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F u n 2 d 6 la serena octubre 2015 ada en 19 Fluvial facies and architecture of the late middle , Mayoan, deposits of Chilean

Hermann Rivas1,2 *, Enrique Bostelmann3, Jacobus Le Roux1,4, Raúl Ugalde5. 1: Departamento de Geología, Facultad de Ciencias Físicas y Matemáticas, Universidad de . Plaza Ercilla #803, Santiago, Chile. 2: Museo Nacional de Historia Natural, Parque Quinta Normal s/n, Santiago, Chile. 3: Red Paleontológica U-Chile, Laboratorio de Ontogenia y Filogenia, Departamento de Biología, 4: Facultad de Ciencias, Universidad de Chile, Avda. Las Palmeras 3425, Ñuñoa, Santiago, Chile. 4: Centro de Excelencia en Geotermia de los Andes. Plaza Ercilla #803, Santiago, Chile. 5: PEDECIBA Geociencias, Facultad de Ciencias, Universidad de la República, Iguá 4225, Montevideo, Uruguay *email: [email protected] interpreted as volcaniclastic, alluvial and mobile-channel Abstract belts of fluvial successions (Suárez et al., 1994, De La Cruz et al., 2003; Quiroz & Bruce, 2010). This study According to their depositional patterns, four provides a new sub-division, a deeper facies analysis and lithostratigraphic units could be recognized in the detailed depositional interpretation of the fluvial deposits Cenozoic deposits located south of the town of within this formation. Based on the analysis, the validity Balmaceda, Aysén region. The basal two are composed of the Oscuro Formation is also questioned. of Eocene and lower/middle(?) Miocene, massive volcaniclastics deposits, with distinct grades of edaphic (paleosols) development. The upper units correspond to late Miocene, cross bedded tabular sandstones and 2. Methods rhythmic sand-mud beds of fluvial epiclastic deposits. Changes in color and competence constitute the main Eight detailed stratigraphic sections were measured differences between the units. Abundant reworked during the summers of 2014-2015, reaching a total of 913 pyroclastic elements attest for intense volcanic activity m. Graphic logs were ploted in SedLog and edited using during deposition. Eight architectural elements of a Adobe Illustrator. Colors of rocks follows the general channel and overbank fluvial environment were ‘Geological Rock Color Chart’ (Munsell, 2009) and identified in these deposits. Geometry and arrangement facies analysis was based on the models of Miall (2006). of these elements allow for interpret shallow to deeper Collected fossil specimens were deposited in the braided fluvial systems with periods of sinuous-wandering Paleontological Area of the National Natural History behavior. U/Pb zircon dating of selected horizons gives 12.3 and 12.18 Ma age, late Miocene (Serravallian), for Museum (MNHN) in Santiago de Chile. the middle of the successions. These fluvial deposits form part of the incorrectly defined Oscuro Formation, and are lithologically correlative with the El Portezuelo Formation 3. Lithostratigraphy in the neighboring Cerro Guenguel, . These beds also represents the Chilean correlatives of the type According to their lithological properties, we recognized lithostratigraphic units in which the Mayoan South four distinct units composing what is presently American Land Mammal Age (SALMA) was originally th considered the Oscuro Formation. A brief description of characterized, at the beginning of the 20 century. each unit is provided below.

Keywords: middle Miocene, fluvial architecture, braided Lower, volcaniclastic units rivers, Oscuro Formation, Mayoan SALMA, Aysén, Chile. Corresponds to a 184 m of volcaniclastic sandstones and 1. Introduction tuffaceous deposits. These rocks are exposed along the Cenozoic sedimentary rocks of easterly Aysén are valley of the Oscuro and Simpson Rivers. Its deposits are represented as discrete outcrops close to the bordering characterized by orangey to yellowish-grey and pinkish with Argentina. These rocks crops out as a bedded massive beds, with diffuse bedding and the presence of succession in the riberbanks and mesa hills, between paleosol horizons. Regarding their lithology and age, the Ministro and Simpson Rivers, 15 km southwest of deposits could be separated in two different units. Unit 1 Balmaceda village (46°S). Similar rocks are represented has been dated by U/Pb at 46.76 Ma, while a in Argentina as adjacent outcrops forming the lower half lower/middle(?) Miocene age for Unit 2 is inferred based of the Meseta Guenguel and Pampa del Chalía, mapped on the fossil content. The existence of a paraconformity as the Río Frías, Río Mayo, Pedregoso and El Portezuelo between these deposits is suggested. formations (Escosteguy et al., 2003). In the Chilean side, Upper, epiclastic units all of these rocks have been grouped and classified as a single formation (Oscuro Formation), which is 812 AT 1 GeoloGía ReGional y Geodinámica andina

These deposits overlie the basal volcaniclastic units with values measured in discrete, thick trough cross-bedded intercalation of sandy bedforms (SB) and overbank Mayoan SALMA. The available evidence suggest that a covered contact. According to the bed color and sandstones evidence a dominant NE direction. elements, chiefly levee (LV) deposits, and in less this SALMA should be temporally bracketed between competence, they are categorized in two different units proportion, crevasse splays (CS), floodplain fines (FF) ≥12.3 and 11.8 Ma, a time span that superposes with the (Unit 3 and 4). and channel abandoned channel fills deposits (CH(FF)). SALMA. The solution of this conundrum clearly demands a thoughtful review of all the available Unit 3 4. Discussion 4.2 Fluvial depositional systems evidence. Exposures are visible in the headwaters of the Oscuro 4.1 Architectural elements Deposits of Unit 3 represent shallow, braided river These deposits evidence an abrupt depositional River and in the base of a series of soft-weathered, gently systems, distal to the sediment source. Some periods of The facies associations are interpreted as common environment change, from a period dominated by sloped “mesa type” hills nearby the river. It consist of wandering behavior are not discarded, forming small pyroclastic deposits, to an active perennial fluvial system. approximately 244 m of tabular sandstones and architectural elements of the channel (elements GB, SB, levees and point bars. Fine sediments prevail as the SG, DA/LA, LS) and overbank fluvial sub-environment The reactivation of the fluvial deposition at the eastern mudstones with sharp and erosive contacts. The base infilling of abandoned channels. This unit could be margin of the arc, is related with a tectonic uplift, starts with 33 m of massive and trough cross-bedded (elements LV, CS, FF, CH(FF)). The interpretation of compared with the low-sinuosity braided river and each association and its corresponding architectural triggered for the subduction of the Chile Ridge at 15-14 tabular sandstones (Sm, St), intercalated with massive shallow perennial braided river models of Miall (2006). Ma (Lagabrielle, 2004). This uplift have led to an element is shown in Table 1. mudstones (Fm) and sand-mud intercalations (Fl). increase in the erosion rate and sediment supply. Changes Unit 4 shows is dominated by a braided river system Between 33-53 m dominates thick, massive sandstones in the fluvial styles reflected in the logs could be related Table 1. Summary of facies associations and correlative pattern, but with periods of wandering-meandering beds (Sm). Following a ~30 m covered section, from the with convergence/uplift fluctuations in the tectonic front, architectural elements (Miall, 2006). FA: Facies behavior. The intercalation of lenticular gravelly deposits 83 m to 141 m, a new succession of tabular massive, Association; LF: Lithofacies (described in text); AE: which had resulted in changes in accommodation and represents high discharge pulses of sedimentation. planar and trough cross-bedded sandstones (Sm, Sp, St) Architectural element; [m/u]: unit where the element was Thicker sandy deposits of this unit represent a more sediment supply. appears; intercalated with massive and sand-laminated interpreted, 3 = unit 3, 4 = unit 4. Light gray: Channel mudstones (Fm, Fl). From 141 m to the top (244 m) elements; Dark gray: Overbank elements. stable, deeper, braided channel pattern. Although, the top governs a sandstones succession, displaying massive and of this unit is marked by thinner beds, representing a cross-bedded layers (Sm, St, Sp). They are interrupted by AE LF Interpretation lower regime, probably distal shallow braided river with Acknowledgements a 7.5 m thick mudstone zone at the 166 m (Fm, Fl). GB Gh, Gp, Gt Gravelly bedforms (2D-3D periods of ephemeral sandy meandering behavior. We want to express our gratitude to Fredy Pedee and Diffuse horizontal bedded sandstones (Sh) and gravel [4] dunes) 4.3 Regional stratigraphy and age of the Mayoan Ximena Pedee of ‘Estancia La Frontera’, Balmaceda. lenses/lag deposits (Gh) appear discreetly along this unit. SB Sp, Sl, St, Sr, Sandy bedforms (ripples, 2D-3D deposits Alvar Dahmen assist us during the 2015 fieldwork. The graphic log of the upper 191 m, measured at one [3,4] dunes). CH SG Sm Sediment gravity-flow deposits Christian Salazar share comments and recommendations mesa hill is shown in Figure 2A. [3,4] (sheet-floods). The fluvial deposits here described are included as part of on an early draft. This research was funded by project the upper levels of the Oscuro Formation (De la Cruz et Unit 4 LS Sh, Sl Laminated sand sheets deposits Fondecyt 1130006. [4] (distal sheet-floods) al., 2003). This formation actually encompasses all the It overlies Unit 3 with a sharp boundary reaching at least LV Fl, Fsm, Fm, Levee deposits (pulses of Cenozoic bedded sedimentary deposits located south of 624 m, composed by tabular sandstones and mudstones [3,4] Sr, Sm flooding into the floodplain) Balmaceda. Paleogene dating and Neogene fossil with sharp and erosive boundaries. It is exposed in the CS Sr, Sm, Fl Crevasse splays (delta-like vertebrates on the two basal pyroclastic units clearly References [3,4] flooding deposits) demonstrates the existence of several unconformities and top of the mesa hills, adyacent to the Oscuro River. The Ameghino, F. 1906. Les formations sédimentaires du Crétacé FF Fm, Fsm Floodplain fines (settling in temporal hiatuses within the formation. Important base consist of 119 m of thick sandstones, displaying [3,4] ponds, swamps) Supérieur et du Tertiaire de Patagonie avec un parallelé changes in the lithological attributes also attest for these entre leurs faunes mammalogiques et celles de l´ancien planar and trough cross-bedded (Sp, St), ripple laminated CH(FF) Fm, Fsm Abandoned channel fills (settling) discontinuities. Nomenclatural priority and historical continent. Anales del Museo Nacional de Buenos Aires (Sr) and massive bodies (Sm). Massive and sand- [3,4] mapping denote that the Oscuro Formation is an (tercera serie) 8: 1-568. laminated mudstone (Fm, Fl) beds are scarce and crop inadequate lithostratigraphic conceptualization Dal Molín, C. & Franchi, M. 1996. Reinterpretación estratigráfica out as tabular beds, up to 2 m thick. From 119 m to 137 de las sedimentitas terciarias del suroeste del Chubut. En m the grain size increase, showing horizontal and cross These elements were recognized in the units as follow: (González, 1967; Escosteguy et al., 2003). We suggest Congreso Geológico Argentino, No. 13 y Congreso de bedded, pebbly-sandstones (Sh, St) beds, and horizontal the abandon of this name and use of valid historical exploración de Hidrocarburos No. 3, Actas 1: 473-478. and cross-bedded, granule-pebble conglomerate (Gh, Gt). Basal part of Unit 3 shows intercalations of thick stacks equivalents to refer to these successions. Based on their Buenos Aires. An approximately 60 m covered area separates the next of sandy bedforms (SB) intercalated with thinner levee lithology and depositional environment Units 3 and 4 are De la Cruz, R; Suárez, M; Belmar, M; Quiroz, D; Bell, M., 2003, (LV), floodplain fines (FF) and abandoned channel fills then correlated with the El Portezuelo Formation Área Coihaique-Balmaceda, Región de Aisén del 94 m (197–291 m) of an intercalation of sandstones (Sm, General Carlos Ibáñez del Campo. Servicio Nacional de St, Sp, Sr, Sh) and mudstone (Fm, Fsm, Fl). From 291 m (CH(FF)) deposits. Thicker muddy beds with thin sandy (Escosteguy et al., 2003), first described at the intercalation (Fl) are interpreted as levee and crevasse Geología y Minería, Carta Geológica de Chile, Serie to 423 m is governed by thicker tabular sand bodies, neighboring Cerro Guenguel, Argentina. Geología Básica, No. 80, 40p. displaying trough cross bedding, over the mudstones splays (CS) deposits, whereas massive thick mudstone Escosteguy, L..; Dal Molín, C.; Franchi, M.; Geuna, S..; Lapido, O. Dating performed gives two U/Pb ages of 12.3 ± 0.16 Ma layers. The following 40 m present sand-mud (Fm) are formed by settling in small ephemeral lakes. Hoja Geológica 4772-II, Lago Buenos Aires, Provincia Last 76 m is dominated by tabular sandy bedforms (SB) and 12.18 ± 0.4 Ma (Serravallian age) for the base and de Santa Cruz. Programa Nacional de Cartas Geológicas intercalations. From 463 m to 557 m thick sandstones 40 39 mid-portion of Unit 4. At Cerro Guenguel, Ar/ Ar de la República Argentina 1:250.000. Boletín del beds (Sm, St, Sr, Sh) dominates, with some intercalations and negligible lenses of gravelly bedforms (GB) and dating of the overlying “Río Mayo” Formation provides Servicio Geológico Argentino; Año: 2003 vol. 339 of mudstone (Fm, Fl). Finally, the last 67 m end with a abandoned channel fills deposits (CH(FF)) within. an age of 11.8 Ma (De Iuliis et al., 2008). Dal Molín & González, R. 1967. Geología de la región de Lago Blanco y Paso thinner sandstone-mudstone tabular beds intercalations Unit 4 basal section is dominated by thick, tabular, sandy Franchi (1996) obtain a 12.18 Ma 40Ar/39Ar age from a Río Mayo (Chubut). Acta Geológica Lilloana IX, 45-65. Tucumán, Argentina. (Sm, St, Sr, Fm, Fl), with bed thickness up to 3 m. Some bedforms (SB) intercalated with thinner abandoned tuff on the top of the El Portezuelo Formation at Arroyo fine deposits (Sm, Fm, Fl, Fsm) in Unit 4 shows edaphic Miall, A. 2006. The Geology of Fluvial Deposits: Sedimentary channel fills deposits (CH(FF)); thicker mudstone-sand Peregoso, in the Meseta del Chalía (incorrectly identified structures. Fossils include root traces, burrowing facies, basin analysis, and petroleum geology. Berlin: intercalation are interpreted as levee deposits (LV). as the Pedregoso Formation by them). These beds also Springer. structures and remains of an unidentified Interatheriinae, Gravelly bedforms (GB) occur as lenses or lag deposits. forms the outcrops at Lago Blanco and the upper Río Quiroz, D.; Bruce, Z. 2010. Geología del Área Puerto Ingeniero presently the first Mayoan fossil mammal of the Chilean At 119 m, sandy bedforms (SB) and gravelly bedforms Fénix, the historical localities from which Ameghino Ibáñez-Villa Cerro Castillo, Región Aisén del General Patagonia (Bostelmann et al., this congress). The graphic Carlos Ibáñez del Campo. Servicio Nacional de Geología (GB) occur. The upper section is controlled by an (1904) and Roth (1924) originally characterized the log of part of this unit is show in Figure 2B. Paleocurrent 813 ST 4 ESTRATIGRAFÍA, ANÁLISIS DE CUENCAS Y PROCESOS SEDIMENTARIOS

intercalation of sandy bedforms (SB) and overbank Mayoan SALMA. The available evidence suggest that elements, chiefly levee (LV) deposits, and in less this SALMA should be temporally bracketed between proportion, crevasse splays (CS), floodplain fines (FF) ≥12.3 and 11.8 Ma, a time span that superposes with the and channel abandoned channel fills deposits (CH(FF)). Laventan SALMA. The solution of this conundrum clearly demands a thoughtful review of all the available 4.2 Fluvial depositional systems evidence. Deposits of Unit 3 represent shallow, braided river These deposits evidence an abrupt depositional systems, distal to the sediment source. Some periods of environment change, from a period dominated by wandering behavior are not discarded, forming small pyroclastic deposits, to an active perennial fluvial system. levees and point bars. Fine sediments prevail as the The reactivation of the fluvial deposition at the eastern infilling of abandoned channels. This unit could be margin of the arc, is related with a tectonic uplift, compared with the low-sinuosity braided river and triggered for the subduction of the Chile Ridge at 15-14 shallow perennial braided river models of Miall (2006). Ma (Lagabrielle, 2004). This uplift have led to an increase in the erosion rate and sediment supply. Changes Unit 4 shows is dominated by a braided river system in the fluvial styles reflected in the logs could be related pattern, but with periods of wandering-meandering with convergence/uplift fluctuations in the tectonic front, behavior. The intercalation of lenticular gravelly deposits which had resulted in changes in accommodation and represents high discharge pulses of sedimentation. Thicker sandy deposits of this unit represent a more sediment supply. stable, deeper, braided channel pattern. Although, the top of this unit is marked by thinner beds, representing a lower regime, probably distal shallow braided river with Acknowledgements periods of ephemeral sandy meandering behavior. We want to express our gratitude to Fredy Pedee and 4.3 Regional stratigraphy and age of the Mayoan Ximena Pedee of ‘Estancia La Frontera’, Balmaceda. deposits Alvar Dahmen assist us during the 2015 fieldwork. Christian Salazar share comments and recommendations The fluvial deposits here described are included as part of on an early draft. This research was funded by project the upper levels of the Oscuro Formation (De la Cruz et Fondecyt 1130006. al., 2003). This formation actually encompasses all the Cenozoic bedded sedimentary deposits located south of Balmaceda. Paleogene dating and Neogene fossil vertebrates on the two basal pyroclastic units clearly References demonstrates the existence of several unconformities and Ameghino, F. 1906. Les formations sédimentaires du Crétacé temporal hiatuses within the formation. Important Supérieur et du Tertiaire de Patagonie avec un parallelé changes in the lithological attributes also attest for these entre leurs faunes mammalogiques et celles de l´ancien discontinuities. Nomenclatural priority and historical continent. Anales del Museo Nacional de Buenos Aires mapping denote that the Oscuro Formation is an (tercera serie) 8: 1-568. inadequate lithostratigraphic conceptualization Dal Molín, C. & Franchi, M. 1996. Reinterpretación estratigráfica de las sedimentitas terciarias del suroeste del Chubut. En (González, 1967; Escosteguy et al., 2003). We suggest Congreso Geológico Argentino, No. 13 y Congreso de the abandon of this name and use of valid historical exploración de Hidrocarburos No. 3, Actas 1: 473-478. equivalents to refer to these successions. Based on their Buenos Aires. lithology and depositional environment Units 3 and 4 are De la Cruz, R; Suárez, M; Belmar, M; Quiroz, D; Bell, M., 2003, then correlated with the El Portezuelo Formation Área Coihaique-Balmaceda, Región de Aisén del (Escosteguy et al., 2003), first described at the General Carlos Ibáñez del Campo. Servicio Nacional de Geología y Minería, Carta Geológica de Chile, Serie neighboring Cerro Guenguel, Argentina. Geología Básica, No. 80, 40p. Escosteguy, L..; Dal Molín, C.; Franchi, M.; Geuna, S..; Lapido, O. Dating performed gives two U/Pb ages of 12.3 ± 0.16 Ma Hoja Geológica 4772-II, Lago Buenos Aires, Provincia and 12.18 ± 0.4 Ma (Serravallian age) for the base and de Santa Cruz. Programa Nacional de Cartas Geológicas 40 39 mid-portion of Unit 4. At Cerro Guenguel, Ar/ Ar de la República Argentina 1:250.000. Boletín del dating of the overlying “Río Mayo” Formation provides Servicio Geológico Argentino; Año: 2003 vol. 339 an age of 11.8 Ma (De Iuliis et al., 2008). Dal Molín & González, R. 1967. Geología de la región de Lago Blanco y Paso Franchi (1996) obtain a 12.18 Ma 40Ar/39Ar age from a Río Mayo (Chubut). Acta Geológica Lilloana IX, 45-65. Tucumán, Argentina. tuff on the top of the El Portezuelo Formation at Arroyo Miall, A. 2006. The Geology of Fluvial Deposits: Sedimentary Peregoso, in the Meseta del Chalía (incorrectly identified facies, basin analysis, and petroleum geology. Berlin: as the Pedregoso Formation by them). These beds also Springer. forms the outcrops at Lago Blanco and the upper Río Quiroz, D.; Bruce, Z. 2010. Geología del Área Puerto Ingeniero Fénix, the historical localities from which Ameghino Ibáñez-Villa Cerro Castillo, Región Aisén del General Carlos Ibáñez del Campo. Servicio Nacional de Geología (1904) and Roth (1924) originally characterized the

814 AT 1y Geolo Minería,Gía CartaReGional Geológica y Geodinámica de Chile, a Seriendina Geología Básica 124: 48 p., 1 mapa escala 1: 100.000. Roth, S. 1925. Investigaciones geológicas en la región norte de la Patagonia durante los años 1897- 1899. Museo de La Plata, Revista, Vol. 27, p. 146-180.y Minería,y Minería, Carta Carta Geológica Geológica de de Chile, Chile, Serie Serie Geología Suárez, M.; Prieto,Básica X.; 124: Básica Belmar 48 124: p.,, 48 M.;1 mapap., Quiroz,1 mapa escala escala D. 1: 100.000.1: 1994. 100.000. Roth,Estratigrafía S. Roth, 1925. S. TerciariaInvestig 1925. Investigaciones del aciones sector geológicas nororiental geológicas en en lade la región región Aysén,norte 44º de-46ºLS,norte la Patagode la Chile. Patagonia duranteniaEn durante Congreso los los años años 1897 1897-- Geológico1899. Chileno,Museo1899. Museo de 7., La de Concepción, Plata, La Plata, Revista, Revista, 17 Vol.- 21 Vol. 27, 27, p. p. 146-180.146 -180. Octubre:Suárez, pp. 533 M.; - Prieto,537. X.; Belmar, M.; Quiroz, D. 1994. Suárez, M.; Prieto,Estratigrafía X.; Belmar Terciaria, M.; del Quiroz, sector nororiental D. 1994. de EstratigrafíaAysén, Terciaria 44º-46ºLS, del sector Chile. nororientalEn Congreso de Aysén, Geológico 44º-46ºLS, Chileno, Chile. 7., Concepción,En Congreso 17-21 GeológicoOctubre: Chileno, pp. 533 -537. 7., Concepción, 17-21 Octubre: pp. 533-537.

Figure 1.! Stratigraphic sections legend used in Figures 2A and 2B.! Figure 1.! Stratigraphic sections legend used in Figures 2A and 2B.!

Figure 1.! Stratigraphic sections legend used in Figures 2A and 2B.!

Figure 2. Two stratigraphic sections of Balmaceda deposits, scale 1:200. Left: Unit 3 (upper part). Right: Unit 4 (lower part)!

Figure 2. Two stratigraphic sections of Balmaceda deposits, scale 1:200. Left: Unit 3 (upper part). Right: Unit 4 (lower part)! Figure 2. Two stratigraphic sections of Balmaceda deposits, scale 1:200. 815 Left: Unit 3 (upper part). Right: Unit 4 (lower part)!