Outer-fan Lobes Research Articles

Deposystem architectures and lithofacies of a submarine fan-dominated deep sea succession in an orogen: A case study from the Upper Triassic Langjiexue Group of southern Tibet

Over thirty stratigraphic sections of the Himalaya orogen Upper Triassic Langjiexue Group in southern Tibet, China, were studied to interpret the environments and lithofacies. The facies associations channel (A), lobe (B), levee-interchannel (C), and basin plain (D) with nine facies (A1–3, B1–3, and C1–3) were distinguished. They form six architectural elements: channel-interchannel, overbank-levee, crevasse-splay, outer fan-lobe, fan-fringe, and basin plain. Taking into account the facies analysis, (sub-) deposystem correlation, paleocurrent dispersal pattern, and restoration of primary stratal width, the Langjiexue Group displays the architecture of a coalescing submarine fan-dominated deep sea deposystem, measuring about 400–500km×600–700km in size or even more, one of the largest pre-Cenozoic submarine fans ever reported. Subdivisionally, four fans, lacking inner fans, could have coalesced laterally within the submarine fan deposystem, and at least six submarine fan developments were vertically succeeded by mid- to outer-fan deposits with progradational to retrogradational successions. According to the range of 30–70% of sandstone content, the fan deposystem is mud- and sand-rich, suggesting a medium-far (over 400–600km) transport of sediment from the source area.

Architectural elements in a high-continuity sand-prone turbidite system, late Precambrian Kongsfjord Formation, northern Norway: Application to hydrocarbon reservoir characterization

Quantifying the geometry of high-continuity (relatively unconfined) sand-prone systems in deep-water sedimentary environments is important both for a better understanding of the intrinsic nature of these systems (volumetrics, stacking patterns, etc.) and for the potential application of this data to modeling the depositional characteristics of such systems for basin analysis and reservoir modeling. A quantitative methodology is presented for defining the geometry of architectural elements within sedimentary systems. This methodology is then applied to a detailed analysis of sand-rich, deep-water systems, with examples from the late Precambrian Kongsfjord Formation, Arctic Norway, and other published outcrop and subsurface data. The efficacy of the scheme is demonstrated by its ability to discriminate effectively between reservoir architectural elements of differing scales within environments (e.g., individual beds, packets of beds) and also between environments (e.g., abyssal plain, outer-fan lobe, and submarine channel deposits). This scheme permits a quantitative, more objective, means of comparing modern and ancient, including subsurface, depositional systems. The methodology and analysis presented here provide important geometrical and geological information on architectural elements at the subseismic, typical interwell scale. The data produced by this methodology should prove useful for reservoir production and development techniques. This methodology also should have applicability in exploration, through its application to architectural elements at the fan and basin scale. In exploration and production (Begin page 1732) in deep-water clastic systems, the available data, which are commonly extremely expensive to collect, typically consist of seismic data (with high aerial but relatively poor vertical resolution) and generally few wells (with high vertical resolution but extremely low aerial resolution). Therefore, information derived from suitable outcrop analogs on the geometry of fan elements augments significantly the typically sparse available industry data from exploration and production. In reservoir development and production, standard industry disciplines such as reservoir engineering and reservoir modeling (both deterministic and stochastic) should benefit from the approach taken in this article. This is because this article provides additional information on internal reservoir heterogeneity and architecture that is directly applicable to these disciplines. The application of standard geostatistical techniques to assess the spatial continuity of individual elements (such as variogram analysis) has the potential to further extend the results derived from this methodology and analysis.

Deep Water Deposits of the Tanqua and Laingsburg Subbasins, Southwest Karoo Basin, South Africa: Analog for the Gulf of Mexico: ABSTRACT

ABSTRACT The Tanqua and Laingsburg subbasins in South Africa had near-contemporaneous formation and filling and contain Permian-age basin-floor and slope fans that display characteristics similar to deposits in the northern Gulf of Mexico. Outcrop area for each subbasin is about 650 km2 and individual fans range from 150 to 450 km2 with lateral continuity of individual fans up to 34 km. With a sandstone/shale ratio of 75-90%, the arenaceous fans vary in thickness from 20 to 60 m with interlying shales ranging from 20 to 75m in thickness. Both subbasins were influenced in their formation and in the architecture of their deposits by structures and events associated with the Cape Fold Belt. Having undergone little tectonic tilting, the highly continuous outcrops in the Tanqua subbasin expose two fans continuing from mid-fan channel complexes with associated levee-overbank deposits to sheet-like outer fan lobes, while three other fans expose outer fan depositional lobes. Paleocurrent directions for four of the fans are from SSW to S and a direction of W to WNW for the fifth. These fans most likely had a,single point source which migrated over the time of basin fill. Unrestricted deposition suggests an open basin depositional setting. The Laingsburg subbasin was strongly influenced by the tectonism associated with the Cape Fold Belt. Deposition occurred in a deeper and narrower basin and the deposits, except for the overlying deltaics cannot be correlated with those of the Tanqua subbasin. The two subbasins, while associated with an active margin, were likely filled at slightly different times. Both had a distant source area which led to deposits exhibiting characteristics of a passive margin depositional environment. Understanding the evolution of the subbasins and the tectonic conditions under which the submarine fans were deposited leads to the determination of the mechanisms that influenced the formation of the fans and their resulting architecture. These fans permit detailed studies on their architecture necessary to 1) increase our understanding of fine-grained, low sandstone/shale ratio fans, 2) determine influences of paleostructures and tectonics on basin fill, 3) carry out detailed reservoir simulation programs, and 4) make predictive models of deep-water sands in the northern Gulf of Mexico. End_of_Record - Last_Page 646-------

Stratigraphy and facies analysis of the Caqueza Group, a sequence of Lower Cretaceous turbidites in the Cordillera Oriental of the Colombian Andes

The thickest and most widely exposed Lower Cretaceous lithostratigraphic unit in the Cordillera Oriental of the Columbian Andes is the Caqueza Group. Along the eastern margin of the Bogota trough, the group is formally subdivided into the Buenavista, Macanal, and Alto de Caqueza Formations. On the basis of ammonite assemblages, the age of the group is late Tithonian-Hauterivian. Six main lithofacies and two subfacies are recognized, based on six detailed cross sections. These form five submarine fan facies associations (inner fan conglomerate, middle fan braided channel, middle fan-mouth channel, outer fan lobe sandstone-mudstone, and outer fan mudstone-siltstone/sandstone). The progression of the fan facies associations shows two main cycles of retrogradation and progradation in the turbidite basin. The Lower Cretaceous turbidites of the Caqueza Group were deposited in different submarine fan environments. The breccia-conglomerates and conglomerates of the Lower Member of the Buenavista Formation were deposited in inner fan environments. During the earliest Cretaceous, basin subsidence increased more rapidly than sediment supply, and channelized massive and pebbly sandstones of the Upper Member of the Buenavista Formation were deposited in braided middle fan environments that developed only in more proximal parts of the area. As a result of retrogradation of the fan system, middle fan-mouth channel deposits were covered by distal outer fan sediments. After the Berriasian, the sediment supply increased more rapidly than basin subsidence, resulting in deposition of an outer fan/outer fan lobe/braided middle fan progradational sequence.

Upper Pleistocene turbidite sand beds and chaotic silt beds in the channelized, distal, outer-fan lobes of the Mississippi fan

Cores from a Mississippi outer-fan depositional lobe demonstrate that sublobes at the distal edge contain a complex local network of channelized-turbidite beds of graded sand and debris-flow beds of chaotic silt. Off-lobe basin plains lack siliciclastic coarse-grained beds. The basin-plain mud facies exhibit low acoustic backscatter on SeaMARC IA sidescan sonar images, whereas high acoustic backscatter characteristic of the lobe sand and silt facies. The depth of the first sand-silt layer correlates with relative backscatter intensity and stratigraphic age of the distal sublobes (i.e., shallowest sand = highest backscatter and youngest sublobe). The high proportion (>50%) of chaotic silt compared to graded sand in the distal, outer-fan sublobes may be related to the unstable, muddy, canyon-wall source areas of the extensive Mississippi delta-fed basin slope. A predominace of chaotic silt in cores or outcrops from outer-fan lobes thus may predict similar settings for ancient fans.

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

Architectural Elements of Fan-Delta Complex in Pennsylvanian Taos Trough, New Mexico: ABSTRACT

Identification of architectural elements within alluvial-fan and subaqueous fan-delta gravel units is fundamental to resolving depositional processes within fan-delta complexes of the Pennsylvanian Taos trough, New Mexico. Subaqueous fan-delta deposits consist of lenticular gravel-body complexes encased by black, basinal shales. Gravel-body complexes are composed of a series of stacked gravel lenses, each of which is enveloped by fifth-order bounding surfaces. The central portion of individual gravel lenses contains a channel complex. Channels are outlined by third- and fourth-order bounding surfaces and are infilled by high-density gravity flow deposits. The fringe of submarine gravel lenses consists of stacked, laterally continuous Bouma sequences separated by second-order bounding surfaces. Bouma sequences were deposited by dilute turbidity flows during evacuation of submarine channels. Subaqueous channel complexes within gravel lenses represent midfan channels, whereas the fringe of lenticular gravel lenses represent outer-fan lobes. Recognition of depositional processes and architectural elements of fan deltas in the Sandia Formation enables distinction between these and other types of coarse-grained deltas in the Taos trough. This, in turn, has implications for resolving evolution of the trough.

Depositional Environments and Controls of Juncal Formation, Southern San Rafael Mountains, California: ABSTRACT

The lower to middle Eocene Juncal Formation, north of the Santa Ynez fault in easternmost Santa Barbara County, represents the onset of marine sedimentation following an approximately 20-m.y. hiatus. The Juncal paraconformably overlies the upper Campanian Unnamed sandstone and is gradationally overlain by the middle Eocene Matilija Formation. Access to the Juncal exposures of this area was considerably enhanced by the Wheeler fire of July 1985. Facies analysis of both the vertical and lateral exposures of the Juncal Formation (approx. 1400 m thick) indicate that the Juncal represents coalescing outer-fan depositional lobes. This interpretation is based on the lateral continuity of sandstone beds, presence of thickening-upward cycles, and high-concentration sediment gravity flows, bathyal fauna, and regional associations. Local channelization (to 10 m deep) represents the extension of a mid-fan channel over its associated depositional lobe. Outcrops are subparallel to depositional strike in the eastern part of the study area and form a broad syncline in the western part. Together, these exposures allow documentation of the depositional lobes vertically, laterally, and longitudinally. Outer-fan deposits of the Juncal Formation are part of a progradational basin-filling episode. The Juncal grades upward into the outer- to mid-fan depositions of the lower Matilija Formation. Themore » upper Matilija Formation shoals upward into deltaic facies. This regressive sedimentary sequence was probably initiated by the major sea level fall which occurred near the early/middle Eocene boundary and coincides with the onset of fan sedimentation elsewhere along the California margin. Within the study area, depositional lobe activity was probably governed by either minor fluctuations in relative sea level or channel switching up-system.« less

Carbonate Submarine Fans and Aprons in Frontier Petroleum Region--Carboniferous Calico Bluff Formation, East-Central Alaska: ABSTRACT

The Calico Bluff Formation (CBF) is a spectacularly exposed 500-m thick sequence of carbonate sediment-gravity-flow deposits and interbedded fetid, organically rich lime muds and siliceous shales. CBF facies record an overall seaward-prograding, shoaling-upward section from noncyclic turbidites deposited in basin plain settings to numerous thickening- and coarsening-upward cycles of turbidites and debris flows representing outer-fan lobes or base-of-slope aprons. These sediment-gravity-flow deposits contain abundant crinoid and bryozoan fragments and lesser amounts of calcareous green algae, other biotic components, and sand-sized detrital quartz. They interpret the flows were derived from a shallow water platform rimmed with bryozoan and crinoid meadows. The presence of detrital quartz in the mass-flow deposits indicates that this platform was probably attached to a continental landmass and was not an isolated offshore carbonate platform. Paleocurrent flow directions, in present-day coordinates, flow toward the south-southeast, toward the craton. These anomalous flow directions may be explained by block rotation or by derivation from a nearby, yet-to-be-discovered platform margin in Lisburne Group carbonates to the north and northwest. The organic content of the CBF lime muds and siliceous shales averages 5 wt. %. TAI and T/sub max/ data indicate the organic matter is well into oil generation but not overmature,more » and S/sub 2//S/sub 3/ ratio suggests that the CBF contains mixed gas/oil prone source rocks. These source-rock data, in conjunction with possible carbonate apron, fan, and platform margin reservoir facies, indicate the CBF is a new potential exploration target in this frontier area of Alaska.« less

Quartzose turbidites of the accretionary complex of Barbados, II: Variations in bedding styles, facies and sequences

Barbados provides the only subaerial exposures of the Barbados Ridge, an accretionary complex lying outboard of the Lesser Antilles volcanic arc. Within nine measured sections of quartzose turbidites, five major types of sequences are recognized: 1. (1) Progradational sequences (basin plain-outer fan) of uniformly fine-grained, flat-based, base-absent turbidites that thicken upward and gradually become coarser-grained and base-present at the top of the sequence. 2. (2) A single rejuvenation (rapid progradational) sequence was identified as an abrupt progradational sequence above abandoned-lobe deposits, and is not characterized by stacked, well-developed, upward-thickening successions as many progradational sequences are. 3. (3) Channelized successions of coarse-grained, base-present turbidites arranged in packets 5–20 or 50–140 m thick which are midfan and inner-fan deposits, respectively. The distinction between predominance of base-present and base-absent turbidites seems to correspond with the interpreted change from midfan channelized to outer-fan sheet flow deposition. 4. (4) One example of a major retrogradational sequence, of midfan deposits above inner-fan (?) sandstones, has been found on Barbados. Minor sequences of facies retrogradation are relatively common in the outer-fan environment where upward-thinning sandstone beds overlie upward-thickening units to define symmetric sequences. 5. (5) Abandonment successions are: major, as indicated by massive accumulations of mudstone; minor, as indicated by abrupt reversals of upward-thickening trends in outer-fan lobe deposits; and cryptic where indicated by thick (> 4 m) bioturbated zones of massive sandstone.

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------------

Facies Analysis of Upper Jackfork Formation (Pennsylvanian), DeGray Dam, Arkansas: ABSTRACT

The DeGray Dam spillway cut displays perhaps the best exposed section of the upper Jackfork formation (Pennsylvanian) in the Ouachita Mountains of Arkansas and Oklahoma. Comprising more than 321 m (1,050 ft) of interbedded sandstone and shale, this deep-water succession was originally interpreted as a sequence of alternating proximal and distal turbidites, and subsequently as outer-fan depositional lobes. Recent detailed facies analysis, however, demonstrates that the succession represents a mid-fan association of channel and interchannel deposits. Channelized intervals consist of stacked thinning-upward and/or amalgamated packages characterized by Mutti and Ricci Lucchi facies A, B, C, and G. Associated interchannel intervals consist of facies C, D, E, and G beds that are randomly interbedded or form thickening-upward packages that superficially resemble depositional lobes. These lobe-like packages, which are generally less than 4 m (13 ft) thick, are interpreted as crevasse-splay deposits. A similar association of channel and interchannel deposits can also be observed in nearby outcrops of the underlying lower Jackfork. A longitudinal submarine fan system, analogous to the present-day Bengal fan, is visualized as the overall depositional setting for the Jackfork formation. End_of_Article - Last_Page 509------------

Slope and Deep-Sea Fan Facies and Paleogeography of Upper Cretaceous Chatsworth Formation, Simi Hills, California

The Upper Cretaceous Chatsworth Formation is a sand-rich deep-sea fan deposit consisting of thick channelized sequences and interchannel deposits typical of middle fan facies. It has poorly developed outer fan lobes and slope facies. Petrographic and x-ray diffraction (XRD) data indicate the arkosic deposits of the Chatsworth Formation were derived from an uplifted granitic terrane. Paleocurrent studies indicate northward sediment transport and an east-trending slope. Regional comparisons of the Simi Hills with the Santa Monica and Santa Ana Mountains suggest these terranes were once contiguous and covered part of a deep-sea fan system that comprised much of the southern California area. Subsequently, these terranes have been offset along the Malibu-Cucamonga and Elsinore faults, and rotated in response to the opening of the Los Angeles basin and to other adjustments in the tectonic framework of the area. The Chatsworth Formation is thought to be part of a forearc-basin deposit similar to the Great Valley sequence and not part of the Salinian borderland block. The original geographic position of the forearc basin was to the south of the Sierra Nevada Mountains and the Great Valley sequence.

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.

Sedimentology of Some Allochthonous Deep-Water Carbonate Reservoirs, Lower Permian, West Texas: Carbonate Debris Sheets, Aprons, or Submarine Fans?: ABSTRACT

During the Wolfcampian, sediment gravity flows were common events at some shelf margins in the Permian basin. These mass flows transported large volumes of shoal-water bank and reef carbonates downslope into the Midland and Delaware basins, forming a wide variety of redeposited lithofacies. For example, along a segment of the Eastern shelf margin at least 40 km (25 mi) long, redeposited carbonates extend into the Midland basin 25 km (16 mi) or more. Within this basin margin setting, several petroleum pay zones occur in mass-transported debris. In designing exploration strategies for these types of frontier deep-water reservoirs, whether within the Permian basin or elsewhere, one must develop appropriate depositional models. Some questions come to mind. Do these deposits represent episodic, widespread, single-pulse debris sheets, debris aprons dominated by numerous but rather random pulses of areally extensive sheet-flow calcarenites, or more systematically developed submarine fan facies having both channelized deposits in inner and mid-fan settings as well as sheet-flow calcarenites deposited as outer-fan lobes? Redeposited Wolfcampian carbonates are subdivided into three major lithofacies. (1) Limestone and dolomite conglomerate debris flows and turbidites with dark interstitial micrite. Individual beds are as much as 8 m (26 ft) thick, normal to massively graded, and some beds are arranged by thinning-upward sequences. These carbonates from on of the reservoir facies with intercrystalline, solution interparticle, fracture, and vuggy porosity. (2) Wackestone to packstone calcarenite turbidites consisting largely of biotic grains. This lithofacies forms the most abundant type of redeposited sediment. The calcarenites occur in beds a few cm to 2.5 m (8 ft) thick that exhibit a variety of Bouma turbidite divisions and in some localities are arranged in thickening-upward units. Calcarenite turbi ite locally form petroleum reservoirs with solution interparticle, intrabiotic, biomoldic, and fracture porosity. (3) Wackestone to packstone calcisiltite and calcarenite turbidites that occur in less than 2 cm (1 in.) thick beds. This facies does not exhibit vertical cycles of bed thickness nor good reservoir qualities. Analyses of cores from 12 wells both within and outside the petroleum fields suggest that these redeposited carbonates may represent a combination of debris sheet and submarine fan depositional processes. The conglomerates could be genetically unrelated to the calcarenites and represent episodic debris sheet pulses; or alternatively, these conglomerates may be channelized deposits in inner fan to mid-fan positions near the basin margin. Some of the thick-bedded calcarenites possibly represent mid-fan channelized deposits whereas the more basinward thickening-upward calcarenites resemble unchannelized outer-fan calcarenite lobes. Thin-bedded calcisiltite turbidites appear to occupy basin plain, outer-fan fringe, and interchannel settings. If these reservoirs are developed within one or more fan facies, the size and spatial arrangement of the individual fans still remain to be determined. A better knowledge of appropriate depositional models should enhance future exploration efficiency. End_of_Article - Last_Page 442------------

Alternating Carbonate and Siliciclastic Deep-Water Facies in Tectonic Evolution of Northwest Palawan Margin (Philippines), South China Sea: ABSTRACT

Shifting Mesozoic and Cenozoic tectonic events controlled the deposition of thick alternating sequences of deep-water carbonate and siliciclastic sediments on the northwest margin of the North Palawan crustal block. Highly deformed Jurassic to Lower Cretaceous rocks at the base of the section may have originated in a fore-arc region along the South China margin. This convergent margin shifted into a broad extensional region in the Late Cretaceous or early Paleogene. The Late Cretaceous through the early Eocene is generally a time of hiatus in the rock record, but rocks of this age may occur in half grabens under the northwest Palawan slope. Late Eocene to mid-Oligocene dolomites and limestones were deposited in restricted to open marine environments as the rifted Mesozoic terrain subsided. During sea-floor spreading from the mid-Oligocene to early Miocene in what is now the central South China Sea, extensive carbonate deposition of reefs, platform lagoons, and deep-water sediments draped the trailing (northwest) edge of the southward-drifting North Palawan block. Over 1 km (3,300 ft) of diverse deep-water carbonate facies was deposited in an upper slope to basin setting. Turbidites were derived from reefal sources dominated by benthic forams, coral, and coralline algae. Mudstones formed by off-bank transport and settling of platform lagoon muds differ from normal pelagic deposits rich in planktonic forams. Deep-water car onate units show a continual up-section decrease in abundance of reef- and lagoon-derived sediment and are abruptly overlain by deep-water siliciclastics. Seismic profiles indicate late early Miocene tilting and partial emergence of the North Palawan block, during which clastic sediments prograded around and over relict carbonate platforms and deposited thick deep-water sequences on the northwest Palawan slope. These siliciclastics were deposited in submarine fan complexes as sand-rich middle to inner fan channels, outer fan lobes, and pelitic interchannel and interlobe deposits. After crossing the submerged carbonate terrain, turbidites were axially confined to a northeast-trending trough and formed a stratigraphic wedge several hundred meters thick against the slope of the relict platforms. Eroded shallow-water carbonate lithoclasts were commonly incorporated within siliciclastic turbidites. Turbidite sandstones are texturally submatu e feldspathic litharenites and subarkoses, and indicate a source terrain of quartzo-feldspathic sediments and metasediments, chert, volcanics, and acid-intermediate plutonic rocks. Petrologic studies thus support seismic and dip-meter interpretations that these sediments were derived from emergent pre-Tertiary rocks of the North Palawan crustal block. Regional uplift in the middle Miocene was followed by mid to upper miocene subsidence, producing additional siliciclastic wedges on the northwest margin. The last regional uplift event, latest (?) Miocene, was characterized by wrench and reverse faulting. Miocene tectonism may have resulted from collisions of the North Palawan block with now adjacent terrains in the South Palawan, Mindoro-Panay, and North Sulu Sea regions. The northwest Palawan margin has been tectonically quiescent since the early Pliocene, marked again by carbonate deposition of reefs and flanking deep-water deposits. End_of_Article - Last_Page 503------------

Hydrocarbon-Bearing Submarine Fan System of Cellino Formation, Central Italy

The Cellino Formation is a lower Pliocene submarine fan system that forms extensive subsurface reservoir rocks. The sequence consists of interbedded sandstones and shales which crop out in central Italy; it exhibits a definite geometric pattern of lateral fan accretion. It is 60 by 20 km (37 by 12 mi), elongate NNW-SSE, and it follows the trend of previous fan systems. The vertical and lateral sequences are interpreted from electric logs and supported by field observations. The Cellino Formation consists of a series of different turbidite facies associations which includes a lower basin-plain fan edge, overlain by outer fan and fringe facies associations. The lower part of the Cellino Formation here has been cut and deeply eroded in the northern part. A new sequence follo s, which activates the eastern migration of the fan system and deposits facies. Postdepositional deformation (compressional faulting and folding) decreases in the area in intensity eastward. Carbonate cements sandstone layers in the southwest part of the area. Associated hydrocarbons were trapped in outer fan lobes which were deformed into an anticline. The thin shale beds provide the cap rocks of the turbidite reservoirs. Many of the depositional lobes have their individual gas-water contacts.

Sedimentology and Reservoir Potential of Matilija Sandstone: An Eocene Sand-Rich Deep-Sea Fan and Shallow-Marine Complex, California

The Matilija Sandstone of the Santa Ynez Mountains of southern California records a major regressive event in the Eocene Santa Ynez basin in which turbidites were deposited and subsequently covered by shallow-marine deposits. Despite thick, favorable source beds and generally good initial reservoir characteristics, the Matilija Sandstone is not a productive unit in the basin. Low reservoir permeability (generally less than 1 md) and porosity (1 to 10%) are interpreted to be the result of early compaction, cementation, and diagenesis. The lower part of the Matilija Sandstone is a 1,300 ft (400 m) thick sand-rich deep-sea fan complex, which overlies basin plain turbidite deposits of the Juncal Formation. The Matilija Sandstone consists of anastomosing outer fan lobes, lobe fringe, and interlobe deposits overlain by channelized middle and inner fan deposits. The upper Matilija Sandstone consists of shallow-marine cross-bedded sandstone, red beds, and carbonate-evaporite-lignite sequences which overlie the turbidites. Deposition of the Matilija Sandstone was terminated by a rapid basin deepening event which culminated in deposition of basin plain, slope, and turbidite deposits of the Cozy Dell Formation. The lower Matilija Sandstone deep-sea fan deposits have a high sandstone to shale ratio (4:1 to 10:1) and consist of submature arkoses (Folk, 1980). The depositional lobes consist of facies B, C, and D turbidites and a channelized sequence of facies A and B turbidites. The average sandstone is medium grained, moderately sorted, subangular to subrounded, and massive, containing 40% quartz, 35% feldspar (about equal amounts of plagioclase and potassium feldspars), 10% lithic fragments (mostly volcanic and metamorphic types), and minor amounts of detrital clay (illite), mica, chert, and heavy minerals. Early compaction, carbonate cementation, and authigenic pore-lining and pore-filling chlorite, albite, potassium feldspar, quartz, and localized laumontite have reduced the initial porosit and permeability. The only remaining porosity is secondary, resulting from the dissolution of detrital feldspars, and minor fracture porosity.

Paleogeography and Sedimentology of Upper Cretaceous Turbidites, San Diego, California

Upper Cretaceous (Campanian and Maestrichtian) marine strata of the Rosario Group in the San Diego area include the Point Loma Formation and overlying Cabrillo Formation. These units contain six facies associations: (1) shelf and lagoonal sandstone, (2) slope and basin-plain(?) mudstone, (3) outer-fan lobe sandstone, (4) middle-fan channel-fill sandstone, (5) middle-and inner-fan interchannel and channel-margin thin-bedded turbidites and mudstone, and (6) inner-fan channel-fill conglomerate and sandstone. The facies associations define a deep-sea fan deposited by westward-flowing sediment gravity flows that transported sediments derived chiefly from batholithic and prebatholithic metamorphic rocks of the Peninsular Ranges. The sedimentary basin initially deepened abruptly, partly aided by eustatic sea-level rise. The fan then prograded westward into the basin, with a retrogradational phase recorded in the uppermost part of the sequence, which is erosionally truncated by transgressive lower Eocene conglomerate. The fan was deposited along the eastern edge of a forearc basin similar to that of the Great Valley sequence in northern California. The western part of the fan, which probably contained mostly outer-fan lobe and associated basin-plain deposits, appears to have been truncated by late Cenozoic strike-slip faulting associated with the San Andreas fault system. The fan and remnants of the western part of the basin and associated subduction complex may be present on the northwest in the Channel Islands region or still farther north.

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.