paleontologic evidence for sedimentary displacement in neogene

Copyright � 2007, SEPM (Society for Sedimentary Geology) 0883-1351/07/0022-0003/$3.00

PALAIOS, 2007, v. 22, p. 3–16

Research Report

DOI: 10.2110/palo.2005.p05-081r

PALEONTOLOGIC EVIDENCE FOR SEDIMENTARY DISPLACEMENT IN NEOGENE FOREARCBASINS 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 Geologıa, Universidad de Chile, Casilla 13518, Correo 21, Santiago, Chilee-mail: [email protected]

FIGURE 1—Study areas in central Chile. A) Navidad Formation. B) Ranquil For-mation. C) Lacui Formation. See Figure 2 for detailed maps and sampled localities.

ABSTRACT

A consensus on the biostratigraphic age and depositional environ-ment of the Navidad, Ranquil, and Lacui formations exposed alongthe tectonic margin of central Chile has been elusive due to conflict-ing evidence. This study resolves this dilemma and gains further in-sight regarding the history of the Chilean coast. Problematic inter-pretations stem primarily from the remarkable similarity betweenthe molluscan fauna of these units with those well documented forthe late Oligocene to early Miocene of Peru. Planktic foraminifers,however, indicate that the Chilean sections accumulated in the lateMiocene to early Pliocene interval following a regional hiatus thatextends into the Eocene. The prevalence of mixed-depth bathyal as-semblages of benthic foraminifers and ostracodes, the majority ofwhich include lower-bathyal (�2000 m) indicators, reveals thatdownslope displacement was a primary mode of deposition in thebasins. Although the molluscan assemblages are dominated by shal-low marine taxa, most include species that range into or are restrictedto deeper waters. Sedimentary features connote rapid subsidence anddeep-water deposition of gravity flows. Although older Tertiary andCretaceous planktic foraminifers in several assemblages indicate re-working of older units, lack of data on pre-Tortonian faunas of thisregion precludes recognition of other age-discordant components thatcould constitute a significant portion of the recovered fauna. Thefindings of this study revise the prevailing conception of the region’sgeologic history that considered these units to be early to middleMiocene shelf deposits and indicate that infilling and uplift havecharacterized the nearshore basins since the late Pliocene.

INTRODUCTION

Along the coast of central Chile, the Navidad Formation and its strati-graphic equivalents, the Ranquil Formation on Peninsula Arauco and theLacui Formation on Chiloe Island (Fig. 1), commonly have been inter-preted as early to middle Miocene shallow-water deposits (see PreviousWork below). Ages of the Navidad Formation presented in the literatureare incongruent, there is little published data for the Ranquil Formation,and no prior data are available on the age of the Lacui Formation. Thisstudy addresses the following questions paramount to other research onthese units. (1) What are the ages of these exposures? (2) How correlativeare they with each other? (3) What are the depositional histories andenvironments represented by these deposits?

PREVIOUS WORK

The first report of Tertiary fossils from central Chile is that ofd’Orbigny (1842), who collected them during his 1826–1833 expeditionto South America. Darwin (1846, 1900) provided the first geologicalstudy of the region aboard the H.M.S. Beagle in 1835. He described theNavidad Formation and noted several coastal outcrops that remain key

*Corresponding author.

localities. He also collected the fossil mollusks that Sowerby (1846) de-scribed in an appendix to Geological Observations on South America(Darwin, 1846). Hupe (1854) named additional species within the mon-umental volumes of Gay (1844–1854). A monograph by Philippi (1887)on the Tertiary and Quaternary fossils of Chile presented a synopsis andrevision of the fauna, including descriptions of many new taxa, including119 new gastropod species. It remains the most complete reference on

4 PALAIOSFINGER ET AL.

FIGURE 2—Sampled localities. A) Navidad Formation. B) Ranquil Formation. C) Lacui Formation.

FIGURE 3—Exposures of the Navidad Formation at Punta Perro. Photo taken approximately 15 m stratigraphically above intertidal platform (PPP).

this fauna. Subsequent studies on the geology (e.g., Bruggen, 1934, 1950;Tavera, 1942; Garcıa, 1968; Tavera et al., 1985) and macropaleontology(e.g., Moricke, 1896; Philippi, 1897; Frassinetti, 1974; Covacevich andFrassinetti, 1980, 1986; Frassinetti and Covacevich, 1981, 1982, 1993)of central Chile did not stray from the common view of the NavidadFormation and its equivalents as being early to middle Miocene, shallowmarine deposits.

Martınez and Osorio (1964) first identified planktic foraminifers anddiscoasters indicative of a Tortonian (late Miocene) age from the NavidadFormation. Correlating with Patagonian molluscan biostratigraphy, Tav-era (1968) placed the Navidad Formation in the Burdigalian (early Mio-cene), which Dremel’s study (in Herm, 1969) of planktic foraminiferafrom Punta Perro supported. Osorio (1978) referred to the ostracodes ofthe Rıo Rapel section as late Miocene and bathyal. It is not clear if he

PALAIOS 5FOSSIL EVIDENCE OF DISPLACEMENT

FIGURE 4—The Ranquil Formation at Peninsula Arauco. The bluff (RAN) consists mostly of poorly defined beds of massive sandstones with intermittent beds of glauconiticsandstone. Height of bluff is approximately 20 m.

←

FIGURE 5—The Lacui Formation south of Cucao, Chiloe Island. Locality CUC isthe sandstone blockfall. Height of bluff is approximately 40 m.

based his interpretations on correlation with Caribbean ostracodes orplanktic foraminifers or both. Osorio (1978) also noted the discrepancybetween previous studies and suggested that it was due to sampling ofdisparate stratigraphic levels within the formation.

Tavera (1979) distinguished three members within the Navidad For-mation, from oldest to youngest: the Navidad, Lincancheo, and Rapel.Although Darwin (1846) named the Navidad Formation based on its low-er part, applying the name Navidad to one of its members presents a caseof homonymy that Encinas et al. (2006) have resolved by elevating allthese members to the rank of formation: Navidad, Licancheu, and RapelFormations.

Martınez-Pardo (1990) referred all of Chile’s Miocene marine depositsto the Navidad Formation, which he correlated with the lower of twotransgressive sequences recognized in Chile and Peru. He referred to thisevent as the Neogene South East Pacific Sequence I (NSEPS-I), demar-cated with significant hiatuses below and above. NSEPS-I consists of alower subcycle extending from 19 Ma (Burdigalian) to 13 Ma (Serraval-lian) with maximum transgression at 14 Ma (Serravallian), and an uppersubcycle extending from 13 Ma to 10 Ma (Tortonian) with maximumtransgression at approximately 11 Ma. It has been difficult to incorporatethese interpretations into the research of others because locality data andsupporting paleontological evidence were not presented.

Ibaraki (1992a) recorded the presence of Neogloboquadrina acostaen-sis (Blow) after examining planktic foraminifers from the Punta Perrosection. In transitional latitudes, this species first appears (FAD) in ZoneMt9 of Berggren et al. (1995), which is correlative with the low-latitudeGloborotalia acostaensis Zone of Bolli and Saunders (1985) and zoneN16 of Blow (1969). Ibaraki’s data added credibility to the Tortonian ageassigned by Martınez and Osorio (1964). Subsequent palynologic analysiscorrelated the Navidad Formation (Meon et al., 1994) with the late middle


FIGURE 6—Coastal exposure of the basal Navidad Formation north of Mostazal, north of locality MOS. Igneous intrusions (not shown) confirm that the numerousdiscontinuous basement-rock exposures in this vicinity are displaced boulders. Height of outcrop is approximately 5 m.

FIGURE 7—Siltstone bed with rip-up clasts in coastal bluffs at locality MAT, approximately 1 km north of Matanzas.

to late Miocene, which neither refined nor contradicted the Tortonianassignment. Nevertheless, recent correlations based on Peruvian mollus-can biostratigraphy (DeVries and Frassinetti, 2003; DeVries and Nielsen,2003) and shark teeth (Suarez et al., 2006) suggest that there is a lateOligocene or early Miocene component in the Navidad Formation.

MATERIALS AND METHODS

This study is based on field observations and samples collected fromthree units that crop out along the central coast of Chile: (1) the Navidad

Formation in an area beginning approximately 130 km southwest of San-tiago, (2) the Ranquil Formation of the Arauco Peninsula, and (3) theLacui Formation of Chiloe Island. Fieldwork involved measuring strati-graphic sections, describing sedimentary features, and collecting inver-tebrate fossils and microfossiliferous sediments from outcrops of the Na-vidad 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 wheregastropods were in highest concentration, were extracted from the sedi-


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.

FIGURE 9—Stratigraphic range of Navidad molluscan species in southern Peru.

mentary rock samples by disaggregating in hydrogen peroxide and wash-ing 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. Sam-pling of the Ranquil Formation on Peninsula Arauco (Fig. 4) and theLacui Formation on Chiloe Island (Fig. 5) yielded macroinvertebrates andmicrofossils similar to those obtained from the Navidad Formation.

SEDIMENTOLOGY

In the Navidad area, approximately 100 to 200 m of the Navidad For-mation overlie the Paleozoic granitic basement or the Upper Cretaceousmarine Punta Topocalma Formation (Cecioni, 1978). Extensive coastalbluffs are characterized by nearly horizontal bedding, but stratification isoften poorly defined and discontinuous. The base of the Navidad For-mation (Fig. 6) consists of a few meters of fossiliferous conglomerate.The conglomerate is clast supported and contains subrounded to angularclasts and fragments of bivalves, gastropods, solitary corals, neritic for-aminifers, and wood. Fossil fragments are usually scarce, but in someplaces they are abundant enough to characterize the conglomerate as acoquina. These shallow-marine deposits mark the initial subsidence ofthe basin.

Overlying the basal conglomerate is an interval of interbedded sand-stone and mudstone, with minor conglomerate. These deposits include adiverse fossil assemblage of bivalves, gastropods, foraminifers, sharkteeth, leaf impressions, pollen, and crabs (Philippi, 1887; Tavera, 1979;Martınez-Pardo and Valenzuela, 1979; Troncoso, 1991; Troncoso andRomero, 1993; Meon 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 (Encinaset al., 2006), as well as the ichnofossils Chondrites, Zoophycos, Thalas-sinoides, and Ophiomorpha identified in the Navidad Formation indicaterapid and deep subsidence (L.A. Buatois, personal communication, 2005).Transport is also evident in small stringers of gastropods (Fig. 8) observedcommonly on beach platforms.

BIOSTRATIGRAPHIC CORRELATIONS

Molluscan Biostratigraphy

Comparison of molluscan assemblages in the Navidad Formation andits equivalents with those from Cenozoic marine formations of southernPeru (Fig. 9) favors a youngest Oligocene to early middle Miocene age


FIGURE 10—Biostratigraphically important planktic foraminifers present in the Neogene of coastal Chile. A) Neogloboquadrina continuosa (Blow). B) Neogloboquadrinaacostaensis (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) Globigerinellaaequilateralis (Brady). K) Orbulina universa Orbigny. L) Reworked globotruncanid. M) Reworked Catapsydrax dissimilis Cushman and Bermudez. Scale bars � 100 �m.

for the Navidad molluscan fauna (DeVries and Frassinetti, 2003; DeVriesand Nielsen, 2003). Several species typical of the Navidad Formationalso occur in the Cenozoic forearc basins of southern Peru, includingFicus distans, Acanthina katzi, Testallium cepa, Olivancillaria claneo-phila (�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 specieswere found in successions dated using radiolaria (Marty, 1989), forami-nifera (Dunbar et al., 1990; Ibaraki, 1992b, 1992c, 1993), diatoms (Ma-chare and Fourtanier, 1987; Machare et al., 1988; Dunbar et al., 1990;DeVries, 1998), 40K-40Ar isotopes (De Muizon and Bellon, 1980; DeMuizon and DeVries, 1985; Noble et al., 1985; Dunbar et al., 1990), and40Ar-39Ar isotopes (DeVries, 1998).

Late Miocene (Tortonian, Messinian) and Pliocene successions fromsouthern Peru contain many of the same molluscan species that occur incoeval Chilean deposits previously thought to be younger than the Na-vidad Formation (Herm, 1969; Guzman et al., 2000; DeVries and Fras-sinetti, 2003). In contrast, middle Miocene (Serravallian) deposits insouthern Peru contain a distinctive suite of molluscan species, some ofwhich also occur in contemporaneous sediments of the Talara Basin innorthern Peru (DeVries, 1997b). None of the middle Miocene mollusksof Peru are present in the Navidad Formation.

Some of the molluscan species in the Navidad Formation range nohigher than the lower middle Miocene (Langhian; basal Pisco Formation)in southern Peru, where they are most common throughout the lowerMiocene (Burdigalian, Aquitanian) Chilcatay Formation (DeVries, 1998).The oldest occurrence of these are two gastropods, Olivancillaria cla-neophila (Duclos) and Testallium cepa (Sowerby), found in latest Oli-

gocene (Chattian) deposits near Caravelı and Nazca (DeVries and Fras-sinetti, 2003); 40Ar-39Ar dating of volcanic ash in the Caraveli beds in-dicates an age of about 25 Ma (Noble et al., 1985).

Microfossil Biostratigraphy

The sampled exposures yielded a foraminiferal fauna comprised of morethan 300 species of foraminifera. The majority of species belong to benthicgenera typical of Neogene offshore deposits at middle latitudes. Temperatemicrofaunas typically include benthic and planktic species also known tooccur in subpolar and subtropical latitudes. Many of the benthic species inthe Chilean Neogene have been recorded living on the Chilean slope (Bandyand Rodolfo, 1964; Ingle et al., 1980; Resig, 1981). Numerous species andhomeomorphs in the Chilean Neogene are constituents of the cosmopolitandeep-water fauna documented by van Morkhoven et al. (1986).

In this study, 17 of 19 assemblages include index species of plankticforaminifera that have late Miocene or early Pliocene datums (Figs. 9–10). Ten of the assemblages yielding biostratigraphic markers can be as-signed to either zone N16, indicated primarily by representatives of theNeogloboquadrina continuosa–Ng. acostaensis–Ng. pachyderma plexus,or zone N19, indicated primarily by Globorotalia puncticulata. Absentfrom outcrop samples were other early Pliocene species of Globorotalia(i.e., Glr. cibaoensis Bermudez, Glr. conoidea Walters, Glr. conomiozeaKennett, Glr. crassula Cushman and Stewart) identified in well samplesfrom the Navidad area on loan from Empresa Nacional del Petroleo(ENAP). Although Martınez and Osorio (1964) reported Candeina nitida(N17b FAD) and Glr. miocenica (N19/20 FAD) in the Navidad Forma-tion, these markers were not seen during this study. Accurate identifica-

[Erratum: Identifications of G and H are transposed.]


FIGURE 11—Biostratigraphy of sampled outcrops based on concurrent ranges of diagnostic planktic foraminifers (as numbered). Framework derived from Kennett andSrinivasan (1983), Bolli and Saunders (1985), and Berggren et al. (1995). Dashed lines demarcate the predominant concurrent range zones in the lower parts of N16 andN19. Most grey segments of the biostratigraphic range bars reflect notable differences in data presented in these references; that for Ng. pachyderma reflects its gradationfrom 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 ofsections studied by Martınez 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 theTortonian and six (MOS, PPP, PTA, FRA, RQK, CUC) are restricted to N19in the Zanclean. Seven assemblages (RAP, PPN, LBZ, FRM, RAN, LEB,CHE) have longer concurrent range zones that include N16, but they lackany Globorotalia species that occur first in the Pliocene, and one assemblage(PNH) is devoid of planktic foraminifera. Faunal similarities suggest thatthese seven localities probably are late Miocene, implying that all sampledlocalities 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 ma-jority are endemic or have been recorded from the Miocene to recent ofthe southeastern Pacific, the southwestern Atlantic, and the SouthernOcean. Monographs on the Foraminifera and Ostracoda found in thisstudy are in preparation.

PALEOENVIRONMENTS

Paleoclimate

Foraminifera and ostracodes in the Navidad and related units typify tem-perate latitudes. Included are many taxa that have considerably wide latitu-dinal ranges, particularly those associated with water masses emanating fromthe Antarctic. In contrast, the mollusk assemblages have many tropical andsubtropical elements that become less frequent from Navidad southward to-ward Chiloe (i.e., from north to south). Many of the gastropod genera, in-cluding Nerita, Strombus, Xenophora, Zonaria, Echinophoria, Olivancillaria,and large Terebra, suggest relatively high water temperatures. Except for

Xenophora and Echinophoria, these probably come from the displacedshallow-water or coastal deposits and represent reworked fossils from oldersuccessions deposited in a warmer climate.

Other gastropods typify cooler and often deeper environments. Theseinclude the trochoids Cantrainea and Bathybembix (Nielsen et al., 2004),the turrid Ptychosyrinx, and the volutes Adelomelon, Miomelon, andPachycymbiola (Nielsen and Frassinetti, 2007). All genera still live offthe Chilean coast.

Molluscan Facies

Interpretation of gastropod facies in Chile is based primarily on de-tailed studies of recent faunas and comparison with early Miocene faunasin southern Peru from intertidal to upper-slope marine environments. Na-vidad mollusks in the Peruvian fauna are most abundant and diversewhere the sedimentology suggests intertidal to inner-neritic deposition.Intertidal deposits include oyster-encrusted igneous boulders and a di-verse assemblage of transported mollusks, shark teeth, and other verte-brate remains. In contrast, deeper-water deposits of southern Peru aretuffaceous and diatomaceous silty sandstones with scattered articulatedvenerid bivalves, clupeid fish vertebrae and scales, and cetacean bones(DeVries and Nielsen, 2003).

Molluscan assemblages characteristic of rocky-shore, inner-neritic, andouter-neritic to upper-slope environments are recognized in the Navidad,Ranquil, and Lacui formations (Fig. 12). Ninety-three gastropod generaor subgenera are distributed as follows: 16% rocky coast, 22% rocky andinner neritic, 33% inner neritic, 5% inner neritic to upper slope, 9% outerneritic to upper slope, and 15% rocky shore to upper slope (see Table 1).The distribution pattern of gastropod genera suggests that most sediment


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 (Hupe). 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 Zinsmeisterand Griffin. P) Exilia alanbeui Nielsen. Q) Borsonella sp. R) Struthiochenopus bandeli Nielsen. S) Xenophora paulinae Nielsen and DeVries. Scale bar � 2 cm.


TABLE 1—Habitat zones indicated by selected gastropod genera represented in sam-pled localities. Inferred minimum depth zones are based on genera restricted to a singledepth zone; inferred maximum depth zones are based on the presence of genera thatrange into the outer neritic-upper bathyal realm.

TABLE 2—Occurrence of selected deep-water microfossils in sampled localities.

packages contain assemblages derived from only one of the three envi-ronments (i.e., assemblages are not mixed-depth associations). Althoughassignment of a genus to more than one setting indicates that at least oneof its species inhabits multiple zones, it may be an artifact of combiningsingle zone ranges of congeneric species (e.g., Calliostoma, Ficus, Ter-ebra, and Turritella are represented by multiple species here). Gastropodsrepresentative of the rocky shore are Nerita chilensis (Philippi), Zonariafrassinettii Groves and Nielsen, and new species of Fissurella and Ra-nella. Shallow-water associations contain the majority of such classicNavidad gastropods as Astele chilensis (Orbigny) (�Trochus laevis Sow-erby; see Nielsen et al., 2004), Sassia armata (Hupe), Lamprodominadimidiata (Sowerby), and Olivancillaria claneophila [�O. tumorifera(Hupe); see Nielsen, 2004]. Much less common in the Navidad are deep-water invertebrates, notably the gastropods Struthiochenopus, Falsiluna-tia, Dalium, Exilia, and Borsonia (e.g., Nielsen, 2005a, 2005b). The onlygastropod species present that confidently ranges from shallow to deepis Testallium cepa (Sowerby); all other genera with this range are rep-


FIGURE 13—Representative bathyal foraminifers from the Neogene of coastal Chile. A) Fontbotia wuellerstorfi (Schwager). B) Pyrgo murrhina Schwager. C) Ehrenberginafyfei 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, butit is known outside its deep-water habitat only by a worn specimen andworn spire.

Microfossil Indications of Bathymetry

Kennett (1982, p. 555) noted, ‘‘Fossil benthonic foraminiferal assem-blages have been widely used for paleobathymetric reconstructions ofsedimentary basins since the 1930s (Natland, 1957). The approach, withits limitations, has been quite sound.’’ The method recognizes that speciesare distributed according to water depth and, to avoid being misled bythose individuals gravitationally displaced from shallow depths, the upperdepth limits of the deepest-dwelling extant species or their homeomorphsindicate the minimum depth of deposition (Bandy, 1961; Ingle, 1980).

All samples referred to in this study yielded mixed-depth assemblagesof benthic foraminifers and ostracodes, indicating the prevalence ofdownslope transport. This phenomenon is not surprising, inasmuch asgravity-driven flows triggered by seismic events characterize slope sedi-mentation along this active tectonic margin (Contardo et al., 2005). Typ-ical offshore-marine foraminiferal assemblages have 30 to 60 species perthousand specimens (Murray, 1973); however, most of the Chilean as-

semblages consist of 60 to 120 species per thousand specimens. Bathy-metric mixing could account for such high species diversities.

The upper-depth limits of several benthic foraminifers in the ChileanNeogene (Table 2) are based on distributions of similar taxa currentlyliving along the Pacific margin of central South America (Bandy andRodolfo, 1964; Ingle et al., 1980). Among the lower middle-bathyal andlower-bathyal indicators in the Chilean Miocene are Bathysiphon gigan-teus Cushman, Melonis pompilioides (Fichtel and Moll), Osangulariabengalensis (Schwager), Pleurostomella elliptica Galloway and Hemin-way, Nodogenerina sagrinensis (Bagg), and large Sphaeroidina bulloidesd’Orbigny (see Fig. 13). These species belong to genera well representedamong 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 southeasternPacific, southwestern Atlantic, Caribbean Sea, and Southern Ocean. Onthe basis of modern-depth distributions, nearly all the recovered assem-blages are mixed associations of sublittoral, neritic, and bathyal taxa,


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) Palmoconchasp. J) Poseidonamicus sp. K) Agrenocythere sp. L) Bradleya sp. Scale bars � 200 �m.

supporting the interpretation of downslope sedimentary transport. Psy-chrospheric species, distinguished by Benson (1972a, 1972b, 1975) asbelonging to a major global fauna characterizing cold, deep waters, arerecognized in the majority of assemblages studied (Table 2; Fig. 14).

EVIDENCE OF REWORKING

Occurrences of a Late Cretaceous globotruncanid in the Ranquil For-mation (FRA) and Catapsydrax dissimilis Cushman and Bermudez (mid-dle Eocene to early Miocene) in the Ranquil Formation (MIB, RAN,RQT) and the Lacui Formation (PCB) are evidence for reworking ofnotably older sediments. Although reworking can exaggerate species di-versities, recognition of all reworked components was not possible be-cause so little is known about older faunas in this region.

Reworking is also evident in the material studied by Martınez andOsorio (1964), which yielded the calcareous nannofossils Discoaster de-flandrei (Bramlette and Riedel) and D. musicus (Stradner), which haveSerravallian LADs, as well as Catapsydrax dissimilis.

DISCUSSION

The Navidad, Ranquil, and Lacui formations contain very similar mi-crofossil and macrofossil faunas. Planktic foraminifers reveal that theyare late Miocene to early Pliocene in age. The predominant role of gravityflows in their deposition and the abundance of benthic taxa characteristicof the rocky shore and neritic zone imply that most of these sedimentshad accumulated on the continental shelf prior to their downslope dis-placement. Bathyal foraminifers are ubiquitous, but not always in asso-ciation with deep-water ostracodes and gastropods (Table 3). The absenceof deep-water species of ostracodes or gastropods at some sample local-ities yielding bathyal foraminifera can be attributed to their relative abun-dances being heavily skewed toward the shallower photic zone, wherehigh primary productivity enriches the overall biota, and grazers canthrive. In contrast, communities inhabiting deeper aphotic substrates ofcoastal basins have trophic pyramids based on the abundant organic de-tritus derived from productive zones upslope and above and the high-standing bacteria crops that it maintains (Lipps, 1983). Although these


TABLE 3—Presence of deep-water indicators for sampled localities. Filled circle �deep, open circle � possibly deep, s � shallow, indet. � indeterminate (poor assem-blage).

trophic resources can support a rich benthic foraminiferal community,they are not the primary food source of most ostracodes, nor are theysufficient in deep water to support a large biomass of gastropods.

Age-discordant specimens in association indicate reworking of signif-icantly older sediments, which usually involves gravitational displace-ment from their provenance. Species diversities are exaggerated for thoseassemblages that bathymetric mixing and reworking have augmented withallochthonous species. These assemblages cannot be readily distinguishedfrom the in situ components.

Although some Navidad mollusks occur in southern Peruvian strataconfidently dated as latest Oligocene to early middle Miocene (DeVriesand Frassinetti, 2003), present knowledge of the Chilean molluscan faunaprecludes identification of early Miocene from late Miocene to early Pli-ocene associations. The diachroneity of 10 myr between the two faunassuggests that several of the Peruvian mollusk species may have shiftedtheir ranges southward during this time, whereas others have been re-worked from older deposits. Without any confirmed lower Miocene strataalong the western margin of central and southern Chile, there is no re-liable means of distinguishing the supposedly reworked component fromthe in situ benthic fauna of mollusks, foraminifers, and ostracodes. Fur-ther 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 purportedto be younger than those of the Navidad Formation, such as Lo Abarca(Covacevich and Frassinetti, 1990), or containing an apparently mixedfauna of Miocene and Pliocene elements, such as Chepu (Watters andFleming, 1972). Further work on well-dated sections in central Chile(e.g., Le Roux et al., 2004, 2006) could constrain the temporal ranges ofbenthic taxa in central Chile.

Alternatively, upwelling currents could have moved deep-water fora-minifers and ostracodes to shallower depths. The sections examined, how-ever, lack evidence (e.g., laminated strata, phosphatic deposits, abundantdiatoms and fish debris) suggesting that the process was locally intense.Furthermore, wind-driven upwelling off the west coasts of continents istoo shallow (�200 m) and too weak to scour middle- to lower-bathyal(�500 m) substrates (see Barber and Smith, 1981). Another plausibleexplanation is that all deep-water indicators are reworked. It is unlikelybecause most species are well preserved, found in many samples, andrelatively abundant.

The occurrence of well-preserved, fragile shells of Xenophora (see Nielsen

and DeVries, 2002) in sediments with mixed-depth microfossil assemblagesindicative of deep-water deposition has been problematic. This could be at-tributed to relatively nonchaotic transport (e.g., slumps or slides), but the fieldstudy did not find sedimentary evidence that supports this interpretation. Onthe other hand, shells relocated by gravity flows may or may not be physicallyabraded during transport, because the potential for such damage depends ongrain size, flow rheology, duration of flow, position in flow, and shell archi-tecture and thickness. Nevertheless, it is not surprising that all of the well-preserved specimens of Xenophora were collected from deep-water siltstonefacies at Punta Perro (PPP).

The sedimentologic and micropaleontologic data presented here for theNavidad Formation implies that the basins off central Chile underwentmajor subsidence in the late Miocene. Deposition of the basal, shallow-marine conglomerate occurred during initial stages of this event and itsassociated marine transgression. The basin floor dropped rapidly to bathy-al depths, as inferred by sedimentology, trace fossils, and benthic micro-fossils. In addition to these deposits, conglomerates with locally abundantbasement clasts up to several meters in diameter and large fossil treetrunks suggest that a narrow continental shelf separated this basin fromthe nearshore.

Northern and central Chile is bordered by a narrow, 5–30 km wide ridgedforearc with a terrestrial uplands structural high, whereas most of Peru isbordered by a shelved forearc characterized by a broad shelf (Dickinson andSeely, 1979; Laursen et al., 2002). The geomorphologic setting for Neogenedeposition off central Chile may be analogous to the modern forearc Val-paraıso Basin, which is one of only a few significant basins along the centralChilean margin (Laursen et al., 2002; Laursen and Normark, 2003). Themodern basins off central Chile, however, are shallower (see Gonzalez, 1989).Although foraminifers from outcrop and offshore well samples indicate con-siderably deeper depths, it is apparent that the basins have been shallowingby infilling and uplift since the late Pliocene.

CONCLUSIONS

This study exemplifies the value of microfossils for identifying re-worked and displaced deposits, which often are not as readily apparentin macrofossil assemblages or sedimentary facies. Foraminifera and os-tracodes, most of which include species indicative of the lower-bathyalzone (2000 to 4000 m), suggest that outcrops of the Navidad Formationand its equivalents along the central coast of Chile are late Miocene toearly Pliocene bathyal deposits. This interpretation fits well with the lateMiocene to early Pliocene scenario of deep-water forearc basins off cen-tral Chile. Downslope mixing of older sediments into the Navidad For-mation is evidenced by the inclusion of mollusks, foraminifera, and os-tracodes characteristic of shallower biofacies. Reworking of older sedi-ments is apparent in robust Late Cretaceous and early Miocene forami-nifera in several younger assemblages and suggested by a molluscanfauna that appears to be correlative with deposits from the late Oligoceneto early Miocene in Peru.

Mixed-depth microfaunal associations and sedimentary features revealdownslope transport of sediments, but gastropods appear to be in originalassociations and usually show no physical evidence of transport or re-working. The random spatial distribution of specimens, however, indi-cates that they were not preserved in situ. Although the presence of gas-tropods restricted to greater depths would confirm downslope displace-ment of the shallower association, it is apparent that none were incor-porated during transportation and redeposition of the shallowerassociation.

No outcrops or subsurface sections from the Oligocene to middle Mio-cene interval have been encountered in the region to account for thereworked microfossils in the Navidad, Ranquil, and Lacui formations.These units unconformably overlie Eocene and Cretaceous strata or gra-nitic basement. This implies that nondeposition and extensive erosionwere coincident with a major regressive phase during the Oligocene.


ACKNOWLEDGMENTS

K.L. Finger received funding for fieldwork and electron microscopyfrom the University of California Museum of Paleontology. S.N. Niel-sen’s work was supported by grants Ba 675/25-1, Ni 699/1-1, and Ni699/4-1 of the Deutsche Forschungsgemeinschaft. A. Encinas’ fieldworkwas supported by Proyecto Fondecyt 1010691, Programa MECE Edu-cacion Superior UCH0010, and Beca PG/50/02 of the Departamento dePostgrado y Postıtulo-Universidad de Chile. P. Vasquez (TechnischeUniversitat, Berlin, Germany) assisted in drafting the maps. Luc Ortlieband an anonymous reviewer provided comments and suggestions thatimproved the manuscript.

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paleontologic evidence for sedimentary displacement in neogene

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