Transgressive Facies Research Articles

Low‐amplitude, synsedimentary folding of a deltaic complex: Roda Sandstone (lower Eocene), South‐Pyrenean Foreland Basin

ABSTRACTThe South‐Pyrenean Foreland Basin is a sedimentary trough developed during Palaeogene times in response Pyrenean orogenesis. The structural development of the chain progressively involved the foreland basin deposits resulting in synsedimentary thrusting and growth of folds through the basin. The lower Eocene Roda Sandstone was deposited in a shallow‐marine environment whose topography (bathymetry) was modified by a series of growing gentle folds. This synsedimentary folding is evidenced in the field (i) thickening of sedimentary units above synclinal structures and thinning over anticlines; (ii) carbonate platform deposits growing on top of anticlines; (iii) the areal distribution of benthic foraminifera in transgressive facies assemblages determined by an irregular, fold‐influenced palaeobathymetry; (iv) variation of sandstone palaeocurrents related to the presence of a sedimentary trough formed by the synsedimentary growth of a syncline. In addition, synsedimentary folding has been evidenced from seismic data. In the Roda Sandstone example, the growth of gentle folds occurred in an area with high sedimentation rates (∼0.21 ± 0.06 mm y−1). Due to the high sedimentation rates, exceeding the folds uplift rate (∼0.10 ± 0.01 mm y−1), there are no noticeable unconformities in the growth strata at outcrop scale. However, the effects on the sedimentation are very significant, because the sediments were deposited close to sea level, and thus were very sensitive to fauna and facies distribution.

Facies and sequence stratigraphy of two Cambrian grand cycles: implications for Cambrian sea level and origin of grand cycles

ABSTRACT The Pika, Arctomys and Waterfowl formations comprise two cycles as originally defined by Aitken (1966, 1978, 1981). A detailed across-strike facies stratigraphic reconstruction based on measured outcrop sections and subsurface data reveal that the Pika and Waterfowl Formations consist of five transgressive-regressive sequences of subtidal facies and tidal flat facies each up to 100 m thick and some 105 to 106 years duration. The bulk of the Arctomys Formation consists of terrestrial to marginal marine playa deposits, correlates with an unconformity in the adjacent subsurface, and was deposited during a sea level low stand. The upper third of the Arctomys Formation records a single minor marine transgressive-regressive cycle. Transgressive facies can extend eastward onto the craton up to 1000 km from the platform to slope transition and record local relative increases in sea level of various magnitudes. Maximum transgressive or regressive phases do not necessarily correspond with the bottoms or tops of grand cycles. A sea level curve derived from our facies stratigraphy is different than the curve proposed by Bond et al. (1989) from R2 analysis, and also different to the sealevel curve of Montanez and Osleger (1993) for the southern Great Basin. End_Page 478------------------------

Lithofacies modelling and sequence stratigraphy in microtidal cool‐water carbonates: a case study from the Pleistocene of Sicily, Italy

ABSTRACT Quaternary carbonates in SE Sicily were deposited in seamount and short ramp settings during glacio‐eustatically driven highstand conditions. They provide an excellent opportunity to investigate the depositional and erosional aspects of cool‐water carbonate sedimentation in a microtidal marine water body. The derived ramp facies model differs significantly from modern‐day, open‐ocean ramp scenarios in projected facies depth ranges and in the preservation of inshore facies. A sequence stratigraphic study of the carbonates has confirmed many established aspects of carbonate sedimentation (e.g. production usually only occurred during highstands). It has also revealed several new features peculiar to water bodies with little tidal influence, including ‘catch‐up’ surfaces taking the place of transgressive facies, second‐order sequence boundary events being most important as triggers for initiating resedimentation and a virtual absence of sediment shedding to the basin during the terminal lowstand. Production in the carbonate factory lasted for about 0·5 Myr. Despite this, carbonate production was considerable and included both bioconstructional and bioclastic‐dominated facies and the production of abundant lime muds. A model for eustatically controlled cool‐water carbonate production and resedimentation in microtidal marine water bodies is presented. This is considered to be more applicable to Neogene and Quaternary strata in the Mediterranean region than are current open‐ocean models.

CONODONT SEQUENCE BIOSTRATIGRAPHY OF THE HERMOSA GROUP (PENNSYLVANIAN) AT HONAKER TRAIL, PARADOX BASIN, UTAH

In cyclical Pennsylvanian strata, conodonts find their greatest stratigraphic utility as biotic signatures of physically defined stratigraphic entities (cycles, parasequences, high frequency sequences, etc.) rather then the primary means of stratigraphic subdivision (e.g., biostratigraphic interval zones). The practice of identifying depositional entities for purposes of regional correlation on the basis of their constituent conodont faunas is herein called conodont sequence biostratigraphy. In this paper, the concept is utilized to successfully correlate Pennsylvanian cycles of the Paradox basin with their Midcontinent counterparts.The Honaker Trail section is the most accessible and well-studied succession of carbonate shelf strata in the Paradox basin. Approximately 350 m of cyclically bedded limestone, sandstone, and shale comprising 53 fifth-order cycles are exposed along the cliffs of the deeply entrenched San Juan River. Maximum transgressive facies of 19 cycles yielded Idiognathodus-Neognathodus- and/or Streptognathodus-dominated conodont faunas. Those from the Chimney Rock, Gothic, LHT-5, UHT-3, UHT-5, UHT-8, unnamed limestone, and Shafer correspond to faunas from the Verdigris, Lower Fort Scott, Altamont (Lake Neosho), Lost Branch, Hertha (Mound City), Swope (Hushpuckney), Dennis (Stark), and South Bend cycles of the Midcontinent, respectively. By extrapolation, all minor cycles of the Marmaton, Pleasanton, and Bronson Groups (except for the Critzer) also appear to have counterparts at Honaker Trail.The position of the Desmoinesian–Missourian boundary in the Honaker Trail section can be approximated using conodonts in conjunction with fusulinids. The highest Desmoinesian conodont fauna, the Idiognathodus nodocarinatus fauna, occurs in cycle UHT-3 in the lower part of the Upper Honaker Trail sequence. The highest occurrence of Beedeina occurs in the same cycle. The lowest conodont fauna with the Missourian species I. eccentricus appears two cycles higher, in cycle UHT-5. Because in the Midcontinent region a small interval of strata separates the first appearance of I. eccentricus from the base of the Missourian, we place the base of the Missourian at the base of cycle UHT-4 (bed 105) at Honaker Trail. The appearance of Streptognathodus firmus and S. pawhuskaensis in the Shafer limestone indicates that the Missourian–Virgilian boundary lies slightly above or below this stratigraphic horizon.

Conodont sequence biostratigraphy of the Hermosa Group (Pennsylvanian) at Honaker Trail, Paradox Basin, Utah

In cyclical Pennsylvanian strata, conodonts find their greatest stratigraphic utility as biotic signatures of physically defined stratigraphic entities (cycles, parasequences, high frequency sequences, etc.) rather then the primary means of stratigraphic subdivision (e.g., biostratigraphic interval zones). The practice of identifying depositional entities for purposes of regional correlation on the basis of their constituent conodont faunas is herein called conodont sequence biostratigraphy. In this paper, the concept is utilized to successfully correlate Pennsylvanian cycles of the Paradox basin with their Midcontinent counterparts.The Honaker Trail section is the most accessible and well-studied succession of carbonate shelf strata in the Paradox basin. Approximately 350 m of cyclically bedded limestone, sandstone, and shale comprising 53 fifth-order cycles are exposed along the cliffs of the deeply entrenched San Juan River. Maximum transgressive facies of 19 cycles yielded Idiognathodus-Neognathodus- and/or Streptognathodus-dominated conodont faunas. Those from the Chimney Rock, Gothic, LHT-5, UHT-3, UHT-5, UHT-8, unnamed limestone, and Shafer correspond to faunas from the Verdigris, Lower Fort Scott, Altamont (Lake Neosho), Lost Branch, Hertha (Mound City), Swope (Hushpuckney), Dennis (Stark), and South Bend cycles of the Midcontinent, respectively. By extrapolation, all minor cycles of the Marmaton, Pleasanton, and Bronson Groups (except for the Critzer) also appear to have counterparts at Honaker Trail.The position of the Desmoinesian–Missourian boundary in the Honaker Trail section can be approximated using conodonts in conjunction with fusulinids. The highest Desmoinesian conodont fauna, the Idiognathodus nodocarinatus fauna, occurs in cycle UHT-3 in the lower part of the Upper Honaker Trail sequence. The highest occurrence of Beedeina occurs in the same cycle. The lowest conodont fauna with the Missourian species I. eccentricus appears two cycles higher, in cycle UHT-5. Because in the Midcontinent region a small interval of strata separates the first appearance of I. eccentricus from the base of the Missourian, we place the base of the Missourian at the base of cycle UHT-4 (bed 105) at Honaker Trail. The appearance of Streptognathodus firmus and S. pawhuskaensis in the Shafer limestone indicates that the Missourian–Virgilian boundary lies slightly above or below this stratigraphic horizon.

Seismic sedimentology and regional depositional systems in Mioceno Norte, Lake Maracaibo, Venezuela

Seismic sedimentology is the use of seismic data to study sedimentary rocks and the processes by which they form. It differs from classic seismic stratigraphy in that it uses mainly the horizontal, instead of the vertical, character of seismic data to provide high-resolution images of seismic attribute patterns that can be related to geomorphology and depositional models. A display of seismic attributes on geologic time surfaces is the basic tool of seismic sedimentology. However, there are strict limitations and conditions under which reservoir geometries can be optimally delineated on time and/or horizon slices. Specifically, the slice must be picked at or parallel to a geologic time-equivalent seismic event. This condition is seldom satisfied in practice because of structural dipping and lateral thickness variations of depositional sequences. Stratal slicing of 3-D seismic data overcomes some shortcomings of time slicing and horizon slicing by proportionally slicing between geologic time-equivalent seismic reference events. This facilitates the seismic mapping of sedimentary features on depositional surfaces. Miocene deposits in Mioceno Norte, Lake Maracaibo, Venezuela, provide an excellent example of this seismic sedimentological approach. A high-resolution stratigraphic framework, based on wireline logs, defines approximately 30 genetic stratigraphic units (GSU) separated by marine flooding surfaces, lacustrine flooding surfaces, paleosols, and floodplain shales (Figure 1). These GSUs, each 50–150 ft thick, were correlated by tracing low-resistivity shale markers on logs. The origin, scale, and hierarchy of GSUs in Mioceno Norte are similar to those of depositional episodes and genetic depositional sequences defined by Galloway (1989). A GSU, typically bounded by flooding surfaces or paleosols, represents a cycle of coastal progradation commonly capped by transgressive facies and a condensed section. Detailed isopach and facies mapping based on conventional core and SP/GR log-facies patterns showed that the GSU origins are shoreface, deltaic, and fluvial, respectively. Figure 1. Stratigraphic framework in Mioceno Norte …

Sequence stratigraphy of the Vaal Reef facies associations in the Witwatersrand foredeep, South Africa

Abstract Facies analysis and sequence stratigraphic methods resolve the correlation and relative chronology of Vaal Reef facies associations at the Great Noligwa and Kopanang mines in the Klerksdorp Goldfield, Witwatersrand Basin. Four depositional sequences, separated by major erosional surfaces, build the Vaal Reef package. Lowstand, transgressive and highstand systems tracts (HST) are identified based on stratal stacking patterns and sedimentological features. The core of the Vaal Reef deposit is represented by a Transgressive Systems Tract (TST), which is built by shoreface (proximal) facies at Great Noligwa and inner shelf (distal) facies at Kopanang. The proximal TST includes the G.V. Bosch reef and the overlying MB4 quartzites. These facies are correlative to the Stilfontein conglomerates and quartzites, respectively, at Kopanang. The overlying Upper Vaal Reef is part of a Lowstand Systems Tract (LST). It is only preserved at Kopanang, with a thickness that equals the difference in thickness between the proximal and distal transgressive facies. This suggests that the Upper Vaal is an erosional remnant that accumulated on a gently dipping topographic profile. Traditionally, the MB4 quartzite has been considered the hanging wall relative to the Vaal Reef. We infer that these quartzites are merely associated with the G.V. Bosch conglomerate, and interpret that they are coeval with the Stilfontein quartzites, and older relative to the Upper Vaal facies. From a genetic viewpoint, the true hanging wall to the Vaal Reef begins with the Zandpan gravel lag that extends across the entire study area. Paleocurrent analysis of shoreface and fluvial systems indicate a curved shoreline and a northern elevated source area. The coastline underwent north–south transgressive and regressive shifts during Vaal Reef time.

Contrasting facies patterns in subtropical and temperate continental slope sediments: inferences from east Australian late Quaternary records

Abstract Studies of latest Quaternary continental slope sediments at two localities on the east Australian margin have revealed markedly different responses to late Quaternary sea level fluctuations. Offshore of Noosa, in the sub-tropics, the sediment is predominantly a mixture of fine metastable carbonate, siliciclastic material, and pelagic carbonate. Important features of the stratigraphy include a siliciclastic-dominated facies deposited relatively slowly during the last glacial lowstand (sedimentation rate ≤8 cm/ka), and a calcareous facies, rich in metastable carbonate, deposited more rapidly during the late post-glacial transgression (sedimentation rates 15–24 cm/ka). Highstand and transgressive sedimentation rates are greater than lowstand rates by a factor of 2.5–6 due to increased shelf carbonate productivity after flooding of the mid-shelf. Off Sydney, in temperate latitudes, continental slope sediment is largely a mixture of fine siliciclastic material and pelagic carbonate. Mean sedimentation rates range from 2 to 5 cm/ka over the last four oxygen isotope stages, with mean glacial/interstadial rates higher than Holocene rates by a factor of ∼1.36. This largely reflects the transfer of siliciclastic mud from the shelf to the slope during sea level regression. In both localities, facies changes on the slope are not related to specific sea level states (e.g. lowstand facies, transgressive facies, etc.), but reflect instead the interaction of changing sea level with shelf morphology.

Continental island from the Upper Silurian (Ludfordian Stage) of Inner Mongolia: Implications for eustasy and paleogeography

An unconformity between the Silurian Xibiehe Formation and Ordovician igneous rocks marks the perimeter of a small paleoisland near Bater Obo in north-central Inner Mongolia, 180 km northwest of the provincial capital of Hohhot. The stratigraphic position of the lower part of the Xibiehe Formation is correlated by means of conodonts with the upper part of the Ancoradella ploeckensis Zone in the basal Ludfordian Stage (corresponds to mid-Ludlovian Epoch, ca. 421 Ma). Elongate in plan (610 m × 200 m), the exhumed diorite core rises 30 m above the lowest elevations of surrounding Silurian strata. Paleoshores along the principal axis of the inlier delineate contrasting facies. Robust stromatoporoids are in growth position within silty limestones, some directly encrusting the unconformity surface of the sheltered southeast margin. A basal conglomerate of diorite cobbles and boulders characterizes the high-energy northwest margin. The depositional constraints and timing of transgressive facies associated with this continental paleoisland have implications for the eustatic and paleogeographic history of the parent Sino-Korean plate. Burial of the island corresponds to the beginning of a global rise in sea level that peaked in late Ludlovian time. Our interpretation of windward and leeward facies requires an approximate 90° clockwise rotation of the parent plate to accommodate the dominant pattern of low-latitude trade winds and storms.

Caledonian Cyclic Unit in the Vendian–Lower Paleozoic Succession of the Ishim–Karatau Lithostructural Zone (Kazakhstan)

The lithofacial study of Vendian–Lower Paleozoic sedimentary sequences of the Greater Karatau and Baikonur synclinorium, which are the constituents of the Ishim–Karatau lithostructural zone, revealed that Vendian, Cambrian, and Ordovician rocks form a continuous succession composing the Caledonian cyclic unit. In duration (220–240 Ma), the latter corresponds to a megacycle. The cyclic unit consists of the transgressive and regressive parts. The upper part of the transgressive succession includes facies of the continental slope, slope foot, near-continental area, and transitional zone between the near-continental and pelagic areas of the abyssal plain. It is shown that an avalanche sedimentation of the second global level occurred twice during the transgressive and regressive phases. The formation of transgressive facies was related to the extension regime during the basin opening, whereas regressive facies accumulated under the compression regime. The Ishim–Karatau zone is located in the western, outer part of Caledoninian structures in Kazakhstan. The Caledonian cyclic unit formed on the Vendian–Early Paleozoic Atlantic-type margin of the Kazakhstan microcontinent. Specific features of the cyclic development of mobile areas are outlined.

Bypass Margins, Basin-Restricted Wedges, and Platform-to-Basin Correlation, Upper Devonian, Canadian Rocky Mountains: Implications for Sequence Stratigraphy of Carbonate Platform Systems

ABSTRACT Carbonate platforms can commonly keep up with relative sea-level rise because of high rates of sediment accumulation and platform aggradation. Surrounding basinal environments are commonly starved but can receive variable extrabasinal siliciclastic input and episodically deposited carbonate sediment. If accumulation rates in basinal settings lag behind those of the platform, a bypass or erosional margin can develop. Under these circumstances platform and basin depositional sequences become physically detached and direct correlation of basinal and platform sequences is hindered. We report here the results of high-resolution stratigraphic analyses of two Upper Devonian isolated carbonate platforms in western Alberta that provide insight into the sequence stratigraphy of bypass margins and criteria for accurate correlation of platform and basinal sequences. The slope and basin sequences surrounding the Miette and Ancient Wall platforms consist of basin-restricted, onlapping wedges of fine-grained background sediment deposited dominantly from suspension and coarse-grained platform-derived sediment redeposited by a variety of gravity-flow mechanisms. Sequence boundaries are identified within the redeposited carbonate intervals. Identification of sequence boundaries and differentiation of highstand and lowstand slope and basinal facies was based on the geometry, mineralogy, and clast content of redeposited carbonate units. Highstand carbonates contain sheet-like debris flows and turbidites with abundant slope-derived clasts and background facies with high total carbonate content. Lowstand carbonates contain sheet-like and channelized debris flows and turbidites with abundant platform-derived clasts and background facies with low carbonate content and locally high amounts of organic carbon. Transgressive facies are dominated by initially carbonate-poor and organic-rich background sediments that display a progressive increase in carbonate content and decrease in organic carbon content. These patterns are interpreted to record abundant background carbonate sedimentation during late transgression and highstand when the carbonate factory was robust. Highstand redeposited carbonates record slope erosion due to oversteepening and slope readjustment processes. Lowstand redeposited carbonates indicate platform and platform-margin erosion and low background carbonate sedimentation when the platform was either exposed or under very shallow peritidal conditions. High siliciclastic and organic contents during lowstand and early transgression may partly be the result of reciprocal sedimentation but alternatively may represent continuous siliciclastic supply during times with little dilution by fine-grained carbonate sediment. Successive stages of platform development at Miette and Ancient Wall were controlled by accommodation changes driven by relative sea-level fluctuations. Backstripping analyses of strata from both platforms confirm that significant differential subsidence was a major control on variations in platform thickness and patterns of slope development. Greater subsidence at Ancient Wall fostered the development of a steeper bypass margin and different slope evolution compared to Miette. Slope oversteepening also initiated a process of slope readjustment that eventually reduced the platform-to-basin gradient and facilitated regressive platform progradation. In conventional siliciclastic sequence stratigraphy, basin-restricted wedges are interpreted as lowstand deposits on the basis of their geometry and position relative to an updip margin. Wedge-shaped basinal units along the Miette and Ancient Wall bypass margins contain both highstand and lowstand facies that straddle sequence boundaries. The results of this study provide objective criteria for differentiating systems tracts in carbonate slope and basin environments through mineralogic and compositional analyses providing more accurate correlation of detached platform and basin sequences. Interpretation of carbonate basin-restricted wedges as purely highstand or lowstand deposits may lead to erroneous conclusions regarding sequence stratigraphy, platform-to-basin correlation, and the volumetric partitioning of sediments deposited in different systems tracts.

THE VENEZUELAN HYDROCARBON HABITAT, PART 1: TECTONICS, STRUCTURE, PALAEOGEOGRAPHY AND SOURCE ROCKS

Venezuela forms part of an important hydrocarbon province, defined by the presence of prolific Cretaceous source rocks, which extends across northern South America. By early 1997, the country had produced 53 billion barrels of oil. Reserves are estimated to total 73 billion barrels of oil and 146 TCF of gas with 250 billion barrels recoverable in the Heavy Oil Belt. Most reserves are located within the intermontane Maracaibo and foreland Barinas‐Apure and Eastern Venezuela Basinx They correspond to more than 1.5 trillion BOE originally in place.The province's hydrocarbon history began with a broad passive margin over which the sea transgressed throughout much of the Cretaceous. Limestones and shales followed basal sands and included rich source rocks. Convergence between the distal part of the area and the Caribbean Plate created an active margin that migrated southwards, so that flysch and wildflysch followed the transgressive facies. The process culminated in Lute Cretaceous to Middle Eocene orogeny with the emplacement of southward‐vergent nappes and the development of northward‐deepening foredeeps. Flysch and wildflysch formed in the north while important deltaic—paralic reservoir sands accumulated in the south. Major phases of hydrocarbon generation from Jurassic‐Cretaceous source rocks occurred across the entire margin of northern South America during the orogeny. They are recorded by Jurassic ‐ Middle Cretaceous graphitic marbles, schists and quartzites (metamorphosed, organic limestones and shales and oil‐bearing sandstones) in the Coastal and Northern Ranges of Venezuela and Trinidad. They probably charged giant fault and stratigraphic traps analogous to today's Oficina‐Temblador and Heavy Oil Belt accumulations.From Late Eocene to Recent times, transpressive interaction between northern South America and neighbouring parts of the Caribbean and the Pacific inverted Mesozoic extensional systems below the remaining passive margin. The area became subdivided into a series of intermontane, foreland and pull‐apart basins bounded by transpressional uplifts, the latter sufering considerable shortening and strike‐slip displacement. Sedimentation progressed from deep marine to deltaic and molassic facies, providing reservoir sands and local source rocks.Inverted faults and foreland flexuring and interplay between structuration and sedimentation produced abundant structural and stratigraphic traps. Hydrocarbons from earlier accumulations suffered further maturation in place, remigrated to younger traps or escaped to the surface. Further hydrocarbon generation, involving Upper Cretaceous source rocks, occurred in local foredeep kitchens. Minor contributions also came from Tertiary source rocks.

Late Quaternary evolution of the upper reaches of the Solent River, Southern England, based upon marine geophysical evidence

Geological evidence suggests that during the Late Quaternary, a river system (the Solent River) drained a large part of central Southern England. Its upper reaches flowed in a west-east direction, flanked to the south by a Chalk ridge (the Purbeck-Isle of Wight Chalk Ridge). Today, only part of the upper reaches of the river's tributary channels remain, as the area was inundated during the Flandrian Transgression, forming an embayment system (Poole and Christchurch Bays). In order to map the offshore buried channels of the upper reaches of the Solent River an extensive set of shallow-marine geophysical data was analysed and interpreted. The results of this investigation show that the Solent River system was disrupted irreversibly by southerly capture of its upstream section before the Flandrian Transgression. This disruption was the result of the fluvial breaching of the southern barrier of the system (the Purbeck-Isle of Wight Ridge) at three points, probably during Late Devensian time. Poole Bay was first to be submerged during the transgression. The estuaries which resulted from the drowning of the fluvial palaeovalleys of Poole Bay were infilled with transgressive facies sequences which have been preserved within the buried palaeovalleys. In contrast, Christchurch Bay was submerged at a later time, but because of the abrupt manner of its inundation, no transgressive facies have been preserved within its buried palaeovalleys.

Sequence stratigraphy of the upper jurassic evaporite-carbonate: Sequence at the western area of Wadi Al-Jawf-Marib basin, Yemen

The Upper Jurassic evaporite-carbonate sequence (Sabatayn Formation and its equivalent Madbi Formation or Transition Beds) and its constituent systems tracts have been recognized at the western side of Wadi Al-Jawf-Marib intracratonic rift basin and its adjacent shelf areas. The Sabatayn Formation was subdivided, from the base to the top, into the Shabwa Salt Member, the Layadim Shale Member, the Ayad Gypsum Member and its equivalent M’qah Sandstone Member. The evaporite depositional systems of the lowstand facies (or systems) tract LST began in Middle Kimmeridgian times after the complete drawdown of the shelf area and basin isolation. The effect of ephemeral flooding events of the marine water and/or surface run-off water on the basinal salt pans resulted in the deposition of four or more parasequences (shallowing upward salinas of the Shabwa Salt Member). When the rift was reconnected to the open ocean in Late Kimmeridgian times, the basinal LST and the formerly exposed carbonate sequence of the shelf area were flooded and an onlapping retrogradational transgressive facies tract (TST) represented by the lagoonal-sabkha Layadim Shale and the Ayad Gypsum members and their equivalent shoals, shallow subtidal to intertidal Madbi Formation were established. The continued relative sea-level rise followed by relative sea-level falls due to orogenic movement during Portlandian times resulted in the deposition of aggradational to progradational offlapping strata of early and late highstand facies tract (HST representing part of the Naifa Formation). On the shelf area, the early HST consists of a subtidal ammonitic limestone which changes westwards into a shallow subtidal-intertidal sandy rippled mudstone containing fossil fish remains and varicolored cross bedded sandy mudstone containing plant remains, while the late HST consists of supratidal to sabkha gypsiferous shale and sandstone with gypsum lenses. However, on the shelf slope, a brecciated ammonitic limestone interbedded with calcareous mudstone containing pelagic fauna represent the forced regressive wedge-systems tracts (FRWST).

Continental, brackish and marine carbonates from the Lower Cretaceous of Kolone–Barbariga (Istria, Croatia): stratigraphy, sedimentology and geochemistry

During the Early Cretaceous, wide areas of the Dinaric–Adriatic Carbonate Platform emerged for long periods. The Hauterivian–Barremian carbonates from Kolone–Barbariga show a few typical examples of lacustrine facies with dinosaur bones and brackish/palustrine facies. The sequence of the platform is made for the most part by subtidal and intertidal limestones. The bone levels are located in a large depression few meters deep in the uppermost Hauterivian marine limestones. The filling facies of this depression are made by oncolitic rudstones and algal boundstones, which represent marginal lacustrine facies, and by laminated limestones, thin stromatolitic levels and distal fringes of rudstones which represent relatively open lacustrine facies. The fossil content is characterized by rare charophyte stems, ostracods, gastropods and plant remains, while typical marine fauna is absent. At the Hauterivian–Barremian boundary a major emersion event has been observed, then a slow transgressive phase occurred. The transgressive facies are primarily made by mudstones with ostracods, charophytes and Spirillina (brackish and probably freshwater facies), wackestones with Ophtalmidiidae and rare dasyclad algae, storm layers with gastropods and miliolids and breccia-like dinoturbated beds. Wackstones, packstones and very rich in dasyclad grainstones outcrop at the top of the section, representing the maximum of the transgression. Trace elements content, carbon and oxygen stable isotope analyses have been performed to aid the palaeoenvironmental interpretation. In this geological setting, Barium seems to discriminate between brackish and freshwater facies. The isotopic values of the marine carbonates appear to depend on early diagenetic processes, meanwhile lacustrine facies seem to show a weak signal of the depositional environment.

Late highstand patterns of shifting and stepping coastal barriers and washover-fans (late Messinian, Sorbas Basin, SE Spain)

The late Messinian Sorbas Member, up to 75 m thick, consists in its type area of a parasequence set of three prograding coastal barriers (sequences I–III), associated with lagoon and washover sediments. Around the town of Sorbas these strata can be studied exceptionally well due to absence of burrowing by raised Messinian salinities and exposure along a network of up to 30 m deep canyons. Fifteen vertical sections were logged and carefully correlated. This permits to reconstruct and discuss patterns of relative sea-level movements between decimetres, up to 15 m within a parasequence. Excellent examples of non-tidal transgressive facies are characterized by lagoon and washover sediments instead of the usual combination of washover and tidal deposits (channel and flood-tidal delta). Implications for the sandstone connectivity are given. The lower two sequences are deposited in a relatively large, tectonically enhanced wedge-shaped accommodation space. They show both fining-up, deepening sequences, followed by prograding coarsening-up shoaling sequences and can be compared to the classical parasequences of the Western Interior Basin (USA). Progradation of sequence II was interrupted by a major slide event (most likely triggered by an earthquake), which caused more than 400 m seaward slumping of a stretch of 1 km of coastal sands. The architecture of sequence III is more complex due to limited accommodation space characteristic for the late highstand, so that this setting was very sensitive to sea-level fluctuations. This resulted in an intricate pattern of juxtaposed and superposed lagoonal muds, washover fans and swash zones. So-called ‘stranded’ coastal barriers occur, which were left behind after seaward jump of the coastline over more than 1.5 km during forced regression. The pattern of reconstructed sea-level positions is well comparable to the sequential pattern shown by the correlated equivalents along the northern basin margin, which belong to the so-called Terminal Carbonate Complex. The problem of ranking the complex sequence III as one or more parasequences and its consequences for cyclostratigraphy are shortly discussed. Two models of washover formation are given, respectively during more rapid and more slow sea-level rise. The influence of synsedimentary folding on the location of barriers is discussed and also the source area of extra- and intraclasts supplied to barriers and washovers. After deposition of the Sorbas Member the sea withdrew from the Sorbas Basin, probably as a result of the major downdrop in the Mediterranean at the maximum isolation during the Salinity Crisis. It is suggested that the semi-enclosed setting of the basin resulted only in limited, localized erosion, in contrast to the deeper adjacent Vera Basin, which was more open to the Mediterranean.

Platform‐to‐basin correlation by high‐resolution sequence stratigraphy and cyclostratigraphy (Berriasian, Switzerland and France)

ABSTRACTBased on detailed analyses of facies evolution and stacking pattern of Berriasian carbonate‐dominated sections in the Swiss Jura Mountains, the Swiss Ultrahelvetic and the French Vocontian Trough, a high‐resolution platform‐to‐basin correlation is proposed. Biostratigraphical tie points are furnished by ammonites, dinoflagellates, calpionellids, and ostracod‐charophyte assemblages. The hierarchical stacking of small‐scale depositional sequences reflects Milankovitch cyclicity: sequences corresponding to the 20‐, 100‐, and, locally, 400‐ka orbital cycles can be identified. Elementary (20 ka) sequences on the platform generally consist of one bed of shallow subtidal to intertidal, high‐ or low‐energy carbonate facies, whereas on the slope and in the basin they are commonly developed as limestone‐marl couplets. These elementary sequences group into small‐scale composite sequences reflecting the first orbital eccentricity cycle (100 ka), which in turn build up large‐scale (3rd‐order) composite sequences. One 3rd‐order sequence has been analysed in detail: according to the cyclostratigraphic interpretation, it took ≃2 Myr to form, which is in accordance with the duration of the corresponding ammonite subzones. Sequence‐stratigraphic and cyclostratigraphic platform‐to‐basin correlation shows that for about 900 ka the platform was exposed or only partly flooded, whereas on the slope and in the basin, lowstand deposits with channel fills and slumps accumulated. With rising sea level, accommodation space gradually increased on the platform and a thickening‐upward sequential pattern with transgressive facies developed, while in the basin the facies still had lowstand characteristics with thick and nodular limestone beds. This situation lasted about 700 ka. The following 300 ka were characterized by sediment starvation and increased bioturbation on the platform, and by more marly, transgressive sediments on the slope and in the basin. The maximum‐flooding phase is more or less isochronous on the platform and in the basin, although the surface with the best‐developed maximum‐flooding features may be displaced by one or two small‐scale composite sequences because of superimposed high‐frequency sea‐level fluctuations, and/or local variations in substrate morphology and sediment distribution. Third‐order highstand conditions prevailed for only about 100 ka. The combination of cyclostratigraphy and high‐resolution sequence stratigraphy, constrained by good biostratigraphy, is thus a powerful tool for detailed stratigraphical correlation over long distances and from one sedimentary environment to another.

Fluvial Response in a Sequence Stratigraphic Framework: Example from the Montserrat Fan Delta, Spain

ABSTRACT Exceptional exposure of the Montserrat fan-delta system (Eocene) in northeastern Spain provides an excellent framework to evaluate the alluvial response to sea-level changes over two different time scales. The alluvial system contains multiple fifth-order cycles ( 104 yr) and eight fourth-order cycles ( 105 yr). Fifth-order cycles are characterized by long-distance shoreline migrations and, occasionally, by incised basal scour surfaces but not by changes in fluvial style, lithofacies, or channel stacking pattern. Fourth-order cycles are composed of stacked fifth-order cycles and have non-erosional basal boundaries. Vertical sedimentation rates and channel-stacking patterns change significantly within fourth-order cycles. The lower parts of these cycles, referred to as the transgressive facies tract, show overall shoreline transgression, and associated alluvial deposits contain abundant overbank materials with isolated (ribbon) channel bodies. During this time the supply of terrigenous material to the shoreline was reduced, as indicated by sediment starvation offshore. The overlying middle part of fourth-order cycles, the lower regressive facies tract, differs only in that shoreline is overall regressive, and there is increased clastic supply to the offshore. In the upper part of hese cycles, the upper regressive facies tract, channel-stacking geometries become denser and more sheet-like, shoreline regression is more pronounced, and vertical aggradation rates are inferred to be reduced. Changes in the alluvial system during fourth-order cycles are most pronounced adjacent to shoreline and die away upstream over just a few kilometers--indicating that the base-level signal decays away over the distance of a few backwater lengths (channel flow depth/slope). Higher-frequency (fifth-order) changes in relative sea level appear to produce the largest shoreline migrations, but lower-frequency (fourth-order) changes have more impact on the channel stacking architecture of the alluvial systems. Observed changes in alluvial stacking pattern may be most commonly found in tectonically active, rapidly subsiding, foreland basins because of their back-tilted geometry. We propose a model in which sediment is trapped in the proximal basin during times of rapid tectonic subsidence and attendant relative sea-level rise. Progradation occurs as erosion rates in the mountain belt increase, and rates of subsidence and relative sea-level rise diminish. Changes in alluvial architecture reflect an increase in sediment flux towards the shoreline as less sediment is trapped upstream. Hence, changes in channel-stacking pattern coincident with transgressions and regressions likely reflect the interplay between subsidence and sediment supply in the proximal part of the basin and are not necessari y driven by eustatic sea-level changes.

Development of Stratigraphy and Sediment Distribution in the Northeastern Yellow Sea During Holocene Sea-level Rise

ABSTRACT A succession of Holocene deposits in the northeastern Yellow Sea were interpreted in a sequence-stratigraphic context, and sedimentology of each deposit was characterized, based on analyses of high-resolution seismic profiles and sediment cores taken along a transect. Thin ( 3 m) transgressive sediments that accumulated through shoreface erosion during the rapid sea-level rise in the early Holocene overlie a subaerially eroded, low-gradient, late Pleistocene platform on the present-day inner shelf region. Cores reveal marine sands over tidal/estuarine sandy muds with an intervening, rather indistinct ravinement surface. In the deepest shelf area, these early Holocene tidal/estuarine deposits remain exposed at the seabottom. In the mid o late Holocene, two contrasting deposits of the highstand type (deltaic clayey mud and tidal sand ridges) began to accumulate in response to slower rates of sea-level rise. A series of tidal sand ridges in the nearshore area migrated offshore over the transgressive sand sheet, and a deltaic watery mud prograded discordantly over consolidated tidal/estuarine deposits in the central shelf. The transgressive facies relationships present a model for macrotidal, epicontinental-sea settings subject to low wave energy and reduced sediment supply.

Eastern Mississippi delta: late Wisconsin unconformity, overlying transgressive facies, sea level and subsidence

Analysis of 92 engineering core logs located in the Balize sector of the eastern Mississippi delta focuses on the late Wisconsin unconformity and lithofacies of strata lying immediately below and above this stratigraphic horizon. This major sequence boundary is a key feature used to correlate strata across shelf and slope to the basin. Observations emphasize lithofacies distributions of the latest Pleistocene sediments underlying the unconformity, the late Pleistocene to early Holocene transgressive facies, and the immediately overlying deltaic deposits. Maps and a cross-section compiled with this information highlight the critical relation between lithofacies distributions and late Wisconsin sea-level oscillations. Core analysis reveals that the transgressive facies comprises distinct environments of deposition, offshore to onshore. Findings identify criteria to determine the approximate position of the shoreline at the late Wisconsin maximum sea-level lowstand and at the extent of early Holocene maximum marine inundation in the Balize complex. Mapping of lithologies along the late Wisconsin unconformity serves to improve correlation of sediment facies with changes in acoustic response along high-resolution seismic profiles. An estimate of long-term averaged land subsidence suggests lowering of at least 1 mm/year near the shelfedge during the past ∼ 18 000 years. This mapping also refines sea-level response models for the Mississippi delta, including criteria to locate key paleogeographic features such as shelfedges and maximum flooding surfaces in other Recent marine deltas and in older, stacked deltaic sequences.