Araucarian woodlands from the Jurassic of Patagonia, taphonomy and paleoecology
Introduction
The study of “petrified forests” has been a matter of interest because the spatial arrangement of trees provides an excellent tool for understanding of paleoecological relationships and structure of past vegetation. In many cases, the use of recent techniques applied to living communities has proven to be effective in the fossil record allowing comparisons between past and modern vegetation arrangements, facilitating a better understanding of the evolution of terrestrial plant ecosystems (Williams et al., 2003). Additionally, several paleoclimatological and paleogeographical implications have emerged from such studies providing a dynamic approach to the study of fossil plants (Pole, 1999; Cúneo et al., 2003).
It is noteworthy that a good number of paleoforest analyses has been directed to studying high paleolatitude examples (Jefferson, 1982; Francis, 1991; Taylor et al., 1992; Basinger et al., 1994; Pole, 1999; Falcon Lang et al., 2001; Cúneo et al., 2003; Williams et al., 2003; Thorn, 2005; among others). This is particularly relevant because present vegetation at high latitudes (above 60°) is uncommon (particularly in the southern land masses), illustrating paleoecological and paleoclimatological shifts in the colonization of land through Earth history. Low/middle latitude studies of fossil forest exist, especially involving Paleozoic examples (Cúneoand Andreis, 1983; Gastaldo, 1986; DiMichele et al., 2007, to summarize a few of them), Mesozoic (McKnight et al., 1990; Keller and Hendrix, 1997; Roberts and Hendrix, 2000; Ash and Creber, 2000; Lehman and Wheeler, 2001; Artabe et al., 2007; Brea et al., 2008; Falaschi et al., 2011) and Cenozoic sites (Mosbruggeret al., 1994). Most of these studies have provided an enormous amount of data that help our understanding of tree communities, in particular lycopods in the Paleozoic, and conifers and to lesser degree angiosperms in the Mesozoic and Cenozoic.
The Araucariaceae is a family of conifers presently distributed only in the southern hemisphere, with most species occurring in the southwest Pacific. Only two species are known in South America (Seward and Ford, 1906). This contrasts with its past distribution when members of the family also lived in the northern hemisphere, and those inhabiting South America were more diverse than today. Presently in South America Araucaria is restricted to SW Brazil (mostly) and NE Argentina where Araucaria angustifolia occurs, and along the Andean ranges of Chile (mostly) and western Argentina with A. araucana. In both areas, human influence has probably distorted in part the original structure of these araucarian stands; however, there are some places where natural relationships probably remain unaltered including the upper Chilean Andean range and the eastern Argentinean slope of the Andes. In the first case, A. araucana represents the uppermost stratum of the forest vegetation (Montaldo, 1974; Veblen, 1982), while on the eastern side this taxon occurs as low-density woodlands (Veblen et al., 1995). This latter distribution probably reflects relicts of earlier forests, that still have some continuous or sporadic regeneration under relatively seasonal xeric conditions (precipitations in the order of 700 mm/year; Burns, 1991).
The fossil case described here refers to one of the classical examples of fossil trees known in the literature: the Jurassic Cerro Cuadrado (among other names) araucarian trees from Patagonia. Original references to this “paleoforest” can be traced back to the early twentieth century when beautifully preserved araucarian seed cones and logs were discovered (Spegazzini, 1924). Following the initial discovery, a series of papers were published that focused on the study of the large amounts of seed cones collected (Wieland, 1935; Calder, 1953). These included cones not only of araucarian affinity (see Stockey, 1982) but also other conifers (Stockey, 1977), although found at a separate locality. However, in spite of the preservation of the spatial arrangement of the trees, perhaps some of the largest known in the fossil record, little research was focused on the taphonomy, structural features and spatial distribution. Preliminary reports by Cúneo (1991) and Cúneo and Panza (2008) offered a first view on the structure and general nature of the “forest”; while Falaschi et al. (2011) provided a detail survey on the architecture of the La Matilde Jurassic “forest” with consideration on the systematic affinities of the trees. The goal of this contribution is to present a quantitative analysis of the araucarian trees preserved in this area of south-central Patagonia, and to reconstruct the stand structure and the taphonomical process responsible of their preservation.
Section snippets
Materials and methods
The area where the fossil trees occur occupies the central portion of the Santa Cruz province in southern Patagonia, geologically known as the Deseado Massif (Fig. 1). Massive volcanic events during the Jurassic formed the Chon-Aike volcanic province (Kay et al., 1989), a region that extends from the Atlantic through the Andes. These events deposited several km3 of mostly acidic lavic and pyroclastic rocks known as the Bahia Laura Group (Pankhurst et al., 1998; Guido, 2004) which is subdivided
Sedimentary geology
Guido (2004) divided the Bahia Laura Group or complex into two main lithofacies: effusive and volcaniclastics, represented by rhyolitic to dacitic volcanics (lavas and ignimbrites) referred to the Chon-Aike Fm., and volcaniclastic deposits included in La Matilde Fm. Even though this division was based on outcrops developed on the eastern border of the Deseado Massif, both facies can also be recognized on the central and western areas, where the current study was performed. Instead, Guido (2004)
Taphonomy of the trees
At TL 1 the preferable orientation of the fallen logs (mean 98° E, Fig. 2) is interpreted as wind-throw as a result of strong winds from the west, probably related to a volcanic blast (Frogattet al., 1981). The trees were partially covered by “dry” ash falls that contributed to their death and burial. These are succeeded by “wet” ash falls that have parallel lamination, rare ripple marks (eastward oriented), deformed contacts due to underlying logs and small accretionary lapilli (cf. Heiken and
Tree-stand structure and design
Even though more than two landscapes were colonized by trees, the present study has focused on the two best-preserved examples (Fig. 2). The first lower level (TL 1) occurs as exposures in vertical sections, and consequently it is only possible to approximate the spatial arrangement of the trees. Tree Level 2, on the contrary, occurs as extensive bedding plane exposures, which enabled a more complete structural analysis. Another episode of colonization is indicated by the presence of a small
Comparisons with other large fossil examples
The size and structural organization of the fossil Patagonian araucarian trees is only comparable with a few other gigantic examples in the fossil record, although most of them preserved under different taphonomic conditions. For example, trees from the Triassic “araucarioid petrified forest” from Arizona (Ash and Creber, 2000) were transported and buried in barforms by meandering high regime streams. Some of these trees exhibit the same size range, the largest up to 59 m long (after
Paleoclimate setting
Southern South America has been divided into two main climatic belts during the Middle and Upper Jurassic. A northern, septentrional belt with drier conditions, probably associated with a “desert zone” as defined by Rees et al. (2000), and a southern, meridional belt probably more seasonal, with dry summers and wet winters (winter-wet zone of Rees et al., 2000). Patagonia in particular occupied a mid-latitude location between 35 and 50°S that is included in the “seasonally dry (winter-wet)”
Concluding remarks
At least two well-defined episodes of forestation were detected from Middle Jurassic deposits in the La Matilde Fm. of Patagonia. A third woodland colonization tree landscape partially developed in the area but was destroyed during an intense ash-fall event. The petrified trees from the Jurassic of southern Patagonia were structurally organized as a low density woodland, with similarities to a modern equivalent, represented by Araucaria araucana stands developed on the eastern Andean slope
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This is an author's personal homage to the late Prof. Renato R. Andreis, who together with the author performed the sedimentological analysis of the La Matilde Fm. at the National Monument of the Petrified Forest in Santa Cruz. For field assistance Gerardo Cladera is fully thanked. Dr. Silvia Gnaedinger is thanked for wood determinations. Prof. Volker Mossbrugger performed the tree height calculations. Suggestions on a previous manuscript from Ignacio Escapa and David Cantrill are appreciated.
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