Quantification of the Bed-scale Architecture of Submarine Depositional Environments and Application to Lobe Deposits of the Point Loma Formation, California

Rosemarie Fryer, Zane Richards Jobe

As of August 2018, this work is in review in "The Depositional Record", a journal from the IAS (International Association of Sedimentologists) Submarine-fan deposits form the largest sediment accumulations on Earth and host significant reservoirs for hydrocarbons. While many studies of ancient fan deposits qualitatively describe lateral architectural variability (e.g., axis-to-fringe, proximal-to-distal), these relationships are rarely quantified. In order to enable comparison of key relationships that control the lateral architecture of submarine depositional environments, we digitized published bed-scale outcrop correlation panels from five different environments (channel, levee, lobe, channel-lobe-transition-zone, basin plain). Measured architectural parameters (bed thickness, bed thinning rates, lateral correlation distance, net-to-gross) provide a quantitative framework to compare lithology architectures between environments. The results show that sandstone and/or mudstone bed thickness alone or net-to-gross do not reliably differentiate between environments. Lobe sub-environments display the most variability in all parameters, which could be partially caused by subjectivity of qualitative interpretations of environment and demonstrates the need for more quantitative studies of bed-scale heterogeneity. These results can be used to constrain forward stratigraphic models and reservoir models of submarine depositional environments.
This work is paired with a case study to refine the depositional environment of submarine lobe strata of the Upper Cretaceous Point Loma Formation at Cabrillo National Monument near San Diego, California. The strike-oriented, laterally-extensive exposure offers a rare opportunity to observe bed-scale architecture in turbidites over 1 km lateral distance. Thinning rates and bed thicknesses are not statistically different between lobe elements. This signifies that the lateral exposure is necessary to distinguish lobe elements and it would be extremely difficult to accurately interpret elements in the subsurface using 1D data (e.g., core). The grain size, mudstone to sandstone bed thicknesses, and element/bed compensation observed in the Cabrillo National Monument exposures of the Point Loma Formation are most similar to values of semiconfined lobe deposits; hence, we reinterpret that these exposures occupy a more medial position, perhaps with some degree of confinement.