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F u n 2 d 6 la serena octubre 2015 ada en 19 Lithostratigraphy, depositional environments and tectonic setting of the Bahía Inglesa Formation west of Copiapó, north-central

Jacobus P. Le Roux1,2, Luciano Achurra3, Álvaro Henríquez4, Catalina Carreño5, Huber Rivera1 1Departamento de Geología/2Andean Geothermal Center of Excellence, Universidad de Chile, Chile; 3Arcadis Consultores, Santiago, Chile; 4SQM. S.A., Calama, Chile; 5Codelco, Calama, Chile

1 Contact e-mail: [email protected]

Abstract. As yet unpublished stratigraphic sections were A total of 32 stratigraphic sections were measured in the used to revise the lithostratigraphy and conduct a Bahía Inglesa Formation since 2003, but they have only geohistory analysis of the Bahía Inglesa Formation south been informally recorded in unpublished Graduate and of Caldera. This indicates that the basin began to subside Masters Theses (Achurra, 2004; Henriquez, 2006; from more than 100 m above present sea-level at 15 Ma to Carreño, 2012). Of these, one section (Le Roux, reach 100 m below sea-level at around 9 Ma. It then unpublished work) lies within a cutting experienced an uplift of 100 m until 7.7 Ma, and then through older strata of the Bahía Inglesa Formation, so that subsided more than 300 m until 6 Ma, before being uplifted the canyon-fill deposits cannot be correlated directly with again by 200 m until 5 Ma. These tectonic events are the rest of the Bahía Formation profiles, even though they ascribed to the southward migration of the Juan Fernández Ridge underneath the continental crust. Differences in the are considered to belong to the same succession. Table 1 migration speed and intensity of uplift-subsidence cycles shows the proposed stratigraphic subdivision, depositional can possibly be attributed to oroclinal bending of the Juan environments and approximate ages. Fernández Ridge due to friction with the overlying continental plate. 3 Metodology Keywords: ; Juan Fernández Ridge; oroclinal bending The sedimentary facies and stratigraphic succession of the Bahía Inglesa Formation reflect sea-level oscillations that can be used, in conjunction with global sea-level curves 1 Introduction (Hardenbol et al., 1998), to deduct relative tectonic We present an interpretation of the , age and movements affecting this part of the southeastern Pacific depositional environment of the Bahía Inglesa Formation coastline during the . The methodology, known in the Caldera Basin west of Copiapó, based on as yet as geohistory analysis, consists of determining the age and unpublished stratigraphic sections. The basin, which depositional depths of the units composing a stratigraphic stretches south along the coastal plain from Caldera to the succession, relating the paleobathymetry to the global sea mouth of the Copiapó River, is filled by a Neogene marine level at the time, and calculating the progressive elevation succession overlying metamorphic rocks and of a reference surface relative to the present . granitoids. The succession is subdivided into the Here, the cumulative thickness of the units must be Angostura Formation, dated at 15.3±0.6 Ma (Henriquez, employed, but in this case the thicknesses of the different 2006), Bahía Inglesa Formation, and the Caldera Beds. members are variable from section to section and in some The Bahía Inglesa Formation is a semi-consolidated cases incomplete, e.g. where the base or top of the member sequence of fossiliferous, clastic and phosphatic rocks was not exposed. Therefore, only the maximum measured characterized by numerous lateral and vertical facies thickness of each particular member, rounded to the change. It has been dated by different authors as middle nearest meter, was used. The cumulative thickness was to Pliocene in age, whereas the depositional measured upward from the reference surface (the contact environment has been interpreted as ranging from the between the Angostura Member and the basement littoral zone to the upper continental slope (Marquardt, at Puerto Viejo) to the middle of each member, thus 1999; Godoy et al., 2003; Achurra, 2004; Henriquez, progressively adding half the maximum thickness of each 2006; Achurra et al., 2009, Carreño, 2012). member to those of the underlying members. As concerns the paleobathymetry, the following mean depositional 2 Stratigraphy depths were assigned to the different facies: supratidal flat – 1 m; foreshore – 0 m; estuary – -10 m; upper shoreface

-, -20 m; middle shoreface – -50 m; lower – shoreface – - 821 ST 4 ESTRATIGRAFÍA, ANÁLISIS DE CUENCAS Y PROCESOS SEDIMENTARIOS

80 m; inner shelf – -120 m; outer shelf – -170 m; upper continental slope – -500 m. Fig. 1 shows the results of this analysis.

4 Results and discussion References

The absence of rocks with an age between 15.3 and about Achurra, L.E. 2004. Cambios del nivel del mar y 10 Ma in the coastal sector between Puerto Viejo and evolución tectónica de la cuenca neógena de Caldera, Caldera indicates a major unconformity, suggesting that III Región. Masters Thesis (unpublished), this part of the coastline was subjected to subaerial erosion Universidad de Chile, Departamento de Geología: during the Langhian to early Tortonian. The Angostura 138 p. Formation was deposited in a shoreline environment, Achurra, L.E.; Lacassie, J.P.; Le Roux, J.P.; Marquardt, probably a paleo-estuary of the Copiapó River, but at an C.; Belmar, M.; Ruiz-del-Solar, J.; Ishman, S.E. elevation of about 100 m above present sea level. Since 2009. Manganese nodules in the Miocene Bahía 15.3 Ma, tectonic subsidence occurred simultaneously Inglesa Formation, north-central Chile. Petrography, with a drop in sea-level, but no marine deposition took geochemistry, genesis and palaeoceanographic place until about 10 Ma. From 10 – 9.0 Ma, the basin significance. Sedimentary Geology 217: 128–139. subsided to 100 m below present sea level, rising about 70 Carreño, C.A. 2012. Ambiente deposicional de la m between 9.0 and 8.0 Ma. This was followed by dramatic Formación Bahía Inglesa (Neógeno) en la Cuenca de subsidence of nearly 300 m, bringing the depositional Caldera, III Región, Chile. Graduate Thesis depths to that of the upper continental slope. However, a (unpublished), Universidad de Chile, Departamento similarly dramatic turn-around at about 7 Ma uplifted the de Geología: 98 p. coastal sector more than 200 m, succeeded by another Godoy, E.; Marquardt, C.; Blanco, N. 2003. Carta Caldera, subsidence of 70 m until 2.3 Ma. Región de Atacama. Serie Geología Básica, No. 76. The seaboard of around 100 m a.s.l. at 15 Ma can possibly Servicio Nacional de Geología y Minería, Santiago: be attributed to the presence of the NE-trending Juan 38 p. Fernández Ridge underneath the continental crust during Hardenbol, J.; Thierry, J.; Farley, M.B.; Jacquin, T.; the Langhian. As the ridge migrated southward, the crust Graciansky, P.-C.; Vial, P.R. 1998. Mesozoic and subsided strongly in its wake, before isostatic rebound sequence chronostratigraphic framework caused renewed uplift during the Zanclean. These events of European basins. In Mesozoic and Cenozoic mirror similar tectonic processes described further south sequence stratigraphy of European basins at Carrizalillo and Tongoy (Le Roux et al., 2005). (Graciansky, P.-C., et al., editors. SEPM (Society for However, there was apparently a slower migration rate and Sedimentary Geology) Special Publication 60, chart more intense uplift-subsidence effects in the Caldera area 1. than further south. This could be attributed to oroclinal Henríquez, A.A. 2006. Variaciones locales del nivel del bending of the Juan Fernández Ridge due to friction with mar en las cuencas neógenas de Caldera, III Región the overlying continental plate, which diminished the y Arauco, VIII Región: Deducción de tasas de angle of incidence and the intensity of the stress field, but alzamiento y subsidencia tectónica. Masters Thesis increased the migration velocity of the ridge relative to the (unpublished), Universidad de Chile, Departamento coastline. de Geología: 170 p. Le Roux, J.P.; Gómez, C.A.; Olivares, D.M.; Middleton, H. 2005. Determining the Neogene behavior of the Acknowledgements Nazca Plate by geohistory analysis. Geology 33: 165- 168. Projects FONDECYT 101691, 1130006, and CONICYT- Marquardt, C. 1999. Neotectónica de la franja costera y FONDAP 15090013, "Andean Geothermal Center of aportes a la geología regional entre Caldera y Caleta Excellence (CEGA) 195, are thanked for financial and Pajonal (27° - 27°45´), III Región de Atacama. logistical support. Masters thesis (unpublished), Universidad de Chile, Departamento de Geología: 134 p.

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Table 1. Stratigraphic subdivision of the Bahía Inglesa Formation (A = Achurra, 2004; H = Henríquez, 2006; C = Carreño, 2012).

New Correlation Lithological characteristics and Depositional Age range subdivision with older maximum recorded thickness environments Mdeian (Members) units age): Ma Quebrada 8, 9 (A); 11 Conglomerate, , (bio)calci- Upper – lower <2.3 Blanca (H) rudite, diatomaceous shale (23) shoreface, backflow Rocas Negras 6, 7 (A); 12 (Bio)calcirudite, (bio)calcarenite, Upper – middle 4.2 (H); 7 (C) minor tuff, conglomerate, sandstone, shoreface (30 m) Mina 4, 5 (A); 8, 9, Sandstone, , shale, minor Upper shoreface – 7.6–6.0 (6.8) Fosforita 10 (H); 4, 5, 6 phosphate-pebble conglomerate, upper continental (C) biocalcarenite (22 m) slope Chorillos 4 (C) Conglomerate with large rip-up clasts Submarine channel, 8.1–6.8 (7.5) (8 m) tsunami backflow La Higuera 4 (A); 8 (H); 2, Shale, sandstone, siltstone, gypsum Outer shelf – upper 9.2–7.0 (8.1) 3, 4 (C) veins (26 m) continental slope 8 (C) Sandstone, siltstone (9 m) Supratidal flat 9.0–6.5 (8.4) Punto Totoral 2, 3, 4 (A); 4, 7 Biocalcarenite, biocalcirudite, Upper – middle 9.9–8.5 (9.2) (H): 2, 3 (C) conglomerate (23 m) shoreface Puerto Viejo 1, 2 (A); 6 (H); Sandstone, siltstone, shale (11 m) Middle – lower 9.7–9.2 (9.5) 1 (C) shoreface El Pimiento 1, 2 (A); 3, 5, 6 Biocalcarenite, (bio)calcirudite, Upper – lower shore- 9.9–9.5 (9.7) (H) sandstone, minor conglomerate, shale, face claystone (24 m)

Ma 20 15 10 5 0 200 150 100 50 0 -50 -100 -150

-200 Paleobathymetry -250 -300 -350

Figure 1. Tectonic uplift and subsidence in the Caldera basin as reflected in the Bahía Inglesa Formation.

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Table 1. Stratigraphic subdivision of the Bahía Inglesa Formation (A = Achurra, 2004; H = Henríquez, 2006; C = Carreño, 2012).

New Correlation Lithological characteristics and Depositional Age range subdivision with older maximum recorded thickness environments Mdeian age): (Members) units Ma Quebrada 8, 9 (A); 11 Conglomerate, sandstone, (bio)calci- Upper – lower <2.3 Blanca (H) rudite, diatomaceous shale (23) shoreface, tsunami backflow Rocas Negras 6, 7 (A); 12 (Bio)calcirudite, (bio)calcarenite, Upper – middle 4.2 (H); 7 (C) minor tuff, conglomerate, sandstone, shoreface shale (30 m) Mina 4, 5 (A); 8, 9, Sandstone, siltstone, shale, minor Upper shoreface – 7.6–6.0 (6.8) Fosforita 10 (H); 4, 5, 6 phosphate-pebble conglomerate, upper continental (C) biocalcarenite (22 m) slope Chorillos 4 (C) Conglomerate with large rip-up clasts Submarine channel, 8.1–6.8 (7.5) (8 m) tsunami backflow La Higuera 4 (A); 8 (H); Shale, sandstone, siltstone, gypsum Outer shelf – upper 9.2–7.0 (8.1) 2, 3, 4 (C) veins (26 m) continental slope Cerro Ballena 8 (C) Sandstone, siltstone (9 m) Supratidal flat 9.0–6.5 (8.4) Punto Totoral 2, 3, 4 (A); 4, Biocalcarenite, biocalcirudite, Upper – middle 9.9–8.5 (9.2) 7 (H): 2, 3 (C) conglomerate (23 m) shoreface Puerto Viejo 1, 2 (A); 6 Sandstone, siltstone, shale (11 m) Middle – lower 9.7–9.2 (9.5) (H); 1 (C) shoreface El Pimiento 1, 2 (A); 3, 5, Biocalcarenite, (bio)calcirudite, Upper – lower shore- 9.9–9.5 (9.7) 6 (H) sandstone, minor conglomerate, shale, face claystone (24 m)

Figure 1. Tectonic uplift and subsidence in the Caldera basin as reflected in the Bahía Inglesa Formation.

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