Progradation Rates Research Articles

Strontium-isotope stratigraphy of Upper Cretaceous platform carbonates of the island of Brač (Adriatic Sea, Croatia): implications for global correlation of platform evolution and biostratigraphy

The chronostratigraphy of Coniacian–Maastrichtian platform carbonates exposed on the island of Brač and the adjacent mainland has been revised, based on numerical ages derived from strontium-isotope stratigraphy (SIS) of low-Mg calcite of rudist shells. The Dol intra-platform basin formed during the mid-Coniacian–early Santonian. The base of the prograding Pučišća Formation is of mid-Santonian age (84.9 Ma) in the southeast, and late Middle Campanian (77.3 Ma) in the northwest of the island, indicating a progradation rate of the platform margin of ca. 2.5 km/myr. Subaerial exposure of the platform occurred during the latest Middle Campanian and is coeval with a major drop in sea level reported from the Boreal Realm, North America, and the southern Tethyan margin. The base of the Sumartin Formation is revised here to the earliest Late Campanian (ca. 75 Ma). At its top, the formation contains rudist-bearing limestones of latest Maastrichtian age (65.4–65.0 Ma). The exact position of the K/T boundary cannot be drawn due to the lack of material suitable for SIS, and to the absence of diagnostic fossils in restricted innermost-platform deposits of the Liburnian Formation, which follows conformably over the rudist-bearing Sumartin Formation. Based on the revised chronostratigraphy of platform evolution, and particularly on the numerical ages that constrain the progradation of the Pučišća Formation, the stratigraphic ranges of characteristic Tethyan rudist bivalves and benthic foraminifers are re-evaluated.

Διαχρονική παρατήρηση των μεταβολών της ακτογραμμής τμήματος της παράκτιας ζώνης του νόμου Πιερίας και εκτίμηση των επιπτώσεων από την μελλοντική άνοδο της θαλάσσιας στάθμης

The aim of this study is to detect shoreline changes along part of the coastal zone of Piena during the time period between 1969 and 2000 using aerial photographs and satellite images. Additionally, a quantitative and qualitative assessment of the future sea-level rise (triggered by the global climate change) implications to the physical and socioeconomic environment of the area is attempted taking into account various sea-level rise scenarios. Retreating as well as prograding regions along the study area were defined and retreating/prograding rates for the time periods 1969-1987 and 1987-2000 were estimated using GIS and Remote Sensing techniques. Building activity rates for the coastal area of Paralia Katerinis were also estimated for the same periods. The coastline of the study area is retreating^ except than the area north of torrent Mavroneri where a progradation rate of 48 cm/year was estimated between 1969 and 1987. Retreating rate of the coast for the northern part of the area (Saltworks) is estimated to be 25 cm/year and 19 cm/yrear for the periods of 1969-1987 and 1987-2000 respectively. The broader study area is particularly vulnerable to a potential future sealevel rise due to the low-lying topography of the coastal zone and intensive socioeconomic activities such as tourism and commerce.

Evolution of the Rhône delta plain in the Holocene

The delta plain of the Rhone shows many sandy beach ridges cropping out at the surface. We propose here a radiocarbon chronology for the accumulation of the sedimentary bodies and an interpretation of the morphology of the mouth lobes that they form. Morphologies of the lobes depend on four principal factors: (1) the variation of the relative sea level, which constitutes the morphogenic base level, (2) the fluvial input (volume of water discharge and sedimentary input, number and position of the mouths), (3) marine dynamics (volume and direction of the longshore drift, dominant wave direction) (4) the accommodation space created by the rise of relative sea level, along with the paleobathymetry onto which the lobe will prograde immediately in front of the delta and at sea. Our conclusions are: the main Holocene paleo-delta lobes of Ulmet and Saint Ferréol were contemporaneous, being deposited from 4000 to 2000 yr BP. However, the fluvial style of their downstream channel (meandering for Ulmet channel and linear for Saint-Ferréol) and the lobe morphologies of both channels are contrasted: rounded in the case of Ulmet and elongated for Saint-Ferréol. The role of the accommodation space is confirmed by two recent drillholes traversing the entire Holocene succession, which reveal a prior stage of sedimentary filling in the case of the Saint-Ferréol lobe, thus limiting the accommodation space and favouring a rapid and pointed progradation of this lobe. The decoupling between hydrological activity and progradation of lobe indicates the complexity of mechanisms in the large catchment basin of Rhône. However, the high rate of progradation of Saint-Ferréol lobe after 2900 BP is probably a consequence of the increase of human activity after this date. The variation of relative sea level remains an important factor controlling the evolution of the Rhone delta after 6000 BP. The local (?) stabilization of the relative sea level between 4585 BP and 3520 BP played an important role in the triggering of coastal progradation (Saint-Ferréol lobe, eastern margin Rhône delta).

Distributary channels and their impact on sediment dispersal

A global analysis of world deltas, with details from a natural and an anthropogenic-influenced delta, demonstrates how distributary channels control the flux of sediment into the coastal ocean. The study addresses the range in the number of distributary channels across world deltas using remote sensing techniques. A power law relationship is found between the number of distributary channels and the length of river and, separately, the delta gradient. These relationships hold for all types of deltas whether controlled or strongly influenced by waves, river discharge, tides or ice (permafrost or sea-ice). Nature-controlled deltas, such as the Klinaklini delta, have distributary channels that act as overflow conduits that become active during flooding events. Anthropogenic-controlled deltas, like the Po delta, have distributary channels that are controlled for flood mitigation or low flow maintenance. Anthropogenic influences greatly impact the natural rate of delta progradation through changes in sediment supply, controlling the position of distributary channels, and impacting subsidence from gas and groundwater extraction. Even with flood controls, the Po delta traps 16% of the sediment load in its channels that are becoming super-elevated at rates of 4 to 10 cm/yr, with respect to the delta plain. A new model is formulated and shown to predict accurately the sediment flux through each channel, along with their hydraulic properties. Deltas with high numbers of distributary channels produce hypopycnal plumes with reduced transport capacity. As a result, sediment diffuses out of the multi-channel deltas as a buoyantly driven plume, rather than as a momentum driven jet.

Salt tectonics in two convergent-margin basins of the Cyprus arc, Northeastern Mediterranean

Seismic reflection profiles from the Adana–Cilicia and Iskenderun–Latakia Basin complexes in the NE Mediterranean Sea show the presence of linked extensional–contractional fault systems, detached at the base of the Messinian evaporites. The extensional domain is developed within the thick proximal part of the Plio-Quaternary delta succession in the inner part of each basin. It is characterized by imbricate fans of listric normal faults, which delineate turtle anticlines and their intervening syncline. The faults sole into the gently southwest-dipping detachment surface. In the inner basins, salt is largely evacuated toward the outer basins, but remnants occur as rollers in the footwalls of the listric extensional faults and small subdued pillows beneath turtle anticlines. The narrow synclines between the two turtle back growth anticlines record a more complicated history of rise and fall of the salt related to switch in vergence of the extensional fault fan. A large salt wall is developed at the toe of the delta complex in both basins. Two wide peripheral sinks are present within the Plio-Quaternary units on both flanks of the salt wall. Locally small segments of salt welds are present near the base of the salt wall. The contractional domain is characterized by a relatively thin Plio-Quaternary cover overlying a relatively uniform, 500–800 ms-thick salt substrate. A series of salt-cored growth folds are commonly associated with thrusts of variable vergence. The halokinetic structures in the Cilicia and Latakia Basins display a remarkable resemblance to detached and linked systems of extensional and contractional fault fans documented in classical salt provinces on passive margins. The study area also resembles scaled experiments which document synchronous development of extensional and contractional fault systems detached in a ductile substrate. However, a number of differences also exist, which may arise from several factors, including the nature of ductility contrast between the ductile portion of the evaporite layer and the supra-salt succession, the thickness ratio of overburden to evaporite unit, and the rate of progradation and the nature of sediment distribution during the evolution of the delta complex. The special aspect of these associations of linked gravity-driven extensional and contractional salt structures is that they are developed as a self-contained system wholly situated within a Plio-Quaternary transtensional basin.

Fluvial and marine controls on combined subaerial and subaqueous delta progradation: Morphodynamic modeling of compound‐clinoform development

Fluviodeltaic systems commonly display a compound‐clinoform geometry that consists of a subaerial/subaqueous delta couplet. The extent of subaqueous delta development varies significantly and, in modern systems, is a function of fluvial input and basin hydrodynamics. We present a model of fluviodeltaic progradation in which the repeated occurrence of characteristic terrestrial floods and large coastal storms drives fluvial and shallow marine morphodynamics, respectively. We couple fluvial and shallow marine sediment dynamics via the surf zone, which we collapse to a shock condition and treat as a moving boundary. With steady sediment supply and sea level and simple basin geometry, our model naturally develops prograding deltas with compound‐clinoform geometries. The subaerial delta grows via fluvial aggradation and shoreface progradation, whereas the subaqueous delta expands through foreset progradation, with only minor topset aggradation. The interplay of fluvial input with the wave/current field controls the basic partitioning of sediment between subaerial and subaqueous deltas and, by extension, the compound‐clinoform geometry. Increasing the frequency or magnitude of coastal storms, decreasing flood frequency or discharge, and reducing grain size all increase the fraction of sediment delivered to the shallow marine environment and the extent of subaqueous delta progradation relative to subaerial delta development. Our model, which emphasizes the intrinsic coupling of fluvial and shallow marine sediment dynamics and downplays the importance of allogenic fluctuations, can explain many of the first‐order morphologic features of natural delta systems, including significant lateral separation of the shoreline and clinoform rollover and differing rates of subaerial and subaqueous delta progradation.

Steep microbial boundstone-dominated platform margins—examples and implications

Seaward progradation of several kilometers has been documented mostly for leeward margin low-angle carbonate slope systems with a dominant platform top sediment source. However, steep and high-relief margins fronting deep basins can also prograde and as such are somewhat perplexing. Characteristics of two prograding Carboniferous examples provide a model which may apply elsewhere: (1) outcrops in Asturias, northern Spain serve as important analogs for (2) hydrocarbon reservoirs in steep-sided isolated platforms of the North Caspian Basin, Kazakhstan, such as Tengiz. Seismic and well data from Tengiz corroborate outcrop patterns for slope development, showing progradation of up to 5 and more than 10 km, respectively, despite the high-relief (up to 600 m) and steep (∼20–35°) nature of these margins. The two examples share a highly productive microbial boundstone factory extending from the platform break down the slope to nearly 300 m (or more) depth and a lower slope dominated by (mega)breccias and grain flow deposits derived from the margin and slope itself. The broad depth range of microbial boundstone increases the potential for production during both lowstands and high stands of sea level and thereby facilitates progradation independent from platform-top-derived sediment. Rapid in situ lithification of the boundstone provides stability to the steep slopes, but also leads to readjustment through shearing and avalanching. What controls the microbial cement boundstone formation remains a debate but its presence is a key factor controlling the progradational geometry of these and possibly other margins. This new model of “slope” shedding has implications for slope readjustment processes and resulting architecture, sequence stratigraphic interpretation, reservoir characterization, and reservoir modeling. Especially the isotropic character of microbial boundstone will reduce the potential for coherent seismic reflections to develop and possibly invoke, under certain stress regimes, shattering and fracturing thereby generating significant non-matrix permeability. Key considerations are the contrasts with the Bahamian high stand shedding depositional model, slope progradation rates that range from 450 to >1500 m/My, and net growth rates of in situ boundstone of ∼1000 m/My, comparable to or higher than accretion rates for metazoan skeletal reef growth.

Shelf-geometry response to changes in relative sea level on a mixed carbonate–siliciclastic shelf in the Guyana Basin

Differences in the shelf-margin geometry for various depositional regimes show how siliciclastic and carbonate margins respond differently to changes in accommodation space. During the Cenozoic, sequences of carbonate and siliciclastic sediment were deposited in the Guyana Basin on the passive continental margin of NE South America. Study of the sequence geometries reveal significantly different geometric responses to changes in relative sea level of intervals dominated by carbonates to intervals dominated by siliciclastics. Using the geometrical shelf break as a reference point, aggradation and progradation rates for the carbonate and siliciclastic depositional systems were measured. In siliciclastics, the lateral position of the margin remains roughly stationary with relative sea-level rises in the order of 30 m/My. At higher rates the margin retreats at lower rates it progrades. Carbonate margins remain stationary or slightly progradational even with relative sea-level rises of up to 100 m/My, the fastest rates observed in this study. This illustrates the strong tendency of carbonate platforms to stack their margins and keep up with relative rises in sea level, rather than gradually retreat landward as do siliciclastics. This observation may explain why carbonate platforms preferentially try to defend a margin prior to ultimate backstepping. The high aggradation potential of carbonate margins also gives onlap and downlap termination patterns on seismic profiles where carbonate platforms develop on sloping siliciclastic shelves. The resulting unconformities are a result of differences in sediment dispersal between the two systems and not necessarily from changes of relative sea level.

Impact of a Quaternary volcano on Holocene sedimentation in Lillooet River valley, British Columbia

Lillooet River drains 3850 km 2 of the rugged Coast Mountains in southwestern British Columbia, including the slopes of a dormant Quaternary volcano at Mount Meager. A drilling program was conducted 32–65 km downstream from the volcano to search for evidence of anomalous sedimentation caused by volcanism or large landslides at Mount Meager. Drilling revealed an alluvial sequence consisting of river channel, bar, and overbank sediments interlayered with volcaniclastic units deposited by debris flows and hyperconcentrated flows. The sediments constitute the upper part of a prograded delta that filled a late Pleistocene lake. Calibrated radiocarbon ages obtained from drill core at 13 sites show that the average long-term floodplain aggradation rate is 4.4 mm a −1 and the average delta progradation rate is 6.0 m a −1. Aggradation and progradation rates, however, varied markedly over time. Large volumes of sediment were deposited in the valley following edifice collapse events and the eruption of Mount Meager volcano about 2360 years ago, causing pulses in delta progradation, with estimated rates to 150 m a −1 over 50-yr intervals. Two of the volcaniclastic units identified in drill core correlate with previously documented strong acoustic reflectors in Lillooet Lake at the downstream end of the basin. The Mount Meager massif constitutes only 2% of the Lillooet River drainage, but lithology counts of Lillooet River channel gravels indicate that a disproportionate percentage of the sediment is derived from the volcano. The data indicate that deposits of large debris flows are important elements of the sedimentary sequence and that Mount Meager dominates the sediment supply to Lillooet River.

Plant succession and dune dynamics on actively prograding dunes, Whatipu Beach, northern New Zealand

Whatipu Beach, north Manukau Harbour, Auckland, has prograded episodically over 1.5 centuries. Sand dune vegetation is quantitatively described, and related to successional stage, local environment, and progradation dynamics. Foredunes, with fine, mobile, infertile sand, occupy the most recently prograded areas and are partially vegetated by native species. Instead of a dune slack, a stream which changed course to flow between dune ridges has created wet “sand river” vegetation. Relict fore‐dunes and a dune slack persist inland of the sand river, disrupting a lineal spatial sequence, as do rear dune wetlands. Less‐mobile grass and shrub communities, derived from Ammophila arenaria dunes, occupy surfaces >50 years old. No climax communities are present. In contrast to spatial analogues, the Whatipu dunes demonstrate how rapidly succession proceeds in response to the formation of new habitat, while freshwater movements generate non‐seral sand‐river communities. Succession is modified by invading exotics and by the absence of many disturbance‐intolerant native shrub species. Coastal progradation rates at Whatipu indicate that shrubland develops within 50 years, suggesting that spatial analogues of dune succession can mis‐represent the longevity of early seral stages. These unusual successional patterns on dateable surfaces, and the presence of rare flora and fauna, make Whatipu Beach important for conservation.

Structure and development of detrital reef deposits in turbid nearshore environments, Inhaca Island, Mozambique

Coral communities are intermittently developed along the seaward margins of wide (up to 500 m) intertidal flats that fringe the western margins of Inhaca Island, southern Mozambique. The coral communities occur in an environment that can be considered as marginal for coral growth in the sense that they are subject to high turbidity levels (which results in rapid light attenuation) and to the effects of periodic smothering and burial associated with a mobile, siliciclastic-dominated sediment substrate. These constraints limit coral community development both spatially and bathymetrically. In addition, historical evidence suggests that the main sites of coral growth shift over decadal timescales due to periodic sediment remobilisation. Such rapid rates of turnover restrict the development of rigid framework deposits and, in contrast, the main depositional product of coral growth is an abundance of branched (mainly Acropora sp.) coral rubble. Rubble production is aided both by intense internal bioerosion (sponge, bivalve and worm traces) and by periodic storm damage. Much of this rubble accumulates along the channel margins and provides a substrate for subsequent coral recruitment. The deposits can be distinguished from those associated with clear water reef environments by (1) the lack of a rigid reef framework either comprising of in-situ coral or encrusted coral rubble, (2) the distinct lack of rubble encrustation/syn-depositional cementation, and (3) the predominance of a siliciclastic-rich sediment matrix. The deposits indicate, however, a long-term history of coral community occurrence in sites dominated by siliciclastic sedimentation. Cores recovered from a transect across the tidal flats identify a coral rubble floatstone facies extensively developed along the western margins of the island. It comprises mainly branched coral rubble (often heavily bioeroded, but sparsely encrusted), along with spheroidal rhodoliths and molluscan fragments set within a poorly sorted, mixed siliciclastic: carbonate sediment matrix. 14C dating indicates that most of the rubble deposits date from the period 6500–4700 years BP, which corresponds to a period of regional sea level highstand. A detailed growth chronology cannot be established due to the lack of in-situ coral, but rapid rubble deposition and reef flat progradation (around 350 m) can be inferred for this period (equivalent to an average progradation rate of around 0.2 m year −1). Over the subsequent 4700 years, progradation of only around 25 m has occurred. This may be attributed to reworking and erosion of the seaward margin of the deposits during periods of fluctuating sea level between 4700 and 900 years BP, or to a lack of seaward accommodation space.

Controls on sequence stratigraphy of a middle Miocene-Holocene, current-swept, passive margin: Offshore Canterbury Basin, New Zealand

The offshore Canterbury Basin exemplifies sequence development on a prograding passive margin influenced strongly by submarine currents. Nineteen middle Miocene–Holocene, regional, sequence-bounding unconformities are interpreted by using high-resolution multichannel seismic data. The sequences can be grouped into larger units based on seismic geometry and facies that reflect different combinations of controls on sequence architecture. Correlation with oxygen isotope records suggests that eustasy controls the timing of sequence boundaries. The number of sequences is similar to that of coeval cycles on a temperature-adjusted, Miocene and early Pliocene δ 18 O record. The late Pliocene–Pleistocene sequence record is of lower frequency than the isotopic record of this period, either because of the limitations of seismic resolution or because of removal of sequence boundaries by erosion associated with high-amplitude eustasy. However, the last two sequence boundaries correlate well with the last two 100 k.y. isotopic cycles. In contrast, sequence architecture is influenced strongly by local processes. Along-strike currents create large, elongate sediment drifts that control sequence thickness; current erosion in drift moats forms diachronous unconformities. Drifts focus deposition on the slope, reducing the rate of basinward advance of the shelf edge, but increasing that of the slope toe, thereby reducing slope inclination. Replacement of along-strike processes by downslope processes increases rates of shelf-edge progradation, and the slope steepens as the reduced accommodation space over the expanded slope is filled. Clinoform geometries along strike from active drifts suggest that currents might influence clinoform formation even in locations lacking seismic evidence of current reworking.

Coastal Change and Beach Ridges along the Northwest Coast of Peru: Image and GIS Analysis of the Chira, Piura, and Colán Beach-Ridge Plains

Abstract Since approximately 5200 cal yrs BP, five sets of eight to nine beach ridges were built and preserved along the northwestern Peruvian coastal desert (3°30′S–9°S). Potential ridge-building mechanisms in the hyper-arid environment of northwest Peru include El Nino floods and storms, seismic activity, and sea-level change, as well as more gradual climate changes that affected coastal morphology. Image processing and Geographic Information System (GIS) methods were used to analyze aerial photographs and measure historic coastal patterns along three beach-ridge plains over a 37-year time period. Coastal features were digitized from image mosaics of each ridge plain at different time intervals from 1946 up to 1983. Progradation rates were examined at ridge locations north of the Chira River and Piura River, as well as at the base of ephemeral stream valleys in Colan. The total change in beach area was measured from historic aerial photographs taken at different time intervals. The resulting measurement...

Annual Layers Revealed by GPR in the Subsurface of a Prograding Coastal Barrier, Southwest Washington, U.S.A.

ABSTRACT The southwest Washington coastline has experienced extremely high rates of progradation during the late Holocene. Subsurface stratigraphy, preserved because of progradation and interpreted using ground-penetrating radar (GPR), has previously been used successfully to document coastal response to prehistoric storm and earthquake events. New GPR data collected at Ocean Shores, Washington, suggest that the historic stratigraphy of the coastal barrier in this area represents a higher resolution record of coastal behavior than previously thought. GPR records for this location at 200 MHz reveal a series of gently sloping, seaward-dipping reflections with slopes similar to the modern beach and spacings on the order of 20-45 cm. Field evidence and model results suggest that thin (1-10 cm), possibly magnetite-rich, heavy-mineral lags or low-porosity layers left by winter storms and separated by thick (20-40 cm) summer progradational sequences are responsible for generating the GPR reflections. These results indicate that a record of annual progradation is preserved in the subsurface of the prograding barrier and can be quantified using GPR. Such records of annual coastal behavior, where available, will be invaluable in understanding past coastal response to climatic and tectonic forcing.

Evidence for storm-dominated early progradation of Castle Neck barrier, Massachusetts, USA

Washovers, dune scarps and flattened beach profiles with concentrations of coarse-grained sediment or heavy minerals are the diagnostic geological signatures of large storms on barriers today. It is clear that storms are a major force driving transgressive barriers onshore, but what is not as well understood is the role these powerful erosive events play in the evolution of prograding barriers. Application of ground-penetrating radar (GPR) and a combination of coring techniques have significantly improved our ability to image barrier architecture. Results of these studies reveal a more complex evolution than previously recognized. It is now possible to precisely locate and map storm deposits within prograded barrier lithosomes. A comprehensive study of northern Castle Neck, Massachusetts was performed using 15 km of GPR surveys, a 120-m-long seismic line, 11 cores, and several radiocarbon dates. Storm-related layers are the most prominent horizons contained in the barrier stratigraphy. The geometry and sedimentology of these layers closely resembles those of a present-day post-storm beach. Twenty closely spaced, curvilinear heavy mineral layers imaged in the landward portion of the barrier suggest that the Castle Neck barrier was heavily influenced by storms during its initial phase of progradation beginning ∼4000 years BP. Approximately 1800 years BP, two intense storms impacted the coast depositing two extensive coarse-grained units. These layers mimic the flat-lying sand and gravel deposits that occur in front of a nearby eroding till outcrop following major storms. The great number of storm deposits in the early history of Castle Neck is related to either a period of greater storm activity and/or a slow rate of barrier progradation. The occurrence and preservation of these earlier storm layers are likely a product of the exposure of nearby drumlins resulting in greater availability of iron oxide and ferromagnesian sands. The supply of heavy-mineral sands has gradually diminished as the barrier prograded and the proximal drumlin source was buried by beach and dune sands.

Evolution of progradation/erosion along the French Guiana mangrove coast: a comparison of mapped shorelines since the 18th century with Holocene data

The problems in mapping the mangrove shoreline of French Guiana are reviewed. The present variability of the coastline due to the shifting mudbanks with Avicennia mangroves on their shoreface-attached upper fringe is extended to the historical period two and a half centuries ago, by means of ancient maps and nautical charts. In selected sites (around the Sinnamary River and the Mana River estuaries, Cayenne Peninsula) the earliest reported shoreline (before 1765), compared with other 18th and 19th century maps, is relatively stable, demonstrating that erosion repeatedly counterbalanced progradation on a centennial time scale. This data contrasts with the 20th century initiation of a sedimentary regime that results in very high local progradation rates. An examination of the entire Holocene sediment record in preserved in French Guiana, including palynological evidence, suggests that most of the Holocene progradation may have occurred by different accretion processes than have formed the historical and recent mudbanks settled by Avicennia mangals.

Material exchange between the continental shelf and mangrove-fringed coasts with special reference to the Amazon–Guianas coast

Mangrove-fringed coasts are a major coastal environment in the tropics that exchange enormous quantities of inorganic and organic materials and dissolved ions with the adjacent ocean in unique ways. The 1,600 km long Guiana coast, from the Amazon River mouth to the Orinoco, the world's most extensive example, is exposed to significant oceanic wave and tidal energy but is almost exclusively composed of mud deposits. The huge alluvial discharge of the Amazon advected along the inner continental shelf is reworked into 20–25 shore-attached and mobile mud-banks 10–40 km in length. Frequent and deep resuspension created by the energetic conditions results in fluid mud suspensions (10–400 g 1 −1) that dewater upon re-deposition, resulting in extreme shoreline progradation rates (up to hundreds of metres per year) that are equally rapidly mobilized by wave attack. Biological assemblages (mangroves, foraminifera, diatoms, etc.) and biogeochemical conditions are strongly controlled by the intense recycling of sediments on the shoreface. Differences in sedimentary, ecological and biogeochemical processes are striking when this coast is compared with mangrove-fringed deltaic and estuarine coasts elsewhere, including those of west Africa and southeast Asia.

Salt tectonics driven by differential sediment loading: stability analysis and finite‐element experiments

AbstractAt many continental margins, differential sediment loading on an underlying salt layer drives salt deformation and has a significant impact on the structural evolution of the basin. We use 2‐D finite‐element modelling to investigate systems in which a linear viscous salt layer underlies a frictional‐plastic overburden of laterally varying thickness. In these systems, differential pressure induces the flow of viscous salt, and the overburden experiences updip deviatoric tension and downdip compression. A thin‐sheet analytical stability criterion for the system is derived and is used to predict conditions under which the sedimentary overburden will be unstable and fail, and to estimate the initial velocities of the system. The analytical predictions are in acceptable agreement with initial velocity patterns of the numerical models.In addition to initial stability analyses, the numerical model is used to investigate the subsequent finite deformation. As the systems evolve, overburden extension and salt diapirism occur in the landward section and contractional structures develop in the seaward section. The system evolution depends on the relative widths of the salt basin and the length scale of the overburden thickness variation. In narrow salt basins, overburden deformation is localised and characterised by high strain rates, which cause the system to reach a gravitational equilibrium and salt movement to cease earlier than for wide salt basins. Sedimentation enhances salt evacuation by maintaining a differential pressure in the salt. Continued sedimentary filling of landward extensional basins suppresses landward salt diapirism. Sediment progradation leads to seaward propagation of the landward extensional structures and depocentres. At slow sediment progradation rates, the viscous flow can be faster than the sediment progradation, leading to efficient salt evacuation and salt weld formation beneath the landward section. Fast sediment progradation suppresses the viscous flow, leaving salt pillows beneath the prograding wedge.

Stratal patterns and lithofacies of an intact seismic-scale Carboniferous carbonate platform (Asturias, northwestern Spain): a virtual outcrop model

Abstract Among the more challenging questions in geology are those concerning the anatomy of sedimentary bodies and related stratal surfaces. Though significant progress has been made on the interpretation of depositional environments, little systematic data are available on their dimensions and geometry. With the recent advances in computer power, software development and accuracy of affordable positioning equipment, it has now become possible to extract high-resolution quantitative data on the anatomy of sedimentary bodies. In Asturias, northwestern Spain, aerial photography provides continuous 2-D cross-sections of a seismic-scale, rotated to vertical, carbonate platform margin of the Early Carboniferous. Digital elevation models, orthorectified aerial photographic imagery and ground verification of stratal surfaces generated the elements that are required to reconstruct the true dimensions, angular relationships of bedding planes and the spatial distribution of facies units in this platform margin. Along with biostratigraphy this provides sufficient constraints to estimate rates of progradation, aggradation, growth and removal of sediments in the slope environment. Here we present a methodology to create outcrop models and integrate complementary types of data that provide new insights in sedimentology that were previously unattainable.

The Barremian-Aptian Evolution of The Eastern Arabian Carbonate Platform Margin (Northern Oman)

ABSTRACT Carbonate platform margins are sensitive recorders of changes in sea level and climate and can reveal the relative importance of global and regional controls on platform evolution. This paper focuses on the Barremian to Aptian interval (mid Cretaceous), which is known for climatic and environmental changes towards more intensified greenhouse conditions. The study area in the northern Oman mountains offers one of the very few locations where the Cretaceous carbonate margin of the Arabian Plate can be studied along continuous outcrops. Our detailed sedimentological and sequence stratigraphic model of the platform margin demonstrates how major environmental and ecological changes controlled the stratigraphic architecture. The Early Cretaceous platform margin shows high rates of progradation in Berriasian to Hauterivian times followed by lower rates and some aggradation in the Late Hauterivian to Barremian. High-energy bioclastic and oolitic sands were the dominant deposits at the margin. Turbidites were deposited at the slope and in the basin. The Early Aptian platform margin shows a marked change to purely aggradational geometries and a welldeveloped platform barrier that was formed mainly by microbial buildups. The sudden dominance in microbial activity led to cementation and stabilization of the margin and slope and, therefore, a decrease of downslope sediment transport by turbidites. In the Late Aptian, large parts of the Arabian craton were subaerially exposed and a fringing carbonate platform formed. Seven Barremian to Early Albian large-scale depositional sequences reflecting relative sea-level changes are identified on the basis of time lines constrained by physical correlation and biostratigraphy. The reconstruction of the margin geometries suggests that tectonic activity played an important role in the Early Aptian. This was most likely related to global plate reorganization that was accompanied by increased volcanic activity in many parts of the world. Along the northeastern Arabian platform the associated global changes in atmospheric and oceanic circulation are recorded with a change in platform-margin ecology from an ooid-bioclast dominated to a microbial dominated margin. Time-equivalent argillaceous deposits suggest an increase in rainfall and elevated input of nutrients onto the platform. This process contributed to the strongly diminished carbonate production by other organisms and favored microbial activity. The platform margin may thus represent a shallow-marine response to the Early Aptian global changes, commonly associated with an oceanic anoxic event in basinal environments.