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Hummocky and cracked coal seams can be useful paleoclimatic indicators for cool climates (Krull and Retallack 1995). This paper discusses a coal seam with marked thickness variation, 10 kilometers southeast of Schroeder Hill, central Transantarctic Mountains (85°23.741'S 174°50.208'W). These strata are in the Falla Formation (Collinson and Elliot 1984), which has been dated by pollen and spores in the Beardmore Glacier region as Late Triassic (Farabee, Taylor, and Taylor 1989).
One coal seam was studied in detail over a distance of 10 meters. The studied sequence consists of a basal silty layer, a thin carbonaceous underclay overlain by the coal, and an uppermost fine- to medium-grained sandstone with shaly lenses (figure 1). The coal seam shows a distinct hummocky topography ( figure 2). These hummocks are localized accumulations of permineralized, thin (3-6-centimeter) compressed logs alternating with coalified horizons. None of the logs was found in growth position, and no distinct rooting horizon was observed. Depressions have few permineralized logs or woody debris and are mainly coal.
Because highly decomposed peat compacts more than dense, woody particles do, differential compaction after burial might have enhanced the observed relief, but it cannot completely account for the ridge-and-swale structures. The overlying sandstones thicken in depressions and thin above the mounds, indicating that this microrelief predated deposition of the cover sequence. Fine, organic-rich clayey layers are restricted to areas above depressions, as if fine sediment settled preferably in lower lying areas. Erosion can also be discounted as a relief-former because individual peat layers can be traced laterally without interruption, and cut-and-fill structures are not associated with development of the ridges and swales. Absence of sand or gravel in association with logs and woody debris rule out transport and deposition within a fluvial system. The process responsible for the recurring scheme of alternating logs and peat layers within distinct hummocks must have taken place in a relatively stable environment because time for formation for these deposits was at least 6,000 to 10,000 years, using normal rates of peat accumulation (Retallack 1990). Therefore, distinct ecological conditions were the primary relief-forming mechanism for development of these structures.
Today, forested fens or bogs constitute the margins of minerotrophic mires (Tallis 1983). Mire-fringes are often invaded by trees that form on elevated, drier tops of tussocks (Tallis 1983). As the weight of the tree increases, the tussocks become unstable and eventually sink or roll over, killing the trees. This zone is also exposed to high winds. Young, shallow-rooted trees at the mire margin are more susceptible to windthrow and are easily uprooted before maturing (Allen 1992; Ehrenfeld 1995). Thus, this zone is characterized by many young dead or dying trees on depressed waterlogged tussocks. These logs from windthrow or drowning provide suitable regeneration sites because of the elevated microtopography (Ehrenfeld 1995) and the increased nutrient availability (Agnew, Wilson, and Sykes 1993). Growth of mosses on the fallen tree in the primarily waterlogged environment forms a new peat-layer. At these sites of greater nutrient availability and drier conditions, vegetation growth is favored (Ehrenfeld 1995). As the initial microrelief increases, floras less-adapted to waterlogging colonize the site as the hummock rises above the water table. Continuous peat accumulation on fallen logs, preferred growth of young trees on elevated sites, and episodic windthrow and drowning lead to the formation of hummocks with alternating peat and wood layers.
Today, similar ecotones of forested peatland and mires outside the zone of continuous or discontinuous permafrost occur at latitudes of 40° to 55° in humid, cool, temperate climate zones. During the late Triassic, Schroeder Hill was situated at comparable latitudes of 57° to 55°S (Scotese 1994), indicating that paleoclimate belts in the midlatitudes of the late Triassic were similar to today's climate zones.
I thank Shaun Norman for assistance in the field and Greg Retallack for helpful comments. Especially appreciated were the skilled pilots of Helicopters New Zealand and the organizational efforts of David Elliot and Kevin Killilea in the Shackleton base camp of 1995-1996. This work was supported by National Science Foundation grant OPP 93-15228.
References
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