Art D. Donovan1
(1) Exxon Ventures, Houston, TX
Abstract: Topographic-keyed Sequence Models: Explaining and Predicting the Development of Basin-floor (Lowstand) Fans within Sequences
Depositional sequences display depositional topography along sequence boundaries (erosional), within sequences (constructional), and as relict physiographic relief. Analysis of sequences deposited in a variety of tectonic settings indicates that lowstand fan development is directly related to depositional relief of the underlying sequence boundary.
In the datasets studied, three distinct types of depositional sequences were identified. Low-relief sequences have sequence boundaries with slopes of <1 degree and relief of < 50 meters. As might be expected, these sequences lack lowstand fans. Moderate-relief sequences have sequence boundaries with slopes of .5 to 3 degrees and relief of 50-150 meters. Although these sequences display distinct clinoform development, they lack lowstand fans. High-relief sequences have sequence boundaries with slopes of 1-3 degrees and relief 180 meters. These sequences contain lowstand fans.
These relationships indicate that there is a Critical Depositional Shelf Break that controls slope stability or failure during relative sea-level falls. In basins where depositional relief is less than the Critical Shelf Break, progradation continues during relative sea-level falls. Resulting low- to moderate- relief sequences lack lowstand fans. In basins where depositional relief is greater than the Critical Shelf Break, slumping, canyon development, fluvial capture, sediment by-pass, and fan development occurs during relative sea-level falls. In the datasets studied, the threshold for the Critical Shelf Break and the development of high-relief sequences is erosional depositional relief of 150-180 meters. Topographic-keyed sequence models provide explorationists with a powerful tool to explain and predict fan development in a wide variety of depositional basins.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana