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Abstract: Continental Erosion and Silici-Clastic Sequence Stratigraphy

Lucazeau, Francis, Chantal Rovri, Isteem

The three major geological variables that determine sequence organisation in sedimentary basins (sediment flux, base level fluctuations and tectonic subsidence) are not totally independent: continental denudation, and consequently sediment flux reaching the ocean, is largely determined by sea-level changes.

This coupling can be investigated conceptually by numerical simulations, but requires a realistic model of continental erosion at geological scales. We present a simple 1-D model that is compatible with both small scale hillslope diffusion processes (sediment flux proportional to local slope) and large scale observations relating average denudation to average elevation. The link between these two scales results from the interaction between hillslope processes and rivers transport that have huge time scale ratio: rivers adjust so rapidly to the base-level that they determine lateral slopes and hence sediment influx. This is simply accounted for in the 1-D model, by calculating the topographic profiles of both river and divide along the main drainage direction, when vertical displacemen caused by active tectonics exist. The elevation difference between ridges and valleys, and the ridge to ridge distance determine lateral slope, and the lateral influx of sediment is proportional to this slope. This assumption can be verified for natural drainage basins (Alpes, Pyrenees, Andes, Himalaya): the order of magnitude for sediment yield determined by this difference is that measured directly in river networks.

The continental erosion/depositional model is combined with a marine deposition model when the base-level is reached. This includes the rapid fluvial energy drop, and possibly some dispersion by wave action or tide currents. The power of the model is therefore to investigate how the stratigraphic record is related to the tectonic/erosion activity inland, how the variations of sea-level affect continental erosion or the partitioning between continental and marine deposits. Lateral variations of facies may be related to transport processes (grain size reduction) and to the tectonic activity (source material) inland. Different geological scenari with various uplift rates, continental hypsometries, tectonic subsidence, eustatic variations, ...are presented in order to illustrate the impor ance of the interactions.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France