Marek Widera

SAND- AND MUD-FILLED FLU VIAL PALAEOCHANNELS

IN THE WIELKOPOLSKA MEM BER OF NEO GENE POZNAÑ

FOR MA TION, CEN TRAL PO LAND

In sti tute of Ge ol ogy, Adam Mickiewicz Uni ver sity, Maków Polnych 16, 61-606 Poznañ, Po land; e-mail: widera@amu.edu.pl

Widera, M., 2013. Sand- and mud-filled flu vial palaeochannels in the Wielkopolska Mem ber of Neo gene Poznañ For ma tion, cen tral Po land. Annales Societatis Geologorum Poloniae, 83: 19–28.

Ab stract: This study fo cuses on the sin gle- and multi-storey flu vial palaeochannel lithosomes en cased in mudrich floodplain de pos its in the al lu vial suc ces sion of the late Neo gene Wielkopolska Mem ber of the Poznañ For -ma tion, cen tral Po land, well-ex posed in the lig nite min ing pits of the re gion. The flu vial lithosomes in clude both sand-filled and mud-filled chan nel va ri et ies. The chan nel-fill fa cies are not di ag nos tic for any par tic u lar type of flu vial sys tem, as the fine- to very fine-grained sandy de pos its are mas sive to trough cross-strat i fied and also the muddy de pos its are mas sive to weakly flat-lam i nated. The scar city of lat eral ac cre tion bed ding pre cludes the pos si bil ity of me an der ing rivers, whereas the low width/thick ness ra tios of the palaeochannels pre clude braided rivers. The width/thick ness ra tio is in the range of 4.5–14 (av er ag ing 7.5) for sand-filled chan nels and in the range of 6–10 (av er ag ing 9) for mud-filled ones, which in di cates nar row rib bons in gen eral clas si fi ca tion of flu vial chan nel belts. The or i gin of the al lu vial suc ces sion is at trib uted to a W-/NW-di rected anastomosing river sys tem char ac ter ized by lat er ally in ac tive cutandfill chan nels with co he sive and veg e tated banks. The sandfilled chan -nels con veyed wa ter and sed i ment dis charges on a pe ren nial ba sis, whereas the mud-filled con duits are thought to have been the cut-and-aban doned branches of the sys tem, filled by overbank flood ing from ad ja cent ac tive chan nels. Mi nor lat eral mi gra tion of chan nels oc curred prob a bly dur ing pe ri ods of min i mum sub si dence rate, when the flu vial sys tem was forced to de velop lat eral ac com mo da tion for its dis charges.

Key words: flu vial lithosomes, floodplain de pos its, chan nel-fill fa cies, chan nel width/depth ra tio, hyperconcen-trated flow, anastomosing river sys tem.

Manu script re ceived 9 February 2013; ac cepted 21 June 2013

IN TRO DUC TION

Al lu vial suc ces sions, as the infill of small and large con ti nen tal bas ins, are gen er ally di vided into flu vial (chan -nel-fill) and floodplain (overbank) sed i men tary do mains (Schumm, 1977; Miall, 1996; Bridge, 2003). The rel a tive pro por tion of flu vial to floodplain de pos its in an al lu vial suc ces sion de pends both on the ba sin sub si dence rate (Leeder, 1977; Bridge & Lee(Leeder, 1979) and the type of river sys -tem, vary ing from > 90% in some braidplain sys tems (Ne-mec, 1992; Miall, 1996) to around 50% and even < 5% in me an der ing or anastomosing river sys tems (Allen, 1965, 1978; Schumm, 1968; Rust, 1978, 1981; Smith, 1983, 1986; Har wood & Brown, 1993; Miall, 1996; Gibling et al., 1998; Makaske, 2001). Flu vial de pos its have gen er ally re ceived more re search in ter est, as they are more cru cial for the rec og ni tion of al lu vial sys tem type and also as po ten tial pe tro -leum res er voirs or their out crop an a logues. A large amount of data on the width and thick ness (depth) of mod ern and an cient flu vial chan nels is avail able in the lit er a ture (Gibling, 2006), col lected pri mar ily as a da ta base for the pos si

-ble pre dic tion of chan nel-belt widths from their thick nesses mea sured in ex plo ra tion wells.

Apart from gravelfilled or mixed sandgravel chan nels, typ i cal of high land and proglacial set tings, the ma jor -ity of stud ies from low land flu vial sys tems have doc u -mented sand-filled chan nels (e.g., Horne et al., 1978; Friend

et al., 1979; Friend, 1983; Stear, 1983; Nemec, 1984;

Lorenz et al., 1985; Gibling & Rust, 1990; Törnqvist et al., 1993; Nadon, 1994; Gradziñski et al., 1995, 2000, 2003; Kraus, 1996; Bridge et al., 2000; Davies-Vollum & Kraus, 2001; Makaske, 2001; Stouthamer, 2001; Gibling, 2006; Doktor, 2007; Gouw, 2007; González-Bonorino et al., 2010; Gross et al., 2011; Widera, 2012). Less com monly re -ported are mud-filled chan nels (e.g., Eberth & Miall, 1991; Gibling et al., 1998; Kraus & Davies-Vollum, 2004; Gruszka & Zieliñski, 2008), prob a bly be cause such palaeochannels – where en cased in muddy floodplain de pos its – are dif fi cult to rec og nize in out crops, bore hole cores and seis mic or GPR sec tions.

The pres ent study re ports on sand- and mud-filled pala-eochannels rec og niz able in the ex ten sive open-pit mine faces of the lig nite-bear ing Wielkopolska Mem ber of the Neo gene Poznañ For ma tion in cen tral Po land. The sed i -men tary suc ces sion has been stud ied since the mid-19th cen -tury, but its mud-filled chan nels were not rec og nized and also its or i gin re mained con tro ver sial (see Piwocki et al., 2004). Three main suc ces sive palaeoenvironmental in ter -pre ta tions were of fered. Un til the early 1960s, the de pos its were at trib uted mainly to a “Plio cene lake” en vi ron ment (e.g., Areñ, 1957, 1964). In the late 1960s to early 1980s, the lac us trine ba sin was con sid ered to have been ma rinein -flu enced, paralic (Dyjor, 1968, 1970; Ciuk & Po¿aryska, 1982). More re cent in ter pre ta tions pos tu late an al lu vial or i -gin for the Wielkopolska Mem ber (Badura & Przybylski, 2004; Piwocki et al., 2004; Widera, 2012) and its strati -graphic equiv a lents (Kasiñski, 2000), and the pres ent study con curs with this no tion. The aims of this study are: (1) to de scribe the sed i men tary char ac ter is tics of chan nelbelt de

pos its; (2) to quan tify the geo met ri cal width/thick ness as pect of sand and mudfilled palaeochannels; and (3) to as sess the style of flu vial drain age sys tem in the ba sin by com par ing the palaeochannels with those of braided, me an der -ing and anastomos-ing rivers.

GEO LOG I CAL SET TING

The Wielkopolska Mem ber is lat er ally ex ten sive, cov -er ing an area of ~75,000 km2 in cen tral Po land (Fig. 1A). The orig i nal ex tent of these de pos its was prob a bly much larger, re duced by the ero sional ef fect of the Pleis to cene Scan di na vian ice-sheet (Piwocki, 1992; Piwocki et al., 2004). The depth of gla cial ero sion and de for ma tion lo cally ex tends to the Neo gene de pos its, as ev i denced in the open-pit lig nite mines Kazimierz North, JóŸwin IIB and Drzewce (Fig. 1B). These min ing ar eas are in shal low tec tonic gra-bens (Fig. 1B), with the first two pits lo cated in the west ern branch of the Kleczew Graben, formed in the early to mid -dle Mio cene, and the last pit lo cated in the cen tral part of the Bilczew-Drzewce Graben, which be gan to form in the early Oligocene (Widera, 2004, 2007).

The Wielkopolska Mem ber rep re sents the up per part of the Poznañ For ma tion, which is the up per most Neo gene stratigraphic unit in cen tral Po land (Fig. 2). The un der ly ing part of the Poznañ For ma tion is the lig nite-bear ing muddy Mid dle-Pol ish Mem ber, ear lier re ferred to as the Grey Clays unit, which con tains the First Mid-Pol ish Lig nite Seam (Piwocki & ZiembiñskaTworzyd³o, 1995, 1997), known ear lier as the Henryk Seam (Dyjor, 1968, 1970). The age es ti -mates for the Wielkopolska Mem ber in di cate a time span from the late mid-Mio cene to the lat est early Plio cene (Pi-wocki & Ziembiñska-Tworzyd³o, 1995, 1997; Wa¿yñska, 1998; Troæ & Sadowska, 2006).

The Wielkopolska Mem ber is pres ently thought to have been de pos ited in an al lu vial en vi ron ment (Badura & Przybylski, 2004; Piwocki et al., 2004; Widera, 2012), with sup port ing ev i dence from min er al og i cal and geo chem i cal stud -ies of chan nel-sur round ing floodplain mudstones (e.g., Wyrwicki & Wiewióra, 1981; Górniak et al., 2001; Ducz-mal-Czernikiewicz, 2010, 2011; and other ref er ences therein). In the late Neo gene, cen tral Po land is thought to have hosted an endohoric al lu vial ba sin, with a cen trip e tal pat tern of flu vial drain age (Fig. 1A; Widera, 2012).

METH ODS AND TER MI NOL OGY

The study is based on field work con ducted in the re -gion’s open-pit lig nite mines in 2004–2012. The out crop walls, up to 40–60 m high, were mapped over a lat eral dis -tance of tens of kilo metres with the use of bin oc u lar and high-res o lu tion photomosaics, and the chan nel-fill and flood- plain de pos its were sam pled and doc u mented in close up de -tail wher ever they were ac ces si ble. In to tal, 25 palaeochan-nels were rec og nized and doc u mented for fur ther anal y sis, 19 of them sand-filled and 6 mud-filled. The de pos its are poorly com pacted and only weakly ce mented, and hence are re ferred to by the soft-sed i ment names sand, silt, mud and clay.

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Fig. 1. Study area. A – Lo ca tion of the study area (frame B) with the pres ent-day ar eal ex tent of the Wielkopolska Mem ber (mod i fied from Piwocki, 1992, 1998; Czapowski & Kasiñski, 2002); B – Lo ca tion of the lig nite opencast min ing pits in the study area

Three main lines of de tailed re search were car ried out in this study. Firstly, the width and thick ness (depth) of the flu vial palaeochannels were mea sured and their width/ thick ness as pect ra tios were cal cu lated. A mod i fied ter mi nol ogy of Friend (1983) and Nadon (1994) is used for chan -nel-belt ge om e try. Chan nel belts with an as pect ra tio of 15–30 are con sid ered to be me dium rib bons, whereas those with a ra tio of < 15 are re ferred to as nar row rib bons and those with a ra tio of > 30 as wide rib bons. As pect ra tios may help to iden tify the plan-form of an an cient flu vial sys tem, such as braided, me an der ing or anastomosing (Makaske, 2001; Gibling, 2006). Sec ondly, the sed i men tary fa cies of chan nelfill and overbank de pos its are iden ti fied and de scribed, us ing the let ter code of Miall (1977). The ter mi nol ogy for sed i men tary struc tures is af ter Collinson & Thomp -son (1982). Thirdly, 79 sed i ment sam ples were col lected for grain-size anal y sis, in clud ing 34 from sand-filled and 5 from mud-filled chan nels and an ad di tional 40 from over-bank de pos its. The stan dard Udden (1914) grain-size scale mod i fied by Wen tworth (1922) is used. Clayey de pos its with a silt ad mix ture of > 50 vol. % are re ferred to as mud (Lundegard & Samuels, 1980).

RE SULTS

On the ba sis of out crop map ping, the rel a tive vol u met -ric pro por tion of flu vial lithosomes in the Wielkopolska Mem ber is es ti mated to be less than 5%, one of the low est re ported from al lu vial suc ces sions. Ob ser va tions on the in -ter nal char ac -ter is tics and cross-sec tional geo met ric as pect of the palaeochannels are re viewed in this sec tion.

Sand-filled chan nels

These are sin gle and multistorey chan nels, filled pre -dom i nantly with fine-grained sand (see chan nels 1–19 in Figs 3–9). They form con cave-up wards lenses en cased in floodplain de pos its. The lat ter com prise mud interlayered with clay, silt and very fineto finegrained sand sheets (Ta -ble 1), in clud ing vari col oured palaeosols with plant-root traces, car bon ate nod ules, gyp sum crys tals, slick en sides and spo radic lig nite lay ers (Figs 3–9). Most of these palaeo-chan nels, in the up per part of the suc ces sion, are af fected by the Pleis to cene glacitectonic de for ma tion and par tial ero -sion (see chan nels 1, 2 and 7 in Figs 3 & 5; chan nels 5 and 6 in Fig. 4; chan nels 3, 4 and 8 in Figs 4 & 6; chan nels 9–12 in Fig. 7A, B; and chan nels 13–19 in Figs 7C & 8).

The sandy chan nel-fill de pos its are poorly strat i fied and ap pear to be mainly mas sive, with the ex cep tion of chan nel 4 (Fig. 4) that is filled with a mod er ately- to very wellsorted sand show ing trough crossstrat i fi ca tion. The strat i fi -ca tion in rec og niz able ow ing to the pres ence of mud and/or fine plant de tri tus on strata in ter faces (Fig. 4B–G) and in di cates trans port to wards the W and NW (Fig. 4). This chan -nel is ex cep tional also in that its infill, al though dom i nated by fine- to very fine-grained sand, in cludes mud at the base and at the top (Fig. 4).

The sandy chan nelfill in most cases shows a fin ingup -wards trend, al though the trend is poorly de fined in fine-grained sand and rec og niz able only by grain-size anal y sis of sed i ment sam ples (e.g., see chan nels 2 and 13 in Figs 3B & 7C). Most of the palaeochannels ap pear to have been filled by ver ti cal ac cre tion (Figs 3, 7 & 8), but some also show sub or di nate lat eral ac cre tion in dic a tive of point bars with

Fig. 2. Com piled lithostratigraphy of the Neo gene Poznañ For ma tion (af ter Ciuk, 1970; Dyjor, 1968, 1970; Piwocki & Ziembiñska-Tworzyd³o, 1995, 1997); note the strati graphic po si tion of the late Neo gene Wielkopolska Mem ber in the right-hand col umn. The let ter code in the left-hand col umn in di cates: O – the Or³owo lig nite seam group; IA – the Oczkowice lig nite seam group; and I – the Mid-Pol ish lig nite seam group

rel a tively steep (15–20°) to gently in clined flanks (Figs 4E–G, 5). Chan nel-bank lev ees in the geo met ri cal form of lat eral “wings” are only lo cally rec og niz able (e.g., see chan -nels 2 and 12, Figs 3B & 7A, B). It is un cer tain whether such lat eral overbank ridges are in re al ity more com mon, as the low-re lief lev ees, com posed of interlayered sand, silt and mud, are poorly dis tin guish able from the sur round ing floodplain de pos its.

The sand-filled chan nels in cross-sec tion are up to 4.5 m thick and 63 m wide (Ta ble 1), with the width/thick ness ra tio rang ing from 4.5 for chan nels 12 and 19 (Figs 7B & 8A) to 14 for chan nel 7 (Fig. 5). The av er age w/t ra tio cal cu lated for the 19 sand-filled chan nels is ~7.5. All these palaeochannels are thus clas si fied as nar row rib bons. Most of them are, in fact, very nar row by com par i son with flu vial chan nel belts in gen eral (Friend, 1983; Nadon, 1994; Bridge, 2003).

Mud-filled chan nels

Six palaeochannels of this type have been doc u mented (Figs 8 & 9), 5 of them filled with mud and one also with a sand ad mix ture. These are multistorey chan nels, com pris ing 2 (Fig. 8) to 4 stor eys (Fig. 9) and gen er ally show ing lit tle or no rec og niz able glacitectonic de for ma tion. Their con -cave-up wards ero sional bases are key fea tures al low ing chan nel iden ti fi ca tion.

The muddy chan nel-fill de pos its are mainly mas sive, only lo cally show ing weak, ir reg u lar, flat lam i na tion (Fig. 9B). Grain-size anal y ses re vealed up ward fin ing, and the basal part of chan nel-fill in some cases is en riched in silt and/or very fine- to fine-grained sand ad mix ture. There is no ev i dence of lat eral ac cre tion, and these chan nels were ap par ently filled by ep i sodic ver ti cal aggradation (Fig. 9B).

The mud-filled chan nels have thick nesses in the range of 1–2 m and their widths range from 7.8 to 16 m (e.g., see chan nels 20, 21 and 23 in Figs 8 & 9). The w/t ra tio of these chan nels var ies from 6 to 10 (Ta ble 1), av er ag ing ~9.0. Ac -cord ingly, the palaeochannels of this cat e gory are sim i larly clas si fied as nar row rib bons.

IN TER PRE TA TION

Field ob ser va tions in di cate that both the sand-filled and mud-filled palaeochannel va ri et ies in the Wielkopolska

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Ta ble 1

Sum mary of data on the chan nel-fill type, chan nel width and chan nel thick ness (depth) in the Wielkopolska

Mem ber Channel number [Figs 3-9] Sediment type Channel-fill width [m] Channel-fill thickness [m] Width/thickness ratio [w/t] 1 v.f. sand, mud 6.5 0.7 9.3 2 f. sand 10 1 10 3 f. sand >4 >0.5 ? 4 v.f. to f. sand, mud, organic material 25 2.5 10 5 v.f. sand 9 0.8 11.3 6 v.f. sand 4 0.6 6.7 7 v.f. to f. sand, mud 63 4.5 14 8 v.f. to f. sand, mud, organic material 30 >3 <10 9 v.f. sand, mud 7.8 1.2 6.5 10 f. sand 6.7 1.1 6.1 11 f. sand 4 0.8 5 12 f. sand 5.7 1.2 4.5 13 f. sand 5.8 0.9 6.4 14 v.f. sand 2.2 0.3 7.3 15 v.f. sand 5.3 0.8 6.6 16 v.f. sand 3 0.5 6 17 v.f. sand 3.6 0.6 6 18 v.f. sand 2.5 0.4 6.3 19 v.f. sand 1.8 0.4 4.5 20 mud, v.f. sand 16 2 8 21 mud 7.8 1.3 6 22 mud 12.5 1.3 9.6 23 mud 10 1 10 24 mud 11.8 1.2 9.8 25 mud 10.7 1.1 9.7

Sand grain-size ab bre vi a tions: f. – fine-grained; v.f. – very fine-grained Fig. 3. Non-de formed, sand-filled sin gle-storey chan nels in the

Wielkopolska Mem ber. A – Palaeochannel ex posed in the Kazi-mierz North lig nite pit (Fig. 1B), view to the east; B – Palaeochan nel with a levee “wing” at tached to its lefthand mar gin, ex -posed in the JóŸwin IIB lig nite pit (Fig. 1B), view to the east. Num bers in cir cles re fer to the num ber ing of palaeochannels in Ta -ble 1; the black squares in B in di cate sed i ment sam pling for grain-size anal y sis

Member are quite nar row rib bons in the gen eral clas si fi ca -tion flu vial chan nel belts. The sandy chan nel-fill de pos its are mas sive and/or trough strat i fied. Trough cross-strat i fi ca tion rep re sents mi gra tion of linguoid 3-D dunes in the up per part of lower flow re gime (Collinson & Thomp son, 1982; Bridge, 2003), whereas the nontrac tional de po -si tion of mas -sive sand is at trib uted to a rapid dump ing of sed i ment di rectly from tur bu lent sus pen sion (Lowe, 1988), which is in dic a tive of hyperconcentrated flows (Pierson, 2005; Nemec, 2009). These sed i men tary fa cies are not di ag nos tic for any par tic u lar river type, as they may oc cur in vir -tu ally all flu vial sys tems (cf. Allen, 1965, 1978; Schumm, 1968; Miall, 1977, 1996; Rust, 1978; Bridge, 2003). Sim i -larly non-di ag nos tic are the in ter nal fa cies of mud-filled

chan nels, which in di cate slack-wa ter de po si tion of mud from sus pen sion fallout in an aban doned con duit, with pos -si ble in cur -sions of heavily mud-laden flows (Baas & Best, 2002; Baas et al., 2009), all of which can be at trib uted to overbank flood ing from ad ja cent ac tive chan nels (Bridge, 2003). In short, the chan nelfill fa cies alone are not di ag nos -tic for any spe cific river type.

The bed ding ar chi tec ture of chan nel-fill de pos its in most cases in di cates sim ple cut-and-fill con duits plugged by ver ti cal sed i ment ac cre tion, but shows ev i dence of mi nor lat eral ac cre tion in some of the sand-filled chan nels (Fig. 4E–G). At least some of the chan nels were thus lat er ally ac -tive, mi grat ing side ways and form ing in cip i ent point bars. There fore, it is the low width/thick ness ra tio of the

palaeo-Fig. 4. Glacitectonically de formed, sandfilled multistorey chan nels in the Wielkopolska Mem ber. A – Broad east ward view of out -crop sec tion show ing at least four palaeochannels in the Kazimierz North lig nite pit (Fig. 1B) with the black squares in di cat ing sed i ment sam pling for grain-size anal y sis; B–D – Pho to graphic out crop de tails; E–G – The cor re spond ing line-draw ings of palaeochannels. Note the ver ti cal chan nelfill ac cre tion in E and the lim ited lat eral ac cre tion in F and G. The ar rows in di cate di rec tion of glacitectonic de for ma -tion in the over ly ing, muddy floodplain de pos its

chan nels that would ap pear to be most di ag nos tic of the flu -vial sys tem type, as dis cussed fur ther in the next sec tion.

DIS CUS SION

Lat er ally in ac tive sand- and mud-dom i nated chan nels with low width/thick ness ra tios and a ver ti cally accreted infill are gen er ally char ac ter is tic of anastomosing river sys -tems (Rust, 1981; Friend, 1983; Smith, 1983, 1986; Nadon, 1994; Makaske, 2001). Such chan nel sys tems re quire for their for ma tion sta ble banks com posed of co he sive and/or veg e tated sed i ment (Mc Car thy et al., 1991; Gibling et al., 1998; Gradziñski et al., 2003), which is con sis tent with the field ev i dence in the pres ent case.

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Fig. 5. Sandfilled chan nel ex posed in the Kazimierz North lig -nite pit (Fig. 1B). A – Broad view of the out crop, look ing to wards the NE, with the dashed line in di cat ing palaeochannel top; B – Close-up de tail show ing a lat eral to ver ti cal pat tern of ac cre tion bed ding, high lighted by muddy in ter ca la tions (darker-col our bands)

Fig. 6. Glacitectonic de for ma tion of palaeochannels in the Wielkopolska Mem ber in the Kazimierz North lig nite pit (Fig. 1B). A – Broad east ward view of a glacitectonically tilted and toptrun cated palaeochannel, with the ar rows in di cat ing the south -ward di rec tion of strain; B – Close-up view of muddy floodplain de pos its with steep Riedel shears (R’), thrust south wards over a palaeochannel lithosome (Fig. 6A)

Fig. 7. Glacitectonically thrust-tilted sand-filled chan nels in the Kazimierz North lig nite pit (Fig. 1B); view to wards the east. A – Palaeochannels sep a rated by a thrust fault, with the ar rows in di cat ing di rec tion of glacitectonic strain; B – Palaeochannels dis -placed by a thrust du plex struc ture, with the ar rows in di cat ing di rec tion of glacitectonic strain; C – Palaeochannel lithosome slightly north-tilted by glacitectonic de for ma tion; the ap par ent oblique bands on out crop wall are scratches left by bucket-wheel ex ca va tion ma chine

The chan nel width/thick ness ra tios re ported from in ter preted an cient anastomosing flu vial sys tems are char ac ter is -ti cally low, but show con sid er able vari a -tion. For ex am ple, the re ported as pect ra tios of sandy palaeochannel rib bons in the Palaeogene Willwood For ma tion of Wy o ming are in the range of 3–13 (Davies-Vollum & Kraus, 2001), but are in the range of 5.5–15 in other cases (Friend et al., 1979; González-Bonorino et al., 2010). The as pect ra tio of anasto- mosing river chan nels may ex ceed 15 and pos si bly even 100 in some cases (Gibling et al., 1998), but these would be in stances in volv ing con sid er able lat eral mi gra tion or braid -ing of the con duit.

For com par i son, the mud-filled chan nels in the Will-wood For ma tion have re ported width/thick ness ra tios in the range of 3–55, with an av er age of less than 20 (Kraus & Davies-Vollum, 2004). In the Tri as sic of south ern Po land, the as pect ra tios of mud-filled chan nels of an in ferred anastomosing river sys tem are in the range of 27–60 (Gruszka & Zieliñski, 2008), which has been at trib uted to a com -bi na tion of such fac tors as the low en ergy of chan nel-fill ing flow; a re sis tant, co he sive sub strate; and a high load of sus pended mud. How ever, the ex act role of these fac tors in ren -der ing the chan nel w/t ra tio high re mains un clear, and the mud-filled chan nels there may rep re sent aban doned bran-ches of a braided river sys tem (cf. Mroczkowski, 1977; Mroczkowski & Mader, 1982).

The width/depth ra tios of mod ern anastomosing flu vial sys tems, such as the Pol ish Narew River, are in the range of

2–10 (Gradziñski et al., 2000, 2003). The anastomosing river chan nels on the Ho lo cene Rhine-Meuse delta plain of north ern Ger many and in the lower Mis sis sippi Val ley of U.S.A. have width/thick ness ra tios sig nif i cantly lower than 25 (Törnqvist et al., 1993; Stouthamer, 2001; Gouw, 2007).

In sum mary, the low width/thick ness as pect ra tios of the palaeochannels in the pres ent study (Ta ble 1) are the stron gest in ter pre tive ar gu ment for an anastomosing flu vial sys tem. There is no ev i dence of sig nif i cant chan nel me an -der ing, whereas the pos si bil ity of a braided flu vial sys tem can be pre cluded, be cause the bound ary be tween me an der ing and braided rivers cor re sponds to an as pect ra tio be -tween 200 and 500 or even higher (Horne et al., 1978; Eberth & Miall, 1991; Törnqvist et al., 1993; Gradziñski et

al., 1995; Makaske, 2001; Gibling, 2006; Doktor, 2007;

Gouw, 2007). Some of the sandfilled chan nels were sub -ject to mi nor lat eral mi gra tion, prob a bly dur ing phases of neg li gi ble sub si dence rate, when the con duits were forced to de velop lat eral ac com mo da tion for their dis charges. The muddy interlayers in some of the sand-filled chan nels (Figs 4E & 5B) in di cate that many con duits prob a bly con veyed sand only dur ing high stages of the flu vial sys tem, with an in ter ven ing trans port of mud dur ing slack-wa ter stages. This kind of al ter na tion of high sand dis charges and ep i -sodic slack-wa ter con di tions in some chan nel branches is con sis tent with the no tion of an anastomosing flu vial sys -tem (Smith, 1983, 1986; Mc Car thy et al., 1991; Nadon, 1994; Miall, 1996; Gibling et al., 1998; Makaske, 2001).

Fig. 8. Multi-storey sand- and mud-filled chan nels in the Kazimierz North lig nite pit (Fig. 1B), af fected by glacitectonic de for ma tion. A – Broad view of out crop sec tion, look ing to wards the east; B, C – Close-up views of mud-dom i nated palaeochannels filled by ver ti cal ac cre tion

CON CLU SIONS

1. The Wielkopolska Mem ber of the Neo gene Poznañ For ma tion in cen tral Po land con sists of sin gle- and multi-storey palaeochannel lithosomes en cased in mud-rich flood- plain de pos its and is in ter preted to have formed in an al lu vial en vi ron ment with a west ward to northwest ward di rec -tion of fluvial drainage.

2. Two types of palaeochannels were dis tin guished: sand-filled and mud-filled. Both show an up ward fin ing trend, al though it is of ten rec og niz able only by the grain-size anal y sis of sed i ment sam ples, rather than at a mac ro scopic level.

3. The fine to very finegrained sandy chan nelfill de pos its are mainly mas sive, but in clude trough crossstrat i fi -ca tion. The muddy chan nel-fill de pos its are mas sive to faintly lam i nated. The or i gin of the chan nel-fill fa cies was in ter preted. The sed i men tary fa cies alone are not di ag nos tic for any par tic u lar type of flu vial sys tem, but the lack of sys -tem atic lat eral ac cre tion bed ding pre cludes the possibility of meandering rivers.

4. The cross-sec tional ge om e try of the palaeochannels was stud ied in terms of their width/thick ness ra tio, which is in the range of 4.5–14 for the sand-filled chan nels and in the range of 6–10 for the mud-filled ones. There fore, the palaeochannels are con sid ered to be nar row rib bons in the gen -eral clas si fi ca tion of flu vial chan nel belts, and this ev i dence pre cludes braided rivers.

5. The or i gin of the Wielkopolska Mem ber is at trib uted

to an anastomosing river sys tem char ac ter ized by cut-and-fill chan nels with co he sive and veg e tated banks. Some of the sandfilled chan nels were lat er ally ac tive, sub ject to lim -ited me an der ing. The mud-filled con duits are thought to have been the cut-and-aban doned branches of the sys tem, whereas the sand-filled con duits were those con vey ing the main dis charge and sand load on perennial basis.

Ac knowl edge ments

The manu script was crit i cally re viewed by Massimiliano Ghi- nassi, Brian Wil lis and an anon y mous re viewer, and was subse-quently ed ited by Wojciech Nemec. Their con struc tive com ments and ed i to rial help con sid er ably im proved the pa per and are much ap pre ci ated by the au thor.

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Fig. 9. Multistorey mudfilled chan nels ex posed in the Kazimierz North lig nite pit (Fig. 1B). A – Broad east ward view of out crop sec -tion, show ing four sin gle-storey iso lated palaeochannels with glacitectonically de formed floodplain de pos its at the out crop top; B – Close-up view of a palaeochannel filled by ver ti cal ac cre tion

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