Zoltán Sylvester

A diverse deep‐marine Ichnofauna from the Eocene Tarcau sandstone of the Eastern Carpathians, Romania

The Paleocene to Middle Eocene Tarcau Sandstone at Buzau Valley, eastern Carpathians, Romania, records sedimentation in a turbidite system. These strata contain a diverse and abundant pre‐ and postdepositional ichnofauna consisting of 35 ichnogenera and 54 ichnospecies. The predepositional assemblage is rich in graphoglyptids and ornate grazing trails; simple grazing trails, resting traces, and feeding structures also occur. The predepositional assemblage includes Acan‐thorhaphe, Belorhaphe, Cardioichnus, Circulichnus, Coch‐lichnus, Cosmorhaphe, Desmograpton, Fustiglyphus, Gordia, Helicolithus, Helminthopsis, Helminthorhaphe, Lorenzinia, Megagrapton, Paleodictyon, Paleomeandron, Protopaleodictyon, Scolicia (S. strozzii), Spirorhaphe, Spirophycus, Treptichnus, and Urohelminthoida. The ich‐nodiversity, composition, ethology, and morphologic complexity of the predepositional association are indicative of the Nereites ichnofacies. The postdepositional association essentially consists of dwelling, feeding, and grazing traces, and is represented by Chondrites, Glockerichnus, Halopoa, Nereites, Ophiomorpha, Phycodes, Planolites, Polykampton, Scolicia(S. prisca. S. striata), Taenidium, Thalassinoides, and Zoophycos. Palaeophycus occurs in both assemblages. Allochthonous Teredolites is present in wood fragments, The postdepositional association includes elements of the Skolithos ichnofacies and facies‐crossing forms that are commonly present in deep‐marine deposits, Elements of the Skolithos ichnofacies are present not only in the most proximal parts of the turbidite system, but also in distal parts. The number of predepositional forms greatly exceeds postdepositional ones, reflecting a dominance of K‐selected over r‐selected population strategies in a stable environment. High levels of ichnodiversity in the Tarcau Sandstone are comparable with deep‐sea ichnofaunas from the Polish Carpathians and with other flysch trace‐fossil assemblages of similar age. This abundant and diverse Eocene ichnofauna supports the idea of extremely rich deep‐sea ichnofaunas in the Cenozoic.

Turbidite Channel Architecture: Recognizing and Quantifying the Distribution of Channel-base Drapes Using Core and Dipmeter Data

Field and simulation studies indicate that channel architecture and the presence of channel-base drapes (CBDs) can have a significant impact on oil recovery and represent key uncertainties in the understanding of a turbidite channel reservoir. Accordingly, understanding the frequency and distribution of CBDs provides valuable insights into reservoir performance. Core and dipmeter data contain information that can be used to recognize channel-base disconformities and associated CBDs. By comparing the observed number of channel-base disconformities to the observed number of disconformities overlain by mudstone, a statistical assessment of their frequency and distribution can be made. In a spatial sense, the fraction observed in the wells represents the average percentage of the channel elements within the reservoir that are overlain by a drape.

Stratigraphic evolution of intraslope minibasins: Insights from surface-based model

Although numerous case studies exist to illustrate the large-scale stratigraphic architecture of salt-withdrawal minibasins, there is no clear understanding of how stratal patterns emerge as a function of the interplay between basin subsidence and sedimentation. Here we present a simple model of mass balance in minibasin sedimentation that focuses on the interaction between long-term sediment supply and basin-wide subsidence rate. The model calculates the sediment flux in three dimensions assuming a simplified basin and deposit geometry. The main model output is a cross section that captures the large-scale stratigraphic patterns. This architecture is determined by the relative movement of the stratal terminations along the basin margin: consecutive pinchout points can (1) be stationary, (2) move toward the basin edge (onlap), or (3) move toward the basin center (offlap). The direction and magnitude of this movement depend on the balance between the volume made available through subsidence, calculated only over the area of the previous deposit, and the volume needed to accommodate all the sediment that comes into the basin. Cycles of increasing-to-decreasing sediment supply result in stratigraphic sequences with an onlapping lower part and offlapping upper part. If the sediment input curve is more similar to a step function, stratigraphic sequences only consist of an onlapping sediment package, with no offlap at the top. Modeling two linked basins in which deposition takes place during ongoing subsidence shows that conventional static fill-and-spill models cannot correctly capture the age relationships between basin fills. In general, lower sediment input rates and periods of sediment bypass result in sand-poor convergent stratal patterns, and episodic but high volumetric sedimentation rates lead to well-defined onlap with an increased probability of high sand content.

High-resolution, millennial-scale patterns of bed compensation on a sand-rich intraslope submarine fan, western Niger Delta slope

Near-seafloor core and seismic reflection-data from the western Niger Delta continental slope document the facies, architecture, and evolution of submarine channel and intraslope submarine fan deposits. The submarine channel enters an 8-km-long by 8-km-wide intraslope basin, where more than 100 m of deposits form an intraslope submarine fan. Lobe deposits in the intraslope submarine fan show no significant downslope trend in sand presence or grain size, indicating that flows were bypassing sediment through the basin. This unique data set indicates that intraslope lobe deposits may have more sand-rich facies near lobe edges than predicted by traditional lobe facies models, and that thickness patterns in intraslope submarine fans do not necessarily correlate with sand presence and/or quality. Core and radiocarbon age data indicate that sand beds southward during the late Pleistocene, resulting in the compensation of at least two lobe elements. The youngest lobe element is well characterized by core data and is sand rich, ∼2 km wide × 6 km long, and >1 m thick and was deposited rapidly over ∼4000 yr, from 18 to 14 ka. Sand beds forming an earlier lobe element were deposited on the northern part of the fan from ca. 25 to 18 ka. Seafloor geomorphology and amplitudes from seismic reflection data confirm the location and age of these two compensating lobe elements. A third compensation event would have shifted sand deposition back to the northern part of the fan, but sediment supply was interrupted by rapid sea-level rise during Meltwater Pulse 1-A at ca. 14 ka, resulting in abandonment of the depositional system.

Development of cutoff-related knickpoints during early evolution of submarine channels

Submarine channels are often thought of as having relatively simple geometries, with significant along-channel morphologic and stratigraphic continuity. Using high-resolution seismic reflection data from offshore Angola and a kinematic model of channel evolution, we present evidence that channels on the seafloor can develop slope variability as a result of meander cutoff events. When cutoffs develop, the shortened flow paths produce locally steep gradients, thus initiating knickpoints. Waves of knickpoint retreat and the related channel incision explain the occurrence of terraces and associated remnant channel deposits above the youngest channel thalweg. The simple processes of meander cutoff followed by knickpoint retreat are intrinsic to submarine channels and result in significant morphologic variability, erosion, and stratigraphic complexity, without any external forcing. These insights highlight the early evolution of submarine channels, a phase with a record that is commonly fragmented or completely absent as a result of subsequent erosion, and allow a better understanding of the autogenic controls on deep-marine stratigraphy.

Facies architecture of submarine channel deposits on the western Niger Delta slope: Implications for grain‐size and density stratification in turbidity currents

High-resolution bathymetry, seismic reflection, and piston core data from a submarine channel on the western Niger Delta slope demonstrate that thick, coarse-grained, amalgamated sands in the channel thalweg/axis transition to thin, fine-grained, bedded sands and muds in the channel margin. Radiocarbon ages indicate that axis and margin deposits are coeval. Core data show that bed thickness, grain size, and deposition rate strongly decrease with increasing height above channel thalweg and/or distance from channel centerline. A 5 times decrease in bed thickness and 1–2 ψ decrease in grain size are evident over a 20 m elevation change (approximately the elevation difference between axis and margin). A simplified in-channel sedimentation model that solves vertical concentration and velocity profiles of turbidity currents accurately reproduces the vertical trends in grain size and bed thickness shown in the core data set. The close match between data and model suggests that the vertical distribution of grain size and bed thickness shown in this study is widely applicable and can be used to predict grain size and facies variation in data-poor areas (e.g., subsurface cores). This study emphasizes that facies models for submarine channel deposits should recognize that grain-size and thickness trends within contemporaneous axis-margin packages require a change in elevation above the thalweg. The transition from thick-bedded, amalgamated, coarser-grained sands to thin-bedded, nonamalgamated, finer-grained successions is primarily a reflection of a change in elevation. Even a relatively small elevation change (e.g., 1 m) is enough to result in a significant change in grain size, bed thickness, and facies.

Global (latitudinal) variation in submarine channel sinuosity: COMMENT

[Peakall et al. (2012)][1] propose that submarine channel sinuosity correlates with latitude, and conclude that this correlation results from modification of the turbidity flow field by the Coriolis force. However, a closer look at the data and simple physical arguments suggest that slope and the

A New Approach for Conditioning Process-Based Geologic Models to Well Data

Generating a realistic earth model that simultaneously fits data observed at multiple well locations has been a long-standing problem in petroleum geology. Two insights are offered for solving this problem in a Bayesian framework. The first is conceptual—it connects geologic inversion to the new field of probabilistic programming and shows that the usual description of a Bayesian problem in terms of a graphical model is inadequate for describing a process-based geologic model due to the dynamics of the generative algorithm. This is a paradigm shift in probabilistic modeling where stochastic generative models are represented using a syntax resembling modern programming languages. Probabilistic programming allows one to generalize this structure to include complex programming concepts, while also simplifying the process of developing new inference algorithms. The second insight is algorithmic and involves using variational inference to derive a simpler, more computationally tractable approximation to the posterior probability density function. If this surrogate distribution is close to the true posterior, it allows for very fast simulation of an arbitrary number of models that all fit the data equally well. This study focuses on the particular geologic formation known as submarine lobes: elongated pancake-like formations which are sequentially laid down, one on top of the other over geologic time, forming potential petroleum reservoirs. The location and orientation of the lobes at each time step are the variables that are optimized so that, at the final time step, all available well data are approximately fit. The methodology is illustrated on synthetic data as a proof-of-concept, and compared to several alternatives. An important conclusion is that, even though the variational approximation is crude, it produces better predictions than any point-based method, including maximum likelihood. The fact that probabilistic programming outperforms conventional Bayesian approaches in the case of lobe models offers the potential for attacking more complicated forward models where multiple geologic processes are simultaneously active.

Submarine Fans and Their Channels, Levees, and Lobes

Submarine fans are complex morphological features that develop on the continental slope, rise and abyssal plain, normally at the mouths of submarine canyons. They are constructed principally from the deposits of sediment gravity flows (mainly turbidity currents and debris flows) as terrigenous and shallow marine sediment is redistributed into deeper water. In this chapter we focus on the most important building blocks of submarine fans: leveed submarine channels and the submarine lobes they feed. Mass transport deposits are also important components of many submarine fans; they are described in the Chapter on “ Submarine Canyons and Gullies”. Submarine channels are the most noticeable geomorphic features on submarine fans, linking net erosional elements like canyons and gullies to net depositional elements like submarine lobes. They develop through both erosional and depositional processes, and have straight to highly sinuous planform geometries. Where they are flanked by aggradational levees or are entrenched into the seabed, they provide stable pathways through which sediment is transported and partitioned into different fan settings. Coarse-grained sediment commonly accumulates on the floors or at the mouths of submarine channels; finer-grained sediment preferentially accumulates on channel banks and on adjacent aggradational levees. In this chapter we describe the wide range of morphological features recognised on the surfaces of submarine fans, and the physical processes that shape the seabed in areas where submarine channels, levees, and lobes develop.

ABSTRACT: Textural Trends in Deposits of Collapsing Turbidity Currents: Sedimentology of Turbidites and Slurry Beds from the Oligocene Flysch of the East Carpathians, Romania

Normal size grading has been considered one of the characteristic features of turbidites. However, there is growing evidence that textural trends in relatively thick sandy turbidites are more complex. In addition to vertical textural trends, the distribution of detrital mud is another important property of sand-rich sediment gravity flow deposits. Lowe and Guy (2000) described ‘slurry beds’ of the Britannia Formation, deposits of mud-rich gravity currents transitional between turbidity currents and debris flows. This report focuses on deep-water sands of the Oligocene flysch of the East Carpathians, Romania. In three outcrops in the Buzau Valley area, that expose deep-water rocks of Oligocene age (Fusaru Sandstone and Lower Dysodilic Shale), layers of mud-rich sand occur not only as individual sedimentation units, but also as parts of sequences with turbidite divisions of Bouma (1962) and Lowe (1982). Mud-poor and mud-rich sedimentation units were selected for quantitative textural study. A total of 87 thin sections were point counted for grain size. Three hundred quartz and feldspar grain lengths were measured in each thin section.