Turbidite Units Research Articles

Late Holocene fine-grained sediments of the Balearic Abyssal Plain, Western Mediterranean Sea

The Late Holocene fine-grained sedimentation in the deep central Balearic Abyssal Plain, Western Mediterranean Sea, has been studied after the analysis of high quality sediment cores from three multicoring stations, named A, B and C. The coring stations are 25 km apart from each other and form a triangle that is at the greatest distance from the nearest landmasses. The sediments consist of foraminifer–pteropod oozes (layers U1 and U4 from bottom to top), two pteropod oozes (U2 and U5) and a turbidite layer (U3). The same sequence has been identified in the three cores. The turbidite layer U3 that lies on top of U2 ( 14C AMS dated at 1814 cal yrs BP) ranges from 5 to 11 cm thick and is clearly distinguished from the dominating hemipelagic sediments by colour variations, higher water content and the absence of planktonic foraminifer shells. Rather uniform values of organic carbon and nitrogen are also characteristic of the U3 turbidite unit, likely because of homogenisation during transport and emplacement. In addition, U3 unit shows higher contents of terrigenous elements such as K and Fe. Otherwise, the presence of zeolites in the turbidite layer supports the hypothesis of the Sardinian continental margin, where pyroclastic deposits are known, as the most likely source area for U3. Our results demonstrate the ability of turbidity currents originating on the margins surrounding the Balearic Abyssal Plain to reach the very centre of the basin during high sea level still stands as the present one.

Turbidites and their association with past earthquakes in the deep Çınarcık Basin of the Marmara Sea

Two gravity cores (CAG-3 and C-15) from the tectonically active, 1,276-m deep Cinarcik Basin of the Marmara Sea each include three sandy turbiditic mud units (1 mm–2 cm thick) with sharp basal contacts. The high benthic foraminifer content of these units suggests that the sediments were transported by turbidity currents from the upper slope region. These units represent the thin edges of turbidites thickening towards the subsiding north-eastern part of the basin, and contain quartz, detrital calcite, intact shells and shell fragments, smectite, pyrite framboids, muscovite, biotite, epidote and garnet. Their clay fractions are more enriched in smectite than those of adjacent layers. AMS 14C ages (957±43 a.d. and 578±31 a.d.) of two upper and middle turbiditic units in core C15 overlap with the historical Istanbul-Thrace (intensity=10) and Istanbul-Kocaeli (intensity=9) earthquakes of 26 October 986 and 15 August 553, respectively. This overlap, together with sedimentological characteristics, strongly suggests that the turbiditic units are related to the tectono-seismic activity of the North Anatolian Fault. The age of the lowest turbiditic unit in core C-3 was found to be 6,573±87 a b.p. (calendar) by AMS 14 C. In terms of chronostratigraphic relationships and lithological composition, the turbiditic units in core CAG-3 cannot be correlated with those in C15. This can be explained by gravity-controlled sedimentation causing wedging out of turbidites towards the edge of the basin.

Evidence of Himalayan erosional event at ∼0.5Ma from a sediment core from the equatorial Indian Ocean in the vicinity of ODP Leg 116 sites

Abstract A sediment core collected from an area ∼100 miles south of the ODP Leg 116 (distal Bengal Fan) in the equatorial Indian Ocean was investigated for microfossils, mineralogy, mineral chemistry, magnetic susceptibility, grain size, major, minor and rare-earth element geochemistry, organic carbon and total nitrogen contents in the bulk sediments. Distinct changes in depositional characteristics (including presence of abundant sand-sized micas and other detrital minerals) occur at two sub-surface depths corresponding to ∼0.5 and probably 0.8 Ma time periods. The detrital mineral suite of this core resembles that of turbidite unit I sediments of ODP cores in the distal Bengal Fan. The core site has received an increased supply of terrigenous sediments at these two time periods, the older pulse (0.8 Ma) stronger than the younger pulse. Several lines of evidence such as the nature of the mineral suite, lower magnetic susceptibility values, Si/Al in mica mineral separates; major element composition; discrimination plots of Ca/Ti versus K/Ti and K2O/Al2O3 and La/Yb ratios suggest a highly metamorphosed source such as higher Himalayan crystalline (HHC) series indicating two events of increased physical weathering and erosion in the Himalayan region. While the erosional event of 0.8 Ma is well known, the episode of 0.5 Ma was not reported earlier.

Taconian mélanges in the parautochthonous zone of the Quebec Appalachians revisited: implications for foreland basin and thrust belt evolution

In the Quebec Appalachians, disruption, imbrication, and thrusting of the Taconian foreland basin sequence are responsible for the development of chaotic units within the turbiditic sequence of the Caradocian Sainte-Rosalie Group, the main lithologic assemblage of the parautochthonous zone. These chaotic units have been termed olistostromes or tectonosomes on the basis of field criteria and following Pini's (1999) classification. Olistostromal units containing blocks of the middle mudstone (Utica Shale) and upper turbidite units (Ste-Rosalie Group) of the foreland basin and spanning the Caradocian N. gracilis, C. americanus, O. ruedemanni, and C. spiniferus graptolite zones were deposited and incorporated into the Sainte-Rosalie Group. Disruption of more competent beds of the flyschic sequence and fault stacking and slicing of older rock units occurred along major thrust faults and now form structurally aligned corridors or tectonosomes. Graptolites and new chitinozoan data from both olistostromes and tectonosomes indicate older ages (early Late Ordovician) than the flysch units of Sainte-Rosalie Group (mid Late Ordovocian). Lithological, stratigraphic, and structural criteria indicate that tectonosome slices are imbricated foreland basin rocks that are correlative to the Black River, Trenton, Utica, Sainte-Rosalie, and Lorraine groups of the Laurentian platform. Thermal maturation data indicates that disruption of the autochthonous sequence, and folding and thrusting of the entire foreland basin sequence, must have occurred shortly after their deposition. Contrary to what had been suggested, blocks in the olistostromes and tectonosomes were not derived from the allochthonous Chaudière thrust sheet, even though it presently marks the southern contact with the parautochthonous zone. Imbrication of the foreland basin sequence must have occurred before emplacement of the Chaudière thrust sheet.

Contemporaneous ultramafic and felsic intrusive and extrusive magmatism in the Archaean Boorara Domain, Eastern Goldfields Superterrane, Western Australia, and its implications

Two komatiite emplacement styles are recognized in the Boorara Domain of the Kalgoorlie Terrane, Eastern Goldfields Superterrane, Yilgarn Craton, Western Australia. (1) Large volume, dominantly cumulate-textured komatiite bodies, interpreted as high-level intrusions based on the presence of symmetrical spinifex-cumulate textural zonation, together with apophysy injection and xenolith incorporation along the top and basal margins of the komatiite. (2) Extrusive komatiite lavas exhibit asymmetrical textural zonation, with the top chilled flow margin preserving a fine-grained, fractured crust that coarsens downward into a supercooled former olivine spinifex zone and then into pseudomorphed olivine (now serpentine) orthocumulate texture above a thin basal chilled margin.The intrusive komatiite unit is observed to be intercalated with quartz- and feldspar-phyric dacite. The two lithologies exhibit primary irregular contacts against each other; komatiite apophyses have been injected into the dacite and magma mingling has occurred. These features are indicative of liquid–liquid interaction between the two units and komatiite magmatism is interpreted to have been contemporaneous with dacitic magmatism. The dacite was emplaced first and the komatiite intruded the dacite before it solidified.The reconstructed stratigraphy of the Boorara Domain comprises a basal unit of quartzofeldspathic turbidite beds, conformably overlain by high Mg tholeiite basalt, the intercalated dacite and komatiite units overlie the lower tholeiitic basalt with an unobserved contact, an upper komatiitic basalt unit overlies the komatiite and at the stratigraphic top of the sequence are felsic volcaniclastic turbidite units with felsic porphyry intrusions. These greenstone lithologies were emplaced within a deep submarine basin as indicated by carbonaceous metapelite units intercalated throughout the sequence.The tectonic-scale processes operating in, and around, the Boorara Domain depocentre include derivation of the komatiite unit from a likely mantle plume source. The contemporaneous dacite magmatism is interpreted to be the result of melting of pre-existing Archaean crust during plume ascension, although the exact composition of the primitive crust is yet to be determined.

Geology and stratigraphy of forearc basin sediments in Hokkaido, Japan: Cretaceous environmental events on the north-west Pacific margin

Litho-, bio-, and chemostratigraphy of the Cretaceous forearc basin sediments exposed in Hokkaido, northern Japan allow a synthesis of the faunal, sedimentological, and environmental history of the north-west Pacific margin. Although the succession, named the Yezo Group, has yielded an abundant record of mid- to late Cretaceous invertebrates, monotonous lithologies of sandstone and mudstone, showing occasional lateral facies changes, have caused confusion regarding the lithostratigraphic nomenclature. Based on our wide areal mapping of the sequence, and analysis of litho- and biofacies, a new lithostratigraphic scheme for the Yezo Group is proposed. In ascending order, the scheme is as follows: the Soashibetsugawa Formation (Lower Aptian mudstone unit); the Shuparogawa Formation (Lower Aptian–lower Upper Albian sandstone-dominant turbidite unit); the Maruyama Formation (lower Upper Albian tuffaceous sandstone unit); the Hikagenosawa Formation (Upper Albian–Middle Cenomanian mudstone-dominant unit); the Saku Formation (Middle Cenomanian–Upper Turonian sandstone-common turbidite unit); the Kashima Formation (Upper Turonian–Lower Campanian mudstone-dominant unit); and the Hakobuchi Formation (Lower Campanian–Paleocene shallow-marine sandstone-conglomerate unit). In addition, we designate two further lithostratigraphic units, the Mikasa Formation (Upper Albian–Turonian shallow-marine sandstone-dominated unit) and the Haborogawa Formation (Middle Turonian–Campanian shelf mudstone/sandstone unit), which correspond in age to the shallower facies of the Saku and Kashima formations, respectively. Despite a lack of so-called “black shales”, because of siliciclastic dilution, our stratigraphic integration has revealed the horizons of oceanic anoxic events (OAEs) in the Yezo Group. The OAE1a horizon in the Soashibetsugawa Formation is characterized by a lack of foraminifers, macrofossils and bioturbation, and a prominent positive excursion of δ 13C org. A significant hiatus during the late Aptian and early Albian removed the OAE1b horizon. The OAE1c horizon in the Maruyama Formation shows a distinct negative excursion of δ 13C org with a concomitant high productivity of radiolarians. The OAE1d horizon in the middle part of the Hikagenosawa Formation consists of weakly laminated, pyrite-rich mudstone. Planktonic and calcareous benthic foraminifers are absent, whereas radiolarians are abundant above the OAE1d horizon. The mid-Cenomanian event (MCE) horizon is identified at the top of the Hikagenosawa Formation. Stepwise extinction of calcareous benthic foraminifers and a decrease in radiolarian diversity become apparent above the MCE horizon. In the study area, the OAE2 horizon has been well documented, and is placed in the middle part of the Saku Formation.

Spatial variability of Hurst statistics in the Castagnola Formation, Tertiary Piedmont Basin, NW ITaly: discrimination of sub-environments in a confined turbidite system

Abstract The purpose of this paper is to investigate the potential of the use of the Hurst test as a statistical tool for quantifying the degree of clustering of high and low values of field measurements in vertical turbiditic sections within a small and confined tectonically mobile basin. The paper (1) documents the fact that significant clustering occurs in almost all sections, (2) shows that there are systematic spatial variations in the degree of clustering such that different parts of the basin have different Hurst statistics, (3) discusses the origin of those spatial trends, and (4) compares Hurst H with Chen and Hiscott’s Hurst K . The case study focuses on one well-exposed reservoir-scale turbiditic unit cropping out in the eastern part of the Tertiary Piedmont Basin (NW, Italy) and representing the lower portion of an Upper Oligocene-Lower Miocene turbidite system (Castagnola Formation). The sedimentological model that has been adopted is based on the description of 41 sedimentological logs (approximately 25–35 m apart) physically correlated bed by bed. Three variables were studied: thickness of sandstones and siltstones, grain size score, and sand and silt thickness percentage (i.e. the ratio of coarse division to overlying mudstones, in percent). The Hurst test has revealed the existence of common clustering for the three studied variables (only 2 of 41 logs failed to pass the test at the 10% significance level for all the studied variables). This clustering is associated with lateral and vertical facies variations in response to changes in depositional sub-environments within the Castagnola Basin. A geostatistical analysis has been performed in order to estimate the spatial distribution of the Hurst statistics within the studied area. The analysis makes possible predictions at unsampled locations.

Partial Ponding of Turbidite Systems in a Basin with Subtle Growth-Fold Topography: Laingsburg-Karoo, South Africa

ABSTRACT The Laingsburg Formation of the South African Karoo foreland basin provides an excellent opportunity to study the interplay between deep-water sedimentation and syndepositional fold growth. The 800 m thick formation, consisting of six sand-rich turbidite units separated by basin-wide mud-dominated intervals, occupies a sub-basin created by the onset of Cape Fold Belt deformation during the Late Paleozoic. Episodic growth of local fold structures is documented by significant changes in the thickness of turbidite deposits. Isopach analysis of successive depositional units indicates that initial shortening in the basin occurred through growth of isolated periclinal antiforms and synforms that underwent progressive linkage along their hinges. Terminations, bifurcations, and regional changes in the plunge of the growth folds created a series of depocenters. Spatial continuity of sand-rich turbidite units suggests that surface topographic relief was not large enough to cause complete confinement of deposits in lows. Pronounced thickness variations, however, indicate the strong influence of topography on the location of deposition from turbidity currents. Although the vertical and lateral facies distribution records an overall progradation of slope deposits, downstream variation between erosional, transitional, and depositional architectural types occurs along the stepped basin profile. This tectonically segmented depositional environment produced highly variable coeval stratigraphic units in different parts of the deepwater basin. The partial ponding of Laingsburg turbidite systems represents the middle part of a spectrum between graded basin margin/plain environments (characterized by development of extensive basin-floor fans followed by slope systems) and the conditions of successive fill and spill (as in the Gulf of Mexico slope minibasins with significant salt-withdrawal-generated topography). Laingsburg turbidites were depositing contemporaneously on both lower-slope and basin-plain areas although the maximum thickness of deposition shows dip-parallel migration. Local variations also resulted from changes in usable accommodation due to renewed deformation of the basin floor.

Sedimentary processes and development of the Zenisu deep-sea channel, Philippine Sea

Zenisu deep-sea channel originated from a volcanic arc region, Izu-Ogasawara Island Arc, and vanished in the Shikoku Basin of the Philippine Sea. According to the swath bathymetry, the deep-sea channel can be divided into three segments. They are Zenisu canyon, E-W fan channel and trough-axis channel. A lot of volcanic detritus were deposited in the Zenisu Trough via the deep-sea channel because it originated from volcanic arc settings. On the basis of the swath bathymetry, submersible and seismic reflection data, the deposits are characterized by turbidite and debrite deposits as those in the other major deep-sea channels. Erosion or few sediments were observed in the Zenisu canyon, whereas a lot of turbidites and debrites occurred in the E-W channel and trough axis channel. Cold seep communities, active fault and fluid flow were discovered along the lower slope of the Zenisu Ridge. Vertical sedimentary sequences in the Zenisu Trough consist of the four post-rift sequence units of the Shikoku Basin, among which Units A and B are two turbidite units. The development of Zenisu canyon is controlled by the N-S shear fault, the E-W fan channel is related to the E-W shear fault, and the trough-axis channel is related to the subsidence of central basin.

The Recycled Orogenic Sand Provenance from an Uplifted Thrust Belt, Betic Cordillera, Southern Spain

The Betic Cordillera of southern Spain represents an uplifted foreland fold–thrust belt. Source rock types of the Betic Cordillera include metamorphic (mainly phyllite, schist, quartzite, and gneiss), sedimentary (siliciclastic and carbonate), volcanic (felsic to intermediate pyroclasts), and mantle-derived (peridotite, gabbro, serpentinite, and serpentine schist) rocks. The fluvial systems range that transect the Betic Cordillera are the major detrital source of sediment along the southern Spanish coast, supplying sand to beaches and offshore depositional systems in the Alboran Sea basin. Three key sand petrofacies derived from the Betic mountain belt reflect the main clastic contribution of known source rocks. All the sands are quartzolithic, ranging from quartz-rich to lithic-rich. Fluvial systems draining the Sierra de Los Filabres, the Sierra Nevada, the Sierra de Gador, and the Ma´laga Mountains, and their related beaches constitute a metamorphic–sedimenticlastic quartzolithic sand petrofacies (Qm34610 F463 Lt6269; Lm72614 Lv264 Ls26613), derived dominantly from the Nevado–Fila´bride, Alpuja´rride, and Mala´guide complexes. This quartzolithic petrofacies extends from northeast of Almeria to Torremolinos (southwest of Ma´laga), and northeast of Algeciras. Only one beach sand sample, east of Cabo de Gata, is volcanolithic. Volcanic detritus (mainly having felsitic textures) is derived from Miocene (15–7 Ma) pyroclastic sequences cropping out in the southeast of the chain. This metamorphic–sedimenticlastic quartzolithic petrofacies changes in the coastal stretch from Torremolinos to Marbella, where drainage systems cut across the Serrania de Ronda. Here source rock types include peridotite, gabbro, and serpentinite of the Ronda Peridotite Massif, and metamorphic rocks of the Ma´laguide and Alpuja´rride units. The fluvial and beach sands of this area are quartzolithic (Qm32612 F1063 Lt58611), and include abundant peridotite and serpentinite grains. The latter quartzolithic petrofacies changes abruptly from Algeciras to Ca´diz, where the sand becomes quartz-rich (Qm7765 F462 Lt1964). This sand petrofacies is derived predominantly from recycling of sedimentary sequences, mainly the quartzarenite turbidite units of the Gibraltar Arc (the Algeciras Flysch). This petrofacies is characterized by higher proportion of quartz grains and abundant sedimentary lithic fragments (Lm163 Lv161 Ls9863). The three onshore petrofacies plot in the recycled-orogen provenance compositional field and the lithic to transitional to quartzose recycled subfields of Dickinson (1985). They vary from lithic, to transitional and quartzose depending on their source lithologies in the Betic foreland fold–thrust belt. These actualistic petrofacies best describe the nature and distribution of sand petrofacies derived from a collisional fold–thrust belt where primary and recycled source rocks are interfingered. Deep-marine turbidites of the Alboran Basin have basinwide quartzolithic sands having close compositional relations with Betic Cordillera onshore sand petrofacies. Comparison of detrital modes from mainland to deep-marine environments provides a suitable basis for interpreting the Miocene to Pleistocene sand dispersal history in the Alboran Basin. These modern quartzolithic petrofacies are used to interpret analogous ancient collisional sandstone petrofacies of the Alpine orogenic belt of the western-central Mediterranean region and of other collisional orogenic systems, as a broader point of view.

Late Quaternary turbidite input into the east Mediterranean basin: new radiocarbon constraints on climate and sea-level control

Abstract The Late Pleistocene-Holocene (0–30 ka bp ) allochthonous sedimentation in the Herodotus Basin of the eastern Mediterranean has been controlled, in part, by a combination of regional climatic change and eustatic sea-level fluctuation. A new series of radiocarbon dates, made on planktonic foraminifers and pteropod shells taken from the pelagic and hemipelagic intervals between individual turbidite units, has given bracketing dates for each major turbidity current event that deposited sand and mud on the Herodotus Basin plain. Two partly independent cycles are evident. Climate-induced cycles have lead to an alternation of periods of turbidites sourced from the Nile delta-fan system with those from the North African shelf and Anatolian rise. These correlate with pluvial and inter-pluvial climatic periods recognized in the Nile hinterland. Sea-level cycles have tended to focus turbidite emplacement, from whatever source, at periods of sea-level fall within the latest Wisconsin and sea-level rise from the Wisconsin-Holocene period. In addition to the Herodotus Basin Megaturbidite (HBM) described previously, six other beds with volumes in excess of 25 km 3 and wide lateral extent across the basin can be termed megaturbidites. There is no simple sea-level or climate control on the timing of these events, so we must conclude that triggering and emplacement of megaturbidites is independent and variable.

Seismostratigraphy and sediment studies of Lake Hestvatn, southern Iceland: implications for the deglacial history of the region

AbstractCore 95‐HV was obtained from Lake Hestvatn, southern Iceland, after a seismic survey had identified a sediment sequence over 45 m thick in the lake. The core penetrated ca. 25 m into the sediment correlating with seismostratigraphic unit 2 of alternating seismic facies, with semi‐parallel reflectors of high amplitude and chaotic reflectors. Detailed sedimentological, chemical, magnetic and chronostratigraphical analyses on core 95‐HV, combined with interpretation of the seismic survey, show considerable changes in the sedimentary environment between ca. 10300 and 9000 14C yr BP (reservoir corrected). The lowermost ca. 12 m of the core was deposited in a marine environment during energy shifts from a very rapid accumulation of sandy turbidite units to slower deposition of suspended fine‐grained sediments. The turbidite units, thought to have originated from catastrophic outburst floods (jökulhlaup), contributed greatly to the high sediment accumulation rate at the Younger Dryas–Preboreal transition. Further change in the sedimentary environments is evidenced by the deposition of organic‐rich lake sediment with abundant tephra layers, starting at ca. 9000 yr BP (seismostratigraphic unit 3). The study shows that the inland glacier did not override the site of Hestvatn during the time of sediment deposition in the lake basin. Copyright © 2001 John Wiley & Sons, Ltd.

The Late Cenozoic sedimentary succession of the Taza-Guercif Basin, South Rifian Corridor, Morocco

The Taza–Guercif Basin is located in the area between the eastern termination of the Rifian front and the Middle Atlas System. It was part of the South Rifian Corridor, the Miocene foredeep of the Rif. Stratigraphic analyses enabled us to distinguish two areas with a different evolution. A north-western area, on the left side of the Melloulou River, presents a Tortonian succession up to 500–600 m thick. The south-eastern area is characterised by a succession at least 1500 m thick, which develops from Tortonian to Pliocene. The examined successions locally begin with continental deposits, followed by transitional and shallow marine deposits and by thick, open marine sediments, developing up again to marginal and continental facies. The stratigraphic analysis, together with a structural study, enabled to point out the most significant steps in the evolution of the basin. The onset of sedimentation in the Taza–Guercif Basin, induced by the advancement of the Rifian front during the Tortonian, is marked by fluviatile deposits of the Draa Sidi Saada Unit. The following transgressive trend, outlined by the deposition of the Ras el Ksar Unit, develops upwards with a deposition of open marine deposits of the Blue Marl Subunit, in the late Tortonian. The intercalation of two turbiditic units, late Tortonian–Messinian in age, may represent the expression of a tectonic uplift of the internal Middle Atlas area. During the early Messinian, thick deposits of the Gypsiferous Marl Subunit indicate that strong subsidence still continued in the south-eastern part of the basin. A final regressive trend during the Messinian–Pliocene is pointed out by the development of transitional to continental sedimentation of the Kef ed Deba and Bou Irhardaiene Units.

Processes of sedimentation on a lacustrine border-fault margin; interpretation of cores from Lake Malawi, East Africa

Large lakes are attractive research sites because they provide ideal spatial scales for understanding the complex relationships between facies distributions and processes of sedimentation. In this paper, we describe deepwater facies and associated environments of deposition, based primarily on cores, from the tectonically active South Rukuru sublacustrine fan system of Lake Malawi, east Africa, where river discharge is high, offshore gradients are steep, and gravity-driven processes are common. The coring program was tied to a simultaneous seismic investigation, which is discussed in a companion paper (Soreghan et al. 1999). All facies, except the hemipelagic drape, contain widespread evidence of gravity-driven sedimentation processes, with turbidity currents being responsible for most of the sediment transport and deposition. Multiple fining-up turbidite units dominate the sandy facies of the canyons, channels, mouth-of-canyon fans, and basin-plain fill environments, and thin distal turbidites are present in cores through most of the muddier facies. Angular gravel and coarse sand are common not only throughout the canyons of the sloping relay ramp region but also as far offshore as the main axial channel and its depocenter, the basin-plain fill facies. Turbidites range in thickness from 0.25 to 2.0 m, are usually stacked, and can be recognized by well-defined Bouma sequences. A hemipelagic drape of highly laminated to structureless mud, which forms an uneven blanket over much of the lake bottom, is commonly interspersed with turbidites. Because lake levels can change rapidly as a result of climatic variations, the rate of sediment mobilization is extremely fast relative to marine systems. Vertical excursions in lake level of 100 m over only a few hundred years may lessen the distinctions between highstand and lowstand deposits. The border-fault setting allows coarse sediment to be delivered to the steep offshore, even during periods of lake high-stand. However, during periods of falling and low lake level it is likely that downcutting and erosion of highstand deltas increase the magnitude and intensity of turbidite deposition in the mouth-of-canyon fan and in the basin-plain fill environments.

Diagenesis and reservoir properties of Campanian–Paleocene sandstones in the GANT#1 well, western Nuussuaq, central West Greenland

As part of a study on reservoir characterisation of western Nuussuaq, central West Greenland (Sønderholm & Dam 1998) the diagenesis and reservoir properties of sandstone units in the GANT#1 well located in north western Nuussuaq have been investigated (Kierkegaard 1998). The GANT#1 well was drilled during the summer of 1995 by grønArctic Energy Inc. (Calgary, Canada) as part of the company’s hydrocarbon exploration activities in the region. These also included the drilling of four other wells: GANW#1, GANE#1, GANK#1 and GRO#3 on the south-western coast of Nuussuaq (Fig. 1). The GANT#1 core was examined in order to elucidate the diagenetic and detritus factors that control the present porosity and permeability variations occurring in the Cretaceous – Lower Paleocene succession of western Nuussuaq. The examinations were carried out with standard petrographic methods, including X-ray diffractometry of mudstones and standard polarisation microscopy of thin sections from sandstones, supported by scanning electron microscopy (Kierkegaard 1998). Porosity and permeability measurements from the GANT#1 core generally reveal poor reservoir quality with a range in porosity from 4.92 to 18.79% and in permeability from 0.106 to 90.7 mD, but both porosity and permeability are greater in post-Campanian sandstones compared to Campanian sandstones (Kierkegaard 1998). The GANE#1 well provides a section through Upper Paleocene turbiditic sandstone units alternating with mudstone units which thus furnishes a complementary and stratigraphically higher section to that found in GANT#1. However, porosity and permeability measurements from the GANE#1 well show low arithmetic average values of 6.4% and 1.46 mD, respectively (Andersen 1996).

Calcite dissolution accompanying early diagenesis in turbiditic deep ocean sediments

The oxidation of marine sedimentary organic carbon (C org) by O 2 diffusing from bottom water produces a characteristic linked loss of C org and CaCO 3. A theoretical treatment is presented, based on carbonate system porewater equilibrium and Redfield C org stoichiometry, which predicts that the molar ratio of the two losses should be 1:1.08. This ratio differs significantly from literature values which ignore porewater equilibria. Calcium carbonate dissolution in well-constrained profiles from vertically-homogenous Atlantic turbiditic sediments exceeds that predicted on the basis of this simple theoretical model, indicating that additional processes must also contribute to CaCO 3 loss. Possible additional diagenetic processes involving Fe 2+, Mn 2+, S, NO 3 −and NH 4 +geochemistry are evaluated to account for this discrepancy. It is demonstrated that (after C org respiration) oxidation of trace FeS 2 is the process most likely to have produced the CaCO 3 loss observed. In addition, loss of alkalinity consequent upon the oxidation of NH 4 + is shown to contribute significantly to CaCO 3 loss. This NH 4 + is supplied to the progressive oxidation front from below by diffusion from a much deeper zone of SO 4 2− reduction and appears quantitatively capable of accounting for the remainder of the observed CaCO 3 loss at a site where a progressive oxidation front is active in a turbidite. In order to investigate later diagenetic effects, three buried turbidite units recovered by ODP drilling were also studied. Units sampled in the depth range 130–230 mbsf (age 3–7 My) still retained clear evidence of the effects of similar fossil oxidation fronts in the solid phase C org and CaCO 3 profiles. Thus the rapid oxidation front process leaves a clear signature which can survive longer-term anoxic C org remineralisation processes.

Embaluh Group turbidites in Kalimantan: evolution of a remnant oceanic basin in Borneo during the Late Cretaceous to Palaeogene

Turbidites crop out in large parts of NW Borneo, including Kalimantan and Sarawak. The strata predominantly comprise the Embaluh Group in Kalimantan and lateral equivalents the Rajang Group in Sarawak (collectively known as the Rajang–Embaluh Group). Previous interpretation of these rocks as the product of deep marine deposition and deformation as part of an accretionary prism during Late Cretaceous to Palaeogene subduction implies south-dipping thrusts, northward stratigraphic younging, the existence of a complete arc to trench system, and significant deformation and metamorphism of the turbidites. However new fieldwork described here established consistent southward stratigraphic younging in Kalimantan, no evidence for southward-dipping thrusts and a lack of metamorphism and associated accretionary complex-related deformation features. Furthermore all available palaeontological and radiometric age data indicate that deposition of the bulk of the Rajang–Embaluh Group postdates in-board subduction-related magmatism and thus a complete arc-trench system cannot be demonstrated. Consequently the Rajang–Embaluh Group turbidites are re-interpreted as having formed in either a post-collisional foreland basin or a remnant ocean basin. The lack of a identifiable mountain belt and linked thrust system, and probable oceanic aYnity of crust beneath the Rajang–Embaluh Group basin favour the latter interpretation. Borneo occupies a pivotal position within southeast Asia. This new model of a remnant oceanic basin setting for extensive turbidite units of Borneo therefore has far-reaching implications for the tectonic evolution of Borneo, the South China Sea and SE Asia. The new model is compared with older, Palaeozoic orogenic belts which also contain thick turbidites sequences.

Seismic identification of depositional processes in a turbidite fan environment, Deepwater Block SB-G, NW Sabah

A seismic interpretation and mapping project of the Intra Stage IV F, Upper Lingan Fan Unit was carried out in the deepwater Sabah area 120 km northwest of Kota Kinabalu. The objective of the. project was to delineate the internal architecture of the Upper Seismic sequence of Lingan Fan Unit in an attempt to understand the environment of deposition and the processes involved. With the understanding of depositional environment, reservoir characteristics could be interpreted. The Upper Lingan Fan Unit is a multi-sequence turbidite unit characterised by a period of active channelling and downcutting of earlier deposits as the Lingan Fan progrades basinward. Three seismic facies were delineated from interpreted seismic paper sections of several vintages. Isochores of these three facies were generated and analysed in relation to a depth structure map on Top Lingan Fan Unit. The mapped turbidite sequence is interpreted to be a combination of: a) channel lobe (Facies I), b) levee-channeVcrevasse splay deposits (Facies II) and c) onlapping distal lowstand prograding wedge (Facies III). Identification and interpretation of these facies gives a better understanding of the possible reservoir quality found in any exploration prospects. The results of this shallow analogue study give a better insight in the complex processes involved in the deposition of a turbidite system. Additionally, it also highlights the complexities and resolution issues to be expected in a seismic interpretation at deeper depths. With the availability of high quality seismic data, it is possible to delineate the internal architecture and constrain the predicted reservoir quality in turbidite prospects.

Petroleum geological investigations in East Greenland: project ‘Resources of the sedimentary basins of North and East Greenland’

The multidisciplinary research project ‘Resources of the sedimentary basins of North and East Greenland’ was initiated in 1995 with financial support from the Danish Research Councils (Stemmerik et al., 1996). In 1996, the hydrocarbon-related studies focused on the sedimentary basins in East Greenland between latitudes 71°N and 74°N (Fig. 1) where nine field teams worked for six weeks in July and August supported by a Hughes 500 helicopter. Within the framework of the project, additional hydrocarbon-related field studies were undertaken in 1996 in western North Greenland, and ore-geological studies were carried out in much of North Greenland (Kragh et al., 1997; Stemmerik et al., 1997). The 1996 field work in East Greenland concentrated on integrated structural, sedimentological and biostratigraphical studies of the Upper Permian and Mesozoic successions. Two Ph.D. projects focused on the sedimentology of the Lower Triassic Wordie Creek Formation and the diagenesis of the Middle and Upper Jurassic succession. Post-doctorate studies were carried out on the Mesozoic–Tertiary structural development of the basin and the mineralisation of the Upper Permian Ravnefjeld Formation. Three student projects on Lower Triassic and Middle Jurassic ammonite stratigraphy, Upper Permian sedimentology, and fault-associated mineralisation were also included in the work. The most important new results arising from the 1996 field work are: 1) Re-interpretation of the Upper Permian Schuchert Dal Formation as a lowstand turbidite unit within the Ravnefjeld Formation; 2) Recognition of Middle Jurassic deposits and thick lowermost Cretaceous sandstones on Hold with Hope; 3) Interpretation of a full spectrum of scarp-derived coarse-clastic mass movement deposits interbedded with Cretaceous shales on eastern Traill Ø; 4) The presence of a thick sand-rich Cretaceous turbidite succession on eastern Traill Ø; 5) Re-interpretation of the Mesozoic–Cenozoic fault systems on Traill Ø and Geographical Society Ø.

Shallow marine event sedimentation in a volcanic arc-related setting: the Ordovician Suri Formation, Famatina Range, northwest Argentina

The Loma del Kilómetro Member of the Lower Ordovician Suri Formation records arc-related shelf sedimentation in the Famatina Basin of northwest Argentina. Nine facies, grouped into three facies assemblages, are recognized. Facies assemblage 1 [massive and parallel-laminated mudstones (facies A) locally punctuated by normally graded or parallel-laminated silty sandstones (facies B] records deposition from suspension fall-out and episodic storm-induced turbidity currents in an outer shelf setting. Facies assemblage 2 [massive and parallel-laminated mudstones (facies A) interbedded with rippled-top very fine-grained sandstones (facies D)] is interpreted as the product of background sedimentation alternating with distal storm events in a middle shelf environment. Facies assemblage 3 [normally graded coarse to fine-grained sandstones (facies C); parallel-laminated to low angle cross-stratified sandstones (facies E); hummocky cross-stratified sandstones and siltstones (facies F); interstratified fine-grained sandstones and mudstones (facies G); massive muddy siltstones and sandstones (facies H); tuffaceous sandstones (facies 1); and interbedded thin units of massive and parallel-laminated mudstones (facies A)] is thought to represent volcaniclastic mass flow and storm deposition coupled with subordinated suspension fall-out in an inner-shelf to lower-shoreface setting. The Loma del Kilómetro Member records regress ive-transgressive sedimentation in a storm- and mass flow-dominated high-gradient shelf. Volcano-tectonic activity was the important control on shelf morphology, while relative sea-level change influenced sedimentation. The lower part of the succession is attributed to mud blanketing during high stand and volcanic quiescence. Progradation of the inner shelf to lower shoreface facies assemblage in the middle part represents an abrupt basinward shoreline migration. An erosive-based, non-volcaniclastic, turbidite unit at the base of this package suggests a sea level fall. Pyroclastic detritus, andesites, and a non-volcanic terrain were eroded and their detritus was transported basinward and redeposited by sediment gravity flows during the low stand. The local coexistence of juvenile pyroclastic detritus and fossils suggests reworking of rare ash-falls. The upper part of the Loma del Kilómetro Member records a transgression with no evidence of contemporaneous volcanism. Biostratinomic, paleoecologic, and ichnologic analyses support this paleoenvironmental interpretations and provide independent evidence for the dominance of episodic sedimentation in an arc-related shallow marine setting. Fossil concentrations were mainly formed by event processes, such as storms and volcaniclastic mass flows. High depositional rates inhibited formation of sediment-starved biogenic concentrations. Collectively, trace fossils belong to the Cruziana ichnofacies. Low diversity, scarcity, and presence of relatively simple forms indicate benthic activity under stressful conditions, most probably linked to high sedimentation rates. Contrasting sedimentary dynamics between ‘normal shelves’ and their volcaniclastic counterparts produce distinct and particular signatures in the stratigraphic record. Arc-related shelves are typified by event deposition with significant participation of sediment gravity flows, relatively high sedimentation rates, textural and mineralogical immaturity of sediments, scarcity and low diversity of trace fossils, and dominance of transported and reworked faunal assemblages genetically related to episodic processes.