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Journal of South American Earth Sciences 29 (2010) 619–626

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Journal of South American Earth Sciences

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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

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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. (2005) adjacent sites. The sampling extended from the water line of the and Rossetti and Toledo (2007) are consistent with the interpreta- river to 6–9 m from the top of the section, to which there was tions presented in Campbell et al. (2006), with the exception of the no access. Surface relief at the top of the section explains the Author's personal copy

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Fig. 3. Generalized geologic section representing the stratigraphic sequence commonly observed in riverine cutbanks in western and central Amazonia. Stratigraphic positions of dated volcanic ashes in left column are approximate. Modiﬁed from Campbell et al. (2001).

difference between the section reported here and that described by up to 400 °C, but not higher because the Pyrex glass melts at higher Campbell et al., 2006; Fig. 7. The Ipururo Formation was also sam- temperatures. pled in the approximate vicinity where Amahuacatherium peruvium was found (12°34.0860S; 70°06.1690W). Oriented block samples were taken wherever the rocks were 4. Results indurated enough to survive that method of sampling. Where the outcrop was too friable, or unconsolidated, for block sampling, As shown by orthogonal demagnetization (‘‘Zidjerveld”) plots we sampled by pushing 2.5-cm diameter Pyrex glass vials into (Fig. 4), nearly all the samples possessed a clear single-component the outcrop and then marked their orientation before removal. A remanence that was oriented north and up (normal in the southern wad of compressed cotton was placed over the open end of the vial, hemisphere) or south and down (reversed in the southern hemi- which was then sealed with tape. Three separately oriented sam- sphere). Some samples (Fig. 4A and B) showed very little response ples were taken at each site. Sites were spaced one meter apart to the AF demagnetization, suggesting that there was a slight over- stratigraphically, except in thick clay/mud horizons where spacing printing resulting from a high-coercivity component such as goe- was at one-half meter intervals. In a few instances sites were sam- thite. Indeed, many of these samples showed a rapid change in pled immediately below and above abrupt changes in lithology. In magnetization after thermal demagnetization at 200 °C (the tem- the laboratory, the original oriented blocks were hardened with so- perature at which the iron hydroxides have all dehydrated to dium silicate before analysis. The glass tubes were carefully un- hematite). Many of these samples had a small amount of rema- sealed, and then Zircar aluminum ceramic was poured into the nence left after thermal demagnetization above the blocking tem- open end and dried in field-free space to form a plug. All samples perature of magnetite (580 °C), suggesting that some hematite was were then measured on the 2G cryogenic magnetometer with Cal- present. Other samples (e.g., Fig. 4C) showed a large drop in inten- tech-style automatic sample changer at Occidental College. They sity during AF demagnetization, consistent with a remanence held were measured at NRM (natural remanent magnetization), then entirely in low-coercivity minerals such as magnetite. After ther- demagnetized in alternating fields (AF) of 2.5, 5.0, 7.5 and mal demagnetization, most of these samples had little or no rem- 10.0 mT (millitesla). After AF demagnetization, block samples were anence left above the blocking temperature of magnetite, so thermally demagnetized at 50 °C steps from 100 °C to 650 °C. The there was no significant amount of hematite or goethite in these samples in the Pyrex glass tubes were thermally demagnetized rocks. Author's personal copy

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Table 1 Most samples showed a clear stable component between 300 Paleomagnetic data from the Cerro Colorado section. Latitude and longitude are given and 500 °C. This component was summarized with the least- for the base of each series of sample sites. N = number of samples; DEC = declination; squares method of Kirschvink (1980) and the site statistics were INC = inclination; K = precision parameter; a95 = ellipse of 95% confidence interval around mean. calculated using the method of Fisher (1953) and classified in the scheme of Opdyke et al. (1977). SITE N DEC INC K a 95 The mean for all normal sites was D = 11.3, I = 28.1, k = 11.6, 12°34.0860S; 70°06.1690W a95 = 6.3, n = 49; for reversed sites, the mean was D = 191.0, Gomphothere Site 3 162.7 34.1 2.8 95.0 I = 27.0, k = 5.4, a95 = 17.0, n = 17. These directions are antipodal 0 0 12°33.946 S; 70°06.273 W within error estimates, showing that the remanence is primary 0 1 3 189.8 26.1 195.5 8.8 0 3 150.4 48.4 4.4 67.9 and that most overprinting is removed (Fig. 5). Inverting the re- 0 + 1 2 355.1 61.4 1.0 180.0 versed sites through the center of the stereonet produces a forma- 12°33.9130S; 70°06.2940W tional mean direction of D = 11.2, I = 27.8, k = 9.1, a95 = 6.1, n = 66. 1 3 212.6 32.5 2.4 111.5 This direction is statistically indistinguishable from the present- 2 3 235.4 63.8 12.1 37.2 day field direction of D = 10.4, I = 23.0, so the unit has not been 3 3 12.2 69.4 123.1 11.2 rotated or translated to any significant degree. 4 3 345.7 44.8 7.8 47.4 5 3 171.5 47.3 5.0 62.5 In the Cerro Colorado section (Fig. 6), the single site in the Ipur- 6 3 342.5 37.4 17.7 30.2 uro Formation was reversed in polarity. The second site in the Ipur- 7 3 338.7 40.6 6.9 50.9 uro Formation, from near where Amahuacatherium peruvium was 8 3 31.6 28.6 4.8 63.8 found, was also reversed in polarity, but this site is not illustrated 9 3 183.5 50.4 8.0 46.8 10 3 237.9 48.7 1.8 180.0 in the section. A total of 18 normal and reversed magnetozones 11 3 359.3 36.2 4.8 63.8 was obtained from the Cerro Colorado outcrop of the Madre de 12 3 23.7 14.5 20.5 28.0 Dios Formation, with only 2 of 62 sites giving indeterminate re- 13 3 357.5 35.6 11.2 38.8 sults. Unit A yielded at least 11 magnetozones in 20 m of section. 14 3 235.1 34.3 1.3 180.0 The basal and capping clays of Unit B were mostly reversed in 15 3 8.3 32.3 3.5 79.2 16 3 10.4 32.4 3.5 80.0 polarity, but, except for a single site in the middle of the unit, 17 3 352.7 49.2 4.1 70.8 the remainder of Unit B was normal in polarity. The entire sampled 18 3 151.2 21.1 4.5 67.2 thickness (20 m) of Unit C was normal in polarity. Nearly all the 19 3 61.7 43.6 24.3 25.6 sites in the Cerro Colorado section were statistically separated 20 2 167.7 10.5 7.2 114.1 21 3 201.3 15.3 13.3 35.3 from a random distribution at the 95% confidence interval, mean- 22 3 219.0 36.4 2.8 95.0 ing they were Class I sites in the classification of Opdyke et al. 23 3 5.2 30.4 21.3 27.4 (1977). 24 3 204.2 35.2 4.0 71.8 25 3 7.7 25.1 4.6 65.4 26 3 81.0 16.7 17.5 30.4 27 2 31.1 15.2 32.0 45.7 5. Discussion 28 3 327.1 58.3 6.1 54.9 29 3 45.3 41.0 4.4 67.4 5.1. Dating the first pulse of the GAFI 30 2 2.4 29.7 4.1 180.0 31 3 1.2 25.1 52.1 17.3 40 39 32 3 354.1 29.0 32.7 21.9 As reported in Campbell et al. (2001), there are Ar/ Ar dates of 9.01 ± 0.28 Ma near the base of the Madre de Dios Formation and 12°33.8800S; 70°06.2500W 3.12 ± 0.02 Ma near the top of the formation (Fig. 2). Based on 33 3 22.9 30.9 72.1 14.6 these dates, we correlate the magnetozones in the Cerro Colorado 34 3 5.9 31.2 63.4 15.6 section with Chrons C4Ar to C2An (9.5–3.0 Ma). Not only do the 35 3 357.9 28.0 9.0 43.7 36 3 3.1 10.4 36.1 20.8 dates at the base and near the top of the section constrain our cor- 37 2 183.1 50.1 14.8 70.5 relation quite well (Fig. 7), but the thickness and spacing of most of 38 3 3.3 31.2 3.6 77.5 the magnetozones in the Cerro Colorado section also match those 39 3 352.3 23.9 2.7 98.5 of the timescale of Lourens et al. (2004) quite well. 40 3 3.7 45.3 7.6 48.2 The late Miocene gomphothere A. peruvium was recovered from 42 3 32.0 37.8 2.3 117.3 43 3 20.0 47.7 4.2 69.3 the moderately consolidated clay deposits of the Ipururo Forma- 44 3 21.4 26.8 5.8 57.0 tion at the base of the outcrop at Cerro Colorado (Romero-Pittman, 45 3 90.2 41.8 1.6 180.0 1996; Campbell et al., 2000; Campbell et al., 2009). Some authors 46 3 183.4 14.9 3.0 89.9 have argued that this gomphothere is a late Pleistocene example 47 3 4.4 22.1 6.8 51.4 of Haplomastodon waringi (e.g., Alberdi et al., 2004; Ferretti, 2008) because they considered the provenance, hence the age, of 12°33.9460S; 70°06.2480W 48 3 0.7 36.9 3.7 76.3 the specimen to be uncertain. Campbell et al. (2009) presented 49 3 8.7 21.2 19.1 29.0 counter-arguments to these claims, and reaffirmed their earlier 50 3 3.5 15.7 71.4 14.7 interpretation as to the stratigraphic position of Amahuacatherium. 51 3 0.9 23.9 28.0 23.7 The magnetostratigraphy of the Cerro Colorado section, and espe- 52 3 6.8 26.8 33.7 21.6 53 3 27.6 47.7 252.7 7.8 cially the reversed paleomagnetic directions of the Ipururo Forma- 54 3 13.4 17.0 128.5 10.9 tion, reported here are consistent with a late Miocene age of 55 3 18.1 28.1 60.1 16.0 9.5 Ma for Amahuacatherium as proposed by Campbell et al. 56 3 19.3 27.2 38.5 20.1 (2001), and they are not consistent with a late Pleistocene inter- 57 3 24.9 25.5 70.2 14.8 58 3 18.3 18.0 10.2 40.8 pretation because the magnetic field of the earth has been normal 59 3 43.1 73.5 1.6 180.0 in polarity for the past 800,000 years. These results add increased 60 3 22.8 21.5 7.2 36.9 confidence to the proposal that A. peruvium is the oldest recorded 61 3 28.6 29.0 5.4 59.1 North American mammal to have entered South America as part of 62 3 25.4 30.3 114.5 11.6 the GAFI. Author's personal copy

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Fig. 4. Orthogonal demagnetization, or ‘‘Zidjerveld” plots, of three representative samples from the Madre de Dios Formation. The north axis is oriented to the right, rather than top, to improve legibility. Each increment is 106 emu. The ﬁrst step is NRM, followed by AF demagnetization steps at 2.5, 5.0, 7.5, and 1.0 mT. The remaining steps are thermal demagnetization steps, starting at 100 °C and increasing by 50 °C increments. Samples A and B show a north and up (normal in the southern hemisphere) orientation; sample C is south and down (reversed polarity in the southern hemisphere), with only slight overprinting.

Also, as noted by Campbell et al. (2006, 2009), fossils of other forth their own interpretations. These include Cozzuol (2006), gomphotheres, peccaries, and tapirs have been recovered from who does not accept the hypothesis of a Pan-Amazonian Ucayali the basal conglomerates of the Madre de Dios Formation immedi- Unconformity; Gingras et al. (2002) and Hovikoski et al. (2005, ately overlying the Ucayali Unconformity. It would appear that 2008), who argue for late Miocene marine deposition in western there were numerous lineages of North American mammals in Amazonia; Westaway (2006), who argued for Mio-Pliocene regio- South America in the late Miocene. These mammals are recognized nal uplift and incision of drainage systems in southwestern Amazo- as comprising the first southward pulse of the GAFI. nia; Latrubesse et al. (2007), who argued for continual deposition in western Amazonia throughout the Neogene; and others. How- ever, a critical factor missing from all of these hypotheses is an 5.2. Implications for the geological evolution of Amazonia accurate chronology for the Neogene deposits found within the basin. Nonetheless, some of these hypotheses could potentially draw The interpretation of the Neogene stratigraphy of Amazonia into question the validity of using the volcanic ash dates reported presented above is questioned by other authors, who have put by Campbell et al. (2001) to anchor the magnetostratigraphic column as we have done herein so it is useful to comment on the most pertinent of these hypotheses. We base our correlation of the Madre de Dios Formation among river systems of southwestern Amazonia on the basis of physical examinations of riverine outcrops that reveal the formation’s rela- tively uniform stratigraphy (i.e., its ready division into three horizons) and the fact that it always overlies the Ucayali Unconformity, as described in Campbell et al. (2006). No structural modifications (i.e., folding or faulting) of the horizontal horizons of the Madre de Dios Formation have ever been observed in exposures of that formation, although large slump blocks in cutbanks are rather com- mon. Although the riverine outcrops are not continuous, they occur often enough to give confidence in the correlation of stratigraphic horizons along river courses. In the case of correlation of the Cerro Colorado outcrop with the Piedras ash, the latter is less than 60 km from the Cerro Colorado outcrop, although much far- ther by river travel. Nevertheless, because the Las Piedras River is a tributary of the Madre de Dios River (Fig. 1), it is possible to trace the Ucayali Unconformity and the horizons of the Madre de Dios Formation from the Cerro Colorado outcrop to the outcrop from which came the Piedras ash using intervening outcrops. Correlation of the Cerro Colorado outcrop with the Cocama ash, which is 240 km distant, could be seen as more problematic by some because the Cocama ash lies within the Purus River drainage Fig. 5. Stereonet of mean normal and reversed directions for the Cerro Colorado system and it is not possible to navigate directly between the two section. Solid dot and solid circle = mean and 95% ellipse of confidence for reversed sites. If the Ucayali Unconformity is not a Pan-Amazonian geologic sites (lower hemisphere); open dot and dashed line = mean and confidence ellipse feature, but rather local unconformities resulting from shifting riv- for normal sites (upper hemisphere). Solid square = projection of reversed mean to upper hemisphere, where it nearly coincides with the normal mean. As is apparent, er valleys as proposed by Santos and Silva (1976) and Cozzuol the sites are antipodal within error limits. (2006), then it would be inappropriate to use an unconformity as Author's personal copy

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Fig. 6. The magnetostratigraphy of the Cerro Colorado section reveals a total of 18 polarity zones, confirming that Amahuacatherium, which came from the base of this section, was not a late Pleistocene gomphothere. The VGP (virtual geomagnetic pole) latitude for the sites is shown, with solid black circles representing sites that are statistically significant (Class I sites of Opdyke et al., 1977). Solid gray circles are Class II sites (one sample missing, so significance cannot be calculated). Open circles are Class III sites of Opdyke et al. (1977), where most samples gave a clear polarity preference, but one sample was divergent. Figure modified from Campbell et al. (2009). a tool for correlation. Nevertheless, Campbell et al. (2006) and Ros- preted from a statement by Cozzuol (2006:197) when, in arguing setti and Toledo (2007) cited numerous independent works in sev- against the Ucayali Unconformity, he stated that the description eral countries describing the Ucayali Unconformity throughout for the Içá Formation of Brazil (=Madre de Dios Formation of Peru) Amazonia. Confirmation of this interpretation can even be inter- in central Amazonia presented by Rossetti et al. (2005) Author's personal copy

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in Campbell et al. (2006). Hovikoski et al. (2005) do not identify the three units (A, B, and C) as we do here, but, nonetheless, it is possible to correlate their major stratigraphic horizons to those we observed. In particular, Hovikoski et al. (2005, Fig. 2, DR1) identify and illustrate what they claim to be cyclic tidal rhythmites representing a marine influence. This horizon, as best as can be determined, would fall at meter 36 or 37 in our section (Fig. 6), which places it near the border between Chron C2An and C3n1n, or 4.3–4.4 Ma. This dating does correspond to approximately the Zanclean high sea level stand (Hardenbol et al., 1998), but this high stand only reached 90 m above modern sea level. The top of the outcrop at Cerro Colorado lies at 270 m elevation, which places the ‘‘tidal” deposits of Hovikoski et al. (2005) at 245 m elevation, far above any Neogene sea level high stand. The same argu- ment against a marine influence at the sections reported by Gingras et al. (2002) (elevation 230 m) and Hovikoski et al., 2008 (mean elevation of several sites, 200 m) applies. A marine influence on deposition at Cerro Colorado, or any other locality in southwestern Amazonia, would have been impossible unless the region has experienced up to 150 m of uplift since the middle Pli- ocene. As stated above, there are no data that support that such a scenario. Campbell et al. (2006) discussed the possibility that the formation of the Ucayali Unconformity was intricately related to the Quechua II tectonic phase of Andean orogeny. Since that time new data and arguments have been presented that date the beginning of this orogenic event to 10 Ma in the Central Andes (Garzi- one et al., 2008), which is similar to the timing reported earlier for the Andes of Ecuador (Hungerbühler et al., 2002). The rise of the Andes would undoubtedly have led to regional changes in climate, perhaps including the development of intense monsoonal conditions that could have led to the formation of the Ucayali Unconfor-

Fig. 7. Correlation of the Cerro Colorado section with the magnetic polarity time mity. If Andean uplift began at 10 Ma as reported, and given a scale of Lourens et al. (2004), showing the calibration by 40Ar/39Ar dates of reasonable lag time for any effects on climatic systems to be felt, Campbell et al. (2001). this timing is consistent with the hypothesis that the development of the Ucayali Unconformity at 9.5–9.0 Ma was a direct result of this tectonism. ‘‘... matches almost perfectly what is found along the Acre River Finally, although a late Miocene date for the origin of the mod- ...” in southwestern Amazonia, thousands of km distant. Given ern Amazon River system is often cited (Hoorn et al., 1995; Figuei- the absence of any structurally-based topographic barriers within redo et al., 2009), that hypothesis is rejected here because there are lowland Amazonia in the Neogene, there does not appear to be no data that directly support such a hypothesis. Furthermore, if the any explanation for the widespread Ucayali Unconformity except Amazon River had been in place in the late Miocene it would have that it is a Pan-Amazonian feature rather than localized intrafor- been physically impossible for deposition of the Madre de Dios For- mational erosion surfaces, in which case it is an acceptable crite- mation to have occurred. The Amazon planalto, or high plain sur- rion for correlation among different drainage systems. face, represents the final surface of deposition of the Madre de Other authors have argued that independent, subsiding basins Dios Formation (=Içá Formation in Brazil) within lowland Amazo- existed in lowland Amazonia in the Neogene (e.g., Räsänen et al., nia, and assuming that deposition of this formation continued until 1987), or, conversely, that the same areas experienced uplift and 3.0–2.5 Ma (Campbell et al., 2001), the formation and entrench- entrenchment of modern river systems (e.g., Westaway, 2006). ment of the modern Amazon River system must post-date that Both of these scenarios conflict with the hypothesis of a Pan-Ama- time (Campbell et al., 2006). This interpretation is consistent with zonian Ucayali Unconformity because the unconformity is always that of Westaway (2006: p. 129), who also described entrenchment exposed at or near the low waterline of rivers throughout Amazo- of the modern Amazon River system as beginning ‘‘...(possibly by nia. If there had been large areas of regional subsidence or uplift, the Middle Pliocene, 3 Ma)”. the unconformity would either be absent (i.e., deeply buried) or highly elevated over large-scale, regional areas. Neither of these conditions has ever been observed or reported. 6. Summary As mentioned above, Gingras et al. (2002) and Hovikoski et al. (2005, 2008) have proposed that marine tidal deposits are repre- A magnetostratigraphic study of the Cerro Colorado outcrop on sented in the Neogene stratigraphic sequence of southwestern the Madre de Dios River reveals 18 polarity zones in the 65 m Amazonia. Latrubesse et al. (2007) presented several cogent argu- thick section, which are correlated with Chrons C4Ar to C2An ments against these proposals and rejected the hypotheses out- (9.5–3.0 Ma) based on A40/A39 dates on volcanic ashes in the re- right. The results of the study presented here allow us to gion. The magnetostratigraphic profile confirms the late Miocene comment further on the possibility of marine deposits occurring age of A. peruvium, a primitive gomphothere, which is the oldest in the Cerro Colorado section, as claimed by Hovikoski et al. known participant in the Great American Faunal Interchange. The (2005). Those authors presented a measured section of 40 m at results of this study indicate the potential of magnetostratigraphy Cerro Colorado, about one-third less than that reported here and as a tool for correlating among the various river systems of lowland Author's personal copy

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Amazonia. Until an accurate chronostratigraphy of the Amazon Ba- Fisher, R.A., 1953. Dispersion on a sphere. Royal Society of London Proceedings sin is achieved there can be no consensus as to the evolutionary A217, 295–305. Frailey, C.D., 1986. Late Miocene and Holocene mammals, exclusive of the history of the basin, which is necessary before a better understand- Notoungulata, of the Rio Acre region, western Amazonia. Contributions in ing of its modern biogeographic patterns and great diversity can be Science, Natural History Museum of Los Angeles County 374, 1–46. understood. Garzione, C.N., Hoke, G.D., Libarkin, J.C., Withers, S., MacFadden, B., Elier, J., Ghosh, P., Mulch, A., 2008. Rise of the Andes. Science 320, 1304–1307. Gingras, M.K., Räsänen, M., Ranzi, A., 2002. The significance of bioturbated inclined Acknowledgments heterolithic stratification in the southern part of the Miocene Solimoes Formation, Rio Acre, Amazonian Brazil. Palaios 17, 591–601. Hoorn, C., Guerrero, J., Sarmiento, G.A., Lorente, M.A., 1995. Andean tectonics as a We thank Dr. Jose Macharé and Dr. Hernando Nuñez del Prado cause for changing drainage patterns in Miocene northern South America. for their support of our field work in Peru during their tenure at Geology 23, 237–240. INGEMMET. We thank S. Bogue and J. Kirschvink for help in main- Hovikoski, J., Räsänen, M., Gingras, M., Roddaz, M., Brusset, S., Hermoza, W., Romero Pittman, L., Lertola, K., 2005. Miocene semidiurnal tidal rhythmites in Madre de taining the Occidental College paleomagnetics lab, and Nigel Pit- Dios, Peru. Geology 33, 177–180. man and staff of the Los Amigos Biological Station (ACCA) for Hovikoski, J., Räsänen, M., Gingras, M., Ranzi, A., Melo, J., 2008. Tidal and their hospitality during our stay at Cerro Colorado. N. 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