What is “typical” Pliocene? An evaluation of new and long known Chilean Pliocene mollusk faunas

Sven N. Nielsen Institut für Geowissenschaften, Christian-Albrechts-Universität zu Kiel, Ludewig-Meyn-Str. 10, 24118 Kiel, Germany

*E-mail: [email protected]

Abstract. Certain mollusk faunas are traditionally regarded 2 Geology of fossil bearing deposits to be “typically” Pliocene. However, since the last extensive review some of these, like the lower part of the Bahía 2.1 Morro Mejillones, Mejillones Peninsula, Inglesa Formation, were shown to be late Miocene in age and others, previously thought to be of Pleistocene age, northern Chile have proven to represent Pliocene faunas. New and little known Pliocene mollusk faunas from Mejillones, Horcón, La The Morro Mejillones fauna (MOM) comes from a densely Cueva and Arauco are analyzed, their age as much as packed coquina at the rim of the uppermost marine terrace possible refined, and problems discussed. Apparently there on Morro Mejillones, Mejillones Peninsula, northern Chile exist at least three different “types” of Pliocene mollusk (S 23°07.126’, W 070°30.815’). Today, coquina and faunas: 1) a late Miocene to Pliocene fauna which is terrace are at an elevation of approximately 410m. The historically considered as “Pliocene”; 2) the extant Chilean deposits were briefly described by Nielsen (2009), but for fauna which can be shown to be fully developed at certain some reason the GPS data given there, taken in the year doubtlessly Pliocene localities; and 3) a typical Pliocene 2002, are completely off-track. This coquina has a fauna of supposedly late Pliocene age. carbonate-rich sandy matrix and consists mainly of Keywords: Chile, Mollusca, Pliocene barnacle clusters and debris; it also contains well- preserved mollusks, indicating an environment of sandy beach and nearby rocky shore. Strontium isotope data 1 Introduction (87 Sr-86 Sr ratios) indicate a Pliocene age of ~3.4 Ma for this coquina (Nielsen 2009; Victor et al. 2011). Biotite from a Pliocene marine mollusk faunas from Chile are mainly nearby ash layer on Morro Mejillones was dated as known from the Coquimbo Formation of central Chile aproximately 3 Ma (Marquardt et al. 2005) and supports (Herm 1969). Four new Pliocene mollusk faunas were this middle Pliocene age. It also suggests tectonic uplift investigated and show little resemblance to the Coquimbo between 3.4 and 3 Ma (Victor et al. 2011). fauna. Because of its faunal composition, the Coquimbo fauna is interpreted to be of late Pliocene to Pleistocene age. The new faunas include several species previously 2.2 Horcón Formation, central Chile known only from southern Peru and some earliest occurrences are reported. Key species include Chlorostoma The Horcón Formation crops out along the coast between quipua DeVries, 2007, Incatella cingulatiformis (Möricke, Horcón (32°42’S, 71°21’W) and Maitencillo (32°40’S, 1896), Concholepas camerata DeVries, 2000, Chorus 71°26’W) and consists of subhorizontal sandstones to doliaris (Philippi, 1887), Chorus grandis (Philippi, 1887), mudstones with few intercalated conglomerates. These and Stramonita zinsmeisteri DeVries, 2007. Some of the unconformably overlie the crystalline basement and are Pliocene faunas can easily be distinguished from the themselves overlain by fossil dunes. The mollusk fauna Miocene Navidad fauna and the younger Plio-Pleistocene from the Horcón Formation was described by Tavera fauna and are endemic to the area between southern Peru (1960), who also assigned a Pliocene age to that unit. The and south central Chile. They reflect recovery of several appearance of Chlamys cf. hupeanus (Philippi, 1887), taxa after the severe regional extinction of the Navidad Panopea coquimbensis (d’Orbigny, 1842), Chorus fauna (see Kiel and Nielsen 2009) which possibly resulted blainvillei (d’Orbigny, 1842), Chorus doliaris (Philippi, from drastic cooling of seawater surface temperatures 1887) and Herminespina mirabilis (Möricke, 1896) which was proposed for the late Miocene. However, many suggests a late Pliocene age. of the typical Pliocene taxa did not survive and have no representatives in the modern fauna. The modern Chilean mollusk fauna has its diversity peak in the south; a feature 2.3 La Cueva Formation, central Chile which developed in the Quaternary after glaciers retreated and left a fractal coastline with a variety of habitats open Most of the fauna from the La Cueva Formation comes for colonization (Kiel and Nielsen 2009). from the Estero El Ganso (EGA), a locality already roughly described by Philippi (1887), who himself was not very lucky in finding fossils there but described several species sent to him by the owner of the hacienda. Möricke 728 (1896) described some new species from this locality, but did not describe it. Both, locality and fauna were briefly Species Peru MOM EGA RQS described by Herm (1969) but sedimentology and Nacella ● ● stratigraphy of the La Cueva Formation were only recently intiforma 2 described in some detail by Encinas et al. (2006). Chlorostoma ● ●3 ● According to these latter authors, the La Cueva Formation atrum 1 consists mainly of sandstone with rare conglomerates and Chlorostoma ● ●4 even less siltstones. It covers either the underlying Rapel quipua 2 Formation with a paraconformity or lies discordantly on Liotia cancellata 1 ●3 the granitoid basement and is covered discordantly by Diodora sp. ● Pleistocene continental deposits (Encinas et al. 2008). The Fissurella (F. ) ● ● ● thickness of the La Cueva Formation is estimated to be spp. about 100 m (Encinas et al. 2008). Encinas et al. (2008) Incatella ● ● ● ? confirmed the Pliocene age given by previous authors cingulatiformis 2 based on macrofossils, mainly mollusks, but even absolute 1 3 dating did not provide better constrained ages (Encinas et Caecum chilense ● Polinices cf. ● al. 2006). 1 panamaensis Neverita ● 2 2.4 Huenteguapi Sandstone, Arauco Peninsula, obtectiformis Trochita cf. ● south-central Chile 1 trochiformis A coarse grey sandstone of the Ranquil formation was Trochita ● 2 called Huenteguapi Sandstone (RQS) and contains large colchaguensis reworked clasts from the underlying deposits (see Finger et Crucibulum ● 1 al. 2007; Le Roux et al. 2008) with equally reworked quiriquinae mollusks like Olivancillaria claneophila , Lamprodomina Crepipatella ● ● 1 dimidiata and Sinum subglobosum , but also the apparently dilatata younger species reported here. The Huenteguapi Sandstone Priene scabrum 1 ● ●3 ● consists of the main sandstone bed of about 5 to more than Macron ●4 ● 30 m thickness, with an erosive basal contact and escalonia 2 sandstone intrusions reaching into the underlying deposits Oliva peruviana 1 ● ● ● at varying angles with a length of up to more than 30 m Mitra orientalis 1 ● and up to 2 m width (Le Roux et al. 2008). This unit was Austrofusus ● interpreted as a tsunami backflow deposit eroding coastal fontainei 1 dunes (Le Roux et al. (2008), which would explain Austrofusus ● reworked fossils and the scarcity of younger specimens. steinmanni 1 The underlying mudstones (RQT, MIB) and sandstones Nassarius cf. ● ● (RAN) of the Ranquil Formation yielded a late Miocene gayi 1 age based on the co-occurrence of Neogloboquadrina Concholepas ● ●3,4 continuosa and N. pachyderma while the overlying camerata 2 calcareous sandstone (RQK) has an early Pliocene age Acanthina ● based on the occurrence of Globorotalia puncticulata unicornis 1 (Finger et al. 2007). Without considering the strong Acanthina ●4 reworking effect and the scarcity of youngest age markers triangularis 2 (Finger et al. 2007) this makes an earliest Pliocene age for Crassilabrum ● ● ● this deposit likely. crassilabrum 1 Xanthochorus ● cassidiformis 1 3 Species and ranges Chorus doliaris 2 ● ● ● Chorus grandis 2 ● ● Pliocene molluscan species from southern Peru, Morro Stramonita ● ●4 Mejillones (MOM), La Cueva (Estero El Ganso, EGA) and zinsmeisteri 2 the Huentequapi Sandstone (RQS) are compared. The Bulla punctulata 1 ● Horcón fauna is still under study and likely late Pliocene in Chilina sp. ● age and therefore not shown here. Anadara ● chilensis 2 Acar pusilla 1 ●3

729 Glycymeris ● References ovatus 1 Crenella sp. ●3 ● DeVries, T.J., and D. Frassinetti. 2003. Range extensions and Retrotapes ● biogeographic implications of Chilean Neogene mollusks found fuenzalidae 2 in Peru. Boletín del Museo Nacional de Historia Natural Chile 52: 119-135. Protothaca ● 1 antiqua Encinas, A., J.P. Le Roux, L.A. Buatois, S.N. Nielsen, K.L. Finger, Amiantis ● E. Fourtanier, and A. Lavenu. 2006. Nuevo esquema domeykoana 2 estratigráfico para los depósitos mio-pliocenos del área de Navidad (33°00’-34°30’S), Chile central. Revista Geológica de Diplodonta ● 1 Chile 33: 221-246. inconspicua 1 Semele solida ● Encinas, A., K.L. Finger, S.N. Nielsen, A. Lavenu, L.A. Buatois, Tagelus dombeii 1 ● ● D.E. Peterson, and J.P. Le Roux. 2008. Rapid and major coastal Chama pellucida 1 ● subsidence during the late Miocene in south-central Chile. Journal of South American Earth Sciences 25: 157-175. Cyclocardia sp. ● 2 nov. Finger, K.L., S.N. Nielsen, T.J. DeVries, A. Encinas, and D.E. Ensis macha 1 ● Peterson. 2007. Paleontologic evidence for sedimentary displacement in Neogene forearc basins of Central Chile. Palaios 1: Recent species, 2: Exclusively Pliocene species, 3: 22: 3-16. Oldest record, 4: Range extension. Herm, D. 1969. Marines Pliozän und Pleistozän in Nord- und Mittel- Chile unter besonderer Berücksichtigung der Entwicklung der Mollusken-Faunen. Zitteliana 2: 1-159.

4 Results Kiel, S., and S.N. Nielsen. 2010. Quaternary origin of the inverse latitudinal diversity gradient among southern Chilean mollusks. Comparison of these faunas with the typical Pliocene fauna Geology 38: 955-958. described by Herm (1969) shows major differences in Le Roux, J.P., S.N. Nielsen, H. Kemnitz, and A. Henriquez. 2008. A composition. Faunas from typical Pliocene localities near Pliocene mega-tsunami deposit and associated features in the Coquimbo and on Mejillones include species like Chlamys Ranquil Formation, southern Chile. Sedimentary Geology 208: simpsoni (Philippi, 1887), Chlamys vidali (Philippi, 1887), 164-180. Chorus blainvillei (d’Orbigny, 1842), Concholepas nodosa Möricke, 1896, Herminespina mirabilis (Möricke, 1896), Marquardt, C., M. Fomari, A. Lavenu, G. Vargas, L. Ortlieb, J.F. Ritz, none of which ranges younger than Pliocene, and Fusinus H. Philip, and N. Marinovic. 2005. Volcanic ash dating from the Mejillones Peninsula (23°S): Implications for the Neogene outer remondi (Philippi, 1887) which may reach the earliest fore-arc stratigraphy , tectonics and volcanic relationships. In 6th Pleistocene (Herm 1969; DeVries and Frassinetti 2003). International Symposium of Andean Geodynamics, Extended This composition agrees in part with that of the Horcón Abstracts: 477-480. Barcelona. Formation. However, none of these species occurs in the above reported localities. It seems likely that typical Möricke, W. 1896. Versteinerungen der Tertiärformationen von Chile. Neues Jahrbuch für Mineralogie, Geologie und Pliocene localities are younger than the here reported ones. Paläontologie Beilage Band 10: 548-612. This implies that the modern Chilean fauna was already established during the time of typical Pliocene faunas, Nielsen, S.N. 2009. Pliocene balanuliths from northern Chile: the suggesting that ecological differences must exist between first report of fossil balanuliths. Palaios 24: 334–335. localities with a typical Pliocene fauna and coeval localities with a modern fauna. To resolve the nature of Philippi, R.A. 1887. Die tertiären und quartären Versteinerungen Chiles. F.A. Brockhaus: 266 p. Leipzig. these ecological differences will be substantial for the understanding of faunal turnover and ecological as well as Tavera J. 1960. El Plioceno de Bahía Horcón en la provincia de stratigraphical interpretation of single localities. Valparaíso. Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Anales 17: 346-367.

Victor, P., M. Sobiesiak, J. Glodny, S.N. Nielsen, and O. Oncken. 2011. Long-term persistence of subduction earthquake segment boundaries – evidence from Mejillones peninsula, N-Chile. Journal of Geophysical Research 116, B02402, doi:10.1029/2010JB007771.

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