AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
The Slope Rise and Run: A Critical Factor in Explaining and Predicting the Development, Distribution, and Scale of Basin-Floor Fans
(1) BP, Houston, TX.
Few terms have been used for such a wide variety of different physiographic features in the geologic literature then the terms shelf break and slope break. In order to provide maximum utility, as well as clarity, the shelf break is simply defined as the inflection point between the topset (shelf) and foreset (slope) portion of a depositional sequence boundary. Similarly, the slope break is defined as the inflection point between the foreset (slope) and bottomset (basin floor) portion of a depositional sequence boundary. In this context, the shelf break marks the down dip limit of sub-aerial erosion produced by the loss of accommodation during relative falls in sea level. The slope rise is the vertical distance between the shelf break and the slope break, while slope run is the horizontal distance. In the datasets studied to date, it appears that the slope rise and run directly control the development as well as distribution of basin-floor fans in the geologic record. Furthermore, slope run may provide insights into the scale of basin floor fans.
Shelf breaks may occur inboard of the continental margin (Epicontinental Shelf Breaks) or coincident with the continental margin (Continental Shelf Breaks). Epicontinental Shelf Breaks have slopes with short runs where the distance between the coeval shelf and slope breaks are just kilometers apart. In these settings, basin-floor fans can develop on the floors of epicontinental seaways when the slope rise exceeds a critical limit of approximately 150m (500’). When developed, however, these basin-floor fans are located just kilometers (miles) from the coeval shelf break due to the short run of the slope. Furthermore, the footprint of basin-floor fans developed outboard of slopes with short runs is relatively small. This appears to be due to limited scale of the fluvial drainage networks which commonly develop within epicontinental seaways. In contrast, continental shelf breaks have long slope runs and robust slope rises. Downdip of the major fluvial drainage systems which rim the continental margins,“large” basin-floor fans commonly develop tens of kilometers (miles) outboard of the coeval continental margins on the ocean floor. Furthermore, the long slope runs in these settings provide a suitable pallet for a robust portfolio of deepwater plays (levee channel, confined channel, and slope fans) to develop on the continental slope.