3 Byrd Polar Research Center and Dept. of Plant Biology, The Ohio State University. ABSTRACT. An interdisciplinary sedimentological and paleobotaoical.
Comptes Rendus XII ICC-P Volume 2: 13-40 Buenos Aires, 1993
THE GLOSSOPTERIS
FLORA FROM ANT ARCTICA: AND PALEOECOLOGY
T APHONOMY
N. Rubén CUNEO 1, John ISBELL 2, Edith L. TAYLOR 3 and Thomas N. TAYLOR 3
1
Byrd Polar Research Center, The Ohio State University 2
3
and CONICET,
Argentina.
Byrd Polar Research Center and Dept. of Geology, The Ohio State University.
Byrd Polar Research Center and Dept. of Plant Biology, The Ohio State University.
ABSTRACT. An interdisciplinary sedimentological and paleobotaoical study is currently being condocted in the Pennian and Triassic of the Transantarctic Mountains in order to reconstruct the paleoecosystems of the Antarctic continent. In this first contribution, a paleoecological analysis was carried oot on several fossil plant localities from southern Victoria Land and the Central Transantarctic Mountains. Phytotaphocoenses are included in fine- to coarse- grained facies deposited in differenl subenvironments of meandering and braided fluvial systems, as well as in lacustrine seuings. Autochthonoos, hypoautochthonous and allochthonous plant assemblages have been identified and their possible bioslratinomic processes are suggesied. Based on this analysis, the original vegetation is considered to have been dominatcd by plant communities that showed a dense and dominant forest physiognomy, although floristically composed of a low generic diversity association dominated by members of the Glossopleridales. The information gained from the paleoecological analysis has been used lo suggest comparisons with the paleovegetation from other Gondwanic regions and has been useful in supporting a phytogeographic differentiation for the Antarctic subcontinent during the Pennian. Finally, some suggestions are made in order to understand the growth of vegetation under polar climatic regimes. .
INTRODUCTION The Glossopteris Flora in Antarctica is part of a unique evolutionary stage in the Gondwana plant kingdom during the Permian. In order to understand the community structure, an interdisciplinary approach based on paleobotanical and sedimentological data was undertaken on fossil plant bearing localities from two different regions of the Antarctic continent, i.e. southern Victoria Land and the Beardmore Glacier are a (Fi8,: 1), Strata in these two areas were deposited in t"{o separate basins. Permian units in the Beardmore area accumulated within a foreland basin that developed along the margin of the East Antarctic Craton, while strata in southe¡n Victoria Land were deposited within a narrow basin 10cated cratonward of the foreland basin. A paleogeographic high, termed the Ross High, separated the two basins throughout the Permian (Collinson et al., in press; Colinson, 1990; Isbell, 1990a). These deposits are widely distributed and represent an important thickness of the whole Permian sedimentary sequence. In Southern Victoria Land, the Permian system is represented by a discontinuous glacial deposit (Metschel Tillite) and the overlying Weller Coal Measures. The two units range in age from Early to lower Late Permian (Collinson, et al., in press). The Weller consists of meandering and braided stream deposits as well as some lacustrine intercalations (Isbell, 1990a, in press). •• • t
N. R. Cúneo et al.
In the Beardmore Glacier región, the Permian deposits consist of, in ascending order, basal glacial deposits of the Pagoda Forrnation, black shales and fine-grained sandstones of the Mackellar Formation, fluvial sandstones of the FairSVL child Formation and fluvial sandstones, shales and coals of the Buckley Formation. Following deglaciation, sandstone at the top of this formation represents deltaic progradation. This event ANTARCTIC was followed by fluvial deposition, which dominated throughout the rest of the Permo-Triassic time (cf. Barrett et al .. 1986). Fine-grained intercalations within the Weller Coal Measures and the Buckley Formation con~Pe"''II.n tain an abundant phytotophocoenosis belonging Woutcrop to the Glossopteris Flora. This flora has been y~oo noted in numerous geological reports from throughout Transantarctic Mountains. A detailed synthesis about the geographical occurrences Figure 1. Distribution of Pennian strata in Antarctica. and composition of the Permian plant asNVL = northern Victoria Land; SVL = southern Victoria Land; CTM = Central Transantarctic Mountains; PM = semblages is provided by Bose et al. (1990) and Pensacola Mountains; EM = Ellsworth Mountains; EC = Taylor and Taylor (1990). English Coast; TM = Theron Mountains; DML = DronIn spite of attempts to link the fossil plant asning Maud Land; PCM = Prince Charles Mountains (Taken from Isbell, 1990a). The arrows indicate the location semblages and the depositional setting of of the study areas. sedimentary rocks (see for example Pyne, 1986; Kyle, 1976), little is known about the origin of the taphocoenoses and the cornposition, distribution and structure of the original plant communities. Based on the analysis of plant taphocoenoses from several localities in the Transantarctic Mountains, this preliminary contribution attempts to outline the major successional processes and the possible dynamics of the Permian communities as well as the influence of paleoclimatic factors that controlled the distribution of the vegetation. These communities may also be used to characterize the Antarctic subcontinent as a separated phytogeographic unit during the Permian based on ecological data. MATERIALS
AND METHODS
During three field seasons in Antarctica, severál stratigraphic sections were measured to identify major sedimentary processes and the paleoenvirónrnental conditions at each locality. Siliciclastic lithofacies were coded using Miall's (1984) facies code for alluvial deposits. Horizons containing plant assemblages wére analized in detail to determine small scale depositional events that incorporated the origin phytomass and gave rise to the biostratinomic processes that culminated in the taphocoenoses found today. Each fine-grained sedimentary unit containing plant assemblages was quantitatively evaluated for plant cover using the method of Scott (1978). This technique provides an idea of the dominant taxa and plant parts that cover individual bedding planes. This level of resolution was undertaken to identify, where possible, both the plant assemblages within the various lithofacies and variations in abundance and composition of plant parts, verticalIy and laterally within each sedimentary body.
14
The Glossopteris
Flora from Antarctica
In special taphonomical cases, such as the petrified forest at Mount Achernar (Upper Buckley Formation), in situ stumps were quantitatively analyzed using the Point-Centered Quarter Method (Cottam and Curtis, 1956), which pro vides several parameters regarding the structure of the forest. The paleoleaf-litter found on the same horizons was measured using the quadrat method by Scott (op. cit.). Leaf impressions were identified al the generic level in the field whereas different leaf morphotypes were identified in the laboratory in order to obtain information about the morphological variations regarding their occurrences in particular sedimentary subenvironments. T APHONOMY OF THE PLANT ASSEMBLAGES Phytotaphocoenoses of Permian strata within the Weller Coal Measures are related to braided and meandering fluvial depositional systerns in southern Victoria Land and the Buckley Formation in the central Transantarctic Mountains. Both types of river deposits occur in the sedimentary sequence at many localities. This Figure 2. Petrified trunk included in channel deposits of intercalation is a consequence of the evolutionbraided streams and oriented downcurrent. Mount Fleary development of each basin. ming, SVL. The Weller Coal Measures were deposited within a narrow basin that contained transverse and longitudinal drainage. In the lower Weller, coarse-grained transverse braided streams were located along the basin margins. These streams graded basinward into coarsegrained meandering streams. A narrow north-northwest to south-southeast trending lacustrine belt occurred along the basin axis. These deposits interfingered with alluvial deposits toward both eastern and western basin margins. In the upver part of the Weller, coarse-grained braided streams flowed transversely into the basin and longitudinally down the axis. Downstream these deposits graded into meandering streams containing fine- to medium-grained sandstones (Isbell, 1991). ~ The plant-bearing Buckley Formation in the Beardmore Glacier region is characterized by two stratigraphic sequences reláted to development and evolution of the approximately 500 km wide foreland basin. The sequences were deposited by transverse streams that flowed into the basin across both orogenic and cratonic basin margins, and by a longitudinal river system that flowed along the topographic drainage axis. Braided stream deposits occur along the margins and grade basinward into low-sinuosity single-channel and meandering stream deposits. Both braided and meandering river deposits occur along the axis. Deposits of these rivers form three major geographically distributed facies patterns: 1) thick sandstones with thin mudstone intercalations along the orogenic basin margin, 2) thick inter-
..
15
N. R. Cúneo et al.
Figure 3. Vertical accretion Hills, SVL. Scale in cm.
bedding
in very fine-grained
sandstone
or siltstone
containing
Vertebraria
root systems.
Allan
stratified mudstones and sandstones along the basinal axis and 3) thick sandstones with thick to thin mudstone intercalations along the cratonic basin margino Upward within each sequence, the orogenic-margin sandstone expands basinward and the axial mudstone/sandstone and cratonic margin sandstones are displaced toward the craton. A concomitant shift in the position of the longitudinal drainage axis toward the craton also occurs (Isbell, 1990; Isbell and Macdonald, in press). Isbell (1990) and Isbell and Macdonald (in press) suggest that these sequences were produced by tectonic loading within the Gondwanide orogenic belt along the paleo-Pacific margin of Antarctica. Sediments that accumulated in the various fluvial subenvironments bear plant taphocoenosis of several preservational modes (impression-compressions, permineralizatíons, casts, coalifications, etc.), suggesting burial histories that resulted in autochthonous, hypoautochthonous or allochthonous assemblages. The fluvial deposits (subenvirohments) included in the taphonomical analysis were sandstone sheets (including lag deposits and longitudinal channel bars), point-bars, levees, -floodplains, swamps, abandoned chañnels and lakes. SANDSTONE
SHEETS
Medium- to coarse-sandstone sheets are 3 to 20 m thick and lateralIy continuous across outcrop surfaces for several kilometers. Sheet dimensions suggests migration and sweeping of the channel belt across the alIuvial surface. These sandstone sheets are characterized by numerous erosional surfaces and sandstone-filled abandoned channels. Horizontal stratification (Sh), trough
..
16
The Glossopteris
Flora from Antarctica
cross-beds (St), and planar cross-bedding (Sp) occur throughout the sandstone sheets. Lag deposits are found along scours and include fine-grained intraclasts and large amounts of transported plant debris, including petrified wood and peat. Channel bars contain large numbers of petrified logs; in some cases (e.g. Mount Fleming and Allan Hills) entire trees with bases, oriented downcurrent, were found (Fig. 2). The origin of this taphocoenosis, as suggested by modern taphonomical studies, is related to the collapse of the river bank and leve es that were colonized by riparian vegetation (Spicer, 1989), or due to erosion of floodplain and swamp deposits by channel migration and/or avulsion. POINT -BAR SANDSTONES Point-bar deposits are characterized by finning-up sandstone cycles that contain large-scale lateral-accretion surfaces. Vertical accretion deposits (Sh) occur at the top of point-bar sequences (Fig. 3). They have been recognized at several localities in the Weller Coal Measures, southern Victoria Land (Kennar Valley, Aztec Mountain, Mount Fleming and Allan Hills), as well as in some localities of the Buckley Formation in the central Transantarctic Mountains (e.g. Mount Achernar, Mount Ropar, Clarkson Peak). Plant taphocoenoses are preserved in the upper parts of the lateral accretion surfaces and in the vertical accretion deposits. They are exclusively dominated by Vertebraria root-systems that, in some localities (e.g. Allan Hills) , can be traced laterally for more than 500 m. This taphocoenosis originated in situ (autochthonous). However, there is no evidence of other identifiable plant remains incorporated in the point-bar deposits. This suggests that the aerial parts produced by the original community that occupied this subenvironment were probably transported downstream during flooding events. Spicer (1989) suggests that point-bar deposits probably represent the best sample of riparian vegetation because both robust and delicate materials can be preserved. On the other hand, Scheihing and Pfefferkorn (l984) found that point bar sands did normally not contain any recognizable plant parts and that only rare occurrences could be encountered at the uppermost or lowermost end of exceptional point bars. The fossil case from the Permian sequence of Allan Hills suggests that only robust materials such as Vertebraria roots were preserved. LEVEES Levee deposits are recognized in most of the Permian localities of Antarctica. They are represented by thin fine-grained sandstone and sandy siltstone beds (Sr, Sm, Sh or Fl) that occur directly above the channel deposits, especially associated with braideJ fluvial streams. The leve es are mostly affected by erosional procésses during major flooding events and/or during stream migration, which normally do not allow for préservation of aerial plant parts produced by the local plant community. Some studies on modern levees show that the preservation of plant material is controlled by root penetfation and oxidation (Gastaldo et al., 1989), However, Burnham (1989) has demonstrated well preserved plant taphocoenoses in modern tropical levees that represent the surrounding vegetation. In the Permian of Antarctica, the plant taphocoenoses of levee facies consist of only rootsystems of the Vertebraria type, and are considered as an autochtonous taphocoenoses originating from the local vegetation. No other plant remains were found in the levee deposits, suggesting that the potential for preservation of plant parts in this subenvironment was quite low and only underground organs had the best chance of being preserved .
.
17
N. R. Cúneo et al.
FLOODPLAINS The best preserved plant taphocoenoses occur within floodplain sediments, due to the ./ prevailing low energy sedimentary processes in this subenvironment, which did not fragment the incorporated plant material. Floodplain deposits / consist of structureless or well laminated carbonaceous and gray shales (Fsc) and massive rooted siltstones (Fr). They vary from a few centimeters up to several meters in thickness, and represent the uppermost part of fining upward meandering or braided fluvial cycles. Although well developed floodplain facies occur with meandering stream deposits, they also occur with deposits of braided streams, some times associated with lacustrine facies. In the latter case WCM floodplain deposits were preserved by high subsidence rates within the depositional basin (Isbell and Collinson, 1991). Plant taphocoenoses in floodplain facies were formed from continued overbank flooding and the incorporation of the plant litter that acFigure 4. Weller Coal Measures Section at Kennar Vacumulated on the floodplain surface. This suglley, SVL, showing fluvial deposits dominated by meandering streams. gests that the plant material was transported only a minimal distance; hence, it can be considered as part of an hypoautochthonous taphocoenosis. Since most of the floodplain plant assemblages identified at particular geographic localities show differences in their composition and abundance, they will be considered individually for taphonomical analysis. In southern Victoria Land, the localities in Kennar Valley and the Allan Hills (and to some extent at Mount Fleming, Fig. 6) show well preserved floodplain plant taphocoenoses. These deposits are associated with both meandering and braided stream systems. In the case of Kennar Valley, phytotaphocoenoses are included in carbonaceous shales (Fsc), immediately above levee deposits (Fig. 4). Here, floodplain sediments are intercalated with crevasse splay (Sr) and swamp (C) deposits. Quantitative measurements (Table 1) indicate that the bedding planes are covered by a dense phytomass (71 %) that consists entirely of the leaves of Cordaites, angamopteris and Glossopteris respectively. In the Allan Hills, two thin floodplain sequences bearing identifiable plant remains, are associated with braided streams (Fig. The first case corresponds to the Plant Section 1 of the Allan Hills, with two distinct plant layers occurring immediately below and above a thick coal seam. The lower one s~ows a plant cover of 80 % and is dominated by Gangamopteris and Glossopteris; associated glossopterid reproductive organs (e.g. Plumsteadia) are ..,..-
-\
5t.
Figure S. Weller Coal Measures Section at Allan Hills, SVL. General sequence showing the transition from deposits dorninated by meandering streams to braided streams in the upper parto a) Stratigraphic microsection of Plant Section 1; b) Idem, Plant Section 11;e) Idem Plant Section III; d) Idem Plant Section IV. E-F = paleoenvironmental and paleoecological reconstructions.
18
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The Glo ssopteris
Sand m e !
Flora from Antarctica
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