Kenneth J. Hsü

Catastrophic Debris Streams (Sturzstroms) Generated by Rockfalls

Large rockfalls commonly generate fast-moving streams of debris that have been called “sturzstroms.” The geometry of sturzstrom deposits is similar to that of mudflows, lava flows, and glaciers. Sturzstroms can move along a flat course for unexpectedly large distances and may surge upward by the power of their momentum. A currently popular hypothesis to account for their excessive distance of transport suggests that sturzstroms slide on air cushions. Contrary to that hypothesis, evidence is herein presented to support Heim9s contention that sturzstroms indeed flow. The flow of a sturzstrom can be compared to flow of a mass of concentrated cohesionless grains in a fluid medium. Frictional resistance to such grain flow is, according to Bagnold, less than that for sliding of rigid bodies because of the buoyancy of an interstitial fluid which serves to reduce the effective normal pressure of the entrained grains. The presence of sturzstrom deposits on the Moon indicates that the interstitial fluid is not necessarily a compressed gas or a wet mud. The dispersion of fine debris and pulverized rock dust among the colliding blocks may have provided an uplifting stress during the motion of some terrestrial and lunar sturzstroms. Scale models to provide kinematic simulation of sturzstroms may have practical application. Preliminary results suggest that a bentonite suspension of a certain consistency is a suitable material for scale models and that the flow of thixotropic liquids is kinematically similar to sturzstroms. The parameter “excessive travel distance” is introduced to replace the expression “equivalent coefficient of friction” as a measure of mobility of sturzstroms. There is, on the whole, a positive semilog correlation of the excessive travel distance to the size of the fallen mass. Exceptions to the rule include on the one extreme the unusual mobile Huascaran rockfall which gave rise to a sturzstrom with a dense interstitial mud and, on the other extreme, the least mobile Vaiont rockslide which remained a sliding block and failed altogether to generate a sturzstrom.

Mesozoic overthrust tectonics in south China

Whether south China was a platform or a para-platform after the mid-Paleozoic deformation or whether the mountains of Huanan (= south China) form a Mesozoic collision-type orogen depends upon whether the ophiolite melange in the Yangtze region is Precambrian basement or was overthrust onto a foreland thrust belt during the Mesozoic. This paper presents evidence of tectonic windows under the melange and basement klippes above the melange in the Yangtze region.

Mass Mortality and Its Environmental and Evolutionary Consequences

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 oceans 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.

Messinian event in the black sea

Abstract Three holes were drilled during the 1975 DSDP Leg 42B drilling the Black Sea. A section from Hole 380, at 2107 m water depth on the western edge of the abyssal plain, is 1074 m thick, and provides the most complete stratigraphic section. Dating of the sediments is based upon (1) fossil evidence from pollen, crustaceans, benthic foraminifera, and diatoms, (2) correlation with climatic changes and with unusual isochronous events that have been dated elsewhere, (3) paleomagnetic data, and (4) estimates of sedimentation rate. The history of Black Sea sedimentation recorded by the DSDP cores includes black shale sedimentation during the Late Miocene, followed by periodic chemical sedimentation from Late Miocene to Early Quaternary, and a change to dominantly terrigenous sedimentation from the Middle Quaternary. These hemipelagic and turbiditic sediments were deposited in lacustrine and brackish marine environments. The Messinian sediments, however, consist of stromatolitic dolomite, oolitic sands, and coarse gravels, deposited in supratidal and intertidal environments. The intercalation of the shallow-water sediments in a deep-water sequence suggests a drastic lowering of the water-level within the Black Sea basin during the Messinian so that the edge of the present abyssal plain was then the edge of a shallow lake. The Messinian draw-down phase of the Black Sea was in existence for about 100,000 years during the Lago-Mare stage of the salinity crisis. The evaporated waters formed an alkaline lake before it was drowned by a brackish marine transgression correlative to the Trubi transgression of the Mediterranean.

Tectonic Evolution of the Mediterranean Basins

Compared to the Atlantic Ocean, the opposite shores of which fit together almost perfectly, the Mediterranean is more complex, being bounded on the north by three peninsulas which run at right angles to the trend of the sea. To make matters worse, the Mediterranean Ridge has proved to be a compres-sional feature quite distinct from a normally extensional mid-ocean ridge. One can hardly postulate a simple splitting apart of the adjacent continents to account for the origin of this inland sea. Unlike the Pacific Ocean, with its circumoceanic orogenic system, the Mediterranean is not rimmed on all sides by coastal mountains. Thus, we cannot invoke circum-Mediterranean subduction to explain the genesis of the Mediterranean.

Mesozoic and Cenozoic Sedimentary History of South China

Mesozoic and Cenozoic strata of South China contain commercial oil and gas, as well as coals, evaporites, oil shales, and sedimentary copper deposits. The pre-Mesozoic history includes Proterozoic to Silurian depositional episodes, Caledonian orogeny, younger depositional phases, and Hercynian events. The Mesozoic and Cenozoic tectonic setting formed from collision-type orogenies involving terranes and associated suture zones. The Yangtze and Huanan (South China) terranes collided during the Indosinian orogeny whereas the Huanan and coastal terranes collided later in the Mesozoic. The pre-Indosinian Triassic shows Paleotethys-type facies, which include basinal, shallow marine, and continental deposits. After the Indosinian, several gulfs formed in basins that were transgressed primarily from the west, and they became swamps, lakes, and alluvial plains. A unified continental region formed in the earliest Jurassic; marine conditions continued to affect only a part of southeastern China. The Jurassic of South China was characterized mainly by a continental basin in the west and volcanism in the east. Overall, the climate changed to more arid conditions. During the Cretaceous, a series of continental, riftlike basins formed in the east as basinal development in the west waned and then effectively ceased. Climate was arid and semiarid, as indicated by red beds and evaporites. The Tertiary basins tend to reflect downfaulting in the Paleogene, with some lacustrine sediments (even local source rock), and downwarping under fluvial conditions during the Neogene. The climate showed areal differentiation and in many areas changed from humid to arid with time.

Swiss lakes as a geological laboratory

ConclusionsVarves are sediments registering annual events. Carbonate varves register annual precipitation of calcium carbonate. Glacio-lacustrine varves record an annual freeze-over. The Swiss lakes provide field sites where annual changes of biologic productivity can be monitored, and where secular climatic variations can be recognized. Our investigations of lake sediments have helped not only to understand the modern lacustrine deposition, but also to develop an insight into the interrelations between climate, productivity, and sedimentation.

Ultrahelvetic Flysch Sedimentation and Deformation Related to Plate Tectonics

Arguments are presented to support the proposition that Ultrahelvetic Flysch basins were present on both sides of the Habkern island arc that was convex to the southeast during Paleocene time. The width of this Ultrahelvetic realm of sedimentation is estimated to have been at least 300 km in a northwest-southeast direction; this estimate is based upon comparison with the geometrical dimensions of modern basins and on paleoecological considerations. The deformational history of the Ultrahelvetic Flysch is interpreted in terms of plate tectonics. Two plates were involved, a Helvetic plate and a Penninic plate. The origin of the Flysch basins was related to the descent of the northward-moving Penninic plate under an island arc which was at the leading edge of the Helvetic plate. During late Eocene and early Oligocene, a plate flip took place and the Helvetic plate then descended below the Penninic plate. This reversal in plate relationships accounts for the occurrence of the discordant Bergell Pluton and concurrent andesitic volcanism, which supplied detritus to the North Helvetic Flysch.

The messinian salinity crisis

Two major discoveries made by the deepsea drilling during the last decade were the discovery of the Late Miocene desiccation of the Mediterranean Sea and of the Late Miocene glacial advance of the Antarctic. The coincidence in timing suggests a causal connection between the two. The desiccation of the Mediterranean may have been the catalytic event which led to a momentous expansion of the Antarctic ice cap. Or conversely, a world-wide lowering of the sea level as a consequence of Antarctic glaciation rriay have closed off a shallow connection between the Atlantic and the Mediterranean and resulted in the isolation and eventual desiccation of the latter. A paper by Adams et al. [1] presented evidence to support a contention that there was indeed a eustatic drop of sea level during the late miocene time, and that this world-wide change was the cause of the Messinian salinity crisis. Stratigraphical data from widely separated regions of Australia, of Fiji, of Marshall Islands, of North and South Florida, and of Spain have been evaluated; they all show a stratigraphic hiatus including the Messinian timespan. My associates and I have discussed various, far-reaching geomorphic, climatic, zoogeographic, and paleobotanical changes of the circum-Mediterranean world, when the water levels within the Mediterranean basins were drastically lowered. An appreciable eustatic lowering of sea level would have world-wide consequences and leave an impressive geological record behind. Perhaps the record is indeed so extraordinary that one Japanese geologist was moved to write a book and spinned a yawn of a 2000-m eustatic sealevel change since the Late Miocene [2]. While I do not for one minute believe in that tall figure, it may nevertheless be worthwhile that we take a closer look at some of his strange tales. Since the precise dating of the numerous events of the Late Miocene is fraught with difficulties, we may never be able to decide if the Messinian Antarctic glaciation was the cause or the consequence of the Mediterranean desiccation. It seems satisfying that the isolation of the Mediterranean could be explained by a slight drop of sea level, so that we need not postulate an orogenic uplift of the Betic Portal at a rate exceeding the erosive power of invading Atlantic waters. On the other hand, we must then look for another cause to explain the explosive expansion of Antarctic ice cap; we could not relate the glaciation which caused the eustatic change to the extraction of 6% of the dissolved salt in the world, or to an increased albedo effect when an appreciable portion of a darkblue sea was replaced by a white salt desert. Of course, the workings of nature is commonly more sophisticated than we normally dare to assume. Cause and effect were perhaps interacting in such a way that the final resonance was the two Messinian catastrophes: the Antarctic glacial advance and the Mediterranean desiccation. K.J. Hsti Swiss Federal Institute of Technology, Zurich

Application of an eutrophic lake model to the origin of ancient organic‐carbon‐rich sediments

In this paper we approach the problem of the origin of organic-carbon-rich sediments by using an integrated isotopic and organic geochemical study of the annual productivity/eutrophication cycle in the water column of Lake Greifen (Switzerland) and the historical expression of such cycles as found in the sediments. The Lake Greifen water column and sediment studies reveal that Δδ13C carbonate-organic matter is correlated with the hydrogen indices (HI) of kerogens, explicable in terms of changing productivity and preservation of the organic matter, and the CO2 budget of the water body. The lake model implies that, if high productivity in CO2 limited surface waters (high nutrient/CO2 ratio) was controlling the preservation of organic matter, a correlation between decreasing Δδ13C carbonate -organic matter and increasing HI values may be observed. In contrast, if low to moderate productivity in CO2 unlimited surface waters (low nutrient/CO2 ratio) where bottom water anoxia promotes the preservation of organic matter, a correlation between increasing Δδ13C carbonate-organic matter and increasing HI values may be observed. Application of this model to two well known Jurassic sequences, gave a correlation of increasing HI values with decreasing Δδ13C carbonate-organic matter for the Kimmeridge Clay Formation (United Kingdom), and increasing HI values with increasing Δδ13C carbonate organic matter for the early Toarcian shales (France). This suggests that the controlling factor of the former deposit was high primary bioproductivity and of the latter deposit anoxic bottom water conditions.