Hybrid event bed character and distribution in the context of ancient deep-lacustrine fan models

Abstract

Hybrid event beds are texturally and compositionally-diverse deposits preserved within deepwater settings. They are deposited by flows exhibiting ‘mixed behaviour’, forming complex successions of sandstone and mudstone, which are often challenging to predict. Hybrid event beds are documented in deep-marine settings, where they have been thoroughly characterized, and are well-known as effective fluid transmissibility barriers and baffles in reservoirs. By comparison, there are far-fewer studies of hybrid event beds from deep-lacustrine settings, where their character and distribution remains relatively under-explored. In order to provide insights into these deposits, this study presents the detailed analysis of three-dimensional seismic data, wireline logs and core from a series of ancient deep-lacustrine fan systems in the North Falkland Basin. Results confirm that deep-lacustrine hybrid event beds comprise the same idealized sequence of the ‘H1–H5’ divisions. However, in this study H3 ‘debrite’ units can be sub-divided into ‘H3a–H3c’, based on: sharp or erosional intra-H3 contacts, bulk lithology, mud-content and discrete sedimentary textures. This study interprets the H3a–H3c sub-units as the products of multiple flow components formed through significant rearward longitudinal flow transformation processes, during the emplacement of a single hybrid event bed. Hybrid event beds are observed within lobe fringes, where flow types, energies, and transport mechanisms diversify as a result of flow transformation. The temporal context of hybrid event bed occurrences is considered in relation to stages of fan evolution, including: the Initiation; Growth (I); Growth (II); By-pass; Abandonment; and Termination phases. Hybrid event beds are mainly found in either the initiation phase where flow interaction and erosion of initial substrates promoted mixed flow behaviour, or in the abandonment phase as facies belt retreated landward. The results of this study have important implications in terms of flow processes of hybrid event bed emplacement, in particular sub-division of the H3 unit, as well as the prediction of hybrid event bed occurrence and character within ancient deep-lacustrine fan settings, in-general.