Paleovegetation studies and growth-ring analysis of a mixed Middle Triassic forest from Argentina.

BREA, M., A. ARTABE Y L.A. SPALLETTI.

Mendoza

Workshop; GONDWANA 12 - "Geological and Biological Heritage of Gondwana":; 2005

situ petrified forests are very scarce in the geological record and their occurrences have contributed significantly to our understanding of Triassic Gondwana ecosystem. This contribution investigates the palaeoecology of an in situ Middle Triassic forest from the Paramillo Formation (= lower section of the Potrerillos Formation) in Agua de la Zorra, northwestern Cuyo Basin, Mendoza province (69° 12' W and 32° 30' S). In 1835, during his historic voyage around the world, Charles Darwin first described these fossil stumps as a monotypic forest of Araucarites. The forest is here reinterpreted based on palaeobotanical, sedimentological and spatial analysis. The in situ fossil forest emerges at three localities: El Sauce, Darwin and Portezuelo. At El Sauce three fossiliferous levels were recognised: 1) FL I with Cladophlebis mesozoica Frenguelli 1947, Cladophlebis mendozaensis (Geinitz) Frenguelli 1947 and Cladophlebis kurtzi Frenguelli 1947; 2) FL III with poor preserved Cladophebis ssp; 3) FL IV with Araucarioxylon protoaraucana Brea 1997 and Cuneumxylon spallettii Artabe and Brea 2003. FL V, at Darwin, consists of A. protoaraucana and C. spallettii. At Portezuelo a further fossiliferous level, FL VI, was found, and contains only A. protoaraucana. The sediments were deposited in a high sinuosity fluvial system, in which channel-filling sand bodies are associated with mud-dominated floodplain deposits. Siliceous permineralised stumps assigned to corystosperms and conifers represent the dominant elements of the large forest. The plant community reconstruction was based on quantitative mapping data (mean separation of trees, basal area per stumps, species distribution), integrated with the taxonomic and sedimentological information. The predicted height of tree fossils was analysed using the equations of Niklas (1993, 1994). Corystosperm wood fossils have a mean stem diameter (D) of 60 cm (30–90 cm), and a mean estimated height (Hest) of  22 m (11–26 m); their mean critical height (Hcrit.) was 68 m (43–89 m). The conifers are comparatively smaller, with D= 36 cm (10–60 cm), Hest= 18 m (10–22 m) and Hcrit= 47 m (21–68 m). The distribution of class diameters has a bimodal curve for the conifers and a normal curve for the corystosperms, suggesting that there was a mature or old-growth corystosperm forest followed by younger conifers. The distribution pattern was continuous with a density of 600–700 trees/ha. There is no spatial autocorrelation for the specific distribution variable, and therefore the species were intermingled. The mixed forest was constituted by 60 % conifers and 40 % corystosperms, developing a c.15–20 m high conifer canopy with emergent corystosperm trees, and an understorey of Cladophlebis ssp. Coniferous wood growth-ring analysis was used to evaluate climatic changes. Twenty-four ring series were obtained, ranging in span from 6–36 rings. The average ring widths were 1.23 mm (0.13–2.92 mm), the thinnest ring was 0.12 mm, and the widest 4.44 mm. A. protoaraucana has narrow growth rings which are slightly demarcated; they are characterised by a relatively wide zone of large, thin-walled earlywood cells terminated by only three or four thick-walled latewood cells. Mean sensitivity (MS) values range between 0.196 and 0.371, with an average of 0.310. Based on the coniferopsid quantitative analysis outlined by Falcon-Lang (2000), the A. protoaraucana growth ring anatomy suggests an evergreen habit. Compared with their nearest living relatives (extant Araucaria araucana), the slightly demarcated fossil conifer growth rings indicate greater leaf longevity and they could not be used as an indicator of growing conditions towards the end of the growing season. The morpho-structural characters present in Cuneumxylon spallettii are also consistent with an evergreen habit. By comparison with extant plants growing in arid regions, the included phloem and associated parenchyma in these stems may be an important adaptive strategy to avoid water stress. The vegetation structure, growth ring analysis and sedimentatary environment suggest that this evergreen forest developed under dry subtropical conditions with strong seasonal changes.