Abstract: Canterbury Plains (New Zealand) and Sequence Stratigraphy: The Role of Tectonism and Wave Climate--Eustasy is Not So Important

Dale A. Leckie

The Canterbury Plains, New Zealand, bounded by the Southern Alps and Pacific Ocean, are 60 km wide and 185 km long, and traversed by four large gravel rivers. The coastline is wave-dominated and microtidal with high rates of north-directed longshore drift. The southern coast is transgressive with 22 m cliffs in Pleistocene gravel. Beaches are gravel and sand. Coastal erosion is approx. 1 m/yr, causing fluvial incision up to 22 m. There are no estuarine conditions within the downcutting valleys. River headwaters in the Southern Alps are also uplifting, causing incision to take place there. Thus, fluvial incision is taking place in the west due to mountain uplift and in the east due to the transgressive shorelines. This valley incision occurs during sea level highstand. Sequence stratig aphic models would suggest that downcutting should occur during sea level lowstands. Banks Peninsula, a resistant volcanic complex acts a large groin to southerly waves, affecting the northern coast which progrades approx. 1 m/yr. Gravel does not reach the sandy coastline. Erosional terraces are not present in lower river reaches. Thus, this coastline within the same basin during sea level highstand is at one locale progradational and elsewhere transgressive. Gravel is only reaching the transgressive coast where a steep gradient is maintained by downcutting. Dominant controls on sedimentation are the high-wave energy coastline, highly variable but extreme rainfall, rising mountains, and the subsiding basin. Although sea level plays a role, more important controls on progradation, retrogr dation, and valley incision are extreme wave energy and longshore drift.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994