The internal sediment architecture of a drumlin, Port Byron, New York State, USA

Abstract

An exposure within the central portion of a large drumlin at Port Byron, New York State, USA, part of the large New York drumlin field, reveals a sequence of steeply dipping cemented sands and gravels of proglacial, ice-contact deltaic origin overlain by a thin till veneer. The sands and gravels appear to have been deposited within the proximal proglacial environment during a late retreat phase of the Laurentide Ice Sheet sometime prior to being overridden by subsequent ice and drumlinized. During deposition of the ice-contact delta, escaping subglacial regelation-meltwater permeated the proximal deltaic sediment pile and calcium carbonate was released, in a series of pulses, to form pore-occluding calcite cement within the sand and gravel porespaces. The calcium carbonate precipitated into the sands and gravels due to a reduction in hydrostatic pressure and CO₂ outgassing of the meltwater as it exited from beneath the ice sheet. Once cemented, these deltaic sediments were considerably stronger and acted afterward as an obstacle around which the future ice advance streamed and, in turn, produced the characteristic drumlin shape. In overriding the ice-contact deltaic sediments, the ice sheet emplaced a thin layer of till which exhibits syndepositional deformation features indicative of being emplaced as a deforming bed layer beneath the advancing ice sheet. Micromorphological analysis of the overlying till shows that no interstitial or intraclastic calcite occurs within the till.

Introduction

An exposure on the western side wall of a drumlin within the village of Port Byron, New York State, USA, reveals a section in places over 35 m high and more than 100 m in length (Fig. 1a–c). Such deep, laterally extensive stratigraphic sections within drumlins are relatively rare and, typically, ephemeral (Fig. 1c). This drumlin is part of the large western New York State drumlin field that stretches from Lake Ontario south to the Valley Heads Moraine (Fairchild, 1929; Slater, 1929; Miller, 1972; Calkin and Muller, 1992; Stahman, 1992). Within this drumlin field bands of calcite cemented, glaciofluvial deposits, mapped as kame deposits, are found transverse to the alignment of the drumlins (Fig. 1b) (Cadwell et al., 1991). The presence of many transverse, steeply dipping thick sand and gravel units in the Port Byron to Weedsport to Wolcott district has been interpreted as indicative of multiple ice frontal stillstands with small prograding deltas in the manner of the Salpausselkä moraines in southern Finland (Fyfe, 1990; Fleisher, 2003). The evidence would therefore suggest that in the final stages of the Late Wisconsinan, 17–13 ka BP, Laurentide Ice traversed this area of New York, first, in a series of slow retreats and stillstands in which transverse marginal deltaic and other glaciofluvial forms were deposited. This was followed by, at least, one advance in which many proglacial deltaic and other outwash ice-contact sediments were streamlined into drumlinoidal forms similar to those described by Krüger, 1987, Krüger, 1994 in front of Myrdalsjökull, Iceland. In many places these steep sided ‘kames’ appear exposed as highly cemented transverse ridges separate from the drumlins. It is unclear as to how many other kames may have been drumlinized since many kames have been worked as aggregate sources and surface cover totally removed. The site at Port Byron, therefore, may be a relatively rare occurrence since it permits investigation into a specific style of drumlin formation.

Over the past 150 years drumlins have been the subject of intense scrutiny and discussion as to their origin, and possible development (cf. Boulton, 1987; Menzies and Shilts 2002, p. 230–2). Within New York State drumlins, the origin and analyses of their styles of formation have been discussed almost continuously since the initial examinations by Martin (1901) and Fairchild, 1907, Fairchild, 1929 and later workers (Admiraal, 1970; Miller, 1972; Calkin and Muller, 1992; Stahman, 1992; Dreger, 1994; Robertson, 1994; Hart, 1997; Menzies et al., 1997).

It appears that till drumlins may form in a dominantly depositional mode where intense soft sediment deformation leads to ‘sticky’ points developing at the ice–bed interface (Boulton, 1987; Menzies, 1989, Hart et al., 1999; Boulton et al., 2001; Knight, 2002; van der Meer et al., 2003; Piotrowski et al., 2004). These ‘sticky’ points subsequently act as obstacles around which further deforming sediment may be plastered. It is now fairly well established that as basal sediment deforms, at the ice–bed interface, a complex set of rheological conditions are necessary for deformation to occur. Since conditions at the ice–bed interface are in continuous variance (cf. Menzies, 1989; Marshall et al., 1996; Boulton et al., 2001; Clarke, 2005) there are times and places at the bed when immobilization occurs (cf. van der Meer et al., 2003; Piotrowski et al., 2004) resulting in ‘sticky’ points developing. Other ‘depositional’ models of drumlin formation invoke various changes in ice basal velocity, basal sediment shear strength (cf. Menzies and Shilts, 2002) but a deforming origin has become the most likely mechanism for drumlin formation.

In contrast to a depositional style of formation, some drumlins may be wholly or in part formed due to basal ice or basal meltwater erosion acting upon pre-existing sediment or boulder piles or bedrock knobs (Menzies, 1984; Krüger, 1987; Benn and Evans, 1998; Sawyer, 2000; Menzies and Shilts, 2002; Kjær et al., 2003; Iverson et al., 2005). Similarly, Shaw and Sharpe (1987) postulated that drumlins formed by direct meltwater erosion of sediment within subglacial cavities.

In most of the above hypotheses it has always been generally accepted that where a pre-existing obstacle such as a bedrock knob or resistant ‘plug’ of sediment existed at the ice–bed interface then a drumlin, as a pseudo-crag and tail, might form under streaming active ice. Such formation could result from either additional deposition around the pre-existing ‘obstacle’ or by erosion of already deposited or emplaced sediment around the ‘obstacle’ (cf. Menzies, 1984, Menzies, 1987; Menzies and Shilts, 2002).

The drumlin at Port Byron (Fig. 1b) lies approximately in the center of the western New York State drumlin field (cf. Stahman, 1992) and is 740 m in length, over 40 m in height at its highest point, and is 520 m at its widest. The large exposure, cut for aggregate extraction, lies on the western flank of the drumlin and provides a rare opportunity to study its internal sediment architecture. The drumlins in New York State were most likely formed over several periods of ice sheet activity and typically exhibit a variety of shapes and forms (cf. Miller, 1972; Ridky and Bindschadler, 1990; Stahman, 1992; Hart, 1997; Menzies et al., 1997). Based upon previous studies, this vast complex field is a mosaic of differing periods and styles of drumlin formation.