Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Integrating Outcrop Sections and Research Boreholes to Understand the Internal Organisation of Submarine Lobe Deposits
(1) University of Liverpool, Liverpool, United Kingdom.
The internal organisation and the lateral distribution of submarine lobe deposits is here documented by using research boreholes and outcrop observations from Fan 3 (a lobe complex of the Tanqua depocentre), and Unit A (a composite sequence set of the Laingsburg depocentre) of the Karoo Basin, South Africa. A hierarchy of depositional elements ranges in thickness from a few centimetres to more than 350 meters and include bed, lobe element, lobe, lobe complex, lowstand systems tract, sequence, composite sequence and composite sequence set. A 550 m long behind-outcrop core allows precise characterisation of fine-grained units that overlie individual sand-prone elements, which is essential to interpret depositional elements. Through integration of the research borehole and the surrounding outcrops, lobe complexes can be correlated over 15 km in a down-dip direction and individual lobes can be correlated for several km. Contrary to several published models, thickening-upward is only one of several stacking patterns in lobes. Internal stacking patterns can change laterally from thickening-upward (apparent progradation), through thinning-upward (apparent retrogradation) and no vertical trend (aggradation), to thickening-then- thinning-upwards (progradational-to-retrogradational).
Mapping of individual lobes also facilitates analysis of the spatial and temporal distribution of sedimentary facies. Four environments of deposition are defined at lobe scale: axis, off-axis, fringe, and distal fringe. Each environment is characterised by a sedimentary facies association, thickness range, and position within the mapped element. They show a finger-like distribution within the lobe body and toward the pinchout, illustrating that several turbidity current flow ‘pathways’ existed during the evolution of a single lobe. Away from the main flow pathways (axes), lobe deposits tend to thin and preserve more bed stratification, and include a greater proportion of siltstone and hybrid beds.
The integration of core and outcrop from the Karoo merits close attention as an outcrop analogue to systems of a similar scale and character (including reservoirs in the Paleogene Wilcox Group, Gulf of Mexico).