Stratigraphic Signals of Shelf-Edge Delta Progradation with Special Reference to Delta-Front Turbidites: Upper Cretaceous Tres Pasos and Dorotea Formations, Magallanes Basin, Chile

Jacob A. Covault, Brian W. Romans, and Stephan A. Graham
Geological and Environmental Sciences, Stanford University, Stanford, CA

This study utilizes superbly-exposed outcrops of the Upper Cretaceous Tres Pasos (delta-fed deep-marine slope) and Dorotea (shallow-marine and deltaic) formations of southern Chile to analyze shelf-edge delta progradation at the resolution of individual beds and packages of beds. Coarse-grained shelf-edge strata at the informally-named Cerro Escondido outcrop are >300 m thick, and include two upward-coarsening successions of turbidites overlain by hummocky cross-stratified beds. The uppermost turbidite-hummocky bed succession is overlain by trough cross-stratified sandstone encased in carbonaceous mudstone interpreted to have been deposited in distributary and interdistributary deltaic environments. The thick section (>50 m) of turbidites between hummocky beds exhibits progressively thinner turbidites and an overall increase in sand richness upwards. Preservation of this section implies relatively abrupt and large-magnitude relative sea-level fluctuations, which prevented extensive reworking of turbidites by the combined storm-wave and unidirectional currents that create hummocky cross stratification. The observed turbidite stacking pattern reflects the increased occurrence of relatively frequent and dilute sediment gravity flows, such as might be initiated by decadal or centennial hyperpycnal fluvial discharge from a delta-channel mouth, and is thus a signal of progradation. This study shows that delta-front turbidite sections are significant components of continental margin-scale prograding shelf-slope systems.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas