Resumo

MATHEOS, Sergio D; BREA, Mariana; GAMUZA, Daniel  e  ZAMUNER, Alba. Sedimentology and paleoecology of the Lower Tertiary in Southern Chubut province, República Argentina. Rev. Asoc. Argent. Sedimentol. [online]. 2001, vol.8, n.1, pp.93-104. ISSN 1853-6360.

This contribution investigates sedimentological and paleobotanical aspects of Szlápelis and Ormachea localities (Chubut province). A lot of big petrified trunks have been found associated with conglomeradic levels of the Salamanca Formation (Danian). The two fossiliferous localities are located to the southward of the Musters and Colhué Huapi lakes between 69° 04´ - 69° 18´ W and 45° 48´ - 45° 57´ S, at SE of Sierra de San Bernardo (Fig. 1). The methodology has consisted of an integrated-detailed profile (scale 1: 100), sampling of sedimentites and measures of diameter, orientation, fragmentation, authoctony grade and quality of preservation of fossil woods. These data were used to estimate diameter, circumference, basal area, cover, estimated biomass and height, in order to reconstruct paleocommunities and paleoecosystems. The sedimentary sequence begins with reddish-brown to greyish pyroclastic sedimentites of the Chubut Group and passed unconformably overlain by the Salamanca Formation (Lesta y Ferello, 1972), in turn followed by the Río Chico Formation of Upper Paleocene age (Feruglio, 1929). Sedimentation is represented by shales, tobaceous shales and yellowish to greenish fine to sabulitic sandstone with conglomeradic beds at their upper yielding silicified wood logs. Cross-bedded tabular to simple tangential stratification are well developed. All the sedimentation is disposed subhorizontally, massively in structures, with intercalation of sandy massive strata normally graduated. The paleoflora described in this unit includes palynomorphs, fossil wood and leaf impression and compressions of leaves. The wood flora found in Szlápelis and Ormachea localities is composed of a large number of petrified trunks of parautocthonous gymnosperms and angiosperms, with a preferential orientation between 45°- 55° SW; only few of them have less than 50 cm in diameter and someone represents stumps. Generally they are compressed and hollow at the core part; occasionally they show signs of biological activity. Diameter oscillate between 27 to 110 cm (59.7 cm in average), the average basal area is 3146.61 cm ² , average circumference is 185.98 cm, exposed length is 9 m and an estimated biomass of 834.04 kg/tree (Brown et al., 1991); they also have values of 401.36 tn/ha of fresh weight (WF) and 179,02 tn/ha of dry weight (WD) (sensu Mosbrugger et al., 1994) (Table 1, Fig. 5). A great number of trees have diameters of 40-80 cm (61.50%), 4.7% of them exceed 100 cm and less than 20% are below 40 cm in diameter. The presence of trees with more than 1m of width suggests an exuberant growth and very good environmental conditions for development. Respect to the classes of diametric distribution, woods can be classified in coetaneous and multietaneous and a degree of types between them (Donoso, 1993). The diametric distribution in these localities shows a two or more tree strata woodland structure. These types of paleocommunities could be arisen because of the aperture of the canopy, from a downfall of old trees, so these fossil communities grow as a multietaneous wood; here the crowns are irregular and different in shapes, with individuals of different classes - this implies permanent regeneration- (Donoso, 1993). As the woodland becomes older, there are a tendency to develop a bimodal distribution due to the arising of subpopulations of big trees and small trees (Fig. 5). Biomass can be measured from the diameter of trunks (Murray, 1927; Mosbrugger et al., 1994; Calow, 1998), and this value is related with the type of woodland (Brown y Lugo, 1984; Brown et al., 1991) and life zones defined by Holdridge (1979). Taking into account estimated biomass value: WD (Table 1) of the studied lignoflora, these paleocommunities could be defined as a tropical-subtropical closed forest. The frequency distribution of estimated biomass (Fig. 6) indicates that this forest could have developed in humid life zones (Brown et al., 1991). Based on Whitmore's (1993) classification of wet tropical forests this fossiliferous association probably conformed a humid, semi-evergreen forest with a dry seasonality and little annual differences in the soil water disposability. The structure of the vegetation and the wood ring analysis of fossil woods indicate that these associations developed under warm temperate and humid climates, with a dry summer and a wet spring time (Brea et al., 2000) forming a mature, stratified, closed, mixed and humid forest. The finding of logs with more than a meter in width and the sedimentation suggest that these Paleocene forests were developed in a warm temperate, humid and stable climate in a fluvial to deltaic environment. It could be concluded that: 1) Diametric distribution of fossil woods of the studied paleocommunity shows a wood structure with two or more tree strata. 2) Based of data base from estimated biomass, the paleocommunity could be defined as a tropical-subtropical closed forest. 3) According to Whitmore's (1993) classification of wet tropical forest, the association conforms a humid, semievergreen woodland, with a dry seasonality and little annual differences in soil water availability. 4) Structure of vegetation and wood ring analysis suggests that the association was developed under warm temperate and humid climates, with a dry summer and a wet spring and matches to a closed, stratified, humid, mixed and mature forest. 5) Associated sedimentation to the lignoflora suggests that these Paleocene forests were developed in a stable environment related with a meander fluvial to deltaic characteristics.

Palavras-chave : Tertiary; Fluvial environment; Chubut province; Paleoecology.