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Exposure ages for Pleistocene periglacial deposits in Australia
ARTICLE IN PRESS
Quaternary Science Reviews 23 (2004) 697–708
Exposure ages for Pleistocene periglacial deposits in Australia Timothy T. Barrowsa,*, John O. Stoneb, L. Keith Fifieldc b
a Research School of Earth Sciences, Australian National University, Canberra ACT 0200, Australia Department of Earth and Space Sciences and Quaternary Research Center, University of Washington, Box 351310, Seattle 98195-1310, USA c Department of Nuclear Physics, Research School of Physical Sciences and Engineering, Australian National University, Canberra, ACT 0200, Australia
Received 1 April 2003; accepted 23 October 2003
Abstract Pleistocene periglacial landforms are widespread in Australia and provide valuable information on past temperatures, but dating the time of their formation has proven difficult. To remedy this, we have explored the use of cosmogenic 36Cl for direct dating of periglacial deposits. We sampled six deposits in four locations in southeastern Australia, ranging from blockstreams and block slopes to former rock glaciers. Eighteen exposure ages reveal a concentration of periglacial activity during the last glacial maximum (LGM) between 16 and 23 ka, with a population having a weighted mean age of 21.970.5 ka. This age is shortly before the time of maximum ice advance during the LGM in southeastern Australia. Exposure dating of block deposits provides a way of extending the chronology of cold climate activity beyond glaciated regions. r 2003 Elsevier Ltd. All rights reserved.
1. Introduction Pleistocene periglacial landforms are far more widespread in Australia than glacial deposits. Galloway (1965) mapped solifluction deposits and estimated that most of southeastern Australia above 600 m in altitude probably experienced a periglacial climate during the coldest phase of the last glaciation. Periglacial processes in alpine climates were therefore an important influence in shaping southeast Australia’s landscape. Modern periglacial activity is limited to a few mountains only. Despite more than a century of research describing Australian glaciation, periglacial landforms have received relatively little attention and have frequently been misinterpreted as glacial deposits (Galloway, 1963). Galloway (1965) showed that periglacial landforms could be more reliable recorders of past temperatures than glaciation because their formation was less sensitive to changes in precipitation. Galloway estimated that the height of the last glaciation was B9–11 C cooler than present in southeastern Australia, despite the snowline *Corresponding author. Current address: Department of Nuclear Physics, Research School of Physical Sciences and Engineering, Australian National University, Canberra, ACT 0200, Australia. Tel.: +61-2-6125-2077; fax: +61-2-6125-2083. E-mail address: [email protected] (T.T. Barrows). 0277-3791/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2003.10.011
being only B800 m lower in altitude, a result of much drier conditions. After Galloway’s (1965) initial study showed the value of Australian periglacial deposits in climatic reconstruction, the next decade of research rapidly expanded knowledge on the subject (e.g., Talent, 1965; Caine and Jennings, 1968; Costin, 1972; Costin and Polach, 1973; Webster, 1974). A significant problem faced by research into Pleistocene periglacial deposits is placing the climatic information derived from landforms into a temporal framework. The existing chronology is limited to a few sites only, where organic soil or wood incorporated within or beneath a deposit has been dated with 14C. These dates provide only maximum limits for the initiation of periglacial activity. Because of these limits and the scarcity of material for radiocarbon dating, there is a need for a method that can directly date the formation of periglacial deposits to harness their potential as climatic indicators. In recent work, we have shown that exposure dating of glacial deposits using cosmogenic nuclides can provide a quantitative chronology for the presence of ice in several Australian locations (Barrows et al., 2001, 2002). For exposure dating to be successful there should be no prior exposure and the surface should remain continuously exposed in the same geometry. Provided these criteria are met, periglacial deposits are candidates for exposure dating. A reliable methodology will allow
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us to directly date cold climate landforms in a nonglacial environment. This paper reviews the chronology of periglacial activity in Australia during the Pleistocene. In particular, we assess the feasibility of exposure dating periglacial deposits at four locations in Australia. Our aim is to evaluate the effects of prior exposure and postglacial modification of the landforms, as limits to deriving direct ages. The new ages for periglacial activity in Australia are compared with the existing glacial chronology from adjacent areas.
36°S
Australia
RavineSection Ridge Mt HothamMt Little Higginbotham
40°S
2. Sampling strategy For exposure dating, we chose deposits consisting of meter-sized blocks generated by frost shattering at outcrops and cliff faces. These landforms are the least likely to be affected by prior exposure or disturbance after deposition. Nonetheless, the cosmogenic nuclide concentration on a block surface is accumulated in three stages: before a block falls from the free face, in transport while the landform is active, and since the deposit became inactive. Therefore the simple exposure age calculated for a block may be much greater than the true age of the deposit if its prior exposure was long, or less if the deposit has not been stable since the periglacial event. The exposure period that we wish to record is bracketed by the onset of block generation from the outcrop and the time when the deposit became inactive. Exposure age measurements on individual blocks will fall in this time range as long as the initial exposure at the outcrop was short and the deposit has remained undisturbed since periglacial activity ceased. Four locations were chosen to cover a range of different deposits and climatic settings (Fig. 1). We were highly selective in only sampling the largest blocks and avoided spalled or weathered surfaces. All of the sites examined have minimal snow cover during the winter months. We were able to identify original joint faces on most dolerite and basalt blocks derived from columns, indicating negligible post-depositional erosion. There is abundant evidence that the deposits are presently inactive: the blocks have weathering rinds; are covered by lichens and mosses; organic debris has accumulated between the blocks at depth; and vegetation is colonizing the edges. Descriptions and dimensions of the deposits are also included to provide context for the interpretation of likely exposure history.
Ben Lomond Plateau Mt Wellington
44°S 140°E
144°E
148°E
Fig. 1. Regional map of southeastern Australia with sample locations.
3.1. Ravine, Snowy Mountains, New South Wales
ideal areas for the formation of blockstreams, some of which were described by Caine and Jennings (1968). For exposure dating, we targeted those on the western aspects of the Section Ridge near Ravine, near from the northern end of the Toolong Range (Fig. 2), because their very low altitude signifies they must have formed during a period of maximum cooling. The Ravine blockstreams are situated from B1100 to 1200 m in altitude, about 500 m lower in elevation than those described by Caine and Jennings (1968), and B800 m lower than last glacial maximum (LGM) glacier equilibrium lines around Mt. Kosciuszko to the south (Barrows et al., 2001). They also have a representative morphology of a common landform in southeast Australia. The tops of the blockstreams are located near free faces at the ridge crest and lie on slope angles of 15–30 . Frost shattering has occurred predominantly along joints in the columnar basalt creating a fairly uniform block size of 30–50 cm but some columns are occasionally longer than 1 m. The thickness of the blocks in the elongate blockstreams as revealed in road cuts is rather shallow, with a maximum depth of B1–2 m. The blockstreams range up to B300 m in length but usually less than 10–20 m in width. Arcuate ridges formed by block flow that run the full width of the blockstreams are well preserved. Of the four main blockstreams at Ravine, we chose the longest for further study (blockstream ‘B’ in Fig. 2). This deposit is narrow and begins in a subdued slope at the top of the ridge where the block size is finest. Sample sites are listed in Table 1. We sampled for exposure dating in four ways:
Basalt outcrops are common in the Toolong Range of the Snowy Mountains and their morphology creates
(1) Three basalt columns were selected in an altitudinal transect (Block 1: RAV-001/RAV-002, Block 2:
3. Sites
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(Webster, 1974). A group of representative deposits was chosen for exposure dating from Mt. Little Higginbotham (1750 m), a basalt-capped mountain located B4 km southeast of Mt. Hotham. There are two main types of deposits (Fig. 3), both common in the region. The first type consists of block aprons on the low-angle flanks around the basalt cap (‘proximal deposits’), similar to those around Mt. Byron at Ravine. There is no preference for aspect, but the most extensive aprons are on the northern and eastern sides. The flanks of the basalt cap can be steep (40–50 ) and high (up to 50 m) and are draped with fine block debris and numerous outcrops. Physical weathering along the wide jointing of the basalt has resulted in large blocks (up to 1–2 m) in the deposits, especially on the southwest side where the jointing is widest. The block deposits have slope angles as low as 5 , but are locally as steep as 30 . Road cuts (such as ‘C’ in Fig. 3) reveal that the deposits are only a few meters thick. The second type of block deposit (‘distal deposits’) lies further down the mountain below the block aprons, separated by a vegetated strip of blocks filled with interstitial soil. The bulk of these deposits are completely free of fine sediments with the exception of longitudinal ‘islands’ consisting of blocks with a soil matrix. The largest distal deposit (‘A’ in Fig. 3) lies on
Fig. 2. Location of Ravine blockstreams and sample sites. Stippled areas are block deposits and bold lines are the Section Ridge crest and the edge of the Mount Byron plateau. Dashed line is the Goobarragandra powerline trail.
RAV-005, Block 3: RAV-006/RAV-007) to determine whether the age of the deposit varies significantly from head to toe. (2) Samples were taken from both ends of two individual columns (Block 1: RAV-001+RAV002, Block 3: RAV-006+RAV-007) to determine whether significant exposure occurred at one end of the column before block production. (3) A whole small block (RAV-010; 11.5 cm thick) was collected from the surface to investigate whether it had the same exposure history as the large blocks, and whether exposure ages of thicker blocks had been affected by overturning during transport. (4) A sample was taken from the top of a free face (RAV013), adjacent to the head of the blockstream to measure the cosmogenic nuclide concentration that has accumulated since the blockstream became inactive. 3.2. Mt. Little Higginbotham, Victorian Alps Basalt block deposits occur in the Mt. Hotham area on Mt. Loch, Mt. Battery and the Bogong High Plains
Fig. 3. Location of Mt. Little Higginbotham block deposits and sample sites. Stippled areas are block deposits and bold line is the edge of the Mt. Little Higginbotham plateau. Dashed line is an access road.
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700 Table 1 Site data Sample
Longitude ( E)
Latitude ( S)a
Altitude (m)
Scaling factor (nucleons)b
Scaling factor (muons)b
Horizon correction
Thickness (correction for spallation)
Thickness (correction for thermal n)
Thickness (correction for epithermal n)
36.78 36.51 35.64 36.68 36.85 36.89 34.39
1170 1170 1100 1190 1190 1190 1200
2.299/2.343 2.299/2.331 2.181/2.172 2.333/2.374 2.333/2.382 2.333/2.384 2.351/2.281
1.541/1.557 1.541/1.552 1.494/1.490 1.554/1.569 1.554/1.572 1.554/1.573 1.561/1.536
0.9869 0.9869 0.9869 0.9869 0.9869 0.9869 1.0000
0.9705 0.9657 0.9633 0.9657 0.9763 0.8887 0.9515
1.254 1.289 1.307 1.288 1.212 1.612 1.377
1.050 1.057 1.061 1.057 1.042 1.112 1.075
Victorian Highlands: Mt. Little Higginbotham LHB-05 147.16 37.00 37.52 1680 LHB-06 147.16 37.00 36.6 1680 LHB-07 147.16 36.99 41.06 1520
3.422/3.461 3.422/3.392 3.049/3.302
1.945/1.956 1.945/1.936 1.817/1.895
0.9916 0.9807 0.9869
0.9468 0.9515 0.9468
1.388 1.386 1.405
1.089 1.082 1.089
Ben Lomond Plateau: Carr Villa BLM-05 147.7272 41.5133 BLM-06 147.7272 41.5133 BLM-08 147.7272 41.5133 BLM-10 147.7272 41.5133
41.99 45.19 44.79 46.14
1360 1310 1290 1250
2.950/2.972 2.841/2.999 2.798/2.936 2.713/2.900
1.776/1.783 1.737/1.792 1.722/1.770 1.692/1.759
0.9812 0.9696 0.9818 0.9970
0.9368 0.9732 0.9633 0.9281
1.500 1.248 1.328 1.539
1.078 1.043 1.055 1.094
Denisons Crag BLM-19 147.7200
41.6167
46.27
1285
2.791/2.986
1.720/1.788
0.9874
0.9678
1.286
1.051
Mount Wellington MTW-29 147.2000 MTW-30 147.2000 MTW-32 147.2000
42.9000 42.9000 42.9000
46.41 47.56 47.13
1220 1220 1240
2.706/2.843 2.706/2.888 2.748/2.917
1.690/1.740 1.690/1.757 1.705/1.766
0.9989 0.9989 1.0000
0.9633 0.9500 0.9656
1.315 1.404 1.296
1.050 1.070 1.049
Latitude ( S)
Snowy Mountains: Ravine RAV-01 148.4167 35.8333 RAV-02 148.4167 35.8333 RAV-05 148.4167 35.8333 RAV-06 148.4167 35.8333 RAV-07 148.4167 35.8333 RAV-10 148.4167 35.8333 RAV-13 148.4167 35.8333
a b
Effective geomagnetic latitude. First number for geographic latitude, second number for effective geomagnetic latitude.
the northeast side of the mountain and is B275 m long and B120 m at the widest point. The surface of the deposit has ridges that can run the width of the deposit with a relief of 0.5–1 m. Arcuate bulges and conical pits lie on the ridges and the main body of the deposit, the latter commonly elongated in the down slope direction. The bulk of the deposit has a slope angle of B25–30 , but has sections above the ridges with a lower angle of B20 . A section towards the terminus of the deposit reveals a 3–4 m thick surface layer entirely of blocks resting on sedimentary bedrock. The presence of a different rock type underneath the deposit and the lack of mixing of the rock types confirm the lateral transport of blocks across the surface. Original columns of basalt at the surface are rare and some blocks are regolith corestones and show very thick weathering rinds. Samples (Table 1) were taken from the two main classes of block deposits as follows: (1) Two samples were taken from the proximal block aprons (LHB-005, LHB-006). These samples were chosen to determine the age of the last major phase of block production.
(2) One sample was taken from the largest distal deposit at Mt. Little Higginbotham (LHB-007). This block had original faces and was one of the largest on the lower end of the slope. We assumed that this block was produced at the free face during the penultimate phase of block production. Due to their weathered condition, few other blocks were suitable for sampling at this site. 3.3. Ben Lomond Plateau, Tasmania The block deposits at Ben Lomond are the best described and most extensive in Australia (Caine, 1983). There is abundant evidence for at least one episode of ice cap glaciation over the Ben Lomond Plateau in northeastern Tasmania. The most recent glaciation was during the LGM when only small cirques were active, including Tranquil Tarn (Barrows et al., 2002). Rock fall since the LGM has produced only small talus cones within the limits of glaciation at Tranquil Tarn. However, within the limits of the ice cap glaciation there is extensive development of blockslopes and talus below the dolerite cliffs, and blockstreams and
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Fig. 4. Northern flank of the Ben Lomond Plateau and sample sites. Stippled areas are block deposits and bold line is the edge of the Ben Lomond Plateau. Dashed lines are roads. Circular symbols are glacially sculpted bedrock (from Caine, 1985) and the tick indicates the direction of ice flow.
blockfields on the plateau. In places, blockstreams originating from vast, low-gradient solifluction slopes issue off the plateau onto the talus slopes. The age of the ice cap glaciation is poorly constrained and hence the length of time the block deposits have been forming is unknown. There are minimum dates on the blockfields of 3080–4700 14C yr BP on organic material in soil profiles (Caine, 1983). Two areas of Ben Lomond were selected for exposure dating, the Carrvilla blockslope on the northern flank (Fig. 4), and the blockslope below Denisons Crag on the southern flank. The blockslopes at both locations consist of high-angle aprons of block debris that encroach on vegetated areas of till. The blockslopes do not lie at the angle of repose, do not show the morphology of gravity-dominated talus slopes nor show sorting of block size with distance from the cliff. Talus cones of classical shape are comparatively rare locally and are restricted to within the LGM cirques and behind large-scale topples (Caine, 1983). The accumulation of blocks is clearly linked to the presence of dolerite cliffs, which are up to 100 m high. The massive jointing of the dolerite contributes to the formation of very large blocks, most frequently on the scale of 1–6 m in length. The depth of the deposits is unknown but road sections through other deposits suggest a thickness of at least 5 m. Block contribution to the existing slopes appears to be presently rare because there are few fresh rock fall scars. The most common surface morphology on the blockslopes is the presence of large transverse ridges roughly parallel to the slope contours and bowing down slope. The blockslopes also commonly show hollows, furrows, a convex form, a bulging lower section, and
upper surface gradients much lower than the angle of repose (Caine, 1983). This resemblance to rock glaciers indicates that although the blockslopes may have originally accumulated as talus slopes, they have subsequently been deformed by slow flow (Caine, 1983). This was most likely the response to a lowering of internal friction due to the presence of interstitial ice and, possibly, fine sediments at depth. Five samples (Table 1) were collected from these blockslopes as follows: (1) Four blocks were sampled along a transect on the Carrvilla blockslope (Block 1: BLM-005, Block 2: BLM-006, Block 3: BLM-008, Block 4: BLM-010). This was to determine whether block production occurred in a single discrete event or whether the age of the deposit increases with distance from the cliff. (2) A single block on the Denisons Crag blockslope (BLM-019) was sampled to date the age of the block slope in which Tranquil Tarn is situated. This sample was taken at about 100 m east of Denisons Crag (Fig. 2 in Barrows et al., 2002). 3.4. Mt. Wellington, Tasmania The most southerly area examined was Mount Wellington, overlooking Hobart in Tasmania. This location is the closest to the coast of any of the sites and, consequently, has a maritime montane climate. This dolerite plateau reaches 1270 m and has a broad, shallow depression across to the Thark Ridge in the west (Fig. 5). Broad depressions on the Mt. Wellington plateau are covered by vast deposits of coarse diamicton consisting
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Fig. 5. Location of Mt. Wellington block deposits and sample sites. Stippled areas are block deposits and bold line are the edge of the Mt. Wellington plateau and the crest of the Thark Ridge.
of weathered blocks supported in clay-rich fine sediment and covered by alpine vegetation. The deposits lie on slope angles as low as 3 and have very smooth profiles. These deposits were originally confused for glacial till but there is no other evidence for glaciation on the plateau in the form of cirques, ice-eroded bedrock or moraines. The deposits were reinterpreted as solifluction slopes (Davies, 1958), similar to those occurring on the top of the Ben Lomond Plateau. The blocks in the solifluction slopes appear to be largely derived from preexisting regolith and some are in an advanced state of decomposition. The fine matrix has been removed locally around the edges of the deposits and along drainage lines to leave block-supported deposits. These block deposits are occasionally elongated into a blockstream shape and can show longitudinal furrows and transverse ridges. Mt. Wellington shows much less extensive talus development below its prominent cliffs (such as ‘The Organ Pipes’) than Ben Lomond. Exposed upstanding columns at the highest elevations tend to be rounded and exfoliated. Aprons of dolerite columns, none of which are very extensive or form blockstreams, often flank outcrops and free faces along the Thark Ridge.
In many places, the most recently fallen blocks encroach upon the weathered diamicton that forms part of the older blockfields. The presence of reactivated older solifluction deposits and the appearance of a single period of subsequent block production suggests at least two major periods of periglacial activity. The first phase was responsible for the extensive solifluction slopes, probably reworking much of the pre-existing regolith and exposing outcrops. The second phase was responsible for local reactivation of the blockfields and also enhanced block production from exposed outcrops by frost shattering. Sampling at Mt. Wellington was designed to determine whether exposure dating of periglacial deposits was possible in an area of low physical erosion and short free faces. Samples were taken from the Thark Ridge (Table 1), as follows, to constrain the age of the last phase of block production: (1) A large column was selected from a deposit that formed part of a block cover encroaching on older deposits below low dolerite outcrops (1–2 m in height). Two samples were taken, from the rounded former top of the column (MTW-029) and from the
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fractured end (MTW-030), which would have been shielded by 2.2 m when the column stood upright. The column was lying approximately 10 m from the free face where it originated. (2) The top of a free face was sampled (MTW-032) from the crest of the ridge above the block aprons to determine the cosmogenic nuclide concentration at the top of a column before block production.
4. Methods All samples for exposure dating from the Ravine and Mt. Hotham sites are basalt. Those from Ben Lomond and Mt. Wellington are dolerite. Exposure ages are calculated as detailed in Barrows et al. (2002), except that production of 36Cl by thermal and epithermal neutron capture is based on the diffusion model of Phillips et al. (2001) rather than the earlier model of Liu et al. (1994). We also use a more recent estimate of 36Cl production rates from titanium and iron (Masarik, 2002). Age calculations are presented in two forms, an ‘apparent’ age and a ‘corrected’ age. The former is a
703
conventional exposure age, calculated using geographic latitude and the site data in Table 1. The corrected age is calculated relative to an effective geomagnetic latitude, and includes an estimate of 36Cl production from titanium and iron. Corrections for geomagnetic changes are made individually on each age. Based on the well preserved columnar morphology of the samples (except MTW-30 and MTW-32), all ages are calculated assuming that there has been no significant erosion since initial exposure. Measurements of 36Cl were made on the 14UD tandem accelerator at the Department of Nuclear Physics at the Australian National University. Target element concentrations and the neutron production and capture properties of samples analyzed for 36Cl (Table 2) are based on X-ray fluorescence analyses of major elements and inductively coupled plasma mass-spectrometric analyses of trace B, Gd, U and Th. We determined chlorine content by isotope dilution on B1 g splits of the leached rock samples analyzed for 36 Cl. Full chemical data are given in Appendix A and production rates for the apparent exposure ages are presented in Appendix B. The Appendices can be obtained from the Australian Quaternary Data Archive (http://rses.anu.edu.au/enproc/AQUADATA/archive.
Table 2 Chemical data Sample
[CaO] (wt%)
[K2O] (wt%)
[Cl] (ppm)
[TiO2] (wt%)
[Fe2O3] (wt%)
Cross section (103 cm2 g1)
(s35N35/S) (103)a
(s39N39/S) (103)b
1.23170.023 1.22170.023 1.14370.023 1.26470.023 1.24870.023 0.91870.023 1.19270.023
33.771.85 34.870.25 27.770.23 40.270.31 40.870.28 48.970.83 37.170.51
1.91870.026 1.90570.026 1.83470.026 1.96870.026 1.91170.026 1.94670.026 1.90170.026
11.62670.017 11.87070.017 12.34470.017 11.52270.017 11.74070.017 10.84870.017 11.72170.017
7.44570.121 7.49270.122 7.54470.19 7.48570.121 7.5170.121 7.33970.121 7.50370.121
2.5470.1455 2.60670.04625 2.0670.05457 3.01470.05414 3.04870.05357 3.73970.08832 2.77570.05893
0.084870.0021 0.083670.00208 0.077770.0025 0.086670.00211 0.085270.00209 0.064170.00192 0.081570.00205
Victorian Highlands: Mt. Little Higginbotham LHB-05 10.93870.01 0.82170.01 25.770.26 LHB-06 10.40070.006 0.66170.023 27.370.37 LHB-07 10.73070.006 0.72370.023 17.770.25
3.13970.02 1.99370.026 2.23570.026
14.63970.00 16.11470.017 16.49870.017
8.73270.116 7.92770.126 8.20170.127
1.65270.02753 1.93370.04042 1.21170.02543
0.048270.00868 0.042870.00164 0.045270.0016
Ben Lomond Plateau: Carr Villa BLM-05 11.61370.00 0.61970.01 BLM-06 10.65970.01 0.85570.05 BLM-08 11.01170.01 0.67070.04 BLM-10 10.49270.02 0.86970.05
4.770.35 6.670.4 4.670.42 12.070.52
0.57070.02 0.75670.03 0.59370.03 0.75770.03
10.24470.02 11.45970.04 10.68470.03 12.17570.04
5.55170.127 6.23170.145 5.90070.131 6.28270.149
0.475170.03802 0.594470.03859 0.437570.04111 1.07270.05295
0.057270.00226 0.070470.00443 0.058270.00371 0.070970.00441
Denisons Crags BLM-19 10.25970.01
0.83370.05
11.870.35
0.87370.05
12.14970.04
6.38870.149
1.03670.03912
0.066970.00431
Mount Wellington MTW-29 7.81870.00 MTW-30 8.93170.006 MTW-32 7.36270.00
1.39470.00 1.31670.023 1.54970.00
14.270.34 20.870.31 18.470.40
0.84270.01 0.86870.026 0.94570.00
5.85470.113 6.34170.118 6.12570.112
1.36170.04181 1.84170.0439 1.68670.04782
0.12270.00235 0.10670.00272 0.1370.00236
Snowy Mountains: Ravine RAV-01 9.31570.006 RAV-02 9.33570.006 RAV-05 9.01670.006 RAV-06 9.48170.006 RAV-07 9.31870.006 RAV-10 9.32070.006 RAV-13 9.48870.006
a b
Fraction of neutrons captured by 35Cl. Fraction of neutrons captured by 39K.
8.08070.01 9.77470.017 8.63370.00
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704 Table 3 36 Cl exposure ages Sample
Lab code
Snowy Mountains: Ravine RAV-01 ANU-W041-06 RAV-02 ANU-W041-07 RAV-05 ANU-W041-08 RAV-06 ANU-W041-09 RAV-07 ANU-W041-10 RAV-10 ANU-W041-11 RAV-13 ANU-W042-06
[36Cl]c ( 105 g1)a 3.26670.158 3.08870.163 2.17670.132 3.38270.17 3.46970.169 3.31470.159 1.82170.124
Victorian Highlands: Mt. Little Higginbotham LHB-05 ANU-W042-02 4.25570.175 LHB-06 ANU-W042-03 3.08470.141 LHB-07 ANU-W042-05 10.2170.339
[36Cl]r ( 103 g1)b
P
P
Production rate (g1 yr1)d
Apparent age (ka)
Corrected age (ka)
13.9270.485 14.8770.54 12.2970.415 14.8370.537 14.6770.525 14.2570.679 14.6670.534
14.8770.494 14.8670.492 12.9270.413 15.7770.545 15.6770.535 15.1370.69 14.9270.519
24.171.5 22.771.5 18.171.3 23.471.5 24.371.5 23.971.7 12.671.0
22.571.4 21.371.4 17.271.2 22.071.4 22.771.4 22.571.5 12.471.0
5.6170.099 8.2370.19 5.5170.13
19.3270.616 18.1570.62 15.9370.494
20.9170.622 19.2870.614 18.5470.533
22.671.2 17.371.0 69.473.4
20.871.1 16.370.9 58.972.8
11.070.64 11.370.23 8.3870.42 13.170.27 12.970.26 16.970.43 11.870.27
Production rate (g yr1)c 1
Ben Lomond Plateau: Carr Villa BLM-05 ANU-W060-08 BLM-06 ANU-W060-09 BLM-08 ANU-W060-11 BLM-10 ANU-W061-04 Denisons Crags BLM-19 ANU-W061-05
3.32370.163 11.7770.438 8.05170.34 47.171.66
0.40170.033 0.78870.052 0.35970.034 1.0970.057
14.6470.439 14.4470.454 13.7870.429 14.2270.459
15.3970.443 15.9970.478 15.1470.45 15.9170.488
23.371.4 90.274.9 62.773.5 624767
22.171.3 80.774.2 56.773.1 498743.5
21.3670.851
1.3270.053
14.4270.46
16.2370.491
181711
15779.5
Mount Wellington MTW-29 ANU-C025-27 MTW-30 ANU-W039-09 MTW-32 ANU-C025-29
12.5670.539 38.1271.33 47.7471.99
1.9170.059 3.4870.091 2.7170.078
14.1570.426 15.4270.492 14.8970.468
15.3970.447 17.0870.524 16.3970.495
99.475.8 366727 582763.9
90.475.2 313721 483745
a
C=cosmogenic component. R=radiogenic component. c Production without Ti and Fe and using geographic latitude. d Production with Ti and Fe and using effective geomagnetic latitude. b
html) or the WDC-A for Paleoclimatology (http:// www.ngdc.noaa.gov/paleo/paleo.html). Calculated exposure ages are presented in Table 3. Radiocarbon dates are calibrated using CALIB 4.3 (Stuiver et al., 1998) if they are younger than B20,650 14C yr BP or by using the polynomial of Bard et al. (1998) if they are older than this. Calibration of dates older than 30,000 yrs is approximate only.
5. Results 5.1. Ravine, Snowy Mountains, New South Wales The exposure ages from two of the columns (RAV001+RAV-002, RAV-006+RAV-007) and the small block (RAV-010) show excellent agreement ranging from 21.371.4 to 22.771.4 ka. The third column (RAV-005), despite being the lowest in the transect has a younger apparent age (17.271.2 ka). Within the age uncertainties, the blockstream can be attributed to a single periglacial episode during the LGM. The deposit
was produced relatively quickly and became stable along its length since at least 17 ka. The youngest age may indicate a section of the deposit that was the last to stabilize or a block that has moved since deposition. Because the ages at each end of the two columns are not significantly different, we infer that these blocks were produced at an outcrop that did not have a significant previous exposure history or were well shielded before their incorporation in the deposit. The sample from the top of the free face has a much younger apparent age (12.471.0 ka). This result indicates that block production had effectively ceased at the free face by the Holocene. Therefore, the concentration of 36Cl on the outcrop will increase between periglacial episodes, but will be significant at shallow depths only. The most significant process currently acting to limit build-up of cosmogenic isotopes is local spalling of the surface. Large blocks are not currently being produced. The smaller block has the same exposure history as the columns. This indicates that transport within this landform is probably not an important influence on the duration of exposure.
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5.2. Mt. Little Higginbotham, Victorian Alps The two blocks from the proximal block aprons around Mt. Little Higginbotham gave exposure ages similar to those from Ravine (16.370.9 and 20.871.1 ka). The results indicate that the basalt cap here is thick enough to shield blocks spalled onto the aprons. With only two ages it is not possible to determine whether this reflects a full age range. Postdepositional movement is unlikely in this setting because of the very low slope angles. The block from the distal block deposit (58.972.8 ka) has an age significantly older than the block aprons. The exposure age implies that this deposit formed during oxygen isotope chronozone (OIC) 4, at a time similar to the Snowy River Advance (59.375.4 ka) in the Snowy Mountains (Barrows et al., 2001). However, it is not possible to eliminate the possibility that excavation of a block with prior exposure during the LGM, has coincidentally resulted in this apparent age. The presence of a large number of weathered corestones in the deposit indicates at least some reworking from the regolith. 5.3. Ben Lomond Plateau, Tasmania The sample transect at Carr Villa shows a dramatic increase in age from 22 to 498 ka with increasing distance from the cliff. The results indicate that the Carr Villa blockslope is a compound feature made up of blocks produced over a long period of time. We infer that the slope has been active episodically as a rock glacier, and that the age of the youngest block at the head of the deposit dates the most recent period of activity. This age of 22.171.3 ka coincides closely with glacial activity at Tranquil Tarn, on the southern rim of the plateau (20.670.9 ka; Barrows et al., 2002). The block on the Denisons Crag blockslope (15779.5 ka) is older than most of the blocks on the Carr Villa blockslope, and significantly older than the Tranquil Tarn moraine. The maximum ages on each of the blockslopes provide limiting ages for plateau glaciation. The maximum age at Denisons Crag places the glaciation at >160 ka and the maximum age at Carr Villa suggests that the glaciation may have occurred before 500 ka. The time-dependent development of weathering rinds on buried clasts gives a relative age estimate of B145 ka for the ice cap (Caine, 1983). Recalibrating this relationship by substituting an age of 20.6 ka for the Tranquil Tarn moraine (Barrows et al., 2002), instead of 10 ka as Caine (1983) assumed, increases the age to B300 ka, closer to our maximum exposure age. Weathering features of the rock surface, such as solution pan size, indicate a long hiatus between the two glaciations (Caine, 1983). However, weathering rinds on subaerially exposed blocks are developed to a similar extent on the LGM cirque moraines and on the blocks in the
705
blockslopes, and both are thicker than the rinds developed on the older ice cap moraines. The wide age range of blocks with similar weathering rind thickness indicates that weathering characteristics on subaerially exposed blocks must be used with great caution as a method of relative age determination. 5.4. Mt. Wellington, Tasmania The fallen column examined has an apparent age of 313721 ka at the rounded end (formerly the weathered column top) and 90.475.2 ka at the former base. The exposure age of the latter sample gives a maximum estimate of the time since the block toppled. It is an upper limit for two reasons: (i) The block may have had its side exposed as a free face for a significant period prior to spalling. (ii) The sample was irradiated 2.2 m below the top of the column for several hundred thousand years before toppling, at a production rate B5% of that at the surface. Hence B15–20% of the apparent ‘age’ of this second sample is the result of 36Cl build-up at depth. Unless this particular block stood for tens of thousands of years at the free face, we can conclude that it was excavated well before the LGM, and probably before 70 ka. The top of the free face yielded a very old exposure age of 483745 ka, strongly contrasting with the free face at Ravine. A high concentration of cosmogenic nuclides is consistent with field observations of low rates of physical weathering. Blocks produced from outcrops on Mt. Wellington are highly likely to contain vestiges of prior exposure due to the low height of the cliffs shedding the blocks, the low erosion rates and the lack of prior physical weathering. Exposure ages from the Mt. Wellington blockfield therefore correspond only to maximum ages, and so a finite age for the first episode of periglacial activity is difficult to determine.
6. Discussion 6.1. Direct dating of periglacial deposits The internal consistency and the reproducibility of the Cl results indicates that quantitative age information can be successfully derived by exposure dating block deposits. This technique provides direct dates on the time of landform formation on material that would be impossible to determine using other techniques. We sampled the sites strategically to determine the magnitude of potential complications. The results allow us to draw conclusions that exposure dating will be most successful where the following criteria are satisfied: 36
(1) The provenance of the blocks is well constrained. The exposure history is simplest where blocks can be traced to an adjacent free face. Complications
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arise where there is significant lateral transport, a long residence time on the talus slope, or reworking from another deposit or from the regolith (cf. Mt. Little Higginbotham distal deposit). It is important to avoid blocks that show features that may indicate prolonged burial. (2) The initial exposure is negligible or well constrained. Blocks must have their origin at steep, high cliffs or be deeply excavated from an outcrop (cf. Ravine, Mt. Little Higginbotham proximal deposits, and Ben Lomond), rather than originating from free faces less than 3 m tall or from the ground surface (cf. Mt. Wellington, Mt. Little Higginbotham distal deposit). Initial exposure will be minimal where the top of the outcrop is eroding rapidly and/or subject to physical weathering during the Pleistocene. A large volume of block production during the periglacial episode (and previous episodes) effectively removes the initial surface material with the longest exposure history. (3) The rate of block production should have been high, block size large, and the rate and duration of frost shattering sufficient to ensure that nuclide accumulation on the free face is negligible between spalling events during block production. For shortlived periglacial episodes (2000–3000 years) this should be negligible. (4) The block deposits are structurally stable and unlikely to have moved since the end of the periglacial episode. Very low-angle block (o5 ) deposits with a large block size are the most likely to yield reliable ages because the high internal friction minimises the chance of post-depositional subsidence. Noting the preservation of the original surface architecture of the deposit, such as ridges, pits, furrows and aligned blocks, minimizes the risk of sampling reactivated blocks. Exposure dating will be least successful where the landforms are composite features that may have formed due to multiple processes and events, making it impossible to constrain the exposure histories of the blocks. Exposure dating is not likely to succeed on landforms such as solifluction slopes or block deposits that recycle regolith and reactivate old deposits, or rock glaciers that cannibalize pre-existing talus. Nor would it succeed on allochthonous blockfields derived from in situ weathering, such as those that occur on plateaus or above glacial trimlines. Limiting ages may be put on palimpsest landforms, such as rock glaciers, in exceptional circumstances where the talus age can be constrained, such as where it occurs within a glaciated catchment.
cold conditions during the LGM in southeastern Australian. The ages confirm that most of the periglacial deposits examined were formed during the Late Pleistocene, which is consistent with field observations that they were formed by cold-climate processes and are currently inactive. The exposure ages younger than 50 ka are plotted as a relative probability curve in Fig. 6. Six ages from Ravine, two ages from Mt. Little Higginbotham and one age from the Ben Lomond group fall within the range 16–23 ka. There is a bimodal distribution, consisting of two groups with ages of 20–23 and 16–17 ka. A normalized w2 -test shows the nine ages are unlikely to constitute a single population with dispersion in the data being due to random error (w2 =n ¼ 6:08). The oldest 7 ages (20–23 ka) are much more likely to be a single population (w2 =n ¼ 0:32) with a weighted mean age of 21.970.5 ka. The similarity between ages from separate cold-climate landforms in different regions is striking, and indicates that these deposits formed about the same time during the LGM. The younger population has a weighted mean age of 16.670.7 ka, but only contains two samples. It is not clear at present whether this constitutes a reactivation event due to cooling after the LGM, although its age is similar to the late glacial Mt. Twynam Advance (16.871.4 ka). The 21.970.5 ka age for peak periglacial activity is similar to the age of the maximum glacial advance during the LGM using 36Cl (20.171.9 ka) and 10Be (17.371.1 ka) (Barrows et al., 2002) and the time of minimum sea-surface temperature (SST) (20.671.0 ka) in the nearby southwest Pacific Ocean (Barrows et al., 2000). This supports the inference that the block deposits formed during a time of maximum cooling on land and that the expansion of ice during the LGM in
0.20
0.15
relative probability
706
0.10
0.05
0.00
6.2. Periglacial chronology
0
5
10
15
20
25
30
35
40
age (ka)
The exposure ages for the periglacial deposits provide quantitative non-glacial ages for the timing of maximum
Fig. 6. Relative probability plot for exposure ages between 0 and 50 ka. The thick black line is the total cumulative age distribution.
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Australia was a response primarily to regional cooling. Although the difference is not large considering the errors and differences in target element composition, the age for the periglacial activity is about 10% older than the corresponding 36Cl exposure ages on moraines. Two factors are likely to contribute to this. First, exposure at the outcrop and during formation is more likely to affect the periglacial blocks rather than the moraine blocks. Second, blocks in till are more susceptible to be being exhumed on moraines. Considering this, the periglacial exposure ages may relate more to the time of maximum cooling and the moraine exposure ages inherently biased more towards the last time the glacier last stood at the moraine. The ages older than the LGM fall into three groups: two ages of 56.773.1 and 58.972.8 ka; another pair at 80.774.2 and 90.475.2 ka; and two older ages of 15779.5 and 483745 ka. The youngest pair is similar in age to the oldest glacier advance known for the Kosciuszko Massif (59.375.4 ka; Barrows et al., 2001) and might represent periglacial activity early in the last glacial cycle. The older ages from Ben Lomond hint that there may have been earlier periglacial episodes at 162710 and 525750 ka, though these ages should be regarded as minimum limits, given the possibility that the blocks sampled rolled, or were covered, at some time during transport to their present positions. From a comparison of the ages presented above and the main glacier advances, two periods seem to include both low-altitude periglacial activity and ice advance: the LGM and OIC 4. Very low temperatures are probably the common factor here, as recorded by deep-sea records of SST in the southwest Pacific Ocean (Barrows et al., 2000, 2001). Other glacier advances during OIC 3 (such as the Headley Tarn Advance (3272.5 ka)) do not at present appear to have periglacial equivalents. These periods represent intervals of low, but not minimal, temperature in SST records (e.g., Barrows et al., 2001). Relatively high precipitation during these time periods probably compensated for the milder temperatures, resulting in lowered snowlines, but not greatly increased periglacial activity. Previous dating of periglacial deposits relied solely on radiocarbon dates. The oldest periglacial deposit previously dated in the Snowy Mountains is a blockstream in the Toolong Range of the Snowy Mountains (Caine and Jennings, 1968). A stem of Nothofagus sp. recovered from beneath the deposit was dated at B40,700 cal yr BP (35,200+1600/2150 14C yr BP) providing a limiting age. This tree is now locally extinct, and the adjacent sediments contained Early Mid Cenozoic pollen (D. Walker in Caine and Jennings, 1968). Given the limits of early radiocarbon dating, this should conservatively be regarded as a minimum age. At lower elevations, wood fragments and organic soil from three solifluction slopes in the Snowy Mountains at Munyang
707
(1370 m), Geehi (1370 m) and Island Bend (1160 m) were dated at between 30,200 and 39,000 cal yr BP (26,500+2650/1990 and 33,700+2200/1730 14C yr BP) (Costin and Polach, 1971; Costin, 1972). The dates range by as much as 6000 yr in the same site, probably due to the large analytical uncertainties. Together, these ages led to a long-held belief that the main period of periglacial activity extended from B26,000 to 35,000 14C yr BP, which we have now shown is not the case. The lowest altitude deposits of potentially periglacial origin are described from Black Mountain (590 m) (Costin and Polach, 1973). Although these slope deposits were originally described as solifluction slopes, they are more likely to represent alluvial fan deposits (Wasson, 1979), but still reflect treeless conditions and high slope instability. Carbonised wood fragments from the fan were dated at between B30,600–36,600 cal yr BP (26,100+3400/2400–31,500+10800/4500 14C yr BP). A similar date B32,600 cal yr BP (27,85071100 14 C yr BP) came from charcoal in a slope deposit at the Wombeyan Caves (Gillieson et al., 1985), northeast of Black Mountain. The series of radiocarbon ages listed above, assuming they are reliable, group between B30,000–41,000 cal yr BP. The radiocarbon dates are all significantly older than the exposure ages directly on block deposits attributed to the LGM. Our new ages indicate that the existing chronology for periglacial deposits based on 14C dating of organic remains is broadly bracketing at best. Exposure dating, in conjunction with the other forms of dating, shows considerable potential for further elucidating the timing of cold climate activity outside glaciated areas.
Acknowledgements We thank Richard Cresswell, Kexin Liu and Mariana di Tada for assistance in running the 36Cl samples. Joan Cowley (RSES) kindly prepared some of the ICP-MS analyses.
References Bard, E., Arnold, M., Hamelin, B., Tisnerat-Laborde, N., Cabioch, G., 1998. Radiocarbon calibration by means of mass spectrometric 230 Th/234U and 14C of corals. An updated data base including samples from Barbados, Mururoa and Tahiti. Radiocarbon 40, 1041–1085. Barrows, T.T., Juggins, S., De Deckker, P., Thiede, J., Martinez, J.I., 2000. Sea-surface temperatures of the southwest Pacific Ocean during the last glacial maximum. Paleoceanography 15 (1), 95–109. Barrows, T.T., Stone, J.O., Fifield, L.K., Cresswell, R.G., 2001. Late Pleistocene glaciation of the Kosciuszko Massif, Snowy Mountains, Australia. Quaternary Research 55, 179–189.
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Barrows, T.T., Stone, J.O., Fifield, L.K., Cresswell, R.G., 2002. The timing of the last glacial maximum in Australia. Quaternary Science Reviews 21, 159–173. Caine, N., 1983. The Mountains of Northeastern Tasmania. A.A. Balkema, Rotterdam, 200pp. Costin, A.B., 1972. Carbon-14 dates from the Snowy Mountains area, southeastern Australia, and their interpretation. Quaternary Research 2, 579–590. Caine, N., Jennings, J., 1968. Some blockstreams of the Toolong Range, Kosciusko State Park, New South Wales. Journal and Proceedings of the Royal Society of New South Wales 101, 93–103. Costin, A.B., Polach, H.A., 1971. Slope deposits in the Snowy Mountains, southeastern Australia. Quaternary Research 1, 228–235. Costin, A.B., Polach, H.A., 1973. Age and significance of slope deposits, Black Mountain, Canberra. Australian Journal of Soil Research 11, 13–25. Davies, J.L., 1958. The cryoplanation of Mt. Wellington. Papers and Proceedings of the Royal Society of Tasmania 92, 151–154. Galloway, R.W., 1963. Glaciation in the Snowy Mountains: A reappraisal. Proceedings of the Linnean Society of New South Wales 88, 180–198. Galloway, R.W., 1965. Late Quaternary climates in Australia. Journal of Geology 73, 603–618.
Gillieson, D., Spate, A., Head, J., 1985. Evidence from cold climate processes at Wombeyan Caves, Southern Tablelands, New South Wales. Search 16, 46–47. Liu, B., Phillips, F.M., Fabryka-Martin, J.T., Fowler, M.M., Stone, W.D., 1994. Cosmogenic 36Cl accumulation in unstable landforms 1. Effects of the thermal neutron distribution. Water Resources Research 30, 3115–3125. Masarik, J., 2002. Numerical simulation of in-situ production of cosmogenic nuclides. Geochimica Cosmochimica acta 66, A491 (Abstract). Phillips, F.M., Stone, W.D., Fabryka-Martin, J.T., 2001. An improved approach to calculating low-energy cosmic-ray neutron fluxes near the land–atmosphere interface. Chemical Geology 175, 689–701. Stuiver, M., Reimer, P.J., Bard, E., Beck, J.W., Burr, G.S., Hughen, K.A., Kromer, B., McCormac, F.G., van der Plicht, J., Spurk, M., 1998. INTCAL98 radiocarbon age calibration, 24,000-0 cal BP. Radiocarbon 40 (3), 1041–1083. Talent, J.A., 1965. Geomorphic forms and processes in the highlands of eastern Victoria. Proceedings of the Royal Society of Victoria 78, 119–135. Wasson, R.J., 1979. The identification of relict periglacial slope mantles. Australian Quaternary Newsletter 13, 26–34. Webster, R., 1974. Block deposits of southeastern Australia. B.A. Honours Thesis, Department of Geography, Monash University, Unpublished.
Quaternary Science Reviews 23 (2004) 697–708
Exposure ages for Pleistocene periglacial deposits in Australia Timothy T. Barrowsa,*, John O. Stoneb, L. Keith Fifieldc b
a Research School of Earth Sciences, Australian National University, Canberra ACT 0200, Australia Department of Earth and Space Sciences and Quaternary Research Center, University of Washington, Box 351310, Seattle 98195-1310, USA c Department of Nuclear Physics, Research School of Physical Sciences and Engineering, Australian National University, Canberra, ACT 0200, Australia
Received 1 April 2003; accepted 23 October 2003
Abstract Pleistocene periglacial landforms are widespread in Australia and provide valuable information on past temperatures, but dating the time of their formation has proven difficult. To remedy this, we have explored the use of cosmogenic 36Cl for direct dating of periglacial deposits. We sampled six deposits in four locations in southeastern Australia, ranging from blockstreams and block slopes to former rock glaciers. Eighteen exposure ages reveal a concentration of periglacial activity during the last glacial maximum (LGM) between 16 and 23 ka, with a population having a weighted mean age of 21.970.5 ka. This age is shortly before the time of maximum ice advance during the LGM in southeastern Australia. Exposure dating of block deposits provides a way of extending the chronology of cold climate activity beyond glaciated regions. r 2003 Elsevier Ltd. All rights reserved.
1. Introduction Pleistocene periglacial landforms are far more widespread in Australia than glacial deposits. Galloway (1965) mapped solifluction deposits and estimated that most of southeastern Australia above 600 m in altitude probably experienced a periglacial climate during the coldest phase of the last glaciation. Periglacial processes in alpine climates were therefore an important influence in shaping southeast Australia’s landscape. Modern periglacial activity is limited to a few mountains only. Despite more than a century of research describing Australian glaciation, periglacial landforms have received relatively little attention and have frequently been misinterpreted as glacial deposits (Galloway, 1963). Galloway (1965) showed that periglacial landforms could be more reliable recorders of past temperatures than glaciation because their formation was less sensitive to changes in precipitation. Galloway estimated that the height of the last glaciation was B9–11 C cooler than present in southeastern Australia, despite the snowline *Corresponding author. Current address: Department of Nuclear Physics, Research School of Physical Sciences and Engineering, Australian National University, Canberra, ACT 0200, Australia. Tel.: +61-2-6125-2077; fax: +61-2-6125-2083. E-mail address: [email protected] (T.T. Barrows). 0277-3791/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2003.10.011
being only B800 m lower in altitude, a result of much drier conditions. After Galloway’s (1965) initial study showed the value of Australian periglacial deposits in climatic reconstruction, the next decade of research rapidly expanded knowledge on the subject (e.g., Talent, 1965; Caine and Jennings, 1968; Costin, 1972; Costin and Polach, 1973; Webster, 1974). A significant problem faced by research into Pleistocene periglacial deposits is placing the climatic information derived from landforms into a temporal framework. The existing chronology is limited to a few sites only, where organic soil or wood incorporated within or beneath a deposit has been dated with 14C. These dates provide only maximum limits for the initiation of periglacial activity. Because of these limits and the scarcity of material for radiocarbon dating, there is a need for a method that can directly date the formation of periglacial deposits to harness their potential as climatic indicators. In recent work, we have shown that exposure dating of glacial deposits using cosmogenic nuclides can provide a quantitative chronology for the presence of ice in several Australian locations (Barrows et al., 2001, 2002). For exposure dating to be successful there should be no prior exposure and the surface should remain continuously exposed in the same geometry. Provided these criteria are met, periglacial deposits are candidates for exposure dating. A reliable methodology will allow
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us to directly date cold climate landforms in a nonglacial environment. This paper reviews the chronology of periglacial activity in Australia during the Pleistocene. In particular, we assess the feasibility of exposure dating periglacial deposits at four locations in Australia. Our aim is to evaluate the effects of prior exposure and postglacial modification of the landforms, as limits to deriving direct ages. The new ages for periglacial activity in Australia are compared with the existing glacial chronology from adjacent areas.
36°S
Australia
RavineSection Ridge Mt HothamMt Little Higginbotham
40°S
2. Sampling strategy For exposure dating, we chose deposits consisting of meter-sized blocks generated by frost shattering at outcrops and cliff faces. These landforms are the least likely to be affected by prior exposure or disturbance after deposition. Nonetheless, the cosmogenic nuclide concentration on a block surface is accumulated in three stages: before a block falls from the free face, in transport while the landform is active, and since the deposit became inactive. Therefore the simple exposure age calculated for a block may be much greater than the true age of the deposit if its prior exposure was long, or less if the deposit has not been stable since the periglacial event. The exposure period that we wish to record is bracketed by the onset of block generation from the outcrop and the time when the deposit became inactive. Exposure age measurements on individual blocks will fall in this time range as long as the initial exposure at the outcrop was short and the deposit has remained undisturbed since periglacial activity ceased. Four locations were chosen to cover a range of different deposits and climatic settings (Fig. 1). We were highly selective in only sampling the largest blocks and avoided spalled or weathered surfaces. All of the sites examined have minimal snow cover during the winter months. We were able to identify original joint faces on most dolerite and basalt blocks derived from columns, indicating negligible post-depositional erosion. There is abundant evidence that the deposits are presently inactive: the blocks have weathering rinds; are covered by lichens and mosses; organic debris has accumulated between the blocks at depth; and vegetation is colonizing the edges. Descriptions and dimensions of the deposits are also included to provide context for the interpretation of likely exposure history.
Ben Lomond Plateau Mt Wellington
44°S 140°E
144°E
148°E
Fig. 1. Regional map of southeastern Australia with sample locations.
3.1. Ravine, Snowy Mountains, New South Wales
ideal areas for the formation of blockstreams, some of which were described by Caine and Jennings (1968). For exposure dating, we targeted those on the western aspects of the Section Ridge near Ravine, near from the northern end of the Toolong Range (Fig. 2), because their very low altitude signifies they must have formed during a period of maximum cooling. The Ravine blockstreams are situated from B1100 to 1200 m in altitude, about 500 m lower in elevation than those described by Caine and Jennings (1968), and B800 m lower than last glacial maximum (LGM) glacier equilibrium lines around Mt. Kosciuszko to the south (Barrows et al., 2001). They also have a representative morphology of a common landform in southeast Australia. The tops of the blockstreams are located near free faces at the ridge crest and lie on slope angles of 15–30 . Frost shattering has occurred predominantly along joints in the columnar basalt creating a fairly uniform block size of 30–50 cm but some columns are occasionally longer than 1 m. The thickness of the blocks in the elongate blockstreams as revealed in road cuts is rather shallow, with a maximum depth of B1–2 m. The blockstreams range up to B300 m in length but usually less than 10–20 m in width. Arcuate ridges formed by block flow that run the full width of the blockstreams are well preserved. Of the four main blockstreams at Ravine, we chose the longest for further study (blockstream ‘B’ in Fig. 2). This deposit is narrow and begins in a subdued slope at the top of the ridge where the block size is finest. Sample sites are listed in Table 1. We sampled for exposure dating in four ways:
Basalt outcrops are common in the Toolong Range of the Snowy Mountains and their morphology creates
(1) Three basalt columns were selected in an altitudinal transect (Block 1: RAV-001/RAV-002, Block 2:
3. Sites
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699
(Webster, 1974). A group of representative deposits was chosen for exposure dating from Mt. Little Higginbotham (1750 m), a basalt-capped mountain located B4 km southeast of Mt. Hotham. There are two main types of deposits (Fig. 3), both common in the region. The first type consists of block aprons on the low-angle flanks around the basalt cap (‘proximal deposits’), similar to those around Mt. Byron at Ravine. There is no preference for aspect, but the most extensive aprons are on the northern and eastern sides. The flanks of the basalt cap can be steep (40–50 ) and high (up to 50 m) and are draped with fine block debris and numerous outcrops. Physical weathering along the wide jointing of the basalt has resulted in large blocks (up to 1–2 m) in the deposits, especially on the southwest side where the jointing is widest. The block deposits have slope angles as low as 5 , but are locally as steep as 30 . Road cuts (such as ‘C’ in Fig. 3) reveal that the deposits are only a few meters thick. The second type of block deposit (‘distal deposits’) lies further down the mountain below the block aprons, separated by a vegetated strip of blocks filled with interstitial soil. The bulk of these deposits are completely free of fine sediments with the exception of longitudinal ‘islands’ consisting of blocks with a soil matrix. The largest distal deposit (‘A’ in Fig. 3) lies on
Fig. 2. Location of Ravine blockstreams and sample sites. Stippled areas are block deposits and bold lines are the Section Ridge crest and the edge of the Mount Byron plateau. Dashed line is the Goobarragandra powerline trail.
RAV-005, Block 3: RAV-006/RAV-007) to determine whether the age of the deposit varies significantly from head to toe. (2) Samples were taken from both ends of two individual columns (Block 1: RAV-001+RAV002, Block 3: RAV-006+RAV-007) to determine whether significant exposure occurred at one end of the column before block production. (3) A whole small block (RAV-010; 11.5 cm thick) was collected from the surface to investigate whether it had the same exposure history as the large blocks, and whether exposure ages of thicker blocks had been affected by overturning during transport. (4) A sample was taken from the top of a free face (RAV013), adjacent to the head of the blockstream to measure the cosmogenic nuclide concentration that has accumulated since the blockstream became inactive. 3.2. Mt. Little Higginbotham, Victorian Alps Basalt block deposits occur in the Mt. Hotham area on Mt. Loch, Mt. Battery and the Bogong High Plains
Fig. 3. Location of Mt. Little Higginbotham block deposits and sample sites. Stippled areas are block deposits and bold line is the edge of the Mt. Little Higginbotham plateau. Dashed line is an access road.
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700 Table 1 Site data Sample
Longitude ( E)
Latitude ( S)a
Altitude (m)
Scaling factor (nucleons)b
Scaling factor (muons)b
Horizon correction
Thickness (correction for spallation)
Thickness (correction for thermal n)
Thickness (correction for epithermal n)
36.78 36.51 35.64 36.68 36.85 36.89 34.39
1170 1170 1100 1190 1190 1190 1200
2.299/2.343 2.299/2.331 2.181/2.172 2.333/2.374 2.333/2.382 2.333/2.384 2.351/2.281
1.541/1.557 1.541/1.552 1.494/1.490 1.554/1.569 1.554/1.572 1.554/1.573 1.561/1.536
0.9869 0.9869 0.9869 0.9869 0.9869 0.9869 1.0000
0.9705 0.9657 0.9633 0.9657 0.9763 0.8887 0.9515
1.254 1.289 1.307 1.288 1.212 1.612 1.377
1.050 1.057 1.061 1.057 1.042 1.112 1.075
Victorian Highlands: Mt. Little Higginbotham LHB-05 147.16 37.00 37.52 1680 LHB-06 147.16 37.00 36.6 1680 LHB-07 147.16 36.99 41.06 1520
3.422/3.461 3.422/3.392 3.049/3.302
1.945/1.956 1.945/1.936 1.817/1.895
0.9916 0.9807 0.9869
0.9468 0.9515 0.9468
1.388 1.386 1.405
1.089 1.082 1.089
Ben Lomond Plateau: Carr Villa BLM-05 147.7272 41.5133 BLM-06 147.7272 41.5133 BLM-08 147.7272 41.5133 BLM-10 147.7272 41.5133
41.99 45.19 44.79 46.14
1360 1310 1290 1250
2.950/2.972 2.841/2.999 2.798/2.936 2.713/2.900
1.776/1.783 1.737/1.792 1.722/1.770 1.692/1.759
0.9812 0.9696 0.9818 0.9970
0.9368 0.9732 0.9633 0.9281
1.500 1.248 1.328 1.539
1.078 1.043 1.055 1.094
Denisons Crag BLM-19 147.7200
41.6167
46.27
1285
2.791/2.986
1.720/1.788
0.9874
0.9678
1.286
1.051
Mount Wellington MTW-29 147.2000 MTW-30 147.2000 MTW-32 147.2000
42.9000 42.9000 42.9000
46.41 47.56 47.13
1220 1220 1240
2.706/2.843 2.706/2.888 2.748/2.917
1.690/1.740 1.690/1.757 1.705/1.766
0.9989 0.9989 1.0000
0.9633 0.9500 0.9656
1.315 1.404 1.296
1.050 1.070 1.049
Latitude ( S)
Snowy Mountains: Ravine RAV-01 148.4167 35.8333 RAV-02 148.4167 35.8333 RAV-05 148.4167 35.8333 RAV-06 148.4167 35.8333 RAV-07 148.4167 35.8333 RAV-10 148.4167 35.8333 RAV-13 148.4167 35.8333
a b
Effective geomagnetic latitude. First number for geographic latitude, second number for effective geomagnetic latitude.
the northeast side of the mountain and is B275 m long and B120 m at the widest point. The surface of the deposit has ridges that can run the width of the deposit with a relief of 0.5–1 m. Arcuate bulges and conical pits lie on the ridges and the main body of the deposit, the latter commonly elongated in the down slope direction. The bulk of the deposit has a slope angle of B25–30 , but has sections above the ridges with a lower angle of B20 . A section towards the terminus of the deposit reveals a 3–4 m thick surface layer entirely of blocks resting on sedimentary bedrock. The presence of a different rock type underneath the deposit and the lack of mixing of the rock types confirm the lateral transport of blocks across the surface. Original columns of basalt at the surface are rare and some blocks are regolith corestones and show very thick weathering rinds. Samples (Table 1) were taken from the two main classes of block deposits as follows: (1) Two samples were taken from the proximal block aprons (LHB-005, LHB-006). These samples were chosen to determine the age of the last major phase of block production.
(2) One sample was taken from the largest distal deposit at Mt. Little Higginbotham (LHB-007). This block had original faces and was one of the largest on the lower end of the slope. We assumed that this block was produced at the free face during the penultimate phase of block production. Due to their weathered condition, few other blocks were suitable for sampling at this site. 3.3. Ben Lomond Plateau, Tasmania The block deposits at Ben Lomond are the best described and most extensive in Australia (Caine, 1983). There is abundant evidence for at least one episode of ice cap glaciation over the Ben Lomond Plateau in northeastern Tasmania. The most recent glaciation was during the LGM when only small cirques were active, including Tranquil Tarn (Barrows et al., 2002). Rock fall since the LGM has produced only small talus cones within the limits of glaciation at Tranquil Tarn. However, within the limits of the ice cap glaciation there is extensive development of blockslopes and talus below the dolerite cliffs, and blockstreams and
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701
Fig. 4. Northern flank of the Ben Lomond Plateau and sample sites. Stippled areas are block deposits and bold line is the edge of the Ben Lomond Plateau. Dashed lines are roads. Circular symbols are glacially sculpted bedrock (from Caine, 1985) and the tick indicates the direction of ice flow.
blockfields on the plateau. In places, blockstreams originating from vast, low-gradient solifluction slopes issue off the plateau onto the talus slopes. The age of the ice cap glaciation is poorly constrained and hence the length of time the block deposits have been forming is unknown. There are minimum dates on the blockfields of 3080–4700 14C yr BP on organic material in soil profiles (Caine, 1983). Two areas of Ben Lomond were selected for exposure dating, the Carrvilla blockslope on the northern flank (Fig. 4), and the blockslope below Denisons Crag on the southern flank. The blockslopes at both locations consist of high-angle aprons of block debris that encroach on vegetated areas of till. The blockslopes do not lie at the angle of repose, do not show the morphology of gravity-dominated talus slopes nor show sorting of block size with distance from the cliff. Talus cones of classical shape are comparatively rare locally and are restricted to within the LGM cirques and behind large-scale topples (Caine, 1983). The accumulation of blocks is clearly linked to the presence of dolerite cliffs, which are up to 100 m high. The massive jointing of the dolerite contributes to the formation of very large blocks, most frequently on the scale of 1–6 m in length. The depth of the deposits is unknown but road sections through other deposits suggest a thickness of at least 5 m. Block contribution to the existing slopes appears to be presently rare because there are few fresh rock fall scars. The most common surface morphology on the blockslopes is the presence of large transverse ridges roughly parallel to the slope contours and bowing down slope. The blockslopes also commonly show hollows, furrows, a convex form, a bulging lower section, and
upper surface gradients much lower than the angle of repose (Caine, 1983). This resemblance to rock glaciers indicates that although the blockslopes may have originally accumulated as talus slopes, they have subsequently been deformed by slow flow (Caine, 1983). This was most likely the response to a lowering of internal friction due to the presence of interstitial ice and, possibly, fine sediments at depth. Five samples (Table 1) were collected from these blockslopes as follows: (1) Four blocks were sampled along a transect on the Carrvilla blockslope (Block 1: BLM-005, Block 2: BLM-006, Block 3: BLM-008, Block 4: BLM-010). This was to determine whether block production occurred in a single discrete event or whether the age of the deposit increases with distance from the cliff. (2) A single block on the Denisons Crag blockslope (BLM-019) was sampled to date the age of the block slope in which Tranquil Tarn is situated. This sample was taken at about 100 m east of Denisons Crag (Fig. 2 in Barrows et al., 2002). 3.4. Mt. Wellington, Tasmania The most southerly area examined was Mount Wellington, overlooking Hobart in Tasmania. This location is the closest to the coast of any of the sites and, consequently, has a maritime montane climate. This dolerite plateau reaches 1270 m and has a broad, shallow depression across to the Thark Ridge in the west (Fig. 5). Broad depressions on the Mt. Wellington plateau are covered by vast deposits of coarse diamicton consisting
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Fig. 5. Location of Mt. Wellington block deposits and sample sites. Stippled areas are block deposits and bold line are the edge of the Mt. Wellington plateau and the crest of the Thark Ridge.
of weathered blocks supported in clay-rich fine sediment and covered by alpine vegetation. The deposits lie on slope angles as low as 3 and have very smooth profiles. These deposits were originally confused for glacial till but there is no other evidence for glaciation on the plateau in the form of cirques, ice-eroded bedrock or moraines. The deposits were reinterpreted as solifluction slopes (Davies, 1958), similar to those occurring on the top of the Ben Lomond Plateau. The blocks in the solifluction slopes appear to be largely derived from preexisting regolith and some are in an advanced state of decomposition. The fine matrix has been removed locally around the edges of the deposits and along drainage lines to leave block-supported deposits. These block deposits are occasionally elongated into a blockstream shape and can show longitudinal furrows and transverse ridges. Mt. Wellington shows much less extensive talus development below its prominent cliffs (such as ‘The Organ Pipes’) than Ben Lomond. Exposed upstanding columns at the highest elevations tend to be rounded and exfoliated. Aprons of dolerite columns, none of which are very extensive or form blockstreams, often flank outcrops and free faces along the Thark Ridge.
In many places, the most recently fallen blocks encroach upon the weathered diamicton that forms part of the older blockfields. The presence of reactivated older solifluction deposits and the appearance of a single period of subsequent block production suggests at least two major periods of periglacial activity. The first phase was responsible for the extensive solifluction slopes, probably reworking much of the pre-existing regolith and exposing outcrops. The second phase was responsible for local reactivation of the blockfields and also enhanced block production from exposed outcrops by frost shattering. Sampling at Mt. Wellington was designed to determine whether exposure dating of periglacial deposits was possible in an area of low physical erosion and short free faces. Samples were taken from the Thark Ridge (Table 1), as follows, to constrain the age of the last phase of block production: (1) A large column was selected from a deposit that formed part of a block cover encroaching on older deposits below low dolerite outcrops (1–2 m in height). Two samples were taken, from the rounded former top of the column (MTW-029) and from the
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fractured end (MTW-030), which would have been shielded by 2.2 m when the column stood upright. The column was lying approximately 10 m from the free face where it originated. (2) The top of a free face was sampled (MTW-032) from the crest of the ridge above the block aprons to determine the cosmogenic nuclide concentration at the top of a column before block production.
4. Methods All samples for exposure dating from the Ravine and Mt. Hotham sites are basalt. Those from Ben Lomond and Mt. Wellington are dolerite. Exposure ages are calculated as detailed in Barrows et al. (2002), except that production of 36Cl by thermal and epithermal neutron capture is based on the diffusion model of Phillips et al. (2001) rather than the earlier model of Liu et al. (1994). We also use a more recent estimate of 36Cl production rates from titanium and iron (Masarik, 2002). Age calculations are presented in two forms, an ‘apparent’ age and a ‘corrected’ age. The former is a
703
conventional exposure age, calculated using geographic latitude and the site data in Table 1. The corrected age is calculated relative to an effective geomagnetic latitude, and includes an estimate of 36Cl production from titanium and iron. Corrections for geomagnetic changes are made individually on each age. Based on the well preserved columnar morphology of the samples (except MTW-30 and MTW-32), all ages are calculated assuming that there has been no significant erosion since initial exposure. Measurements of 36Cl were made on the 14UD tandem accelerator at the Department of Nuclear Physics at the Australian National University. Target element concentrations and the neutron production and capture properties of samples analyzed for 36Cl (Table 2) are based on X-ray fluorescence analyses of major elements and inductively coupled plasma mass-spectrometric analyses of trace B, Gd, U and Th. We determined chlorine content by isotope dilution on B1 g splits of the leached rock samples analyzed for 36 Cl. Full chemical data are given in Appendix A and production rates for the apparent exposure ages are presented in Appendix B. The Appendices can be obtained from the Australian Quaternary Data Archive (http://rses.anu.edu.au/enproc/AQUADATA/archive.
Table 2 Chemical data Sample
[CaO] (wt%)
[K2O] (wt%)
[Cl] (ppm)
[TiO2] (wt%)
[Fe2O3] (wt%)
Cross section (103 cm2 g1)
(s35N35/S) (103)a
(s39N39/S) (103)b
1.23170.023 1.22170.023 1.14370.023 1.26470.023 1.24870.023 0.91870.023 1.19270.023
33.771.85 34.870.25 27.770.23 40.270.31 40.870.28 48.970.83 37.170.51
1.91870.026 1.90570.026 1.83470.026 1.96870.026 1.91170.026 1.94670.026 1.90170.026
11.62670.017 11.87070.017 12.34470.017 11.52270.017 11.74070.017 10.84870.017 11.72170.017
7.44570.121 7.49270.122 7.54470.19 7.48570.121 7.5170.121 7.33970.121 7.50370.121
2.5470.1455 2.60670.04625 2.0670.05457 3.01470.05414 3.04870.05357 3.73970.08832 2.77570.05893
0.084870.0021 0.083670.00208 0.077770.0025 0.086670.00211 0.085270.00209 0.064170.00192 0.081570.00205
Victorian Highlands: Mt. Little Higginbotham LHB-05 10.93870.01 0.82170.01 25.770.26 LHB-06 10.40070.006 0.66170.023 27.370.37 LHB-07 10.73070.006 0.72370.023 17.770.25
3.13970.02 1.99370.026 2.23570.026
14.63970.00 16.11470.017 16.49870.017
8.73270.116 7.92770.126 8.20170.127
1.65270.02753 1.93370.04042 1.21170.02543
0.048270.00868 0.042870.00164 0.045270.0016
Ben Lomond Plateau: Carr Villa BLM-05 11.61370.00 0.61970.01 BLM-06 10.65970.01 0.85570.05 BLM-08 11.01170.01 0.67070.04 BLM-10 10.49270.02 0.86970.05
4.770.35 6.670.4 4.670.42 12.070.52
0.57070.02 0.75670.03 0.59370.03 0.75770.03
10.24470.02 11.45970.04 10.68470.03 12.17570.04
5.55170.127 6.23170.145 5.90070.131 6.28270.149
0.475170.03802 0.594470.03859 0.437570.04111 1.07270.05295
0.057270.00226 0.070470.00443 0.058270.00371 0.070970.00441
Denisons Crags BLM-19 10.25970.01
0.83370.05
11.870.35
0.87370.05
12.14970.04
6.38870.149
1.03670.03912
0.066970.00431
Mount Wellington MTW-29 7.81870.00 MTW-30 8.93170.006 MTW-32 7.36270.00
1.39470.00 1.31670.023 1.54970.00
14.270.34 20.870.31 18.470.40
0.84270.01 0.86870.026 0.94570.00
5.85470.113 6.34170.118 6.12570.112
1.36170.04181 1.84170.0439 1.68670.04782
0.12270.00235 0.10670.00272 0.1370.00236
Snowy Mountains: Ravine RAV-01 9.31570.006 RAV-02 9.33570.006 RAV-05 9.01670.006 RAV-06 9.48170.006 RAV-07 9.31870.006 RAV-10 9.32070.006 RAV-13 9.48870.006
a b
Fraction of neutrons captured by 35Cl. Fraction of neutrons captured by 39K.
8.08070.01 9.77470.017 8.63370.00
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704 Table 3 36 Cl exposure ages Sample
Lab code
Snowy Mountains: Ravine RAV-01 ANU-W041-06 RAV-02 ANU-W041-07 RAV-05 ANU-W041-08 RAV-06 ANU-W041-09 RAV-07 ANU-W041-10 RAV-10 ANU-W041-11 RAV-13 ANU-W042-06
[36Cl]c ( 105 g1)a 3.26670.158 3.08870.163 2.17670.132 3.38270.17 3.46970.169 3.31470.159 1.82170.124
Victorian Highlands: Mt. Little Higginbotham LHB-05 ANU-W042-02 4.25570.175 LHB-06 ANU-W042-03 3.08470.141 LHB-07 ANU-W042-05 10.2170.339
[36Cl]r ( 103 g1)b
P
P
Production rate (g1 yr1)d
Apparent age (ka)
Corrected age (ka)
13.9270.485 14.8770.54 12.2970.415 14.8370.537 14.6770.525 14.2570.679 14.6670.534
14.8770.494 14.8670.492 12.9270.413 15.7770.545 15.6770.535 15.1370.69 14.9270.519
24.171.5 22.771.5 18.171.3 23.471.5 24.371.5 23.971.7 12.671.0
22.571.4 21.371.4 17.271.2 22.071.4 22.771.4 22.571.5 12.471.0
5.6170.099 8.2370.19 5.5170.13
19.3270.616 18.1570.62 15.9370.494
20.9170.622 19.2870.614 18.5470.533
22.671.2 17.371.0 69.473.4
20.871.1 16.370.9 58.972.8
11.070.64 11.370.23 8.3870.42 13.170.27 12.970.26 16.970.43 11.870.27
Production rate (g yr1)c 1
Ben Lomond Plateau: Carr Villa BLM-05 ANU-W060-08 BLM-06 ANU-W060-09 BLM-08 ANU-W060-11 BLM-10 ANU-W061-04 Denisons Crags BLM-19 ANU-W061-05
3.32370.163 11.7770.438 8.05170.34 47.171.66
0.40170.033 0.78870.052 0.35970.034 1.0970.057
14.6470.439 14.4470.454 13.7870.429 14.2270.459
15.3970.443 15.9970.478 15.1470.45 15.9170.488
23.371.4 90.274.9 62.773.5 624767
22.171.3 80.774.2 56.773.1 498743.5
21.3670.851
1.3270.053
14.4270.46
16.2370.491
181711
15779.5
Mount Wellington MTW-29 ANU-C025-27 MTW-30 ANU-W039-09 MTW-32 ANU-C025-29
12.5670.539 38.1271.33 47.7471.99
1.9170.059 3.4870.091 2.7170.078
14.1570.426 15.4270.492 14.8970.468
15.3970.447 17.0870.524 16.3970.495
99.475.8 366727 582763.9
90.475.2 313721 483745
a
C=cosmogenic component. R=radiogenic component. c Production without Ti and Fe and using geographic latitude. d Production with Ti and Fe and using effective geomagnetic latitude. b
html) or the WDC-A for Paleoclimatology (http:// www.ngdc.noaa.gov/paleo/paleo.html). Calculated exposure ages are presented in Table 3. Radiocarbon dates are calibrated using CALIB 4.3 (Stuiver et al., 1998) if they are younger than B20,650 14C yr BP or by using the polynomial of Bard et al. (1998) if they are older than this. Calibration of dates older than 30,000 yrs is approximate only.
5. Results 5.1. Ravine, Snowy Mountains, New South Wales The exposure ages from two of the columns (RAV001+RAV-002, RAV-006+RAV-007) and the small block (RAV-010) show excellent agreement ranging from 21.371.4 to 22.771.4 ka. The third column (RAV-005), despite being the lowest in the transect has a younger apparent age (17.271.2 ka). Within the age uncertainties, the blockstream can be attributed to a single periglacial episode during the LGM. The deposit
was produced relatively quickly and became stable along its length since at least 17 ka. The youngest age may indicate a section of the deposit that was the last to stabilize or a block that has moved since deposition. Because the ages at each end of the two columns are not significantly different, we infer that these blocks were produced at an outcrop that did not have a significant previous exposure history or were well shielded before their incorporation in the deposit. The sample from the top of the free face has a much younger apparent age (12.471.0 ka). This result indicates that block production had effectively ceased at the free face by the Holocene. Therefore, the concentration of 36Cl on the outcrop will increase between periglacial episodes, but will be significant at shallow depths only. The most significant process currently acting to limit build-up of cosmogenic isotopes is local spalling of the surface. Large blocks are not currently being produced. The smaller block has the same exposure history as the columns. This indicates that transport within this landform is probably not an important influence on the duration of exposure.
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5.2. Mt. Little Higginbotham, Victorian Alps The two blocks from the proximal block aprons around Mt. Little Higginbotham gave exposure ages similar to those from Ravine (16.370.9 and 20.871.1 ka). The results indicate that the basalt cap here is thick enough to shield blocks spalled onto the aprons. With only two ages it is not possible to determine whether this reflects a full age range. Postdepositional movement is unlikely in this setting because of the very low slope angles. The block from the distal block deposit (58.972.8 ka) has an age significantly older than the block aprons. The exposure age implies that this deposit formed during oxygen isotope chronozone (OIC) 4, at a time similar to the Snowy River Advance (59.375.4 ka) in the Snowy Mountains (Barrows et al., 2001). However, it is not possible to eliminate the possibility that excavation of a block with prior exposure during the LGM, has coincidentally resulted in this apparent age. The presence of a large number of weathered corestones in the deposit indicates at least some reworking from the regolith. 5.3. Ben Lomond Plateau, Tasmania The sample transect at Carr Villa shows a dramatic increase in age from 22 to 498 ka with increasing distance from the cliff. The results indicate that the Carr Villa blockslope is a compound feature made up of blocks produced over a long period of time. We infer that the slope has been active episodically as a rock glacier, and that the age of the youngest block at the head of the deposit dates the most recent period of activity. This age of 22.171.3 ka coincides closely with glacial activity at Tranquil Tarn, on the southern rim of the plateau (20.670.9 ka; Barrows et al., 2002). The block on the Denisons Crag blockslope (15779.5 ka) is older than most of the blocks on the Carr Villa blockslope, and significantly older than the Tranquil Tarn moraine. The maximum ages on each of the blockslopes provide limiting ages for plateau glaciation. The maximum age at Denisons Crag places the glaciation at >160 ka and the maximum age at Carr Villa suggests that the glaciation may have occurred before 500 ka. The time-dependent development of weathering rinds on buried clasts gives a relative age estimate of B145 ka for the ice cap (Caine, 1983). Recalibrating this relationship by substituting an age of 20.6 ka for the Tranquil Tarn moraine (Barrows et al., 2002), instead of 10 ka as Caine (1983) assumed, increases the age to B300 ka, closer to our maximum exposure age. Weathering features of the rock surface, such as solution pan size, indicate a long hiatus between the two glaciations (Caine, 1983). However, weathering rinds on subaerially exposed blocks are developed to a similar extent on the LGM cirque moraines and on the blocks in the
705
blockslopes, and both are thicker than the rinds developed on the older ice cap moraines. The wide age range of blocks with similar weathering rind thickness indicates that weathering characteristics on subaerially exposed blocks must be used with great caution as a method of relative age determination. 5.4. Mt. Wellington, Tasmania The fallen column examined has an apparent age of 313721 ka at the rounded end (formerly the weathered column top) and 90.475.2 ka at the former base. The exposure age of the latter sample gives a maximum estimate of the time since the block toppled. It is an upper limit for two reasons: (i) The block may have had its side exposed as a free face for a significant period prior to spalling. (ii) The sample was irradiated 2.2 m below the top of the column for several hundred thousand years before toppling, at a production rate B5% of that at the surface. Hence B15–20% of the apparent ‘age’ of this second sample is the result of 36Cl build-up at depth. Unless this particular block stood for tens of thousands of years at the free face, we can conclude that it was excavated well before the LGM, and probably before 70 ka. The top of the free face yielded a very old exposure age of 483745 ka, strongly contrasting with the free face at Ravine. A high concentration of cosmogenic nuclides is consistent with field observations of low rates of physical weathering. Blocks produced from outcrops on Mt. Wellington are highly likely to contain vestiges of prior exposure due to the low height of the cliffs shedding the blocks, the low erosion rates and the lack of prior physical weathering. Exposure ages from the Mt. Wellington blockfield therefore correspond only to maximum ages, and so a finite age for the first episode of periglacial activity is difficult to determine.
6. Discussion 6.1. Direct dating of periglacial deposits The internal consistency and the reproducibility of the Cl results indicates that quantitative age information can be successfully derived by exposure dating block deposits. This technique provides direct dates on the time of landform formation on material that would be impossible to determine using other techniques. We sampled the sites strategically to determine the magnitude of potential complications. The results allow us to draw conclusions that exposure dating will be most successful where the following criteria are satisfied: 36
(1) The provenance of the blocks is well constrained. The exposure history is simplest where blocks can be traced to an adjacent free face. Complications
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arise where there is significant lateral transport, a long residence time on the talus slope, or reworking from another deposit or from the regolith (cf. Mt. Little Higginbotham distal deposit). It is important to avoid blocks that show features that may indicate prolonged burial. (2) The initial exposure is negligible or well constrained. Blocks must have their origin at steep, high cliffs or be deeply excavated from an outcrop (cf. Ravine, Mt. Little Higginbotham proximal deposits, and Ben Lomond), rather than originating from free faces less than 3 m tall or from the ground surface (cf. Mt. Wellington, Mt. Little Higginbotham distal deposit). Initial exposure will be minimal where the top of the outcrop is eroding rapidly and/or subject to physical weathering during the Pleistocene. A large volume of block production during the periglacial episode (and previous episodes) effectively removes the initial surface material with the longest exposure history. (3) The rate of block production should have been high, block size large, and the rate and duration of frost shattering sufficient to ensure that nuclide accumulation on the free face is negligible between spalling events during block production. For shortlived periglacial episodes (2000–3000 years) this should be negligible. (4) The block deposits are structurally stable and unlikely to have moved since the end of the periglacial episode. Very low-angle block (o5 ) deposits with a large block size are the most likely to yield reliable ages because the high internal friction minimises the chance of post-depositional subsidence. Noting the preservation of the original surface architecture of the deposit, such as ridges, pits, furrows and aligned blocks, minimizes the risk of sampling reactivated blocks. Exposure dating will be least successful where the landforms are composite features that may have formed due to multiple processes and events, making it impossible to constrain the exposure histories of the blocks. Exposure dating is not likely to succeed on landforms such as solifluction slopes or block deposits that recycle regolith and reactivate old deposits, or rock glaciers that cannibalize pre-existing talus. Nor would it succeed on allochthonous blockfields derived from in situ weathering, such as those that occur on plateaus or above glacial trimlines. Limiting ages may be put on palimpsest landforms, such as rock glaciers, in exceptional circumstances where the talus age can be constrained, such as where it occurs within a glaciated catchment.
cold conditions during the LGM in southeastern Australian. The ages confirm that most of the periglacial deposits examined were formed during the Late Pleistocene, which is consistent with field observations that they were formed by cold-climate processes and are currently inactive. The exposure ages younger than 50 ka are plotted as a relative probability curve in Fig. 6. Six ages from Ravine, two ages from Mt. Little Higginbotham and one age from the Ben Lomond group fall within the range 16–23 ka. There is a bimodal distribution, consisting of two groups with ages of 20–23 and 16–17 ka. A normalized w2 -test shows the nine ages are unlikely to constitute a single population with dispersion in the data being due to random error (w2 =n ¼ 6:08). The oldest 7 ages (20–23 ka) are much more likely to be a single population (w2 =n ¼ 0:32) with a weighted mean age of 21.970.5 ka. The similarity between ages from separate cold-climate landforms in different regions is striking, and indicates that these deposits formed about the same time during the LGM. The younger population has a weighted mean age of 16.670.7 ka, but only contains two samples. It is not clear at present whether this constitutes a reactivation event due to cooling after the LGM, although its age is similar to the late glacial Mt. Twynam Advance (16.871.4 ka). The 21.970.5 ka age for peak periglacial activity is similar to the age of the maximum glacial advance during the LGM using 36Cl (20.171.9 ka) and 10Be (17.371.1 ka) (Barrows et al., 2002) and the time of minimum sea-surface temperature (SST) (20.671.0 ka) in the nearby southwest Pacific Ocean (Barrows et al., 2000). This supports the inference that the block deposits formed during a time of maximum cooling on land and that the expansion of ice during the LGM in
0.20
0.15
relative probability
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0.10
0.05
0.00
6.2. Periglacial chronology
0
5
10
15
20
25
30
35
40
age (ka)
The exposure ages for the periglacial deposits provide quantitative non-glacial ages for the timing of maximum
Fig. 6. Relative probability plot for exposure ages between 0 and 50 ka. The thick black line is the total cumulative age distribution.
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Australia was a response primarily to regional cooling. Although the difference is not large considering the errors and differences in target element composition, the age for the periglacial activity is about 10% older than the corresponding 36Cl exposure ages on moraines. Two factors are likely to contribute to this. First, exposure at the outcrop and during formation is more likely to affect the periglacial blocks rather than the moraine blocks. Second, blocks in till are more susceptible to be being exhumed on moraines. Considering this, the periglacial exposure ages may relate more to the time of maximum cooling and the moraine exposure ages inherently biased more towards the last time the glacier last stood at the moraine. The ages older than the LGM fall into three groups: two ages of 56.773.1 and 58.972.8 ka; another pair at 80.774.2 and 90.475.2 ka; and two older ages of 15779.5 and 483745 ka. The youngest pair is similar in age to the oldest glacier advance known for the Kosciuszko Massif (59.375.4 ka; Barrows et al., 2001) and might represent periglacial activity early in the last glacial cycle. The older ages from Ben Lomond hint that there may have been earlier periglacial episodes at 162710 and 525750 ka, though these ages should be regarded as minimum limits, given the possibility that the blocks sampled rolled, or were covered, at some time during transport to their present positions. From a comparison of the ages presented above and the main glacier advances, two periods seem to include both low-altitude periglacial activity and ice advance: the LGM and OIC 4. Very low temperatures are probably the common factor here, as recorded by deep-sea records of SST in the southwest Pacific Ocean (Barrows et al., 2000, 2001). Other glacier advances during OIC 3 (such as the Headley Tarn Advance (3272.5 ka)) do not at present appear to have periglacial equivalents. These periods represent intervals of low, but not minimal, temperature in SST records (e.g., Barrows et al., 2001). Relatively high precipitation during these time periods probably compensated for the milder temperatures, resulting in lowered snowlines, but not greatly increased periglacial activity. Previous dating of periglacial deposits relied solely on radiocarbon dates. The oldest periglacial deposit previously dated in the Snowy Mountains is a blockstream in the Toolong Range of the Snowy Mountains (Caine and Jennings, 1968). A stem of Nothofagus sp. recovered from beneath the deposit was dated at B40,700 cal yr BP (35,200+1600/2150 14C yr BP) providing a limiting age. This tree is now locally extinct, and the adjacent sediments contained Early Mid Cenozoic pollen (D. Walker in Caine and Jennings, 1968). Given the limits of early radiocarbon dating, this should conservatively be regarded as a minimum age. At lower elevations, wood fragments and organic soil from three solifluction slopes in the Snowy Mountains at Munyang
707
(1370 m), Geehi (1370 m) and Island Bend (1160 m) were dated at between 30,200 and 39,000 cal yr BP (26,500+2650/1990 and 33,700+2200/1730 14C yr BP) (Costin and Polach, 1971; Costin, 1972). The dates range by as much as 6000 yr in the same site, probably due to the large analytical uncertainties. Together, these ages led to a long-held belief that the main period of periglacial activity extended from B26,000 to 35,000 14C yr BP, which we have now shown is not the case. The lowest altitude deposits of potentially periglacial origin are described from Black Mountain (590 m) (Costin and Polach, 1973). Although these slope deposits were originally described as solifluction slopes, they are more likely to represent alluvial fan deposits (Wasson, 1979), but still reflect treeless conditions and high slope instability. Carbonised wood fragments from the fan were dated at between B30,600–36,600 cal yr BP (26,100+3400/2400–31,500+10800/4500 14C yr BP). A similar date B32,600 cal yr BP (27,85071100 14 C yr BP) came from charcoal in a slope deposit at the Wombeyan Caves (Gillieson et al., 1985), northeast of Black Mountain. The series of radiocarbon ages listed above, assuming they are reliable, group between B30,000–41,000 cal yr BP. The radiocarbon dates are all significantly older than the exposure ages directly on block deposits attributed to the LGM. Our new ages indicate that the existing chronology for periglacial deposits based on 14C dating of organic remains is broadly bracketing at best. Exposure dating, in conjunction with the other forms of dating, shows considerable potential for further elucidating the timing of cold climate activity outside glaciated areas.
Acknowledgements We thank Richard Cresswell, Kexin Liu and Mariana di Tada for assistance in running the 36Cl samples. Joan Cowley (RSES) kindly prepared some of the ICP-MS analyses.
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