QUANTIFYING PROCESS REGIME IN ANCIENT MIXED-ENERGY SYSTEMS

A deltaic system is defined as a sedimentary body that progrades into a standing body of water, built by fluvial processes in combination with wave and tidal processes. Deltas are classified as fluvial-, wave- and tide-dominated. However, the analysis of numerous examples shows that those basic types can be merely considered as relatively unusual end-members of a continuum. Sedimentary structures produced by fluvial, wave, and tidal processes can act simultaneously in mixed energy systems and produce complex stratal packages. To successfully predict ancient coastal morphology and sand-body geometry, it is necessary to disentangle the signals of waves, tidal and river processes in the stratigraphic record, and to understand what mechanisms control the latero-vertical partitioning of fluvial-, wave-, and tide-influenced facies.

We propose that, once all facies characteristics are collected, each sedimentary structure can be associated with a percentage (probability) of being the result of waves, tidal or river processes, based on the frequency of its association in the literature (i.e. experimental and modern studies). The sum of all process probabilities through time (vertically) creates process probability curves. This has been tested on a 22 m thick deltaic parasequence of the Jurassic Lajas Formation, in the Neuquén Basin (Argentina). The process probability curves constructed for the parasequence studied show that from bottom to top (i.e. from more distal settings to more proximal settings) wave energy decreases while river influence increases. Tidal influence decreases in the most distal settings, but persists in the most proximal ones, where it co-exists with river processes. However, this general trend is not linear. In the middle part of the parasequence (corresponding to the distal delta-front setting) all three processes co-exist, creating a complex pattern.

The process probability curves identify the complexity of mixed-energy systems, but at the same time they offer an alternative way to classify these systems. The correct recognition of all the processes involved in the evolution of a deltaic system (at any scale) is of paramount importance for the correct understanding of sand partitioning, paleogeographic reconstructions, and a correct evaluation of the basin history.