Spatial Variability in Facies and Mineralogy in Deep-Water Lobes of the Point Loma Formation, San Diego, CA

Stammer, Jane and Pyles, David
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The Cretaceous Point Loma Formation, San Diego, California contains laterally extensive, world-class outcrops of deep-water lobe strata. Quantitative analysis along an axis-to-margin transect of the basal bed of a lobe element and the entire lobe element documents discrete and non-linear axis-to-margin changes in facies, bed thickness, grain size, and mineralogy.

Axis-to-margin changes in bed thickness, grain size, and facies are evident. In the axial position of the lobe, the basal bed is 1 m thick and consists of medium-grained, structureless sandstone. It maintains a similar character for approximately 1.0 km, and then decreases abruptly to 0.5 m in thickness and consists of fine-grained argillaceous sandstone, organic matter, argillaceous sandstone with abundant shale clasts, and organic-rich, sandy argillite. The bed thins to 20 cm in thickness at its margin, and consists of organic-rich, sandy argillite and rippled siltstone.

Mineral proportions within the basal bed change along the axis-to-margin profile. The percentages of K-feldspar, plagioclase, biotite, and organic material relative to quartz increase toward the lateral margin, while the percentage of hornblende relative to quartz decreases toward the lateral margin. Additionally, minerals of different sizes and shapes are juxtaposed along this transect. This variability is interpreted to reflect hydrodynamic fractionation of grains based on size, mineral density and grain shape.

The lobe element follows a similar pattern to the basal bed. The element consists of 5 m of structureless, medium-grained sandstone in the axis. In the off-axis location, individual beds begin to de-amalgamate, and facies consist of finer-grained argillaceous sandstones with flame structures and abundant shale clasts, and organic-rich, sandy argillite. At the margin, the lobe element is only 0.5 m thick, and facies are dominated by organic-rich argillites and mud-rich, co-genetic debrite-turbidites.

AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013