Relationships between shelf‐edge trajectories and sediment dispersal along depositional dip and strike: a different approach to sequence stratigraphy

Analysis of shelf‐edge trajectories in prograding successions from offshore Norway, Brazil, Venezuela and West Africa reveals systematic changes in facies associations along the depositional dip. These changes occur in conjunction with the relative sea‐level change, sediment supply, inclination of the substratum and the relief of the margin. Flat and ascending trajectories generally result in an accumulation of fluvial and shallow marine sediments in the topset segment. Descending trajectories will generally result in erosion and bypass of the topset segment and deposition of basin floor fans. An investigation of incised valley fills reveals multiple stages of filling that can be linked to distinct phases of deepwater fan deposition and to the overall evolution of the margin. In the case of high sediment supply, like the Neogene Niger and Orinoco deltas, basin floor fans may develop systematically even under ascending trajectory styles. In traditional sequence stratigraphic thinking, this would imply the deposition of basin floor fans during a period of relative sea‐level highstand. Facies associations and sequence development also vary along the depositional strike. The width and gradient of the shelf and slope show considerable variations from south to north along the Brazilian continental margin during the Cenozoic. During the same time interval, the continental shelf may display high or low accommodation conditions, and the resulting stacking patterns and facies associations may be utilized to reconstruct palaeogeography and for prediction of lithology. Application of the trajectory concept thus reveals nuances in the rock record that would be lost by the application of traditional sequence stratigraphic work procedures. At the same time, the methodology simplifies the interpretation in that less importance is placed on interpretation and labelling of surface boundaries and systems tracts.     

Tectono‐sedimentary Evolution of Early Pennsylvanian Alluvial Systems at the Onset of the Alleghanian Orogeny,Pocahontas Basin,Virginia

Foreland basin strata provide an opportunity to review the depositional response of alluvial systems to unsteady tectonic load variations at convergent plate margins. The lower Breathitt Group of the Pocahontas Basin, a sub‐basin of the Central Appalachian Basin, in Virginia preserves an Early Pennsylvanian record of sedimentation during initial foreland basin subsidence of the Alleghanian orogeny. Utilizing fluvial facies distributions and long‐term stacking patterns within the context of an ancient, marginal‐marine foreland basin provides stratigraphic evidence to disentangle a recurring, low‐frequency residual tectonic signature from high‐frequency glacioeustatic events. Results from basin‐wide facies analysis, corroborated with petrography and detrital zircon geochronology, support a two end‐member depositional system of coexisting transverse and longitudinal alluvial systems infilling the foredeep during eustatic lowstands. Provenance data suggest that sediment was derived from low‐grade metamorphic Grenvillian‐Avalonian terranes and recycling of older Palaeozoic sedimentary rocks uplifted as part of the Alleghanian orogen and Archean‐Superior‐Province. Immature sediments, including lithic sandstone bodies, were deposited within a SE‐NW oriented transverse drainage system. Quartzarenites were deposited within a strike‐parallel NE‐SW oriented axial drainage, forming elongate belts along the western basin margin. These mature quartzarenites were deposited within a braided fluvial system that originated from a northerly cratonic source area. Integrating subsurface and sandstone provenance data indicates significant, repeated palaeogeographical shifts in alluvial facies distribution. Distinct wedges comprising composite sequences are bounded by successive shifts in alluvial facies and define three low‐frequency tectonic accommodation cycles. The proposed tectonic accommodation cycles provide an explanation for the recognized low‐frequency composite sequences, defining short‐term episodes of unsteady westward migration of the flexural Appalachian Basin and constrain the relative timing of deformation events during cratonward progression of the Alleghanian orogenic wedge.     

Can sediment supply variations create sequences? Insights from stratigraphic forward modelling

Classic sequence stratigraphy suggests depositional sequences can form due to changes in accommodation and due to changes in sediment supply. Accommodation‐dominated sequences are problematic to define rigorously, but are commonly interpreted from outcrop and subsurface data. In contrast, supply‐dominated sequences are much less commonly identified. We employ numerical stratigraphic forward modelling to compare stratal geometries forced by cyclic changes in relative sea level with stratal geometries forced by sediment discharge and water discharge changes. Our quantitative results suggest that both relative sea‐level oscillations and variations in sediment/water discharge ratio are able to form sequence‐bounding unconformities independently, confirming previous qualitative sequences definitions. In some of the experiments, the two types of sequence share several characteristics, such as an absence of coastal‐plain topset deposits and stratal offlap, something typically interpreted as the result of falling relative sea level. However, the stratal geometries differ when variations in amplitude and frequency of relative sea‐level change, sediment/water discharge ratio, transport diffusion coefficient and initial bathymetry are applied. We propose that the supply‐dominated sequences could be recognised in outcrop or in the subsurface if the observations of stratal offlap and the absence of coastal‐plain topset can be made without any strong evidence of relative sea‐level fall (e.g. descending shoreline trajectory). These quantitative results suggest that both supply‐dominated and accommodation‐dominated sequences are likely to occur in the ancient record, as a consequence of multiple, possibly complex, controls.     

Palaeovalleys in foreland ramp settings: what happens as accommodation decreases down dip?

Recent advances in our understanding of palaeovalleys are largely guided by examples from passive margins, in which accommodation increases down depositional dip. This study tests these models against a dataset from the Pennsylvanian Breathitt Group of the central Appalachian foreland basin, USA. This fluvio‐deltaic succession contains extensive erosionally based fluvio‐estuarine sand bodies that can be tracked over 80 km down depositional dip from a proximal zone of high accommodation close to the orogenic margin to a distal, lower accommodation zone close to the cratonic margin of the basin. The sand bodies are up to 25 m thick, multi‐storey and characterized in their lower parts by strongly amalgamated storeys containing sandy fluvial to estuarine bar accretion elements, and in their middle to upper parts by more fully preserved storeys up to 10 m thick and laterally extensive over 100s of metres. The upper storeys include abandonment channel‐fills of heterolithic marine or marginal marine deposits or muddy to sandy point‐bar elements. Three major regional‐scale architectures include: (i) Tabular sand bodies that everywhere incise open marine prodelta and mouth bar facies and are interpreted as palaeovalleys formed during falling stage and lowstand systems tracts, when eustatic sea‐level fall outpaced tectonic subsidence across the entire study area. (ii) Sand bodies that incise genetically related floodplain lake and/or bay‐fill minor mouth bar deposits up depositional dip and open marine prodelta and mouth bar facies down dip. These stacked distributary channel deposits map down dip into palaeovalleys and formed when up dip subsidence rate resulted in positive, but reduced rate of accommodation creation, while lower tectonic subsidence rate down‐dip resulted in incision. (iii) Sand bodies that incise genetically related floodplain, lake and/or bay‐fill minor mouth bars up dip and pass down‐dip into genetically related unconfined floodplain, prodelta and mouth bar deposits. These sand bodies represent stacked distributary channel fills and channel amalgamation was the product of high rates of lateral migration, typical of the behaviour of channels above their backwater reach. Case (2) sand bodies demonstrate that in rapidly subsiding foreland basins, cross‐shelf palaeovalleys may form down depositional dip from aggradational, distributive fluival strata. Additionally, the genetic relationship between stacked distributary channels and palaeovalleys supports recent models for palaeovalley formation that emphasize diachronous, cut‐and‐fill during falling stage and lowstands of relative sea level.     

Effects of large sea‐level variations in connected basins: the Dacian–Black Sea system of the Eastern Paratethys

Sea‐level changes provide an important control on the interplay between accommodation space and sediment supply, in particular, for shallow‐water basins where the available space is limited. Sediment exchange between connected basins separated by a subaqueous sill (bathymetric threshold) is still not well understood. When sea‐level falls below the bathymetric level of this separating sill, the shallow‐water basin evolution is controlled by its erosion and rapid fill. Once this marginal basin is filled, the sedimentary depocenter shifts to the open marine basin (outward shift). With new accommodation space created during the subsequent sea‐level rise, sediment depocenter shifts backwards to the marginal basin (inward shift). This new conceptual model is tested here in the context of Late Miocene to Quaternary evolution of the open connection between Dacian and Black Sea basins. By the means of seismic sequence stratigraphic analysis of the Miocene‐Pliocene evolution of this Eastern Paratethys domain, this case study demonstrates these shifts in sedimentary depocenter between basins. An outward shift occurs with a delay that corresponds to the time required to fill the remaining accommodation space in the Dacian Basin below the sill that separates it from the Black Sea. This study provides novel insight on the amplitude and sedimentary geometry of the Messinian Salinity Crisis (MSC) event in the Black Sea. A large (1.3–1.7 km) sea‐level drop is demonstrated by quantifying coeval sedimentation patterns that change to mass‐flows and turbiditic deposits in the deep‐sea part of this main sink. The post‐MSC sediment routing continued into the present‐day pattern of Black Sea rivers discharge.     

Lateral variability in clinoform trajectory,process regime,and sediment dispersal patterns beyond the shelf‐edge rollover in exhumed basin margin‐scale clinothems

Sediment supply rate and accommodation regime represent primary controls on the depositional architecture of basin margin successions, but their interaction is commonly inferred from 2D dip profiles and/or with limited constraints on sedimentary facies. In this study, three parallel (>40 km long) 2D depositional oblique‐dip profiles from outcrops of the lower Waterford Formation (Karoo Basin, South Africa) have been correlated. This data set provides a rare opportunity to assess the lateral variability in the sedimentary process record of the shelf‐to‐slope transition for eight successive clinothems over a 900 km² area. The three profiles show similar shelf‐edge rollover trajectories, but this belies significant along‐margin variability in sedimentary processes and down‐dip sediment supply. The depositional architecture of three successive clinothems (WfC 3, 4 and 5) also show along‐shelf physiographic differences. The reconstructed shelf‐edge rollover position is not straight, and a westward curve to the north coincides with an area of greater sand supply to the slope beyond a shelf dominated by wave and storm processes. All the clinothems thicken northwards, indicating an along‐margin long‐term increase in accommodation that was maintained through multiple shoreline transits across the shelf. The origin of the differential subsidence cannot be discriminated confidently between tectonic or compaction processes. The interplay of basin margin physiography, differential subsidence rate and process regime resulted in significant across‐strike variability in the style and timing of sediment dispersal patterns beyond the shelf‐edge rollover. This study highlights the challenge for accurate prediction of the sediment partitioning across the shelf‐edge rollover in subsurface studies.     

100 Myr record of sequences, sedimentary facies and sea level change from Ocean Drilling Program onshore coreholes, US Mid-Atlantic coastal plain

We analyzed the latest Early Cretaceous to Miocene sections (～110–7 Ma) in 11 New Jersey and Delaware onshore coreholes (Ocean Drilling Program Legs 150X and 174AX). Fifteen to seventeen Late Cretaceous and 39–40 Cenozoic sequence boundaries were identified on the basis of physical and temporal breaks. Within‐sequence changes follow predictable patterns with thin transgressive and thick regressive highstand systems tracts. The few lowstands encountered provide critical constraints on the range of sea‐level fall. We estimated paleowater depths by integrating lithofacies and biofacies analyses and determined ages using integrated biostratigraphy and strontium isotopic stratigraphy. These datasets were backstripped to provide a sea‐level estimate for the past ～100 Myr. Large river systems affected New Jersey during the Cretaceous and latest Oligocene–Miocene. Facies evolved through eight depositional phases controlled by changes in accommodation, long‐term sea level, and sediment supply: (1) the Barremian–earliest Cenomanian consisted of anastomosing riverine environments associated with warm climates, high sediment supply, and high accommodation; (2) the Cenomanian–early Turonian was dominated by marine sediments with minor deltaic influence associated with long‐term (10⁷ year) sea‐level rise; (3) the late Turonian through Coniacian was dominated by alluvial and delta plain systems associated with long‐term sea‐level fall; (4) the Santonian–Campanian consisted of marine deposition under the influence of a wave‐dominated delta associated with a long‐term sea‐level rise and increased sediment supply; (5) Maastrichtian–Eocene deposition consisted primarily of starved siliciclastic, carbonate ramp shelf environments associated with very high long‐term sea level and low sediment supply; (6) the late Eocene–Oligocene was a starved siliciclastic shelf associated with moderately high sea‐level and low sediment supply; (7) late early–middle Miocene consisted of a prograding shelf under a strong wave‐dominated deltaic influence associated with major increase in sediment supply and accommodation due to local sediment loading; and (8) over the past 10 Myr, low accommodation and eroded coastal systems were associated with low long‐term sea level and low rates of sediment supply due to bypassing.     

Tectonic vs. climate forcing in the Cenozoic sedimentary evolution of a foreland basin (Eastern Southalpine system, Italy)

This paper discusses the Cenozoic interaction of regional tectonics and climate changes. These processes were responsible for mass flux from mountain belts to depositional basins in the eastern Alpine retro‐foreland basin (Venetian–Friulian Basin). Our discussion is based on the depositional architecture and basin‐scale depositional rate curves obtained from the decompacted thicknesses of stratigraphic units. We compare these data with the timing of tectonic deformation in the surrounding mountain ranges and the chronology of both long‐term trends and short‐term high‐magnitude (‘aberrant’) episodes of climate change. Our results confirm that climate forcing (and especially aberrant episodes) impacted the depositional evolution of the basin, but that tectonics was the main factor driving sediment flux in the basin up to the Late Miocene. The depositional rate remained below 0.1 mm year^?1 on average from the Eocene to the Miocene, peaking at around 0.36 mm year^?1, during periods of maximum tectonic activity in the eastern Southern Alps. This dynamic strongly changed during the Pliocene–Pleistocene, when the basin‐scale depositional rate increased to an average of 0.26 mm year^?1 (Pliocene) and 0.73 mm year^?1 (Pleistocene). This result fits nicely with the long‐term global cooling trend recorded during this time interval. Nevertheless, we note that the timing of the observed increase may be connected with the presumed onset of major glaciations in the southern flank of the Alps (0.7–0.9 Ma), the acceleration of the global cooling trend (since 3–4 Ma) and climate variability (in terms of magnitude and frequency). All these factors suggest that combined high‐frequency and high‐magnitude cooling–warming cycles are particularly powerful in promoting erosion in mid‐latitude mountain belts and therefore in increasing the sediment flux in foreland basins.     

Quantifying faulting and base level controls on syn‐rift sedimentation using stratigraphic architectures of coeval,adjacent Early‐Middle Pleistocene fan deltas in Lake Corinth,Greece

Quantification of allogenic controls in rift basin‐fills requires analysis of multiple depositional systems because of marked along‐strike changes in depositional architecture. Here, we compare two coeval Early‐Middle Pleistocene syn‐rift fan deltas that sit 6 km apart in the hangingwall of the Pirgaki‐Mamoussia Fault, along the southern margin of the Gulf of Corinth, Greece. The Selinous fan delta is located near the fault tip and the Kerinitis fan delta towards the fault centre. Selinous and Kerinitis have comparable overall aggradational stacking patterns. Selinous comprises 15 cyclic stratal units (ca. 25 m thick), whereas at Kerinitis 11 (ca. 60 m thick) are present. Eight facies associations are identified. Fluvial and shallow water facies dominate the major stratal units in the topset region, with shelfal fine‐grained facies constituting ca. 2 m thick intervals between major topset units and thick conglomeratic foresets building down‐dip. It is possible to quantify delta build times (Selinous: 615 kyr; Kerinitis: >450 kyr) and average subsidence and equivalent sedimentation rates (Selinous: 0.65 m/kyr; Kerinitis: >1.77 m/kyr). The presence of sequence boundaries at Selinous, but their absence at Kerinitis, enables sensitivity analysis of the most uncertain variables using a numerical model, ‘Syn‐Strat’, supported by an independent unit thickness extrapolation method. Our study has three broad outcomes: (a) the first estimate of lake level change amplitude in Lake Corinth for the Early‐Middle Pleistocene (10–15 m), which can aid regional palaeoclimate studies and inform broader climate‐system models; (b) demonstration of two complementary methods to quantify faulting and base level signals in the stratigraphic record—forward modelling with Syn‐Strat and a unit thickness extrapolation—which can be applied to other rift basin‐fills; and (c) a quantitative approach to the analysis of stacking patterns and key surfaces that could be applied to stratigraphic pinch‐out assessment and cross‐hole correlations in reservoir analysis.     

Hinterland basin development and infilling through tectonic and eustatic processes: latest Messinian‐Gelasian Valdelsa Basin,Northern Apennines,Italy

This article reports a stratigraphic and structural analysis of the Neogene‐Quaternary Valdelsa Basin (Central Italy), filled with up to 1000 m of uppermost Miocene to lower Pleistocene strata. The succession is subdivided into seven unconformity‐bounded stratigraphic units (synthems, or large‐scale depositional sequences) that include fluvio‐deltaic and shallow‐marine deposits. Structures related to basin shoulders and internal boundaries controlled the Neogene location and geometry of different depocentres. During the Tortonian‐Messinian, a buried NE‐trending high related to regional, basin‐transverse lineaments separated two adjacent sub‐basins. During the lower Pliocene, compressional displacement along NW‐trending, thrust‐related highs controlled the distribution of depocentres and dispersal of sediment. Extensional tectonics, although previously considered the dominant deformation style affecting the rear of the Northern Apennines since the late Miocene, is no longer considered a dominant control on tectono‐sedimentary development of the Valdelsa basin. Instead, the Valdelsa Basin shares features with continental hinterland basins of orogenic belts where compression, extension, and transcurrent stress fields determine a complex spatial and temporal record of accommodation and sediment supply. In the Valdelsa Basin tectonics and eustatic sea‐level fluctuations were dominant in forcing the deposition of sedimentary cycles at several scales. Zanclean and Gelasian large‐scale depositional sequences were mainly controlled by crustal shortening, whereas a eustatic signal was preferentially recorded during the Piacenzian. Smaller scale depositional sequences, common to most synthems, were controlled by orbitally forced glacio‐eustatic cycles.     

Clinoform architecture and along‐strike facies variability through an exhumed erosional to accretionary basin margin transition

Exhumed basin margin‐scale clinothems provide important archives for understanding process interactions and reconstructing the physiography of sedimentary basins. However, studies of coeval shelf through slope to basin‐floor deposits are rarely documented, mainly due to outcrop or subsurface dataset limitations. Unit G from the Laingsburg depocentre (Karoo Basin, South Africa) is a rare example of a complete basin margin scale clinothem (>60 km long, 200 m‐high), with >10 km of depositional strike control, which allows a quasi‐3D study of a preserved shelf‐slope‐basin floor transition over a ca. 1,200 km² area. Sand‐prone, wave‐influenced topset deposits close to the shelf‐edge rollover zone can be physically mapped down dip for ca. 10 km as they thicken and transition into heterolithic foreset/slope deposits. These deposits progressively fine and thin over tens of km farther down dip into sand‐starved bottomset/basin‐floor deposits. Only a few km along strike, the coeval foreset/slope deposits are bypass‐dominated with incisional features interpreted as minor slope conduits/gullies. The margin here is steeper, more channelized and records a stepped profile with evidence of sand‐filled intraslope topography, a preserved base‐of‐slope transition zone and sand‐rich bottomset/basin‐floor deposits. Unit G is interpreted as part of a composite depositional sequence that records a change in basin margin style from an underlying incised slope with large sand‐rich basin‐floor fans to an overlying accretion‐dominated shelf with limited sand supply to the slope and basin floor. The change in margin style is accompanied with decreased clinoform height/slope and increased shelf width. This is interpreted to reflect a transition in subsidence style from regional sag, driven by dynamic topography/inherited basement configuration, to early foreland basin flexural loading. Results of this study caution against reconstructing basin margin successions from partial datasets without accounting for temporal and spatial physiographic changes, with potential implications on predictive basin evolution models.     

Impact of structural and autocyclic basin-floor topography on the depositional evolution of the deep-water Valparaiso forearc basin, central Chile

The Valparaiso Basin constitutes a unique and prominent deep‐water forearc basin underlying a 40‐km by 60‐km mid‐slope terrace at 2.5‐km water depth on the central Chile margin. Seismic‐reflection data, collected as part of the CONDOR investigation, image a 3–3.5‐km thick sediment succession that fills a smoothly sagged, margin‐parallel, elongated trough at the base of the upper slope. In response to underthrusting of the Juan Fernández Ridge on the Nazca plate, the basin fill is increasingly deformed in the seaward direction above seaward‐vergent outer forearc compressional highs. Syn‐depositional growth of a large margin‐parallel monoclinal high in conjunction with sagging of the inner trough of the basin created stratal geometries similar to those observed in forearc basins bordered by large accretionary prisms. Margin‐parallel compressional ridges diverted turbidity currents along the basin axis and exerted a direct control on sediment depositional processes. As structural depressions became buried, transverse input from point sources on the adjacent upper slope formed complex fan systems with sediment waves characterising the overbank environment, common on many Pleistocene turbidite systems. Mass failure as a result of local topographic inversion formed a prominent mass‐flow deposit, and ultimately resulted in canyon formation and hence a new focused point source feeding the basin. The Valparaiso Basin is presently filled to the spill point of the outer forearc highs, causing headward erosion of incipient canyons into the basin fill and allowing bypass of sediment to the Chile Trench. Age estimates that are constrained by subduction‐related syn‐depositional deformation of the upper 700–800 m of the basin fill suggest that glacio‐eustatic sea‐level lowstands, in conjunction with accelerated denudation rates, within the past 350 ka may have contributed to the increase in simultaneously active point sources along the upper slope as well as an increased complexity of proximal depositional facies.     

The long-term thermal consequences of rifting: implications for basin reactivation

The attenuation of the continental crust during rifting and the subsequent filling of the rift‐related accommodation alter the long‐term thermal and mechanical state of the lithosphere. This is primarily because the Moho is shallowed due to density contrasts between the sediment fill and the crust, but also reflects the attenuation of the pre‐existing crustal heat production and its burial beneath the basin, as well the thermal properties of the basin fill. Moho shallowing and attenuation of pre‐existing heat production contribute to long‐term cooling of the Moho and thus lithospheric strengthening, as has been pointed out in many previous studies. In contrast, basin filling normally contributes to significant Moho heating allowing the possibility of long‐term lithospheric weakening, the magnitude of which is dependent on the thermal properties of the basin‐fill and the distribution of heat sources in the crust. This paper focuses on the thermal property structure of the crust and basin‐fill in effecting long‐term changes in lithospheric thermal regime, with particular emphasis on the distribution of heat producing elements in the crust. The parameter space appropriate to typical continental crust is explored using a formalism for the heat production distributions that makes no priori assumptions about the specific form of the distribution. The plausible parameter space allows a wide range in potential long‐term thermal responses. However, with the proviso that the accommodation created by the isostatic response to rifting is essentially filled, the long‐term thermal response to rift basin formation will generally increase average crustal thermal gradients beneath basins but cool the Moho due to its reduction in depth. The increase in the average crustal thermal gradient induces lateral heat flow that necessarily heats the Moho along basin margins, especially in narrow rift basins. Using coupled thermo‐mechanical models with temperature sensitive creep‐parameters, we show that such heating may be sufficient to localise subsequent deformation in the vicinity of major basin bounding structures, potentially explaining the offset observed in some stacked rift basin successions.     

Sequence architecture and depositional evolution of the Ordovician carbonate platform margins in the Tarim Basin and its response to tectonism and sea‐level change

The sequence architecture and depositional evolution of the Ordovician carbonate platform margins in the Tarim Basin, China, were formed in response to the interplay of tectonism and sea‐level change, their history being documented by the integrated analysis of many seismic lines, drilling and outcrop data. The Ordovician carbonate system in the basin is divided into four composite sequences defined by major unconformities. Each sequence consists of a regional depositional cycle from transgression with an onlapping transgressive systems tract (TST) to regression with a prograding highstand systems tract (HST), and can be further subdivided into 10 third‐order sequences based on subordinate discontinuous boundaries at the carbonate platform marginal zones. Constrained by the marginal slope of the early‐rifted Manjiaer aulacogen, the carbonate platform margins of the Lower and Middle Ordovician that prograded eastward in an arcuate belt extending generally north‐south across the northern part of the basin. The development of the Tazhong uplift due to compression resulted in an extensive paleokarst hiatus between the Middle and the Upper Ordovician in the south‐central basin, and subsequently constrained the formation of a peninsula‐shaped carbonate platform whose margins were controlled by marginal thrust‐fault belts of the paleo‐uplift during the Late Ordovician. In the northern basin, the Late Ordovician carbonate platform margin developed around the marginal slope of the Tabei paleouplift. The transgressive–regressive cycles of the carbonate system are comparable and seem to have occurred simultaneously across the entire basin, suggesting that the cyclic sequence architecture was fundamentally controlled by eustatic fluctuations. Stacking patterns of the composite sequences varied due to the interplay between the accommodation produced by tectonism and sea‐level change, and the carbonate production rate. The reef–shoal facies complexes that developed along the platform margins, with paleokarst development at unconformities, constitute the major reservoir of large petroleum reserves in the basin.     

Using detrital zircon U‐Pb ages to calculate Late Cretaceous sedimentation rates in the Magallanes‐Austral basin,Patagonia

Determining both short‐ and long‐term sedimentation rates is becoming increasingly important in geomorphic (exhumation and sediment flux), structural (subsidence/flexure) and natural resource (predictive modelling) studies. Determining sedimentation rates for ancient sedimentary sequences is often hampered by poor understanding of stratigraphic architecture, long‐term variability in large‐scale sediment dispersal patterns and inconsistent availability of absolute age data. Uranium–Lead (U‐Pb) detrital zircon (DZ) geochronology is not only a popular method to determine the provenance of siliciclastic sedimentary rocks but also helps delimit the age of sedimentary sequences, especially in basins associated with protracted volcanism. This study assesses the reliability of U‐Pb DZ ages as proxies for depositional ages of Upper Cretaceous strata in the Magallanes‐Austral retroarc foreland basin of Patagonia. Progressive younging of maximum depositional ages (MDAs) calculated from young zircon populations in the Upper Cretaceous Dorotea Formation suggests that the MDAs are potential proxies for absolute age, and constrain the age of the Dorotea Formation to be ca. 82–69 Ma. Even if the MDAs do not truly represent ages of contemporaneous volcanic eruptions in the arc, they may still indicate progressive‐but‐lagged delivery of increasingly younger volcanogenic zircon to the basin. In this case, MDAs may still be a means to determine long‐term (≥1–2 Myr) average sedimentation rates. Burial history models built using the MDAs reveal high aggradation rates during an initial, deep‐marine phase of the basin. As the basin shoaled to shelfal depths, aggradation rates decreased significantly and were outpaced by progradation of the deposystem. This transition is likely linked to eastward propagation of the Magallanes fold‐thrust belt during Campanian‐Maastrichtian time, and demonstrates the influence of predecessor basin history on foreland basin dynamics.     

Comment on ‘Clinoform quantification for assessing the effects of external forcing on continental margin development’

The application of geometric modeling to shelf‐margin stratigraphy has the potential to constrain interpretations of external forcings on margin development. Here we apply such a model to the Ebro margin in order to complement the analysis of Kertznus & Kneller (2009) . Our results suggest that increased mass wasting in the Pleistocene was unlikely to have been a factor in the observed long‐term shelf‐edge trajectory, and that the trajectory can be explained by the interaction of sediment flux, relative sea‐level rise, and basin shape.     

Supradetachment basin evolution unravelled by detrital apatite fission track analysis: the Gediz Graben (Menderes Massif,Western Turkey)

The Menderes Massif is a Tertiary metamorphic core complex tectonically exhumed in the late Oligocene–Miocene during coeval development of a series of E–W‐trending basins. This study analyses the source‐to‐sink evolution of the Gediz Graben and the exhumation pattern of the Central Menderes Massif at the footwall and hanging wall of the Gediz Detachment Fault. We use a comprehensive approach to detrital apatite fission track dating combining analysis of modern river sediments, analysis of fossil sedimentary successions and mineral fertility determinations. This approach allowed us to: (i) define the modern short‐term erosion pattern of the study area, (ii) unravel the long‐term exhumation history, (iii) identify major exhumation events recorded in the sedimentary basin fill and (iv) constrain the maximum depositional age of the sedimentary succession. Three main exhumation events are recorded in the analysed detrital samples: (i) a late Oligocene/early Miocene exhumation event involving the whole Menderes Massif; (ii) a late Miocene event involving the northern edge of the Central Menderes Massif; (iii) a Plio‐Quaternary more localized event involving only the western part of the southern margin of the basin (Salihli area) and bringing to the surface the Gediz Detachment and its intrusive footwall (Salihli granodiorite). The modern short‐term erosion pattern closely reflects this latter Plio‐Quaternary event. Single grain‐age distributions in the sedimentary basin fill highlight drainage pattern reorganizations in correspondence of the transition between different stratigraphic units, and allowed to better constrain the depositional age of the sedimentary units of the basin pointing to a possible onset of sedimentation in the basin during the middle Miocene.     

Assembling the stratigraphic record: depositional patterns and time-scales in an experimental alluvial basin

Our understanding of sedimentation in alluvial basins is best for very short and very long time‐scales (those of bedforms to bars and basinwide deposition, respectively). Between these end members, the intermediate time‐scales of stratigraphic assembly are especially hard to constrain with field data. We address these ‘mesoscale’ fluvial dynamics with data from an experimental alluvial system in a basin with a subsiding floor. Observations of experimental deposition over a range of time‐scales illustrate two important properties of alluvial systems. First, ephemeral flows are disproportionately important in basin filling. Lack of correlation between flow occupation and sedimentation indicates that channelized flows serve mainly as conduits for sediment, while most deposition occurs via short‐lived unchannelized flow events. Second, there is a characteristic time required for individual depositional events to average to basin‐scale stratal patterns. This time can be scaled in terms of the time required for a single channel‐depth of aggradation, and in this form is constant through a four‐fold variation of experimental subsidence rate.     

Long-term Callovian–Oxfordian sea-level changes and sedimentation in the Iberian carbonate platform (Jurassic, Spain): possible eustatic implications

Facies analysis across the carbonate platform developed during the Callovian–Oxfordian in the northern Iberian basin (Jurassic, Northeast Spain) is used to characterize successive stages of sedimentary evolution, including palaeoenvironmental reconstructions showing the distribution of a wide spectrum of facies, from ferruginous oolitic, peloidal, spongiolithic to intraclastic. The studied successions consist of two long‐term transgressive–regressive cycles bounded by a major unconformity with a major gap, comprising at least the upper Lamberti (Callovian) and Mariae (Oxfordian) Zones. Major transgressive peaks of these two cycles occurred at the end of the Early Callovian (late Gracilis Zone) and at the end of the Middle Oxfordian. The Callovian and Oxfordian successions were further divided into three and seven higher frequency cycles, respectively. The modelling of two sections (i.e. Ricla and Tosos) located 40 km apart in the more subsident open platform areas, allows the reconstruction of two curves showing a similar evolution of long‐term sea‐level changes that are in theory eustatic, though subject to uncertainties derived form the assumptions required for their construction. The changes affecting the northern Iberian basin seem to reflect nearly homogeneous subsidence (rates around 2 cm kyr^?1) combined with possible eustatic changes including an Early Callovian rise, a fall at the middle Callovian–earliest Oxfordian (i.e. the Anceps–Mariae Zones), with average long‐term rates around 2 cm kyr^?1 (total fall of 40–60 m), a period of lowstand at the Early–Middle Oxfordian transition and a long‐term rise at the Middle–Late Oxfordian transition (Transversarium and Bifurcatus Zones). Facies distribution across the Iberian platform indicates a progressive Middle–Late Callovian relative sea‐level fall rather than a rapid relative sea‐level fall at the end of the Callovian. After this falling episode, the progressive onlap over the swell areas during the Early Oxfordian and at the beginning of the Middle Oxfordian indicates a period of accommodation gain, which is explained by the combined effects of continuous subsidence across the platform and reduced sedimentation rates in spite of the possible eustatic lowstand. Eustatic lowstand, combined with other factors (ocean water circulation, volcanism) could help to explain the loss of carbonate production during the latest Callovian–Early Oxfordian, previous to the widespread eustatic rise and warning recorded at the onset of the Transversarium Zone (Middle Oxfordian).     

Stratigraphic signatures of translation of thrust-sheet top basins over low-angle detachment faults

Abstract Low‐angle detachment faults and thrust‐sheet top basins are common features in foreland basins. However, in stratigraphic analysis their influence on sequence architecture is commonly neglected. Usually, only eustatic sea level and changing flexural subsidence are accounted for, and when deformation is considered, the emphasis is on the generation of local thrust‐flank unconformities. This study analyses the effects of detachment angle and repetitive detachment activation on stratigraphic stacking patterns in a large thrust‐sheet top basin by applying a three‐dimensional numerical model. Model experiments show that displacement over low‐angle faults (2–6°) at moderate rates (～5.0 m kyr^?1) results in a vertical uplift component sufficient to counteract the background flexural subsidence rate. Consequently, the basin‐wide accommodation space is reduced, fluvio‐deltaic systems carried by the thrust‐sheet prograde and part of the sediment supply is spilled over towards adjacent basins. The intensity of the forced regression and the interconnectedness of fluvial sheet sandstones increases with the dip angle of the detachment fault or rate of displacement. In addition, the delta plain is susceptible to the formation of incised valleys during eustatic falls because these events are less compensated by regional flexural subsidence, than they would be in the absence of fault displacement.