Submarine Fan Deposits Research Articles

Distal sedimentation in a peripheral foreland basin: Ordovician black shales and associated flysch of the western Taconic foreland, New York State and Ontario

Black shale is characteristic of early siliciclastic fill along the distal (western) flank of the Middle to Late Ordovician Taconic peripheral foreland basin. This facies, referred to as the Utica black-shale magnafacies, includes at least five intervals bounded by unconformities and/or condensed beds. Each unit records a pulse of subsidence and subsequent siliciclastic sedimentation in the foreland basin. Progressive episodic shifts from carbonate to organic-rich mud deposition may be due to steepening and subsidence of the western carbonate ramp followed by onlap of axially dispersed sediment. In general, silt-poor, organic-rich mud was deposited on a subsided carbonate ramp that sloped eastward toward the nearest siliciclastic sediment source. This mud may have accumulated from the pelitic tails of axial fan turbidites derived from source areas to the east, and possibly the south. The resultant black shale does not necessarily represent the deepest part of the basin; the deepest part of the basin (the basin axis) contained coarser-grained, axial, submarine fan deposits. Throughout the later part of the Middle Ordovician and the first half of the Late Ordovician, the basin axis migrated westward over 100 km, yet the orientation of the axis and the basin margins were persistent. This basin orientation (generally parallel to the orogenic belt), along with the large-scale basin shape, resulted from a combination of deformational loading of the continental margin and progressive flexure in the foreland. Smaller-scale structural elements, normal fault-bounded basement blocks, were superposed on large-scale Taconic foreland-basin geometry. Abrupt shifts from carbonate ramp sedimentation to deeper-water, organic-rich mud deposition may reflect movement along these faults during the orogeny.

Structure of the southern Falkland Islands continental shelf: initial results from new seismic data

Acquisition of a modern marine dataset has revealed the internal structure of sedimentary basins to the south of the Falkland Islands for the first time. The new data reveal a variety of structural styles and indicate the presence of a sedimentary section ranging from 2 to 10 km in thickness developed in three linked sub-basins. (1) The eastern part of the Malvinas Basin lies south-west of the islands. A prominent regional unconformity developed throughout the area marks the top of gently folded rocks of probable Palaeozoic age and their cover of Late Jurassic volcanics. To the north-west of the island, the underlying strata approach the surface in a structural high which forms a prominent gravity low and may record the continuation of the Palaeozoic basin exposed on the islands themselves. Strata of probable Mesozoic and Tertiary age rest on the unconformity throughout the area and show a regional southward dip. These deposits increase in thickness south-westwards towards the depocentre of the Malvinas Basin midway between the islands and Argentina. (2) The South Falkland Basin and Burdwood Bank lie south of the islands. Thicknesses increase towards an area of southward-dipping extensional fault blocks within the South Falkland Basin. Downdip these are buried beneath a prograding and north-vergent stack of blind and emergent thrusts at the northern margin of the Burdwood Bank, which is a structural highly lying a shallow water south of the islands. Thrust loading has resulted in the development of a foreland basin north of the thrust front. Complex antiformal structures are associated with the thrusting, whereas a shallow sedimentary basin developed on the crest of the Burdwood Bank is interpreted as a piggy-bank basin riding on, and sourced dominantly from within the thrust pile. (3) The Falkland Plateau Basin lies to the east and south-east of the islands and is characterized by a varied and extensive Mesozoic to Tertiary succession of submarine fan deposits thickening eastwards into deeper water. Igneous intrusives deep in the section may correlate with Jurassic volcanic rocks present within the Malvinas Basin to the west, or may record younger rifting associated with the opening of the South Atlantic. These initial results indicate considerable structural heterogeneity in this part of the Falkland Islands Continental Shelf. In the light of the hydrocarbon discoveries already made further west, significant economic potential is likely. Stratigraphic traps may occur at regional unconformities and within submarine fan deposits, whereas potential structural traps include Mesozoic extensional fault blocks and thrust and sub-thrust structures generated by Tertiary and Quaternary compression.

Architectural Characteristics of Fine-Grained Submarine Fans: A Model Applicable to the Gulf of Mexico: ABSTRACT

ABSTRACT Submarine fan deposits in the Gulf of Mexico and in many areas of the Atlantic Ocean fall into the category of fine-grained, low overall sand/shale ratio basin-floor fans. Models published over the years that have been applied to either exploration or development are primarily based on sand-rich fans, most of which were deposited in active margin settings and are not applicable to the areas mentioned above. Observations from the excellent outcrops of the Permian Tanqua Karoo in the southwestern part of the Karoo Basin, South Africa, the Jackfork Formation (Pennsylvanian) in Arkansas, and the Deep Sea Drilling Project Leg 96 on the Mississippi Fan make it possible to construct a macro-scale model that addresses the architecture of both the macro- and meso-scale depositional features of fine-grained submarine fans deposited in generally unconfined settings. At the entrance to the basin floor in the base-of-slope area, the large conduit that was initially carved out across the basin slope, may start to widen. Most of the high- and low-density flows moving from the submarine canyon in the upslope area through the conduit (fan valley or upper fan channel), will be too small to occupy the entire width of the lower part of the conduit. The result is a complex build-up of channel fills incised into each other, forming amalgamated sandstone bodies interbedded with remnants of shale-rich levee-overbank deposits. Once in the basin proper the degree of erosion of flows will be reduced and channel-levee complexes result. The channel fills are primarily amalgamated sandstones whereas the levee-overbank deposits are often thin-bedded, low-contrast, low-resistivity sandstones with high porosity and permeability. The interbedded shales reduce vertical conductivity of fluids but proper production techniques make these deposits potentially very productive. The channels gradually become smaller and loose their influence to govern the direction of the head of density flows. A gradual widening of the deposits results, known as sheet sands or depositional lobes. They have high sand/shale ratios and the overall sheet sand deposition suggests lateral switching resulting from seafloor compensation rather than frequent channel progradation.

Heterogeneous Distribution of Calcite Cement at the Outcrop Scale in Tertiary Sandstones, Northern Apennines, Italy

Calcite cement derived intraformationally in seven stratigraphic units of marine origin (five submarine-fan deposits and two shelf deposits) is distributed heterogeneously at the outcrop scale. Sandstone beds intercalated with calcareous shale older than Pliocene tend to be completely cemented, whereas stacked sandstone beds that lack shale interbeds have calcite cement in the form of tightly cemented concretions that make up only 10-30% of a bed. The abundance and distribution of concretions, with few exceptions, are irregular and unpredictable. Concretion shapes include spheres (<1 m diameter), oblate and prolate spheroids (<1.5 m), tabular forms (to 8 m long), and irregular forms. Patterns of concretions within beds are remarkably varied and include both random and uniform spacing; preference for either the top, middle, or bottom of beds; preference for faults that cut bedding at a high angle; and localization around shale rip-up clasts. There is no preference of concretions for shell-rich layers. Some formations have cement patterns specific to that formation, whereas other formations have different patterns at different outcrops. Most formations have more than one cement pattern in an outcrop. The lack of strong textural (grain size, graded bedding) or compositional controls on the localization of calcite cement suggests the preeminence of highly localized hydrologic factors in determining the spatial distribution of authigenic pore-filling calcite. Spherical concretions grew by diffusive supply of intraformationally derived components, whereas prolate and elongate concretions grew chiefly under the influence of advective supply. Faults apparently served as fluid conduits and were selectively cemented. In general, only sandstones intercalated with shale are totally cemented. This indicates that shales were a major source of cement components for these sandstones at least.

Exploration Strategies and Possible Submarine Fan Complexes in the Rough Creek Graben, Western Kentucky: ABSTRACT

The Rough Creek Graben is a deep, east-west-oriented rift basin more than 160 km long and 40 to 70 km wide in western Kentucky. It is a north-dipping half graben bounded on the north by the Rough Creek Fault Zone and on the east by a presumed tectonic inversion structure associated with reactivation of the East Continent Rift Basin. The half graben is filled with up to 5.5 km of dominantly marine sediments of the Cambrian pre-Knox Group that thin away from the Rough Creek Fault Zone. Most of the pre-Knox oil and gas exploration in the graben has taken place near the Rough Creek Fault Zone. The boundary fault has a polyphase movement history and is characterized by extensive fracturing in and near the fault zone. Both the mechanical drilling problems and the lack of exploration success that have marked past exploration attempts are likely related to the high degree of fracturing associated with the northern edge of the graben. The pre-Knox rocks south of the boundary-fault zone are virtually untested and include viable exploration targets. Recent investigations using limited seismic reflection data provide strong evidence for the presence of basin-floor submarine fan deposition at least in the western,more » basal part of the graben. Several fan complexes up to 2 km in aggregate thickness have been interpreted in the northern part of the basin. To the south, thinner, less continuous fan deposits are inferred. If confirmed by drilling, these fan complexes could represent important future hydrocarbon reservoirs in many areas of the graben.« less

シーケンス層序学 その可能性を求めて 海水準変動に起因した海底扇状地の発達様式 新潟県北蒲原郡，中蒲原郡，西蒲原郡および阿賀沖海域の椎谷期を例として （その１）

The Shiiya Age successions (6.0Ma to 3.5Ma), northren part of the Niigata oil fields are mainly consisted of radial-type submarine fan deposits. Using many seismic profiles and wells, we could detect five high sea-level events during the Shiiya Age, based on analysis of foraminiferal assemblages, and make clear that the deposition of the submarine fans are controlled by eustatic sea-level changes, under a stable tectonic environment of the back-arc basin of Japan Arc through the time.The eustatic controls during the Shiiya Age are as follows:1) the sand lobes of submarine fan mainly develop in a basin center at low sea-levels.2) each sand lobe retreated gradually from a basin center to the margin with sea-level rise.3) the successions of the submarine fan change upward from proximal to distal facies.Those three things are fit well to the basic concepts of Mutti's (1985) model.

Submarine fan and channel levee deposits in the Lower Cretaceous Bogota trough, Colombian Andes

Bogota trough in the Cordillera Oriental of Colombia was an elongate NE/SW oriented Lower Cretaceous basin. It was one of the main depocentres in the northern Andes during earliest Cretaceous time. Six lithofacies and three facies associations are recognised, based on four detailed stratigraphic sections situated in the western portion of the trough. Sedimentation in the basin comprised channel levee, fan lobe and outer fan-basin plain (?) depositional environments. Channel-levee complexes without a well-developed fan-growth pattern were more proximal to the sediment supply. In more distal settings, the migrating channels evolved into suprafan lobes. Their sediments were covered by outer fan-basin plain (?) deposits at a time of retrogradation of the fan system. The evolution in the basin continued with progradation and development of depositional lobes and channel-fill middle fan environments. The western portion of the trough was situated near to the zone of convergence between the Nazca and the South American plates and the main sediment supply derived from the Paleo-central Cordillera, which in early Cretaceous time consisted of an uplifted Jurassic plutonic arc. This active margin defined the coexistence of small sand-rich radial fans and channel-levee complexes. In the eastern portion of the basin main source area, the stable Guayana shield determined the existence of a larger fan system of a hybrid type between radial and elongate fans. Despite the differences between the two fan systems in eastern and western portions of the trough, the observed retrogradational and progradational sequences are similar.

Fluvially Dominated Deltaic and Submarine Fan Deposits in Two Adjacent Karoo Subbasins, South Africa.

Fluvially Dominated Deltaic and Submarine Fan Deposits in Two Adjacent Karoo Subbasins, South Africa.

Reservoir Characterization and Diagenesis of the Oligocene 64-Zone Sandstone, North Belridge Field, Kern County, California

The Oligocene (Zemorrian) 64-Zone sandstone is an important oil and gas reservoir in North Belridge field, Kern County, California. The 64-Zone is a submarine-fan deposit that ranges from 83 to 137 m in thickness. Although diagenesis has played an important role in modifying porosity and permeability, the overall variations in reservoir quality reflect an upward increase in grain size associated with depositional processes. Three diagenetic events have had a significant impact on reservoir quality: (1) compaction, which has reduced intergranular volume to an average of 20%, (2) quartz cement, which has reduced porosity by an average of 6.2%, and (3) feldspar dissolution. Although an average of 2.7% porosity is directly associated with leached feldspar grains, mass balance calculations indicate that secondary porosity is roughly balanced by formation of authigenic kaolinite, resulting in little or no net gain in porosity. Three episodes of calcite cementation reflect various stages of burial history. Petrographic and isotopic data demonstrate that calcite I formed at shallow depths in the zone of bacterial sulfate reduction. Fluid inclusion data indicate that calcite II precipitated at temperatures of 92 to 167°C from fluids less saline than seawater. Fracture-filling calcite III postdates calcite II, but formed at lower temperatures (85 to 125°C). The results of isotopic modeling indicate that calcite II precipitated in equilibrium with waters expelled by shales during I/S diagenesis (^dgr18OSMOW = +2 to +8^pmil) and that calcite III precipitated during tectonic uplift from a mixture of shale-derived and meteoric waters (^dgr18OSMOW = 0 to +4^pmil) Most fluid inclusions in calcite II yield temperatures greater than present bottom-hole values (^sim110°C). Assuming that fluid inclusions in calcite II record maximum burial and a geothermal gradient of 33°C/km, tectonic uplift of 0.6 to 1.7 km (^sim2000-5700 ft) would be required to explain the fluid inclusion data.

Recent Advances in Sequence Stratigraphy: The Lowstand and Transgressive Systems Tracts

On basin margins characterized by a ramp physiography, relative sea level fall induces basinally isolated, shelf-perched, forced-regression shoreline and deltaic deposition rather than deep-water submarine-fan turbidite deposition. These deposits comprise the early lowstand systems tract. In proximal locations, the sequence boundary at the base of these lowstand deposits is expressed as an erosional unconformity, whereas in distal settings this surface occurs as a correlative conformity. The stratigraphic discontinuity at the top of these lowstand deposits is a ravinement surface and in places can be more striking than the sequence-bounding surface at the base. significant erosion due to fluvial processes during forced regression and subsequent shoreface processes during transgression may be common at the tops of these lowstand deposits. The transgressive systems tract commonly comprises, proximally to distally, backstepped barrier beach deposits, sheet-like lag deposits, and [open quotes]healing phase[close quotes] deposits. This latter unit has not been described widely, but volumetrically may contain a significant part of the transgressive systems tract. Healing phase deposits comprise a wedge of sediment that onlaps the last clinoform of the underlying progradational phase. These sediments commonly are derived by cannibalization from the top or edge of the subjacent progradational phase deposits (i.e., either highstand ormore » lowstand), during and immediately after transgression. These deposits are referred to as the healing phase because their deposition tends to [open quotes]heal over[close quotes] the relatively steep clinoform slope of the underlying progradational phase after transgression has resulted in landward shift of the depocenter. Although sometimes misinterpreted as lowstand deposits, these depositional units commonly do not contain significant reservoir facies.« less

Sedimentology of the Archean Doré metasediments, Arliss Lake area, southern Michipicoten greenstone belt, Superior Province

Doré metasediments in the Arliss Lake area of the Michipicoten greenstone belt predominantly represent submarine-fan sedimentation in mid- to outer-fan settings; subaerial deposition was rare, occurring in a sandy braided stream environment and as mud-poor debris flows in a probable fan-delta setting. The Arliss Lake subbasin was likely small, and of an irregular configuration which restricted down-fan sediment distribution, promoting aggradational lobe development; it existed on a convergent plate margin. Felsic pyroclastic volcanism was periodically synchronous with epiclastic sedimentation but only rarely dominated. Paleogeographic development, addressed by comparing juxtaposed thrust-fault-bounded lithological packages of similar facing, suggests that the subbasin experienced several episodes of instability, which caused dramatic changes in relative sea level.

Tectonostratigraphy of Mino terrane: Jurassic accretionary complex of southwest Japan

The Mino terrane, a disrupted accretionary terrane in central Japan, consists of four fault-bounded mappable units of Mesozoic and Paleozoic unmetamorphosed sedimentary rocks. The four units are described as (1) Permian greenstone-carbonate-chert unit, (2) Lower Triassic to lowest Cretaceous siliceous pelagite-distal turbidite unit, (3) Middle Jurassic proximal turbidite unit, and (4) upper Lower Jurassic to lowest Cretaceous olistostrome-slump unit. The first unit is interpreted as a sedimentary cover which was formed on and around a seamount in an open-ocean setting. The second unit records the accumulation of deep-water radiolarian-rich sediments in a pelagic realm and a tapering wedge of trench turbidites. The third unit contains submarine fan sediments deposited in a possible trench-slope basin. The forth unit, labeled as sedimentary melange containing blocks derived from accretionary prisms, is considered to be submarine deposits which accumulated in a lower trench-slope basin to trench floor setting. An originally southward-vergent imbricate stacking of structural wedges of these four units characterizes the highly complicated structure of the Mino terrane rocks. Collision and subsequent offscraping accretion of Permian to Middle Jurassic oceanic rocks and Lower Jurassic to lowest Cretaceous clastic rocks provide the most satisfactory explanation for juxtaposition of the structural wedges of the four units which were originally laid down in largely different depositional sites. These tectonic events most probably took place during Early Jurassic to Early Cretaceous time along an active continental margin of East Asia.

Palynology of the James Ross Island area

The James Ross Island area is recognized as one of the geological treasures of Antarctica. It exposes a section of over 6 km of Cretaceous and Tertiary marine sedimentary strata with an important content of reworked Upper Jurassic rocks near the base. This sequence has produced some of the most remarkable fossil finds in the Southern Hemisphere. The list is extensive and includes a wealth of marine invertebrates, plesiosaurs, dinosaurs, land mammals and birds, giant penguins, whales, and fossil leaves and wood. The sediments record a long history of submarine fan, shelf and deltaic deposition in a back-arc environment, and the contained fossils provide valuable information on Cretaceous and Tertiary climates, and biological evolution at high palaeolatitudes.

Glacially influenced sedimentation in the Later Proterozoic of the Paraguay belt (Mato Grosso, Brazil)

Late Proterozoic (probably early Vendian, 610-590 Ma) glaciogenic and turbiditic sediments and metasediments are present along the transitional zone between the Amazonian Craton to the northwest and the Paraguay Belt to the southeast, in the Cuiabá region, Mato Grosso, Brazil. A few exposures of diamictites are associated with conglomerates and graded pebbly sandstones on the cratonic fringe and pass southeastwards into fine-grained sediments deposited in the deeper part of the probably marine basin. In the Paraguay Belt, these rocks have been affected by the Brasiliano Orogeny, possibly around 570 Ma. They show increasing deformation and metamorphism from the craton towards the fold belt. The sedimentation scheme proposed for the western border of the Paraguay Belt and the eastern edge of the Amazonian Craton involves three, probably marine, main depositional systems. To the west, on the craton, the platform domain can be subdivided into a western inner shelf and an eastern outer shelf. The deposits on the inner shelf subdomain show an alternation of dominant massive diamictites, sandstones and fine-grained sediments with few dropstones. This diamictite association is tentatively interpreted as glaciomarine in origin. In the outer shelf subdomain, the massive diamictites are progressively replaced southeastwards by an association of massive diamictites, stratified diamictites and fine-grained sediments. This diamictite association is interpreted, as a whole, as resedimented glacial debris deposited by subaqueous debris flows. The slope domain, located at the transition between craton and fold belt, is characterized by strong reworking of the glaciomarine sediments by gravity flows responsible for a type of submarine fan deposition. The coarse clastic facies, which result from a reworking by rapid downslope transport of cohesive debris-flows and gravity flows with high viscosity, fill feeder channels in the upper fan. Eastward, inside the belt, a progressive sorting in the deeper portions of the fan is shown by the transition from diamictites, massive gravels, sandstone intercalations with occasional inverse and/or normal grading (debris flow + turbidity current) to normal graded, finer sediments (turbidity current). Sandstone and siltstone intercalations represent interchannel deposits formed by turbidity currents. East of the slope, well-bedded, fine-grained sediments were generated by low-density turbidity currents. Direct glacial influence is reduced to the presence of a few isolated clasts or dropstones. Two distinct Upper Proterozoic glaciogenic units are known in Brazil. The older and more widespread one, dated at about 950 Ma, is located on the São Francisco Craton and in the adjacent Brasiliano fold belts. The younger one (≈600 Ma) is restricted to the Paraguay Belt and the adjacent border of the Amazonian Craton. In southern Western Gondwana, Late Proterozoic glaciation is of mountain-type and developed at latitudes of 60° on highs probably generated by Pan African-Brasiliano rifting. At the same time, in its northern part, a widespread ice sheet covered the West African Craton.

Paleogene-Neogene depositional history of the middle U.S. Atlantic continental rise: mixed turbidite and contourite depositional systems

The construction of the middle U.S. Atlantic continental rise from late Paleogene to late Pliocene time included downslope sediment gravity flows and alongslope contourite deposition. Some previous studies have suggested that contour-following bottom currents were the dominant control on depositional patterns. Depositional relief from two large contourite drifts, largely built in late Miocene to late Pliocene time, continue to dominate present-day lower rise morphology. Detailed seismic sequence mapping presented here suggests that significant turbidite (fan) systems also were active and are preserved in the stratigraphic section. A single thick depositional sequence is mapped for Oligocene to middle Miocene sediments deposited when inferred fan systems off the Norfolk-Washington canyons were reworked by bottom currents along the lower rise to form the proto-Hatteras Outer Ridge. Detailed mapping of three late Miocene to late Pliocene depositional sequences indicates that fan-channel systems were active at the same time large contourite drifts were being constructed. Sediment delivered to the lower rise by these elongate turbidite systems undoubtedly was an important local source for construction of the Hatteras Outer Ridge and, through entrainment and resuspension, was an important source for regional contourite deposition controlled by the deep Western Boundary Undercurrent. We suggest that submarine fans and current-controlled sediment drifts may develop simultaneously as companion systems. Types of fan/drift interaction include current-only modification of fans, transitional fan-to-drift, and adjacent or overlapping submarine fan and sediment drift deposition.

Locations and Characteristics of Thin-Bedded Turbidites in Passive-Margin-Setting Submarine Fans

ABSTRACT Thin beds form an important part of submarine fan deposits. They can be found in five different subenvironments: top of a channel fill, channel margin, levee-overbank area, sandwiched between thick layers in the updip non-channelized depositional lobes, and fan fringe. At the present we are not able to distinguish clearly one depositional subenvironment from the other, using thin beds as a criterion. In the passive margin or margin setting of the Gulf of Mexico and in a number of areas in the Atlantic Ocean, massive sands belonging to a submarine fan can be wet while thin-bedded sandstones may be oil prone. Such a condition makes it important to be able to determine the depositional setting needed to provide information about reservoir geometry, connectivity between successive sandstone layers, porosity/permeability trends, and orientation of the reservoir sands. Depending on the layer thickness of the thin-bedded turbidites and the sand/shale ratios, it might be difficult to recognize the sandstones on conventional well logs. A good seismic survey, however, may permit indication or detection of a channel cross section from which inferences can be drawn as to where thin-bedded sections can be expected. Initial studies on outcrops have shed some light on characteristics of the levee-over-bank and channel top fill thin bedded series. Using existing depositional models, circular reasoning is often involved as to the identification of each depositional environment.

Early Tertiary submarine fan deposits in the Porcupine Basin, offshore Ireland

Abstract The Porcupine Basin, offshore western Ireland, contains up to 10 km of Mesozoic and Cenozoic sediments. Seismic sequence analysis of the Palaeocene-Eocene strata on the southwestern and southeastern margins of the basin reveals a mounded and draped geometry. Four seismic sequences are defined and are coeval with a series of deltaic deposits in the northern and central parts of the basin. In the southwestern area these are interpreted as stacked submarine fan deposits. In the southeastern area the lower two sequences are interpreted as shelf-slope deposits which are overlain by two submarine fan complexes. Funnelling of the sediment was controlled by erosive channelling through interfan topographic lows. The seismic response indicates that the fans are dominated by lower fan sheet sand deposits with minor amounts of upper fan channellized sandstones. They probably developed in response to an interplay of eustatic sea level changes, tectonic uplift on the basin margins, and thermal subsidence towards the basin centre.

Structural development of an Archean Orogen, Western Point Lake, Northwest Territories

The Point Lake orogen in the central Archean Slave Province of northwestern Canada preserves more than 10 km of structural relief through an eroded antiformal thrust stack and deeper anastomosing midcrustal mylonites. Fault restoration along a 25 km long transect requires a minimum of 69 km slip and 53 km horizontal shortening. In the western part of the orogen the basal decollement places mafic plutonic/volcanic rocks over an ancient tonalitic gneiss complex. Ten kilometers to the east in the Keskarrah Bay area, slices of gneiss unroofed on brittle thrusts shed molasse into several submerged basins. Conglomerates and associated thinly bedded sedimentary rocks are interpreted as channel, levee, and overbank facies of this thrust‐related sedimentary fan system. The synorogenic erosion surface at the base of the conglomerate truncates premetamorphic or early metamorphic thrust faults formed during foreland propagation, while other thrusts related to hinterland‐progressing imbrication displace this unconformity. Tightening of synorogenic depositional troughs resulted in the conglomerates' present localization in synclines to the west of associated thrust faults and steepening of structural dips. Eastern parts of the orogen consist of isoclinally folded graywackes composed largely of Mutti and Ricci‐Lucchi turbidite facies B, C, and D, interpreted as submarine fan deposits eroded from a distant volcanic arc. Thrust faults in the metasedimentary terrane include highly disrupted slate horizons with meter‐scale duplex structures, and recrystallized calcmylonites exhibiting sheath folds and boudin trains with very large interboudin distances. The sequence of fabric development and the overall geometry of this metasedimentary terrane strongly resembles younger forearc accretionary prisms. Conditions of deformation along the thrusts parallel the regional metamorphic zonation: amphibolite facies in the basal decollement through greenschist facies shear zones to cataclastic crush zones in the region of emergent thrusts in Keskarrah Bay. Depth differences can account for only half of the metamorphic gradient; thermal profiles which increased downwards in obducted greenstone belts and synthrusting plutonism explains other high metamorphic gradients. A tectonic model involving the collision of an accretionary prism with a continental margin best explains the structural and sedimentological evolution of the orogen.

The Mississippian Antler Foreland and Continental Margin in Southern Nevada: The Eleana Formation Reinterpreted

Rocks mapped as the Mississippian Eleana Formation at the type locality on the Nevada Test Site appear to comprise two completely different, but coeval, sedimentary units. In the Eleana Range (Western Eleana Formation), the strata are siliciclastic and carbonate turbidites of Mississippian age. From immediately east of the Eleana Range to Syncline Ridge (Eastern Eleana Formation), the strata are Devonian-Mississippian mudstone and quartzite conformably overlying Devonian limestone and underlying Pennsylvanian limestone. Although the contact between the two sedimentary packages is not exposed, small-scale structures document an east-dipping fault contact and reverse motion. Sandstone petrography and stratigraphic considerations support the age data in identifying two separate Mississippian units. Sandstones from the Western Eleana are chert litharenites with significant amount of feldspar and both volcanic and sedimentary lithic grains. These rocks are interpreted to be a submarine fan deposit; southwest-directed paleocurrent indicators suggest that they were deposited in an elongate trough, filled axially from the northeast. The source of the sediments was the antler allochthon and foreland basin. The authors tentatively correlate this section with the Dale Canyon-Chainman-Diamond Peak section near Eureka, Nevada. Sandstones from the Eastern Eleana are quartz arenites with rare chert and detrital heavy minerals. These strata are tentativelymore » interpreted to be a shallow shelf deposit, with sediments derived from the continent to the east. They tentatively correlate this section with the Guilmette-Pilot-Scotty Wash-Chainman section of eastern Nevada. These sedimentary systems are initially separated an unknown distance across the late Paleozoic continental margin.« less

A Passive Margin-type Submarine Fan Complex, Permian Ecca Group, South Africa

A submarine fan complex, comprising five arenaceous fan systems separated by basinal shale units, occurs in the southwestern part of the intracratonic Karoo basin in South Africa. Although basin development is related to a subduction zone bordering the palaeo-Pacific ocean to the south of Gondwanaland and the evolution of the Cape Fold Belt, the entire Lower Permian Ecca Group basin-fill succession reflects depositional characteristics of a passive-margin setting. The submarine fan complex, 250 m thick, originated from sediments supplied by Mississippi-type deltas dominating the Ecca coastline. The fine grain-size and low sand/shale ratio of the submarine fan and deltaic deposits reflect the maturity of the ancient river systems. Outcrops of the fan complex are well exposed and cover an area of 650 km{sup 2}. The strata are not affected by folding, and deep erosion allows three-dimensional viewing of mid-fan to outer-fan deposits. Features of interest include stacked lobe deposits displayed along 2.5 km of a 60 m high cliff section, and a transverse cliff section through channel-fill deposits 500 m wide. Paleocurrent directions reveal that each sequence had its own main source area located to the northwest and south of its present geographic location. The cyclic nature of the fanmore » complex is attributed to relative sea-level changes; deposition took place on the basin floor in water depths that do not exceed 500 m. Shoaling of the basin to wave base depths is reflected in the pro-delta and delta front deposits overlying the uppermost fan sequence. Major factors in controlling direction of fan progradation were delta switching and basin floor topography.« less