Accepted Manuscript
Pennsylvanian glacimarine sedimentation in the Cushamen Formation, western North Patagonian Massif
Paulo Marcos, Daniel A. Gregori, Leonardo Benedini, Mercedes Barros, Leonardo Strazzere, Cecilia Pavón Pivetta
PII: S1674-9871(17)30101-9 DOI: 10.1016/j.gsf.2017.05.005 Reference: GSF 567
To appear in: Geoscience Frontiers
Received Date: 20 October 2016 Revised Date: 4 May 2017 Accepted Date: 21 May 2017
Please cite this article as: Marcos, P., Gregori, D.A., Benedini, L., Barros, M., Strazzere, L., Pavón Pivetta, C., Pennsylvanian glacimarine sedimentation in the Cushamen Formation, western North Patagonian Massif, Geoscience Frontiers (2017), doi: 10.1016/j.gsf.2017.05.005.
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MANUSCRIPT
ACCEPTED ACCEPTED MANUSCRIPT
Pennsylvanian glacimarine sedimentation in the Cushamen Formation, western
North Patagonian Massif
Paulo Marcos a,* , Daniel A. Gregori b, Leonardo Benedini b, Mercedes Barros b,
Leonardo Strazzere c, Cecilia Pavón Pivetta a
a Cátedra de Geofísica, Departamento de Geología, Universidad Nacional del Sur and INGEOSUR, San Juan 670, 8000 Bahía Blanca, Argentina. b Cátedra de Geología Argentina, Departamento de Geología, Universidad
Nacional del Sur and INGEOSUR, San Juan 670, 8000 Bahía Blanca, Argentina. c Cátedra de Geología Minera, Departamento MANUSCRIPT de Geología, Universidad Nacional del Sur and INGEOSUR, San Juan 670, 8000 Bahía Blanca, Argentina.
*Coresponding author e-mail address: [email protected]; [email protected]
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ACCEPTED MANUSCRIPT
Abstract
The metasedimentary sequence of the Cushamen Formation in the western North
Patagonian Massif is 540 m thick and comprises six sedimentary lithofacies associations related to a glacimarine environment. Four of these lithofacies represent distal glacimarine environments, whereas another one was deposited in proximal glacimarine environments, and the last includes subglacial environments.
The organization and configuration of these lithofacies associations represent the advance and retreat of the glacier masses. The maximum glacial advance is correlatable with the G2 glacial interval of the Pennsylvanian Pampa de Tepuel,
Las Salinas and Valley Chico, formations of the Extraandean Chubut, and the southern part of Neuquén Cordillera. ContemporaneouMANUSCRIPTsly, in southern Chile there are marine and glacimarine sediments. The chronostratigraphic relationships between the Silurian to Permian units allow five paleogeographic stages to be distinguished. The middle Silurian–late Devonian igneous rocks represent the first magmatic stage. The second stage, which is transitional to the first, is represented by a marine basin that includes the late Devonian–early Carboniferous Esquel and
Río Pescado formations and the Llanquihue Complex. The third stage (early–late
Carboniferous) includes granitoids of the second magmatic event that partially overlapped theACCEPTED first magmatic igneous belt. The fourth stage belongs to the late Carboniferous sedimentation of the Cushamen and equivalent formations. The extended early Permian magmatism was the last Paleozoic event in the studied area. ACCEPTED MANUSCRIPT
Key words: Pennsylvanian, glacimarine sedimentation, Cushamen Formation,
North Patagonian Massif.
1. Introduction
In the western corner of the North Patagonian Massif, the metamorphic rocks are named the Cushamen Formation, which is composed of metasediments and metavolcanites of low to medium metamorphic degree according to Volkheimer
(1964), Ravazzoli and Sesana (1977), Cerredo and López de Luchi (1998, 1999)
(Fig.1). The outcrops of this unit have a scattered distribution within an elongated belt in a N–S direction between Paso Flores MANUSCRIPT(Río Negro province) and Gualjaina (Chubut province). In the southern part of the belt, the outcrops are located near
Gualjaina, Cushamen, and Río Chico localities, whereas in the northern part, outcrops are continuous between Zimmermann Resta and Paso Flores (Fig. 1).
The analysis of the sedimentary protolith facies of Cushamen Formation is important to understand the processes and sedimentary environments involved during its sedimentation stages. Also, these analyses allow for understand the evolution of the sedimentary basins along the middle to late Paleozoic times. However, littleACCEPTED is known about the Cushamen Formation in the north part of the belt. There are several studies related to the metamorphic, structural, and geochronological characteristics of the rocks located in the southern part of the belt
(Volkheimer and Lage, 1981; Dalla Salda et al., 1994; Cerredo and López de ACCEPTED MANUSCRIPT
Luchi, 1998; Ostera et al., 2001; Duarth et al., 2002; Hervé et al., 2005). On the other hand, previous studies of the north belt of the basement are related only to regional structural and metamorphic characteristics (Nullo, 1979; Gonzalez et al.,
2003; von Gosen, 2009). At this time, the protolith settings and the arrangement of facies in the northern outcrops are unknown. Also, the stratigraphic and paleogeographic relationships with other units of the basement in the southwest corner of Gondwana have not yet been studied.
The aim of this paper is to characterize the sedimentary lithofacies of the
Cushamen Formation in the Comallo–Neneo Ruca area to determine the processes and sedimentary environments that were active during its deposition.
A second aim is to define the temporal range of the unit and to establish correlations with units of the North PatagonianMANUSCRIPT Massif, Extraandean Chubut, Neuquén Cordillera, and metamorphic complexes of southern Chile.
In order to produce a coherent model for the development of the middle–late
Paleozoic marine basins on the southwest edge of Gondwana, we also analyze the temporal and spatial arrangement of the Silurian to Permian units of this region.
2. Geological setting of the Paleozoic units
In the region,ACCEPTED between 39° and 44°S and between the Atlantic and Pacific Oceans, the basement of the continent of Gondwana, (Neoproterozoic to Permian), crops ACCEPTED MANUSCRIPT
out in the North Patagonian Massif, the Neuquén Cordillera, the Extraandean
Chubut, and southern Chile (Fig. 1).
The oldest units are located in the northeastern sector of the North Patagonian
Massif (Fig. 1). In this region there are slates, phyllites, metagreywackes, gneisses, and marbles of Ediacaran–Cambrian age, that form the Mina Gonzalito Complex and the El Jaguelito and Nahuel Niyeu formations (Varela et al., 1997, 1998; Basei et al., 2002; Gonzalez et al., 2002) (Fig.1). In the central sector of the North
Patagonian Massif (Fig. 1), foliated leucogranites, shales, amphibolites, gneisses, and marbles are part of the Yaminué Complex. Cambrian to Ordovician ages were obtained in granitoids of this complex by Rapalini et al. (2013) and Pankhurst et al.
(2014).
The Silurian to Devonian igneous-metamorphicMANUSCRIPT units are widely distributed in southern Chile, the Neuquén Cordillera, and the western part of the North
Patagonian Massif (Fig. 1).
In southern Chile, the Devonian magmatism is located near Chaitén (Duarth et al.,
2009; Hervé et al., 2016). There are tonalities with crystallization ages of 388 ± 6
Ma (Duarth et al., 2009) and 384 ± 3 Ma (Hervé et al., 2016). In Pichicolo, 100 km to the north of Chaiten, Hervé et al. (2016) obtained an age of 383 ± 2 Ma for zircons of tonalities (Fig. 1). Leucocratic intrusives near Chaiten and Yelcho lake have zircon agesACCEPTED between ca. 364 ± 2 and 361 ± 7 Ma (Hervé et al., 2016). In this zone there are also metapsammites and metapelites, belonging to the Llanquihue
Complex, with borehole samples that carry detrital zircons with U/Pb Devonian ACCEPTED MANUSCRIPT
ages (ca. 385 Ma). In addition, the metasedimentary rocks of the Trafun and Bahía
Mansa Complexes carry early Carboniferous detrital zircons with ages between ca.
355 and 330 Ma (Hervé et al., 2016) (Fig. 1).
Within the Neuquén Cordillera, the igneous-metamorphic rocks cropping out north of the Limay river were considered to belong to the Colohuincul Complex.
Geochronological studies of these rocks were conducted by Varela et al. (2005),
Pankhurst et al. (2006) and Hervé et al. (2016) who obtained U/Pb zircon ages of
419 ± 27 Ma and 395 ± 4 Ma in granitoids near San Martín de los Andes and 374 ±
3 Ma in a granodiorite near Curruhué Chico lake. Martinez et al. (2012) established a metamorphic peaks at 392 ± 4 Ma in migmatites outcrops near San Carlos de
Bariloche. Serra Varela et al., 2016, obtained the maximum sedimentation age ca. 506 ± 12 Ma in detrital zircon of the metasedimentsMANUSCRIPT part corresponding of the Colohuincul Complex.
In the western part of the North Patagonian Massif, prior to the radiometric dating, several authors assigned a Precambrian age to the basement rocks, whereas others considered Precambrian–middle Paleozoic ages (Nullo, 1979, and
Volkheimer and Lage, 1981). The oldest ages recorded in the igneous- metamorphic rocks of this area are Silurian to Devonian (U/Pb zircons ages: ca.
425 to 387 Ma) and were obtained by Varela et al. (2005) in granitoids near Sañicó. In theACCEPTED southwestern part of the Massif, Devonian ages (ca. 394 ± 4 to 371 ± 2 Ma) were obtained by Pankhurst et al. (2006) in Gastre and Colan Conhué
(Fig. 1). ACCEPTED MANUSCRIPT
The Cushamen Formation consists of metapelites, metasandstones, metagreywakes, metadiamictites, metavolcanites, schists, gneisses, and migmatites (Volkheimer, 1964; Ravazzoli and Sesana, 1977; Nullo, 1979;
Volkheimer and Lage, 1981; Dalla Salda et al., 1994; Cerredo and López de Luchi,
1999; Duarth et al., 2002). The rocks are affected by greenschist–amphibolite metamorphism and four deformation events (D1–D4) (Cerredo and López de
Luchi, 1998). The previous studies about the age of sedimentation and metamorphic events of Cushamen Formation recorded different time intervals.
Ravazzoli and Sesana (1977) conducted the first K/Ar dating and obtained a
Pennsylvanian age (ca. 300 Ma) in micacites. Later, using Rb/Sr, Ostera et al.
(2001) obtained late Devonian ages (371 ± 33 and 362 ± 10 Ma) for the age of the metamorphism in metagreywackes located in the province of Chubut. Finally, a Visean maximum age of sedimentation is considered MANUSCRIPT based on U/Pb ages in detrital zircon obtained by Hervé et al. (2005).
The Permian magmatic event is made up of a series of S and I type granites, widely distributed along the western North Patagonian Massif (Fig. 1) (Pankhurst et al., 2006). These bodies assigned to the Mamil Choique Formation intruded the
Cushamen Formation (Figs. 1 and 2). Studies carried out by Varela et al. (2005) established that the ages of the granites south of Limay River are between ca. 286 ± 13 and 272ACCEPTED ± 2 Ma. The geochemistry of these rocks indicated a volcanic-arc setting for their emplacement according to Varela et al. (2015). Other units in this area are Mesozoic–Cenozoic volcanic and sedimentary rocks that cover the
Paleozoic rocks unconformably (Fig. 1). ACCEPTED MANUSCRIPT
3. Methods and terminology
The studied profile is located at 41°06'S between 7 0°23'W and 70°24'W, where the
Cushamen Formation is very well exposed. A few granitic and pegmatitic dikes cut the sequence (Fig. 2). The studied profile is 1 km long and follows the railway track and a ravine tributary of the Arroyo Comallo (Fig. 2).
The terminology of the lithofacies follows the classification codes of Eyles et al.
(1983) and Miall (1996). However, there are no specific codes for the greywackes and sandstones with hummocky cross-stratified lithofacies, so we added a specific nomenclature for these lithofacies (Table 1). The 2016 chronostratigraphic chart version was used for time of sedimentation and intrusion of the units (Cohen et al.,
2013).
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4. Results
The Cushamen Formation constitutes a sequence 540 m thick with a stratification predominantly N327°/30° SW and sometimes N15°/30° N W in the studied area.
Along the profile, the lithological characteristics remain homogeneous.
Metamorphic minerals are quartz, biotite, muscovite, and garnet. The incipient orientation of the muscovites and biotites mimetically copies the original stratification ACCEPTEDof the sedimentary rocks. Some quartz grains exhibit recrystallization textures with development of subgrains. Garnet crystals are mostly fractured and present a few inclusions of feldspar and quartz without preferential orientation. ACCEPTED MANUSCRIPT
Due to the fact that the metamorphic overprint is very soft, it is possible to distinguish the different sedimentary structures and lithologies that form this sequence. Using textural features and sedimentary structures, we can recognize eight lithofacies in the studied profile.
4.1. Lithofacies
4.1.1 Conglomerate and diamictite lithofacies
These lithofacies are formed of clasts of varying size between granules and boulders immersed in a sandy matrix. Clasts vary in size between 8 mm and 5 m
(Fig. 3a and c). These lithofacies represent the maximum grain size recorded and show high variability when compared with otherMANUSCRIPT lithofacies. The level of selection of the clasts, the variability in size and the massive or layered appearance of the outcrops allow two sub-lithofacies to be differentiated.
4.1.1.1 Lithofacies A: Massive matrix support diamictites (Dmm)
This lithofacies is composed of clasts of granitoid immersed in a poorly stratified matrix (Fig. 3a). The clasts, 5 cm to 5 m long, form 10% of the diamictite deposit. The frequencyACCEPTED and average size of the clasts allow them to be divided into three groups. The smaller ones are between 5 and 20 cm. Clasts between 40 and 70 cm are less abundant, and boulders between 2 and 5 m are scarce. ACCEPTED MANUSCRIPT
Coinciding with the criteria employed by Eyles et al. (1983) for the characterization of massive diamictite facies (Dm), this lithofacies shows stratification in less than
10 % of the sequence (Fig. 3a). The deformational features observed in the matrix are developed close to the boulders. This deformation consists of conical fold series up to 25 m long (Fig. 3b). This lithofacies appears between 230 and 250 m above the base of the profile (Fig. 2b).
4.1.1.2 Lithofacies B: Conglomerates (Gcs)
The conglomerates are composed of clasts of K-feldspar, plagioclase, and minor quartz in a stratified arrangement (Figs. 2b and 3c). The clasts are spherical to ovoid with sizes in the range of granule to pebble. The matrix is composed of quartz and K-feldspar grains < 1mm (Fig. 3c MANUSCRIPT and d). These lithofacies are represented by two banks located between 150 and 170 m from the base of the profile (Fig. 2b). On the upper surface of the banks appear several lineations, which are interpreted as sole marks, such as longitudinal ridges and furrows (Fig.
3d). Measuring these lineations provides paleocurrent directions near to N 80° E
(Fig. 3d).
4.1.2. SandstoneACCEPTED lithofacies
This lithofacies includes sandstones with grain sizes that vary between 0.2 and 2 mm and greywackes with clasts up to 5 cm long as well as variable quantities of ACCEPTED MANUSCRIPT
quartz, K-feldspar, and biotite. In matrix-rich sandstones, there are abundant biotites distributed between the clasts of plagioclase, K-feldspar, and quartz. The
K-feldspar-rich sandstones are formed by orthoclase, plagioclase, and quartz with limited proportions of matrix (Fig. 4a). The following division into sub-lithofacies was performed based on the granulometric variations the recognition of sedimentary stratified structures.
4.1.2.1 Lithofacies C: Parallel stratified sandstones (Sh)
This lithofacies is constituted mainly by matrix-rich sandstones and minor quantities of K-feldspar-rich sandstones with abundant clasts of K-feldspar arranged in stratified banks 30 to 50 cm thick (Fig. 4a). Muscovites, biotites, quartz and feldspars imprint a weak foliation mimetic MANUSCRIPT to the stratification (Fig. 4a). The frequency of the parallel stratified, matrix-rich sandstone along the profile is considerably higher than that of the K-feldspar sandstones (Fig. 2b).
4.1.2.2 Lithofacies D: Cross-stratified sandstones (St)
This lithofacies includes matrix-rich sandstones with festoon structures that are concave upwards with an erosive base. Toward the top, these concave structures tend to horizontalizeACCEPTED (Fig. 4b).These structures form isolated lenticular bodies (Fig.
2b). A clearly defined lenticular body of 1.5 m thick and 10 m long appears at 170 ACCEPTED MANUSCRIPT
m from the base of the profile (Fig. 4b). Two other lenticular bodies about 3 m thick appear at 260 m from the base of the profile, but the structure is not complete.
4.1.2.3 Lithofacies E: Hummocky-type cross-stratified sandstones (Hcs)
This lithofacies consists of matrix-rich sandstones arranged in layers 20 to 40cm thick (Fig. 4d). The structure is 2 m long (Fig. 2b) and is located 300 m from the base of the profile. At the top of the sequence, these structures are 40 cm long.
4.1.2.4. Lithofacies F: Parallel-stratified greywackes (Swh).
This lithofacies contains subspherical to subrounded crystals of K-feldspar and clasts of quartz with grain size between pebbles MANUSCRIPT and granules, dispersed in a fine- grained matrix (Fig. 4 c). This lithofacies forms beds 10 cm to 1 m thick that are more common at the base and top of the profile, whereas in the middle part, there are few stratified levels of greywackes with thickness of less than 10 cm. Clasts of
K-feldspar are more abundant than the quartz and are 0.7 to 1 cm in size in the lower part. On the other hand, there are clasts 0.4 to 5 cm in size in the upper part of the profile.
ACCEPTED
4.1.3 Pelite lithofacies ACCEPTED MANUSCRIPT
This lithofacies includes a pelite laminated levels with strong developing of micaceous foliation. The muscovite and biotite are oriented parallel to the stratified sequence and compound around the 80% and 90% of the minerals in this lithofacies. Quartz and K-feldspar are located between the micaceous foliations.
The pelite lithofacies is divided in two sub-lithofacies taken to account the presence or inexistence of blocks.
4.1.3.1 Lithofacies G: Pelites with dropstones (Fld)
This lithofacies is defined by the presence of 0.30 to 2 m long elliptical or subspherical blocks of granitoids scattered in a stratified pelite matrix (Fig. 5a). The blocks in this lithofacies are isolated, forming less than the 1% of these deposits. This lithofacies appears in two sectors of the MANUSCRIPT profile. One is located between 190 and 230 m from the base of the profile (Fig. 2b), and the other one, which is 3 m thick, at 250 m.
4.1.3.2 Lithofacies H: Pelites (Fl)
This lithofacies presents a marked millimetric foliation (Fig. 5b), formed mostly by micaceous minerals that are mimetic of the stratification. This lithofacies appears at the base of theACCEPTED profile, forming 40 cm thick strata. However, the best expositions are located 130 and 190 m above the base of the profile (Fig. 2b). In this part, the pelites form banks more than 40 cm thick. In the middle and upper parts of the ACCEPTED MANUSCRIPT
profile, this lithofacies is limited to a few banks with similar thickness to those located at the base of the profile.
4.2. Lithofacies associations
The distribution of the lithofacies along the profile indicates that it can be divided into three distinct sections. The lower and upper sections are constituted by sandstones, greywackes, and pelites, while the middle one includes diamictites and sandstones (Figs. 2 and 6).
Six lithofacies associations were established through the stratigraphic relationship between different lithofacies. These relationships are transitional and indicated by the disappearance or appearance of one or several lithofacies. MANUSCRIPT
4.2.1 Lithofacies association AFA: Parallel stratified greywackes, sandstones, and pelites (Swh–Sh–Fl)
This lithofacies association is composed mostly by greywackes and sandstones with minor quantities of pelites (Fig. 6) arranged as concordant stratified banks.
The arrangement of these banks shows grain and strata fining-upwards successions,ACCEPTED 5 and 8 m thick. The first 90 m of the profile is constituted by the lithofacies association AFA, transitionally passing to sandstones and greywackes corresponding to the lithofacies association BFA (Fig. 6). To the west, the lithofacies association AFA constitutes the top of the sequence, between 485 and ACCEPTED MANUSCRIPT
540 m from the base of the profile (Fig. 6). At that point, the lithofacies association
AFA is distinguished from the same lithofacies of the lower section by the greater predominance of greywackes and major variations in grain sizes.
4.2.2 Lithofacies association BFA: Parallel stratified sandstone and greywackes
(Sh–Swh)
The BFA association is composed of sandstone with interbedded greywackes in similar proportions (Fig. 6). They form sets with concordant tops and bases, 45 m thick in the middle part of the lower section. At 135 m from the base of the profile, the BFA passes transitionally to banks of pelites corresponding to the CFA lithofacies association. MANUSCRIPT In the upper section of the profile, 390 m from the base, the BFA association forms a sedimentary package 95 m thick (Fig. 6). The greywackes begin to dominate above 485 m in the profile, where the transition between the AFA and BFA lithofacies associations appears.
4.2.3 Lithofacies association CFA: Parallel stratified pelites, sandstones, and conglomeratesACCEPTED (Fl–Sh–Gcm) This association is composed mostly of pelites, low proportions of sandstone, and only two levels of conglomerates, forming a stratified sequence with concordant surfaces (Fig. 6). The CFA is located in the upper part of the lower section of the ACCEPTED MANUSCRIPT
profile, between 135 and 190 m (Fig. 6), concordantly on the BFA. Its upper limit is determined by the presence of cross-stratified sandstone of the DFA association.
4.2.4 Lithofacies association DFA: Parallel and cross-stratified sandstones and pelites (Sh–St–Fl–Fld)
This association is mainly composed of sandstones and minor pelites located 250 m from the base of the profile, above the association EFA. The sandstones are commonly arranged in banks with parallel stratification, and only in places with cross-stratification (Figs. 4b and 6). This lithofacies association is 40 m thick at the base of the middle section (Fig. 6). In this part of the profile, the succession is made up, at the base, of cross-stratified sandstone lateral to pelites with dropstones. Upwards there are parallel stratified MANUSCRIPT sandstones and isolated levels of pelites with dropstones.
This lithofacies association forms a sequence 50 m thick at the top of the middle section (Fig. 6). Unlike the base of this section, this sequence starts with pelites with dropstones, followed by cross- and parallel-stratified sandstones and finally parallel-stratified pelites.
4.2.5. LithofaciesACCEPTED association EFA: Massive diamictites and parallel-stratified sandstones (Dmm–Sh) ACCEPTED MANUSCRIPT
This association is constituted mostly by massive diamictites with interbedded stratified sandstones and is located between the two sequences corresponding to lithofacies association DFA (Fig. 6).
The contrasting textural features between associations DFA (Fld) and EFA (Dmm) are the grain sizes and abundance of the clasts. These differences show a coarsening pattern at 230 m from the base of the profile, associated with the transition between the associations DFA and EFA. On the other hand, 250 m from the base of the profile, appear fining-upwards sequences from massive diamictites to pelites with dropstones of the association DFA.
4.2.6. Lithofacies association FFA: Parallel and hummocky-type stratified greywackes, pelites, and sandstones (Swh–Fl–Sh–Hcs) MANUSCRIPT.
The FFA association is composed of sandstone, greywackes, and pelites that are arranged in stratified banks. In this association, the sandstones are predominant, followed by greywackes and pelites (Fig. 6). At 300 m from the base of the profile, the pelites and sandstones of the association DFA pass transitionally to levels of sandstones with hummocky-type structures of association FFA (Figs. 4d and 6).
This association also appears on the base of the upper section, forming a 90 m thick sequenceACCEPTED that transitionally passes to sandstones and greywackes of association BFA.
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5. Discussion
5.1. Interpretation of lithofacies association
5.1.1. Lithofacies associations AFA–BFA–CFA–FFA
The limit between associations AFA and BFA is transitional and marked by the appearance or disappearance of pelites. Both associations form sequences 135 m thick in the lower section and 150 m thick in the upper section. The fining-upwards pattern of the associations is assigned to multi-episodic events of detrital materials supplied by turbulent currents. In these currents, detrital coarse-grained fractions are mobilized on the substrate (Swh–Sh), while the fine material (Fl) forms a buoyant plume (Eyles et al., 1985; Powell and Molnia, 1989; Lonne, 1995; Visser,
1997). The abundance of pelites in the association CFA is related to lower energy stages of sedimentation. This fact was used MANUSCRIPTby Powell and Molnia (1989) and Visser (1997) to explain the presence of fine-grained facies in glacial deposits.
From the characteristics listed above, it can be established that the AFA, BFA, and
CFA lithofacies associations form debris flow deposits (Fig. 7).
The hummocky-type stratified sandstones of the association FFA are indicative of flows related to storm episodes and the processes of remobilization and redeposition, as was indicated by Le Heron et al. (2005) for Ordovician glacimarine deposits locatedACCEPTED in northwestern Africa. Several studies emphasize the abundance of episodes of redeposition in glacimarine environments (Powell and Molnia, 1989;
Eyles et al., 1985; Aitchison et al., 1988; Hart and Roberts, 1994; Lonne, 1995). ACCEPTED MANUSCRIPT
The debris materials that form the sedimentary successions of the associations
AFA, BFA, CFA, and FFA were probably derived by melting of glaciers. This link was established by Powell and Molnia (1989) in modern glacimarine environments and in the Dwyka Group (Carboniferous–Permian) glacial deposits in the Karoo–
Kalahari basin of Africa (Visser, 1997).The ablation that drives the melt-out streams can be derived from subglacial, englacial, or supraglacial sectors, as established by Powell and Molnia(1989) and Lonne (1995) (Fig. 7).
The lithofacies associations AFA, BFA, CFA, and FFA are comparable with periglacial facies of marine deltaic proglacial (II-E-1) and extraglacial (II-F) environments characterized by Brodzikowski and Van Loon (1987). On the other hand, the sedimentary processes assigned to these lithofacies associations can be correlated with current examples of proglacialMANUSCRIPT glacimarine and distal paraglacial environments observed by Powell and Molnia (1989). In addition, sedimentary structures associated with storm deposits of Ordovician sandstones located in western Africa were interpreted by Ghienne (2003) and Le Heron et al. (2005) as distal glacimarine facies. Therefore, we suggest that the sedimentary succession of the lower and upper sections of the studied profile was developed in distal glacimarine environments (Fig. 7).
5.1.2 LithofaciesACCEPTED association DFA
This association, represented by isolated granitic blocks and cross-stratified sandstones, appears in the middle section of the profile and represents extra- ACCEPTED MANUSCRIPT
basinal rocks transported as supraglacial material (Fig. 7). The isolated distribution of granitic blocks is associated with sporadic discharge of detritus generated by melting of icebergs. A similar relationship was established by Hart and Roberts
(1994) in glacimarine Quaternary deposits located in West Runton, England.
Likewise, Eyles et al. (1985) recognized that isolated clasts within fine facies of open marine environments come from debris falling from icebergs. No striated clasts were found in this area.
Besides, the cross-stratified sandstones of lithofacies St possibly represent systems of underwater paleo-channels (Figs. 4b and 7). These paleo-channels are used for the material transported by unidirectional melt-out flows, which probably comes from the subglacial area (Powell and Molnia, 1989; Brodzikowski and Van Loon, 1987). The resedimentation of the detritusMANUSCRIPT possibly occurred during the development of these channelized systems, as indicated by Brodzikowski and Van Loon (1987) during their analysis of glacimarine facies. It is clear that the presence of detrital fragments of block size denotes the progradation of proximal glacial environments in relation to the distal glacimarine environment (CFA association). In addition, the studies carried out by Aitchison et al. (1988) in diamictite deposits of the Buckeye Formation in the region of Ohio, Antarctica, indicate that the frequency and size of the clasts are higher when the glacial mass is closer to the deposition area.ACCEPTED The association DFA is transitional to the EFA association and defines a coarsening-upward pattern that is related to gradual advance of the glacier masses ACCEPTED MANUSCRIPT
towards the depositional area. In addition, fining-upwards sequences from EFA to
DFA represent a retreat of the ice towards the continent.
According to the facies classification proposed by Brodzikowski and Van Loon
(1987), the DFA lithofacies association represents a terminoglacial marine sub- environment (II-D), where the deposits derived from the melting of the iceberg are more frequent than in distal glacimarine areas. The presence of paleo-channels of the lithofacies St can be linked to tunnel-mouth (II-D-2-b) environments of the glacimarine deposit described by Brodzikowski and Van Loon (1987). From the stratigraphic relationships, sedimentary processes, and comparisons with previous works on glacial deposits, it may be considered that the DFA lithofacies association belongs to a proximal glacimarine environment (Fig. 7).
MANUSCRIPT 5.1.3 Lithofacies association EFA
As indicated above, at 230 m from the base of the profile, the association DFA of a proximal glacimarine environment transitionally passes to EFA (Fig. 6). The abundance and diversity of block sizes suggest a still closer location of the glacial masses relative to the underlying DFA association. This deduction is coincident with the analyses carried out by Frakes and Crowell (1969), Aitchison et al. (1988), Eyles et al. (1985),ACCEPTED Powell and Cooper (2002), and Lang et al. (2012) in different glacimarine deposits. In addition, we considered the deformation of the lithofacies association EFA to be related to glaciotectonic deformation, where the glacial mass was in contact with the unconsolidated substrate. ACCEPTED MANUSCRIPT
5.1.3.1 Deformation inside the lithofacies association EFA
As indicated in section 3.1.1.1, the lithofacies A contains irregular and conical folds
(Fig.8). There is a clear relationship between the conical folds and the presence of meter sized blocks. Note that the deformation produces folding of the stratified sandy matrix near the biggest blocks, but these bodies are undeformed (Figs. 3a and 3b). The distribution and dimension of these folds are comparable with those located in the Pampa de Tepuel Formation (Pennsylvanian) recorded by González
Bonorino et al. (1988). These autors suggest that these features are related to a synsedimentary deformation coetaneous with the glacial advance. These characteristics are similar to the folding located in the study area (Figs. 3b and 8b). A similar origin and process were suggestedMANUSCRIPT by Ghienne (2003) for tight folds observed in an Ordovician glacial sequence located in the northwestern part of
Africa. Le Heron et al. (2005) suggested a subglacial deformation for the same deposit.
Hart and Boulton (1991), Roberts and Hart (2005), and Waller et al. (2011) described similar behaviors in diamictites affected by glaciotectonic deformation in
Quaternary deposits. One of the most studied examples of diamictite deposits with evidence of glaciotectonic deformation is located in West Runton, England (Hart and Boulton,ACCEPTED 1991; Hart and Roberts, 1994; Roberts and Hart, 2005). There, Hart and Boulton (1991) established that the folds with conical geometry in diamictite deposits are associated with subglacial deformation. The dimensions of folds with ACCEPTED MANUSCRIPT
conical geometry in the Cushamen Formation are comparable with those observed by Hart and Roberts (1994) and Roberts and Hart (2005) in the folded West
Runton diamictites.
All the features described above, as well as comparisons with other ancient glacial deposits, allow it to be established that the lithofacies association EAF was affected by glaciotectonic deformation. This type of deformation may develop in proglacial or subglacial environments (Hart and Boulton, 1991). However, the conical geometry, dimensions, and design of the folds are rather comparable with the subglacial deformation described by Hart and Boulton (1991) and Le Heron et al. (2005). Therefore, the analysis of the lithofacies association EFA allows it to be considered that this association was deposited in a subglacial environment (Fig. 7).
MANUSCRIPT 5.3. Paleoenvironmental evolution.
Several facts, such as the tectonic settings, climatic variations, eustatic changes of the sea level, morphology of the continental shelf, thermal regime, and glacial front type are the main factors that regulate the production of glacial debris (Eyles et al.,
1985; Powell, 1984; Powell and Molnia, 1989; Visser, 1997; Powell and Cooper,
2002).
No matter whatACCEPTED the processes that trigger the production of detritus in a glacimarine environment are, the pattern of coarsening and fining-upward sequences has been linked with the advance and retreat of glacial masses
(Aitchison et al., 1988; González Bonorino et al., 1988; Visser, 1997; Powell and ACCEPTED MANUSCRIPT
Cooper, 2002; Ghienne, 2003; Le Heron et al.2005; Limarino and Spalleti, 2006).
In addition, there is a close association between these episodes and eustatic sea level changes (Visser, 1990, 1997).
These ideas allow it to be considered that the coarsening-upwards arrangement recognized between the lower section and the first 60 m of the middle section is related to a glacial advance period and decrease of the sea level. On the other hand, the stratigraphic succession observed at 250 m of the profile indicates a fining-upwards pattern related to a period of retreat of the ice towards the main land simultaneously with a period of eustatic rise.
5.3.1. First stage (advance of the glaciers) MANUSCRIPT The outcrops of the Cushamen Formation in the study area do not allow the base of the unit or the relationships with older units to be recognized. However, it is possible to consider two alternatives, taking into account the lithofacies associations recognized along the profile. One of them is that the diamictite deposit recognized in this profile represents a single glacial event in the Cushamen
Formation. On the other hand, this deposit possibly represents one of various glacial episodes, although only one was recorded in the study area.
The transitionACCEPTED of CFA to DFA associations registered in Neneo Ruca area represents the initiation of the advance of the glacier masses into the marine basin
(Fig. 9). Upwards, the presence of granitic blocks, which are related to glaciotectonic deformation, indicates the maximum glacial advance towards the ACCEPTED MANUSCRIPT
basin (Fig. 9). Similar considerations were already presented by Powell and
Cooper (2002), Ghienne (2003), and Lang et al. (2012) in other glacimarine sequences. It is likely that the maximum drop in the sea level was coincident with the episode of maximum glacial advance.
5.3.2. Second stage (retreat of the glaciers)
The beginning of the stage of glacial retreat is registered in the transition from a sub-glacial environment (EFA) to a proximal glacimarine environment (DFA) (Fig.
9) located between 250 and 300 m above the base of the profile. Other evidence of this glacial regression is the passage from diamictites to sandstones and pelites
(Fig. 6). MANUSCRIPT The advanced stage of glacial retreat is the transition between proximal glacimarine association (DFA) and the distal glacimarineone (FFA) located in the upper section (Fig. 9). We believe that this change of deposition environment was accompanied by a sea level rise. However, the predominance of greywackes and sandstones above the pelites precludes determination of whether this section represents the definitive disappearance of the glacial systems in the western North
Patagonian Massif or simply represents an interglacial stage.
ACCEPTED
5.4. Lithofacies correlation and age of the Cushamen Formation ACCEPTED MANUSCRIPT
The existence of granitic pebbles in pelitic-psammitic matrix in outcrops of the
Cushamen Formation near its type locality was previously reported by Cagnoni et al. (1997). In addition, quartzitic and granitic pebbles forming diamictites in the
Cushamen Formation was reported by Duarth et al. (2002) in outcrops located 100 km south of the studied area. However, paleoenvironmental considerations were not evaluated by those authors.
The rocks of the Cushamen Formation were studied by Hervé et al. (2005) near to
Río Chico locality, providing a U/Pb age of ca. 335 Ma in detrital zircons, indicating a Visean maximum depositional age. According to those authors, the unit is younger than the Silurian–Devonian and middle Carboniferous magmatic events recorded between Colan Conhué and Bariloche (Fig. 1).
On the other hand, the intrusive relationship MANUSCRIPTbetween the early Permian granitoids and the Cushamen Formation constrains the sedimentation and deformation of the latter to a Pre-Permian period (Varela et al., 2005; Pankhurst et al., 2006).
Therefore, we believe that the event of sedimentation of the Cushamen Formation must be restricted to the Pennsylvanian, as was previously indicated by Hervé et al. (2005).
5.5. CorrelationsACCEPTED of the Cushamen Formation with other units Similar temporal and lithological features of the unit described here are recognized in several units of the Neuquén Cordillera, Extraandean Chubut, and southern
Chile. ACCEPTED MANUSCRIPT
5.5.1. Correlations with units of the Neuquén Cordillera
5.5.1.1. Valle Chico formation
Near the locality of Esquel, the sedimentary successions constituted by diamictites, sandstones, and pelites correspond to the Valle Chico Formation (Cucchi, 1980).
This unit unconformably overlies the sandstones and pelites of the Esquel
Formation, which is 600 m thick (Cucchi, 1980; González Bonorino and González
Bonorino, 1988; González Bonorino, 1992). The deformation of the sandstones and diamictites of the Valle Chico Formation is related to glaciotectonic processes according to observations made by González Bonorino and González Bonorino
(1988) and González Bonorino (1992). Those authors considered a proximal marine environment of sedimentation influenced MANUSCRIPT by glaciers. The same environments of sedimentation and glacial processes have been interpreted for the lithofacies associations DFA and EFA of the Cushamen Formation near Neneo
Ruca.
González Bonorino (1992) assigned the Valle Chico Formation to the late
Carboniferous, contemporaneously with the age assigned to the Cushamen
Formation by Hervé et al., 2005. According to the temporal and lithological similarities, weACCEPTED consider that both formations are correlatable.
5.5.1.2. Piedra Santa Formation ACCEPTED MANUSCRIPT
In the central eastern part of the Neuquén Cordillera, Digregorio and Uliana (1980) distinguished the metasedimentary rocks of the Piedra Santa Formation, as a younger unit than the high-degree metamorphic rocks of the Colohuincul Complex.
The latter unit was considered as pre-Carboniferous according to the dating of
Varela et al. (2005), Pankhurst et al (2006), Martinez et al. (2011), Hervé et al.
(2016) and Serra Varela et al., (2016).
Franzese (1995), using K/Ar in schist of Piedra Santa, established a post-Devonian metamorphic evolution (ca. 300 Ma). From the lithological point of view, the interpretation put forward by Digregorio and Uliana (1980) was that this unit, formed by sandstones, pelites, and scarce amounts of conglomerates, was deposited in a marine environment.
According to the age and marine environmentMANUSCRIPT of deposition of the Piedra Santa Formation, it is possible to correlate this unit with the Cushamen Formation.
However, it should be noted that, so far, there is no concrete evidence for late
Paleozoic glacial events in this unit.
5.5.2. Correlations with units of the Extra-Andean Chubut
5.5.2.1. Pampa de Tepuel Formation
The late PaleozoicACCEPTED units of the Tepuel Group were correlated with the Esquel and
Valle Chico formations (Cucchi, 1980; Lopez Gamundi, 1980; González Bonorino and González Bonorino, 1988; González Bonorino, 1992). The glacial sedimentary ACCEPTED MANUSCRIPT
sequence of the Tepuel Group was named Pampa de Tepuel Formation by Lesta and Ferrello (1972) and is 2900 m thick. The first 1200 m comprises a succession of diamictites, conglomerates, sandstones, and pelites (González Bonorino et al.,
1988; González Bonorino, 1992).
In Ap-Iwan ranch, located 40 km to the southeast of Esquel locality (Fig.1), the conglomerates of the Pampa de Tepuel Formation lie unconformably on the metasedimentary rocks of the Río Pescado Formation (Rolleri, 1970; Spikermann,
1977; Gonzalez et al., 1995). However, in the Sierra de Tecka and Tepuel the sedimentary sequence of this unit appears concordantly over the sandstones of the
Jaramillo Formation (González Bonorino et al., 1988) (Fig.10).
Like the Valle Chico and Cushamen formations, the diamictites of the Pampa de Tepuel Formation have structures related to MANUSCRIPTglaciotectonic deformation (González Bonorino et al., 1988; González Bonorino, 1992).The stratigraphic analyses of the last unit (González Bonorino, 1991, 1992; Limarino and Spalleti, 2006) indicate a late Carboniferous age of sedimentation, concurrently with the Cushamen
Formation (Hervé et al., 2005). The considerations indicated above allow us to suggest that the Cushamen and Pampa de Tepuel formations are temporally and environmentally correlatable.
ACCEPTED 5.5.2.2. Las Salinas and Menuco Negro formations ACCEPTED MANUSCRIPT
Units equivalent to the Tepuel Group were identified in the eastern sector of the
Extraandean Chubut, north of Sierra de Languiñeo and southwest of Pampa de
Agnia (Robbiano, 1971; Gonzalez, 1972; Gonzalez Bonorino, 1992) (Fig. 1).
In the northern part of Sierra de Languiñeo appears a 2400 m thick sequence of diamictites, sandstones, and pelites assigned to the Las Salinas Formation
(González, 1972). González Bonorino (1992) correlated this unit with the Pampa de Tepuel Formation. The structures due to synsedimentary deformation were linked to resedimentation processes caused by active tectonics (Frakes and
Crowell, 1969; Gonzalez, 1972). However, Gonzalez Bonorino (1992) indicated that these deposits are genetically linked to glaciotectonic deformation. On the other hand, the paleontological content of the Las Salinas Formation indicates a late Carboniferous age (González, 1972). MANUSCRIPT Southwest of the Pampa de Agnia, the Menuco Negro Formation consists of diamictites, sandstones, and pelites lying unconformably over the Catreleo Granite
(Robbiano, 1971). The unit, which is 86 m thick, shows structures related to synsedimentary deformation similar to those of the Cushamen, Pampa de Tepuel, and Las Salinas formations (Frakes and Crowell, 1969; González Bonorino, 1992).
Although the origin of the deformation of the Las Salinas and Menuco Negro formations is still debated, the presence of diamictites, as well as the age of the fossils, allowsACCEPTED us to correlate these formations with the Cushamen Formation.
5.5.3. Correlations with units of southern Chile ACCEPTED MANUSCRIPT
5.5.3.1. Trafun Metamorphic Complex
The Trafun Metamorphic Complex is located in the southern region of Chile and is mostly constituted by metapelites, metasandstones, and locally metadiamictites
(Hervé et al., 2016). In the Isla Huapi (Chile), located 150 km northwest of Neneo
Ruca (Fig. 1), the igneous blocks of the metadiamictite lithofacies carry zircons with Devonian ages (ca. 371 ± 6 Ma; Hervé et al., 2016). These authors also establish that the youngest age grouping of detrital zircon in the matrix surrounding the blocks has Devonian–Carboniferous ages (ca. 357 ± 4 Ma). According to these characteristics, the Trafun Metamorphic Complex can be correlated with the
Cushamen Formation.
5.5.3.2. Bahía Mansa Metamorphic Complex MANUSCRIPT
The Bahía Mansa Metamorphic Complex is located in the southern region of Chile and is composed of pelitic schists, quartzites, ultramafic bodies, and occasionally pillow lavas (Duarth et al., 2009; Hervé et al., 2013).Those authors established the youngest detrital zircon populations (ca. 320 to 353 Ma) in metasediments located in the vicinity of Chaitén, Yelcho lake, north of the Chiloé Island and Los Muermos
(Fig. 1). Therefore, the maximum sedimentation age of the Bahía Mansa MetamorphicACCEPTED Complex is similar to those published by Hervé et al. (2005) for the metamorphic rocks of the Cushamen Formation (ca. 335 Ma). Even though there are notable lithological and metamorphic differences between both units, they are contemporaneously and probably have the same sources of sediment. ACCEPTED MANUSCRIPT
As in the Piedra Santa Formation, diamictite deposits were not found in the Bahía
Mansa Metamorphic Complex. Therefore, although the previously mentioned units present a series of geological features in common with the Cushamen Formation, is not possible to specify an exact stratigraphic correlation as with the Extraandean
Chubut units.
5.5.4. Regional stratigraphic correlation between units of the Neuquén Cordillera,
Extra-Andean Chubut, and North Patagonian Massif (Neneo Ruca, Esquel, Tecka-
Tepuel, and Languiñeo)
The detailed analysis of the Carboniferous units allows us to draw stratigraphic correlations between units of the Neuquén Cordillera, Extraandean Chubut, and North Patagonian Massif (Fig. 10). MANUSCRIPT
Firstly, it should be noted that, unlike the sequence described for the Valle Chico,
Pampa de Tepuel, and Las Salinas formations, which show three diamictite sections (González, 1972; González Bonorino and González Bonorino, 1988;
González Bonorino, 1992), the Cushamen Formation studied near Neneo Ruca displays only one.
These diamictite deposits represent three glacial intervals according to the analysis of González ACCEPTEDBonorino (1992). The diamictite deposit located at the base of the Valle Chico, Pampa de Tepuel, and Las Salinas formations represents the beginning of the first glacial interval (Fig. 10). This deposit is located unconformably over the Esquel and Río Pescado formations, which present a ACCEPTED MANUSCRIPT
notably different deformation and metamorphism from the glacial deposits (Rolleri,
1970; Gonzalez, 1972; Spikermann, 1977; Cucchi, 1980; Gonzalez et al., 1995).
This relationship has not been recorded in the Neneo Ruca area, since the diamictites of the middle section of the profile are concordant with the lower section, without differences in metamorphism or deformation. For this reason, we consider that the diamictites of the Cushamen Formation near Neneo Ruca do not represent the first glacial interval proposed by González Bonorino (1992).
Therefore, the previously mentioned characteristics allow the diamictites of the study profile to be correlated with the G2 or G3 glacial intervals of González
Bonorino (1992). However, we consider that the abundance of greywackes in the upper section of the studied profile comes from the ablation of the glacier masses, which is indicative that the lower and upper sections represent interglacial stages before and after the diamictite deposition that MANUSCRIPT formed the G2 glacial interval (Fig. 10).
5.6 Silurian – Permian paleogeographic evolution
The isotopic dating carried out by different authors in the middle and late Paleozoic rocks of southern Chile, Neuquén Cordillera, and North Patagonian Massif (Varela et al., 2005; Hervé et al., 2005, 2013, 2016; Pankhurst et al., 2006, Duarth et al.,
2009; MartinezACCEPTED et al., 2012) makes possible to establish five tectonostratigraphic events between Silurian and Permian times. ACCEPTED MANUSCRIPT
The older event includes middle Silurian to late Devonian granitoids (ca. 425 to 361
Ma) (Varela et al., 2005; Pankhurst et al., 2006; Duarth et al., 2009; Martinez et al.,
2012; Hervé et al., 2013, 2016). Later, there is a first sedimentary stage (late
Devonian–early Carboniferous), which includes units from the Extra-Andean
Chubut and southern Chile (Hervé et al., 2016). The limit between early-late
Carboniferous times (ca.330 to 314 Ma) is represented by granodiorites, tonalities, and leucogranites that form the second magmatic event (Pankhurst et al., 2006).
The second sedimentary stage is late Carboniferous and is widely distributed in the studied area (Hervé et al., 2005). Finally, an extensive magmatism occurred in the early Permian (ca. 286 to 272 Ma) (Varela et al., 2005, 2015; Pankhurst et al.,
2006).
According to the late Silurian–CarboniferousMANUSCRIPT paleogeographic reconstruction of Hervé et al. (2013), late Silurian–early Carbonifer ous rocks formed a topographical barrier from Colan Conhué (Chubut province) in the south to Lake Curruhué Chico
(Neuquén province) in the north. However, because the Cushamen and equivalent formations crop out east and west of the late Silurian–Carboniferous topographic highs, we consider it necessary to generate a new paleogeographic model.
5.6.1 First magmatic event middle Silurian–late Devonian (ca. 425 to 361 Ma) ACCEPTED As indicated above, there is enough evidence of isotopic ages from the granitic and metamorphic rocks of the Colohuincul Complex which confirm the existence of a
Chanic orogeny (Silurian–Devonian) in the Neuquén Cordillera (Varela et al., 2005, ACCEPTED MANUSCRIPT
2015; Pankhurst et al., 2006; Martinez et al., 2012; Hervé et al., 2013, 2016) (Fig.
11).
In the Chubut province, the dating by Pankhurst et al. (2006) in the Cacares
Granite near Gastre and the Colan Conhué Granite determined the existence of a
Devonian magmatism in the southwestern part of the North Patagonian Massif
(Fig. 11). These units represent the extension of the Chanic Orogen to the south of the studied area (Varela et al., 2005; Hervé et al., 2013, 2016) and may represent the topographic high of Hervé et al. (2013). In addition, Devonian tonalites and leucogranites occur near Chaitén and Pichicolo in southern Chile (Duarth et al.,
2009; Hervé et al., 2016) (Figs.1 and 11).
5.6.2. First sedimentary stage: late Devonian?–earl MANUSCRIPTy Carboniferous (ca. 350 Ma)
The stratigraphic and paleontological studies of the Esquel and Río Pescado formations established a pre-Pennsylvanian age for these sedimentary sequences
(Rolleri, 1970; Spikermann, 1977; Cucchi, 1980; González Bonorino and González
Bonorino, 1988; González Bonorino, 1991, 1992; González et al., 1995; Limarino and Spalleti, 2006).
Cucchi (1980) observed K-feldspar and volcanic clasts in the Esquel Formation. In addition, SpikermannACCEPTED (1977) interpreted the tuffaceous sandstones interbedded in the metapelites and metapsammites of the Río Pescado Formation as due to the presence of an active volcanism coeval with the sedimentation. The lithological features and the stratigraphic relationship previously established for both units ACCEPTED MANUSCRIPT
suggest that Devonian volcanism that developed in southern Chile and Argentina could be active in the supply area (Fig. 11).
As indicated in the section on regional settings, the schist of the Llanquihue
Complex located at the subsurface in Los Muermos (Chile) contains youngest detrital zircon populations of ca 385 ± 3 and 387 ± 3 Ma (Hervé et al., 2016).
Therefore, we considered that the Llanquihue Complex is contemporaneously with the Esquel and Río Pescado formations (Fig. 11).
In our paleogeographic reconstruction, the above mentioned units form a marine basin between late Devonian– early Carboniferous times in the Extraandean
Chubut and southern Chile (Fig. 11). However, evidence of such a basin was not found, up to now, in the western North Patagonian Massif. MANUSCRIPT
5.6.3. Second magmatic event: early–late Carboniferous (ca. 330 to 314 Ma)
The igneous rocks belonging to this event were registered from Bariloche (Río
Negro province) to Colan Conhué (Chubut province) (Figs. 1 and 12). The ages vary between ca. 330 ± 4 Ma to 314 ± 2 Ma (Pankhurst et al., 2006). In the northern part of this belt, between Bariloche and El Maitén, the ages were obtained in tonalites and granodiorites from Cordón del Serrucho (Pankhurst et al., 2006).
The youngerACCEPTED ages were recorded in the Extraandean Chubut and form a belt parallel to the Chubut River. This indicates that the distribution of the early-late
Carboniferous magmatism follows a belt nearly parallel to and partly overlapping ACCEPTED MANUSCRIPT
the middle Silurian–late Devonian magmatism, according to the paleogeographic reconstructions of Varela et al. (2005) and Hervé et al. (2013, 2016).
During the second magmatic event, the metasedimentary rocks of the Esquel and
Rio Pescado formations and possible Llanquihue Complex were deformed and metamorphosed (Fig. 12). This event of deformation and magmatism produced the closure of the late Devonian–early Carboniferous marine basin and the uplifting of the metasedimentary units located in the Extra-Andean Chubut and southern Chile region (Fig. 12). A similar conclusion was already reached by Spikermann (1977) and Cucchi (1980) during their stratigraphic and petrographic analyses of the sequences cropping out in the Esquel and Ap-Iwan ranch localities.
5.6.4 Second sedimentary stage: late Carboniferous MANUSCRIPT (ca. 314 to 300 Ma)
As indicated above, the temporal and lithofacies correlation between the units of the western North Patagonian Massif, Neuquén Cordillera, and Extra-Andean
Chubut suggests a Pennsylvanian age for the sedimentation of the Cushamen,
Valle Chico, Pampa de Tepuel, Las Salinas, and Mogote Negro formations. This second stage of sedimentation is represented by Trafun and Bahia Mansa
Complex in the southern Chile region.
If the first andACCEPTED second magmatic stage rocks were a discontinuous topographic high for the Pennsylvanian sedimentation stage, then it is possible to distinguish three marine basins (Fig. 12). ACCEPTED MANUSCRIPT
The late Carboniferous sedimentation along the western part of the North
Patagonian Massif was developed in the Cushamen basin (Fig. 12). The results of the geochemical analysis for the Cushamen Formation in its type locality show that this unit derived from the erosion of felsic and a few intermediate igneous rocks
(Cagnoni et al. 1997). In the same region, detrital zircons obtained by Hervé et al.
(2005) in metasediments of the Cushamen Formation yield Silurian, Devonian and
Visean ages peaks (ca. 400, 425, 335 Ma) with a few zircons derived from older units. In the Neneo Ruca area, our measurements of paleocurrents indicate a provenance from the west.
All of the above features cited suggesting that rocks of the first magmatic stage were located to the west of the Cushamen basin and were the most important area for sediment supply. Therefore, in our PennsylvaniaMANUSCRIPTn paleogeographic configuration, the majority of relief located westw ards was formed by granitoids of the Colohuincul Complex. The Visean detrital zircons population was probably derived from the second magmatic stage, which formed a discontinuous topographic high between Bariloche and Colan Conhué (Fig. 12). The pre-middle
Silurian detrital zircons recognized in the Cushamen Formation were probably derived from the eastern region. If Piedra Santa Formation is considered
Pennsylvanian, it is possible that the Cushamen basin extends to the Aluminé locality locatedACCEPTED 210 km to the north of Neneo Ruca (Figs. 1 and 12). The measurements of paleocurrents and facies analyses in the Extraandean
Chubut units indicate two areas of provenance for the input of the material to the
Tepuel Basin (Suero, 1962; González Bonorino and González Bonorino, 1988; ACCEPTED MANUSCRIPT
González Bonorino et al., 1988; González Bonorino, 1992). The western area was possibly located in Chile and composed of Devonian rocks (ca. 390 to 360 Ma) that crop out near Chaitén (Duarth et al., 2009; Hervé et al., 2016) (Fig. 12). The eastern area of provenance was possibly formed by Devonian and Carboniferous igneous rocks located between El Maitén and Colan Conhué (Pankhurst et al.,
2006) (Fig. 12).
In Chile, the Bahia Mansa and Trafun complexes are located between Isla Huapi and Lake Yelcho. Both complexes were part of the southern Chile basin during the late Carboniferous sedimentation stage (Fig. 12). In the southern part of this basin, the rocks of Bahía Mansa Complex derive from the first and second magmatic events (Duarth et al., 2009; Hervé et al., 2016). The low-degree metamorphic facies, which include metasandstones and metapeliteMANUSCRIPTs, are considered as distal facies of the Tepuel basin. In addition, the diamic tite lithofacies of the Trafun Metamorphic Complex located near Isla Huapi show that the glaciers were active in the northern part of the southern Chile basin. The provenance of the detritus of these diamictite deposits was probably located eastwards, where the granitoids of the Colohuincul Complex formed a topographic high (Hervé et al., 2016) (Fig. 12).
According to the evidences cited above, the first and second magmatic events do not form a continuous topographic high, as suggested by Hervé et al. (2013), but instead form ACCEPTEDa discontinuous topographic barrier that allows the passage of the proto-Pacific ocean to the western North Patagonian Massif (Fig. 12). In addition, in our paleogeographic configuration of the late Carboniferous stage, we ACCEPTED MANUSCRIPT
considered that the three marine basins were interconnected across the discontinuous relief developed by the first and second magmatic belts (Fig. 12).
5.6.5 Third magmatic event: early Permian (ca. 286 to 272 Ma)
In the southwestern edge of the continent of Gondwana, the Permian records a large magmatic event. In the western North Patagonian Massif, structural, geochemical, and petrologic studies were carried out in order to understand the different stages of this magmatism and the effect on previous units (Caminos and
Llambías, 1984; Varela et al., 1991, 1999, 2005; 2015; Cerredo and López de
Luchi, 1998; Pankhurst et al., 2006; López de Luchi and Cerredo, 2008; von
Gosen, 2009). The U/Pb ages obtained by Varela et al. (2005) and Pankhurst et al. (2006) established that the Permian magmatic MANUSCRIPT event is distributed eastward of the first and second magmatic events (Figs. 1 and 13).
Near the studied area, the Comallo granodiorite (281±17 Ma, Varela et al., 2005) intrudes into the eastern part of the profile (Fig. 2). In the Río Chico area, López de
Luchi and Cerredo (2008) established that the metamorphic peak of the Cushamen
Formation occurred in the late Devonian. However, von Gosen (2009) determined that the metamorphic peak in the sedimentary rocks of the Cushamen Formation was synchronousACCEPTED with an early Permian magmatic event. Taking into account that the Cushamen Formation carries Visean detrital zircons, the metamorphic event affecting this unit must be post-Pennsylvanian.
ACCEPTED MANUSCRIPT
6. Conclusions
In the outcrops near Neneo Ruca, Río Negro province, Argentina, there is a 540 m metasedimentary sequence corresponding to the Cushamen Formation. It is possible to distinguish six sedimentary lithofacies associations related to a glacimarine environment.
Four of these lithofacies are sandstone and pelite sequences deposited by debris flow currents over distal glacimarine environments (AFA–BFA–CFA–FFA). The
DFA lithofacies association includes parallel stratified pelites and sandstones, pelites with dropstones, and cross-stratified sandstones corresponding to paleo- channel flows and iceberg-debris deposits in proximal glacimarine environments.
The local conical folding observed in the lithofacies association EFA is related to glaciotectonic deformation. These features wereMANUSCRIPT originated in subglacial environments.
The arrangement of the lithofacies along the profile seems to be related with the advance and retreat of glacier masses. The maximum glacial advance is related to the EFA lithofacies association, located between 230 and 250 m from the base of the profile. The regression stage is related with the transition from subglacial deposits to proximal and distal glacimarine lithofacies association at 250 m from the base to the end of the profile. The maximum glacial advance is correlatable with the G2 glacialACCEPTED interval established by Gonazález Bonorino (1992) in the
Pennsylvanian Valley Chico, Pampa de Tepuel, and Las Salinas formations of the
Extraandean Chubut and the southern part of Neuquén Cordillera. The lithofacies ACCEPTED MANUSCRIPT
associations of the glacimarine distal environments, located in the lower and upper sections of the profile, correspond to the interglacial intervals.
After the analysis of the chronostratigraphy and distribution of the basement units in southern Chile, the North Patagonian Massif, Neuquén Cordillera, and
Extraandean Chubut, we distinguished five stage of paleogeographic evolution between Silurian–Permian times.
The existence of Pennsylvanian deposits in the northwestern part of the North
Patagonian Massif indicates that the middle Silurian–Devonian and Carboniferous magmatism units did not form a continuous topographic high.
MANUSCRIPT 7. Acknowledgements
This study is part of the PhD thesis of Paulo Marcos. The Configuración Geológica y Geodinámica del sector central de la Comarca Nordpatagónica and Gondwánico y Patagonídico del Macizo Nordpatagónico occidental research projects, granted by the Universidad Nacional del Sur were used during this study.
Many thanks to CONICET for the research project Significado y evolución de los eventos tectonomagmáticos Gondwánicos y Patagonídicos del norte de Patagonia. We would likeACCEPTED to thank the people of the Comallo locality that allowed us access to their lands, gave us shelter and helped throughout many field trips. We are also deeply grateful for the comments and suggestions made by Associated Editor Stijn ACCEPTED MANUSCRIPT
Glorie, Paula Castillo and by anonymous reviewer who provided a detailed critique of the manuscript which considerably improved its quality.
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Figure captions
Figure 1: Geological sketch map showing the basement outcrops between 38°S and 44°S, and the Atlantic and Pacific oceans. modi fied after Volkheimer (1964),
Coira et al. (1975), Ravazzoli and Sesana (1977), Nullo (1979), Volkheimer and
Lage (1981), Varela et al. (1991, 1997, 1998), Cerredo and López de Luchi (1998),
Ostera et. al. (2001), Basei et al. (2002), Gonzalez et al., (2003), Hervé et al.
(2005), Varela et al. (2005), Pankhurst et al. (2006), Duhart et al. (2009), Hervé et al. (2013, 2016) andRapalini et al. (2013).
Figure 2: Geological setting of the Cushamen Formation. (a) Geological map of the northwestern corner of the Ingeniero Jacobacci chart (modified from González et al., 2003). Location of figure 2b is shown. (b) Profile of the Cushamen Formation. References for the profile are also displayed. MANUSCRIPT
Figure 3: Conglomerate and diamictite lithofacies. (a) Five meters diameter block in the lithofacies A (Dmm). (b) Anticline and sincline conical fold inside poorly stratified psamitic lithofacies A. (c) Thin section of conglomerate of lithofacies B.
Note the K-feldspar clast and the matrix around. (d) Top surface of the conglomerate strata with clasts and ridges and furrows structures.
Figure 4: SandstoneACCEPTED lithofacies (a) Outcrop and thin sections of lithofacies C (Sh).
Compare the color differences between different banks. Abundance of biotite in matrix rich sandstones is higher than in K-feldspar sandsotnes. (b) 1.5 m thick solitary paleochannel of lithofacies D (St). (c) Strata from lithofacies F (Swh). Most ACCEPTED MANUSCRIPT
of the clasts are 0.8 cm to 1.2 cm diameter. (d) Wavy surfaces of hummocky cross stratification in lithofacies E on upper section of the profile (person as scale at the left side). Detailed part of this structure is showed on the upper right corner.
Figure 5: Pelite lithofacies. (a) Granitic block inside pelite with fine lamination of the lithofacies G. (b) Images of lithofacies H. On top of the picture, note thin lamination typical of this lithofacies.
Figure 6: Lithofacies associations distinguished along the lower, middle and upper sections of the profile. Lithofacies references are as the figure 2. See the text for more detail of the lithofacies associations.
Figure 7: Glacimarine paleoenvironments distinguish MANUSCRIPTed along the profile of the Cushamen Formation. Upper and lower sections have lithofacies associations of distal glacimarine environments. Middle section profile has lithofacies association corresponding to proximal and subglacial environments (for further details and discussion, see the text).
Figure 8: Meso-scale folds in lithofacies association EAF. (a) N–S view of the conical folds. Note that the upper part of the sequence is not folded. (b) Sketch of the conical foldsACCEPTED along a transversal section. Note the difference between tight anticline fold and the open sincline.
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Figure 9: Block diagrams showing the paleoenvironmental evolution of the
Cushamen Formation. The advance and retreat stages are shown. Note the different positions of the ice glacier mass respect to the coast line and the sea level changes.
Figure 10: Stratigraphic correlation between Pennsylvanian units from the North
Patagonian Massif and Extraandean Chubut. (1) Profile of Cushamen Formation in
Comallo-Neneo Ruca area. (2) González Bonorino and González Bonorino (1988) and González Bonorino (1992) profile of the Valle Chico Formation in Esquel. (3)
González Bonorino et al. (1988) and González Bonorino (1992) profile of the
Pampa de Tepuel Formation in Tecka-Tepuel region. (4) González (1972) profile of the Las Salinas Formation in Languiñeo. Glacial intervals according to Gonzalez Bonorino (1992). MANUSCRIPT
Figure 11: Paleogeographic sketch of the southwestern margin of the Gondwana continent during the first magmatic stage (middle Silurian–late Devonian) and the first sedimentary stage (late Devonian–early Carboniferous).
Figure 12: Paleogeographic sketch of the southwestern margin of the Gondwana continent during the second magmatic stage (early–late Carboniferous) and the second sedimentaryACCEPTED stage (late Carboniferous).
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Figure 13: Paleogeographic sketch for the southwestern margin of the Gondwana continent during the third magmatic stage (early Permian). For more detail about each U/Pb age see Varela et al. (2005) and Pankhurst et al. (2006).
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Table 1: Lithofacies identified in the Cushamen Formation. Nomenclatures after
Eyles et al., (1983), Miall (1996) and the proposed in this paper.
Facies Lithofacies Sedimentary structures code
Dmm Matrix supported diamictite. Horizontal stratification (<10%)
Gcs Clast supported-stratified gravel. Horizontal stratification.
Sand, very fine to very coarse, may be Horizontal lamination, parting Sh pebbly. lineation. Sand, medium to very coarse, may be St Solitary trough cross-beds. pebbly.
Hcs Fine sand and mud. Hummocky cross-stratification.
Fine sand, silt or mud with more than Swh Horizontal stratification. 25% of pebbly clast.
Fld Sand, silt, mud with dropstones. Fine lamination. MANUSCRIPT Fl Sand, silt, mud. Fine lamination.
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• The Cushamen Formation comprises 8 lithofacies in a 540 m thick
sequence.
• Four lithofacies association represent distal glacimarine environments.
• Other two represent proximal glacimarine and subglacial environments.
• Diamictite deposits are coeval with Pennsylvanian units in Chubut, Neuquén
and Chile.
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