Depositional setting and paleobotany of Permian and ... - Science Direct

International Journal of Coal Geology, 12 (1989) 657-679

657

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Depositional setting and paleobotany of Permian and Triassic permineralized peat from the central Transantarctic Mountains, Antarctica EDITH L. TAYLOR', THOMAS N. TAYLOR' and JAMES W. COLLINSON 2

~Byrd Polar Research Center, Department of Botany, and 2Byrd Polar Research Center, Department of Geology and Mineralogy, The Ohio State University, Columbus, OH 43210, U.S.A. (Received March 25, 1988; revised and accepted October 11, 1988)

ABSTRACT Taylor, E.L., Taylor, T.N. and Collinson, J.W., 1989. Depositional setting and paleobotany of Permian and Triassic permineralized peat from the central Transantarctic Mountains, Antarctica. In: P.C. Lyons and B. Alpern (Editors), Peat and Coal: Origin, Facies, and Depositional Models, Int. J. Coal Geol., 12: 657-679. Silicified peat is known from two sites in the central Transantarctic Mountains. Both are within a 2-km-thick Permo-Triassic sandstone-shale sequence that was deposited by braided streams in a rapidly subsiding foreland basin along the paleo-Pacific margin of Antarctica. Upper Permian permineralized peat occurs as scattered boulders just above a channel-form sandstone in the upper part of the Buckley Formation on Skaar Ridge overlooking the Beardmore Glacier. These boulders are erosional remnants of fine-grained deposits that accumulated in shallow lakes or swamps on a flood plain. At Fremouw Peak, the peat occurs near the top of the Fremouw Formation in strata that are probably Middle to Late Triassic in age. The peat consists of large blocks that were rafted into a sandy braided stream channel during a flood and then stranded and buried as flood waters receded. Both sites are characterized by exceptionally well-preserved plant material, although the composition and diversity of the two floras are very different. Permineralization apparently took place rapidly and was enhanced by the dissolution of siliceous volcanic detritus that is abundant in both formations.

GENERAL GEOLOGIC AND STRATIGRAPHIC SETTING

The Transantarctic Mountains occupy the entire length of the western margin of East Antarctica, which comprises a stable cratonic area (Fig. 1 ). The Ellsworth Mountains constitute one of several micro-plates that make up West Antarctica (Dalziel and Elliot, 1982 ). In the Transantarctic Mountains, postOrdovician sedimentary rocks are subhorizontal and form a series of plateaus. In the Ellsworth Mountains, Paleozoic rocks were intensely folded and thrust 0166-5162/89/$03.50

© 1989 Elsevier Science Publishers B.V.

658 taulted during the Late Permian-Triassic Gondwanian orogeny (Elliot, 1975 ). Very little is known about the Paleozoic-early Mesozoic history of West Antarctica because rocks of that age are exposed in only a few places such as in the Ellsworth Mountains. Situated nearer the paleo-Pacific margin, Paleozoic sequences in the Ellsworth Mountains are much thicker and may be more complete than their East Antarctica counterparts. The dispersal of calc-alkaline volcanic sediments into the Transantarctic Mountains from a West Antarctic source during the Permo-Triassic suggests the presence of a volcanic arc and subduction zone along the paleo-Pacific margin (Collinson, in press). Plant-bearing rocks of late Paleozoic to early Mesozoic age occur throughout the Transantarctic and Etlsworth Mountains. The area is occupied by four basins, each defined by the continuity of stratigraphic units within them: (1) Northern Victoria Land; (2) Southern Victoria Land; (3) Central Transantarctic Mountains; and (4) Ellsworth Mountains. Similarities in stratigraphy from basin to basin permit general correlation and suggest that an overall genetic relationship exists between basins. The presence of an orogenic belt in -~o70°S- ~ ~

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TABLE 1 VictoriaGroup stratigraphyin variousAntarcticbasins (data in part from Barrett et al. (1986); Collinsonet al. (1983 ),Collinson and Elliot(1984), Collinsonet al. (1986),and McElroy and Rose (1987 ) Ellsworth Mountains

C. Transantarctic Mountains

S. Victoria Land

N. Victoria Land

Triassic Upper

F a l l a Fm.: 160-530 m. Volcaniclastic sandstone and tuff. Dicroidium flora. Braided channels and flood plain,

L a s h l y Fm.: 325 + m. Upper: Volcaniclastic and quartzose sandstone. Dicroidium flora. Meandering channels and flood plain.

Section Peak Fm.: 160 m. Quartzose sandstone. Dicroidium flora. Braided channels

Middle

F r e m o u w Fro.: 620750 m. Volcaniclastic and quartzose sandstone, Dicroidium flora.

Lower: Volcaniclastic sandstone. Braided channels and flood plain.

Cynognathus and Lystrosaurus faunas,

Lower

Braided channels and flood plain, Permian Upper

Lower

P o l a r a t a r Fm.: 1,000 q- m. Upper: Volcaniclastic sandstone, carbonaceous shale, and coal. Glossopteris flora. Fluvial-deltaic.

B u c k l e y Fm.: 750 m. Volcaniclastic sandstone, carbonaceous shale and coal in upper part. Arkosic sandstone, carbonaceous shale and coal in lower part. Glossopteris flora throughout. Braided channels and flood plain.

Lower: Argillite and fine volcaniclastic

F a i r c h i l d Fm.: 130-220 m. Arkosic sandstone. Braided channels M a c k e l l a r Fm.: 60-140 m. Dark shale and fine arkosic sandstone. Non-marine trace fossils. Fresh-water deltaic and basinal P a g o d a Fm.: 126-200 m. Diamictite. Terrestrial glacial.

sandstone. Marine (?) tracefossils. Marine (?) basinal.

Whiteout Congl.: 1,000+ m. Diamictite.Marine (? ) glacial.

Feather Conglomerate: 215 m. Quartzose sandstone. Braided channels.

Weller Coal Measures: 222 m. Arkosic sandstone. Glossopteris flora. Meandering channels and flood plain.

T a k r o u n a Fm. 280 + m. Arkosic sandstone. Glossopteris flora. Braided fluvial

M e t a c h e l Tillite: 0-85 m. Diamictite. Terrestrial glacial.

U n n a m e d unit.: 0-350 m. Diamictite. Terrestrial glacial.

660 West Antarctica, as demonstrated by deformed Permian rocks in the Ellsworth Mountains and provenance and paleocurrent data from the Transantarctic Mountains, suggests that these basins constituted part of an extensive foreland basin that developed in response to compressive forces along the paleoPacific margin. The general stratigraphy of Antarctic basins is summarized in Table 1. Permo-Carboniferous glacial diamictites initiate the sequence throughout the foreland basin; these are of marine origin in the Ellsworth Mountains, as compared to a terrestrial origin in the Transantarctic Mountains (Ojakangas and Matsch, 1981 ). A Lower Permian postglacial black shale, representing an interior seaway with more open-marine connections toward the Ellsworth Mountains, pinches out at the southern end of the central Transantarctic Mountains (Miller et al., 1988; Collinson et al., in press). After filling of the seaway by deltaic progradation by Late Permian, widespread coal measures containing the Glossopteris flora dominated the foreland basin. Similar Triassic fluvial deposits occur from the Nilsen Plateau (central Transantarctic Mts. ) to northern Victoria Land (Fig. 1 ). Collinson et al. ( 1987 ) hypothesized that a major stream system flowed along the axis of the foreland basin toward northern Victoria Land. The Triassic rests disconformably on Permian rocks in the central Transantarctic Mountains and southern Victoria Land and nonconformably on Ordovician granite in northern Victoria Land (Collinson et al., 1986). The Lower Triassic is generally noncarbonaceous and in the central Transantarctic Mountains it contains diverse vertebrate faunas of the Lystrosaurus and Cynognathus zones (Hammer and Cosgriff, 1981; Colbert, 1982; Hammer et al., 1987 ). Locally, poorly preserved fossil logs are found in association with vertebrates. The Middle and Upper Triassic contains carbonaceous shale with the typical Dicroidium flora and rare coat beds in the central Transantarctic Mountains and Victoria Land. PEAT-BEARING FORMATIONSIN THE BEARDMOREGLACIERREGION

Stratigraphy The stratigraphy in the Beardmore Glacier region is summarized in Figure 2. Permineralized peat deposits occur in the upper Buckley Formation at Skaar Ridge and in the uppermost part of the type section of the Fremouw Formation at Fremouw Peak (Fig. 3). Grindley ( 1 9 6 3 ) n a m e d the Buckley Coal Measures for Buckley Island, a small nunatak in the upper reaches of the Beardmore Glacier. Barrett (1969) assigned the lower part of Grindley's Buckley Coal Measures to the Fairchild Formation, a cross-bedded, noncoal-bearing sandstone, and retained the upper coal-bearing sequence in the Buckley Formation. A complete and continuous section is not exposed anywhere in the region, but Barrett estimated a thick-

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ness of 750 m by combining two sections (Barrett, 1969). Sandstone predominates in the lower part of the formation; upward the amount of shale in the section increases (Barrett et al., 1986). In the upper part of the Buckley Formation where the permineralized peat occurs, channel-form sandstones are

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663 most 180 ° reversal in paleocurrent directions occurs at the same horizon (Isbell, in press). The Fremouw Formation was named by Barrett (1969) for a 614-m-thick sequence at Fremouw Peak. The formation is thicker in the Shackleton Glacier area (Fig. 1) where it is in excess of 750 m (Collinson and Elliot, 1984). An erosional disconformity with several meters of local relief separates carbonaceous beds in the Buckley from noncarbonaceous beds in the overlying Fremouw. The Fremouw Formation, particularly the lower resistant part, has been reported at many localities from the Disch Promontory (83 ° 34' S, 162 ° 52'E ) to the Nilsen Plateau (86°20'S, 158°00'W) (Fig. 1), a distance of 475 km (Collinson and Elliot, 1984; Barrett et al., 1986). The formation has been divided into three informal members throughout its extent. The lower member, a resistant sandstone, ranges from 80 to 120 m thick. It is predominantly quartzose around the Beardmore Glacier, but the volcaniclastic component becomes dominant toward the Shackleton Glacier area (Vavra et al., 1981 ). The middle member, consisting of nonresistant mudstone and fine-grained volcaniclastic sandstone, ranges from 180 to 250 m thick. Highly altered volcanic tufts occur in this part of the sequence. The upper Fremouw member consists of slopeforming fine- to medium-grained volcaniclastic sandstone and minor carbonaceous shale. The upper member is 320 m thick at Fremouw Peak (Barrett et al., 1986 ) and more than 500 m thick in the Shackleton Glacier area (Collinson and Elliot, 1984). Vertebrate fossils of the Lystrosaurus zone in the lower member and the Cynognathus zone in the basal upper member indicate an Early Triassic age for much of the formation (Hammer et al., 1987 ). Kyle and Schopf (1982) noted subzone A or B palynomorphs (Alisporites zone, Kyle, 1977) in the middle member and subzone C types at the top of the upper member. These would suggest that the upper member includes sediments of Middle and possibly Late Triassic age. The lower part of the overlying Falla Formation contains palynomorphs of subzone C and possibly D.

Depositional environment of the Buckley Formation The Buckley Formation was initially interpreted as a meandering stream deposit with the extensive channel-form sandstones representing laterally migrating point-bar deposits and the fine-grained beds representing floodplain and flood-basin deposits (Barrett et al., 1986). In a more recent study, IsbeU (in press) has interpreted the laterally extensive channel-form sandstone bodies as representing broad, low-sinuosity braided streams that migrated across the flood plain by avulsion. Some of the evidence for this interpretation includes the lateral continuity of sandstone bodies, the presence of sand-filled rather than mud-filled abandoned channels, and the rarity of lateral accretion surfaces typically associated with point-bar deposition. Isbell (in press) compares the Buckley river system to humid, low-gradient, alluvial fans in the

664

Himalayan foreland, such as the Brahmaputra River ( Coleman, 1969 ) and the Kosi River (Wells and Dorr, 1987). Fine-grained sediments including the peat accumulated in swamps on the flood plain. Thinly laminated shales at several localities are suggestive of lacustrine deposition (Barrett et al., 1986). Permineralization of the peat (see Schopf, 1975, for a definition of permineralization) at Skaar Ridge was probably induced early by silica-charged waters due to dissolution of silicic volcanic detritus, which is abundant in the upper part of the Buckley Formation. Because the preservation of permineralized peat is such a rare phenomenon, the circumstance may be related to a rare event, such as a volcanic tuff blanketing the swamp and inducing almost immediate permineralization. No volcanic tufts have been reported from the Buckley, but many of the highly altered mudstone units are likely candidates.

Depositional environment of the Fremouw Formation The Fremouw Formation was deposited by low-sinuosity braided streams (Barrett, 1970; Collinson et al., 1981 ). Channel-form sandstone bodies in the lower member are similar to those in the Buckley Formation, but are not generally as laterally extensive. Flood-plain sediments consist of greenish-gray mudstone that locally contains small root casts. The abundance of small roots and the complete absence of larger roots and fossil wood in the flood-plain facies suggest that initially the Fremouw flood plain was dominated by a herbaceous plant cover. A low-gradient fan similar to that in the Buckley, but formed under less humid conditions, is hypothesized for lower and middle Fremouw deposition. Conditions changed with the deposition of the upper member. An influx of sand blanketed the basin. The preservation of carbonaceous material in the upper member suggests that the change in sedimentation may have been in part climatically induced. These braided stream deposits are more typical than those in the Buckley and lower Fremouw, because only a small proportion of the fine-grained flood-plain facies is preserved. Small root casts are common in greenish-gray mudstones, which suggests a herbaceous cover on flood plains, but an in situ fossil forest in the uppermost part of the formation at the head of the Gordon Valley and at several localities in the Shackleton Glacier area indicate the existence of local stands of trees. As in the Buckley, permineralized peat in the Fremouw has been found only at a single locality. This locality is situated approximately 30 m below the top of the formation on Fremouw Peak. The occurrence is restricted to a bedding surface upon which several blocks of permineralized peat and large fossil logs occur. One such log is 0.6 m in diameter and 22 m long (fig. 33a in Barrett et al., 1986). The peat occurs as rectangular-shaped blocks, the largest measuring 1 X 2.5 X 2.5 m (Fig. 4 ). A 40-cm-thick, crudely laminated carbonaceous shale

665

Fig. 4. Section through peat clastat Fremouw Peak. Lighter material at top ispermineralized peat; dark layer at base (arrow) represents carbonaceous shale (paleosol).Bar scale=30 cm.

containing rootlets (a paleosol) comprises the base of this block. Blocks are slightly discordant with bedding and were apparently buried by migrating channel bars. Paleocurrents as indicated by directions of trough axes converge upstream of blocks, which suggests that stream currents were diverted around blocks. Blocks of peat were probably rafted into the area during a flood and became stranded on sand bars as flood waters waned. A possible scenario that would also account for the abundance of logs at the same horizon would be the destruction of a series of swampy, tree-covered islands during a major flood. Permineralization was enhanced by the dissolution of siliceous volcanic detritus that is abundant in the upper Fremouw. PERMINERALIZEDPEAT - AN INTRODUCTION Permineralized peat has been an important source of paleobotanical data since the discovery of Carboniferous coal balls in the Lancashire coal fields of Great Britain in the mid-1800's (Hooker and Binney, 1855). Preserved within calcium carbonate, these fossil peats provided a system that was relatively easy to study, initially via thin sections and nitrocellulose peels, and more recently using the cellulose acetate peel technique (Joy et al., 1956). As a result, the

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number of contributions on the anatomy and morphology of Late Carboniferous plants over the last hundred years has numbered over 1,000 (Scott and Rex, 1985 ), with the result that the floras from Late Carboniferous coal swamps are probably the most completely known from any time period. More recently, it has been possible to examine other parameters of this flora, such as paleoenvironment and changes in floral composition through time, based on a relatively complete knowledge of the individual components of the flora (e.g., Phillips and DiMichele, 1981; Phillips and Peppers, 1984 ). These carbonate-permineralized peats (i.e., "coal balls") occur only in Carboniferous rocks, but silicified peats are known from many other time periods (see Table 2). For example, the Middle Eocene permineralized peat deposit near Allenby, British Columbia (e.g., Basinger and Rothwell, 1977) has yielded a number of exceptionally well-preserved angiosperm and gymnosperm fossils, including delicate floral parts (e.g., Basinger, 1976; Stockey, 1987). In the southern hemisphere, Gould (1970) and Gould and Delevoryas (1977) described a silicified peat of Permian age from the Bowen Basin in Queensland, Australia, that included permineralized reproductive organs assignable to the Glossopteridales. Although the Rhynie chert (Middle Devonian) deposit of silicified plants has been described as a permineralized peat (e.g., Chaloner, 1985; Knoll, 1985 ), Edwards (1986) has provided evidence that the plants were silicified in growth position by successive inundations and thus did not represent a peat-like deposit. Although some authors have utilized a very broad definition of fossilized peat to include almost any silicified deposit rich in plant material (e.g., Knoll, 1985 ), by analogy with modern peats such definitions should distinguish true permineralized peats from other chert deposits that include plant material, TABLE 2 Known worldwide silicified peat deposits (arranged in stratigraphic order) Locality

Age and reference

Allenby Fm., British Columbia Clarno Fro., Oregon Deccan Intertrappean beds, India Sentinel Butte Fm., N. Dakota Fremouw Fm., Antarctica Serian Fm., Malaysia Autun, France Bowen Basin, Australia Buckley Fm., Antarctica Rive de Gier Fm., France

Eocene (Basinger and Rothwell, 1977 ) Eocene (Arnold, 1945) Eocene (Sahni and Rode, 1937) Paleocene (Ting, 1972) Triassic (Schopf, 1978) Triassic (Gastony, 1969) Permian (Renault, 1893, 1896; Galtier, 1980) Permian (Gould, 1970) Permian (Schopf, t970a) Carboniferous ( Grand'Eury, 1877; Galtier and Phillips, 1977) Carboniferous (Galtier et al., 1986)

Tazekka, Morocco

667 such as a number of Proterozoic sites (Knoll, 1985 ) and in situ growth deposits like the Rhynie chert. Some of the difficulty in determining what constitutes a fossilized peat deposit originates from the fact that the term peat is generally defined based on modern deposits, i.e., as accumulations of unconsolidated plant debris with certain levels of moisture and carbon (Schopf, 1966; Bates and Jackson, 1980). Although these definitions cannot be applied directly to fossilized peats, certain aspects are comparable. For example, fossilized peats are unconsolidated deposits consisting largely of plant debris that exists in various stages of preservation or decay. Based on modern peats, autochthonous fossilized peats would be expected to be unstratified, whereas allochthonous ones should exhibit various levels of stratification. Cohen (1970) and Cohen and Spackman (1977) also noted that modern autochthonous peat usually contains ingrown rootlets and plants which have not been compressed to any appreciable degree. However, it is important to note that there have been no comparative studies to date on the deposition and stratification in autochthonous vs. allochthonous fossilized peats. One of the most important characteristics in distinguishing fossilized peat deposits from other occurrences of permineralized plants is that peats normally exhibit heterogeneity in their plant composition. By this criterion, the Rhynie chert is clearly not comparable to other fossilized peats (Edwards, 1986). In addition, fossilized peats probably represent a different stage of development than do most modern peats. It is clear from the paleobotanical evidence that permineralized peats were preserved at a very early stage in their formation. This inference is based partially on the excellent preservation of the plant tissues, especially very delicate and ephemeral stages in plant growth. In addition, studies on modern peats (e.g., Cohen and Spackman, 1980) have shown much more extensive decomposition of plant parts in the process of modern peat formation than that commonly seen in fossilized peat. Although the paleobotanical evidence points to early permineralization of fossilized peats, analysis of organic matter in coal balls (Hatcher et al., 1982; Lyons et al., 1983) has shown that the plant matter is coalified to about the same level as the surrounding high-volatile bituminous coal, i.e., that there is no cellulose or lignin remaining in the plants as in modern peat. In contrast, Morey and Morey (1969) found evidence of both original cellulose and lignin in a Miocene lignitized wood of lower rank, indicating that these components may be preserved during the lower stages of coalification. Ultrastructural studies on Carboniferous coal ball plants indicate that the cell walls of some of these plants contain fibrils of a similar size and shape to cellulose wall fibrils (Purelis, 1962; Smoot and Taylor, 1984) and possible subcellular details have been reported in some plants (e.g., "starch" grains [Baxter, 1964] and nuclei [Millay and Eggert, 1974] ). At present, it is not understood what these discoveries represent chemically, or how a chemical transformation occurred in coal balls with little structural (or possibly ultrastructural) change. Chemical

668

studies have not yet been completed on any silicified peats, so the degree oi~ chemical alteration in these plants continues to remain unknown. Little work has been addressed specifically to the formation of silicified peats (see Schopf, 1971 ), and there has been continued controversy over the means of formation of coal balls almost since their discovery (e.g., Hooker and Binney, 1855; Stopes and Watson, 1908; Mamay and Yochelson, 1962; Perkins, 1976; Scott and Rex, 1985). Many authors have proposed a marine incursion into the peat swamp as the source of the calcium or magnesium carbonate (e.g., Mamay and Yochelson, 1962; Cross, 1969; Perkins, 1976). Anderson et al. (1980), based on an isotopic study of the coal balls from the Herrin Coal in Illinois, concluded that only the topmost peat reflected carbonate with a carbon composition of marine origin. Scott and Rex (1985) have recently completed a detailed, petrologic analysis of coal balls from many different sites and found that the carbonate structure is very similar in all those examined. The coal balls they examined were preserved at various stages of decay and there is evidence that plant cell walls may have acted as nucleation centers for calcite crystal growth. A continuing problem in accounting for the exquisite preservation of the plants is how the permineralizing or pore fluid penetrated the mass of plant debris rapidly enough to insure preservation. Scott and Rex (1985) concluded that crystallization occurred first within natural spaces in the peat and then continued to infiltrate the compacted peat via aerenchymatous root tissues. PERMINERALIZED PEATS FROM ANTARCTICA

Although Carboniferous coal ball deposits generally occur within coal beds, many of the silicified peat deposits throughout the world appear to be closely associated with volcanic activity (Chaloner, 1985), which suggests that this proximity provided the source of silica for the permineralizing fluid. This is also true of the permineralized peats from Antarctica (see "Depositional environment" section). Schopf (1971) suggests that three types of silica deposition are represented in the Permian peat from Skaar Ridge. Spaces are often filled with coarse, prismatic quartz (e.g., Fig. 10, bottom and lower right); however, fibrous microcrystalline quartz, i.e., chalcedony, appears to be the basis for the delicate preservation of the plant tissues at this site. Schopf considers that chalcedony deposition occurred first, with plant cell walls probably acting as the nucleation centers. The evidence for this initial deposition is the presence of a layer of chalcedony around all plant parts. Schopf suggests that silica was distributed throughout the peat by capillary action involving plant cell walls and membranes. In a second stage of silicification, more fibrous silica filled the remaining small interstices in the matrix and finally, prismatic, slowgrowing quartz filled the largest spaces (e.g., within Vertebraria roots, Fig. 10) in the peat.

669

Triassic peat from Fremouw Peak Schopf firstreported the occurrence of permineralized peat within the Frem o u w Formation from a col (saddle) just north of Fremouw Peak in the Queen Alexandra Range (Schopf, 1978). As noted above (see "Depositional environment of the Fremouw Formation"; Taylor et al.,1986b; Smoot et al.,1987), the peat probably is not in situbut rather consists of blocks that were transported into the paleostream channel. Thus, it is not possible to make direct observations on the paleoenvironment of the peat-forming plants. However, it is possible to make a number of inferences about the nature of the deposit and the plant associations therein. For example, although the deposit as it exists now is certainly allochthonous, it is clear that the blocks of peat were not transported far.There is littleevidence of decay within the peat, and the blocks are irregularin their sizeand shape (Fig.4). It seems likelythat the forested island where the blocks originated represented the originallocation of the plants contained within the peat, i.e.,the peat on the island was autochthonous. This is based firstof all on the excellent preservation of the plants (similarto autochthonous Carboniferous peats), but also on a lack of stratificationwithin the peat and a lack of orientation of any of the plant organs present. The permineralized peat represents two differentplant associations. Possibly these associationsrepresented differentpeat islands initially.Certain blocks contain a diverse fern flora (Fig. 5) (Millay, 1987) and abundant remains of the cycad Antarcticycas (Smoot et al.,1985), including stems and numerous roots (Fig. 6). Other blocks are dominated by foliage of the Dicroidium type and these have never been found to contain Antarcticycas remains. Instead, they include a relatively diverse flora consisting of small sphenophyte axes, various seeds (e.g.,Fig. 8), gymnospermous wood, and conifer-likeleaves.Also present is a cupulate reproductive organ containing several small triangular seeds and pollen sacs with bisaccate pollen grains (Fig. 9). Although several large logs and upright stumps are present in the col at Fremouw Peak (fig.33a, Barrett et al.,1986), no large blocks of wood are present in the permineralized peat, which suggests that the Dicroidium foliage found at this site was borne on a relativelysmall tree. The overall size of the axes and the delicate foliage suggests that the Dicroidium plant bore relatively small (probably less than 0.5 m) fronds. M a n y of the blocks of peat appear to represent a rooting zone, since they contain abundant roots of all sizes (Millay et al.,1987). Roots of Antarcticycas are present both attached to the stems and in the matrix. Other gymnospermous roots are also abundant. The cycad Antarcticyaswas probably a small and scrambling, prostrate plant. The m a x i m u m stem diameter known is about 4-5 cm. This ancient member of the Cycadales probably did not resemble the arborescent cycads that are most c o m m o n today, but may have looked like some of the small and partially subterranean extant forms such as Zamia and Bowenia. N o cycad petioles with

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671

attached foliage have been found yet at Fremouw Peak, so it is not possible to speculate on the overall size of the plant. The presence in this peat of a true cycad, a group which today is restricted to tropical and subtropical environments, is somewhat of an enigma because Retallack (1987) has interpreted similar aged floras from New Zealand as living in a humid, cool temperate environment. In contrast, Parrish et al. (1986) believe that the evidence is strong from both plants and amphibians that the polar regions were relatively warm during the Triassic. Their interpretation is supported by the reptileamphibian faunas (e.g., Hammer et al., 1987) and the presence of coal in the Fremouw Formation, as well as the lack of tillite deposits in the polar regions. They suggest that there was no significant ice or snow cover at this time and that a general warming trend extended from the Permian into the Jurassic. Thus, while the Fremouw cycads were probably not living in a tropical environment as cycads do today, their existence and abundant growth certainly suggests an amelioration of the climate since Permian time. There is evidence of ample moisture present in the environment, both because of the occurrence of the peat itself and also because of the composition of the peat. The abundant Dicroidium foliage present is often thin and parenchymatous. In addition, pollen-bearing organs that are similar to Dicroidium in their internal anatomy are found within the peat and they are often only a few cell layers thick.

Permian peat from Skaar Ridge The peat deposit within the Buckley Formation at Skaar Ridge near Mt. Augusta in the central Transantarctic Mountains was first reported by Schopf (1970a,b). Although he made no detailed systematic study of the plants from this site, he provided an initial overview of the flora (Schopf, 1970a) and made several detailed studies on the mode of preservation of the plants (e.g., Schopf, 1971, 1977). The discovery was especially significant because it represented Fig. 5. Cross section through Triassic fern stem showing cellular preservation in central stele and thickened cortex. X 38. Bar scale = 0.5 ram. Fig. 6. Section of Triassic peat with large Antarcticycas root (center) surrounded by smaller rootlets and organic debris. X25. Bar scale= 0.5 ram. Fig. 7. Section through Permian peat showing numerous, matted Glossopteris leaves in cross section. Arrow indicates vascular bundles (veins) within leaf. Bar scale = 2.0 ram. Fig. 8. Longitudinal section of seed and several small roots (arrows) within Triassic peat. Note other plant parts in various stages of degradation. X 25. Bar scale = 0.5 mm. Fig. 9. Pollen sac (Triassic) containing bisaccate pollen grains. X 130. Bar scale = 0.1 mm.

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the first example of petrified peat from a Gondwana locality. The peat boulders occur scattered on the surface along with numerous dolerite boulders (Schopf, 1970a; Smoot and Taylor, 1986; Taylor et al., 1986a). Due to the location of the boulders of peat near the top of a ridge, it is not possible to determine the origin of this deposit because it could have been deposited at its present locality by glacial action. However, the presence of mixed sandstone-peat boulders suggests that initial permineralization took place within a fluvial sandstone (Taylor et al., 1986a). Compared to the peat from Fremouw Peak, the flora of the Permian peat is monotonous, consisting almost exclusively of glossopterid remains, including several types of leaves (Figs. 7, 11 ) (Pigg and Taylor, 1987), stems, roots of the Vertebraria type (Figs. 10, 12), and a number of reproductive organs. One of the interesting features of this flora, as well as that of the Triassic peat, is the variety and abundance of fungi (e.g., Fig. 13 ). Wood-decaying fungi have been reported at both sites which exhibit patterns of decay identical to those formed by modern white-rot and white-pocket rot fungi (Stubblefield and Taylor, 1986 ). While it is not yet possible to correlate these mycofloras with specific paleoenvironmental parameters, further studies may provide a basis for more accurately defining environments of peat deposition based on tolerances of the mycofloras. It has been suggested that almost all fossil peat deposits represent evolutionarily conservative floras with low diversity (e.g., Phillips and DiMichele, 1981; Knoll, 1985; Scott and Rex, 1985 ). Chaloner (1985), however, takes issue with this concept and suggests that instead of representing "relict" floras, these deposits could be described more accurately as consisting of plants especially adapted to the swamp habitat. Although they may not have had the capacity to colonize other types of habitats, they do not a priori represent conservative elements. In the case of the peat from Skaar Ridge, the almost monodominant occurrence of glossopterid remains is typical of many Gondwana floras of Late Permian age. Although the environmental and evolutionary constraints that Fig. 10. Cross section of Vertebraria axis (root of Glossopteris) showing growth rings (right arrow) and degraded wood caused by fungal attack (upper arrow). Triangular area at lower right (air spaces in the living plant) is filled with prismatic quartz. X 1. Bar scale= 1.0 cm. Fig. 11. Portions of three Glossopteris leaves in cross section showing numerous vascular strands within them. X 32. Bar scale = 0.5 mm. Fig. 12. Longitudinal section of Vertebraria wood showing excellent preservation of tracheid pitting. X 120. Bar scale=0.1 mm. Fig. 13. Fungal chlamydospore with attached stalk {Triassic ). X 535. Bar scale = 0.02 mm. Fig. 14. Section of moss leaves (Merceria) showing several midribs and a delicate, single-celled lamina (arrow). X 140. Bar scale=0.1 mm.

¢.0

67,1

produced this unique plant community throughout much of Gondwanaland are not tully understood at present, some authors have suggested that the dominance of the relatively low diversity Glossopteris flora can be attributed to an increasingly seasonal and, therefore, stressful environment in the southern hemisphere at this time (e.g., Rigby, 1971 ). There is evidence from sedimentary features that the climate was cool to cold temperate (e.g., Martini and Johnson, 1987 ). Wood that is present in the permineralized peat contains welldefined growth rings. Although it is not known whether these represent annual rings, Glossopteris leaves have been found in other Gondwana sites in varved sediments, which suggests that the plant was seasonally deciduous. The presence of deposits of abundant, densely matted Glossopteris leaves, not only in the Skaar Ridge peat (e.g., Figs. 7, 11 ), but also in the nearby shales provides additional support for a temperate, seasonal climate. On the basis of the sedimentary evidence (see "Depositional environment of the Buckley Formation" ), the permineralized peat at Skaar Ridge probably formed in swamps and lacustrine areas on the flood plain. Plant evidence to support this scenario includes the presence in the peat of stacked layers of wellpreserved entire leaves that could have formed by settling to the bottom of a quiet body of water (Figs. 7, 10). Also present are very delicate remains such as leaf-like glossopterid reproductive structures with ovules still attached. As is true of the Fremouw Peak site, the preservation of the plant material at Skaar Ridge is very similar overall to that seen in autochthonous Carboniferous coal balls. The plant axes present in the Permian peat are generally larger (e.g., decorticated Vertebraria axes > 11 cm) than those from Fremouw Peak, which suggests the presence of a forest of more sizeable trees. Perhaps the permineralized peat represents the remains of "tree islands" similar to those known from modern peats in temperate climates (e.g., Gleason et al., 1977). One of the more interesting plant types present in this deposit is the bryalean moss, Merceria (Fig. 14) (Smoot and Taylor, 1986). Although mosses are known to occur in modern peats that are dominated by vascular plants (e.g., Hulme and Durno, 1980; Cohen, 1983), the association predominantly involves sphagnalean rather than bryalean mosses. In addition, mosses have not been described from any of the comparatively better-known Carboniferous age peats. SUMMARY AND CONCLUSIONS

The Permian and Triassic silicified peats from Antarctica represent the first occurrences of this type of preservation recorded from the continent. As such, they are an important source of information about peat formation and the composition of swamp floras at high southern latitudes during these time periods. On a worldwide basis, only one other site has yielded permineraIized peat of Permian age (in the Bowen Basin of Queensland, Australia, Gould, 1970;

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Gould and Delevoryas, 1977), and the only similar Triassic deposit is the poorly known peat from Sarawak, Malaysia (Table 2) (Gastony, 1969). At this stage, information about the floral components from these two sites is still incomplete, but there are a number of generalizations that can be noted. The Triassic flora exhibits considerably greater diversity as compared with the Permian peat. However, it is uncertain how this diversity compares to that found in floras from other paleoenvironments because the compressional floras from Antarctica are still relatively poorly known. The increased diversity of the Triassic permineralized peat as compared with the Permian peat may only represent a relatively warmer and more equitable climate during this time in the region. The relatively low diversity within the Permian peat is comparable to that found in many Late Permian floras from other Gondwanan continents and can probably be partially attributed to cooler, seasonal climatic conditions throughout the southern hemisphere during the late Paleozoic. At present, it is not clear whether this low diversity is due to the presence of a relict population of plants, as has been suggested in Carboniferous peat-swamp floras. Further quantitative work comparing the floras from these unique, peat-forming paleoenvironments to those from other depositional environments in Antarctica during these time periods is needed to address this problem. ACKNOWLEDGEMENTS

This work was supported in part by the National Science Foundation (DPP8418354 and 8611884). Helicopter support in the field was provided by the U.S. Navy VXE-6 Squadron. This represents Contribution No. 617 of the Byrd Polar Research Center.

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