AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Stratigraphic Architecture and Hierarchy of Lobes in Distributive Deepwater Settings: An Outcrop Study of the Point Loma Formation, California
(1) Chevron Center of Research Excellence Department of Geology and Geological Engineering, Colorado School of Mines, Golden, CO.
(2) Clastic Stratigraphy R&D, Chevron ETC, Houston, TX.
(3) Geology and Geological Engineering, Colorado School of Mines, Golden, CO.
Deepwater, distributary channel-lobe systems are common on all of Earth’s continental margins. They contain large amounts of sand and are excellent reservoirs for oil and gas. Previous studies document characteristics of lobes such as their fan-shaped plan form and cross sectional shape which is thickest and sandiest in the axis and thins and become finer grained toward the lateral and distal margins. This study uses outcrops of the Cretaceous Point Loma Formation, San Diego, California to build upon these earlier findings.
An analysis of these distributary lobes deposits, allows this deepwater system to be subdivided into its hierarchical components (lobe element, lobe complex, and lobe system). Lobe elements contain predictable vertical and lateral associations of beds and bed sets that systematically stack to build a body that has a flat to weakly erosional lower bounding surface and a convex upward upper bounding surface. They have thick sandy axes and strata thin and become finer grained toward the lateral and distal margins. The boundaries between vertically adjacent lobe elements are characterized by abrupt changes in: 1) the location of the axis, 2) facies associations, 3) bedding style, and 4) paleocurrent direction. Lobe complexes are composed of multiple genetically related lobe elements which exhibit a similar depositional trend. The boundaries between vertically adjacent lobe complexes have the same characteristics as those described above but reflect a larger scale shifting of the depositional system. The >6 km long outcrop belt contains 27 lobe elements, organized into four lobe complexes which collectively build one lobe system.
Several characteristics systematically decrease along axis-to-margin and proximal-to-distal transects at all hierarchical levels. They are: 1) ratio of amalgamated sandstone, 2) grain size, 3) net:gross, 4) ratio of erosion to unit thickness, and 5) facies diversity. Genetically related lobes stack compensationally to build lobe complexes. The amount of lateral offset between vertically adjacent lobe elements decrease within each complex reflecting decreased compensation through time. Associated with this pattern is increased grain-size, amount of erosion, and element thickness through time. These data are interpreted to reflect progradation of lobe elements within each complex. This progradation is also reflected in the vertical stacking of lobe complexes within the lobe system, which stack basinward through time.