Lobe Facies Research Articles

Ichnofossils Characteristics in Pelagic Siliciclastic Carbonate Turbidites of Weda Formation, Halmahera Island

The Weda Formation in Lili River, Dorosagu, East Halmahera consists of siliciclastic and pelagic carbonates deposited in a submarine fan environment. Research on the Weda Formation in the northern part of Halmahera Island is relatively minimal because most of the research about this formation has been carried out in the southern part of Halmahera Island. The lack of road access and its remote location have resulted in the lack of intensive research on this formation. This study aims to determine the relationship between the intensity and diversity of trace fossils in the submarine fan facies association of the Weda Formation. Sedimentological studies include sedimentary texture, sedimentary structure, turbidite material type, and layer thickness. While the study of trace fossils includes the identification of trace fossil types, classification of behavior, intensity, and diversity in sedimentary layers, and the factors that influence them during deposition. There are eight types of ichnofossil observed in the Lili River, Ophiomorpha, Thalassinoides, Rhizocorallium, Palaeophycus, Zoophycos, Chondrites, Lorenzinia, and Spirorhaphe, which are found in three facies associations (AF): submarine fan channel facies association (AF1), submarine fan lobe facies association (AF2), and submarine fan distal facies association (FA3). The submarine fan channel facies is characterized by the presence of Ophiomorpha and Thalassinoides which present in low intensity and low diversity due to unstable environmental conditions-high stress due to intensive turbidite currents. The submarine fan lobe facies shows a higher density and diversity where ichnofossil is classified into two based on the substrate in the flysch deposits. Rhizocorallium, Thalassinoides, and Palaeophycus ichnofossil were formed in the sand substrate, while Zoophycos and Chondrites ichnofossil were formed in the shale substrate. Palaeophycus, Zoophycos, Chondrites, Lorenzinia, and Spirorhaphe ichnofossil were found in the submarine fan distal facies associated with high intensity indicating low energy levels and low sedimentation rates.

Submarine lobe deposits of the Point Loma Formation, California: Quantifying event‐bed architecture and lateral heterogeneity

AbstractOver the last several years, numerous outcrop localities have been revisited to add quantitative detail to submarine lobe facies models that previously focussed on facies relationships in a qualitative sense. This study utilises well‐exposed submarine lobe deposits of the Point Loma Formation in Cabrillo National Monument (San Diego, CA) to provide quantitative and statistical insights into the lateral variability of event‐bed (i.e. turbidite) architecture within and between lobe elements. Within a lobe element (defined as a surface‐bounded, genetically related package), event beds compensationally stack, thinning over subtle sea floor topography created by the previous event bed. Between lobe elements, larger‐scale compensation is observed, with clearly defined stratal surfaces and facies architecture distinguishing the four lobe elements. A lateral facies transition is observed for one lobe element, where sandstone beds pinch out and the element thickness halves over a distance of 100 m. However, architectural parameters of event beds (e.g. bed thickness, thinning rate, fining rate) are not appreciably different between these elements, suggesting that the observed stratal architecture does not readily translate into vertical bed thickness (i.e. stacking) patterns that could be easily recognised in common subsurface data types like borehole‐derived core. While the data derived from this outcrop study are valuable for improving the construction of realistic geological and reservoir models, caution is necessary when interpreting lobe element boundaries from borehole data. The lobe deposits measured in this study have event‐bed thicknesses and thinning rates most similar to semi‐confined proximal lobes, suggesting a more proximal position and more confined than previously interpreted. Based on the relationships between sandstone and mudstone thicknesses and thinning rates, bed and lobe‐element compensation and minimal evidence of erosion, the Point Loma Formation at Cabrillo National Monument is reinterpreted as a medial lobe environment with some degree of lateral and/or frontal confinement.

Facies Heterogeneity and Lobe Facies Multiscale Analysis of Deep-Marine Sand-Shale Complexity in the West Crocker Formation of Sabah Basin, NW Borneo

Deepwater lobes constitute a significant volume of submarine fans and are primarily believed to exhibit a simple sheet geometry. However, recent studies interpret the geometries of these deep-marine lobes as distinct with respect to the complexity of the facies and their distribution. Hence, a conceptual model of deep-marine sediments is essential to discuss the deep-marine sediments associated with the fan and lobe architecture. The present study highlights the facies heterogeneity and distribution of various lobe elements at a multiscale level by considering a case study of the West Crocker Formation of Sabah in northwest Borneo. The formation was logged on a bed-to-bed scale from recently well-exposed sections, with a total vertical thickness of more than 300 m. The lithological characteristics, bed geometry, sedimentary textures and structures of individual beds were used to categorize the rock units into nine sedimentary lithofacies: five sandstone lithofacies (S1–S5), one hybrid bed facies (H), two siltstone facies (Si1 and Si2) and one shale or mudstone facies (M). These facies were grouped into four facies associations (FA1–FA4), which were interpreted as lobe axis (FA1), lobe off-axis (FA2), lobe fringe (FA3) and distal fringe to interlobe (FA4) facies associations. This study is applicable for the distribution of lobes and their subseismic, multiscale complexities to characterize the potential of hydrocarbon intervals in deep-marine sand-shale system around the globe.

Intra-oceanic submarine arc evolution recorded in an ~1-km-thick rear-arc succession of distal volcaniclastic lobe deposits

Abstract International Ocean Discovery Program (IODP) Expedition 351 drilled a rear-arc sedimentary succession ~50 km west of the Kyushu-Palau Ridge, an arc remnant formed by rifting during formation of the Shikoku Basin and the Izu-Bonin-Mariana arc. The ~1-km-thick Eocene to Oligocene deep-marine volcaniclastic succession recovered at Site U1438 provides a unique opportunity to study a nearly complete record of intra-oceanic arc development, from a rear-arc perspective on crust created during subduction initiation rather than supra-subduction seafloor spreading. Detailed facies analysis and definition of depositional units allow for broader stratigraphic analysis and definition of lobe elements. Patterns in gravity-flow deposit types and subunits appear to define a series of stacked lobe systems that accumulated in a rear-arc basin. The lobe subdivisions, in many cases, are a combination of a turbidite-dominated subunit and an overlying debris-flow subunit. Debris flow–rich lobe-channel sequences are grouped into four, 1.6–2 m.y. episodes, each roughly the age range of an arc volcano. Three of the episodes contain overlapping lobe facies that may have resulted from minor channel switching or input from a different source. The progressive up-section coarsening of episodes and the increasing channel-facies thicknesses within each episode suggest progressively prograding facies from a maturing magmatic arc. Submarine geomorphology of the modern Mariana arc and West Mariana Ridge provide present-day examples that can be used to interpret the morphology and evolution of the channel (or channels) that fed sediment to Site U1438, forming the sequences interpreted as depositional lobes. The abrupt change from very thick and massive debris flows to fine-grained turbidites at the unit III to unit II boundary reflects arc rifting and progressive waning of turbidity current and ash inputs. This interpretation is consistent with the geochemical record from melt inclusions and detrital zircons. Thus, Site U1438 provides a unique record of the life span of an intra-oceanic arc, from inception through maturation to its demise by intra-arc rifting and stranding of the remnant arc ridge.

Bottomset and foreset sedimentary processes in the mixed carbonate-siliciclastic Upper Jurassic-Lower Cretaceous Vaca Muerta Formation, Picún Leufú Area, Argentina

The Upper Jurassic-Lower Cretaceous Vaca Muerta Formation at the Picún Leufú area constitutes bottomset and foreset marine deposits comprising open bay, siliciclastic basin and mixed carbonate-siliciclastic shelf facies. Detailed sedimentological, ichnological and sequence stratigraphic analysis of six stratigraphic sections allows establishing two depositional sequences. Depositional Sequence 1 (DS1) begins with lowstand eolian deposits included in the Quebrada del Sapo Formation, and continues with thin, retrograding, transgressive open bay facies of the Vaca Muerta Formation encompassing coastal sand sheet and dune, marginal marine, bay margin and distal bay facies. Above the transgressive deposits, the regressive hemicycle of DS1 consists of bottomset deposits representing a siliciclastic basin facies association. Lobe and lobe fringe hyperpycnal flow sedimentation alternated with hemipelagic basinal deposition in an oxygen-deficient environment. A relative sea-level fall generated a sequence boundary, which coincides with an angular unconformity that marks the base of Depositional Sequence 2 (DS2). This sea-level fall triggered the formation of an extensive lowstand channel-fill and lobe complex at the base of the slope, followed by retrogradational lobe facies during the subsequent transgression. The regressive hemicycle of DS2 represents foreset mixed carbonate-siliciclastic shelf facies, forming a slope mud belt and slope sand bodies. Near the top, sedimentation in a foreset-topset transition resulted in the accumulation of bioturbated mixed slope and sandy shoal deposits. Sand bars and lagoonal facies occur on top of all sections and are truncated by a sequence boundary. The combined analysis provides insights into the sedimentary processes affecting bottomset and foreset deposition, underscoring the role of wave-influenced hyperpycnal flows, and the effect on organic matter dilution in one of the most important unconventional reservoirs from Argentina.

Dispersal pathways in the early Messinian Adriatic foreland and provenance of the Laga Formation (Central Apennines, Italy)

The early Messinian Laga Formation represents a turbidite complex deposited in the Late Neogene foreland basin system of the growing Apenninic chain. While the stratigraphy and physiography of the Laga Basin are well known, the source of its sediments is contentiously claimed to be either recycled Apenninic or primary Alpine. Furthermore, a shift in paleocurrent was proposed as a marker of provenance change around 6Ma. By combining double-dating of detrital zircons (fission-track and U-Pb dating) with compositional analyses, the sedimentary provenance of the lower Laga arenites and differences between the proximal channelized and distal lobe facies are addressed. Due to sediment sorting processes, the lobe facies shows a reduced heavy mineral spectrum relative to the channelized facies. Hence, proximal deposits reflect their hinterland lithologies better than their distal counterpart and should be preferred in provenance analyses. The petrographic composition of the Laga units implies a major metamorphic source combined with an additional dolomite and carbonate source. No compositional difference spanning the shift in paleocurrents is observed, which therefore likely reflects the evolving topography of the foreland due to syn-sedimentary tectonics. Detrital zircon fission-track data reveal youngest age populations at ~16–17Ma and lag times in the range of 9 to 11Ma that can be related to modern fission-track ages observed in the Central Alps. The two major 238U/206Pb age populations, centered at 277.5 and 37.5Ma, represent (post-)Variscan events and the Paleogene magmatic activity in the Central Alps, specifically the Adamello complex. The Central and Southern Alps are thus inferred as the major source for the early Messinian Laga arenites. The pathways of the sediments from the Alps to the Laga Basin crossed the Alps-Apennines foreland and passed on the outer Apenninic wedge-top along elongated and tectonically controlled basins and channels that entered the basin from the north and northwest. The transfer could have been direct along the Apenninic depozones. Additionally, late Tortonian sediments from the Alps could have been temporarily stored on top of the Apenninic wedge, e.g. in the Marnoso-arenacea Basin, and then seamlessly cannibalized into the Laga Basin.

High-resolution, millennial-scale patterns of bed compensation on a sand-rich intraslope submarine fan, western Niger Delta slope

Near-seafloor core and seismic-reflection data from the western Niger Delta continental slope document the facies, architecture, and evolution of submarine channel and intraslope submarine fan deposits. The submarine channel enters an 8 km long x 8 km wide intraslope basin, where more than 100 m of deposits form an intraslope submarine fan. Lobe deposits in the intraslope submarine fan show no significant downslope trend in sand presence or grain size, indicating that flows were bypassing sediment through the basin. This unique dataset indicates that intraslope lobe deposits may have more sand-rich facies near lobe edges than predicted by traditional lobe facies models, and that thickness patterns in intraslope submarine fans do not necessarily correlate with sand presence and/or quality. Core and radiocarbon age data indicate that sand beds progressively stack southward during the late Pleistocene, resulting in the compensation of at least two lobe elements. The youngest lobe element is well characterized by core data and is sand-rich, ~ 2 km wide x 6 km long, > 1 m thick, and was deposited rapidly over ca. 4,000 yr, from 18-14 ka. Sand beds forming an earlier lobe element were deposited on the northern part of the fan from ca. 25 to 18 ka. Seafloor geomorphology and amplitudes from seismic reflection data confirm the location and age of these two compensating lobe elements. A third compensation event would have shifted sand deposition back to the northern part of the fan, but sediment supply was interrupted by rapid sea level rise during Meltwater Pulse 1-A at ca. 14 ka, resulting in abandonment of the depositional system.

Proposta de novas unidades litoestratigráficas para o Devoniano da Bacia do Rio do Peixe, Nordeste do Brasil

Subsurface stratigraphic analysis of Devonian strata in the Rio do Peixe Basin, newly recognized by palynological studies, has resulted in the identification of two new formations assembled in the newly established Santa Helena Group. These units were characterized using cores, sidewall and cuttings samples, conventional well logs, image log and 3D seismic data. The Piloes Formation, the lower unit, is composed mainly of dark mudstones and medium-to-very fine-grained sandstones, with minor conglomerates and breccias, and the Triunfo Formation, the upper unit, comprises whitish grey, kaolinitic, coarse-grained to conglomeratic, cross stratified sandstones and conglomerates, with interbedded mudstones and fine-grained sandstones. The Piloes Formation is represented by prodeltaic lagoonal-lacustrine facies, with lesser associated subaqueous talus, debrite and sandy turbidite lobe facies, part of fandelta and delta systems. The Triunfo Formation is interpreted as braided fluviodeltaic facies. The Santa Helena Group corresponds to the Lower Devonian tectono-sequence deposited in a NW-SE-trending graben during a transgressive-regressive cycle. With 343 m of thickness (isochore) in well 1-PIL-1-PB (Piloes 1), this sequence has a non-conformity at the lower boundary and its upper boundary is an angular unconformity that represents a hiatus of about 265 Myr, at the base of the Lower Cretaceous tectono-sequence (Rio do Peixe Group). Ignimbrites and co-ignimbrite breccias (Poco da Jurema volcanic breccia) were recognized at the northern margin of the Sousa half-graben. There is evidence that the pyroclastic event is coeval with the Lower Devonian graben filling. The results of this study indicate a polyhistorical tectono-volcano-sedimentary evolution of the basin. This lithostratigraphic update brings new perspectives for geological research in the Rio do Peixe Basin, as well as in other inland basins of the Northeast of Brazil.

Depositional environments and ichnology of the deep-marine succession of the Amiran Formation (upper Maastrichtian–Paleocene), Lurestan Province, Zagros Fold–Thrust Belt, Iran

The paleoenvironmental significance of deep marine trace fossil assemblages in the upper Maastrichtian–Paleocene Amiran Formation has been done for the first time in Lurestan Province (NW Zagros) from the Zagros Fold–Thrust Belt. A semi-quantitative study of trace fossil abundance (bioturbation indices) in the Amiran Formation shows that trace fossil assemblages can be very powerful discriminators of deep-marine clastic fan and related environments. The Amiran Formation is a sand-rich, thick-bedded, and coarse-grained turbidite succession. Deep marine paleoenvironments from basin slope to basin floor settings are preserved. These strata contain a diverse and abundant pre- and post-depositional ichnofauna. Proximal and axial parts of the sandy systems show low-diversity and low-density trace fossil assemblages dominated by post-depositional trace fossils, but distal environments show a general increase in trace fossil diversity, abundance and number of pre-depositional trace fossils. The post-depositional association essentially consists of dwelling, feeding, and grazing traces. The pre-depositional assemblage is rich in graphoglyptids and grazing trails, and feeding structures also occur.The ichnodiversity, composition, ethology, and morphologic complexity of the pre-depositional association of the heterolithic successions of thin to thick-bedded turbidite sandstone and inter-turbidite mudstone are characteristic of three ichnosubfacies of the Nereites ichnofacies. These ichnosubfacies are identified in Amiran Formation deposits, the typical succession of ichnosubfacies can express a bathymetric trend from shallower to deeper parts and from higher-to-lower hydrodynamic conditions. They are: (1) the Ophiomorpha rudis ichnosubfacies (the proximal and axial parts with thick bedded channel and lobe related environments), (2) the Paleodictyon ichnosubfacies (channel-lobe transition in the distal lobe facies), and (3) the Nereites ichnosubfacies (distal part of off-axis lobe or fan fringe–basin floor transition and overbank settings). There is also mixed O. rudis to Paleodictyon ichnosubfacies (channel–margin, lobe–fringe in the proximal lobe facies). The dominance of Zoophycos and Chondrites in poor-oxygen basin floor facies may represent Zoophycos ichnofacies. The number of graphoglyptids increases from distal (basin floor) to proximal (fan fringe–basin floor transition, fan fringe, lobe complex fringe/lobe complex off-axis) settings. They are absent in the most proximal/axial lobe environments due to increased volume, frequency, erosive power of turbidity current events and sedimentation rate, grain size, decreased preservation potential of shallow tier, and pre-turbidite trace fossils. The ichnosubfacies method used here, as utilized by other authors, has the potential to improve paleoenvironmental analysis of other deep marine depositional settings, and in outcrop investigation of turbidite systems.

The Ophiomorpha rudis ichnosubfacies of the Nereites ichnofacies: Characteristics and constraints

The Ophiomorpha rudis ichnosubfacies of the Nereites ichnofacies contains a low diversity (commonly 1–4 ichnotaxa) trace fossil assemblage that besides the eponymous ichnospecies commonly includes Ophiomorpha annulata and Scolicia, and rarely Nereites irregularis, Chondrites, graphoglyptids or other ichnotaxa. This ichnosubfacies occurs in channel and proximal depositional lobe facies of deep-sea fans or in thick-bedded facies of deep-sea clastic ramps. When considered with the Nereites and Paleodictyon ichnosubfacies, it can be used for determination of bathymetric trends. Specimens of O. rudis can also occur in overbank or fan-fringe facies in the Paleodictyon ichnosubfacies, in which ichnodiversity is distinctly higher. Ophiomorpha rudis ranges from the Tithonian but is rare in the Lower Cretaceous. It is distinctly abundant since the Turonian with a pronounced peak of abundance in the Eocene. After the Eocene, its abundance apparently declines. The occurrence of the O. rudis ichnosubfacies at present is stratigraphically limited to the Upper Cretaceous–Miocene. Ophiomorpha rudis is a very deep-feeding burrow system composed of oblique to vertical shafts and horizontal galleries. Probably, it was produced by shrimp-like crustaceans, which possibly fed by means of farmed microbes on decomposed phytodetritus close to or below the redox boundary. Tunnels of O. rudis were reinforced against collapsing, with irregular muddy sand granules. In more cohesive sediment, the burrows are smooth, without a wall, and in firmground sediment they are covered locally with casts of scratch ornament produced by the legs of the tracemaker.

Inflow and outflow facies from the 1993 tsunami in southwest Hokkaido

We describe in detail the sedimentary facies, provenance and grain-size distributions of the 1993 tsunami deposits in the lower reaches of Usubetsu River. Only two lithofacies were recognized, a gravel lobe facies (GLF) and a sand-sheet facies (SSF). The gravel lobe facies was deposited rapidly by a hydraulic jump. In contrast, the sand-sheet facies, which contains both current ripples and dunes, was deposited by a traction current under subcritical flow conditions. We identified four stratigraphic units: Unit 1, Unit 2, Unit 3, and Unit 4, in ascending order. We interpreted Units 1 and 2 as having been deposited by the inflow and outflow events of the first run-up and Units 3 and 4 by the inflow and outflow events of the second run-up, respectively. Units 3 and 4 were more widely distributed and coarser grained than Units 1 and 2 because the second run-up was larger than the first run-up. Furthermore, Unit 4 was more widely distributed than Unit 3 because the outflow eroded the inflow deposits. Grain-size distributions of the inflow and outflow deposits were clearly different. Unit 4 deposits had a peak at 2.5 phi (P-1 population). In contrast, Unit 3 deposits had a bimodal distribution with peaks at − 0.5–1.5 phi (P-2 population) and at 2.5 phi (P-1 population). A comparison of the tsunami deposits with other sedimentary deposits near the study area showed that the tsunami deposits were mainly derived from the seabed at depths below 5.5 m in the offshore area, where the marine sands are about 2–2.5 phi in size and thus are the source of the P-1 population. The P-2 population is derived from coarse (0.5–2 phi) fluvial materials eroded during the tsunami run-up. According to our calculations, benthic foraminiferal tests were entrained from the comparatively deep seabed (at depths up to 100 m) during the tsunami run-up and included in the tsunami deposits.

Characterization and recognition of deep-water channel-lobe transition zones

The channel-lobe transition zone (CLTZ) is an important, but commonly overlooked, element of many deep-water turbidite systems. Recognizing this zone is difficult in both modern and ancient environments and depends largely on the quality and resolution of the data obtained. In this article, three case studies of modern CLTZs are presented, largely based on high-resolution side-scan sonar imagery. These data are then compared to other well-defined CLTZs, both modern and ancient, and the common characteristics identified. CLTZs occur at canyon/channel mouths and are commonly associated with a break of slope. Most sediment bypasses this zone, and consequently only coarse sands and gravels are deposited, although these are commonly patchily distributed and extensively reworked. The CLTZ is characterized by abundant erosional features, including isolated spoon- and chevron-shaped scours up to 20 m deep, 2 km wide, and 2.5 km long. In areas of more widespread erosion, these merge to form amalgamated scours several kilometers across. Depositional bed forms include sediment waves with wavelengths of 1-2 km and wave heights up to 4 m. The presence or absence of a CLTZ has important implications for hydrocarbon exploration and development, especially in terms of the connectivity between sandy channel-fill and lobe facies.

Estrellichnus jacaensis nov. igen., nov. isp. - a large radial trace fossil from Eocene flysch (Hecho Group, northern Spain)

Estrellichnus jacaensis igen nov., isp. nov. is a large radial trace fossil preserved as hypichnial semireliefs. It occurs in Eocene flysch deposits (depositional lobe facies) of the Hecho Group, north of Jaca in northern Spain. It is interpreted as a subsurface burrow composed of a central shaft and radiating tubular tunnels, produced by an unknown tracemaker. Estrellichnus jacaensis is similar to some extent to Capodistria vettersi Vialov. Estrellichnus jacaensis igen nov., isp. nov. est une grande trace fossile radiale qui se présente sous forme d'hypichnia en semi-relief. Il se rencontre dans les flyschs éocènes (faciès de lobe) du Groupe de Hecho, au nord de Jaca (Nord de l'Espagne). Il est interprété comme un terrier de subsurface composé d'une cheminée centrale et de tunnels tubulaires radiaux, produit par un organisme inconnu. Estrellichnus jacaensis assez relativement semblable à Capodistria vettersi Vialov.

ABSTRACT: Stochastic 3D Geological Modeling of Deep Water Reservoirs: Methodology and Case Studies

Abstract Various turbidite facies architectures have been observed in deep water fields offshore Gulf of Mexico including lobe systems, amalgamated channel systems, sheet sand systems, and channel levee systems. The geometric anisotropy and petrophysical properties of the individual facies in the individual depositional systems are different. As such, they need to be captured in the geological model for accurate monitoring of reservoir production performance. Integrated geological, geophysical, and petrophysical studies provide information of reservoir architecture and heterogeneity at different scales. In this study various stochastic techniques have been successfully used to model the varied reservoir architecture in deep water systems. Different case studies presented here demonstrate the ability of using available stochastic techniques in geological modeling of deep water systems. General Marked Point Process, an object-based modeling technique, with specially designed facies shapes and 3D intensity function are used in a prograding turbidite lobe system to model the feeder channel, lobe and barrier facies. In the channelized turbidite system, fluvial algorithm and a specially designed grid is used to model the channel and overbank facies with curved paleo-flow directions. For petrophysical modeling, pixel-based Sequential Gaussian Simulation algorithm with a specific data transformation and trends can help to capture the variations of petrophysical properties in different facies. During the modeling process, great efforts were made to optimize the workflow, algorithms and input parameters for each depositional system. Careful quality control was performed to insure that the major facies architecture and petrophysical features are preserved and input data are honored.

Abstract: 3D Synthetic Seismic Modeling of Turbidite Sandstones 

ABSTRACT 3-D synthetic seismic models were generated for a fine-grained, sand-rich turbidite system to illustrate lateral variability in acoustic impedance and seismic response as a function of turbidite facies architecture. The models represent a 250 meter exposed section containing 4 stacked turbidite fan complexes from the Karoo Basin, South Africa. The ultimate goal of this work is to aid in seismic interpretation of deepwater turbidite prospects and reservoirs. Model generation consisted of: 1) construction of the 3-D lithostratigraphic framework from detailed outcrop descriptions and gamma ray measurements, 2) calculation and assignment of rock properties representative of consolidated subsurface reservoir sandstones and shales, and 3) generation of the 3-D synthetic seismic trace volume. Acoustic impedance values were calculated using an empirical relationship to estimate P-wave velocity from porosity and clay volume. Synthetic seismic volumes were generated by convolving the reflectivity function derived from the acoustic impedance volume with Ricker wavelets having different peak-frequencies. These models illustrate: 1) the seismic response to changes in thickness and acoustic impedance of the turbidite channel, lobe and overbank facies without hydrocarbon effects, and 2) the ability to resolve the individual fan complexes using different model pulses End_of_Record - Last_Page 2-------

Depositional facies in Late Cretaceous-?Early Tertiary deposits in northwestern Maranhão, Brazil

Late Cretaceous-?Lower Tertiary deposits from northwestern Maranhao State (northern Brazil) were investigated using facies analysis in outcrops in combination with subsurface data provided from gamma-ray profiles. Four facies associations were recognized in this area and attributed to: a) mouth bar (facies association 1); b) braided fluvial/distributary channel (facies association 2); c) distal mouth bar/crevasse lobe (facies association 3); and d) interdistributary bay/prodelta (facies association 4). Mouth bar deposits consist of well-sorted, fine- to very-fine grained sandstone with a variety of sedimentary structures, mostly including: convolute lamination; overturned foresets; sigmoidal, trough, and tabular cross stratifications; and massive bedding. Growth faults and slumpings are common features associated with these deposits. Braided fluvial/distributary channel deposits are characterized by poorly-sorted, coarse-grained cross-stratified sandstone with sharp, locally concave-up, basal surfaces mantled by lags. Distal mouth bar/crevasse lobe deposits consist of a succession of strata formed by coarsening upward cycles represented by massive and cross-laminated sandstones that are interbedded with mudstones containing abundant plant remains. Finally, interdistributary bay/prodelta deposits include mudstones with abundant plant remains and streaks of silt and very-fine grained sandstones. The four facies associations described above characterize depositional environments typical of delta settings, an interpretation consistent with the funnel-shaped gamma-ray patterns observed in well logs. Correlation of these deposits with a wave-dominated estuarine system (i.e., Cujupe Formation) exposed in the eastern of Sao Luis Basin led to attribute them to a bayhead delta complex located in the innermost portion of this estuary. The sedimentological and stratigraphic data presented in this paper support the continuity of the Cujupe estuarine system southward of the Ferrer-Urbano Santos Arch. These results thus suggest a paleogeographic reconstruction in which the Sao Luis and Grajau basins behaved as a unique basin at least from the late Cretaceous on.

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

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

Seismic Stratigraphic Interpretation of a Suspected Fan-Complex in Central San Joaquin Basin

The pre-drill interpretation of a suspected submarine fan complex (SFC) included seismic sequence analysis, seismic facies analysis, isopach mapping of lobes, and structural mapping. The seismic sequence analysis interpreted an eroded sand-rich shelf updip from the SFC and placed the SFC in a lowstand sequence. However, downdip well control showed little sand present near the SFC and sand-richness of the system was a concern. Seismic facies analysis was performed on a mile by mile grid of seismic data for the purpose of identifying facies of higher probable sand content within the SFC. Due to low seismic resolution, only features larger than 400 ft horizontally and 150 ft vertically were identifiable. The SFC was divided into six chronostratigraphic units using onlap relationships and the principle of superposition. Within each chronostratigraphic unit, interpretation of channel and lobe systems were made. The postulated channels identified as updip were interpreted to bifurcate downdip with channel fill interpreted as depositional and more likely sand prone. The initial well location was chosen to optimize the number of channels to be intersected, while still drilling structurally updip. An isopach thick was also drilled in the event that the lobe facies were sand prone. A second well wasmore » drilled into a downdip isopach thickness suspected of containing only lobe facies. Post-drill evaluations show the SFC to be fine grained and sand poor. However, two zones containing sand were encountered in the first well at the levels where channels were predicted. Given this result the value of these analyses are readily apparent.« less

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

Comparison of turbidite facies associations in modern passive-margin Mississippi fan with ancient active-margin fans

Our comparison of the modern passive-margin Mississippi fan (DSDP Leg 96) with ancient active-margin fans (e.g. Eocene Hecho Group, Spain) reveals major differences in turbidite facies associations (Mutti and Ricci Lucchi scheme) and in seismic characteristics in the lower fan area. The lower (outer) Mississippi fan is composed of channel (Facies B and F) and non-channel facies (C? and D), whereas ancient fans are characterized by non-channelized, thickening-upward, depositional lobe facies (C and D). An absence of depositional lobes in the lower Mississippi fan is also suggested by a lack of convex-upward (mounded) seismic reflections with bidirectional downlap. Continuous seismic reflections of the lower Mississippi fan may represent “sheet sands”, but not those of true depositional lobes with mounded character. Extensive channelization in modern passive-margin fans appears to be a product of the lateral shifting of a major sinuous distributary system, developed as a consequence of low gradients and the transport of sediment with a relatively low sand/mud ratio. In contrast, channels in active-margin fans are short and of low sinuosity as a result of high gradients and the transport of sediment with a relatively high sand/mud ratio. The turbidite facies association scheme, which was developed exclusively from ancient active-margin fans, should be applied to mature passive-margin fans with qualifications because of the differences in spatial distribution of turbidite facies and their associations.