Jeff Peakall¹, Mike Leeder², Jim Best¹, Phil Ashworth³

(1) University of Leeds, Leeds, United Kingdom
(2) University of east Anglia, Norwich
(3) Brighton University, Brighton

ABSTRACT: River Response to Lateral Ground Tilting

River channels are affected by lateral ground tilting in a number of tectonic settings, most notably in active half-grabens, and respond to lateral ground tilting by either undergoing avulsion or by progressively migrating downtilt.

A number of alluvial architecture models of half-grabens have incorporated an avulsive response to lateral tilting, but have not included progressive downtilt movement because of A) a lack of a quantitative criterion for differentiating these two processes, and B) conflicting models of gradual lateral migration. A quantitative criterion for differentiating between these two styles of channel movement is proposed, based on the rate of lateral tilting. A new model of progressive lateral migration is also presented that unifies the previously disparate models.

Two recent field tests of the affects of lateral tilting on alluvial deposits have produced diametrically opposed views on the validity of existing architectural models. Results are presented here from stream table experiments subjected to repeated increments of lateral tilting. Aggradation took place throughout each run, and subsequent sectioning of the deposits has allowed the spatial distribution of architectural elements to be examined. The experiments show that the areal extent of coarse-grained material increases towards the area of maximum subsidence. Similarly, both mean and maximum bed lengths increase towards the locus of subsidence. These results are in agreement with existing architecture models and suggest that they correctly model the gross sedimentary responses to lateral tilting, despite the need to incorporate progressive downtilt migration.

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