Basal Unconformity Research Articles

Late Miocene to Plio-Pleistocene fluvio-lacustrine system in the Karacasu Basin (SW Anatolia, Turkey): Depositional, paleogeographic and paleoclimatic implications

The sedimentary record of the late Cenozoic Karacasu Basin, a long-lived continental half-graben from southwestern Turkey, is characterized by siliciclastic and carbonate deposits. Sedimentation was controlled by an active NW–SE trending major normal fault along the basin's southern margin and by climatically-induced lake-level changes. Detailed facies analysis subdivides the entire Neogene–Quaternary basin-fill into three distinct litostratigraphic units representing paleogeographic changes and sedimentation patterns throughout the basin evolution.Sedimentation commenced in the late Miocene with the deposition of proximal–medial alluvial fan and fluvial facies (Damdere Formation; FA1). At this stage, alluvial fans developed in elevated areas to the south, prograding towards the basin center. At the beginning of the Pliocene, fresh to slightly alkaline, shallow lake deposits (FA2a) of the Karacaören Formation formed. The lake became open and meromictic conditions developed (FA2b). Pollen data from the FA2b facies show that climate was arid to humid. Climate probably changed cyclically through time producing alternation of Artemisia steppe (cold and dry periods) and more forested vegetation (warm and wet). The open lake facies passes upwards into lake margin facies (FA2c), but it was still dominated by alkaline to slightly saline lake conditions. Sedimentation was almost continuous from the late Miocene to Pleistocene.In the early Quaternary, the basin was dissected by the re-activation of basin bounding faults. The unconformable base of the overlying Quaternary deposits (Karacasu Formation; FA3) reflected the basin's transformation from a half-graben into a full-graben system. Oxygen isotope data from carbonates show an alternation of humid climatic periods, when freshwater settings predominated, and semiarid/arid periods in which the basin hosted alkaline and saline water lakes. Neotectonic activity has rejuvenated many of the basin-bounding faults, causing development of talus aprons and local alluvial fans. The basin was progressively incised by modern rivers that have largely smoothed out the topographic relief of the graben margins.

Regional Sequence Stratigraphy, Biostratigraphy, and Facies of the Upper Cretaceous Austin Chalk in South and Central Texas

This regional study constructs a sequence stratigraphic framework for the Upper Cretaceous Austin Chalk in South and Central Texas, based on new nannofossil biostratigraphy, core descriptions, correlations of wireline logs, and synthesis of outcrop, core, and paleontological descriptions from literature. The study area extends along strike from the Rio Grande to Austin, and along dip from the outcrop belt to the limit of downdip well control at the Lower Cretaceous shelf edges. New biostratigraphic control enables correlations across the region and ties the stratigraphic nomenclature from the outcrop to the major producing petroleum fields on either side of the San Marcos Arch. The upper Turonian–lower Campanian Austin Chalk consists of three unconformity-bounded sequences, with a basal unconformity, two internal unconformities, and an overlying unconformity. Sequence boundaries are recognized by nannofossil biostratigraphic hiatuses, which coincide with hardgrounds, Glossifungites ichnofacies, and coarse-grained glauconitic and phosphatic skeletal lags in cores and outcrops. Glauconite-rich beds have regionally extensive log character that overlie angularly truncated section. Facies within sequences can be distinguished as inner ramp (light colored skeletal wackestone, with diverse burrowers and macrofossils), middle ramp (medium gray horizontally burrowed skeletal wackestone), and outer ramp (interbedded dark horizontally burrowed and laminated skeletal wackestone), based on a depositional model of increasing oxygen content in shallower water. Regional cross sections show that the three sequences (AC–I to AC–III) in the Austin Chalk have an extremely asymmetric distribution across the San Marcos Arch. The upper Turonian–lower Coniacian section (AC–I) is very thick west of the San Marcos Arch but is almost completely eroded in the east. The upper Coniacian–Santonian section (AC–II) is very thick east of the San Marcos Arch but is almost completely eroded over the San Marcos arch, and is thin on the west side. The upper Santonian–lower Campanian sequence (AC–III) is more balanced, but similar to the middle sequence. Downdip, the Austin Chalk thins dramatically toward the relict Lower Cretaceous Edwards margin. The Austin Chalk was deposited as a downdip thinning wedge, due to greater carbonate productivity updip and non-deposition or erosion downdip. This interpretation is based on the geometry of the overlying Anacacho Formation, which thickens as the Austin Chalk thins, and internal facies relationships and markers within the Austin Chalk. Petroleum reservoirs in the Austin Chalk occur across a range of inner to outer ramp depositional environments but generally are older on the west side of the San Marcos Arch compared to the east side.

Various styles of deposition of Pleistocene glacigenic deposits in Essex, England

Late Ordovician siliciclastic glacial and related deposits on the North Gondwana continental shelf comprise striking incised features referred to as large-scale channel-fill structures. These are described from exposures and aerial photographs in two areas (Adrar and Hodh) in Mauritania. The channel-fill structures are up to several kilometres in length and several hundred metres wide. They are slightly sinuous narrow sandstone bodies deeply incised into the Late Ordovician glacial drift and the Cambro–Ordovician bedrock. Palaeogeographical reconstructions indicate that these structures were located in the ice-marginal zone. They are preferentially oriented parallel to palaeo-ice-flow directions. Sedimentary facies analysis reveals three vertically stacked architectural units infilling a U-shaped erosional basal unconformity. Unit 1 is thin and made up of conglomeratic sandstones of various origins comprising debris-flows, deltaic-like progradational foresets and trough cross-stratified sands. In Adrar, architectural unit 2 shows sheet-like, evenly laminated, fine- to medium-grained, well sorted sandstones characteristic of a high-energy marine environment. In contrast, in the Hodh area, unit 2 displays vertically stacked successions of coarse-grained sandstones that indicate high-discharge sediment-laden flows; these are interpreted as meltwater sediments. In both Adrar and Hodh, architectural unit 3 is made up of gravelly coarse-grained, trough cross-stratified sandstone emplaced in a braided, low sinuosity fluvial environment. The three-stage infilling history and the palaeogeographic location combined with size and shape criteria make it possible to compare the channel-fill structures with Pleistocene tunnel-valleys found in areas of low-relief in cool temperate and low Arctic zones. Such features are incised by high-pressure subglacial meltwater and later infilled by proglacial to postglacial deposits. Based on this comparison, architectural unit 1 represents subglacial or proglacial outwash, high sediment discharge in unit 2 took place in a proglacial environment in the Hodh area while an isostatically downwarped shelf was responsible for a marine incursion in the Adrar channels, and deposits in unit 3 are fluvial postglacial sediments. Identification of preserved channel-fill structures may be useful in reconstructing Late Ordovician ice-sheet dynamics.

Solute transport across basement/cover interfaces by buoyancy-driven thermohaline convection: Implications for the formation of unconformity-related uranium deposits

Previous studies have suggested that buoyancy-driven convection could only have developed in thick sandstone aquifers within Proterozoic intracratonic sedimentary basins that host unconformity-related uranium deposits. On the other hand, oxygen and hydrogen isotopic and fluid inclusion studies of quartz and carbonate veins have shown that basinal brines have interacted with basement rocks and basement-derived fluids pervasively. Finite element modeling was conducted to investigate the potential mechanisms driving fluid interaction across a basement/cover unconformity. We firstly constructed a simplified conceptual model by integrating the known features shared by the Athabasca, Thelon and Kombolgie basins and their Phanerozoic counterparts. Based on this conceptual model, various numerical scenarios were designed to examine buoyancy-driven (heat and solute transport are coupled) fluid flow patterns and the corresponding solute transport. The results show that thermohaline convection may have penetrated into the basement for up to 1 to 2 km below the unconformity, when typical hydrological parameters for these Proterozoic hydrogeological units were used. Fluid flow velocities in the sandstone sequence were several orders of magnitude larger than those in the basement. If a uranium source (a pore fluid with 500 mg/l uranium) was assumed to be located in the center of the basin below the unconformity, uranium was able to gradually spread into the sandstone aquifer through thermohaline convection without considering any contribution from fluid-rock interaction. The uranium concentration of basinal fluids above the uranium source approached 15 and 24 mg/l after 1 and 5 m.y. of modeling time, respectively. If the uranium source was initially located at the center of the aquifer, a uranium plume developed and percolated down to 2 km below the unconformity at 5 m.y. The location of the uranium source also affects the solute transport efficiency. A uranium source located around the sloping basal unconformity, either in the basin fill or basement, close to the basin margin, led to a wider uranium plume than if it was located near the center of the basin. Given appropriate hydrological conditions, thermohaline convection could have caused widespread interaction of basinal brines with basement rocks or basement-derived fluids in Proterozoic basins where unconformity-related uranium deposits have developed, and that enough uranium could have been leached from the uranium-rich basement to form large, high-grade unconformity-related uranium deposits.

Palaeoenvironments of Early Devonian fish and other aquatic fauna of the Campbellton Formation, New Brunswick, Canada

The palaeoecology and palaeoenvironments of the classic fish-bearing strata of the Campbellton Formation are here described. A well-documented fossil assemblage includes acanthodian, placoderm, chondrichthyan, and cephalaspid fishes, tracheophyte remains, eurypterids, ostracods, and molluscs. As part of the Old Red Sandstone continent, the Emsian Campbellton Formation occupied a small basin within the mountainous terrain of the Acadian Orogen, and may have connected through a narrow seaway to a closing foreland basin. The highest diversity of vertebrate fossils was found above a basal unconformity with Val d'Amour Formation volcanics, marked by fissure-fills and unstratified breccia representing a craggy, irregular palaeosurface that provided shelter for some fauna and sediment for rapid burial of skeletal remains. Overlying prodeltaic calcareous siltstones are also highly fossiliferous, and represent a low energy habitat. Occurrences of pterygotid eurypterids, cephalaspids, and chondrichthyans in deltaic sandstones, place these taxa in a habitat that may have been variably fresh or brackish. The surrounding landscape was vegetated with primitive tracheophytes, and abundant plant debris is preserved throughout the basin. The diverse assemblage of fish, arthropods, gastropods, plants and microbial growths suggests that a trophically complex ecosystem was in existence.

Tectono‐stratigraphic signature of multiphased rifting on divergent margins (deep‐offshore southwest Iberia, North Atlantic)

Regional 2D multichannel seismic, borehole, dredge and outcrop data, together with burial models for strata in southwest Iberia, are used to investigate the tectono‐stratigraphic signature of multiphased rifting on divergent margins. Our burial model reveals that Mesozoic extension occurred during three main phases, each comprising distinct subsidence pulses separated by short‐lived periods of crustal uplift. The importance of the three phases varies across discrete sectors of the margin, each one revealing similar depositional architectures and associated tectonic systems tracts: 1) the Rift Initiation phase, characterized by incipient subsidence and overall aggradation/progradation over a basal unconformity, 2) the Rift Climax phase, which marks maxima of tectonic subsidence and is characterized by retrogradation‐progradation, and 3) the Late Rift phase, recording the progradational infill of the basin and the effects of eustasy. The Rift Initiation systems tracts comprise Sinemurian and late Callovian‐early Oxfordian strata. Marine units in the Pliensbachian and Late Oxfordian‐Kimmeridgian represent the Rift Climax phase, a period marked by the development of Maximum Flooding Surfaces. Late Rift deposits were identified in the Rhaetian‐Hettangian, Toarcian‐Bathonian and Kimmeridgian‐Berriasian. The results of this work are important to the economic exploration of deep‐offshore rift basins, as they reveal that sequence stratigraphy can be used to predict sedimentary facies distribution in more distal segments of such basins. Significantly, this work recognizes that multiple tectonic‐stratigraphic (rift) cycles can occur on deep‐offshore rift basins, from the onset of rift‐related extension until continental break‐up, a character that contrast to what is known from deep‐sea drilling data from the distal margin of Northwest Iberia.

Forced regressive tidal flats: response to falling sea level in tide-dominated settings

Abstract During forced regression, the coastline advances seaward irrespective of sediment supply and relocates to a topographically lower position. Forced-regressive deposits are a component of the falling-stage systems tract. They are recognized by a basal unconformity which records erosion during the seaward facies shift, and are in turn capped by another unconformity due to subaerial exposure or wave ravinement during subsequent transgression. In wave-dominated settings, the basal “regressive surface of marine erosion” is a scoured surface cut by wave action, because erosion is needed to maintain the seaward-sloping bathymetric profile in equilibrium with the wave energy. The response to falling sea level in tide-dominated settings has not been as thoroughly discussed. The stratal architecture of the Lower Cambrian Gog Group of the southern Canadian Rocky Mountains reveals a new mechanism for the formation of this surface landward of the lever point of balance between sedimentation and erosion in the subtidal environment. Subtidal parasequences of the Lake O’Hara Member are typically composed of thin-bedded mudstone and ripple cross-laminated sandstone, followed by Skolithos pipe rock and thin- to medium-bedded cross-stratified sandstone overlain by thick-bedded cross-stratified sandstone. However, at the top of the Lake O’Hara Member, an erosional surface truncates a pipe-rock interval and the expected capping cross-stratified sandstones are absent. Overlying this surface, the Lake Oesa Member comprises tidal-flat deposits composed mainly of thinly interbedded mudstone and sandstone. This jump in facies at the erosional surface can be explained as a response to a fall in sea level. As the shoreline is forced to regress, the laterally continuous tidal flats advance and the pre-existing shallow-subtidal compound dunes are scoured by strong tidal currents, along with waves, that gradually carve a new equilibrium profile. We argue that the accretion of intertidal flats on top of subtidal sands is an overlooked yet predictable component of falling-stage systems tracts in tide-dominated settings.

Forebulge migration in late Cenozoic Western Taiwan Foreland Basin

In the distal part of the Western Taiwan Foreland Basin, the forebulge is caused by emplacement and loading of the Taiwan orogenic belt onto a flexed Eurasian plate. Examining reflection seismic data collected from the shelf region in the Taiwan Strait, we present the merging of successive unconformities which is the result of regional uplift due to upwarp of flexural forebulge, erosion on elevated highs, and subsequent lateral shift of forebulge. The intersection points occurring along westward merging unconformities overstepped toward Chinese craton, representing cratonward migrations of a forebulge in the Western Taiwan Foreland basin. A hiatus map across foreland basal unconformity shows not only cross-strike increase in chronostratigraphic gap, but also a S-shaped distribution of similar hiatus intervals in the along-strike direction. Foreland basin sequence in the Taiwan Strait can thus be divided into two zones on the basis of seismic reflection patterns. The distal zone comprises flat-lying layers, while the proximal zone comprises feather-edging of reflection intervals and onlapping reflections. The current position of forebulge crest is suggested to be farther cratonward than that of the middle part of the Taiwan Strait.

Far-field impact and coastal sedimentation associated with the 2006 Java tsunami in West Australia

A detailed assessment of the impact of a far-field tsunami on the Australian coastline was carried out in the Steep Point region of Western Australia following the July 17 2006 Java tsunami. Tsunami inundation and run-up were mapped on the basis of eyewitness accounts, debris lines, vegetation damage and the occurrence of recently deposited fish, starfish, corals and sea urchins well above high-tide mark. A topographic survey using kinematic GPS with accuracies of 0.02 m in the horizontal and 0.04 m in the vertical recorded flow depths of between 1 and 2 m, inundation of up to 200 m inland, and a maximum recorded run-up of 7.9 m AHD (Australian Height Datum). The tsunami impacted the sparsely populated Steep Point coastline close to low tide. It caused widespread erosion in the littoral zone, extensive vegetation damage and destroyed several campsites. Eyewitnesses reported three waves in the tsunami wave train, the second being the largest. A sand sheet, up to 14 cm thick and tapering landwards over 200 m, was deposited over coastal dunes. The deposits are predominantly composed of moderately well-sorted, medium-grained carbonate sand with some gravel and organic debris. A basal unconformity defines the boundary between tsunami sediments and underlying aeolian dune sand. Evidence for up to three individual waves is preserved as normally graded sequences mantled by layers of dark grey, organic-rich fine silty sand. Given the strong wind regimes in the area and the similarity of the underlying dune deposits to the tsunami sediments, it is likely that seasonal erosion will remove all traces of these sediment sheets within years to decades.

The Osa Group: an auriferous Pliocene sedimentary unit from the Osa Peninsula, southern Costa Rica

Recent geological mapping in the Osa Peninsula and micropaleontological studies have led to the recognition of a new lithostratigraphic unit- the Osa Group. This group replaces the Charco Azul and the Armuelles Formation in the Osa Peninsula as shown on the DGMP 1:200,000 geological map, and includes tje informally defined Punta La Chancha formation. It is defined as a sequenceof semi-consolidated to lithified, greenish-grey, graded, graywacke-type conglomerates, sandstones, siltstones and claystones of Mid to Upper Pliocene age lying unconformably on a Nicoya Complex basement, and unconformably overlain by unconsolidated Quaternary sediments of the Puerto Jimenez Group. The Osa Group was deposited in a wide variety of sedimentary enviroments ranging from fresh-water fluviatile; brackish lagoonal-estuarine-deltaic; shallow shelf down to deep marine basin. Economically important placer gold is widespread and tendsto be concentrated locally in the conglomeratic members, especially at the basal unconformity. It is suggested that the Osa Group sediments were deposited in intermittently subsiding pull-apart basins governed by wrench faulting in a geological setting similar to that on and around the Osa Peninsula at present. The present-day Panama- Coiba Fracture Zone in the Gulf of Panama can be extrapolated northwarsd via three submarine canyons on the continental slope, and thence into a system of braided northwest-trending transcurrent faults parallel to Costa Rica's Pacific coastline. The landforms and bathymetry of the region surrounding the Osa Peninsula are considered to be largely controlled by (intermittent) dextral strike-slip movement on the main fractures. Within this tectonic setting the Golfo Dulce basin and the Coto Colorado-Chiriqui lowlands form pull-apart or tipped-wedge basins, whereas the Osa and the Burica Peninsula, and the offshore Coiba Ridge, are tilted horst blocks.

The role of glacio-eustasy in sequence formation: Mid-Atlantic Continental Margin, USA

Previous studies of the Mid-Atlantic Continental Margin had shown the lack of correlation between Pleistocene glacio-eustatic oscillations and sequence boundaries. This study documents late Pleistocene to Holocene paleoenvironments within a seismic stratigraphic sequence in order to characterize sequence formation and the development of unconformities in response to glacio-eustatic oscillations. The data, high-resolution CHIRP subbottom profiling and 28 vibracores, were collected in water depths of 38 m to 145 m as part of the R/V Endeavor cruise 370 survey to the Mid-Atlantic Continental Margin. Sediment sources, sedimentary and seismic facies associated with the last major glaciation, and oxygen isotope stages OIS 5 to OIS 1 were examined and a chronology was constructed from mollusk assemblages of intertidal origin that allowed tracking the paleoshoreline from 19 to 6 ka BP. The maximum extent of the Last Glacial Maximum (LGM) paleoshoreline at 17 ka BP was documented near the 120 m modern isobath. The shoreline was marked by sediment mounds up to 5 m high and troughs 15 m deep and 1 km wide (interpreted as shoreface and tidal inlet or lagoons) formed from 17 to 15 ka BP. Five kilometers seaward from the LGM shoreline and the head of the Hudson Canyon is a wedge-shaped feature interpreted as a lowstand delta. Sediments older than the last glacial, possibly OIS 3 or OIS 5, onlap a basal unconformity. We correlate this unconformity to a previously identified sequence boundary. Approximately 20 m to 30 m of sediment accumulated on the outer shelf NE of the Hudson Canyon during the last glacial and little deposition has occurred since ~ 11 ka BP. The modern outershelf is covered by ~ 1 m of mollusk-rich glauconitic sand deposited from latest Pleistocene to early Holocene, 15 to 11 ka BP. These results showed a close correspondence between depositional environments and glacio-eustasy but sequence development and sequence boundary formation appeared to be influenced by the incomplete filling of accommodation space due to poor preservation during OIS 2 and low accumulation during the Holocene.

Facies architecture of the Stuttgart Formation (Schilfsandstein, Upper Triassic), central Germany, and its comparison with modern Ganga system, India

The Stuttgart Formation (Schilfsandstein) is approximately 50 m thick in Thuringia, representing deposition during the “Mid-Carnian Wet Intermezzo”. Stratigraphically it occurs between the Grabfeld and Weser formations, which formed under arid conditions . It comprises NNE–SSW-trending elongate, anastomosing channelized sand-rich bodies with erosional bases (channel belts) that are several kilometres wide and pass laterally into predominantly mudstones deposited in interfluve areas. The source area of these clastics was the uplifted Norwegian Caledonides. Muddy interfluve facies is dominant in exposures in Thuringia, Central Germany. The Lower Stuttgart Formation has an unconformable base that is locally overlain by metre-thick “Basal Beds”. These consist of grey mudstones and thin sandstones deposited under humid conditions in predominantly shallow brackish water environments after a marine ingression via the Eastern Carpathian/Upper Silesian Gate. The following 30–40 m-grey, fine-grained sandstones, siltstones and mudstones were deposited in fluvial environments in channel belts and interfluve areas under humid conditions. These are followed by predominantly reddish mudstones and sandstones of mainly fluvial origin, deposited under somewhat drier conditions with seasonal droughts. The Upper Stuttgart Formation may be more than 16 m thick; it comprises reddish and grey sandstones and mudstones that were mostly deposited in lake delta settings by recurring flash floods. During the deposition of this unit climate was weakly humid with less prominent seasonal draughts. The modern Ganga Plain of India is an analogue for the depositional setting of the Stuttgart Formation. Climatic conditions in Ganga Plain are humid monsoonal with seasonal droughts and roughly comparable with those interpreted for Mid-Carnian times in Germany. The sandy deposits of incised channel belts and channels and muddy deposits of interfluve areas in the Ganga Plain are comparable with the sandstone-dominated channelized facies and mudstone dominated interfluve facies of the Stuttgart Formation, respectively.

Deposition in the Kuznetsk Basin, Siberia: Insights into the Permian–Triassic transition and the Mesozoic evolution of Central Asia

This paper describes the Permian–Mesozoic stratigraphy of the Kuznetsk Basin, southern Siberia, which is adjacent to the vast and hydrocarbon-rich West Siberian Basin and on the edge of the Siberian flood basalts. The basin fill is Permian to Cretaceous in age, and is dominated by non-marine siliciclastics up to ∼ 7 km thick. Palaeocurrent indicators show dominant flow to the north/northeast during the Permian to Jurassic. Fourteen lithofacies are grouped in three facies associations: fluvial channel belt, overbank and floodplain/floodplain pond. Coal-bearing Permian siliciclastics are interpreted as meandering river deposits in a foreland basin, with subsidence generated by thrust-sheet loading from at least three basin margins. These sediments pass abruptly but conformably upwards into coal-barren sandstones and conglomerates and siltstones, interpreted as braided river deposits. Two basalt flows occur within the coal-barren succession. A recently-published, precise Ar-Ar age of 250.3 ± 0.7 Ma for the lower of these basalts, < 50 m above the sedimentary transition, suggests that the Permian–Triassic boundary occurs just above this flow. We relate the loss of coal-producing flora and the increase in mean sediment grain size to vegetation loss, in turn triggered by the eruption of the Siberian flood basalts to the north. End-Permian and Lower Triassic(?) strata are overlain by Lower Jurassic fluvial siliciclastics via a gentle angular unconformity. Conglomerates punctuate a sandstone-dominated succession that continues in to the Middle Jurassic. Both the basal unconformity and the rejuvenation in sedimentation may result from intracontinental thrusting at the basin margins and beyond; this thrusting was triggered by orogenies at the Eurasian margin. Lower and mid Cretaceous siliciclastics are poorly exposed and crop out only locally: field relations indicate an angular unconformity at their base. The end-Permian stratigraphy in the Kuznetsk Basin documents the environmental crisis at the time of the Siberian flood basalts, and reinforces the link between these eruptions and climatic and environmental deterioration. The Mesozoic sedimentary record highlights how episodic deformation influenced sediment supply to the West Siberian Basin, and is an example of the record of Eurasian assembly and deformation preserved within the continental interior.

Sedimentology, architecture, and sequence stratigraphy of coarse-grained, submarine canyon fills from the Pleistocene (Gelasian-Calabrian) of the Peri-Adriatic basin, central Italy

The Plio-Pleistocene stratigraphic record of the Peri-Adriatic basin (eastern central Italy) is well exposed along the uplifted western margin of the basin and consists of a series of coarse-grained slope canyon fills encased in a thick succession of hemipelagic mudstones. This study deals with the detailed sedimentology, stratal architecture, and sequence-stratigraphic interpretation of two of these submarine canyon-fills (namely CMC1 and CMC2) exposed at Colle Montarone. These strata contain widespread evidence of gravity-driven sedimentation processes, with high- and low-density turbidity currents, slumps and cohesive debris flows being responsible for most of the sediment transport and deposition. Beds are organised into four recurrent lithofacies, each corresponding to a specific deep-water depositional element: (i) clast-supported conglomerates (channel complexes); (ii) thin-bedded sandstones and mudstones (levee-overbank); (iii) very thinly-bedded mudstones (tributary channels); (iv) pebbly mudstones and chaotically bedded mudstones (mass-transport complexes). One of the most conspicuous attributes of the canyon-fill sediments exposed at Colle Montarone is their pronounced cyclic packaging, resulting from several pulses of erosion and fill. The lithofacies listed above combine to form a number of discontinuity-bound stratal packages, or high-frequency depositional sequences, each characteristically having an overall fining-upward character. Within sequences, systematic vertical trends in their constituent lithofacies define two main stratigraphic motifs, namely Motif-1 and Motif-2, which are typical of CMC1 and CMC2, respectively. Each sequence is deemed to reflect cyclical changes in volume, composition, and rheology of sediment gravity flows entering the canyons and comprises a basal unconformity (sequence boundary), succeeded by a high net-gross channel-levee complex (lowstand systems tract), overlain by a succession of either a low net-gross heterolithic deposits (early transgressive systems tract, Motif-1 sequences) or mass-transport deposits (transgressive to falling stage systems tract, Motif-2 sequences). Given the severe Icehouse climatic regime during the Pleistocene, the cyclic arrangement is most reasonably attributed primarily to repeated glacio-eustatic fluctuations in global sea level, which regulated the flux of terrigenous clastic sediment to the slope and basin-floor settings. Results from this paper are directly applicable to the oil and gas industry when making predictions of reservoir and seal relationships within slope turbidite systems developed during periods of high-frequency, eustatic sea-level fluctuations.

Sedimentary record of Late Neoproterozoic rifting in the NW Tarim Basin, China

Upper Neoproterozoic sediments which have been exhumed in the NW Tarim Basin, China, comprise a 600–900 m thick non-marine to marine synrift succession that reveals an early phase of extension. This study presents new data from two measured and interpreted stratigraphic sections near Aksu and Wushi. The synrift succession unconformably overlies high-grade metamorphic basement and consists of the non-marine Sugaitebulake Fm and the marine Qegebulake Fm. The Sugaitebulake Fm is characterised by a basal (alluvial fan) conglomerate that passes upwards into fluvial and lacustrine facies sandstones and siltstones. The stratigraphic position of the basal unconformity and the thickness of the basal conglomerate is locally variable, which reflects the formation of small, isolated depocentres during the onset of extension, in addition to a pre-existing topography. The Sugaitebulake Fm also contains a series of concordant, extrusive basalts which record episodic volcanism, and together with the fluvial and lacustrine facies indicate the development of the rift system and the growth, interaction and linkage of extensional faults. The conformable Qegebulake Fm is characterised by foreshore to intertidal facies dolomites that contain abundant stromatolites, representing a basin-wide marine transgression and the rift climax stage. This synrift sequence is shown to correlate with a similar succession in NW Australia, and corresponds to at least partial separation of the Tarim Block from East Gondwana during the latest Neoproterozoic to Early Palaeozoic.

Application of modern 2-D and 3-D seismic-reflection techniques for uranium exploration in the Athabasca BasinThis article is one of a series of papers published in this Special Issue on the themeLithoprobe — parameters, processes, and the evolution of a continent.

Seismic-reflection techniques have been applied in several studies over the last 20 years as a uranium-exploration tool within the Athabasca Basin and have been utilized to provide the larger structural context for known uranium deposits within the basin. At the crustal scale, deposits within the eastern Athabasca Basin are shown to be associated with deep-seated shear zones that originated during Trans-Hudson orogeny and have subsequently been reactivated during and subsequent to deposition of the basin-fill sandstones. Seismic properties of the Athabasca sandstones and underlying basement have been determined through in situ borehole measurements. Reflectivity within the sandstones is generally weak. Seismically recognizable signatures are primarily associated with variations in fracture density, porosity, and degree of silicification. The basement unconformity and regolith, a prime target of exploration, is widely imaged as it is characterized by variable but generally distinct reflectivity. Results from the McArthur River mine site suggest that the spatial coincidence of seismically imaged high-velocity zones and deep-seated faults that offset the unconformity may be a more broadly applicable exploration targeting tool. Three-dimensional (3-D) seismic imaging near existing ore zones can define the local structural controls on the mineralization and point the way to new targets, thus leading to more efficient exploration drilling programs. Furthermore, seismically generated structural maps of the unconformity and rock competence properties may play a significant role at the outset of mine planning.

Ordovician deformations in the Pyrenees: new insights into the significance of pre-Variscan (‘sardic’) tectonics

AbstractTwo deformational events which developed prior to the Variscan structures can be characterized in the Palaeozoic rocks of the Pyrenees: a Middle (?) Ordovician folding event and a Late Ordovician fracture episode. The Middle (?) Ordovician folding event gives rise to NW–SE- to N–S-oriented, metric- to hectometric-sized folds, without cleavage formation or related metamorphism. These folds can account for the deformation and uplift of the pre-Upper Ordovician (Cambro-Ordovician) sequence and for the formation of the Upper Ordovician unconformity. Ordovician folds control the orientation of the Variscan main-folding-phase minor structures, fold axes and intersection lineation in the Cambro-Ordovician sediments. The Late Ordovician fracture episode gave rise to normal faults affecting the lower part of the Upper Ordovician series, the basal unconformity and the underlying Cambro-Ordovician metasediments. Displacement of some of these faults diminishes progressively upwards of the series and tapers off in the upper part of the Upper Ordovician rocks, indicating that the faults became inactive during Late Ordovician times before deposition of the Ashgillian metasediments. Normal faults can be linked to the Upper Ordovician volcanic activity, which has been extensively described in the Pyrenees. The aforementioned deformation episodes took place after the Early Ordovician magmatic event, which gave rise to a large volume of plutonic rocks in the Pyrenees as in other segments of the European Variscides. This Middle Ordovician contractional event separated two extensional events in the Pyrenees from Early Ordovician to Silurian times. This event prevents us from assuming the existence of a continuous extensional regime through Ordovician and Silurian times, and suggests a more complex evolution of this segment of the northern Gondwana margin during the Ordovician.

Channel structures formed by contour currents and fluid expulsion: significance for Late Neogene development of the central North Sea basin

Abstract Channel horizons, delimiting a major Pliocene sedimentary prism in the central North Sea basin, have been investigated in the context of oceanographic, climatic and tectonic controls, using 3D seismic and well data from the UK and Norwegian sector. Aggrading channel structures, forming 700–2500 m wide and 75–150 m deep linear to arcuate troughs, superimpose the truncated distal part of the prism in the northern Central Graben. The origin of the aggradational channel and prism complex is related to differential deposition and erosion by contour currents that intensified as tectonic subsidence formed a deep marine basin (&gt;600 m) with open connections to the Atlantic and Norwegian Sea. The largest channel troughs appear to be filled with consolidated sediments and emerge from areas where the Neogene strata is pierced by salt diapir chimneys. From Early Pleistocene ( c . 2.5 Ma) the channels were subject to progressive burial by rapid clinoform progradation. Based on the seismic observations, a depositional model is proposed that relates contourite channel development to fluid expulsion from salt diapir structures and fracture zones extending from lower Miocene strata. The sedimentary prism accumulated over a Late Miocene/Early Pliocene Unconformity marked by incised channels that are reminiscent of a northward diverging drainage system. This erosive low-stand development is probably related to late Alpine compression, which promoted uplift in the British Isles and the Channel region. The ensuing subsidence of the central North Sea, associated with concomitant uplift of the Norwegian–Danish Basin, generated the present southwestward dip of 0.5–0.8° of the basal unconformity. The Late Miocene compressional phase followed by rapid basin depending during Pliocene to early Pleistocene suggests that present concepts of North Sea basin development have to be re-evaluated.

Palaeodrainage systems at the basal unconformity of the Bohemian Cretaceous Basin: roles of inherited fault systems and basement lithology during the onset of basin filling

Palaeodrainage systems at the basal unconformity of the Bohemian Cretaceous Basin: roles of inherited fault systems and basement lithology during the onset of basin filling

Numerical modeling of hydrothermal fluid flow in the Paleoproterozoic Thelon Basin, Nunavut, Canada

This paper presents the first computational modeling of fluid flow and heat transport associated with the formation of unconformity-type uranium deposits in the Thelon Basin, Nunavut, Canada. A paleo-hydrostratigraphic model is developed for the time of uranium mineralization through integration of existing geological, geophysical and geochemical data, based on which a series of scenarios are designed to examine the roles of various factors in controlling ore-forming fluid flow. Our numerical experiments suggest that free convection may develop throughout the Thelon Formation at geothermal gradients of 25 to 35 °C/km, and that this may be an attractive mechanism for uranium transport. Changes to the assumed geothermal gradient, the presence of basal unconformities, or of diagenetic aquitards do not modify the fluid flow patterns at the basin scale. The highest fluid fluxes are generated at the unconformity, which supports the fact that uranium is concentrated within or around the unconformity. A change of water table slope leads to complicated streamline configurations, depending on the scale change. However, it is not the dominant driving force accounting for the formation of unconformity-type uranium deposits. The importance of faults depends on their scale, spatial distribution density, and relationship to the aquifer. Basement-derived fluids are not observed to flow through the basement faults as was expected, probably because active faulting (deformation) during flow was not included in the models.