Abstract

Recent improvements in seismic resolution and more detailed outcrop mapping of basin-floor 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 a stratal element. These elements stack to form lobes, which in turn stack to form lobe complexes and ultimately the fan. The arrangement of the building blocks on the fan is driven by channel avulsion, and the location of the avulsion defines the hierarchy.

Basin-floor deposits of the Upper Kaza Group at Castle Creek comprise five stratal elements: scours fills, avulsion splays (interpreted to be genetically related with scours, and here combined into “avulsion complexes”), distributary channels, terminal splays, and fine-grained deposits. Using a Markov-chain statistical analysis the vertical stacking pattern of these elements was found to be non-random, which suggests an internal control or forcing on the system. Most notably, the results show that in spite of their thicker nature and high abundance in the studied section, sand-rich terminal splays are superimposed less frequently than expected in a random distribution of stratal elements. This reluctance of terminal splays to stack vertically suggests that it is not only deposition in the splay, but also the temporal stability of that deposition that drives upflow avulsions, and accordingly the observed stacking of the other associated stratal elements. This may be the result of the accumulated thickness of previous deposits forcing the system to episodically shift its depositional axis into bathymetrically lower areas. The consequence is that although the overall sedimentary pile contains abundant terminal splays with their characteristic high sandstone-to-mudstone ratio (∼ 75%), amalgamated terminal-splay deposits are thinner and the overall stratigraphic architecture more complex and heterogeneous than would otherwise be predicted in a random distribution of elements.