Geological controls on the geometry of incised-valley fills: Insights from a global dataset of late-Quaternary examples

Incised valleys that develop due to relative sea-level change are common features of continental shelves and coastal plains. Assessment of the factors that control the geometry of incised-valley fills has hitherto largely relied on conceptual, experimental or numerical models, else has been grounded on case studies of individual depositional systems. Here, a database-driven statistical analysis of 151 late-Quaternary incised-valley fills has been performed, the aim being to investigate the geological controls on their geometry. Results of this analysis have been interpreted with consideration of the role of different processes in determining the geometry of incised-valley fills through their effect on the degree and rate of river incision, and on river size and mobility. The studied incised-valley fills developed along active margins are thicker and wider, on average, than those along passive margins, suggesting that tectonic setting exerts a control on the geometry of incised-valley fills, likely through effects on relative sea-level change and river behaviour, and in relation to distinct characteristics of basin physiography, water discharge and modes of sediment delivery. Valley-fill geometry is positively correlated with the associated drainage-basin size, confirming the dominant role of water discharge. Climate is also inferred to exert a potential control on valley-fill dimensions, possibly through modulations of temperature, peak precipitation, vegetation and permafrost, which would in turn affect water discharge, rates of sediment supply and valley-margin stability. Shelves with slope breaks that are currently deeper than 120 m contain incised-valley fills that are thicker and wider, on average, than those hosted on shelves with breaks shallower than 120 m. No correlation exists between valley-fill thickness and present-day coastal-prism convexity, which is measured as the difference in gradient between lower coastal plains and inner shelves. These findings challenge some concepts embedded in sequence stratigraphic thinking, and have significant implications for analysis and improved understanding of source-to-sink sediment route-ways, and for attempting predictions of the occurrence and characteristics of hydrocarbon reservoirs

Search for the lepton-family-number nonconserving decay \mu -> e + \gamma

The MEGA experiment, which searched for the muon- and electron-number violating decay \mu -> e + \gamma, is described. The spectrometer system, the calibrations, the data taking procedures, the data analysis, and the sensitivity of the experiment are discussed. The most stringent upper limit on the branching ratio of \mu -> e + \gamma) < 1.2 x 10^{-11} was obtained

Incised valley paleoenvironments interpreted by seismic stratigraphic approach in Patos Lagoon, Southern Brazil

<div><p>ABSTRACT: The Rio Grande do Sul (RS) coastal plain area (33,000 km 2 ) had its physiography modified several times through the Quaternary, responding to allogenic and autogenic forcings. The Patos Lagoon covers a significant area of RS coastal plain (10,000 km 2 ), where incised valleys were identified in previous works. About 1,000 km of high resolution (3.5 kHz) seismic profiles, radiocarbon datings, Standard Penetration Test (SPT) and gravity cores were analyzed to interpret the paleoenvironmental evolution as preserved in incised valley infills. Seismic facies were recognized by seismic parameters. The sediment cores were used to ground-truth the seismic interpretations and help in the paleoenvironmental identification. Key surfaces were established to detail the stratigraphical framework, and seismic facies were grouped into four seismic units, which one classified in respective system tracts within three depositional sequences. The oldest preserved deposits are predominantly fluvial and estuarine facies, representing the falling stage and lowstand system tracts. The Holocene transgressive records are dominated by muddy material, mainly represented by estuarine facies with local variations. The transgression culminated in Late Holocene deposits of Patos Lagoon, representing the highstand system tract. The depositional pattern of the vertical succession was controlled by eustatic variations, while the autogenic forcing (paleogeography and sediment supply) modulated the local facies variation.</p></div

Letter to H.B. Stenzel from H.N. Fisk on 1941-01-18

Jackson School of Geoscience

Letter to H.B. Stenzel from H.N. Fisk on 1953-05-28

Jackson School of Geoscience

Letter to H.B. Stenzel from H.N. Fisk on 1940-12-09

Jackson School of Geoscience

Fill characteristics of abandoned channels and resulting stratigraphy of a mobile sand‐bed river floodplain

Floodplains are diverse sedimentary environments where infill processes of abandoned channels interact with overbank sedimentation and bank erosion. The result, particularly in river systems with high suspended load and rapid channel migration, is a complex three‐dimensional mosaic of deposits with spatial variability in terms of grain‐size, age, organic carbon content and resistance to erosion. Abandoned channels represent a significant deposition volume in fluvial systems that can accommodate large proportions of the equivalent material mobilised during their abandonment. However, time scales and fill processes vary between different kinds of abandoned channels and the sediment calibre involved and are not fully understood, particularly in respect to highly dynamic sand‐bed rivers. This study investigates time scales and spatio‐temporal patterns of infill of abandoned chute channels and abandoned channel segments left behind following neck cutoff of meander bends. The study focuses on the Rio Beni, a large, tropical, sand‐bed river in the Bolivian Amazon basin. Electrical resistivity ground imaging is used to elucidate the stratigraphy of floodplains and satellite imagery is employed to investigate contemporary fill processes and rates. Given suitable bend migration patterns, chute channels may remain stable for several years but are eventually abandoned and rapidly filled with bed material during a single flood season. Smaller scroll sloughs can convey coarse bedload across point bars and, when filled, present stratigraphic bodies similar to chute fills. Abandoned meander bends tend to develop plug bars at both ends immediately after cutoff. Of these bars, the downstream plug aggrades at a faster rate due to the often larger diversion angles with the main channel and efficiently seals off the bend. The subsequent infill of the channel is a function of hydraulic connectivity and distance to the active channel as well as rate of lake deposition. Considerable overbank deposition can increase the spatial sedimentological heterogeneity of these floodplains, which needs to be taken into account in floodplain evolution models