The role of climate variation in delta architecture: lessons from analogue modelling

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Abstract

Sequence-stratigraphic models for fourth to sixth order, glacio-eustatic sequences based only on relative sea-level variations result in simplified and potentially false interpretations. Glacio-eustatic sea-level variations form only one aspect of cyclic climate variation; other aspects, such as variations in fluvial water discharge, vegetation cover, weathering and sediment supply can lead to variable sediment yield, thus adding complexity to sequence-stratigraphic patterns normally attributed to sea-level variations. Analogue flume models show a significant impact of water discharge on the timing and character of sequence boundaries, and on changes in the relative importance of systems tracts, as expressed in sediment volumes. Four deltas, generated under the influence of an identical sea-level curve, and affected by different water-discharge cycles were generated in the Eurotank facility: (1) constant discharge; (2) high-frequency discharge variations (HFD); (3) discharge leading sea level by a quarter phase; (4) discharge lagging sea level by a quarter phase. HFD shift the parasequence stacking pattern consistently but do not alter large-scale delta architecture. Water-discharge changes that lead sea-level changes result in high sediment yield during sea-level rise and in the poor development of maximum flooding surfaces. Delta-front erosion during sea-level fall is expressed by multiple, small channels related to upstream avulsions, and does not result in an incised valley that efficiently routs sediment to the shelf edge. When water-discharge changes lag sea-level changes, sediment yield is high during falling sea level and results in rapid progradation during forced regression. Erosion from incised valleys is strong on the proximal delta top and dissipates towards the delta front. The combination of high discharge and sea-level fall provides the most efficient mode of valley incision and sediment transport to the shelf edge. During sea-level rise, low water discharge results in sediment starvation and well-developed maximum flooding surfaces. Water-discharge variations thus alter sequence-stratigraphic patterns and provide an alternative explanation to the amplitude of sea-level fall for generating either type 1 or 2 erosional unconformities.

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