Facies architecture of the fluvial to tidal transition of mixed-influence deltas

Abstract

Coastal deposits are often classified based on the depositional processes (wave, tide, fluvial) which operate in the depositional environment. The mix of processes operating during deposition have an effect on the size, shape, orientation and internal heterogeneities of sand- and sandstone bodies. It is important to know these properties for predicting and modelling fluid flow through aquifers. Facies models for fluvial-dominated deltas and wave-dominated shorefaces are well established and widely used. However, although there are many modern tide-influenced deltas, research in facies models for tide-influenced deltas lags behind to their better studies fluvial- and wave-dominated counterparts. This is partially because tide-dominated deposits are often interpreted in a sequence stratigraphic framework as transgressive tide-dominated estuaries. The aim of this study is to present a facies model for mixed-influence deltas, with emphasis on the preservation of channelised fluvial- and tidal channels in a progradational setting. Two case studies are part of this thesis. Firstly, outcrops of the Upper Cretaceous Sego Sandstone (Utah, USA) have been studied in order to investigate the facies architecture of a mixed tide- and wave influenced deltaic deposit. Secondly, cores of the mixed influence Lower to Middle Jurassic Ror, Tofte and Ile formation from the subsurface of the Halten Terrace (offshore mid-Norway) have been examined. Both case studies were deposited in structurally controlled embayments which favoured amplification of tides. Both case studies show an initial progradational phase with little evidence for tidal processes. In contrast, deposits on the delta plain show abundant evidence for tidal currents. In this study, it has been interpreted that these channelised tidal channels are part of continuous progradation of a tide-influenced delta. This is in contrast to sequence stratigraphic models in which tide-dominated deposits are interpreted as part of a transgressive phase. During progradation, these tidal and fluvial channels erode down in underlying deposits and they are deposited in progressively more basin-ward positions. More tide-dominated channels deposited in relatively distal or off-axis locations have a more planar geometry. More fluvial-dominated channels deposited in relatively proximal or on-axis location are more lenticular.