Holocene sedimentation of a wave-dominated barrierisland shoreline: Cape Lookout, North Carolina

The sedimentary record of 130 km of microtidal (0.9 m tidal range) high wave energy (1.5 m average wave height) barrier island shoreline of the Cape Lookout cuspate foreland has been evaluated through examination of 3136 m of subsurface samples from closely spaced drill holes. Holocene sedimentation and coastal evolution has been a function of five major depositional processes: (1) eustatic sea-level rise and barrier-shoreline transgression; (2) lateral tidal inlet migration and reworking of barrier island deposits; (3) shoreface sedimentation and local barrier progradation; (4) storm washover deposition with infilling of shallow lagoons; and (5) flood-tidal delta sedimentation in back-barrier environments.

Twenty-five radiocarbon dates of subsurface peat and shell material from the Cape Lookout area are the basis for a late Holocene sea-level curve. From 9000 to 4000 B.P. eustatic sea level rose rapidly, resulting in landward migration of both barrier limbs of the cuspate foreland. A decline in the rate of sea-level rise since 4000 B.P. resulted in relative shoreline stabilization and deposition of contrasting coastal sedimentary sequences. The higher energy, storm-dominated northeast barrier limb (Core and Portsmouth Banks) has migrated landward producing a transgressive sequence of coarse-grained, horizontally bedded washover sands overlying burrowed to laminated back-barrier and lagoonal silty sands. Locally, ephemeral tidal inlets have reworked the transgressive barrier sequence depositing fining-upward spit platform and channel-fill sequences of cross-bedded, pebble gravel to fine sand and shell. Shoreface sedimentation along a portion of the lower energy, northwest barrier limb (Bogue Banks) has resulted in shoreline progradation and deposition of a coarsening-up sequence of burrowed to cross-bedded and laminated, fine-grained shoreface and foreshore sands. In contrast, the adjacent barrier island (Shackleford Banks) consists almost totally of inlet-fill sediments deposited by lateral tidal inlet migration. Holocene sediments in the shallow lagoons behind the barriers are 5–8 m thick fining-up sequences of interbedded burrowed, rooted and laminated flood-tidal delta, salt marsh, and washover sands, silts and clays.

While barrier island sequences are generally 10 m in thickness, inlet-fill sequences may be as much as 25 m thick and comprise an average of 35% of the Holocene sedimentary deposits. Tidal inlet-fill, back-barrier (including flood-tidal delta) and shoreface deposits are the most highly preservable facies in the wave-dominated barrier-shoreline setting. In the Cape Lookout cuspate foreland, these three facies account for over 80% of the sedimentary deposits preserved beneath the barriers. Foreshore, spit platform and overwash facies account for the remaining 20%.     

Systematic vertical and lateral changes in quality and time resolution of the macrofossil record: Insights from Holocene transgressive deposits,Po coastal plain,Italy

In siliciclastic marine settings, skeletal concentrations are a characteristic feature of transgressive intervals that provide insights into biological and sequence-stratigraphic processes. To investigate taphonomic signatures of transgressive intervals, we analysed three cores along a depositional profile from the high resolution chrono- and stratigraphic framework of the Holocene Po coastal plain, in northern Italy. Coupled multivariate taphonomic and bathymetric trends delineated spatial and temporal gradients in sediment starvation/bypassing, suggesting that quality and resolution of the fossil record vary predictably along the studied depositional profile. Moreover, integration of taphonomic, bathymetric, and fossil density trends across the study area reveals distinctive signatures useful in characterizing facies associations and determining surfaces and intervals of sequence-stratigraphic significance. Within the southern Po plain succession, taphonomic degradation of macroskeletal remains increases from proximal/nearshore to distal/offshore locations. This trend is discernible for both biologically-driven (bioerosion) and physically-driven (e.g., dissolution, abrasion) shell alterations. Compared to the up-dip (most proximal) core, the down-dip core is distinguished by shell-rich lithosomes affected by ecological condensation (co-occurrence of environmentally non-overlapping taxa) and by higher taphonomic alteration. The onshore-offshore taphonomic trend likely reflects variation in sediment-accumulation along the depositional profile of the Holocene Northern Adriatic shelf, with surface/near-surface residence-time of macroskeletal remains increasing down dip due to lower accumulation rates. These results indicate that, during transgressive phases, changes in sea level (base level) are likely to produce down-dip taphonomic gradients across shelves, where the quality and resolution of the fossil record both deteriorate distally. Radiocarbon-calibrated amino acid racemisation dates on individual bivalve specimens and the chronostratigraphic framework for this profile suggest jointly that the high levels of taphonomic degradation observed distally developed over millennial time scales (∼8ky). Whereas in proximal setting overall low taphonomic degradation and geochronologic constrains point to centennial-scale time-averaging during the late transgression phase. Patterns documented in the Holocene transgressive (and lowermost regressive) deposits of the southern Po Plain may be characteristic of siliciclastic-dominated depositional systems that experience high-frequency, base-level fluctuations.     

Linking diagenesis to sequence stratigraphy in fluvial and shallow marine sandstones: Evidence from the Cambrian-Ordovician lower sandstone unit in southwestern Sinai, Egypt

By integrating diagenesis and sequence stratigraphy, the distribution of diagenetic alterations and their impact on reservoir quality was investigated within a sequence stratigraphic framework using the fluvial and shallow marine sandstones in the Cambrian-Ordovician succession of southwest Sinai. The perographic and geochemical analysis of the studied sandstone revealed that the eogenetic alterations display fairly systematic spatial and temporal distribution patterns within the lowstand system tract and transgressive system tract, as well as along the sequence stratigraphic surfaces (i.e., sequence boundaries, transgressive surfaces and parasequence boundaries). During relative sea-level fall, percolation of meteoric waters through sandstones of the LST and below sequence and parasequence boundaries resulted in extensive dissolution of detrital grains and formation of kaolinite, authigenic K-feldspar and feldspar overgrowths as well as formation of mechanical infiltrated clays around the detrital grains. During relative sea-level rise, invasion of marine water into the sandstones as a consequence of landward migration of the shoreline, as well as low sedimentation rates encountered in the TST, resulted in the formation of glauconite, apatite and pyrite. Development of pseudomatrix, which was formed by mechanical compaction of mud intraclasts, is mostly abundant along transgressive surfaces and parasequence boundaries of the TST, and is related to the abundance of mud intraclasts in the transgressive lag deposits.The types and extent of eogenetic alterations have an important impact on the distribution of the mesogenetic alterations, including the formation of quartz overgrowths and dickite.Distribution of mesogenetic quartz overgrowths in the sandstones was controlled by the distribution of mechanically infiltrated clays and the presence of eogenetic cement. Sandstones that remained poorly cemented during eodiagenesis and that have thin or discontinuous infiltrated clay rims around the detrital grains were cemented during mesodiagenesis by quartz. The absence of extensive eogenetic cements in the sandstones suggested that the partial deterioration of porosity was mainly due to mechanical compaction. Partial transformation of kaolinite to dickite, which indicates neomorphic change to a better-ordered and more stable crystal structure at the elevated temperatures during mesodiagenesis, is partially a function of distribution of kaolinite during eodiagenesis.The conceptual model developed in this study shows the diagenetic evolutionary pathways in the reservoir sandstones within a sequence stratigraphic context, which in turn provides some insights into the controls on reservoir heterogeneity.     

Seismic and sequence stratigraphy of the central western continental margin of India: late-Quaternary evolution

Seismic and sequence stratigraphic architecture of the central western continental margin of India (between Coondapur and south of Mangalore) has been investigated with shallow seismic data. Seismic stratigraphic analysis defined nine seismic units, that are configured in a major type-1 depositional sequence possibly related to fourth-order eustatic sea-level changes, comprising regressive, lowstand, transgressive and highstand systems tracts. The late-Quaternary evolution of the continental margin took place under the influence of an asymmetric relative fourth-order sea-level cycle punctuated by higher frequency cycles. These cycles of minor order were characterised by rapid sea-level rises and gradual sea-level falls that generated depositional sequences spanning different time scales. During the regressive periods, dipping strata were developed, while erosional surfaces and incised valleys were formed during the lowstands of sea level. Terraces, v-shaped depressions, lagoon-like structures observed on the outer continental shelf are the result of the transgressive period. In the study area we have recognised a complex erosional surface that records a long time span during the relative sea-level fall (regressive period) and the following sea-level lowstand and has been reworked during the last transgression. We also infer that sedimentation processes changed from siliciclastic sedimentation to carbonate sedimentation and again to siliciclastic sedimentation, marking an important phase in the late-Quaternary evolution of the western continental shelf of India. We attribute this to an abrupt climate change at the end of the oxygen isotope stage 2, between the Last Glacial Maximum and the Bølling-Allerod event (14?000 yr BP). This sensitive climate change (warming) favoured the formation of reefs at various depths on the shelf, besides the development of Fifty Fathom Flat, a carbonate platform on the outer shelf off Bombay developed prior to 8300 yr BP. The highstand systems tracts were deposited after the sea level reached its present position.     

Integration of High and Very High-resolution Seismic Reflection Profiles to Study Upper Quaternary Deposits of a Coastal Area in the Western Gulf of Lions, SW France

High resolution (HR – sparker) and very high resolution (VHR – boomer) seismic reflection data acquired in shallow water environments of the Roussillon coastal area are integrated to provide an accurate image of the stratigraphic architecture of the Quaternary deposits. The complementary use of the two systems is shown to be of benefit for studies of shallow water environments. The HR sparker data improved the landward part of a general model of Quaternary stratigraphy previously established offshore. They document an incised valley complex interpreted as the record of successive late Quaternary relative sea-level cycles. The complex is capped by a polygenetic erosional surface developed during the last glacial period (>18 ky) and variably reworked by wave ravinement during the subsequent post-glacial transgression. The overlying transgressive systems tract is partly preserved and presents a varying configuration along the Roussillon coastal plain. The VHR boomer data provide information on the architecture of the uppermost deposits, both in the near-shore area and in the lagoon. These deposits overlie a maximum flooding surface at the top of the transgressive systems tract and constitute a highstand systems tract composed of two different architectural elements. In the near-shore area, a sandy coastal wedge is subdivided into a lower unit and an upper unit in equilibrium with present day dynamics. In the Salses-Leucate lagoon area, the sedimentary architecture is highly complex due to the closure of a former embayment and the formation of the present beach barrier.     

Sedimentological response to neotectonics and sea-level change in a delta-fed,complex graben: Gulf of Amvrakikos,western Greece

The Gulf of Amvrakikos is one of several E–W grabens developed in the Middle Quaternary almost orthogonal to the tectonic grain of the external Hellenides in western Greece and is critically located at the termination of the Kefallinia transform fault linking the Hellenic subduction zone to the Adriatic-Dinaride collision lineament. The tectonic evolution of the Gulf and the response of sedimentation have been investigated based on 3.5 kHz and sparker profiles. The Holocene stratigraphy results from rising eustatic sea-level breaching the tectonic sill north of Preveza and inundating the braid plain of the Arachthos river, which then prograded across the northern Gulf following the formation of the maximum flooding surface. Older major transgressive surfaces are recognised in sparker profiles. Interpretation of these transgressions in terms of eustatic sea-level changes allows a chronology to be established and estimates to be made of neotectonic rates of subsidence. Two sets of neotectonic faults are present: reactivated NNE-trending mid-Tertiary thrusts of the Ionian zone and more active E–W trending faults. The sedimentary succession in the Gulf of Amvrakikos is the result of a complex interplay between eustatic sea level changes, the neotectonic creation of accommodation, and autocyclic delta distributary switching. The details of this process are resolved for the Holocene and the general sedimentation pattern established back to marine isotope stage (MIS) 8 (245–300 ka). In general, MIS 8 shows greater evidence of terrestrial deposition, compared with MIS 6 (130–190 ka) and especially MIS 2–4 (10–80 ka), when there was lacustrine deposition and some marine incursions at intermediate eustatic sea levels. This implies that regional subsidence has been taking place at least since MIS 8. In addition, changing sediment supply plays a role in overall sediment architecture, but is only well resolved for the Holocene, with higher rates of fluvial supply recognised in the mid Holocene.     

Geochemical and stratigraphic indicators of late Holocene coastal evolution in the Gythio area, southern Peloponnese, Greece

The study of past changes in sea level, and of historical and pre-historical coastal evolution, using coastal sediment stratigraphies is well-established over a range of geographic areas, in both seismic and aseismic settings. In the eastern Mediterranean, however, such studies are less common, and, notably, the use of sediment geochemistry, and its combination with lithostratigraphic studies to analyze palaeoenvironmental and palaeo-sea-level change, has not been explored to any significant extent, despite the fact that geochemical data have been successfully used elsewhere to aid in the identification of sea-level changes. Here, we use a combined geochemical, stratigraphic and microfossil approach to reconstruct late Holocene coastal evolution and sea-level change at two sites near Gythio in the southern Peloponnese, Greece. The sites show stratigraphic and geochemical evidence of the presence in Late Helladic times (ca. 1500 BC) of barrier-protected coastal lagoonal/wetland environments, which have gradually infilled over the last ca. 3500 yr. Archaeological remains and ceramic and charcoal-bearing horizons within the sediment sequences indicate Late Roman occupation of the area, although there is no sedimentary evidence of significant pre-Roman activity at the study sites. An apparent brackish wetland peat deposit at − 3.4 m (overlain by anoxic lagoonal clays) at Kamares (Kato Vathi) Bay shows a calibrated radiocarbon age of 1640–1440 BC, suggesting a relative sea-level rise of 0.8–1 mm/yr in this area over the past 3500 yr, in good agreement with previous archaeological and sea-level modelling studies. There is no evidence, based on the stratigraphic, microfossil or geochemical record, of sudden marine flooding events related to local or regional seismic activity, despite the presence of the area in a seismically active zone known to be subject to periodic earthquakes and tsunami. The data highlight the utility of combining geochemical and stratigraphic studies in the reconstruction of coastal evolution and the study of palaeo-sea-level changes, particularly in sequences (such as those described here) where microfossils are poorly preserved.     

Delta formation at the southern entrance of Istanbul Strait (Marmara sea, Turkey): a new interpretation based on high-resolution seismic stratigraphy

A detailed stratigraphic investigation based on high-resolution seismic profiles revealed that the delta at the southern entrance of the Istanbul Strait consists of three parasequence sets. The lowermost parasequence shows a sea-level stillstand at the beginning of the lowstand systems tract, possibly at 11,000±1,100 a b.p., whereas the upper two parasequences reflect deposition at lowstand and during the subsequent transgression. A maximum flooding surface may be developing on the delta at present. The delta is located on the eastern side of the Istanbul Strait canyon, with east–west prograding parasequences. The development of the delta is clearly associated with the Kurbağalı Stream on the east coast, and not with the Black Sea outflow through the strait. The geometry of the delta indicates a radial architecture arranged from northeast to southwest.     

Rift sequence stratigraphy

Conventional sequence stratigraphy has been developed primarily for passive-margin basins. Despite the conceptual advances within the last 30 years, a suitable model for rift basins has not yet been devised. Many authors have attempted to adapt the passive-margin model to all other tectonic settings, including rifts, despite the fundamental differences in terms of the mechanisms controlling the formation and evolution of these sedimentary basins. Passive margins have their stratigraphic framework controlled largely by cyclic sea-level fluctuations superimposed on long-term thermal subsidence. By contrast, rift basins have their accommodation history strongly related to their mechanical subsidence regime, with episodic pulses of extension that create space for sediment accumulation at very fast rates. Stages of rapid mechanical subsidence are typically followed by longer periods of tectonic quiescence, when sediment supply gradually consumes and fills the available accommodation. This cyclicity results in depositional sequences that display overall progradational trends and coarsening-upward vertical stacking patterns. Sequence boundaries are often marked by sharp flooding surfaces related to the transgression of lacustrine or marine systems in response to rapid tectonic subsidence and the consequent ‘instantaneous’ generation of accommodation. As such, a typical rift depositional sequence starts with a flooding surface overlain by a relatively thin transgressive systems tract and a much better developed highstand systems tract. A renewed subsidence pulse leads to the drowning of the previous deposits and the start of a new depositional sequence. The strong asymmetry of the base-level curve resembles the shape of glacio-eustatic cycles, with fast transgressions followed by longer term regressions, although at potentially different temporal scales.     

Facies architecture of the mixed carbonate/siliciclastic inner continental shelf of west-central Florida: implications for Holocene barrier development

Sediment vibracores and surface samples were collected from the mixed carbonate/siliciclastic inner shelf of west–central Florida in an effort to determine the three-dimensional facies architecture and Holocene geologic development of the coastal barrier-island and adjacent shallow marine environments. The unconsolidated sediment veneer is thin (generally <3 m), with a patchy distribution. Nine facies are identified representing Miocene platform deposits (limestone gravel and blue–green clay facies), Pleistocene restricted marine deposits (lime mud facies), and Holocene back-barrier (organic muddy sand, olive-gray mud, and muddy sand facies) and open marine (well-sorted quartz sand, shelly sand, and black sand facies) deposits. Holocene back-barrier facies are separated from overlying open marine facies by a ravinement surface formed during the late Holocene rise in sea level. Facies associations are naturally divided into four discrete types. The pattern of distribution and ages of facies suggest that barrier islands developed approximately 8200 yr BP and in excess of 20 km seaward of the present coastline in the north, and more recently and nearer to their present position in the south. No barrier-island development prior to approximately 8200 yr BP is indicated. Initiation of barrier-island development is most likely due to a slowing in the Holocene sea-level rise ca. 8000 yr BP, coupled with the intersection of the coast with quartz sand deposits formed during Pleistocene sea-level highstands. This study is an example of a mixed carbonate/siliciclastic shallow marine depositional system that is tightly constrained in both time and sea-level position. It provides a useful analog for the study of other, similar depositional systems in both the modern and ancient rock record.     

Regional stratigraphic framework linking continental shelf and coastal sedimentary deposits of west-central Florida

A regional study of the Holocene sequence onlapping the west-central Florida Platform was undertaken to merge our understanding of the barrier-island system with that of the depositional history of the adjacent inner continental shelf. Key objectives were to better understand the sedimentary processes, sediment accumulation patterns, and the history of coastal evolution during the post-glacial sea-level rise. In the subsurface, deformed limestone bedrock is attributed to mid-Cenozoic karstic processes. This stratigraphic interval is truncated by an erosional surface, commonly exposed, that regionally forms the base of the Holocene section. The Holocene section is thin and discontinuous and, north or south of the Tampa Bay area, is dominated by low-relief sand-ridge morphologies. Depositional geometries tend to be more sheet-like nearshore, and mounded or ridge-like offshore. Sand ridges exhibit 0.5–4 m of relief, with ridge widths on the order of 1 km and ridge spacing of a few kilometers. The central portion of the study area is dominated nearshore by a contiguous sand sheet associated with the Tampa Bay ebb-tidal delta. Sedimentary facies in this system consist mostly of redistributed siliciclastics, local carbonate production, and residual sediments derived from erosion of older strata. Hardground exposures are common throughout the study area. Regional trends in Holocene sediment thickness patterns are strongly correlated to antecedent topographic control. Both the present barrier-island system and thicker sediment accumulations offshore correlate with steeper slope gradients of the basal Holocene transgressive surface. Proposed models for coastal evolution during the Holocene transgression suggest a spatial and temporal combination of back-stepping barrier-island systems combined with open-marine, low-energy coastal environments. The present distribution of sand resources reflects the reworking of these earlier deposits by the late Holocene inner-shelf hydraulic regime.     

Seismic evidence for the preservation of several stacked Pleistocene coastal barrier/lagoon systems on the Gulf of Valencia continental shelf (western Mediterranean)

The focus of this study is the analysis of coastal sand barriers and associated coastal lagoons on the inner continental shelf of the Gulf of Valencia (western Mediterranean), based on two W–E seismic profiles recorded seaward of the Albufera de Valencia coastal lagoon. Seismic facies identified include a number of coastal sand barriers with landward lagoons draped by contemporary continental shelf deposits. The barrier systems have been grouped into two sedimentary systems tracts, the older one corresponding to a prograding/aggrading highstand systems tract involving at least four paleo-coastal sand barrier/lagoon systems, followed landward by a transgressive systems tract comprising three such systems. All the systems have been allocated a Tyrrhenian age, the formation of individual barrier systems having been associated with successive sea-level stillstands, and their present-day position being explained by the very high regional subsidence rate. In summary, this study demonstrates that the Quaternary stratigraphic record of the Gulf of Valencia inner continental shelf is composed of littoral sand facies, in particular coastal sand barrier and lagoon deposits. These findings are in agreement with corresponding observations on other continental shelves of the western Mediterranean, showing that the formation of coastal sand barriers was a characteristic feature of this region during the Quaternary.     

Sea-level and subsidence data from a Late Holocene back-barrier lagoon (Valle Standiana, Ravenna, Italy)

Late Holocene back-barrier sediments have been studied in a former lagoon reclaimed during the last century. Several shallow boreholes were cored, from which nine organic-rich samples were selected for radiocarbon dating, and others for palynological and palaeontological analysis. This enabled the reconstruction of the evolution of a back-barrier coastal basin during the late Holocene coastline progradation. Since late Roman times, the rate of relative sea-level rise (RSLR) has increased from 1.1 mm/yr to over 2 mm/yr, even after removal of the main human impacts during of the last century. This change is ascribed to sediment compaction related to changes in the hydrological regime, and possibly to an isostatic subsidence wave accompanying the northward shift of the Po Delta during the last 2000 years.     

The late-Holocene Gargano subaqueous delta,Adriatic shelf: Sediment pathways and supply fluctuations

The Gargano subaqueous delta formed on the eastern and southeastern sides of the Gargano promontory, in the western Adriatic. This subaqueous deposit represents the southernmost portion of the late-Holocene highstand systems tract (HST) growing along the western side of the Adriatic as an extensive wedge of deltaic and shallow-marine mud. The late-Holocene HST rests above a regional downlap surface that marks the time of maximum landward shift of the shoreline attained around 5.5 cal. kyr BP, at the end of the late-Pleistocene–Holocene sea-level rise. High-resolution seismic–stratigraphic and tephra correlation indicate the presence of a thin basal unit recording condensed deposition between 5.5 and 3.7 cal. kyr BP over much of the basin. Above this unit, sediment accumulation rates increased to high values (as much as 1.5 cm yr⁻¹) reflecting the stabilisation of relative sea level and the forcing from high frequency climatic or anthropogenic changes affecting river dynamics. The late-Holocene mud wedge, of which the Gargano subaqueous delta is a significant component, reaches up to 35 m in thickness and has a volume of ca 180 km³. The shore-parallel thickness distribution of the mud wedge reflects the dominant oceanographic regime of the basin and the asymmetric location of the mostly western sediment sources (with a combined modern delivery of 51.7×10⁶ t yr⁻¹ of mean suspended load). In sections perpendicular to the coast the late-Holocene mud wedge appears composed of forestepping clinoforms with gently dipping foresets (typically 0.5°). The Gargano subaqueous delta is characterised by a submarine topset in water depths shallower than 25–28 m, and accounts for about 1/7th of the total volume of the late-Holocene mud wedge, despite the absence of direct river supply to the Gargano area. In the area of maximum interaction between shore-parallel currents and basin morphology, progradation occurs onto a flat and barren bedrock outcrop in about 50–80 m water depth. The rapid transition from a thickness of 30 m of late-Holocene mud to nil is a good indication of the role of southward-flowing bottom-hugging shelf currents in causing the redistribution of sediment along the Adriatic inner shelf. Additional evidence of this regime comes from: (1) the most recent sigmoid (defined at seismic–stratigraphic scale) deposited since the onset of the Little Ice Age, showing a shore-parallel thickness distribution and a main depocentre to the southeast of the Gargano promontory; (2) the maximum values of sediment accumulation rates over the last century (documented by ²¹⁰Pb measurements) defining a narrow shore-parallel belt immediately seaward of the depocentre of the most recent sigmoid. The Gargano subaqueous delta grows from the outbuilding of progressively younger progradational sigmoids that tend to parallel the previous ones. The Gargano subaqueous delta differs from other documented late-Holocene subaqueous deltas because its growth reflects: (1) sediment transport dominated by bottom currents sub-parallel to the strike of the composing clinoforms; (2) a complex supply regime including the Po delta (350 km to the north) and several coalescing Apennine rivers acting as ‘line source’; (3) several alternating intervals of enhanced outbuilding and condensed deposition; and (4) an in-phase growth of the most recent sigmoid with the major progradation of the Po delta during the Little Ice Age.     

Continental shelf architecture and sea-level cycles: Late Quaternary high-resolution stratigraphy of the Gulf of Cádiz, Spain

 The stratal architecture of the Gulf of Cádiz continental margin (SW Spain) has been analyzed by using single-channel, very high-resolution seismic reflection profiles. An evolutionary scheme of asymmetrical depositional sequences is proposed that was governed by the Late Pleistocene–Holocene sea-level fluctuations. Stratigraphic analysis defined 14 seismic units, that are configured into two major type-1 depositional sequences related to 4th-order eustatic sea level changes (100–110 ka). Within these sequences, minor asymmetrical depositional sequences have been recognized related to 5th-order eustatic cycles (22–23 ka) superimposed and modulated by the regressive trends of 4th-order cycles. In 5th-order depositional sequences, the forced regressive and lowstand deposits are volumetrically dominant. They cause the main progradation of the margin in such a way that they form the margin structure almost entirely. Received: 6 April 1995 / Revision received: 8 March 1996     

Post-Calabrian sequence stratigraphy of the northwestern Alboran Sea (southwestern Mediterranean)

The post-Calabrian sedimentary column of the northwestern Alboran Sea comprises three depositional sequences. The two older depositional sequences are defined by lowstand systems tracts (shelf-margin deltas, slope, base-of-slope, and basin deposits, and the Guadiaro channel-levee complex). In contrast, the most recent depositional sequence also includes transgressive (relict shelf facies) and high-stand (the Guadalmedina-Guadalhorce prodelta and hemipelagic facies) systems tracts. The stratigraphic architecture of these depositional sequences is controlled by the synchronism between high frequency sea-level changes, variations in sediment supply, and sedimentary processes. The configuration of the depositional sequences is variable and their distribution is complex, as a result of the relative importance played by sea-level changes and tectonism through the area.

The sequence boundaries are represented by polygenetic surfaces in the proximal margin, and by monogenetic surfaces in the distal margin and basin. Each polygenetic surface results from the interaction between the sequence boundary with the lowstand erosional truncation surface and the transgressive surface, both developed during the previous sea-level cycle. The monogenetic surfaces correspond to unconformities and their correlative conformities, formed during sea-level lowstands. This pattern of depositional sequences developed in the margin and basin of the northwestern Alboran Sea shows differences with the Exxon Sequence Stratigraphy Model as traditionally applied: sea-level change control is essentially recognized through lowstand systems tracts, and sequence boundary coincides with lowstand erosional truncation surface and transgressive surface, both developed during the previous sea-level cycle.     

The transgressive infill of an inherited-valley system: The Springhill Formation (lower Cretaceous) in southern Austral Basin, Argentina

The Berriasian-Valanginian Springhill Formation of the Austral Basin of southern South America comprises fluvial to marine deposits. In order to interpret depositional systems and unravel the stratigraphic architecture of this unit in the southern region of the basin (Tierra del Fuego Province, Argentina), 500 m of cores combined with well-log data from 41 wells were studied. Facies associations corresponding to fluvial (A1-A6), estuarine (B1-B5) and open-marine (C1-C4) depositional environments were identified. These facies associations succeed each other vertically across the entire study area (6800 km²) forming a ∼120-m-thick transgressive succession. This unit filled a north-south-oriented valley system, developed in the underlying Jurassic volcanic complex.Lowstand fluvial deposits of the first stage of the valley-system fill occur in downdip segments of the system above a sequence boundary (SB). These fluvial deposits are overlain by coastal-plain and tide-dominated estuarine strata across an initial transgressive surface (ITS). In the northern sector the earliest valley infill is characterized by a transgressive fluvial succession, overlying a merged SB/ITS that is probably time-equivalent of marginal-marine deposits of the southern sector. The fluvial strata in the north are overlain by wave-dominated estuarine deposits. A drastic change to open-marine conditions is marked by a marine flooding surface, with local evidence of marine erosion (FS-RS). Open-marine strata are thin (<10 m) and dominated by lower-shoreface and offshore-transition deposits. They are capped by a younger flooding surface (FS), which represents the onset to offshore conditions across the study area due to a continuous long-term transgression that persisted until the Barremian.Although the interpreted depositional systems and stratigraphic architecture of the Springhill Formation resemble transgressive incised-valley-fill successions, the greater thickness and larger size of the Springhill valleys suggest inherited rift topography rather than valley development during a relative sea-level fall.     

Tectonism: the dominant factor in mid-Cretaceous deposition in the Jeanne d'Arc Basin, Grand Banks

The development of stratigraphic sequences has been demonstrated to be controlled by a set of factors including variations in subsidence, sediment input, eustatic sea level and physiography. Well and seismic data from the Jeanne d'Arc Basin, Grand Banks indicate that mid-Cretaceous tectonism controls at least three of these factors, namely subsidence, sediment input and physiography. North Atlantic rift tectonism was therefore the dominant factor in controlling the migration of coastal to shallow marine environments and the development of sequence stratigraphy in this basin during the mid-Cretaceous. The Avalon Formation respresents a mainly Barremian to Early Aptian regressive phase of clastic, marine to marginal marine sedimentation. This followed the deposition of a thick sequence of mainly marine limestones and shales of the Whiterose Formation above a mid-Valanginian sequence-bounding unconformity. The increased clastic input and northward progradation of coastal environments represented by the Avalon Formation occurred during uplift of a basement arch to the south with subsidence of the basin increasing to the north, accompanied by only relatively minor faulting. These features indicate that a period of epeirogenesis was initiated during the Barremian. Continuing uplift over an expanding area at the southern end of the basin is interpreted to have resulted in the development of an angular unconformity with incised valleys. This mid-Aptian unconformity defines the top of the Whiterose/Avalon sequence. Initiation of brittle fracturing of the sedimentary package and underlying basement (i.e. rifting) in mid-Aptian times resulted in rapid fault-controlled subsidence and fragmentation of the Jeanne d'Arc Basin. This great increase in subsidence rate caused retrogradation of coastal environments across the previously developed sequence-bounding unconformity, despite continuing high rates of sediment input from the uplifted basin margins. The transgressive, siliciclastic Ben Nevis Formation comprises two separate but related facies associations. A locally preserved basal association represents interfingering back-barrier environments and is herein defined as the Gambo Member. An upper, ubiquitous facies association comprises tidal-inlet channel, shoreface and lower shoreface/offshore transition sandstones. This upper facies association onlapped marine ravinement diastems above the laterally equivalent back-barrier facies. The rapid fault-controlled subsidence and high sediment input rate of this mid-Aptian to late Albian rift period resulted in the accumulation and preservation of very thick shoreface sandstones. The transgressive sandstones were buried by laterally equivalent offshore shales of the Nautilus Formation. Flooding of the basin margins induced by the onset of thermal subsidence in latest Albian or early Cenomanian times marks the top of the Ben Nevis/Nautilus syn-rift sequence.