Submarine Fan Environment Research Articles

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.

Transition from Slope to Fan Facies, Lower Cretaceous Sediments, Andean Foothills, Southern Chile: ABSTRACT

In southern South America, mid-Cretaceous compressional deformation transformed an Early Cretaceous back-arc basin-slope-platform setting into a protocordillera-foreland basin. The Lower Cretaceous sedimentary rocks exposed in the Andean foreland fold and thrust belt at lat. 51°S record an abrupt transition from back-arc basin-slope facies to foreland-basin submarine-fan facies. The Zapata Formation of Portlandian to Albian age represents deposition on the slope of a coeval back-arc basin which formed to the west. The 600-m section consists predominantly of rhythmically interbedded and extensively bioturbated mudstone and siltstone. The siltstone beds are laterally continuous and contain sedimentary structures suggesting deposition by turbidity currents. Sporadic turbidite sandstones occur as shallow channel-fill sequences or as thin, laterally persistent graded beds rich in coarse detrital mica. Slumped intervals are common, and show a northwest-southeast trend of slump fold axes, which parallels the axis of the back-arc basin to the west. The overlying Punta Barrosa Formation of Albian to Cenomanian age was deposited contemporaneously with deformation in the cordillera to the west. It represents a rapid change in depositional regime, signaled by an abrupt increase in sandstone beds. The thick sandstone beds are lenticular, commonly amalgamated, and show thickening/coarsening-upward and thinning/fining-upward cycles indicative of deposition in a submarine-fan environment. NNW to SSE paleocurrent trends indicate a longitudinal basin-fill pattern; this persists through the Upper Cretaceous flysch sequence. The Punta Barrosa Formation thus represents the establishment of submarine-fan deposition associated with the initiation of a foreland basin on the site of the preexisting back-arc basin slope. End_of_Article - Last_Page 643------------

Geology of Mesozoic Basement Rocks from Well Cores in Santa Maria Basin, Santa Barbara County, California: ABSTRACT

The Mesozoic rocks underlying the Tertiary cover in the Santa Maria basin, California, consist of an Upper Cretaceous sedimentary sequence overlying dismembered ophiolitic rocks of undetermined age. The ophiolitic rocks are ophicalcites, basalts, greenstones, dike and sill rocks, gabbros, and serpentenized peridotites in a Franciscan assemblage. The Upper Cretaceous rocks are sandstones, mudstones, and shales that were deposited in a submarine fan environment. They are characterized by tectonic disruption in the form of brecciation and chaotic mixing. These basement rocks of the Santa Maria basin were compared to rocks of similar age surrounding the basin in terms of petrography, sedimentary structures, and facies relations. Two major groups of strata were compared: relat vely undisturbed sedimentary rocks correlated to the Great Valley sequence and the disrupted Franciscan rocks of the Cambria, Point San Luis, and Pfeiffer Beach slabs. The Santa Maria basin basement rocks are similar to the latter group. Mesozoic rocks underlying the Santa Maria basin were formed in a converging plate boundary regime on the inner trench-slope. The ophiolitic rocks and the overlying deep-water cherts and pelagic sediments were accreted in fold and thrust segments as the downgoing oceanic plate was subducted. The accreted material formed linear ridges which trapped younger sediments in a trench-slope basin. Sediments from several different source areas accumulated in these localized basins and were tectonically deformed with continuation of subduction in the trench. The rocks resulting from this process are called the Franciscan melange. The Santa Maria basin formed above these Franciscan rocks following collision of the Farallon-Pacific spreading center with the trench off California in Oligocene time. The subsequent transform regime produced major right-lateral strike-slip fault movements and associated downwarping of the Santa Maria area. Deep-water, middle Miocene sediments were deposited in this newly formed basin in contact with the Mesozoic rocks. End_of_Article - Last_Page 932------------

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

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

Stratigraphy of an Archean submarine fan

This paper describes the occurrence of iron-formation in a submarine fan environment of the Archean of Ontario. A section, over 500 m thick, of greywacke turbidite, shale, siltstone, sandstone, conglomerate, and three iron-formations is well preserved. The sedimentary rocks unconformably overlie mafic flows and felsic pyroclastics of a typical Archean volcanic cycle. Sedimentary structures such as graded-bedding, cross-bedding, delicate laminations in shale and iron-formation, load casts, slump balls, ripples, starved ripples, Bouma sequences, sedimentary dikes are well preserved. Magnetite iron-formation occurs as laminated units in sandstone, conglomerate, between turbidites, and interbedded with shale. Evidence for a similar previous deposit is the presence of well-rounded pebbles of siltstone, sandstone (quartzite), and siliceous iron-formation in the conglomerate units. A submarine fan depositional environment is suggested. The existence of a second fan was indicated during preliminary investigation about 8 km west of the area described. Iron-formation terminates by facies change into coarse clastics. The iron-formations are sufficiently thick through original deposition or subsequent folding to make them of economic interest.

Sedimentology and Paleobathymetry of Neogene Trench-Slope Deposits, Nias Island, Indonesia

The Neogene strata exposed on Nias Island, Indonesia consist of Miocene marls, sandstones, and conglomerates of the Nias beds that overlie the Oyo Complex, a tectonic mélange of pre-Miocene age. They provide an excellent opportunity to study ancient trench-slope sedimentation. Lithofacies and vertical sedimentary sequences in the Nias beds are similar to those described from ancient submarine fan environments. The overall vertical sedimentary sequences in the Nias beds describe a coarsening-and thickening-upward trend. The basal portion of the sections consists dominantly of thin-bedded and fine-grained turbidites and marls of Facies D and G. Sandstone-to-shale ratios show a general up-section increase, which is observed as a corresponding up-section increase in Facies A, B, and C sandstones and a decrease in Facies D siltsones and marls. Within the large-scale thickening-and coarsening-upward cycles are smaller-scale thickening- and coarsening-upward and thinning- and fining-upward cycles characteristic of deposition in various submarine fan environments. The general vertical progression of fan facies is basin plain $$\rightarrow$$ outer fan $$\rightarrow$$ midfan. Benthic Foraminifera, generally common and well-preserved throughout all but the basal portions of the Nias beds, provide clear evidence of uplift of the section from lower bathyal/abyssal depths. Samples from the oldest Nias beds contain either a very meager fauna or no calcareous microfossils at all, indicating deposition below the CCD. Lower Miocene assemblages are indicative of depths greater than 2000 meters. Middle bathyal (500-2000 m) species represent the in situ bottom fauna within the upper Miocene strata. The Nias beds are overlain by Pliocene shelf deposits and a Pleistocene coral cap. This implies that the lower Miocene strata were uplifted 4000 meters in about 20 million years, yielding an average uplift rate of $$20 cm/10^{3}$$ yrs. A model of trench-slope sedimentation on Nias suggests that near the base of the trench slope, hemipelagic sedimentation took place below the CCD in basins a few kilometers in length and width. Because the basins were shallower than the trench floor, they were isolated from trench turbidites. As the basins were uplifted tectonically, pelagic carbonate became a more conspicuous component. Terrigenous turbidites began to be delivered to the basins as submarine canyons cut down through the slope. Continued terrigenous sedimentation led to progradation of submarine fan deposits over the older basin-plain deposits. The basin sediments were folded and faulted penecontemporaneously with sedimentation. The dominant source area for the Nias beds was the arc terrane of mainland Sumatra.

Use of clast shape in determining the sedimentary history of the late devonian keepit conglomerate, Australia

Clasts of the Late Devonian Keepit Conglomerate of northeastern New South Wales, Australia, are predominantly andesites of isotropic nature that lack any apparent initial shape controls. Given adequate time in a particular environment, these clasts could be reasonably expected to develop shapes characteristic of abrasion within that environment. Clasts from both fluvial and marine resedimented conglomerates of the Keepit Conglomerate possess shape characteristics indicative of shaping within a fluvial environment. Maximum projection sphericity, and a plot of sphericity against either the oblate-prolate index or the flatness parameter C/A, were the most useful in indicating fluvial shaping processes. In addition, the OP Index and the dominant form classes of the Sneed and Folk Sphericity-Form diagram were in support of a fluvial history for the clasts. The occurrence of clasts exhibiting typical fluvial shapes in marine resedimented conglomerates indicates a lack of time and/or insufficient energy in a beach environment for reshaping of the clasts prior to resedimentation into deeper-water submarine-fan environments. This interpretation is consistent with and lends support to the interpretation, based on all other available data, for the Keepit Conglomerate of a history of rapid progradation of fluvial gravels into a relatively low-energy coastline environment, with periodic resedimentation of coarse sediment into deeper water.

Reservoir Quality, Pliocene-Pleistocene Sandstones, Offshore Gulf of Mexico: ABSTRACT

Significant variations in the quality of sandstone reservoirs commonly reflect the amount of clay content which is controlled by the environment of deposition and diagenesis. In the Pliocene-Pleistocene sandstones, offshore Gulf of Mexico, the clay content varies significantly between sands deposited in delta and submarine fan environments. The overall shapes of the gamma ray and SP curves for both environments are commonly similar, End_Page 1555------------------------------ but differences between high and low resistivities can reflect differences in original environment of sand deposition. Diagenesis can add significant quantities of clay to a rock through chemical precipitation in pore spaces. Diagenetic (authigenic) clays are of importance because they can significantly effect electric-log response (SP, gamma ray, neutron, density), and can largely control the reaction of a sand reservoir to well-bore fluids. It is often forgotten that diagenesis can also remove clays from a sandstone, thereby cleaning-up an originally dirty sandstone. The composition of drilling and stimulation fluids also has a significant effect on reservoir quality, especially when the sandstone pores are lined with diagenetic clays. Use of incorrect drilling or stimulation fluids can make a potentially good reservoir nonproductive. In the Pliocene-Pleistocene section of the Gulf of Mexico, variations in environment of deposition, diagenesis, and the composition of drilling fluids can play havoc with interpretations based solely on log characteristics. End_of_Article - Last_Page 1556------------

Forties Field, United Kingdom Continental Shelf, North Sea: ABSTRACT

The Forties field, located about 180 km east-northeast of Aberdeen, mainly in UK Licence Block 21/10 in the North Sea, was discovered in October 1970 in Paleocene sandstones of the Forties Formation. Four appraisal wells drilled during 1971-72 proved the existence of a giant oil field with an area of about 90 sq km and estimated oil in place of 4,600 million bbl. Additional geologic data from the 50 development wells drilled to date show rapid facies variations over the field, with some sand bodies having a cross section less than the prime well spacing of 700 m. A large, partly isolated sand body, the Charlie sand, is recognized in the upper, western part of the reservoir. The sandstones and shales of the Forties Formation are considered to have been deposited in a middle and lower submarine-fan environment. A mixture of sedimentary processes including grain flow, debris flow, and turbidity currents have been identified and four broad facies defined from cores. The facies types form significant vertical associations that are related to specific depositional environments and recognized by petrophysical log patterns. Detailed lithofacies studies, together with pressure-decline data and log-pattern analyses, provide a practical means of correlating and mapping the complex sand geometry. These techniques provide a method for well location and a better understanding of the reservoir performance. End_of_Article - Last_Page 466------------

Longshore Drift, Submarine Canyons, and Submarine Fans in Development of Niger Delta

The southwesterly prevailing wind of the Gulf of Guinea strikes symmetrically on the nose of the Niger delta, causing divergent longshore drifts which meet opposing drifts near Lagos and Fernando Poo. Submarine canyons channel about 1 million cu m of sand a year from each pair of opposing drifts to feed submarine fans on either side of the delta foot. In times of low sea level axial distributaries of the Niger feed a third, now inactive, submarine fan in front of the delta. At the time of the last low sea level numerous submarine canyons formed on the front of the Niger delta, and their heads cut back into the Benin Formation continental sands. As the sea rose, these canyon heads formed wide estuaries which have since been filled. All Quaternary canyons except three currently scoured by resedimented longshore drift material have been filled. Because the Niger delta has prograded toward the southwest throughout the Tertiary and because the prevailing wind has blown persistently from the southwest, the longshore drift pattern long has been as it is now, and the two corners of the delta have been areas of high concentrations of submarine canyon formation. There may have been a third area of high concentration of submarine canyons between the Cross River and Niger deltas when these were separate. Recognition of the submarine fan environment leads to a new symmetrical five-layer interpretation of the structure of the Niger delta: (5) top continental sand unit (Benin Formation); (4) transitional sand/shale unit (Agbada Formation); (3) marine shale unit (Akata Formation); (2) transitional shale-sand unit (newly distinguished); (1) bottom submarine fan sand unit (newly distinguished). Other deltas feeding into waters of oceanic depths may have a comparable structure.