548


Journal of Paleontology 92(4):546–567


Figure 2. Gran Bajo San Julián localities. (1) Barda Blanca locality showing carbonaceous pelitic levels (arrow); (2) wood “in situ” from Barda Blanca locality; (3) Cerro Conito locality; (4) wood “in situ” from Cerro Conito locality.


the first time from the Gran Bajo de San Julián sector (northern part of Estancia Meseta Chica, Barda Blanca, Cerro Conito, Laguna La Guadalosa, and the south-east border of Laguna del Carbón) and the central and south-western sector (Bajo El Puma), collected from the Middle Jurassic of the La Matilde Formation in Argentina.


In this contribution, Agathoxylon species are described for Geological setting


The La Matilde Formation is part of the Bahía Laura Group and is widely distributed in the province of Santa Cruz (De Barrios et al., 1999, fig. 28, p. 513) (Fig. 1). The specimens analyzed come from the Gran Bajo San Julián, central, and south-western areas (Fig. 1.2). In the Gran Bajo San Julián sector, the La Matilde Formation comprises ~70–80m of tuffs, sometimes sandy tuffites, lithic tuffs (tufolites), and chonites of greatly varied coloration, generally in tabular strata; also present are very subordinate amounts of fine-grained conglomerates. There are also carbonaceous pelitic levels, up to true coals,


which are generally located near the base of the sequence cropping out in the Gran Bajo de San Julián sector, particularly in the Laguna del Carbón and Barda Blanca localities (Figs. 1.3, 2). Woods and some other fossil conchostracans, pelecypods, insects, and anurans (Stipanicic and Reig, 1957; Gallego, 1994;


Morton and Herbst, 2001; Baez and Nicoli, 2003) are associated in these levels. The rest of the abundant wood is distributed throughout the formation. The base of the Bahía Laura Group is either the Roca Blanca Formation (Lower Jurassic) or the Bajo Pobre Formation (probably Aalenian–Bajocian) (Panza and Irigoyen, 1995). The top is constituted by a series of brown clays (San Julián Formation), which, because they contain a fossil flora of angiosperm leaves, would be assigned to the Cenozoic (De Giusto, 1955; Bertels, 1977; De Giusto et al., 1980).


The age of the Bahía Laura Group is still subject to some


dispute. In modern times, by means of radiometric dating, dif- ferent authors have offered different age data, but all coincide between 157 Ma and 162 Ma±5–10 Myr; this means the La Matilde Formation can be assigned to the Bathonian interval up to the Callovian (Spalletti et al., 1982; De Barrios, 1993; Echeveste et al., 2001). Paleoenvironmentally, the La Matilde Formation sedi-


ments indicate that it is a continental sequence, characteristic, in part, of a low-energy fluvial environment; in the flood plains, some bodies of water (lagoons) formed in some areas that partly came to be of marsh conditions (lentic and reducing). In the region, an intense volcanism developed, producing the exten- sively distributed ash and other pyroclastic products that con- stitute the bulk of the sediments (Mazzoni et al., 1981; De Barrios et al., 1999).


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