Evolution of High-Relief Slope Clinoforms: Comparing Cretaceous Strata of the North Slope, Alaska and the Magallanes Basin, Southern Chile

Slope profile and shelf-edge trajectory analysis of high-relief (>500 m) slope clinoforms provides insight into the stratigraphic evolution and growth of basin margins. In many instances, over-steepened slopes within an overall graded system are associated with increased delivery of coarse-grained material to the toe-of-slope and basin-floor. The objective of this study is to outline two slope systems of similar relief and clinoform evolution that developed over-steepened slopes and corresponding submarine fan systems: 1) subsurface strata of the Cretaceous Colville Basin, North Slope, Alaska; and 2) outcropping Cretaceous-Paleogene units of the Magallanes Basin, Chile.

Both the Colville and Magallanes foreland basins were associated with high rates of subsidence and sediment supply, providing suitable conditions for basin axial, high-relief clinoform propagation. Colville Basin strata, ~3500 m thick, is examined within a 1500 km² 3D seismic volume. Slope lengths are 7-20 km with heights (relief) of 400-700 m yielding slope angles of 2.1-4.2°. The Magallanes Basin outcrop analogue is characterized by exposure of ~3500 m of strata along a 100 km² depositional-dip oriented outcrop belt. Slope lengths are 30-40 km with 850-1000 m relief (slope angles 1.3-1.8°).

Similarly, these graded shelf-margin systems have an initial phase of rising to flat shelf-edge trajectories (~600 m relief clinoforms) associated with poorly developed submarine fan systems. A major phase of retrogradation followed, likely associated with substantial basin-scale subsidence, and subsequent progradation of relatively lower relief clinoforms (~400 m) across the drowned, relict shelf. Upon reaching the relict shelf-edge, over-steepened high-relief slopes developed, initiating slope readjustment characterized by abundant mass-wasting and widespread shelf-edge incision, as well as accumulation of thick Deepwater sands. Eventually, the over-steepened slopes reached equilibrium and the graded, high-relief clinoform systems resumed progradation.

Comparison of these two datasets promotes insight into basin margin evolution, benefitting from the large-scale geometry and imagery afforded by the high-quality 3D seismic volume and the detailed sedimentology and stratigraphic context of the Magallanes Basin outcrops. The transition from an equilibrium system to an over-steepened interval during slope evolution imparts a strong control on sediment delivery to the deep-sea.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California