Dale A. Leckie¹

(1) Nexen Inc, P.O. Box 2727, AB

ABSTRACT: Climatic Controls on Nonmarine Sedimentation - Maximum Aggradation During Lowstand; Incision During Transgression and Highstand

Sedimention and erosion on the Canterbury Plains, New Zealand do not respond to eustatic sea level fluctuations as generally predicted because of extremes of sediment supply, wave conditions, tectonic uplift and precipitation. Inland, on the western Canterbury Plains, maximum nonmarine sediment aggradation and progradation occurred during the peak of glaciation and lowstand. However, rather than incision and sediment bypass, large alluvial megafans were constructed. Incision of the megafans occurred during the subsequent sea-level rise and highstand, due to climate changes. Sea level had a minimal role except downstream. Deposition of the Canterbury gravels on the inner plains is attributed to sedimentation during glacial episodes, with subsequent incision and terracing being a deglacial (rising eustatic sea level) and interglacial (eustatic highstand) phenomena.

The major points are: 1)during glacial maxima, when sea level was lowest - nonmarine sediment aggradation and progradation was at its maxima. Alluvial megafans constructed at this time are thousands of square kilometres in area. 2)nonmarine valley incision, which took place during the sea level rise, was the result of climatic amelioration, landscape stabilization by vegetation and reduction of sediment supply. A zone of sediment bypass was created in the incised valley, when sea level was rising. 3)climate was the dominant control on sediment deposition and erosion. Climate affected vegetation, thus erosion; it also affected precipitation thus erosion, transport and deposition. 4) Sea level/base level during all this was "a long way away - at the shoreline" with minimal effect on the nonmarine sediment record.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado