Turbidite Basin Research Articles

Hydrocarbon source rock sampling in Peary Land 1980

The Peary Land region in North Greenland (fig. 31) contains a sequence of Lower Palaeozoic sediments which is probably more than 4 km thick (Dawes, 1976; Christie & Peel, 1977; Hurst, 1979; Christie & Ineson, 1979; Hurst & Surlyk, 1980; Ineson & Peel, 1980; Surlyk, Hurst & Bjerreskov, 1980). From Early Cambrian to Wenlock the area was divided into a northern turbidite trough and a southern, mainly carbonate platform (fig. 32). The platform seems to have undergone several phases of backstepping to the south, accompanied by expansion of the turbidite basin (fig. 32) (Surlyk et al., 1980). The region was affected by an orogeny of assumed Devonian - early Carboniferous age (Dawes, 1976). Deformation is most intense in northern Johannes V. Jensen Land (fig. 31), where an amphibolite facies is attained along the north eoast (Dawes & Soper, 1973) and decreases southwards, leaving the platform earbonates virtually undeformed. A separate, strongly block-faulted sedimentary basin, the Wandel Sea Basin is present in eastern Peary Land and farther to the south-east (Dawes & Soper, 1973; Håkansson, 1979). It eontains a sequenee of Upper Palaeozoie carbonates and Upper Palaeozoic - Mesozoic mainly coarse clastics more than 3 km thick.

Back-arc extension in the southern Andes: a review and critical reappraisal

The interpretation that the mafic ‘rocas verdes’ (green rocks) complex of the southern Andes represents part of the uplifted floor of a Late Jurassic to Early Cretaceous back-arc basin has proved particularly useful in understanding the geological evolution of the southern Andes, the north Scotia Ridge and the Antarctic Peninsula. Clear field evidence of the back-arc setting of the 'rocas verdes’ gabbro-sheeted dyke - pillow lava ophiolitic assemblages has encouraged fruitful petrological and geochemical comparison with mid-ocean ridge and marginal basin basalts, other onshore ophiolite complexes, and Archaean greenstone belts. Uncertainty still surrounds estimates of the original width and depth of the basin, as well as the proportion of new mafic crust, compared with relict sialic crust, in the basin floor. These questions are unresolved, owing mainly to the considerable Lower Cretaceous turbiditic basin infill and the effects of mid-Cretaceous compressional deformation. While the field relations clearly indicate that the ‘rocas verdes’ basin is not an older piece of ocean floor ‘trapped’ behind a volcanic arc, it is not yet clear whether the basin is directly subduction-related or falls in the category of back-arc ‘leaky transforms’ like the proto-Gulf of California or apparent ‘rip-off’ features like the Andaman Sea.

Hydrocarbon Potential of Matilija Sandstone, an Eocene Sand-Rich, Deep-Sea Fan and Shallow-Marine Complex, California: ABSTRACT

The Matilija Sandstone Member, exposed in the Santa Ynez Mountains, California, records a major regressive event in the Eocene Santa Ynez basin in which turbidites were deposited in the basin and subsequently covered by shallow-marine complexes. Despite thick favorable source beds and generally good initial reservoir characteristics, the Matilija sandstone is not a productive unit in the basin. Lowered reservoir rock permeability (< 1 md) and porosity (0-10%) are due to early compaction, cementation, and diagenesis. The lower part of the Matilija sandstone is a 700-m-thick sand-rich deep-sea fan complex which overlies basin plain and turbidite deposits (Juncal shale). The Matilija sandstone consists of anastomosing outer-fan depositional lobes overlain by channelized middle- and inner-fan deposits. Cross-bedded sandstone, red-bed, and carbonate-evaporite sequences overlie the turbidites. Matilija sandstone deposition closed with rapid transgression which culminated in the deposition of basin plain and turbidite deposits (Cozy Dell shale). The Matilija sandstone lower deep-sea fan complex has a high sandstone and shale ratio (4:1) and consists of submature arkoses of facies B. The average sandstone is medium grained, moderately sorted, subangular, massive, and contains 40% quartz, 35% feldspar (about equal amounts of potassium and plagioclase feldspars), 10% lithic fragments (mostly granitic and volcanic types), and smaller amounts of mica, chert, and heavy minerals. Early compaction, carbonate cementation, and authigenic pore-lining chlorite, albite, and quartz have reduced the initial porosity and permeability. Minor secondary fractures are the only effective porosity in these rocks now. End_of_Article - Last_Page 740------------

Comparison of Sand-Layer Geometry on Flat Floors of 10 Modern Depositional Basins

A comparative study of 10 deep turbidite basins indicates that sand-layer thickness, frequency, and continuity can be related to their basin geometries, tectonics, and source areas. This study is based on piston-core data from the flat floors of four oceanic-crust abyssal plains (Sohm, Hatteras, Blake-Bahama, and Silver), two Bahama-Plateau reentrant basins (Columbus basin and Tongue of the Ocean), three subduction zone--island arc basins (Hispaniola-Caicos, Navidad, and St. Croix basins), and one continental-borderland basin (Santa Monica basin). With increasing tectonic activity, sand layers on flat basin floors tend to be thinner and more frequent. Except in elongated narrow basins such as Navidad, source-area size is directly related to proximal sand-layer thickness. Continuity of individual sand layers is a function of drainage-basin size and depositional-basin shape. For example, on the Hatteras Abyssal Plain long-distance continuity (up to 500 km) of individual sand layers is achieved because most of the turbidites are large and are introduced only at the north upstream end. In the small horseshoe-shaped Columbus basin, sand-layer continuity is limited because flows are small and are introduced from three sides. In most basins, sands thin and the percentage of sand layers in the sediment column decreases away from sour e areas. However, in basins that are small relative to the typical turbidity-current size (e.g., Hispaniola-Caicos basin), the differences between basin-edge and basin-center sand layers are slight.

Paleogene Depositional Systems, Western Transverse Ranges, Southern California: ABSTRACT

Paleogene strata of the western Transverse Ranges include varied clastic facies that prograded southwestward from arkosic sediment sources in granitic and metamorphic terranes of the Salinian and Mojave blocks. Deposition occurred within a complex fore-arc basin whose bathymetry was modified by pronounced uplift of a structural high along the trench slope break, and by internal deformation associated with oblique subduction during shallow plate descent beneath the Laramide cordillera. Integrated depositional systems produced intertonguing deep-marine, shallow-marine, marginal-marine, and nonmarine deposits. Exposed transitions between basinal turbidite successions, deltaic strandline complexes, and alluvial-plain sequences are common at several horizons. Mappable formatio s and local members are distinguished chiefly by their shaly, sandy and conglomeratic, or mixed lithologic character. Each such stratigraphic unit typically includes several facies associations. Shale-rich facies were deposited on terrestrial flood plains, prodelta and basin-flank slopes, overbank surfaces of subsea fans, and distal basin plains. Sand-rich intervals include thickening-upward cycles of sheet-flow depositional lobes on subsea fans, thinning-upward cycles of turbidite channels on subsea fans, coarsening-upward cycles of prograding shelf breaks, fining-upward cycles of migratory shelf and shoal-water bars, coarsening-upward cycles of distributary-mouth bars, and fining-upward cycles of fluviodeltaic End_Page 698------------------------------ channels. Conglomeratic strata occur both as piedmont-fan deposits and within proximal turbidite channel complexes. Sparse nonclastic facies include local algal carbonate platforms built on isolated submarine banks formed by tectonic uplift, and minor lagoonal beds of algal carbonate rocks and gypsiferous evaporites associated with delta platforms. End_of_Article - Last_Page 699------------

Keweenawan turbidites in Michigan (deep borehole red beds): A foundered basin sequence developed during evolution of a Protoceanic Rift System

A 1538‐m pre‐Mt. Simon (Upper Cambrian) red bed sequence, encountered in the Gratiot County deep borehole, consists of undeformed recurring beds of submature and immature arkosic sandstone, 5–85 cm thick, that grade upward into mudstone units, 3 cm to 3.8 m thick. Similar facies are recorded by a red bed sequence penetrated in a well on Beaver Island, 260 km to the north, and both sequences occur within the mid‐Michigan gravity high. We suggest penecontemporaneous deposition by sediment gravity flows, low‐velocity clear water currents, and pelagic fallout on submarine fans forming in an elongate, broadly subsiding protoceanic basin which foundered isostatically following arrestment of Keweenawan rifting and production of oceanic lithosphere. We further suggest that protoceanic rifting in the Lake Superior region and that along the mid‐Michigan gravity anomaly were approximately contemporaneous and that both areas underwent a similar sequence of thermotectonic volcanism, deformation, and sedimentation, followed by passive foundering and shallow marine and basinal turbidite sedimentation.

The Bucegi Conglomerate: a Romanian Carpathian submarine slope deposit

THE Bucegi Conglomerate, a thick conglomerate-sandstone unit of Albian age (Cretaceous), is well exposed in the Eastern Carpathian Chain near Sinaia, Romania. This unit occupies more than 5,000 km2 in the vicinity of the change in structural–stratigraphic trend of the mobile belt from E–W to N–S (Fig. 1a). The thickness of this remarkably coarse series is regionally variable and locally exceeds 2,000 m. To the west the conglomerate overlies, with angular unconformity, Middle and Upper Jurassic platform carbonate rocks which in turn unconformably overlie crystalline basement. To the east, over a distance of approximately 10 km, the Bucegi rests with local unconformity on Neocomian and Barremian–Aptian flysch several kilometres thick (Fig. 1b and c). This unconformity post-dates pre-upper Aptian to post-lower Aptian folding 1,2. Previous work has alluded to the shallow marine and nearshore origin3 of this formation, and it has been frequently referred to as a molasse, or para-molasse4, mainly due to its coarse nature, its pebble shape3,5, its stratigraphic position above sandstone-shale turbidite-rich formations, and presumably also to its ‘post-tectonic’ position within the mobile belt. Here we consider an alternative, that is that the Bucegi Conglomerate accumulated in an infra-neritic to deep marine environment and that it records progradation of coarse material directly onto the slope and base of slope of a turbidite basin. The field observations we made in conjunction with Romanian colleagues in selected sections of the Sinaia region (Jepi Valley, Chindichiu road cuts) and an evaluation of previous reports provide a plausible basis to reconsider the depositional origin of the Bucegi.

Sedimentation and Trapping Mechanism in Upper Miocene Stevens and Older Turbidite Fans of Southeastern San Joaquin Valley, California

Most of the stratigraphic oil production in the southern San Joaquin Valley comes from the sandstones of the upper Miocene turbidites. Updip, time-equivalent rocks of the shallow-marine environment are second in productivity. Third in importance are the deep-marine, fractured, siliceous shales deposited beyond distal margins of the fan complex. Many underexplored areas remain in the San Joaquin Valley despite many years of field development and wildcat drilling. The Miocene of the Bakersfield arch is representative of the classic arcuate type of turbidite deposits characteristic of modern submarine fans. Further paleogradient of the fans has been accentuated and not reversed through geologic time; stratigraphic oil fields are numerous. As in most California deep-marine troughs, the turbidite facies most easily reached by the drill, and in which most of the oil reserves have been developed, are in the mobile zone of basin-margin wedging. Two additional trapping aspects related primarily to depositional stratigraphy within the midfan facies are (1) intrabasin contemporaneous faults (analogous to Gulf Coast growth faults) resulting in expanded section and reverse drag on the downthrown block, and (2) compaction anticlines formed by post-sediment ry accommodation over submarine channel sands resulting in structural inversion with depth. South of the Bakersfield arch is an earlier (pre-late Mohnian) turbidite basin, virtually unexplored and referred to informally as the Maricopa subbasin, where drilling depths to potential Miocene pays range from 12,000 to 20,000 ft (3,600 to 6,000 m). Basic stratigraphic relations recognized in the Bakersfield arch may be applied in the subbasin to develop new hydrocarbon reserves.

Paleobathymetry of Cretaceous Turbidite Basins of the East Alps Relative to the Calcite Compensation Level

The paleobathymetry of ancient turbidite basins has traditionally been dealt with in terms of paleontologic depth indicators. The calcite compensation depth now provides an additional bathymetric reference level. In this study the concept of carbonate compensation levels (CCL) has for the first time been applied in detail to Cretaceous turbidite basins. Out of the four East Alpine basins studied two (i.e., Flysch Zone of the East Alps and Kossen Gosau Basin) have calcite-free hemipelagic green claystones of Campanian-Early Maestrichtian age and were below the CCL. Alternative causes such as climatic variations and terrigenous influx of carbonate-free detritus are unlikely to be major controlling factors for the absence of calcite in the hemipelagic claystones. Distinction between hemipelagic and turbiditic mudstones is facilitated by the high calcium carbonate content of the latter. Hemipelagic and pelagic layers of the same age in the other two basins (Reichenhall Gosau Basin and Ultrahelvetic flysch) co...

The Rio Dell Formation: a Plio‐Pleistocene basin slope deposit in Northern California

ABSTRACTThe Rio Dell Formation (Pleistocene and Pliocene), exposed south of Eureka, California, is a prograded sequence of basinal turbidites overlain by basin slope and shelf deposits. The slope deposits studied in the Centerville Beach section accumulated in a steadily shallowing environment delineated by analysis of palaeobathymetrically significant benthonic foraminiferal biofacies in turn suggesting deposition at depths of 1000–100 m. Lower slope deposits interfinger with basinal turbidites derived from the Eel River delta to the north. Slumped blocks of silty mudstone, and associated silt and mud beds, are common. The middle slope deposits are mudstones; coarser sediments bypassed this zone. Mudstones and muddy siltstones alternate on the upper slope. Shallow depressions, probably slump scars, that have been rapidly filled by upper slope sediment are common. The transition to shelf deposits is marked by an increase in sediment grain size, in the degree of oxidation, and in the abundance of megafossils. High percentages of benthonic foraminifera displaced from shelf depths indicate that resedimentation processes are most important on the upper slope.

Turbiditic and non‐turbiditic mudstone of Cretaceous flysch sections of the East Alps and other basins

ABSTRACTRecognition 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.

Crustal rupture and the initiation of imbricate thrusting in the Peru-Chile Trench

A detailed survey of parts of the Peru-Chile Trench between lat 6° and 10°S shows that it has undergone extensive deformation within the past several thousand years. The tholeiitic basalt crust and overlying sedimentary deposits have ruptured, forming ridges within the trench axis and uplifted turbidite basins of former trench fill on the seaward wall of the trench. Several displaced terrigenous turbidite units show that 700 to 910 m of uplift has occurred within the past 3,155 yr on the most prominent ridge, while 700 m of uplift has occurred during the past 5,100 yr in the basins on the seaward wall of the trench. Average rates of relative vertical movement may range from 14 to 22 cm/yr in this area. This study suggests that the oceanic plate is fractured into three segments (N11°W, segment 1; N24°W, segment 2; and N31°W, segment 3) between lat 6° and 10°S. The most intense deformation is found at lat 7°40′S and 9°20′S, between segments 1 and 2 and 2 and 3, respectively; major tear faults are postulated for these two regions of extreme crustal rupture. The bathymetry, structure, nature of vertical movements, sediments of the Peru-Chile Trench and lower continental slope, and first motions of earthquakes on the oceanic plate suggest that imbricate thrusting plays an important role in the development of the features observed at the convergent boundary off Peru. The structural elements are best explained by a five-stage imbricate thrust model: (1) normal faulting of the descending oceanic plate resulting from tensional stress along the line of flexure; (2) reverse faulting resulting from a change from tensional to compressional stress within the trench; (3) initiation of thrust faults within the trench; (4) extensive rupture forming axial ridges and basins within the trench; and (5) accretion to the lower slope of the thrust sheet as it moves forward. Reactivation of movement along older imbricate thrusts in the continental slope may occur, particularly in stages 3, 4, and 5. The prominent benches on the continental slope probably develop as a consequence of the imbricate thrusting. The onshore and offshore region from lat 6° to 10°S is marked by a seismic gap (absence of shallow-focus earthquakes with magnitude > 7.7) and the onshore region by a paucity of late Cenozoic plutonic and volcanic activity. We postulate that the seismic gaps occur because extensive fracturing of the descending oceanic plate and motion along old imbricate thrust faults in the continental slope preclude the build-up of the large stress fields necessary to produce large-magnitude earthquakes. If imbricate thrusting and accretion is more prevalent than subduction in this region, it may also help to explain the lack of late Cenozoic volcanism in the area of the Huancabamba deflection.

Seismic profiles of the eastern Indian Ocean and deep-sea drilling

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.

Petroleum Potential on Continental Rise Off Central California: ABSTRACT

The JOIDES program has demonstrated the technical feasibility of drilling in the deep ocean. However, except for petroleum shows in the Sigsbee Knolls in the Gulf of Mexico, there is no firm evidence of petroleum reservoirs in the deep ocean beyond the limits of the continental slope to require economic use of such technology. Emery suggested the possibility of vast petroleum resources in deep water adjacent to the continents in the areas of hemipelagic sedimentation. One such area adjacent to the United States is the continental rise off California between Point Conception and Cape Mendocino, which covers 200,000 sq km and consists of three major submarine fans, the Arguello, Monterey, and Delgada, at depths of about 3,000 m to 4,500 m. The fans are composed of continental debris carried down submarine canyons and deposited on the fan through a system of branching and meandering submarine End_Page 1461------------------------------ channels. During the depositional growth of the fan, the active channelway shifted periodically, producing abandoned channels. The coarser sediment naturally confined to the channel became covered, after abandonment, by finer grained material. The interchannel deposits, composed chiefly of green hemipelagic muds, are potential petroleum source beds because of their significant carbon content. Thus abandoned channels after sufficient burial and invasion from source beds make excellent stratigraphic traps, especially near the mouth of tributary submarine canyons where the grain size would be larger and the channel width and depth greater. Off central California, potential stratigraphic traps would be seaward of the mouths of the Arguello, Sur-Partington-Lucia, Monterey-Carmel, Ascension, Pioneer, Farallon, Bodega, and Delgada canyons as shown by subbottom profiling. The proximity of such potential reserves to the United States makes buried channels on large-scale deep-sea fans particularly attractive prospects similar to prospects in turbidite basins of the California borderland. End_of_Article - Last_Page 1462------------

Uplifted Turbidite Basins on the Seaward Wall of the Peru Trench

Hemipelagic sediment layers inter-bedded with silt turbidite units were recovered from an elongate basin seaward of the axis of the Peru Trench. The basin is elevated 700 m above the shallowest depth of the present trench axis between lat 7°20′ and 8°30′ S. Micro-faunal content, mineralogy, and topography point to a source for this turbidite on the continental shelf to the northeast of the basin. A radiocarbon date of 5,100 B.P. was obtained for the youngest turbidite in the core, which dates the beginning of uplift in the basin. Although the nature of vertical motion is difficult to determine, the data suggest that there has been a combination of uplift of the seaward wall of the trench and subsidence of the trench axis. These rapid vertical movements may be a consequence of rupturing and decoupling of the Nazca plate into two segments that are being subducted at differential rates or different angles. If the seaward wall is under compressional stress as we suggest, we have an apparent contradiction with earthquake first motions, which indicate extensional stress on the seaward wall of this and other trenches.

Sequential Analysis of Turbidite Basins in North-Central Apennines: ABSTRACT

The Miocene Marnoso-arenacei and Laga Formations form two of the largest outcropping turbidite units of the north-central Apennines,where extensive exposures make facies analysis feasible. Following a more general model proposed by Mutti and Ricci Lucchi (which utilizes bed geometry), sedimentary structures and texture, facies association (slope, submarine fan, basin plain) and sedimentary facies (conglomeratic sandstones, sandstones, sandstones with shale interbeds, shales with sandstone interbeds, chaotic deposits, hemipelagic pelites) were recognized. The value of sequential analysis for comparing stratigraphic subunits within a submarine fan system, and particularly for differentiating between inner fan, mid-fan, outer fan, and basin-plain environments, is emphasized. Recurrent patterns of sedimentation (megasequences or megarhythms, 10-100 m thick) are composed of sets of thick, mostly arenaceous, massive, graded, or crudely laminated layers, with alternate finer grained beds, which are thinly laminated in the lower part. Two main types of ordered megasequences are recognized: (A) positive, or thinning upward, interpreted as filling of fan valleys or channels; and (B) negative, or thickening upward, considered to be prograding outer fans along nonchanneled distributaries. Three monotonous, or random, sequences composed of finer grained sediments are also present: (C) slope deposits (thin layers, with a sandstone/shale ratio less than 1); (D) interdistributary fan deposits (thin to medium layers, sand/shale ratio less than 1; and (E) basin-plain dep sits (medium to thin layers, with sporadic isolated thick turbidite units, sandstone/shale ratio greater than 1). The vertical association of these subunits, and especially those of the A and B megasequences, show both similarities and differences in the sedimentary history and tectonic control of the Miocene basins. Further application of these criteria to other turbidite basins probably will aid in predicting the location and geometry of sandstone bodies. End_of_Article - Last_Page 801------------

Late Pleistocene Deformation of Cascadia Basin Turbidites along Washington Continental Margin: ABSTRACT

A minimum rate of underthrusting of the Juan de Fuca plate beneath North America for the late Pleistocene has been calculated to be approximately 0.7 ± 0.3 cm/yr. This estimate was determined in each of 18 CSP crossings of the continental slope off the coast of Washington by dividing the amount of shortening within the westernmost anticlinal ridge by an approximation of the time elapsed since the beginning of the ridge deformation. The latter estimate was obtained by tracing the late Pleistocene discontinuity as described by Leg 18 of the Deep Sea Drilling Project from site 174 on Astoria fan into the deformed Cascadia basin turbidites of the continental slope. Recent plate-tectonics theories predicting northwesterly movement of the Pacific plate parallel with the Sa Andreas fault at 6 cm/yr, and the spreading of the Juan de Fuca plate away from the Pacific plate at 6 cm/yr, require an underthrusting rate of 2 cm/yr measured perpendicular to the trend of the Washington continental margin. If the age of the late Pleistocene discontinuity is assumed to be a half-million years, the deformation End_Page 768------------------------------ of the marginal ridge can account for nearly 40 percent of the theoretical crustal shortening between the Juan de Fuca and North American plates. Seismic profiles also indicate that accretion of Cascadia basin turbidites along the continental margin occurred during the Pliocene-Pleistocene and has continued to the present. End_of_Article - Last_Page 769------------

Transport and Deposition of Coarse Clastics in Turbidite Basin in French Alps: ABSTRACT

Marine breccias of Jurassic to Early Cretaceous ages are present in the Breccia Nappe of the French Prealps. Breccia types Ia and Ib are restricted to the lower part of the sequence in the Lower Shale, Lower Breccia, and Upper Shale formations. Type Ia breccias occur in beds from a few centimeters to tens of meters thick. They contain clasts up to more than 1 m in diameter, and are sometimes graded. Sole markings occur but are not common. Tops of some beds have large scale cross-stratification or parallel bedding, usually in granule-pebble grade material. Individual beds are of limited lateral extent--of the order of 1-2 km along the depositional strike and in places up to 7-8 km across it. The breccias have a clast framework and interstitial material is usually pebble or granule size. There is a continuous spectrum, with change in relative proportions of gravel and sand, from the breccias to pebbly turbidite sandstones. Type Ib is much less common than Ia. It has clasts of the same composition and size but it is characterized End_Page 342------------------------------ by more than 50% sand-grade matrix. Beds are less than 200 cm thick, parallel-sided, and have an uppermost sand-silt layer with convolute lamination. Type Ia is interpreted as a deposit from mass flow of coarse granular debris where internal shear was extensive enough to allow development of grading. For the transport of Type Ib beds, a slide mechanism is favored. End_of_Article - Last_Page 343------------

Sedimentation of Pliocene Sandstones in Santa Barbara Channel, California: ABSTRACT

Pliocene rocks in the Ventura basin, including the part under the Santa Barbara Channel, provide an excellent area to study a strongly deformed but essentially intact turbidite basin. Conglomeratic beds, containing typical turbidites, are present along the northern margin of the basin. Isopach, sand-isolith, and sand-percentage maps of the Pliocene Repetto and Pico Formations in the eastwest Central basin show thick deposits bounded on both sides by thinner deposits in the Rincon trend on the north and the Montalvo trend on the south. Deposition was controlled by partly effective fault barriers on the north and south margins of the Central basin. Sand-isolith and sand-percentage maps indicate local increases in sand in the north on the Rincon trend suggesting the presence of ancient subsea fans in the Repetto and lower Pico. The overall decrease in sand on the west denotes a major influx of sand from the east down the Central basin axis. The first influx of sand from the southern margin of the basin is found on the sand-percentage map of the middle Pico A. Deposit on of sand was also affected by topographic highs associated with growing faults and anticlines. Distribution, textural properties, primary structures, and microfauna of sandstones in the Ventura basin are comparable to the modern deposits in the nearby Santa Monica basin. Stratigraphic maps reveal Pliocene subsea fans, and observations of outcrops and cores show the same type of beds as described in the Santa Monica basin. Hence, sedimentation was controlled by sand deposition in and at the foot of submarine canyons in the form of subsea fans. End_of_Article - Last_Page 335------------

Classification and Interpretation of Disturbed Bedding Types in Jackfork Flysch Rocks (Upper Mississippian), Ouachita Mountains, Arkansas

ABSTRACT The Jackfork Group in the Ouachita Mountains of Arkansas is an Upper Mississippian flysch succession that displays readily distinguishable lithofacies. The frontal Ouachitas contain a preponderance of disturbed beds which are interbedded with turbidite sandstones. These disturbed and disrupted beds developed at the foot of a northern unstable slope, the resultant bedding dependent upon the type of material involved, its state of consolidation, and the distance moved. Information gathered concerning the progressive changes from one type of sedimentary structure to another gives a qualitative measure of downslope distance. Muds which undergo submarine deformation upon a sloping sea floor first become flowage shales which change to blocky mudstones with continued failure. Failure of plas ic muds and cohesive sandstone progressively form contorted, chaotic, and rubble bedding. Deformation of muds and sands form slurried bedding and breccias. Movement of flowing, creeping, or sliding clean sand may have occurred but is not considered to be a type of disruptive bedding. The deformation of mobilized, liquefied sand under some sediment cover produced load structures, sandstone dikes, and intrastratal flow structure. The Ouachita trough possibly consisted of an offsetting series of unstable shale slopes deepening southward to a gentle, west-dipping abyssal plain. The muds and channel sands upon the slope frequently slumped southwards, sometimes flowing out over the abyssal floor at right angles to basinal turbidites flowing westward down the trough axis. Belts where disturbed bedding comprises the greatest percentage of the entire section define the unstable margins of the flysch trough while the highest sand-shale ratios parallel the axis of the trough.