Middle-fan Channel Research Articles

Climatic and tectonic controls on the sedimentary processes of an alluvial fan of the western Ganga Plain, India

AbstractDetailed facies analysis and morphotectonic investigations of the Malin River's alluvial fan in the western Ganga Plain, India, reveal that the morphology of the fan is largely tectonically controlled whereas the sedimentary processes are mainly climatically controlled. The sedimentation occurred in two distinct evolutionary cycles which are separated by a time gap. The older cycle deposited thick gravelly units up to the distal-fan area, whereas the sediment fill of the younger cycle is gavel-dominated in the proximal-fan area, gravel–sand dominated in the middle-fan area and sand–mud dominated in the distal-fan area. The gravels of the older cycle were emplaced by intense sediment gravity flows during periods of strengthened monsoon and steeper regional gradient. During the younger cycle, the proximal to distal parts of the fan were dominated by different sedimentary processes. This was a time of relatively weaker monsoon and gentler regional slopes, when gravels could travel only up to the middle-fan area. The gravels in the proximal-fan area have mainly been deposited by sediment gravity flows and channel processes; in the middle-fan area channel processes, sheetfloods and sediment gravity flows have been the main sedimentary processes; and in the distal-fan area fluvial processes of channel migration and overbank deposition have been the main sedimentary processes.

Downfan Transformation from Turbidity Currents to Debris Flows at a Channel-to-Lobe Transitional Zone: The Lower Pleistocene Otadai Formation, Boso Peninsula, Japan

Transformation of turbidity currents into debris flows across a channel-to-lobe transition zone was recognized on the basis of lateral mapping of a falling-stage-systems-tract deposit in the lower Pleistocene Otadai Formation submarine-fan sequence on the Boso Peninsula, Japan. The most proximal outcrops comprise 1 to 4 m thick turbidites, which are laterally equivalent to debrites encased in turbidites in outcrops farther down flow. Such turbidites and encased debrites were deposited from single flow events. The most distal outcrops comprise only thinner (2 to 40 cm thick) turbidites. Debrites contain many siltstone clasts and finer-grained sediments, which are characterized by deep-water faunas and clay-mineral composition similar to those of interbedded hemipelagites. Therefore, the transformation of turbidity currents into debris flows is interpreted to have occurred in response to the incorporation of many siltstone clasts and finer-grained sediment particles into the precursor turbidity currents through erosion of muddy substrates. Subsequently, turbulence in the precursor turbidity currents was suppressed and near-bed flow with higher sediment concentration developed. Intense erosion of muddy substrates is interpreted to have occurred in response to the increase in intensity of turbulence in the precursor turbidity currents at the mouths of middle-fan channels. The precursor turbidity currents (or turbidity currents generated by dilution of debris flows) produced a turbidite deposit, which was subsequently overrun by the debris flow in a proximal lobe environment. The debris flows are interpreted to have subsequently been transformed into turbidity currents, which are recorded by turbidite deposition in the more distal-lobe environment. Debrites can characterize the proximal portion of an attached-lobe deposit in a channel-to-lobe transition zone. Such debrites can be significant heterogeneities for fluid flows in sand-prone successions from middle-fan channel to attached-lobe deposits.

深海底堆積物研究の進展：低海水準期堆積モデルの再構築

A falling-stage deep-water succession is characterized by sandstone-dominated channel-and-overbank deposits and age-equivalent depositional-lobe deposits. In contrast, a lowstand deep-water succession is muddier and is characterized by a thickening-upward pattern of distal overbank deposits. The base of the falling-stage deposits is characterized by a distinct erosional surface compared with the base of the lowstand deposits, which is commonly represented by a gradational contact to the underlying falling-stage deposits.Deep-water channels commonly exhibit meandering patterns similar to fluvial channels, and the channel-fill deposits are encased in muddy overbank deposits. The distribution patterns and volumes of coarse-grained channel-fill deposits developed mainly during falling- and lowstand stages can be estimated from outcrop and seismic data on the basis of several empirical relationships among width, depth, and sinuosity of deep-water channels. Hyperpycnal flows have recently been interpreted to be one of the important mechanisms for developing thick to very thick-bedded, deep-water massive sandstones (DWMSs). DWMSs commonly exhibit sheet-like geometry and abruptly thin out and fine to the downslope direction. These abrupt changes in thickness and grain size of DWMSs are interpreted to be a result of deposition of a sufficient volume of sand particles from precursor hyperpycnal flows, which are subsequently transformed into buoyant plumes with finer-grained sediment particles associated with residual lower-density turbidity currents flowing to the further downslope direction. Modulation and transformation of turbidity currents into debris flows at a channel-to-lobe transition zone were recognized. The transformation of turbidity currents into debris flows is interpreted to have occurred in response to incorporation of many siltstone clasts and finer-grained sediment particles into the precursor turbidity currents from muddy substrates. The finding indicates that a laterally continuous sandstone body from middle-fan channel to depositional lobe deposits internally contains muddy baffles, which develop heterogeneity of fluid flows in reservoir sands and sandstones.

Physiography and recent sediment distribution of the Celtic Deep-Sea Fan, Bay of Biscay

The Celtic Deep-Sea Fan located in the northwestern part of the Bay of Biscay is a middle sized fan with a surface area of more than 30,000 km 2 . The whole system is a mature mud/sand-rich submarine fan on a passive margin. Multi-beam echo sounder data, 3.5 kHz seismic and 12 Kullenberg cores were examined to define the fan morphology, the lithological characteristics, the sedimentary processes and the relationship between the evolution of the fan deposits and the environmental conditions on the Celtic continental shelf. The upper fan is characterised by the presence of two distinct tributary systems: (1) the Whittard system with a large, persistent, slightly sinuous channel, which is linked to the southern end of the Irish Sea River system; and (2) the Shamrock system, with a moderate sized channel, which is linked to the western end of the English Channel River system. The middle and lower fan corresponds to divergent braided secondary channels and associate lobes. Successive lobe elements, without impor- tant relief, were generated during periodic avulsions of middle fan channels. The lithological, palaeontological, and geochemical analyses on cores show the evolution of sedimentation since the last glaciation. During the last lowstand and rise of sea-level frequent low-density turbidity currents were predominant and deposited sediments throughout the whole fan system. They were initiated at the front of a deltaic environment on the Celtic outer-shelf. During the high sea-level conditions, occasional high-density turbidity currents and/or non-cohesive debris flows occur and were responsible for sand deposition in the middle-lower fan. They are derived from reworked sands due to the high- energy conditions on the outer shelf. Thus for the Celtic Fan, the variations of the hydrodynamic conditions on the outer Celtic Shelf seem to be the primary control on facies shift and fan growth. q 2000 Elsevier Science B.V. All rights reserved.

Morphology and Acoustic Character of the Antarctic Wilkes Land Turbidite Systems: Ice-Sheet-Sourced Versus River-Sourced Fans

The Wilkes Land continental slope contains an intricate network of submarine canyons that on the continental rise develop into a series of channel and overbank deposits of turbidite systems. We can define upper-fan, middle-fan, and lower-fan provinces. The Wilkes Land upper fans are characterized by large channels with relief up to 900 m, distances between levee crests up to 18 km, and channel-floor widths up to 6 km. Middle-fan channels also have high relief (∼300 m), and locally, interchannel areas exhibit mounded contourite-style deposits with high relief (up to 490 m). Within middle-fan sediment mounds there are acoustic facies of channel-overbank deposits from turbidity currents, and of sediment waves from contour-current sedimentation. The lower rise is characterized by small, shallow channels (50-75 m relief) and by interchannel areas of low relief, both characteristic of a lower-fan environment. The Wilkes Land turbidite systems show the following significant morphological differences compared to most river-sourced fans: (1) multiple large tributary channels across the upper and middle fan, (2) channel relief several times greater (900 m) than typical relief (100-200 m) for channels on fans less than 300 km in diameter, and (3) steep middle-fan and lower-fan gradients. We interpret the differences in channel network patterns, channel size, and middle-fan and lower-fan gradients between the Wilkes Land fans and other fans to result from the continental ice sheet feeding glacial ice streams that reached the outer continental shelf at times of glacial maxima. The Wilkes Land canyon-channel network patterns are comparable with the high-latitude Laurentian Fan and Labrador Sea channels. The Laurentian Fan also has large upper-fan channels with larger relief (800 m) than typical relief (200-300 m) expected for fans that are 600 km in diameter. Both the Labrador Sea and Laurentian Fan are fed by continental ice sheets at the shelf edge.

Upper Campanian and Lower Maestrichtian Depositional Systems and Gas Production, Southern Sacramento Basin, California: ABSTRACT

Upper Campanian and lower Maestrichtian strata of the southern Sacramento basin include four west- and southwest-prograding submarine-fan/slope/delta systems. The Winters, Lathrop, Tracy, and Blewett formations consist of submarine-fan and related slope/basin-plain deposits that were fed by various deltaic complexes of the Starkey Formation. Four major basinwide transgressive shale units (Sacramento Shale, Sawtooth Shale, Ragged Valley Shale, and H and T Shale) help intrasystem correlations. The Winters, Tracy, and Blewett fans are small, radial, coalescing sand-rich systems that contain the following principal facies: (1) sandstone-filled inner fan channel deposits, (2) mudstone-dominated inner fan interchannel deposits, (3) middle-fan amalgamated suprafan-type sandstone-rich channel deposits, and (4) mudstone-dominated outer fan deposits. The Lathrop fans are larger, elongate, mixed-sediment systems that contain basin-plain, outer fan lobe, middle fan-channel, levee, interchannel, and inner fan channel facies. The Sierran-derived fluvio-deltaic Starkey Formation can be divided into six sand-rich deltaic cycles that can be subdivided on the basis of log signaturres and spatial distribution into prodelta, delta-front, lower delta-plain, and upper delta-plain/fluvial facies. More than 50 gas fields produce from these systems. Stratigraphic traps include updip pinchouts of submarine canyon/gullies and inner fan channels into slope shale, especially in the many overlapping and coalescing sand-rich systems. Lateral pinchoutsmore » of outer fan lobes and middle-fan suprafan-type bodies are also productive. Structural traps generally characterize production from deltaic deposits because of the more continuous nature of these bodies.« less

Role of Diagenesis in Development of Upper Morrow Fan-Delta Reservoirs in Anadarko Basin: ABSTRACT

Upper Morrow chert conglomerates are prolific natural gas reservoirs in the deeper part of the Anadarko basin. Based on data obtained during the course of this investigation, the conglomerates display several diagnostic features of a fan-delta depositional environment, with various facies representative of middle-fan and distal parts of the delta-fan complex. These facies were calibrated to suites of wireline logs, characteristic log signatures were identified for each facies, and a map was prepared showing the distribution of facies within the fan-delta complex. Major lithology types of conglomerates are chert, quartz, feldspar, and a combination of these three constituents. A rather complex diagenetic history includes at least nine cementation episodes reflecting various subsidence stages and thermal regimes. Reservoir-quality lithofacies are present mainly in the middle-fan channel facies, which consists of stacks and repetitive sequences of braided-stream deposits. Both intergranular primary and secondary porosity types are present in the chert conglomerates. Although intergranular porosity is very common, it was modified by cementation and dissolution. Complete preservation of original porosity is rare. Remnant primary porosity has provided avenues for fluid migration, which resulted in partial to complete dissolution of rock constituents. Siliceous detrital matrix, chert, and feldspar are the major constituents affected bymore » the dissolution processes. A geochemical model emphasizing the role of feldspar hydrolysis is proposed to explain the dissolution of chert and the development of productive reservoirs in the chert conglomerates.« less

Reservoir Geometry and Trapping Mechanisms, Lindsey Slough Gas Field, Southern Sacramento Basin: ABSTRACT

Multiple reservoir units, numerous unconformities and stratigraphic pinch-outs, and extensive faulting make the Lindsey Slough field area (T5N, R2E, MDBM) geologically intriguing and economically attractive. The petroleum geology class of California State University, Northridge, undertook a group mapping project in November 1985, to delineate producing pools, determine reservoir geometries and trapping mechanisms, and identify potential exploration and development locations. More than 12 separate pools, producing from at least one of the 13 reservoir zones, are present. Reservoirs are of two main types: (1) extensive sheet sandstones deposited in delta-front and shelf(.) environments, and (2) lenticular channel sandstones deposited in submarine canyon, slope, and submarine fan environments. Among the sheet sandstones, the upper and lower Petersen members of the Upper Cretaceous Starkey formation are especially productive, with pay zones exceeding 100 ft. Both members pinch out to the southwest, along the paleoshelf edge. Of the lenticular sandstones, reservoir quality and thickness appear to be greatest in delta-mouth slope channels that fed sand basinward to submarine fans. Net pay in the K-1 and related upper Cretaceous channels exceeds 200 ft in some wells. Middle-fan channels of the Upper Cretaceous channels exceeds 200 ft in some wells. Middle-fan channels of the Upper Cretaceousmore » Winters Fan are locally productive, and a small amount of gas has been produced from thin, poorly sorted sandstones within the Paleocene Martinez Submarine Canyon.« less

The Jersey Shale Formation—a late Precambrian deep-water siliciclastic system, Jersey, Channel Islands

The late Precambrian Jersey Shale Formation, with an estimated thickness of 2500 m, is described in terms of six facies: (I) laminated mudstone; (II) ripple-laminated very fine-grained sandstone; (III) graded medium to fine-grained sandstone; (IV) cross-stratified medium-grained sandstone; (V) clast-supported pebble/cobble conglomerate; and (VI) chaotic beds. Five facies associations are recognised: Association I—canyon or inner fan channel fill; Association II—middle fan channel fill; Association III—lower middle fan/outer fan deposits; and Association IV—divided into lobe, lobe-fringe and fan-fringe deposits in an outer fan. The Jersey Shale Formation and the overlying, approximately 2300 m thick, andesitic-rhyolitic Volcanic Group are believed to have accumulated in an active, convergent plate margin setting.

Stratigraphy, Depositional Rates, and Other DSDP Leg 96 Conclusions: Mississippi Fan: ABSTRACT

The Quaternary Mississippi fan consists of at least seven seismically mappable fan lobes. Eight sites were drilled into the youngest fan lobe during DSDP Leg 96. The Holocene (Ericson Zone Z) is capped by a marly foraminiferal ooze. Assuming an age of 12,000 yr for the Holocene/Pleistocene boundary, a minimum accumulation rate of 3-30 cm/1,000 yr (1-12 in./1,000 yr) is computed for the Holocene. The youngest fan lobe was deposited during the upper part of Ericson Zone Y (late Wisconsin glacial, 12,000-85,000 y.B.P.). Deposition rates for the Y Zone are extremely high for the middle fan sites, averaging 12 m/1,000 yr (39 ft/1,000 yr). Lower fan accumulation rates are 7 m/1,000 yr (23 ft/1,000 yr) for the channel sites and 6 m/1,000 yr (20 ft/1,000 yr) for the channel-mouth depositional lobes. These rates are not corrected for compaction. Foraminifera are scarce; the occurrence of shallow-water benthic species indicates a displaced inner and middle neritic origin for the sediments. Drilling on the youngest fan lobe shows that most of the silts and sands were transported through the upper and middle fan channel onto the lower fan, producing a 6 to 10-km (4 to 6-mi) wide, 135-m (443-ft) thick aggradational channel deposit. Much of the fine-grained sediment spilled out of the channel onto the adjacent overbank areas, constructing a broad marginal plain. The channels on the lower fan are smaller and shift position frequently. Channel-mouth depositional lobes are constructed at the end of the active channels and are composed of more than 50% net sand. End_of_Article - Last_Page 463------------

Turbidite Facies and Facies Associations of Cretaceous and Paleocene Gottero Sandstone, Northern Italy: ABSTRACT

Turbidites of the Gottero Sandstone were deposited as a small deep-sea fan in a trench-slope basin. The Gottero is as thick as 1,500 m (4,900 ft), had an original radius of 30-50 km (19-30 mi), and in the early and middle Cenozoic was thrust northeastward onto the Italian peninsula. The Gottero Sandstone is stratigraphically part of the Vara Supergroup, which has an ophiolite at its base and forms one of several stacked allochthonous sheets in the Ligurian Apennines southeast of Genoa. The Gottero, which contains foraminifers ranging in age from Albian to Paleocene that indicate deposition at bathyal depths, consists chiefly of feldspathic sandstone thought to be derived from the largely plutonic Corsican-Sardinian continental block to the southwest. The Gottero rests on ectonically disrupted shale that contains intercalated olistostromes. To the northwest, northeast, and southeast, it pinches out into and is overlain by basin-plain End_Page 512------------------------------ turbidites. An outward-radiating paleocurrent pattern suggests northeastward transport of sand through a single major inner-fan-channel complex, succeeded by radial downfan dispersal to smaller middle-fan channels, outer-fan lobes, and fan-fringe deposits. Sandstone-to-shale ratios are 99:1 in inner-fan-channel deposits, 10:1 in middle-fan-channel deposits, 2.5:1 in middle-fan-levee deposits, 1.3:1 in middle-fan-interchannel deposits, 11:1 in outer-fan-lobe deposits, 2.6:1 in fan-fringe deposits, and 0.25:1 in basin-plain deposits. Orderly and progressive downfan changes in most sedimentary parameters, such as maximum clast size, thicknesses of fining- or coarsening-upward cycles, facies, and facies associations permit the growth history of the fan to be determined. The Gottero Sandst ne provides a useful model for petroleum exploration in small deep-sea fans characterized by a mixture of grain sizes and deposited adjacent to tectonically active, convergent continental margins. End_of_Article - Last_Page 513------------

Characteristics of Mississippi Fan Sediments, DSDP Leg 96: ABSTRACT

Sedimentologic, paleontologic, geochemical, and geotechnical studies were conducted on cores drilled at eight sites on the Mississippi fan during the Deep Sea Drilling Project Leg 96. Together with seismic and well log data, these studies allow development of a number of depositional facies within an overall fan-lobe model. The central middle-fan channel of the youngest Mississippi fan-lobe was an effective conduit for the transport of coarse-grained material; only clays and minor amounts of silt spilled over the channel margins. The channel fill deposit is basically an upward-fining sequence, commencing with coarse-grained sands and gravels, overlain by sands, sandy-silty muds, and muds. The basal coarse-grained sediment interval is approximately 134 m (450 ft) thick. Th swale deposits, the overbank deposits adjacent to the meandering channel, and the marginal overbank deposits, are characterized by fine-grained turbidites and hemipelagics. Basically, both sites contain a minor upward-coarsening sequence. Deposits on the lower fan, in the area where the channel shifts position frequently, show alternating sequences of channel fill, levee, and overbank End_Page 455------------------------------ deposits. Sediments near the channel terminus have upward-coarsening sequences (i.e. channel-mouth depositional lobes) and contain 47 to 65% sand. Most of the sparse microfauna in both sands and muds are benthic species characteristic of inner and middle neritic origin. Traces of biogenic methane and other hydrocarbons were found in the underlying lobes but not in the youngest lobe. All sediments are underconsolidated, resulting from the extremely high accumulation rates of 6-12 m/1,000 yr (20-40 ft/1,000 yr). End_of_Article - Last_Page 456------------

Submarine-fan facies associations of the Upper Cretaceous and Paleocene Gottero Sandstone, Ligurian Apennines, Italy

The Upper Cretaceous and Paleocene Gottero Sandstone was deposited as a small deep-sea fan on ophiolitic crust in a trench-slope basin. It was thrust northeastward as an allochthonous sheet in Early and Middle Cenozoic time. The Gottero, as thick as 1500 m, was probably derived from erosion of Hercynian granites and associated metamorphic rocks in northern Corsica. Outcrops of inner-fan channel, middle-fan channel and interchannel, outer-fan lobe, fan-fringe, and basin-plain facies associations indicate that the depositional model of Mutti and Ricci Lucchi for mixed-sediment deep-sea fans can be used. The original fan had a radius of 30 to 50 km.

Delgada Fan: Preliminary interpretation of channel development

The Delgada Fan, an irregularly shaped turbidite deposit extending more than 350 km offshore from northern California, consists of two large leveed-valley units each fed by a separate complex of coalescing submarine canyons and slope gullies. Although the leveed-valley units head within 25 km of each other, both appear to have developed independently during fan growth. The larger southern leveed-valley system has not developed middle-fan distributary channels and appears to illustrate a period of progressive valley abandonment. Although the lower-fan area is underlain by sandy sediments, little sand has been recovered in piston cores from the leveed-valley unit.

The Astoria Fan: An elongate type fan

The Astoria Fan, a modern system, is located on a subducting oceanic crust and fills a north-south-trending trench along the Oregon continental margin. Well-developed channels cross the entire fan length; they display classic inner-fan leveed profiles but evolve into distributaries in the midfan area where the gradient decreases sharply. During periods of low sea level, inner- and middle-fan channels funnel sand to distal depositional sites in the outer-fan area where the sand/shale ratios are highest. This pattern of sand displacement and efficiency of transport appears to be characteristic of elongate fans fed by a major river and submarine canyon.

Slope and Deep-Sea Fan Facies of Miocene Castaic Formation and Lower Part of Ridge Route Formation, Ridge Basin, Southern California: ABSTRACT

The late Miocene marine Castaic Formation in Ridge basin is over 2,000 m (6,600 ft) thick and consists mostly of slope and deep-sea fan facies. The Castaic Formation is vertically transitional into the overlying Marple Canyon Sandstone Member of the Ridge Route Formation and laterally interfingers with the Violin Breccia to the southwest. The slope facies consists of poorly bedded mudstone interbedded with sandstone, conglomerate, and coquina deposits interpreted as turbidite-filled slope channels. The slope facies follows the northwest trend of the basin and occurs on the northeast, north, and southwest sides of the basin. The slope channels have laterally adjacent levee deposits. Paleocurrents in the channels are to the west-northwest and southwest-southeast, whereas pa eocurrents in the levee deposits are to the northwest-southeast. Large slide blocks, slump-folded strata, and breccia beds are common in the slope facies. Deep-sea fan deposits consist of inner and middle fan channel and interchannel facies and outer fan depositional-lobe facies. They are confined to the center of the basin, interfinger into slope facies to the southwest and northeast, and are overlain by nonmarine fan-delta complexes to the north. The channels contain thick sandstone deposits that thin and fine upward and are laterally discontinuous, whereas the adjacent interchannel deposits consist of thin-bedded sandstone and mudstone sequences which form inclined wedges of highly slump-folded strata. Depositional-lobe facies thicken and coarsen upward and consist of interbedded sandstone and mudstone which form laterally continuous deposits with minor channeling and slump folding. Paleocurrents in these deposits are to the south-so theast and suggest sediment transport down the axis of the basin from the north-northeast. Ridge basin was a relatively shallow-marine trough about 6 km (4 mi) wide and 10 to 20 km (6 to 12 mi) long. The deep end of the basin was to the southwest and connected to the Ventura basin across the San Gabriel fault. Typical deep-sea fans did not develop in Ridge basin during Castaic Formation time; instead extensive slope deposits formed along the margins of the basin and thick but narrowly confined turbidite-filled channels and depositional lobes filled the valley or trough of this basin. End_of_Article - Last_Page 504------------

Depositional Environment and Reservoir Morphology of Canyon Sandstones, Central Midland Basin, Texas: ABSTRACT

Canyon sandstones in the Midland basin were originally thought to be part of the Canyon Group, but are actually downdip equivalents of fluvial and deltaic sandstones in the Cisco and Wolfcamp Groups. The repetitive, ordered sequence of sedimentary structures and the texture and composition of these basinal sandstones indicate that they are deposited by turbidity currents. Canyon sandstones were deposited as constructive turbidite channels. Bedset associations and reservoir morphology of constructive turbidite channels exhibit changes from upper-fan channels at Jameson field to middle-fan channels at Rock Pen field and to distal-fan channels and interchannel areas at Burnt Rock field. The upper-fan channels at Jameson field are commonly more than 50 ft (15 m) thick and consist of incomplete AE bedsets up to 6 ft (2 m) thick. The middle-fan channel at Rock Pen field averages 30 ft (9 m) thick, but displays similar incomplete AE sequences up to 3 ft (1 m) thick. Distal channels are thinner, have limited lateral extent compared to upper- and middle-fan channels, and consist of better End_Page 244------------------------------ developed Bouma sequences. Few sequences exceed thicknesses of 1 ft (0.3 m) in distal channels. Canyon sandstones are low-permeability reservoirs, with mean porosity and permeability of 10% and 0.2 md, respectively. Isoporosity and isopermeability maps indicate that porosity and permeability are greatest in turbidite channels and decrease laterally. Interchannel sandstones have limited lateral extent and make poor reservoirs. End_of_Article - Last_Page 245------------

Two Types of Outer Fan Lobe Sequence, from the Late Precambrian Kongsfjord Formation Submarine Fan, Finnmark, North Norway

ABSTRACT A comparison of outer fan sections from different parts of the Kongsfjord Formation Submarine Fan has revealed two types of outer fan lobe sequence, designated Type I and Type II. Both types of lobe sequence contain lobes, including lobe fringe deposits, that are between 2 m and 15 m thick, although Type I lobes are generally less than 10 m thick; there are no obvious differences in bed thickness, grain size, or other sedimentary attributes. The distinction between Type I and Type II lobes is based primarily upon their vertical frequency within fan fringe deposits. Type I lobes are separated by lobe fringe plus fan fringe deposits of similar thickness; therefore, in vertical sections the lobes are regularly spaced. Type II lobes occur sporadically within sections where lobe fringe plu fan fringe deposits vary from tens of centimeters to 70 m. In the two representative sections that illustrate the typological differences, Type I lobes, plus lobe fringe deposits, comprise 60 percent of the section, compared to 22 percent for the Type II lobe sequences. Type I lobe sequences are interpreted as the result of regular lobe switching immediately downfan from the middle fan channels, within a relatively proximal outer fan environment. Type II lobe sequences are interpreted as a consequence of sporadic progradation of lobes in a relatively distal fan environment that usually received fan fringe sedimentation. Type I lobes represent regular and stable deposition controlled by intrafan (intrabasinal) processes, whereas Type II lobes developed because of relatively catastrophic intrafan (e.g., major channel relocation) or extrafan (source control) processes. This distinction may serve as a proximality indicator in outer fan successions.

Reservoir Properties of Submarine-Fan Facies: Great Valley Sequence, California

Submarine-fan sandstones of the Great Valley sequence west of the Sacramento Valley, California, have low porosities and permeabilities (64 samples averaged 10.1% porosity and 0.87 millidarcies permeability). However, petrography and scanning electron microscope studies indicate that most sands in almost all submarine fan environments are originally porous and permeable. Thin turbidite sandstones deposited in areas dominated by shale in outer-fan, basin-plain, and overbank environments are cemented mainly by calcite; shale dewatering is inferred to contribute to rapid cementation early in the burial process. Sands deposited in inner- and middle-fan channels within interchannel and fan-fringe environments that contain only thin shale beds have small percentages of intergranular matrix or cement. The original porosity is substantially reduced mechanically at shallow depths and by pressure solution at deeper levels. Permeability decreases systematically with increasing age of the rocks, presumably as a result of increasing burial depths. Computer-run stepwise regression analyses show that the porosity is inversely related to the percentage of calcite cement. Such parameters as the contents of quartz, feldspar, and unstable rock fragments have no correlative effect on either porosity or permeability. The results reported here indicate original porosity and permeability can be high in deep-water submarine fans and that fan environments dominated by sand (with high sand/shale ratios) are more likely to retain higher porosity and permeability to greater depths than sand interbedded with thick shale sequences.

Variation in Turbidite Sand Facies and Application to Petroleum Geology: ABSTRACT

Turbidite sand in canyon, slope base, fan valley, and depositional lobe deposits interbedded with hemipelagic mud provides an ideal environment for petroleum accumulation. The geometry of sand facies, composition of source mud, and petroleum potential vary with basin size and setting. In the smallest restricted basins (i.e., carbonate platform troughs <10 km diameter) with multiple sources of extremely coarse-grained material, channelized turbidite facies do not develop. Thick, coarse-grained sediment gravity flows accumulate in slope valleys and base-of-slope settings associated with potentially organic-rich slope mud. At greater distances from the slope base, turbidite sand beds become fewer, thinner, and more widely dispersed in basin mud. Restricted basin fans of intermediate size (i.e., California borderland basins <100 km) fed by canyons intercepting littoral drift cells have excellent petroleum reservoir potential. Low-matrix turbidite sand is channeled to mid-fan lobes and may be interbedded with organic-rich hemipelagic mud. Sandstone continuity extends laterally from inner fan channels to suprafan lobes and vertically within lobe sequences. Tectonic overprint in restricted basin settings commonly permits preservation of turbidite reservoir beds and enhances thermal maturation of organic-rich mud. Turbidite sand of large, open basin fans fed from major river sources is: (1) finer grained and contains more matrix and (2) channeled to more distal depositional sites than those of restricted basin fans. The organic content of interbedded pelagic or hemipelagic mud is low because of oxidation and infaunal activity. Continuity is poor laterally between lower fan depositional lobes and upper to middle fan channel sand or vertically within mud-rich depositional lobe sand beds. Subduction of deep-sea floor fan complexes may destroy turbidite sand bodies as reservoir sites, just as it may tectonize linear turbidite sand bodies of trench fill and deep-sea channel systems. Similar sand bodies of submarine canyons however may have good petroleum potential where they are enclosed by organic- ich slope mud. End_of_Article - Last_Page 961------------