- Miospores and chlorococcalean algae from the Los Rastros Formation, Middle to Upper Triassic of central-western Argentina
Fieldwork at the Río Gualo section indicated five lacustrine-deltaic cycles each with two facies assemblages.
Lake facies assemblage
This assemblage is dominated by dark gray to black carbonaceous claystones. Iron mudstones and very fine-grained sandstones are interbedded. The beds are tabular. The clayshales are horizontally laminated, whereas the ironstones may be massive or laminated. Conchostracans, plant debris, insect impressions and rare fish body fossils are found in this assemblage, but with different preservation conditions in the different facies. The clayshales were deposited by suspension in distal-lake conditions whereas the ironstones were deposited by distal turbiditic currents (Mancuso, 2003).
Deltaic facies assemblage
This assemblage includes stream load sediments. The assemblage begins with alternative green siltstones and gray claystones, which form rhythmic packages of massive individual beds containing plant debris. Fine- to medium-grained sandstone beds are intercalated, and are more abundant and thicker upward. Small ripple cross-laminations characterize these sandstones, which also contain plant debris. Medium- to coarse-grained sandstones are dominant upsection. They begin with horizontal lamination, passing to planar cross-bedded and culminate with trough cross-bedded lamination. The sequence coarsens and thickens upwards. Fossil fish bodies and poorly preserved wood fragments occur in the horizontally laminated sandstones while only fossil wood remains were found in the planar and trough cross-bedded sandstones (Mancuso, 2003). In its top, the facies assemblage features dark gray mudstones and very fine-grained sandstones interbedded with planar and trough cross-bedded sandstones. They are internally massive or pass upward from plane to rippled-lamination, and contain a very high percentage of organic matter, mainly carbonized fragments. This facies assemblage represents the progradation of the fluvial system into the lake body (Mancuso, 2003).
Palynological results and discussion
Kerogen is generally abundant and it is composed by a variable amount of wood, cuticle, amorphous organic matter and terrestrial palynomorphs. Fiftyeight species of miospores and three species of Chlorococcalean algae have been recorded at the Río Gualo section (figure 3). Palynomorphs are fairly well preserved, with the exception of those found in samples 1434, 1441 and 1442, which are commonly crushed or corroded.
The palynoflora of the Los Rastros Formation at Río Gualo section is dominated by disaccate pollen grains of corystosperm affinity, especially Alisporites australis de Jersey. Also abundant are inaperturate pollen (mainly Inaperturopollenites reidi de Jersey, a probable Araucariaceae) and diploxylonoid disaccate pollen grains ( Platysaccus spp., Podocarpaceae?), together with spores of pteridophytes ( Cadargasporites spp., Clavatisporites conspicuus Playford, Converrucosisporites cameronii (de Jersey) Playford and Dettmann, Osmundacidites spp., Polypodiisporites ipsviciensis (de Jersey) Playford and Dettmann, Rugulatisporites spp.), lycophytes (mostly Aratrisporites compositus Volkheimer and Zavattieri, of pleuromeian affinity) and sphenophytes ( Calamospora impexa Playford). Ginkgo biloba -like monocolpates ( Cycadopites spp.), monocolpates with alveolar ektexine ( Brachysaccus sp. A), monosaccates ( Variapollenites spp., Coniferales), striate pollen grains ( Lueckisporites spp., Lunatisporites spp., Protohaploxypinus spp., Striatopodocarpites pantii (Jansonius) Balme), ephedroid pollen grains ( Equisetosporites cacheutensis Jain) and Cedrus -like pollen grains ( Cedripites spp.) are also present. Phytoplankton is represented by Botryococcus sp., Plaesiodictyon mosellanum ssp. Perforatum Wille and Plaesiodictyon mosellanum ssp. Symmetricum Brenner and Foster.
Comparison of the Río Gualo fossil palynoflora with local fossil macroflora is hindered by the fact that only scarce and generally unidentifiable plant macrofossil remains are present. However, plant macrofossil assemblages from other localities of the basin has strong affinities with the Río Gualo microflora. The commonest elements from the Los Rastros Formation are the Corystospermales (9 species), together with a consistent representation of Sphenophyta (6 species) and, in decreasing importance, Osmundaceae (3 species), Peltaspermales, Cycadales, Voltziaceae and Czekanowskiales (Zamuner et al ., 2001).
The palynoflora of the Río Gualo section resembles those Late Triassic assemblages grouped into the cool temperate ?Ipswich microflora? (Dolby and Balme, 1976). This microfloral province has been recognized in the south of Australia, New Zealand, Antarctica (Victoria Land), South Africa and Argentina (Zavattieri and Batten, 1996), and is believed to represent plant communities characteristic of high latitudes (40º-70º S lat.).
The palynoflora of the Río Gualo section is, in general, comparable with those described from other Middle to Upper Triassic sequences of Argentina (Zavattieri and Batten, 1996), and particularly, from the Ischigualasto-Villa Unión Basin (Herbst, 1965, 1970, 1972; Yrigoyen and Stover, 1970; Zavattieri and Melchor, 1999; Zavattieri and Milana, 2000). Botryococcus was previously cited in the Triassic, from the fluvio-lacustrine Las Cabras and Casa de Piedra formations, Cuyo Basin (Zavattieri, 1991b; Barredo et al ., 1999; Rodríguez Amenábar and Ottone, 2002, 2003) while Plaesiodictyon is known from lacustrine levels of the Ischichuca Formation, Ischigualasto-Villa Unión Basin (Zavattieri and Melchor, 1999). The presence of planktonic forms, mainly Botryococcus which has extant representatives of known ecological preferences, facilitate palaeoenvironmental interpretations about the lacustrine system at the Río Gualo area (Batten and Grenfell, 1996; Guy-Ohlson, 1992).
Botryococcus Kützing 1849, is an euryhaline, freshwater chlorococcalean that grows preferently in shallow, oligotrophic lakes or ponds (Tyson, 1995; Rodríguez Amenábar and Ottone, 2003). It is relatively abundant (about 5-10% of total palynomorphs content) in levels BAFC-Pl 1433, 1437, 1444. In samples 1431, 1432, 1435, 1436, 1438, 1439, 1440, 1443 and 1445, Botryococcus occurs in moderate quantities (less than 5% of total palynomorphs content). In levels 1441 and 1442, no Botryococcus has been recorded (figure 2). Structureless mass was found in the whole section (figure 5.K) suggesting that the colonies growth either in a stressed environment or with restricted access to oxygen during deposition (Guy-Ohlson, 1992, 1998; Guy-Ohlson and Lindström, 1994; Rodríguez Amenábar and Ottone, 2003). Level 1433 contains simple, globular colonies (40-70 µm in diameter), together with compound colonies showing branching processes and compound colonies with botryoidal form. In level 1437, there are large, compound colonies, with or without branches, with size range of 80-145 µm; and young simple colonies of about 60 µm in diameter (figures 5.G-H). The variable forms found in samples 1433 and 1437 suggest varying seasonal conditions of environment and/or climate (Guy-Ohlson, 1992, 1998; Guy-Ohlson and Lindström, 1994; Rodríguez Amenábar and Ottone, 2003). In level 1444, numerous large, botryoidal compound colonies are present, suggesting a uniform seasonal growth. Botryococcus is unusually abundant in this sample, making up about 99% of the palynoflora. In general, the occurrences of phytoplankton blooms follow occasional freshening stages in the water body, due to heavy rains or changes in water inflow that increased the availability of dissolved phosphorous in the basin (Warren, 1986).
Plaesiodictyon Wille 1970 is a freshwater form probably related to the Hydrodictyaceae (Wood and Benson, 2000). It is relatively abundant in sample 1431 (about 5% of total palynomorphs content), where is the most consistently represented planktonic form, and is also present, as a minor constituent in samples 1432, 1433 and 1435 (less than 5% of total palynomorphs content) (figure 2, figures 5.E-F, I). The Río Gualo coenobia of Plaesiodictyon lack any dehiscent slit in its cells. This feature, in extant planar coenobia of Hydrodictyaceae, currently reflects a decrease in ability of cell division due to changes in the environment, rising salinity, stressed conditions or competition for nutrients (Brenner and Foster, 1994). Plaesiodictyon is strikingly similar to Pediastrum Meyen 1829, both genera display planar coenobia commonly with holes, and marginal cells usually with lobes and processi. The main difference between this two genera lies in the cell arrangement, which determine the shape of the coenobia. While Pediastrum is circular in outline and has a circular arrangement of cells, Plaesiodictyon displays rectangular coenobia with cells in perpendicular rows (Brenner and Foster, 1994; Komárek and Jankovská, 2001). However, an extant species of Pediastrum , ( P . orbitale Komárek 1983) includes circular and oval coenobia. It is difficult to know, simply by analogy with living Pediastrum , the precise ecological needs of Plaesiodictyon , mostly considering that extant Pediastrum includes many species and subspecies which display different ecological preferences (Komárek and Jankovská, 2001).
Changes in relative abundances of Botryococcus and Pediastrum (Tyson, 1995), or by roughly analogy, Botryococcus and Plaesiodyction , are considered a consequence of ecological changes. Therefore, the variable percentages of these coccal forms throughout the Río Gualo section could indicate that depositional conditions varied through time. The low Botryococcus / Plaesiodictyon ratio at the base of the sequence and its increase towards upsection would presumably reflect changing environmental conditions. Initially, a stretch of freshwater of probably eutrophic conditions would develop, mainly at the level represented by sample 1431, where the eutrophic character appears reinforced by the presence of abundant microscopic, ill preserved filaments of probable algal affinity (A. Godeas, personal communication). This situation would change, through the middle and upper part of the Río Gualo section, to a milieu of oligotrophic conditions.
From a sedimentological viewpoint, each cycle was inferred in the Río Gualo section (figure 2), according to the parasequence criterion (Vail et al ., 1991), as shallowing-upward. Gore (1988) noted that the lakes gradually infilled by sediment from deltaic input and shoreline progradation, produce regressive, coarsening-upward, lacustrine sequences overlain by a fluvial system. This is apparently the case with the Ischigualasto-Villa Unión Basin, wherein each cycle starts with fine sediments deposited in a lacustrine environment and finishes with sandstones of fluvial origin. The palynological analyses indicates that the richest assemblages are those associated with the lacustrine (pelitic) facies. Moreover, the most abundant phytoplankton occurr in the upper distallake facies of each cycle. This facies is characterized by black shales which suggest an anoxic bottom; furthermore, the local absence of grazing trails reinforce this hypothesis. The anoxic conditions might result from stratification of the water column, since the circulation of the oxygen water would be blocked by the stratification, thus preventing the oxygenation of the bottom (Wetzel, 1981). Bottom anoxic conditions are typically produced by a significant lake deepening, however, if there are high organic productivity in saline and shallow waters, the black shales may be accumulated (Gore,1988).
The scarce records of phytoplankton blooms through the section appear to indicate that the lake was probably deficient in dissolved phosphorous (Warren, 1986). The chemical composition of the lake could explain the lack of fish bones in the distal-lake facies.
The conspicuous record of conchostracans through the section indicate the presence of temporary freshwater bodies with development of alternating dry and wet bottom conditions (Vannier et al ., 2003). Adaptations to environmental stress conditions of growth are also reflected in the morphology of the phytoplanctonic forms (structureless mass in Botryococcus , lack of dehiscent slits in Plaesiodictyon ).
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