http://basin.earth.ncu.edu.tw/download/courses/seminar_MSc/2012/1018_1_1.pdf

Marine hyperpycnal flows: initiation, behavior and related deposits. A review

A marine hyperpycnal plume is a particular kind of turbidity current occurring at a river mouth when the concentration of suspended sediment is so large that the density of the river water is greater than the density of sea water. The plume can then plunge and possibly erode the seafloor to become self-maintained for a particular period of time (hours to weeks). Frequency of hyperpycnal plumes emanating from river discharge can be predicted with knowledge of rating curve characteristics, particularly during flood conditions. Examples of these curves are shown for middle-sized North American rivers. Semi-empirical relationships among average discharge, average sediment concentration, and the discharge during flood are proposed and applied to 150 world rivers. Results show the importance of small and medium sized rivers in their ability to trigger underflow at their mouth. There are at least nine "dirty" rivers that may trigger underflows during one or more periods of the year. Most other rivers are cleaner...

Turbidity Currents Generated at River Mouths during Exceptional Discharges to the World Oceans

Although it is well known that metabolism, feeding, and behaviour of lotic organisms is influenced by various flow characteristics, hydraulic variables usually are not accurately measured in lotic ecology studies. Using an approach we call "hydraulic stream ecology", we link organismic responses to a more comprehensive treatment of the physical environment. Since a unified analytical solution for all important hydraulic variables in running waters does not exist at the moment, we advocate a simpler view of the physical system. We demonstrate methods for estimating complex hydraulic key characteristics, like turbulence in the free flow, turbulence close to the stream bottom, and the force of flow prevailing at the bottom. Calculations of these complex key characteristics require measurement of simple hydraulic characteristics like mean velocity, water surface slope, depth, bottom roughness, kinematic viscosity, and density of the water. The hydraulic environment shows characteristic patterns within whole c...

Hydraulic Stream Ecology: Observed Patterns and Potential Applications

Carbonate minerals are a common constituent of lacustrine sediments. The great variability with respect to geological setting, climatic environment, water chemistry, and biological activity limits generalizations about mechanisms of carbonate sedimentation. On the whole, in constrast to an oceanic environment, the bulk of primary lacustrine carbonates are inorganic chemical precipitates. Two distinctly different geological settings can be recognized: (1) carbonate and evaporite deposition in brine lakes or on playas in arid regions (see Chap. 8, this volume), and (2) carbonate sedimentation in fresh- and brackish-water lakes in humid regions. Occurrences of lacustrine chalks and marls in young geological formations of temperate regions have been described since the time of Lyell (1830). In northern America and Europe, Late Quaternary chalks were found near still existing lakes and their genesis was related to a postglacial period of climatic amelioration (e.g., Heim, 1919). Typically, these deposits are fine grained, either rhythmically laminated or massive, white to dull-yellowish gray chalks to marls. This distinction is qualitative based on a bulk carbonate content boundary around 60%. The dominant mineral is calcite. Similar varve-like carbonate or marl sediments were found in older lacustrine deposits (e.g., Bradley, 1929). Nipkow (1920) described recent analogies to laminated lacustrine carbonates in Lake Zurich; the light laminae are rich in CaCO3. Forel (1901) early recognized a biological role, but Minder (1922, 1926) developed the concept of inorganic, biogenically induced calcite precipitation. Meanwhile lacustrine marls in regions of Quaternary glaciation around the Great Lakes in North America were studied (e.g., Davis, 1901; Pollock, 1918). Calcite precipitation in those biologically active hardwater lakes (e.g., Halbfass, 1923; Ruttner, 1962; Pia, 1933; Ohle, 1952; Hutchinson, 1957; Wetzel, 1975) was soon related to extraction of Co2 during photosynthesis by algae.

Freshwater Carbonate Sedimentation

Summary
The literature on the structure and behaviour of gravity currents is reviewed, with emphasis on some recent studies, and with particular attention to turbidity currents, though reference is also made to comparable behaviour in pyroclastic flows. Questions of definition are discussed, in particular the distinction between dense currents, which may deposit en masse, and more dilute currents. High-density dispersions may exist as a discrete, independently moving layer beneath a more dilute flow, as the basal part of a continuous density distribution or possibly as a transient depositional layer. Existing theory appears inadequate to explain the behaviour of some high-density dispersions. Surge-type currents are contrasted with quasi-steady currents, which may be generated by a variety of mechanisms including direct feed by rivers in flood. Such fluvially generated currents provide one means of generating currents with reversing buoyancy. Geologically significant turbidity currents are impractical for direct study owing to their large scale and (often) destructive nature. Small-scale laboratory currents offer a wealth of insights into turbidity current behaviour. This paper summarizes recent experimental studies that focus on the physical structure of gravity currents, with emphasis on the velocity and turbulence structure, the vertical density distribution and the stability of stratification. Preliminary quantification of the turbulence structure (including controls on turbulent entrainment, turbulent kinetic energy, Reynolds stresses and turbulence production) has been facilitated by recent technological developments that have allowed the measurement of instantaneous fluctuations in both velocity and concentration. Laboratory models, however, generally involve substantial simplification, and require compromises in some parameters to achieve adequate scaling of the parameters of most interest. Mathematical modelling also provides important insights into turbidity current behaviour. We discuss various approaches to modelling, ranging from simple hydraulic equations to systems of partial differential equations that explicitly treat conservation of momentum, fluid and sediment mass, and turbulent kinetic energy. The application for which the model is designed (i.e. to calculate mean head velocity or to create an instantaneous two-dimensional contour plot of downstream velocity in a current) determines the complexity of the mathematical model required. The behaviour of suspension currents around topography is complex and depends upon the relative height of the topography, and upon the density and velocity structure of the current. Many interactions with topography are well described by the internal Froude number, Fri. Both reflection and deflection of currents may occur on the upstream side of topography, depending upon Fri. On the downstream side of topography, flow separation, lee waves or hydraulic jumps may occur.

The structure and fluid mechanics of turbidity currents: a review of some recent studies and their geological implications

The latest Mesozoic and earliest Tertiary sediments at Deep Sea Drilling Project site 524 provide an amplified record of environmental and biostratographic changes at the end of Cretaceous. Closely spaced samples, representing time intervals as short as 102 or 103 years, were analyzed for their bulk carbonate and trace-metal compositions, and for oxygen and carbon isotopic compositions. The data indicate that at the end of Cretaceous, when a high proportion of the ocean9s planktic organisms were eliminated, an associated reduction in productivity led to a partial transfer of dissolved carbon dioxide from the oceans to the atmosphere. This resulted in a large increase of the atmospheric carbon dioxide during the next 50,000 years, which is believed to have caused a temperature rise revealed by the oxygen-isotope data. The lowermost Tertiary sediments at site 524 include fossils with Cretaceous affinities, which may include both reworked individuals and some forms that survived for a while after the catastrophe. Our data indicate that many of the Cretaceous pelagic organisms became extinct over a period of a few tens of thousands of years, and do not contradict the scenario of cometary impact as a cause of mass mortality in the oceans, as suggested by an iridium anomaly at the Cretaceous-Tertiary boundary.

https://science.sciencemag.org/content/sci/216/4543/249.full.pdf

Mass mortality and its environmental and evolutionary consequences.

Reconnaissance air-gun seismic reflection profiles and sonobuoy refraction measurements were recorded in 17 perialpine lakes. Quaternary fill velocities derived from sonobuoy refraction records suggest a stratigraphic sequence consisting of two general elements. The upper interval with velocities between 1.5 and 1.9 km/sec may represent normal sedimentation of glaciolacustrine sediments, whereas higher velocities of 1.9 to 2.7 km/sec in a lower element probably result from glacier overriding and compaction. The refraction data also show bedrock velocities from 3.6 to 6.7 km/sec in concert with underlying geological formations. Reflection sections from the deep basins show modest overdeepening of the lakes north of the Alps and deep incision of the southern perialpine lakes. The data are consistent with an interpretation of glacial overdeepening of fluvially cut bedrock valleys, overriding of older glaciolacustrine deposits (Riss?), and limited erosion by the Wurm glacier streams that seem to scoop but not downcut the basins. We found no evidence for late Quaternary faults.

Seismic stratigraphy and bedrock forms in perialpine lakes

Speed and direction of bottom currents induced by density underflow of two sediment-laden rivers were measured by oceanographic current meters in the Walensee (= Lake of Walenstadt), Switzerland. The apparently shooting flow of currents (up to 30 cm/s in this study) is suggested as an explanation for laminations in turbidite sequences. The current speed apparently stabilizes on slopes around 2°; this angle seems to correspond to the critical slope where the flow of the measured currents becomes steady. Current direction is controlled by bottom topography and direction of river inflow. Reversal of current direction observed at two sites is probably due to the underflow-induced backward motion of the overlying lake water. Underflow activity in Walensee is correlative with density peaks of the river water input. The currents are compared to Lake Mead (Southwestern U.S.) underflows and sporadic currents in some submarine canyons.

Measurements of Density Underflows from Walensee, Switzerland

Turbidites After Ten Years of Deep Sea Drilling Wringing Out the Mop

An ash bed has been recognized as a chronostratigraphic horizon in the Ventura basin, California, at a level near the Pliocene-Pleistocene contact. Suites of samples just above and below the ash bed were taken for facies analyses from seven outcrop sections and one well. The rock types recognized on the basis of petrographic and microfaunal analyses are (1) tuff, (2) laminated shales, (3) mudstones, (4) cobbly mudstones, (5) sandstones, with graded bedding, (6) sandstones and siltstones, without graded bedding and commonly cross-laminated, and (7) conglomerates with mud matrix. These sediments are believed to be the products of four different kinds of sedimentary agents: (1) catastrophic sediment-gravity flows which deposited muddy conglomerates near the source, (2) turbidity currents which deposited graded sands distant from the source, (3) bottom currents which sorted and reworked turbidites and deposited cross-laminated silty sediments, and (4) surface currents which deposited laminated shales in the anoxic zone, and mudstones where bioturbation wa active. The sediment types are present in four characteristic associations: (1) the conglomerate facies characterized by an interbedded sequence of conglomerates, cobbly mudstones, mudstones, and sandstones; (2) the graded-sand facies characterized by sequences of turbidite sands with graded bedding, separated by laminated-shale intercalations; (3) the thin-bedded sand facies characterized by an interbedded sequence of thin-bedded, laminated, or cross-laminated sandstones, mudstones, and shales; and (4) the mudstone facies characterized by monotonous sequences of mudstones, with rare intercalations of laminated shales and cross-laminated siltstones. The paleogeography, paleobathymetry, paleooceanography, and geometry and trends of the sediment bodies led to the identification of those four associations, or facies, as sediments of (1) submarine canyons and fans, (2) basin trough, (3) basin flanks or continental rise, and (4) basin slopes or escarpments respectively of the Pliocene-Pleistocene Ventura basin. The Ventura facies model emphasizes longitudinal transport and deposition of turbidite sands as potential reservoirs in a basin-trough setting, and is thus different from the submarine-fan model which postulates deposition of fine sediments there.

Kerry Kelts

Papers

Mass mortality and its environmental and evolutionary consequences.

Seismic stratigraphy and bedrock forms in perialpine lakes

Measurements of Density Underflows from Walensee, Switzerland

Turbidites After Ten Years of Deep Sea Drilling Wringing Out the Mop

Resedimented Facies in Ventura Basin, California, and Model of Longitudinal Transport of Turbidity Currents