Sediment supply and climate change: implications for basin stratigraphy

The rate of sediment supply from erosional catchment to depositional basin depends primarily upon climate, relief, catchment slope and lithology. It varies in both time and space. Spatial changes in erosion rates due to variations in lithology are illustrated by contrasting rates of drainage divide migration away from faults of known ages. Time variations in relative sediment supply are extremely complex and vary widely according to the direction and magnitude of climate change. In many parts of the Great Basin and south-western USA, glacial maximum climates were characterized by higher effective moisture and the altitudinal downward spread of woods and forests. Sparse data from alluvial fans indicate reduced sediment supply, despite the increased runoff evident from higher lake levels. The situation in Mediterranean areas is less clear, with rival climatic scenarios for vegetation ecotypes predicting contrasting runoff. In order to test these latter we run Cumulative Seasonal Erosion Potential [CSEP] experiments for present-day and a variety of full-glacial Mediterranean candidate climates. The results indicate the likelihood of enhanced sediment supply and runoff compared to the present day during full-glacial times for a cool wet winter climate and a reduction in sediment supply and runoff for a full-glacial cool dry winter climate. We then explore the consequences of such phase differences in sediment supply, and sea and lake levels for the stratigraphy of sedimentary basins. Highstands and lowstands of sea or lake may be accompanied by greater or lesser sediment and water supply, as determined by the regional climate and the direction of climatic change. Thus marine lowstands are not necessarily periods of great transfer of coarse clastic sediments to shelves and deep water basinal environments. Unsteady sediment supply has greatest implications for alluvial systems, in particular the effect that changing relative supplies of water and sediment have upon river and fan channel incision. basin-bordering catchments, minus any sediment stored INTRODUCTION or otherwise exported, e.g. by wind. It is determined, in complex ways, by catchment geology, relief, slope and The rate of sediment supply exerts major controls on a basin’s stratigraphic record (Galloway, 1989; Schlager, by the role of climate in determining vegetation and runoff. 1993). In the past few years geologists have tended to concentrate on exploring tectonic controls of relief and Here we: $ outline major spatial and time controls on sediment slope upon sediment supply. At the same time the occurrence of climate change during Quaternary times production rates; $ review data on the variation of sediment supply and has been well established, together with increasing appreciation of its effects on the sedimentary record erosion rates with changing semiarid climate at different Quaternary time-scales; during earlier geological epochs (e.g. Perlmutter & Mathews, 1989). In particular, with the advent of $ present results of modelling experiments on predicted sediment supply and runoff for full glacial and sequence stratigraphic concepts, it is a matter of great importance to explore whether ups and downs of sea or Mediterranean climates; $ indicate the likely stratigraphic consequences of lake level are accompanied by more or less sediment supply from continental catchments, i.e. do sea level and variations in sediment supply and their implications for basin modelling; sediment supply change in-phase or out-of-phase? The rate of sediment supply to a sedimentary basin depends $ establish the role of sediment supply on the evolution of axial rivers upon the rate of surface soil and rock erosion in the © 1998 Blackwell Science Ltd 7