Regressive delta

Abstract: Many modern deltas show complex morphologies and architectures related to the interplay of river, wave and tidal currents. However, methods for extracting the signature of the individual processes from the stratigraphic architecture are poorly developed. Through an analysis of facies, palaeocurrents and stratigraphic stacking patterns in the Jurassic Lajas Formation, this paper: (i) separates the signals of wave, tide and river currents; (ii) illustrates the result of strong tidal reworking in the distal reaches of deltaic systems; and (iii) discusses the implications of this reworking for the evolution of mixed-energy systems and their reservoir heterogeneities. The Lajas Formation, a sand-rich, shallow-marine, mixed-energy deltaic system in the Neuquen Basin of Argentina, previously defined as a tide-dominated system, presents an exceptional example of process variability at different scales. Tidal signals are predominantly located in the delta front, the subaqueous platform and the distributary channel deposits. Tidal currents vigorously reworked the delta front during transgressions, producing intensely cross-stratified, sheet-like, sandstone units. In the subaqueous platform, described for the first time in an ancient outcrop example, the tidal reworking was confined within subtidal channels. The intensive tidal reworking in the distal reaches of the regressive delta front could not have been predicted from knowledge of the coeval proximal reaches of the regressive delta front. The wave signals occur mainly in the shelf or shoreface deposits. The fluvial signals increase in abundance proximally but are always mixed with the other processes. The Lajas system is an unusual clean-water (i.e. very little mud is present in the system), sand-rich deltaic system, very different from the majority of mud-rich, modern tide-influenced examples. The sand-rich character is a combination of source proximity, syndepositional tectonic activity and strong tidal-current reworking, which produced amalgamated sandstone bodies in the delta-front area, and a final stratigraphic record very different from the simple coarsening-upward trends of river-dominated and wave-dominated delta fronts.

Abstract: We investigate the influence of three local controls (sediment supply, subsidence, and physiography) on delta development and sequence variability, using the three-dimensional numerical model of deltaic deposition presented in a companion paper (Ritchie et al. 2004). Independently varying a single local control within geologically constrained limits produces marked differences in three-dimensional morphology, cross-sectional stratal geometry, and delta evolution during a cycle of sea-level change. Sediment supply strongly influences not only the timing of transgressive and maximum flooding surface development during sea-level rise, and hence the diachroneity of lowstand, transgressive, and highstand systems tracts, but also the timing of onset of fluvial incision and the characteristics of incised valleys and forced regressive wedges. Low sediment supply leads to early drowning of forced regressive and lowstand prograding wedges, high-magnitude transgressions, and the late development of maximum flooding surfaces. During relative sea-level fall, low sediment supply results in early initiation of fluvial incision and development of few, but major incised valleys that route sediment to elongate forced regressive lobes at their mouths. In contrast, high sediment supply leads to early onset of normal regression and late transgression during relative sea-level rise, and numerous, poorly developed incised channels and a broad, laterally continuous forced regressive apron during relative sea-level fall. On basin margins with a shelf-slope morphology, base-of-slope deepwater deposits may occur at times of sea-level highstand due to sediment bypass across the slope. During relative sea-level fall fluvial incision is enhanced by the relatively steep gradient of the slope, leading to the early onset of incision and rapid headward propagation. As a result, major, deep incised valleys develop that capture sediment and feed it directly to thick shelf-edge forced regressive and lowstand prograding wedges. The greater water depth ahead of the shelf edge and the steep slope gradients result in limited forced and normal regression compared to a ramp setting. In contrast to other controls, tectonic subsidence (or uplift) leads to modification of accommodation, so that relative sea-level change may vary significantly along a basin margin. In high-subsidence settings, relative sea level may continue to rise even at times of high rates of background sea-level fall. In such settings, deltas lack incised valleys, forced regressive delta lobes, and prominent sequence boundaries. Furthermore, normal regression and even transgression in high-subsidence locations can be contemporaneous with forced regression and incised-valley development in adjacent lower-subsidence locations. The results of this sensitivity analysis suggest that systems tracts and their bounding surfaces are likely diachronous along many basin margins and may locally be absent. More than one control can produce a similar stratigraphic response, making interpretation of controlling processes from the stratigraphic product equivocal. The models provide a framework for understanding the stratigraphy of natural basin fills, but further research on the interplay between the controls is required in order to understand the climatic, tectonic, and sea-level signals concealed in the stratigraphic record.

Abstract: Lake Pannon, covering the Pannonian Basin (Hungary) during the Late Miocene, had a complex lake bottom with deeper sub-basins and intrabasinal basement highs, sometimes emerging above the lake level as islands and peninsulas. Above structural highs, the basin fill sequence usually commenced with deposition of transgressional, locally sourced coarse-grained deltas. These deposits are overlain by distally-sourced deltaic bodies associated with the prograding delta system that gradually filled up the entire basin. The transition between these two distinct delta systems was studied in a large outcrop on the edge of a former basement high (Mecsek Mts., southern Hungary). The transgressional phase is represented by the deposition of a mass flow dominated fan delta body, fed by local material from a granitic catchment area. An overlying fossil-rich, clayey unit records an episode for which sediment delivery into the basin was curtailed, possibly due to submergence of the granite body. The deposition of these two units took place between 8 and 6.8 Ma. The onset of sedimentation associated with the arrival of the distally sourced regressive delta system (around 6.8 Ma) is represented by deposition of shoreface sediments. This unit is characterized by distinctive bioturbation and storm related, sand filled scour-fills. Resedimented local material that forms mm thick, coarse laminas in the scour fills is indicative of denudation due to tectonic events and implies coupled storm-flood sedimentation. The deposition of the two distinct delta bodies and the interplay between tectonic events and lake-level changes occurred during a relatively short, ca. 1.5 Ma long time interval.

Abstract: Two explicit landscape simulation models were used to investigate habitat shifts in coastal Louisiana due to varying river forcing and sea level rise scenarios. Wetland conversion to open water and yearly shifts of marsh habitats in two contrasting estuarine regions were examined; the Atchafalaya delta which is a prograding delta area with strong riverine input, and the Barataria Basin is a regressive delta with high wetland loss which is isolated from riverine input. The models linked several modules dynamically across spatial and temporal scales. Both models consisted of a vertically integrated hydrodynamic model coupled with process-based biological modules of above and below ground primary productivity and soil dynamics. The models explored future effects of possible sea level rise and river diversion plans for 30 and 70-year projections starting in 1988. Results showed that increased river forcing had large land preservation impacts, and indicated that healthy functioning of the Mississippi Delta depends largely on inputs of freshwater, nutrients, and sediments in river water. These types of models are useful for research and as management tools for predicting the effects of regional impacts on structural landscape level changes.