DOI: 10.1515/squa-2015-0007
ICE-SHEET DY NAM ICS OF WARTA GLA CI ATION (SAALE)
IN THE MAR GINAL ZONE OF KNYSZEWICZE AREA,
NORTH EAST ERN PO LAND
Joanna Rychel1, Barbara Woronko2, Miros³aw T. Karasiewicz3, Pawe³ Szymczuk4, Marcin Morawski1 1 Pol ish Geo log i cal In sti tute – Na tional Re search In sti tute, Rakowiecka 4, 00-975 War saw, Po land,
e-mail: joanna.rychel@pgi.gov.pl 2
Fac ulty of Ge ol ogy Uni ver sity of War saw, ¯wirki i Wigury 93, 02-089 War saw, Po land, e-mail: bworonko@uw.edu.pl
3
Fac ulty of Earth Sci ences, Nicolaus Co per ni cus Uni ver sity, Lwowska 1, 87-100 Toruñ, Po land, e-mail: mtkar@umk.pl
4
Fac ulty of Ge og ra phy and Re gional Stud ies, Uni ver sity of War saw, Krakowskie Przedmieœcie 30, 00-927 War saw
Ab stract
The pa per pres ents a re search on a mar ginal zone near Knyszewicze in the south ern part of Sokó³ka Hills (north east ern Po land). Ter mi nal mo raine hills are ar ranged amphitheatrically in a lobal pat tern. Dy nam ics of the Knyszewicze fron -tal ice-sheet lobe dur ing the Saale Gla ci ation and suc ces sive stages of the mar ginal zone near the vil lage of Knysze-wicze were re con structed based on sed i men tary and geomorphological anal y sis, us ing a dig i tal el e va tion model and morpholineaments. Three main phases of the Knyszewicze gla ciallobe ac tiv ity were iden ti fied in clud ing ac cu mu la -tion of glaciofluvial de pos its, ad vances of the ice mar gin and ice-lobe re treat. Mo raine hills de vel oped at a sta ble ice-lobe ter mi nus, ini tially as short end-mo raine fans with the fol low ing se quence of lithofacies GhÞSGhÞSh or GmÞGhÞSh. Such a se quence in di cates cy clic sheetfloods. Dur ing a small but dy namic ad vance of the ice sheet ter mi nus, these de pos its were moved for ward and monoclinally folded, then fur rowed with slop ing faults due to hor i zon -tal pres sure. Typ i cal thrust-block push mo raines de vel oped in this way. Ice sheet ad vance took place when per ma frost was pres ent in the sub stra tum and very high wa ter pres sure oc curred at gla cial ter mi nus. In side a lobal con fig u ra tion of mo raines, there is a rich in ven tory of gla cial forms with a clas sic ter mi nal de pres sion in the cen tral part. Based on this landform pat tern, their shape, rhythm and glaciotectonic dis tur bances, the land re lief may be re ferred to as a hill-hole pair. The struc ture of Horczaki Knoll, de pos ited on the sub-Qua ter nary tec tonic struc ture, sig nif i cantly con trib uted to a de vel op ment of this mar ginal zone.
Key words: ice lobe, mar ginal zone, glaciotectonic dis tur bances, thrust-block push mo raines, morpholineaments. Manuscript treceived 22 july 2015, accepted
IN TRO DUC TION
Gla cial lobes rep re sented a clear mor pho log i cal el e ment in a mar ginal part of the Pleis to cene ice sheets in Eu rope, Asia and North Amer ica. Such sit u a tion is best known from the Vistulian Gla ci ation area in Po land (e.g. Galon and Roszko, 1961; Roszko, 1968; Kozarski, 1995; Karasiewicz, 2006; Kasprzak, 2007; Morawski, 2009b; Wysota and Molewski, 2011; Narloch et al., 2013) and other re gions of Eu rope (e.g. Boulton et al., 2001; Houmark-Niel sen, 2004; Johansson et al., 2011), as well as North Amer ica (e.g. Punkari, 1997; Kovanen and Slaymaker, 2004; Mickelson and Colgan, 2004) and Asia (e.g. Grosswald, 1998). Gla cial lobes were more dy namic com pared to the main body of ice,
whereas the fron tal icesheet lobes could be the ef fect of gla -cial surges or ice streams as de scribed by sev eral au thors (Echelmeyer and Har ri son, 1990; Echelmeyer et al., 1991; Patterson, 1998; Marks, 2002, 2005; Hol land et al., 2008; Wysota and Molewski, 2011).
Mech a nisms that fa vour de vel op ment and func tion ing of the Pleis to cene gla cial lobes on the Eu ro pean Plain were de scribed in de tail by Narloch et al. (2013). The au thors point out to com plex re la tion ships be tween the ice sheet, its sub -strate, cli mate and wa ter cir cu la tion within and be neath the ice sheet body, as well sev eral other fac tors.
The course and func tion ing of the out let lobes are of ten de ter mined by sub strate to pog ra phy and rhe o log i cal con di -tions of subglacial sed i ments. Ac cord ing to Piotrowski
(2006) and Boulton (2006), subglacial meltwaters were of ma jor im por tance to the mech a nism of ice move ment and dy -nam ics. Un der fa vour able con di tions, pres ence of wa ter at the ice-sub stra tum in ter face could re sult in de vel op ment of a wa ter film, which con se quently may lead to décollement of an icesheet from its base (Piotrowski and Kraus, 1997; Ar nold and Sharp, 2002; Wood ward et al., 2003) and con se
-quently, to basal slid ing (Hermanowski and Piotrowski, 2009; Narloch et al., 2013). Basal slid ing may cause an in creased rate of ice sheet ad vance (Iverson et al., 1995). Sci -en tific stud ies show ing the in flu -ence of basal slid ing on the ice-move ment ac cel er a tion were con ducted based on the anal y sis of gla cial sed i ments and land re lief in dif fer ent mar -ginal zones (Patterson, 1997; Piotrowski and Kraus, 1997;
Piotrowski, 2006; Piotrowski and Tulaczyk, 1999; Stokes and Clark, 2001; Hermanowski and Piotrowski, 2009). Such stud ies were also con ducted in con tem po rar ily gla ci ated ar -eas (e.g. Tulaczyk et al., 2000; Zwally et al., 2002).
In the 1970s, Mojski (1972) de scribed a geo mor phol ogy of the Podlasie Low land in east ern Po land, draw ing at ten tion to a lobal pat tern of ter mi nal mo raines south of Sokó³ka. He pre sented an ice sheet gate near Knyszewicze as a pe cu liar landform in a mar ginal zone that could arise as a re sult of a gla cial lobe or a small out let gla cier de vel oped from the ice sheet of the Saale Gla ci ation (MIS 6).
The ob jec tive of the field work car ried out in the mar -ginal zone of the Knyszewicze lobe was to re con struct: (1) the ac cu mu la tion con di tions in the fore land of a gla cial lobe, (2) the na ture of glaciotectonic dis tur bances af fect ing the sed i ments, (3) the dy nam ics of the lobe ter mi nus and (4) the na ture of post-de po si tion dis tur bances and the con di tions of their or i gin.
GEO LOG I CAL SET TING
The study area is lo cated in the North Podlasie Low land, in the area of Sokó³ka Hills (Kondracki, 2000), at a dis tance of ca. 40–50 km to the north east of Bia³ystok. The re lief of this area re sults from a gla cial mo tion dur ing the Saale Gla ci -ation (Boratyn, 2003), which was slid ing over the area of the Sokó³ka Hills from two di rec tions – the ad vance from the north was con nected with the Biebrza lobe and the ad vance from the north east was cor re lated with the Niemen lobe (Ber
et al., 2012). Small fron tal ice sheet lobes de vel oped in a mar
-ginal zone of both ice streams, in clud ing the lobe of Knysze-wicze (Banaszuk, 2010).
The di rec tion of ice lobe ad vances dur ing the Saale Gla -ci ation (MIS 6) was re con structed based on the ge ol ogy and geo mor phol ogy and anal y sis of gla cial morpholineaments. The de scribed zone con sists of sev eral dis tinct mo raine ridges, the struc ture of which was thor oughly in ves ti gated in the ex po sure at Knyszewicze. The site Knyszewicze is lo -cated in the arch-shaped zone of ter mi nal mo raines open to the north east. It forms a lobe-like pat tern that stretches from the en vi rons of Wojnowiec in the north east, through the vil -lage Horczaki Dolne to Knyszewicze in the south west, and then it turns to wards the vil lage of Mienkowce in the south east (Fig. 1). The lobe stretches over ca. 9 km in the SW di -rec tion from the state bor der and has a width rang ing from ca. 4 km in the most west ern part to 7.5 km in the east. The length of the mo raine se quence is ca. 17.5 km. It con sists of 27 dis -tinct forms, mostly ter mi nal ac cu mu la tion mo raines and push mo raines, par tic u larly in the south ern part of the lobe (Boratyn, 2006), but also deadice mo raines. These forms are ar -ranged in two and in the north-west ern part of the ridge, even three se quences par al lel to each other (Fig. 1). The high est point of these mo raines is lo cated north west of the Wojno-wiec vil lage (240 m a.s.l.) and the low est one near Knysze-wicze (ca. 180 m a.s.l.). Their morphometry is var ied, the width at the base of the small est ter mi nal mo raine is ca. 200–300 m and the width of the larg est one ranges from 200 to 800 m, with a length of ca. 3.5 km. Most of the ter mi nal mo raines have asym met ri cal slopes; prox i mal slopes are steeper than the dis tal ones – from 2 to 15–20 (Fig. 2). Mo
raines near Knyszewicze and Horczaki Dolne have the steep -est slopes.
In the north, the mo raines are en closed by a long ridge re -ferred to as Horczaki Knoll, ris ing above the sur round ing land in the NE–SW di rec tion, i.e. par al lel to the mo raines mark ing the north ern and the south ern range of the Knyszewicze lobe (Fig. 2). The Horczaki Knoll had a very sig nif i cant im pact on the palaeo ge ogra phy of this area and ad -vances of the ice sheets, the tec tonic set tings of which are con nected with the ac tiv ity of the Qua ter nary sub stra tum (Fig. 3). Prob a bly al ready dur ing the Elster Gla ci ation, and cer tainly dur ing the trans gres sion of the Saale ice sheet, the Horczaki Knoll rep re sented a con sid er able bar rier in the ter rain, caus ing the ac cu mu la tion of ice sheet masses and sed i -ments or di ver sion of the ad vanc ing ice sheet (Boratyn, 2006).
Outwash plains are lo cated south east of the mo raines, while dead ice mo raines and less fre quently kames oc cur in the hin ter land of the mo raines. A large ter mi nal de pres sion filled with silt de posit is lo cated near the vil lage of Suchy-nicze (Fig. 2).
In the southwesternmost part of the mo raine, near the Knyszewicze vil lage, a rel a tively large gate (up to 20 m wide) with steep slopes cuts the mo raines. Ac cord ing to Mojski (1972), it is a clas si cal ex am ple of a gla cial gate (Figs 1, 2). A two-level ex ca va tion lo cated in the hill ad ja cent to the gla cial gate pro vided ac cess to the in ter nal struc ture of the mo raine (Fig. 1). The ex ca va tion rose ca. 25 m above the sur -face of the sur round ing ter rain. The width of the hill is ca. 200 m, and the ab so lute height be fore the ex ploi ta tion was over 192.5 m a.s.l., while in cli na tion of slopes in the south -west ern part was up to 10–15.
MA TE RIAL AND METH ODS
The land re lief near Knyszewicze and sed i ments ex -posed in this area were ana lysed in de tail to re con struct the spa tial and tem po ral dy nam ics of the ice sheet in the area of Sokó³ka Hills and Bia³ystok Pla teau.
Anal y sis of the land re lief
The anal y sis of the land re lief in the en vi rons of Knysze-wicze con sisted in the de ter mi na tion of gla cial morpholinea-ments over an area of 1447 km2. This has be come an in creas -ingly ver sa tile tool for in te grated palaeogeomorphological and palaeo geo graphi cal re con struc tion (cf. Morawski, 2005). The first stage of the study con sisted in iden ti fi ca tion of lin ear el e ments of the postglacial re lief ac cord ing to the clas si fi -ca tion of Rychel and Morawski (2014): mo raines (Mo), cre-vasses (Crev), ter mi nal de pres sions (Depr), val leys (Val), closed de pres sions with no out flow (Pit), outwash fans (Fan) and hills on the outwash plains, up lands and dead ice mo raines (Hu). The anal y sis was per formed by the GIS soft -ware, us ing a shaded and col oured Dig i tal Ter rain El e va tion Data (DTED 2), an ex ag ger ated dig i tal el e va tion model (res -o lu ti-on -of 30 m), t-op -o graphic maps (WMS) and ge-o l-og i cal maps, in par tic u lar the Sokó³ka sheet of the De tailed Geo log i cal Map of Po land in scale 1:50,000. The length of lin ea -ments was counted within the groups of ge net i cally and
chro no log i cally in ter re lated land forms, at in ter vals of ev ery 5°. Re sults are pre sented in ro sette di a grams (Fig. 4).
The sec ond stage of the anal y sis con sisted of iden ti fi ca -tion of re gions where ge net i cally iden ti cal groups of gla cial lin ea ments had sim i lar ori en ta tion, i.e. in di cated dif fer ent dy nam ics of ice sheet body, in clud ing a di rec tion of the ice sheet ad vance. This in turn could be used to de ter mine a range of po ten tial ice lobes (cf. Morawski, 2003, 2009a, b).
Struc tural anal y sis of sed i ments
A de tailed lithofacial anal y sis of sed i ments was con -ducted, fol low ing the code of Pisarska-Jamro¿y and Zie-liñski (2012). Mea sure ments of struc tures in the strat i fied sed i ments and reg is tered faults were made. In ad di tion, the max i mum par ti cle size (MPS) was mea sured in grav els. Mea sure ments of 10 clasts were made di rectly from the out
crop wall and in the case of de pos its fill ing a subglacial chan -nel from a scree lo cated at the base of the chan -nel.
RE SULTS Palaeo geo graphi cal do mains
Groups of postglacial elon gated forms (e.g. ter mi nal mo -raines and eskers) were dis tin guished on the ba sis of gla cial morpholineaments near the Knyszewicze vil lage, and con se quently the di rec tion of icesheet trans gres sion was de ter -mined (Fig. 4). Two do mains, in clud ing the Horczaki Knoll and Knyszewicze lobe mo raines were dis tin guished, dif fer -ent from each other in the ori en ta tion of the same groups of gla cial lin ea ments, i.e. in di cat ing di rec tion of the ice sheet move ment. The az i muth of the axis of the stron gest thrust of the con ti nen tal ice sheet was equal to 171°, ob tained for the do main of the Horczaki Knoll, re curred through out the ana -lysed area and prob a bly cor re sponded to the main di rec tion of the ice sheet ad vance dur ing the max i mum range of the Saale Gla ci ation (MIS 6) from the north-north west. On the other hand, the az i muth of the thrust within the do main of the Knyszewicze lobe mo raine was 23° and was con sis tent with mea sure ments of the overthrust faults at the east ern wall of the out crop. This in di cated a move ment of the lobe from the north-north east (Fig. 4).
Sed i ments – de scrip tion and in ter pre ta tion
A se ries of sand and gravel de pos its with a to tal thick ness of ca. 10 m is ex posed at two lev els of the ex ca va tion pit at
Knyszewicze. The dis tin guished lithofacies were grouped into three units (U1–U3), con tact ing lat er ally with one an other.
Unit 1 (U1)
It con sists of hor i zon tally-strat i fied gravel (Gh), mas sive gravel (Gm), hor i zon tally strat i fied sand with gravel (SGh) and sand (Sh). These sed i ments form gravel and sand se -quences: GhÞSGhÞSh and GmÞGhÞSh. The strata are char ac ter ised by a large strike and tiled shape (Figs 5 and 6). Mas sive gravel beds (Gm) are 5–20 cm thick, whereas sand-with-gravel beds (SGm) from 15 to 25 cm. The depth of wash outs of oc ca sion ally re corded trough cross-strat i fied sandy and grav elly sed i ments (SGt) does not ex ceed 40 cm. Mea sure ments of max i mum par ti cle size (MPS) of the ten larg est clasts from hor i zon tally strat i fied gravel lithofacies (Gh) and mas sive gravel (Gm) were 52–81 mm. The MPS value in creased to wards the sur face, both in the east ern and north ern part of the out crop. The lon ger axes of clasts were per pen dic u lar to the wa ter flow di rec tion.
Unit 2 (U2)
Hor i zon tally strat i fied fine-grained sand and fine-grai-ned gravel (Sh, Gh), and sands with rip ple-cross lam i na tion (Sr) were re corded in the east ern wall of the out crop with N–S ori en ta tion. The sed i ments were ob served in a nar row zone (up to 3 m) with a thick ness of up to ca. 3.5 m. Sed i -ments of the unit 2 are in a di rect con tact with the strat i fied gravelsand sed i ments (unit 1) from the south, and with sed i -ments fill ing the chan nel (unit 3) from the north.
Unit 3 (U3)
In the north ern and south ern wall of the up per part of the out crop, hor i zon tally strat i fied sand-with-sand beds are cut by chan nels. The chan nel ob served on the north ern wall is ca. 5 m deep and 7 m wide in a lower part and ca. 15 m wide in the up per part. The chan nel on the south ern wall is ca. 4 m deep and 11 m wide. In both cases, the chan nel’s slopes are very steep, in clined at 60–90° (Fig. 7). The bot tom of the chan nel vis i ble on the north ern wall is lo cated lower than the chan nel on the south ern wall and the wash out sur face is in clined north wards, i.e. to wards the in ner parts of the Knyszewicze lobe. The bot tom part of the north ern chan nel was filled with mas sive gravel (Gm; ca. 1 m thick) with an gu lar sandy clasts of strat i fied fine-grained sand de pos its with a di am e ter of up to 20 cm and boul ders with a di am e ter of up to 40 cm. This de posit is over lain by mas sive ma trix-sup ported grav els (Gm) and tab u lar cross-strat i fied grav els (Gp) with poorly-marked strat i fi ca tion. In the up per most layer, a rhythmite of mas sive silty diamicton (Dm) and poorly sorted, hor i zon tally and high an gle crossstrat i fied grav els (Gh, Gp) was re
-Fig. 4. Dis tri bu tion of gla cial morpholineaments within the Knyszewicze mar ginal zone Black ar row –main overthrust di rec tion dur ing the last gla ci ation in the study area, blue ar row di rec tion of ice flow in the Knyszewicze lobe, n – num ber of mea sure ments, r – ra dius of length.
Fig. 5. Glaciotectonically dis turbed sed i ments of the endmo -raine fan at Knyszewicze?????????????????????????????????
Fig. 6. Lithological log of the end-mo raine fan at Knyszewicze af ter Szymczuk et al. (2014).
corded, with a bed thick ness of 1.5 m, which turned into wellsorted, hor i zon tally crossstrat i fied gravel de pos its to -wards the cen tre of a wash out (high an gle – Gp). MPS for the re corded gravel de pos its in the floor of the chan nel’s in fill ing was 180 mm.
On the south ern wall, mas sive gravel (Gm) with a thick -ness of ca. 0.7 m was found be low the chan nel, which cuts across the gravel-sand sed i ments at a right an gle in re la tion to sed i ment strat i fi ca tion.
Sed i ment dis tur bances – de scrip tion and in ter pre ta tion
All rhyth mi cally strat i fied grav elly and sandy sed i ments ob served in the out crop were strongly dis turbed and mono-clinally po si tioned at an an gle of 40–55° north wards. Two types of faults were noted: nor mal, steep and re verse faults. The nor mal faults were steep and their fault planes dip 46–83° north wards. They were ob served mainly in the north -ern part of the wall (Figs 5, 6). The re verse faults with their planes dip ping 11–44° north-northeastwards in di cated a north-north east ern di rec tion of the ice-sheet thrust. They were ob served in the east ern wall of the up per out crop only, mainly in sed i ments of the unit 2. De pos its fill ing the chan nel (U3) were in volved in dis con tin u ous de for ma tions (Fig. 7). A to tal thick ness of the dis turbed sed i ments in the out crop was equal at least 30 m.
On the north ern wall of the out crop, a sed i ment flex ure oc curred in the sur face zone (up to 0.7 m), which was a re sult of mass move ments. It rep re sented the so-called downslope bend ing of strata. They de vel oped at the time when per sis -tent, long-term per ma frost re treated and sed i ments were plas tic. This is sup ported by a steep in cli na tion of the slopes of this landform.
PHASES OF GLA CIAL-LOBE AC TIV ITY
Based on the con ducted study, three phases of the Kny-szewicze gla cial lobe ac tiv ity were dis tin guished: phase I with de po si tion of glaciofluvial de pos its in the ice sheet
front, phase II with ad vances of the ice mar gin and phase III with the ice lobe re treat. The phases re flect a dy nam ics of the ice front in the mar ginal zone near Knyszewicze.
Phase I: ac cu mu la tion of glaciofluvial de pos its
The first phase of the mar ginal zone of the Knyszewicze lobe starts from the ac cu mu la tion of glaciofluvial sed i ments (U1 and U2) build ing the ter mi nal mo raine (Fig. 8). The pres -ence of lithofacies se qu-ences GhÞSGhÞSh and GmÞGh ÞSh in di cates that their de po si tion oc curred dur ing non-channelized, ex ten sive and shal low sheet-flows. This flow was char ac ter ised by short-term surges, re flect ing an abla-tional rhythm of the ice-sheet (Zieliñski, 1993; Krzysz-kowski and Zieliñski 2002; Zieliñski, 2015). Dur ing the flow max i mum, strong aggradation pre vailed and mas sive (Gm) and hor i zon tally strat i fied grav els (Gh) oc curred. Hor i zon tally strat i fied sandwithgravel and sand (SGh or Sh) de vel -oped dur ing lower flow stages. Ac cord ing to Krzyszkowski and Zieliñski (2002), this type of sed i ment de vel op ment in di cated lo ca tion of sed i ments in a dis tal zone of the end mo -raine fan (ice-mar ginal fan) of the type B2b. Sed i ments were de pos ited in the ice-con tact zone, on steep fan slopes (Blair and McPherson, 1994; Krzyszkowski and Zieliñski, 2002; Zieliñski, 2015). The same type of de po si tion was ob served in a south ern part of the Knyszewicze lobe, at Babiki lo cated ca. 2.5 km NE from Knyszewicze (Figs 1, 2). Thus, it ap pears that this type of sed i men ta tion was com mon in a fore land of the Knyszewicze lobe. The end mo raine fans were small and over lap ping. A sim i lar type of de po si tion was re corded in var i ous parts of the Pol ish Low lands (Kasprzak and Ko-zarski, 1984; Krzyszkowski et al., 1999; Krzyszkowski and Zieliñski, 2002). The end mo raine fan ac cu mu la tion took place when the ice front was sta bi lized or slowly os cil lat ing (Krzyszkowski and Zieliñski, 2002). In the case of the end mo raine fan of Knyszewicze, a thick ness of sed i ments sligh-tly in creased to wards the up per most layer (the roof), which in di cated that de po si tion took place dur ing very slow ad -vance of the ice front.
This ice sheet ad vance was fol lowed by a re treat of the Knyszewicze lobe front. Sed i ments of the push mo raine lost their sup port, which led to de vel op ment of many nor mal faults. Based on the col lected data, it is not pos si ble to de ter -mine. how far the ice sheet front re treated.
Phase II: ad vances of the ice mar gin
Three par al lel se quences of hills can be iden ti fied in the land scape of the north ern and west ern part of the Knysze-wicze lobe. They re flect sub se quent ad vances or re treats of the Knyszewicze lobe, in ter rupted by ep i sodes of ice front sta bi li za tion. On the other hand, a 15 m high sin gle crest par -al lel to the ice mar gin oc curs in the vi cin ity of Knyszewicze (Fig. 1A). The ad vance was fol lowed by de for ma tions in the end mo raine fan, par tic u larly in the east ern and cen tral part of the out crop (Figs 2, 8). De for ma tions oc curred as thrustings of semi-co her ent blocks along dis crete fault planes. This stage of de for ma tion was con nected with a sys tem of slop ing faults, with a fault plane in clined at 11–44° north wards. They were par tic u larly well vis i ble on the east ern wall of the out
-Fig. 7. Con tact zone of sed i ments fill ing a deep chan nel (U3) and dis turbed sed i ments of the end-mo raine fan (U1) af ter Szymczuk et al. (2014).
crop (Figs 5, 6). In cli na tion of the fault planes in di cates a north-north east ward di rec tion of the ice sheet thrust that was re spon si ble for de po si tion. Nev er the less, the ad vanc ing ice sheet did not pass through the ana lysed form and the folded struc tures did not de velop ei ther. The dis turbed sed i ments were cut through by a deep chan nel, the bound ing sur face of which was dip ping at low an gle to wards the north (up stream dip ping beds). This proved high en ergy of the wa ter flow, con cen tra tion (chan nel on the north ern wall) and low dis per -sion of wa ters (chan nel on the south ern wall).
The ana lysed form is a re sult of the for ward ad vance of the ice mar gin into the end mo raine fan, which caused lat eral com pres sion of the mo raine fan. The gla cial stress field was gen er ated by push-from-the-rear (Bennett, 2001). This type of de for ma tion was char ac ter is tic for the thrust-block push mo raines (Bennett, 2001) or thrust-dom i nated mo raines (Boulton et al., 1999). Boulton et al. (1999) iden ti fied four ridge types in re spect of com pres sive stress gen er ated dur ing de for ma tion. The form at Knyszewicze rep re sents large (>5 m high) push mo raines, with a sin gle crest ori en tated par al lel to the ice mar gin. Ac cord ing to Boulton (1986) and Boulton et
al. (1999), a de vel op ment of the outwash fan is a prereq ui
-site for push mo raine for ma tion. Steeply in clined mo raine slopes at Knyszewicze may in di cate a sud den ad vance of the ice sheet or even gla cier surges (surge-type overthrusts). How ever, this ap proach is chal lenged by Boulton et al. (1999). Given the shape of the mo raine hills, es pe cially a steep ness of the slopes and, in some cases, their clear asym me try, most mo raines can be as sumed to have been com -posed of glaciotectonically dis turbed sed i ments. Clarification of this is sue is ad di Clarification ally hin dered by a lack of out -crops in the mo raines. The au thors of the De tailed Geo log i cal Map of Po land did not pro vide suf fi cient an swers ei ther (Boratyn, 2006).
Nu mer ous ob ser va tions of the thrust-dom i nated push mo raines around the mar gins of the Laurentide and Eu ro pean ice sheets in di cated im por tance of per ma frost in proglacial thrust ing (e.g. Clay ton and Moran, 1974; Astakhov et al., 1996). Ac cord ing to Boulton et al. (1999), per ma frost is nec -es sary for de for ma tion de vel op ment, al though not ev ery one shares this view (e.g. Aber, 1988). Most prob a bly, the per ma -frost was also pres ent in the sub stra tum dur ing the ad vance of the Knyszewicze lobe. The prev a lence of such con di tions may be ev i denced by pres ence of an gu lar sandy clasts of strat i fied sand de pos its in the bot tom sed i ments fill ing a chan nel (Unit 3). This fact is also ev i denced by the pre served pri mary struc ture of sandy clasts; a frozen sed i ment only can pre serve an in ter nal struc ture of loose sed i ments in cluded in the flu vial trans port (Mycielska-Dowgia³³o, 1998; Pisarska-Jamro¿y and Zieliñski, 2012; Weckwerth, 2010, Weckwerth and PisarskaJamro¿y, 2014). Most likely, pres ence of per -ma frost in the sub stra tum led to a high pres sure of wa ter in the ice sheet ter mi nus, which con se quently broke through and carved a deep chan nel, the ero sion of which re sulted from the in ten sive drain age un der the ice sheet. In such con -di tions, the N-type chan nel was formed (Nye, 1973, 1976) in sed i ments of the end mo raine fan. Fur ther more, the slopes of the chan nel are ver ti cal, which could be a fur ther proof that sed i ments of the end mo raine fan were frozen at the time of ero sion. One can ar rive at sim i lar con clu sions based on the
net work of faults in ter sect ing the sed i ments, which also sug -gests that the ma te rial ex posed to dis tur bances was frozen at that time. A sim i lar sit u a tion was de scribed by Ga³¹zka et al. (2009) from the site at Ro¿ental in north-cen tral Po land and Piotrowski (1994, 1997) from Ger many.
A large de pres sion filled with fine-grained sed i ments is lo cated in the hin ter land of the ter mi nal mo raine se quence of the Knyszewicze lobe (Fig. 2; Boratyn, 2006). Ac cord ing to van der Wateren (1995, 2005) and Beerten (2014), a push mo raine al ways seems to be as so ci ated with a ter mi nal ba sin. The ex is tence of gla cial bas ins can be ex plained by glacio-tec tonic pro cesses, i.e. re moval of at tached frozen sed i ment/ bed rock from the un der neath of the ice sheet.
At the sec ond stage, small os cil la tions of the Knyszewicze lobe front could oc cur. This hy poth e sis may be con
-Fig. 8. Ge netic model for sed i ment suc ces sion and landform for -ma tion at Knyszewicze. A – sta bi lized ice front and sed i men ta tion of the end mo raine fan; B – re treat of the ice lobe, C – ad vance of the ice mar gin, D – re treat of the ice lobe
firmed by the fact that sed i ments fill ing the eroded chan nel in the north ern wall of the out crop in the dis turbed sed i ments (Unit 2) of the end mo raine fan are in volved in the de for ma -tions (Figs 7, 8). Sed i ments fill ing the chan nel were most likely frozen dur ing the de for ma tion. This fact is sup ported by brit tle de for ma tions which af fected sed i ments of the unit 2. Phase II was prob a bly the last stage of the out er most mar ginal zone for ma tion by the ice-sheet in the Knyszewicze lobe.
Phase III – ice-lobe re treat
The icesheet front re treated and con se quently, the pre -vi ously dis turbed sed i ments lost their sup port. Most likely, the sec ond gen er a tion of steep nor mal faults is con nected with this stage of the mar ginal zone de vel op ment. The pro longed stag na tion of the ice sheet front re sulted in a de vel op -ment of the ter mi nal de pres sion in the hin ter land of ter mi nal mo raines. Slope pro cesses oc curred at the end of the fi nal stage.
DIS CUS SION
The end-mo raine zone near Knyszewicze is an ex am ple of glaciotectonic dis tur bances re corded in the re lief as a hill-hole pair when a se quence of hills of sim i lar size and shape oc curs along a rel a tively small dis tance of a ter mi nal de pres sion (Bluemle and Clay ton, 1984; Boulton, 1986; Benn and Ev ans, 2010). This type of sit u a tion was de scribed by Kasprzak (2007) from the Wielkopolska re gion near S³awa where an ice lobe de vel oped from the Vistulian ice sheet with the ice-con tact fans at the ter mi nus, sub se quently folded as a re sult of the ice sheet front os cil la tion. Os cil la tory move ments were in duced by win ter changes in the ice bal -ance. The ad vance was fol lowed by the ice sheet stag na tion and de vel op ment of re ces sional mo raine se quences.
Prob a bly, the Knyszewicze lobe was most ac tive and caused large dis tur bances in the sed i ments ac cu mu lated in the ice-con tact zone near Knyszewicze as well as in other parts of the south ern mo raine se quence. This hy poth e sis may be sup ported by the fact that three arcs of mo raines are pres -ent in the re lief of the north ern and west ern part of the lobe; they are not ob served in the south, i.e. near Knyszewicze and Babiki (Fig. 1). The thus de vel oped re lief may in di cate that the Horczaki Knoll could be a tec toni cally ac tive and slowly lifted form (Boratyn, 2005). There fore, the north ern wing of the lobe was “slid ing down” from the Horczaki Knoll, while the south ern part was com pressed.
The anal y sis of morpholineaments clearly shows that a sub se quent ad vance as so ci ated with a de vel op ment of the Knyszewicze lobe fol lowed from NNE or NE (Fig. 8). The Horczaki Knoll (a large ridge with NNE–SSW course) played a rel a tively im por tant role in its course and di rec tion (Fig. 1). The knoll is prob a bly a huge subQua ter nary el e va -tion with tec tonic set tings (Figs 1, 2, 4). Dur ing ice sheet overthrusts, the knoll was an un sta ble (la bile) zone and the load of the ad vanc ing ice sheet re sulted in in den ta tion. The un sta ble struc ture could af fect frag men ta tion of the ice body and de vel op ment of ice sheet lobes. Pres ence of three mo -raine arcs in the north ern part of the lobe could be caused by iso static move ments of the Horczaki Knoll and thus the “slip
-page” of the lobe in the south ern di rec tion. Af ter reach ing the max i mum range, the ice “push side ways”, most likely mainly to wards south or south west, that is in agree ment with sed i -ment de for ma tions at the site at Knyszewicze.
The flex ure of glaciotectonically dis turbed sed i ments of the end mo raine fan at the site Knyszewicze, on the east ern wall in the sur face layer (at a depth of ca. 0.7 m from the sur -face) fol lows a slope in cli na tion, i.e. ca. 10° (Fig. 5). The struc ture of sed i ments rep re sents an ex am ple of bent rock lay ers de vel op ing in pres ence of per ma frost. The flex ure of strata is an ef fect of mass move ments, in this case – solifluction, which most likely took place dur ing the Vistulian Gla ci ation. It ap pears that mass move ments cov ered the en tire ac -tive layer of the long-term per ma frost, i.e. a thick ness of about 0.7 m. This pro cess oc curred dur ing one-sided freez ing when solifluction con sisted of two pro cesses: (1) frost creep that re peated di ur nal frost through out a year and (2) geli-fluction dur ing a spring (Jahn 1970; French, 2007). The solifluction lobes be ing an ef fect of mass move ments dur ing the Vistula Gla ci ation, were re corded at the site of Ja³ówka in north east ern Po land (Woronko et al., 2013; Rychel et al., 2014).
CON CLU SIONS
The de pos its of the mar ginal zone in the Knyszewicze re corded the dy nam ics of the small fron tal ice-sheet lobe, which was iso lated from the com pact ice sheet of the Saale Gla ci ation (MIS 6) at the time of its re treat. The la bile struc -ture of the Qua ter nary sub stra tum, which is man i fested by the Horczaki Knoll, had prob a bly a ma jor im pact on for ma tion of the fron tal ice sheet lobe in the vi cin ity of Knyszewicze and, at the same time, a di rec tion of its ad vance. The mar ginal zone at Knyszewicze was formed dur ing three phases:
1. The first one was as so ci ated with the de vel op ment of the end-mo raine fan, a de po si tion of which oc curred within a con tact zone of the ice sheet ter mi nus. At that time, the ice sheet front was sta tion ary, with only mi nor os cil la tions.
2. The next stage was con nected with a dy namic ad vance of the ice sheet front and dis tur bance in the end-mo raine fan de pos its. Dis tur bances re sulted in de vel op ment of thrust-block push mo raines. The ad vance of the lobe took place at pres ence of per ma frost in the sub stra tum of its ter mi nus and fore land. The per ma frost was re spon si ble for a very high wa ter pres sure in a subglacial zone and ero sion of a deep chan -nel which cuts the dis turbed mo raine de pos its.
3. The last phase is as so ci ated with a with drawal of the lobe front and de vel op ment of a net work of nor mal faults.
Such a sce nario of events re sulted in de vel op ment of a het er o ge neous mar ginal zone of hill-hole pair type, with well-de vel oped ter mi nal mo raines and large, well-de vel oped ter mi nal de pres sion in the hin ter land.
The col lected data did not an swer a ques tion on a type of the ter mi nal lobe. Was it a re sult of surge ad vance the ice sheet, con nected e.g. with a sud den hy dro log i cal change in the ice body? Or was it an ice-stream ter mi nus con nected with iso static ef fect of the tec tonic struc ture near the Hor-czaki Knoll. At this stage of the re search, it is im pos si ble to an swer these ques tions.
Ac knowl edge ments
The re search was funded by the grant of the Na tional Sci ence Cen tre no. UMO-2013/09/B/ST10/02118.
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