Classical Turbidites Research Articles

Facies and facies association of the siliciclastic Brak River and carbonate Gemsbok formations in the Lower Ugab River valley, Namibia, W. Africa

The Neoproterozoic Zerrissene Turbidite Complex of central-western Namibia comprises five turbiditic units. From the base to the top they are the Zebrapüts Formation (greywacke and pelite), Brandberg West Formation (marble and pelite), Brak River Formation (greywacke and pelite with dropstones), Gemsbok River Formation (marble and pelite) and Amis River Formation (greywacke and pelites with rare carbonates and quartz-wacke). In the Lower Ugab River valley, five siliciclastic facies were recognised in the Brak River Formation. These are massive and laminated sandstones, classical turbidites (thick- and thin-bedded), mudrock, rare conglomerate and breccia. For the carbonate Gemsbok River Formation four facies were identified including massive non-graded and graded calcarenite, fine grained evenly bedded blue marble and calcareous mudrock. Most of these facies are also present in the other siliciclastic units of the Zerrissene Turbidite Complex as observed in other areas. The vertical facies association of the siliciclastic Brak River Formation is interpreted as representing sheet sand lobe to lobe-fringe palaeoenvironment with the abandonment of siliciclastic deposition at the top of the succession. The vertical facies association of the carbonate Gemsbok Formation is interpreted as the slope apron succession overlain by periplatform facies, suggesting a carbonate slope sedimentation of a prograding depositional shelf margin. If the siliciclastic–carbonate paired succession would represent a lowstand relative sea-level and highstand relative sea-level, respectively, the entire turbidite succession of the Zerrissene Turbidite Complex can be interpreted as three depositional sequences including two paired siliciclastic–carbonate units (Zebrapüts-Brandberg West formations; Brak River–Gemsbok formations) and an incomplete succession without carbonate at the top (Amis River Formation).

Bed geometry used to test recognition criteria of turbidites and (sandy) debrites

The origin of thick-bedded deep-water sandstones has generated much controversy in recent years. Two fundamentally different models have been proposed for beds with the same internal sedimentary characteristics: (1) progressive particle settling from the base of a turbulent flow—the “turbidity current” model and (2) en-masse freezing of a higher-concentration flow—the “sandy debris flow” model. These models predict beds with very different geometries; turbidites thin gradually whereas debrites have abrupt terminations. Previous studies have relied upon sedimentary recognition criteria (i.e., sedimentary features in small-scale outcrop or core) to interpret depositional mechanism. In this study, depositional mechanism is deduced from bed geometry gained from extensive correlations of individual sandstones preserved in a classic turbidite system (Marnoso-arenacea Formation, Italy). This approach allows recognition criteria for turbidites and submarine debrites to be independently tested. We find that tabular and tapered sandstones (turbidites) have distinctly different internal characteristics to beds with abrupt margins (debrites). Turbidites are relatively well sorted, often exhibit grading and traction structures and have relatively low matrix mud contents. They may also contain massive division, floating clasts and inverse grading. Debrites are moderate-to-poorly sorted, ungraded, structureless, contain floating clasts and have elevated matrix mud contents. These findings have implications for the assessment of submarine gravity flows deposits and reservoir rock characterization.

Geometry and petrophysical parameters of a calcarenitic turbidite lobe (Barremian-Aptian, Pas-de-la-Cluse, France)

Several bioclastic resedimentation systems, late Barremian to early Aptian in age, outcrop in southeastern France. The upslope portion of a calcarenitic turbidite lobe outcrops as a cliff, 2.3 km wide and up to 45 m high, oriented roughly perpendicular to the direction of palaeoflow. It has been studied through 31 sections. The studied body is point-sourced and corresponds to a channel/lobe transition located at the toe-of-slope of a Urgonian carbonate platform on the western edge of the Vocontian Trough (French Subalpine Chains). The internal geometry of the lobe is the result of repeated erosion and filling of channels, creating well-defined stacked units. Filling phases result in lobe-shaped morphology. Units evolve from base to top in an ordered way. First, the sediments exported from the platform and deposited in the basin are represented by decimetre-thick, sheet-like and fine-grained Bouma a-e and ab-e sequences. In this first unit, erosion is limited and represented by shallow and symmetrical channels. This initial phase of deposition corresponds to the formation of a lobe with a positive morphology. The next phase of system development is more erosional and this is dominated by migrating, sinuous channels several metres deep filled with coarse-grained, poorly graded turbidites. These channels are first filled through lateral accretion and then sediment spills over to create a sub-lobe. The mixture of channel and lobe features could be considered as a carbonate equivalent of the channel–levee system of siliciclastic deposits. Finally, the system ends with the deposition of a flat-based unit. This unit extends for more than 2 km and is composed of the southwards lateral accretion of hundreds of metres wide sequences each composed of a debris-flow deposit capped by a coarse-grained calcarenite bed. These sigmoidal bodies could be the ultimate forms of a spreading out phenomenon of the lobe. Overall, the Pas-de-la-Cluse system is a coarsening-upward body. As the grain-size is related to the nature of the material, this coarsening-upward implies either an evolution in the nature of the carbonate material produced on the platform (from grainy to bioconstructed), a displacement of the source sediment (from inner platform to margin) or a displacement of the channel head through time (from distal to proximal). The upward evolution from classic turbidites to debris-flow deposits can be explained either by the way the platform facies evolved with time in a depositional sequence or through retrogressive erosion by successive slope failures that cut deeper and deeper into the platform slope. Although its present-day porosity is low, the size of the system and the fact that it is encased in marls make it an interesting outcrop analogue for hydrocarbon exploration.

The Late Carboniferous Glacial to Postglacial Transition: Facies and Sequence Stratigraphy, Western Paganzo Basin, Argentina

Namurian-early Westphalian glacial-postglacial deposits have been documented in the Paganzo Basin, Argentina, but integrated sequence stratigraphic and sedimentological analyses of the glacial-postglacial transition are rare. In the Cuesta de Huaco section the glacial-postglacial transition encompasses the lower sequence of the existing schemes. Glacioeustatic changes in sea-level controlled the architecture and vertical arrangement of facies. The basal sequence boundary is coeval with the onset of glaciation when sea level fell, contemporaneous with glacial erosion and bypass of sediment. The upper sequence boundary is a high-frequency sequence boundary. The basal deposits are interpreted as an early transgressive systems tract (ETST) initiated during glacial retreat. The interval fines upward, contains current reworked diamictites, conglomerates and sandstones deposited in a subglacial to proglacial environment (Facies I-II) and ice-rafted glaciomarine rhythmites (Facies III). Postglacial brackish-marine black shales (Facies IV) represent the late transgressive systems tract (LTST). Micritic-limestone beds were deposited during a period of sediment starvation coeval with the maximum marine inundation in the basin (MFZ). It marks a turnaround in the stacking pattern that became coarsening and thickening upward during the overlying highstand systems tract (HST). It consists of fine-grained turbidites (Facies V) and classical turbidites with wave reworked beds at the top (Facies VI) deposited in a shallow deltaic systems. Intervening postglacial brackish deposits contain an ichnofauna dominated by trackways of arthropods and minor grazing traces, preserved in bedding planes, that resemble nonmarine ichnocoenoses and differ from the archetypal brackish ichnofaunas, even when palynological data support the saline-depleted environment.

A new depositional model for the classical turbidite locality at San Clemente State Beach, California

The Miocene turbidite system exposed in the beach cliffs at San Clemente State Beach, California, has been used by industry and academia alike as a field laboratory. It has been used as an analog for petroleum reservoirs in the Los Angeles Basin and other areas. We interpret the turbidite system at San Clemente State Beach to represent the fill of a single turbidite channel that aggraded subvertically. This interpretation is based on construction of a detailed photomosaic, mapping of three-dimensional facies distributions, measurement of representative sections, and collection of new paleocurrent data; we also extended this analysis to outcrops not previously described in the literature. The channel had a minimum width of 1 km and a paleotransport direction toward the northwest. Our new paleocurrent measurements indicate an average transport direction of 321 degrees for the channel deposits, approximately perpendicular to previously published paleocurrent data. Our paleocurrent data are in agreement with the trend of the channel as defined by facies mapping. The lateral and vertical facies changes along the sea cliffs at San Clemente are a result of interfingering between axial and marginal facies within a single turbidite channel. We interpret the turbidite channel to have been cut into a low-gradient continental slope (less than 1 degrees), rather than representing a channel on a submarine fan. We propose that the Gollum channel system is a modern analog of the turbidite system at San Clemente State Beach.

50 years of the turbidite paradigm (1950s—1990s): deep-water processes and facies models—a critical perspective

Under the prevailing turbidite paradigm, the term turbidite (i.e., deposits of turbidity currents with Newtonian rheology and turbulent state) is used very loosely and is commonly applied to deposits of debris flows with plastic rheology and laminar state. For example, because high-density turbidity currents are defined on the basis of three different concepts (i.e., flow density, grain size, and driving force), there are no consistent criteria for recognition of their deposits. As a result, deep-water massive sands of debris-flow origin are routinely misinterpreted as high-density turbidites. The concept of waxing flow as a type of turbidity current is problematic because waxing flows are defined on the basis of velocity, not on fluid rheology and flow state. The waxing-flow concept allows inversely graded sands to be misinterpreted as turbidites. Perhaps, the most problematic issue is the use of alluvial channel traction bed forms observed in flume experiments as the analog for the five divisions of the Bouma Sequence (i.e., classic turbidites deposited from suspension). This is because flume experiments were conducted under equilibrium flow conditions, whereas natural turbidity currents deposit sediment under disequilibrium waning flow conditions. This and other problems of deep-water processes and facies models are addressed in this paper from the authors personal perspective. Classification of sediment-gravity flows into Newtonian flows (e.g., turbidity currents) and plastic flows (e.g., debris flows), based on fluid rheology and flow state, is a meaningful and practical approach. Although popular deep-water facies models are based on transport mechanisms, there are no standard criteria in the depositional record to reliably interpret transport mechanisms. According to existing turbidite-facies models, an ideal turbidite bed, which has normal grading, with gravel- to mud-size particles should contain a total of 16 divisions. However, no one has ever documented a complete turbidite bed with 16 divisions in modern or ancient deposits. Recognition of units deposited by deep-water bottom currents (also referred to as contour currents) is difficult. Traction structures are good indicators of bottom-current reworking, but distinguishing deposits of bottom currents from deposits of overbanking turbidity currents is difficult even though it has important implications for developing depositional models for hydrocarbon exploration and production. I consider sandy debris flows to be the dominant process responsible for transporting and depositing sands in the deep sea. Experiments on sandy debris flows suggest that low clay content (as little as 1%) is sufficient to provide the strength necessary for sandy debris flows. Deposits of experimental sandy debris flows are characterized by massive sand, sharp upper contacts, floating clasts, inverse grading, normal grading with clasts, and water-escape structures. As a counterpart to turbidite-dominated fan models suited for basinal settings, a slope model is proposed that is a debris-flow dominated setting with both non-channelized and channelized systems. Contrary to popular belief, deposits of sandy debris flows can be thick, areally extensive, clean (i.e., mud poor), and excellent reservoirs. High-frequency flows tend to develop amalgamated debris-flow deposits with lateral connectivity and sheet-like geometry. Submarine-fan models with turbidite channels and lobes have controlled our thinking for nearly 35 years, but I consider that these models are obsolete. The suprafan lobe concept was influential in both sedimentologic and sequence-stratigraphic circles because it provided a basis for constructing a general fan model and for linking mounded seismic facies with sheet-like turbidite sandstones. However, this concept recently was abandoned by its proponent, which has left the popular sequence-stratigraphic fan models with a shaky foundation. A paradigm shift is in order in the 21st century. This shift should involve the realization that thick deep-water massive sands are deposits of debris flows, not high-density turbidites. However, there are no standard vertical facies models that can be applied universally for either turbidites, contourites, or sandy debris flows. Science is a journey, whereas facies models terminate that journey and become the final destination.

Abstract: A 90- degree turn on the interpretation of the classical turbidite locality at San Clemente State Beach, California

Abstract: A 90- degree turn on the interpretation of the classical turbidite locality at San Clemente State Beach, California

The Fernie-Kootenay transition, Banff National Park, Canada: depositional characteristics of density-induced and combined flows

The Fernie—Kootenay transition (Late Jurassic-Early Cretaceous) occurs in the east of Banff exit, Canada Highway 1 and consists of deformed sequence of dark gray shale alternated with thin-bedded sandstone. The thin-bedded sandstone comprises sharply based-and-topped massive, laminated, or wavy cross-laminated units that mimic classical turbidites. The sharp base of each sandstone bed is commonly characterized by flute and tool marks of plant debris and fluted burrows that show consistent northwestward paleoflow directions. These observations suggest that deposition of the sandstone/shale couplets occurred in well-aerated shallow water environments adjacent to a landmass where occasional storm-generated density-induced undercurrents were prevalent. The multiple beds that comprise Bouma-type units are perhaps due to multiple strokes of storms that traveled across the shelf.

Basin-Floor Fans in the North Sea: Sequence Stratigraphic Models vs. Sedimentary Facies: Reply

Examination of nearly 12,000 ft (3658 m) of conventional core from Paleogene and Cretaceous deep-water sandstone reservoirs cored in 50 wells in 10 different areas or fields in the North Sea and adjacent regions reveals that these reservoirs are predominantly composed of mass-transport deposits, mainly sandy slumps and sandy debris flows. Classic turbidites are extremely rare and comprise less than 1% of all cores. Sedimentary features indicating slump and debris-flow origin include sand units with sharp upper contacts; slump folds; discordant, steeply dipping layers (up to 60°); glide planes; shear zones; brecciated clasts; clastic injections; floating mudstone clasts; planar clast fabric; inverse grading of clasts; and moderate-to-high matrix content (5-30%). Many f the cored reservoirs either have been previously interpreted as basin-floor fans or exhibit seismic (e.g., mounded forms) and wireline-log signatures (e.g., blocky motif) and stratal relationships (e.g., downlap onto sequence boundary) indicating basin-floor fans within a sequence stratigraphic framework. This model predicts that basin-floor fans are predominantly composed of sand-rich turbidites with laterally extensive, sheetlike geometries. However, calibration of sedimentary facies in our long (400-700 ft) cores with seismic and wireline-log signatures through several of these basin-floor fans (including the Gryphon-Forth, Frigg, and Faeroe areas) shows that these features are actually composed almost exclusively of mass-transport deposits consisting mainly of slumps and debris flo s. Distinguishing deposits of mass-transport processes, such as debris flows, from those of turbidity currents has important implications for predicting reservoir geometry. Debris flows, which have plastic flow rheology, can form discontinuous, disconnected sand bodies that are harder to delineate and less economical to develop than deposits of fluidal turbidity currents, which potentially produce more laterally continuous, interconnected sand bodies. Our core studies thus underscore the complexities of deep-water depositional systems and indicate that model-driven interpretation of remotely sensed data (i.e., seismic and wireline logs) to predict specific sedimentary facies and depositional features should proceed with caution. Process sedimentological interpretation of conventional cor is commonly critical for determining the true origin and distribution of reservoir sands.

Deposition of megabeds (megaturbidites) and sea-level change in a proximal part of the Eocene-Miocene flysch of central Dalmatia (Croatia)

Some carbonate megabeds of the proximal part of the Dalmatian Eocene-Miocene flysch basin (eastern Adriatic Sea) are underlain and overlain by conglomeratic fan deltas, whereas thinner-bedded turbidites interfinger with fan-delta conglomerates. Fan deltas and interfingering turbidites are interpreted as having been deposited during lowstand(s) and early sea-level rise(s), whereas deposition of megabeds is attributed to periods of accelerated sea-level rises that aborted growth of fan deltas. Attribution of megabeds to periods of sea-level rise is based on the following observations: (1) they overlie some conglomeratic fan deltas, (2) skeletal and lithic debris in megabeds originated by subaerial weathering of older sediments, and (3) their large thickness indicates considerable primary accumulation 0f debris and low frequency of Hows. Sea-level changes inferred from mega- beds are different from those related to thinner classical turbidites, and distinction of the two is important for sequence stratigraphy in deep-marine sediments.

Basin-Floor Fans in the North Sea: Sequence Stratigraphic Models vs. Sedimentary Facies

Examination of nearly 12,000 ft (3658 m) of conventional core from Paleogene and Cretaceous deep-water sandstone reservoirs cored in 50 wells in 10 different areas or fields in the North Sea and adjacent regions reveals that these reservoirs are predominantly composed of mass-transport deposits, mainly sandy slumps and sandy debris flows. Classic turbidites are extremely rare and comprise less than 1% of all cores. Sedimentary features indicating slump and debris-flow origin include sand units with sharp upper contacts; slump folds; discordant, steeply dipping layers (up to 60°); glide planes; shear zones; brecciated clasts; clastic injections; floating mudstone clasts; planar clast fabric; inverse grading of clasts; and moderate-to-high matrix content (5-30%). Many f the cored reservoirs either have been previously interpreted as basin-floor fans or exhibit seismic (e.g., mounded forms) and wireline-log signatures (e.g., blocky motif) and stratal relationships (e.g., downlap onto sequence boundary) indicating basin-floor fans within a sequence stratigraphic framework. This model predicts that basin-floor fans are predominantly composed of sand-rich turbidites with laterally extensive, sheetlike geometries. However, calibration of sedimentary facies in our long (400-700 ft) cores with seismic and wireline-log signatures through several of these basin-floor fans (including the Gryphon-Forth, Frigg, and Faeroe areas) shows that these features are actually composed almost exclusively of mass-transport deposits consisting mainly of slumps and debris flo s. Distinguishing deposits of mass-transport processes, such as debris flows, from those of turbidity currents has important implications for predicting reservoir geometry. Debris flows, which have plastic flow rheology, can form discontinuous, disconnected sand bodies that are harder to delineate and less economical to develop than deposits of fluidal turbidity currents, which potentially produce more laterally continuous, interconnected sand bodies. Our core studies thus underscore the complexities of deep-water depositional systems and indicate that model-driven interpretation of remotely sensed data (i.e., seismic and wireline logs) to predict specific sedimentary facies and depositional features should proceed with caution. Process sedimentological interpretation of conventional cor is commonly critical for determining the true origin and distribution of reservoir sands.

Lithofacies and depositional processes from a Carboniferous lake, Sierra de Narváez, Northwest Argentina

Thick and widespread Late Carboniferous lacustrine successions occur in the Sierra de Narváez area, northwest Argentina. A detailed lithofacies analysis forms the basis for the definition of fourteen lithofacies clustered into three assemblages. The delta assemblage consists of channelized cobble to pebble conglomerates, quartz-rich sandstones, and carbonaceous mudstones. This assemblage is present at the top of the lake succession and records the progradation of a short-headed stream delta. Wave- and current-rippled sandstones, and a heterolithic facies make up the shallow-lake assemblage. The nearshore area was probably composed of small bars, shoreline sand flats and low-energy sand-mud flats. The deep-lake assemblage is represented by two types of coarsening- and thickening-upward lithofacies sequences. The first type consists, from bottom to top, of laminated mudstones, thin-bedded turbidites, classic turbidites and massive sandstones. Each sequence is thought to represent turbidite lobe progradation in a basinal setting. The second type of sequence consists, from base to top, of underflow current sandstones and mudstones, thin-bedded and classic turbidites, deformed sandstones and hummocky cross-stratified sandstones. Each sequence records progradation of turbidite lobes on the delta front. The analyzed lithofacies record deposition in a hydrologically open lake. Final lake infill was related to the progradation of a deltaic system. It is suggested that four distinct processes controlled sediment transport and deposition in the Carboniferous lake: (1) delta progradation, (2) episodic sediment density surges, (3) river plume dispersion, and (4) wave action. During periods of high discharge, fine to very fine sand and silt bypassed mouth-bar zones and were deposited farther into the lake by underflow currents. Sands supplied by the delta system accumulated in the upper delta front in unstable conditions and were later redeposited by episodic turbidity currents. Wave action was able to rework shallow-water sands supplied by the delta system or emplaced by turbidity currents.

Neoproterozoic (Vendian) ichnofossils from Lower Alcudian strata in central Spain

AbstractSimple trace fossils are reported from three localities in central Spain within a monotonous succession of shale and greywacke and classical turbidites attributed to the Neoproterozoic (Riphean) Lower Alcudian megaunit. The Lower Alcudian strata are believed to have a complex tectonic history including deformation during the Pan-African Orogeny. Chronostratigraphic control is not available for this structurally complex succession and fossil evidence is sparse. Here we report on a low-diversity ichnofossil association including the ichnospecies (isp.)Gordia marinaEmmons, 1844,Gordiaisp.,G.aff.arcuataKsiążkiewicz, 1977. The stratigraphically inferred Proterozoic age of the rocks, added to the present ichnofossil association, suggests that colonization of deeper waters started before the early Cambrian. We further consider that the plausible oxygen requirements of marine invertebrate(s) which produced the present ichnofossil taxa may well have exceeded the oxygen levels in a largely dysaerobic environment. A circulation model proposed by one of us (Palacios, 1989), implying the presence of extensive upwelling and descending oxygen-rich waters, helps to explain the continuous colonization of deep-water settings in Lower and Upper Alcudian strata. We argue that the ichnofaunal record of strata older than 650 Ma is both rare and inconclusive. Moreover, the present ichnofauna, being similar to ichnofaunal associations in environmentally comparable Upper Alcudian strata, is largely documented from Neoproterozoic (Vendian) and early Palaeozoic associations elsewhere. We conclude that the new ichnofossil evidence is consistent with a Vendian or younger age for the Lower Alcudian megaunit. This view is also consistent with a recently published maximum deposition age of 565 Ma inferred from U–Pb datings of detrital zircons from the supposedly much older (Middle Riphean) Tentudia Group or ‘Serie Negra’, that appears roughly time-equivalent with Lower Alcudian strata. It thus may appear that basin formation and flysch deposition in central Iberia encompass a minor segment of Neoproterozoic time, being probably limited to the Vendian. By default, formerly postulated Pan-African deformation of Neoproterozoic Lower Alcudian strata appears implausible. We further conclude that the complex pre-Phanerozoic structural history of Alcudian strata is probably restricted to Cadomian and younger deformation. Former structural models need substantial revision.

Stratigraphie et paléogéographie d'un bassin d'avant-arc ordovicien, Estrie-Beauce, Appalaches du Québec

Two new formations, the Frontière and Etchemin formations, have been found to lie below the Beauceville and Saint-Victor formations, the two known formations of the Magog Group. The Frontière Formation, at the base of the group, is made up of centimeter-thick beds of medium- to coarse-grained litharenite and of greyish green mudstone; the sandstone, greyish green, contains abundant felsic volcanic rock fragments and chromite grains. The Etchemin Formation is composed mostly of centimeter-thick dusky yellow green siliceous mudstone; at the base, there is also a purple mudstone, and meter-thick beds of dusky green volcaniclastic rocks rich in intermediate to felsic volcanic rock fragments and crystals of feldspar and quartz occur near its top. The Beauceville Formation consists of interbedded centimeter-thick beds of black clayslate and centimeter- to meter-thick beds of black volcaniclastic rocks. The Saint-Victor Formation consists of classic turbidite beds with few meter-thick yellowish volcaniclastic rock beds similar to those of the Beauceville Formation; the sandstone is a litharenite rich in quartz grains and sedimentary rock fragments. Most rocks of the Frontière and Etchemin formations as well as the volcaniclastic rocks of the Beauceville and Saint-Victor formations were derived from a magmatic arc located to the southeast. However, the shale of the Beauceville Formation and the turbidites of the Saint-Victor Formation were derived from an orogenic source located to the northwest. The Magog Group is located between the Saint-Daniel Mélange and the Ascot Complex interpreted as remnants of an accretionary prism and a magmatic arc, respectively. The sediments of this group were thus deposited in a fore-arc basin active during the Taconian orogeny of the Middle to Late Ordovician.

Debris Flow/Slump Emplacement of a Classic Turbidite"Sequence, Pennsylvanian Jackfork Group, Ouachita Mountains (Arkansas and Oklahoma): ABSTRACT"

Debris Flow/Slump Emplacement of a Classic Turbidite"Sequence, Pennsylvanian Jackfork Group, Ouachita Mountains (Arkansas and Oklahoma): ABSTRACT"

The mid‐Palaeozoic turbiditic Mathinna Group, northeast Tasmania

Palaeocurrents, sandstone compositions and sedimentologic profiles in the turbiditic Mathinna Group of northeastern Tasmania have enabled the geometry and dynamics of the Late Silurian‐Early Devonian part of the depositional basin to be defined. The oldest rocks in the Mathinna Group are Early Ordovician or older, and comprise a thick‐ to thin‐bedded turbiditic arenite succession ∼ 1 km thick (Stony Head Sandstone) overlain by a 1–2 km thick pelite (Turquoise Bluff Slate) containing the only known Ordovician fossil in the succession — an Early Ordovician graptolite. This pelite succession passes conformably upwards by influx of gradually increasing proportions of siltstone and fine‐grained sandstone into a thin‐ to thick‐bedded interval of classical turbidites at least several kilometres thick containing Early Devonian fossils in its upper part. The lower part of this upper turbiditic succession (Bellingham Formation) contains the oldest palaeocurrent pattern measured and indicates lobes on a submarine fan system prograding to the east‐northeast. Dominant sandstone composition is quartzose sublitharenite, consistent with derivation from stable platform areas inferred to have lain to the southwest. The age of this fan system is uncertain, but could be Late Silurian. The upper part of the Mathinna Group is a several kilometre thick sand‐rich succession of sublitharenite and litharenite with variable feldspar content entering a north‐northwest‐elongate basin. Measurement of several hundred cross‐laminations and sole marks has enabled recognition of two large fan systems with mutually opposed palaeocurrent directions parallel to the basin axis. These strata contain a minor, but significant, proportion of feldspar which is not present in the older east‐northeast‐flowing turbidites. The south‐southeast‐prograding fan system has twice the feldspar content of the north‐northwest‐directed system, reflecting probable derivation of the former from the wide area of Late Silurian‐lowermost Devonian silicic igneous rocks in southeastern Australia. Graptolites and fragmentary plant remains indicate that this basin geometry is, at least in part, Early Devonian. Comparison of palaeocurrent directions from cross‐laminations in Bouma C divisions of both A/BC(D)E and C(D)E graded beds with directions from sole marks (predominantly flute and groove casts) on beds in the same outcrops, shows that Bouma C cross‐laminations can give the same vector‐mean azimuth, within statistical error, but with a higher variance, as the sole marks. When the palaeocurrent patterns are considered at the highest summary levels, simple and consistent palaeodrainage patterns emerge, including coincidence of the probable Early Devonian basin axis with the regional fold trend.

The Goldenville–Halifax transition, Mahone Bay, Nova Scotia: relative sea-level rise in the Meguma source terrane

In the Meguma Group of Nova Scotia, the transition from sand-rich turbidites of the Goldenville Formation to overlying mud-rich turbidites of the Halifax Formation shows unusually rapid vertical facies variations. The Tancook member of the Goldenville Formation is divided into four laterally continuous units. Unit 1 consists of generally thinly bedded classical turbidites interpreted as lobe facies. Unit 2 comprises mottled (bioturbated) and laminated muds. Units 3 and 4 contain packets of thick amalgamated sandstone beds, alternating with more thinly bedded muddy sections. A unique bioclastic bed occurs near the top of unit 3. In unit 4, burrowed sandstone packets traced along strike (across paleocurrent) are not significantly incised into underlying muds. Laminated slates and siltstones of the overlying Mosher's Island member (Halifax Formation) are enriched in manganese and trace metals and contain diagenetic concretions of manganoan carbonate rimmed by metamorphic garnet. The overlying Cunard member comprises dark grey to black, strongly cleaved, carbon-rich slates, interbedded with thin, pyrite-rich, graded siltstones.Changes in the transition zone can be explained by a relative sea-level rise in the Gondwanaland source area of the Meguma Group. During deposition of the Tancook member, turbidite sand supply was progressively and intermittently interrupted as sand became trapped on the shelf. In unit 1, deposition occurred in a submarine-fan system; in higher units, deposition of sandstone packets was probably controlled by transgressions and regressions in the source area. Expanded shelves, briefly colonized by a shelly fauna, led to high organic productivity and eventual anoxia. During deposition of the Mosher's Island member, manganese was concentrated in reducing waters, delivered to the sea floor close to the oxic–anoxic boundary, and trapped as carbonate during early diagenesis. Expansion of fully anoxic conditions led to deposition of black, sulphide-rich shales of the Cunard member.

Application of turbidite sedimentology to determination of thrust fault displacements in the Carolina Slate Belt

Discontinuities in the lateral variations of grain size, bed thickness and internal sedimentary structures of ‘classic turbidites’ are used to calibrate thrust fault displacements in the Carolina Slate Belt. The Ugly Creek Thrust, just east of Albemarle, North Carolina, has been determined to have a displacement of 11.7–19.6 km, while the Uwharrie Thrust in the same area has a displacement of 19.6–29.5 km. The method relies on the predictability of lateral change in a down-paleocurrent direction of sedimentological parameters. Values of the various parameters from opposite sides of the thrust fault are cross-plotted against their lateral down-current distance from a common reference point or base line. Data from one side of the fault are shifted relative to that of the other along the lateral distance axis so as to construct by eye a curve that mimics a predetermined curve form for lateral change of the parameter. The amount of shifting necessary to achieve this is equal to the apparent thrust displacement in the down-current direction. This is then trigonometrically related to the actual thrust displacement.

Sedimentation and volcanism in a Permian arc-related basin, southern New Zealand

An exceptionally well-exposed, ancient, intra-arc basin in the Permian Takitimu Group of New Zealand contains 14 km of interbedded primary volcanic and marine volcaniclastic rocks of basaltic to rhyodacitic composition. These are the products of subaerial and submarine arc volcanism and closely associated turbidite sedimentation. The Takitimu oceanic arc/basin setting formed a dynamic closed sedimentary system in which large volumes of volcaniclastic material generated at the arc was rapidly redeposited in marine basins flanking the eruptive centres. Volcanism probably included (1) moderate- to deep-water extrusion of lava and deposition of hyaloclastite, (2) extrusive and explosive eruptions from shallow marine to marginally emergent volcanoes in or on the margin of the basin, and (3) Plinian and phreato-Plinian eruptions from more distant subaerial vents along the arc. Much of the newly erupted material was rapidly transported to the adjacent marine basin by debris flows, slumping and sliding. Hemipelagic sedimentation predominated on the outer margin of the basin, infrequently interrupted by deposition of ash from the most explosive arc volcanism and the arrival of extremely dilute turbidites. Turbidite sedimentation prevailed in the remainder of the basin, producing a thick prograding volcaniclastic apron adjacent to the arc. The volcaniclastic strata closely resemble classic turbidite deposits, and show similar lateral facies variations to submarine fan deposits. Study of such sequences provides insight into poorly understood processes in modern arc-related basins.

Diagenetic Evolution of Cretaceous-Tertiary Turbidite Reservoirs, Campos Basin, Brazil

Three sandstone turbidite sequences of the Campos basin (offshore, state of Rio de Janeiro, Brazil) were petrologically studied: (1) the Albian-Cenomanian Namorado Sandstone on the Macae Formation, (2) the Upper Cretaceous Carapebus Member of the Campos Formation, and (3) the Eocene Carapebus Member of the Campos Formation. The sequences represent deep marine deposits consisting mostly of massive sandstones rather than classical turbidites, indicating sand-rich submarine fans were the main depositional system of these sequences. The framework composition of the sandstones averages for quartz, feldspar, and lithics are Q60F40Ltr for the Cretaceous rocks and Q71F29Ltr for the Eocene rocks, plotting granitic r ck fragments at the feldspar pole. The main diagenetic phases that affected the sandstones studied were (1) development of a clay matrix due to compaction of rip-up mud clasts, (2) partial replacement of the matrix by opal, (3) precipitation of small pyrite framboids, (4) widespread direct precipitation or replacement of different materials by calcite, (5) intense generation of secondary porosity, (6) localized kaolinite development, (7) minor precipitation of quartz and feldspar overgrowths, (8) development of dolomite, ferroan dolomite, and ankerite, and (9) replacement of different materials with minor direct precipitation of late pyrite. Geologic and geochemical evidence lets us infer the main processes that controlled diagenetic transformations and mass transfer within the sequence studied. The principal source of carbonate cements was pressure solution of the underlying Macae Formation. Most of the diagenetic evolution of the sandstones was apparently controlled by the relative balance between the activity of CO2 and carboxylic acid species in the formation waters, both related to organic matter transformations within adjacent marine shales. The timing of hydrocarbon (generated in deeper lacustrine shales) migration to the reservoirs was an important factor in preserving diagenetically enhanced porosity.