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Ameghiniana v.45 n.1 Buenos Aires ene./mar. 2008


Ichnology of the Lower Miocene Chenque Formation, Patagonia, Argentina: animal - substrate interactions and the Modern Evolutionary Fauna

Noelia B. Carmona1, Luis A. Buatois2, María Gabriela Mángano2 and Richard G. Bromley3

1Laboratorio de Geología Andina. Centro Austral de Investigaciones Científicas. CONICET. B. Houssay 200, 9410 Ushuaia. Tierra del Fuego. Argentina.
2Department of Geological Sciences. University of Saskatchewan. 114 Science Place. Saskatoon. SK S7N 5E2, Canada.
3Department of Geography and Geology. University of Copenhagen. Øster Voldgade 10. 1350 Copenhagen K. Denmark.

Abstract. The lower Miocene Chenque Formation of Patagonia contains superbly preserved and diverse ichnofaunas, including the ichnogenera Asterosoma, Balanoglossites, Chondrites, Gastrochaenolites, Gyrolithes, Helicodromites, Macaronichnus, Nereites, Ophiomorpha, Palaeophycus, Phycosiphon, Planolites, Protovirgularia, Rhizocorallium, Rosselia, Schaubcylindrichnus, Scolicia, Siphonichnus, Skolithos, Spongeliomorpha, Taenidium, Teichichnus, and Thalassinoides. Wave-influenced open-marine deposits are characterized by intense bioturbation, complex tiering structures and a very high diversity. Restricted, brackish-water, tide-dominated deposits display lesser degrees of bioturbation, less complex tiering structures and lower ichnodiversity. Both marginal- and open-marine deposits are commonly punctuated by discontinuity surfaces delineated by firmground ichnofaunas. The complex tiering structure of open marine ichnofaunas shows the development of a finely partitioned infaunal niche and an increment in bioturbation by the Neogene, which is consistent with trends revealed by body fossils. However, further expansion of the Cenozoic ichnologic database is necessary in order to evaluate the role of evolutionary and latitudinal controls.

Resumen. Icnología De La Formación Chenque (Mioceno Inferior), Patagonia, Argentina: Interacciones Sustrato-Animal y La Fauna Evolutiva Moderna. La Formación Chenque (Mioceno temprano) se caracteriza por presentar icnofaunas extremadamente diversas, que incluyen los icnogéneros Asterosoma, Balanoglossites, Chondrites, Gastrochaenolites, Gyrolithes, Helicodromites, Macaronichnus, Nereites, Ophiomorpha, Palaeophycus, Phycosiphon, Planolites, Protovirgularia, Rhizocorallium, Rosselia, Schaubcylindrichnus, Scolicia, Siphonichnus, Skolithos, Spongeliomorpha, Taenidium, Teichichnus y Thalassinoides. Los depósitos marinos normales con influencia de oleaje se caracterizan por presentar un intenso grado de bioturbación, el desarrollo de estructuras de escalonamiento muy complejas y una alta icnodiversidad. Por el contrario, los depósitos restringidos, de aguas salobres, dominados por mareas, presentan un menor grado de bioturbación, una estructura de escalonamiento menos compleja y una menor icnodiversidad. Tanto los depósitos marinos marginales como los marinos abiertos presentan icnofaunas características de sustratos firmes, las cuales delimitan superficies de discontinuidad. La compleja estructura de escalonamiento observada en las icnofaunas marinas normales refleja el desarrollo de una fina partición del ecoespacio infaunal y un incremento en la bioturbación durante el Neógeno, lo cual concuerda con las tendencias globales reflejadas a partir del registro de cuerpos fósiles. Sin embargo, se requiere un mayor número de estudios sobre icnofaunas cenozoicas para discriminar entre controles evolutivos y latitudinales.

Key words. Trace fossils; Miocene; Chenque Formation; Patagonia; Shallow marine.

Palabras clave. Trazas fósiles; Mioceno; Formación Chenque; Patagonia; Marino somero.


Lower Miocene deposits of the Chenque Formation, outcropping in the southeast of Chubut and northeast of Santa Cruz provinces, Argentina, contain extremely abundant and diverse trace fossils. Although the body fossils and sedimentary facies of this formation have received considerable attention (Frenguelli, 1929; Feruglio, 1949; Expósito, 1977; Cione, 1978; Bellosi, 1987, 1995; Bellosi and Barreda, 1993; Paredes, 2002; del Río, 2002), its ichnology has not yet been analyzed. Only recently, some papers provided information on some of these well-preserved trace fossil assemblages (Carmona et al., 2002; Buatois et al., 2003a; Carmona and Buatois, 2003; Carmona, 2005; Carmona et al., 2006). Therefore, the main purposes of this paper are: (1) to describe and illustrate the trace fossils of the Chenque Formation, (2) to evaluate their paleoecology and ethology, and (3) to briefly discuss the paleoenvironmental distribution of the trace fossils and their significance with respect to secular changes in bioturbation linked to the development of the Modern Evolutionary Fauna.

Geological setting

The Chenque Formation crops out in the San Jorge Basin, Central Patagonia, Argentina (figure 1) and consists mainly of shallow-marine deposits bearing abundant and diverse invertebrate body fossils and trace fossils. The Chenque Formation comprises five shallowing-upward depositional sequences (figure 2), deposited during the Leonense (26-21 Ma) and Superpartagoniense (19-18 Ma) Atlantic transgressions (Bellosi, 1987, 1995). The age of the Chenque Formation is based on the sequence stratigraphic framework (Bellosi, 1990a; Bellosi and Barreda, 1993), dinoflagellates (Palamarczuk and Barreda, 1998; Barreda and Palamarczuk, 2000a,b), pollen and spores (Barreda, 1996; Barreda and Palamarczuk 2000a,b), foraminifers (Bertels and Ganduglia, 1977), vertebrates (Caviglia, 1978; Cione, 1978), and isotope dating (Riggi, 1979; Bellosi, 1990b). The first two sequences are dominated by sandy, muddy, and tuffaceous shoreface deposits, with abundant body fossils (specially oysters and other mollusks), and dominance of shoreface environments, while the upper sequences of this formation (sequences III-V), are sandier, with less abundant body fossils and ichnofossils, reflecting deposition in restricted, marginal-marine, tide-influenced environments (Bellosi, 1987, 1995, 2000; Bellosi and Barreda, 1993).

Figure 1. Map showing the distribution of the Chenque Formation and the studied localities / mapa de distribución de la Formación Chenque, mostrando la ubicación de las localidades estudiadas.

Figure 2. Sequences of the Chenque Formation and proposed depositional environments (modified from Bellosi and Barreda, 1993). This study focused on sequences I-III (marked with grey shadow) / secuencias de la Formación Chenque y diferentes ambientes depositacionales propuestos (modificado de Bellosi y Barreda, 1993). El presente estudio se centró en el análisis de las secuencias I-III (marcadas con color gris).

Field work was especially concentrated on deposits outcropping on the Atlantic coast of Chubut and Santa Cruz provinces, because these deposits show the best preservation of trace fossils and the highest ichnodiversity. Trace fossils were analyzed at 14 localities (figure 1), the majority of which consist of vertical cliff sections and extensive horizontal surfaces that represent the abrasion platform exposed during low tide. Superb preservation allows threedimensional reconstructions of the trace fossils. The analyzed localities include deposits belonging mostly to the lower sequences (I-III) which, as mentioned above, record the highest abundance and diversity of biogenic structures of this formation.

Systematic ichnology

This section focuses on the systematic description of the trace fossils identified in the Chenque Formation. Trace fossils were mainly studied in the field, and ichnotaxonomic analysis was complemented with drawings, photographs and collected specimens, which are housed at the Collection of Paleontology of Invertebrates of CADIC (Centro Austral de Investigaciones Científicas), Ushuaia, Argentina. Ichnotaxa are arranged alphabetically, and their analysis includes a brief discussion about ichnotaxonomy, environmental and stratigraphic range, and probable ethology of the tracemaker.
The majority of the specimens were identified at ichnospecific level. However, some ichnofossils show morphological variation that makes it difficult to assign them to defined ichnospecies. In addition, some beds contain abundant biogenic structures that intersect with complex cross-cutting relationships, a fact that further complicates identification of ichnofossils. In these cases, open nomenclature is used. Table 1 summarizes additional information about the recognized ichnogenera in the Chenque Formation.

Table 1. Ichnogenera identified in the Chenque Formation. Information includes environmental distribution, stratigraphic range, ethology and probable trace makers / icnogéneros identificados en la Formación Chenque. La tabla incluye información sobre la distribución ambiental, rango estratigráfico, etología y posibles organismos productores.

Ichnogenus Asterosoma von Otto, 1854

Asterosoma isp. A Figure 3.1-2

Figure 3. 1-2, Asterosoma isp. A. 1, Bedding plane view, Playa Las Cuevas. 2, Cross-section view and Schaubcylindrichnus coronus (Sch), Punta Delgada. 3-4, Asterosoma isp. B. 3, Specimen showing predominantely horizontal bulbs, Caleta Olivia. 4, Cross-section view, Cerro Hermitte. 5, Balanoglossites isp. in the tuffaceous beds, Punta Delgada. The arrow indicates the presence of apparent scratch ornament. 6, Chondrites isp. reworking previous trace fossils, Playa Las Cuevas. 7-8, Gyrolithes isp., bedding plane view, Punta Delgada. In 7, the fill of Gyrolithes isp. is reworked by pale Chondrites isp. (Ch). In 8, two specimens of Gyrolithes isp. (Gy) indicated by arrows / 1-2, Asterosoma isp. A. 1, Vista en planta, Playa Las Cuevas. 2, Vista en sección de Asterosoma isp. A y Schaubcylindrichnus coronus (Sch), Punta Delgada. 3-4, Asterosoma isp. B. 3, Vista en planta de un ejemplar mostrando predominio de bulbos horizontales, Caleta Olivia. 4, Vista en sección, Cerro Hermitte. 5, Balanoglossites isp. en depósitos tobáceos, Punta Delgada. La flecha indica la presencia de aparente ornamentación. 6, Chondrites isp. retrabajando estructuras biogénicas previas, Playa Las Cuevas. 7-8, Gyrolithes isp., vista en planta, Punta Delgada. En la figura 7, el relleno de Gyrolithes isp. se encuentra retrabajado por Chondrites isp. (Ch) claros. En la figura 8 se indican con flechas dos especimenes de Gyrolithes isp. (Gy).

Description. Horizontal and subhorizontal bulbs arranged radially or in a fan-like manner. The fill of the bulbs comprises concentrically laminated sediment enclosing a central to subcentral tube. Bulb diameter is 15-25 mm and maximum length is 200 mm. The concentric structures are composed of mud and very fine-grained sand. Intersection of bulbs is commonly observed in cross section. Preserved as full relief.
Remarks. The bulb walls of some Cretaceous (Otto, 1854) and Jurassic (Schlirf, 2000) occurrences show longitudinal furrows or striae. In the Miocene specimens identified as Asterosoma isp. A, however, this feature was not recognized, probably because their external surface is not visible. They also lack the typical tapering of the bulbs, and associated shafts (e.g. Bromley and Uchman, 2003) were not observed. Specimens of Asterosoma isp. A resemble Asterosoma radiciforme von Otto, 1854 for its radial or star-like orientation of the bulbs. However, poor definition of the general morphology and absence of views of the external surfaces of the bulbs preclude ichnospecific assessment.

Localities. Playa Las Cuevas, Punta Delgada and Rada Tilly.

Asterosoma isp. B Figure 3.3-4

Description. Subvertically arranged bulbs, showing concentric lamination and subcentral inner tube. Bulbs do not overlap and generally taper at their distal ends. Bulb diameter is commonly 25-30 mm and length is 90-200 mm. Preserved as full relief.
Remarks. Asterosoma isp. B differs from Asterosoma isp. A in having a subvertical arrangement of the bulbs and tapering ends. At only one locality (roadcut on National Route 3), do the bulbs show furrows on their external surfaces. However, these specimens are preserved as concretions and this preservation could have caused the irregularities and striae seen on their surfaces. Therefore, this feature probably should not be regarded as a character reflecting the original morphology of this trace fossil. Asterosoma isp. B differs from A. radiciforme von Otto, 1854 by the radial and horizontal arrangement of the bulbs in the latter. These specimens also differ from A. ludwigae Schlirf, 2000 by the subvertical orientation of the bulbs and absence of the typical ramification of this ichnospecies (Schlirf, 2000).

Localities. Caleta Olivia, Cerro Antena, Cerro Hermitte and roadcut on National Route 3.

Ichnogenus Balanoglossites Mägdefrau, 1932

Balanoglossites isp. Figure 3.5

Description. Connected U-shaped burrows, 15-19 mm wide. Maximum depth observed is approximately 140 mm. Some specimens have tunnels with smaller diameters that form blind ends, diverging from the principal U-shaped burrow. Striations occur on the wall of some specimens. Burrows are passively filled with sand. Preserved as full relief.
Remarks. Specimens of Balanoglossites isp. from the Chenque Formation differ from the type specimens in the presence of tunnels that go downward and laterally from the main burrow. In contrast, specimens from the Middle Triassic of Poland and Germany show ramifications that diverge from the U-shape burrow in an upward direction (Kazmierczak and Pszczólkowski, 1969).
The studied specimens of Balanoglossites isp. resemble Diplocraterion parallelum var. arcum (Ekdale and Lewis, 1991) in overall morphology of the causative burrow. However, absence of spreiten distinguishes Balanoglossites isp. from D. parallelum. In addition, the ichnogenus Balanoglossites characteristically has ramifications, which are absent in D. parallelum. Kazmierczak and Pszczólkowski (1969) observed that the material from the Middle Triassic of Poland presents net outlines and smooth walls without any evidence of bioglyphs. These authors also suggested that the tracemakers were not able to live in consolidated or firm substrates and suggested emplacement in a soft substrate instead based on the deformation observed in some of the burrows. In contrast, specimens of Balanoglossites from the Chenque Formation rework a tuffaceous substrate that was firm at the time of colonization. This interpretation is supported by the presence of bioglyphs (figure 3.5), the net outline and the ichnofauna associated with Balano- glossites isp. Furthermore, Knaust (1998) described Balanoglossites in association with the boring Trypanites in hardgrounds, suggesting a firm, pre-cementation substrate for Balanoglossites.

Locality. Punta Delgada.

Ichnogenus Chondrites von Sternberg, 1833

Chondrites isp. Figures 3.6-7, 4.5, 5.10

Figure 4. 1-3, Gastrochaenolites ornatus. 1, Lateral view of several specimens with the internal casts and external moulds of the bivalve tracemakers. 2, Lateral view, showing in the upper portion the bivalve cast well preserved. 3, Basal view of several specimens showing different sizes. 4-5, Helicodromites mobilis. 4, Bedding plane view with H. mobilis reworking Scolicia isp. (Sc). Thalassinoides isp. (Th) is in the upper right corner. 5, Helicodromites mobilis reworked by Chondrites isp. (Ch). 6-7, Macaronichnus segregatis, Caleta Olivia. 6, Bedding plane view of M. segregatis. This trace fossil constitutes a dominant element in these beds. 7, Cross section view of M. segregatis (Ma). 8, Palaeophycus tubularis, Cerro Hermitte / 1-3, Gastrochaenolites ornatus. 1, Vista lateral de varios especimenes de G. ornatus con los moldes internos y secundarios de los bivalvos productores. 2, Vista lateral, mostrando en la porción superior, el molde secundario del bivalvo productor con excelente preservación. 3, Vista basal de varios ejemplares de diferentes tamaños. 4-5, Helicodromites mobilis. 4, Vista en planta de H. mobilis retrabajando Scolicia isp. (Sc). Thalassinoides isp. (Th) en el extremo superior derecho. 5, Helicodromites mobilis retrabajado por Chondrites isp. (Ch). 6-7, Macaronichnus segregatis, Caleta Olivia. 6, Vista en planta de M. segregatis. Esta traza fósil constituye un elemento dominante en estos estratos. 7, Vista en sección de M. segregatis (Ma). 8, Palaeophycus tubularis, Cerro Hermitte.

Figure 5. 1-2, Nereites missouriensis preserved in the interface between sandy and muddy beds, Caleta Olivia. 3-5, Ophiomorpha nodosa. 3, 4, Cerro Hermitte. In 3, well developed pellets and sandy filling. In 4, pellets have been weathered out, leaving empty spaces along the vertical shaft. 5, Specimen from Playa Alsina, showing a thick wall and the fill mainly composed of coarse sand and bioclastic fragments. 6-10., Ophiomorpha isp., Punta Delgada. 6 and 7 correspond to vertical sections of Ophiomopha isp. In 6, the arrow indicates the apparent bilobate morphology of pellets. In 7 the characteristic conical morphology of the pellets is visible. 8, Cross section of Ophiomorpha isp., showing greater development of those pellets located on the roof of the burrow. The arrow indicates the absence of pellets on the floor. 9, Specimen at the turnaround point, showing that ramifications form sharp angles. 10, Ophiomorpha isp. in oblique section, showing specimens of Chondrites isp. (Ch) reworking its fill / 1-2, Nereites missouriensis preservado en la interface arena fango, Caleta Olivia. 3-5, Ophiomorpha nodosa. 3, 4, Cerro Hermitte. En 3, ejemplar de O. nodosa con pellets bien desarrollados y relleno arenoso. En 4, los pellets no se preservaron, dejando espacios vacíos a lo largo del tubo vertical. 5, Especimen de Playa Alsina, mostrando una pared gruesa y relleno principalmente compuesto por arenas gruesas y fragmentos bioclásticos. 6-10, Ophiomorpha isp., Punta Delgada. Las figuras 6 y 7 muestran secciones verticales de Ophiomorpha isp. En 6, la flecha indica la morfología aparentemente bilobada de los pellets. En 7 se puede reconocer la morfología cónica características de estos pellets. 8, Sección de Ophiomorpha isp. mostrando mayor desarrollo de los pellets en el techo de la excavación. La flecha indica la ausencia de pellets en la base. 9, Vista en planta de un ejemplar de Ophiomorpha isp. con ramificaciones formando ángulos rectos. 10, Ophiomorpha isp. en un corte oblicuo, mostrando especimenes de Chondrites isp. (Ch) retrabajando su relleno.

Description. A tree-like system of tunnels that branch downward. Width of tunnels constant within each specimen (1.0-1.8 mm). Angle of branching commonly less than 45º. Almost all specimens show second-order branches. Color of the sediment fills always different from the color of the host rock. Preserved as full relief.
Remarks. The studied specimens differ from the ichnospecies Chondrites intricatus by commonly having slightly curved branches and tube diameters larger than 1 mm. The slightly curved branches resemble C. targionii (Brongniart, 1828), although in this ichnospecies, tunnels are commonly a few millimeters wide (Uchman, 1999). Chondrites patulus Fischer- Ooster, 1858 has simple branches perpendicular to the primary stems (Uchman, 1999) and C. recurvus (Brongniart, 1823) shows branches arising only from one side of the masterbranch, bending in a single direction. These characteristics allow the distinction of both C. recurvus and C. targionii from the studied specimens. Chondrites isp. also differs from C. caespitosus (Fischer-Ooster, 1858) by the dense and short winding branches that characterized the latter. Finally, these specimens differ from C. stellaris Uchman, 1999 in the smaller size and radiating branching of the latter (Uchman, 1999). Tunnel fill of Chondrites isp. may be either lighter or darker than the host rock (figure 3.6).

Localities. Playa Alsina, Playa Las Cuevas, Punta Borja, Punta Delgada and Rada Tilly.

Ichnogenus Gastrochaenolites Leymerie, 1842

Gastrochaenolites ornatus Kelly and Bromley, 1984 Figure 4.1-3

Specimens. CADIC PI 39-45.

Description. Clavate structures with bioglyphs in their deeper portions. Diameters of the main chambers are highly variable (2-25 mm). When the neck is preserved, specimens are up to 95 mm long. The bioglyphs show two preferred orientations: oblique and parallel to bedding. Some specimens have a small, shallow subcentral depression at their base.
Remarks. The presence of bioglyphs relates these lower Miocene specimens to the ichnospecies Gastrochaenolites ornatus. This feature distinguishes Gastrochaenolites ornatus from the other ichnospecies of Gastrochaenolites (e.g. G. ampullatus Kelly and Bromley, 1984, G. cor Bromley and D'Alessandro, 1987, G. dijugus Kelly and Bromley, 1984, G. lapidicus Kelly and Bromley, 1984, G. orbicularis Kelly and Bromley, 1984, G. cluniformis Kelly and Bromley, 1984, G. turbinatus Kelly and Bromley, 1984 and G. torpedo Kelly and Bromley, 1984).
The studied specimens were constructed in a firm, tuffaceous substrate and are connected to the boundary surface between the continental Sarmiento Formation and the shallow-marine Chenque Formation (Carmona et al., 2006). Although the studied specimens were emplaced in firm substrates and the ichnogenus Gastrochaenolites was primarily defined to designate clavate borings developed in hard substrates (Kelly and Bromley, 1984), these specimens are assigned to this ichnogenus because their morphology is identical to the specimens constructed in lithified substrates (Carmona et al., in press).

Localities. Astra, Infiernillo and West of Bahía Solano.

Ichnogenus Gyrolithes de Saporta, 1884

Gyrolithes isp. Figure 3.7-8

Description. Vertical, unbranched spiral burrows. Width of the burrows is 45-65 mm and radius of the whorls is 69-95 mm. Whorls sinistrally coiled and wall structure commonly consisting of a thin muddy layer (1-2 mm) or with few, irregularly distributed conical pellets. Preserved as full relief.
Remarks. Specimens from the Chenque Formation have the diagnostic characteristics proposed by Bromley and Frey (1974) for this ichnogenus. However, the studied specimens clearly differ from the type ichnospecies Gyrolithes davreuxi of Saporta, 1884, from G. saxonicus (Häntzschel, 1934) and from G. krymensis Vialov, 1969 by their larger size, orientation of the whorls and characters of the wall. In addition, incomplete preservation of these specimens precludes an ichnospecific designation. Preservation restricted to bedding planes also precludes measurement of height of the burrows and height of the whorls.
Specimens of Gyrolithes isp. form a dense aggregation, covering approximately a surface of 2 m2. Some burrows have been reworked by Chondrites isp. (figure 3.7). The same characteristic was observed in the fillings of Ophiomorpha isp., which occur in the same bed with Gyrolithes isp. These observations, and the presence of sparsely distributed conical pellets in the Gyrolithes specimens, support the idea that these two ichnogenera were produced by the same organism. Mayoral and Muñiz (1993) also recorded intergrading Gyrolithes and Ophiomorpha in Late Miocene- Pliocene deposits of Huelva. Hester and Prior (1972) interpreted that these intergradations could represent adaptative responses of the organisms to live in substrates with differences in cohesiveness, building Ophiomorpha-like burrows in sandier substrates and Gyrolithes-like burrows in muddier deposits.

Locality. Punta Delgada.

Ichnogenus Helicodromites Berger, 1957

Helicodromites mobilis Berger, 1957 Figures 4.4-5, 7.2

Description. Horizontal spiral burrows, with pale fill contrasting with the host-rock. Diameter of tubes with two different sizes; smaller specimens are 2.6- 3.2 mm wide and larger ones are 5.4-9.8 mm wide. Length is highly variable and some structures reach up to 300 mm long. Distance between successive whorls is 10-13 mm in the smaller specimens and 26- 29 mm in the larger examples. Preserved as full relief.
Remarks. Analysis of the specimens from the Chenque Formation indicates a relationship between tube diameters and the distance between successive whorls. In all the studied specimens this relationship is 0.2-0.3, suggesting that specimens of Helicodromites mobilis were constructed by the same organism and that differences in sizes reflect ontogenetic variations. Tiering analysis shows that the Helicodromites tracemaker was a relatively deep bioturbator, cutting almost all the other biogenic structures in these deposits, being only reworked by Chondrites isp. (figure 4.5).

Localities. Playa Las Cuevas and Punta Delgada.

Ichnogenus Macaronichnus Clifton and Thompson, 1978

Macaronichnus segregatis Clifton and Thompson, 1978 Figure 4.6-7

Description. Predominately horizontal or sub-horizontal, cylindrical trace fossil that interpenetrates randomly. Diameter is 4-6 mm. Trace fossil fill lighter than the host rock. It occurs gregariously. Preserved as full relief.
Remarks. Pemberton et al. (2001) and Gingras et al. (2002b) mentioned that modern polychaetes produce structures similar to the ichnospecies Macaronichnus segregatis, ingesting sand to consume bacteria and organic material attached to the grains, and excreting the clean sand that fills the core of the burrow (Pemberton et al., 2001).
Macaronichnus segregatis segregatis differs from the other three ichnosubspecies by its randomly interpenetrating burrow configurations (Bromley et al., in press). The specimens from the Chenque Formation are straight or slightly curved and in some cases, approaching the morphology of M. s. lineiformis show interpenetrations producing "false branchings" (sensu D'Alessandro and Bromley, 1987). In the Chenque Formation M. segregatis is the dominant ichnotaxon in deposits that also contain vertical components of Ophiomorpha nodosa.

Localities. Caleta Olivia, Playa Alsina and Punta Delgada.

Ichnogenus Nereites MacLeay, 1839

Nereites missouriensis (Weller, 1899) Figure 5.1-2

Description. Predominantely horizontal, winding to meandering trace fossil with a dark, central tunnel and lateral lobes of reworked, paler sediment. Measured on horizontal surfaces, the diameter is 5.2- 6.5 mm, and the central tunnel, composed of alternating muddy and sandy laminae, is 3.0-4.0 mm wide. Maximum length observed is 64 mm. On vertical sections, the same measurements were 5.5-6.9 mm and 2.2-2.5 mm respectively. The thin lateral lobes are not clearly visible in horizontal sections, especially if there is a dense association of structures, but they are much clearer in vertical section, where they resemble giant Phycosiphon. Preserved as epirelief.
Remarks. Proportions between width of central channel and the reworked zone fall within the range proposed by Uchman (1995) for Nereites missouriensis. This ichnospecies differs from Nereites cambrensis (Murchinson, 1839) in the absence of lanceolate lateral lobes and from Nereites imbricata Mángano, Buatois, West and Maples, 2000, by having transverse scalariform ridges (Mángano et al., 2002) instead of the imbricated subspherical pads characteristic of the latter. Nereites missouriensis is also distinguished from N. irregularis (Schafhäutl, 1851) by having the central backfilled tunnel and the envelope zone of similar thickness, and also by the absence of coiled, closely packed meanders, typical of the last ichnospecies (Uchman, 1998).

Locality. Caleta Olivia and Playa Las Cuevas.

Ichnogenus Ophiomorpha Lundgren, 1891

Ophiomorpha nodosa Lundgren, 1891 Figure 5.3-5

Description. Large burrow systems, with vertical and horizontal components, having walls composed of dense, regularly or irregularly distributed ovoid pellets. Branching T- and Y-shaped. The fill is passive and diameter range is 20-40 mm. Maximum depth observed is approximately 640 mm. Preserved as full relief.
Remarks. The studied specimens show the diagnostic characteristics of Ophiomorpha nodosa, although they show some variations in size, three-dimensional configuration and pellet composition in the different specimens studied. For example, in the upper deposits of Playa Alsina locality specimens are composed principally of vertical components, with large and ovoid pellets (approximately 7.5 mm wide). These pellets are muddy and form a continuous wall. Burrow diameter is approximately 40 mm and the fill is composed of alternating muddy and sandy laminae with bioclast fragments (figure 5.5). In other localities (e.g. roadcut on National Route 3; Cerro Antena and Cerro Hermitte), the specimens are smaller, reaching diameters up to 25 mm (figure 5.3).

Localities. Caleta Olivia, Cerro Antena, Cerro Chenque, Cerro Hermitte, Cerro Viteau, Infiernillo, Playa Alsina, Punta Borja, Rada Tilly and roadcut on National Route 3.

Ophiomorpha isp. Figures 5.6-10, 6.7

Figure 6. 1-2, Phycosiphon incertum. 1, Cross section view, Punta Delgada. 2, Cross section, Caleta Olivia. 3, Abundant Planolites beverleyensis, Cerro Viteau. 4, Protovirgularia isp., Caleta Olivia. Epichnial preservation in a muddy layer. 5, Cross section view of Rhizocorallium isp. in the upper tuffaceous deposit from Punta Delgada. Arrows indicate specimens of Thalassinoides isp. (Th). 6-8, Rosselia socialis. 6, R. socialis in tidal channel deposits, showing erosive truncation (indicated by arrows), Cerro Hermitte. 7, Rosselia socialis, reworked by Ophiomorpha isp. in its basal portion. Rada Tilly. 8, Rosselia socialis, Caleta Olivia. The specimen shows erosive truncation (indicated by the arrow), and renewed building up of the structure / 1-2, Phycosiphon incertum. 1, Vista en sección, Punta Delgada. 2, Vista en sección, Caleta Olivia. 3, Abundantes especímenes de Planolites beverleyensis, Cerro Viteau. 4, Protovirgularia isp., Caleta Olivia. Preservación epicnia en un sustrato pelítico. 5, Vista en sección de Rhizocorallium isp. en el nivel tobáceo superior de Punta Delgada. Las flechas señalan ejemplares de Thalassinoides isp. (Th). 6-8, Rosselia socialis. 6, R. socialis en depósitos de canales mareales, mostrando truncamiento erosivo (indicado por flechas), Cerro Hermitte. 7, Especimen de R. socialis retrabajado por Ophiomorpha isp. en su porción basal, Rada Tilly. 8, Rosselia socialis, Caleta Olivia. Este ejemplar muestra truncamiento erosivo (indicado por la flecha), y nuevo crecimiento de la estructura biogénica.

Figure 7. 1, Schaubcylindrichnus coronus, showing closely bundled tubes. 2, Schaubcylindrichnus freyi, Helicodromites mobilis (He), Scolicia isp. (Sc) and Planolites beverleyensis (Pl), Playa Las Cuevas. 3-5, Scolicia isp., Playa Las Cuevas. 3, Upper bedding plane view of Scolicia isp. with its possible tracemaker (spatangoid echinoid). 4, Cross section of Scolicia isp. showing its two drain tubes (dt) at the base of the trace fossil. 5, Bedding plane view of a deposit completely reworked by Scolicia isp. These specimens are commonly associated with the body fossils of their tracemakers. 6, Cross section of Siphonichnus eccaensis associated to Thalassinoides suevicus (Th), Playa Alsina. 7, Specimen of Spongeliomorpha isp., Playa Alsina. The arrow indicates the scratch ornament (so) sculpted in the firm host sediment / 1, Schaubcylindrichnus coronus, mostrando el agrupamiento ceñido de los tubos. 2, Schaubcylindrichnus freyi, Helicodromites mobilis (He), Scolicia isp. (Sc) y Planolites beverleyensis (Pl), Playa Las Cuevas. 3-5, Scolicia isp., Playa Las Cuevas. 3, Vista en planta de Scolicia isp. con su posible organismo productor (equinodermo espatangoideo). 4. Vista en sección de Scolicia isp. mostrando los dos canales de drenaje (dt) en la parte basal de la traza fósil. 5, Vista en planta de un nivel completamente retrabajado por Scolicia isp. Comúnmente estas estructuras biogénicas se encuentran asociadas a los cuerpos fósiles de sus organismos productores. 6, Vista en sección de Siphonichnus eccaensis asociado a Thalassinoides suevicus (Th), Playa Alsina. 7, Ejemplar de Spongeliomorpha isp., Playa Alsina. La flecha señala los bioglifos (so) generados en un sustrato firme.

Description. Burrow systems having T- and Y-shaped branches, characterized by the presence of conical pellets, with a square base facing to the interior of the burrow. Some pellets apparently have a bi-conical morphology. Although size of pellets is variable (5-11 mm width and 4.0-11.5 mm height), most commonly they are 8-10 mm wide and 7-9 mm high. Pellets are regularly arranged in the walls. Burrows are 40-100 mm wide and the fill is passive. Preserved as full relief.
Remarks. These specimens were described in detail by Carmona and Buatois (2003) and are classified at the ichnogeneric level because the overall morphology of the pellets distinguishes this material from all formally defined ichnospecies of Ophiomorpha. These Lower Miocene specimens differ from the ichnospecies Ophiomorpha nodosa by the absence of ovoid pellets and by having a more regular distribution in the walls. These specimens differ from Ophiomorpha annulata (Ksiazkiewicz, 1977) because, although having the elongate section of the pellets oriented perpendicular to the long axis of the burrows, they are not smooth in their external surface.
Ophiomorpha irregulaire Frey, Howard and Pryor, 1978 may have elongated conical, ovoid or mastoid pellets, irregularly distributed on burrow walls and a characteristic "meander maze" morphology (e.g. Bromley and Ekdale, 1998; Pedersen and Bromley, 2006). However, the specimens from the Chenque Formation show a much more organized distribution of the pellets than O. irregulaire, and we consider that this character is diagnostic. Ophiomorpha puerilis Gibert, Netto, Tognoli and Grangeiro, 2006, is characterized by having distinctive elongated round-ended fecal pellets, which are absent in the specimens of Ophiomorpha isp. described here. Finally, Ophiomorpha borneensis Keij, 1965 is characterized by regularly distributed bilobate pellets (Frey et al., 1978), similar to the examples from the Chenque Formation. However, these bilobate pellets are rounded and not conical as in the studied material. In summary, the overall morphology of the pellets distinguishes Ophiomorpha isp. from all formally defined ichnospecies of Ophiomorpha and we consider that these lower Miocene burrows could correspond to a new ichnospecies of Ophiomorpha and will be further analyzed elsewhere.

Localities. Punta Delgada and Rada Tilly.

Ichnogenus Palaeophycus Hall, 1847

Palaeophycus heberti (de Saporta, 1872) Not figured

Description. Straight to slightly curved, inclined to horizontal smooth-walled burrows. Wall lining is relatively thick (1.5-2.5 mm). Tube diameter remains constant along the burrows (6-10 mm). Maximum length observed is 33 mm. Preserved as full relief.
Remarks. Some of these specimens are similar in burrow diameter and lining width to Schaubcylindrichnus Frey and Howard, 1981 and they commonly occur in the same beds. This ichnospecies differs from Palaeophycus tubularis by having a thicker lining and differs from other ichnospecies of Palaeophycus (e.g. P. striatus Hall, 1852 or P. alternatus Pemberton and Frey, 1982) by the absence of ornamentations or annulations.

Localities. Playa Alsina, Playa Las Cuevas and Punta Delgada.

Palaeophycus tubularis Hall, 1847 Figure 4.8

Description. Straight to slightly sinuous, horizontal, smooth, thinly lined burrows. Width is 4-7 mm and maximum length observed is 50 mm. Burrow lining is 0.5-1.0 mm thick. Burrow fill is massive and similar to the host rock. Preserved as full relief.
Remarks. Palaeophycus tubularis is the type ichnospecies of Palaeophycus. This ichnospecies has a thinner wall lining than P. heberti and is distinguished from other ichnospecies of Palaeophycus (e.g. P. striatus) by the absence of striae and annulations.

Localities. Cerro Hermitte, Playa Las Cuevas, Punta Delgada and Rada Tilly.

Ichnogenus Phycosiphon Fischer-Ooster, 1858

Phycosiphon incertum Fischer-Ooster, 1858 Figure 6.1-2

Description. Spreite trace fossils formed by recurving U-lobes in bedding planes. In cross section, the dark core is 3.7-4.7 mm wide whereas the surrounding pale mantle is 1.3-1.8 mm wide. Preserved as full relief.
Remarks. The studied specimens do not clearly show the presence of spreiten, but show the pale mantle and the general configuration of Phycosiphon incertum and, therefore, they are assigned to this ichnospecies. The analyzed specimens are similar in preservation to those studied by Uchman (1995, Plate 8, Fig. 7 and 8).

Localities. Caleta Olivia, Playa Alsina, Playa Las Cuevas, Punta Delgada and Rada Tilly.

Ichnogenus Planolites Nicholson, 1873

Planolites beverleyensis (Billings, 1862) Figures 6.3, 7.2

Description. Relatively large, unlined, smooth, straight to slightly curved structures, oriented predominantely horizontally. Diameter is 4-7 mm.
Maximum length observed is 50 mm. Some specimens possess diagenetic halos. Preserved as full relief.
Remarks. These specimens differ from Planolites annularis Walcott, 1890 in the absence of annulations characteristics of that ichnospecies. Planolites beverleyensis also differs from P. montanus Richter, 1937 by its larger size, better defined boundaries and by being more continuous along the bedding plane. Absence of small striae distinguishes these specimens from Planolites terraenovae Fillion and Pickerill, 1990.

Localities. Caleta Olivia, cerro Antena, cerro Chenque, cerro Hermitte, cerro Viteau and roadcut on National Route 3.

Planolites montanus Richter, 1937 Not figured

Description. Relatively small horizontal to subhorizontal, unlined structures, 3-5 mm wide and 25-50 mm long. Burrow fill is massive and lighter than host rock. Preserved as full relief.
Remarks. These specimens differ from Planolites beverleyensis in their small size and more curved course. Localities. Playa Alsina, Playa Las Cuevas, Punta Delgada and Rada Tilly.

Ichnogenus Protovirgularia M'Coy, 1850

Protovirgularia isp. Figure 6.4

Description. Predominantly horizontal trace fossil characterized by the presence of closely spaced, Vshaped ornamentation along the structure. If the internal structure is preserved, it is formed by successive sediment pads. Specimens are 7-18 mm wide and the maximum length observed is 188 mm. Traces are mostly straight, locally curved. The chevron ornamentation is variable in morphology, being closely spaced and having a defined morphology in some specimens while showing an irregular outline in others. In epichnial specimens, chevron patterns are formed mainly by mud, although in endichnial preservations they are formed of alternating sandy and muddy packets.
Remarks. The analyzed Protovirgularia isp. shows a wide morphologic variability, probably related to differences in substrate consistency. Mángano et al. (2002 and references therein) mentioned morphological variations of Protovirgularia in Carboniferous deposits from Kansas, and attributed them to substrate properties and position of the bivalve within the substrate. The specimens from the Chenque Formation are preserved as endichnia and, more rarely, as epichnia. Some specimens show affinities with Protovirgularia dichotoma M'Coy, 1850, based on the morphology of the chevron markings and a few specimens have a Lockeia-like structure at the beginning or end of the locomotion structures, a characteristic that could relate this material to Protovirgularia rugosa (Miller and Dyer, 1878). However, the endichnial and epichnial preservations do not allow to clearly observe the morphology of the chevron patterns and, therefore, it is not possible to classify this material at ichnospecific level.

Locality. Caleta Olivia.

Ichnogenus Rhizocorallium Zenker, 1836

Rhizocorallium isp. Figure 6.5

Description. U-shaped burrows having a spreite, oriented oblique to the bedding plane. Diameter of the limbs is approximately 5-7 mm and total width of the structure is 40-80 mm. Limbs are filled with sand. Preserved as full relief.
Remarks. These specimens resemble Rhizocorallium jenense Zenker, 1836, in their oblique orientation and their general morphology (a simple U-shaped burrow). However, absence of a bedding plane view precludes assignement of this material to a defined ichnospecies. Abundance of specimens of Rhizocorallium isp. is low to moderate, being restricted only to a tuffaceous level in the Punta Delgada locality.

Locality. Punta Delgada.

Ichnogenus Rosselia Dahmer, 1937

Rosselia socialis Dahmer, 1937 Figure 6.6-8

Description. Vertical to inclined funnel-shaped burrow with a central tube filled with sandy sediment, surrounded by concentric muddy lamination. Size is highly variable; diameter is 30-100 mm and length is 50 to 400 mm. Specimens showing erosional truncations are common. Preserved as full relief.
Remarks. Rosselia socialis differs from R. chonoides Howard and Frey, 1984 by the absence of spiral swirls of sediment and from R. rotatus McCarthy, 1979 by having concentric lamination and lacking of helicoid structures in the backfill, which are produced by the rotational movement of the central tube in R. rotatus (Fillion and Pickerill, 1990). Alternatively, Uchman and Krenmayr (1995) synonymized R. rotatus with R. socialis, considering that features of R. rotatus constitute intraspecific variations of R. socialis related to high-energy environments. Wide variations in length and diameter found in the studied specimens may be related to variable environmental conditions. Specimens of Rosselia socialis are isolated or form crowded clusters. The isolated specimens show a vertical orientation in the finegrained deposits of the lower shoreface, whereas an inclined orientation is typical in sandier tidal channel deposits (figure 6.7 and 8).

Figure 8. 1, Taenidium isp., bedding plane view, Playa Las Cuevas. 2-3, Cross section views of Teichichnus rectus, Caleta Olivia. 4-5, Teichichnus zigzag, Playa Las Cuevas. 4, Oblique cross-section of T. zigzag. 5, Bedding plane view of T. zigzag and associated trace fossils. 6, ?Teichichnus isp., showing the causative burrow with arcuate filling, Rada Tilly. 7-8, Thalassinoides suevicus. 7, Cerro Chenque, note the large size of this structure and its development in the horizontal plane. 8, High abundance of T. suevicus, Playa Alsina. 9, Bedding plane view of Thalassinoides isp., Punta Borja, showing apparent Y-ramification pattern. The boundary of this structure is not clearly visible and the filling seems to correspond to the arcuate passive type / 1, Taenidium isp., vista en planta, Playa Las Cuevas. 2-3, Vista en sección de Teichichnus rectus, Caleta Olivia. 4-5, Teichichnus zigzag, Playa Las Cuevas. 4, Sección oblicua de T. zigzag. 5, Vista en planta de T. zigzag y trazas fósiles asociadas. 6, ?Teichichnus isp., mostrando el tubo causativo con relleno arqueado, Rada Tilly. 7-8, Thalassinoides suevicus. 7, Ejemplar del Cerro Chenque. Note el gran tamaño de esta estructura y el desarrollo que presenta en el plano horizontal. 8, Gran abundancia de T. suevicus, Playa Alsina. 9, Vista en planta de Thalassinoides isp., Punta Borja, mostrando aparentemente un patrón de ramificación en Y. El límite de este ejemplar no se distingue claramente y el relleno parece corresponder al tipo pasivo arqueado.

Localities. Caleta Olivia, cerro Antena, cerro Chenque, cerro Hermitte, cerro Viteau, Infiernillo, Playa Las Cuevas, Punta Borja, Punta Delgada, Rada Tilly and roadcut on National Route 3.

Ichnogenus Schaubcylindrichnus Frey and Howard, 1981

Schaubcylindrichnus coronus Frey and Howard, 1981 Figures 3.2, 7.1

Description. Oblique to horizontal bundles of congruent, pale-lined tubes. Diameter is 5.9-6.6 mm and lining is 1.3-1.6 mm thick. Tubes are mainly circular in cross-section and some specimens show cross-cutting relationships between successive tubes. Exterior surfaces of the tubes are smooth, without ornamentation. Preserved as full relief.
Remarks. Schaubcylindrichnus coronus is characterized by the presence of closely-packed bundles of tubes, this feature being the main difference between this ichnospecies and S. freyi (Miller, 1995). Number of tubes in each specimen is variable, although commonly they comprise 3-4 tubes. Schaubcylindrichnus coronus also differs from S. formosus Löwemark and Hong, 2006, by being smaller and by the absence of interconnections and ramifications in the central part of the sheaves, typical of this last ichnospecies.

Localities. Playa Las Cuevas, Punta Delgada and Rada Tilly.

Schaubcylindrichnus freyi Miller, 1995 Figure 7.2

Description. Vertical to oblique burrows, rarely subhorizontal, and having a pale lining. The burrows form loose bundles. Diameter is highly variable (4.5- 12.0 mm), although tubes from the same specimens maintain a constant diameter. Lining is 0.8- 2.0 mm thick. Some specimens show flattened tubes. Preserved as full relief.
Remarks. The specimens from the Chenque Formation form loose bundles, which is distinctive of Schaubcylindrichnus freyi and allow differentiation from S. coronus and S. formosus. In addition, these specimens are smaller than S. formosus (Löwemark and Hong, 2006).
The analyzed specimens from the Chenque Formation show a wide size range. Miller (1995) considered that size variations observed in specimens of S. freyi are related to grain size. He concluded that in coarsening-upward sequences, the basal deposits with finer-grained sediments contain the largest specimens, while in the upper coarser-grained deposits, the specimens of S. freyi show variable sizes. His conclusion is consistent with the relations observed in the Chenque Formation (e.g. in Playa Las Cuevas and Punta Delgada localities).
Recently, Nara (2006) considered Schaubcylindrichnus freyi as a junior synonym of S. coronus. This author analyzed the type material of these two ichnospecies and concluded that except for their mode of occurrence, size and construction of the tubes are similar in both ichnospecies. However, we consider that our material should be included in S. freyi because it displays the morphological characters of this ichnospecies, being clearly distinguished from specimens of S. coronus.

Localities. Caleta Olivia, Playa Alsina, Playa Las Cuevas, Punta Delgada and Rada Tilly.

Ichnogenus Scolicia de Quatrefages, 1849

Scolicia isp. Figures 4.4, 7.2-5

Description. Horizontal, sinuous or meandering trails with a bilobate backfill and two parallel strings located at the base. The strings are 7-8 mm wide and are interpreted as traces of drain canals. The region between these canals is slightly concave upward. In cross section, burrow outline is normally oval to somewhat square, 42-44 mm wide and 20-22 mm high. Preserved as full relief.
Remarks. The studied specimens were not assigned to any defined ichnospecies of Scolicia because these are distinguished by characters present on their bases, which are observed only in hypichnial preservations. In longitudinal section, some specimens have a small V-shaped depression in the upper part of the backfill, being similar in morphology to the Laminites preservational variant as illustrated by Plaziat and Mahmoudi (1988, figure 3). The backfill is invariably composed of alternate sandy and muddy laminae.

Localities. Playa Alsina, Playa Las Cuevas, Punta Borja, Punta Delgada and Rada Tilly.

Ichnogenus Siphonichnus Stanistreet, Le Blanc Smith and Cadle, 1980

Siphonichnus eccaensis Stanistreet, Le Blanc Smith and Cadle, 1980 Figure 7.6

Description. Vertical structures containing a backfill of concave-downward menisci. The laminae that form the backfill are cut through centrally by a vertical tube, filled with pale, massive sand. In bedding plane view, burrow diameter is 20-28 mm and central tube diameter is approximately 9-11 mm.
Remarks. In bedding plane views, specimens of Siphonichnus have a rounded transverse cross-section. This feature distinguishes this ichnogenus from aggradational Lingulichnus, which are characterized by having an elliptical transverse cross-section (Zonneveld and Pemberton, 2003). Most of the specimens of Siphonichnus eccaensis show a straight vertical orientation, although some examples may present a slightly sinuous course. Stanistreet et al. (1980) postulated that siphon length of the bivalve tracemakers should be equal to the length of backfill laminae. In their material, the minimum length proposed is 12.5 cm. However, the specimens from the Chenque Formation are longer, recording a maximum length of 35 cm, which renders the bivalve model unlikely. Alternatively, this may record the increase in burrowing depth by bivalves that resulted from the Mesozoic Marine Revolution.

Localities. Playa Alsina, Playa Las Cuevas, Punta Borja, Punta Delgada and Rada Tilly.

Ichnogenus Skolithos Haldemann, 1840

Skolithos linearis Haldemann, 1840 Not figured

Description. Simple, vertical tubes. Diameter is 3-4 mm and length of complete specimens is 200-350 mm. The majority of specimens are preserved as fragments. Burrow fill is sandy and the walls are thinly lined by fine sediment. Preserved as full relief.
Remarks. The specimens of Skolithos linearis are associated with sandy deposits. The wall of these specimens is commonly affected by diagenesis, enhancing their visibility. Skolithos lineraris is associated with Ophiomorpha nodosa, suggesting moderate to high energy conditions.

Locality. Cerro Chenque.

Skolithos verticalis (Hall, 1843) Not figured

Description. Vertical to inclined, straight to slightly curved tubes. Diameter is 2.3-4.0 mm and length is 50-70 mm. Burrow fill is finer-grained than the host rock and is mainly composed of massive mud. Preserved as full relief.
Remarks. This ichnospecies is poorly represented in the studied deposits. However, in those localities where it was observed, the abundance of structures was relatively high, being dominant and forming monospecific associations.
Diameters obtained from the studied specimens of Skolithos verticalis are coincident with the values from the Paleozoic examples studied by Fillion and Pickerill (1990), although the Miocene specimens are longer. Skolithos verticalis is distinguished from S. linearis because the former is smaller and shorter. They also differ in the type of fill, being muddy in S. verticalis and mainly sandy in S. linearis.

Localities. Playa Alsina and roadcut on National Route 3.

Ichnogenus Spongeliomorpha de Saporta, 1887

Spongeliomorpha isp. Figure 7.7

Description. Burrow with an ornament of short ridges covering the surface of the structure. Burrow diameter is 25 mm. Burrow fill is massive, composed of sandy sediment. Preserved as full relief.
Remarks. The studied specimen constitutes a burrow fragment and therefore it was not possible to identify the general morphology of the structure, precluding ichnospecific assessment. The short scratches are oriented mainly parallel to the axis of the burrow. This specimen is distinguished from Spongeliomorpha sublumbricoides (Azpeitia Moros, 1933), S. orviense (Ksiazkiewicz, 1977), S. milfordiensis Metz, 1993 and S. carlsbergi (Bromley and Asgaard, 1979), these ichnospecies having obliquely oriented scratch ornament (D'Alessandro and Bromley, 1995). It also differs from Spongeliomorpha sicula D'Alessandro and Bromley, 1995, in the absence of longitudinally extended ridges and ovoid chambers. These specimens are also distinguished from S. chevronensis Muñiz and Mayoral, 2001, by the absence of regularly distributed, oblique ridges, and from S. sinuostriata Muñiz and Mayoral, 2001, by lacking the long and sinuous scratch ornament that characterizes this ichnospecies.

Localities. Infiernillo and Playa Alsina.

Ichnogenus Taenidium Heer, 1877

Taenidium isp. Figure 8.1

Description. Cylindrical and unlined sinuous trace fossils, 10-15 mm wide. Length is variable, generally 80-100 mm. Fill consists of meniscate segments alternately composed of fine- and coarse-grained sediments. Preserved as full relief.
Remarks. The taxonomic status of Taenidium is uncertain because the type material is lost and at its type locality no true specimens of Taenidium have been subsequently found. The name Taenidium is provisionally adopted here, pending a comprehensive review of meniscate ichnofossils.
Taenidium isp. from the Chenque Formation commonly occurs in intensely bioturbated beds. This high bioturbation does not allow clear observations of the relevant morphologic characteristics and, therefore, precludes ichnospecific designation. Specimens found in Playa Las Cuevas and Rada Tilly localities show hemispheric menisci, similar to those described for T. barretti (Bradshaw, 1981). However, specimens of T. barretti have a more constant diameter than the specimens from the Chenque Formation. These specimens also differ from Taenidium crassum Bromley, Ekdale and Richter, 1999 by the absence of parabolic- or chevron-like menisci that characterize this ichnospecies (Bromley et al., 1999).

Localities. Playa Las Cuevas and Rada Tilly.

Ichnogenus Teichichnus Seilacher, 1955

Teichichnus rectus Seilacher, 1955 Figure 8.2-3

Description. Horizontal to slightly inclined, unlined, simple structures, with a retrusive spreite. Causative burrow is 17.5-27.3 mm in diameter and the length of the spreite is up to 200 mm. In lateral view, the specimens are mainly horizontal or they can display a flattish U-shape. Preserved as full relief.
Remarks. Schlirf (2000) observed that Late Jurassic specimens from the Boulonnais, France, have the causative burrow rarely preserved. This kind of preservation is commonly observed in the majority of the studied specimens, except those from Caleta Olivia where T. rectus displays excellent causative burrows in cross section (e.g. figure 8.3).

Localities. Caleta Olivia, Playa Las Cuevas, Punta Delgada and Rada Tilly.

Teichichnus zigzag Frey and Bromley, 1985 Figure 8.4-5

Description. Teichnichnus having a spreite showing not only a vertical migration vector, but also a lateral vector, producing a zig-zag pattern. Width of the spreite is 15-40 mm, a range that exceeds the diameter of the causative burrow (10-12 mm). Size and morphology of the specimens are highly variable. The causative burrow fill is preferentially sandy, while the dark spreite laminae are muddy. Preserved as full relief.
Remarks. This ichnospecies differs from the other defined ichnospecies of Teichichnus by the zig-zag pattern of the spreiten. In the original diagnosis of this ichnospecies, Frey and Bromley (1985) considered that the spreite observed in these structures typically shows an arcuate course, developed in the vertical plane. However, they also mentioned that in some specimens the spreite course is horizontal, as is the case of specimens from the Chenque Formation. Frey and Bromley (1985) suggested that the general morphology of T. zigzag could correspond to a shallow U-shaped burrow. In Playa Las Cuevas locality, the specimens of T. zigzag are well preserved and they are only reworked by Chondrites isp.

Locality. Playa Las Cuevas.

?Teichichnus isp. Figure 8.6

Description. Large horizontal structures with a retrusive spreite and a terminal, passively filled tube. Some specimens seem to branch. Spreite height is 60- 70 mm and maximum length observed is 550 mm. Preserved as full relief.
Remarks. The analyzed specimens are asigned to Teichichnus with doubts, because they apparently present ramifications, making interpretation of the threedimensional morphology of these trace fossils uncertain. In cross-section, the specimens are similar to large Techichnus rectus (with dimensions resembling T. pescaderoensis Stanton and Dodd, 1984, an ichnospecies considered to be a junior synonym of T. rectus by Fillion and Pickerill, 1990, and Schlirf and Bromley, 2007). However, the apparent branching pattern seen in the bedding plane view prevents assignment of this material to the ichnospecies T. rectus. These structures also resemble Teichichnus patens Schlirf (2000) because this ichnospecies presents ramifications. However, the Miocene material differs from T. patens in the more clearly defined morphology and constant diameter of the branches of the latter. Structures similar to those found in the Chenque Formation were observed in lower Miocene deposits from Las Grutas, Patagonia, Argentina (Olivero, pers. com., 2005). These specimens show a comparable general morphology, although the lithologic constrast is greater in these deposits, allowing identification of some features such as possible scratch traces. These specimens from Las Grutas are interpreted as having been constructed by decapods (Olivero, pers. com., 2005).

Localities. Rada Tilly.

Ichnogenus Thalassinoides Ehrenberg, 1944

Thalassinoides suevicus (Rieth, 1932) Figures 7.6, 8.7-8

Description. Predominantly horizontal, passively filled, large and regularly branching, cylindrical burrow systems. Diameter is 60-120 mm. T-shaped and Yshaped bifurcations are common. Swellings occur at branching points. In horizontal components, maximum observed length is 1.5 m. Preserved as full relief. Remarks. The studied specimens present lengths and diameters larger than those mentioned for other specimens of T. suevicus (e.g. Frey and Bromley, 1985). The burrows display T-shaped and Y-shaped ramifications. These specimens differ from Thalassinoides paradoxicus (Woodward, 1830) in having ramifications more regularly arranged and in the presence of predominantly horizontal components.

Localities. Cerro Chenque, Playa Alsina, Playa Las Cuevas and Punta Delgada.

Thalassinoides isp. Figures 4.4, 8.9

Description. Large burrow systems with horizontal and vertical components. Diameter is highly variable (17-76 mm). Passive fill is mainly composed of structureless sand or subhorizontal, alternating muddy and sandy laminae. Preserved as full relief.
Remarks. In many of the studied deposits, the specimens assigned to Thalassinoides isp. are incompletely preserved, mainly consisting of short fragments; branching points are not observed. This partial preservation precludes ichnospecific identification. The fill of Thalassinoides isp. is mainly sandy, except in those deposits interpreted as tidal flat facies, where the fill consists of alternating sandy and muddy laminae, most likely reflecting tidal cyclicity. In some cases, burrows are reworked by Chondrites isp. and Phycosiphon incertum.

Localities. Astra, Caleta Olivia, cerro Antena, cerro Hermitte, Infiernillo, Playa Las Cuevas, Punta Delgada, Rada Tilly and West of Bahía Solano. Structures produced by the bivalve Atrina Figure 9.1-3

Figure 9. 1-3, Equilibrium structures of Atrina, Caleta Olivia. 1-2, Close-up views of two different specimens, with the body fossils of their tracemakers at the end of the structures, and also showing the traces left by the byssal threads (indicated by arrows). 3, General view of the equilibrium trace fossils. 4-5, Concave upward structures from Caleta Olivia. These biogenic structures are probably feeding traces of rays / 1-3, Estructuras de equilibrio de Atrina, Caleta Olivia. 1-2, Vista detallada de dos ejemplares que presentan los cuerpos fósiles de los organismos productores preservados al final de las estructuras. Note también las marcas dejadas por el biso (indicadas con flechas). 3, Vista general de las estructuras de equilibrio. 4-5, Estructuras cóncavas observadas en depósitos de Caleta Olivia. Estas trazas fósiles son comparables a estructuras de alimentación de rayas.

Description. Large vertical to slightly oblique, V-shaped spreiten structures. Vertical length is variable (120-240 mm). Maximum length observed is 500 mm. The Vshaped spreite is always retrusive. Commonly, the shells of the bivalve tracemakers are preserved at the upper end of the structures. Exceptionally preserved specimens display some pale, very thin sandy threads, radiating from the basal part of these trace fossils.
Remarks. These biogenic structures represent equilibrichnial trace fossils produced by Atrina bivalves during their upward movement, in response to aggradational events. The retrusive backfill is composed of alternating sandy and muddy V-shaped laminae. The thin sandy structures that radiate from the basal part of the ichnofossils are interpreted as the traces left by the byssal threads of these bivalves (figures 9.1 and 2). Preservation of these specimens is restricted to vertical sections, so bedding plane views are not available. These equilibrium trace fossils co-occur with specimens of Teichichnus rectus in the same beds.
Hanken et al. (2001) defined the ichnogenus Scalichnus to include large, vertically oriented, bottleshaped structures, formed during retrusive and pro- trusive movement of the bivalve Panopea. As in the case of the trace fossils generated by Atrina in the Chenque Formation, Scalichnus is also regarded as an equilibrichnial trace fossil. However, this ichnogenus differs from the present material by its general sacklike morphology and its thick lining.

Locality. Caleta Olivia. Large, concave-upward structures Figure 9.4-5

Description. Concave upward structures, 250-320 mm wide in the upper portion and 80-100 mm in the lower portion. Cross sections of these structures generally show asymmetric U-profiles, with one slope steeper than the other. Fill differs markedly from the host rock. Preserved as full relief.
Remarks. The specimens observed in these Miocene deposits resemble biogenic structures generated by rays while feeding, assigned to the ichnogenus Piscichnus Fiebel, 1987. The ichnogenus Piscichnus was originally defined by Fiebel (1987) to include shallow depressions interpreted as nesting traces produced by bony fishes. In his original description, Fiebel (1987) recognized only one ichnospecies, Piscichnus brownii. Later, Gregory (1991) described a new ichnospecies of Piscichnus, P. waitemata, to include pits much deeper and more cylindrical than P. brownii. Piscichnus waitemata also cut the internal structure of the substrate and is interpreted as formed by hydraulic jetting generated by rays while feeding (Gregory, 1991). Although the specimens from the Chenque Formation are similar to Piscichnus, the low abundance of structures and the absence of bedding plane views preclude a definite assignment.

Locality. Caleta Olivia.

Environmental distribution of trace fossils The Chenque Formation comprises shallowmarine deposits (figure 2), encompassing both open marine and marginal marine environments (Bellosi, 1987, 1995, Buatois et al., 2003a, Carmona, 2005). In turn, tide-dominated marginal marine successions include both estuarine and deltaic deposits. Some of these deposits are truncated by surfaces that are associated with erosional exhumation of the substrate (firmgrounds). Table 2 summarizes the occurrence of trace fossils in the different localities.

Table 2. Occurrence of trace fossils in the Chenque Formation. X indicates the presence of the different ichnotaxa. Localities: AS: Astra, BS: West of Solano Bay, CA: Cerro Antena, CC: Cerro Chenque, CH: Cerro Hermitte, CO: Caleta Olivia, CRN: roadcut on National Route 3, CV: Cerro Viteau, IN: Infiernillo, LC: Playa Las Cuevas, PA: Playa Alsina, PB: Punta Borja, PD: Punta Delgada, RT: Rada Tilly / Ocurrencia de las trazas fósiles en la Formación Chenque. La X indica la presencia de los diferentes ichnotaxones. Localidades: AS: Astra, BS: Oeste de Bahía Solano, CA: Cerro Antena, CC: Cerro Chenque, CH: Cerro Hermitte, CO: Caleta Olivia, CRN: corte de la Ruta Nacional 3, CV: Cerro Viteau, IN: Infiernillo, LC: Playa Las Cuevas, PA: Playa Alsina, PB: Punta Borja, PD: Punta Delgada, RT: Rada Tilly.

Open marine environments

Open marine successions include upper, middle and lower shoreface deposits. Lower shoreface deposits are recognized in the Playa Las Cuevas, Rada Tilly and Punta Delgada localities. These deposits consist of thoroughly bioturbated very fine-grained silty sandstone. Only locally parallel lamination and discrete shell layers are observed. Trace fossils are abundant and diverse. These lower shoreface deposits are characterized by the presence of the archetypal Cruziana ichnofacies (Buatois et al., 2003a).
Common elements include Chondrites isp., Phycosiphon incertum, Thalassinoides suevicus, Teichichnus zigzag and T. rectus, Palaeophycus heberti, Scolicia isp., Helicodromites mobilis, Nereites missouriensis, Planolites montanus, Schaubcylindrichnus coronus and S. freyi, Taenidium isp., Asterosoma isp. A, and Rosselia socialis.
In the Rada Tilly and Punta Delgada localities, a slight upward increase in grain size and the presence of elements of the Skolithos ichnofacies suggest progressive shallowing and deposition in a middle shoreface environment. This ichnofauna is dominated by Ophiomorpha isp. and Asterosoma isp. A. Subordinate elements comprise Thalassinoides suevicus, Phycosiphon incertum, Schaubcylindrichnus freyi, Chondrites isp., Scolicia isp., Rosselia socialis, Palaeophycus heberti, Palaeophycus tubularis, Planolites montanus and Teichichnus rectus. This trace fossil suite is interpreted as a proximal expression of the Cruziana ichnofacies.
Upper shoreface deposits are recognized in the upper interval of the Playa Alsina section. These deposits consist of glauconitic sandstones with trough cross-stratification and planar lamination. The trace fossil suite represents the Skolithos ichnofacies, and is characterized by heavily lined, deeply penetrating vertical structures such as Ophiomorpha nodosa, and mobile intrastratal deposit feeding burrows, such as Macaronichnus segregatis.

Tide-dominated estuarine environments

Estuarine deposits occur at Cerro Antena, Cerro Chenque, Cerro Hermitte, Cerro Viteau, and the roadcut on National Route 3. At almost all of these localities, intertidal flat and subtidal sandbar and channel deposits are present. The intertidal flat deposits mainly consist of heterolithic beds with a low to moderate degree of bioturbation and well preserved physical sedimentary structures. Depositfeeder trace fossils, such as Asterosoma isp. B and Planolites beverleyensis, are dominant components of these assemblages.
The subtidal sandbar and channel deposits display planar and trough cross bedding. The observed ichnodiversity is low to moderate and monospecific assemblages are common. Macaronichnus segregatis, Ophiomorpha nodosa and Rosselia socialis dominate this ichnofauna.
Overall, these characteristics (e.g. low to moderate ichnodiversity, monospecific associations, generally small sizes, presence of an impoverished Cruziana- Skolithos ichnofacies) suggest a stressful environment, affected by salinity fluctuations. The Chenque estuarine ichnofauna is similar to other brackish-water estuarine ichnofaunas described from the fossil record (e.g. Pemberton and Wightman, 1992; Mac- Eachern and Pemberton, 1994; Mángano and Buatois, 2004b; Buatois et al., 2005).

Tide-dominated deltaic environments

Tide-dominated deltaic deposits occur in outcrops from the Caleta Olivia locality. Two main subenvironments are recognized: prodelta and deltafront, stacked forming a progradational coarseningupward succession. The prodelta is characterized by lenticular and wavy-bedded heterolithic strata, with abundant synaeresis cracks. These deposits typically display low to moderate bioturbation intensities, although a high degree of bioturbation occurs locally. The trace fossil assemblage is dominated by deposit feeder structures, such as Planolites beverleyensis, Teichichnus rectus and Phycosiphon incertum; subordinate and rare elements include Asterosoma isp. B, Nereites missouriensis, Rosselia socialis, Schaubcylindrichnus freyi, and Thalassinoides isp. This assemblage is considered a stressed expression of the archetypal Cruziana ichnofacies.
The delta-front succession shows two main facies representing distal to proximal deposits. The distal delta-front facies consists of flaser-bedded, heterolithic muddy sandstone, almost completely obliterated by equilibrium trace fossils of Atrina.
Subordinately, Teichichnus rectus, Thalassinoides isp. and Schaubcylindrichnus freyi are also present. Proximal delta-front facies are sand-dominated with thin siltstone interbeds. The trace fossil suite is dominated by large Rosselia socialis and Macaronichnus segregatis in the sandier beds, whereas Nereites missouriensis and Protovirgularia isp. are commonly associated to the mud drapes blanketing the sandstone foresets. The intensity of bioturbation is commonly low, although some intervals may show relatively higher values. The trace fossil suite found in the delta-front facies corresponds to an impoverished expression of the proximal Cruziana ichnofacies.
The described ichnofaunas show features typical of deltaic environments (MacEachern et al., 2005). These include alternation of unburrowed and bioturbated intervals, juxtaposition of stressed and relatively diverse suites, opportunistic colonization of substrates and suppression of the Skolithos ichnofacies.

Firmground surfaces

Firm substrates with development of the Glossifungites ichnofacies are recognized at Astra, West of Solano Bay and Infiernillo. At these localities the Glossifungites ichnofacies develops on the boundary surface between the Sarmiento Formation and the Chenque Formation and contains Gastrochaenolites ornatus and Thalassinoides isp. (Carmona et al., 2006). This suite occurs in a co-planar surface that results from amalgamated lowstand and transgressive erosion. Firm substrates containing the Glossifungites ichnofacies also occur in the lower interval of the Playa Alsina section and in the upper part of the Punta Delgada section. Thalassinoides suevicus, Siphonichnus eccaensis and Spongeliomorpha isp. are present in Playa Alsina, while Thalassinoides suevicus, Balanoglossites isp. and Rhizocorallium isp. dominate in the Punta Delgada firmground. These two surfaces correspond to transgressive surfaces of erosion. At both sections, the surfaces are overlain by conglomeratic lags and the trace fossil suites show dominance of suspension feeder structures. However, at the Playa Alsina section, the presence of Siphonichnus eccaensis (a deposit feeding structure) may suggest emplacement of the discontinuity surface in a more distal setting than that recorded in the Punta Delgada section (see MacEachern and Burton, 2000, and Pemberton et al., 2004).
Finally, a third example of the Glossifungites ichnofacies is recorded in the upper interval of the Playa Las Cuevas section, where post-omission Thalassinoides isp. penetrate deeply into the upper interval of the lower shoreface deposits, from the overlying regressive surface of marine erosion (Buatois et al., 2003a). This surface probably records rapid shallowing during a forced regression.

The Chenque ichnofauna and the Modern Evolutionary Fauna

Secular changes in bioturbation through the Phanerozoic, based on comparative ichnologic analysis are receiving increasing attention in recent years (e.g. Thayer, 1983; Uchman, 2004; Buatois et al., 2005). Comparisons of the ichnofauna from the Chenque Formation with other Mesozoic and Cenozoic shallow- marine trace fossil assemblages reveal some similarities, mainly in trace fossil composition, although differences in tiering structure and partitioning of the infaunal niche are evident also. Cretaceous siliciclastic shallow-marine ichnofaunas are well known and have been analyzed in detail in a number of studies (Howard and Frey, 1984; Frey and Howard, 1990; MacEachern and Pemberton, 1992). Some characteristics and patterns identified in these studies (e.g. the composition of archetypal Cruziana ichnofacies in the lower shoreface settings) are similar to those recognized in the Chenque Formation.
The ichnology of Cenozoic shallow-marine deposits is comparatively less documented (especially for the Paleogene). However, a number of studies serve for comparison (e.g. Ting et al., 1991; Pickerill et al., 1996; Uchman and Krenmayr, 2004; Uchman and Gazdzicki, 2006). Early Eocene, shallow-marine ichnofaunas are known from the La Meseta Formation, Seymour (Marambio) Island, Antarctica (Uchman and Gazdzicki, 2006). These authors documented an ichnoassemblage consisting of Diplocraterion, Lockeia, Polykladichnus, Teichichnus, Scolicia, Ophiomorpha, Parataenidium, Protovirgularia, Rhizocorallium, Skolithos and Taenidium; and considered that these deposits accumulated in a foreshore-offshore environment. In a previous study of this ichnofauna, Porebski (1995) suggested that the low ichnodiversity observed could be indicating lowered salinity conditions. Nevertheless, Uchman and Gazdzicki (2006) consid- ered that although fluctuations in salinity were possible, the presence of trace fossils produced by stenohaline organisms (e.g. Scolicia) indicates that the salinity was normal, at least locally. In addition, these authors mentioned that the ichnodiversity recorded in the La Meseta Formation is not different from that documented in other Cenozoic shallowmarine formations. However, the present study shows a higher ichnodiversity for the Lower Miocene, fully-marine deposits of the Chenque Formation. Uchman and Krenmayr (2004) analyzed Lower Miocene ichnofaunas from Austria that are compositionally very similar to those from the Chenque Formation.
These authors described some characteristics that were also identified in the Chenque Formation (e.g. presence of isolated Rosselia specimens in highenergy, tide-dominated deposits, identification of Macaronichnus- or Teichichnus-monospecific ichnofabrics; occurrence of Macaronichnus in thin sandstone beds of mud dominated successions).
Furthermore, these authors identified the presence of Macaronichnus-Ophiomorpha ichnofabrics in relation to subtidal sandbar facies. This ichnofabric has been previously described from Jurassic and Eocene deposits of NW Europe by Pollard et al. (1993).
However, the tiering structure in some open-marine facies from the Chenque Formation (e.g. in Playa Las Cuevas and Punta Delgada; see Buatois et al., 2003a, b) reveals a greater complexity than that recorded by the Austrian ichnofaunas. Notably, the tiering structure of the Austrian ichnofaunas closely resembles that of the tide-dominated, deltaic ichnofaunas from the Chenque Formation (e.g. Caleta Olivia), which is consistent with the abundance of tidal indicators in the example documented by Uchman and Krenmayr (2004).
Studies in Middle Miocene to Late Pliocene deposits from Taiwan (Ting et al., 1991) have also shown similarities in trace fossil composition with the Chenque Formation. In particular, lithofacies C from Taiwan is similar to the lower shoreface facies studied in the Playa Las Cuevas, Rada Tilly and Punta Delgada localities. Lithofacies C is characterized by a high degree of bioturbation, with complete homogenization of the beds, and dominance of Zoophycos isp., Scolicia isp., Thalassinoides paradoxicus and Ophiomorpha nodosa. However, the study from Taiwan does not provide information about tiering structure, and therefore, it cannot be further compared to the ichnofauna from the Chenque Formation. Pickerill et al. (1996) studied the ichnologic content of the Pliocene Bowden Formation from Jamaica and recognized the dominance of deposit-feeder structures (e.g. Chondrites isp., Planolites isp., Phycosiphon incertum, Teichichnus rectus). This ichnofauna characterizes low-energy and low sedimentation rates in a relatively deep-water environment (Pickerill et al., 1996). Comparison between this Pliocene ichnofauna and that from the Chenque Formation suggests that the former is less diverse and has a more simple tiering structure, and indicates that the Bowden ichnofauna may illustrate a transition between a distal Cruziana ichnofacies and a Zoophycos ichnofacies.
Modern ichnofaunas seem to have been well established in shallow-marine, open environments since the Mesozoic. This is particularly well exemplified in the Chenque Formation, where finely tuned climax communities display vertical niche partitioning and a remarkable use of the infaunal ecospace. For example, in the lower shoreface deposits analyzed, up to six ichnoguilds and nine tiers have been recognized (Buatois et al., 2003b).
This tiered ichnocoenosis includes vagile, depositfeeder structures that produce a mottled texture in the shallowest tiers, a Thalassinoides-Asterosoma- Rosselia ichnoguild that includes semi-vagile, deposit- feeder traces in the shallow tiers, a Schaubcylindrichnus-Palaeophycus ichnoguild consisting of vagile, suspension- and deposit-feeder structures in the middle tiers, a Scolicia- Phycosiphon-Helicodromites-Teichichnus-Taenidium ichnoguild comprising vagile, deposit-feeder structures in the middle tiers, a Thalassinoides ichnoguild that consists of stationary, deposit-feeder structures in the deep-tiers, and a Chondrites ichnoguild that includes non-vagile, deposit-feeder or chemosymbiont structures in the deepest tiers. This complex tiering structure reflects a higher partitioning of the infaunal niche and represents a departure with respect to Mesozoic and Paleogene ichnofaunas in siliciclastic settings, being only equivalent to the tiering structure documented for Cretaceous chalk of northern Europe (Ekdale and Bromley, 1984) and southern United States (Frey and Bromley, 1985). During the Mesozoic, the development of the Modern Evolutionary Fauna led to important ecological changes in marine communities (Sepkoski, 1990). Some of these changes involved the acquisition of additional ecologic guilds that were not present in the Cambrian and Paleozoic Evolutionary Faunas, particularly with respect to the exploitation of the deep infaunal ecospace (Thayer, 1983; Bambach, 1983; Sepkoski, 1990). The complex tiering structure found in the Chenque Formation shows the development of a finely partitioned infaunal niche and an increment in bioturbation by the Neogene. This is consistent with trends revealed by body fossils, which show that by the late Cenozoic, marine paleocommunities have a much greater representation of infaunal organisms and higher proportion of motile animals than mid- Paleozoic communities (Bush et al., 2007).

Paleoclimatic significance

Recent research is starting to explore the importance of latitudinal and climatic controls on shallowmarine ichnofaunas (e.g. Aitken et al., 1988; Mángano et al., 2002; Goldring et al., 2004; Pemberton et al., 2006; Gingras et al., 2006). Aberhan et al. (2006) noted that the proliferation of deep burrowers seems to be most evident in mid and high latitudes. Bush et al. (2007) also recognized a greater proportion of deep infauna in temperate Cenozoic paleocommunities than in tropical Cenozoic assemblages. Interestingly, the mid-latitude Chenque ichnofauna reveals a dominance of deep burrowers when compared with the ichnologic data available for Cenozoic low-latitude shallow-marine settings, further supporting the pattern reflected by the body fossil record. Goldring et al. (2004) suggested that Cenozoic and Pleistocene occurrences of echinoid and crustacean burrows present a climatic control and recognized three latitudinal zones for shallow-marine settings: a tropical and subtropical zone (with occurrences of pellet-forming thalassinideans and burrowing echinoids), a temperate zone (with echinoid burrows and Thalassinoides, but not Ophiomorpha), and an arctic zone (where neither group is present).
The lower Miocene Chenque Formation was formed in mid latitudes and its body fossil content suggests temperate to cold climates. Both Ophiomorpha and Scolicia are abundant in the lower shoreface deposits, these occurrences being anomalous to the latitudinal division suggested by Goldring et al. (2004). Consequently, further expansion of the Cenozoic ichnologic database is required in order to discriminate between latitudinal and evolutionary controls.


Valuable exchange with participants of the Ichnia 2004 field trip is greatly acknowledged, particularly J. de Gibert, R. Netto, D. Knaust, A. Uchman and A. Wetzel. B. Aguirre-Urreta, E. Olivero, J.J. Ponce, D. Lazo, M.I. López Cabrera, M. Nara, L. Löwemark, D. Martinioni and N. Pearson provided useful feedback. J.J. Ponce, M.S. Alvarez, E. Bellosi and J. Paredes helped in the field. Alvar Sobral is thanked for preparation of thin sections and polished slabs. We thank A. Uchman and M. Verde for their detailed reviews and suggestions. Financial support for this study was provided by the Antorchas Foundation, a Doctoral Grant from the Argentinean Research Council (CONICET) and an International Association of Sedimentologists Postgraduate Grant Scheme to Carmona, PIP-CONICET 5100, Natural Sciences and Engineering Research Council (NSERC) Discovery Grants 311727-05 and 311726-05 awarded to Mángano and Buatois, respectively, and University of Saskatchewan Start-up funds to Buatois.


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Recibido: 3 de octubre de 2006.
Aceptado: 27 de noviembre de 2007.

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