The importance of ancient fluvio-deltaic systems dominated by catastrophic flooding in tectonically active basins

Ancient fluvio-deltaic systems deposited in tectonically active basins are essentially built up by inter-gradational fan-delta and river-delta systems dominated by catastrophic flooding. These systems and their component depositional elements cannot therefore be described and interpreted in terms of current sedimentological models based on "normal" fluvial and deltaic processes, facies and geomorphic settings derived from the study of modern environments. Direct and indirect sedimentological and stratigraphic evidence indicates that "normal" sedimentation occurred also in these systems, but its preservation potential appears to be very small with the exception of tidal diffusion in estuarine settings. Facies and facies associations of ancient flood-dominated depositional systems include a very broad spectrum of essentially poorly described and understood sediments that vary from thick-bedded and disorganized conglomerates to thin-bedded graded mudstones via a great variety of pebbly-sandstone and sandstone facies. Despite this variability, all these sediments are characteristically composed of graded flood units in both alluvial and marine environments. The greatest preservation potential of individual flood units is found in the final marine depositional zones of each system considered. Ancient flood-dominated fluvio-marine systems comprise huge accumulations of conglomerates, sandstone and mudstone facies whose origin and stratigraphic importance have been essentially overlooked in previous literature. These depositional systems can be understood only in terms of tectonically-controlled physiographic settings characterized by small and medium-sized fluvial systems with high-elevation drainage basins and high-gradient transfer zones located close to marine basins. In settings of this type, sediment flux to the sea can dramatically increase when climatic conditions provide sufficient amounts of water to produce catastrophic floods. These floods generate mixtures of water and sediment that can enter sea waters with sufficient velocity and sediment concentration to produce hyperpycnal flows and related, self-sustained turbidity currents. The resulting depositional settings are thus dominated by flood-related facies that can develop in shelfal or deeper marine regions. Thick and laterally extensive successions of shelfal sandstone lobes with flood-generated HCS are the fundamental depositional element of both fan-delta and river-delta systems considered in this study. These lobes are essentially similar to deeper-water turbidite sandstone lobes in terms of geometry, facies tracts, and high-frequency cyclic stacking patterns. Shelfal sandstone lobes probably represent the only possible expression of fluvial-dominated delta-front sandstone facies, since, in the absence of flood-generated hyperpycnal flows, river-borne sands can only be redistributed in marine environments by waves and tides. As indicated by their overall stacking patterns, the evolution of ancient flood-dominated fluvio-deltaic systems with time is apparently controlled by the initial uplift of the drainage basin, the rate of denudation, the gradient of each system, and the volume and sediment concentration of individual floods, the latter being a function of the amount of water and sediment made available to the system considered. A flood-dominated system of this type comes to an end when the sediment flux to the sea is progressively reduced to "normal" conditions. This occurs when relief and elevation of drainage basins and related sediment availability, as well as the gradient of transfer zones, have been substantially reduced through progressive denudation and sediment exportation to marine depositional zones. The occurrence of cyclic stacking patterns developed at different hierarchical orders is one of the most striking aspects of flood-dominated fluvio-deltaic systems. The most complete record of this cyclicity is preserved in the final depositional zone of each system. These stacking patterns are apparently very similar to those which are thought to be characteristic of sequence-stratigraphic models. Despite this apparent similarity, we suggest that the overall vertical evolution of flood-dominated systems is primarily controlled by Davisian-type cycles produced by alternating periods of orogenic uplift and denudation. In their most complete development, these cycles are ideally recorded by an overall forestepping-backstepping succession recorded by a basal turbidite system (basin floor fan of sequence stratigraphy) overlain by a flood-dominated fluvio-deltaic system which passes upward and landward into a "normal" fluvial or fluvio-deltaic system with time. Higher-frequency stacking patterns developed within each of the above stages are essentially produced by forestepping-backstepping episodes of sand deposition which are essentially controlled by cyclic climatic variations. The relationships between Davisian-type and higher-frequency climatic cycles and eustasy-driven cycles of relative sealevel variations remain to be explored through careful stratigraphic, sedimentological and structural studies carried out without preconceived ideas. It is likely, however, that the eustatic control on flood-dominated sedimentation patterns of high-gradient, tectonically active settings cannot generally compare with the importance of tectonism and related cycles of uplift and denudation.