THE FLUVIAL TO MARINE TRANSITION ZONE AND ITS STRATIGRAPHIC SIGNIFICANCE

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Abstract

The seaward end of modern rivers is characterized by the interactions of marine and fluvial processes within a tract known as the fluvial to marine transition zone (FMTZ), which varies between systems due to the relative strength of fluvial, tidal and wave processes. River deltas also show a range of different morphologies and architectures linked to the balance between these depositional processes. However sedimentological interpretations of fluvial to shallow-marine deposits from the rock record commonly relate sedimentary structures to single depositional processes and give greater importance to facies-based observations rather than to architectural style. To better understand how fluvial and tidal process interactions are preserved and distributed in the rock record, deltaic deposits of the Middle Jurassic Lajas Formation (Neuquén Basin, Argentina) have been investigated in large-scale outcrops. The degree of tidal influence has been evaluated using both facies-scale observations and architecture analysis, and critically testing the application of the FMTZ concept in ancient deposits. The characteristics of the reconstructed FMTZ together with the architectural elements described from the Lajas Formation are consistent with an interpretation of a fluvial-dominated, tide-influenced delta, rather than a tide-dominated system, as previously proposed. The results presented herein suggest that highstand systems tract progradational deltas of the Lajas Fm. accumulated largely under microtidal conditions. Comparison of facies and architecture between modern tide dominated deltas and published examples from ancient successions shows a fundamental mismatch; modern systems are mud-dominated with laterally accreting tidal bars while ancient examples are sand-rich and dominated by forward accreting deposits. This thesis argues that the majority of ancient sand-rich successions interpreted as tide-dominated deltas might have formed in large parts in a fluvial-dominated, tide-influenced delta system and that tide-dominated deltas may have not been adequately described from the rock record yet. The improved identification of process interactions presented in this study is crucial to refining classifications of shallow-marine successions and to understanding the recorded spatial and temporal evolution of ancient depositional systems. This can have fundamental implications in refining reservoir models and predicting correct geometries in hydrocarbon-bearing successions.

Keyword(s)

Argentina; Neuquen Basin; fluvial-dominated; fluvial-marine transition; fluvial-tidal interactions; tide-influenced

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