Resumo

SAGASTI, Guillermina  e  POIRE, Daniel Gustavo. Asociaciones icnolitológicas de la porción basal de la Formación Agrio, arroyo Loncoche, provincia de Mendoza. Rev. Asoc. Argent. Sedimentol. [online]. 1998, vol.5, n.2, pp.105-118. ISSN 1853-6360.

Sediments comprising the Agrio Formation (Weaver, 1931) were deposited in the Neuquén Basin (Argentina) during the late Tithonian - early Barremian cycle embracing the Mendoza Group (Stipanicic et al. 1968). This formation extends throughout the Neuquén Basin, and it is characterised by different facies associations. In the Neuquén Province the Agrio sediments are siliciclastic and skeletal sandstones, sandy limestones, micritic limestones and carbonate shales. In the Mendoza Province they are mainly shales, marlstones, mudstones and wackestones; bituminous shale is the most common lithology (Mombrú et al., 1978). The study area is located in the south of Mendoza Province, about 18 km southwest of Malargüe. A detailed section through the Agrio Formation was measured along the Loncoche creek (Fig. 1). Ammonites present throughout the sequence demonstrate its late Valanginian to late Hauterivian age. At this locality, the Agrio Formation overlies oyster bank facies of the Chachao Formation, and it is succeeded by shallow marine limestones of the Chorreado Member (Huitrín Formation). The complete sedimentary succession (222 m thick) forms a monotonous alternation of shales, marls and micritic limestones (mudstones and wackestones) deposited in the outer zone of a carbonate ramp. Because sea level oscillations do not exert direct control on outer ramp sedimentation (Haq, 1991), definition of depositional sequences is difficult in this area. However, based on facies stacking pattern, faunal content and bioturbation it is possible to distinguish different stages within the eustatic cycle. We employ these variables to discriminate these in the basal interval of the Agrio Formation. Sedimentary facies of the Agrio Formation identified in this study are laminated shales, marlstones, mudstones, massive wackestones and cross stratified wackestones. Bioturbation is a distinctive feature and Thalassinoides, Planolites, Rhizocorallium, Palaeophycus, Teichichnus and Scolicia occur throughout the section. References to ichnofacies, trace fossil assemblages, ichnocenosis, ichnofabric and trace fossil suites are common in the ichnological literature, but no constant relationship between trace fossils, lithology and sedimentary structures has been established in the definition of these terms. The term ichnofacies (Seilacher, 1964; 1967a) refers to an association of trace fossils developed under specific energy conditions (referred originally to bathymetric control). However, the use of ichnofacies refers not only to the energy, but also to the nature of the substrate, oxygenation levels, and feeding resources. In many cases this term is useful in defining recurring facies on the basis of their contained trace fossil assemblages. The term trace fossil assemblage embraces a group of trace fossils corresponding to a single benthic community, or they may represent several overprinted bioturbation events (cf. Bromley, 1990). Dörjes & Hertweck (1975) defined ichnocoenosis as covering an assemblage of traces produced in one definite environment by members of a single biocoenose. Ichnofabric embraces all aspects of the nature and internal structure of sediment resulting from bioturbation and bioerosion at all scales (Bromley & Ekdale, 1986; Bromley, 1990). Finally, in many cases, an assemblage of trace fossils can be seen to comprise different subgroups that appear to represent the work of separate endobenthic communities. Bromley (1975) refers to these subgroups as suites; for example, preomission suite, omission suite, prelithification suite, etc. Here we introduce the concept of an ichnolithological association, and we define it as the "genetically and spatially related trace fossil and sedimentary facies". This expression is intended to summarise combined lithological and ichnological features and to integrate their sedimentological interpretation. It is important to differentiate an ichnolithological association from the facies. A facies is a body of rock with specific characteristics. It may be a single bed, or a group of multiple beds. Ideally, it should be a distinctive rock that formed under certain conditions of sedimentation, reflecting a particular process, set of conditions, or environment. Facies can be defined on the basis of colour, bedding, composition, texture, fossils and sedimentary structures (Reading & Levell, 1996). Although facies could embrace ichnolithological associations, we consider it more useful to separate the use of these terms. This is because ichnogenera are rarely considered when rock facies are defined, and only the adjective bioturbated is employed to denote the presence of trace fossils. In the basal succession of the Agrio Formation we recognise three ichnolithological associations (Fig. 4): 1- Massive wackestones with coarse-filled Thalassinoides (Fig. 4A). The ichnofossils occur within limestone beds (endichnia).  Their matrix is micritic, while abundant foraminifers and ostracods comprise the filling. A thin lamina of the infilling material typically covers the bioturbated layer. Typical diameters of the tubes and galleries are between 1 and 3 cm (Fig. 5). Interpretation: Thalassinoides producers bioturbated a carbonate substrate under moderate energy and well oxygenated conditions (cf. Savrda & Bottjer, 1986). These organisms were forced to leave their dwellings because of sea-level rise, and passive fill of the structures then took place. However, the absence of other trace fossils makes it impossible to determine the nature of the substrate (firmground versus softground). One criterion to differentiate between burrowing and boring behaviour is whether the excavator cuts through, rather than remove or avoid individual sediment grains. But such a distinction is difficult to make in clays or micrites, unless shells or lithoclasts are present (cf. Frey & Pemberton, 1984). 2- Mudstones with Thalassinoides  filled by mud (Fig. 4B). Thalassinoides appear in both marlstone and limestone dominated intervals. Trace fossils are endichnia. Tubes and galleries are filled by micritic mud, and typical diameters are between 1 and 2 cm (Fig. 6). Small Planolites (2 mm diameter) and Rhizocorallium accompany Thalassinoides. Interpretation: As in Association 1, this group represents Thalassinoides producers. However, the micritic nature of both the facies and the infill suggest deeper and lower energy conditions of sedimentation. 3- Cross-stratified wackestones with Palaeophycus and Teichichnus (Fig. 4C). Large diameter specimens (up to 2 cm) of these two ichnogenera are present in the base (hypichnia) of amalgamated cross-stratified wackestones (Fig. 7). Tube infill texture is similar to the limestone matrix texture. Teichichnus clearly displays its internal structure. Small Scolicia  (0.5 cm diameter) are occasionally present in some interbedded marlstones. Interpretation: trace fossils of this association represent large excavations developed under well oxygenated conditions (Savrda & Bottjer, 1986), preserved at the base of small subtidal bars (0.40 m maximum thickness and up to 5 m in lateral extent). This association appears when tidal influence becomes a factor. Four intervals have been recognised through the basal succession of the Agrio Formation (77.6 m thick): 1. Bioturbated massive wackestones comprise the first unit. Foraminifers and ostracods are abundant, accompanied by sponge spicules, bivalves, gastropods and ammonites. Shales occur between the limestone beds, also containing foraminifers, bivalves and ammonites. A very rich foraminifer bed denotes the top of the interval, below which there is a bioturbated limestone bed with coarse infill Thalassinoides (ichnolithological association 1). This interval was deposited under moderate energy and well oxygenated conditions on the proximal outer ramp. It represents the early phase of a transgressive systems tract. 2. Unbioturbated shales with a few skeletal remains of foraminifers, bivalves and ammonite impressions dominate the second unit (13.70 m thick). It was deposited on the distal outer ramp during  flooding of the basin. 3. The third unit (34.40 m thick) begins with shale - mudstone alternations. Sponge spicules dominate the faunal content. Bioturbation is poor, represented by ichnolithological association 2. Towards the top of the sequence marlstones replace the shales, and the faunal content and diversity increase. Bioturbation also increases and ichnolithological association 2 becomes very common. This interval was deposited during an early highstand systems tract, in a shallower environment by comparison with unit 2, between the distal and proximal outer ramp. 4. Mudstones, massive wackestones and cross-stratified wackestones with thin interbedded marlstones comprise the last unit (26.50 m thick). Ichnolithological associations 1 and 2 intercalated in the lower bed denote a transitional passage from the distal to the proximal outer ramp. In this interval minor oscillations of sea level determine one or the other situations. In the uper beds, Teichichnus and Palaeophycus are preserved in the base of cross-stratified wackestones (ichnolithological association 3). Because of the absence of skeletal-oolitic grainstone and packstone facies, it is difficult to determine whether this sequence was deposited in an inner ramp environment. An alternative model derived from siliciclastic platforms (see Johnson & Baldwin, 1996) would interpret these deposits as small-scale offshore tidal bars developed on the proximal outer ramp. Within the eustatic cycle this part of the sequence represents a later highstand systems tract. The complete section constitutes a single depositional sequence. The thickness (77.6 m) and interval of deposition of the sequence (approximately 1.5 ma) suggest that it corresponds to a third order cycle (Vail et al., 1977). The lower boundary is at the base of the Agrio Formation, and the upper boundary is above the last unit, below the next marlstone - mudstone alternating sequence. The transgressive systems tract is characterised by ichnolithological association 1 deposited in a proximal outer ramp environment. Deeper marine deposits with scarce fauna and without bioturbation correspond to the distal outer ramp. This sequence developed over the transgressive unit is attributed to the maximum flooding zone The highstand systems tract shows two stages of development: the first corresponds with a deep, well oxygenated distal to proximal outer ramp environment, where shales, marlstones and mudstones of ichnolithological association 2 were deposited. The second marks the timing of highest carbonate productivity. It begins with interbedding of the first and second ichnolithological associations, representing a transition from the distal to the proximal outer ramp, and ends with ichnolithogical association 3, denoting periods of tidal influence on the proximal outer ramp.

Palavras-chave : Carbonate deposits; Outer ramp; Ichnolitological associations; Depositional sequences; Lower Cretaceous; Neuquén Basin.

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