Recognition criteria, characteristics and implications of the fluvial to marine transition zone in ancient deltaic deposits (Lajas Formation, Argentina)

The seaward end of modern rivers is characterized by the interactions of marine and fluvial processes, a tract known as the fluvial to marine transition zone, which varies between systems due to the relative strength of these processes. To understand how fluvial and tidal process interactions and the fluvial to marine transition zone are preserved in the rock record, large-scale outcrops of deltaic deposits of the Middle Jurassic Lajas Formation (Neuquen Basin, Argentina) have been investigated. Fluvial–tidal indicators consist of cyclically distributed carbonaceous drapes in unidirectional, seaward-oriented cross-stratifications, which are interpreted as the result of tidal modulation of the fluvial current in the inner part of the fluvial to marine transition zone. Heterolithic deposits with decimetre-scale interbedding of coarser-grained and finer-grained facies with mixed fluvial and tidal affinities are interpreted to indicate fluvial discharge fluctuations (seasonality) and subordinate tidal influence. Many other potential tidal indicators are argued to be the result of fluvial–tidal interactions with overall fluvial dominance or of purely fluvial processes. No purely tidal or tide-dominated facies were recognized in the studied deposits. Moreover, fluvial–tidal features are found mainly in deposits interpreted as interflood (forming during low river stage) in distal (delta front) or off-axis (interdistributary) parts of the system. Along major channel axes, the interpreted fluvial to marine transition zone is mainly represented by the fluvial-dominated section, whereas little or no tide-dominated section is identified. The system is interpreted to have been hyposynchronous with a poorly developed turbidity maximum. These conditions and the architectural elements described, including major and minor distributary channels, terminal distributary channels, mouth bars and crevasse mouth bars, are consistent with an interpretation of a fluvial-dominated, tide-influenced delta system and with an estimated short backwater length and inferred microtidal conditions. The improved identification of process interactions, and their preservation in ancient fluvial to marine transition zones, is fundamental to refining interpretations of ancient deltaic successions.Centro de Investigaciones Geológica

Decoupling seasonal fluctuations in fluvial discharge from the tidal signature in ancient deltaic deposits: an example from the Neuquén Basin, Argentina

Fluvial discharge fluctuations are a fundamental characteristic of almost all modern rivers and can produce distinctive deposits that are rarely described from ancient fluvial or mixed-energy successions. Large-scale outcrops from the Middle Jurassic Lajas Formation (Argentina) expose a well-constrained stratigraphic succession of marginal-marine deposits with a strong fluvial influence and well-known tidal indicators. The studied deposits show decimetre-scale interbedding of coarser- and finer-grained facies with mixed fluvial and tidal affinities. The alternation of these two types of beds forms non-cyclic successions that are interpreted to be the result of seasonal variation in river discharge, rather than regular and predictable changes in current velocity caused by tides. Seasonal bedding is present in bar deposits that form within or at the mouth of minor and major channels. Seasonal bedding is not preserved in channel thalweg deposits, where river flood processes were too powerful, or in floodplain, muddy interdistributary-bay, prodelta and transgressive deposits, where the river signal was weak and sporadic. The identification of sedimentary facies characteristic of seasonal river discharge variations is important for accurate interpretation of ancient deltaic process regime

Sedimentological and Paleoclimate Modeling Evidence for Preservation of Jurassic Annual Cycles in Sedimentation, Western Gondwana

Abstract The Lajas Formation in the Neuquén Basin, Argentina, consists of a succession of mainly deltaic deposits. In the Middle Jurassic (170 million years ago), the basin was in western Gondwana roughly at the same paleolatitude as its present location (32°–40°S). Decimeter-scale, interbedded, coarser-grained and finer-grained beds in channelized and nonchannelized deltaic deposits have been interpreted as a product of variability in river discharge. The coarser-grained sandstone beds have erosional bases and contain mudstone clasts; internal cross bedding is commonly directed paleoseawards. These beds are interpreted as deposition during river-flood conditions. In contrast, the finer-grained beds are composed of interlaminated sandstone and mudstone, deposited during interflood periods. Bidirectional ripples and millimeter-scale sand–mud laminae suggest the influence of tides. This sedimentological evidence raises the question of whether these cycles represent annual variability in fluvial input. To answer this question, a simulation using the Fast Ocean Atmosphere Model for the Middle Jurassic was run to equilibrium. The model shows that the paleoclimate of the Neuquén Basin was characterized by a strong seasonal cycle, with a wet winter and a dry summer. Model runs suggest that February mean temperatures were 28°C with 4-mm precipitation (±4 mm standard deviation) per month, whereas August mean temperatures were 8°C with 34-mm precipitation (±17 mm standard deviation) per month. The strong seasonal cycles in the simulation, representing 24% of the variance in the precipitation time series, suggest that the sedimentological cycles represent annual variations. The simulation also suggests a Middle Jurassic climate where increased seasonality of precipitation occurred farther poleward than previously thought.</jats:p

Bioindication and biomonitoring studies carried out in rural and urban sites by ozone sensitive clover clones.

Results coming from open-air experiments of bioindication and biomonitoring performed on ozone resistant and sensitive clover clones are showed

Application of fluvial to marine transition zone concepts to ancient deltaic deposits, Lajas Fm., Neuquén basin, Argentina

The seaward end of modern rivers is marked by the fluvial to marine transition zone (FMTZ), which is characterized by the interaction of marine and fluvial processes. In the study of ancient deposits, facies are only rarely interpreted as the result of process interactions, often leading to over-emphasis of a single process and potentially misleading depositional models. Because deltas show a range of different morphologies, architectures, facies and grain size distributions that are linked to the relative strength of fluvial, tidal and wave processes, understanding process interactions in ancient deposits is crucial to improve prediction of 3D depositional architecture and reservoir connectivity.This study provides an example where the FMTZ concept has been applied to an ancient deltaic succession. The Middle Jurassic Lajas Formation has been investigated in large-scale outcrops, 40 km south of Zapala (Figure 1). These deposits accumulated in different fluvial and shallow marine settings during the back-arc, post-rift phase of the Neuquén Basin. Sedimentological logging, correlation panel construction, and architectural element and facies analysis were used to constrain the depositional features and to evaluate the degree of tidal influence during deposition.Fil: Gugliotta, Marcello. University of Manchester; Reino UnidoFil: Flint, Stephen S.. University of Manchester; Reino UnidoFil: Hodgson, David M.. University of Leeds; Reino UnidoFil: Veiga, Gonzalo Diego. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigaciones Geológicas. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Centro de Investigaciones Geológicas; Argentina9th International Conference on Tidal SedimentologyPuerto MadrynArgentinaAsociación Argentina de Sedimentologí

Sedimentology of late Holocene fluvial levee and point-bar deposits from the Cambodian tract of the Mekong river

The study of channel margin deposits is crucial to better understand fluvial systems and has significant social and economic implications. This paper describes sedimentological features of Late Holocene levee and point-bar deposits from five excellent outcrop exposures along the Cambodian tract of the Mekong River. Point-bar deposits show a typical fining-upward trend with low-angle inclined bedding and consist of gravelly sand, sand and mud beds. Levee deposits show a typical coarsening-upward or no clear vertical grain-size trend and consist of mud and very fine sand beds. Channel margin deposits formed in the outer river bend show a simple levee element, whereas inner bend deposits show more complex architecture with combinations of point-bar and levee elements. Water-level fluctuations play a fundamental role in the construction of point-bar and levee architecture; the resulting deposits show evidence of strong river currents with rapid sedimentation alternating with evidence of subaerial exposure with no sedimentation. This combination of sedimentary features can also form in other environments, but is distinctive of channel margin deposits forming in climates with a pronounced seasonality, including large and perennial rivers in tropical savannah climate areas

Tidal sedimentary dynamics of the Early Pleistocene Messina Strait (Calabria, southern Italy) based on its modern analogue

International audienceThe Messina Strait excursion opens a series of geological field trips associated with the 10th International Congress of Tidal Sedimentology (Tidalites), Matera, Italy, 5-7 October 2021. This guide aims at documenting a number of selected outcrops located along the eastern margin of the modern Messina Strait, in order to illustrate the sedimentary dynamics of the Early Pleistocene tide-dominated Messina Strait. Since the Pliocene, this extensional basin separated Sicily from Calabria, forming a wide non-tidal seaway. Successively, this basin turned into a ca. 10-15 km-wide and 40 km-long, tide-dominated strait during the Early Pleistocene, prior to its definitive closure following a Middle Pleistocene phase of tectonic uplift. As for today in its modern analogue, the ancient strait acted as a major conduit for marine water exchanges between the Ionian and the Tyrrhenian seas. Semi-diurnal, reverse bidirectional tidal currents flowed in phase opposition parallel to the strait margins, being subject to tidal amplification due to bathymetric restriction across the shallower strait-centre zone. This oceanographic setting partitioned the strait into specific environments. Nowadays, their sedimentary record is exposed in a series of outcrops across the western (Sicily) and eastern (Calabria) margins of the modern strait. A series of stops along a south-to-north transect covers a total distance of ca. 20 km. Outcrops of the first day show coarse-grained deposits lying adjacent to a block-faulted central horst and transgressively overlain by cross-stratified, mixed bioclastic-siliciclastic arenites. These strata record bypass and residual sedimentation in the strait-centre zone of the ancient system. The second day, large- and medium-scale cross-stratification exhibiting a variety of tidal sedimentary indicators are observed, interpreted as the ancient northern tidal dune field. The third day focuses on one major section representing the north-eastern flank of the ancient strait, where subaqueous canyon-fill strata, mass-wasting deposits and tide-influenced delta front-facies are exposed