Glacial stress field orientation reconstructed through micromorphology and mX-ray computed

tomography of till

F. FASANO*, C. BARONI*†, F.M. TALARICO‡, M. BETTUZZI§ and A. PASINI§

*Università degli Studi di Pisa, Dip.Scienze della Terra, via S.Maria, 53–56126–Pisa, Italy (e-mail: fasano@unisi.it)†CNR, Istituto di Geoscienze e Georisorse, Pisa, Italy

‡Università degli Studi di Siena, Dip.Scienze della Terra, via Laterina 8 – 53100 – Siena, Italy§Università di Bologna, Dipartimento di Fisica, viale C.B. Pichat, 6/2 – 40127 – Bologna, Italy

ABSTRACT

Detailed laboratory analyses largely support field studies on the petrology of glacial materials. Inparticular, detailed structural analysis of glacial sediments helps reconstruct the presence, orienta-tion and intensity of glacial stress fields. This pilot study compares thin-section micromorphology(two-dimensional observation under a microscope) and mX-ray computed tomography (mXR-CT,three-dimensional observation involving data collection and subsequent processing) to evaluate thepotential of the latter technique to yield reliable data. The location and stratigraphy of samplingsites are reported for a complete presentation. Results are encouraging: compared with thin-section micromorphology, mX-ray computed tomography yielded results that are more detailed,reproducible and unaffected by cutting orientation. Moreover, mXR-CT is easily applied to inco-herent materials, brittle samples and unique specimens.

Keywords till, XR-CT, micromorphology, glaciotectonic.

INTRODUCTION

This study aims to develop µXR-CT applicationsin the microstructural investigation of continentalglacigenic sediments. The correct genetic interpreta-tion of glacigenic sediments is indispensable fortheir use as palaeoclimatic indicators and dependson the thorough and accurate collection of textural,microstructural and micromorphological data.These characteristics, which vary considerably inglacial deposits and in different stratigraphic levelswithin the same unit (van der Meer, 1996), reflectthe complex interplay among local ice dynamics,depositional mechanisms and associated post-depositional processes.

For this pilot study samples were selected fromthe Ricker Hills nunatak, along the inland marginof the Transantarctic Mountains (Victoria Land,Antarctica). It is located between 75°45′ and 75°35′S and 158°50′ and 159°20′ E. We analyzed two

samples collected in the middle of the nunatak atLake Depression (Fig. 1).

The Ricker Hills geomorphology is complexand records several phases of the glacial history ofthe East Antarctic Ice Sheet. In particular, palaeo-iceflow directions cannot be easily reconstructed onthe basis of field data alone. The Cenozoic ‘RickerHills Tillite’ is exposed in several outcrops and hasbeen mapped by Capponi et al. (1999).

To increase our understanding of tillite micro-textures and their use as palaeoenviromental indicators in the Ricker Hills area, we carried out µX-ray tomography analysis of two selected samples. The results of these investigations are herebriefly summarised to highlight the potential of thistechnique for the detailed textural characterisa-tion of tillites. µX-ray tomography is an analyticalmethod that has only recently become available to the geosciences (Ketcham and Carlson, 2001; van Geet et al., 2001), and its application to the

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study of glacigenic sediments has only just begun(Ketcham, 2005).

MATERIALS AND METHODS

The Ricker Hills Tillite is composed of a massivematrix-supported diamictite with a porphyriccoarse/fine (bimodal) related distribution, lowsorting, low rounding and medium to high angu-larity. The Tillite is interpreted as a tectomict sensu van der Meer et al. (2003). In this tectofacies,deformation structures are very common andtheir orientation is clearly related to stress-field orientation.

Fig. 1 Location map of the Ricker Hills. The LakeDepression outcrop is indicated.

Samples were impregnated under vacuum witha thin bi-component epoxy resin. Thin sections (9 × 6 cm) were cut following the methods out-lined by Murphy (1986) and van der Meer (1997,1993). Thin sections were studied using a petro-graphic microscope with a magnification range of 2× to 40×.

The instrument for µXR-CT analysis is com-posed of an X-ray source with a microfocus beam(spot size of less than 10 µm), a mechanical systemfor moving the sample and an XCCD camera asdetector. The current is generally set to less than300 µA, with a voltage of 140 kV. Samples are coresof about 2 cm in diameter and 1.5 cm long. The sample is rotated 360°, and one or more images are collected for every 1° step. All the data are pro-cessed through specific software to obtain 1) a 3Ddataset, 2) a 3D rendering, and 3) image analysis.Rendering and processing were completed usingfree software such as MRicro and AMIDE. Theimage usually obtained is linked to Beer’s law:

I = I0 e−µχ

where I0 is the integral current of incident X-rays, I is the integral current transmitted by the sample, µ is the linear attenuation coefficient of thematerial and χ is the sample width. There is a directlink between the transmitted current revealed bythe detector and the linear attenuation. In positiveimages, this means that brightness and attenua-tion are directly linked. In other words, the lightercoloured areas indicate greater attenuation (lowerattenuation in negative images).

SEDIMENTOLOGY

In this study we focus on the ‘lake depression’ out-crop, since it is a key position in the middle of the Ricker Hills (Fig. 1). Disrupted sandstones ofthe Beacon Supergroup lie at the base of the pro-file (Fig. 2). Several high-angle, westward-dippingshear planes were identified; these are generallyrelated to rotational structures marked by a senseof shear to the west. Several injection veins up tosome tens of centimetres long and 1–2 cm thick were observed. A massive yellow to greyish-yellow,lithified diamictite overlies the fractured bedrock.It consists of mud-supported clasts (mainly dolerites

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of the Ferrar Supergroup, with some sandstones).The overall texture ranges from sandy to cobbly. Thesand fraction is mainly derived from weatheringof the Takrouna Sandstone (Beacon Supergroup).In the section described here, this layer is 2 mthick, but it is estimated to reach 8–10 m in thenearby outcrops. A large, erratic dolerite boulderlies within the diamictite. A lens with a laminatedstructure is present above the boulder. Above theboulder the massive diamictite is interrupted by a 30 cm-thick layer of sorted, roughly stratifiedgravel. This layer is clast-supported, and the siltymatrix is sparse. A yellow, uncemented diamictoncontaining striated pebbles belongs to a LatePleistocene glacial drift and forms the upper portion of the outcrop.

In thin section, the diamictite is composed pri-marily of grains 200–300 µm in size. Quartz is thedominant mineral, with minor lithic fragments ofdolerite, plagioclase and K-feldspar. Lithic sand-stone fragments are generally scarce and the graindistribution is typically homogeneous. The matrixis mainly composed of quartz fragments andphyllosilicates, and its grain size ranges from silt

to clay. The pale yellow to brown colour indicatesoxidation. The coarse fraction is equal to or greaterthan the fine fraction, and the estimated size limitis 50 µm. Deformational structures such as rotationalstructures, pressure shadows and grain fractur-ing are widespread. In particular, rotational struc-tures are common and better developed in samples LD3 and LD6. Although a periglacial origin can-not be completely ruled out, the low porosity, theabsence of secondary zeolites and of mammillatedvugs, deformational structures and profile settingshighlight the glaciotectonic origin of rotationalstructures. Pressure shadows are present in sampleLD6. Linear features marked by clast alignmentsare clearly visible in sample LD2 (Fig. 3). Thin sections were prepared on oriented samples, whoseorientation is reported in Figure 3. The alignmentsclearly dip to the west. In sample LD1, laminatedsandstones are deformed and reoriented (Fig. 4).Coal layers mark oriented planes with a westwarddip direction.

µXR-CT observations reveal the heterogene-ous composition of the diamictite. In sample LD5 (Fig. 5) different attenuation domains (the imageis positive) are visible at the top of the core. Thelighter domains are 50–500 µm in size and show arandom distribution. They probably represent thedoleritic clasts visible both in the hand specimenand in thin section. The very light clast at the centerof the upper surface is a highly attenuated body.Its lithology is unknown. In the cut-out of the core

Fig. 3 Sample LD2, thin section. Clast alignment (right)and rotational structures (left) are indicated with dottedlines. E and W are indicated at the bottom of the picture.

Fig. 2 Lake depression lithological log profile withsampling points. Lithologies: (a) sandstone; (b) disruptedsandstone; (c) diamictite; (d) erratic; (e) laminated lens; (f) clast-supported gravel; (g) uncemented diamictite.

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some planar structures are visible. They cut acrossthe entire core and dip roughly 30–45° WNW.These structures mark some planes of weaknessprobably related to the presence of low-attenuationmaterial concentrated along shear planes. To obtaina more precise plane orientation, the core image wasrotated through rendering with 1° steps around thesupposed orientation. For every step the core wascut (sequential cutting at one-degree intervals ofrotation) and the dip angle was measured. Sincethe apparent dip angle is always less than the realmaximum dip plane, rotation, cutting and measure-ment operations were repeated until the maximumdip angle was found.

In sample LD5 the maximum angle of 40° wasmeasured on the cut oriented N265°, which prob-ably represents the direction of maximum glacialstress.

In sample LD 1 (Fig. 6), representing disruptedsandstone in the bedrock, there are several different

Fig. 4 Sample LD1, thin section. Linear features markedby coal alignment are indicated with dotted lines. Eastand west are indicated at the bottom of the picture.

A

B

Fig. 6 Sample LD1, 3D image. Two consecutive cuts,same cut direction. Cut (B) is more central, while cut (A) is more lateral. Note the light coal layer severalmillimetres thick that dips to the west.

Fig. 5 Sample LD5, 3D image. Note the clasts on the top(dolerite) and the planes dipping to the west. North isindicated.

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attenuation domains. These follow a planar dis-tribution, with a coherent westward dip direction.The images are negative (Fig. 6). The light coloured,westward-dipping planes some tens of microns in thickness represent the low- attenuation coal levels also visible in thin section (Fig. 4). Throughsequence rotation cutting, the maximum measuredangle was 43°, with a cut direction of N273° relatedto the direction of maximum shear stress.

A cut in sample LD1 also reveals a very light-coloured, round domain some tens of microns in diameter (Fig. 6b). This domain has very lowattenuation since the image is negative, and isprobably related to a pore.

DISCUSSION

The application of µXR-CT analyses to the RickerHills Tillite indicates that the adopted instrumentconfiguration and operating conditions, as well as the compositional and textural features of theinvestigated samples can reveal sufficiently con-trasting attenuation domains. Resulting 3D imagescan greatly increase our ability to reconstruct thegeometry of shear planes in oriented glacigenic rock samples.

Based on these preliminary studies, we areconfident that similar results can be obtained in anumber of onshore (e.g. Sirius Formation) or off-shore (e.g. diamictites recovered at Cape Robertsdrill sites (Barrett, this volume)). These Antarcticglacigenic deposits are known to be composition-ally akin to the Ricker Hills Tillite.

Micromorphological investigations performedon selected thin sections document a local E–Wdirection of glacier movement (Baroni and Fasano,2006).

Comparison of micromorphological data with µX-ray tomography yields useful complement-ary data for interpreting the glacial stress field.Planar and linear features related to deforma-tion, reorientation and movement resulting from shear-strain, dip uniformly westwards at variousdip angles. These features indicate that the stressfield is oriented E-W, like the sense of movementof the glacier responsible for till deposition. Bothmethods yield data that support this hypothesis,although µX-ray tomography data is more preciseirrespective of the cut direction. Measurements

reveal a coherent E-W direction of ice movement.A slight shift in the maximum stress directiontoward the WSW is documented in the upper por-tion of the outcrop.

CONCLUSIONS

1 Microtextural data collected through µXR-CTinvestigations and those provided by conventionalmicromorphological studies consistently indicate awestward palaeoflow direction for the glacier respons-ible for the Ricker Hills Tillite.2 Compared with micromorphology, µXR-CT yieldsmore coherent results and better quantitative 3Ddata. Since structural elements visible in thin sectionvary according to the predetermined cut direction,µXR-CT is a better tool for investigating structural orientation.3 The study of these materials benefits from µXR-CTfor various reasons: (i) larger quantities of higher-quality microstructural and micromorphological dataare acquired than would be possible through 2Danalysis of just a few large-format thin sections; (ii) 3Dquantitative models can be constructed; (iii) poorlyconsolidated specimens can be analysed; (iv) ana-lyses are non-destructive and preserve the specimenintact.

REFERENCES

Baroni C. and Fasano F. (2006) Micromorphologicalevidence of warm-based glacier deposition from the Ricker Hills Tillite (Victoria Land, Antarctica).Quatern. Sci. Rev., 25, 976–992.

Capponi, G., Crispini, L., Meccheri, M., Musumeci, G.,Pertusati, P.C., Baroni, C., Delisle, G. and Orsi, G.(1999) Antarctic Geological 1:250.000 map series,Mount Joyce Quadrangle (Victoria Land), MuseoNazionale dell’Antartide, Sez. Scienze della Terra,via Laterina, 8 (53100) Siena.

Ketcham R.A. and Carlson W.D. (2001) Acquisition,optimization and interpretation of X-ray computedtomographic imagery: applications to the geosciences.Comput. and Geosci., 27, 381–400.

Ketcham R.A. (2005) Three-dimensional grain fabricmeasurements using high-resolution X-ray com-puted tomography. J. Struct. Geol., 27, Issue 7,1217–1228.

Murphy, P.C. (1986) Thin section preparation of soil andsediments. AB Academic, Berkhamsted, 149 pp.

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van der Meer, J.J.M. (1993) Microscopic evidence ofsubglacial deformation. Quatern. Sci. Rev., 12, 553–587.

van der Meer, J.J.M. (1997) Particle and aggregatemobility in till: microscopic evidence of subglacial pro-cesses. Quatern. Sci. Rev., 16, 827–831.

van der Meer J.J.M. (1996). Micromorphology. In:Menzies J. (Ed.) Glacial environments. Vol 2. Butter-worth & Heinemann, Oxford, 335–355.

van der Meer, J.J.M., Menzies, J. and Rose, J. (2003)Subglacial till: the deforming glacier bed. Quatern. Sci.Rev., 22, 1659–1685.

van Geet M., Swennen R. and Wevers M. (2001)Towards 3-D petrography: application of microfocuscomputer tomography in geological science. Comput.and Geosci., 27, 1091–1099.

SOFTWARE REFERENCES

Mricro http://www.psychology.nottingham.ac.uk/staff/cr1/mricro.html

AMIDE http://amide.sourceforge.net/

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