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PALAIOS, 2007, v. 22, p. 3–16
Research Report
DOI: 10.2110/palo.2005.p05-081r
PALEONTOLOGIC EVIDENCE FOR SEDIMENTARY DISPLACEMENT IN NEOGENE FOREARC
BASINS OF CENTRAL CHILE
KENNETH L. FINGER,1* SVEN N. NIELSEN,2 THOMAS J. DEVRIES,3 ALFONSO ENCINAS,4 and DAWN E. PETERSON1
1
University of California Museum of Paleontology, 1101 Valley Life Sciences Building, Berkeley, California 94720-4780, USA; 2 GeoForschungsZentrum Potsdam,
Section 3.1, Telegrafenberg, 14473 Potsdam, Germany; 3 Burke Museum of Natural History and Culture, University of Washington, Seattle, Washington 98195,
USA; 4 Departamento de Geologı́a, Universidad de Chile, Casilla 13518, Correo 21, Santiago, Chile
e-mail: [email protected]
ABSTRACT
A consensus on the biostratigraphic age and depositional environment of the Navidad, Ranquil, and Lacui formations exposed along
the tectonic margin of central Chile has been elusive due to conflicting evidence. This study resolves this dilemma and gains further insight regarding the history of the Chilean coast. Problematic interpretations stem primarily from the remarkable similarity between
the molluscan fauna of these units with those well documented for
the late Oligocene to early Miocene of Peru. Planktic foraminifers,
however, indicate that the Chilean sections accumulated in the late
Miocene to early Pliocene interval following a regional hiatus that
extends into the Eocene. The prevalence of mixed-depth bathyal assemblages of benthic foraminifers and ostracodes, the majority of
which include lower-bathyal (⬎2000 m) indicators, reveals that
downslope displacement was a primary mode of deposition in the
basins. Although the molluscan assemblages are dominated by shallow marine taxa, most include species that range into or are restricted
to deeper waters. Sedimentary features connote rapid subsidence and
deep-water deposition of gravity flows. Although older Tertiary and
Cretaceous planktic foraminifers in several assemblages indicate reworking of older units, lack of data on pre-Tortonian faunas of this
region precludes recognition of other age-discordant components that
could constitute a significant portion of the recovered fauna. The
findings of this study revise the prevailing conception of the region’s
geologic history that considered these units to be early to middle
Miocene shelf deposits and indicate that infilling and uplift have
characterized the nearshore basins since the late Pliocene.
localities. He also collected the fossil mollusks that Sowerby (1846) described in an appendix to Geological Observations on South America
(Darwin, 1846). Hupé (1854) named additional species within the monumental volumes of Gay (1844–1854). A monograph by Philippi (1887)
on the Tertiary and Quaternary fossils of Chile presented a synopsis and
revision of the fauna, including descriptions of many new taxa, including
119 new gastropod species. It remains the most complete reference on
INTRODUCTION
Along the coast of central Chile, the Navidad Formation and its stratigraphic equivalents, the Ranquil Formation on Peninsula Arauco and the
Lacui Formation on Chiloé Island (Fig. 1), commonly have been interpreted as early to middle Miocene shallow-water deposits (see Previous
Work below). Ages of the Navidad Formation presented in the literature
are incongruent, there is little published data for the Ranquil Formation,
and no prior data are available on the age of the Lacui Formation. This
study addresses the following questions paramount to other research on
these units. (1) What are the ages of these exposures? (2) How correlative
are they with each other? (3) What are the depositional histories and
environments represented by these deposits?
PREVIOUS WORK
The first report of Tertiary fossils from central Chile is that of
d’Orbigny (1842), who collected them during his 1826–1833 expedition
to South America. Darwin (1846, 1900) provided the first geological
study of the region aboard the H.M.S. Beagle in 1835. He described the
Navidad Formation and noted several coastal outcrops that remain key
*Corresponding author.
FIGURE 1—Study areas in central Chile. A) Navidad Formation. B) Ranquil Formation. C) Lacui Formation. See Figure 2 for detailed maps and sampled localities.
Copyright 䊚 2007, SEPM (Society for Sedimentary Geology)
0883-1351/07/0022-0003/$3.00
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FIGURE 2—Sampled localities. A) Navidad Formation. B) Ranquil Formation. C) Lacui Formation.
this fauna. Subsequent studies on the geology (e.g., Brüggen, 1934, 1950;
Tavera, 1942; Garcı́a, 1968; Tavera et al., 1985) and macropaleontology
(e.g., Möricke, 1896; Philippi, 1897; Frassinetti, 1974; Covacevich and
Frassinetti, 1980, 1986; Frassinetti and Covacevich, 1981, 1982, 1993)
of central Chile did not stray from the common view of the Navidad
Formation and its equivalents as being early to middle Miocene, shallow
marine deposits.
Martı́nez and Osorio (1964) first identified planktic foraminifers and
discoasters indicative of a Tortonian (late Miocene) age from the Navidad
Formation. Correlating with Patagonian molluscan biostratigraphy, Tavera (1968) placed the Navidad Formation in the Burdigalian (early Miocene), which Dremel’s study (in Herm, 1969) of planktic foraminifera
from Punta Perro supported. Osorio (1978) referred to the ostracodes of
the Rı́o Rapel section as late Miocene and bathyal. It is not clear if he
FIGURE 3—Exposures of the Navidad Formation at Punta Perro. Photo taken approximately 15 m stratigraphically above intertidal platform (PPP).
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FIGURE 4—The Ranquil Formation at Peninsula Arauco. The bluff (RAN) consists mostly of poorly defined beds of massive sandstones with intermittent beds of glauconitic
sandstone. Height of bluff is approximately 20 m.
based his interpretations on correlation with Caribbean ostracodes or
planktic foraminifers or both. Osorio (1978) also noted the discrepancy
between previous studies and suggested that it was due to sampling of
disparate stratigraphic levels within the formation.
Tavera (1979) distinguished three members within the Navidad Formation, from oldest to youngest: the Navidad, Lincancheo, and Rapel.
Although Darwin (1846) named the Navidad Formation based on its lower part, applying the name Navidad to one of its members presents a case
of homonymy that Encinas et al. (2006) have resolved by elevating all
these members to the rank of formation: Navidad, Licancheu, and Rapel
Formations.
Martı́nez-Pardo (1990) referred all of Chile’s Miocene marine deposits
to the Navidad Formation, which he correlated with the lower of two
transgressive sequences recognized in Chile and Peru. He referred to this
event as the Neogene South East Pacific Sequence I (NSEPS-I), demarcated with significant hiatuses below and above. NSEPS-I consists of a
lower subcycle extending from 19 Ma (Burdigalian) to 13 Ma (Serravallian) with maximum transgression at 14 Ma (Serravallian), and an upper
subcycle extending from 13 Ma to 10 Ma (Tortonian) with maximum
transgression at approximately 11 Ma. It has been difficult to incorporate
these interpretations into the research of others because locality data and
supporting paleontological evidence were not presented.
Ibaraki (1992a) recorded the presence of Neogloboquadrina acostaensis (Blow) after examining planktic foraminifers from the Punta Perro
section. In transitional latitudes, this species first appears (FAD) in Zone
Mt9 of Berggren et al. (1995), which is correlative with the low-latitude
Globorotalia acostaensis Zone of Bolli and Saunders (1985) and zone
N16 of Blow (1969). Ibaraki’s data added credibility to the Tortonian age
assigned by Martı́nez and Osorio (1964). Subsequent palynologic analysis
correlated the Navidad Formation (Méon et al., 1994) with the late middle
←
FIGURE 5—The Lacui Formation south of Cucao, Chiloé Island. Locality CUC is
the sandstone blockfall. Height of bluff is approximately 40 m.
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FIGURE 6—Coastal exposure of the basal Navidad Formation north of Mostazal, north of locality MOS. Igneous intrusions (not shown) confirm that the numerous
discontinuous basement-rock exposures in this vicinity are displaced boulders. Height of outcrop is approximately 5 m.
to late Miocene, which neither refined nor contradicted the Tortonian
assignment. Nevertheless, recent correlations based on Peruvian molluscan biostratigraphy (DeVries and Frassinetti, 2003; DeVries and Nielsen,
2003) and shark teeth (Suárez et al., 2006) suggest that there is a late
Oligocene or early Miocene component in the Navidad Formation.
MATERIALS AND METHODS
This study is based on field observations and samples collected from
three units that crop out along the central coast of Chile: (1) the Navidad
Formation in an area beginning approximately 130 km southwest of Santiago, (2) the Ranquil Formation of the Arauco Peninsula, and (3) the
Lacui Formation of Chiloé Island. Fieldwork involved measuring stratigraphic sections, describing sedimentary features, and collecting invertebrate fossils and microfossiliferous sediments from outcrops of the Navidad Formation (sensu Tavera, 1979; Figs. 1–2). Macroinvertebrates,
predominantly gastropods, were collected directly from the outcrops,
whereas foraminifera and ostracodes, found to be most abundant where
gastropods were in highest concentration, were extracted from the sedi-
FIGURE 7—Siltstone bed with rip-up clasts in coastal bluffs at locality MAT, approximately 1 km north of Matanzas.
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FIGURE 8—Stringer of Turritella trilirata Philippi in siltstone on the intertidal platform (PPP) along Punta Perro. Part B is a close-up photograph of part of Part A.
mentary rock samples by disaggregating in hydrogen peroxide and washing over a U.S. Standard No. 230 (63 ␮m openings) mesh screen.
Coastal bluffs and wave-cut benches expose the Navidad Formation
(Fig. 3). Inland outcrops are the younger Licancheu and Rapel formations
(Encinas et al., 2006), which yield poorly preserved fossils only. Sampling of the Ranquil Formation on Peninsula Arauco (Fig. 4) and the
Lacui Formation on Chiloé Island (Fig. 5) yielded macroinvertebrates and
microfossils similar to those obtained from the Navidad Formation.
SEDIMENTOLOGY
In the Navidad area, approximately 100 to 200 m of the Navidad Formation overlie the Paleozoic granitic basement or the Upper Cretaceous
marine Punta Topocalma Formation (Cecioni, 1978). Extensive coastal
bluffs are characterized by nearly horizontal bedding, but stratification is
often poorly defined and discontinuous. The base of the Navidad Formation (Fig. 6) consists of a few meters of fossiliferous conglomerate.
The conglomerate is clast supported and contains subrounded to angular
clasts and fragments of bivalves, gastropods, solitary corals, neritic foraminifers, and wood. Fossil fragments are usually scarce, but in some
places they are abundant enough to characterize the conglomerate as a
coquina. These shallow-marine deposits mark the initial subsidence of
the basin.
Overlying the basal conglomerate is an interval of interbedded sandstone and mudstone, with minor conglomerate. These deposits include a
diverse fossil assemblage of bivalves, gastropods, foraminifers, shark
teeth, leaf impressions, pollen, and crabs (Philippi, 1887; Tavera, 1979;
Martı́nez-Pardo and Valenzuela, 1979; Troncoso, 1991; Troncoso and
Romero, 1993; Meón et al., 1994; Finger et al., 2003). Bouma cycles,
debris flows, synsedimentary breccia, slides, slumps, rip-up clasts (Fig.
7), water-escape structures, sheared flames, and armored balls (Encinas
et al., 2006), as well as the ichnofossils Chondrites, Zoophycos, Thalassinoides, and Ophiomorpha identified in the Navidad Formation indicate
rapid and deep subsidence (L.A. Buatois, personal communication, 2005).
Transport is also evident in small stringers of gastropods (Fig. 8) observed
commonly on beach platforms.
BIOSTRATIGRAPHIC CORRELATIONS
Molluscan Biostratigraphy
FIGURE 9—Stratigraphic range of Navidad molluscan species in southern Peru.
Comparison of molluscan assemblages in the Navidad Formation and
its equivalents with those from Cenozoic marine formations of southern
Peru (Fig. 9) favors a youngest Oligocene to early middle Miocene age
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FIGURE 10—Biostratigraphically important planktic foraminifers present in the Neogene of coastal Chile. A) Neogloboquadrina continuosa (Blow). B) Neogloboquadrina
acostaensis (Blow). C) Neogloboquadrina pachyderma (Ehrenberg). D) Globoturborotalita apertura (Cushman). E) Globoquadrina dehiscens (Chapman, Parr, and Collins).
F) Globigerinella obesa (Bolli). G) Globorotalia puncticulata (Deshayes). H) Globorotalia sphericomiozea Walters. I) Globigerina venezuelana Hedberg. J) Globigerinella
aequilateralis (Brady). K) Orbulina universa Orbigny. L) Reworked globotruncanid. M) Reworked Catapsydrax dissimilis Cushman and Bermúdez. Scale bars ⫽ 100 ␮m.
[Erratum: Identifications of G and H are transposed.]
for the Navidad molluscan fauna (DeVries and Frassinetti, 2003; DeVries
and Nielsen, 2003). Several species typical of the Navidad Formation
also occur in the Cenozoic forearc basins of southern Peru, including
Ficus distans, Acanthina katzi, Testallium cepa, Olivancillaria claneophila (⫽O. tumorifera; see Nielsen, 2004), and Glycymeris ibariformis
(DeVries and Vermeij, 1997; Vermeij and DeVries, 1997; DeVries, 1997a,
2003; Nielsen and Frassinetti, 2003). Peruvian specimens of these species
were found in successions dated using radiolaria (Marty, 1989), foraminifera (Dunbar et al., 1990; Ibaraki, 1992b, 1992c, 1993), diatoms (Macharé and Fourtanier, 1987; Macharé et al., 1988; Dunbar et al., 1990;
DeVries, 1998), 40K-40Ar isotopes (De Muizon and Bellon, 1980; De
Muizon and DeVries, 1985; Noble et al., 1985; Dunbar et al., 1990), and
40Ar-39Ar isotopes (DeVries, 1998).
Late Miocene (Tortonian, Messinian) and Pliocene successions from
southern Peru contain many of the same molluscan species that occur in
coeval Chilean deposits previously thought to be younger than the Navidad Formation (Herm, 1969; Guzmán et al., 2000; DeVries and Frassinetti, 2003). In contrast, middle Miocene (Serravallian) deposits in
southern Peru contain a distinctive suite of molluscan species, some of
which also occur in contemporaneous sediments of the Talara Basin in
northern Peru (DeVries, 1997b). None of the middle Miocene mollusks
of Peru are present in the Navidad Formation.
Some of the molluscan species in the Navidad Formation range no
higher than the lower middle Miocene (Langhian; basal Pisco Formation)
in southern Peru, where they are most common throughout the lower
Miocene (Burdigalian, Aquitanian) Chilcatay Formation (DeVries, 1998).
The oldest occurrence of these are two gastropods, Olivancillaria claneophila (Duclos) and Testallium cepa (Sowerby), found in latest Oli-
gocene (Chattian) deposits near Caravelı́ and Nazca (DeVries and Frassinetti, 2003); 40Ar-39Ar dating of volcanic ash in the Caraveli beds indicates an age of about 25 Ma (Noble et al., 1985).
Microfossil Biostratigraphy
The sampled exposures yielded a foraminiferal fauna comprised of more
than 300 species of foraminifera. The majority of species belong to benthic
genera typical of Neogene offshore deposits at middle latitudes. Temperate
microfaunas typically include benthic and planktic species also known to
occur in subpolar and subtropical latitudes. Many of the benthic species in
the Chilean Neogene have been recorded living on the Chilean slope (Bandy
and Rodolfo, 1964; Ingle et al., 1980; Resig, 1981). Numerous species and
homeomorphs in the Chilean Neogene are constituents of the cosmopolitan
deep-water fauna documented by van Morkhoven et al. (1986).
In this study, 17 of 19 assemblages include index species of planktic
foraminifera that have late Miocene or early Pliocene datums (Figs. 9–
10). Ten of the assemblages yielding biostratigraphic markers can be assigned to either zone N16, indicated primarily by representatives of the
Neogloboquadrina continuosa–Ng. acostaensis–Ng. pachyderma plexus,
or zone N19, indicated primarily by Globorotalia puncticulata. Absent
from outcrop samples were other early Pliocene species of Globorotalia
(i.e., Glr. cibaoensis Bermúdez, Glr. conoidea Walters, Glr. conomiozea
Kennett, Glr. crassula Cushman and Stewart) identified in well samples
from the Navidad area on loan from Empresa Nacional del Petróleo
(ENAP). Although Martı́nez and Osorio (1964) reported Candeina nitida
(N17b FAD) and Glr. miocenica (N19/20 FAD) in the Navidad Formation, these markers were not seen during this study. Accurate identifica-
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FIGURE 11—Biostratigraphy of sampled outcrops based on concurrent ranges of diagnostic planktic foraminifers (as numbered). Framework derived from Kennett and
Srinivasan (1983), Bolli and Saunders (1985), and Berggren et al. (1995). Dashed lines demarcate the predominant concurrent range zones in the lower parts of N16 and
N19. Most grey segments of the biostratigraphic range bars reflect notable differences in data presented in these references; that for Ng. pachyderma reflects its gradation
from Ng. acostaensis and that for sample locality RQT is due to the uncertain identification of Ng. acostaensis in that assemblage.
tion of Glr. miocenica implies a late Pliocene age for at least part of
sections studied by Martı́nez and Osorio (1964).
Of the 19 sample localities that yielded rich foraminiferal assemblages (Fig.
11), five (MAT, RQT, MIB, CHO, PCB) are restricted to lower N16 in the
Tortonian and six (MOS, PPP, PTA, FRA, RQK, CUC) are restricted to N19
in the Zanclean. Seven assemblages (RAP, PPN, LBZ, FRM, RAN, LEB,
CHE) have longer concurrent range zones that include N16, but they lack
any Globorotalia species that occur first in the Pliocene, and one assemblage
(PNH) is devoid of planktic foraminifera. Faunal similarities suggest that
these seven localities probably are late Miocene, implying that all sampled
localities are within the late Miocene to early Pliocene interval.
Approximately 150 ostracode species have been identified in this study.
Although they include some cosmopolitan deep-water species, the majority are endemic or have been recorded from the Miocene to recent of
the southeastern Pacific, the southwestern Atlantic, and the Southern
Ocean. Monographs on the Foraminifera and Ostracoda found in this
study are in preparation.
PALEOENVIRONMENTS
Paleoclimate
Foraminifera and ostracodes in the Navidad and related units typify temperate latitudes. Included are many taxa that have considerably wide latitudinal ranges, particularly those associated with water masses emanating from
the Antarctic. In contrast, the mollusk assemblages have many tropical and
subtropical elements that become less frequent from Navidad southward toward Chiloé (i.e., from north to south). Many of the gastropod genera, including Nerita, Strombus, Xenophora, Zonaria, Echinophoria, Olivancillaria,
and large Terebra, suggest relatively high water temperatures. Except for
Xenophora and Echinophoria, these probably come from the displaced
shallow-water or coastal deposits and represent reworked fossils from older
successions deposited in a warmer climate.
Other gastropods typify cooler and often deeper environments. These
include the trochoids Cantrainea and Bathybembix (Nielsen et al., 2004),
the turrid Ptychosyrinx, and the volutes Adelomelon, Miomelon, and
Pachycymbiola (Nielsen and Frassinetti, 2007). All genera still live off
the Chilean coast.
Molluscan Facies
Interpretation of gastropod facies in Chile is based primarily on detailed studies of recent faunas and comparison with early Miocene faunas
in southern Peru from intertidal to upper-slope marine environments. Navidad mollusks in the Peruvian fauna are most abundant and diverse
where the sedimentology suggests intertidal to inner-neritic deposition.
Intertidal deposits include oyster-encrusted igneous boulders and a diverse assemblage of transported mollusks, shark teeth, and other vertebrate remains. In contrast, deeper-water deposits of southern Peru are
tuffaceous and diatomaceous silty sandstones with scattered articulated
venerid bivalves, clupeid fish vertebrae and scales, and cetacean bones
(DeVries and Nielsen, 2003).
Molluscan assemblages characteristic of rocky-shore, inner-neritic, and
outer-neritic to upper-slope environments are recognized in the Navidad,
Ranquil, and Lacui formations (Fig. 12). Ninety-three gastropod genera
or subgenera are distributed as follows: 16% rocky coast, 22% rocky and
inner neritic, 33% inner neritic, 5% inner neritic to upper slope, 9% outer
neritic to upper slope, and 15% rocky shore to upper slope (see Table 1).
The distribution pattern of gastropod genera suggests that most sediment
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FIGURE 12—Selected gastropods and their habitats. A–E ⫽ coast; F–J ⫽ shallow water; K–M ⫽ all environments; N–S ⫽ deep water (outer neritic and bathyal). A)
Zonaria frassinettii Groves and Nielsen. B) Ficus gayana Covacevich and Frassinetti. C) Fissurella sp. D) Nerita (Heminerita) chilensis Philippi. E) Austrocominella obesa
(Philippi). F) Sassia armata (Hupé). G) Olivancillaria claneophila (Duclos). H) Lamprodomina dimidiata (Sowerby). I) Terebra undulifera Sowerby. J) Astele chilensis
(Orbigny). K) Testallium cepa (Sowerby). L) Sinum subglobosum (Sowerby). M) Echinophoria monilifer (Sowerby). N) Dalium sp. O) Struthiochenopus philippii Zinsmeister
and Griffin. P) Exilia alanbeui Nielsen. Q) Borsonella sp. R) Struthiochenopus bandeli Nielsen. S) Xenophora paulinae Nielsen and DeVries. Scale bar ⫽ 2 cm.
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TABLE 1—Habitat zones indicated by selected gastropod genera represented in sampled localities. Inferred minimum depth zones are based on genera restricted to a single
depth zone; inferred maximum depth zones are based on the presence of genera that
range into the outer neritic-upper bathyal realm.
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TABLE 2—Occurrence of selected deep-water microfossils in sampled localities.
packages contain assemblages derived from only one of the three environments (i.e., assemblages are not mixed-depth associations). Although
assignment of a genus to more than one setting indicates that at least one
of its species inhabits multiple zones, it may be an artifact of combining
single zone ranges of congeneric species (e.g., Calliostoma, Ficus, Terebra, and Turritella are represented by multiple species here). Gastropods
representative of the rocky shore are Nerita chilensis (Philippi), Zonaria
frassinettii Groves and Nielsen, and new species of Fissurella and Ranella. Shallow-water associations contain the majority of such classic
Navidad gastropods as Astele chilensis (Orbigny) (⫽Trochus laevis Sowerby; see Nielsen et al., 2004), Sassia armata (Hupé), Lamprodomina
dimidiata (Sowerby), and Olivancillaria claneophila [⫽O. tumorifera
(Hupé); see Nielsen, 2004]. Much less common in the Navidad are deepwater invertebrates, notably the gastropods Struthiochenopus, Falsilunatia, Dalium, Exilia, and Borsonia (e.g., Nielsen, 2005a, 2005b). The only
gastropod species present that confidently ranges from shallow to deep
is Testallium cepa (Sowerby); all other genera with this range are rep-
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FIGURE 13—Representative bathyal foraminifers from the Neogene of coastal Chile. A) Fontbotia wuellerstorfi (Schwager). B) Pyrgo murrhina Schwager. C) Ehrenbergina
fyfei Finlay. D) Neouvigerina hispida (Schwager). E) Bathysiphon giganteus Cushman. F) Nodogenerina sagrinensis (Bagg). G) Laticarinina pauperata (Parker and Jones).
H) Osangularia bengalensis (Schwager). I) Pleurostomella elliptica Galloway and Heminway. J) Sphaeroidina bulloides Orbigny. K) Melonis pompilioides (Fichtel and Moll).
Scale bars ⫽ 200 ␮m.
resented by multiple species. Xenophora may also share this range, but
it is known outside its deep-water habitat only by a worn specimen and
worn spire.
Microfossil Indications of Bathymetry
Kennett (1982, p. 555) noted, ‘‘Fossil benthonic foraminiferal assemblages have been widely used for paleobathymetric reconstructions of
sedimentary basins since the 1930s (Natland, 1957). The approach, with
its limitations, has been quite sound.’’ The method recognizes that species
are distributed according to water depth and, to avoid being misled by
those individuals gravitationally displaced from shallow depths, the upper
depth limits of the deepest-dwelling extant species or their homeomorphs
indicate the minimum depth of deposition (Bandy, 1961; Ingle, 1980).
All samples referred to in this study yielded mixed-depth assemblages
of benthic foraminifers and ostracodes, indicating the prevalence of
downslope transport. This phenomenon is not surprising, inasmuch as
gravity-driven flows triggered by seismic events characterize slope sedimentation along this active tectonic margin (Contardo et al., 2005). Typical offshore-marine foraminiferal assemblages have 30 to 60 species per
thousand specimens (Murray, 1973); however, most of the Chilean as-
semblages consist of 60 to 120 species per thousand specimens. Bathymetric mixing could account for such high species diversities.
The upper-depth limits of several benthic foraminifers in the Chilean
Neogene (Table 2) are based on distributions of similar taxa currently
living along the Pacific margin of central South America (Bandy and
Rodolfo, 1964; Ingle et al., 1980). Among the lower middle-bathyal and
lower-bathyal indicators in the Chilean Miocene are Bathysiphon giganteus Cushman, Melonis pompilioides (Fichtel and Moll), Osangularia
bengalensis (Schwager), Pleurostomella elliptica Galloway and Heminway, Nodogenerina sagrinensis (Bagg), and large Sphaeroidina bulloides
d’Orbigny (see Fig. 13). These species belong to genera well represented
among the cosmopolitan deep-water taxa (van Morkhoven et al., 1986).
Cyclammina, Eggerella, Textularia, Bulimina, Cassidulina, Cibicidoides,
Ehrenbergina, Eponides, Globocassidulina, Hoeglundina, Laticarinina,
Oridorsalis, and Pullenia are other common bathyal genera (see Murray,
1991, p. 290) represented in Chilean assemblages.
Many Chilean Neogene ostracode species are extant in the southeastern
Pacific, southwestern Atlantic, Caribbean Sea, and Southern Ocean. On
the basis of modern-depth distributions, nearly all the recovered assemblages are mixed associations of sublittoral, neritic, and bathyal taxa,
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FIGURE 14—Psychrospheric (bathyal) ostracodes from the Neogene of coastal Chile. A) Dutoitella sp. B) Krithe producta Brady. C) Legitimocythere acanthoderma (Brady).
D) Bythoceratina cf. B. scabra Bold. E) Penneyella dorsoserrata (Brady). F) Krithe reversa Bold. G) Cardobairdia glabra Bold. H) Bradleya normani (Brady). I) Palmoconcha
sp. J) Poseidonamicus sp. K) Agrenocythere sp. L) Bradleya sp. Scale bars ⫽ 200 ␮m.
supporting the interpretation of downslope sedimentary transport. Psychrospheric species, distinguished by Benson (1972a, 1972b, 1975) as
belonging to a major global fauna characterizing cold, deep waters, are
recognized in the majority of assemblages studied (Table 2; Fig. 14).
EVIDENCE OF REWORKING
Occurrences of a Late Cretaceous globotruncanid in the Ranquil Formation (FRA) and Catapsydrax dissimilis Cushman and Bermúdez (middle Eocene to early Miocene) in the Ranquil Formation (MIB, RAN,
RQT) and the Lacui Formation (PCB) are evidence for reworking of
notably older sediments. Although reworking can exaggerate species diversities, recognition of all reworked components was not possible because so little is known about older faunas in this region.
Reworking is also evident in the material studied by Martı́nez and
Osorio (1964), which yielded the calcareous nannofossils Discoaster deflandrei (Bramlette and Riedel) and D. musicus (Stradner), which have
Serravallian LADs, as well as Catapsydrax dissimilis.
DISCUSSION
The Navidad, Ranquil, and Lacui formations contain very similar microfossil and macrofossil faunas. Planktic foraminifers reveal that they
are late Miocene to early Pliocene in age. The predominant role of gravity
flows in their deposition and the abundance of benthic taxa characteristic
of the rocky shore and neritic zone imply that most of these sediments
had accumulated on the continental shelf prior to their downslope displacement. Bathyal foraminifers are ubiquitous, but not always in association with deep-water ostracodes and gastropods (Table 3). The absence
of deep-water species of ostracodes or gastropods at some sample localities yielding bathyal foraminifera can be attributed to their relative abundances being heavily skewed toward the shallower photic zone, where
high primary productivity enriches the overall biota, and grazers can
thrive. In contrast, communities inhabiting deeper aphotic substrates of
coastal basins have trophic pyramids based on the abundant organic detritus derived from productive zones upslope and above and the highstanding bacteria crops that it maintains (Lipps, 1983). Although these
14
PALAIOS
FINGER ET AL.
TABLE 3—Presence of deep-water indicators for sampled localities. Filled circle ⫽
deep, open circle ⫽ possibly deep, s ⫽ shallow, indet. ⫽ indeterminate (poor assemblage).
trophic resources can support a rich benthic foraminiferal community,
they are not the primary food source of most ostracodes, nor are they
sufficient in deep water to support a large biomass of gastropods.
Age-discordant specimens in association indicate reworking of significantly older sediments, which usually involves gravitational displacement from their provenance. Species diversities are exaggerated for those
assemblages that bathymetric mixing and reworking have augmented with
allochthonous species. These assemblages cannot be readily distinguished
from the in situ components.
Although some Navidad mollusks occur in southern Peruvian strata
confidently dated as latest Oligocene to early middle Miocene (DeVries
and Frassinetti, 2003), present knowledge of the Chilean molluscan fauna
precludes identification of early Miocene from late Miocene to early Pliocene associations. The diachroneity of 10 myr between the two faunas
suggests that several of the Peruvian mollusk species may have shifted
their ranges southward during this time, whereas others have been reworked from older deposits. Without any confirmed lower Miocene strata
along the western margin of central and southern Chile, there is no reliable means of distinguishing the supposedly reworked component from
the in situ benthic fauna of mollusks, foraminifers, and ostracodes. Further investigation could fill this void, inasmuch as Le Roux and Elgueta
(2000) reported an Oligocene to Miocene sequence in the Valdivia Basin.
It is likely the Navidad fauna will also be present in sections purported
to be younger than those of the Navidad Formation, such as Lo Abarca
(Covacevich and Frassinetti, 1990), or containing an apparently mixed
fauna of Miocene and Pliocene elements, such as Chepu (Watters and
Fleming, 1972). Further work on well-dated sections in central Chile
(e.g., Le Roux et al., 2004, 2006) could constrain the temporal ranges of
benthic taxa in central Chile.
Alternatively, upwelling currents could have moved deep-water foraminifers and ostracodes to shallower depths. The sections examined, however, lack evidence (e.g., laminated strata, phosphatic deposits, abundant
diatoms and fish debris) suggesting that the process was locally intense.
Furthermore, wind-driven upwelling off the west coasts of continents is
too shallow (⬍200 m) and too weak to scour middle- to lower-bathyal
(⬎500 m) substrates (see Barber and Smith, 1981). Another plausible
explanation is that all deep-water indicators are reworked. It is unlikely
because most species are well preserved, found in many samples, and
relatively abundant.
The occurrence of well-preserved, fragile shells of Xenophora (see Nielsen
and DeVries, 2002) in sediments with mixed-depth microfossil assemblages
indicative of deep-water deposition has been problematic. This could be attributed to relatively nonchaotic transport (e.g., slumps or slides), but the field
study did not find sedimentary evidence that supports this interpretation. On
the other hand, shells relocated by gravity flows may or may not be physically
abraded during transport, because the potential for such damage depends on
grain size, flow rheology, duration of flow, position in flow, and shell architecture and thickness. Nevertheless, it is not surprising that all of the wellpreserved specimens of Xenophora were collected from deep-water siltstone
facies at Punta Perro (PPP).
The sedimentologic and micropaleontologic data presented here for the
Navidad Formation implies that the basins off central Chile underwent
major subsidence in the late Miocene. Deposition of the basal, shallowmarine conglomerate occurred during initial stages of this event and its
associated marine transgression. The basin floor dropped rapidly to bathyal depths, as inferred by sedimentology, trace fossils, and benthic microfossils. In addition to these deposits, conglomerates with locally abundant
basement clasts up to several meters in diameter and large fossil tree
trunks suggest that a narrow continental shelf separated this basin from
the nearshore.
Northern and central Chile is bordered by a narrow, 5–30 km wide ridged
forearc with a terrestrial uplands structural high, whereas most of Peru is
bordered by a shelved forearc characterized by a broad shelf (Dickinson and
Seely, 1979; Laursen et al., 2002). The geomorphologic setting for Neogene
deposition off central Chile may be analogous to the modern forearc Valparaı́so Basin, which is one of only a few significant basins along the central
Chilean margin (Laursen et al., 2002; Laursen and Normark, 2003). The
modern basins off central Chile, however, are shallower (see González, 1989).
Although foraminifers from outcrop and offshore well samples indicate considerably deeper depths, it is apparent that the basins have been shallowing
by infilling and uplift since the late Pliocene.
CONCLUSIONS
This study exemplifies the value of microfossils for identifying reworked and displaced deposits, which often are not as readily apparent
in macrofossil assemblages or sedimentary facies. Foraminifera and ostracodes, most of which include species indicative of the lower-bathyal
zone (2000 to 4000 m), suggest that outcrops of the Navidad Formation
and its equivalents along the central coast of Chile are late Miocene to
early Pliocene bathyal deposits. This interpretation fits well with the late
Miocene to early Pliocene scenario of deep-water forearc basins off central Chile. Downslope mixing of older sediments into the Navidad Formation is evidenced by the inclusion of mollusks, foraminifera, and ostracodes characteristic of shallower biofacies. Reworking of older sediments is apparent in robust Late Cretaceous and early Miocene foraminifera in several younger assemblages and suggested by a molluscan
fauna that appears to be correlative with deposits from the late Oligocene
to early Miocene in Peru.
Mixed-depth microfaunal associations and sedimentary features reveal
downslope transport of sediments, but gastropods appear to be in original
associations and usually show no physical evidence of transport or reworking. The random spatial distribution of specimens, however, indicates that they were not preserved in situ. Although the presence of gastropods restricted to greater depths would confirm downslope displacement of the shallower association, it is apparent that none were incorporated during transportation and redeposition of the shallower
association.
No outcrops or subsurface sections from the Oligocene to middle Miocene interval have been encountered in the region to account for the
reworked microfossils in the Navidad, Ranquil, and Lacui formations.
These units unconformably overlie Eocene and Cretaceous strata or granitic basement. This implies that nondeposition and extensive erosion
were coincident with a major regressive phase during the Oligocene.
PALAIOS
FOSSIL EVIDENCE OF DISPLACEMENT
ACKNOWLEDGMENTS
K.L. Finger received funding for fieldwork and electron microscopy
from the University of California Museum of Paleontology. S.N. Nielsen’s work was supported by grants Ba 675/25-1, Ni 699/1-1, and Ni
699/4-1 of the Deutsche Forschungsgemeinschaft. A. Encinas’ fieldwork
was supported by Proyecto Fondecyt 1010691, Programa MECE Educación Superior UCH0010, and Beca PG/50/02 of the Departamento de
Postgrado y Postı́tulo-Universidad de Chile. P. Vásquez (Technische
Universität, Berlin, Germany) assisted in drafting the maps. Luc Ortlieb
and an anonymous reviewer provided comments and suggestions that
improved the manuscript.
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ACCEPTED JULY 3, 2006