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Facies relationships between deep and shallow marine to continental deposits in the Eocene-Oligocene sequence of the Santa Ynez Mountains, California, have been studied in detail to (i) determine interrelationships between shallower and deeper marine sands (coastal and turbidite); (2) establish criteria for the recognition of genetic sand types; (3) determine factors influencing facies distribution; and (4) propose hypotheses of marine sand deposition. The rocks studied include the Anita, Sierra Blanca, Juncal (with the Camino Cielo Member), Matilija, Cozy Dell, and “Coldwater” formations but not the topmost unit of the Eocene-Oligocene sequence, the non-marine Sespe formation. In landward sequence, the facies recognized include: Turbidites and marine lutites, proximal turbidites, shallow marine, coastal and continental facies. These occur in two major regressive sequences. In the first, the Juncal-Matilija sequence, thin-bedded turbidites and marine lutites are overlain by, and are laterally equivalent to, very thick proximal turbidites which pass upward into shallow marine and coastal sands. The major sand accumulations are in the basin margin shallow marine, coastal, and proximal turbidite facies. The second regression, the Cozy Dell-Sespe sequence, lacks significant proximal turbidite deposits but has extensive shallow marine and coastal deposits. Facies distribution and stratigraphical sequence are explained as responses to the interplay of depositional and structural processes. Detailed stratigraphical mapping has clarified correlation in the Eocene sequence to the Santa Ynez Mountains. Micropaleontology is used in support of correlations and bathy-metric interpretations.

Seventeen piston cores were collected at 25-km intervals following the 3500-m isobath along a 400-km portion of the continental rise off the southeastern United States. The area extends between the Hatteras Canyon System and the tip of the Blake Outer Ridge. This study evaluates the mechanisms of rise formation over a large continental-rise area. From north to south, the calcium-carbonate content increases, minerals indicative of a northerly source are partially replaced by minerals more representative of the southern U.S. and Caribbean, and the sedimentation rate decreases. No identifiable turbidites were noted in any of the cores. Coarse-grained layers are infrequent; most are less than 2 cm in thickness and none showed grading. Radiography of several cores revealed mottling throughout. Bottom photographs show that present bottom-current movement closely parallels contours and flow is in a general southerly direction. The results indicate that contour-following bottom currents are primarily responsible for shaping this part of the U.S. Atlantic rise. Turbidity currents may be important only as a source of sediment, via overflows, to be transported south by bottom currents.

Several gentle hills close to the crest of the East Pacific Rise are covered with extensive Fe-Mn-rich deposists. The setting of the sediments precludes the possibility that turbidite flows, slumping or downhill winnowing of material can have been responsible for the accumulation of these sediments, as is also indicated by the lack of reworked faunas or crushed shells. Palaeontological dating and the magnetic anomalies of the crustal substratum show that the sediments must be very young, that is, the sedimentation rates are very high; thus, 8–10-m-thick sediments were formed in 600,000 yeas. The accumulation rates on these hills for several elements, such as Fe, Mn, Cu and Ba, are much higher than in ordinary pelagic regions. The element deposition is best described as an exhalative-sedimentary process. The areal extent of these sediments is considerably larger than that of any other currently forming analogous deposits. The results have far-reaching implications for the discussion of how sedimen...

A sequence of graded ophiolitic sandstones, 120 m thick (Sanguigna Formation), outcrops within the Messinian Evaporite Group in a limited area near Gabbro, Fine Basin, western Tuscany. The formation lies between fine-grained sediments laid down under a thin water-cover. The graded beds show, on the other hand, many features typical of proximal turbidites, such as amalgamated layers, scour-and-fill structures, clay flakes and lumps, top-truncated Bouma sequences. Grain-size analyses suggest that they were deposited from high-density turbidity flows. The petrographic composition and the grain fabric indicate that the feeding was from the northeast across the Fine Basin. From the inferred dimensions and depth of the basin, the mean slope should have been less than 1°. The turbidity currents probably originated at a river mouth during flood stages. The Sanguigna graded beds are compared with occurrences of shallow-water turbidites.

SummaryThe sedimentology of a key locality in the Caradoc of SW Wales is discussed for the first time. Five facies are described and interpreted: mudstones and siltstones (A), turbidite arenites (B), rudites (C), a diamictite of massflow origin (D), and muddy arenites rich in shaleclasts (E) which show a transition from massflow deposits to immature turbidites. Closely adjacent sections illustrate considerable lateral variations within an assemblage suggested to be of proximal turbidite character and to represent deposits high on a Middle Fan environment.

SummaryConglomerate beds occur in an Upper Ordovician—Lower Silurian clastic sequence on the northeastern side of the Newfoundland Central Paleozoic Mobile Belt. They contain scattered pebbles and cobbles dispersed in a finely laminated sandstone and siltstone matrix. Laminations are generally 5–20 mm thick and the clasts 10–300 mm across. The laminations are locally disrupted by the clasts. These laminations have subsequently been tectonically flattened into augen around the clasts and locally disrupted by boudinage. It is concluded that these conglomerate beds represent icerafted glacio-marine deposits interbedded with turbidites which were probably deposited in a locally restricted marine environment. The deposits are interpreted on the basis of scattered fossil localities as being of Late Ordovician–Early Silurian age. They are compared with deposits of similar age elsewhere in the North Atlantic region.

Measurements of upper unlithified sediment layer interval velocities using the sonobuoy technique were made on several expeditions in the Bay of Bengal and adjacent areas, the Bering Sea, the Japan Sea, and the North Pacific. The results are presented in figures, regression equations, and tabulated station data by geographical areas. In each area the sediment surface velocity was based on measurements in cores that were corrected to in situ values. These sediment surface velocities ranged from 1454 m/s in the Tatar Strait to 1585 m/s in the Aleutian trench. An average linear velocity gradient was computed from 0 to 0.5 s (one-way travel time) for each of 13 areas in which the sediments were mostly turbidites. The values at each 0.1-s interval were then averaged. These averaged gradients decreased from about 1.32 s−1 at t = 0 to 0.77 s−1 at t = 0.5 s. Such average values of velocity gradients can be used with measured or predicted sediment surface velocities and one-way travel times (measured from reflection records) to compute predicted true sediment layer thicknesses for areas in which no interval velocity data are available. One of the most important uses of interval velocity information is in computations of true thicknesses of sediment layers. When no interval velocity measurements are available, there are usually three alternatives in computing sediment thicknesses: (1) use equations and curves from an area of similar stratigraphy and sediment type, (2) use a predicted linear velocity gradient and sediment surface velocity (as was done above), or (3) assume an interval velocity. Of these three alternatives the last is most commonly used and is the least accurate. There is a tendency in marine geophysical literature to assume interval velocity values for upper unlithified sediment layers that are usually too high (e.g., 2000 or 2150 m/s). In 15 areas covered by Houtz and his colleagues and this report the average interval velocities for layers in which one-way travel times are 0.1 and 0.2 s are 1619 and 1719 m/s.

Summary A detailed reinvestigation of rock relationships in the western Richtersveld led to a new stratigraphic classification of the Late Precambrian deposits, which were found to belong to one continuous cycle of geotectonic activity in the Orange River area and which are now grouped together in the Gariep Group. The development of a geosynclinal basin started with basement shearing and subsequent deposition of the mixed sedimentary-volcanic Stinkfontein Formation. Further subsidence led to the accumulation of mixed clastic-carbonate shoreline and shelf deposits of the Hilda Formation, followed by the clastic Numees and Helskloof formations. At the same time basic to andesitic volcanics of the Grootderm Formation were extruded in the eugeosynclinal basin, accompanied and followed by turbidite deposits of the Holgat and Oranjemund formations. Gravity deformation took place during and after deposition of the Gariep strata and was succeeded by orogenic activity which culminated in compressional eastward-di...

The interpretation of sedimentological and geochronological results lead us to the conclusion that the sedimentary levees flanking the deep-sea channels of the Rhone deep-sea fan are made up of various kinds of turbidites originating from different coastal areas and were deposited during the Quaternary glacial epochs.

Apart from Bouma sequences, recurrent patterns of sedimentation can be recognized at two hierarchic levels in turbidite formations of the northern Apennines. First order cycles (turbidite suites) encompass most of the basin fill, second order cycles (megasequences) occur as organized groups of beds 2-70 m thick within the suites. The Marnoso-arenacea (Lower-Upper Miocene) is an example of a progradational or offlapping suite with thickening and coarsening upward trend. The Laga Formation (Upper Miocene-Lower Pliocene) is characterized by a "transgressive" (retrogradational) or onlapping trend. Vertical facies changes go either way through the following steps: basin plain outer fan midfan inner fan slope deposits ( = prograding, = receding). Both suites are enclosed by hemipelagic pelites and chaotic deposits, the start of turbidite sedimentation being gradual in the progradational case, and abrupt (large scale erosion) in the reverse one. Second order cycles follow each other or are separated by monotonous (uniform or irregularly bedded) sequences of extremely variable thickness (2 to more than 1,000 m) composed either of fan (overbank, fringe) or basin plain deposits (with key beds laterally continuous up to 175 km). The trend of cycles may be asymmetric (termed positive when beds thin upward, negative when they thicken upward), symmetric, composite or not definable. Thick to massive beds with sand/shale ratio >> 1 or > 1 and well developed coarse divisions (products of high density turbidity currents, grain flows, debris flows, fluidized flows) form the thicker bedded portion of cycles and sometimes the whole cycle. One hundred and thirty-two cycles were analyzed and subdivided into simple and complex (= multiple or composite). By splitting multiple cycles into simple components, a total of 170 cycles was reached. Thirty-nine of them were measured in channelized deposits, 131 in non-channelized turbidites, laterally continuous exposures enabling the distinction to be made. Results of sequence analysis are illustrated by columnar profiles and bed thickness diagrams, i.e. CD (coarse division) and L (layer) diagrams. Eighty percent of channelized cycles show a positive trend, whereas a negative trend characterizes 60% of non channelized cycles. This strengthens the assumption that most positive cycles reflect filling of fan channels, while negative cycles are the expression of prograding depositional lobes, i.e. localized areas of preferred sand accumulation in front of distributary channels. Among progradational cycles, a distinction was made between "fast" and "slow" accretion types by means of relative number of beds (L/T ratio), relative number of coarse divisions (L/CD ratio), and frequence of interbedded hemipelagites. The possible non-prograd tional nature of some "slower" cycles is discussed; they could alternatively reflect sporadic phases of sand accumulation in the basin plain environment due to huge flows triggered by tectonic events and bypassing the fan system (or independent from a fan system). Attempts of correlation of negative cycles both downcurrent and across current are shown.

Numerical analyses were made of the sequential ordering of divisions within the turbidite succession of the β1 Member, Cloridorme Formation, logged at the type section at Grand Étang in the Gaspé Peninsula, Quebec. The sequence is satisfactorily described by a first order Markov chain model, whose transition structure includes and corresponds in part to a Bouma model of sedimentation. Additional second order Markov modifications differentiate two types of parallel-laminated division associated with high (a minority) and ?low flow regimes (the majority). Substitutability analysis reinforces many of the genetic interpretations based on the Markov model. The results of autocorrelation and correlation of logarithmically transformed division thicknesses suggest the deposition of successive turbidites as essentially discrete events and the operation of two different depositional mechanisms within the history of at least two types of turbidite unit. However, there appears to be a positive association between successive basal clastic division thicknesses, possibly indicating a longer term control in the budgeting of coarse sediment available for deposition. The succession is characterized by three types of unit with distinctive internal division thickness relationships, lithology and associated bedforms.

The upper part of the Lower Palaeozoic sequence west of the Leinster Granite in eastern Ireland is shown, by means of chitinozoa, to be Llandovery in age, with some of it perhaps younger. These Silurian rocks are part of a turbidite succession which extends down to the lower Ordovician west of the granite and down to the lower Cambrian on the east.

Turbidites, belonging to the β1, member, Cloridorme Formation, are exposed on the north shore of the Gaspé Peninsula, Quebec. Their structural attitude is such that vertical sections through turbidite beds are exposed on the wave-cut platform and their strike is approximately parallel to the paleocurrent direction, as shown by sole marks on the bases of beds.Certain thick turbidite beds, in a distal position, display a sequence of sedimentary structures which differs from the sequence defined by Bouma. Three broad divisions are recognized: a basal division consists of either limestone or quartz granule to pebble conglomerate (0–4 cm thick) or coarse sand graywacke or calcareous wacke (0–15 cm thick). Basal divisions of calcareous wacke frequently display ripple-lamination, parallel lamination, or upstream-inclined laminae. Where the upstream inclined laminae form a single set, they occur below a sinuous profile (wavelength 40–80 cm, and amplitude 2–5 cm).A second division (0–330 cm thick) consists in most places of spindle- or globular-shaped calcareous nodules scattered in an argillaceous host. In some beds, streaking and lobing of light colored, carbonate bearing material is associated with these nodules. Internal hemi-ellipsoid structures, arranged en echelon and convex towards the base of the bed, are displayed from the second division. The upper division consists of fine grained siltstone and shale.The upstream-inclined laminae in the basal division of calcareous wacke beds are interpreted as being the result of the upstream migration of antidunes. The nodules within the second division developed as 'pseudo-nodules'. The hemi-ellipsoid structures resemble damped, large scale (macroturbulent) eddies associated with the flow of dense grain dispersions.Correlation of these beds has been achieved over a distance of 12 km. Basal divisions of granule and pebble conglomerate persist over this distance and show that coarse particles may be transported by turbidity currents over long distances. The sedimentary structures of the basal divisions of several calcareous wacke beds might be interpreted as the result of either an increase in flow regime downcurrent, or of nonpreservation of structures at up-current localities.The beds were probably deposited from turbidity currents composed largely of mud and fine sand, but containing a zone of coarse grains concentrated near the bed. The basal division was deposited from this lower zone and a period of traction formed rippled, flat, or antidune bed forms. Stratification in the basal division was preserved by the rapid deposition on top of sediment that settled en masse from the subsequent high concentration body of the current. The formation of a succession of 'quick' beds led to the sedimentation of the second division. The flows responsible for the sequence of structures observed and the downcurrent persistence of the beds probably approached closely a state of 'autosuspension'.

To present a model for flysch deposition involves the simultaneous representation of many parameters. The most significant of these are arenite-lutite bed thickness ratio, mean and variance of arenite-bed thickness, mean palaeocurrent direction and the primary directional structures yielding them. Field and flume data suggest that arenite-bed thickness decreases exponentially away from source, with a complementary increase in siltstone bed thickness in a turbidite fan. If HA, HL. are arenite and lutite bed thicknesses in a couplet (the unit of bedding at a flysch outcrop), then where m and R are constants of the outcrop in the turbidite wedge. Field evidence suggests m= 1, so that R-values allow for a classification of couplets at an outcrop or within a zone. Maximum HA, HL. values are indicative of proximal deposition from turbidites, and usually provide transverse directional structures, as does (HA) max. Arenite bed thickness can be considered as thick beds or thin beds according to whether they exceed a chosen thickness, say t cm. If 15 cm be chosen, the percentage thick arenite-bed thickness HA1 and thin arenite bed thickness percentage, HA2 can be represented with HL. (siltstone bed thickness percentage) over twenty-five couplets, as one point on a triangular diagram. The mean palaeocurrent direction Nφ° E is represented as an arrow through the point, a colour (point or arrow) being used to distinguish the directional structure (flute, ripple, groove, foreset, etc.) producing it. Flysch examples from Ordovician, Silurian and Devonian in Central Victoria are thus interpreted in terms of proximal and distal emplacement. In cases where bedding detail is clear, A + B, C and D + E divisions substitute for HA1HA2 and HL. respectively, for a unit of 1 m thickness of outcrop on the triangular diagram. If thick arenite bed thickness percentage be plotted against total arenite bed thickness percentage, the linear plot of points for outcrop samples results on log-log paper. Limiting lines of the form log HA1= m log HA+ log C (m, C constants) demarcate the field. Parallel and equally spaced lines with values C1, C2, C3,…Cn subdivide this into sub-fields. If the determinations for HA1 in any one sub-field be plotted on a locality map considering only MAXIMUM HA1 values, and the HA1H values conveniently contoured, trends emerge which closely agree with the modal palaeocurrent directions in the case of the Llandovery Deep Creek Flysch 25 km NNW of Melbourne, Australia.

Two depositional processes control the mud accumulation on the southern Balearic Abyssal Plain: pelagic settling at a rate of 10 cm/1000 years, and turbidity currents at an average frequency of > 3 per 2000 years. Thermo-haline bottom flow has little effect on the abyssal sediment distribution. Just over half of the Late Quaternary section is made up of turbidite mud. Distinctive properties of turbidite mud are: structural, textural, and compositional continuity from the underlying turbidite sand-silt layer into the overlying mud, grading within the mud layer, a ratio of carbonate percent with the underlying turbidite sand-silt layer of about 0.5, and a proportion of sand of > 1%. Those of (hemi)pelagic mud are: bioturbation, an average of 8% of sand consisting largely of remains of foraminifera and pteropods, a grain size distribution which is virtually normal with a median around 9 φ, and very poor sorting; in general, the properties of (hemi)pelagic muds are the same in widely separated localities and depths in cores. In some instances the clay mineral ratios of the turbidite mud layer are markedly different from those of the overlying (hemi)pelagic mud layer.

A series of sediment cores were obtained from the Whiting Basin southeast of Puerto Rico to investigate the factors affecting the velocity of sound in marine carbonate deposits. The cores indicated that the deposits in the Whiting Basin are similar to abyssal-plain deposits with lenticular turbidite sequences alternating with pelagic sediments. The sediment, comprised of highly porous sands and silts, averaged 80% calcium carbonate consisting of aragonite, low-Mg calcite and high-Mg calcite. Normal methods for predicting sound velocity from the physical properties of the deposits were found to be inaccurate for these samples. The established relationships of grain size and porosity to sound velocity were invalid because the sands found in the cores consisted of hollow-foram tests, causing high porosity independent of grain size. The rigidity of the deposit was the most significant factor determining sediment sound velocity and was itself controlled by the sediment source, transportation effects and the packing of the deposit. Future work is needed to accurately measure the effect of these factors on the rigidity modulus.

Mud layers of turbidite origin are readily distinguished from those of hemipelagic origin in X-radiographs of dominantly muddy cores collected in the southern part of the Balearic Abyssal Plain, western Mediterranean Sea. Radiocarbon dates based on the carbonate sand fraction sieved from hemipelagic muds indicate a hemipelagic rate of sedimentation of 10 cm 1000 yr and a frequency of turbidity current incursions of > 3 2000 yr during the past 15,000 years.

Turbidity currents are formed in Fayetteville Green Lake by sediment shmlping on the sides of the lake basin. Deposition from these currents accounts for about 50% of all sediment accumulation on the floor of the main basin. Rate of sediment loss from the basin sides through the formation of turbidity currents approximately equals nonturbidite scdimentation rates on the gently sloping basin floor. Major turbidity currents arise most frequently where sources of terrestrial sediment are present. Major turbidity currents involve about 200-300 m3 (dry volume) sediment and cxtcnd over about 6 x 10’ m2. Turbidity currents may play an important role in lake sedimentation. Not only do they affect sedimentation rates but they apparcntly contribute to the development of both lateral and vertical variation in the sediments of larger lakes. Turbidity currents have apparently played a major role in sedimentation in l?ayetteville Green Lake (43”03’03”N, 75’57’59”W ) in north-central New York State. The sedimentary column is exccptionally well preserved showing varves composed of a dark and an overlying pale lamina and turbidites ( Ludlam 1969). The meromictic condition of this lake, along with its small size (0.258 km”), simple and regular morphology, and small, uniform watershed ( Brunskill and Ludlam 1969)) combine to eliminate or moderate many processes that would otherwise affect sedimentation. The lake, therefore, provides an cxccllent system in which to study the role of lacustrine turbidity currents in the deposition and distribution of sediment.

Recognition of the occurrence and extent of hemipelagic and pelagic deposits in turbidite sequences is of considerable importance for environmental analysis (palaeodepth, circulation, distance from land, hemipelagic or pelagic versus turbidite sedimentation rates) of ancient basins. Differentiation between the finegrained parts (E-division) of turbidites and the (hemi-) pelagic layers (F-division of turbidite-pelagite alternations) is facilitated in basins where carbonate turbidites were deposited below the carbonate compensation depth (CCD) such as the Flysch Zone of the East Alps but may be difficult in other basins where less compositional contrast is developed between the fine-grained turbidites and hemipelagites. This difficulty pertains particularly in Palaeozoic and older basins. For Late Mesozoic-Cenozoic oceans with a relatively deep calcite compensation level three other types of turbidite basins may be distinguished for which differentiation becomes increasingly more difficult in the sequence from (1) to (3): (1) terrigenous turbidite basins above the CCD; (2) carbonate turbidite basins above the CCD; (3) terrigenous turbidite basins below the CCD. Criteria and methods useful for the differentiation between turbiditic and hemipelagic mudstone in the Upper Cretaceous of the Flysch Zone of the East Alps include calcium carbonate content, colour, sequential analysis, distribution of bioturbation, and microfaunal content. In modern turbidite basins clay mineral content, organic matter content, plant fragments, and grain-size (graded bedding, maximum grain diameter) have reportedly also been used as criteria (see Table 3). Deposition of muddy sediment by turbidity currents on weakly sloping sea bottoms such as the distal parts of deep-sea fans or abyssal plains is not only feasible but may lead to the accumulation of thick layers. Contrary to earlier speculation it can be explained by the hydrodynamic theory of turbidity currents, if temperature differences between the turbidity current and the ambient deep water as well as relatively high current velocities for the deposition of turbiditic muds (an order of magnitude higher on mud surfaces than commonly assumed) are taken into consideration. The former add to the capacity of turbidity currents to carry muddy sediment without creating a driving force on a low slope.

The Oligocene Balleny Group of Chalky Island, southwestern Fiordland, comprises a typical continental margin sequence 900 m in thickness. Thin nearshore traction deposited sediments at the base are overlain by submarine canyon and fan lithofacies that were deposited by the full range of subaqueous mass-transport processes. A steep-walled channel within Balleny Group is interpreted as a fossil proximal fan-channel. The sedimentary fill of the channel is texturally similar to sediments moving by slump-creep in Recent submarine canyons and fan-valleys. The field data presented indicate (1) that a small canyon complex at Sealers Bay was initially cut by subaqueous debris-flows derived from an adjacent cliffed continental coast; (2) that transport within the upper parts of the canyon and fanchannel complex was primarily by inertia-flow and slump-creep ; and (3) that these more proximal types of mass-transport gave way gradationally and successively to fluxoturbidity and turbidity currents at locations further down-slope, with consequent deposition of sediment in more distal fan-channel and fan-surface environments as fluxoturbidites and turbidites, with lesser contributions from inertia-flows.

Polished slabs and thin sections of lower Silurian graptolitic mudstones and interbedded barren mudstones show three main lithologies are present (a) unfossili-ferous green mudstones, sometimes with indistinct silt laminae, (b) a similar black pyritic lithology, with rare graptolites, (c) striped mudstones, with prominent carbonaceous and silt laminae, and common graptolites. The abundance of silt laminae suggests that the striped mudstone facies represents the highest energy depositional conditions. Comparison of sedimentary structures with modern deep sea muds suggests this facies is distal turbidite. Graptolite preservation was favoured by rapid burial.

Ab Upper Ordovician sandstone—shale sequence in Asturias, Northwest Spain, is analysed with particular regard to lithology, sedimentary structures and trace fossils. It is deduced that deposition of the sandstones was from axially flowing turbidity currents in a deep-water trough. Trace fossils are rare but there is a predominance of deep-water turbidite-facies types including Helminthopsis, Spirophycus, Cosmorhaphe, and Protopaleodictyon. Arenicolites, normally regarded as typical of shallow water, is, however, also recorded. The low abundance and diversity of trace fossils in this succession is in marked contrast to the situation in many younger turbidite sequences. The initiation of turbidite deposition followed a long period of quiet-water mud sedimentation. Recognition of this abrupt facies change in two sections separated by a down-current distance of 40 km, provides a reference level from which vertical and lateral facies changes are quantitatively documented. In each section, deposition starts with a fining and thinning-upwards cycle and continues with several similar cycles. The onset of turbidite deposition, abrupt facies transition, and repetition of the cycles, are probably largely the result of fault block uplift at source, while initial development of the deep-water trough may have resulted from associated downthrow. The lateral facies variations, which include downcurrent reduction in sandstone/shales ratio and the Walker ABC index, probably result from a combination of factors, including reduced flow velocity and decreased sediment load.

The Greenland Group of the Reefton area, of probable pie-Devonian age, consists of alternating beds of indurated mudstone and sandstone, and is interpreted as a turbidite succession. Sedimentary features of the sandstone beds include common graded bedding, sharp basal contacts, directional sole marks, load casts, flame structures, internal cross- and parallel-laminations, and top surface ripple marks. The Reefton sediments cannot be differentiated on either lithology or facies from the Greenland Group succession in the Paparoa Range to the west, and the rocks in both areas are considered to form part of the same sequence. They probably formed proximal deposits on a submarine fan or fans. A secondary lithological layering is developed parallel to the slaty cleavage of some beds. This layering is interpreted as the result of grain movement in wet, unconsolidated sediments undergoing compression. The possible geological significance of these results is discussed.

SUMMARY Many, but not all, current ripples show log-normal grain-size distribution and good sorting (P. D. Trask). A range in medians was measured from fifty to several hundred microns. Turbidite ripples are log-normal, very well-sorted and are restricted to medians of very fine sand to silt (< 130 μ) because the particles have been supplied in suspension at rippling velocity. Experimental suspension current ripples formed at about 50–60 cm/sec are similar in composition and structure to turbidite ripples. The continuous supply results in climbing ripples. The restricted grain size and the internal structure of turbidite ripples both form strong evidence against the action of normal currents. The number of grain-size analyses is still rather small.

Results are presented of mineralogical and size-frequency analyses of over one hundred samples from nine spatially distinct lithostratigraphic members within the constituent formations of the Plynlimon Group. The majority of the samples are from deposits of “proximal” turbidite character and are mineralogically rather mature greywackes with much evidence of polycyclic material. Variation within textural parameters is large and for given values of median grain size there is large variation in the amounts of mineralogical components and, by corollary, sorting and skewness. These variations are illustrated by scatter fields and serve as a strong warning of the importance of textural control on the interpretation of mineralogical analyses. Field evidence (not separately discussed) is thought to indicate that the arenites were not deposited or reworked by ocean bottom currents and petrographic variability is suggested to be largely the result of derivation from a single heterogeneous source.

Navy Fan is a deep-sea fan forming as a result of the overflow of sediment from San Diego Trough into San Clemente Basin. It appears to have preserved its growth pattern unmodified by the effects of the Holocene transgression or of tectonic activity. Since the beginning of the last glaciation about 56 $$km^{3}$$ of sediment has been deposited on the fan. The fan is fed by Navy Channel, a deep gorge which cuts through the basement volcanic rock of a threshold 1,500 m deep which separates San Clemente Basin from San Diego Trough. Although it is situated 50 km offshore, Navy Channel contains gravels. The upper fan is crossed by a leveed depositional fan-valley which opens out onto the suprafan: an area of rapid sedimentation characterized by many shallow, shifting channels. Seaward of the suprafan, the lower fan passes into two small ponded basins within San Clemente Basin. The sediments of the fan comprise turbidite sands and muds and hemipelagic muds. Near-surface distribution of the sediments, determined ...

SUMMARY Turbidity currents are thought to be the agency responsible for the transport and deposition of the sediments known as turbidites. The way in which such currents may arise has been studied experimentally in a glass-walled tank. The starting point of the experiments was setting the sediments in motion by raising their pore pressure via a shock or vibration. This was achieved in the experiments by striking the metal bottom of the tank with a mallet. The response of a heap of homogeneous sand to such a shock was merely to change the angle of its slope. If a layer of sand was draped over a core of coarser, homogeneous sand, sliding occurred along the plane of permeability contrast and the less coarse sand was deposited at the toe of the slope. When the layer consisted of kaolin, which has a much lower permeability than sand, sliding again took place, but the clay subsequently mixed with the water and was moved along the bottom of the tank in the manner attributed to turbidity currents. An attempt is made to set up a scaling rule for experiments of this type and the consequences that such a rule may have in subsequent study are briefly discussed.

The distribution, thickness and textural properties of turbidites off the east coast of the United States are defined and discussed. Results of 195 granulometric analyses of turbidite layers are presented. Properties of deep-sea sands within a single basin are functions of availability of sediment and physiographic setting. In the Hatteras Abyssal Plain turbidity-current activity has been at a moderate level, there are no obstructions to flow, and sediment patterns are well-defined. Increasing distance from main route of flow (laterally and downcurrent) is matched by decrease in mean grain size, maximum particle size, range and thickness of units. Muddy sands are restricted to areas off the ends of submarine canyons and along principal avenues across the plain. The situation of the Sohm Abyssal Plain is different in that there has been an ample supply of sediment, more vigorous turbidity-current activity, seaward flow has been obstructed by many seamounts and seaknolls, and the overall result is a haphazard dispersal of sand by bottom-seeking flows. Many of the sands are muddy, poorly sorted and lack obvious grading. They have the textural properties of wackes. It is speculated that these abyssal plains represent modern sedimentary basins filled with flysch. If the results are combined with similar findings from other areas ( Horn et al., 1971a, b), the major conclusion is that deep-sea sands and silts of the outer continental rise and adjacent abyssal plains have many properties of flysch and are emplaced by turbidity currents. These results corroborate the ideas of Kuenen (e.g., 1967). Inspection of the cores reveals nothing to support the concept that the sands are products of normal bottom-current activity.

Acoustic reflection profiles in the Gulf of Alaska afford a new geologic dimension that allows tests of many previous speculations and hypotheses about the marine geology of the three abyssal plains that surround a central area of hills and seamounts. The bulk of sediments forming the two eastern abyssal plains (Alaskan and Tufts) have been transported by turbidity currents from continental sources through many leveed channels. The undulating, partly buried topography beneath the plains is of volcanic origin, and the central hilly area is a high that antedated the deposition of the plains. The moats, or depressions, around hills and seamounts were formed by differential deposition; ponding behind ridges is common. The distinctive features of fourteen channels crossed in the northern Tufts plain are the flat beds within the channel proper, a lower levee on the east side, and a very wide higher levee on the west side (downstream, right-hand side); some channels were originally controlled by lows in the underlying topography; channel migration upward and away from the high-leveed side is apparent. These channels are depositional rather than erosional in origin. Turbidite thicknesses decrease with distance from continental sources. In the southeast part of the Alaskan plain sediment thickness is 680 meters; in the northern Tufts plain maximum thickness is 470 meters, and in the southwest part it is 145 meters thick. The Aleutian abyssal plain, west of the central hilly area, has no present-day source of bottom-transported sediments; it is a fossil turbidite plain (maximum thickness, 282 meters) covered, in the west-central part, by 96 meters of pelagic material. Sediment thicknesses, probable rates of deposition, and known geologic history in adjacent areas, indicate the following geologic history: During Late Cretaceous to middle Tertiary the southward extension of deep-water flysch deposits (from the Alaskan peninsula area) formed the fossil turbidite Aleutian abyssal plain. In early to middle Tertiary, the first down-bowing of the Aleutian trench cut off turbidite deposition, and pelagic deposition began; the East Pacific rise was elevated. In middle Tertiary, the East Pacific rise subsided and great effusions of volcanic material from crestal areas covered the adjacent sea floor; turbidite deposition began over the Alaskan and Tufts plains (formerly the west flank of the rise). Pelagic deposition on the Aleutian plain continued to the present; turbidite deposition continued through the Pleistocene on the Alaskan and Tufts plains. It is concluded that the East Pacific rise in the Gulf of Alaska is in an advanced stage of rise evolution, and that the whole area has been quiescent since middle Tertiary.

This paper shows with two examples, from localities of unquestionable turbidite emplacement, that all of the pelitic zone (zone ρ of Bouma, 1962) is deposited by turbidity processes. This zone is richly fossiliferous in the Ordovician strata of Victoria, Australia, and the graptolites can be shown to be oriented, to the highest horizon, in a direction equivalent to the direction of emplacement of the underlying turbidite (as determined from sole markings and internal structures). The fossils can also be subdivided into models which show a direct order of superposition explainable along the lines of a waning current.

Single-set cross-stratification, considered to have been produced by the upstream migration of antidunes, is present in the basal division of certain thick turbidite beds. The basal divisions of these turbidites consist of very coarse to medium sand with a smooth or fluted base and sinusoid top. Internal laminae are inclined upstream at a shallow angle (less than 15°), opposing the local paleocurrent direction shown by flutes and ripple-drift cross-lamination. The sinusoid (antidune) bed forms and their associated internal cross-strata were preserved by the rapid fall-out of sediment from suspension normal to the bed. A paleohydraulic analysis suggests that during the final stages of antidune growth and migration, the currents moved at approx. 1 m/sec over very low slopes (much less than 1°).

A geosynclinal main phase, characterized by submarine volcanism, is succeeded by a late phase of turbidite deposition and by decrease or cessation of volcanism. This succession of events and the evidence of detritus composition and pebble shape of the turbidites, leads to the following model of geosyncline history. The main phase is considered to be tensional. The onset of compression results in closure of volcanic vents, active uplift of part of the preceding zone of geosynclinal deposition and active downwarping of other parts. The area of uplift provides the mass of turbidite material, the area of active downwarping the space for its deposition. Pebbles, in passing the coastline of the uplifted land, are shaped by the surf. Surf-shaped pebbles are less faceted than fluviatile pebbles. Beach pebbles below the low water line are highly spherical,pebbles higher up on the beach are flatter. Geosyncline-derived detritus of the turbidites is supplied from the flank of the turbidite basins, which are not coextensive with the previous basin of main phase deposition. Accumulation of detritus along the coast of the uplifted land, and tilting between areas of uplift and areas of downwarping, result in local oversteepening and triggering of turbidity currents, conglomerate flows, slides and slumps. Long-shore mixing explains the heterogeneity of the detritus. A distinction is made between source area, area of accumulation and area of deposition of the turbidites. Neither the volcanic main phase nor the phase of turbidite deposition agree with modern steady-state concepts of island arcs. It is discussed that geosynclines could differ both from zones of upwelling and from zones of crustal disappearance, and that they could mark the boundary between two global crustal plates moving in the same general direction, with attenuation through tension in a first phase and subsequent side-swiping compression.

A sequence of alternating siltstone-claystone couplets in the Pennsylvanian Haymond formation was studied in order to determine which parameters might be useful in correlating turbidite strata. Analyses of thickness, composition, and paleocurrent direction in 768 couplets showed no significant variations with time, although it was found that the pattern of thickness variation could be used for correlation. Using thickness variations, two outcrops of siltstone-claystone couplets, separated by 10.5 km., were correlated in the field. This field correlation was then illustrated by a sliding correlation coefficient and correlation diagrams. A comparison of the two series revealed a 46 per cent decrease in the number of couplets in a basinward direction with only the thicker siltstone layers present in both outcrops. Siltstone thickness decreased 46 per cent and claystone thickness decreased 26 per cent in a direction away from the source.

The Ordovician and Silurian rocks of central Murrisk, County Mayo, Ireland, are described with reference to their stratigraphy, provenance, and conditions of deposition; 36 000 ft of Ordovician rocks are overlain, unconformably, by 5600 ft of Silurian strata. The Arenig rocks are divided into two series: the Owenmore Series (2000 ft) below and the Murrisk Series (15 000 ft) above. The Owenmore Series (boulder-slides, turbidites, and black slates) was derived entirely from the north; the Murrisk Series (tuffs and turbidites) was derived from the east and south. The Glenummera Series (4000 ft of green slates with thin psammitic turbidites) follows and is overlain by the Partry Series (15 000 ft). The Partry Series is divided into two groups; the Mweelrea Group below, and the Maumtrasna Group above. The Mweelrea Group (7000 ft of current-bedded deltaic grits) contains six beds of ignimbrite, deposited by nuees ardentes with a south-easterly provenance. Periodic westward tilting caused the eastward recession of the delta-front with the concurrent development of boulder-slides and turbidites. The Maumtrasna Group is similar to the Mweelrea Group but contains no ignimbrites, and the grits are coarser, with many interbedded conglomerates. The Silurian rocks are divided into the Lower Owenduff Group (Upper Llandovery, 0 to 20 ft) below, and the Croagh Patrick Series (Wenlock, 5600 ft) above. The Lower Owenduff Group was deposited in a complex littoral environment and contains a large amount of pyroclastic material. The Croagh Patrick Series is divided into six groups (Cregganbaun Group, 500 ft; Pollanoughty Group, 450 ft; Oughty Group, 360 ft; Fiddaunarinnia Group, 600 ft; Lough Nacorra Group, 1500 ft; Owenwee Group, 2000 ft), the Oughty Group containing five tuff bands. The Croagh Patrick Series was deposited in a northwardly transgressive littoral and sub-littoral belt and was derived exclusively from the north and east. The Silurian rocks rest unconformably upon the Owenmore Series as the result of strong pre-Upper Llandovery thrusting, concentric folding, and uplift. During most of Ordovician times, the northern margin of an e-w trough (the South Mayo trough) was denned by a normal fault. The southern margin was defined throughout most of Ordovician times and the whole of the Silurian by a rising Cordillera in Conne-mara. Dal radian rocks provided the source for nearly all the clastic material.

Cretaceous turbidite sequence, lamination, slump structures, turbid lobes, flute casts, paleogeographic significance, southeast France

Twelve seismic reflection profile sections illustrate the variation in sea floor topography and shallow subbottom structure along the length of Bowers ridge. The configuration of the sediments on the ridge appears to be that of depositional draping over irregular relief by some mechanism that tended to concentrate sedimentation along the western flank. Two profile sections of Shirshov ridge reveal thick accumulations of sediment contained in steep valleys between basement highs. The sediments on the flank of Bowers and Shirshov ridges extend below a younger highly reflective turbidite sequence in the adjacent basins. The ends of the ridges may be structurally connected at shallow depth below the turbidites in the interplain channel that separates them.

The 5000 ft. of pre-Downtonian or pre-Dittonian Silurian rocks of the Hagshaw Hills inlier of the Midland Valley of Scotland, have been redivided into nine formations, three more than recognised by Peach and Horne (1899). A poorly preserved shelly fauna from 900 ft. of marine turbidites shows lower beds of the inlier to be Upper Llandoverian-Lower Wenlockian, and not Salopian as previously reported. Comparison with other recently revised inliers of this region suggests an uppermost Valentian age for these beds. Current direction data show an anomalous southern derivation for the turbidites. Higher beds in the inlier are paralic or even terrestrial. Fossils from the Fish Bed Formation, previously thought to be the bryozoan Glauconomella disticha (Goldfuss), are described and referred to incertae sedis. A new locality of the Fish Bed is recorded. A large number of faults and igneous intrusions complicate the overturned asymmetrical anticline, and it is suggested that the bases of conglomerates have acted as detachment horizons during tectonism.

SummaryStatistical sampling of the Aberystwyth Grits, using a method previously employed in an analogous study of Tyee Formation, Oregon, shows an overall decrease in the ratio of sandstone to shale from south-west to north-east, away from the supposed source of depositing currents. Comparison with other methods of proximality evaluation in turbidite sequences indicates that at least in this case the ratio of sandstone to shale is the most sensitive indicator of palaeogeography.

The cyclothem discussed in this paper is 150 m. thick and is assigned to the Upper Carboniferous Northam Formation, north Devon, England. A regressive sequence of facies, including turbidites, is particularly well developed. The lower part of the cyclothem consists of quiet-water black mudstones. One hundred forty graded siltstone-turbidites represent the first incoming of coarser clastic material into the basin. Above the turbidites, the facies gradually show more and more evidence of basin agitation, and siltstones give way to cross-laminated sandstones. Near the top is a 15-m.-deep channel partially filled with turbidites-the channel presumably acted as a feeder for turbidity currents flowing farther out into the basin. Above the channel are more cross-laminated siltstones and sandstones, and the regressive sequence is topped by a 20-m.-thick large-scale cross-bedded sandstone body. The top of the sandstone is bioturbate and passes very abruptly into black mudstones of the next cyclothem. The sequence ...

The results of continuous sub-bottom reflection measurements in the Bering Sea show the upper 1 km of sediments in the central Aleutian basin to consist of stratified horizontal layers. These markedly undisturbed sediments demonstrate the stability of the basin and give evidence that tectonic effects that occurred in the Aleutian area after commencement of the sediment filling are restricted to a very narrow zone. The horizontality of the layers denotes a highly efficient leveling process in their deposition. A transition from fairly homogeneous underlying sediments to stratified sediments indicates a change in sedimentary regime, possibly associated with the beginning of the Pleistocene sedimentary cycle. The presence of stratified sediment on the flanks of Bowers ridge, presumably the same stratification as that in the basal section of stratified sediments in the adjacent area, suggests that elevation of Bowers ridge continued during deposition of some of the stratified sediments. The total thickness of sediments in the central basin is too large for measurement by the reflection technique but is judged, from previously reported seismic refraction measurements, to be about 4 km, in contrast with a thickness of about 300 m of transparent sediment found south of the ridge. The basin filling appears to be almost entirely turbidites deposited after the formation of a continuous barrier.

Summary The term “diamicton” has recently been proposed as a non‐genetic name for any non‐calcareous terrigenous sediment containing a wide variety of grain sizes; the lithified equivalent is a “diamictite.” The present paper deals with the origin and classification of water‐borne diamictons. The latter have been divided by the writer into three groups: Group I. Diamictons with the coarse and fine fractions deposited simultaneously. Group II. Diamictons with the coarse and fine fractions deposited separately. Group III. Diamictons with the fine fraction derived from the coarse fraction by diagenetic alteration in place. Group I diamictons have been divided into the following types: (i) P‐diamictons, in which the fine fraction was deposited from true suspension (the coarser material being transported by suspension, saltation, or traction), (ii) M—diamictons, in which the fine (and coarse) fractions were deposited from mudflows. (iii) K‐diamictons, in which the fine fractions were deposited in the form of d...

Summary The E 1a –E 2a (inclusive) succession of the North Staffordshire Basin exhibits a crudely rhythmic sequence with units of calcareous siltstone lithofacies, units of siderite-bearing protoquartzitic sandstone lithofacies and typical marine bands of thicker-shelled goniatite faunal phase. The eastern and western margins of the basin are marked by total or partial disappearance of the silt-stone and protoquartzite lithofacies; the southern margin is unpreserved. The protoquartzite lithofacies exhibits sandstone-rich and sandstone-poor sub-types, interpreted as proximal and distal turbidites respectively. The flows originated to the south of the basin, probably on the fronts of periodically advancing deltas. The calcareous siltstones were deposited in a more fully marine environment by intermittent waning currents, similar in some, but not all, respects to turbidity currents. Supply was probably from the west and north-west. The rhythmic sequence is explained as a response to sea-level oscillation during a period of unusually strong differential movement between the North Staffordshire Basin and its margins.

Much of the terrain occupied by the Devonian-Dinantian (lower Carboniferous) in the southern Vosges mountains of eastern France belongs to the Markstein and Odern complexes. These are separated by a thrust fault whose trace is marked by a line of klippen represented by a string of exotic erosional massifs. The Markstein complex of shales and greywackes is allochthonous Culm (lower Carboniferous) comprising a series of flysch type turbidites. The laminites of the series exhibit the microfigures of sedimentation such as micro-erosion, micro-current ripple-marks, micro-sinking by thixotropy, micro-carpet traction, micro-slumping, microboudinage, and micro-cross lamination. Major sedimentary rhythms are clearly shown.

The sedimentology of the Simla Series is practically unstudied. In connection with structural and stratigraphic work, Ghosh et al. (1966) gave a brief sedimentological description of the rocks and suggested that the Simla Series represent a geosynclinal sediment and that the pebbly shale horizons represented an intraformational conglomerate formed by either a turbidity current or a mud flow. Pandey and Narayan (1966) suggested that the Simla Series rocks represent flysch-type sediments. In the present study the authors describe the primary sedimentary structures, vertico-lateral variation of the sediments, etc., in greater detail. Plane laminae and the predominance of finer sediments (shale, siltstone and fine-grained sandstone) suggest a deep-water origin of the sediments. The lithological character and the sedimentary structures strongly suggest that the Simla Slate Series are turbidite deposits belonging to flysch facies.

The experiments involved pouring aqueous starting suspensions of pumice sand and kaolinite silt into a submerged trough. The steepness of the bottom slope of the trough was varied from run to run. Heights of sand and silt were recorded at 10-cm intervals away from source. The results: (1) support earlier suggestions that the ratio of sandstone to shale is a sensitive indicator of proximality in ancient turbidites, but do not support clearly the suggestion that the rate of decrease in that ratio away from source may indicate steepness of bottom slope; (2) suggest that the rate of decrease in sandstone thickness away from source in ancient turbidites may be inversely proportional to steepness of bottom slope. The possible application of the results to the geological record is discussed.

Turbidite sandstones interbedded with argillites are cut by veins composed of quartz and ankerite derived from host sandstones during successive phases of deformation.The existence of the oldest phase of deformation (F1) is established by the presence of its widespread vein system, which is folded by the F2 folds and cut by quartz veins of the later deformation phases.The nature of the veining, its mode of origin and regional significance are considered.

SUMMARY Eleven facies of the (Lower Westphalian) Abbotsham Formation and upper portion of the Northam Formation of north Devon are described: black mudstone, silty mudstone, “turbidite”, silty streak, sandy streak, three kinds of oscillatory beds, cross-stratified sandstones and mudstones, fining-upwards units and major sandstones. Their relationships to each other are determined and they are shown to recur in a cyclic pattern. Six cycles are distinguished in the 1,200 ft. of the Abbotsham Formation. Each passes upwards gradually from black mudstone into coarser beds, the top of which is sharply separated from the black mudstone of the succeeding cycle. We suggest that the environment of deposition of these cycles ranges from a moderately deep basin, in which mudstones and occasional turbidites were deposited, through the sediments of an advancing coastline, fronted by deltas, to coastal plains in which fluviatile sediments were laid down.

SummaryMany deep water marine muds, including lower Palaeozoic mudstones from Britain, have thin graded beds in which mud and silt laminae alternate, with the silt becoming finer and less abundant upwards. Of the known deep-sea depositional processes, turbidity currents are the most likely cause of such graded laminated beds. The lamination may be produced by alternating cohesive and granular bed conditions. Much more careful examination of laminated fine grained terrigenous sediment is needed.

SUMMARY Two pyroclastic sheet-like deposits were found by the author between the stratigraphic sequence of marine Oligocene sediments of the Vati Group (Rhodes Island, Greece). The lower one is described in this paper. Its uniform thickness over a large area suggests an origin from submarine ash flow. This assumption is also supported by some welding phenomena observed in several thin sections. Submarine eruption of ignimbritic type is advocated to account for these properties. This type of volcanism agrees well with tectonic and paleogeo-graphic evolution of the Vati Group basin. Strong analogies and relationship between submarine ash flows (gaseous suspension) and turbidity currents (watery suspension) are pointed out. The new term of “igniturbidites” is proposed to characterize some pyroclastic turbidity current deposits genetically related to submarine ash flows. The paleogeographic reconstruction of the Vati Group basin gives some information about the location of the volcanic eruption area and about the submarine travelling of the ash flows.