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Amazonian magnetostratigraphy: Dating the first pulse of the Great AmericanFaunal 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, USAb Department of Geology, Occidental College, Los Angeles, CA 90041, USAc Instituto Geológico, Minero y Metalúrgico (INGEMMET), San Borja, Apartado 889, Lima 41, Perud Department of Biology, California State University, 18111 Nordhoff Street, Northridge, CA 91330, USA

a r t i c l e i n f o

Article history:Received 8 May 2009Accepted 29 November 2009

Keywords:AmazonGAFIMagnetostratigraphyCerro ColoradoAmahuacatheriumMadre de Dios FormationPeru

a b s t r a c t

The chronostratigraphy of the youngest Neogene deposits of the Amazon Basin, which comprise the Mad-re de Dios Formation in eastern Peru, remains unresolved. Although 40Ar/39Ar dates on two volcanic ashesfrom 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 chronostratigraphyof the Madre de Dios Formation, we report here the magnetostratigraphy of an outcrop on the Madre deDios River in southeastern Peru. A total of 18 polarity zones was obtained in the �65-m-thick Cerro Col-orado section, which we correlate to magnetozones Chrons C4Ar to C2An (9.5–3.0 Ma) based on the prior40Ar/39Ar dates. These results confirm the late Miocene age of a gomphothere recovered from the IpururoFormation, which underlies the late Miocene Ucayali Unconformity at the base of the Cerro Colorado out-crop. The results also support earlier interpretations of a late Miocene age for other fossils of North Amer-ican mammals recovered from basal conglomeratic deposits of the Madre de Dios Formation immediatelyabove the Ucayali Unconformity. These mammals include other gomphotheres, peccaries, and tapirs, andtheir presence in South America in the late Miocene is recognized as part of the first pulse of the GreatAmerican Faunal Interchange.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Correctly interpreting Earth history is dependent upon accuratechronologies. However, for some regions and for some time peri-ods, obtaining accurate chronologies has proven very difficult.One such region for which accurate chronologies are sorely lackingis the Amazon Basin, where vast tropical lowlands are covered byNeogene sediments of undocumented age. As a consequence thereis considerable debate regarding the geologic history of lowlandAmazonia (reviewed in Campbell et al., 2006). To date, there areonly two 40A/39A dates on volcanic ashes within lowland Amazoniaand a paucity of paleomagnetic data that support a chronology forthe region (Campbell et al., 2001). To further document the chro-nostratigraphy of Neogene sediments of Amazonia, we report herethe magnetostratigraphy of a section through the Madre de DiosFormation in southeastern Peru. These data confirm the value ofmagnetostratigraphy in dating Neogene sediments of Amazoniaand suggest a potential for correlating deposits widely withinAmazonia. The data are also consistent with prior interpretations

of a late Miocene age for the oldest known suite of North Americanmammals to have arrived in South America in the first pulse of theGreat American Faunal Interchange (GAFI).

The paleomagnetic data reported by Campbell et al. (2001) werethe result of initial tests to determine the feasibility of using thistechnique for dating the unconsolidated late Neogene sedimentsof Amazonia. To expand on those limited test sites, the Cerro Colo-rado section (centered at approximately 12�33.980S; 70�06.190W)(Figs. 1 and 2) on the Madre de Dios River was selected because itis one of the largest sections through the Madre de Dios Formationexposed in southeastern Peru, it is readily accessible, and the prim-itive late Miocene gomphothere Amahuacatherium peruviumRomero-Pittman (1996) came from the Ipururo Formation thatcrops out at the base of the outcrop in the dry season (Campbellet al., 2000, 2009). Doubts regarding the age assigned to this gom-phothere had been raised (e.g., Alberdi et al., 2004; Ferretti, 2008),and these doubts could best be addressed through independentdating methods such as magnetostratigraphy.

2. Neogene stratigraphy of Amazonia

The Neogene stratigraphy of Amazonia, as interpreted byCampbell et al. (2000, 2001, 2006), comprises a sequence of older

0895-9811/$ - see front matter � 2009 Elsevier Ltd. All rights reserved.doi:10.1016/j.jsames.2009.11.007

* 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](F. Hertel), [email protected] (N. Rivera).

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

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Journal of South American Earth Sciences

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formations that are separated from overlying younger sedimentsby the Pan-Amazonian Ucayali Unconformity (Fig. 3). The olderdeposits are primarily moderately to well-consolidated, fine-grained, terrigenous deposits that predate �10.5–9.5 Ma, whereasthe younger deposits are unconsolidated silts, sands and claysreferable to the Madre de Dios Formation (=Içá Formation in Brazil)that post-date�9.5 Ma. Deposition of the Madre de Dios Formationis estimated to have begun at �9.5–9.0 Ma, based on the 40A/39Adate on the Cocama ash (9.01 ± 0.28 Ma) (Campbell et al., 2001),which occurs �4 m above the Ucayali Unconformity. The base ofthe Madre de Dios Formation is often a very fossiliferous clay-peb-ble or clay-ball conglomerate that yields many significant verte-brate fossils that date the conglomerate to the HuayquerianSouth American Land Mammal Age, or 9–6 Ma (Frailey, 1986;Campbell et al., 2006). Above the conglomerate are three horizons,informally referred to as Units A, B, and C, from the bottom up. UnitA is most often a horizon of medium to coarse sands with extensivecross-bedding. Unit B is often marked by thick basal and cappingbeds of clay, with alternating thin beds of fine sands, silts, and claysbetween the thick clay beds. Unit C is primarily medium to finesands 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,Campbell et al. (2001) estimated that deposition of Unit C of theMadre de Dios Formation ceased �2.5 Ma. Where preserved, thetop of Unit C forms the Amazon planalto. The types of sedimentsand sedimentary structures of the Madre de Dios Formation aretypical of sedimentary deposits accumulating in modern fluvial,fluvio-lacustrine, and lacustrine environments of Amazonia.

As demonstrated and reviewed in Campbell et al. (2006), thestratigraphy of the Madre de Dios Formation is consistent acrosssouthern Amazonia, and the same stratigraphic relationships havebeen described throughout lowland Amazonia. The recent descrip-tions of the Içá Formation in central Brazil by Rossetti et al. (2005)and Rossetti and Toledo (2007) are consistent with the interpreta-tions presented in Campbell et al. (2006), with the exception of the

age assigned to the deposits. Of significance here, the UcayaliUnconformity is almost uniformly visible at the water line duringthe dry season because the underlying, well-consolidated horizonsof the older Miocene beds form local base levels for rivers andstreams. Occasionally the Ucayali Unconformity rises above or fallsbelow the water level for a few meters, which is interpreted asresulting from irregularities in the paleotopography. These diver-gences from the low water line are localized, however, and donot represent large-scale, or regional, uplift or depression.

3. Materials and methods

Following the procedures outlined in Campbell et al. (2001), wesampled nearly the entire Madre de Dios Formation at the �65-m-thick section at the �750 m-long Cerro Colorado exposure on theMadre de Dios River (Figs. 1 and 2). In addition, the subjacent Ipur-uro Formation, beneath the Ucayali Unconformity, was sampled.Slumping and vegetation cover prevented exposure of the entiresection at a single point, so the section was sampled using four clo-sely adjacent sites at the west end of the outcrop (see Table 1; 62sites). Distinctive horizons allowed ready correlation betweenadjacent sites. The sampling extended from the water line of theriver to �6–9 m from the top of the section, to which there wasno access. Surface relief at the top of the section explains the

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. Modifiedfrom Campbell et al. (2001).

Fig. 2. The Cerro Colorado outcrop as it appeared in 1986, illustrating thedistinctiveness of the various horizons within the Madre de Dios Formation. Units‘‘A,” ‘‘B,” and ‘‘C” are indicated. The Ipururo Formation appears in the lower rightcorner (I), rising just above the water line during the dry season. The sampling wereport on herein occurred to the left of this view because this portion of the outcrophas experienced considerable slumping since 1986 and is not amenable to samplingat this time.

620 K.E. Campbell Jr. et al. / Journal of South American Earth Sciences 29 (2010) 619–626


difference between the section reported here and that described byCampbell et al., 2006; Fig. 7. The Ipururo Formation was also sam-pled in the approximate vicinity where Amahuacatherium peruviumwas found (12�34.0860S; 70�06.1690W).

Oriented block samples were taken wherever the rocks wereindurated enough to survive that method of sampling. Where theoutcrop was too friable, or unconsolidated, for block sampling,we sampled by pushing 2.5-cm diameter Pyrex glass vials intothe outcrop and then marked their orientation before removal. Awad of compressed cotton was placed over the open end of the vial,which was then sealed with tape. Three separately oriented sam-ples were taken at each site. Sites were spaced one meter apartstratigraphically, except in thick clay/mud horizons where spacingwas at one-half meter intervals. In a few instances sites were sam-pled immediately below and above abrupt changes in lithology. Inthe laboratory, the original oriented blocks were hardened with so-dium silicate before analysis. The glass tubes were carefully un-sealed, and then Zircar aluminum ceramic was poured into theopen end and dried in field-free space to form a plug. All sampleswere then measured on the 2G cryogenic magnetometer with Cal-tech-style automatic sample changer at Occidental College. Theywere measured at NRM (natural remanent magnetization), thendemagnetized in alternating fields (AF) of 2.5, 5.0, 7.5 and10.0 mT (millitesla). After AF demagnetization, block samples werethermally demagnetized at 50 �C steps from 100 �C to 650 �C. Thesamples in the Pyrex glass tubes were thermally demagnetized

up to 400 �C, but not higher because the Pyrex glass melts at highertemperatures.

4. Results

As shown by orthogonal demagnetization (‘‘Zidjerveld”) plots(Fig. 4), nearly all the samples possessed a clear single-componentremanence that was oriented north and up (normal in the southernhemisphere) or south and down (reversed in the southern hemi-sphere). Some samples (Fig. 4A and B) showed very little responseto the AF demagnetization, suggesting that there was a slight over-printing resulting from a high-coercivity component such as goe-thite. Indeed, many of these samples showed a rapid change inmagnetization after thermal demagnetization at 200 �C (the tem-perature at which the iron hydroxides have all dehydrated tohematite). Many of these samples had a small amount of rema-nence left after thermal demagnetization above the blocking tem-perature of magnetite (580 �C), suggesting that some hematite waspresent. Other samples (e.g., Fig. 4C) showed a large drop in inten-sity during AF demagnetization, consistent with a remanence heldentirely in low-coercivity minerals such as magnetite. After ther-mal demagnetization, most of these samples had little or no rem-anence left above the blocking temperature of magnetite, sothere was no significant amount of hematite or goethite in theserocks.

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

K.E. Campbell Jr. et al. / Journal of South American Earth Sciences 29 (2010) 619–626 621


Most samples showed a clear stable component between 300and 500 �C. This component was summarized with the least-squares method of Kirschvink (1980) and the site statistics werecalculated using the method of Fisher (1953) and classified in thescheme of Opdyke et al. (1977).

The mean for all normal sites was D = 11.3, I = �28.1, k = 11.6,a95 = 6.3, n = 49; for reversed sites, the mean was D = 191.0,I = 27.0, k = 5.4, a95 = 17.0, n = 17. These directions are antipodalwithin error estimates, showing that the remanence is primaryand that most overprinting is removed (Fig. 5). Inverting the re-versed sites through the center of the stereonet produces a forma-tional mean direction of D = 11.2, I = �27.8, k = 9.1, a95 = 6.1, n = 66.This direction is statistically indistinguishable from the present-day field direction of D = 10.4, I = �23.0, so the unit has not beenrotated or translated to any significant degree.

In the Cerro Colorado section (Fig. 6), the single site in the Ipur-uro Formation was reversed in polarity. The second site in the Ipur-uro Formation, from near where Amahuacatherium peruvium wasfound, was also reversed in polarity, but this site is not illustratedin the section. A total of 18 normal and reversed magnetozoneswas obtained from the Cerro Colorado outcrop of the Madre deDios Formation, with only 2 of 62 sites giving indeterminate re-sults. Unit A yielded at least 11 magnetozones in 20 m of section.The basal and capping clays of Unit B were mostly reversed inpolarity, but, except for a single site in the middle of the unit,the remainder of Unit B was normal in polarity. The entire sampledthickness (20 m) of Unit C was normal in polarity. Nearly all thesites in the Cerro Colorado section were statistically separatedfrom a random distribution at the 95% confidence interval, mean-ing they were Class I sites in the classification of Opdyke et al.(1977).

5. Discussion

5.1. Dating the first pulse of the GAFI

As reported in Campbell et al. (2001), there are 40Ar/39Ar datesof 9.01 ± 0.28 Ma near the base of the Madre de Dios Formation and3.12 ± 0.02 Ma near the top of the formation (Fig. 2). Based onthese dates, we correlate the magnetozones in the Cerro Coloradosection with Chrons C4Ar to C2An (9.5–3.0 Ma). Not only do thedates at the base and near the top of the section constrain our cor-relation quite well (Fig. 7), but the thickness and spacing of most ofthe magnetozones in the Cerro Colorado section also match thoseof the timescale of Lourens et al. (2004) quite well.

The late Miocene gomphothere A. peruvium was recovered fromthe moderately consolidated clay deposits of the Ipururo Forma-tion at the base of the outcrop at Cerro Colorado (Romero-Pittman,1996; Campbell et al., 2000; Campbell et al., 2009). Some authorshave argued that this gomphothere is a late Pleistocene exampleof Haplomastodon waringi (e.g., Alberdi et al., 2004; Ferretti,2008) because they considered the provenance, hence the age, ofthe specimen to be uncertain. Campbell et al. (2009) presentedcounter-arguments to these claims, and reaffirmed their earlierinterpretation as to the stratigraphic position of Amahuacatherium.The magnetostratigraphy of the Cerro Colorado section, and espe-cially the reversed paleomagnetic directions of the Ipururo Forma-tion, reported here are consistent with a late Miocene age of�9.5 Ma for Amahuacatherium as proposed by Campbell et al.(2001), and they are not consistent with a late Pleistocene inter-pretation because the magnetic field of the earth has been normalin polarity for the past 800,000 years. These results add increasedconfidence to the proposal that A. peruvium is the oldest recordedNorth American mammal to have entered South America as part ofthe GAFI.

Table 1Paleomagnetic data from the Cerro Colorado section. Latitude and longitude are givenfor the base of each series of sample sites. N = number of samples; DEC = declination;INC = inclination; K = precision parameter; a95 = ellipse of 95% confidence intervalaround mean.

SITE N DEC INC K a95

12�34.0860S; 70�06.1690WGomphothere Site 3 162.7 34.1 2.8 95.0

12�33.9460S; 70�06.2730W0 � 1 3 189.8 26.1 195.5 8.80 3 150.4 48.4 4.4 67.90 + 1 2 355.1 �61.4 1.0 180.0

12�33.9130S; 70�06.2940W1 3 212.6 32.5 2.4 111.52 3 235.4 63.8 12.1 37.23 3 12.2 �69.4 123.1 11.24 3 345.7 �44.8 7.8 47.45 3 171.5 47.3 5.0 62.56 3 342.5 �37.4 17.7 30.27 3 338.7 �40.6 6.9 50.98 3 31.6 �28.6 4.8 63.89 3 183.5 50.4 8.0 46.810 3 237.9 48.7 1.8 180.011 3 359.3 �36.2 4.8 63.812 3 23.7 �14.5 20.5 28.013 3 357.5 �35.6 11.2 38.814 3 235.1 34.3 1.3 180.015 3 8.3 �32.3 3.5 79.216 3 10.4 �32.4 3.5 80.017 3 352.7 �49.2 4.1 70.818 3 151.2 21.1 4.5 67.219 3 61.7 �43.6 24.3 25.620 2 167.7 10.5 7.2 114.121 3 201.3 15.3 13.3 35.322 3 219.0 36.4 2.8 95.023 3 5.2 �30.4 21.3 27.424 3 204.2 35.2 4.0 71.825 3 7.7 �25.1 4.6 65.426 3 81.0 �16.7 17.5 30.427 2 31.1 �15.2 32.0 45.728 3 327.1 �58.3 6.1 54.929 3 45.3 �41.0 4.4 67.430 2 2.4 �29.7 4.1 180.031 3 1.2 �25.1 52.1 17.332 3 354.1 �29.0 32.7 21.9

12�33.8800S; 70�06.2500W33 3 22.9 �30.9 72.1 14.634 3 5.9 �31.2 63.4 15.635 3 357.9 �28.0 9.0 43.736 3 3.1 �10.4 36.1 20.837 2 183.1 50.1 14.8 70.538 3 3.3 �31.2 3.6 77.539 3 352.3 �23.9 2.7 98.540 3 3.7 �45.3 7.6 48.242 3 32.0 �37.8 2.3 117.343 3 20.0 �47.7 4.2 69.344 3 21.4 �26.8 5.8 57.045 3 90.2 �41.8 1.6 180.046 3 183.4 14.9 3.0 89.947 3 4.4 �22.1 6.8 51.4

12�33.9460S; 70�06.2480W48 3 0.7 �36.9 3.7 76.349 3 8.7 �21.2 19.1 29.050 3 3.5 �15.7 71.4 14.751 3 0.9 �23.9 28.0 23.752 3 6.8 �26.8 33.7 21.653 3 27.6 �47.7 252.7 7.854 3 13.4 �17.0 128.5 10.955 3 18.1 �28.1 60.1 16.056 3 19.3 �27.2 38.5 20.157 3 24.9 �25.5 70.2 14.858 3 18.3 �18.0 10.2 40.859 3 43.1 �73.5 1.6 180.060 3 22.8 �21.5 7.2 36.961 3 28.6 �29.0 5.4 59.162 3 25.4 �30.3 114.5 11.6



Also, as noted by Campbell et al. (2006, 2009), fossils of othergomphotheres, peccaries, and tapirs have been recovered fromthe basal conglomerates of the Madre de Dios Formation immedi-ately overlying the Ucayali Unconformity. It would appear thatthere were numerous lineages of North American mammals inSouth America in the late Miocene. These mammals are recognizedas comprising the first southward pulse of the GAFI.

5.2. Implications for the geological evolution of Amazonia

The interpretation of the Neogene stratigraphy of Amazoniapresented above is questioned by other authors, who have put

forth their own interpretations. These include Cozzuol (2006),who does not accept the hypothesis of a Pan-Amazonian UcayaliUnconformity; Gingras et al. (2002) and Hovikoski et al. (2005,2008), who argue for late Miocene marine deposition in westernAmazonia; Westaway (2006), who argued for Mio-Pliocene regio-nal uplift and incision of drainage systems in southwestern Amazo-nia; Latrubesse et al. (2007), who argued for continual depositionin western Amazonia throughout the Neogene; and others. How-ever, a critical factor missing from all of these hypotheses is anaccurate chronology for the Neogene deposits found within the ba-sin. Nonetheless, some of these hypotheses could potentially drawinto question the validity of using the volcanic ash dates reportedby Campbell et al. (2001) to anchor the magnetostratigraphic col-umn as we have done herein so it is useful to comment on the mostpertinent of these hypotheses.

We base our correlation of the Madre de Dios Formation amongriver systems of southwestern Amazonia on the basis of physicalexaminations of riverine outcrops that reveal the formation’s rela-tively uniform stratigraphy (i.e., its ready division into three hori-zons) 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 deDios Formation have ever been observed in exposures of that for-mation, although large slump blocks in cutbanks are rather com-mon. Although the riverine outcrops are not continuous, theyoccur often enough to give confidence in the correlation of strati-graphic horizons along river courses. In the case of correlation ofthe Cerro Colorado outcrop with the Piedras ash, the latter is lessthan 60 km from the Cerro Colorado outcrop, although much far-ther by river travel. Nevertheless, because the Las Piedras River isa tributary of the Madre de Dios River (Fig. 1), it is possible to tracethe Ucayali Unconformity and the horizons of the Madre de DiosFormation from the Cerro Colorado outcrop to the outcrop fromwhich 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 bysome because the Cocama ash lies within the Purus River drainagesystem and it is not possible to navigate directly between the twosites. If the Ucayali Unconformity is not a Pan-Amazonian geologicfeature, but rather local unconformities resulting from shifting riv-er valleys as proposed by Santos and Silva (1976) and Cozzuol(2006), then it would be inappropriate to use an unconformity as

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, ratherthan top, to improve legibility. Each increment is 10�6 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 arethermal 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.

Fig. 5. Stereonet of mean normal and reversed directions for the Cerro Coloradosection. Solid dot and solid circle = mean and 95% ellipse of confidence for reversedsites (lower hemisphere); open dot and dashed line = mean and confidence ellipsefor normal sites (upper hemisphere). Solid square = projection of reversed mean toupper hemisphere, where it nearly coincides with the normal mean. As is apparent,the sites are antipodal within error limits.



a tool for correlation. Nevertheless, Campbell et al. (2006) and Ros-setti and Toledo (2007) cited numerous independent works in sev-eral countries describing the Ucayali Unconformity throughoutAmazonia. Confirmation of this interpretation can even be inter-

preted from a statement by Cozzuol (2006:197) when, in arguingagainst the Ucayali Unconformity, he stated that the descriptionfor the Içá Formation of Brazil (=Madre de Dios Formation of Peru)in central Amazonia presented by Rossetti et al. (2005)

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 statisticallysignificant (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 ofOpdyke et al. (1977), where most samples gave a clear polarity preference, but one sample was divergent. Figure modified from Campbell et al. (2009).



‘‘. . . matches almost perfectly what is found along the Acre River. . .” in southwestern Amazonia, thousands of km distant. Giventhe absence of any structurally-based topographic barriers withinlowland Amazonia in the Neogene, there does not appear to beany explanation for the widespread Ucayali Unconformity exceptthat it is a Pan-Amazonian feature rather than localized intrafor-mational erosion surfaces, in which case it is an acceptable crite-rion for correlation among different drainage systems.

Other authors have argued that independent, subsiding basinsexisted in lowland Amazonia in the Neogene (e.g., Räsänen et al.,1987), or, conversely, that the same areas experienced uplift andentrenchment of modern river systems (e.g., Westaway, 2006).Both of these scenarios conflict with the hypothesis of a Pan-Ama-zonian Ucayali Unconformity because the unconformity is alwaysexposed at or near the low waterline of rivers throughout Amazo-nia. If there had been large areas of regional subsidence or uplift,the unconformity would either be absent (i.e., deeply buried) orhighly elevated over large-scale, regional areas. Neither of theseconditions has ever been observed or reported.

As mentioned above, Gingras et al. (2002) and Hovikoski et al.(2005, 2008) have proposed that marine tidal deposits are repre-sented in the Neogene stratigraphic sequence of southwesternAmazonia. Latrubesse et al. (2007) presented several cogent argu-ments against these proposals and rejected the hypotheses out-right. The results of the study presented here allow us tocomment further on the possibility of marine deposits occurringin the Cerro Colorado section, as claimed by Hovikoski et al.(2005). Those authors presented a measured section of 40 m atCerro Colorado, about one-third less than that reported here and

in Campbell et al. (2006). Hovikoski et al. (2005) do not identifythe three units (A, B, and C) as we do here, but, nonetheless, it ispossible to correlate their major stratigraphic horizons to thosewe observed. In particular, Hovikoski et al. (2005, Fig. 2, DR1) iden-tify and illustrate what they claim to be cyclic tidal rhythmites rep-resenting a marine influence. This horizon, as best as can bedetermined, 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 approximatelythe Zanclean high sea level stand (Hardenbol et al., 1998), but thishigh stand only reached �90 m above modern sea level. The top ofthe outcrop at Cerro Colorado lies at �270 m elevation, whichplaces the ‘‘tidal” deposits of Hovikoski et al. (2005) at �245 m ele-vation, far above any Neogene sea level high stand. The same argu-ment against a marine influence at the sections reported byGingras et al. (2002) (elevation �230 m) and Hovikoski et al.,2008 (mean elevation of several sites, �200 m) applies. A marineinfluence on deposition at Cerro Colorado, or any other locality insouthwestern Amazonia, would have been impossible unless theregion has experienced up to 150 m of uplift since the middle Pli-ocene. As stated above, there are no data that support that such ascenario.

Campbell et al. (2006) discussed the possibility that the forma-tion of the Ucayali Unconformity was intricately related to theQuechua II tectonic phase of Andean orogeny. Since that timenew data and arguments have been presented that date the begin-ning of this orogenic event to �10 Ma in the Central Andes (Garzi-one et al., 2008), which is similar to the timing reported earlier forthe Andes of Ecuador (Hungerbühler et al., 2002). The rise of theAndes would undoubtedly have led to regional changes in climate,perhaps including the development of intense monsoonal condi-tions that could have led to the formation of the Ucayali Unconfor-mity. If Andean uplift began at �10 Ma as reported, and given areasonable lag time for any effects on climatic systems to be felt,this timing is consistent with the hypothesis that the developmentof the Ucayali Unconformity at �9.5–9.0 Ma was a direct result ofthis tectonism.

Finally, although a late Miocene date for the origin of the mod-ern Amazon River system is often cited (Hoorn et al., 1995; Figuei-redo et al., 2009), that hypothesis is rejected here because there areno data that directly support such a hypothesis. Furthermore, if theAmazon River had been in place in the late Miocene it would havebeen physically impossible for deposition of the Madre de Dios For-mation to have occurred. The Amazon planalto, or high plain sur-face, represents the final surface of deposition of the Madre deDios Formation (=Içá Formation in Brazil) within lowland Amazo-nia, and assuming that deposition of this formation continued until3.0–2.5 Ma (Campbell et al., 2001), the formation and entrench-ment of the modern Amazon River system must post-date thattime (Campbell et al., 2006). This interpretation is consistent withthat of Westaway (2006: p. 129), who also described entrenchmentof the modern Amazon River system as beginning ‘‘. . .(possibly bythe Middle Pliocene, �3 Ma)”.

6. Summary

A magnetostratigraphic study of the Cerro Colorado outcrop onthe Madre de Dios River reveals 18 polarity zones in the �65 mthick section, which are correlated with Chrons C4Ar to C2An(9.5–3.0 Ma) based on A40/A39 dates on volcanic ashes in the re-gion. The magnetostratigraphic profile confirms the late Mioceneage of A. peruvium, a primitive gomphothere, which is the oldestknown participant in the Great American Faunal Interchange. Theresults of this study indicate the potential of magnetostratigraphyas a tool for correlating among the various river systems of lowland

Fig. 7. Correlation of the Cerro Colorado section with the magnetic polarity timescale of Lourens et al. (2004), showing the calibration by 40Ar/39Ar dates ofCampbell et al. (2001).



Amazonia. Until an accurate chronostratigraphy of the Amazon Ba-sin is achieved there can be no consensus as to the evolutionaryhistory of the basin, which is necessary before a better understand-ing of its modern biogeographic patterns and great diversity can beunderstood.

Acknowledgments

We thank Dr. Jose Macharé and Dr. Hernando Nuñez del Pradofor their support of our field work in Peru during their tenure atINGEMMET. We thank S. Bogue and J. Kirschvink for help in main-taining the Occidental College paleomagnetics lab, and Nigel Pit-man and staff of the Los Amigos Biological Station (ACCA) fortheir hospitality during our stay at Cerro Colorado. N. Rivera wassupported by an Occidental College summer research fellowshipduring the laboratory analysis of these samples. The commentsof two anonymous reviewers helped us improve the quality ofthe paper. Field work was supported by Alison Stenger.

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