Major Erosional Surface Research Articles

Early post-rift depositional systems of the Central Atlantic: Lower and Middle Jurassic of the Essaouira-Agadir Basin, Morocco

Passive margins are traditionally regarded as tectonically quiescent, however the increasing recognition of significant post-rift tectonic uplift along their flanks offers an important control on sediment delivery. The most extensive record of the early post-rift succession of the Central Atlantic Margin (CAM) is found in the Lower and Middle Jurassic outcrops of the Essaouira-Agadir Basin (EAB). This important succession is characterised by alternating deposition of marine carbonates and paralic siliciclastics that correlate with periods of tectonic activity along the margin, rejuvenating sediment input to the basin. Field observations, well data and petrographic analysis are integrated into a coherent sedimentological model, correlated across the basin within a sequence stratigraphic framework. Comparison is drawn with equivalent dated units in the Central High Atlas, which allows a constraint on the regional versus local tectonostratigraphic evolution.In the EAB, Upper Sinemurian to Lower Pliensbachian open marine ramp carbonates record an initial transgression. They are only preserved locally in the north of the basin, below a major fluvial erosion surface that is regionally traceable across the basin and incisive into the Pliensbachian, CAMP basalts or Triassic sediments. In the Central High Atlas (CHA), the correlative fluvial erosive event has been dated as Toarcian in age. This influx of siliciclastic sediments is interpreted to have been sourced from the Meseta and/or the Anti-Atlas, supporting recent apatite-fission track thermochronology that indicates erosional exhumation at this time.During the Upper Toarcian, a regional carbonate platform, dominated by peritidal deposits, developed across the EAB in response to renewed marine transgression. Facies include oolitic and bioclastic grainstones, crystalline dolomite, stromatolites and dissolution breccias or evaporites. Overlying Middle Jurassic shallow-marine and fluvial siliciclastics encroached from south of the basin (possibly related to a potential source area in the Anti-Atlas), while to the north shallow marine carbonates dominated. These observations evidence the role of tectonic movements of the hinterland during a passive margin phase as a mechanism to trigger forced regressions, compensating the effect of eustasy.

Tectonic motion in oblique subduction forearcs: insights from the revisited Middle and Upper Pleistocene deposits of Rhodes, Greece

Abstract: The sedimentary model of coastal deposits in eastern Rhodes over the last 2 Ma is refined and improved in accuracy. New field investigations and U/Th dating of Spondylus bivalve shells, combined with micropalaeontological and sedimentological data, allow the recognition of four synthems separated by major erosional surfaces. We present here evidence for two of these erosional surfaces. This new model allows the identification and quantification of the vertical movements recorded by the studied exposures. The history of these vertical motions is characterized by two periods of uplift and two periods of subsidence. Such an evolution is unique at the regional scale in the eastern Hellenic forearc. We interpret these results as reflecting the individualization of Rhodes as a single tectonic block during increasing trench bending. This trench bending is accommodated by an increase in the curvature of the forearc during the last 2 Ma. Supplementary material: U/Th dating on spondylid shells, index calcareous nannofossils and Khallithea and Faliraki Road sections are available at https://doi.org/10.6084/m9.figshare.c.4211357

Evolution of a volcanic island on the shoulder of an oceanic rift and geodynamic implications: S. Jorge Island on the Terceira Rift, Azores Triple Junction

The S. Jorge Island in the Azores lies on a peculiar setting, the southern shoulder of the Terceira Rift (TR), which raises a series of questions that we address in this study. We first established the main volcanic stratigraphy by recognizing, in the field, the main unconformities/discontinuities and their meaning (major erosion surfaces and faults), then we collected critical samples, and finally dated them by K/Ar to calibrate the stratigraphy and the age of inferred large-scale flank collapses. Based on field, geochronological and marine geophysical data: (1) we found much older rocks in S. Jorge than in previous studies (ca. 1.85 Ma), and established a new volcanic stratigraphy (from bottom to top): Old Volcanic Complex (ca. 1.9–1.2 Ma), cropping out in the eastern third of the island; Intermediate Volcanic Complex (ca. 0.8–0.2 Ma), cropping out in the western two thirds of the island and separated from the underlying complex by a major fault; Young Volcanic Complex (<ca. 0.1 Ma), unconformable on both older units. (2) We discuss the most probable mode of destruction of the successive volcanic complexes, and we conclude that a major landslide occurred between ca. 1.2 and 0.8 Ma, which was responsible for the major lateral discontinuity between the Old and the Intermediate Volcanic complexes. (3) We use previous palaeomagnetic data and new seismic data to evaluate the effects of uplift of the TR's southern shoulder on S. Jorge Island, and conclude that south-westward tilting of the oldest lava flows occurred between ca. 1.2 and 0.8 Ma due to rotation of the TR's southern shoulder during uplift. (4) Finally, we used all data to constrain the age of TR initiation to be ca. 1 Ma in this sector of the TR.

Processes and typology in Gilbert-type delta bottomset deposits based on outcrop examples in the Corinth Rift

Abstract Middle Pleistocene Gilbert-type delta in the Gulf of Corinth, Greece, has been investigated combining field methods and photo acquisition by drone to generate a high-resolution 3D model. This study case can be used to document four different dynamics in Gilbert-type bottomset deposits, each one of which is characterized by a specific range of facies, facies associations and geometries: (1) the sandy-gravelly bottomset, (2) the erosional-bypass stage, (3) the fine-grained bottomset and (4) the massive-sandy bottomset. By comparing the typologies of the bottomset, we propose a conceptual model that predicts the occurrence of these four different bottomset stage dynamics depending on the stratigraphic context. During highstand normal regression, the gravelly bottomset develops under subcritical flow. The supercritical flow undergoes a stationary hydraulic jump in the toeset due to the slope break. As a result, a low-relief channel-levees system is formed in the bottomset. The channels are filled/reworked by backstepping conglomeratic lenses interbedded with silty concave-up and concave-down levees. During normal regression, the foreset beds are steeper than during previous stage and scoured in the upper part. In the bottomset, significant erosion recording sediment bypass downstream toward the pro-delta can be observed. During lowstand normal regression, a starved fine silt to shale bottomset onlaps onto the major erosional surface. The bulk of the coarse-grained sediments is stored in the delta topset and foreset. During the transgressive to highstand stage, the former topset and foreset are eroded by high-density turbidity currents and massive-sandy turbidites are deposited in the bottomset, which onlap onto the foreset beds and form a slope apron geometry in the delta toe. The stratigraphic model improves the prediction for the sand distribution within the various parts of the bottomset. This approach is particularly relevant for clastic depositional systems with high sediment discharge and a high accommodation rate.

Development of submarine canyons after the Mid-Pleistocene Transition on the Ebro margin, NW Mediterranean: The role of fluvial connections

After the Messinian Salinity Crisis (MSC), the Ebro Margin, like other Mediterranean deltaic margins, rebuilt through progradation and increasingly significant aggradation. The Plio-Quaternary transition from a ramp-like system to a “new” margin with large clinoforms is an opportunity to understand the processes that govern canyon initiation and evolution. We used a 3D seismic data set located at the outer shelf - upper slope of the Ebro margin. We tied major bounding surfaces to chrono-stratigraphic constraints from borehole data or, for the most recent interval, from averaged accumulation rates derived from borehole stratigraphy. The major shelfal erosion surfaces are interpreted as sequence boundaries, tied to major glacial maxima. Along these surfaces, seismic attributes characterize the fluvial/canyon connection, viewed as one of the key factors in canyon development. The first appearance of “proto-canyons” (dense networks of channels and gullies 50–100 m deep and 1–2 km wide) occurs in the late Zanclean. Their size increased progressively throughout the Pliocene and early Quaternary, in relation to the increase in clinoform heights. “True canyons” (with distinct interfluves, more than 200 m deep and 3–4 km wide) appeared during the Mid-Pleistocene Transition (MPT, 1250–700 ka BP). The first evidence of direct connection between canyon heads and fluvial systems is observed during Marine Isotope Stage 22, one of the most pronounced glacial stages of the Quaternary. Similar connections also existed, at least, during MIS 16 and MIS 12, which are also stages of pronounced low sea level. The topography of fluvial channels in the outer shelf is not imaged in detail in the picked horizons at the resolution of our seismic data, but sinuous fluvial systems are very well imaged through amplitude and coherency attributes. Direct connection of fluvial systems during and after MIS 22 also favored headward erosion and the formation of shelf-indenting canyons, probably during early phases of sea-level rise.

Evidences of “Lago-Mare” episode around the Messinian-Pliocene boundary in eastern Tunisia (central Mediterranean)

Eight stratigraphic sections, located in northeastern part of the Sahel area of Tunisia recorded evidences of “Lago-Mare” episode and events related to the Messinian-Late Pliocene interval. A comparison with previous studies carried on sections from neighboring areas and boreholes data drilled within the Gulf of Hammamet and the Gulf of Gabès, is conducted and gives useful information to characterize the Late Messinian to Late Pliocene events. The most notable feature distinguished in the studied area consists on the lack of gypsum, commonly recorded in relation with the crucial event of the Messinian Salinity Crisis. However, only lagoonal deposits, bearing messinian brackish fauna, are encountered. These sediments are usually attributed to the “Segui” formation or the so called “Mio-Pliocene continental”. Thin sections samples and field observations have recognized sands, marls, clays, lacustrine limestone, some gypsum lenses, mud-cracks, lignite and Messinian brackish fauna. Similar deposits were previously described in the Kechabta basin from the Northern Tunisia and in some wells from the Gulf of Gabès and the Gulf of Hammamet. We suggest that all these facies belong to the coeval of the “Lago-Mare” facies within Eastern and Western Mediterranean basins (e.g. Sicily, Mallorca, Libya and Cyprus). Finally, four major erosional surfaces have been recorded within the Late Messinian-Late Pliocene deposits, aged post-Tortonian, intra-Messinian, Late Messinian and intra-Pliocene times. They seem to be the result of local tectonic uplifts and eustatic fluctuations.

A newly discovered Pliocene volcanic field on the western Sardinia continental margin (western Mediterranean)

A previously unknown submerged volcanic field offshore western Sardinia (western Mediterranean Sea), has been identified based on swath bathymetric data collected in 2009, 2010 and 2013, and high-resolution seismic profiles collected in 2011 and 2013. About 40 conical-shaped volcanic edifices (maximum width of about 1600 m and maximum height of about 180 m) and several lava outcrops (up to 1,200 m wide) were recognized at 20 to 150 m water depth over an area of 800 km2. The volcanic edifices are mainly eruptive monogenic vents, mostly isolated with a rather distinct shape, or grouped to form a coalescent volcanic body in which single elements are often still recognizable. High-resolution seismics enabled identifying relationships between the volcanic bodies and continental margin successions. The edifices overlie a major erosional surface related to the margin exposure following the Messinian salinity crisis, and are overlain by or interbedded with an early Pliocene marine unit. This seismo-stratigraphic pattern dates the volcanic activity to the early Pliocene, in agreement with the radiometric age of the Catalano island lavas (4.7 Ma) reported in earlier studies. The morphometry of the volcanic bodies suggests that cone erosion was higher at shallow water depths. Indeed, most of the shallow edifices are strongly eroded and flattened at 125 to 130 m water depth, plausibly explained by recurrent sub-aerial exposure during Pleistocene sea-level lowstands, whereas cones in deeper water are much better preserved. Volcanic vents and lava deposits, hereafter named the Catalano volcanic field (CVF), are emplaced along lineaments corresponding to the main directions of the normal fault system, which lowered the Sinis Basin and the western Sardinia continental margin. The CVF represents a volumetrically relevant phase of the late Miocene – Quaternary anorogenic volcanic cycle of Sardinia, which is related to the first stage of the extensional tectonics affecting the island since the late Miocene.

Estratigrafía de Sr y evolución sedimentaria de los depósitos marinos del Mioceno temprano en el antepaís del norte de la Cuenca Austral (o Magallanes), Argentina.

Early Miocene shallow marine deposits in the region of Lago Posadas-Meseta Belgrano (Argentina) represent part of the “Patagoniense” transgression, an Atlantic marine incursion that flooded large part of Patagonia, including the Austral (foreland) Basin (southern Patagonia). These deposits, referred as El Chacay (Argentina) or Guadal (Chile) formations, and the transition to the overlying Santa Cruz Formation were divided into seven facies: subtidal sand-bars, shallow marine sandy deposits, muddy shelf deposits, estuarine complex deposits, fluvial channels and fluvial floodplains. These are arranged in a general transgressive-regressive cycle, subdivided into two stratigraphic sequences, separated by a major erosional surface. 87Sr/86Sr ages from shell carbonate in eight oysters yielded an age range of 20.3 to 18.1 Ma for these “Patagoniense” deposits. Correlation with other dated “Patagoniense” sections in southern Patagonia, like those at Lago Argentino or Comodoro Rivadavia, indicates that they belong to a single transgression that flooded several Patagonian basins approximately at the same time. Eustasy, flexural subsidence created by tectonic loading in the adjacent fold-and-thrust belt, and basin floor paleo-topography controlled the duration of the depositional event and the sedimentation style of these shallow marine deposits.

Displaced/re-worked rhodolith deposits infilling parts of a complex Miocene multistorey submarine channel: A case history from the Sassari area (Sardinia, Italy)

In the Sassari area (north-western Sardinia, Italy), the Miocene Porto Torres sub-basin sequences represent the complex multistorey mixed carbonate-siliciclastic submarine feature called the Sassari Channel. During the late Burdigalian–early Serravallian, repeated terrigenous supplies from uplifted Paleozoic crystalline substrata fed the Sassari Channel system by means of turbidity and locally hyper-concentrated turbidity flows. Shelfal areas were the source of terrigenous clasts, but open shelf rhodalgal/foramol carbonate areas were very productive and largely also contributed to the channel infilling. Re-worked sands and skeletal debris were discontinuously re-sedimented offshore as pure terrigenous, mixed and/or carbonate deposits. Major sediment supply was introduced between the latest Burdigalian and the start of the middle Langhian, during which a large amount of carbonate, mixed and siliciclastic sediments reached the Porto Torres Basin (Sassari Channel I). Contributions from shallow proximal source areas typify the lower intervals (Unit A) in marginal sectors of the channel. Upward, these evolve into autochthonous rhodolith deposits, winnowed by strong currents in relatively shallow well lit settings within a complex network of narrow tidally-controlled channels (Unit D) locally bearing coral assemblages. Conversely, re-sedimented rhodoliths from the Units B and C accumulated under conditions of higher turbidity. In deeper parts of the channel taxonomically diversified rhodoliths point to the mixing of re-deposited skeletal components from different relatively deep bathmetric settings. In the latest early Langhian, major re-sedimentation episodes, resulting in large prograding bodies (Unit D), triggered by repeated regression pulses in a frame of persistent still stand. During these episodes photophile assemblages dwelled in the elevated margin sectors of the channel. A significant latest early Langhian drop in relative sea-level resulted in impressive mass flows involving early cemented channel-margin and levee blocks and culminated in the formation of major erosional surface (ER-E). Such events seemingly correlate with the long-term global cooling trend of the mid-Miocene climatic transition. Episodes of middle Langhian re-sedimentation concluded with the channel abandon phase after which new erosive episodes followed. Overall, this led to a shift in the Sassari Channel II, with phases presumably started during the earlymost Serravallian, subsequent to the major sea-level drop at the Langhian–Serravallian boundary.

Crustal structure of the eastern Algerian continental margin and adjacent deep basin: implications for late Cenozoic geodynamic evolution of the western Mediterranean

The Karukera spur is an easternmost submerged rise of the Lesser Antilles fore-arc in the Guadeloupe archipelago, culminating about 4000 m above the fore-arc basin 150 km west of the deformation front. The analysis of 3500 km 2D high-resolution multichannel seismic reflection and multibeam bathymetric data, and the study of 14 core samples, allow for the first time to reconstruct its sedimentary anatomy and tectonic evolution. Seven seismic units (US1 to US7) are evidenced, organised into four major depositional sequences separated by erosional surfaces. Units US1 to US4 thicken to the south-southeast on the spur. The sedimentary deposits rest upon a deeply eroded metamorphic basement of the Caribbean Plate as in La Desirade Island. Sequence 1 (units US1 and US2) remains unknown in its lower part because it does not outcrop. Its upper part comprises early Miocene shallow water carbonates. Sequence S2 (Unit US3) only comprises an upper Miocene transgressive systems tract deposited into basinal environment. It is bounded by major erosional surfaces SB2 and SB3. Sequence 3 (US4 to US6) is Zanclean to Pleistocene. It is dominantly composed of shallow water, reefal deposits. Sequence 4 corresponds to recent deposits. The Karukera spur experienced three main extensional episodes with alternations between uplift and subsidence periods. Uplifts are responsible for emersions and erosions. Subsidence severely drowned shallow water deposits. The first extensional episode led to the formation of horsts and grabens in the basement, prior to early Miocene deposits. The second extensional episode occurred during upper Miocene, in accordance with data in surrounding islands. The third tectonic episode developed since Calabrian. Noticeable is the constant reactivation of N130°–N150°, N150°–N180° and N40° trending faults since early Miocene and the neoformation of E–W trending faults since Calabrian. Our results suggest that at the latitude of Guadeloupean archipelago the Lesser Antilles subduction zone is an erosional margin because of the successive sweepings of the Santa Lucia then Tiburon Ridges.

Tectonic and sedimentary architecture of the Karukéra spur: A record of the Lesser Antilles fore-arc deformations since the Neogene

The Karukéra spur is an easternmost submerged rise of the Lesser Antilles fore-arc in the Guadeloupe archipelago, culminating about 4000 m above the fore-arc basin 150 km west of the deformation front. The analysis of 3500 km 2D high-resolution multichannel seismic reflection and multibeam bathymetric data, and the study of 14 core samples, allow for the first time to reconstruct its sedimentary anatomy and tectonic evolution. Seven seismic units (US1 to US7) are evidenced, organised into four major depositional sequences separated by erosional surfaces. Units US1 to US4 thicken to the south-southeast on the spur. The sedimentary deposits rest upon a deeply eroded metamorphic basement of the Caribbean Plate as in La Désirade Island. Sequence 1 (units US1 and US2) remains unknown in its lower part because it does not outcrop. Its upper part comprises early Miocene shallow water carbonates. Sequence S2 (Unit US3) only comprises an upper Miocene transgressive systems tract deposited into basinal environment. It is bounded by major erosional surfaces SB2 and SB3. Sequence 3 (US4 to US6) is Zanclean to Pleistocene. It is dominantly composed of shallow water, reefal deposits. Sequence 4 corresponds to recent deposits. The Karukéra spur experienced three main extensional episodes with alternations between uplift and subsidence periods. Uplifts are responsible for emersions and erosions. Subsidence severely drowned shallow water deposits. The first extensional episode led to the formation of horsts and grabens in the basement, prior to early Miocene deposits. The second extensional episode occurred during upper Miocene, in accordance with data in surrounding islands. The third tectonic episode developed since Calabrian. Noticeable is the constant reactivation of N130°–N150°, N150°–N180° and N40° trending faults since early Miocene and the neoformation of E–W trending faults since Calabrian. Our results suggest that at the latitude of Guadeloupean archipelago the Lesser Antilles subduction zone is an erosional margin because of the successive sweepings of the Santa Lucia then Tiburon Ridges.

Origin and architecture of a Mass Transport Complex on the northwest slope of Little Bahama Bank (Bahamas): Relations between off-bank transport, bottom current sedimentation and submarine landslides

The analysis of the sedimentary dynamics of the carbonate slope of the northwest part of Little Bahama Bank (LBB, Bahamas) reveals a complex interaction between slope destabilisations, off-bank sediment export and longitudinal transport, the latter being driven by the Antilles and the Florida currents, at the northern end of the Florida Strait. Their combined action since the middle Miocene resulted in an extensional growth slope, previously called ‘LBB Drift’ (Mullins et al., 1980). Deposition within this extensional growth slope is dominated by either platform-derived downslope sedimentation or bottom current sedimentation. The latter induces the formation of a plastered drift, showing both upslope and downslope migrations, which do not correspond to the ‘LBB Drift’ as described by Mullins et al. (1980). Interestingly, a large submarine landslide affects the upper part of this plastered drift, and displays a complex and striking geomorphology on the seafloor. A new high-quality multibeam echosounder and seismic dataset allowed a detailed characterisation of the architecture of this Mass Transport Complex (MTC). A 44km-long circular incision at 275m and 460m water depths, with a steep external edge (from 40 to 70m high), forms the only present day evidence of this ancient MTC. It comprises confined Mass Transport Deposits (MTDs), which are delimited by frontal and lateral edges that developed inside the plastered drift. The top of this plastered drift is marked by a major erosional surface, most likely induced by an increase in oceanic current circulation. Channelised geometries, laterally associated with overspill deposits, developed within the depression induced by the MTC, and are an additional evidence of bottom current activity in this area. In addition, recent pockmarks are visible on the seafloor in front of the circular scarp of the MTC and probably relate to fluid escape, originating from the underlying MTDs' compressional area.All these features seen on the northwest slope of LBB bring new understanding on MTC sedimentary processes and associated morphologies in carbonate slope settings. Furthermore, this study highlights the interplay between off-bank transport, oceanic circulation and mass flow processes, which are seen as key processes in the shaping of Bahamian slopes and in their sedimentary dynamics.

Sedimentology, Stratigraphic Architecture, and Depositional Context of Submarine Frontal-Lobe Complexes

Abstract Frontal lobes develop during discrete periods of progradation in deep-water systems, and commonly form on the lower slope to base of slope. In reflection seismic datasets, they are identified as high-amplitude reflectors that are cut as the feeder channel lengthens. Here, an exhumed sand-prone succession (> 80% sandstone) from Sub-unit C3 of the Permian Fort Brown Formation, Laingsburg depocenter, Karoo Basin, South Africa, is interpreted as a frontal-lobe complex, constrained by its sedimentology, geometry, and stratigraphic context. Sub-unit C3 crops out as a series of sand-prone wedges. Individual beds can be followed for up to 700 m as they thin, fine, and downlap onto the underlying mudstone. The downlap pattern, absence of major erosion surfaces or truncation, and constant thickness of underlying units indicates that the wedges are non-erosional depositional bodies. Their low aspect ratio and mounded geometry contrasts markedly with architecture of terminal lobes on the basin floor. Furthermore their sedimentology is dominated by decimeter-scale sinusoidal stoss-side preserved bedforms with a range of low-angle to high-angle climbing-ripple laminated fine-grained sandstones. This indicates that the flows deposited their load rapidly close to, and downstream from, an abrupt decrease in confinement. The sedimentology, stratigraphy, cross-sectional geometry, and weakly confined setting of a sand-prone system from the Giza Field, Nile Delta, is considered a close subsurface analogue, and their shared characteristics are used to establish diagnostic criteria for the identification and prediction of frontal-lobe deposits. In addition, deposits with similar facies characteristics have been found at the bases of large external levee deposits in the Fort Brown Formation (Unit D). This could support models in which frontal lobes form an initial depositional template above which external levees build, which provides further insight into the initiation and evolution of submarine channels.

Hydrocarbon evaluation aspects in neogene clastic reservoirs, Vlora-Elbasan Region, Albania

The Neogene clastic deposits contain more than half of the reserves discovered in the Albanian territory, from which more than 25÷30% are still undeveloped. This paper describes the spatial distribution and the reservoir characteristics of these molasse deposits in relation with the regional geological and structural evolution. These reservoirs constitute transgressive sequences deposited on the top of underlying Mesozoic and Paleogene limestones above a major erosional surface. The paleorelief, mainly shelf, controlling the distribution of the molasse, is made up of terraces and paleo-valleys, which developed by erosion of the continental margin of the Ionian plate during episodes of thrust-related tectonic uplift. The links between tectonics, erosion and sedimentation have controlled boththe distribution of the main molasse depocenters, and the coeval depositional environments. The molasse deposits were initially devoided of organic matter precluding any in situ generation of hydrocarbons. As a consequence, the oil currently found in the Neogene reservoirs results from a secondary migration from underlying limestone reservoirs at places where the intervening Oligocene flysch has been entirely removed by erosion. The reservoir characteristicsof the Neogene clastics are mainly related to lateral and vertical lithological changes controlled by the paleo-environments and tectonics. From the regional viewpoint, the deposits relate to the presence of bays and tectonics uplifts, with erosion, transport and sedimentation processes operating both at regional and local scales.The oil accumulations are concentrated in dominantly stratigraphic traps within the bays and outside them, as a result of lithological changes that made possible their preservation. The main migration pathways are located along the transgressive contacts in Patos-Verbas, Kuçove and Selenica fields. In general, sandstone thickness increases from south to north and from east to west. The reserve estimates made until now for the hydrocarbon potential of the Neogene molasse deposits, have been based using the former Eastern European classification, but they lack a modern economical evaluation. Often, the analogy with other, better known and more mature Petroleum provinces has been used. When re-evaluating the hydrocarbon potential of Albania the reserve estimates of the Neogene clastic deposits have been determined in accordance with the requirements of the western classification into proven, probable and possible reserves with increasing reliability. Attention has been paid also to account for lateral and vertical shifts of the water-oil contacts operating during the deformation of the reservoirs due to regional tilting, folding and/or thrusting.

Denudation of the continental shelf between Britain and France at the glacial–interglacial timescale

The erosional morphology preserved at the sea bed in the eastern English Channel dominantly records denudation of the continental shelf by fluvial processes over multiple glacial–interglacial sea-level cycles rather than by catastrophic flooding through the Straits of Dover during the mid-Quaternary. Here, through the integration of multibeam bathymetry and shallow sub-bottom 2D seismic reflection profiles calibrated with vibrocore records, the first stratigraphic model of erosion and deposition on the eastern English Channel continental shelf is presented. Published Optical Stimulated Luminescence (OSL) and 14C ages were used to chronometrically constrain the stratigraphy and allow correlation of the continental shelf record with major climatic/sea-level periods. Five major erosion surfaces overlain by discrete sediment packages have been identified. The continental shelf in the eastern English Channel preserves a record of processes operating from Marine Isotope Stage (MIS) 6 to MIS 1. Planar and channelised erosion surfaces were formed by fluvial incision during lowstands or relative sea-level fall. The depth and lateral extent of incision was partly conditioned by underlying geology (rock type and tectonic structure), climatic conditions and changes in water and sediment discharge coupled to ice sheet dynamics and the drainage configuration of major rivers in Northwest Europe. Evidence for major erosion during or prior to MIS 6 is preserved. Fluvial sediments of MIS 2 age were identified within the Northern Palaeovalley, providing insights into the scale of erosion by normal fluvial regimes. Seismic and sedimentary facies indicate that deposition predominantly occurred during transgression when accommodation was created in palaeovalleys to allow discrete sediment bodies to form. Sediment reworking over multiple sea-level cycles (Saalian–Eemian–early Weichselian) by fluvial, coastal and marine processes created a multi-lateral, multi-storey succession of palaeovalley-fills that are preserved as a strath terrace. The data presented here reveal a composite erosional and depositional record that has undergone a high degree of reworking over multiple sea-level cycles leading to the preferential preservation of sediments associated with the most recent glacial–interglacial period.

Subglacial to proglacial depositional environments in an Ordovician glacial tunnel valley, Alnif, Morocco

This paper presents the sedimentary analysis of an exceptional Ordovician glacial tunnel valley in the eastern part of the Anti-Atlas. The valley infill comprises two major glacial erosion surfaces (striated pavements) each overlain by a fining-upward glacial unit. These units are composed of five distinct facies associations, recording the evolution from subglacial to proglacial environments, and an additional sixth facies association, overtopping the tunnel valley infill, and associated with post-glacial environments. The tunnel valley infill also records a transitional environment between the subglacial and proglacial settings, which is compared with the Antarctic ice-sheet margin. These three environments are defined by the position of the grounding line and the coupling line. The new proposed depositional model also differs from usual Ordovician depositional models in which the main tunnel valley infill is interpreted as essentially proglacial outwash deposits, in a range of glaciomarine to glaciofluvial environments. Overall, a substantial part of the valley infill (~50% of volume) was deposited in a subglacial setting. The sedimentary bodies could form potentially thick and laterally extended, although these were limited by the shape and extent of the subglacial accommodation space. Finally, the sedimentary record, when compared with regional analogues, also provides information for the palaeogeographic reconstruction of the Ordovician ice-sheet in this region.

Facies and integrated stratigraphy of the Upper Turonian (Upper Cretaceous) Großberg Formation south of Regensburg (Bavaria, southern Germany)

ABSTRACT Niebuhr, B., Richardt, N. and Wilmsen, M. 2012. Facies and integrated stratigraphy of the Upper Turonian (Upper Cretaceous) Grosberg Formation south of Regensburg (Bavaria, southern Germany). Acta Geologica Polonica, 62 (4), 595-615. Warszawa. The Upper Turonian Grosberg Formation of the Regensburg area (Danubian Cretaceous Group, Bavaria, southern Germany) has a mean thickness of 20-25 m and consists of sandy bioclastic calcarenites and calcareous sandstones which are rich in bryozoans, serpulids and bivalves (oysters, rudists, inoceramids). Eight facies types have been recognized that characterize deposition on a southward dipping homoclinal ramp: the inner ramp sub-environment was characterized by high-energy sandwave deposits (sandy bioclastic rud- and grainstones, bioclastic sandstones) with sheltered inter-shoal areas. In mid-ramp settings, bioturbated, glauconitic, calcareous sand- and siltstones as well as bioturbated, bioclastic wacke- and packstones predominate. The carbonate grain association of the Grosberg Formation describes a temperate bryomol facies with indicators of warm-water influences. An inferred surplus of land-derived nutrients resulted in eutrophic conditions and favoured the heterozoan communities of the Grosberg Ramp. Carbon stable isotope geochemistry cannot significantly contribute to the stratigraphic calibration of the Grosberg Formation due to the depleted and trendless bulk-rock δ13 C values, probably resulting from a shallow-water aquafacies with depleted δ13 C DIC values and low δ13 C values of syndepositional and early diagenetic carbonate phases. However, strongly enriched skeletal calcite δ13 C values support a correlation of the Grosberg Formation with the mid-Late Turonian positive Hitch Wood isotope event (Hyphantoceras Event of northern Germany). This interpretation is supported by biostratigraphic data and a range from the Mytiloides striatoconcentricus Zone into the lower My. scupini Zone is indicated by inoceramid bivalves. Both the base and top of the Grosberg Formation are characterized by unconformities. Sequence boundary SB Tu 4 at the base is a major regional erosion surface (erosional truncation of the underlying Kagerhoh Formation in the Regensburg area, fluvial incision at the base of the Seugast Member of the Roding Formation in the Bodenwohr area towards the north and northeast). It is suggested that this unconformity corresponds to a major sea-level drop recognized in many other Cretaceous basins below the Hitch Wood or Hyphantoceras Event. The transgression and highstand of the Grosberg Formation is concomitant to the deposition of the fluvial Seugast Member and the onlap of the marginal-marine “Veldensteiner Sandstein” onto the Frankische Alb. The unconformity at the top of the Grosberg Formation (late Late Turonian SB Tu 5) is indicated by a ferruginous firm-/ hardground and an underlying zone of strongly depleted δ13 C values. The abrupt superposition by deeper marine marls of the lower Hellkofen Formation (uppermost Turonian-Lower Coniacian) may be connected with inversion tectonics at the southwestern margin of the Bohemian Massif.

Sedimentology, palaeoenvironments and biostratigraphy of the Pliocene–Pleistocene carbonate platform of Grande‐Terre (Guadeloupe, Lesser Antilles forearc)

AbstractPliocene and Pleistocene deposits from Grande‐Terre (Guadeloupe archipelago, French Lesser Antilles) provide a remarkable example of an isolated carbonate system built in an active margin setting, with sedimentation controlled by both rapid sea‐level changes and tectonic movements. Based on new field, sedimentological and palaeontological analyses, these deposits have been organized into four sedimentary sequences (S1 to S4) separated by three subaerial erosion surfaces (SB0, SB1 and SB2). Sequences S1 and S2 (‘Calcaires inférieurs à rhodolithes’) deposited during the Late Zanclean to Early Gelasian (planktonic foraminiferal Zones PL2 to PL5) in low subsidence conditions, on a distally steepened ramp dipping eastward. Red algal‐rich deposits, which dominate the western part of Grande‐Terre, change to planktonic foraminifer‐rich deposits eastward. Vertical movements of tens of metres were responsible for the formation of SB0 and SB1. Sequence S3 (‘Formation volcano‐sédimentaire’, ‘Calcaires supérieurs à rhodolithes’ and ‘Calcaires à Agaricia’) was deposited during the Late Piacenzian to Early Calabrian (Zones PL5 to PT1a) on a distally steepened, red algal‐dominated ramp that changes upward into a homoclinal, coral‐dominated ramp. Deposition of Sequence S3 occurred during a eustatic cycle in quiet tectonic conditions. Its uppermost boundary, the major erosion surface SB2, is related to the Cala1 eustatic sea‐level fall. Finally, Sequence S4 (‘Calcaires à Acropora’) probably formed during the Calabrian, developing as a coral‐dominated platform during a eustatic cycle in quiet tectonic conditions. The final emergence of the island could then have occurred in Late Calabrian times.

Würmian deglaciation of western Lake Geneva (Switzerland) based on seismic stratigraphy

Western Lake Geneva (le Petit-Lac) was filled during the Quaternary over a major erosion surface truncating the cemented, folded and thrusted Tertiary sediments of the foreland Alpine basin. The carving of the lake occurred during Quaternary glaciations with ice originating from the Rhone valley catchment basin flowing in two branches oriented SW and NE over the Swiss Plateau. Lake Geneva is situated on the South-Western branch of this paleo ice-cap. For the first time, a dense grid of high-resolution seismic profiles (airgun 5-inch 3, airgun 1-inch 3 and echosounder) has imaged the whole Quaternary sequence, providing a paleoenvironmental interpretation and a detailed reconstruction of the Rhone glacier retreat stages during glacial events that led to the formation of western Lake Geneva. The Quaternary sequence filling up the bedrock valley is exceptionally thick with up to 220 m of deposits and consists of glacial, glacio-lacustrine and lacustrine sediments. Fourteen seismic units have been defined (units U1–U14). Unit U1 represents the remnants of glacial deposits older than the last glacial cycle, preserved in the deepest part of the lake and in secondary bedrock valleys. Unit U2 represents gravel and sands deposited by meltwater circulation at the bottom of the glacial valley. Unit U3 is a thick, stratified unit marking the beginning of the deglaciation, when the Rhone glacier became thinner and buoyant and allowed the formation of a subglacial lake. Younger glacial units (units U4, U5, U7, U9, U11) are acoustically chaotic sediments deposited subglacially under the water table (undermelt tills), while the glacier was thinning. These glacial units are bounded by synform erosion surfaces corresponding to readvances of the glacier. The transition from a glacial to a glacio-lacustrine environment started with the appearance of a marginal esker-fan system (unit U6). Esker formation was followed by a small advance–retreat cycle leading to the deposition of unit U7. Then, the ice front receded and stratified sediments were deposited in a glacio-lacustrine environment (units U8, U10 and U12). This retreat was punctuated by two readvances – Coppet (unit U9) and Nyon (unit U11) – producing large push moraines and proglacial debris flows. Finally, a lacustrine environment with a characteristic lake current pattern and mass movement deposits took place (units U13 and U14). Except for unit U1, the sedimentary sequence records the Würmian deglaciation in a fjord-like environment occupied by a tidewater glacier with a steep, calving ice front. The presence of an esker-fan system reveals the importance of subglacial meltwater flow in continental deglaciation. Push-moraines and erosion surfaces below the glacier indicate at least 5 readvances during the deglaciation thus revealing that oscillations of ice front are the key process in deglaciation of perialpine fjord-lakes. The dating of these continental glacier fluctuations would allow correlation with oceanic and ice records and help to understand the climatic mechanisms between oceans and continents.

Earliest Triassic microbialites in Çürük Dag, southern Turkey: composition, sequences and controls on formation

AbstractThe Permian–Triassic Boundary sequence at Çürük Dag, near Antalya, Turkey, begins with a major erosion surface interpreted as being the Late Permian lowstand, on which lies ca 0·4 m of grainstone/packstone composed of ooids, peloids and bioclasts. Most ooids are superficial coats on fragments of calcite crystals presumed to be eroded from crystal fans which are no longer present. The erosion surface is smooth and shows no evidence of dissolution; the grainstone/packstone contains intraclasts of the underlying wackestone, proving erosion. Next are 15 m of microbialite comprised of interbedded stromatolites, thrombolites, plus beds of planar limestones with small‐scale erosion. The latter comprise a complex interlayering of stromatolitic, thrombolitic and peloidal fabrics and precipitated crystal fans, which form a hybrid of microbialite and inorganic carbonate, together with bioclastic debris and micrite. The Çürük Dag microbialite sequence is repetitious; the lower part is more complex, with abundant stromatolites and hybrid microbialites. Some of the stromatolites are themselves hybrids composed of peloids and crystal fans. In the upper part of the sequence stromatolites are missing and the rock is composed mostly of recrystallized thrombolites that develop upwards from tabular to domal form. The domes form directly below small breaks in microbialite growth where very thin shelly micrites and grainstones/packstones are deposited. Repetition of facies may be controlled by sea‐level change; a deepening‐up model is consistent with the evidence. Stromatolites (with abundant crystal fans) dominate in shallower water, deepening through hybrid microbialite and interlayered sediments to thrombolite, probably no more than a few tens of metres deep, followed by breaks and renewal of microbialite growth. An interpretation of open marine fully oxygenated waters for microbialite growth is consistent with ongoing parallel work that has identified Bairdioid ostracods in the microbialite, a group known to be open marine. However, other researchers have proposed low oxygen conditions for Permian–Triassic boundary facies globally, so work continues to confirm whether the Çürük Dag microbialite grew in dysoxic or normally oxygenated conditions. The principal stimulus for post‐extinction microbialites is likely to be carbonate supersaturation of the oceans. The microbialite sequence is overlain by a further 25 m of grainstone/packstone (without microbialite), followed by Early Triassic shales. Overall, microbialites form a thin aggradational sequence during an overall relative sea‐level rise, consistent with global eustatic rise following the Late Permian lowstand.