to Magnetostratigraphic and Environmental Magnetism researches. a a t fl C c p a f p c a c q t t w m w A c A i s A s p t r t d G o B f fi b [ p t d f M E g t a t m s o c C P F T s d r t l w b s t h a t n c f R h t u 1 nd even through landslides in the hill areas. According to Zárate nd Blasi [78] , the loess in the South of the plateau, including he study area, was mainly blown from Colorado and Negro rivers oodplains. Furthermore, due to the explosive volcanic activity in ordillera, the contribution of volcanic glass has been more or less onstant in geological terms but does not necessarily follow the reviously described sedimentary pathway. Volcanic glass is usu- lly mixed with the rest of the loess minerals, although sometimes orm true tephra layers. There are serious impediments to dating the pampean loess, es- ecially in the timespan that exceeds the radiocarbon and lumines- ence methods. This is due to the scarce available materials suit- ble for dating. A few radiometric datings were performed, mainly onditioned to the finding of impact glasses (see [77] ). Conse- uently, the paleomagnetic records became the most suitable tool o establishing a firm age for these deposits. These techniques in he pampean loess have been heavily-used since the early 1980’s ith magnetostratigraphic purposes [21,46,48,73] . Great achieve- ents were obtained from the use of these techniques, among hich we should highlight the great impulse given to the South merican biostratigraphy, since it allowed to establish more pre- isely the geochronological framework of mammalian associations. lthough there are many studies related to magnetostratigraphy n the pampean loess, these are not evenly distributed. The main tudied areas correspond to the north of the province of Buenos ires [6,8,11,17,21,45] and to the cliffs of the Atlantic coast in the ame province [10,14,49,55,59] . Moreover, other areas where im- ortant magnetostratigraphic analysis were carried out correspond o the Paraná River banks, in Entre Rios Province [6,7] , and Las Car- eras, in Tucumán province [62,63] . On the basis of the above-note reasoning, it should be men- ioned that the paleomagnetic studies in Argentinian loess allowed etermining the Brunhes, Matuyama, Gauss and eventually the late ilbert chrons. While the oldest ages correspond to some profiles f the Atlantic coast [49,59] , in most of the studies carried out only runhes and Matuyama chrons were determined. An interesting act with regards to Argentinian loess paleomagnetic studies is the nding of paleosols levels coincident with the Brunhes–Matuyama oundary (BMB) [6,45] , which has great regional importance (see 15,16] ). These paleosols were found in the north of Buenos Aires rofiles, but also were mentioned in the Atlantic coast [14] . Al- hough the aforementioned authors agree that these paleosols i eveloped during the MIS19, this proposal should be more care- ully discussed. For example, BMB in marine records is located in IS19, but in Chinese loess from MIS 20 to 21 [39,40] and in the uropean loess, from MIS 20 to 22 ( [44] ). From the 1990’s, often simultaneously to the magnetostrati- raphic investigations, the measurement of magnetic parame- ers were carried out, which were used to perform paleoclimate nd paleoenvironmental interpretations [10,11,13,47,50] . Similarly o what happens with magnetostratigraphic studies, the environ- ental magnetism studies are not evenly distributed either. These tudies were also developed mainly in the north of the province f Buenos Aires [13,33,47,50] and on the cliffs of the Atlantic oast [10,32,55] . Moreover, studies were carried out in Corralito, órdoba province [58] , Caleu-Caleu to Necochea transect, in La ampa and Buenos Aires provinces [3,5] and Las Carreras [63] . ig. 1 shows the mentioned localities closest to the study area. he rock magnetic data, reported for the loess–paleosol sequences tudied in the North of Buenos Aires and the coastal cliffs, in- icate that the susceptibility values behaviour should be closely elated to the wind-vigor model ( [10,13] ), although, according to he authors, affected by pedogenesis. Due to that, less weathered oess shows the highest susceptibility values while the intensive eathered horizons the lowest. This behaviour is mostly found, oth in soils and paleosols, but in a few cases, as the recent oils of Caleu-Caleu to Necochea transect (South of loess plateau), he magnetic susceptibility values increase in the most edaphized orizons. The Balcarce Hills area is a poorly studied site and it is in n intermediate place between areas already heavily studied. For his reason, the knowledge of stratigraphy and their magnetic sig- ature, including geochronological and environmental knowledge, an contribute to the better understanding of South America loess ormation mechanisms. For these purposes, Paleomagnetism and ocks Magnetism and their corresponding conventional techniques ave been used as the main supports. Besides, taken into account he Blomendal [19,20] investigation, chemical analyses were also sed as complementary tools. .1. Regional setting The late Cenozoic sediments under consideration are cover- ng, partially, the lower Paleozoic and Precambrian rocks of the 68 M.L.G. Samus et al. / GeoResJ 13 (2017) 66–82 Fig. 2. Photographs showing the sedimentary section in Sierra Bachicha. After Gómez Samus et al. (2016). Fig. 3. Photograph showing the sedimentary section in Ramos Otero. After Gómez Samus et al. (2016). 2 2 s s B 1 o a t s Tandilia Range Zone. The oldest rocks correspond to the igneous- metamorphic basement integrated by gneisses, migmatites, am- phibolites and granitic plutons, being of paleo-proterozoic age. Subsequently, on laying the basement, there are carbonatic and siliciclastic deposits attributed to a shallow marine origin of Neo- proterozoic age and also orthoquartzites of the lower Paleozoic. A review of the geological background of Tandilia System was per- formed by Cingolani [23] . The most relevant studies of the late Cenozoic deposits of Tandilia correspond to Teruggi [71] and Rabassa [53] . In these works, the authors pointed out that both texture and mineralogy are similar to the sediments mentioned by Teruggi [70] , that is, the typical mineralogy of Argentinian loess, but with a moderate influence of the outcropping rocks of the surroundings. There are not yet firm lithostratigraphic schemes to be re- ferred to the sedimentary sequences of Sierras de Balcarce. The only stratigraphic scheme that was published for the area cor- responds to Rabassa [53] , which was proposed for the sedi- ments of the Tandil area, situated 100 km Northwest from Balcarce (see Fig. 1 ). The aforementioned author defined four main units, named from the younger to the oldest as Tandileufú - Las Ánimas, Vela and Barker formations. The Las Ánimas and Tandileufú forma- tions should be of late Pleistocene-Holocene age, the former are primary loess deposits and the latter are fluvial. The Vela Forma- tion is loess reworked by fluvial activity and was assigned to the middle Pleistocene ( [27,53] ). The Barker Formation is also consid- ered loess reworked by fluvial activity, with an age greater than 3.1 Ma ( [27,28] ). In Balcarce, Teruggi [71] carried out work that refers to the sed- iments exposed in a quarry in the locality of Sierra Bachicha (Bal- carce), a place very close to one of the sections studied in the present contribution. The aforementioned authors identified differ- ent stratigraphic levels, corresponding to loess and reworked loess. Gómez Samus [29] indicated that two levels, towards the top of the profile, are equivalent to the Las Ánimas and Vela Formations, the deposits that underlie such units do not yet have a formal definition, but nevertheless, correspond to loessoid deposits bear- ing early Pleistocene fossils (for example Ctenomys Chapadmalensis [68] ). s . Materials and methods .1. Stratigraphic sections Two geological sections were taken into consideration, where edimentary and stratigraphic characteristics were partially de- cribed in previous contributions [27] and [29] . The Sierra achicha section, located at 37 °49 ′ 37, 58 ′′ S and 58 °12 ′ 12,71 ′′ W, at 45 m.a.s.l, and the Ramos Otero section, located 38 km Northwest f the previous section, at 37 °35 ′ 37, 96 ′′ S and 58 ° 29 ′ 59, 40 ′′ W, nd 126 m.a.s.l. Figs. 2 and 3 show the Sierra Bachicha and Ramos Otero sec- ions. Figs. 4 and 5 show the sedimentary columns and the corre- ponding description of the sedimentary units. The Sierra Bachicha ection ( Fig. 4 ) comprises three lithostratigraphic units and shows M.L.G. Samus et al. / GeoResJ 13 (2017) 66–82 69 Fig. 4. Sierra Bachicha sedimentary column, main units identified and their characteristics. 70 M.L.G. Samus et al. / GeoResJ 13 (2017) 66–82 Fig. 5. Ramos Otero sedimentary column, main units identified and their characteristics. t P 2 p p r w ( g i v s l M a a a S g o m c l s o ( 2 a great similarity with the one described by Teruggi [71] in a nearby quarry. The Ramos Otero section ( Fig. 5 ) comprises only two lithostratigraphic units. The younger units of the upper part in both localities have a great similarity, so it is possible to correlate them and place them in the same litostratigraphic unit (L.U.1). The latter corresponds to primary loess, with the development of current soil in the Ramos Otero section (pedogenetic cycle 1), which is classified as tipic Argiudoll [67] , with horizons A-Bt-BC-C. In Sierra Bachicha, the an- thropogenic activity does not allow the observation of the current soil. Due to the sedimentary characteristics and stratigraphic posi- tion, Gómez Samus [29] assigned them to the Las Ánimas Forma- tion and related this unit to the cycle late Pleistocene – Holocene. In both localities, the L.U.1 lies discordantly on units that also have a great similarity, and constitute the same lithostratigraphic unit (L.U.2). The L.U.2 is mainly silty sands, with cross-bedding and horizontal stratification. The reworking is attributed to fluvial en- vironment. Another distinctive feature of L.U.2 is the abundance of CaCO 3 (related to the Tandil paleosurface [71] ) that from the pe- dogenetic point of view corresponds to 2Ckm and 2Ck horizons (pedogenetic cycle 2). Gómez Samus [27] assigned the L.U.2 to the Vela Formation and attributed this unit to the Bonaerian Stage (120–450 Ka) by regional correlations. The base of L.U.2 was not reached in Ramos Otero, but was recorded in Sierra Bachicha, by an erosional unconformity, on a set of tabular layers with loessoid appearance (L.U.3), these are partly sandy and partly silty, massive, with scattered psephitic clast of rocks (gneiss), without identifiable primary structure. The re- worked material is due to local landslides. Four pedogenetic cycles were identified in L.U.3 (pedogenic cycles 3–6). The main devel- oped pedogenetic cycle is at the top of the L.U.3, corresponding to a conspicuous argillic horizon with little calcium carbonate con- cretions (3Btkb). The other paleosols are less developed, two Bw horizons (4Bwb and 6Bwb) and one Bt horizon (5Btb). According l o Gómez Samus [29] , this unit correlates to the unit bearing early leistocene fossils [68] . .2. Paleomagnetic sampling and measurement The lithological, paleomagnetic and rock-magnetic studies were erformed in a sequential order according to the methodology ap- lied until now, giving consistent results with several works car- ied out in the country (e.g. [10,17,55] ). For this work, eighty-one paleomagnetic samples were used, here the sampling was carried out by a hidrobronz piston core designed by Mörner and improved for sampling in loess by Bide- ain) oriented with a geological compass. The collected material s placed in cubic boxes of 8 cm 3 and a non-magnetic adhesive inyl is added to avoid the movement of particles. The vertical ampling distance varied between 5 and 20 cm. Each of the pa- eomagnetic samples was analyzed at the LEMIT laboratory by a olspin Ltd magnetometer and a demagnetizer (also Molspin), by lternating fields (AF). The alternate field demagnetization began t a low field of 2.5 mT, reaching peaks of 80 and 100 mT in lmost of the samples. The data were processed in the software UPER IAPD20 0 0 [72] , and represented by means of stereographic rids, demagnetization curves and end point diagrams. Examples f these latter are given in the Fig. 6 . The characteristic remanent agnetization (ChRM) and the angular deviation (MAD) were cal- ulated by using the Kirschvink method. The paleomagnetic dec- ination (D), inclination (I) and intensity values (J °) were repre- ented in relation with the sedimentary columns, thus the sectors f normal and reverse polarity were defined as the magnetozones Figs. 7–8 ). .3. Rock magnetism and chemical sampling and measurement Representative sediment samples (21 bulk samples) were col- ected in order to carry out chemical analysis as well for measuring M.L.G. Samus et al. / GeoResJ 13 (2017) 66–82 71 Fig. 6. End point diagrams for the litostratigraphics units studied in Sierra Bachicha and Ramos Otero Sections. t d i m d s i d d T t t i c I r d i K t F d f c a r c o m P d 8 a 4 v he magnetic parameters. This material was sampled at different istances depending on the characteristics of the layers, the spac- ng varied between 30 and 60 cm from each other, the amount of aterial was about ½ kg. At the laboratory, each sample was air- ried, homogenized, and grounded in a mortar, for making them uitable for the measurement of magnetic parameters and chem- cal analysis. Additionally, 94 samples were collected at a vertical istance of 5 to 10 cm; the sediment was placed in plastic cylin- ers of 10 cm 3 for measuring the magnetic susceptibility. The chemical analyses were performed at LEMIT Laboratory. he samples were heated up to 110 °C during 24 hours to remove he moisture and were incinerated at 550 °C during 3 hours and he lost on ignition (LOI 550 °) was calculated. Afterwards, the chem- cal analysis of major elements was performed. This procedure was arried out by means of an X-ray fluorescence device, SPECTRO IQ I. Briefly, to a small portion of each sample (5 g), a binding mate- ial was added (BM-0 0 02, fluxana), in ratio 5:1 by weight, in or- er to prevent the movement of grains during the handling. Thus, t was possible to determine and quantify the presence of Ca, Na, , Fe, Al, Si and Mg, expressed as oxides. Figs. 9 and 10 indicate he percentages of LOI 550 °, CaO, Na 2 O, MgO, K 2 O, Al 2 O 3 , SiO 2 y e 2 O 3 . The percentage of organic matter (O.M.) on the recent soil, in ried specimens (110 °C), was also calculated ( Fig. 10 ). It was per- ormed by the wet combustion method of Walkey and Black [74] , onsisting in the oxidation by potassium dichromate in a sulfuric cid environment. The residual dichromate is titrated with a fer- ous salt (FeSO 4 • 7H 2 O). The correction factor used was 1.32, which orresponds to the organic carbon, to estimate the percentage of rganic matter. The values obtained are multiplied by the Van Be- melen factor (1.724). The magnetic parameters were measured at the Institute of hysics Arroyo Seco (IFAS). Prior to the measurements, the air- ried bulk samples were milled and placed in plastic boxes of cc., in order to fix the grains analytical sodium silicate was dded. The magnetic susceptibility was measured in low ( χ lf ; 70 Hz) and high frequency ( χhf ; 4700 Hz) by a MS Bartington de- ice with MS2B sensor and the frequency dependent susceptibility 72 M.L.G. Samus et al. / GeoResJ 13 (2017) 66–82 Fig. 7. Paleomagnetic profile and magnetozones in Sierra Bachicha. Intensity of NRM (J °), declination (D), inclination (I) and maximum angular deviation (MAD). w u p c m [F% = 100x( χ lf - χhf )/ χ lf ], was calculated. By the pARM device at- tached to the AF demagnetizer (100 mT), ARM records for 50μT and 90μT respectively were obtained and the χARM was calcu- lated. With a pulse magnetizer (AC Scientific model IM-10-30) and a magnetometer (minispin Molspin Ltd) the IRM acquisition curves ere obtained ( Fig. 11 ). Reaching the SIRM of the samples, the val- es of coercivity of remanence (H cr ) were also obtained by the ap- lication of reverse field, and the S-ratios (SIRM/IRM -300 ) were cal- ulated. Figs. 12 and 13 indicate the values of magnetic parameters easured represented versus depth. M.L.G. Samus et al. / GeoResJ 13 (2017) 66–82 73 Fig. 8. Paleomagnetic profile and magnetozone in Ramos Otero Section. Intensity of NRM (J °), declination (D), inclination (I) and maximum angular deviation (MAD). Fig. 9. Chemical analyses corresponding to the samples of Sierra Bachicha Section. w m P e 3 3 a Complementarily, magnetic concentrates of coarse silt fraction ere studied in the CINDECA laboratory by an electron-scanning icroscope (Philips SEM 505) with microsound system (EDAX DX RIME). Table 1 summarizes the rock magnetism data and the most rel- vant chemical parameters. . Analysis and interpretation of results .1. Paleomagnetism and magnetostratigraphy The ChRM was obtained after erasing the viscose components t fields between 2.5 and 10 mT ( Fig. 6 ). At higher AF applied 74 M.L.G. Samus et al. / GeoResJ 13 (2017) 66–82 Fig. 10. Chemical analyses corresponding to the samples of Ramos Otero Section. Fig. 11. IRM acquisition curves and back field of Sierra Bachicha and Ramos Otero samples. l - p t t d a b 3 l t o c h i s z 7 t c p w T t c m F ( h A S t o 1 C t h ( the decay of intensity was right towards the origin. The MAD val- ues were lower than 15 ° in most of the samples. The paleomag- netic analysis allowed obtaining normal and reverse polarity sam- ples that were referred to the international polarity zonation [30] . ( Figs. 7 and 8 ). The intensity of the natural remanent magnetization (J °) ranged between 3.2 and 102.2 × 10 −6 Am 2 /kg. The average value for the unit labeled L.U.1 was 41.7 × 10 −6 Am 2 /kg in Sierra Bachicha and 48.1 Am 2 /kg in Ramos Otero, while for the unit L.U.2 was 56 × 10 −6 Am 2 /kg and 34.8 × 10 −6 Am 2 /kg respectively. In the L.U.3 it was 36.7 × 10 −6 Am 2 /kg. The L.U.1 and L.U.2 units, in the two stratigraphic sections, showed normal polarity directions that were assigned to the Brunhes chron ( < 0.78 Ma). In the L.U.3 four polarity boundaries were defined, apparently without evidence of transitional sectors ( Fig. 7 ). The younger polarity change is recorded in the upper part of the unit, and is related to the middle of a paleosol (pedoge- netic cycle 3). According to available stratigraphic data, the mini- mum age for this polarity change should correspond to BMB (0.78 Ma). In addition, this paleosoil containing a paleomagnetic reversal could be correlated with the BMB paleosols considered as a guide horizon of regional significance, previously mentioned in the north of the Buenos Aires province [6,15,16,18,45] . According to this, the paleosoil with reverse polarity recorded in Balcarce, opens the pos- sibility of a huge correlation of polarity changes in the pampean region. A first approach to this is shown in Fig. 14 . Following this proposal, the other paleomagnetic boundaries are referred to Up- per Matuyama - Jaramillo (0.988 Ma) (beginning of pedogenetic cycle 4), the Jaramillo - middle Matuyama (1.077 Ma) should be re- ated to the pedogenetic cycle 5, and finally the middle Matuyama Olduvai (1.778 Ma) should be related to parent material of the edogenetic cycle 6. Consequently, the whole sequence exposed in he profile would have an age less than 1.945 Ma. As additional data, it is possible to calculate the rate of deposi- ion of unit L.U.3 considering the period comprised between Mid- le Matuyama-Olduvai and Brunhes-Matuyama (997 Ky) bound- ries, for a thickness of 5.65 m. By this way, the average rate would e on the order of 1.76 cm/Ka. .2. Geochemistry The LOI 550 ° presented the highest values in the samples col- ected from the Bt and A horizons of current soil ( Fig. 10 ) and in he 3Btkb horizon ( Fig. 9 ). The lower values correspond to samples f C horizons (see Table 1 ). The loss on ignition is a parameter onditioned by the O.M and also by the crystallographic water of ydrated minerals, mainly clays. The content of organic matter is mportant in the A and partially in the Bt horizons of the current oils ( Fig. 10 ). The other samples analyzed from the BC and C hori- ons showed very low values. On the basis of the colour (Value –3 and Chroma 3–4) and no reaction to H 2 O 2 , it can be inferred hat rest of the samples have negligible concentrations of O.M. From the chemical analysis carried out on several elements, it is lear that the calcium varies considerably in abundance along the rofiles. The highest values of CaO correspond to the L.U.2 unit; ith averages of 14.9% (Sierra Bachicha) and 17.6% (Ramos Otero). hese values correspond to levels with higher CaCO 3 concentra- ion. The rest of the elements present a minor variation than the orresponding to calcium carbonate. All of the chemical ele- ents were determined as oxides, clearly, Al and Fe (Al 2 O 3 and e 2 O 3 ) showed a pattern of behaviour conditioned by pedogenesis Table 1 ). The increasing occurs in almost all of the pedogenetic orizons, except in the A horizon of the recent soil. The highest l 2 O 3 content corresponds to the Bt horizon (L.U.1; Ramos Otero ection) ( Fig. 10 ) and the 3Btkb horizon (L.U.3; Sierra Bachicha Sec- ion) ( Fig. 9 ). The concentration of Al 2 O 3 increases noteworthy in B horizons f paleosols in the unit L.U.3, with an average enhancement of 4.8% [100 x ( �Al 2 O 3 B-Horizons - �Al 2 O 3 C –Horizons) / �Al 2 O 3 –Horizons] than the parent material (C horizons). Similarly, the otal Fe 2 O 3 presented the highest values in the B horizons; the ighest value corresponds to the paleosoil in the top of the L.U.3 3Btkb). The factor of average increasing iron in comparison to the M.L.G. Samus et al. / GeoResJ 13 (2017) 66–82 75 Fig. 12. Magnetic parameters of samples corresponding to the Sierra Bachicha Section. Fig. 13. Magnetic parameters of samples corresponding to the Ramos Otero Section. p t A L p c i S c o e h s s arent material is higher than for Al 2 O 3 (20%); on the other hand, he A horizon of the recent soil presents a low concentration of l 2 O 3 as Fe 2 O 3 . Fig. 15 shows bivariate plots between the Fe 2 O 3 , Al 2 O 3 and OI 550 ° of the all bulk samples, indicating the lithostratigraphic and edogenetic horizons, where it is possible to observe the positive orrelations between them. The increase in Al and Fe determined s in agreement with the one reported in previous works ( [26,63] ). uch increasing in paleosols would be linked to the illuviation pro- esses, and the corresponding eluviation of A horizon, with a loss f Al and Fe. Consequently the coefficient of correlation of these lements with the LOI550 ° -with the exception of samples with igh concentration of organic matter- was strongly positive to very trong. On the other hand, weathering [(Al 2 O 3 + Fe 2 O 3 )/(K 2 O + Na 2 O)], alinity [(K 2 O + Na 2 O)/(Al 2 O 3 /SiO 2 )] and clayeyness (Al 2 O 3 /SiO 2 ) 76 M.L.G. Samus et al. / GeoResJ 13 (2017) 66–82 T a b le 1 A v e ra g e , m in im u m a n d m a x im u m v a lu e s o f ch e m ic a l a n d ro ck m a g n e ti c a n a ly si s o f sa m p le s fr o m li to st ra ti g ra p h ic s a n d p e d o g e n e ti c u n it s o f Q u a te rn a ry se d im e n ts o f B a lc a rc e , A rg e n ti n a . L O I 5 5 0 ° A l 2 O 3 F e 2 O 3 χ lf χ fd % S IR M H c r S -r a ti o χ A R M χ A R M / χ lf χ A R M /S IR M S IR M / χ lf L .U .1 A h z v a lu e 8 .1 1 4 .3 5 .3 4 0 4 .2 4 .4 4 6 .7 3 5 .2 0 .9 5 2 5 17 .9 6 .2 5 3 .9 11 .6 B t h z v a lu e 6 .1 1 8 .6 7. 3 3 6 7. 6 4 .1 4 0 .3 3 7. 3 0 .9 5 17 7 7. 5 4 .8 4 4 .1 11 .0 C h zs av e ra g e 3 .1 1 5 .5 6 .1 3 5 1 .7 2 .4 4 6 .4 3 8 .6 0 .9 5 1 4 7 0 .3 4 .2 3 1 .6 1 3 .2 v a ri a ti o n 3 .0 -3 .2 1 5 .4 -1 5 .6 5 .9 -6 .2 3 3 0 -3 7 4 2 .2 -2 .6 4 4 .1 -4 8 .7 3 7. 6 -3 9 .5 0 .9 5 -0 .9 6 11 3 7 -1 6 0 3 4 .1 -4 .3 3 0 .4 -3 2 .9 1 3 .0 -1 3 .4 L .U .2 C k h zs av e ra g e 2 .9 1 2 .3 5 .8 2 7 4 .2 2 .5 4 3 .4 4 3 .2 0 .9 2 11 4 1 .8 4 .2 2 6 .8 1 5 .8 v a ri a ti o n 2 .3 -3 .5 1 0 -1 3 .6 5 .5 -6 .3 2 4 3 -3 2 0 1. 8 -3 .8 3 6 .2 -5 0 .1 3 5 .5 -4 5 .5 0 .9 0 -0 .9 3 8 9 7 -1 4 7 8 3 .7 -5 .9 2 3 .4 -4 0 .8 1 4 .4 -1 6 .7 L .U .3 B h zs av e ra g e 4 .3 1 6 .2 7. 5 3 3 7. 6 4 .4 4 0 .9 3 2 .8 0 .9 5 17 1 9 .7 5 .2 4 6 .9 1 2 .0 v a ri a ti o n 3 .0 -5 .7 1 4 .6 -1 7. 4 6 .9 -8 .6 2 6 6 -3 9 5 2 .3 -7 .1 2 3 .2 -5 3 .4 2 8 .7 -3 8 .3 0 .9 3 -0 .9 6 1 4 7 2 -2 3 2 3 4 .3 -7 .6 2 2 .7 -8 7. 2 8 .7 -1 5 .4 C h zs av e ra g e 3 .0 1 3 .8 6 .0 3 3 9 .7 2 .6 5 3 .9 3 8 .9 0 .9 3 1 5 1 2 4 .4 2 8 .2 1 5 .9 v a ri a ti o n 2 .5 -3 .6 1 0 -1 5 .5 5 .9 -6 .4 3 1 5 -3 6 4 1. 5 -3 5 0 .7 -5 6 .8 3 7. 1 -4 4 .8 0 .9 1 -0 .9 4 11 8 4 -1 7 8 1 3 .7 -5 .1 2 1. 1 -3 3 .5 1 4 .6 -1 7. 7 % % % 1 0 -8 K g /m 3 % 1 0 -3 A m 2 /K g m T a d im 1 0 -8 K g /m 3 a d im 1 0 -5 m /A k A /m Fig. 14. Magnetostratigraphic correlation of Sierra Bachicha Section with represen- tative magnetostratigraphic profiles in North of Buenos Aires and Atlantic coast. c h a R f g a a e t t t g u h e k b c i c t hemical indexes were calculated. Since the amounts of CaCO 3 are ighly variable, these relations were considered without calcium, nd these indexes were chosen taking into account the works of etallack [54] , Sheldon [64,65] , Adamova [1] and Buggle [22] . Fig. 16 shows the bivariate graphs combining various indexes or the samples corresponding to the lithostratigraphic and pedo- enetic horizons. Fig. 16 a shows the relation between weathering nd salinity. Almost all of the samples belonging to the paleosols re grouped towards the bottom right, indicating a greater weath- ring and a lower salinity. In agreement with the pointed above, he values corresponding to the A horizon depart from the rest of he values corresponding to the samples of paleosols. As stated, his is interpreted as a product of the eluviation process, which enerated the loss of some elements, mainly Al and Fe. The values of samples collected from L.U.2 tend to gather in the pper left sector which should indicate a minor weathering and a igher salinity. Fig. 16 b shows the kind of relation between weath- ring vs. clayeyness indexes; it is clearly direct, higher clay content eep relation with higher weathering rates. The highest values of oth indexes correspond to highest pedogenenized horizons (ex- ept A). Fig. 16 c shows the relationship between the clayeyness ndex vs. LOI 550 °C ; it should be pointed out the liaison between lay content and loss on ignition, i.e. the higher the clay content, he higher the loss on ignition. The influence of O.M is only no- M.L.G. Samus et al. / GeoResJ 13 (2017) 66–82 77 Fig. 15. Bivariate plots for Al 2 O 3 , Fe 2 O 3 and LOI 550 ° . (a) Fe 2 O 3 vs. Al 2 O 3 , (b) LOI 550 ° vs. Al 2 O 3 , (c) LOI 550 ° vs. Fe 2 O 3 . Fig. 16. Bivariate plots for different chemical indexes applied. (a) Salinitization vs. Weathering, (b) Clay + O.M. vs. Clayeyness, (c) Clayeyness vs. Weathering, (d) Clayeyness vs. Total iron. t v t t 3 a t r T t n m A d i o ( t able in the A horizon. Finally, in the correlation between total Iron s. clayeyness ( Fig. 16 d), the highest values of Fe 2 O 3 match with he greatest clayeyness, and the coefficient of correlation is posi- ive and very strong (R = 0.82). .3. Magnetic parameters The values of H cr (29–46 mT), S-ratio (0.92–0.96) as well s the shape of the curves of IRM acquisition, particularly for he saturation close to 30 0–40 0 mT ( Fig. 16 ) fit with the fer- imagnetic behaviour, but with a slight antiferromagnetic signal. he graph for the relation H cr vs SIRM/ χ lf according to Pe- ers and Dekkers [52] was also used for studying the mag- etic composition; hence, the data is indicating the dominance of agnetite-titanomagnetites-maghemite as carrier agents ( Fig. 17 ). ccordingly, the magnetic concentrates observations indicate the ominance of titanomagnetite in the silt fraction, with low Ti, as ndicated in the Fig. 18 . This is in agreement with the mineral- gy of loess and loess-like materials established by several authors [3–5,9–13,15–17,32,37,41,47,50,51,55,58,69,75] ). The grains present diameters smaller than 1 μm, according to he diagram χ lf vs χARM of King [38] ( Fig. 19 ); most of the values 78 M.L.G. Samus et al. / GeoResJ 13 (2017) 66–82 Fig. 17. Magnetic composition of samples according to Peters and Dekkers diagram. Fig. 18. SEM-EDAX image showing a titanomagnetite of representative sample of Quaternary loess of Balcarce. Fig. 19. Grain size analysis of samples analyzed according to King et al. (1982). o o m u q L l c i B p i g ( e m t c p y n t t a g t w m a C t h s s a d i 4 4 i p i a p m c t t p p t P w t t t v w f the samples fall around the straight of 0.2 μm, however, some f them would present diameters less than 0.1 μm. Table 1 shows the mean values and the range of variation of the agnetic parameters as function of paleosols and lithostratigraphic nits. The values recorded in the parent material (C horizons) were uite similar in the lithostratigraphic units, however, in the unit .U.2., the extensive parameters ( χ lf ; SIRM; χARM ) were somewhat ower, which seems to be due to the higher content of CaCO 3 . The records of the extensive parameters in paleosols showed a omplex behaviour. Susceptibility values ( χ lf ) showed a decrease n the 3Btkb, 4Bw and 5Btb horizons and an enhancement in A, t, 3Btb and 6Bwb. The SIRM values presented a better-defined attern, with a tendency to decreasing in B horizons and increas- ng in the A horizon. While χARM values tend to increase in pedo- enetic horizons, the highest value corresponds to the A horizon Fig. 13 ). The frequency dependent susceptibility ( χ fd% ) showed the low- st values in C horizons and the highest in paleosols; reaching a aximum in the 3Btkb horizon (7.1%). Owing to this, it is suitable o think that the soil forming process contributes with SP parti- les. In order to study the magnetic grain size we found appro- riate the use of inter-parametric relations χARM / χ lf ; χARM /SIRM SIRM/ χ lf ; applied as a recommended way to eliminate the mag- etic concentration effect. Such relations are widely used for de- ermining the magnetic fine grains, mainly SD particles and also he grains in the boundary SP-SD [18,24] . In agreement with the forementioned authors, the results indicated that the smaller the rains the larger the values of χARM / χ lf and χARM /SIRM, as well he lowest the SIRM/ χ lf values. Another parameter showing a particular behaviour in relation ith the pedogenesis in the studied area is the coercivity of re- anence. The highest H cr values corresponded to the C horizons nd the lowest were recorded in horizons affected by pedogenesis. onsequently, H cr values are also related to those horizons where here are decreasing grain size ( Fig. 20 ). A similar pattern of be- aviour was mentioned by Avramov [2] ; and also by Bartel [5] who tudied current soils in the South of Buenos Aires province. Fig. 21 hows the bivariate plots that link this parameter with the LOI 550 ° nd chemical indexes (weathering, the salinity and clayeyness in- exes). Clearly, the H cr values decrease when the weathering, salin- ty and clayeyness increase. . Discussion .1. Detrital magnetic signal and source areas The parameters measured and the chemical elements identified n the parent material, have a high degree of homogeneity. This is a oint to highlight, because, despite of the local reworking of orig- nal loess, it has not generated a great variation in the magnetic nd chemical composition. Moreover, taking into account the pro- osed ages, it is possible to establish that the source area and the agnetic composition of the parent materials have been relatively onstant during the last 1.9 Ma, or even from before, considering hat these are minimum ages. On the other hand, it is worth men- ioning that Zárate and Blasi [78,79] proposed that the wind-blown articles during the late Pleistocene came mainly from the alluvial lains of the Negro and Colorado rivers ( Fig. 22 ), but the particles ransported by these rivers came mainly from the Andean zone in atagonia. Supporting the interpretation of the South and South- est contribution of wind-blown magnetic particles, it is of value o consider the rock magnetic research performed in recent soils in he southern of pampean region [3,5] . The reported data indicate hat the parent materials (C horizon) showed lower susceptibility alues to the east (Necochea: 450 × 10 −8 m 3 /kg) and higher to the est (East of La Pampa: 600 × 10 −8 m 3 /kg); the latter is the sector M.L.G. Samus et al. / GeoResJ 13 (2017) 66–82 79 Fig. 20. Coercivity of remanence related to the magnetic grain size parameters. Fig. 21. Coercivity of remanence related to chemical indexes. o P t s i s C r p s w i i m a n c c f such transect closest to the source area of magnetic minerals in atagonia. With the exception of those levels with high CaCO 3 , the suscep- ibility values, of the parent materials of the Balcarce sedimentary equences covering the hills, are higher than 300 × 10 −8 m 3 /kg, be- ng the highest values close to 400 × 10 −8 m 3 /kg. These records are imilar to those obtained in the parent materials (siltstones) of the oastal cliffs in Centinela del Mar (near Miramar city), with values anging between 300 and 380 × 10 −8 m 3 /kg [10] . An important as- ect to be highlighted, considering the similarity in the magnetic ignal, is that the costal cliffs are integrated by successions of re- orked loess with distance of source very similar to Balcarce. Tak- ng into account the huge areal distribution of pampean loess, it s useful to compare the magnetic records obtained in the parent aterials of the different areas of the pampean region (see Fig. 22 ). For instance, in the locality of Corralito (Córdoba province) nd also in the North of Buenos Aires province the data of mag- etic susceptibility are lower than in the localities studied in Bal- arce ( Fig. 22 ). In Corralito, even though the locality studied is lose to the source area (the floodplain of Desaguadero river), the 80 M.L.G. Samus et al. / GeoResJ 13 (2017) 66–82 Fig. 22. Average magnetic susceptibility values in parent materials of different sites of south of South American loess plateau (Argentina). a c e t A a b T p c s s i w t w h r 5 i b e s p m n m t b c a g m l d f l t t p t c N p A L f v t R susceptibility values are lower than 300 × 10 −8 m 3 /kg, hence lower than Balcarce values. The highest records obtained in the same lo- cality were at the top, while the lowest were recorded at the base of the profile (100 × 10 −8 m 3 /kg), according to Rouzaut [57] . A plausible hypothesis is that the source area for Córdoba province contains originally fewer amount of titanomagnetites re- spect to the source areas located South of the 34–35 ° S, with a higher influence of basic volcanism, namely the olivinic alkaline basalts. These rocks are exposed in the South of Mendoza province, West of La Pampa and North of Neuquén provinces (see [42] ) ( Fig. 21 ), and were originated through several volcanic pulses in the retro-arc during the Cenozoic, especially from the late Miocene ( [25,31,34,37,43] ). They would have been eroded and transported mainly by fluvial action and re-deposited following the courses of the Negro and Colorado rivers ( Fig. 22 ). Conversely, the Corralito loess has been influenced by outcropping rocks of Sierras Pam- peanas, integrated mainly by granitoids and metamorphic rock of acidic composition [42,56] . As pointed above, in the North of the Buenos Aires province, the loess/loessoid deposits present lower susceptibility values than those obtained in the South of the province and the Balcarce area; the values are generally lower than 200 × 10 −8 m 3 /kg [13,15– 17,50,51] . This would be related to the distal position of the source rocks, as well as with different com position in this area. A simi- lar situation has been found in the loess of Entre Rios and Santa Fe province [6] , even further north, with lowest values than those ones of the North of Buenos Aires. 4.2. Magnetic parameters and pedogenesis It was not possible to establish a simple pattern of variation for the magnetic concentration parameters and the pedogenesis, as it was reported in other loess areas (e.g. [13] ). However, an enhance- ment in the recent soil should be mentioned, with the exception of the SIRM values that decrease in the Bt horizon. In paleosols the differences with the obtained records in the parent material re not noticeable, but in the most developed one, there is a de- reasing trend. Besides, the χARM values increase in particular lev- ls of paleosols (3Btkb). In contrast, the grain size parameters and heir interparametric relations presented a pattern of behaviour. ccording to the interparametric relations ( χARM / χ lf ; χARM /SIRM nd SIRM/ χ lf ) an increase of SD particles is interpreted, and possi- ly SP-SD boundary particles, would occur in the B and A horizons. he latter is in agreement with the increase of the frequency de- endent susceptibility factor, commonly applied for indicating the ontribution of SP particles. It is important to point out that the coercivity of remanence hows one of the most stable behaviours respect to the pedogene- is. The highest values of H cr correspond to the C horizon, indicat- ng a decrease towards the solum, consequently, the lowest values ere obtained in the B and A horizons. As mentioned in 3.3 sec- ion, this behaviour of H cr in relation with the horizons of soils as previously mentioned [2,5] having also relation with the en- ancement of the magnetic susceptibility in the solum which was eferred to the presence of fine to ultrafine maghemite. . Summary and conclusions Younger units labeled as L.U.1 and L.U.2 were deposited dur- ng the Brunhes chron ( < 0.781 Ma). Meanwhile, L.U.3 would have een deposited mainly during Matuyama (0.781 - 2.588 Ma). Old- st sediments showing normal polarity directions have been as- igned to Olduvai (1.778 – 1.945 Ma). The BMB was found in a aleosoil; this allows establishing regional correlations. The magnetic signal is homogeneous. According to the rock agnetic data, the main carriers of remanence are ferrimag- etic minerals as titanomagnetites with a probable contribution of aghemite in paleosols. The magnetic grain size would correspond o the SD grains; an increase of fine to ultrafine particles has also een registered in relation to the pedogenetic horizons. The coer- ivity of remanence (H cr ) shows the best-defined behaviour among ll applied parameters. The lowest records are related to the pedo- enesis, where the magnetic grain size decreases. The magnetic composition and the concentration in the parent aterial seem to have been relatively constant during at least the ast 1.9 Ma. The latter also should indicate that the source area id not change significantly during this time. As an approximation or building a regional magnetic model, it is important to high- ight that the magnetic parameters, mainly the susceptibility, seem o have been conditioned by large scale factors; such as sedimen- ation mechanisms, the different source rocks, and the distance of rovenance close to the Cordillera de los Andes. In agreement with hat, it should be pointed out that the sedimentary deposits of Bal- arce show higher susceptibility values than those located to the orth but lower than the others located to the South, in the pam- ean region. cknowledgments This work was carried out with the financial support of the EMIT, CIC and CONICET. The authors are grateful to IFAS-UNICEN or allowing the use of its facilities and equipment, and to the re- iewers whose comments and suggestions significantly improved he quality of this study. eferences [1] Adamova M , Havlicek P , Sibrava V . Mineralogy and geochemistry of loesses in southern Moravia. Bull Czech Geol Surv 2002;77:29–41 . 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