Tidal modulation of river-flood deposits: How low can you go?

Quantification of the interaction between river discharge and tides is vital to characterize fluvio-deltaic systems, to identify diagnostic elements of tidal signatures in the rock record, and to reconstruct paleogeographies. In modern systems, even microtides can significantly influence delta morphodynamics; yet, many fundamental processes, particularly in prodeltaic settings, remain elusive. Here, by combining a unique process-product data set acquired during a flood event of the Po River (Italy) with numerical modeling, we show that tidal signatures are recorded in the open-water prodelta zone of a microtidal system. Based on the analyses of box-cores collected before and after a flood off the main distributary channel, we interpreted storm beds, tide-modulated flood strata of alternating normal and inverse graded beds, and rapid bioturbation. Modeling of the river discharge indicates that, at the peak of the flood, the steepening of the water-surface profile forced by 0.15 m lowering of sea level during low tides generated an 8% increase in river flow velocity. The alternation of profile steepness and associated cyclicity in flow strength during consecutive tidal cycles controlled the sediment load of the plume and, consequently, led to the deposition of tidal-modulated strata. Formation of microtidal signals appears to be enhanced in fluvio-deltaic successions characterized by multiple distributaries and in basins where river floods are out of phase with storm-wave activity. Bioturbation of sediment, which can start during the waning stage of the flow, and erosion by storm waves hamper the preservation of tidal signals, unless rapid burial occurs. The recognition of tidal-modulated strata in river-dominated settings may facilitate the characterization of mudstone reservoirs and reconstruction of paleogeographic conditions during deposition

How to make a 350-m-thick lowstand systems tract in 17,000 years: The Late Pleistocene Po River (Italy) lowstand wedge

The 350-m-thick succession of the Po River lowstand wedge (Italy) associated with the Last Glacial Maximum (deposited over ~17 k.y) contains stratal architecture at a physical scale commonly attributed to much longer time scales, with complex, systematically varying internal clinothem characteristics. This study investigated clinothem stacking patterns and controls through the integration of seismic reflection data with sediment attributes, micropaleontology, regional climate, eustacy, and high-resolution age control possible only in Quaternary sequences. Three clinothem types are differentiated based on topset geometry, shelf-edge and onlap-point trajectory, internal seismic facies, and interpreted bottomset deposits: type A has moderate topset aggradation, ascending shelf-edge trajectory, and mass-transport bottomset deposits; type B has eroded topset, descending shelf-edge trajectory, and bottomset distributary channel-lobe complexes; and type C has maximal topset aggradation, ascending shelf-edge trajectory, and concordant bottomsets. Type A and C clinothems exhibit reduced sediment bypass and delivery to the basin, whereas type B clinothems are associated with short intervals of increased sediment export from the shelf to deeper water. Clinothems individually span a range of 0.4–4.7 k.y., contemporaneous with significant eustatic and climate changes, but their stacking patterns resemble those found in ancient successions and ascribed to significantly longer durations, indicating that (1) the response time of ancient continental margin–scale systems to high-frequency variations in accommodation and sediment supply could be as short as centuries, (2) even millennial- to centennial-scale stratal units can record substantial influence of allogenic controls, and (3) sandy deposits can be compartmentalized even in a short-duration lowstand systems tract

The Late Pleistocene Po River lowstand wedge in the Adriatic Sea: Controls on architecture variability and sediment partitioning

Although facies and stratal geometries of continental margin successions can be defined in detail based on subsurface and outcrop studies, most studies lack the high-resolution age control needed to constrain the time scale of formation of such successions and infer their external forcing mechanisms. Our work on the Po River Lowstand Wedge (PRLW) indicates that deposition rates are surprisingly high with the entire 350-m-thick succession being deposited in less than 17,000 years, and with individual clinothems recording time periods ranging from 400 to 4700 years. The PRLW preserves a high-resolution record of stacked, deltaic shelf-edge clinothems deposited during the Last Glacial Maximum (31.8–14.4 ky BP) in the Adriatic basin (Mediterranean Sea). We investigated clinothem internal geometry, stacking patterns, and facies distributions to infer the main controls on their growth by integrating seismic reflection data with seismic facies attributes and paleoenvironmental proxies. The stratigraphic framework of the shelf-edge clinothems was then related to major paleoenvironmental shifts during the last glacial cycle and driven by eustatic and climatic changes. Within the PRLW, we recognized three distinctive types of 100's-m-high shelf-edge clinothems, type A, type B and type C, each with diagnostic topset geometries, shelf-edge trajectories, and associated distal basin-fill deposits. These elemental clinothem types stack into two Clinothem Sets. Clinothem Set 1, with essentially flat to slightly descending shelf-edge trajectory, is composed of stacked types A and B clinothems, and records the direct influence of river flux leading to dysoxic conditions on the bottom of the basin. In particular, clinothem accumulation rates were as much as 200 km3/ky in some of the type B clinothems. Clinothem Set 2, showing ascending shelf-edge trajectory, records an aggradational stacking coupled with a retreat of the river-entry points with benthic fauna assemblages that reflect the influence of peaks in freshwater discharge. Whereas Clinothem Set 1 developed under perturbations of river supply linked to the multi-scale waxing and waning of glaciers during an interval dominated by eustatic fall, Clinothem Set 2 reflects the main thawing of glaciers concomitant to the first phase of the eustatic rise. From a sequence stratigraphic perspective, Clinothem Set 1 is interpreted as staked high-frequency sequences, while Clinothem Set 2 represents a stack of high-frequency parasequences. The high-resolution age control from boreholes and seismic data enabled us to relate stratal character to independently constrained environmental proxies: this revealed how the evolution of a shelf-edge system intricately convolves the influences of both global (eustacy) and regional (climate-driven supply fluctuations) controls, both at sub-Milankovitch scales. Finally, the thickness, geometry, and stacking patterns of the centennial to millennial clinothems of the PRLW vary in systematic ways resulting in geometries that closely resemble those of ancient shelf-edge systems, and offering the PRLW as a sub-modern analogue. Our observations also reinforce the focus of the classic sequence-stratigraphic approach on analyzing surfaces and their geometric relations and not on time duration or formation mechanisms

Reconstructions of deltaic environments from Holocene palynological records in the Volga delta, northern Caspian Sea

This article was made available through open access by the Brunel Open Access Publishing Fund.New palynological and ostracod data are presented from the Holocene Volga delta, obtained from short cores and surface samples collected in the Damchik region, near Astrakhan, Russian Federation in the northern Caspian Sea. Four phases of delta deposition are recognized and constrained by accelerated mass spectrometry (AMS) radiocarbon ages. Palynological records show that erosive channels, dunes (Baer hills) and inter-dune lakes were present during the period 11,500–8900 cal. BP at the time of the Mangyshlak Caspian lowstand. The period 8900–3770 cal. BP was characterized regionally by extensive steppe vegetation, with forest present at times with warmer, more humid climates, and with halophytic and xerophytic vegetation present at times of drought. The period 3770–2080 cal. BP was a time of active delta deposition, with forest or woodland close to the delta, indicating relatively warm and humid climates and variable Caspian Sea levels. From 2080 cal. BP to the present-day, aquatic pollen is frequent in highstand intervals and herbaceous pollen and fungal hyphae frequent in lowstand intervals. Soils and incised valley sediments are associated with the regional Derbent regression and may be time-equivalent with the ‘Medieval Warm Period’. Fungal spores are an indicator of erosional or aeolian processes, whereas fungal hyphae are associated with soil formation. Freshwater algae, ostracods and dinocysts indicate mainly freshwater conditions during the Holocene with minor brackish influences. Dinocysts present include Spiniferites cruciformis, Caspidinium rugosum, Impagidinium caspienense and Pterocysta cruciformis, the latter a new record for the Caspian Sea. The Holocene Volga delta is a partial analogue for the much larger oil and gas bearing Mio-Pliocene palaeo-Volga delta.Funding for the data collection and field work was provided from the following sources: 1 – IGCP-UNESCO 2003–2008 (Project 481 CASPAGE, Dating Caspian Sea Level Change); 2 – NWO, Netherlands Science Foundation and RFFI, Russian Science Foundation 2005–2008 (Programme: ‘VHR Seismic Stratigraphy and Paleoecology of the Holocene Volga Delta’); and 3 – BP Exploration (Caspian Sea) Sea Ltd. (Azeri-Chirag-Gunashli) 2005–2008 (‘Unravelling the Small-Scale Stratigraphy and Sediment Dynamics of the Modern Volga Delta Using VHR Marine Geophysics’). The palynological work was funded jointly by BP Exploration (Caspian Sea) Ltd., Delft University of Technology and KrA Stratigraphic Ltd. Ostracod analyses were funded by StrataData Ltd. and funding for two additional radiocarbon dates provided by Deltares