Abstract Seacliff outcrops in the Paleogene Talara forearc basin of northwestern Peru provide a nearly continuous exposure of a 290-m (951-ft)-thick succession of coarse-grained turbidites of upper Eocene age. Lithofacies include thick, amalgamated conglomerate layers, thick-bedded sandstone layers, and interbedded sandstone and mudstone layers. The basal 210 m (690 ft) is a coarse-grained succession composed of fining-upward packages, averaging 20 m (66 ft) thick, with each package including conglomerates at the base and sandstones at the top. The top 80 m (262 ft) is a finer grained succession of fining-upward packages, each composed of amalgamated sandstone layers at the base and interbedded thin-bedded sandstones and mudstones at the top. Coarse-grained units range from pebble conglomerate to coarse-grained sandstone, are up to 7 m (>3 ft) thick, and often pinch out laterally as a result of erosion by subsequent flows. Unlike most thick-bedded, coarse-grained turbidites, these units are dominated by current-structured, SI turbidite divisions that are often capped by intervals of large-scale cross-stratification (Tt turbidite divisions). The abundance of traction structures, coarse grain size, and abundant scoured contacts indicate that the depositing high-density flows were highly erosive and long-lived, and that large amounts of finer grained sediment probably bypassed to more basinward settings. Overall, the succession fines upward and is interpreted to represent multistory, aggradational, coarse-grained channel-fill deposits overlain by finer grained, smaller channel and associated out-of-channel deposits.

Abstract Geophysical and sedimentological data indicate that bottom currents have played a fundamental role in deposition along the Campos continental slope, especially during the Neogene and Quaternary. Sediment drifts are clearly observed in 2D and 3D seismic data. The external geometry and internal reflection pattern of these drifts suggest the predominant action of northward-flowing currents (Southern Ocean Current) along the middle and lower slope (650-1200 m). Upper Quaternary sediments within that zone are composed of highly bioturbated, silty to sandy mud, with rare lamination and no other primary structures. On the upper slope, below the southward flowing Brazil Current, sub-bottom profiles and side-scan sonar records indicate the development of several bedform styles. On the uppermost slope, between 200-300 m, longitudinal lineations and transverse bedforms (2D and 3D dunes) are observed. Sediments are siliciclastic to mixed silty to muddy sand, with rare primary traction structures preserved. Downslope, from 300 to 650 m, linear crested bedforms, a few metres high (2-7 m) are developed. In this zone, an alongslope similarity in the depositional style over more than 50 km suggests bottom current control on sedimentation. The deposits are composed of 1 m of silty muds overlying a decimetric layer of silty sand that grades downslope to a highly oxidized, bioturbated, fine-grained interval. The variability of sediment accumulation rate (2 to 30 cm ka _1 ) is related to high frequency temporal and local modifications

Abstract Lithofacies features of bottom current influenced deep-sea sandy deposits (sandy contourites) have been studied by detailed outcrop analyses of the Plio–Pleistocene infill of the Kazusa forearc basin on Boso Peninsula, Japan. Sandy contourites are characterized by traction structures, such as parallel lamination and ripple cross-lamination, together with minor inverse grading and wave ripple-lamination. Ripple cross-lamination is commonly associated with mud drapes and flaser and lenticular bedding. These sedimentary structures do not display any regular vertical sequences. Sandy contourites are commonly associated with turbidites and comprise turbidite-to-contourite continuums. In general, sandy contourites are better sorted than associated turbidites and have sharp or gradational basal contacts with underlying turbidites or hemipelagites, and sharp upper contacts with overlying hemipelagites. The framework composition of sandy contourites, in contrast, is largely equivalent to that of the associated turbidites. Palaeocurrent directions of sandy contourites are variable in time and space and alongslope-and upslope-directed palaeocurrents are dominant. Turbidite-to-contourite continuums show spatial and temporal variation in bed thickness and lithofacies features. These variations suggest varying intensity of deep-sea bottom currents in the Kazusa forearc basin. Analogous flow conditions have been documented from the modern, deep-water mass of the Kuroshio Current. Thus, the observed variations in lithofacies features and palaeocurrents of turbidite-to-contourite continuums can best be interpreted in terms of fluctuating strength of the deep-water palaeo-Kuroshio bottom current during the Pliocene through Pleistocene

Abstract Ordovician proglacial deposits form gas reservoirs in the In Amenas field, Illizi Basin, Algeria. Depositional models were developed to understand the context and disposition of the main reservoirs through an evaluation of core and analogous outcrops from the Tassili N'Ajjer. Tunnel valleys initially accumulated sandstones with tractional structures. Subsequent failures of subaqueous grounding line sediment deposited proglacial debrites comprising poorly sorted argillaceous sandstone with granules. These were interbedded with high-density turbidity sandstones; their fine grain size indicates they were dynamically disconnected from the lithologically varied debrites. A lobate geometry has been defined for one subsurface composite turbidite. Periodic catastrophic outflows, possibly evacuating subglacial lakes, incised the network of subglacial tunnels and in the process delivered sand to the turbidite outwashes. Bedforms indicate high-energy, transcritical to supercritical outflows that were stable for extended periods. During ice retreat, a period of ice margin stability may have occurred due to grounding over the In Amenas granitic palaeohighs. Outwash fan apices were located along this grounding line with feeder channels developing where the substrate was more easily eroded such as between the palaeohighs. Following further ice retreat, deposition evolved to variably sinuous channels and thence to pelagic fines with dropstones.

To most geologists a turbidite is a graded bed consisting of a sandstone/mudstone couplet which has been deposited by a turbidity current and is commonly overlain by a hemipelagic mudstone containing deep-water fossil assemblages. The Bouma sequence typifies this ideal kind of deposit (Fig. 2a). Bouma (1962) developed his classical model essentially from observations made in upper Paleogene and Neogene turbidite strata of the western Alps (Annot Sandstone) and the northern Apennines (Macigno and Marnoso-arenacea formations). Although commonly restricted to the internal sequence of depositional divisions observed within an individual turbidite bed, the Bouma model actually includes the fundamental concept of “depositional cone” (Fig. 2b), implying the depletive character of turbidity currents, i.e. their deceleration in space away from their origin. On the basis of this concept, Parea (1965) and Walker (1967) developed their models of proximal vs. distal turbidite deposition — an approach which in many respects still has considerable potential for future research. Bouma (1962) interpreted his sequence as the deposit of a typical turbidity current without discussing hydrodynamic processes in detail; Harms and Fahnestock (1965) and Walker (1967) reinterpreted the sequence in terms of laboratory flow regime postulating an upper flow regime for the basal “a” division (Fig. 3). In a very influential paper, Middleton and Hampton (1973) suggested that the entire Bouma sequence was the deposit of a fully turbulent turbidity current and envisaged the “a” division as the result of very rapid deposition from suspension preventing the formation of traction structures. Subsequent work has shown that the Bouma