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This version was published on June 1, 2009
GSA Bulletin; June 2009; v. 121; no. 7-8; p. 1172-1178; DOI: 10.1130/B26359.1
© 2009 Geological Society of America
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Geologic and taphonomic context of El Bosque Petrificado Piedra Chamana (Cajamarca, Peru)

Deborah Woodcock1,{dagger}, Herbert Meyer2, Nelia Dunbar3, William McIntosh3, Isabel Prado4 and Guillermo Morales4

1 Marsh Institute of Clark University, 950 Main Street, Worcester, Massachusetts 01610, USA
2 U.S. National Park Service, Florissant Fossil Beds National Monument, P.O. Box 185, Florissant, Colorado 80816, USA
3 New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, New Mexico 87801, USA
4 Museo de Historia Natural, Universidad Nacional Mayor de San Marcos, Av. Arenales 1256, Lima 14 Peru

Correspondence: {dagger}E-mail: dwoodcock{at}clarku.edu


   FOOTNOTES
 
GSA Data Repository item 2009023, New Mexico Geochronology Laboratory 40Ar/39Ar dating results, is available at http://www.geosociety.org/pubs/ft2008.htm or by request to editing{at}geosociety.org. Back


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 GEOLOGICAL AND STRATIGRAPHIC...
 DATING
 GEOLOGIC AND TAPHONOMIC...
 THE FOSSIL MATERIAL
 DISCUSSION AND CONCLUSIONS
 REFERENCES CITED
 
Volcanic and volcaniclastic rocks in the northern Peruvian Andes (central Cajamarca, 79°10'W, 6°35'S) contain a diverse assemblage of permineralized woods known as El Bosque Petrificado Piedra Chamana. The fossil forest and associated paleosol are preserved in ash-fall and lahar deposits of the Huambos Formation. Dating of plagioclase from the ash-fall deposit using 40Ar/39Ar methods yields a middle Eocene age of 39.35 ± 0.21 Ma. Accuracy of this age determination is supported by a more robust sanidine age of 39.52 ± 0.11 Ma from an underlying welded ignimbrite. Fossil wood and leaves associated with the ash-fall deposit include vertical trees rooted in the paleosol and buried in situ by the ash. Fossil wood is also present in high abundance and diversity in the overlying lahar. The fossils are significant as a low-latitude assemblage including a diversity of both monocots and dicots and in having fossil leaves occurring in close proximity to fossil woods. Preliminary analyses of wood and leaf characters suggest a megathermal climate with some limitations on plant growth associated with limited (seasonal) moisture availability. The assemblage represents lowland tropical forest that was probably growing near sea level and subsequently uplifted to the current elevation at the site (~2400–2600 m).


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
GEOLOGICAL AND STRATIGRAPHIC...
 DATING
 GEOLOGIC AND TAPHONOMIC...
 THE FOSSIL MATERIAL
 DISCUSSION AND CONCLUSIONS
 REFERENCES CITED
 
The small village of Sexi in the northern Peruvian Andes (Fig. 1) is home to a fossil forest (El Bosque Petrificado Piedra Chamana) that became known to science in the early 1990s and was declared part of the Cultural Patrimony of Peru in 1997. Fossil wood and leaves preserved in ash-fall and overlying lahar deposits of the Huambos Formation provide a detailed record of Neotropical vegetation and climate during a time (middle Eocene) when conditions were warm worldwide and prior to the uplift of the modern-day Andes. The Piedra Chamana fossil-wood assemblage is notable for the diversity of taxa present; its occurrence in the New World tropics, where such records are sparse; and for its association with fossil leaves, an uncommon occurrence in the rock record. The plant fossils are also significant with respect to timing and extent of Andean uplift.


Figure 01
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  		Figure 1. Maps showing: (A) Peru and surrounding countries, (B) site location.

 
The study of the fossil forest has been a collaborative United States–Peru project since the 1990s. This paper, based largely on fieldwork carried out in May 2005, describes the stratigraphic and taphonomic context of the fossils, reports new dates for rocks closely associated with the fossil material, and provides a general description of the fossils, together with preliminary paleoclimatic interpretations.

Site Description
El Bosque Petrificado Piedra Chamana is on the Pacific slope of the northern Peruvian Andes (Upper Chancay River Valley; Department of Cajamarca; 79°10' W, 6°35' S; Fig. 1) in a low region along the Andean chain in southern Ecuador and northern Peru. Highest peaks in the area range to ~4000 m; elevations near Sexi and the fossil locality are ~2400–2600 m. The dominant landscape features locally are the Sexi plateau, with its caldera-like basin; the high mountains of the surrounding Cordillera Occidental; and the deep canyon of the Rio Chancay to the east and south. The fossil exposures occur around the rim of the basin within which the town of Sexi is situated. Other research at the site has included a survey of the existing vegetation, which is evergreen sclerophyllous forest with significant shrub, epiphyte, and mat-layer components (Aragon-Carrasco et al., 2006); the area is within the biogeographic zone Amotape-Huancabamba, a region of higher biodiversity and endemism than other parts of the Andes (Young and Reynel, 1997; Weigand et al., 2002).


   GEOLOGICAL AND STRATIGRAPHIC CONTEXT
TOP
 ABSTRACT
 INTRODUCTION
 GEOLOGICAL AND STRATIGRAPHIC...
DATING
 GEOLOGIC AND TAPHONOMIC...
 THE FOSSIL MATERIAL
 DISCUSSION AND CONCLUSIONS
 REFERENCES CITED
 
The geological history of the Central Andes between 4° and 14° S has been related to subduction of the Nazca Plate and extensional tectonics during the Mesozoic, subsequent uplift associated with compression and foreland deformation, and the development of flat-plate subduction since 5 Ma (Ramos, 1999). South America has drifted north over the course of the Cenozoic; the estimated paleolatitude at the time the fossil forest was growing (ca. 39 Ma) is ~13° S (Smith et al., 1994). The volcanic rocks near Sexi belong to the Huambos Formation, which is part of the Calipuy Group, an extensive series of volcanic rocks occurring on both sides of the Andes that are Eocene to Miocene in age (Wilson, 1985; Noble et al., 1990; Garver et al., 2005). Locally, the Huambos Formation is underlain by volcanic rocks of the Llama Formation, which rests unconformably on Cretaceous bedrock.

Noble et al. (1990) give the thickness of the Huambos Formation as 450 m in the type area near Sexi and Huambos (15 km NE of Sexi). The fossiliferous pyroclastic and lahar deposits in the vicinity of Sexi occur in the middle part of this formation. The main fossil exposures cover a 1.0 x 0.5 km area bounded on the north and south by NW-trending faults. A third, NW-trending fault crosses the fossil area. The northeastern blocks appear to be displaced downward in each case, although some antithetic faults may also be present. Much of the fossil wood has weathered out of the rock and is exposed on the surface, in addition to material still in place in the rocks.

A generalized stratigraphic sequence in the study area is presented in Figure 2. Underlying the sequence is a 150-m-thick volcanic unit containing phenocrysts of sanidine, quartz, plagioclase, and biotite, as well as abundant flattened pumice (fiamme). The presence of flattened pumice allows identification of this unit as an ignimbrite, consistent with the interpretation of Noble et al. (1990), who further identify it as a major regional unit. Although neither the upper nor lower contact of this ignimbrite is exposed in the vicinity of the fossil forest, it was observed and sampled for dating at a nearby location.


Figure 02
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  		Figure 2. Generalized stratigraphic profile of the volcanic sequence preserving the Piedra Chamana fossils.

 
Fine-grained, pumice-rich sediments underlie the fossil horizons. These deposits are poorly sorted with distinct beds that are 0.5–2.0 m thick and thin upsection. A layer formed on the top surface is interpreted as a paleosol on the basis of colored horizons, blocky structure, occurrence of leaf fragments (mostly palm fronds at the one location sampled) at the contact with the overlying ash-fall deposit, and the presence of vertically oriented trees with bases, and in some cases rooting structures, near the top surface (Fig. 3A). This feature reaches a thickness of 10 cm or more and is alternately greenish or reddish with some blackened horizons in places.


Figure 03
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  		Figure 3. (A) Specimen with root structure extending into the paleosol; (B) leaf from the ash-fall layer; (C) tree stump extending up into the ash fall, which is partially eroded at this location; (D) largest log present at the site (diameter 0.75 m, length 10 m).

 
Overlying the paleosol is a fine-grained, white deposit, ~1 m thick, that mantles the topography. This deposit contains highly altered but identifiable relict glass shards with well-defined triple junctions, consistent with ash-fall origin (Heiken and Wohletz, 1984). This deposit also contains abundant accretionary lapilli, again consistent with an ash-fall origin. A crystal-rich horizon is present at or near the base of the deposit, containing biotite, quartz, and plagioclase feldspar crystals ~0.5 mm in diameter. Plagioclase crystals in the ash, and in overlying lahar deposits, have predominantly altered to clay, although some unaltered feldspars are present. The upper part of the ash is more crystal poor and contains highly distinctive accretionary lapilli up to 8 mm in diameter. The lapilli occur sparsely throughout the deposit and are also concentrated in some areas. Impressions of leaves have been found at some locations and are more common in the lower half of the ash (Fig. 3B). The fossil wood associated with the ash includes stumps to 30 cm in diameter that extend to or into the underlying paleosol and represent trees in growth position (Figs. 3A and 3C), logs or branches oriented more or less horizontally, and smaller wood fragments.

The ash-fall deposit is overlain by a 3-m-thick, nonbedded, accretionarylapilli–rich layer consisting of identical material to the ash-fall deposit, but lacking any primary bedding. This deposit is fine grained and contains crystals of biotite, quartz, and feldspar and abundant accretionary lapilli, composed completely of volcanic ash and 0.5–2 cm in diameter, which occur throughout and are concentrated in some horizons (Fig. 4). This layer is the main wood-bearing horizon and the source of the large amount of fossil wood that has weathered out of the rocks and is scattered across the surface at the site. The fossil material is highly variable in size but contains large elements (to 0.75 m in diameter and 5–10 m long; Fig. 3D). In situ logs are concentrated in certain areas, having the appearance of chaotic tangles or logjams. However, despite the disorganized appearance of the fossil wood, a preferred orientation of NW/SE was measured (Fig. 5). In many locations, logs occur at high frequency at the surface of the deposit. Although most of the stumps in the ash-fall deposit are broken off at or below the contact with the overlying unit, one small trunk (9 cm in diameter) could be traced upward through the ash into the overlying material.


Figure 04
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  		Figure 4. (A) Accretionary lapilli in lahar; (B) backscattered electron image of epoxy-impregnated, polished sample from an accretionary lapillus. Relict glass shards are abundant, most notably one large, Y-shaped shard near the center of the image (see arrow). Scale is 100 µm.

 


Figure 05
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  		Figure 5. Orientation of logs in lahar deposit (n = 61). Outer circle = 12%.

 
Based on the above observations, we interpret the unit overlying the ash-fall unit to be an ashy lahar. The size, abundance and orientation of the fossil wood in the deposit indicate high energy, but directed deposition. The volcanic component of the deposit is homogeneous and identical to the underlying ash fall, but the lack of primary bedding, plus the energetic nature of deposition indicate a fundamentally different depositional mechanism, and likely one that remobilized previously deposited volcanic ash. Furthermore, the presence of abundant accretionary lapilli in the ash fall and overlying deposit suggest that rain was falling during the eruption, providing the water to mobilize ashy mudflows. No charring of the wood is observed, suggesting cold deposition.

A similar deposit above the lower lahar attains a thickness of ~10 m and is also fine grained and whitish with crystals of biotite, quartz, and feldspar. This layer is also interpreted as a lahar and differs from the one below in the lower frequency of accretionary lapilli and fossil wood. The sparse wood in this unit consists of small-diameter pieces and fragments.

In some places where the top of the upper lahar is present, it is overlain by a poorly sorted, non-bedded deposit ~3 m thick that contains pumice up to 1.5 cm in diameter, lithic clasts less than 2 cm in diameter, and quartz and altered feldspar crystals. The deposit, interpreted as a nonwelded ignimbrite, is highly altered and clay rich.


   DATING
TOP
 ABSTRACT
 INTRODUCTION
 GEOLOGICAL AND STRATIGRAPHIC...
 DATING
GEOLOGIC AND TAPHONOMIC...
 THE FOSSIL MATERIAL
 DISCUSSION AND CONCLUSIONS
 REFERENCES CITED
 
40Ar/39Ar dating of rock specimens was carried out at the New Mexico Geochronology Laboratory with an all-metal automated extraction line and MAP 215-50 mass spectrometer using the single-crystal laser fusion method (GSA Data Repository Item DR11). Sparse plagioclase from the ash layer yielded a date of 39.35 ± 0.21 Ma (middle Eocene). This date is interpreted as the best determination of the age of leaves and wood in the ash and lahar deposits. The accuracy of this age determination is supported by the 39.52 ± 0.11 Ma age of sanidine from the welded ignimbrite underlying the fossiliferous sediments (Table 1). These results are consistent with an earlier, less precise 40Ar/39Ar date of 39.3 ± 1.0 for the basal welded ignimbrite of the Huambos Formation obtained by Noble et al. (1990). The dates place the fossil assemblage at a time after the period of maximum warmth at the Paleocene-Eocene boundary when conditions worldwide were still relatively warm but when cooling may have begun at the high latitudes of North America (Wolfe, 1994; Myers, 2003).


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  		TABLE 1. SUMMARY OF 40Ar/39Ar RESULTS

 

   GEOLOGIC AND TAPHONOMIC INTERPRETATIONS
TOP
 ABSTRACT
 INTRODUCTION
 GEOLOGICAL AND STRATIGRAPHIC...
 DATING
 GEOLOGIC AND TAPHONOMIC...
THE FOSSIL MATERIAL
 DISCUSSION AND CONCLUSIONS
 REFERENCES CITED
 
The fossil wood and leaves associated with the paleosol and overlying ash fall represent a forest growing in an area where there was abundant volcanically derived material. Upsection fining could indicate a topographic depression filling up with sediments. The presence of the paleosol reflects a period of time in which conditions were stable enough for soil development, although perhaps not a long period based on the degree of development of the paleosol. The leaves found at the surface of the paleosol may be part of a litter layer in which more sclerophyllous (thick, hard-leaved) elements such as palm leaves are preferentially represented.

The volcanic event began as an ash-rich eruption. The fine-grained nature of the ash, the thickness of the deposit, and the presence of a basal, crystal-rich layer suggest that the eruption occurred at least several kilometers from the site and material was gravitationally sorted from a high eruptive cloud. Wilson (1985) placed the eruptive center for the Huambos Formation northwest of the town of Llama, which is ~10 km NW of Sexi. Atmospheric moisture during the ash fallout contributed to the formation of accretionary lapilli (McPhie et al., 1993; Schumacher and Schmincke, 1995), which were deposited concurrently with fine ash. The ash fall either stripped the trees of leaves or caused leaf abcission but left many trees standing. Gentle deposition of fine ash preserved the leaves in fairly intact form.

Subsequent rainfall events over the wider region where ash had been deposited saturated and destabilized the slopes and created a lahar that moved through the ash-covered, but possibly still living, forest (at Mount Saint Helens, forest trees remained alive after being buried by several centimeters of ash; Spicer, 1989). One interpretation is that rain beginning during the ash fall intensified, creating the lahar. Presence of layers of accretionary lapilli in an undeformed state indicates that internal stratification was maintained from the source region and that flow was largely nonturbulent. It also suggests a nearby source area. Absence of lithic material is also consistent with the interpretation that the flow involved mobilization of an ash-fall deposit and movement over an ash-covered surface to the deposition site.

The dense flow of rain-soaked ash and accretionary lapilli snapped off many trees as it moved through the area. Other trees were upended and/or uprooted, causing deformation of the surface or creating scouring features where they were dragged along by the flow. The flow also moved around some elements, as, for example, one small tree that could be traced upward through the lahar. Trees were also knocked down and carried downslope; this material became entombed in the lahar, where it was mixed with more locally derived elements. The massive nature of the flow may have allowed for transportation of large trees. Trees that were uprooted together may have been transported and deposited together. Concentrations of wood at the surface of the deposit also suggest that there were logjams of wood floating on the surface. Similarly, lahars associated with the Mount Saint Helens eruptions both transported fragile clasts (at the surface and mixed into the flow) and floated large logs on the surface (Janda et al., 1981).

Preservation conditions appear to have varied somewhat between the ash and the lahar. In general the woods in the ash are less well preserved, probably because the ash, being dry and low density, was a less favorable preservation environment than the water-saturated lahar, which had a cementlike consistency when consolidated. The trees left standing in growth position in the ash layer may also have preserved differently from the horizontally oriented material due to differences in percolation rates and mineral replacement depending on whether the pores were oriented parallel or perpendicular to vertical. The massive nature of the overlying lahars and ignimbrite was certainly important in protecting the ash from weathering and erosion and allowing for preservation of the fossils. Bell and House (2007) have described an association of ash-fall deposits with overlying lahars and suggest a link between storms associated with Plinian-type eruptions and subsequent debris flows that bury and preserve the tephra beds.

A second lahar flow was similar to the first but transported fewer accretionary lapilli and smaller amounts of wood. There may have been decreased availability of wood at this time, most of the larger elements having been swept away by the earlier flow. Alternatively, the source area for this flow may have been less wood rich. A pyroclastic flow then covered the area, possibly from the same eruptive event. The ash fall, lahar flows, and pyroclastic flow probably occurred in close succession, and may have been nearly synchronous, based on the absence of soil formation or significant erosional features.


   THE FOSSIL MATERIAL
TOP
 ABSTRACT
 INTRODUCTION
 GEOLOGICAL AND STRATIGRAPHIC...
 DATING
 GEOLOGIC AND TAPHONOMIC...
 THE FOSSIL MATERIAL
DISCUSSION AND CONCLUSIONS
 REFERENCES CITED
 
Although the quality of preservation and the extent to which anatomical details are observable varies widely, in general the woods preserved in the lahar include the best-preserved specimens. Figure 6 shows examples of well-preserved dicot and monocot specimens. In the present context, the salient distinction between these two major divisions of flowering plants is that arboreal monocots such as the palms have only primary vascular tissue and lack true wood, whereas woody dicots have secondary xylem (wood) produced by a vascular cambium. The difference is readily apparent in Figure 6, where the distinct groupings evident on the left represent the vascular bundles typical of primary growth, and the rowlike alignment of elements (fibers and vessels) on the right represents secondary growth (i.e., wood).


Figure 06
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  		Figure 6. Cross sections of representative monocot (left) and dicot (right) taxa. Scale bar is 250 µm.

 
Although the woods preserved in the ash-fall and lahar deposits might be expected to present somewhat different pictures of the vegetation, with the ash-fall material being a more localized sampling, the poorer preservation makes it difficult to study the ash-fall woods as a distinct assemblage.

A notable aspect of the fossil assemblages is the high diversity of both monocots and dicots. The monocot material (Fig. 7) includes palm stems (some in situ as in Fig. 7B), leaf fragments, and petioles (Fig. 7C). Because the monocot fossils preserve the surface features in many cases—as contrasted with the dicots, in only the inner portion of the trunk (the wood) is preserved (not the bark or phloem)—morphological characters can be used in conjunction with anatomical characters to distinguish the monocot taxa. These characters include features such as stem diameter, internode length, presence or absence of persistent leaf bases, and presence and appearance of leaf scars. The monocot material falls into a range of diameter size classes: a small specimen with a diameter of 7.5 cm is shown in Figure 7D; the largest specimens are 22–26 cm in diameter.


Figure 07
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  		Figure 7. Representative monocot material: (A) palm specimen with horizontal leaf scars (the diameter is 13 cm along shortest axis, 19 cm along widest axis); (B) in situ palm with leaf bases; (C) palm petiole or leafstalk; (D) small-diameter palm.

 
The largest specimens among the dicot material are several large logs, the biggest of which is 0.75 m in diameter and over 10 m long. Among the larger specimens are many that are long (7–11 m) but with diameters ≤0.4 m, suggesting that tall but relatively small-diameter trees were common, as might be expected in a tropical forest setting.

A minimum of 30–35 dicot and 10 monocot taxa are represented among the 300 wood and stem specimens collected, although the diversity of monocots is probably considerably higher given that this material was not collected as extensively (since the initial focus of the study was on interpreting climate based on the dicot woods). Analysis of the anatomical characteristics of the dicots shows the presence of taxa with vessel diameters ranging from large to small; vessel density ranging from high to low; a large range in specific gravity; and growth rings both present and absent, with some taxa showing semi-ring–porous or graduated-porous anatomy. Aspects of the woods of particular significance with respect to paleoclimate estimates are listed below.

(1) Taxa with rings are present at a low frequency. Rings that are anatomically well defined occur in virtually all temperate woods and are present in the tropics at much lower frequencies (but are difficult to estimate precisely because of problems with determinations; Wheeler and Baas, 1991; Woodcock, 2002; Wheeler and Manchester, 2002).

(2) Woods with graduated-porous wood (variation of a factor of five in vessel size across the ring) are represented by several taxa. In North America, this wood type occurs mainly in mid-latitude seasonal climates, being most prevalent in drier areas; occurrence in the tropics may be 5%–15% (Woodcock, 1994; Wheeler and Manchester, 2002). Presence here indicates a deciduous component to the flora.

(3) Woods with wide vessels (>200 µm) occurring at low density (<10 cm2) are represented. Wheeler and Manchester (2002) cite this feature as prevalent in lowland tropical rainforest trees (and probably not occurring elsewhere).

(4) Some of the woods are wide vesseled with a very low degree of lignification. Forest elements with wood of this sort are typified by Bombacacous taxa or species of Erythrina (Leguminosae) and occur in both wet and dry tropical (megathermal) climates.

These various aspects of the wood assemblages, in particular the species diversity, the diversity of palm taxa, and the wood characters of the represented taxa, show a general correspondence to modern lowland tropical forest and suggest that the area was near sea level when the forest was growing and has experienced uplift of as much as 2500–2600 m since the late Eocene.

Approximately 25 morphotypes can be recognized among the 230 leaf specimens examined (most of these are dicots, but some monocots are represented). Almost all the dicot leaves are entire margined and predominantly microphyllous to notophyllous in size, with few mesophylls. Preliminary analysis based on leaf size and margin characteristics (Wolfe, 1995; Wilf, 1997; Wilf et al., 1998) suggests a megathermal climate (>25 °C) with some limitations on plant growth associated with moderate or seasonal precipitation.


   DISCUSSION AND CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 GEOLOGICAL AND STRATIGRAPHIC...
 DATING
 GEOLOGIC AND TAPHONOMIC...
 THE FOSSIL MATERIAL
 DISCUSSION AND CONCLUSIONS
REFERENCES CITED
 
Our dates provide additional evidence that the volcanic rocks of the Huambos Formation are older than had been thought prior to the work of Noble et al. (1990) and predate the elevation of the modern Andes Mountains. A variety of types of evidence puts the time of greatest Andean uplift subsequent to 10 Ma and elevations less than a third of present values prior to 25 Ma (Gregory-Wodzicki, 2000; Graham et al., 2001; Garzione et al., 2008). Noble et al. (1990) suggested that the eruptions that produced the Huambos Formation occurred subsequent to a period of tectonic activity in northern Peru at ca. 42 Ma. The volcanic rocks near Sexi (Huambos and Llama Formations, the latter dated at 44.2 ± 1.2 Ma by Noble et al., 1990) are considered part of the Calipuy Group (Wilson, 1985; Noble et al., 1990) but appear to be significantly older than Calipuy Group volcanics occurring farther south (Garver et al., 2005).

Volcaniclastic depositional settings similar to those reported here have been described for other fossil wood assemblages, including Florissant Fossil Beds in Colorado (Evanhoff et al., 2001; Meyer et al., 2004) and the Yellowstone fossil forests (Fisk, 1976). There are also parallels to the recent Mount Saint Helens and El Chichon eruptions, where ash falls and lahars buried forest remains (Spicer, 1989). The Piedra Chamana fossils are notable, however, in comparison with other fossil wood assemblages, in being a low-latitude assemblage with a diversity of both monocots and dicots. They are also unusual in that both leaves and woods are represented. Co-occurrence of wood and leaves in the same stratum is uncommon in the rock record—one of the few examples is the Eocene Clarno Nut Bed Flora (Wheeler and Manchester, 2002)—and presents a valuable opportunity to study differential taphonomic effects and make critical comparisons between two different environmental and climate proxies.

The monocots represented in the flora may have been readily preserved owing to the degree of lignification and silicification that they show when alive. Although there are many descriptions of fossil monocots, here they occur as part of a diverse forest assemblage. The Neotropical Microfloral Province recognized by Romero (1993) for the Eocene of northern (tropical) South America is characterized by a mixture of palm and dicotyledonous taxa, with palms less prevalent than in the Paleocene but still present in some diversity. The Piedra Chamana assemblage appears to conform to this community structure.

The fossils are consistent with interpretations that a considerable amount of uplift has occurred in the area since 39 Ma. Paleoclimatic interpretations of the woods are preliminary in nature; better quantitative climate estimates will be possible when the woods are analyzed in more detail and we can utilize recently developed techniques for analysis of fossil wood (Wiemann et al., 1998, 1999, 2002) to generate climate estimates. This approach to climate reconstruction does not rely on taxonomic determinations, being based instead on statistical representations of the response of morphological or anatomical characters to climate.

The scientific significance of El Bosque Petrificado Piedra Chamana establishes the fossil site as a premier paleontological locality and makes preservation of the site and its fossils a matter of pressing concern. It is the authors' hope that local and national authorities will give prompt attention to further protection of the fossil forest. The beauty of the setting, at the edge of the canyon of the Rio Chancay with views looking out across the Andes and the Maranon Valley toward the Amazon basin, enhances the potential of the site to be developed as a resource for education and tourism to benefit the people of the area.


   ACKNOWLEDGMENTS
 
We would like to thank the people of Sexi for their interest and support. Special thanks to Elba Davila, Rogelio Davila, Santiago Asenjo, Luis Valverde, Oeler Rojas, Susan Aragon-Carrasco, Jennifer Young, and the Club de Madres. We appreciate comments from Judith Parrish, two anonymous reviewers, and journal editors. This research was supported by an American Philosophical Society grant to D. Woodcock and National Science Foundation grant 0403510 to D. Woodcock and H. Meyer.


   REFERENCES CITED
TOP
 ABSTRACT
 INTRODUCTION
 GEOLOGICAL AND STRATIGRAPHIC...
 DATING
 GEOLOGIC AND TAPHONOMIC...
 THE FOSSIL MATERIAL
 DISCUSSION AND CONCLUSIONS
 REFERENCES CITED
 

Aragon-Carrasco, S., Rimarchin, L., Ayasta, J., and Woodcock, D. 2006, Inventario preliminar de la flora del Distrito de Sexi, Cajamarca: Arnaldoa, v. 13, p. 358– 367.

Bell, J.W., and House, P.K. 2007, Did Plinian eruptions in California lead to debris flows in Nevada?: An intriguing stratigraphic connection: Geological Society of America Bulletin, v. 35, p. 219– 222.

Evanhoff E., Gregory-Wodzicki K.M., Johnson K.R. eds., 2001, Fossil flora and stratigraphy of the Florissant Formation, Colorado: Proceedings of the Denver Museum of Nature and Science, ser. 4, no. 1, 218 p.

Fisk, L.H. 1976, The Gallatin "petrified forest": A review: Montana Bureau of Mines and Geology Special Publication 73, p. 53– 72.

Garver, J.I., Reiners, P.W., Walker, L.J., Ramage, J.M., and Perry, S.E. 2005, Implications for timing of Andean uplift from thermal resetting of radiation-damaged zircon in the Cordillera Huayhuash, Northern Peru: The Journal of Geology, v. 113, p. 117– 138, doi: 10.1086/427664.[CrossRef][Web of Science][GeoRef]

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RECEIVED FOR PUBLICATION November 5, 2007

REVISED MANUSCRIPT RECEIVED September 17, 2008

MANUSCRIPT ACCEPTED October 2, 2008





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