David Mohrig¹, Carlos Pirmez¹, Christine Rossen¹

(1) ExxonMobil Upstream Research Co, Houston, TX

ABSTRACT: Application of Climbing Dune Stratification to Reconstructing the Filling Histories of Deep-Water Turbidite Channels

Some of the thickly bedded turbidites filling paleochannels in the Permian Brushy Canyon Formation of western Texas and in the Carboniferous Ross Formation of western Ireland possess an internal stratification that we interpret as the product of trains of climbing dunes. Sand that had settled out from suspension in these turbidity currents continued to be transported as bedload and was worked into dunes that migrated down-current as the channel base aggraded. We use the stratification produced by these bedforms, together with textural data, to estimate properties of the depositing currents such as duration, average velocity and rates of channel filling. These reconstructions are based on the known mechanics of turbidity currents and of dunes in unidirectional flows. Quantifying aspects of the depositing currents helps us to project the character of associated deposits both up- and down-dip from the studied outcrops, improving our understanding of the channel filling in these deep-water systems.

The deposits described here are composed of fine to very fine sand and can be several meters thick. Dune forms preserved within them are three-dimensional in plan form, with heights and lengths that are typically 5 to 50 centimeters and 50 to 300 centimeters, respectively. Reconstructed values for deposition rate and the duration of depositional events are on the order of 10 cm/hr and 1 day, respectively. Despite their thickness, these beds could constitute as little as 1 percent of the sand that moved over an outcrop location, suggesting comparably thick deposits for significant distances in the down-dip direction.

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