Fan-fringe Deposits Research Articles

Bed type and flow mechanism of deep water sub-lacustrine fan fringe facies: an example from the Middle Permian Lucaogou Formation in Southern Junggar Basin of NW China

Submarine or sub-lacustrine lobe deposits are important reservoirs, but the fan fringe deposits form heterogeneities within deep water fan deposits. Fan fringe facies records the complex sediment gravity flow types. By understanding of the bed types and flow mechanisms, we can identify the fan fringe deposit, which aids in the reconstruction of deep water fan and reservoir evaluations. The Jiucaiyuanzi and Dalongkou sections in the West Bogda Mountains preserve well-exposed 536-m and 171-m thick successions, respectively, of a deep water lacustrine depositional system from the Middle Permian Lucaogou Formation. Bed types of the Lucaogou Formation include high-density turbidite, low-density turbidite, incomplete Bouma-type turbidite, hybrid event beds, and slump deposits. The Lucaogou Formation is interpreted here as a fan fringe facies due to the thin bed thickness that characterize turbidites and hybrid event beds, as well as the predominance of the isolated sheet architecture. Previous studies suggest that these deposits were considered as deposited in a deep water setting due to the absence of wave-related structures. The presence of abundant mud clasts in massive medium-coarse grained sandstone beds reflects the significant erosional capability and interactions between high-density turbidity currents and lake floor. The fan fringe facies here contains amalgamated and thick-bedded homolithic facies (~ 30%) and thin-bedded heterolithic facies (~ 70%). The examination of the bed type is of wider significance for facies prediction and reservoir heterogeneity in the sub-lacustrine fan fringe facies.

Assessing controls on the distribution of ichnotaxa in submarine fan environments, the Basque Basin, Northern Spain

The Cretaceous to Eocene aged Basque Basin in northern Spain preserves a variety of submarine fan related environments of deposition in well exposed outcrops. Here, quantitative trace fossil data is calibrated to sedimentologically defined environments of deposition. Nine environments of deposition are assigned according to sedimentary facies associations, depositional architecture and stratigraphic context. The preserved trace fossil assemblages are interpreted in terms of extrinsic palaeoecological, and intrinsic taphonomical and depositional environment controls. In channel related environments, diverse pre-depositional dominated assemblages are prevalent in marginal settings. These are replaced by low diversity post-depositional dominated assemblages in more axial positions within the system. Lobe related environments display a higher level of diversity and bioturbation intensity in more distal/off axis (lobe fringe) environments compared to the most axial lobe environments that preserve low diversity, and exclusively post-depositional assemblages (dominated by Ophiomorpha). Diverse pre- and post-depositional assemblages are common in fan fringe deposits, with less diverse assemblages dominated by post-depositional ichnotaxa (particularly Zoophycos) in basin floor fan deposits. The use of ‘sub-ichnofacies’ is found to only aid characterisation of trace fossil assemblages based on a general (proximal–distal) position within a submarine fan system. Tier preservation of trace fossil assemblages is almost exclusively determined by the depth of erosion that a substrate is subjected to. As such, colonisation style (pre- versus post depositional) and ethological groupings are proposed as being the most powerful tool in assisting sedimentological observations in assigning environments of deposition.

The reservoir development of the Late Maastrichtian–Early Paleocene Ormen Lange gas field, Møre Basin, Mid-Norwegian Shelf

The giant Ormen Lange Gas Field is situated in the Møre Basin, Norwegian Sea, at water depths around 1000 m. The reservoir interval consists of a lower heterolithic unit mainly of Maastrichtian age (Jorsalfare Formation) and an upper sandstone unit of Danian age (Egga Member, Våle Formation), separated by a widely distributed mudstone unit (‘Våle Tight’), and consists of both high and low density turbidites in association with fine-grained background sediments. The reservoir interval spans the Cretaceous–Tertiary boundary, without any major stratigraphic breaks. The main architectural elements of the reservoir are: channel-dominated deposits, channelized lobe deposits and non-channelized frontal splay or fan fringe deposits. The spatial distribution of these facies elements indicates a dynamic system with changing sediment supply through time. Core and log studies from the wells have been integrated with high-resolution biostratigraphy to develop a dynamic depositional model. During latest Cretaceous and Paleocene times, a series of subtle sub-basins developed along the southeastern Møre Basin margin as a result of differential subsidence above Jurassic extensional faults. The sub-basins created a stepped slope topography that influenced the flow pattern of turbidity currents. In a landward position, east of the Ormen Lange Field, the Cretaceous–Tertiary boundary is characterized by a major unconformity, implying erosion and sediment by-pass. In early Danian time, uplift and rotation of the provenance area to the east (Norwegian mainland) led to extensive erosion and redistribution of sandy sediments into the Møre Basin and caused deposition of the Egga Member. The turbidites filled the intraslope basins initially in a basinwards-stepping sense, but later sedimentation switched landward. Onshore structural, provenance and geomorphological data, combined with offshore structural, seismic and sedimentological data, yield an integrated interpretation of the deep-water depositional system in the greater Ormen Lange area. Present fjords at Møre, controlled by the Møre–Trøndelag fault zone, were valley systems that fed sands to a point source updip of the Ormen Lange area. The point source was located at the transition/relay zone between a narrow shelf area in the south and a broad shelf area in the north. In addition to being a key transition zone for both Caledonian and Jurassic structures, this zone area is also the ‘landing’ point of the major oceanic Jan Mayen Fracture Zone.

Stream‐dominated alluvial fan and lacustrine depositional systems in Cenozoic strike‐slip basins, Denali fault system, Yukon Territory, Canada

ABSTRACTAncient stream‐dominated (‘wet’) alluvial fan deposits have received far less attention in the literature than their arid/semi‐arid counterparts. The Cenozoic basin fills along the Denali fault system of the northwestern Canadian Cordillera provide excellent examples of stream‐dominated alluvial fan deposits because they developed during the Eocene‐Oligocene temperate climatic regime in an active strike‐slip orogen. The Amphitheatre Formation filled several strike‐slip basins in Yukon Territory and consists of up to 1200 m of coarse siliciclastic rocks and coal. Detailed facies analysis, conglomerate: sandstone percentages (C:S), maximum particle size (MPS) distribution, and palaeocurrent analysis of the Amphitheatre Formation in two of these strike‐slip basins document the transition from proximal, to middle, to distal and fringing environments within ancient stream‐dominated alluvial‐fan systems.Proximal fan deposits in the Bates Lake Basin are characterized by disorganized, clast‐supported, boulder conglomerate and minor matrix(mud)‐supported conglomerate. Proximal facies are located along the faulted basin margins in areas where C:S = 80 to 100 and where the average MPS ranges from 30 to 60 cm. Proximal fan deposits grade into middle fan, channelized, well organized cobble conglomerates that form upward fining sequences, with an average thickness of 7 m. Middle fan deposits grade basinward into well‐sorted, laterally continuous beds of normally graded sandstone interbedded with trough cross‐stratified sandstone. These distal fan deposits are characteristic of areas where C:S = 20 to 40 and where the average MPS ranges from 5 to 15 cm. Fan fringe deposits consist of lacustrine and axial fluvial facies. Palaeogeographic reconstruction of the Bates Lake Basin indicates that alluvial‐fan sedimentation was concentrated in three parts of the basin. The largest alluvial‐fan system abutted the strike‐slip Duke River fault, and prograded westward across the axis of the basin. Two smaller, coarser grained fans prograded syntaxially northward from the normal‐faulted southern basin margin.Facies analysis of the Burwash Basin indicates a similar transition from proximal to distal, stream‐dominated alluvial fan environments, but with several key differences. Middle‐fan deposits in the Burwash Basin define upward coarsening sequences 50 to 60 m thick composed of fine‐grained lithofacies and coal in the lower part, trough cross‐stratified sandstone in the middle, and conglomerate in the upper part of the sequence. Upward‐coarsening sequences, 90–140 m thick, also are common in the fan fringe lacustrine deposits. These sequences coarsen upward from mudstone, through fine grained, ripple‐laminated sandstone, to coarse grained trough cross‐stratified sandstone. The upward‐coarsening sequences are basinwide, facies independent, and probably represent progradation of stream‐dominated alluvial‐fan depositional systems.Coal distribution in the Amphitheatre Formation is closely coupled with predominant depositional processes on stream‐dominated alluvial fans. The thickest coal seams occur in the most proximal part of the basin fill and in marginal lacustrine deposits. Coal development in the intervening middle and distal fan areas was suppressed by the high frequency of unconfined flow events and lateral channel mobility.

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.

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

Transitional submarine fan deposits from the late Precambrian Kongsfjord Formats submarine fan, NE Finnmark, N. Norway

ABSTRACT Three transitional submarine fan environments are recognized in the late Precambrian, 3‐2 km thick Kongsfjord Formation in NE Finnmark, North Norway, namely: (1) middle to outer fan; (2) fan lateral margin, and (3) fan to upper basin‐slope deposits. Middle to outer fan deposits have a high proportion of sandstones, typically showing Bouma Tbede with Ta in the thicker beds. Deposition was mainly from sheet flows with rare shallow channels. Middle to outer fan deposits are an association of sandstone packets less than 10 m thick but commonly only a few metres thick, interpreted as channels or lobes. Interchannel and fan fringe deposits occur as discrete packets of beds between the thicker bedded and coarser grained channel or lobe deposits. Fan lateral margin deposits are recognized on the basis of their stratigraphic position adjacent to inner/middle fan deposits. They are characterized by: (a) a relatively high proportion of fine‐grained sandstone/siltstone turbidites compared to other major fan environments; (b) relatively small channels oriented at various angles to the regional basin slope; (c) lobes associated with channels, and (d) abundant clastic dykes and other soft‐sediment deformation. Fan lateral margin deposits are distinguished from the outer fan/basin plain successions on account of the very high proportion of siltstone turbidites comparable with middle fan inter‐channel deposits. Fan to upper basin‐slope deposits occur at the top of the formation as an alternation of sandstone turbidites, most of which are laterally discontinuous, and very thin‐bedded upper basin‐slope siltstones with slide deposits.

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.

Deep-Sea Fan Deposits in the Macigno Formation (Middle-Upper Oligocene) of the Gordana Valley, Northern Apennines, Italy

ABSTRACT Facies characteristics are described for a 2300 m thick turbidite succession of the Macigno Formation (middle-upper Oligocene), which crops out as a continuously exposed section in the Gordana Valley (northern Apennines). In the study area the Macigno Formation consists of two sandy members separated by a shaly member. The sandy members contain lithic associations characteristic of submarine fan deposits and form distinct fan complexes. The middle shaly member represents a period of basin starvation. Three main facies associations are recognized in the Macigno fan deposits, fan-fringe, outer-fan lobe, and braided midfan facies associations. The fan-fringe deposits consist of monotonous or small-scale cyclically arranged thin-bedded turbidites. The outer-fan lobe deposits are composed of medium-to very thick-bedded classical turbidites internally organized in thickening-upward cycles with minor intercalations of thin-bedded sandstone beds. Two distinct types of outer-fan lobe deposits, distal and proximal, are recognized on the basis of rock-type, internal organization, and vertical facies relationships. The midfan deposits consist of amalgamated massive sandstone and pebbly sandstone which display repeated thinning-upward cycles. The three main facies associations are superposed t define major thickening and coarsening-upward cycles reflecting distinct episodes of fan progradation. The relationships between the facies associations of the Macigno deposits and the existing models of submarine fan sedimentation are discussed. After the deposition of the fan complexes, fan sedimentation in this part of the basin was replaced by movements of allochthonous units heralded by large olistostromes.

Synsedimentary Deformation in Fossil Accretionary Prism, Greece: ABSTRACT

End_Page 740------------------------------Structural and stratigraphic evidence demonstrates that Paleogene turbidites of the Politses Formation were progressively deformed during sedimentation. The Politses Formation is preserved in internally undeformed thrust sheets that are tectonically intercalated with sheets of severely deformed blocks of the formation. This imbricate stack is interpreted as a fossil accretionary prism. The Politses Formation typically consists of hemipelagic red shales and limestones (basin-plain facies association) followed by thin-bedded terrigenous turbidites (fan fringe, interlobe and distal depositional lobe facies associations) which are overlain then by predominantly thick-bedded turbidites (depositional lobe facies association). This normal progradational sequence is interrupted by incised channel complexes which lie above basin-plain sediments and beneath fan-fringe deposits in some thrust sheets. Atypical facies organization suggests tectonic activity during sedimentation. Small-scale soft sediment deformation in the form of convolute lamination exists in about 20% of turbidite sandstone beds. Assuming that the deformation resulted from seismic activity, sedimentation ra es and the distribution of structures indicate that seismic shocks affected the depositional area every 10 to 100 years on the average. The most important evidence for synsedimentary deformation lies in the distribution of marl marker beds among thrust sheets. The stratigraphic distribution of marls corresponds uniquely to individual thrust sheets. This and other sedimentological relations demonstrate that marl and turbidite accumulation was controlled by progressive tectonic removal of thrust sheets from active deposition. The absence of unconformities within thrust sheets suggests that the deep sea fans represented in the Politses Formation were deposited in a trench. End_of_Article - Last_Page 741------------