Submarine Ramp Research Articles

Initiation and evolution of an epicontinental shelf‐slope margin in an actively contracting deep‐water basin: The Eocene Aínsa Basin, southern Pyrenees (Spain)

AbstractThe shelf‐slope margin is a geomorphic zone with a change in gradient between subaqueous shelves and slopes, which extends towards the submarine basin‐floor. It is important because it partitions distinct sedimentary and biogenic processes between the shallow and deep‐water realms. The initiation of a shelf‐slope profile from pre‐existing conditions, and the evolution of shelf margins in space and time has been the focus of numerous studies, particularly from seismic data sets on passive margins, although markedly less‐so from active tectonic settings. This study documents the initiation and evolution of a shelf‐slope margin in the well‐studied Eocene Aínsa Basin (Spanish Pyrenees) through the segmentation of a mixed carbonate‐siliciclastic ramp via contractional tectonics and differential subsidence. The basinward propagation of a series of thrusts through the ramp allowed the maintenance of shallow‐water, predominantly carbonate sedimentation on their uplifted hanging wall anticlines. Conversely, the deepened foot wall synclines became muddy slope environments, and their axes became the main loci of siliciclastic turbidity current bypass and deposition. The deflection of turbidity currents around uplifted areas towards the synclinal lows allowed for the continuation of carbonate production at the bathymetric highs, which kept pace with subsidence. The interface between shallow‐ and deep‐water sedimentation (i.e. the shelf‐slope margin) was an erosional and composite submarine scarp surface generated by several phases of large‐scale mass wasting of the aggrading shelf carbonates, and healing by onlap of slope turbidites against the scarp. Continued thrust propagation and basin deepening led to the progressive headward degradation of the surfaces, resulting in an apparent retrogradation of the shelf‐slope margin and onlapping slope deposits. This model for the tectonically controlled conversion of a submarine ramp into a shelf‐slope profile contrasts with conventional models that consider shelf‐slope margins to be inherently progradational after initiation. This study also challenges the notion that large‐scale degradational surfaces and thick successions of submarine landslides are inherently diagnostic of canyons and their fill.

Tectonic control on the evolution of depositional systems in a fossil, marine foreland basin: Example from the SE Austral Basin, Tierra del Fuego, Argentina

Abstract Combined research on the structural and stratigraphic evolution of the southeastern Austral Basin identifies three major depositional systems strongly influenced by the tectonic stages of the Fuegian Andes between the Late Cretaceous and the Miocene. During orogenic growth, a submarine ramp system characterized by oblique progradational clinoforms with transverse sediment dispersal, formed adjacent to the deformation front in the wedge-top or proximal foredeep. In moments when the tectonic load and flexural subsidence were reduced, this system was replaced by a progradational-aggradational system with oblique to sigmoidal clinoforms. This occurred during a short span in the middle Eocene, but especially and definitively after contractional deformation ceased in the early Miocene. The third depositional system was long lived, and comprised axial turbidite channel systems located at the clinoform bases, which re-routed sediment toward the deeper foredeep of the western Malvinas Basin. Distinct facies types develop in each depositional system, and their recognition allows evaluating sediment fairways in the foreland basin system.

Advanced surface logging technology for unconventional plays: well-site applications in tight reservoirs

Advanced surface logging technology for unconventional plays: well-site applications in tight reservoirs

Neoarchaean banded iron formation within a ∼2620 Ma turbidite-dominated deep-water basin, Slave craton, NW Canada

Banded iron formations (BIFs) are iron- and silica-rich chemical metasedimentary rocks that span much of the Archaean and early Palaeoproterozoic rock record. Given that BIFs precipitated from ambient seawater, they provide an important archive of Earth’s ancient marine environments. Within the Slave craton occurs one of the largest and well-preserved Achaean turbidite basins in the world. Interstratified within the greywacke-mudstone turbidites is BIF that is commonly associated with ferruginous sediments and locally with a felsic tuff bed. Field investigations were conducted at four BIF locations from west to east across the Slave craton. These BIFs are composed of micro- and mesobands of quartz and magnetite, with various amounts of metamorphic grunerite, ferro-actinolite, ferro-hornblende as well as greenalite and stilpnomelane, the latter two are interpreted as retrograde metamorphic phases. As the BIF was deposited in a deep-water turbidite-dominated basin(s), the chemical component was occasionally diluted by clastic detritus, as illustrated by highly variable Al2O3 content, and Al2O3/TiO2 and Cr/Ni that are similar to the clastic sediments. On a layer-by-layer basis, the iron content in the BIF ranges from as low as 4wt.% and up to ∼75wt.% Fe2O3, whereas the SiO2 content ranges from 20wt.% to as high as ∼94wt.%. This, together with a hyperbolic germanium/silicon distribution, suggests that continentally-sourced silica comprised the background precipitation, which was periodically interrupted by pulses of hydrothermally-sourced iron. High-temperature hydrothermal fluids contributed, with an average shale-normalized positive Eu anomaly of 1.92, which indicates input of reduced Eu2+ to the ambient seawater. The BIF shows characteristic aqueous fluid-derived rare earths+yttrium fractionation (Pr/Yb<1) and positive La and Y anomalies. A negative Ce anomaly in parts of the BIF indicates the presence of an oxidizing agent able to fractionate Ce3+ to Ce4+. The age and trace element signature of the felsic tuff is similar to arc-type volcanic rocks, potentially linking the BIF and turbidite basin(s) to the Defeat Suite plutons (ca. 2630–2620Ma). The slow chemical rainout behind BIF deposition presumably took place on the basin plain adjacent to a steep submarine ramp where turbidite currents and slump-generated debris flows, sourced and triggered by the tectonically active arc-terrain, episodically delivered clastic sediment.

Sedimentation and volcanism in the Panamanian Cretaceous intra-oceanic arc and fore-arc: New insights from the Azuero peninsula (SW Panama)

Abstract The Azuero Peninsula, located in SW Panama, is a region characterized by a long-lived intra-oceanic subduction zone. Volcanism began in Late Cretaceous time, as the result of subduction of the Farallon plate beneath the Caribbean plate. Usually, ancient volcanic arcs related to intra-oceanic subduction zones are not preserved, because they are in areas with difficult access or covered by modern volcanic arc material. However, on the Azuero peninsula, a complete section of the volcanic arc together with arc basement rocks provides the opportunity to study the sedimentation and volcanism in the initial stages of volcanic arc development. The lithostratigraphic unit which records fore-arc evolution is the “Río Quema” Formation (RQF), a volcanic apron composed of volcanic and volcaniclastic sedimentary rocks interbedded with hemipelagic limestones, submarine dacite lava domes, and intruded by basaltic-andesitic dikes. The “Río Quema” Formation, interpreted as a fore-arc basin infilling sequence, lies discordantly on top of arc basement rocks. The exceptionally well exposed arc basement, fore-arc basin, volcanic arc rocks and arc-related intrusive rocks provide an unusual opportunity to study the relationship between volcanism, sedimentation and magmatism during the arc development, with the objective to reconstruct its evolution. The “Río Quema” Formation can be divided into three groups: 1) proximal apron, a sequence dominated by lava flows, interbedded with breccias, mass flows and channel fill, all intruded by basaltic dikes. The rocks represent the nearest materials to the volcanic source, reflecting a coarse sediment supply. This depositional environment is similar to gravel-rich fan deltas and submarine ramps; 2) medial apron, characterized by a volcanosedimentary succession dominated by andesitic lava flows, polymictic volcanic conglomerates and crystal-rich sandstones with minor pelagic sediments and turbidites. These rocks were deposited from high-density turbidity currents and debris flows, directly derived from erupted material and gravitational collapse of an unstable volcanic edifice or volcaniclastic apron; 3) distal apron, a thick succession of sandy to muddy volcaniclastic rocks, interbedded with pelagic limestones and minor andesitic lavas, intruded by dacite domes and by basaltic to andesitic dikes. Bedforms and fossils suggest a quiet, relatively deep-water environment characterized by settling of clay and silt (claystone, siltstone) and by dilute turbidity currents of reworked volcaniclastic detritus. The timing of the initial stages of the volcanic arc has been constrained through a biostratigraphic study, using planktonic foraminifera and radiolarian species. The fossil assemblage indicates that the age of the “Río Quema” Formation ranges from Late Campanian to Maastrichtian, providing a good constraint for the development of the volcanic arc and volcaniclastic apron, during the initial stages of an intra-oceanic subduction zone.

Stages of sedimentary prism development on a convergent margin — Eocene Tyee Forearc Basin, Coast Range, Oregon, USA

Architecture of ancient forearc basin successions can be difficult to reconstruct because of the widespread syn-depositional and post-depositional deformations experienced by many forearc basin-fills. For this reason various techniques have been used for reconstructing forearc basin-infill geometry, including geochemical correlation. The Tyee Basin succession exposed in Coast Range of Western Oregon, USA, is an Eocene forearc-fill that includes genetically related non-marine, shallow marine and deepwater clastic deposits and is gently deformed. Reconstruction of the depositional geometry of the Tyee Basin succession from detailed outcrop and subsurface data reveals two distinct stages of development for this active basin-margin. These stages are characterized by two different basin-margin clinoform architectures and also by a pronounced change in the character of the associated deepwater deposits. During the initial stage, the basin-margin clinoforms are smaller (<250m clinoform height) and strongly progradational, with clinoform topset dominated by fluvial deposits. At this stage thick sand-rich unconfined turbidite beds accumulated on the slope segment of the clinoforms and extended out onto the basin-floor. Large scale slope conduits such as slope channels or canyons, are notably absent in this stage. The second stage is characterized by larger clinoform height (>500m), a greater degree of topset aggradation with repeated fluvio-deltaic cycles on the shelf, and well-organized, large turbidite channels on the slope. The turbidite channels supplied medium-grained sands to the extensive, stacked basin-floor fans. The first stage described above marks the early development of a shelf-slope prism on the Tyee continental margin, and has been interpreted by some earlier workers as an unique category of basin-margin architecture, termed as a ‘submarine ramp’. However, this was only the initial stage of development of the Tyee margin and it was followed by a period of basin-filling when repeated fluvial and shallow marine shelf-transit cycles fed well-organized turbidite channels on the slope as well as Tyee Basin floor fans. The large volume of sediment deposited during the initial stage, resulted from of the unique geometry of the Tyee Basin, as influenced by the presence of pre-existing topography on the accreted oceanic basement underlying the Tyee succession.

Using sediment geochemistry and detrital zircon geochronology to categorize eroded igneous units: An example from the Mesoarchean Birch-Uchi Greenstone Belt, Superior Province

Sediments deposited in volcanically active areas preserve a record of the geochemistry of eroded igneous units. In some instances this record can be more complete than the remaining volcanic rocks. Whole rock geochemistry and detrital zircon geochronology were performed on sedimentary rocks of the Balmer assemblage in the Birch-Uchi Greenstone Belt, western Superior Province, to investigate the types and ages of igneous rocks that were present in the source area at approximately 2.9 Ga. Spatially juxtaposed Mesoarchean volcanic rocks were also analyzed for comparative purposes. The detrital zircon geochronology revealed that the sandstones studied are significantly younger than the adjacent volcanic rocks, inferring that their stratigraphic position is the result of tectonic interleaving and they do not belong in the Balmer assemblage. As well, the majority of the detrital zircons were older than the adjacent volcanic rocks severely limiting the potential role of these volcanic units as a source terrain. Extreme geochemical heterogeneity within the sandstone sample set implies the depositional environment was supplied by multiple sediment fed systems, which probably delivered detritus to a submarine ramp environment. A subset of the sandstone samples are HFSE enriched and had, in part, an alkalic igneous source. This infers that the ocean plateau tholeiitic basalts, which are abundant in northwest Superior Province, may have been associated with alkalic eruptive units, similar to modern intra-plate igneous activity. Either erosive or tectonic processes have removed these alkalic igneous units from the preserved rock record, but their detritus is present and identifiable in the sedimentary succession.

Latest Cretaceous forearc basin development along an accretionary convergent margin: South-central Alaska

Upper Cretaceous sedimentary strata exposed in south-central Alaska provide insight on tectonic processes that shaped the northern Pacific margin following accretion of the Wrangellia composite terrane, the largest addition of crust to North America over the past 100 m.y. Sedimentologic, geochronologic, biostratigraphic, and petrographic data from the Matanuska Formation permit reconstruction of the tectono-sedimentary history of strata in a forearc basin constructed upon accreted oceanic-arc crust. The Matanuska Formation consists of >3 km of sedimentary strata exposed in the northern Chugach Mountains, Matanuska Valley, and southern Talkeetna Mountains of interior south-central Alaska. Measured stratigraphic sections and lithofacies analyses demonstrate that mass slumps and slides, debris flows, and turbidity currents deposited Campanian–Maastrichtian sandstone, conglomerate, and mudstone on a gullied, trenchward-dipping submarine ramp. Benthic foraminifera, inoceramid bivalves, and Nereites ichnogenera indicate deposition mainly at bathyal water depths. Sandstone and conglomerate petrofacies are characterized by monocrystalline quartz, plagioclase feldspar, and volcanic lithic fragments (Q39F40L21, Qm29F40Lt32, Lm25Lv42Ls32, and Qm42P54K4). Jurassic–Cretaceous arc plutons exposed north of the basin were an important sediment source, based on U-Pb zircon ages of granitoid clasts from conglomerate and detrital zircons from sandstone. Coeval arc plutons were unroofed relatively quickly, judging by the presence of 77–71 Ma detrital zircons in sandstone and 79–77 Ma granitic clasts in conglomerate, together with Maastrichtian (71–65 Ma) ammonite and foraminifera fossils. Sparse Paleozoic–Triassic detrital zircons indicate minor sediment contribution from inboard sources, including the Yukon-Tanana composite terrane and recycled Jurassic–Cretaceous sedimentary strata (Kahiltna assemblage). New data from the upper Matanuska Formation, together with recent studies from age-equivalent strata exposed in the Alaska Range and Wrangell Mountains, provide an exceptional example of basin development along a subduction margin shortly following accretion of an oceanic arc. Forearc basin development was dominated by subsidence and sediment gravity flow deposits enriched in plutonic and volcanic clasts eroded from both remnant- and coeval-arc plutons. Within the arc, newly recognized conglomerate in the northern Talkeetna Mountains records erosion of coeval- and remnant-arc source terranes to the south and Precambrian–Paleozoic sources to the north. Farther inboard, syndepositional shortening prompted thrust-top basin development and accumulation of alluvial-lacustrine strata derived from both the former continental margin to the north and accreted oceanic rocks to the south. Regional subsidence and basin development terminated during late Maastrichtian–early Paleocene time, coincident with subduction of progressively younger oceanic lithosphere inboard of an oceanic spreading center.

Turbidite Systems in the Inner Forearc Domain of the Hikurangi Convergent Margin (New Zealand): New Constraints on the Development of Trench-Slope Basins

The development of trench-slope basins on subduction wedges is strongly related to subduction processes. Theseconfined basins form on the lower trench slope, and their edges consist of structurally controlled linear bathymetric highs. In this particular setting, sedimentary processes may be controlled both by tectonism and by sea-level changes. Thus, the study of the sedimentary record may permit evaluation of the spatial and temporal contribution of tectonic activity on the development and stratigraphic evolution of trench-slope basins. The recent evolution of the Hikurangi subduction wedge (North Island of New Zealand) is characterized by the uplift and subaerial emergence of the trench­slope break, which has provided good exposure of the Miocene Akitio trench-slope basin. Identification of the main sedimentary discontinuities, reconstruction of the geometry of the sedimentary units, and paleocurrent studies were undertaken in the Akitio basin in order to propose a stratigraphic and structural scheme for the evolution of this emerged trench­slope break. Deep-sea facies associations are representative of three main fine-grained gravity-driven systems that may develop during various stages of development of mature trench-slope basins. These are, from base to top, large submarine slides (olistostrome deposits), fine-grained sand-rich submarine fans, and low-gradient submarine ramps (sheet-like turbidites). This basin-fill megasequence constitutes a lowstand systems tract, which can be subdivided into basin-floor fan, slope fan, and progradingwedge complex. Slope facies associations are common and highlight the important contribution of slope processes to the basin sediment budget. Rapid shifts of facies and important paleoenvironmental changes, reflecting slope creation or modification, characterize five major discontinuities that are related to regional tectonic events (i.e., onset of subduction at 25 Ma, beginning of regional subsidence at 15 Ma) or local tectonic events (i.e., local uplift of structural edges of the Akitio trench-slope basin at 17.5 Ma and 16.5 Ma, acceleration of subsidence at 13.2 Ma). Moreover, we show that the timing of development of some lowstand system tracts (i.e., of slope fans and of prograding-wedge complexes) may also be controlled largely by changes in style and/or amplitude of tectonic activity.

Neoarchean trans-subprovince sediment transport in southwestern Superior Province: sedimentological, geochemical, and geochronological evidence

At ~2700 Ma volcanism spawned by northward-directed subduction under Wabigoon subprovince, Canadian Shield, fed sediment into an adjacent fore-arc basin – trench system. Volcanically active island arcs to the south (Wawa subprovince) were transported northward during this interval and at ~2690 Ma collided with the accretionary complex (Quetico subprovince). Metamorphosed sedimentary units intercalated with the ocean floor and island-arc volcanic rocks of northern Wawa subprovince provided the opportunity to amass data useful in extending the paleogeographic interpretation of the area. This study examines the McKellar Harbour Formation, a several kilometre thick assemblage of graded metasandstone beds outcropping to the north of Lake Superior. Its sedimentology, geochemistry, and detrital U–Pb zircon geochronology were investigated in conjunction with the same attributes of possible correlative units in the region. Similarities between the McKellar Harbour Formation and sedimentary rocks composing the fore-arc basin and Quetico trench to the north indicate that by 2696 Ma the trench had been overwhelmed by sediment, to the point where turbidity currents were overflowing onto the abyssal ocean plain, building a prograding submarine ramp–fan that was to become the McKellar Harbour Formation. The turbiditic ramp–fan assemblage comprising most of this extensive depositional system crosses much of southern Superior Province, highlighting the interrelated genesis of rocks formed during amalgamation of the individual subprovinces.

Geochemistry and petrology of a Middle Tithonian limestone-marl rhythmite in the Neuquén Basin, Argentina: depositional and burial history

Abstract The Middle Tithonian Los Catutos Member (Vaca Muerta Formation, Neuquén Basin), is lithologically and geochemically similar to limestone-marl alternations from the Late Jurassic of the northern hemisphere. Both marls and limestones are pelbiomicrites and biopelmicrites principally composed of pellets, radiolaria, forams, ostracods, equinoids, spicules of sponges and gastropods, cemented by several generations of calcite cement. Smectite and interlayers are the major epiclastic components of the fraction below 2 μm, reflecting pedogenic processes developed on volcanogenic source rocks. More abundant kaolinite in some marls reflects stronger humid conditions in the source area and enhanced terrigenous supply. Al 2 O 3 content is demonstrated to be a reliable indicator of clastic input. The same is not true for silica, often related to high biogenic productivity of siliceous organisms. Rocks show total organic carbon (TOC) contents up to 1.95% and constitute regular to good sources for hydrocarbons, although thermally immature. Rhythmites formed gently sloping mounds accumulated in a distal submarine ramp under low-energy and poorly oxygenated open-sea conditions. Sedimentation rates were high due to high productivity on the sea surface, and supply of terrigenous and carbonate sediments transported by suspension plumes originated in shallow, photic areas. δ 13 C values correspond well with the published curves for the Tithonian sea water and with other records from Tethyan limestones. A preliminary analysis of negative excursions of δ 13 C point to a productivity crisis or a mixture of water layers in a stratified sea with a periodicity of 400 ka, which could be a result of changes in the orbital eccentricity of the Earth. Light isotopic composition of O in bulk rocks is the result of diagenetic neomorphism and cement precipitation. Calculated palaeotemperatures from δ 18 C are consistent with those derived from measured vitrinite reflectance ( Ro% ) and burial history reconstruction. Data indicate initial burial during the Tithonian extending up to the Lower Cretaceous, a short period of uplift (Intravalaginian tectonic phase), and renewed uplift during the Cenomanian followed by significant Late Cretaceous sedimentation and Pliocene thrusting.

Sandy submarine canyon-mouth lobes on the western margin of Corsica and Sardinia, Mediterranean Sea

Long-range, low-resolution and deep-towed, high-resolution side-scan sonar records, high-resolution seismic profiles and core samples were used to study the relatively small canyon fed turbidite systems west of Corsica and Sardinia. The margin west of Corsica is dissected by deep (up to 1500 m), straight canyons that have steep axial gradients (10° slopes are common) and that extend from land to sea without a break in gradient. The submarine canyon axes are readily mapped by their stronger acoustic backscatter. The axes have scour holes and trains of gravel or pebble waves. Canyoned slopes have widespread, shallow sediment failures. Five separate depositional lobes are recognised, extending beyond the canyon mouths. Deep-towed, high-resolution seismic profiles across part of one lobe show stacked sedimentary sheets, a few tens of kilometres wide. Cores from these sheets contain coarse to medium sand beds that are up to 3 m thick, with some mud clasts in the middle of the beds and up to 3% clay in the sand matrix. A drape of nannofossil ooze on top of cores indicates that the main activity through the canyons is at times of low sea level. The lobes tend to appear as weak backscatter, with fringes of a braid-like pattern of stronger backscatter. The reasons for this acoustic pattern are not fully understood, though in general the sand bodies are found where backscatter is relatively weak. The size of the canyon-mouth lobes is proportional to the size of the subaerial drainage basins. The limited sediment supply accounts for the absence of a well-developed submarine ramp despite the sand-dominated input from multiple sources.

Revised stratigraphy of the Trenton Group in its type area, central New York State: sedimentology and tectonics of a Middle Ordovician shelf-to-basin succession

This paper presents new stratigraphic correlations of the middle and upper parts of the Trenton Group in the type area, near Trenton Falls, New York, based on detailed bed by bed matching, of all outcrop sections. This work, in conjunction with newly revised biostratigraphy and geochemical fingerprinting of K-bentonites, has been used to establish a high resolution chronology for these deposits. Our revised correlations reveal that published stratigraphic–geochronologic schemes are largely in error, resolve several long-standing dilemmas, and have important implications for interpreting sedimentological and tectonic history of the Taconic foreland basin. Key new conclusions/revisions include: (1) The lowermost part of the Trenton type section at Trenton Falls is laterally equivalent to the Rathbun Member of the Sugar River Limestone (lower Shermanian) in the Newport–Herkimer, New York area. (2) The medial Trenton (Denley Formation), dated primarily within the Corynoides americanus graptolite Zone, can be divided in ascending order into two distinctive units, the Poland, Russia members, each of which is further subdivisible into component shallowing-upward cycles and condensed beds. As such, the Poland is completely exposed at Trenton Gorge (contrary to assertions by previous authors) and is about 10.5 m-thick. At its type section, also Trenton Gorge, the overlying Russia Member, comprising four shallowing-upward cycles, extends upward from the Kuyahoora K-bentonites for about 24 m to its sharp upper contact with another distinctive and fingerprinted K-bentonite, the High Falls ash bed. (3) Both the Poland and Russia members thin southeastward from Trenton Falls and become condensed in downslope sections near Middleville. However, the Poland section then thickens and passes eastward into basinal dark gray shales (lower-medial part of the Flat Creek Formation) in central Mohawk Valley sections, whereas the Russia remains thin and relatively carbonate-rich throughout this area. (4) A third unit, the Rust Limestone is elevated to formation status and subdivided into members. The lower part of the Rust Formation (Mill Dam Member) thins dramatically to the southeast from about 12 m at Trenton Falls to 1.5–2 m in the Middleville–Herkimer area before thickening again into basinal black shale facies. (5) The upper Rust and Steuben formations (coarse skeletal pack-to grainstone facies) of the Trenton Falls area apparently thin by condensation into the Newport area before expanding again into turbiditic slope facies of the Dolgeville Formation (essentially corresponding to the Orthograptus ruedemanni graptolite zone) beginning in the Middleville–Herkimer area. The new correlations imply that the lower-middle Rust interval belongs to the Corynoides americanus graptolite Zone, and that the upper Rust–Steuben interval probably belongs in the O. ruedemanni Zone, rather than the Climacograptus spiniferus or even to the lower Geniculograptus pygmaeus Zone, as previously inferred. (6) The Dolgeville carbonate turbidite facies is found to extend eastward to the vicinity of the Hoffmans Fault, east of Amsterdam. (7) Slumped breccia-filled channels in shelf-margin facies of the upper Rust and Steuben limestones may have served as feeder conduits to submarine fans now represented by the Dolgeville Formation. These observations indicate that a sediment-starved east-facing submarine ramp was developed across the study area during Shermanian time. Regional lithospheric flexure coupled with westward retreat of the shelf, explains the distribution of condensed facies and discontinuities. The widespread distribution of many marker beds plus the observation of spectral facies gradations at many levels, suggests that submarine faulting was usually a minor process superimposed on larger-scale diastrophic and eustatic patterns.

The deep-sea Armorican depositional system (Bay of Biscay), a multiple source, ramp model

The deep-sea Armorican depositional system, located in the central part of the Bay of Biscay, is a medium-sized turbidite system with a surface area of more than 30,000 km2. The whole system is a mud/sand-rich submarine ramp on a passive margin. The medial ramp is characterised by the presence of six distinct tributary channels which form three systems: the Guilcher, Crozon and Cornouaille systems. The distal ramp corresponds to divergent braided secondary channels and associated lobes. Variations in hydrodynamic conditions on the outer Armorican Shelf during the last climatic cycle are the major factor controlling facies shifts and system growth. Thus, the Armorican depositional system is a delta-fed submarine ramp during low sea-level glacial conditions, and an outer shelf-fed submarine ramp at times of sea-level rise.

Delta-Fed Turbidites Infilling Topographically Complex Basins: A New Depositional Model for the Annot Sandstones, SE France

The Eocene-Oligocene Annot Sandstones of southeast France record the predominantly deep-water, siliciclastic infill of the early French Alpine foreland basin. They accumulated in a topographically complex basin as recorded by variable onlap relationships with the underlying Globigerina Marls. Onlap configurations and paleo-bathymetric data from the western outcrops of the Annot Sandstones enables a contour map of the pre-Annot Sandstone basin-floor topography to be reconstructed. The basin comprised a northern and a southern sub-basin, which were separated by a ridge, through which a trough was cut (the Coyer Trough) linking the two sub-basins during the latter stages of infilling. Data on provenance and paleocurrents suggest that the sandstones were sourced from two main feeder systems: an eastern source from the Alps fed the northern sub-basin, and a southern source fed the southern sub-basin. There is evidence of mixing of provenance in the lower parts of the southern sub-basin. Integration of the reconstructed basin-floor topography, sediment-dispersal data, and facies and stratal architectures suggests a depositional model comprising sand-rich, delta-fed submarine ramps and/or aprons whose development was strongly influenced by the basin-floor topography. The southern sub-basin initially had a steeper delta front, characterized by debris flows; this contrasted with the lower-gradient northern sub-basin. The southern sub-basin comprises approximately 800 m of thick-bedded sandstones interpreted as the deposits of sustained and possibly ponded turbidity currents. The upper part of the southern sub-basin records increased channeling and scour related to bypass of material after the basin was filled; these channels transported material northwards through the Coyer Trough, and capped the northern sub-basin. This development of the southern sub-basin is comparable to the fill and spill development of perched, intra-slope basins in the Gulf of Mexico.

Sequence Stratigraphy, Facies, and Sea Level Change of the Hauterivian Productive Complex, Priobskoe Oil Field (West Siberia)

The productive complex of the Priobskoe field formed during the Hauterivian in the central part of the West Siberian basin as part of the Neocomian (Early Cretaceous) progradational complex. Its depositional setting was a shelf-break margin with well-developed clinoform units and a 1-2° slope gradient. Two depositional sequences have been identified and consist of distinct systems tracts. The lowstand systems tract of the lower sequence (AC12.2-4) formed as a result of rapid high-amplitude relative sea level fall, and is clearly differentiated into lowstand fan and lowstand prograding wedge deposits. Sea level fall of lower amplitude (and rate) led to deposition of a lowstand systems tract in the upper sequence (AC10.1-2) in which no lowstand fan unit was deposited. A thin transgressive systems tract of the lower sequence (AC11.0) formed during a short-term sea level rise. In contrast, prolonged high-amplitude sea level rise led to deposition of a thick transgressive deposit in the upper sequence (AC10.0-AC8). Deposition of the sandiest units is related to shelf (shoal-water) deltas and shelf-edge deltas on the shelf, and to submarine fans in basinal portions. Deposition of shelf-edge deltas was accompanied by abundant gravity-driven processes and formation of shingled turbidites, during which the basin margin significantly prograded and infilled accommodation space. In many cases, sediment supply into the deep-marine systems of Priobskoe was provided from multiple feeder entry points that led to coalescence of individual submarine fans into thick submarine ramps. Because it is based on copious drilling and seismic data, the sequence stratigraphic framework developed for the Priobskoe field may serve as a useful analog model for exploration and prospect generation in Hauterivian clinoforms, as well as in the entire West Siberian Neocomian. There is good correspondence between the relative sea level change interpreted for Priobskoe to the quantified eustatic curve (QEC) generated from Russian platform stratigraphy. This implies that eustasy played an important role in controlling sedimentation in the Priobskoe field. In addition, it supports the validity for the Hauterivian interval of the QEC.

Submarine-fan systems i: characterization and stratigraphic prediction

Submarine-fan and related deep-marine clastic systems such as submarine ramps and slope aprons are important hydrocarbon reservoirs in many parts of the world. The origin and character of these systems reflect a complex interplay between a range of autocyclic and allocyclic controls including sea-level fluctuations, basinal tectonics and the rate, type and nature of sediment supply. These controls are commonly interdependent. This paper reviews the characteristics which distinguish different submarine-fans and their subsurface prediction, then proposes a classification framework for fans which discriminates between different systems on the basis of their fundamental controls. The proposed classification is discussed in terms of its value as a predictive tool in hydrocarbon exploration, development and production. A modus operandi is developed which uses the classification for reservoir description and prediction and involves three stages of investigation: basin screening, fan delineation and fan characterization. Each stage demands the integration of a range of tools and techniques including sequence stratigraphy, seismic facies analysis, seismic redisplay and geophysical modeling. These stages of analysis are linked, in turn, with core and wireline log data to constrain the range of possible fan types within a basin-fill and assess the implications for reservoir seal and trap development. This structured approach to stratigraphic analysis is also valuable in constraining the range of reservoir architectural options in appraisal, development and production.

A Delta Submarine Ramp Alternative to the Canyon-Fed Depositional Model of the Stevens Submarine Fan System, Southeastern San Joaquin Basin, Kern County, California: ABSTRACT

Deep-marine sands of the Upper Miocene Stevens Sandstone, one of the most important hydrocarbon-producing units in the United States, were deposited by sediment-gravity flows in the Bakersfield Arch area of the southern San Joaquin basin. The Stevens Sandstone has historically been considered to be a thick turbidite succession shed off the southern Sierra Nevada as four fans in a long-lived submarine fan system fed by several large submarine canyons. Access to previously unavailable proprietary 2-D and 3-D seismic data sets, carefully calibrated by well-log and core data, permits a more complete understanding of the depositional architecture of this highly petroliferous, deep-marine depositional system. This study concludes that these units were deposited in a delta-fed, line- sourced deep-sea system, whose distribution was structurally-controlled. Seismic lines examined in this study show evidence for a large fault-controlled slump feature in the area that has been referred to as [open quotes]Rosedale Canyon,[close quotes] and no evidence supports the existence of submarine canyons feeding the system. The highly progradational Stevens interval consists of thick siliciclastic units separated by thin, intervening biosiliceous shales. Seismically, the upper bounding surfaces of these biosiliceous shales represent major downlap surfaces. As sands were deposited by high-density turbidity currents, the areamore » of the present Bakersfield Arch developed into a deep-sea braid plain. Smaller-scale linear features detected on horizon slices through the 3-D seismic data cube have been interpreted in this study as braided channelform features deposited on the deep-sea braid plain. Hydrocarbon production along these linear trends may be associated with porosity and permeability variations resulting from channelized versus non-channelized sedimentation.« less

Depositional framework and controls on mixed carbonate‐siliciclastic gravity flows: Pennsylvanian‐Permian shelf to basin transect, south‐western Great Basin, USA

ABSTRACTMixed carbonate‐siliciclastic sediment gravity flow deposits of Late Pennsylvanian to Early Permian age are exposed in the Death Valley ‐ Owens Valley region of east‐central California. The Mexican Spring unit constitutes the upper part of the Keeler Canyon Formation and is characterized by turbidites, debris flow deposits and megabreccias, all of mixed carbonate‐siliciclastic composition. The mixed composition of the Keeler Canyon Formation provides an opportunity to link facies architecture to controls on depositional system development. Depositional relationships indicate that the deposits represent a non‐channellized base of slope carbonate apron system with inner, outer and basinal facies associations. These gravity flow deposits are characterized by repeated stacked, small scale (&lt;15 m) coarsening and thickening upward cycles with superimposed medium scale (&gt;100 m) coarsening and thickening upward cycles.Contemporaneous outer shelf and upper slope deposits of the Tippipah Limestone are exposed at Syncline Ridge on the Nevada Test Site. The deposits consist of carbonate buildups directly overlain by cross bedded, quartz‐rich sandstone and conglomerate which filled channels that traversed across the previously existing carbonate shelf. Detritus was transported to the west, down the upper slope by gully systems that fed the temporally persistent base of slope apron of the upper part of the Keeler Canyon Formation. This style of deposition differs from point‐sourced siliciclastic submarine fan depositional systems. However, the Keeler Canyon system has lithofacies similar to some sandy siliciclastic turbidite systems, such as the delta‐fed submarine ramp facies model, which is a line‐sourced, shelf‐fed system that is not supply limited.The mixed clastic apron systems of the Keeler Canyon Formation differ from classical carbonate aprons in that the former is characterized by an abundance of sedimentary cycles. Controls on the development of these cycles and of the facies distribution may have resulted from changes in type and rate of sediment supply, relative sea level changes and/or tectonic events. Interpretation of the data is focused on relative changes in sea level as the most significant control on development of the depositional system. Relative sea level changes serve two important functions: (1) they provide a mechanism for bringing coarse siliciclastic and bioclastic grains together on the outer shelf, and (2) shelf margin collapse may be initiated during relative lowstands allowing for transport of the sediment to the deep basin and development of deep basinal cycles. Therefore, an abundance of mixed clastic gravity flow deposits such as these in the rock record may be an indicator of periods of high frequency changes in relative sea level, which is a characteristic of Late Palaeozoic sea level history.

Turbidite Systems in Deep-Water Basin Margins Classified by Grain Size and Feeder System

Depositional systems in deep-water basin margins can be classified on the basis of grain size and feeder system into 12 classes: mud-rich, mud/sand-rich, sand-rich, and gravel-rich point-source submarine fans; mud-rich, mud/sand-rich, sand-rich, and gravel-rich multiple-source submarine ramps; and mud-rich, mud/sand-rich, sand-rich, and gravel-rich linear-source slope aprons. The size and stability of channels and the organization of the depositional sequences decreases toward a linear source as does the length:width ratio of the system. As grain size increases, so does slope gradient, impersistence of channel systems, and tendency for channels to migrate. As grain size diminishes, there is an increase in the size of the source area, the size of the depositional sys em, the downcurrent length, the persistence and size of flows, fan channels, channel-levee systems, and in the tendency to meander and for major slumps and sheet sands to reach the lower fan and basin plain. The exact positioning of any one depositional system within the scheme cannot always be precise, and the position may be altered by changes in tectonics, climate, supply, and sea level. However, the models derived from each system are sufficiently different to significantly affect the nature of petroleum prospectivity and reservoir pattern. Understanding and recognizing this variability is crucial to all elements of the exploration-production chain. In exploration, initial evaluations of prospectivity and commerciality rely on the accurate stratigraphic prediction of reservoir facies architecture, and trapping styles. For field appraisal and reservoir development, a similar appreciation of variability aids reservoir description by capturing the distribution and architecture of reservoir and nonreservoir facies and their impact on reservoir delineation, reservoir behavior, and production performance.