Glacial geology of the Cobb valley, northwest Nelson

Abstract

Investigations of the Cobb valley in northwest Nelson, New Zealand, yield evidence of at least two and possibly as many as four phases of glaciation in the valley and adjacent areas. (1) The oldest of these advances is undated, but is probably mid Quaternary, as it is preserved on old valley floor remnants c. 100 m above the modern valley. Before the next recorded glacial advance, periglacial conditions occurred at least briefly in the valley. (2) The valley was then overrun by a glacier that deposited two basal tills and probably extended into the modern fluvial gorge. The extensive roche moutonnee fields through much of the valley and the basal tills near the Cobb Dam may relate to this advance. (3) The moraine underlying the Cobb Dam is inferred to be an end moraine of Last Glacial Maximum age (LGM). A suite of recessional moraines extends upvalley from the head of the Cobb Reservoir. The second of these has a minimum age of 17 000 radiocarbon years and is consistent with an LGM interpretation for the Cobb Dam moraine. It is possible that the LGM and deglacial sequence outlined in (3) is the retreat phase of advance (2). (4) Deglaciation was rapid, but there is circumstantial evidence for a temporary readvance from some cirques during the deglaciation. There are no known Holocene glacial deposits. © 2001 Taylor & Francis Group, LLC.