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Metamorphosed palaeosols associated with Cretaceous fossil forests, Alexander Island, Antarctica

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Journal of the Geological Society, London, Vol. 162, 2005, pp Printed in Great Britain. Metamorphosed palaeosols associated with Cretaceous fossil forests, Alexander Island, Antarctica J. HOWE
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Journal of the Geological Society, London, Vol. 162, 2005, pp Printed in Great Britain. Metamorphosed palaeosols associated with Cretaceous fossil forests, Alexander Island, Antarctica J. HOWE 1,2 &J.E.FRANCIS 1 1 Earth Sciences, School of Earth and Environment, The University of Leeds, Leeds LS2 9JT, UK ( 2 British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK Abstract: Fossil soils are present within mid-cretaceous fluvial sediments of the Fossil Bluff Group, Alexander Island, Antarctica. The palaeosols contain in situ fossil trees and rooted plants. These palaeosols are typically composed of two dominant horizons: an upper organic-rich horizon with identifiable plant remains, and a lower horizon with well-developed blocky ped structures, clay cutans and mottling. These features are typical of modern soils in seasonally dry climates. Although evidence of flooding is apparent from interbedded fluvial sandstones and mudstones, the palaeosols indicate that this high-latitude mid- Cretaceous environment was predominantly seasonally dry. The presence of the zeolites laumontite, prehnite and pumpellyite indicates a later metamorphic overprint on the palaeosols, resulting from burial diagenesis of this region at depths of about km and temperatures of C. Keywords: Cretaceous, Antarctica, palaeosols, fossil forests, zeolites. Fossil soils (palaeosols) are excellent indicators of ancient terrestrial environments. They not only provide evidence for ancient vegetation and ecosystems, but also contain signals of palaeoclimates that are independent of evidence from other associated sediments and fossils (Retallack 2001). Palaeosols are often difficult to identify because the soil features can be subtly defined, and obvious features such as in situ plants and diagnostic soil structures such as peds are often not well preserved. In addition, later diagenetic and metamorphic processes can destroy or modify original soil fabrics. Fossil soils of mid-cretaceous age on Alexander Island, Antarctica, are exceptionally well preserved. They contain large upright fossil trees in their original growth positions with their roots within the fossil soils. Distinctive soil structures such as peds and soil horizons are apparent, and even though the palaeosols have suffered some burial metamorphism, the original soil structures have not been destroyed. Despite this good preservation, the palaeosols have not been well described nor their important palaeoenvironmental record properly deciphered, even though the fossil plants and associated sediments have received attention (e.g. Jefferson 1982a; Moncrieff & Kelly 1993; Falcon-Lang et al. 2001; Nichols & Cantrill 2002). This paper presents detailed descriptions of the palaeosols and their environmental significance, plus deductions about regional burial regimes from the metamorphic overprint. Geological setting Alexander Island is situated off the west coast of the Antarctic Peninsula at c. 708S, 708W (Fig. 1a). The island contains a suite of rocks deposited in a forearc setting to the west of a magmatic arc, formed as the Pacific plate was subducted beneath Antarctica (Barker et al. 1991). The sequence of Mesozoic rocks on Alexander Island includes the Upper Jurassic Lower Cretaceous Fossil Bluff Group, a series of marine, fluviatile and deltaic sediments that formed as the infill of the subsiding forearc basin (Butterworth et al. 1988). They are dominated by volcaniclastic components, derived from the eroding volcanic arc. The fossil forests and their palaeosols are preserved within the m thick Triton Point Formation of the Fossil Bluff Group in the SE of Alexander Island (Fig. 1b). These sediments represent braided river systems (Citadel Bastion Member) and meandering river systems (Coal Nunatak Member) with broad river belts and extensive floodplains (Cantrill & Nichols 1996; Nichols & Cantrill 2002; Fig. 2). Both the floodplain and midchannel bars of the wide river channels supported diverse vegetation, the remains of which are now preserved as in situ fossil tree stumps and rooted plants. In southeastern Alexander Island the Citadel Bastion Member crops out on Citadel Bastion, Titan Nunataks and the basal 60 m of Coal Nunatak (Fig. 1c). The Coal Nunatak Member crops out on the upper sections of Coal Nunatak. The strata dip c to the SE. Both the Citadel Bastion and Coal Nunatak members are dominated by sequences of channel and over-bank sandstones (Fig. 3). The over-bank sandstones are up to 10 m thick, are typically featureless and have sharp, non-erosive bases. These are interpreted as sheet flood and crevasse splay deposits and indicate that the river systems were prone to flooding (Nichols & Cantrill 2002). These sandstones encase the trunks of upright fossil trees (Fig. 4a), their roots preserved in palaeosols below. Individual channel sandstones range from 1 to 4 m thick and form units up to 15 m thick. They characteristically have scoured erosional bases with rip-up clasts. In the Citadel Bastion Member the channel sandstones form ribbon deposits characteristic of braided river systems but in the overlying Coal Nunatak Member they have lenticular morphologies typical of meandering river systems (Reading 1986; Cantrill & Nichols 1996). No in situ fossilized trees were found preserved within channel sandstone bodies. Between the sandstone units are carbonaceous-rich finegrained sandstones and siltstones (Fig. 3) that represent deposition from sediment-laden floodwaters on the floodplains. The 951 o o 952 J. HOWE ET AL. Alexander Island Antarctic Peninsula Palmer Land (a) Southeastern Nunataks 72 00'S Hall Cliffs KG4906 KG4914,15 & 4917 KG4904 KG4911, 4912 Citadel Bastion (c) 72 00'S 0 km 500 East Antarctica KG4918 Corner Cliffs 70 o S 75 o W 72 o S 70 o W Alexander Island Fossil Bluff Group Southeastern Nunataks George VI Sound (b) Titan Nunatak (west) KG4938 & 4943 Hyperion Nunataks KG4939 KG km Titan Nunatak (north) KG4942 KG4935 KG4930 KG4919 KG4922 KG4936 Titan Nunatak (east) Coal Nunatak KG W George VI Sound Fig. 1. Location maps. (a) Map of Antarctica, showing location of Alexander Island off the west coast of the Antarctic Peninsula. (b) Alexander Island. The grey area shows the outcrop of the Fossil Bluff Group and the box identifies the location of the southeastern nunataks. (c) The southeastern nunataks, showing the position of localities mentioned in the text and the sites where palaeosols were sampled (e.g. KG.4915). Age Group Formation Member Environment Albian Fossil Bluff Group Neptune Glacier (2200m) Triton Point ( m) Neptune Glacier (2200m) Mars Glacier ( 900m) Coal Nunatak ( 135m) Citadel Bastion ( 700m) Deimos Ridge ( m) Nearshore/ paralic Meandering fluvial Braided fluvial Nearshore/ paralic Fig. 2. The stratigraphy of the upper units of the Fossil Bluff Group, Alexander Island (after Nichols & Cantrill 2002) m thick sandstones are finely laminated and contain abundant plant debris and some fossil tree trunks. The siltstones are also finely laminated, the laminations often draped over fossil plant material that is preserved in situ in palaeosols below. Palaeosol horizons are interbedded with these units, representing the pedogenic alteration of exposed floodplain sediments. Sediments of the Triton Point Formation contain volcaniclastic components with palaeocurrent structures indicative of palaeoflow towards the SW, suggesting sediment supply from the magmatic arc. During the mid-cretaceous a phase of plutonism in Palmer Land caused uplift and erosion that supplied sediment from the arc. Coeval volcanism supplied air-fall ashes that now occur as beds of tuff within the sediments (Vaughan 1995). The Fossil Bluff Group is considered late Albian in age, being under- and overlain by marine sequences containing faunas that indicate a late Albian age (Kelly & Moncrieff 1992). At this time Alexander Island was situated at about 708S palaeolatitude (Lawver et al. 1985). The palaeosols Field and petrographic descriptions Over 90 palaeosol horizons were identified throughout the strata on Citadel Bastion, Titan Nunataks and Coal Nunatak on Alexander Island. Palaeosol horizons were identified using the following criteria: (1) the presence of in situ tree trunks and other plants with rootlets in the palaeosols; (2) recognizable palaeosol features such as ped structures; (3) distinctive organicrich layers containing plant material. The palaeosols have a generally consistent structure throughout all exposures, although some profiles appear less mature than others. Where well developed, the palaeosols consist of two distinct horizons, an upper and a lower horizon. Upper horizon. The upper horizon is black or dark brown in colour, and varies in thickness from 0.05 to 0.6 m with lateral undulations (Fig. 4b). These horizons are mainly massive in structure but with a crumbly texture. Occasional fine laminations of aligned organic material are present but in general all sedimentary structures have been lost. The upper horizon is frequently mottled (mottled areas average 3 4 mm long and 2 6 mm wide) by structures that could be burrows or rootlets, filled with white, fine-grained sandstone or brown clay. The matrix is dominantly organic- or clay-rich, with either an agglomeroplasmic fabric (local matrix areas surrounding skeletal grains; terminology of Brewer 1976) or porphyroskelic fabric (larger grains set within a fine matrix; Brewer 1976). In places the matrix contains the clay mineral smectite, which replaces some mineral grains. Detrital grains of quartz, altered feldspars, laumontite, chlorite, volcanic glass shards and rock fragments make up 5 45% of this horizon. The chlorite crystals are iron- CRETACEOUS PALAEOSOLS, ANTARCTICA 953 Fig. 3. A sedimentary log showing a typical sequence of sediments within which the palaeosols occur. (For logs of the whole sequence see Nichols & Cantrill 2002.) Fig. 4. (a) KG.4923, a tree fossilized in situ (arrow), preserved within a sheet flood sandstone. (b) KG.4912, an undulating palaeosol with an upper mudstone horizon and a weathered lower sandstone horizon. The hammer in both photographs is 35 cm long. rich and are often split and contorted. In addition, they are also sometimes aligned (skelsepic fabric), the result of realignment caused by percolating water. Organic matter (5 40%) occurs either as brown amorphous material or as identifiable plant structures, such as branched rootlets (Fig. 5a). In most of the upper horizon large pieces of plant debris are coated by laumontite. Laumontite also occurs as small particles within the matrix. Lower horizon. The lower horizon typically consists of lightcoloured, fine- to coarse-grained sandstone. This horizon is massive, up to 2 m thick and occasionally banded (20 mm wide bands) as a result of variations in grain size. These sandstones are similar to the fluvial sandstones common throughout the sequence. The upper parts of the lower horizon are commonly darker than the sediment below and are cracked and jointed, forming well-defined ped structures (Fig. 6a). The joints around the peds extend vertically for mm. Within the palaeosol horizons on Alexander Island two types of ped structures were seen: (1) prismatic peds, taller than they are wide, with flat tops and of medium size with diameters of mm (Fig. 6a); (2) angular blocky peds that are more Fig. 5. Microscopic features within the palaeosols. (a) KG , delicate branching rootlet within the mudstone matrix of a palaeosol upper horizon. (b) KG , split chlorite crystals (arrow) with laumontite infilling the gap. Scale bar in both photographs represents 1 mm. 954 J. HOWE ET AL. Fig. 6. Palaeosol features seen in the field. (a) KG.4915, ped structures formed on the surface of a lower horizon. (b) KG.4922, roots extending down and branching out (arrow) into a flood sandstone deposit. The lens cap in both photographs is 5.5 cm in diameter. irregular in shape but have interlocking faces, occurring as both fine in size with a diameter of 5 10 mm and very coarse with a diameter.50 mm (using the classification scheme of Retallack 2001). Blocky peds probably resulted from both cracks that formed around rootlets and the presence of swelling clays. Mottling (irregular patterns of two or more colours with sharp boundaries) is also common in the lower horizon, occurring as grey or white patches and elongate interconnected areas. Using the classification of the Soil Survey Staff (1975), the mottles can be described as prominent (outstanding feature of the horizon), many (occupying more than 20% of the exposed surface) and medium (5 15 mm) to coarse (15 20 mm) in size. The lower horizon is generally dominated by mineral grains and has a granular fabric (skeletal grains touching with little matrix; Brewer 1976) to intertextic fabric (skeletal grains dominant with intergranular braces of matrix; Brewer 1976). Quartz is the dominant mineral, commonly forming 40 50% of the unit. The feldspars (30 40%) are often weakly to moderately altered to smectite with the loss of the original crystal shape. Other minor components of the lower horizon with variable abundances include organic matter, volcanic rock fragments, chlorite, a laumontite or calcite matrix, and the metamorphic minerals laumontite and prehnite. The organic matter is often scattered throughout the rock, forming fine (,1 mm) discontinuous laminae and occasionally forming concentrations. Fossil plant material and rootlets are also scattered throughout the finer-grained matrix, with cell anatomy visible in some plant fragments. Rootlets commonly occur as black carbonaceous material extending for,10 mm, either down from the upper surface or running parallel to bedding. Volcanic rock fragments are sub-angular to sub-rounded in shape. The chlorite crystals are iron-rich and occur in a hydrated state, contorted and exfoliated and regularly split, with laumontite growing between the split sections (Fig. 5b). In places they occur parallel to bedding, giving the rock a laminated appearance, or concentrated around rootlet traces and voids. The laumontite or calcite matrix is a minor component of the lower horizon and fills gaps between grains or occurs as plugs and coatings. Within the matrix, clay minerals (smectite, illite and chlorite) occur with a skelsepic microfabric (highly birefringent clay crystals growing parallel to mineral grains; Brewer 1976), occasionally forming embedded grain or ped cutans (concentration of a soil constituent or in situ modification forming a skin around a grain or ped surface; Brewer 1976). Significant environmental features The palaeosols described above contain important features comparable with those of modern soils, such as rootlet horizons, ped structures and mottling. These features provide important evidence for environmental conditions and their significance is discussed in more detail below. Rootlet horizons. Nearly all palaeosols examined on Alexander Island are associated with large in situ tree stumps, finer rootlets or rootlet traces. The tree roots branch and extend down into the palaeosols to a depth of 1 m (commonly m) (Fig. 6b), and radiate from single tree trunks for up to 2 m, frequently on a horizontal plane. Finer rootlets, up to 10 mm in diameter, were found concentrated in rootlet masses (150 mm thick) directly beneath the upper horizon, commonly visible as dark, organicrich material within the lighter-coloured lower horizon. The presence of such a diverse range of roots confirms that these palaeosols were fertile and supported substantial vegetation. Plant roots need oxygen for the uptake of water and nutrients and so do not penetrate below the water table (apart from those such as mangrove types with special adaptations, not seen here). Fossil roots penetrating to a depth of 1 m suggest that the Cretaceous water table was at least 1 m below the surface. Waterlogging as a result of flooding clearly did not last for such significant periods of time that plant growth was inhibited. Ped structures. Peds are blocky structures formed in soils as a result of wetting and drying processes under seasonal climate regimes (Retallack 2001). The peds have clay cutans (clay coatings), which were formed either as material washed down into cracks or from alteration of the ped surface, the former process being most likely. Clay cutans are characteristic of soils formed above the water table and are common in well-drained soils (Brewer 1976), indicating that these palaeosols were not waterlogged but received water input mostly as precipitation. The presence of ped structures indicates that the Cretaceous climate in which these soils developed consisted of periods of relatively wet conditions during which the rainwater infiltrated the soil and percolated downwards, carrying material into cracks. The wet season was followed by prolonged dryness, similar to mid-latitude temperate climates today. Mottling. Mottling is present within the upper and lower horizons of the palaeosols. The mottling does not have the classic red and grey colours of gley soils, typically formed in waterlogged soils (Retallack 2001). These drab-haloed root traces (Retallack 2001) present here are areas of reduction owing to anaerobic bacterial decay of organic matter in the rootlets. The extensive occurrence of these mottles, found up to 1 m beneath the upper horizon, indicates that there was a significant amount of vegetation and biological activity deep within the soil profile, indicating that the soil was rich and fertile. CRETACEOUS PALAEOSOLS, ANTARCTICA 955 Clay minerals. The clay minerals illite, chlorite and montmorillonite were identified within the palaeosols by X-ray diffraction. Montmorillonite is a swelling clay formed from the degradation of silicates such as feldspars, volcanic minerals, chlorite and illite, and is commonly found in alkaline soils today (Retallack 2001). It occurs in well-drained soils where leaching is not a dominant process, suggesting that there was no significant transport of soluble minerals in these palaeosols. Smectite is also a major component of modern soils that receive,500 mm mean annual rainfall (Retallack 2001). Illite and chlorite are more stable clay minerals, formed mainly during diagenesis from the dehydration of smectite minerals (Dunoyer de Segonzac 1970). Hydrated chlorite. Iron-rich chlorite was identified within the palaeosols but its optical properties make it difficult to distinguish from weathered biotite, and it is likely that both are present. The chlorite frequently shows signs of alteration, such as exfoliation (caused by expansion as a result of hydration) and contortion, reflecting the in situ weathering processes that had occurred within the palaeosol profile. In these palaeosols laumontite intergrowths are commonly present within split chlorites (Fig. 5b). Chlorite is found as aligned grains, a result of the realignment of clay particles during wet phases; this skelsepic fabric is seen in modern temperate soils that are subject to expansion and contraction during seasonal wet and dry periods (Kemp 1985). Classification of the palaeosols The Alexander Island palaeosols have several features that are characteristic of certain types of modern soils, although because of the metamorphic overprint and burial effects a definite comparison with a modern soil type cannot be made. In the most well-developed palaeosols the upper horizon (A horizon) resembles a mollic epipedon, a dark brown or black organic-rich (but not peaty) layer.25 cm in thickness. There is a range from this to less well-developed palaeosols that have an upper layer with a high detrital (volcanic) mineral component, classified as a melanic epipedon. The lower horizon (E/B horizon) is probably a cambic horizon, with relic bedding and features of the underlying bedrock but more weathered, and with peds and mottling in the upper part of this horizon. There is li
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