--> The Control of Terminal Splay Sedimentation on Depositional Patterns and Stratigraphic Evolution in Avulsion Dominated, Deep-Marine Basin-Floor Systems

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The Control of Terminal Splay Sedimentation on Depositional Patterns and Stratigraphic Evolution in Avulsion Dominated, Deep-Marine Basin-Floor Systems

Abstract

Recent improvements in seismic resolution and more detailed outcrop mapping of basin-floor terminal fans has shown that they are made up of a hierarchy of discrete building blocks. The most fundamental building block, made up of one or several event beds, is termed an architectural element. These elements stack and make up the bodies of terminal lobes, which, in turn, stack to form the terminal fan. The arrangement of the building blocks on the fan is driven by channel avulsion, and the location of the avulsion defines the hierarchy. In this study a combined sedimentological and statistical approach is used to describe and interpret the role of the various architectural elements in avulsion on the basin-floor. Data for this study comprise ∼1 km detailed sedimentological logs and maps of proximal mid-fan basin-floor deposits of the Upper Kaza Group, Neoproterozoic Windermere Supergroup. At the Castle Creek study area (Cariboo Mountains, B.C., Canada) vertically dipping strata are continuously exposed in a single ∼1 km wide and 100 m thick outcrop. Here, basin-floor deposits comprise five architectural elements: scours and their fills, avulsion splays (interpreted to be genetically related with scours, and here combined into “avulsion complexes”), distributary channels, terminal splays and fine-grained deposits. Terminal splays are the most common and make up 41% of all the elements by number of occurrences. Markov chain analysis is used to describe and test the observed vertical succession of architectural elements for statistical significance. The results of the analysis show that the vertical succession is non-random, which then indicates an internal control or forcing on the system. Most importantly, the statistical test shows that in spite of their thicker nature and high abundance in the studied section, terminal splays are superimposed less frequently than expected in a random distribution of architectural elements. This reluctance of terminal splays to stack suggests that it is not only deposition in the splay, but also the temporal stability of that deposition that drives the much more frequent upflow avulsions, and accordingly the observed stacking of the other associated architectural elements. This may be the result of sedimentation rates in excess of accommodation creation on the basin floor. The consequence is that amalgamated sand bodies are thinner than expected, and reservoir architecture more complex and heterogeneous.