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Authors requiring further information regarding Elsevier’s archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright Author's personal copy Journal of South American Earth Sciences 29 (2010) 619–626 Contents lists available at ScienceDirect Journal of South American Earth Sciences journal homepage: www.elsevier.com/locate/jsames Amazonian magnetostratigraphy: Dating the first pulse of the Great American Faunal Interchange Kenneth E. Campbell Jr. a,*, Donald R. Prothero b, Lidia Romero-Pittman c, Fritz Hertel d, Nadia Rivera b a Vertebrate Zoology, Natural History Museum of Los Angeles County, 900 Exposition Boulevard, Los Angeles, CA 90007, USA b Department of Geology, Occidental College, Los Angeles, CA 90041, USA c Instituto Geológico, Minero y Metalúrgico (INGEMMET), San Borja, Apartado 889, Lima 41, Peru d Department of Biology, California State University, 18111 Nordhoff Street, Northridge, CA 91330, USA article info abstract Article history: The chronostratigraphy of the youngest Neogene deposits of the Amazon Basin, which comprise the Mad- Received 8 May 2009 re de Dios Formation in eastern Peru, remains unresolved. Although 40Ar/39Ar dates on two volcanic ashes Accepted 29 November 2009 from this formation in Peru provide critical baseline data points, stratigraphic correlations among scat- tered riverine outcrops in adjacent drainage basins remain problematic. To refine the chronostratigraphy of the Madre de Dios Formation, we report here the magnetostratigraphy of an outcrop on the Madre de Keywords: Dios River in southeastern Peru. A total of 18 polarity zones was obtained in the 65-m-thick Cerro Col- Amazon orado section, which we correlate to magnetozones Chrons C4Ar to C2An (9.5–3.0 Ma) based on the prior GAFI 40Ar/39Ar dates. These results confirm the late Miocene age of a gomphothere recovered from the Ipururo Magnetostratigraphy Cerro Colorado Formation, which underlies the late Miocene Ucayali Unconformity at the base of the Cerro Colorado out- Amahuacatherium crop. The results also support earlier interpretations of a late Miocene age for other fossils of North Amer- Madre de Dios Formation ican mammals recovered from basal conglomeratic deposits of the Madre de Dios Formation immediately Peru above the Ucayali Unconformity. These mammals include other gomphotheres, peccaries, and tapirs, and their presence in South America in the late Miocene is recognized as part of the first pulse of the Great American Faunal Interchange. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction of a late Miocene age for the oldest known suite of North American mammals to have arrived in South America in the first pulse of the Correctly interpreting Earth history is dependent upon accurate Great American Faunal Interchange (GAFI). chronologies. However, for some regions and for some time peri- The paleomagnetic data reported by Campbell et al. (2001) were ods, obtaining accurate chronologies has proven very difficult. the result of initial tests to determine the feasibility of using this One such region for which accurate chronologies are sorely lacking technique for dating the unconsolidated late Neogene sediments is the Amazon Basin, where vast tropical lowlands are covered by of Amazonia. To expand on those limited test sites, the Cerro Colo- Neogene sediments of undocumented age. As a consequence there rado section (centered at approximately 12°33.980S; 70°06.190W) is considerable debate regarding the geologic history of lowland (Figs. 1 and 2) on the Madre de Dios River was selected because it Amazonia (reviewed in Campbell et al., 2006). To date, there are is one of the largest sections through the Madre de Dios Formation only two 40A/39A dates on volcanic ashes within lowland Amazonia exposed in southeastern Peru, it is readily accessible, and the prim- and a paucity of paleomagnetic data that support a chronology for itive late Miocene gomphothere Amahuacatherium peruvium the region (Campbell et al., 2001). To further document the chro- Romero-Pittman (1996) came from the Ipururo Formation that nostratigraphy of Neogene sediments of Amazonia, we report here crops out at the base of the outcrop in the dry season (Campbell the magnetostratigraphy of a section through the Madre de Dios et al., 2000, 2009). Doubts regarding the age assigned to this gom- Formation in southeastern Peru. These data confirm the value of phothere had been raised (e.g., Alberdi et al., 2004; Ferretti, 2008), magnetostratigraphy in dating Neogene sediments of Amazonia and these doubts could best be addressed through independent and suggest a potential for correlating deposits widely within dating methods such as magnetostratigraphy. Amazonia. The data are also consistent with prior interpretations 2. Neogene stratigraphy of Amazonia * Corresponding author. Tel.: +1 213 763 3425; fax: +1 213 765 8179. E-mail addresses: [email protected] (K.E. Campbell), [email protected] (D.R. Prothero), [email protected] (L. Romero-Pittman), [email protected] The Neogene stratigraphy of Amazonia, as interpreted by (F. Hertel), [email protected] (N. Rivera). Campbell et al. (2000, 2001, 2006), comprises a sequence of older 0895-9811/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsames.2009.11.007 Author's personal copy 620 K.E. Campbell Jr. et al. / Journal of South American Earth Sciences 29 (2010) 619–626 Fig. 1. Index map showing location of the Cerro Colorado section (solid triangle) and the location of the 40A/39A dated Cocama and Piedras volcanic ashes. Modified from Campbell et al. (2001). formations that are separated from overlying younger sediments by the Pan-Amazonian Ucayali Unconformity (Fig. 3). The older deposits are primarily moderately to well-consolidated, fine- Fig. 2. The Cerro Colorado outcrop as it appeared in 1986, illustrating the grained, terrigenous deposits that predate 10.5–9.5 Ma, whereas distinctiveness of the various horizons within the Madre de Dios Formation. Units the younger deposits are unconsolidated silts, sands and clays ‘‘A,” ‘‘B,” and ‘‘C” are indicated. The Ipururo Formation appears in the lower right referable to the Madre de Dios Formation (=Içá Formation in Brazil) corner (I), rising just above the water line during the dry season. The sampling we report on herein occurred to the left of this view because this portion of the outcrop that post-date 9.5 Ma. Deposition of the Madre de Dios Formation has experienced considerable slumping since 1986 and is not amenable to sampling 40 39 is estimated to have begun at 9.5–9.0 Ma, based on the A/ A at this time. date on the Cocama ash (9.01 ± 0.28 Ma) (Campbell et al., 2001), which occurs 4 m above the Ucayali Unconformity. The base of the Madre de Dios Formation is often a very fossiliferous clay-peb- age assigned to the deposits. Of significance here, the Ucayali ble or clay-ball conglomerate that yields many significant verte- Unconformity is almost uniformly visible at the water line during brate fossils that date the conglomerate to the Huayquerian the dry season because the underlying, well-consolidated horizons South American Land Mammal Age, or 9–6 Ma (Frailey, 1986; of the older Miocene beds form local base levels for rivers and Campbell et al., 2006). Above the conglomerate are three horizons, streams. Occasionally the Ucayali Unconformity rises above or falls informally referred to as Units A, B, and C, from the bottom up. Unit below the water level for a few meters, which is interpreted as A is most often a horizon of medium to coarse sands with extensive resulting from irregularities in the paleotopography. These diver- cross-bedding. Unit B is often marked by thick basal and capping gences from the low water line are localized, however, and do beds of clay, with alternating thin beds of fine sands, silts, and clays not represent large-scale, or regional, uplift or depression. between the thick clay beds. Unit C is primarily medium to fine sands and silts that grade upward into clays of the soil profiles. Based on the 40A/39A date of 3.12 ± 0.02 Ma on the Piedras ash, 3. Materials and methods Campbell et al. (2001) estimated that deposition of Unit C of the Madre de Dios Formation ceased 2.5 Ma. Where preserved, the Following the procedures outlined in Campbell et al. (2001),we top of Unit C forms the Amazon planalto. The types of sediments sampled nearly the entire Madre de Dios Formation at the 65-m- and sedimentary structures of the Madre de Dios Formation are thick section at the 750 m-long Cerro Colorado exposure on the typical of sedimentary deposits accumulating in modern fluvial, Madre de Dios River (Figs. 1 and 2). In addition, the subjacent Ipur- fluvio-lacustrine, and lacustrine environments of Amazonia. uro Formation, beneath the Ucayali Unconformity, was sampled. As demonstrated and reviewed in Campbell et al. (2006), the Slumping and vegetation cover prevented exposure of the entire stratigraphy of the Madre de Dios Formation is consistent across section at a single point, so the section was sampled using four clo- southern Amazonia, and the same stratigraphic relationships have sely adjacent sites at the west end of the outcrop (see Table 1;62 been described throughout lowland Amazonia. The recent descrip- sites). Distinctive horizons allowed ready correlation between tions of the Içá Formation in central Brazil by Rossetti et al.

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