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Supercritical Flow Signatures and Erosional Features in Juncal Formation Basin Floor-Fans, California, US


A drone datasets from the Juncal and Point Loma Formation basin floor fans in California suggests a large degree of channelization even in the outer fan. The Juncal formation dataset further shows a significant deposition from Froude supercritical flow in the outer fan. Outer fan channels and a lower volumetric proportion of lobes has been suggested from some modern and recent systems. For example, the Navy and the Congo fan datasets suggest that submarine fans consist of a multitude of channels and related smaller heterolithic lobes, with sands dominantly deposited in channels, even on outer fan. Sedimentary structures originating from supercritical flow conditions have been identified in other ancient and modern submarine fans. Therefore, these findings raise questions about our understanding of basin floor fan deposits. The objective of this study is to test the hypothesis that a high degree of channelization and supercritical flow deposits are a common feature even in more distal parts of the basin floor fans. We further aim to document the relationship between channels and lobes in basin floor fans.

Drone photography survey of the Middle Eocene Juncal Formation in in Alyson Canyon in Los Padres National Forest, Santa Barbara, Ojai and in Ojala, California, was motivated by a previous study of approximately coeval slope succession in the Frazier Park area that identified a high proportion of supercritical flow deoposits in the slope section. Previous studies interpreted the Alyson Canyon area as proximal depositional lobe complexes in a basin-floor fan (Thompson, 1987), and the Ojala area along Highway 33 as outer-fan setting of depositional lobes (Obligado, 2003). In the Alyson Canyon area the facies are sandstone dominated and the sandstone facies occur as amalgamated, erosionally bound lenticular deposits. The Ojai outcrops are also sandstone prone but the small size of the outcrop does not allow to observe larger bounding erosion surfaces. In the Ojala area a proportion of heterolithic facies is larger and some heterolithic facies have a tabular architecture suggesting deposition in sheets or lobes, whereas sandstone-prone facies occur as erosionally bound lenticular deposits. Comparison to Froude supercritical flow experimentally produced data suggest that Froude supercritical flow deposits are common in the Juncal Formation basin floor fans. The erosion surfaces and the association of sandstone prone facies with the erosionally bound deposits suggest that the proportion of channelized deposits is high, and present even in outer-fan setting. Drone photography survey of the Point Loma Formation in the Sunset Cliffs confirm the latter observations and further documents considerable lithological variability in the outer fan succession, with mudstone-prone and thinly bedded facies occur as interbedded repeated packages that culminate with lenticular sandstones. This stratigraphic architecture suggest presence of relatively small heteroliyhic lobes and sand-prone channels.