Directional Sedimentary Structures Research Articles

Landscape configuration of eastern South China during the late Neoproterozoic: New constraints from sedimentary indices and zircon U-Pb-Hf isotopes of the southeastern margin of the Yangtze Block

The study of the paleogeographic landscapes of South China can contribute to understanding its paleo-position within supercontinents. We conducted a combined study of the Nanhuan-Sinian sedimentary records from the southeastern margin of the Yangtze Block (SMYB) in South China. The results reveal the following. (1) 18 datasets from directional sedimentary structures in the strata of the northern and southern SMYB indicate that the siliciclasts were transported towards the SE-SSE and the NWW-NW-NNW-NNE-NE, respectively. (2) 1670 analyses of 18 zircon samples collected from or near the directional structures have concordant U-Pb ages of 3243–563 Ma. 15 detrital zircon samples have a major peak within the range of 850–730 Ma, and 7 detrital zircon samples have εHf(t) values ranging from − 38.7 to + 13.7, and two-stage modal ages of 4.1–0.9 Ga. These new data reveal three important paleogeographic features of eastern South China during the late Tonian to Ediacaran, as follows. (a) The SMYB was a sedimentary basin (called the “Zhe-Gan-Wan Basin”) with opposing paleotopographic dips on two sides. Its northern part was mainly inclined to the SW-S-SE and belonged to the foreslope of the eastern Yangtze Platform. Conversely, its southern part was mainly inclined to the NW-N-NE and was the submerged part of the eroded area in the eastern Cathaysia Block (ECB) extending to the north. (b) The eroded area (called “Zhe-Min Land”) was probably located within the northeastern ECB and was surrounded by depositional areas on at least three sides. The ECB was likely connected to a Grenville-age orogen in the south direction. (c) The paleogeographic landscape configuration of eastern South China may have comprised five domains, from north to south: the eastern Yangtze Platform, the Zhe-Gan-Wan Basin, the Zhe-Min Land in the northeastern ECB, the Gan-Min-Zhe Basin, and a Grenville-age orogen. According to the landscape configuration, during the supercontinental cycles from Rodinia to Gondwana, the Cathaysia Block was likely located on the margin of the supercontinents, close to North India, West Australia, and East Antarctica.

A lithofacies terrain model for the Blantyre Region: Implications for the interpretation of palaeosavanna depositional systems and for environmental geology and economic geology in southern Malawi

The Blantyre City Area is part of the African savanna in southern Malawi. Sedimentological, geomorphological, chemical and mineralogical studies were conducted to create a lithofacies terrain model. The project involves mapping, cross-sectioning, grain size, heavy mineral analysis, XRD and the study of sedimentary textures under the petrographic microscope. These classical techniques were combined with GIS-based field and office works. The combined efforts led to 2-D maps and 3-D block diagrams that illustrate the geomorphological and sedimentological evolution of the landscape in southern Malawi during the late Mesozoic and Cenozoic. The results obtained through integrated geomorphological–sedimentological studies form the basis for land management (planning of residential areas, waste disposal sites, assessment of bearing capacity of rocks), geohazard prediction (delineation of high risk zones in terms of mass flow and inundation) and the evaluation of high-place (ceramic raw materials) and high-unit value (placers of precious metals and gemstones) mineral commodities in the study area. The study addresses regional and general aspects alike. In regional terms, the study aimed at unraveling the evolution of landforms at the southern end of the East African Rift System during the most recent parts of the geological past. Four stages of peneplanation were established in the working area. Planation was active from the Cretaceous to the Quaternary (stage I: early to mid-Cretaceous, stage II: early Tertiary, stage III: early to mid-Tertiary, stage IV: mid- to late Tertiary). During the most recent parts of the Quaternary, strong fluvial incision was triggered by the base-level lowering of the Shire River. Geomorphological alteration of the landscape goes along with a phyllosilicate–sesquioxide transformation from minerals indicative of more acidic meteoric fluids (e.g., gibbsite, kaolinite) to those typical of more alkaline conditions (e.g. smectite, vermiculite, hydrobiotite). In addition, the investigation is to provide some characteristic features suitable for the recognition and interpretation of terrestrial environments resembling the present-day savanna in the ancient sedimentary record: (1) Conspicuous bimodality in the grain-size distribution. (2) High degree of gravel roundness. (3) Boulder fields (basal conglomerates) with fitting breccias and no rotation of structural elements. (4) Poor to very poor sorting of siliciclastics. (5) Stratification is rare; if present only in sandy beds with antidune and tabular cross-bedding at low angle prevailing over cross-bedding at high angle. (6) Grading rare; in arenaceous deposits poorly developed FU sequences, in rudites poorly developed CU sequences, locally with surface armoring. (7) Directional sedimentary structures in coarse-grained deposits are unimodal. Fabric types: a(t)b(i) and a(p)b(i); shear planes indicative of slide may be present. (8) Ribbon-shaped architectural elements prevail over channel-like types. Bounding surfaces/unconformities are uneven to wavy and more widespread than scour-and-pool structures. (9) Fine-grained carbonaceous interbeds are restricted to shallow depressions. (10) Etch marks such as tafonis, flutes, honeycomb textures are common. (11) Placer deposits of metallic and non-metallic mineral commodities are abundant. (12) Alucretes and ferricretes of bog iron (ferrihydrite > goethite) and lateritic (goethite > ferrihydrite) types are common, calcitic freshwater limestones and gypcretes are scarce. (13) Kandite-group minerals “in” (relic), smectite-group minerals “in”, vermiculite “in” (recent), mica-group minerals “out”.

Stratigraphy and geochemistry of volcanic mass flows in the Stac Fada Member of the Stoer Group, Torridonian, NW Scotland

ABSTRACTThe Stac Fada Member, which forms part of the Stoer Group, contains the only definitive evidence of volcanic activity within the thick Torridonian succession. At the type area in the Bay of Stoer, four volcanic-rich sandy mudstone units are recognised in the Stac Fada. An irregular erosional contact at the base of the main Stac Fada unit truncates two thinner layers of similar volcanic-rich material that were formerly interpreted as intrusions. Directional sedimentary structures associated with the Stac Fada mass flows, and the distribution and thickness of an accretionary or armoured lapilli tuff, support the existence of at least two volcanic sources. Geochemical evidence suggests that the thick Stac Fada unit at the Bay of Stoer and comparable units to the SW were derived from the same volcanic centre. The chemical composititon of volcanic fragments in the main Stac Fada unit indicates that their potassic nature is not a primary feature but is due to alteration and subsequent metasomatism.

The palynology and provenance of the coal fragments contained in the late‐Pleistocene Lleiniog gravels of Anglesey, North Wales

AbstractThe numerous coal clasts in the late‐Pleistocene fluvioglacial Lleiniog Gravels of eastern Anglesey contain a varied assemblage of Westphalian A (and possibly also early Westphalian B) spores. Directional sedimentary structures indicate that the meltwater currents which deposited the gravels came mainly from the north and east, while glacial striae and erratic trains indicate that the parent (? Devensian) glacier flowed from the northeast. Consideration is given to the possibility that the coal clasts were derived from the breaking‐up, fairly locally, of far‐travelled rafts of Coal Measures. It is concluded, however, that the most probable explanation is still Greenly's (1919) postulate that the source is a yet‐to‐be‐identified submarine outcrop situated only a few kilometres northeast of Lleiniog.

Uranium Deposits in Channel Curves of Salt Wash Sandstone Member of Morrison Formation (Late Jurassic), Utah: ABSTRACT

The Salt Wash Member of the Morrison Formation in southeastern Utah is characterized by fluvial sandstone in distinct paleochannels. The deposits are part of a large delta-fan with an apex in the vicinity of the western Grand Canyon. The exhumed channel fills, which excellently expose directional sedimentary structures, were deposited by streams of slight to moderate sinuosity, with occasional sharp bends. Such curves were loci of more conspicuous entrapment and burial of carbonaceous material. Later these areas became highly reducing environments where uranium ores precipitated from ground water percolating through permeable interconnecting channels. Major uranium deposits are more likely to occur in areas where such conditions existed. To determine areas with strong randomness of paleocurrent directions, the vector summation method was applied to the analysis of 2,638 paleocurrent measurements. Because this method is a sensitive measure of dispersion, it was possible to outline areas displaying high and low vector strength values. Areas of high scatter commonly yield low values and polymodal distribution, and areas with low scatter are characterized by high values and unimodal distribution. Thus, areas of high channel sinuosity were successfully outlined. Major known uranium deposits occur most commonly in areas of low vector strength values. Such areas were compared with those containing known ore deposits and those considered favorable for future discoveries, and the results support the conclusion. Although the ve tor method has been used in many paleocurrent studies, this is the first time it has been applied in conjunction with other geologic criteria to delineate and select areas favorable for uranium exploration in sandstone deposits. The results show an important relationship among low paleocurrent vector strength values, carbonaceous material, channel curves, and uranium occurrences in the sandstone channels. End_of_Article - Last_Page 1696------------

Les paleocourants marins du bathonien moyen au bathonien superieur dans le nord de la campagne de caen (Normandie)

In the northern half of the Campagne de Caen (Normandy), the Bathonian deposits show a succession of four marine carbonate cross-bedded formations: Calcaire de Reviers, Calcaire de Blainville (Middle Bathonian), Calcaire de Ranville and Calcaire de Langrune (Upper Bathonian). The directional sedimentary structures are compiled, described and compared. The evolution of size, shape and direction of the main current-marks are followed through the lithological succession. The close association of large-scale planar cross-bedding and trough-bedding characterized the Calcaire de Reviers, at the base of the column: these structures are interpretated as vestigial patterns of large submarine sand dunes, more than 100 m in wavelength, cut off by meandering stream channels. The formation was laid down under shallowing water conditions on the northeastern margin of the Armorican Massif and resulted from the action of both wave- and tidal-currents. Earlier in Middle Bathonian times, the dimensions of the sand-bodies decreased as well as the depth of the depositional environment, while stream velocities strengthened. During the Late Bathonian, a minor deepening of the basin followed a non-sequence and led to a recurrence of the sedimentary evolution. From the Middle to the Late Bathonian, the resultants of the direction of the main currents reversed following a dextral rotation, so by successive steps it drifts from N 40 to N 180. In the palaeogeographical evolution of this area, this drastic change of the direction of main marine currents appears to be linked to epeirogenic movements and regional climatic modifications, which gave rise to a new pattern of marine currents at the beginning of the Late Bathonian times. By submergence of a swell (or a barrier), epeirogeny would be able to favour the southern spread of colder boreal water on the northern part of the Anglo-Paris Basin, contemporaneous with a distension phase which took place on the North Sea floor at this time.

Grain Orientation and Paleocurrent Systems in the Thorold and Grimsby Sandstones (Silurian), Ontario and New York

ABSTRACT Directional data obtained from sedimentary structures and grain orientations suggest that two major paleocurrent systems exist in the Thorold and Grimsby Sandstones (Silurian) along the Niagara Escarpment (Ontario and New York State). The two systems are documented by an analysis of directional data at both an outcrop and a regional level At both levels of analysis, the total sampled populations of the quartz grain orientations were found to he inhomogeneous. Homogeneous grain orientation subpopulations were found which agree with and complement the data from directional sedimentary structures. One paleocurrent system trends northwest, as shown by cross-bedding and the grain orientations of cross-bedded units of the New York area. The statistical characteristics of the directional data from this system suggest the clastics of the Medina Formation were derived from the Appalachian region. A second paleocurrent system trends southwest, and contains common paleocurrent reversals to the northeast. This system is recorded in the azimuths of cross-bedded units of Ontario, and in grain orientations of most of the beds that contain parallel laminations or ripple marks from New York. The behaviour of the directional data of this second paleocurrent system suggests formation by a longshore or tidal paleocurrent.

Devonian Alluvial Fans, Prince of Wales Island, Arctic Canada

ABSTRACT Lower-Middle Devonian continental conglomerates, with minor sandstones, form a belt 150 miles long and up to 10 miles wide in eastern Prince of Wales Island, bordering an uplifted area of Lower Paleozoic and Precambrian rocks (Boothia Uplift). The succession forms part of the Peel Sound Formation; it reaches 1000 feet in thickness and towards the west the coarse clastics intertongue into sandstones, which in turn grade into shales and carbonates. Conglomerate clast types and directional sedimentary structures indicate derivation from the Boothia Uplift. The conglomerates are poorly sorted and mean grain size lies in the pebble range, comparing closely with modern alluvial fan deposits. Areal variations in clast lithologies are interpreted in terms of dispersal on eleven alluvial fans. Areal variations in grain size and clast roundness are interpreted as inter-fan in origin (gross differences between adjacent fans). Measurements of maximum clast size revealed intra-fan changes, namely decrease with distance of transport. Clast changes in vertical section are attributed to varying rates of uplift and erosion in the source area. Comparison of grain size and textural characteristics of the conglomerates with those of modern deposits suggests a debris flood mode of origin.

Upper Cretaceous-Paleocene Synorogenic Conglomerates of Southwestern Montana

The sequence of pebble types and pattern of sediment deposition observed in the thick (10,000+ ft) Late Cretaceous and Paleocene Beaverhead and Monida Formations in the Lima area of southwestern Montana permit determination of source areas, approximate carrying capacity and slope of streams transporting debris to the depositional site, and character of the depositional environments. These data lead, in turn, to an accurate reconstruction of the paleogeography in the Lima-Monida area during the Late Cretaceous and early Tertiary. Directional sedimentary structures and variations in maximum boulder size and pebble-cobble composition suggest the presence of two source areas. Both source areas, the Blacktail-Snowcrest uplift and ancestral Beaverhead Range, were domal uplifts and/or high-angle fault blocks, as indicated by the simple inverted sequence of pebble-cobble lithologic types, sandstone lithologic types, and heavy-mineral suites. Uplift of the source areas preceded emplacement of Laramide low-angle thrust plates, which probably originated as late-stage gravity slides off the ancestral Beaverhead Range. This sequence of Laramide events conflicts in part with the sequences suggested by workers in other areas of the Rocky Mountain thrust belt where synorogenic conglomeratic rocks are present. The coarseness, graded bedding, channeling, imbrication, bed geometry, and bedding characteristics indicate that the conglomerates of the Beaverhead Formation were deposited as alluvial-fan debris. The cross-bedded sandstone of the Monida Formation on the southeast may represent deposition on the distal parts of alluvial fans or possibly on adjacent fan aprons or floodplains.

Grain Orientation in a Berea Sandstone Channel at South Amherst, Ohio

ABSTRACT A particulate study of the orientation of elongate quartz sand grains in horizontal and vertical planes was performed on a well documented, sand-filled stream channel preserved in the Berea Sandstone, a protoquartzite, in Buckeye Quarry at South Amherst, northern Ohio. A mean compass orientation of 263 degrees (standard deviation = 18.8 degrees) with a mean magnitude of 17.7% was obtained for the imbricated, horizontally oriented grains. This direction is in good agreement with paleocurrent directions inferred from channel trend and crossbedding measurements. Paleocurrent studies of sandstones based on grain orientations have much broader application than those based on megascopic directional sedimentary structures.

Magnetic fabric of sediments from the La Jolla submarine Canyon and Fan, California

The results of measurements of the anisotropy of magnetic susceptibility of 24 box cores from the La Jolla Submarine Canyon and Fan are described and compared with magnetic fabrics produced under controlled conditions and with visible directional sedimentary structures. Many cores have a fabric similar to that produced during deposition from running water in the laboratory, and it is concluded that most of the sediment had a fabric of this type immediately after deposition. Most of the samples from the upper canyon have a “primary” magnetic fabric, characterised by a near horizontal magnetic foliation plane, susceptibility maxima well grouped in this plane and approximately parallel to the canyon axis, and q values ranging about 0.4. The sediments from the fan-valley and from the fan show, at least, traces of a similar type of primary fabric. In general, maximum susceptibility axes trend approximately parallel to the fan-valley axis and to the crests of the adjacent levees. The effects of two types of deformation were distinguished: the first of these, recognized by a tendency of the susceptibility minima to be streaked out toward the horizontal and by variable q values, is explained as being the result of forces tangential to the bedding plane, possibly the component of gravity acting tangentially to the side slopes of the channel. The second type of deformation, reflected in the magnetic fabric by low total anisotropy, high q values, and scattered directions of susceptibility axes, is thought to be due to the randomizing action of burrowing organisms.

Sedimentary structures in Huronian rocks of Ontario

A study of cross-bedding in the southern part of the Huronian outcrop belt indicates that the sediments were transported in a southerly direction (vector mean 160°). Symmetrical ripple marks show a similar preferred orientation (178°), and, together with dessication cracks, indicate shallow water conditions. Dimensional fabric of embedded clasts in massive polymictic conglomerates of the Gowganda Formation probably reflects glacial transport in a southerly direction. Clastic dykes in the Espanola and Serpent Formations are interpreted as injection structures from the underlying Bruce conglomerate. The dykes and ball and pillow structures may indicate tectonic instability during sedimentation. In the Cobalt-Gowganda area both directional sedimentary structures and preferred orientation of clasts in the Gowganda conglomerates indicate a southerly direction of transportation. A study of cross-bedding in the region around the Sudbury Basin suggests the presence of a positive area there in early Huronian times. In the Bruce Mines-Blind River area dimensional fabric of the Gowganda conglomerates suggests that the glaciers moved in a northeast-southwest direction in response to a regional palaeoslope. However, directional structures in associated waterlaid sediments indicate transport to the southeast, possibly reflecting a local southeasterly slope in the Huronian sedimentary basin. The presence of flute casts, groove casts, and graded bedding in the waterlaid sediments of the Gowganda Formation suggests the possibility of the activity of turbidity currents, but many other lines of evidence support a glacial origin for the conglomerates and laminated argillites.

Grain-Orientation and Paleocurrent Systems in Medina Formation: ABSTRACT

Several sedimentological parameters were examined to define the paleoenvironments of the Formation (Lower Silurian; outcrops along the Niagara Escarpment in Ontario and New York State). The diverse sedimentological parameters change regionally as well as vertically in a quasi-regular manner. The trends of such changes are similar in all the properties examined. The consistency within the basin of the variations of the variables suggests a deltaic paleoenvironment for the sediments. Two of the sedimentological parameters, grain orientation and directional sedimentary structures, were studied in the Thorold and Grimsby Sandstone Members of the Formation. Determination of grain orientation yielded a complex pattern of directions hardly explainable except by a mixed environment model. It was found that the total sampled population of the grain orientations of the Thorold and Grimsby Sandstones was nonhomogeneous. Utilizing a trial and error methodology homogeneous subpopulations were distinguished: 1. Subpopulations related to the type of sedimentary structure present in the sampled bed: (a) cross-bedded (subpopulation); (b) laminated (subpopulation); and (c) massive (subpopulation). 2. Subpopulations related to the stratigraphic location of the samples: (a) Thorold plus upper third of the Grimsby; and (b) lower two thirds of the Grimsby. 3. Subpopulations related to the geographic location of the samples: (a) east of the Niagara River (New York region); and (b) west of the Niagara River (Ontario region). The various subpopulations evidently are interrelated and they are partly duplicated in the behavior of the azimuths of the cross-beds and other directional sedimentary structures. The directional data obtained from the Thorold and Grimsby Sandstones suggest the existence of two major paleocurrent systems: 1. A first system, trending northwest, was measured in the azimuths of cross-beds and in the cross-bedded grain orientation subpopulation of primarily the New York region. 2. A second system, trending northeast, was measured in the azimuths of cross-beds in the Ontario region and in the laminated grain orientation subpopulation. The characteristics of the paleocurrent systems and the results obtained from other sedimentary properties which better define the subenvironments composing the Medina delta, suggest that the first paleocurrent system is related to distributary channel complexes and the second to marine currents. End_of_Article - Last_Page 540------------

The sedimentology and diagenesis of Silurian turbidites in South-east Wigtownshire, Scotland

Synopsis Sedimentary features of a marine Silurian sequence in south-west Scotland indicate transport and deposition by turbidity currents. Directional sedimentary structures at the base and top of graded greywackes show respectively that early erosive phases of the currents travelled from the north-east, whereas later depositional phases came from the north-west. Structures discussed in relation to their possible origin include sand volcanoes and current crescents. The effects of widespread carbonate replacement and local hematite enrichment are described. There is little evidence in the greywackes for a diagenetic matrix, which is concluded to be largely detrital in origin.

VI.—The Petrology and Sedimentation of Upper Ordovician Rocks in the Rhinns of Galloway, South-west Scotland

SynopsisThe petrology of a group of geosynclinal rudites and arenites of Upper Ordovician age from the Rhinns of Galloway, south-west Scotland, is described. The rudites are confined to the oldest formation, the Corsewall Group, and consist of pebbly microbreccias and “boulder-beds” interbedded with coarse greywackes and platy siltstones. The composition of the microbreccias and boulder-beds varies little with stratigraphic level.The arenites comprise lithic, coarse-grained greywackes with occasional quartzose or feldspathic varieties. They carry, in aggregate, a wide range of igneous, metamorphic and sedimentary rock-fragments, quartz of variable source, a variety of feldspars, ferromagnesian minerals, micas, chlorites and a common to conspicuous argillaceous matrix. The greywackes are well defined stratigraphically as regards both overall composition and the character of the large mineral grains and rock-fragments (“granules”). Moreover composition appears to be related to grain-size in a consistent manner. The size-distribution, sorting and skewness of the greywackes based on thin section measurements show little stratigraphic variation.The aggregate suite of heavy minerals is meagre and only minor differences exist in the individual formation-suites.The character of the rock-fragments and mineral grains suggests that the earlier formations were derived from a nearby source-area in which plutonic and hypabyssal acid and basic igneous rocks were extensively exposed, together with spilitic lavas and glaucophane schists. The petrology of the upper formations reveals that a metamorphic terrain of Southern Highlands aspect became an important additional source of material. However, the highest beds were derived from a region dominated by andesitic and spilitic tuffs and lavas.The principal directional sedimentary structures are described briefly and it is suggested that the lower part of the Corsewall Group, derived mainly from the north and north-east, was deposited in a sheltered pro-delta environment, but that the boulder-beds and associated rocks were formed within a more boisterous region. The Kirkcolm Group is a turbidite-sequence deposited in a marine trough of moderate depth by currents which at first flowed dominantly in opposing directions, towards either north-east or south-west. However, transport from the north-east became progressively established. The Portpatrick Group is another turbidite-sequence but this formation was deposited by currents flowing from the west or south-west.

Paleogeographic Evolution of the Lake Superior Region from Late Keweenawan to Late Cambrian Time

A combined study of regional stratigraphy, petrology, and paleocurrents was made of the Freda sandstone, Jacobsville sandstone, Bayfield group, Dresbach formation, and Franconia formation. Data pertaining to the location and nature of the source of the sediments were obtained primarily from petrology and directional sedimentary structures. Environmental reconstructions were based on patterns of lithologic variations, kinds of sedimentary structures, and heavy minerals. This information indicates that the Northern Michigan highland extended through northern Wisconsin and northern Michigan and acted as a source of sediment from Late Keweenawan through Dresbachian time. The Freda formation accumulated in the Keweenawan basin, which was north of the Northern Michigan highland approximately in the present site of Lake Superior but extended considerably farther to the southwest. Deposition took place in a flood-plain and lacustrine environment. Prior to the deposition of the Jacobsville-Bayfield sediments, the Keweenawan sequence was deformed and eroded, but the Northern Michigan highland persisted as a positive area, and the shape and extent of the basin remained much the same. A lacustrine environment predominated in the central part of the basin, but much of the Jacobsville sandstone and Bayfield group undoubtedly represents fan deposits which merged northward into sediments of an alluvial plain. During Dresbachian time the Northern Michigan highland remained as a positive area, but shallow seas invaded the Lake Superior region from the northwest and central Wisconsin from the south. Most of the Dresbach sediments accumulated in a beach environment, but in southern Wisconsin an offshore neritic environment predominated. Prior to Franconian time there was a widespread regression of the seas, and most of the region was subjected to subaerial erosion. By Franconian time the Northern Michigan highland was reduced to a surface of low relief, and the seas re-advanced across the entire area from the southwest. An appreciable amount of the Franconia sediments accumulated in an offshore environment.

PALEOCURRENT STRUCTURES AND PALEOGEOGRAPHY OF THE ULTRAHELVETIC FLYSCH BASINS, SWITZERLAND

The Ultrahelvetic Flysch of the Swiss Prealps ranges from Late Cretaceous to Eocene in age. It occurs as isolated outcrops which are known by various local names—Waggital Flysch in the Glarus Alps, Schlieren Flysch in central Switzerland, Leissigen Flysch in the Internal Prealps southwest of Lake Thun, and Gurnigel Flysch in the Bernese External Prealps. These Flysch formations are characterized by thick sequences of interbedded sandstone, conglomerate, shale, and silty limestone. The sandstone of the Flysch exhibits sedimentary structures typical of turbidity-current deposition— e.g., graded bedding, flute casts, groove casts. About 400 measurements of the directional sedimentary structures were made from the various Flysch formations. Paleocurrent information thus obtained was interpreted within the framework provided by the stratigraphic and tectonic studies of alpine geologists. The major conclusions of this paper are: (1) The Ultrahelvetic Flysch basins of the central Swiss Alps were created as a result of isostatic adjustment subsequent to an episode of pre-Maestrichtian folding. During the Maestrichtian to middle Eocene time, the Ultrahelvetic region was characterized by a topography typical of block-faulted terranes. Rugged land mass adjoining deep-sea basins resulted in conditions favorable for periodic turbidity-current deposition in those basins. (2) The Schlieren Flysch, the Leissigen Flysch, and the Sandstone Flysch near Adelboden were deposited in a row of near-shore deep-sea basins just south of a shallow shelf sea where carbonates of the Helvetic facies accumulated. These Flysch formations derived their sediments from the south or southwest. They were subsequently overthrust as the nappes of the Internal Prealps. (3) The Waggital Flysch and the Gurnigel Flysch were deposited in a row of deep-sea basins farther offshore. These were separated from the near-shore basins by an island chain which furnished sediments to both units. Subsequently, the Gurnigel Flysch was overthrust as a nappe of the External Prealps.