Abstract: Canyon Evolution in Active Continental Margins: Potential Sea Level Change Control

Xinhua Deng

Two typical but structurally opposite forearc basins were selected to analyze the potential sea level change control on canyon evolution: the Middle American Trench off Guatemala (extensional-convergent), and Nankai Trough (compressional-convergent). Because of structural complexity and the varied shelf-slope geometry, considerably varied sizes of mature canyons, young canyon-gullies and chutes have developed in the active margins. DSDP data and piston core analysis show that slumping and mass flow sediments of the Pleistocene exist in gully floors, and that slopes were flooded by turbidity currents during the global sea level lowstand (LST) in the Pleistocene. The Holocene hemipelagic and pelagic sediments overlay the sandy materials in those gullies, and also cover some mature canyo s which lost their shelf-derived sources during the highstand period (HST). It appears that mass wasting due to high sedimentation rates and steep slopes in the LST caused initial scars or chutes which could enlarge as gullies later with headward erosion, and that the development of the gullies which had not broken the shelfbreak would abort with the subsequent Holocene sea level rises. The sedimentation rates and turbidite occurrence in the mature canyons have decreased even though they are still active in the HST.

It seems that sea level changes correspond well with gully and some canyon developments in the active extensional-convergent margins (with broad slopes and shelfs). Nevertheless, eustasy effect on canyon development was totally masked by tectonic activities in the opposite end member -- Nankai Trench.

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