Jurassic conifer woods from the Lajas Formation (Neuquén Basin), Argentina. Anatomical, systematic and palaeoecological inferences

Abstract

The Neuquén Basin is located on the eastern side of the Andes in Argentina and Chile, between 32° and 40°S latitude. It comprises an almost continuous record from Late Triassic to Early Cenozoic stratigraphy. The Bajocian ? Callovian Lajas Formation is a 600 m thick unit composed of several unconformity-bounded tide-dominated sequences. Sequence boundaries are marked by stacked fluvial channel fills developed on widespread incision surfaces. Though fossil plants, especially those bearing fossil woods, are scarce in the Lajas Formation, trunks from three localities (Rincón del Águila, Paraje los Molles and Cerro Lotena) were recovered and allowed anatomic and systematic studies. All samples collected come from large logs, with a length between 1 and 13 m. To study fossil woods, petrographic slides in three sections (cross, radial and tangential) were made, and analyzed under light microscopy and SEM. The fossils were assigned to three conifer genera: Podocarpoxylon Gothan, Agathoxylon Hartig, and Cupressinoxylon Goppert. In order to obtain palaeoecological data, the growth rings were analyzed, and combined with sedimentological studies. All fossil woods have growth rings, and in most cases marked only by one or two rows of latewood; this condition suggests seasonality. The mean sensibility index was employed to obtain palaeoecological data, with values between 0.31 and 0.70, indicating that they were ?sensitive? to local climate conditions. All these data suggest that trees grew under several fluctuations of environmental conditions. The preliminary conclusions propose that conifers were the most diverse large trees in the studied localities, with taxa related to extant families (e.g. Araucariaceae, Podocarpaceae, and Cupressaceae s.l.). The abundance of large and well preserved fossil wood could suggest that those plants numerically dominated the forest communities of the Neuquén Basin during the Middle Jurassic.