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Micromorphological evidence of warm-based glacier deposition from the Ricker Hills Tillite (Victoria Land, Antarctica)
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Quaternary Science Reviews 25 (2006) 976–992
Micromorphological evidence of warm-based glacier deposition from the Ricker Hills Tillite (Victoria Land, Antarctica) Carlo Baronia,, Francesco Fasanob a
Dipartimento di Scienze della Terra, CNR, Istituto di Geoscienze e Georisorse, Universita` di Pisa, via S.Maria, 53, 56126, Pisa, Italy b Dipartimento di Scienze della Terra, Universita` di Siena, via Laterina, 8, 53100 Siena, Italy Received 2 April 2004; accepted 30 November 2004
Abstract Thin sections of impregnated samples were used for micromorphological analysis of the ‘‘Ricker Hills Tillite’’ in southern Victoria Land, Antarctica. The tillite is composed of massive matrix-supported diamicton with a porphyric coarse/fine related distribution, low sorting, low rounding and medium to high angularity. Glacial deposits are completely represented by lodgement till. Phyllosilicate reorientation patterns (plasmic fabrics) are visible in most samples, especially associated skelsepic and lattisepic patterns. Syndepositional pervasive till shearing (glacially induced) is evidenced by clast alignments, shear planes, rotational structures and pressure shadows. Boudinage, ductile shear zones, and rotational structures are also widespread in the deformed bedrock. Water-escape structures are clearly evident in the tillite and in the bedrock. Shear planes infilled by injection veins in lodgement till and at the till-bedrock interface testify to the presence of water during deposition. Secondary features like clay and silt coatings, sparite calcite, and oxidation products were found in all the examined outcrops. They formed under phreatic conditions due to saturated water circulation. A warm-based ice sheet was responsible for the deposition of the tillite. Past ice-flow directions were similar to present ones. r 2005 Elsevier Ltd. All rights reserved.
1. Introduction A number of outcrops of a semi-lithified to lithified diamicton, informally named Ricker Hills Tillite, were mapped in the Ricker Hills area, in southern Victoria Land (Baroni, in Capponi et al., 1999). The tillite unconformably overlies the sandstone and black shales of the Takrouna Formation (Beacon Supergroup, Permian to Late Triassic) and the Ferrar Dolerite (Jurassic). A massive, matrix-supported diamicton with a sandy silt matrix that ranges in colour from graybrown to olive-gray covers the tillite in all the studied outcrops. It is correlated with the Late Pleistocene ‘‘younger drift’’ recognised by Stuiver et al. (1981) in the Dry Valleys and with the ‘‘Terra Nova Drift’’ described Corresponding author.
E-mail addresses: [email protected] (C. Baroni), [email protected] (F. Fasano). 0277-3791/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2004.11.011
by Orombelli et al. (1991) in the Terra Nova Bay area. Field surveys, profile descriptions, and fabric measurements were carried out on these glacial deposits. Considering its geomorphologic and stratigraphic position, fabric and lithofacies, the Ricker Hills Tillite can be correlated to the Sirius Group. This group comprises a suite of consolidated glacial deposits (subglacial deposits, ablation, and flow tills as well as glaciomarine and glaciolacustrine sediments, and ice marginal deposits; Stroeven, 1997) that are widespread in the Transantarctic Mountains. It is widely understood that the sediments of the Sirius Group derive from temperate glaciers (Stroeven, 1997; Wilson et al., 2002; and references therein); however, the age, environmental evolution and palaeoclimatic significance of this group are still a matter of debate. In particular, although the presence of Pliocene microfossils within the Sirius Group raised some doubts (Harwood, 1983; Webb and Harwood, 1991; Harwood
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and Webb, 1998; Webb et al., 1996), geomorphologic, stratigraphic and volcanic data, and exposure ages suggest an age older than Late to Mid-Miocene (Sugden et al., 1993, 1995a, b; Marchant and Denton, 1996; Sugden, 1999; Schaefer et al., 1999; Stroeven and Kleman, 1999). The value of micromorphology in understanding glacial processes has been widely demonstrated (van der Meer, 1987; van der Meer and Laban, 1990, Menzies and Maltman, 1992; van der Meer, 1993, 1997; Menzies et al., 1997; Boulton and Dobbie, 1998; van der Meer and Hiemstra, 1998; van der Meer et al., 1998, 2003; Menzies, 1998, 2000, Menzies and Zaniewski, 2002; Hiemstra and Rijsdijk, 2003). Indeed the analysis of microstructures and textures, and their relationships provide key information for identifying depositional processes and facies, and reconstructing post-depositional histories. Based on field observations and descriptions, and in order to investigate the processes responsible for its glacial deposition, the Ricker Hills Tillite was sampled for micromorphological analysis. The role of water during deposition, deformation and post-depositional modification of the tillite was also carefully investigated. Water content, one of the most important factors in till differentiation, is a function of glaciodynamics and subglacial temperatures. Water can strongly influence sediment mineral assemblage and calcite precipitation (Hallet, 1975, 1976; Aharon, 1988). Moreover, the presence of pore water can strongly influence sediment rheology (Menzies, 1989; van der Meer et al., 2003); in fact, water, together with clay content of the deforming bed is the major factor controlling deformation (van der Meer et al., 2003).
2. Study area and sampling The Ricker Hills (751400 S, 1591200 E) are located in the southern Prince Albert Mountains (Fig. 1), at the southern margin of the David Glacier basin, about 100 km from the Ross Sea coast. The hills, ranging in elevation from 900 to 1830 m a.s.l., constitute a nunatak at the boundary of the East Antarctic Ice Sheet (EAIS). A complex basin-and-range topography characterises the nunatak. The highest elevations are found at the western and southern margins; two main depressions are located along the southeastern and northern borders. The first depression is presently occupied by a frozen lake and will be hereafter indicated as the ‘‘Lake Depression’’, while the second is named the Morris Basin. Steep rock walls (Ferrar Dolerites, Kirkpatrick Basalt and Sandstone of the Beacon Supergroup) define the western margins of the depressions. At least five glacial drifts are documented in the area, the Ricker Hills tillite being the oldest on the basis
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Fig. 1. Location map for Ricker Hills sampling sites. Arrows indicate reconstructed ice-flow directions.
of its stratigraphic position, degree of lithification and superficial weathering. The Ricker Hills Tillite represents the northernmost outcrop of lithified glacial sediments in southern Victoria Land. Previous studies on the micromorphology of glacial sediments in Victoria Land were carried out by van der Meer et al. (1992), van der Meer and Hiemstra, (1998), Zaniewski (1997), Hiemstra (2001), Stroeven et al. (2002), Wilson et al. (2002) and Lloyd Davies (2004). This paper provides a detailed description of samples collected from three outcrops of the Ricker Hills Tillite (Fig. 1). The outcrops are located 100 m NE of Benson Knob (at 1250 m a.s.l.), at the western margin of the Lake Depression (at 1150 m a.s.l.) and at the western margin of the Morris Basin (at 960 m a.s.l.). Sandstones intercalated with black shales (a few centimetres to few tens of centimetres thick) from the Takrouna Formation outcrop at the base of the Benson Knob profile (Figs. 2a, 3). The top of the bedrock is characterised by a deformed layer about 0.5 to 1 m in thickness. The fractures inside the bedrock contain small amounts of sand. This upper part was interpreted in the field as a glaciotectonised layer. The black shales are particularly fragmented and disrupted. A yellow mud-supported diamictite (1.5 m thick) overlies the fragmented bedrock. The large clasts within the diamictite are typically dolerites and occasionally quartzites. Several injection veins, infilled by coarse sand to fine gravel with a mud matrix, dissect this diamictite. The mean dip of 13 measured veins is to the north (Fig. 2). This observation is fundamental in assessing
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Fig. 2. Stratigraphic section (see Fig. 1 for location). (a) Benson Knob profile; the injection veins in the till are in evidence. Stereographic projection and rose diagram of injection veins (n ¼ 13) are in evidence (square). Dip direction is expressed in 101 classes. (b) Lake Depression profile; note the glacially-disrupted bedrock. (c) Morris Basin profile; the bedrock is not visible. Measurements are in centimetres. Open circles indicate sample locations.
Fig. 3. Benson Knob profile. At the base a dark disrupted bedrock (Black shales) is visible. The light material on top is the Ricker Hills Tillite. An injection vein crosscut the tillite. Note the film container for scale (diameter: 3.5 cm).
that the stress pattern experienced by the tillite has a N–S trending major axis, with a N sense of movement. The formation of injection veins (also described as clastic dikes) is only possible with high confining pressure associated with high pore water content; they originate as extension fractures in simple shear conditions, i.e. at right angles with the axis of principal shear dip consequently upward in the direction of ice movement (Menzies, 2000). The Lake Depression profile is the most complex of the three described (Fig. 2b). The Takrouna Formation
sandstone, disrupted and with a typical blocky structure, lies at the base of the profile. Blocks range in size from 10 to 50 cm. Several high-angle shear planes dipping to the west, in several cases related to rotational structures marked by a sense of shear to the west, were identified. Injection veins up to some tens of centimetres long and 1–2 cm thick are widespread. Their random orientation in places follows fractures and planes of weakness in the deformed bedrock. The injected material has a sand to clay texture, and the same mineralogical composition as the Takrouna Sandstone, as confirmed by microscopic analysis. A massive lithified diamictite, yellow to greyish-yellow, overlies and penetrates the fractured bedrock with small-scale fracture infills. It consists of mud-supported clasts (mainly dolerites with some sandstones). Skeleton texture ranges from sandy to cobbly. The sand fraction mainly derives from weathered Takrouna Sandstone. This layer is 2 m thick in the section described here, but is estimated to reach 8–10 m. A large dolerite erratic rests above this layer. On the upper side of the boulder there is a 50 cm long and 8–10 cm thick lens of sandy silt. The lens has a laminated structure, with a fining upward texture. A layer of sorted gravel, 30 cm thick and with crude stratification, interrupts the massive diamictite. Clasts (sand to cobble) are exclusively dolerite. This layer is clast-supported, and silty matrix is nearly absent. A yellow uncemented diamicton belonging to a Late Pleistocene glacial drift (correlated to ‘‘younger drift’’ described in Stuiver et al., 1981 and ‘‘Terra Nova Drift’’ in Orombelli et al., 1991) with striated pebbles lies above the outcrop.
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A pale-yellow to brownish-yellow, massive, over 10 m thick matrix-supported diamictite crops out widely at Morris Basin (Fig. 2c). The skeleton texture ranges from gravel to boulder, with mainly dolerite and occasionally Takrouna sandstone clasts. Rare carbon flakes are also present. No primary structure is present at the macro scale, except for some crudely-oriented elongated clasts. In one case, a primary sedimentary structure was identified consisting of a north-dipping lens of quartzitic sand. It shows a boudinage structure elongated in a N–S direction. Other secondary structures are fracture surfaces (generally oriented E–W and dipping to the north), which sometimes penetrate clasts. Clast circles indicative of rotational movement are also evident. In order to fully describe the outcrops, samples were collected on cleaned surfaces or in excavated trenches. Samples for micromorphological observation and assessment of lithological composition were collected on the cleaned surfaces. Most samples are partly consolidated, but some are brittle. Kubiena boxes were often used to collect samples, which are all oriented. In order to protect samples during transport to Italy, they were preliminarily superficially impregnated under vacuum with a thick epoxy resin at the Italian Antarctic base in Terra Nova Bay.
3. Laboratory methods Samples were air-dried at 40 1C and impregnated under vacuum with polyester resin thinned with acetone, hardened with the aid of an accelerator and, in a few cases, with a thin bi-component epoxy resin. Thin sections (9 6 cm) were cut following the methods outlined by Murphy (1986) and van der Meer (1987, 1993). Thin sections were studied using a petrographic microscope with a magnification between 2 and 40 , in order to describe millimetre-scale structures and plasmic fabric in detail. The descriptions follow the pedological nomenclature suggested by Brewer (1976), Bullock et al. (1986) and Fitzpatrick (1984), and the glossary of micromorphological terms supplied by van der Meer (1993).
4. Micromorphological description According to van der Meer (1996), the description of thin sections should take into account four characteristics: texture, structure, plasmic fabric, and pedofeatures. In our samples, the latter are not clearly originated by pedological processes; for this reason we classify cutans, pendants, neoformation minerals and translocations as ‘‘secondary features’’. A summary of
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micromorphological description is listed in Tables 1–3 following Carr (1999), with some modifications. 4.1. Benson Knob profile (Fig. 2a) Samples from Benson Knob come from the tectonised top of bedrock and from the overlying tillite. Thin sections are uniformly coloured from pale brown to pale yellow. Sample BK4 (Fig. 4) contains an injection vein: the dark vertical band visible in the middle of the thin section. All the samples are rich in coal fragments Fig. 5 refers to the bedrock (see Section 4.1.5). 4.1.1. Texture Skeleton grains range from 1 cm to 30 mm, with a dominant grain size of 500 mm (Fig. 6). Roundness is medium to low for grains smaller than 500 mm, and medium to high for larger ones; angularity is well expressed in clasts smaller than 500 mm, while larger ones are more rounded. Quartz is the dominant mineral, but lithic fragments (dolerite and black shale) are also widely present; plagioclase and K-feldspar are rare. The larger clasts are represented by black shale and dolerite. The distribution of skeleton grains is homogeneous. The matrix consists of silt, clay and fine sands (quartz fragments). The colour varies from pale yellow to pale brown according to composition and oxide content. The ratio between coarse and fine is generally 1:1. The C/F related distribution ranges from closed to single-spaced porphyric, i.e. skeleton clasts are in contact or as each other distant as their size. 4.1.2. Structure No sedimentary structures were detected in these samples. Porosity, represented by planes and channels, is always less than 10%. Only sample BK2 shows some vughs. Several rotational structures, both with and without core stones, are visible. Shear planes are also clearly visible, as are some linear features constituted by spatial arrangement of particles (Fig. 7). Some larger clasts exhibit pressure shadows marked by finer material. Sample BK4 is characterised by the presence of an injection vein containing several rotational features (Fig. 4). The sample also shows a weakly-developed ‘‘pebble type I’’ structure (cf. van der Meer, 1993: ‘‘pebble type I’’ are nodules ‘‘which consist of till and which do not have an internal plasmic fabric’’). Fine angular grains shown in Fig. 8 are most likely to be the result of grain crushing process. Sample BK2 contains a well-rounded aggregate composed of homogeneous fine material; structure consists of isolated elements of either till or fine grained sediments encased in the till body (‘‘pebble type III’’; van der Meer, 1993) (Fig. 9).
Staining
P+Po Po+Ch Po P P
Neoformation Impregnation Depletion
L L L L L
, abundant/well developed (H).
Infillings
Water escape
Linear features
Skelsepic Lattisepic Omnisepic
Fractured coats
Crushed grains
Coats (grains+voids)
4500 mm
Pressure shadow
Plasmic fabric
L M L L L
o500 mm
Rotation
Deformation structures
Post depositional
5000 500 500 500 501
4500 mm
Angularity
Voids
Keys for the table content: , rare/poorly developed (L); , common/moderately developed (M); For voids: L ¼ presence of lab induced voids; P ¼ planes; Po ¼ pores; Ch ¼ channels.
BK1 BK2 BK3 BK4 BK5
BK1 BK2 BK3 BK4 BK5
Dominant grain Roundness size mm+sorting o500 mm
Skeleton
Masepic
Pebble I
Insepic
Pebble II Pebble III
980
Sample
Table 1 Summary of micromorphological description of samples from Benson Knob profile (see Figs. 1 and 2 for location)
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, abundant/well developed (H).
Infillings
L L
L
Linear features
Ch+Po Ch+P P P P P P+Ch
Neoformation Impregnation Depletion
Water escape
Skelsepic Lattisepic Omnisepic
Staining
Crushed grains
Fractured coats
4500 mm
Pressure shadow
Coats (grains+voids)
o500 mm
Rotation
Deformation structures
Plasmic fabric
L L M H M L M
4500 mm
Angularity
Voids
Post depositional
1000 200 300 300 300 200 300
Dominant grain Roundness size mm+sorting o500 mm
Skeleton
Keys for the table content: , rare/poorly developed (L); , common/moderately developed (M); For voids: L ¼ presence of lab induced voids; P ¼ planes; Po ¼ pores; Ch ¼ channels.
LD1 LD2 LD3 LD4 LD5 LD6 LD7
LD1 LD2 LD3 LD4 LD5 LD6 LD7
Sample
Table 2 Summary of micromorphological description of samples from Lake Depression profile (see Figs. 1 and 2 for location)
Masepic
Pebble I
Insepic
Pebble II Pebble III
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P Po Po+Ch P Po+P Po
L L L L L L
Neoformation Impregnation Depletion
Staining
, abundant/well developed (H).
Infillings
Water escape
Linear features
Skelsepic Lattisepic Omnisepic
Fractured coats
Crushed grains
Coats (grains+voids)
4500 mm
Pressure shadow
Plasmic fabric
L M M L/M L M
o500 mm
Rotation
Deformation structures
Post depositional
500 3000 100 200 500 200
4500 mm
Angularity
Voids
Keys for the table content: , rare/poorly developed (L); , common/moderately developed (M); For voids: L ¼ presence of lab induced voids; P ¼ planes; Po ¼ pores; Ch ¼ channels.
MB1 MB2 MB3 MB4 MB5 MB6
MB1 MB2 MB3 MB4 MB5 MB6
Dominant grain Roundness size mm+sorting o500 mm
Skeleton
Masepic
Pebble I
Insepic
Pebble II Pebble III
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Sample
Table 3 Summary of micromorphological description of samples from Morris Basin profile (see Figs. 1 and 2 for location)
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Fig. 4. Benson Knob. Sample BK4. Thin section of an injection vein (dark, vertical band), annotated with dashed line. Field of view 9 5 cm.
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Fig. 7. Benson Knob. Sample BK5. Linear features are visible from top left to bottom right. Elongation axis of aligned clasts are annotated. Field of view 5 mm, PPL.
Fig. 5. Benson Knob. Sample BK1. Black shales of the glaciotectonised bedrock. Some injected fine material is visible (arrow on the left). The sedimentary lamination of the black shale is folded (dashed line). Field of view 9 5 cm.
Fig. 6. Benson Knob. Sample BK3. The tillite shows heterogeneous grain size and distribution. The dominant size of the sand grains is 500 mm. Field of view 5 mm, Plane polarised light (PPL).
Fig. 8. Benson Knob. Sample BK4 (a) and BK5 (b). An example of edge-to-edge crushing (a) and some crushed grains (b). Field of view 2.5 mm (a) and 0.5 mm (b), PPL.
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Fig. 10. Benson Knob. Sample BK5. A skel-lattisepic plasmic fabric. Note the perpendicular orientation of surfaces of the lattisepic pattern. Main lattice direction are indicated by the dashed lines. Field of view 0.5 mm, XPL.
shales. The skeleton grains (sample BK1, Fig. 5) consist of lithic fragments of the black shale with prevalent coarse portion. Their size ranges from a few hundred microns to several centimetres. Injected fine material is widespread and folded clasts are locally visible (Fig. 5). Masepic plasmic fabric characterises the thin section where the matrix is concentrated. 4.2. Lake Depression profile (Fig. 2b) Fig. 9. Benson Knob. Sample BK5. A well-rounded mud pebble with plasmic fabric, easily distinguished from the surrounding till (‘‘type III pebble’’, van der Meer, 1993). Field of view 2.5 mm, PPL (a), Cross polarised light (XPL) (b).
4.1.3. Plasmic fabric The fine material is diffusely reoriented in domains (Fig. 10). The widespread pattern is the skelsepic (Brewer, 1976) (granostriated, Bullock et al., 1986) plasmic fabric. This feature also marks some veins. A lattisepic plasmic fabric (cross striated) is also visible and shows incident directions of reorientation. An insepic pattern is also present in places. 4.1.4. Secondary features All the samples are characterised by the widespread presence of cutans (coatings of clay and silt), especially within veins and planes (infillings), and also around clasts (pendants). Some relict, reworked cutans (papules) are locally present. Mineral translocations are also diffuse, with staining and impregnation by oxides around elongated voids. 4.1.5. Disrupted bedrock Sample BK1 (Fig. 5) refers to the bedrock, a dark green to brown rock containing fragmented black
Samples from the Lake Depression come from a variety of facies related to glacial deposition and from the glaciotectonised bedrock (Fig. 11). The samples are yellow to pale yellow; lithic fragments, coal fragments, and impregnated domains due to secondary features are clearly evident. Some sedimentary features, such as laminae or a faint stratification, are also visible (samples LD3, LD4 and LD5; Fig. 12). 4.2.1. Texture The dominant skeleton clasts range in size from 200 to 300 mm. Roundness is medium to low, and generally higher for clasts 4500 mm, while angularity is medium to low for this size range and medium for smaller clasts. Samples LD4 and LD6 are characterised by a high degree of roundness, especially clasts 4500 mm. Quartz is the dominant mineral, but lithic fragments of dolerite are also widely present; plagioclase and K-feldspar are visible in places. Sandstone lithic fragments are generally scarce. The skeleton grain distribution is typically homogeneous; only sample LD4 is divided into separate domains, all of which are characterised by a high sorting index. The matrix is mainly composed of silt and clay with some quartz fragments. The colour ranges from pale yellow to brown, reflecting
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Fig. 12. Lake Depression. Sample LD4. (a) Folded laminae, annotated with dashed line. Field of view 5 cm. (b) Photograph of the layer represented above in detail (square). A pen cap on the right for scale.
Fig. 11. Lake Depression. Sample LD1. Deformed bedrock consisting of fragmented blocks of Beacon sandstones. Coal fragment alignment is indicated by dashed lines. Field of view 9 3 cm.
the oxide content. The coarse fraction is equal to or greater than the fine one; only in sample LD6 is the matrix more abundant than the skeleton. The coarse/fine related distribution is close porphyric (grain-supported), especially in samples LD4 and LD6. Skeleton grains are rare in sample LD6, with a single-spaced porphyric coarse-fine related distribution (the distance between clasts is close to or greater than their size, Bullock et al., 1986). In sample LD5 several highly-sorted bands show a unimodal distribution (quartz).
4.2.2. Structure Primary structures such as laminae (some millimetres thick) are visible in sample LD5, and are deformed in centimetre folds (Fig. 12). Sample LD3 shows a well-sorted silt bed; there are no visible primary internal features. Porosity, commonly represented by planes and some minor channels, is never higher than 5–6%. Secondary structures are widespread in this section. Rotational structures are common and better developed in samples LD3, LD6 and LD8. Sample LD5 also shows a ‘‘milky-way’’ or ‘‘galaxy’’ structure, marked by wings of different composition (Fig. 13). Pressure shadows are present in samples LD6 (Fig. 14), while results of grain fracturing mechanisms are widespread in almost all sections. Samples LD6 is characterised by several poorly-sorted silt and fine sand bands of various thickness. Considering their flame-shaped margins and flattening, these features can be described as water escape structures (Fig. 15). Linear features marked by clast alignments are clearly visible in sample LD2 (Fig. 16).
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Fig. 13. Lake Depression. Sample LD5. A milky-way structure with wings (annotated) is well visible on the right of the core stone, but only for a single wing. Field of view 5 mm, PPL.
Fig. 14. Lake Depression. Sample LD5. The left side of the clast on the right is surrounded by fine material concentrated in its pressure shadow. The shape is underlined by dashed lines. Field of view 5 mm, PPL.
4.2.3. Plasmic fabric This fabric is widespread in nearly all samples. The skelsepic plasmic fabric is best developed; lattisepic plasmic fabrics are present locally. Some samples do not show plasmic fabric. Care must be taken in assessing this feature, given the common presence of calcite which, because of its strong birefringence, can mask clay reorientation (van der Meer, 1993).
4.2.4. Secondary features Coatings, either caps or pendants, some fractured (Fig. 17), are clearly visible in these sections. Water circulation is clearly responsible for several infillings, due to the presence of clay and calcite deposition
Fig. 15. Lake Depression. Sample LD1. Water escape structure. Reconstructed water flow directions are suggested by the arrows. Field 5 mm, PPL.
Fig. 16. Lake Depression. Sample LD2. Linear features marking shear planes within the till. Orientation is from top left to bottom right, as indicated by dashed line. Rotational structure in circle. Field 5 mm, PPL.
(Fig. 18). Calcite is widespread as a neo-formation mineral (Fig. 19). 4.2.5. Disrupted bedrock Sample LD1, collected in the deformed bedrock, shows dominant skeleton grain more than 1 cm in size (Fig. 11). The angularity is low for all the grain-size classes. The coarse fraction is four times the fines and their related distribution is close porphyric (grain-supported). Sandstone lithic fragments, disrupted and rotated, are the main component of sample LD1. Aligned coal fragments (markers for linear features) are also visible. Pressure shadows are also present, as well as poorly-sorted silt and fine sand bands of various thickness, indicating a water escape process (Fig. 15). Two perpendicular sections
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Fig. 19. Lake Depression. Sample LD1. Calcite neoformation close to coal fragments (centre and top left). Field of view 5 mm, XPL.
4.3. Morris Basin profile (Fig. 2c) Samples from the Morris Basin come from the tillite and from the previously described vein. They are pale brown to yellow coloured, and locally banded or characterised by staining along planar voids. Coal fragments are widely spread within samples. Secondary structures such as linear features are visible in sample MB4, where they are marked by the alignment of coal fragments.
Fig. 18. Lake Depression. Sample LD2. Calcite deposition in a planar void, from top left to bottom right. Field of view 0.5 mm, XPL.
4.3.1. Texture The size of the skeleton is greater than 50 mm, with a dominant class of 500 mm. Some lithic fragments are up to a few centimetres in size. Roundness is medium to high for clasts 4500 mm while it is low to medium for smaller clasts. The angularity of clasts o500 mm is medium to high, while it is medium to low for larger ones. Only samples MB3 and MB4 show a medium-low angularity for clasts o500 mm and a low angularity for the larger ones. Sorting is appreciable in these two samples. The composition is a mixture of monocrystalline and polycrystalline quartz, dolerite lithic fragments, pyroxene and some sandstone. Some type II (fine grained material still part of the original sediment host) and III ‘‘pebbles’’ (van der Meer, 1993) can be observed (Fig. 20). The matrix consists of silt, clay and fine sand; the latter is constituted by quartz and pyroxene fragments. The colour varies from yellow to pale brown according to the oxide content. The coarse/fine ratio ranges from 1:1 to 2:1 in some tillite domains. The coarse/fine related distribution is close porphyric, in some cases single-spaced (Bullock et al., 1986).
from sample LD1 highlight the alignment of coal fragments dipping 15–301 to the north (Fig. 11). The skelsepic plasmic fabric is well developed.
4.3.2. Structure Sedimentary (primary) structures are not visible in these samples. Porosity, determined by planes, channels
Fig. 17. Lake Depression. Sample LD3. Clay and silt cutans. (a) clast on the top: the cutan (grey material indicated by the arrow) is a pendant. (b) clay cutan. Field of view 0.5 mm (a) and 5 mm (b), PPL.
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Fig. 20. Morris Basin. Sample MB2 (a, b) and MB4 (c, d). Examples of type III ‘‘pebbles’’ according to van der Meer (1993). Note the sharp contrast with the ground mass, especially under crossed-polars. Field of view 2.5 mm, PPL (a, c), XPL (b, d).
and pores, is less than 5%. Secondary structures, due to deformation, are widespread. Stone circles and turbate structures are clearly visible in samples MB6 and MB1; linear features, such as clast alignments and elongated type III ‘‘pebbles’’ (van der Meer, 1993), are well represented in sample MB4 (Fig. 20). Other structures, such as pressure shadows and crushed grains, were observed in samples MB3, MB5 and MB6. Some nodules are worthy of additional observation. Texturally, they are very well rounded and have a core stone with a diameter significantly smaller than the nodule size (Fig. 21). The external part consists of fine material that surrounds the core stone in concentric layers or skins with a well developed skelsepic plasmic fabric. This well recognisable structure can be used for identifying glacier bed deformation, induced by shear stress condition in wet environment. Similar nodules have been described as ‘‘argiquartzosepules’’ by Brewer (1976), as ‘‘fine-grained casings’’ by Hiemstra (1999), and as ‘‘clast haloes’’ by Lachniet et al. (2001). Moreover, sample MB6 reveals several water-escape structures. The lower portion of samples MB3 and MB4
shows an interesting feature: the lens described in the field shows a wide presence of till pebbles, all of which are deformed and strongly elongated (Figs. 20 and 21). 4.3.3. Plasmic fabric Plasmic fabric in these samples is not uniform. The skelsepic plasmic pattern, particularly well developed in sample MB7, is the most common. Samples MB3 and MB4, collected in the same lens, are slightly different; the ‘‘pebbles’’ described above are immediately obvious in these samples because the strong skelsepic plasmic fabric involves the clay material of the concentric layers. Lattisepic plasmic fabrics, with perpendicular reorientation direction, are also present, especially in samples MB5 and MB6. In some domains sample MB5 also shows a masepic plasmic fabric. 4.3.4. Secondary features Staining and iron oxides impregnation is diffuse in samples from the tillite. Translocation of minerals is not uniform, but concentrated around the voids (Fig. 22).
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Fig. 21. Morris Basin. Sample MB3. Examples of rolling-ball structures (see Section 5 for description). Field of view 0.5 mm, PPL (a, c), XPL (b, d).
Fig. 22. Morris Basin. Sample MB5. Mineral translocation is mainly due to water circulation; note the stained material around a void (the limit is marked by the dashed line). Field of view 2.5 mm, PPL.
Fig. 23. Morris Basin. Sample MB2. Cutans and their reworked portions. Other reworked cutans are on the bottom right (arrow). Field of view 2.5 mm, PPL.
A banded distribution of oxidation products was also observed. Coatings are common in samples from the vein (MB3 and MB4). Sample MB7 reveals fragmented
coatings (papule or pedorelict) interpreted as the product of intense mechanical reworking of the cutans (Fig. 23).
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Calcite neo-formation is visible in all samples except MB7. In sample MB1 calcite neo-formation is diffuse and optically disturbs the birefringence of the clay minerals, thereby masking the plasmic fabric. Loose incomplete infillings are also present. In some cases the infillings are enriched in coal fragments.
5. Discussion and conclusions Thin-section analysis confirms that the diamictite informally named Ricker Hills Tillite is linked to glacial action. Grain crushing induced by glacial action produces fine angular and irregular clasts. As a result, in the Ricker Hills Tillite, the larger clasts are rounder and smoother. The more widespread facies is a massive matrixsupported diamicton attributed to lodgement processes in the field but classified as ‘‘tectomict’’ according to van der Meer et al. (2003) on the basis of micromorphological analysis. Plasmic fabric is visible in most samples, especially skelsepic and lattisepic patterns, which are often associated. This feature was not only observed in samples with low clay content but also in those with numerous large skeleton clasts. Syndepositional pervasive till shearing (glacial-induced) is evidenced by the occurrence of clast alignments, shear planes, rotational structures and pressure shadows. Shear planes exploited as drainage channels, infilled by injection veins in lodgement till and at the till–bedrock interface (Benson Knob and ‘‘Lake Depression’’ profiles) would confirm the presence of water at an unknown time after deposition. As a consequence of syn- to post-depositional reworking processes, nodules (‘‘pebble structure’’ according to van der Meer, 1993) are widespread (though to varying degrees) in tectomict. Widespread ‘‘pebble’’ presence is particularly evident in a lens of the Morris Basin profile (Fig. 2). Unusual nodules (up to 1 mm in size) are commonly distributed in this lens (Fig. 21). They are well rounded and composed of a core stone enveloped by clay. The clay envelope exhibits a well-developed granostriated plasmic fabric. Considering the unusual structure and appearance of these nodules, with their clearly identifiable core stone, we propose to name them ‘‘rolling-ball structures’’. This structure is attributed to the prolonged rotation of the core stone and the progressive accretion of clay around the inner clast. According to Menzies (2000), these structures probably formed in an environment characterised by high pore-water and clay contents. As a conclusion, the lens containing ‘‘rollingball structures’’ is a primary structure related to the syndepositional presence of water. Water-escape structures are clearly evident in the tillite and bedrock. Some of the bands which developed
during this process are well sorted, while others are poorly sorted. These structures can be attributed to sediment sorting operated by a slow dewatering process. Alternatively, they can be ascribed to concentration of water in primary fine-grained sorted levels, on which the glacial-induced stress has lead to the squeezing of water content. We favour the second interpretation, considering the unexpected lack of finer and larger clasts, which are otherwise randomly present in the tillite. According to our interpretation, glacier-induced stress squeezes fine-grained, water-enriched sediments along planes of weakness. Poorly sorted structures, on the other hand, result from forced dewatering of low-viscosity sediments. In conclusion, the Ricker Hills Tillite was deposited under wet conditions (with high pore water content) during prevalent ductile deformation. Boudinage, ductile shear zones, and rotational structures are also widespread in the bedrock and confirm this conclusion (Fig. 3). In the ‘‘Lake Depression’’ profile, samples LD3 and LD5 document a layer originated and sorted by water circulation, about 40 cm thick; small clasts are prevalent, and the clay content is low. Further evidence comes from sample LD4 (in the same outcrop) that was clearly deposited under running water, as demonstrated by the primary structures visible in the field and under the microscope (Fig. 12). These two layers are compatible with subglacial deposition in the presence of melt waters. Nevertheless, pressure shadows and galaxy structures as well as deformation of laminated sediments in sample LD4 document a post-depositional deformation induced by a younger glacial advance. On the basis of oriented samples, the ice-flow direction was clearly from the south at Benson Knob and from the east in the ‘‘Lake Depression’’. This further suggests that, also at the time when the Ricker Hills Tillite deposited, the Lake Depression existed and drew a converging glacial flow in that local topographic basin. The micromorphological deformation features of the Ricker Hills Tillite (lattisepic-skelsepic plasmic fabric, water escape, ‘‘type I pebbles’’, van der Meer, 1993), and injection veins in weak joint planes suggest that a warm-based glacier was responsible for the deposition and the deformation of the till. Considering (1) the location of the Ricker Hills nunatak at the margin of the EAIS, (2) the position of the till outcrops, located in depressions of the nunatak over deepened before till deposition, (3) the paleo iceflow directions reconstructed in two sections, (4) the elevation of the highest erosional trimline located at about 1780 m (at the western margin of the nunatak) and at 1520 m (in the central part of the nunatak; Baroni et al., unpublished data), we conclude that the Ricker Hills Tillite was deposited by a warm-based ice sheet with a maximum value of ice thickness evaluated at 600–900 m.
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Syn- to post-depositional processes are mainly represented by secondary features produced by clay and silt migration and oxide impregnation. They document that oversaturated water (i.e. rich in calcium carbonate) circulated during and/or after deposition. Indeed, clay to silty coatings, either caps or pendants, are documented in all the outcrops; some void infilling show lamination structures and have variable grain size composition. Calcite neoformation on the walls of the pores or as infillings is also widespread. Sparitic calcite crystallisation took place under phreatic condition, confirming the circulation of water saturated in calcium carbonate. Some of the secondary features are clearly syn-depositional (i.e. the papule) while others (i.e. pendants and calcite infillings) are most likely postdepositional. In any case, in the Ricker Hills Tillite, all the structures related to water circulation indicate that the subglacial environment was wetter and warmer than the present one during its deposition.
Acknowledgements This work was carried out within the framework of the Italian Programma Nazionale di Ricerche in Antartide (PNRA) and was financially supported through a joint research programme on Geology with the University of Siena. We are very grateful to Jaap van der Meer, John Menzies and Mauro Cremaschi for valuable discussion and helpful suggestions on a preliminary version of the manuscript. Finally, we wish to thank John Hiemstra and Damien Gore whose constructive and helpful comments improved an earlier version of this paper. References Aharon, P., 1988. Oxygen, carbon and U-series isotopes of aragonites from Vestfold Hills, Antarctica: Clues to geochemical processes in subglacial environments. Geochimica et Cosmochimica Acta 52, 2321–2331. Boulton, G.S., Dobbie, K.E., 1998. Slow flow of granular aggregates: the deformation of sediments beneath glaciers. Philosophical Transaction of the Royal Society of London 356, 2713–2745. Brewer, R., 1976. Fabric and Mineral Analyses of Soils. Krieger, Huntingdon, NY 482pp. Bullock, P., Federoff, N., Jongerius, A., Stoops, G., Tursina, T., 1986. Handbook for Soil Thin Section Description. Waine Research Publications 152pp. Capponi, G., Crispini, L., Meccheri, M., Musumeci, G., Pertusati, P.C., Baroni, C., Delisle, G., Orsi, G., 1999. Antarctic Geological 1:250.000 map series, Mount Joyce Quadrangle (Victoria Land), Museo Nazionale dell’Antartide, Sez. Scienze della Terra, via Laterina 8 (53100), Siena. Carr, S., 1999. The micromorphology of Last Glacial Maximum sediments in the Southern North Sea. Catena 35, 123–145. Fitzpatrick, E.A., 1984. Micromorphology of Soils. Chapman & Hall, London, New York 433pp. Hallet, B., 1975. Subglacial silica deposits. Nature 254, 682–683.
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van der Meer, J.J.M., Laban, C., 1990. Micromorphology of some North Sea till samples, a pilot study. Journal of Quaternary Science 5 (2), 95–101. van der Meer, J.J.M., Mu¨cher, H.J., Ho¨fle, H.C., 1992. Micromorphological observations on till samples from Shackleton Range and North Victoria Land, Antarctica. Polarforschung 62 (1), 57–65. van der Meer, J.J.M., Hiemstra, J.F., Stroeven, A.P., 1998. Micromorphology of two Sirius Group core diamict samples from Mt. Feather, Dry Valleys, Antarctica. In: Wilson, G.S., Barron, J.A. (Eds.), Mount Feather Sirius Group Core Workshop and Collaborative Sample Analysis. BPRC Report No. 14, Byrd Polar Research Centre. Ohio State University, Columbus, OH, pp. 96–108. van der Meer, J.J.M., Menzies, J., Rose, J., 2003. Subglacial till: the deforming glacier bed. Quaternary Science Reviews 22, 1659–1685. Webb, P., Harwood, D.M., 1991. Late Cenozoic glacial history of the Ross Embayment, Antarctica. Quaternary Science Reviews 10, 215–223. Webb, P., Harwood, D.M., Mabin, M.C.G., McKelvey, B.C., 1996. A marine terrestrial Sirius Group succession, middle Beardmore Glacier-Queen Alexandra Range, Transantarctic Mountains, Antarctica. Marine Micropaleontology 27, 273–297. Wilson, G.S., Barron, J.A., Ashworth, A.C., Askin, R.A., Carter, J.A., Curren, M.G., Dalhuisen, D.H., Friedmann, E.I., FydorovDavidov, D.G., Gilichinsky, D.A., Harpen, M.A., Harwood, D.M., Hiemstra, J.F., Janecek, T.R., Licht, K.J., Ostroumov, R.D., Powell, R.D., Rivkina, E.M., Rose, S.A., Stroeven, A.P., Stroeven, P., van der Meer, J.J.M., Wizevich, M.C., 2002. The Mount Feather Diamicton of the Sirius Group: an accumulation of indicators of Neogene Antarctic glacial and climatic history. Palaeogeography, Palaeoclimatology, Palaeoecology 182, 117–131. Zaniewski, K., 1997. Micromorphology of Antarctic Samples; selected samples II. Fysisch Geografisch Bodemkundig Laboratorium, Universiteit van Amsterdam, p. 70.
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Micromorphological evidence of warm-based glacier deposition from the Ricker Hills Tillite (Victoria Land, Antarctica) Carlo Baronia,, Francesco Fasanob a
Dipartimento di Scienze della Terra, CNR, Istituto di Geoscienze e Georisorse, Universita` di Pisa, via S.Maria, 53, 56126, Pisa, Italy b Dipartimento di Scienze della Terra, Universita` di Siena, via Laterina, 8, 53100 Siena, Italy Received 2 April 2004; accepted 30 November 2004
Abstract Thin sections of impregnated samples were used for micromorphological analysis of the ‘‘Ricker Hills Tillite’’ in southern Victoria Land, Antarctica. The tillite is composed of massive matrix-supported diamicton with a porphyric coarse/fine related distribution, low sorting, low rounding and medium to high angularity. Glacial deposits are completely represented by lodgement till. Phyllosilicate reorientation patterns (plasmic fabrics) are visible in most samples, especially associated skelsepic and lattisepic patterns. Syndepositional pervasive till shearing (glacially induced) is evidenced by clast alignments, shear planes, rotational structures and pressure shadows. Boudinage, ductile shear zones, and rotational structures are also widespread in the deformed bedrock. Water-escape structures are clearly evident in the tillite and in the bedrock. Shear planes infilled by injection veins in lodgement till and at the till-bedrock interface testify to the presence of water during deposition. Secondary features like clay and silt coatings, sparite calcite, and oxidation products were found in all the examined outcrops. They formed under phreatic conditions due to saturated water circulation. A warm-based ice sheet was responsible for the deposition of the tillite. Past ice-flow directions were similar to present ones. r 2005 Elsevier Ltd. All rights reserved.
1. Introduction A number of outcrops of a semi-lithified to lithified diamicton, informally named Ricker Hills Tillite, were mapped in the Ricker Hills area, in southern Victoria Land (Baroni, in Capponi et al., 1999). The tillite unconformably overlies the sandstone and black shales of the Takrouna Formation (Beacon Supergroup, Permian to Late Triassic) and the Ferrar Dolerite (Jurassic). A massive, matrix-supported diamicton with a sandy silt matrix that ranges in colour from graybrown to olive-gray covers the tillite in all the studied outcrops. It is correlated with the Late Pleistocene ‘‘younger drift’’ recognised by Stuiver et al. (1981) in the Dry Valleys and with the ‘‘Terra Nova Drift’’ described Corresponding author.
E-mail addresses: [email protected] (C. Baroni), [email protected] (F. Fasano). 0277-3791/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2004.11.011
by Orombelli et al. (1991) in the Terra Nova Bay area. Field surveys, profile descriptions, and fabric measurements were carried out on these glacial deposits. Considering its geomorphologic and stratigraphic position, fabric and lithofacies, the Ricker Hills Tillite can be correlated to the Sirius Group. This group comprises a suite of consolidated glacial deposits (subglacial deposits, ablation, and flow tills as well as glaciomarine and glaciolacustrine sediments, and ice marginal deposits; Stroeven, 1997) that are widespread in the Transantarctic Mountains. It is widely understood that the sediments of the Sirius Group derive from temperate glaciers (Stroeven, 1997; Wilson et al., 2002; and references therein); however, the age, environmental evolution and palaeoclimatic significance of this group are still a matter of debate. In particular, although the presence of Pliocene microfossils within the Sirius Group raised some doubts (Harwood, 1983; Webb and Harwood, 1991; Harwood
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and Webb, 1998; Webb et al., 1996), geomorphologic, stratigraphic and volcanic data, and exposure ages suggest an age older than Late to Mid-Miocene (Sugden et al., 1993, 1995a, b; Marchant and Denton, 1996; Sugden, 1999; Schaefer et al., 1999; Stroeven and Kleman, 1999). The value of micromorphology in understanding glacial processes has been widely demonstrated (van der Meer, 1987; van der Meer and Laban, 1990, Menzies and Maltman, 1992; van der Meer, 1993, 1997; Menzies et al., 1997; Boulton and Dobbie, 1998; van der Meer and Hiemstra, 1998; van der Meer et al., 1998, 2003; Menzies, 1998, 2000, Menzies and Zaniewski, 2002; Hiemstra and Rijsdijk, 2003). Indeed the analysis of microstructures and textures, and their relationships provide key information for identifying depositional processes and facies, and reconstructing post-depositional histories. Based on field observations and descriptions, and in order to investigate the processes responsible for its glacial deposition, the Ricker Hills Tillite was sampled for micromorphological analysis. The role of water during deposition, deformation and post-depositional modification of the tillite was also carefully investigated. Water content, one of the most important factors in till differentiation, is a function of glaciodynamics and subglacial temperatures. Water can strongly influence sediment mineral assemblage and calcite precipitation (Hallet, 1975, 1976; Aharon, 1988). Moreover, the presence of pore water can strongly influence sediment rheology (Menzies, 1989; van der Meer et al., 2003); in fact, water, together with clay content of the deforming bed is the major factor controlling deformation (van der Meer et al., 2003).
2. Study area and sampling The Ricker Hills (751400 S, 1591200 E) are located in the southern Prince Albert Mountains (Fig. 1), at the southern margin of the David Glacier basin, about 100 km from the Ross Sea coast. The hills, ranging in elevation from 900 to 1830 m a.s.l., constitute a nunatak at the boundary of the East Antarctic Ice Sheet (EAIS). A complex basin-and-range topography characterises the nunatak. The highest elevations are found at the western and southern margins; two main depressions are located along the southeastern and northern borders. The first depression is presently occupied by a frozen lake and will be hereafter indicated as the ‘‘Lake Depression’’, while the second is named the Morris Basin. Steep rock walls (Ferrar Dolerites, Kirkpatrick Basalt and Sandstone of the Beacon Supergroup) define the western margins of the depressions. At least five glacial drifts are documented in the area, the Ricker Hills tillite being the oldest on the basis
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Fig. 1. Location map for Ricker Hills sampling sites. Arrows indicate reconstructed ice-flow directions.
of its stratigraphic position, degree of lithification and superficial weathering. The Ricker Hills Tillite represents the northernmost outcrop of lithified glacial sediments in southern Victoria Land. Previous studies on the micromorphology of glacial sediments in Victoria Land were carried out by van der Meer et al. (1992), van der Meer and Hiemstra, (1998), Zaniewski (1997), Hiemstra (2001), Stroeven et al. (2002), Wilson et al. (2002) and Lloyd Davies (2004). This paper provides a detailed description of samples collected from three outcrops of the Ricker Hills Tillite (Fig. 1). The outcrops are located 100 m NE of Benson Knob (at 1250 m a.s.l.), at the western margin of the Lake Depression (at 1150 m a.s.l.) and at the western margin of the Morris Basin (at 960 m a.s.l.). Sandstones intercalated with black shales (a few centimetres to few tens of centimetres thick) from the Takrouna Formation outcrop at the base of the Benson Knob profile (Figs. 2a, 3). The top of the bedrock is characterised by a deformed layer about 0.5 to 1 m in thickness. The fractures inside the bedrock contain small amounts of sand. This upper part was interpreted in the field as a glaciotectonised layer. The black shales are particularly fragmented and disrupted. A yellow mud-supported diamictite (1.5 m thick) overlies the fragmented bedrock. The large clasts within the diamictite are typically dolerites and occasionally quartzites. Several injection veins, infilled by coarse sand to fine gravel with a mud matrix, dissect this diamictite. The mean dip of 13 measured veins is to the north (Fig. 2). This observation is fundamental in assessing
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Fig. 2. Stratigraphic section (see Fig. 1 for location). (a) Benson Knob profile; the injection veins in the till are in evidence. Stereographic projection and rose diagram of injection veins (n ¼ 13) are in evidence (square). Dip direction is expressed in 101 classes. (b) Lake Depression profile; note the glacially-disrupted bedrock. (c) Morris Basin profile; the bedrock is not visible. Measurements are in centimetres. Open circles indicate sample locations.
Fig. 3. Benson Knob profile. At the base a dark disrupted bedrock (Black shales) is visible. The light material on top is the Ricker Hills Tillite. An injection vein crosscut the tillite. Note the film container for scale (diameter: 3.5 cm).
that the stress pattern experienced by the tillite has a N–S trending major axis, with a N sense of movement. The formation of injection veins (also described as clastic dikes) is only possible with high confining pressure associated with high pore water content; they originate as extension fractures in simple shear conditions, i.e. at right angles with the axis of principal shear dip consequently upward in the direction of ice movement (Menzies, 2000). The Lake Depression profile is the most complex of the three described (Fig. 2b). The Takrouna Formation
sandstone, disrupted and with a typical blocky structure, lies at the base of the profile. Blocks range in size from 10 to 50 cm. Several high-angle shear planes dipping to the west, in several cases related to rotational structures marked by a sense of shear to the west, were identified. Injection veins up to some tens of centimetres long and 1–2 cm thick are widespread. Their random orientation in places follows fractures and planes of weakness in the deformed bedrock. The injected material has a sand to clay texture, and the same mineralogical composition as the Takrouna Sandstone, as confirmed by microscopic analysis. A massive lithified diamictite, yellow to greyish-yellow, overlies and penetrates the fractured bedrock with small-scale fracture infills. It consists of mud-supported clasts (mainly dolerites with some sandstones). Skeleton texture ranges from sandy to cobbly. The sand fraction mainly derives from weathered Takrouna Sandstone. This layer is 2 m thick in the section described here, but is estimated to reach 8–10 m. A large dolerite erratic rests above this layer. On the upper side of the boulder there is a 50 cm long and 8–10 cm thick lens of sandy silt. The lens has a laminated structure, with a fining upward texture. A layer of sorted gravel, 30 cm thick and with crude stratification, interrupts the massive diamictite. Clasts (sand to cobble) are exclusively dolerite. This layer is clast-supported, and silty matrix is nearly absent. A yellow uncemented diamicton belonging to a Late Pleistocene glacial drift (correlated to ‘‘younger drift’’ described in Stuiver et al., 1981 and ‘‘Terra Nova Drift’’ in Orombelli et al., 1991) with striated pebbles lies above the outcrop.
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A pale-yellow to brownish-yellow, massive, over 10 m thick matrix-supported diamictite crops out widely at Morris Basin (Fig. 2c). The skeleton texture ranges from gravel to boulder, with mainly dolerite and occasionally Takrouna sandstone clasts. Rare carbon flakes are also present. No primary structure is present at the macro scale, except for some crudely-oriented elongated clasts. In one case, a primary sedimentary structure was identified consisting of a north-dipping lens of quartzitic sand. It shows a boudinage structure elongated in a N–S direction. Other secondary structures are fracture surfaces (generally oriented E–W and dipping to the north), which sometimes penetrate clasts. Clast circles indicative of rotational movement are also evident. In order to fully describe the outcrops, samples were collected on cleaned surfaces or in excavated trenches. Samples for micromorphological observation and assessment of lithological composition were collected on the cleaned surfaces. Most samples are partly consolidated, but some are brittle. Kubiena boxes were often used to collect samples, which are all oriented. In order to protect samples during transport to Italy, they were preliminarily superficially impregnated under vacuum with a thick epoxy resin at the Italian Antarctic base in Terra Nova Bay.
3. Laboratory methods Samples were air-dried at 40 1C and impregnated under vacuum with polyester resin thinned with acetone, hardened with the aid of an accelerator and, in a few cases, with a thin bi-component epoxy resin. Thin sections (9 6 cm) were cut following the methods outlined by Murphy (1986) and van der Meer (1987, 1993). Thin sections were studied using a petrographic microscope with a magnification between 2 and 40 , in order to describe millimetre-scale structures and plasmic fabric in detail. The descriptions follow the pedological nomenclature suggested by Brewer (1976), Bullock et al. (1986) and Fitzpatrick (1984), and the glossary of micromorphological terms supplied by van der Meer (1993).
4. Micromorphological description According to van der Meer (1996), the description of thin sections should take into account four characteristics: texture, structure, plasmic fabric, and pedofeatures. In our samples, the latter are not clearly originated by pedological processes; for this reason we classify cutans, pendants, neoformation minerals and translocations as ‘‘secondary features’’. A summary of
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micromorphological description is listed in Tables 1–3 following Carr (1999), with some modifications. 4.1. Benson Knob profile (Fig. 2a) Samples from Benson Knob come from the tectonised top of bedrock and from the overlying tillite. Thin sections are uniformly coloured from pale brown to pale yellow. Sample BK4 (Fig. 4) contains an injection vein: the dark vertical band visible in the middle of the thin section. All the samples are rich in coal fragments Fig. 5 refers to the bedrock (see Section 4.1.5). 4.1.1. Texture Skeleton grains range from 1 cm to 30 mm, with a dominant grain size of 500 mm (Fig. 6). Roundness is medium to low for grains smaller than 500 mm, and medium to high for larger ones; angularity is well expressed in clasts smaller than 500 mm, while larger ones are more rounded. Quartz is the dominant mineral, but lithic fragments (dolerite and black shale) are also widely present; plagioclase and K-feldspar are rare. The larger clasts are represented by black shale and dolerite. The distribution of skeleton grains is homogeneous. The matrix consists of silt, clay and fine sands (quartz fragments). The colour varies from pale yellow to pale brown according to composition and oxide content. The ratio between coarse and fine is generally 1:1. The C/F related distribution ranges from closed to single-spaced porphyric, i.e. skeleton clasts are in contact or as each other distant as their size. 4.1.2. Structure No sedimentary structures were detected in these samples. Porosity, represented by planes and channels, is always less than 10%. Only sample BK2 shows some vughs. Several rotational structures, both with and without core stones, are visible. Shear planes are also clearly visible, as are some linear features constituted by spatial arrangement of particles (Fig. 7). Some larger clasts exhibit pressure shadows marked by finer material. Sample BK4 is characterised by the presence of an injection vein containing several rotational features (Fig. 4). The sample also shows a weakly-developed ‘‘pebble type I’’ structure (cf. van der Meer, 1993: ‘‘pebble type I’’ are nodules ‘‘which consist of till and which do not have an internal plasmic fabric’’). Fine angular grains shown in Fig. 8 are most likely to be the result of grain crushing process. Sample BK2 contains a well-rounded aggregate composed of homogeneous fine material; structure consists of isolated elements of either till or fine grained sediments encased in the till body (‘‘pebble type III’’; van der Meer, 1993) (Fig. 9).
Staining
P+Po Po+Ch Po P P
Neoformation Impregnation Depletion
L L L L L
, abundant/well developed (H).
Infillings
Water escape
Linear features
Skelsepic Lattisepic Omnisepic
Fractured coats
Crushed grains
Coats (grains+voids)
4500 mm
Pressure shadow
Plasmic fabric
L M L L L
o500 mm
Rotation
Deformation structures
Post depositional
5000 500 500 500 501
4500 mm
Angularity
Voids
Keys for the table content: , rare/poorly developed (L); , common/moderately developed (M); For voids: L ¼ presence of lab induced voids; P ¼ planes; Po ¼ pores; Ch ¼ channels.
BK1 BK2 BK3 BK4 BK5
BK1 BK2 BK3 BK4 BK5
Dominant grain Roundness size mm+sorting o500 mm
Skeleton
Masepic
Pebble I
Insepic
Pebble II Pebble III
980
Sample
Table 1 Summary of micromorphological description of samples from Benson Knob profile (see Figs. 1 and 2 for location)
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, abundant/well developed (H).
Infillings
L L
L
Linear features
Ch+Po Ch+P P P P P P+Ch
Neoformation Impregnation Depletion
Water escape
Skelsepic Lattisepic Omnisepic
Staining
Crushed grains
Fractured coats
4500 mm
Pressure shadow
Coats (grains+voids)
o500 mm
Rotation
Deformation structures
Plasmic fabric
L L M H M L M
4500 mm
Angularity
Voids
Post depositional
1000 200 300 300 300 200 300
Dominant grain Roundness size mm+sorting o500 mm
Skeleton
Keys for the table content: , rare/poorly developed (L); , common/moderately developed (M); For voids: L ¼ presence of lab induced voids; P ¼ planes; Po ¼ pores; Ch ¼ channels.
LD1 LD2 LD3 LD4 LD5 LD6 LD7
LD1 LD2 LD3 LD4 LD5 LD6 LD7
Sample
Table 2 Summary of micromorphological description of samples from Lake Depression profile (see Figs. 1 and 2 for location)
Masepic
Pebble I
Insepic
Pebble II Pebble III
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P Po Po+Ch P Po+P Po
L L L L L L
Neoformation Impregnation Depletion
Staining
, abundant/well developed (H).
Infillings
Water escape
Linear features
Skelsepic Lattisepic Omnisepic
Fractured coats
Crushed grains
Coats (grains+voids)
4500 mm
Pressure shadow
Plasmic fabric
L M M L/M L M
o500 mm
Rotation
Deformation structures
Post depositional
500 3000 100 200 500 200
4500 mm
Angularity
Voids
Keys for the table content: , rare/poorly developed (L); , common/moderately developed (M); For voids: L ¼ presence of lab induced voids; P ¼ planes; Po ¼ pores; Ch ¼ channels.
MB1 MB2 MB3 MB4 MB5 MB6
MB1 MB2 MB3 MB4 MB5 MB6
Dominant grain Roundness size mm+sorting o500 mm
Skeleton
Masepic
Pebble I
Insepic
Pebble II Pebble III
982
Sample
Table 3 Summary of micromorphological description of samples from Morris Basin profile (see Figs. 1 and 2 for location)
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Fig. 4. Benson Knob. Sample BK4. Thin section of an injection vein (dark, vertical band), annotated with dashed line. Field of view 9 5 cm.
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Fig. 7. Benson Knob. Sample BK5. Linear features are visible from top left to bottom right. Elongation axis of aligned clasts are annotated. Field of view 5 mm, PPL.
Fig. 5. Benson Knob. Sample BK1. Black shales of the glaciotectonised bedrock. Some injected fine material is visible (arrow on the left). The sedimentary lamination of the black shale is folded (dashed line). Field of view 9 5 cm.
Fig. 6. Benson Knob. Sample BK3. The tillite shows heterogeneous grain size and distribution. The dominant size of the sand grains is 500 mm. Field of view 5 mm, Plane polarised light (PPL).
Fig. 8. Benson Knob. Sample BK4 (a) and BK5 (b). An example of edge-to-edge crushing (a) and some crushed grains (b). Field of view 2.5 mm (a) and 0.5 mm (b), PPL.
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Fig. 10. Benson Knob. Sample BK5. A skel-lattisepic plasmic fabric. Note the perpendicular orientation of surfaces of the lattisepic pattern. Main lattice direction are indicated by the dashed lines. Field of view 0.5 mm, XPL.
shales. The skeleton grains (sample BK1, Fig. 5) consist of lithic fragments of the black shale with prevalent coarse portion. Their size ranges from a few hundred microns to several centimetres. Injected fine material is widespread and folded clasts are locally visible (Fig. 5). Masepic plasmic fabric characterises the thin section where the matrix is concentrated. 4.2. Lake Depression profile (Fig. 2b) Fig. 9. Benson Knob. Sample BK5. A well-rounded mud pebble with plasmic fabric, easily distinguished from the surrounding till (‘‘type III pebble’’, van der Meer, 1993). Field of view 2.5 mm, PPL (a), Cross polarised light (XPL) (b).
4.1.3. Plasmic fabric The fine material is diffusely reoriented in domains (Fig. 10). The widespread pattern is the skelsepic (Brewer, 1976) (granostriated, Bullock et al., 1986) plasmic fabric. This feature also marks some veins. A lattisepic plasmic fabric (cross striated) is also visible and shows incident directions of reorientation. An insepic pattern is also present in places. 4.1.4. Secondary features All the samples are characterised by the widespread presence of cutans (coatings of clay and silt), especially within veins and planes (infillings), and also around clasts (pendants). Some relict, reworked cutans (papules) are locally present. Mineral translocations are also diffuse, with staining and impregnation by oxides around elongated voids. 4.1.5. Disrupted bedrock Sample BK1 (Fig. 5) refers to the bedrock, a dark green to brown rock containing fragmented black
Samples from the Lake Depression come from a variety of facies related to glacial deposition and from the glaciotectonised bedrock (Fig. 11). The samples are yellow to pale yellow; lithic fragments, coal fragments, and impregnated domains due to secondary features are clearly evident. Some sedimentary features, such as laminae or a faint stratification, are also visible (samples LD3, LD4 and LD5; Fig. 12). 4.2.1. Texture The dominant skeleton clasts range in size from 200 to 300 mm. Roundness is medium to low, and generally higher for clasts 4500 mm, while angularity is medium to low for this size range and medium for smaller clasts. Samples LD4 and LD6 are characterised by a high degree of roundness, especially clasts 4500 mm. Quartz is the dominant mineral, but lithic fragments of dolerite are also widely present; plagioclase and K-feldspar are visible in places. Sandstone lithic fragments are generally scarce. The skeleton grain distribution is typically homogeneous; only sample LD4 is divided into separate domains, all of which are characterised by a high sorting index. The matrix is mainly composed of silt and clay with some quartz fragments. The colour ranges from pale yellow to brown, reflecting
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Fig. 12. Lake Depression. Sample LD4. (a) Folded laminae, annotated with dashed line. Field of view 5 cm. (b) Photograph of the layer represented above in detail (square). A pen cap on the right for scale.
Fig. 11. Lake Depression. Sample LD1. Deformed bedrock consisting of fragmented blocks of Beacon sandstones. Coal fragment alignment is indicated by dashed lines. Field of view 9 3 cm.
the oxide content. The coarse fraction is equal to or greater than the fine one; only in sample LD6 is the matrix more abundant than the skeleton. The coarse/fine related distribution is close porphyric (grain-supported), especially in samples LD4 and LD6. Skeleton grains are rare in sample LD6, with a single-spaced porphyric coarse-fine related distribution (the distance between clasts is close to or greater than their size, Bullock et al., 1986). In sample LD5 several highly-sorted bands show a unimodal distribution (quartz).
4.2.2. Structure Primary structures such as laminae (some millimetres thick) are visible in sample LD5, and are deformed in centimetre folds (Fig. 12). Sample LD3 shows a well-sorted silt bed; there are no visible primary internal features. Porosity, commonly represented by planes and some minor channels, is never higher than 5–6%. Secondary structures are widespread in this section. Rotational structures are common and better developed in samples LD3, LD6 and LD8. Sample LD5 also shows a ‘‘milky-way’’ or ‘‘galaxy’’ structure, marked by wings of different composition (Fig. 13). Pressure shadows are present in samples LD6 (Fig. 14), while results of grain fracturing mechanisms are widespread in almost all sections. Samples LD6 is characterised by several poorly-sorted silt and fine sand bands of various thickness. Considering their flame-shaped margins and flattening, these features can be described as water escape structures (Fig. 15). Linear features marked by clast alignments are clearly visible in sample LD2 (Fig. 16).
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Fig. 13. Lake Depression. Sample LD5. A milky-way structure with wings (annotated) is well visible on the right of the core stone, but only for a single wing. Field of view 5 mm, PPL.
Fig. 14. Lake Depression. Sample LD5. The left side of the clast on the right is surrounded by fine material concentrated in its pressure shadow. The shape is underlined by dashed lines. Field of view 5 mm, PPL.
4.2.3. Plasmic fabric This fabric is widespread in nearly all samples. The skelsepic plasmic fabric is best developed; lattisepic plasmic fabrics are present locally. Some samples do not show plasmic fabric. Care must be taken in assessing this feature, given the common presence of calcite which, because of its strong birefringence, can mask clay reorientation (van der Meer, 1993).
4.2.4. Secondary features Coatings, either caps or pendants, some fractured (Fig. 17), are clearly visible in these sections. Water circulation is clearly responsible for several infillings, due to the presence of clay and calcite deposition
Fig. 15. Lake Depression. Sample LD1. Water escape structure. Reconstructed water flow directions are suggested by the arrows. Field 5 mm, PPL.
Fig. 16. Lake Depression. Sample LD2. Linear features marking shear planes within the till. Orientation is from top left to bottom right, as indicated by dashed line. Rotational structure in circle. Field 5 mm, PPL.
(Fig. 18). Calcite is widespread as a neo-formation mineral (Fig. 19). 4.2.5. Disrupted bedrock Sample LD1, collected in the deformed bedrock, shows dominant skeleton grain more than 1 cm in size (Fig. 11). The angularity is low for all the grain-size classes. The coarse fraction is four times the fines and their related distribution is close porphyric (grain-supported). Sandstone lithic fragments, disrupted and rotated, are the main component of sample LD1. Aligned coal fragments (markers for linear features) are also visible. Pressure shadows are also present, as well as poorly-sorted silt and fine sand bands of various thickness, indicating a water escape process (Fig. 15). Two perpendicular sections
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Fig. 19. Lake Depression. Sample LD1. Calcite neoformation close to coal fragments (centre and top left). Field of view 5 mm, XPL.
4.3. Morris Basin profile (Fig. 2c) Samples from the Morris Basin come from the tillite and from the previously described vein. They are pale brown to yellow coloured, and locally banded or characterised by staining along planar voids. Coal fragments are widely spread within samples. Secondary structures such as linear features are visible in sample MB4, where they are marked by the alignment of coal fragments.
Fig. 18. Lake Depression. Sample LD2. Calcite deposition in a planar void, from top left to bottom right. Field of view 0.5 mm, XPL.
4.3.1. Texture The size of the skeleton is greater than 50 mm, with a dominant class of 500 mm. Some lithic fragments are up to a few centimetres in size. Roundness is medium to high for clasts 4500 mm while it is low to medium for smaller clasts. The angularity of clasts o500 mm is medium to high, while it is medium to low for larger ones. Only samples MB3 and MB4 show a medium-low angularity for clasts o500 mm and a low angularity for the larger ones. Sorting is appreciable in these two samples. The composition is a mixture of monocrystalline and polycrystalline quartz, dolerite lithic fragments, pyroxene and some sandstone. Some type II (fine grained material still part of the original sediment host) and III ‘‘pebbles’’ (van der Meer, 1993) can be observed (Fig. 20). The matrix consists of silt, clay and fine sand; the latter is constituted by quartz and pyroxene fragments. The colour varies from yellow to pale brown according to the oxide content. The coarse/fine ratio ranges from 1:1 to 2:1 in some tillite domains. The coarse/fine related distribution is close porphyric, in some cases single-spaced (Bullock et al., 1986).
from sample LD1 highlight the alignment of coal fragments dipping 15–301 to the north (Fig. 11). The skelsepic plasmic fabric is well developed.
4.3.2. Structure Sedimentary (primary) structures are not visible in these samples. Porosity, determined by planes, channels
Fig. 17. Lake Depression. Sample LD3. Clay and silt cutans. (a) clast on the top: the cutan (grey material indicated by the arrow) is a pendant. (b) clay cutan. Field of view 0.5 mm (a) and 5 mm (b), PPL.
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Fig. 20. Morris Basin. Sample MB2 (a, b) and MB4 (c, d). Examples of type III ‘‘pebbles’’ according to van der Meer (1993). Note the sharp contrast with the ground mass, especially under crossed-polars. Field of view 2.5 mm, PPL (a, c), XPL (b, d).
and pores, is less than 5%. Secondary structures, due to deformation, are widespread. Stone circles and turbate structures are clearly visible in samples MB6 and MB1; linear features, such as clast alignments and elongated type III ‘‘pebbles’’ (van der Meer, 1993), are well represented in sample MB4 (Fig. 20). Other structures, such as pressure shadows and crushed grains, were observed in samples MB3, MB5 and MB6. Some nodules are worthy of additional observation. Texturally, they are very well rounded and have a core stone with a diameter significantly smaller than the nodule size (Fig. 21). The external part consists of fine material that surrounds the core stone in concentric layers or skins with a well developed skelsepic plasmic fabric. This well recognisable structure can be used for identifying glacier bed deformation, induced by shear stress condition in wet environment. Similar nodules have been described as ‘‘argiquartzosepules’’ by Brewer (1976), as ‘‘fine-grained casings’’ by Hiemstra (1999), and as ‘‘clast haloes’’ by Lachniet et al. (2001). Moreover, sample MB6 reveals several water-escape structures. The lower portion of samples MB3 and MB4
shows an interesting feature: the lens described in the field shows a wide presence of till pebbles, all of which are deformed and strongly elongated (Figs. 20 and 21). 4.3.3. Plasmic fabric Plasmic fabric in these samples is not uniform. The skelsepic plasmic pattern, particularly well developed in sample MB7, is the most common. Samples MB3 and MB4, collected in the same lens, are slightly different; the ‘‘pebbles’’ described above are immediately obvious in these samples because the strong skelsepic plasmic fabric involves the clay material of the concentric layers. Lattisepic plasmic fabrics, with perpendicular reorientation direction, are also present, especially in samples MB5 and MB6. In some domains sample MB5 also shows a masepic plasmic fabric. 4.3.4. Secondary features Staining and iron oxides impregnation is diffuse in samples from the tillite. Translocation of minerals is not uniform, but concentrated around the voids (Fig. 22).
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Fig. 21. Morris Basin. Sample MB3. Examples of rolling-ball structures (see Section 5 for description). Field of view 0.5 mm, PPL (a, c), XPL (b, d).
Fig. 22. Morris Basin. Sample MB5. Mineral translocation is mainly due to water circulation; note the stained material around a void (the limit is marked by the dashed line). Field of view 2.5 mm, PPL.
Fig. 23. Morris Basin. Sample MB2. Cutans and their reworked portions. Other reworked cutans are on the bottom right (arrow). Field of view 2.5 mm, PPL.
A banded distribution of oxidation products was also observed. Coatings are common in samples from the vein (MB3 and MB4). Sample MB7 reveals fragmented
coatings (papule or pedorelict) interpreted as the product of intense mechanical reworking of the cutans (Fig. 23).
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Calcite neo-formation is visible in all samples except MB7. In sample MB1 calcite neo-formation is diffuse and optically disturbs the birefringence of the clay minerals, thereby masking the plasmic fabric. Loose incomplete infillings are also present. In some cases the infillings are enriched in coal fragments.
5. Discussion and conclusions Thin-section analysis confirms that the diamictite informally named Ricker Hills Tillite is linked to glacial action. Grain crushing induced by glacial action produces fine angular and irregular clasts. As a result, in the Ricker Hills Tillite, the larger clasts are rounder and smoother. The more widespread facies is a massive matrixsupported diamicton attributed to lodgement processes in the field but classified as ‘‘tectomict’’ according to van der Meer et al. (2003) on the basis of micromorphological analysis. Plasmic fabric is visible in most samples, especially skelsepic and lattisepic patterns, which are often associated. This feature was not only observed in samples with low clay content but also in those with numerous large skeleton clasts. Syndepositional pervasive till shearing (glacial-induced) is evidenced by the occurrence of clast alignments, shear planes, rotational structures and pressure shadows. Shear planes exploited as drainage channels, infilled by injection veins in lodgement till and at the till–bedrock interface (Benson Knob and ‘‘Lake Depression’’ profiles) would confirm the presence of water at an unknown time after deposition. As a consequence of syn- to post-depositional reworking processes, nodules (‘‘pebble structure’’ according to van der Meer, 1993) are widespread (though to varying degrees) in tectomict. Widespread ‘‘pebble’’ presence is particularly evident in a lens of the Morris Basin profile (Fig. 2). Unusual nodules (up to 1 mm in size) are commonly distributed in this lens (Fig. 21). They are well rounded and composed of a core stone enveloped by clay. The clay envelope exhibits a well-developed granostriated plasmic fabric. Considering the unusual structure and appearance of these nodules, with their clearly identifiable core stone, we propose to name them ‘‘rolling-ball structures’’. This structure is attributed to the prolonged rotation of the core stone and the progressive accretion of clay around the inner clast. According to Menzies (2000), these structures probably formed in an environment characterised by high pore-water and clay contents. As a conclusion, the lens containing ‘‘rollingball structures’’ is a primary structure related to the syndepositional presence of water. Water-escape structures are clearly evident in the tillite and bedrock. Some of the bands which developed
during this process are well sorted, while others are poorly sorted. These structures can be attributed to sediment sorting operated by a slow dewatering process. Alternatively, they can be ascribed to concentration of water in primary fine-grained sorted levels, on which the glacial-induced stress has lead to the squeezing of water content. We favour the second interpretation, considering the unexpected lack of finer and larger clasts, which are otherwise randomly present in the tillite. According to our interpretation, glacier-induced stress squeezes fine-grained, water-enriched sediments along planes of weakness. Poorly sorted structures, on the other hand, result from forced dewatering of low-viscosity sediments. In conclusion, the Ricker Hills Tillite was deposited under wet conditions (with high pore water content) during prevalent ductile deformation. Boudinage, ductile shear zones, and rotational structures are also widespread in the bedrock and confirm this conclusion (Fig. 3). In the ‘‘Lake Depression’’ profile, samples LD3 and LD5 document a layer originated and sorted by water circulation, about 40 cm thick; small clasts are prevalent, and the clay content is low. Further evidence comes from sample LD4 (in the same outcrop) that was clearly deposited under running water, as demonstrated by the primary structures visible in the field and under the microscope (Fig. 12). These two layers are compatible with subglacial deposition in the presence of melt waters. Nevertheless, pressure shadows and galaxy structures as well as deformation of laminated sediments in sample LD4 document a post-depositional deformation induced by a younger glacial advance. On the basis of oriented samples, the ice-flow direction was clearly from the south at Benson Knob and from the east in the ‘‘Lake Depression’’. This further suggests that, also at the time when the Ricker Hills Tillite deposited, the Lake Depression existed and drew a converging glacial flow in that local topographic basin. The micromorphological deformation features of the Ricker Hills Tillite (lattisepic-skelsepic plasmic fabric, water escape, ‘‘type I pebbles’’, van der Meer, 1993), and injection veins in weak joint planes suggest that a warm-based glacier was responsible for the deposition and the deformation of the till. Considering (1) the location of the Ricker Hills nunatak at the margin of the EAIS, (2) the position of the till outcrops, located in depressions of the nunatak over deepened before till deposition, (3) the paleo iceflow directions reconstructed in two sections, (4) the elevation of the highest erosional trimline located at about 1780 m (at the western margin of the nunatak) and at 1520 m (in the central part of the nunatak; Baroni et al., unpublished data), we conclude that the Ricker Hills Tillite was deposited by a warm-based ice sheet with a maximum value of ice thickness evaluated at 600–900 m.
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Syn- to post-depositional processes are mainly represented by secondary features produced by clay and silt migration and oxide impregnation. They document that oversaturated water (i.e. rich in calcium carbonate) circulated during and/or after deposition. Indeed, clay to silty coatings, either caps or pendants, are documented in all the outcrops; some void infilling show lamination structures and have variable grain size composition. Calcite neoformation on the walls of the pores or as infillings is also widespread. Sparitic calcite crystallisation took place under phreatic condition, confirming the circulation of water saturated in calcium carbonate. Some of the secondary features are clearly syn-depositional (i.e. the papule) while others (i.e. pendants and calcite infillings) are most likely postdepositional. In any case, in the Ricker Hills Tillite, all the structures related to water circulation indicate that the subglacial environment was wetter and warmer than the present one during its deposition.
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