INTRODUCTION

Although Gondwana glaciation has long been hypothesized as an important force driving the evolution of Earth systems during the late Paleozoic (Crowley and Baum, 1991; Montañez et al., 2007), basic questions concerning the geographical extent, timing, and character of the glaciation remain unanswered.

Antarctica is a key component in many paleogeographic and paleoclimatic models of the ice age since reconstructions show a massive ice sheet emanating from an ice center in Victoria Land and extending outward over much of Gondwana (Fig. 1; e.g., Lindsay, 1970; Barrett, 1991; Veevers, 2001; Ziegler et al., 1997). Only recently has this model been challenged (Isbell et al., 2003a, 2003b; Jones and Fielding, 2004).

59

Isbell, J.L., Koch, Z.J., Szablewski, G.M., and Lenaker, P.A., 2008, Permian glacigenic deposits in the Transantarctic Mountains, Antarctica, in Fielding, C.R., Frank, T.D., and Isbell, J.L., eds., Resolving the Late Paleozoic Ice Age in Time and Space: Geological Society of America Special Paper 441, p. 59–70, doi: 10.1130/2008.2441(04). For permission to copy, contact editing@geosociety.org. ©2008 The Geological Society of America. All rights reserved.

The Geological Society of AmericaSpecial Paper 441

2008

Permian glacigenic deposits in the Transantarctic Mountains, Antarctica

John L. IsbellZelenda J. Koch

Gina M. SzablewskiPaul A. Lenaker

Department of Geosciences, University of Wisconsin–Milwaukee, Milwaukee, Wisconsin 53021, USA

ABSTRACT

In Antarctica, late Paleozoic glacigenic strata occur throughout the Transantarctic, Ellsworth, and Pensacola Mountains and in the Shackleton and Heimefront Ranges. The most laterally and stratigraphic continuous exposures occur in the central Trans-antarctic and Darwin Mountains. These strata were deposited within two topograph-ically expressed basins. The larger of the two basins was a trough-shaped basin that extended between the present locations of the Darwin and Amundsen Glaciers. Base-ment highs surrounded the basins and formed uplands onto which preglacial, glacial, and postglacial strata onlapped. An examination of late Paleozoic glacigenic units in the Darwin Mountains and the central Transantarctic Mountains reveals that Permian glacio marine sediments were deposited within the basins, and that subglacial diamictites and proximal glaciomarine sediments were deposited along basin margins. This is in marked contrast to earlier reports that identifi ed glacigenic strata in the Transantarctic Mountains as the deposits of a terrestrial glacial system. On some highs, the occurrence of paleosols overlain by postglacial strata suggests that ice-free areas occurred locally along basin margins. A correlation of fossil spores and pollen with Australian palyno-morph zones suggests that the Antarctic glacigenic strata are restricted to the Lower Permian. These fi ndings suggest that glaciation was less widespread (temporally and spatially) than previously hypothesized. It is thus unlikely that a single, massive ice sheet covered Antarctica continuously at any time during the Carboniferous and Permian.

Keywords: Antarctica, Permian, late Paleozoic glaciation, glaciomarine, Gondwana.

Early workers identifi ed diamictites throughout Antarctica as lodgment and melt-out tills deposited from the terrestrial por-tions of an ice sheet as it waxed and waned across Gondwana (e.g., Lindsay, 1970; Barrett and Kyle, 1975; Barrett et al., 1986; Miller, 1989). Interstratifi ed conglomerates, sandstones, and mud-rocks were interpreted as remnants of eskers, glaciofl uvial out-wash, and lacustrine sediments deposited during glacial retreat and/or during interglacial periods. The occurrence of multiple diamictites within the succession led Lindsay (1970), Frakes et al. (1971), and Miller (1989) to suggest that four to six major glacial-interglacial cycles occurred during the late Paleozoic, with possibly as many as 13 minor advances and retreats of the ice front across the Transantarctic Mountains (Fig. 2). A number of striated and grooved surfaces cut on crystalline basement, Paleo-zoic strata, and Permian glacigenic deposits have been identifi ed. These features have traditionally been interpreted as the result of subglacial erosion and abrasion and have been used to determine ice-fl ow directions (e.g., Aitchison et al., 1988).

Although upper Paleozoic glacigenic strata are known to crop out throughout much of the Transantarctic Mountains and adjacent areas, many of these areas have not been examined in the fi eld in the last 20–45 yr. Even further, many of the existing studies only investigated the gross stratigraphy of a particular region. Signifi cant advances in linking glacial deposits to gla-cial and glacio marine processes and the development of glacial sequence stratigraphy postdate most of these investigations. Recently, glacio marine and associated sediment gravity fl ow deposits have been recognized from Permian Antarctic exposures (Ohio Range, Aitchison et al., 1988; Ellsworth Mountains, Matsch and Ojakangas , 1992; Darwin Glacier region, Lenaker, 2002),

raising doubts about previous terrestrial glacial interpretations. Woodworth-Lynas and Dowdeswell (1994) also suggested that some striated and grooved surfaces within the Permian succession resulted from iceberg scour rather than subglacial abrasion.

Important summaries of upper Paleozoic glacigenic strata in Antarctica include: Long (1964), Minshew (1967), Schmidt and Williams (1969), Lindsay (1970), Frakes et al. (1971), Barrett and Kyle (1975), Barrett and McKelvey (1981), Laird and Bradshaw (1981), Coates (1985), Barrett et al. (1986), Collinson et al. (1986, 1994), Aitchison et al. (1988), Miller (1989), Larsson et al. (1990), Barrett (1991), Matsch and Ojakangas (1992), Seegers (1996), Isbell et al. (1997a, 1997b, 2001, 2003a, 2003b), Isbell (1999), Tessensohn et al. (1999), and Lenaker (2002). These studies form the framework for this paper, which summarizes the present state of knowledge regarding the upper Paleozoic glacial strata in the central Trans-ant arctic Mountains (Byrd Glacier to the Ohio Range) and the Darwin Glacier region (Darwin Glacier to the Byrd Glacier). This transect forms a 1300-km-long expanse in the Trans-antarctic Mountains that stretches from the Darwin Glacier to the Ohio Range (Fig. 2). We also highlight recent interpretations and identify questions that still need resolution. Because Ant-arctica has been previously hypothesized to have been the center of late Paleozoic glaciation, a better understanding of the Ant-arctic glacial record is critical for determining the nature, dura-tion, extent, and possible drivers of late Paleozoic glaciation.

STRATIGRAPHY AND GENERAL GEOLOGY

Thick, late Paleozoic glacigenic deposits are widespread throughout Antarctica and occur in the Transantarctic (north-ern and southern Victoria Land, Darwin Glacier region, and the central Transantarctic Mountains), Pensacola, and Ellsworth Mountains (Fig. 2; Barrett, 1991; Collinson et al., 1994). Out-liers also occur in the Shackleton Range and in the Heimefront Range (Heimefrontfjella; Larsson, et al., 1990; Tessensohn et al., 1999). However, the most laterally and stratigraphically continu-ous exposures of late Paleozoic strata in Antarctica extend from the Darwin Glacier across the central Transantarctic Mountains to the Ohio Range. These strata are described here.

Central Transantarctic Mountains

In the central Transantarctic Mountains, basement and older Paleozoic sedimentary rocks underlie late Paleozoic glaci genic strata. Basement rocks consist of Precambrian meta-morphic rocks of the Nimrod Group; graywacke, mudrock, and metasedimentary rocks of the Neoproterozoic to Cambrian Beardmore Group; folded and faulted Cambrian limestones of the Byrd Group; and Cambrian-Ordovician silicic metavolcanic rocks of the Liv Group (Collinson et al., 1994). These rocks were intruded and deformed during the Cambrian-Ordovician Ross orogeny. The basement rocks are cut by a regional uncon-formity, the Kukri erosion surface, which varies from a planar

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Figure 1. Gondwana reconstruction at 290 Ma showing the glacial re-construction of Ziegler et al. (1997) and the location of an ice-spreading center in Victoria Land (VL) from Lindsay (1970) and Barrett (1991). The pole position is that of Powell and Li (1994), and the plate re-construction was furnished by the PLATES Project at the University of Texas–Austin. Hypothetical ice-fl ow directions are from Lindsay (1970), Barrett (1991), and Veevers (2001). The 290 Ma South Pole is labeled on the map. Because this is a reconstruction centered over the South Pole, all directions are north.

60 Isbell et al.

to a high-relief surface (Fig. 3; Isbell, 1999). The uncon formity is overlain by fossil-bearing Devonian sandstones and mud-rocks in the Ohio Range (Horlick Formation; Bradshaw et al., 1984), arkosic to quartzitic sandstones of unknown age between the Ramsey and Byrd Glaciers (Alexandra Formation and other unnamed units), and unfossiliferous mudrocks (Castle Crags Formation) at the head of the Starshot Glacier (Fig. 2; Laird et al., 1971; Barrett et al., 1986; Isbell, 1999). Isbell (1999) reported that these strata onlap basement rocks and were depos-ited in discontinuous, isolated, topographically expressed basins surrounded by basement highs (Fig. 3).

Late Paleozoic glacigenic strata overlie basement and sedi-mentary rocks and onlap a regional unconformity, the Maya ero-sion surface (Fig. 3). This unconformity truncates sedimentary rocks within the basins and merges with the Kukri surface across basement highs along the basin margins (Fig. 3). Relief on this surface locally exceeds several hundred meters (Isbell, 1999). A sharp surface separates glacigenic strata from postglacial rocks throughout the central Transantarctic Mountains. However, post-glacial strata also onlap and overstep the basement highs along the basin margins (Isbell et al., 1997a; Isbell, 1999). Glacigenic strata in the central Transantarctic Mountains are assigned to

the Pagoda (Byrd to Amundsen Glacier), Scott Glacier (Scott Glacier), and Buckeye (Ohio Range) Formations and consist of massive and stratifi ed diamictite, conglomeratic sandstone, sand-stone, mudrocks, and dropstone-bearing mudrocks.

Darwin Glacier Region

In the Darwin Glacier area (Fig. 1), the glacigenic Darwin Tillite unconformably overlies the 450-m-thick Hatherton Sand-stone (Fig. 2; Lenaker 2002), except locally where granitic basement rocks underlie the glacigenic strata (e.g., Roadend Nunatak). In the Cook Mountains (Fig. 2), 20 km to the northeast, the Hatherton Sandstone is overlain by an unnamed 30-m-thick quartzose sandstone that contains Middle to Upper Devonian plant fossils (Bradshaw et al., 1990), indicating that the Hather-ton Sandstone is no younger than Middle Devonian in age. The Darwin Tillite is <148 m thick and consists of massive diamic-tite, stratifi ed diamictite, sandstone, mudrock, and dropstone-bearing mudrock (Frakes et al., 1968; Barrett and Kyle, 1975; Lenaker, 2002). A black dropstone-free shale at the top of the Darwin Tillite is unconformably overlain by Glossopteris-bearing fl uvial-deltaic strata of the Misthound Coal Measures.

Figure 2. Map showing locations in the central Transantarctic Mountains (Byrd Glacier to the Ohio Range) and the Darwin Glacier area (Darwin to the Byrd Glacier). Inset map shows locations in Antarctica. The Transantarctic Mountains consist of mountain ranges (Ra.) in the central Trans antarctic Mountains (CTM), Darwin Glacier region (DG), southern Victoria Land (SVL), and northern Victoria Land (NVL). Gl.—glacier.

Permian glacigenic deposits in the Transantarctic Mountains, Antarctica 61

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Figure 3. (A) Map of the central Transantarctic Mountains (CTM) and the Darwin Glacier region showing isopachs, paleocurrent orientations, and the locations of the glacigenic basins. (B) Cross section of preglacial, glacial, and postglacial rocks parallel to the trend of the central Trans-antarctic Mountains. (C) Cross section of preglacial, glacial, and postglacial rocks oriented perpendicular to the trend of the central Transantarctic Mountains. Cross section is located in the Nimrod Glacier region. Data are from Isbell et al. (1997a) and Isbell (1999).

Depositional Basins

Glacigenic strata in the Darwin Glacier and central Trans-antarctic Mountains occur in two depositional basins (Fig. 3) that are defi ned by isopachs, onlap of the strata onto basement highs, and paleocurrent orientations (Isbell et al., 1997a; Isbell, 1999). Paleocurrent orientations have been derived primarily from stri-ated surfaces and the orientation of sole marks and cross-strat-ifi cation contained within glacigenic sandstones. One elongate, trough-shaped basin was located in the area between the Darwin and Amundsen Glaciers, while strata in the other basin are now exposed between the Scott Glacier and the Ohio Range (Isbell et al., 1997a; Isbell, 1999). Strata in these basins are a maximum of 440 and 300 m thick, respectively. Paleocurrent orientations from strata exposed between the Darwin and Amundsen Glaciers suggest transverse fl ow across basin margins and longitudinal fl ow down the axis of the basin (Fig. 3). However, these fl ow directions need to be confi rmed in light of Woodworth-Lynas and Dowdeswell’s (1994) suggestion that some of the striated sur-faces were produced by iceberg scour.

Age

Late Paleozoic glacigenic strata have been dated in only a few places in Antarctica. Intrusion of thick dolerite sills during the Jurassic caused degradation of fossil palynomorphs through-out much of the Transantarctic Mountains. Samples from the Darwin Glacier (Darwin Tillite), Nimrod Glacier (Pagoda For-mation), Ohio and Wisconsin Ranges (Buckeye Formation), and Heimefront Range (Fig. 2) areas contain fossil pollen and spores that correlate with Foster and Waterhouse’s (1988) Western Australian Asselian-Sakmarian (Lower Permian) Pseudoreticu-latispora confl uens Oppel-zone (Barrett and Kyle, 1975; Kyle, 1977; Kyle and Schopf, 1982; Lindström, 1995; Askin, 1998). Unpublished reports also suggest a Permian age for palyno-morphs recovered from the Beardmore Glacier and northern Victoria Land areas (Fig. 2; Laird and Bradshaw, 1981; R.A. Askin, 2007, personal commun.). A fossil conchostracan fauna in basal glacigenic rocks in the Shackleton Glacier region also suggests an Early Permian age (Isbell et al., 2001; Babcock et al., 2002). Further work is needed to better defi ne the age of the Ant-arctic strata and to correlate the strata with global biozones.

FACIES ASSOCIATIONS

Glacigenic strata in the Darwin Glacier and central Tran-santarctic Mountains consist of two distinctly different litho-facies associations. A basin-margin facies association occurs adjacent to basement highs along the basin margins, whereas a basinal facies association occurs along the center of the outcrop belt. The following discussion focuses on these two facies asso-ciations as seen between the Darwin and Shackleton Glaciers (Fig. 2) and also describes and discusses the postglacial strata in this region.

Basin-Margin Facies Associations

Basin-margin facies are well exposed at the head of Nimrod Glacier, near Shackleton Glacier (e.g., lower part of the Mt. Butters section, Mt. Munson), and in the Darwin Glacier area (Figs. 2 and 4). These facies overlie an undulating erosion surface with up to several hundred meters of relief cut into crystalline base-ment rocks. However, in the Darwin Glacier area, both granite and Devonian sandstone occur locally below the unconformity. Glacigenic strata onlap onto the unconformity, while postglacial strata onlap and overstep the basement highs (cf. Minshew, 1967; Isbell, 1999). In places (e.g., Mt. Munson, Mt. Butters, Nilsen Plateau), the basement rocks are polished and striated (Minshew, 1967; Coates, 1985; Isbell et al., 1997a). Roche moutonnée–like features have been reported in several areas (e.g., Mt. Butters, Wisconsin Range, Scott Glacier; Minshew, 1967; Coates, 1985), whereas Coates (1985) reported diamictite dikes injected into fi ssures in the underlying granite near the head of Scott Glacier (Mt. Blackburn). At other sites (e.g., Roadend Nunatak, Geolo-gist Range, Mt. Butters, Nilsen Plateau, Wisconsin Range; Figs. 2 and 4), meter-thick soil profi les that developed on basement rocks underlie both glacigenic and postglacial strata (Minshew, 1967; Coates, 1985; Isbell et al., 2001, 2003a, 2003b). Along the margins of a high located at Mt. Butters, the weathering pro-fi le is up to 2 m thick and consists of unweathered granite that grades progressively upward initially into poorly consolidated granite with feldspar grains, containing sparse clay coatings, and then into breccia beds. The breccias are up to 50 cm thick and inter fi nger laterally with thin ostracode-bearing mudrocks (Isbell et al., 2001; Babcock et al., 2002). At several sites on Nilsen Plateau, postglacial sandstone rests directly on brecciated and decomposed granite (Coates, 1985). A similar scenario also occurs in northern Victoria Land (Fig. 2), where postglacial coal measures rest directly on a thick weathered horizon (up to 20 m thick) developed on basement rocks (Collinson et al., 1986).

Lithofacies along basin margins are highly variable and con-sist of highly deformed units of massive and weakly stratifi ed diamictite, thick units of deformed sandstone, dropstone-bearing mudrocks, and clinoform-bearing sandstone beds (Fig. 4). Typi-cally, either sheared diamictite or sheared sandstone overlain by diamictite overlies the striated basement pavements. Within the sheared deposits, structural features (e.g., folding, faulting, folia-tion, and homogenization) indicate an upward increase in strain. Multiple shear horizons and/or striated boulder pavements typi-cally occur upward within stratigraphic sections located along basin margins (Fig. 4; e.g., Geologists Range, Darwin Glacier; Lenaker, 2002).

Much of the basin-margin facies association is composed of massive to weakly stratifi ed diamictite containing deformed sand-stone bodies and discontinuous beds of sandstone (Fig. 4). Con-tacts between diamictites and between diamictites and mudrocks are typically gradational. However, sharp and deformed contacts also occur. The diamictites range from clast-rich to clast-poor and often display internal folding. Thin discon tinuous, structureless

Permian glacigenic deposits in the Transantarctic Mountains, Antarctica 63

sandstone beds and laminae typically defi ne stratifi cation. Com-monly, these sandstones are contained within slide and slump blocks. In the Darwin Glacier, large sandstone bodies up to sev-eral tens of meters across and up to 10 m thick occur as large-scale load and pseudonodule-like structures within massive and strati-fi ed diamictites (Fig. 4; cf. Stow, 2005); diamictite diapirs pene-trate upward into the sandstone bodies (Fig. 4; Lenaker, 2002). Although less common, stratifi ed diamictite facies consisting of alternating millimeter- to decimeter-thick beds of massive diamic-tite and dropstone-bearing mudrock (dropstones pierce and deform underlying stratifi cation) also occur (e.g., Darwin Glacier region; Lenaker, 2002). Laterally continuous sandstone units also are abundant. These units are up to 15 m thick, wedge-shaped, thin over hundreds of meters in a direction parallel to paleo current orientations, and grade laterally into diamictites or mudrocks. The tops of some sandstone units also grade upward into mudrocks. Laterally within the sandstones, parallel to paleo current orien-

tations, there is a progressive change from initial trough cross-bedding (dunes), to trough cross-laminations (ripples), and fi nally to thinly bedded, massive sandstone (some with dewatering pipes) alternating with thin beds and drapes of mudrock and/or diamic-tite. These sandstones are also contained within slide and slump structures (e.g., Arrowhead Nunatak).

The onlap of glacigenic and postglacial rocks onto base-ment highs suggests that glacigenic rocks fi lled topographically expressed basins. The presence of soils suggests that some of the highs were ice-free throughout the late Paleozoic, which in turn suggests that a massive ice sheet was not centered over this por-tion of Gondwana. Breccia beds suggest formation as colluvium along the margins of some highs (Isbell et al., 2001).

The occurrence of striated basement pavements and roche moutonnées attests to the importance of subglacial erosion and abrasion along the basin margins. Diamictite dikes also sug-gest subglacial injection of fl uidized till into joints in the under-

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SandstonesSt---Trough cross-bedded--Traction depositSr---Ripple cross-laminated--Traction depositSh---Horizontally laminated--Traction deposit (PCL) or settling from suspension.Sm---Massive--Rain out or strongly coherent debris flow

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Figure 4. Stratigraphic columns from the central Transantarctic Mountains and the Darwin Glacier region showing examples of the basin-margin and basinal facies associations. Facies codes are modifi ed from Benn and Evans (1998), and the generalized interpretations for facies in this paper are given.

64 Isbell et al.

lying granite (Coates, 1985). Sheared diamictites and deposits containing structures indicative of upward-increasing strain provide additional evidence for subglacial deposition and deformation. Such deposits suggest deposition as lodgment and deformation tills (cf. Van der Wateren, 2002).

The occurrence of massive, weakly stratifi ed, and stratifi ed diamictites; dropstone-bearing mudrocks; and sandstones sug-gests subaqueous ice-proximal deposition due to various glacio-marine processes (Powell and Domack, 2002). Massive and weakly stratifi ed diamictites likely were the result of a combina-tion of settling of fi nes from meltwater plumes, release of par-ticles from melting icebergs, and deposition from debris fl ows. Dropstone-bearing mudrocks were deposited from settling of muds from suspension and the introduction of clasts from melt-ing icebergs. Formation of grounding-line fans from effl uent fl ow is indicated by the occurrence of wedge-shaped sandstone bodies that grade laterally into diamictites of mudrocks and by a change in sedimentary structures that indicates an outward decrease in fl ow velocity. Massive sandstones may have resulted from sediment gravity fl ows and settling of sand from suspension out of interfl ow and overfl ow plumes in an ice-proximal setting (cf. Marr et al., 2001; Lenaker, 2002; Powell and Domack, 2002). Evidence that suggests the occurrence of buoyant meltwater plumes includes: (1) sedimentary structures within the wedge-shaped sandstones that suggest a lateral change from tractive (cross-stratifi cation) to buoyant fl ow and rapid settling from sus-pension (massive sandstone; cf. Powell, 1990); (2) diamic tites with sand laminae, suggesting settling of sand and mud from suspension; (3) gradational contacts between mudrocks and diamictites, which suggest that sedimentation of the diamicts occurred, in part, from rain out; and (4) occurrence of clast-poor diamictites, which suggest that processes other than ice-rafting contributed to rain-out deposition. The occurrence of low-density meltwater plume deposits suggests deposition in a glaciomarine setting rather than in a lacustrine environment. An abundance of soft-sediment deformational structures, including both small- and large-scale load structures, diamictite diapirs, dewatering pipes, slide blocks, and slumped units indicates high sedimentation rates and the occurrence of unstable water-saturated substrates during deposition. Following deposition, these deposits were sub-jected to remobilization in the form of loading, sliding, slumping, and/or creeping (cf. Stow, 2005).

Basinal Facies Association

Basinal facies associations are well exposed along Tillite Glacier and in the upper portion of the glacigenic succession at Mt. Butters (Figs. 2 and 4). Throughout much of the central Transantarctic Mountains, this association rests on sandstone of unknown age (Alexandra Formation and other unnamed units) and reaches a maximum thickness of 440 m within the deposi-tional basin. However, this thickness may represent local over-deepening of the basin due to glacial erosion, as the glacigenic strata within the basin are generally less than 200 m thick.

Facies in this association consist of massive and weakly strati-fi ed diamictite, sandstone, dropstone-bearing mudrock, and mudrock (Fig. 4). Although sheared diamictites, cross-stratifi ed sandstones, and slump blocks of interstratifi ed sandstones and mudrocks also occur, these facies make up a relatively small percentage of the succession (Fig. 4).

Massive and weakly stratifi ed diamictite units are a few meters to several tens of meters thick and have gradational upper and lower contacts with underlying and overlying diamictite and mudrock units. Diamictite units consist of a clay- to fi ne-grained sand matrix and contain clasts up to 2 m in diameter. Laminae and thin wisps of silt and sand, which are better sorted than the sur-rounding matrix, defi ne faint stratifi cation, which is commonly pierced by clasts. Within the diamictites, isolated and scattered centimeter- to meter-thick and decimeter- to meter-wide pods of contorted and massive (with dewatering structures) sandstone occur. Lenses, beds, and wedges of massive and cross-stratifi ed sandstone up to 4 m thick also occur. The sandstones show abun-dant internal soft-sediment deformation, including dewatering structures, folding, and soft-sediment faulting. Abundant load structures occur at the base of these sandstones, while dropstones deform their upper surfaces. Dropstone-bearing mudrocks up to a few meters thick typically separate diamictite units (Fig. 4).

Mudrock units are tens of meters thick and often contain intervals of dropstone-bearing mudrock and beds of fi ne-grained, ripple cross-laminated, internally folded, and/or massive sandstone. Some intervals of interstratifi ed sandstones and mudrocks are incorporated into slump and slide blocks. Dropstone-bearing mudrocks also occur at the base and at the top of thick mudrock units.

A single sheared siltstone and sandstone interval occurs at the top of the glacigenic section at Tillite Glacier (Figs. 2 and 4). This interval begins with a meter-thick siltstone unit that con-tains shear structures (e.g., folding, thrust faults, boudins) that show increasing strain upward. A 3-m-thick sandstone with shear laminations caps the deformation zone. A wedge-shaped sand-stone body with distinctive grooves on its upper surface occurs at Mt. Butters (Fig. 2). The grooves are up to 15 m long and up to 0.5 m deep. The grooves pass laterally into unmodifi ed sandstone.

Massive and weakly stratifi ed diamictites suggest deposi-tion from a combination of suspension settling from meltwater plumes along with the introduction of iceberg-rafted debris (cf. Cowan and Powell, 1991; Dowdeswell et al., 2000). Stratifi ca-tion within the diamictites may be the result of winnowing by currents, periodic remobilization of the deposits as sediment gravity fl ows, or fl uctuations in sediment fl ux near the edge of effl uent jets (Powell and Domack, 2002). The occurrence of sandstones suggests deposition from meltwater outfl ow across the distal portions of grounding-line fans (cross-stratifi ed, wedge-shaped sandstones) and deposition as strongly coherent debris fl ows (massive sandstone with dewatering structures; cf. Powell, 1990; Marr et al., 2001). Distal glaciomarine conditions are suggested by mudrocks with dropstones, whereas open-marine conditions are indicated by thick dropstone-free mud-

Permian glacigenic deposits in the Transantarctic Mountains, Antarctica 65

rocks. The occurrence of sheared deposits suggests subglacial deposition and deformation as deformation till. However, these deposits are rare within strata of the basinal facies association. Although grooved surfaces have been interpreted as products of subglacial abrasion and erosion (Aitchison et al., 1988), grooved and striated surfaces that grade laterally into unmodi-fi ed deposits are more likely the result of scour from iceberg keels (Woodworth-Lynas and Dowdeswell, 1994).

Postglacial Facies Association

Postglacial mudrocks and sandstones are perhaps the most extensive strata exposed in the Transantarctic Mountains, and they occur in a near-continuous belt that extends from the Darwin Gla-cier to the Ohio Range (Figs. 2 and 3). The contact that separates glacigenic diamictites and sandstones below from post glacial dark shales and sandstones above is a sharp, generally planar surface that can be traced throughout the central Trans antarctic Mountains (Figs. 3 and 4). An equally sharp contact near the top of the Darwin Tillite also separates diamictites below from dark shales above. The postglacial strata are gradationally to discon-formably overlain by fl uvial sandstones and coal-bearing fl uvial Lower and Upper Permian strata (Collinson et al., 1994).

In the central Transantarctic Mountains, postglacial rocks consist of multiple, 15–50-m-thick coarsening-upward suc-cessions of mudrock and sandstone. Black to dark-gray shale (1–5 m thick) occurs at the base of each coarsening-upward succession and grades progressively upward, initially into inter-stratifi ed and interlaminated mudrock and sandstone (7–30 m thick), and then into medium-grained sandstones (5–10 m thick) containing clinoform surfaces at the top of each succession. The number of coarsening-upward successions (1–5) varies through-out the central Transantarctic Mountains (Seegers, 1996). Within the interstratifi ed and interlaminated sandstone and mudrock facies, centimeter- to meter-scale sandstone beds fi ne upward (graded bedding) and are commonly horizontally bedded (with primary current lineation) and cross-laminated (trough cross-lamination and climbing ripple lamination). Clinoform surfaces within the medium-grained sandstone dip from 1° to 29° and contain trough cross-laminations. The bottoms of the dipping beds are tangential and interfi nger with underlying mudrock and sandstone lithofacies. Channel-form sandstone bodies over-lie the uppermost postglacial sandstone.

Rare matrix-supported diamictites occur as lenses within 1.5 m of the base of the lowest coarsening-upward succes-sions. These lenses are 8–30 cm thick, 1–15 m long, and con-sist of structureless, medium-grained sandstone with scattered pebbles . At the same stratigraphic level, rare pebbles can be found penetrating laminae within mudrocks. Although rare, these pebbles are more abundant along basin margins than in the center of the depositional basin (Seegers, 1996). Rare trace fossils occur on the tops of beds throughout the postglacial interval and are characterized primarily by horizontal, bilobed trails (Isopodichnus; Miller and Collinson, 1994).

The sharp contact between glacial and postglacial rocks in the Darwin Glacier and central Transantarctic Mountains marks an abrupt change in depositional conditions from high sedimenta-tion rates to sediment-starved conditions. Diamictite lenses were likely the result of iceberg dump, while dropstones indicate depo-sition from ice rafting (cf. Powell and Domack, 2002). However, the rarity and distribution of these features suggest that icebergs only entered the postglacial basin early in its history, and even then icebergs were rare (Seegers, 1996). Structures within the interstratifi ed sandstone and mudrock facies suggest that under-fl ows were an important process in the deposition of the coars-ening-upward successions. Sandstones containing clinoform surfaces capping these coarsening-upward successions are inter-preted as the product of prograding deltas (Miller and Collinson, 1994; Seegers, 1996). Fluvial processes deposited the overlying channel sandstones (Barrett et al., 1986; Collinson et al., 1994; Isbell et al., 1997b).

TIMING AND STYLE OF GLACIATION

The preservation of Permian preglacial soils on basement rocks beneath the glacigenic strata, as well as the presence of soils on basement highs that are onlapped and overstepped by post glacial rocks, raises questions as to the duration, extent, style (alpine ice cap, ice sheet), and glacial thermal regime of late Paleozoic glaciation in Antarctica. Two different hypotheses can be used to explain these features. The fi rst hypothesis states that a polar ice sheet glaciated Antarctica. Under this scenario, little erosion would have occurred during glacial advance and retreat cycles, thereby preserving the soil horizons (cf. Miller et al., 2005). However, this assumption may be untenable, because Atkins et al. (2002) have recently shown that erosion and depo-sition can occur beneath cold-based glaciers. This hypothesis is inconsistent with evidence that suggests deposition occurred from wet-based temperate glaciers (see following discussion). The other scenario is that glaciers did not advance over this region of Antarctica until the Permian and that some areas were protected from erosion. In this scenario, soils on top of paleogeographic highs covered by postglacial rocks suggest that some areas of Antarctica remained ice-free throughout the late Paleozoic. The occurrences of glacial striations on basement pavements, deformation tills, and meltwater plume and grounding-line fan deposits attest to the importance of subglacial meltwater during deposition of glacigenic strata in the Darwin Glacier and central Transantarctic Mountains. These deposits strongly suggest that these glaciers were characterized by temperate (wet-based) ther-mal conditions. Further study is required to ascertain the timing and style of glaciation in other areas of Antarctica.

OVERALL DEPOSITIONAL SETTING

Detailed stratigraphic and sedimentologic studies of glaci-genic strata in Antarctica have been conducted at only a few sites. Despite this limitation, recent studies suggest that deposition in

66 Isbell et al.

the Darwin Glacier and central Transantarctic Mountains occurred primarily in subaqueous settings rather than in terrestrial settings, as was previously thought. Strata in these areas were deposited in at least two large basins bordered by basement highs. Sub-glacial and glacially infl uenced basinal strata interfi nger along basin margins. In these areas, multiple, sheared diamictites sug-gest that grounded ice advanced into the basin while ice-proximal glaciomarine deposition occurred basinward of the grounding line. Ice-proximal deposits include grounding-line fans, possible morainal banks, diamictites resulting from settling of fi nes from meltwater plumes in association with debris rafted by icebergs, debrites, slides, and slumps. These deposits grade basinward into meltwater plume and iceberg-rafted deposits, deposits reworked by iceberg scour, and fi nally into mudrocks. Mudrocks free of ice-rafted debris may record the retreat of glaciers out of the depositional basin and onto land. Although multiple subglacial diamictites have been identifi ed along basin margins, the number varies from site to site. Therefore, the current state of knowledge prevents identifi cation of widespread advance and retreat cycles across the depositional basins.

The sharp contact between glacigenic deposits and post-glacial mudrocks records an abrupt change in environmental conditions in the Darwin Glacier and central Transantarctic Mountains. Although this surface has been previously identi-fi ed as a fl ooding surface (Isbell et al., 1997a), the contact can also be explained by glacial retreat out of the depositional basin and into a terrestrial setting without a change in water levels (cf. Powell and Cooper, 2002). However, the presence of paleo-sols on basement highs directly overlain by postglacial strata suggests fl ooding of the depositional basin and submergence of previously exposed areas. The scarcity of ice-rafted debris and iceberg dump structures within the postglacial mudrocks is consistent with glacial retreat beyond the basin margins. Such postglacial strata fi rst record a basin starved of coarse clastics, then deposition by underfl ow currents, and fi nally sediment deposited by deltaic progradation.

Lindsay (1970) suggested that the postglacial succession was deposited in an isostatically depressed basin that formed by glacial loading from a terrestrial ice sheet that fl owed out of an ice center in Victoria Land and across the central Transantarctic Mountains toward the Ohio Range. Under such a scenario, the greatest amount of subsidence would have been directly under the thickest ice near the center of the ice sheet. According to the model, the postglacial basin formed during fl ooding of the basin as the ice front retreated across the central Trans antarctic Mountains. Deltaic progradation and a return to nonmarine conditions were postulated to have occurred due to isostatic rebound (cf. Lindsay, 1970; Barrett et al., 1986; Collinson et al., 1994). This model places the Darwin Glacier near the center of the ice sheet and the central Transantarctic Mountains located close to an ice margin. Under such circumstances, the greatest amount of isostatic subsidence would have occurred in the Darwin Glacier , with subsidence decreasing in magnitude across the central Transantarctic Mountains toward the Ohio

Range. However, thick glacial and postglacial rocks occur in the Ohio Range, while equivalent strata in the Darwin Glacier and near the Byrd Glacier in the central Transantarctic Moun-tains are relatively thin (Seegers, 1996; Lenaker, 2002), which is opposite to the trend predicted by that model.

Interpretations of the salinity of waters within the deposi-tional basin during glacial and postglacial times are contradic-tory; reports have proposed the occurrence of fresh, brackish, and marine waters within the basin. Interpretations are based on the occurrence or absence of arthropods, acritarchs, and trace fos-sils; diversity of the fossils; carbon/sulfur ratios; iron content; presence or absence of pyrite; thickness and homogeneity of the deposits; and sedimentary structures (Minshew, 1967; Frakes and Crowell, 1975; Matsch and Ojakangas, 1992; Bradshaw, et al., 1984; Collinson et al., 1994; Miller and Collinson, 1994). For the glacial deposits, the occurrence of thick homogeneous suc-cessions of diamictites with intervening dropstone-bearing mud-rocks is used to suggest that marine conditions prevailed in the Ellsworth Mountains, while possible marine acritarchs and low iron abundances have been used to suggest glaciomarine condi-tions in the Ohio Range (Frakes and Crowell, 1975; Matsch and Ojakangas, 1992; Aitchison et al., 1988). Throughout much of the central Transantarctic Mountains, the association of diamictites with striated surfaces has been used as an indication of glacial terrestrial conditions, while dropstone-bearing mudrocks have been interpreted as lacustrine deposits (Lindsay, 1970; Miller, 1989). In contrast, this paper describes an abundance of deposits associated with meltwater plumes, suggesting that glacial melt-water was interacting with higher-density waters by entering a glaciomarine setting as buoyant freshwater plumes. Although ostracod and conchostracan fossils at the base of the glacigenic succession at Mt. Butters indicate freshwater conditions, the occurrence of these fossils can be explained by postmortem accumulation in low-salinity waters close to abundant glacial meltwater, or by evoking lacustrine deposition within the basin prior to fl ooding and development of glaciomarine conditions. Interpretations of the salinity during deposition of the postglacial mudrocks are also contentious. A low-diversity trace fossil fauna and carbon/sulfur ratios suggest that fresh to brackish waters occurred in the Beardmore Glacier area (Miller and Collinson, 1994), whereas the occurrence of the trace fossil Paleodictyon and pyrite in the Scott Glacier area suggest that deposition occurred under marine conditions. One possible explanation that may satisfy the various observations is that deposition occurred within a long narrow marine embayment subparallel to the present trend of central Transantarctic Mountains, where waters became more brackish up the embayment. Such conditions occur today in the Baltic Sea and in fjords (cf. Lindsay, 1970; Syvitski, 1989). The salinity of the waters in the depositional basins, and hence the trace fossil assemblages and diversity, was likely infl uenced by an abundance of glacial meltwater. Although the emplacement of the Jurassic sills altered some of the strata in the central Trans-antarctic Mountains, further geochemical work is needed to help resolve the salinity debate.

Permian glacigenic deposits in the Transantarctic Mountains, Antarctica 67

For much of the Darwin Glacier and central Transantarctic Mountains, facies and paleocurrent data suggest that glaciers transversely crossed the basin margins and fed sediment and meltwater into a basin elongate parallel to the present trend of the central Transantarctic Mountains. Throughout much of gla-cial time, waters covered the Darwin Glacier and central Trans-antarctic Mountains areas and only rarely did glaciers advance beyond the basin margins. Striated surfaces within the basin proper were likely the result of iceberg scour (Woodworth-Lynas and Dowdeswell, 1994). The occurrence of soils on basement highs overlain by postglacial rocks suggests that not all of the central Transantarctic Mountains were covered by ice during the late Paleozoic, thus further calling into question whether glacial loading played a signifi cant role in the formation of the basins in the Darwin Glacier and central Transantarctic Mountains, and whether this area was ever entirely overlain by an ice sheet during the late Paleozoic. The onlapping of preglacial, glacial, and post-glacial strata onto basement highs indicates that the basin was a long-standing feature (Isbell, 1999).

CONCLUSIONS

Only a few areas containing late Paleozoic strata in Antarc-tica have received intense scrutiny using modern sedimentologi-cal and stratigraphic techniques. Despite this limitation, recent studies suggest that deposition occurred in a glaciomarine setting rather than under terrestrial conditions, as previously reported. Glacigenic strata in the Darwin Glacier and central Trans-antarctic Mountains record deposition in at least two large basins surrounded by basement highs. Along basin margins, subglacial deposits interfi nger with proximal glaciomarine deposits; within the depositional basin, distal glaciomarine and basinal deposits dominate. The occurrence of paleosols and colluvial deposits over-lain by glaciomarine strata and their occurrence on uplands elevated a few hundred meters above the basin fl oor suggest that glaciation in Darwin Glacier and central Transantarctic Moun-tains did not begin until the Permian. These features also suggest that an ice sheet did not cover some areas during the late Paleozoic as previously hypothesized. Although palynological data iden-tify the glacigenic rocks as Lower Permian, additional data are needed to reliably correlate these strata with glacigenic strata in other areas of Antarctica and Gondwana. Further study of the rocks in the Darwin Glacier, central Transantarctic Mountains, and other areas of Antarctica is needed to help constrain regional variations and to test the hypotheses presented in this paper.

ACKNOWLEDGMENTS

We thank Lindsey Henry, Rosemary Askin, Molly Miller, Pete Flaig, Tim Cully, Sue Giller, John Roberts, and Shaun Norman for discussions and/or help in the fi eld. Comments by Larry Krissek and Chris Fielding on an earlier draft of this paper are greatly appreciated. The National Science Founda-tion, Raytheon Polar Services, the New York Air National Guard,

Kenn Borek Air Ltd., Trans World Logistics, and Petroleum Heli-copters Incorporated provided logistic support for fi eld work in Antarctica. This work was supported by National Science Foun-dation grants OPP-9909637, ANT-0126086, and ANT-0440919, and a grant from the University of Wisconsin–Milwaukee.

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