Hierarchical Organization and Spatial Variations of Lobes in Distributive Submarine Fans: Outcrop Study of the Point Loma Formation, California, USA

Abstract

This article uses measurements from outcrops of the Point Loma Formation to define the hierarchical organization of a distributive submarine fan and spatial changes in its constituent parts. A four-tier hierarchy for lobes is documented: bed, element, complex, and system. Across each hierarchical tier, there is an increase in size, duration of deposition, number of cross-cutting relationships, number of superposed stratal units, and degree of compensational stacking. Lobe elements contain systematic axis-to-margin and longitudinal decreases in amalgamation ratio, erosion, net sand content, proportions of sand-rich facies, and maximum grain size, with the exception of shale clasts in fringe position. The cross-sectional aspect ratios of lobe elements in the Point Loma are ~1000, a similar value to those measured in other systems, although aspect ratios are slightly higher at the distal reaches of lobe elements than in proximal locations. The key longitudinal patterns in lobe complexes are decreases in proportion of sand-rich facies, maximum grain size, amalgamation ratio of elements, net sand content, and amount of interelement erosion and a longitudinal increase in the degree of the amount of compensational stacking. Lobe complexes stack laterally and progradationally to build a lobe system.

Intra-oceanic submarine arc evolution recorded in an ~1-km-thick rear-arc succession of distal volcaniclastic lobe deposits

“…Channel facies are debrite-dominated features and contain a fining-upward top. These definitions are consistent with more recent 2-to 3-dimensional outcrop-and seismic-based studies of lobe systems and complexes (e.g., Prélat et al, 2009;Pyles et al, 2019;Sweet et al, 2020), which further develop the architectural hierarchy from the bed to the lobe to the lobe system scale within a lobe complex. Within such lobe systems and complexes, lobes may stack vertically or laterally migrate (autocyclic switching) and overlap, constructing thick fan successions (see previously cited publications for details).…”

Submarine lobe deposits of the Point Loma Formation, California: Quantifying event‐bed architecture and lateral heterogeneity

“…Using foraminiferal faunal assemblages, Units 2 and 3 are interpreted to have palaeobathymetric water depths of 850-1,000 m and 600-700 m, respectively (Figure 1; Nilsen & Abbott, 1981;Sliter, 1979Sliter, , 1984. Within Units 2 and 3, Fleming (2010) and Pyles et al (2019) documented four submarine lobe complexes that compensationally stack and display a systematic proximal-to-distal decrease in thickness in vertically amalgamated sandstone ratio, net sandstone to gross sediment thickness ('net-to-gross', also known as net sandstone proportion) and erosion. Stammer (2014) studied an axis-to-fringe transect of a lobe element that lies within Lobe Complex 2 of Fleming (2010).…”