Subglacial conditions in a branching Saalian esker in north-central Poland Amir Mokhtari Fard a , Beata Gruszka b, ⁎ a Geological and Geotechnical Monitoring Hydro Research, S-183 16 Täby, Box 1608, Sweden b Institute of Geology, Adam Mickiewicz University, Maków Polnych 16, 61-606 Poznań, Poland Abstract The results of a sedimentological study of a branching esker system near Uniszki, north-central Poland, provide a deeper insight into the depositional processes that took place within a subglacial tunnel formed during the Wartanian (= Late Saalian) glaciation. The internal structure of the esker deposits is complex, and coarse-grained debris intercalates with silt and clay layers. Ten main lithofacies types are recognized, including one formed in a tunnel-mouth environment; and three other lithofacies, which must have been deposited in a truly subglacial environment. A “subglacial-tunnel association” is defined. Fluctuations in meltwater discharge through the tunnels resulted in cyclicity in the sedimentary succession. Deposition of fine-grained facies took place during low discharge, whereas boulder and gravel lithofacies formed in the tunnel-mouth during high discharge. Changes in the hydrostatic conditions finally resulted in the collapse of the tunnel roof, so that the succession became covered with supraglacial deposits. The Uniszki deposits record deposition under highly variable sedimentary hydrostatic conditions, which is typical of environments in the Polish Lowlands. The results of the present study provide some criteria for the interpretation of subglacial meltwater debris and thus may contribute to the current discussion on subglacial-tunnel sedimentation. © 2006 Elsevier B.V. All rights reserved. Keywords: Subglacial-tunnel deposits; Esker; Facies analysis; Pleistocene; North Poland 1. Introduction Subglacial drainage systems and their deposits are commonly used to investigate the interaction between flow mechanisms and the shape of subglacial tunnels (Hebrand and Amark, 1989; Russell, 1994; Brennand, 1994, 2000). Sedimentological and morphological studies may unravel the origin of an esker system, which in turn provides information about the regional ice-mass behavior during the deglaciation events. Several models of esker sedimentation exist. Most of them assume that sedimentation takes place either within the tunnel, which is usually considered to be entirely filled with water, or directly at the tunnel mouth. Other conceptions of ‘ose centras’ built by deltaic deposits (De Geer, 1940) or esker beads (Banerjee and McDonald, 1975) are still noteworthy. It is not always realized that meltwater can find its way through an ice mass via ice-walled open channels. Sedimentation in such channels can thus take place not only under subglacial, but also (in earlier stages) under englacial or even supraglacial conditions. At the tunnel mouth, sedimentation can take place subaqueously – as commonly reported (Warren and Ashley, 1994; Mokh- tari Fard et al., 1997; Lundqvist, 1997, 1999; Mokhtari Fard, 2001) – or subaerially (Hebrand and Amark, 1989). In the latter case, the resulting landforms (eskers) Sedimentary Geology 193 (2007) 33 – 46 www.elsevier.com/locate/sedgeo ⁎ Corresponding author. Fax: +48 61 8296074. E-mail address: bgruszka@amu.edu.pl (B. Gruszka). 0037-0738/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2006.03.029