Geological Quarterly, 1999,43 (2): 241 - 250

Advantages and disadvantages of petrographic analyses of glacial

Maria GORSKA

G6rska M. (1999) - Advantages and disadvantages of petrographic analyses ofgJacial sediments. Geo!. Quart. , 43 (2): 241 - 250. Warszawa.

Petrographic studies are a helpful tool for glacial , because they may supplement lithofacial analysis. They introduced information about dynamics and thermic conditions in a glacial sole, about different alimentary zones, and routes of an ice sheet and individual glacial streams that deposited tills in central and western Wielkopoiska, western . Apart from advantages, the petrographic analyses have also disadvantages, e.g. ability to recognize 200 types of indicator erratics, which follows in pointing to respective source areas. A sample volume must be represented statistically, i.e. it should consist of not less than 1000 erratics from tills and not less than 300 pebbles from gravel.

Maria G6rslw., Quaternary Research Institute, Adam Mickiewicz University, Wieniawskiego 17119, PL·61 -713 Poznan, Poland; e-mail: [email protected](received: November 11 , 1998; accepted: December 15, 1998).

Key words: Vistulian, glacial deposits, petrographic analysis, erratics.

INTRODUCTION clasts in tills. Studies currently conducted on the erratic com­ position of tills indicate (Fig. 1) that a petrographic spectrum results the most frequently from mixing of the Scandinavian Erratics present in the heterogeneous glacial material have material, transported directly from the alimentary area, with aroused curiosity of nature scientists who attempted to reveal material transported from various parts of Scandinavia, e.g. their odgin based on macroscopic features and deposition drained by rivers and derived from tills of previous (G. location. A systematic petrographic analysis, both qualitative Gillberg, 1977; P. U, Clark, 1987). A local material perhaps and quantitative, have been initiated (S. Konieczny, 1956; J, overlap this, already complex picture, which effectively chan­ Nunberg, 1971). It was aimed to indicate correlation between ges the original content of Scandinavian rocks (A. Dreimanis, the moraine layers and specific glaciations. Numerous and 1990; R. Puranen, 1990), Both issues were also approached very diversified factors influencing the petrographic charac­ in Poland (e,g. Z. Lamparski, 1970; J. Rzechowski, 1971, teristics of ti lls (Fig. 1), did not allow presenting the same 1979; cf M. G6rska, 1998b). mineralogic-petrographic characteristics of glacial deposits representative for a larger study area, for example northwe­ stern Poland. This difficulty results from the extremely dyna­ mically diversified glacier margin, which advancing REVIEW OF METHODS southward covered the morphologically diversified pre-Qu­ aternary surfaces (R. Galan, 1967; E. RUhle, 1968), Thermics of a glacial sale was, however, responsible for a type and Two methods, presented by V, Milthers (e.g. 1909, 1934) and J. Hesemann (e.g. 1931, 1935), are among the first quan­ intensity of the basement destruction and incorporation of titative studies on erratics, and index erratics are considered material, which was moved from its original position. A in both of them. This term introduced by J, Korn (1927) geologic structure of the area, over which a glacier was includes rocks from a single alimentary area, which is descri­ moving, thus both areas of Scandinavia and the Baltic Basin, bed by a latitude and a longitude of its central part, Correlation as well as source areas and foreland of the maximum extent with appropriate outcrops is detennined, based on macrosco­ of an ice sheet, are considered the most important factor, pic characteristics. Both methods exclusively analyse a crys- however, which diversifies petrographic composition of 242 Maria G6rska

SPECIFIC PLACE AND DERIVA nON OF VARIETES OF TRANSPORT TYPE OF GLACIAL DEBRIS GLACIAL DEPOSITION DEPOSITS - Extraglacial processes supra- Jsupraglacial I gla- flowlill I --i -supragJacial derivation I cial - melt-out tilt I

Outcrop above -H m,,, f -size -age shear f------I movements ~'ll"hthO"'~1and flow till -relief -fissuring plane .I ingJaciaJ I -physical features of ., zone sub- ~ allochthonous f flows -----1 flow till I roc'" 1 I inShear~ H in icc I ·1 plane .1melt-OU1lilll I J subglacial I gls- Subglacial processes I zone I lodgement I -redeposion of the debris. initiall ---1 lodging till '-' located in ice cia! -contamination of older till layers ~ glaciodislocation defonnation I -bedrock incorporation :-: beneath ice Sheet: - metamorphism till f----mechanical abrasion

Fig. 1. Factors affecting genetic type and mineral composition of tills (modified after A. Dreimanis, 1982; P. U. Clark, 1987)

talline material, which is considered to be more resistant to In contrast to V. Milthers (1909, 1934), J. Hesemann weathering. P. Smed (1993) questions this thesis, however, (1931,1935) saw additional information in every detail, thus saying that continental glaciers are able to transport less extended a spectrum of analysed erratics to about 200 speci­ resistant rocks, such as limestones or Palaeozoic shales, even mens, considering only 11 types (J. Nunberg, 1971), giving a at significant distances without their entire destruction. secondary role to others. Analysing samples of varying sizes Roundness of surfaces and observed, very gradual decre­ (50--100 specimens of index rocks), he obtained an initial ase of their content in the entire population of erratics. are material to introduce a four-digit index. Each of the digits traces of indisputable mechanic destruction in the glacier represented. after rounding to nearest tens, a number of crys­ interior (M. G6rska, 1997). talline erratics within a one out of four (V. Milthers, 1909, V. Milthers (1909,1934) suggests the analysis of only six, 1934). varying in terms of sizes and alimentary areas. relatively easily recognizable index rocks, which originate These averaged twice values, thus affected by a large from the three alimentary areas, varying in terms of area sizes. error, depart significantly from a real percentage content of This causes, that P. Smed (1993) reproached him with neglect index erratics, and it considerably decreases a quality of the of majority of possible occurrences of Swedish erratics in tills, method. Lack of infonnation about a presence or absence of and causing subsequently a greater significance of the rom­ a specific rock type. which in many cases may have a signifi­ bohedral porphyry from Oslo and rapakivi granites from the cant importance. is the next disadvantage of the J. Hese­ Aland Islands. V. Mi lthers (1909,1934) did not distinguish a mann's method. All types are included into a single till, deposited by the ice sheet advancing from the north at all. alimentary area, knowing, however, that all samples associa­ I.t eventually resulted in incorrect conclusions of V . Milthers ted with this area, could have not been transported together (1909, 1934) about predominance of the ice sheet advancing by the same ice sheet or its individual ice streams. directions from the north-west and the Baltic Basin. G. Ltittig (1958) introduced a new method of determina­ In order to avoid simi1ar mistakes and in result to recon­ tion of the alimentary area of a rock debris in tills, and named struct, relatively precisely, directions ofthe ice sheet advance . it the theoretical erratic centre TGZ (in Gennan: Teoretisches into the European Lowland, possibly numerous rock samples Geschiebe-Zentrum). Based on the analysis of 400 types of not limiting their size. for example. to few subjectively selec­ Scandinavian sedimentary and crystalline erratics from parent ted erratics, should be analysed. areas of known geographic coordinates, G. LUttig (1958) Pctrographic analyses of glacial sediments 243

Tab I e

Characteristc features of important Scandinavian crystalline indicator erraties after J. Hesemann (1975); sketch K.-D. Meyer (1980)

Rock Si7.e Colour Feldspates Quartz Basic elements Texture, other

Bredvad porphyry few, in colour of very few, green very fine light red - partly fluidal Dalarna matrix, 1- 5 mm spots, 1-2 mm few, in colour of numerous, angular, very few, green Red Baltic porphyry very fine brick-red matrix, 0.S-2 mm grey, 0.S-2 mm spots, mm-cm - ohen white Brown Baltic red-brown, many, reddish, few, green-black very fine many. 0.5- 2 mm elongated feldspates grey-brown l-Smm po rphyry spots (weathered) Gronkliu porphyrite reddish-brown, many, red, brown, amany. spots glitter very fine - - Dalarna violet grey, I-S mm (augite, hornblende) few, sometimes in fine -granite, very numerous, PMcallavik grey-reddish- many, up to 2 cm, form of elongated sometimes fluidal, very fine white-grey-blue, porphyry SmAland violet round, white-red stripes or zones, "mock brown" 1-3mm up to I cm structure sometimes "rapakivi" type: numerous, round potassium Aland quartz. brick-red, few reddish, very few, very fine up to O.S cm, fe ld!;pates porphyry red-brown up to 1 cm green spots round, grey surrounded by I mm fringe of plagioclascs many red spots ofbiotites, red brownish-red orthoclases with few, grey, round, Aland Rapakivi middle-coarse homblendes, rapakivi structure! to red-brown greenish 1-3mm 1- lOmm plagioclases, 1-2 cm many red few homblendes sometimes orthoclases, few Aland granite middl e-fine red brownish-red few, grey, 1-3 mm and biotites, hieroglyphic, plagioclases, small spots granular up to 0.5 cm Red Vtixjo granite many, red, numerous, grey- "Smdlaml granite fine -middle-coarse red-light red few, partly spots Smdland upto 1 cm blue, 1-3 mm with blue quartzes"

many, glitter, partly similar 10 Stockholm granitc fine light grey, grey grey-white to white few , grey, 1-3 mm brown-black gneiss whitish, up to I cm, grey, sometimes hornblende, biotite sometimes structure Uppsala granite middle-coarse light grey, grey seldom reddish blue, I-S mm in shape of spots of lenses

clusters of spots, sometimes - augite, 1-5 mm Ki nne diabase middle-fine greenish-grey feldspates, glitter like silk up to 1 cm rusty-brown grains, Scnnia basalt very fine black, dark grey only in matrix - augite, olivine often hollows after weathered olivines momb in shape, porphyry tic, Rhomb porphyry brown, grey, very fine many, pink-while, - feld spates in shape Oslo red-violet - 1-2cm ofrhombs

obtained two values, which are mathematical1y calculated Kozarski et al. (1985. 1987) was confirmed by petrographic averages of a geographic latitude and longitude of the alimen­ examination of glacial sediments considering a theoretical tary area of the entire erratic spectrum. This method has been erratic centre (M. Bose. M. G6rska. 1995; M. G6rska. 1995b. successfully applied till present time by K.-D. Meyer (1965. 1997. I 998a). First of all. changes of TGZ. calculated for 1983.1990.1991.1995). R. Vinx (1996.1998) and T. Geisler specific lithostratigraphic layers in the studied location . pro­ (1996) introduced new rock types from southwestern Sweden. ved a previously suggested distinct direction of the ice sheet including all requirements which have to be filled by index advance during the Chodziei Phase (17.7 ka BP according to erratics. S. Kozarski. 1995). Diversification of a local accumulative sequence at Uj­ A certain disadvantage of this method is the fact. that the scie. obtained earlier by S. Kozarski (1991. 1995) and S. value of TGZ does not correspond to a real past centre of 244 Maria G6rska

Table 2

Characteristic features of important Scandinavian sedimentary rocks after J. Hesemann (1975); sketch K.-D. Meyer (1980)

Rock Age Grain size Colour Significant features

with features of tuff and luffite, very hard, Digcrberg sandstone Dalarna Precambrian fine-middle-coarse red-brown-grey-violet arkose or aplit in character (typical of acid soi l rocks) as above and with big Digerberg conglomerate Precambrian up to coarse conglomerate red-brown-grey-violct pebbles and fragments of Dalarna Dala porphyry sometimes quartzitic. rarely Dala sandstone Precambrian flne-middle-coarse light bricks- brown-red-violet conglomeratic. oOcn spots, rarely carbonate apart from cross-bedding Kalmarsund sandstone Eocambrian yellowish, light grey, also quasi -stratification "Chiasma", western coast of fine to middle (Lower Cambrian) violet-red-brown stripes (coloured belts. stripes). tHand , and the Kalmar Straits sometimes violet-red ScuUt/IOS 1- 3 mm tubes perpendicular ro Scolithus sandstone Scania Lower Cambrian middle light grey, yellowish bedding of transversal Scolithos quartzitic, partly glassy, sometimes Hardeberga sandstone Scania Lower Cambrian fine to middle light grey, white- grey conglomeratic with quartz; variant with Fucoidae flat , light carbonate, hard; undulated layer-top Tessini sandstone, western fine sandy to coarse-grained light grey, weathered- Middle Cambrian (rippiemarks), layer-bottom with coast of Oland silty shale yellowish traces of Paradoxides paradoxissimu,f (ressini) quasi spherical concretions, "Batl" sandstone, bOllom of light grey, yellowish, Devonian (Old Red) fine-middle reflected fre sh broken surface, the Baltic Sea, NE of Golland reddish, greenish glitter; fragments of fi sh Lower Permian Conglomerate ofrhomb with fragments and pebbles of (New Red fine to coarse violet-red, brown, grey porphyry S of Oslo rhomb porphyry, less hard Sandstone) sometimes visible algae Palaeozoic limestone, Ordovician. Silurian very fine, dense grey, greenish, yellowish Palaeoporella and cryst

glacial erosion. It is only mathematically determined, central­ group, are not omitted. Moreover, placed on a map, they ly located relatively to other alimentation centres, from which inform about possible contamination with older tills due to basement deposits were incorporated. their incorporation during younger ice sheet advances (P. Erratics the most commonly occurring in the European Smed, 1993). This last feature has not been considered in the Lowland are these which are included in the Tables 1 and 2. earlier presented methods of V. Milthers (1909, 1934), J. Relatively recently P. Smed (1993, 1994) tried to determine Hesemann (1931,1935) orTGZ. In the method ofP. Smed paths of ice sheet advances using a new presentation with (1993, 1994), the erratics having some index properties are circle maps. Percentage content of index erratics corresponds considered (in German: statistische Leitgeschiebe). They are to a circle diameter, which changes accordingly to a popula­ rocks which are numerous in a sample, possible for explicit tion size of the rocks determined. The circle centre is located identification, but having more than a single alimentary area. in the centre of the alimentary area. Although the Palaeozoic limestones have been taken under This new method of a graphic presentation (p. Smed, consideration in the earlier petrographic studies, they had no 1993, 1994) seems most thoroughly to include the corrected index significance. Because they are common, a significance methodological errors of the previously applied methods, and of information which they contribute into the entire picture is represents a broad spectrum of information. A consideration indisputable. That is why including all erratics in a complex of even individual rocks is a significant advantage of this characteristics of a till layer is a great advantage in a petro­ method. Index erratics, which are not sufficiently abundant, graphic analysis of a pebble fraction. and also these, which do not represent all adjacent types in a Petrographic analyses of glacial sediments 245

Table 3

Petrographic groups of gravel fraction and their diagnostic criteria (modified after G. Pettersson, 1995)

Symbols Petrographic group Diagnostic criteria

crystalline rocks consisting of more or less than 50% quartz respective and containing dark minerals; K acid and basic crystalline metamorohic rocks and mylonitic sedimentarv rocks sandstone. also lothnian . which break between the individual grains. multi ~co loured; lothnian: pink~rcddish - violet S quartzitic sandstone uartzitic sandstone sandstones of dense dis!!cnetic texture, break throullh the CrYstals, white. nale. ninkish Palaeozoic silt, shale and TU dark grey. very soft plates. possible to scratch with a nail other claystones grey. black, while. brownish, reddish, yellowish amorphous silica, with typical conchoidal fracture and sharp F flint edges KK Cretaceous limestone soft white limestone with loose texture composed of numerous fossils grey. red limestones, sometimes with dense fracture. may contain fossil algae Paiaeoporella. react strong to PK Palaeozoic limestone acid white, yellowish. pinkish sedimentary rocks. do not scratch glass. react weakly to acid, blue colour replaces D dolomite violet in reaction with Ma2neson I

L lydite hard, brittle siliceous rock with laminae of chalcedony and quartz. break perpendicular 10 layers Q quartz pure quartz with conchoidal or sugar-like fracture, hard (7 in the Mohs scale) WQ milk quartz as above, and the quartz is distinctly not transparent, white

P. Smed (1993) also considers a problem of the sample These disadvantages caused that since then. a spectrum of size. He suggests that as little as 50 determined index rocks the analysed petrographic material was extended to the entire of 20-60 mm fraction may be sufficient to present a clear rock inventory included in a gravel fraction. picture. Taking into consideration that only 10% crratics from Despite severa1 years of experience in a petrographic till meets requirements for index properties (K.-D. Meyer, segregation of gravel, unfortunately it was not possible to 1983), a sample should consist of at least 1000 specimens determine a uniform fraction , characteristic for this skeletal together with flints and limestones, and it is undoubtedly a material. In Germany, where petrographic analyses belong to disadvantage of this method. Such large population may be standard methods used for examination of tills, A. G. Cepek difficult to collect if a petrographic analysis is limited to a (1962, 1967, 1969) defined a fraction 4-10 mm as the most skeletal till material. R. Puranen (1990) indicated, however, representative one. The same till fraction was analysed by G. that a basal, lodgment-type till provides the best information Ltittig (1957, 1958, 1995) and W.-A. Panzig (1989, 1992). about the origin of erratics. Their record is not subjected to However, K.-D. Meyer (1983) and M. Bose (1979, 1989,. such large post-depositional changes, as for example a supra­ 1995) suggest a broader fractional spectrum of analysis, divi­ glacial material transported on icebergs in proglacial basins. ding it into 4-6.3 and 6.3- 12.5 mm intervals, respectively. Glaciofluvial deposits, transported at larger distances than the Similarly 1. Ehlers (1979, 1980, 1983) and G. Ltittig (1995) corresponding tills (M. Lillieskold, 1990), are subjected to postulate that a larger fractional interval of a skeletal material additional aqueous sorting, which also makes explicit identi­ should be analysed, i.e. 2- 3.15 and 3.1 5-5 mm (1. Ehlers, fication of a parent area of a pebble fraction less possible. 1979), or 2-3.5, 3.5- 5, 5- 8 and 8-13 mm (G. Ltittig, 1995). Petrographic studies of erratics of glaciofluvial facies and a In Sweden petrographic analyses are conducted in various till are conducted in Poland by 1. Rutkowski (1995a-c). fractions. K. Malmberg-Persson, E. Lagerlund (1994), and E. A progressing examination of the Scandinavian erratics Lager1und et al. (1995) selected a fraction 3-8 mm, G. Petter­ made that morc frequently, an attention was diverted to met­ so n (1995, 1997, 1998) and S. Eriksson (1998) a fraction hodologic errors. The following ones are listed among them: 2.8-4, 4-5.6 and 5.6-8 mm. The most recent petrographic - too small quantity of easily identified index erratics analysis of a till is limited to the fraction 4-10 mm (1. (o nly 6 specimen in the analysis of V. Milthers), thus sub­ Albrecht, 1995). sequently, possible omitting a majority of erratics of other W. Schultz (1996) summarized, in terms of a historic derivation; view, selected fractions included in a study on till petrography - non-uniform sample in terms of population (50-100 in Western Europe, referring to a corresponding author and specimen of index erraties in the method of l Hesemann); the area. - uncertain classification of the Scandinavian erraties to Great contribution to methodology of petrographic studies an appropriate group among 200 known index erratics (1. in Poland was made by 1. Trembaczowski (1961,1967). who Dudziak, 1970); tried to standardize a sample popUlation size and its fraction - certain and unquestionable identification of the average subjected to a detailed analysis. He proposed an analysis of 3 pebble spectrum of 10% only (K.-D. Meyer, 1983). till of a volume at least 0.015 m in a fraction 4- 10 mm. He 246 Maria G6rska suggested that a smaller fraction, e.g. 2-3 mm, obtained from than the known one is not taken into account. Also different geologic drillings (J. Gol~b, 1933; A. Dreimanis, 1939; T. than present locations of alimentary areas as well as larger Bartkowski, 1950, 1956;J. Trembaczowski, 1967; A. Linden, outcrops are rarely taken into consideration, however, it is 1975) does not represent fine rock fragments, but only their known (J. E. Mojski, 1995) that sediments 25- 150 m thick compositional components. mainly quartz. were removed from a bottom of the Baltic Basin and the Baltic The Polish Geological Institute advises to use the fraction Sea itself. There are still the areas in Scandinavia which have 5-10 mm, which is selected from a till, collected mostly from not been studied thoroughly and in detail, or their is drillings. This method is used in Poland commonly (e.g. K. not widely distributed because of political-economic reasons, Choma-Moryl et al., 1991; S. Lisicki, 1993, 1998a, b; K. e.g. the Baltic Sea, is an area of navy manoeuvres and pro­ Kenig, 1998). spectiveforresources of natural oil and gas (ej. W. K. Gudelis, Taking into consideration a petrographic analysis introdu­ J. M. Jemielianov, 1982). ced by A. Jaroszewicz-Klyszynska (1938), R. Blachowski (1938), and later applied successfully by J. Ehlers (1978, 1979, 1980, 1982) and M. Bose (1979, 1989), 10 groups of CONCLUSIONS rock material in a till are distinguished (M. G6rska, 1992, 1995a, b, 1997, 1998a; cf Table 3). All who undertake carrying out a petrographic analysis of Different types of petrographic analyses of gravel fraction glacial deposits have to be still open to face its numerous applied are aimed at computing the so-called pebble indexes, shortcomings. that are relationships between specific rock groups. Petro­ Varying transport environments influence a final mineral­ graphic composition of glacial deposits is characterized best petrographic composition of a till, thus an appropriate selec­ by ratio of crystalline rocks (K) to Palaeozoic limestones tion of a lithofacies is necessary. (PK), because both of these components are of Scandinavian A sample population has to be statistically representative. derivation and occur in similar quantities. This type of as­ In case of gravel fraction about 300 specimen (B. Krygowski, sumption was brought out by Z. Lamparski (1971), who 1955; A. Gaigalas, 1963; J. Nunberg, 1971 ; M. Bose, 1989), questioned, in terms of methodology, an application of inde­ and in case of pebble fraction at least 1000 specimen (K.-D. xes based on the Scandinavian and local material, and on Meyer, 1983) are needed. Moreover, the area of occurrence specific rock groups occurring in different quantities. should be appropriately sampled. Considering geology of the northern Europe, it is easier to Determination of zones, where glacial erosion occurs , correlate the entire erratic material of tills occurring in the depends mostly on a proper classification of the index erratic European Lowland with their source areas in Scandinavia and to one of the four source areas in Scandinavia and the Baltic the Baltic Basin. Subsequently, alimentary centres of tills may Basin, and its reference to the appropriate term. be indicated and direction of a long-distance transport, glacier Studies are laborious and long-lasting. Large samples advance paths and/or its individualized ice streams determi­ should be analysed only in the field where water is available ned. to allow an appropriate classification after they have been In the process of segregation of erratic material in gravel flushed. and pebble fractions, errors resulting from subjective asses­ Petrographic analyses of gravels, because of a smaller sment of macroscopic erratic features may occur. Facial va­ volume of samples, can be successfully conducted in a labo­ riability of the same rock type, observed in an alimentary area ratory, especially as a fresh rock fracture can be observed with even at a short distance, may influence decisively a different a use a microscope or a magnifying glass. Selection of lime­ classification of the same erratic, which has been transported stones has to be confirmed by a visible reaction to hydrochlo­ several hundred kilometres apart. According to P. Smed ric acid. Similarly alteration of colour from violet to blue, (1994, and pers. inform.), however, the problem does not triggered by reaction with the Magneson I indicator (A. G. focus on identification of a single or two erratics. It is impor­ Cepek, 1969), allows to distinguish dolomites. tant, which is indicated by R. Vinx (1993), to determine in Despite the listed disadvantages, a study of petrographic case of index rocks a presence of possibly the largest number composition of tills have been accepted among standard re­ of rocks from the same alimentary area (so-called series, suite search methods of these deposits in Western Europe (G. or index sequence), features of which are easily identifiable. Ltittig, 1958, 1995; M. Houmark-Nielsen, 1987; W.-A. Pan­ As the alimentary centre is recognized more thoroughly by zig, 1989, 1992; M. Bose, 1995). Increase of interest in correlation with appropriate erratics, information about a petrographic analysis has been observed lately also in Poland parent area of rocks and their transport path becomes more (Z. Siliwonczuk, 1985; D. Krzyszkowski, 1988, 1990, 1994; convincing than a presence of individual erratics from the W. Stankowski, D. Krzyszkowski, 1991; K. Choma-Moryl et entire Scandinavia. al., 1991; W. Gogolek,1991a, h, 1994; K. Kenig, 1991, 1998; The least amount of problems in terms of classification R. Racinowski, 1991; H. Klatkowa, 1993; S. Dobrzynski, among erratics of pebble fraction are caused by porphyries, 1995; C. Seul, 1995; P. Klysz, 1995; S. Lisicki, 1993, 1997, because of their characteristic fabric; however, sandstones are 1998a- c; P. Czubla, 1998). the most difficult (M. G6rska et aI., 1998). Index erratics are Petrography of the erratic material in gravel fraction 4- classified, based on the present knowledge (R. Vinx, 1996, 12.5 mm (K.-D. Mcyer, 1983;M.Bose, 1979,1989)andindex 1998; T. Geisler, 1996; P. Smed, pers. inform.). A possible erratics 20-60 mm (K.-D. Meyer, 1983) are the ideal exten- derivation of a specific index erratic from another outcrop Petrographic analyses of glacial sediments 245

Table 3

Petrographic groups of gravel fraction and their diagnostic criteria (modified after G. Pettersson, 1995)

Symbols Petrographic group Diagnostic criteria

crystalline rocks consisting of more or less than 50% quartz respective and containing dark minerals; K acid and basic crystalline metamorphic rocks and mylonitic sedimentary rocks sandstone, also lothnian sandstones, which break between the individual grains, multi-coloured; lothnian: pink-reddish-violet S IQuartzitic sandstone Iquartzitic sandstone sandstones of dense diagenetic texture. break through the crystals. white, Dale, pinkish Palaeozoic silt, shale and TU dark grey. very soft plates. possible to scratch with a naU other claystones grey, black, white. brownish. reddish. yellowish amorphous silica. with typical conchoidal fracture and sharp F flint edE:es KK Cretaceous limestone soft white limestone with loose texture composed of numerous fossils grey. red limestones. sometimes with dense fracture, may contain fossil algae Paiaeoporeila, rcact strong to PK Palaeozoic limestone acid white, yellowish. pinkish sedimentary rocks, do not scratch glass. react weakly to acid. blue colour replaces D dolomite violet in reaction with Maeneson I L Iydite hard, brittle siliceous rock with laminae of chalcedony and quartz, break perpendicular to layers Q quartz pure quartz with conchoidal or sugar-like fracture, hard (7 in the Mohs scale) WQ milk quartz as above, and the quartz is distinctly not transparent, white

P. Smed (1993) also considers a problem of the sample These disadvantages caused that since then, a spectrum of size. He suggests that as little as 50 determined index rocks the analysed petrographic material was extended to the entire of 20-60 mm fraction may be sufficient to present a clear rock inventory included in a gravel fraction. picture. Taking into consideration that only 10% erratics from Despite several years of experience in a petrographic till meets requirements for index properties (K.-D. Meyer, segregation of gravel, unfortunately it was not possible to 1983), a sample should consist of at least 1000 specimens determine a uniform fraction, characteristic for this skeletal together with flints and limestones, and it is undoubtedly a material. In Germany, where petrographic analyses belong to disadvantage of this method. Such large population may be standard methods used for examination of tills, A. G. Cepek difficult to collect if a petrographic analysis is limited to a (1962, 1967, 1969) defined a fraction 4-10 mm as the most skeletal till material. R. Puranen (1990) indicated, however, representative one. The same till fraction was analysed by G. that a basal, lodgment-type till provides the best information Uittig (1957,1958, 1995) and W.-A. Panzig (1989, 1992). about the origin of erratics. Their record is not subjected to However, K.-D. Meyer (1983) and M. Bose (1979, 1989,. such large post-depositional changes, as for example a supra­ 1995) suggest a broader fractional spectrum of analysis, divi­ glacial material transported on icebergs in proglacial basins. ding it into 4-6.3 and 6.3- 12.5 mm intervals, respectively. Glaciofluvial deposits, transported at larger distances than the Similarly 1. Ehlers (1979, 1980, 1983) and G. Liittig (1995) corresponding tills (M. Lillieskold, 1990), are subjected to postulate that a larger fractional interval of a skeletal material additional aqueous sorting, which also makes explicit identi­ should be analysed, i.e. 2-3.15 and 3.15-5 mm (1. Ehlers, fication of a parent area of a pebble fraction less possible. 1979), or 2-3.5, 3.5-5, 5-8 and 8-13 mm (G. Liittig, 1995). Petrographic studies of erratics of glaciofluvial facies and a In Sweden petrographic analyses are conducted in various till are conducted in Poland by 1. Rutkowski (l995a- c). fractions. K. Malmberg-Persson, E. Lagerlund (1994), and E. A progressing examination of the Scandinavian erratics Lagerlund et al. (1995) selected a fraction 3- 8 mm, G. Petter­ made that more frequently, an attention was diverted to met­ son (1995, 1997, 1998) and S. Eriksson (1998) a fraction hodologic errors. The following ones are listed among them: 2.8-4, 4- 5.6 and 5.6- 8 mm. The most recent petrographic - too small quantity of easily identified index erratics analysis of a till is limited to the fraction 4-10 mm (1. (only 6 specimen in the analysis of V. Milthers), thus sub­ Albrecht, 1995). sequently, possible omitting a majority of erratics of other W. Schultz (1996) summarized, in terms of a historic derivation; view, selected fractions included in a study on till petrography - non-uniform sample in terms of population (50--100 in Western Europe, referring to a corresponding author and specimen of index erratics in the method of 1. Hesemann); the area. - uncertain classification of the Scandinavian elTatics to Great contribution to methodology of petrographic studies an appropriate group among 200 known index erratics (1. in Poland was made by 1. Trembaczowski (1961,1967), who Dudziak, 1970); tried to standardize a sample population size and its fraction - certain and unquestionable identification of the average subjected to a detailed analysis. He proposed an analysis of 3 pebble spectrum of 10% only (K.-D. Meyer, 1983). till of a volume at least 0.015 m in a fraction 4-10 mm. He 246 Maria G6rska suggested that a smaller fraction, e.g. 2-3 mm, obtained from than the known one is not taken into account. Also different geologic drillings (J. Gohlb, 1933; A. Dreimanis, 1939; T. than present locations of alimentary areas as well as larger Bartkowski, 1950, 1956;J. Trembaczowski, 1967;A. Linden, outcrops are rarely taken into consideration, however, it is 1975) does not represent fine rock fragments, but only their known (J. E. Mojski, 1995) that sediments 25- 150 m thick compositional components, mainly quartz. . were removed from a bottom ofthe Baltic Basin and the Baltic The Polish Geological Institute advises to use the fractIOn Sea itself. There are still the areas in Scandinavia which have 5- 10 mm, which is selected from a till, collected mostly from not been studied thoroughly and in detail, or their geology is drillings. This method is used in Poland commonly (e.g. K. not widely distributed because of political-economic reasons, Choma-Moryl et aI., 1991; S. Lisicki, 1993, 1998a, b; K. e.g. the Baltic Sea, is an area of navy manoeuvres and pro­ Kenig, 1998). spectiveforresources of natural oil and gas (ef W. K. Gudelis, Taking into consideration a petrographic analysis introdu­ J. M. Jemielianov, 1982). ced by A. Jaroszewicz-Klyszynska (1938), R. Blachowski (1938), and later applied successfully by J. Ehlers (1978, 1979, 1980, 1982) and M. Bose (1979, 1989), 10 groups of CONCLUSIONS rock material in a till are distinguished (M. G6rska, 1992, 1997, ef Table 3). 1995a, b, 1998a; All who undertake carrying out a petrographic analysis of Different types of petrographic analyses of gravel fraction glacial deposits have to be still open to face its numerous applied are aimed at computing the so-called pebble indexes, shortcomings. that are relationships between specific rock groups. Petro­ Varying transport environments influence a final mineral ­ graphic composition of glacial deposits is characterized best petrographic composition of a till, thus an appropriate selec­ by ratio of crystalline rocks (K) to Palaeozoic limestones tion of a lithofacies is necessary. (PK), because both of these components are of Scandinavian A sample population has to be statistically representative. derivation and occur in similar quantities. This type of as­ In case of gravel fraction about 300 specimen (B. Krygowski, sumption was brought out by Z. Lamparski (197 1), who 1955; A. Gaigalas, 1963; J. Nunberg, 1971; M . Bose, 1989), questioned, in terms of methodology, an application of inde­ and in case of pebble fraction at least 1000 specimen (K.-D. xes based on the Scandinavian and local material, and on Meyer, 1983) are needed. Moreover, the area of occurrence specific rock groups occurring in different quantities. should be appropriately sampled. Considering geology of the northern Europe, it is easier to Determination of zones, where glacial erosion occurs, correlate the entire erratic material of tills occurring in the depends mostly on a proper classi fication of the index erratic European Lowland with their source areas in Scandinavia and to one of the four source areas in Scandinavia and the Baltic the Baltic Basin. Subsequently, alimentary centres of tills may Basin, and its reference to the appropriate term. be indicated and direction of a long-distance transport, glacier Studies are laborious and long-lasting. Large samples advance paths and/or its individualized ice streams determi­ should be analysed only in the field where water is available ned. to al10w an appropriate classification after they have been In the process of segregation of erratic material in gravel flushed. and pebble fractions, errors resulting from subjective asses­ Petrographic analyses of gravels, because of a smaller sment of macroscopic erratic features may occur. Facial va­ volume of samples, can be successfully conducted in a labo­ riability of the same rock type, observed in an alimentary area ratory, especially as a fresh rock fracture can be observed with even at a short distance, may influence decisively a different a use 0 microscope or a magnifying glass. Selection of lime­ classification of the same erratic, which has been transported stones has to be confirmed by a visible reaction to hydrochlo­ several hundred kilometres apart. According to P. Smed ric acid. Similarly alteration of colour from violet to blue, (1994, and pers. inform.), however, the problem does not triggered by reaction with the Magneson I indicator (A. G. focus on identification of a single or two erratics. It is impor­ Cepek, 1969), allows to distinguish dolomites. . tant, which is indicated by R. Vinx (1993), to determine in Despite the listed disadvantages, a study of petrographIc case of index rocks a presence of possibly the largest number composition of ti1ls have been accepted among standard re­ of rocks from the same alimentary area (so-called series, suite search methods of these deposits in Western Europe (G. or index sequence), features of which are easily identifiable. Llittig, 1958, 1995; M. Houmark-Nielsen, 1987; W.-A. Pan­ As the alimentary centre is recognized more thoroughly by zig, 1989, 1992; M. Bose, 1995). Increase of interest in correlation with appropriate erratics, information about a petrographic analysis has been observed lately also in Poland parent area of rocks and their transport path becomes more (Z. Siliwonczuk, 1985; D. Krzyszkowski, 1988, 1990, 1994; convincing than a presence of individual erratics from the W. Stankowski, D. Krzyszkowski, 1991; K. Choma-Moryl et entire Scandinavia. aI., 199 1;W.Gogolek, 1991a,b, 1994;K.Kenig, 199 1,1998; The least amount of problems in terms of classification R. Racinowski, 1991; H. Klatkowa, 1993; S. Dobrzynski, among erratics of pebble fraction are caused by porphyries, 1995; C. Seul, 1995; P. Klysz, 1995; S. Lisicki, 1993, 1997, of their because characteristic fabric ; however, sandstones are 1998a-c; P. Czubla, 1998). the most difficult (M. G6rska et at., 1998). Index erratics are Petrography of the erratic material in gravel fraction 4- classified, based on the present knowledge (R. Vinx, 1996, 12.5 mm (K.-D. Meyer, 1983;M.Bose, 1979, 1989) and index 1998; T. Geisler, 1996; P. Smed, pers. inform.). A possible erratics 20-60 mm (K.-D. Meyer, 1983) are the ideal exten- derivation of a specific index erratic from another outcrop Petrographic analyses of glacial sediments 247

sion of a lithofacial analysis (M. G6rska, 1997) which is its analyses are a sedimentologic-petrographic complement. significant advantage. because: Therefore. it may set in order a stratigraphic classification - they complement data on dynamics and thermics of a only in conjunction with the other complementary Iithologic­ glacier sole. alimentary area and a path of ice sheet advance; stratigraphic methods. - basing on pebble indexes, they support conclusions Acknowledgements. I would like to express my deep about diversified alimentary centres of tills; gratitude to all who contributed to this paper, first of all to - they confirm a long-distance transport. determine rou­ Margot Bose (Free University, Berlin), Klaus-Dieter Meyer tes of ice sheets andlor their individual ice streams. (NLfB, Hannover), Jacek Rutkowski (Academy of Mining It should be clearly emphasized and very well remembe­ and Metallurgy, Cracow), Roland Vinx (Hamburg Universi­ red, what has been many a time underlined in the subject ty) and Per Smed (Denmark). literature (G. LUttig, 1995; and others) that the petrographic

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ZALETY I WADY ANALIZ PETROGRAFICZNYCH OSAD6w LODOWCOWYCH

Streszczenie

Pierwsze systematyczne i metodologicznie przygotownne anali zy skladu przebiegala cgzaracja, zalezy w dutej mierLe od poprawnego zaklasyfikowa­ petrograficznego glin lodowcowych pochodz~ z pocZ