versión On-line ISSN 1851-8044
Ameghiniana v.45 n.1 Buenos Aires ene./mar. 2008
Paleoecological and paleoenvironmental implications of a high-density Chondrites association in slope deposits of the Neogene Santo Domingo Formation, Valdivia, south-central Chile
1Departamento de Ciencias de la Tierra, Universidad de Concepción, Casilla 160-C, Chile. email@example.com
2Department of Geological Sciences, University of Saskatchewan, 114 Scence Place, Saskatoon, SK S7N 5E2, Canada. firstname.lastname@example.org
3University of California Museum of Paleontology, 1101 Valley Life Sciences Building, Berkeley, CA 94720-4780, USA. email@example.com
Abstract. Neogene marine strata of the Santo Domingo Formation crop out in the vicinity of Valdivia, south-central Chile (40°S, 73°W). The succession is characterized by dark-gray sandy siltstone with abundant Chondrites isp. The occurrence of this ichnotaxon and lower-bathyal benthic foraminifers is consistent with a slope depositional environment. The abundance of Chondrites and the low ichnodiversity reveal poorly oxygenated bottom waters. The Santo Domingo Formation most likely accumulated in fault-controlled, intraslope silled minibasins, which werw formed during a major event of Neogene subsidence of the Chilean margin.
Resumen. Implicancias Paleoecológicas Y Paleoambientales De Una Asociación De Alta Densidad De Chondrites En Depósitos De Talud De La Formación Santo Domingo, Neógeno De Valdivia, Centro-Sur De Chile. Estratos marinos Neógenos pertenecientes a la Formación Santo Domingo afloran en los alrededores de Valdivia, Chile centro-sur (40°S, 73°W). Dicha sucesión se caracteriza por la presencia de limolitas arenosas de color gris oscuro con abundantes Chondrites isp. La presencia de este icnotaxón y de foraminíferos bentónicos característicos de profundidades batiales inferiores indicarían que la sedimentación de esta unidad se produjo en un ambiente de talud. La abundancia de Chondrites y la baja icnodiversidad sugiere que las aguas del fondo marino estaban pobremente oxigenadas. Se propone que la Formación Santo Domingo se depositó en pequeñas cuencas de talud controladas por fallas que se formaron como consecuencia de un importante evento de subsidencia del margen chileno.
Key words. Ichnology; Trace fossils; Chondrites; Slope; Neogene; Chile.
Palabras clave. Icnología; Trazas fósiles; Chondrites; Talud; Neógeno; Chile.
Neogene marine strata crop out around the city of Valdivia, in the coastal area of south-central Chile (40°S, 73°W) (figure 1). These deposits were first studied by Brüggen (1950) who correlated this succession with the Navidad Formation, the reference unit for the marine Neogene of Chile (Cecioni, 1980). Subsequently, Martínez-Pardo and Pino (1979) defined these strata as the Santo Domingo Formation in their study of the homonym roadcut section located approximately 19 km southeast of Valdivia.
Figure 1. Location map, showing localities cited in the text and the outline of the outcrops of the Santo Domingo Formation. Map modified from Sernageomin 1998 / mapa de ubicación en que semuestran las localidades citadas en el texto y los afloramientos de la Formación Santo Domingo. Mapa modificado de Sernageomin 1998.
The Valdivia area is one of several Chilean coastal localities where Neogene marine strata crop out (e.g., Cecioni, 1980; Le Roux et al., 2005; Encinas et al., 2006). These strata have also been recognized in boreholes drilled on the continental shelf of southcentral Chile (Mordojovich, 1981). In the area located approximately between the cities of Temuco and Puerto Montt (38°30'-41°30'S), Neogene marine strata also crop out in the Central Valley and even in the westernmost part of the Main Andean Cordillera (Osorio and Elgueta, 1990; Elgueta et al., 2000). Strata belonging to the Santo Domingo Formation have been interpreted as having been deposited in deep-marine embayments (Chirino-Gálvez, 1985; Le Roux and Elgueta, 2000; Elgueta et al., 2000).
However, Cecioni (1970) and Martínez-Pardo and Pino (1979) mentioned the presence of bathyal species of benthic foraminifera, and Covacevich et al. (1992) noted the occurrence of Chondrites isp., an ichnogenus that, although not exclusive, is commonly abundant in outer shelf and continental slope deposits (Frey and Pemberton, 1984). In addition, studies recently carried out on correlative Neogene units exposed in the Navidad (34°S), Arauco (37°S) and Chiloé (42°S) areas, and previously considered as shallow-marine successions (e.g., Cecioni, 1978; Cecioni, 1980), indicate that they were deposited at lower bathyal (2000-4000 m) depths (Finger et al., 2007; Encinas et al., in press). This encourages us to carry out sedimentologic, ichnologic, and micropaleontologic analyses to try to unravel the depositional environment and paleobathymetry of the Santo Domingo Formation.
Since the pioneering studies of Seilacher (1967), ichnologists have utilized trace fossils as paleobathymetric indicators. Yet, such interpretations must be made with caution, as water depth is only a secondorder controlling factor in the distribution of trace fossils and ichnofacies (Frey et al., 1990). It is well known that other parameters, such as hydrodynamic energy, substrate, oxygen content, and food supply, effectively modify Seilacher's classical scheme (Frey et al., 1990; Pemberton et al., 1992). Fortunately, these factors generally vary in accordance with water depth, which renders ichnology a very useful tool in the determination of paleobathymetry and onshoreoffshore trends, particularly when combined with sedimentologic and paleontologic analyses (e.g. Mac- Eachern et al., 1999; Mángano et al., 2005). In the present study, we use ichnologic and micropaleontologic evidence from the type locality of the Santo Domingo Formation in order to evaluate the paleoenvironmental implications of abundant Chondrites in these deposits.
The area located around Valdivia has a basement of Paleozoic metamorphic rocks and minor Cretaceous granitoids overlain by an Oligocene?-early Miocene? continental-paralic coal-bearing succession of the Pupunahue-Catamutún Formation, Miocene marine deposits of the Santo Domingo Formation, and Pleistocene-Holocene glacial, marine, and fluvial deposits (Sernageomin, 1998).
The Santo Domingo Formation unconformably overlies the Paleozoic metamorphic rocks of the Bahía Mansa complex (Sernageomin, 1998). It also overlies the Pupunahue-Catamutún formation, although it is not clear whether the contact with this unit is gradational (Elgueta et al., 2000) or if there is a discontinuity, as suggested by the abrupt transition from a coal-bearing unfossiliferous succession to a silty, fossil-rich unit that can be observed in boreholes drilled at the Catamutún mining area southeast of Valdivia (Alfaro et al., 1990). Although the maximum thickness of this unit at Cuesta Santo Domingo (figure 1) is 110 m, this must be considered as a minimum estimate because there is no place where a complete section can be measured. The basal part of the Santo Domingo Formation can only be observed in a limited number of places, such as the coastal cliffs located immediately north of Corral (figure 1). It consists in a succession of schist breccia with minor sandstone. The basal breccia is overlain by a succession of dark-gray sandy siltstone, which constitute the most characteristic facies of this unit, and minor sandstone and breccia. At Cuesta Santo Domingo, neither the basal contact nor the basal interval are visible and only a succession of sandy siltstone is exposed. The Santo Domingo Formation contains a fossil biota that includes bivalves, gastropods, brachiopods, bryozoans, crustaceans, echinoids, fishes, foraminifers, ostracodes, radiolarians and leaves (Chirino-Gálvez, 1985; Pino and Beltrán, 1979; Martínez- Pardo and Pino, 1979; Covacevich et al., 1992). Martínez-Pardo and Pino (1979) assigned a late middle Miocene (N13-N15 zones) to this unit based on their study of foraminifers from Cuesta Santo Domingo (figure 1). However, this age determination, considered to be a problematic approach because benthic foraminifers biostratigraphic ranges are facies controlled and therefore time-transgressive as opposed to those of fossil plankton (Ingle, 1980).
Martínez-Pardo and Pino (1979) recorded eight species of planktic foraminifers, but all were identified with uncertainty as indicated by the "cf." (confer with) modifier. It is probably safe to assume that the forms they recorded as Globigerinoides cf. trilobus and Globorotalia cf. continuosa belong to those species, which have a concurrent range from zone N9 to zone N16 (Middle to Late Miocene). In the Catamutún area, approximately 40 km south of Valdivia, marine beds stratigraphically equivalent to the Santo Do- mingo Formation were assigned a middle Miocene (Langhian-Serravallian, zones N10-N12) age by Marchant and Pineda (1988) and Marchant (1990). Although their determinations also were based partially on benthic foraminifers, they recorded the planktic foraminifer Globigerina pachyderma, which has its first appearance datum in N16 and thus indicates a maximum age of Tortonian (Late Miocene). This correlates with the late Miocene-early Pliocene ages determined for the Navidad, Ranquil, and Lacui formations exposed in the areas of Navidad, Arauco, and Chiloé, respectively (Finger et al., 2007; Encinas et al., in press). We therefore ascribe a probable late Miocene age to the Santo Domingo Formation, although we do not discard a broader age range. Additional data is needed to accurately constrain the age of this unit.
The Cuesta Santo Domingo succession: outcrop description, associated trace fossils and foraminiferal assemblages
Our study was focused on the sedimentary succession that crops out at Cuesta Santo Domingo (39°56'S-73°07'W) (figure 1), considered as the type locality of the homonym Formation (Martínez-Pardo and Pino, 1979). Studies were focused on this locality due to the relatively good preservation of invertebrate trace and body fossils. Other roadcut and coastal outcrops west and south of Valdivia where also examined. They show similar sedimentary facies, trace fossils and microfossil content but with a poorer preservation.
The Cuesta Santo Domingo is located along the road between Valdivia and Paillaco, approximately 19 km southwest of Valdivia (figure 1). It starts approximately 300 m southwest of the road exit to Corral and continues along 2 km. Exposures along the edge of the road are intermittent due to high vegetation cover. They are 2 to 6 m in height, except for one of the of the basal part of the section that is 25 m in height on the hillside of a farm near the deviation to Corral (figure 2). The sedimentary succession of Cuesta Santo Domingo consists almost exclusively of sandy siltstone; a few medium-grained sandstone beds are also present. The siltstone is dark gray when fresh and light gray to orangebrown when weathered. It is slightly fissile, massive, and usually breaks in large blocks. No clear bedding planes were observed. Bivalves, gastropods, crustaceans, and echinoderms are common. Although the metamorphic basement is observed in some outcrops near the base of the succession, the basal contact of the Santo Domingo Formation is not exposed.
Figure 2. Typical aspect of sandy siltstone facies of the Santo Domingo Formation at Cuesta Santo Domingo. Cliff is approximately 25 m high / aspecto típico de las facies de limolitas arenosas de la Formación Santo Domingo en la Cuesta Santo Domingo. El escarpe mide aproximadamente 25 m de altura.
The benthic foraminiferal assemblage that we recorded from this unit is much more diverse than that recorder in Martínez-Pardo and Pino's (1979) list, and it includes both shallow- and deep-water indicators. The bathyal component of the assemblage includes Bathysiphon sp., Bulimina spicata, Favulina hexagona, Glandulina laevigata, Globobulimina galliheri, Hansenisca soldanii, Hanzawaia concentrica, Hoeglundina elegans, Oridorsalis umbonatus, Pullenia bulloides, Martinottiella communis, Melonis pompilioides, Nodosaria longiscata, Pseudonodosaria torrida, Rectuvigerina transversa, and Sphaeroidina bulloides. The assemblage also includes many shallow-water indicators; thus, it is mixed-depth association that indicates downslope displacement. Melonis pompilioides is generally considered to be a cosmopolitan indicator of lower-bathyal (2000 to 4000 m) depths (e.g., Ingle, 1980; Morkhoven et al., 1986; Finger et al., 2007); Bathysiphon sp. and Nodosaria longiscata are also indicative of this depth zone off Chile (see Bandy and Rodolfo, 1964; Ingle et al., 1980).
Chondrites isp. is extremely abundant in these deposits. This ichnotaxon was observed after breaking siltstone blocks along fissile planes. No clear bedding planes were observed jutting out as the result of differential erosion, which likely would have facilitated the preservation of traces. The trace fossils are clearly distinguishable only in weathered rocks because they conserve the original gray color that contrasts with the more altered brown matrix. Partially preserved spreiten structures similar to Zoophycos are also present, but their accurate identification is not possible due to their fragmentary preservation. Chondrites isp. consists of straight to slightly curved, regularly branching systems. The tunnels are approximately 0.8 mm wide (figure 3). The angle of branching is less than 45°. Only second-order branches were observed. Specimens are abundant and can be clearly distinguished in oxidized siltstone because the internal filling is pristine and preserves its original gray color. No attempts have been made to classify Chondrites at ichnospecific level because the status of most of its ichnospecies is still uncertain. Some curved segments resemble Nereites missouriensis, but lack the typical backfill of this ichnotaxon.
Figure 3. Chondrites isp. from the Santo Domingo Formation. Scale bar = 1 cm. A) Typical Chondrites branching is observed below the scale. B) Radial pattern and typical branching is observed below the scale / Chondrites isp. de la Formación Santo Domingo. Barra de escala = 1 cm. A) La ramificación característica de Chondrites se puede observar bajo la escala. B) El diseño radial y la ramificación característica se puede observar bajo la escala.
Chondrites is generally considered to be a feeding system of unidentified infaunal deposit-feeders (e.g., Osgood, 1970). However, according to Seilacher (1990) and Fu (1991), the tracemaker of Chondrites may be able to live in the aerobic/anoxic interface as a chemosymbiotic organism that pumps methane and hydrogen sulphide from the sediments, whereas Kotake (1992) suggested the possibility that this trace is merely a "cess-pit" for fecal pellets produced by the excretory behavior of surface deposit feeders. Monospecific assemblages of Chondrites suggest poorly oxygenated bottom waters (e.g., Fu, 1991; Bromley, 1996). For example, Chondrites is the dominant ichnospecies on the northern slope of the Iceland- Faero Ridge, where sluggish bottom currents, organic-rich sediments, and oxygen deficiency prevails (Fu and Werner, 1994). Baas et al. (1998) found massive occurrences of Chondrites during halted or reduced thermohaline circulation leading to lowered oxygenation levels of the bottom waters in the eastern North Atlantic. Löwemark et al. (2004) observed the massive occurrence of this ichnogenus in Holocene sediments of the southwestern Iberian continental slope during the onset of low current velocities and enhanced deposition of particulate organic matter leading to low pore-water oxygen levels. Uchman et al. (2003) found Chondrites in laminated micrite that fills deep submarine cavities in peri-reefal biocalcarenite and calcirudite of the Tithonian-Berriasian Stramberg Limestone (Czech Republic) in a very stressful, low-oxygen environment. In shallow-water settings, extremely dense concentrations of Chondrites were linked to burial of high quantities of organic matter during storm events (Vossler and Pemberton, 1988).
The association of Chondrites and forms resembling Zoophycos compare favorably with the Zoophycos ichnofacies, which is typical, although not exclusive, of outer shelf to slope settings (Frey and Pemberton, 1984). Although this ichnofacies is placed between the sublittoral and the abyssal zones in Seilacher's (1967) popular bathymetric scheme, it also occurs in shallower-water, epeiric deposits, particularly during the Paleozoic (e.g., Miller, 1991). This is because one of the main environmental controls of this ichnofacies is lowered oxygen levels associated with abundant organic material in quiet-water settings (Frey and Seilacher, 1980). However, uncertainties regarding an accurate identification of Zoophycos complicate ichnofacies assessments. Regardless of ichnofacies identification, benthic foraminifers in the Santo Domingo succession also support a deep-marine depositional environment.
A trace fossil association similar to that described here for the Santo Domingo Formation is recognized in the correlative Navidad Formation (Encinas et al., in press). However, there are some differences between the trace fossil assemblages that characterize both units. Two different ichnofacies have been recognized in the Navidad Formation: the Zoophycos ichnofacies, found in siltstone and very fine-grained sandstone, and the Skolithos ichnofacies that occurs mostly in massive, medium- to coarse-grained sandstone. The Zoophycos ichnofacies characterizes calm, low sedimentation intervals whereas the Skolithos ichnofacies reflects short-term, high-energy conditions associated with the sudden deposition of thick packages of sand (Encinas et al., in press). In contrast, the Skolithos ichnofacies has not been recognized in the Santo Domingo Formation. This is probably due to the scarcity of sandy intervals that this unit shows in contrast with the Navidad Formation that presents abundant coarsed-grained intervals. Another difference between these units is that the Zoophycos ichnofacies in the Navidad Formation is a more diverse association that includes Chondrites isp., Zoophycos isp., Lophoctenium isp., Diplocraterion parallelum and Planolites isp. (Encinas et al., in press). In addition, Chondrites is not as abundant as in the Santo Domingo Formation. However, this could be due to differences in outcrop quality. The Navidad Formation occurs as continuous and well-preserved exposures along several kilometers of coastal bluffs, while the Santo Domingo Formation crops out in a more limited number of roadcuts and coastal outcrops because the wetter climate of the Valdivia area supports a thicker vegetational cover. Nevertheless, the consistent and widespread occurrence of Chondrites-bearing gray siltstone facies in most outcrops of the Santo Domingo Formation, some of them several tens of meters thick, suggests that the ichnologic and lithologic differences between this unit and the Navidad Formation reflect more than just differences in outcrop quantity and quality.
The abundance of Chondrites and the presence of lower-bathyal benthic foraminifers in the type locality of the Santo Domingo Formation are consistent with a deep-marine depositional environment for this unit. Sedimentologic and paleontologic studies carried out in Neogene marine deposits that crop out in several areas along the Chilean coastline also indicate bathyal depositional depths for at least part of these successions (Le Roux et al., 2005; Finger et al., 2007; Encinas et al., in press). Deep-marine sedimentation in these areas has been attributed to forearc subsidence caused by a major event of tectonic erosion that affected the Chilean margin during the Neogene (Encinas et al., in press). Hence, the Santo Domingo Formation provides evidence of this tectonically- induced transgression for the Valdivia area. The ichnofauna of the Santo Domingo Formation suggests deposition in poorly oxygenated bottom waters. Further evidence of low-oxygen conditions are the abundance of black siltstone intervals, the occurrence of pyrite in this succession (Chirino-Gálvez, 1985) and geochemical analysis that show V/(V/Ni) rates of 0.75 (Bravo, 2006). A possible explanation for anoxic conditions during sedimentation of the Santo Domingo Formation is that this unit was deposited in fault-controlled, silled, intra-slope minibasins similar to those of the California Borderland and the modern Guaymas Basin in the Gulf of California (Ingle, 1981).
If the depth of the sill is within the oxygen-minimum layer, all of the water below sill depth will be oxygen deficient regardless of the maximum depth of the basin floor (Ingle, 1981). Silled intra-slope minibasins commonly display limited deep-water circulation, leading to depauperate trace fossil assemblages due to oxygen-depleted conditions (Mángano and Buatois, 1997). In addition, open-ocean surface currents bearing planktic foraminifers may be restricted from some silled basins, which could explain relatively small ratios of planktic vs. benthic foraminifers (Martínez- Pardo and Pino, 1979). In contrast, partially ponded intra-slope minibasins contain more diverse ichnofaunas, although the Nereites ichnofacies is absent (e.g. Schultz and Hubbard, 2005).
The Neogene Santo Domingo Formation that crops out near Valdivia contains abundant Chondrites isp. The presence of this ichnogenus and a mixed-depth assemblage of benthic foraminifers that includes lower- bathyal indicators indicate a deep-water depositional environment. In addition, the low diversity of the trace association and the abundance of Chondrites characterize poorly-oxygenated bottom waters. It is suggested that the Santo Domingo Formation was deposited in fault-controlled, silled, deep-marine basins during a significant event of forearc subsidence resulting from tectonic erosion of the margin.
AE was supported by Proyecto Fondecyt No. 3060051 of Conicyt and the IRD (Institut de Recherche pour le Développment). LB was supported by a NSERC Discovery Grant 311726-05. We gratefully thank their financial help to these institutions. We also would like to thank G. Hérail, M. Pino and S. Villagrán for their help. We finally thank E.B. Olivero and M. Verde for reviewing the paper.
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Recibido: 24 de mayo de 2007.
Aceptado: 11 de diciembre de 2007.