Paleoecological and paleoenvironmental implications of a high-density Chondrites association in slope deposits of the Neogene Santo Domingo Formation, Valdivia, south-central Chile
Alfonso Encinas1, Luis A. Buatois2 and Kenneth L. Finger3
1Departamento de Ciencias de la Tierra, Universidad de
Concepción, Casilla 160-C, Chile. alfonso.encinas@gmail.com
2Department of Geological Sciences, University of Saskatchewan, 114 Scence Place, Saskatoon, SK S7N 5E2, Canada.
luis.buatois@usask.ca
3University of California Museum of Paleontology, 1101 Valley
Life Sciences Building, Berkeley, CA 94720-4780, USA.
kfinger@berkeley.edu
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.
Introduction
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.
Geologic setting
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.
Discussion
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).
Conclusions
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.
Acknowledgments
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.