Abstract

Series of long, sinuous and subparallel furrows on bedding planes of rhythmites of the Iararé Subgroup (Early Permian), cropping out in the State of Santa Catarina, Brazil are interpreted as iceberg scours. Troughs are associated with abundant ice-rafted clasts, pellets and mounds of dumped debris melted out from floating icebergs, and lenses of debris probably released from grounded icebergs. Rhythmites were deposited in a sizeable, relatively deep fresh water body, at least in part as varves, during the last episode of deglaciation of the late Paleozoic glaciation. Iceberg scours are common in modern and Pleistocene glaciated shelves, but are not known from the ancient record despite selective preservation of glaciomarine rocks. This paper presents the first detailed description of iceberg scours from pre-Pleistocene glacial strata.

Introduction

Ice scours caused by the interaction of floating ice masses with basin floor sediments are widely reported from the floors of the Pleistocene lakes and are a marked feature of cold climate shallow marine shelves (e.g. Belderson and Wilson, 1973; Harrison and Jolleymore, 1974; Dredge, 1982; Hélie, 1983; Barnes et al., 1984; Thomas and Connell, 1985; Wood worth-Lynas et al., 1985; Eyles and Clark, 1988).

There is much microgeomorphological data regarding iceberg scours available from seismic and sonar surveys, and submarine diving observations (Barnes et al., 1973; Woodworth-Lynas et al., 1985), but little structural data to aid interpretation of these structures in the pre-Pleistocene strata. With exception of the brief mentions of Rattigan (1967) and von Brunn (1977), no fossil ice scour structures have been identified from the Earth's pre-Pleistocene glacial record despite the widespread, selective preservation of glaciomarine rocks.