Studia Quaternaria, vol. 31, no. 2 (2014): 73–81. DOI: 10.2478/squa-2014-0007 AP PLI CA TION OF GEOELECTRICAL PRO FIL ING IN THE DELIN EA TION OF SHAL LOW PERIGLACIAL STRUCTURES ON THE DROHICZYN PLA TEAU Rados³aw Mieszkowski1, Jan Dzier¿ek1, Dominik Stañczuk2 1 Faculty of Geol ogy, Uni ver sity of War saw, ¯wirki i Wigury 93, 02-089 Warsaw, Po land; e-mails: [email protected]; [email protected] 2 Polish Geolog i cal Insti tute – Na tional Re search Insti tute; Rakowiecka 4, 00-975 Warsaw, Po land; e-mail: [email protected] Ab stract The pa per de scribes the re sults of geoelectrical sounding ap pli ca tion for the delin ea tion of periglacial struc tures in Wierzchuca Nagórna within the Drohiczyn Pla teau (east ern Po land). The method re cords the vari a tion of ap par ent re - sistiv ity of depos its along a se lected hori zontal direc tion. The sur vey uti lizes a distinct geoelectrical contrast between gla cial till and struc tures com posed of sand that have been rec og nized ear lier in the gravel pit wall. The studies have been con ducted in direct hin ter land of the gravel pit. Rect an gular and con cen tric ar rays were ap plied in the survey. The re sults have been pre sented as graphs and re sistance distri bu tion maps. The ob tained geo log i cal im age is not com - plete, al though it shows an ori ented course of the larg est lin ea ments and traces of a net work with a small mesh size (1.5–2 m). The rec og nized pat tern of struc tures points to harsh cli matic con di tions dur ing their for mation. Gen eral ori - en ta tion of the struc tures cor re sponds to the ba sic el e ments of the con tem po rary land scape in the area (slope, val ley axes). Geoelectrical sounding us ing the rectan gu lar array gave better results than in the concen tric array, whereas in- tro duc tion of brine into the system did not en hance the res o lu tion of the read ings. The pre sented re sistance distri bu tion maps with a charac teris tic rhythm of varia tions may serve as a benchmark for recog niz ing periglacial structures in ar- eas with out ex po sures. Key words: east ern Po land, Pleis to cene, periglacial struc tures, palaeo ge ogra phy, geoelectrical pro fil ing Manuscript re ceived 13 May 2014, accepted 5 November 2014 2p PRIN CI PLES OF GEOELECTRICAL K = æ 1 1 1 1 ö PRO FIL ING ç - - + ÷ ç r r r r ÷ Geoelectrical pro fil ing re fers to the re cord of ap par ent è AM BM AN BN ø re sis tiv ity vari a tions of dif fer ent lithologies along a se lected hor i zon tal di rec tion (pro file). The se lected elec trode spac ing where: A, B – cur rent elec trodes, allows to regis ter the magni tudes at certain depths, thus to M, N – po ten tial elec trodes, conclude on the course of geo logi cal struc tures below the rAM; rBM; rAM; rAN; rBM – dis tances be tween the elec trodes; sur face. Di rect magni tudes that are mea sured are: the trans- DV – measured poten tial [mV]; mitted cur rent in the sup ply ing cir cuit, the po ten tial in the I – trans mitted current [mA]. measur ing cir cuit and the dimen sions of the entire measur ing The K co effi cient re mains un changed during the survey, sys tem (Fajklewicz, 1972; Stenzel and Szymanko, 1973). be cause AB and MN dis tances are con stant. The method is Tran si tion from these mag ni tudes to ap par ent re sis tiv ity r is par tic u larly ef fec tive for me dia with highly di verse geoelec- expressed by the follow ing equa tion: trical proper ties (Fig. 1). The main assets of such survey are: its non-inva sive charac ter, low cost, and rela tively high reso - DV lution of the measure ments. Geoelectrical pro fil ing has been r = K I first applied to recog nize the pattern of pseudomorphs after frost wedges for the SW part of the Drohiczyn Pla teau where K is the geomet ri cal fac tor ex pressed as: (Mieszkowski and Porze¿yñski, 2004, 2006). The geo logi cal 74 R. MIESZKOWSKI et al. Fig. 1. Scheme of a non-symmet ric measure ment ar ray. struc ture of the sub-surface ho rizons in the area assured good nu merous fis sures and veins filled with sand and gravel, condi tions to test the method. occa sion ally with thin hori zons of sandy mud. The sand struc tures ter minate in the till or in the sand-gravel ho ri zon GEOMORPHOLOGICAL AND GEO LOG I CAL beneath. Their forma tion is con nected with the ac tiv ity of CHAR AC TER IS TICS OF THE SW PART Pleisto cene frost pro cesses within the per ma frost (Dzier¿ek and Stañczuk, 2006). The contrac tion fissures were pri marily OF THE DROHICZYN PLA TEAU filled with ice. Af ter per mafrost deg ra da tion the ice was re - The south-western part of the Drohiczyn Plateau is placed by sand and gravel, and the structures at tained the bounded by the Bug River Val ley (Podlasie Bug Gorge), form of pseudomorphs after ice wedges. Some fissures could which in this area rap idly changes di rection from WNW to be filled with sand al ready in periglacial con di tions, re sult ing NNW (Fig. 2). The lev elled surface of this part of the plateau in the forma tion of a pri mary filling (Murton et al., 2000; lies at 135–150 m a.s.l. The bot tom of the Bug Val ley is lo- Dzier¿ek and Stañczuk, 2006). cated at 105–110 m a.s.l. The erosional slope of the pla teau is high and steep and is best observed in the Drohiczyn escarp - GEO LOG I CAL RE CORD OF PERIGLACIAL ment. Straight valleys with gentle slopes, running al most per- STRUC TURES IN THE WIERZCHUCA pen dic u larly to the Bug Val ley, are a char ac ter is tic el e ment NAGÓRNA GRAVEL PIT WALL of the landscape. The dom inat ing ori enta tions of the valley axes are close to W–E and NNW–SSE; this fact has been The phe nom enon of the Wierzchuca Nagórna site is the taken into account in the palaeogeographic recon struc tion of presence of at least 51 pseudomorphs after ice wedges in an the area (Dzier¿ek, 2009). over 150 m long gravel pit wall (Fig. 3). High den sity of the The main hori zon in the topmost part of the geolog i cal struc tures (ev ery 1.7 m) may reflect harsh clima tic con di tions suc ces sion in the Drohiczyn Pla teau is gla cial till, one to sev - exist ing during their forma tion (Maizels, 1986; Mackay, eral meters thick, occa sion ally cov ered with a thin hori zon of 1993; Barry and Gan, 2011). The pseudomorphs are gener - sands with gravels. The till derives from the Wartanian Gla- ally about 1 m long, although shorter structures, 0.6-0.8 m ciation (Marks, 2005; Marks and Pavlovskaja, 2004; Nity- long, are also pres ent. The width of the upper parts of the choruk, et al. 2008a, b), at present referred to the Wartanian pseudomorphs varies within 0.2–0.5 m, however, in some Stadial (Lindner and Marks, 2012). Below the till occur grav- cases it reaches even 1 m. Some struc tures at tain a typ ical els and sands that are over 4 m thick (Dzier¿ek and Stañczuk, wedge shape, although nar row veins and wide shafts with 2006). The geo logi cal set ting of the Drohiczyn Plateau is verti cal walls have also been noted (Washburn, 1973; Jahn, well vis i ble in the walls of the gravel pit at Wierzchuca 1977; French, 2007). The record of periglacial frost pro- Nagórna (Fig. 3). Soil is under lain by silty sands with con- cesses is also evi denced by: the ver ti cal distri bu tion of peb- centra tions of gravel. Between 0.3 and 0.5 m lies the top of bles in the upper part of the glacial till, the convex top of the brown till with a thickness of up to 1.5 m. The till contains till between the pseudomorphs, a blurred sed imen tary struc- AP PLI CA TION OF GEOELECTRICAL PRO FIL ING 75 Fig. 2. Lo ca tion of the study area on a geo log i cal map (Nitychoruk, et al. 2008a). Fig. 3. Pseudomorphs after ice wedges in the gravel pit wall at Wierzchuca Nagórna (mod i fied from Dzier¿ek and Stañczuk, 2006). ture of the surface ho rizons and the presence of pebble con - pla teau top. Re con struc tion of its char ac ter, in clud ing: sha- centra tions (frost segre ga tion) close to the pseudomorph top pes of the poly gons, mesh size, network gener a tions and bound aries (Dzier¿ek and Stañczuk 2006).