Problemas De Datación De La Formación Navidad Y Possibles

U N I V E R S I D A D D E C O N C E P C I Ó N DEPARTAMENTO DE CIENCIAS DE LA TIERRA 10° CONGRESO GEOLÓGICO CHILENO 2003 TOWARDS AN UNDERSTANDING OF THE AGE OF THE NAVIDAD FORMATION NIELSEN, S. N. 1, DeVRIES, T. J. 2, ENCINAS, A. 3, FINGER, K. L. 4, PETERSON, D. 5 1Geologisch-Paläontologisches Institut und Museum, Universität Hamburg, Bundesstrasse 55, 20146 Hamburg, Germany. [email protected] 2Burke Museum of Natural History and Culture, University of Washington, Seattle, WA 98195 USA (Mailing address: Box 13061, Burton, WA 98013 USA). [email protected] 3Universidad de Chile, Departamento de Geología, Casilla 13518, Correo 21, Santiago, Chile. [email protected] 4University of California Museum of Paleontology, 1101 Valley Life Sciences Building, Berkeley, CA 94720-4780 USA. [email protected] 5California Academy of Sciences, Dept. of Invertebrate Zoology and Geology, Golden Gate Park, San Francisco, California 94118-4599 USA. [email protected] INTRODUCTION Ages of the Navidad Formation presented in the literature differ markedly from each other. The first micropaleontological data came from Martínez and Osorio (1968), who dated the Navidad Formation as Tortonian based on foraminifera and discoasters. Dremel (in Herm 1969) reported Burdigalian foraminifera from the coast at Punta Perro. This coastal locality is part of the basal member of the Navidad Formation in the scheme of Tavera (1979). Using the model of Tavera (1979) in the modified version of Martínez-Pardo (1990) and Valenzuela (1990), sedimentation of the Navidad Formation began during the upper Burdigalian (planktic foraminiferal zone N5/6, 19 Ma) and persisted until the lowermost Tortonian (N15, 10 Ma). Encinas and Suárez (2002) reported the presence of Carcharoides totuserratus, an elasmobranch from the late Oligocene- early Miocene, in the basal member of Navidad Formation. Meón et al. (1994), studying pollen in the same levels used by many of the authors that studied foraminifers and ostracodes, dated the basal member of the Navidad Formation as late early Miocene-late Miocene. In contrast, Martínez and Osorio (1968) and Ibaraki (1992a) dated the basal unit at Punta Perro as Tortonian (N16/17, 10-5 Ma). Based on the evidence of younger ages, the Miocene deposits of central Chile would represent a relatively brief interval rather than almost the entire Miocene. For this study, we investigated the coastal outcrops of the Navidad Formation (Figure 1), including most localities of previous workers. We measured sections, described sedimentary features, and collected samples for macro- and microfossils. All of these outcrops are referred to to the Navidad Member, which is the basal member of the Navidad Formation (Tavera 1979). Outcrops farther inland are heavily weathered and our samples from them were devoid of fossils. For comparison with the Navidad Member, we also examined material from the Ranquil Formation at Peninsula Arauco, and the Lacui Formation at Chiloé Island (Figure 1). Todas las contribuciones fueron proporcionados directamente por los autores y su contenido es de su exclusiva responsabilidad. Figure 1. Sampled localities of the Navidad Formation and comparable units. SEDIMENTOLOGY The lower member of the Navidad Formation has a fossiliferous marine conglomerate at its base that marks the beginning of a relative rise in sea level. The succession continues with mudstone, siltstone, sandstone and conglomerate beds with sedimentary features such as massive sandstones with water-escape structures, sheared mud flames and basal rip-up mudclasts, Bouma sequences, sedimentary breccia beds, slumps and slides. These features reveal that the Navidad Member was formed by gravity-driven flows (e.g., sandy debris flows, turbidity currents, debris flows, slides and slumps) alternating with hemipelagic deposits (Encinas et al. in press, this volume). PALEONTOLOGY Planktic foraminiferal assemblages in many of our samples reveal a Tortonian age (N16) for the Navidad Member (Finger et al. this volume). Mixed depth associations in the Navidad, Ranquil, and Lacui Formations (Encinas et al. in press; Finger et al. this volume) indicate displacement of sediments into bathyal depths greater than 1500 meters (Finger et al. this volume). Some Cretaceous and early and middle Miocene foraminifera in these deposits also reveal reworking of older deposits. Comparison of molluscan assemblages in the Navidad Formation with those of southern Peru favors a youngest Oligocene to early middle Miocene age for the Navidad molluscan fauna (DeVries and Frassinetti 2003; DeVries and Nielsen this volume). Several distinctive Navidad species occur in forearc basins of southern Peru, including Ficus distans Sowerby, 1846, Acanthina katzi (Fleming, 1972), Testallium cepa (Sowerby, 1846), Olivancellaria tumorifera (Hupé, 1854), and Glycymeris ibariformis Frassinetti and Covacevich, 1984 (DeVries 1997a and in press; DeVries and Vermeij 1997; Vermeij and DeVries 1997). The Peruvian specimens of these species were found in sediments that have been dated using radiolaria (Marty 1989), foraminifera (Dunbar et al. 1990; Ibaraki 1992a, 1992b, and 1993), diatoms (Macharé and Fourtanier 1987; Macharé et al. 1988; Dunbar et al. 1990; DeVries 1998; H. Schrader, unpublished data), 40K-40Ar isotopes (Muizon and Bellon 1980 and 1985; Noble et al. 1985; Muizon and DeVries 1985; Dunbar et al. 1990), and 40Ar-39Ar isotopes (DeVries 1998; DeVries, unpublished data). Upper Miocene (Tortonian, Messinian) and Pliocene sediments from southern Peru contain many molluscan species that are the same as those found in Chilean deposits traditionally assigned to post-Navidad, upper Miocene and Pliocene depositional units (Herm 1969; DeVries and Frassinetti 2003; Guzmán et al. 2000). Middle Miocene (Serravallian) deposits in Peru contain a distinctive suite of molluscan species, some being the same as middle Miocene species from the Talara Basin of northern Peru (DeVries 1997b), but none occurring in the Navidad Formation. The youngest specimens of 'Navidad' species appear in southern Peru in basal deposits of the Pisco Formation (Langhian), but they are most common throughout the lower Miocene (Burdigalian, Aquitanian) deposits of the Chilcatay Formation (DeVries 1998). The oldest occurrences of Navidad species (Olivancellaria tumorifera, Testallium cepa) are found in uppermost Oligocene (Chattian) deposits near Caravelí and Nazca (DeVries 2000). The Caravelí fossils are in beds correlative with volcanic ashes dated at about 25 Ma (Noble et al. 1985). The lower Miocene sediments of southern Peru were deposited in a variety of marine environments, ranging from intertidal to upper slope. The intertidal deposits include oyster- encrusted boulders of igneous rock and a diverse transported assemblage of mollusks, shark teeth, and other vertebrate remains. In contrast, the deeper water deposits are tuffaceous, diatomaceous silty fine-grained sandstones bearing cetacean bones, sardine and anchovy vertebrae and scales, and scattered articulated venerid bivalves (DeVries and Nielsen this volume). Accumulations of 'Navidad' mollusks are typically found where sedimentary structures and textures suggest intertidal to inner neritic environments. IN SITU OR MIXED FAUNAS? The different sampled localities from Chile (Figure 1) can be divided among three environmental settings based on their sedimentological and paleontological data (Table 1). We interpret these as representing original settings prior to subsequent displacement. Assigning 83 gastropod genera to these environments (Figure 2) results in a distribution pattern that suggests the sediment Figure 2. Percentage distribution of gastropod genera in different depositional environments. Table 1. Division of localities among environmental settings. Int: Intertidal, Sw: shallow water, Dw: deep water. NAVIDAD FM RANQUIL FM LACUI FM P T LBZ MA MA PPG PPN PPP PPS PPT PTA PUY RAP FRA FRM LEB RAN RQK RQS RQT CHO CUC PCT Int X X X X X X Sw X X X X X X X X X X D X X X X X w packages only contain unmixed assemblages rather than mixed associations resulting from reworking and displacement. Occurrence in two or all three environments is mainly due to the very rough divisions or, for some genera (e.g., Terebra, Turritella), the presence of more than one species. Among the representative gastropods of the intertidal zone are new species of Fissurella, Pilosabia, and Zonaria. Shallow-water associations contain the majority of classic Navidad mollusk taxa such as ‘Trochus laevis’, Sassia armata, Lamprodomina dimidiata, and Olivancillaria tumorifera. Much less common in the Navidad are deep-water taxa, including articulated brittle stars of the genus Ophiomusium (Kutscher et al. this volume) and the gastropods Struthiochenopus philippii, Falsilunatia, Dalium, Exilia, and Borsonia. The small group of gastropods represented in all three environments consists of Sinum subglobosum, Echinophoria, Testallium cepa, and Ptychosyrinx. Some of the relatively large mollusks are fragile (e.g. Xenophora, Nielsen and DeVries 2002) and their preservation suggests that, if they were transported downslope, it would have involved relatively nonchaotic displacement (e.g., slumping) of a large sedimentary package. CONCLUSIONS (1) Planktic foraminifera indicate that the Navidad Formation was deposited during the lower part of planktic foraminiferal Zone N16 (about 10 to 11 Ma), which correlates with the Tortonian interval of the Late Miocene. (2) Reworking and mixing of older sediments

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