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ABSTRACT: Slump axial trends and palaeoslope orientation within the submarine fan environment: the value of image logs

Thompson, Jill and Christian Ottessen , Baker Atlas, Perth, Australia

Image logs provide an invaluable means of identifying and measuring deformational structures developed within the submarine fan environment. Generated by the coherent, down-slope mass-wasting of sediment, a well-developed slump structure will be characterised by high angle dip patterns (mostly >30 degrees) and local bipolar dip azimuths indicative of slump fold closures. These features can be qualitatively observed within dip tadpole plots, and quantitatively evaluated using interactive stereogram interpretation. Using a Wulff projection stereogram, genetically-related slump surfaces will plot on a great circle (with some natural scatter) and the pole (or axis) to that great circle will define the overall trend of the slump structure.

As with fold axis analysis in structural geology, transport direction will normally be perpendicular to the axial trend. In the case of unstable slopes prone to slumping, this would mean that the mean axial trend is parallel to the mean strike of the slope, hence providing a means of palaeoslope analysis within successions often devoid of palaeocurrent indicators.

More thorough analysis shows that slumps not only occur on a variety of scales, but also within a broad range of depositional settings and host lithologies. Distinguishing between regional palaeoslopes and those associated with more local slopes, in particular those associated with channel-fills (e.g. channel bank caving or levee failure) may be important for deciphering apparently chaotic axial trend orientations. It may also impact upon the evaluation of sandbody geometry and internal heterogeneity.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia