--> Froude Supercritical Flow Bedforms in Deepwater Slope Channels, Eocene Forearc Basin, California: Recognition Criteria and Field Examples in Conglomerates, Sandstones and Fine-Grained Deposits, Their Morphological Changes of Bedforms From Shelf-Edge Channels to Middle Slope Channels

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Froude Supercritical Flow Bedforms in Deepwater Slope Channels, Eocene Forearc Basin, California: Recognition Criteria and Field Examples in Conglomerates, Sandstones and Fine-Grained Deposits, Their Morphological Changes of Bedforms From Shelf-Edge Channels to Middle Slope Channels

Abstract

There is a growing body of evidence for Froude supercritical flow bedforms from modern steep deltaic slopes to deepwater continental slopes. Morphodynamics of supercritical flow bedforms and their deposits are well established by experiments (e.g., Alexander et al., 2001; Spinewine et al., 2009; Cartigny et al., 2014). Comparison of the experimental results to field data from deepwater slopes is, however, still relatively rare. Their recognition in the field is further complicated by the various scales of supercritical flow bedforms, where multi-meter to tens of meters thick bedforms are built by smaller-scale bedforms on centimeter to a meter scale. Furthermore, large-scale supercritical flow bedforms are characteristically of 10's to 100 meter long wavelength and thus the complete bedforms are hard to see unless the outcrop scale is large. These bedforms have commonly erosional set boundaries, as well as contain internal discordances, and are therefore easily confused with channels. Similarly, long-wavelength low-angle lenticular bedforms can be misinterpreted as individual lobe elements. This paper aims to describe supercritical flow sedimentary structures from ancient active margin deepwater continental slopes, and discuss their morphodynamics based on comparison to the recent experimental results. The outcrop examples are from the Juncal Formation and the La Jolla Group in the Eocene forearc basin, and the Capistrano Formation in the Miocene-Pliocene strike-slip basin, Southern California. The examples of supercritical flow bedforms range from centimeters to hundreds of meters scale, and occur in conglomeratic, sandy as well as heterolithic fine-grained deposits. The paper exemplifies differences between the erosionally bound large-scale bedforms and their host channels that are an order of magnitude larger; and documents how the bedforms in the channels change from the shelf edge to the confined and weakly confined middle slope settings. The dominance of supercritical flow bedforms in the studied datasets suggests that active margin continental slopes are likely to experience frequent supercritical flow due to steep gradients and high sediment supply. The latter is also significant for preservation of the deposits. The occurrence of sandstones in channels is strongly controlled by supercritical flow processes, which potentially have a critical impact on the slope channel reservoir quality.