The Middle–Late Jurassic forced regression and disconformity in central Portugal: eustatic, tectonic and climatic effects on a carbonate ramp system

Abstract Successions across the Middle–Upper Jurassic disconformity in the Lusitanian Basin (west‐central Portugal) are highly varied, and were probably developed on a large westward‐inclined hangingwall of a half‐graben. The disconformity is preceded by a complex forced regression showing marked variations down the ramp, and provides an example of the effects of rapid, relative sea‐level falls on carbonate ramp systems. In the east, Middle Jurassic inner ramp carbonates (‘Candeeiros’ facies) are capped by a palaeokarstic surface veneered by ferruginous clays or thick calcretes. In the west, mid‐outer ramp marls and limestones (‘Brenha’ facies) are terminated by two contrasting successions: (1) a sharp‐based carbonate sandbody capped by a minor erosion surface, overlain by interbedded marine–lagoonal–deltaic deposits with further minor erosion/exposure surfaces; (2) a brachiopod‐rich limestone with a minor irregular surface, overlain by marls, lignitic marls with marine and reworked non‐marine fossils and charophytic limestones, with further minor irregular surfaces and capped by a higher relief ferruginous erosional surface. The age ranges from Late Bathonian in the east to Late Callovian in the west. This disconformity assemblage is succeeded by widespread lacustrine–lagoonal limestones with microbial laminites and evaporites (‘Cabaços’ facies), attributed to the Middle Oxfordian. Over the whole basin, increasingly marine facies were deposited afterwards. In Middle Jurassic inner‐ramp zones in the east, the overall regression is marked by a major exposure surface overlain by continental sediments. In Middle Jurassic outer‐ramp zones to the west, the regression is represented initially by open‐marine successions followed by either a sharp marine erosion surface overlain by a complex sandbody or minor discontinuities and marginal‐marine deposits, in both cases capped by the major lowstand surface. Reflooding led to a complex pattern of depositional conditions throughout the basin, from freshwater and brackish lagoonal to marginal‐ and shallow‐marine settings. Additional complications were produced by possible tilting of the hangingwall of the half‐graben, the input of siliciclastics from westerly sources and climate change from humid to more seasonally semi‐arid conditions. The Middle–Late Jurassic sea‐level fall in the Lusitanian Basin is also recorded elsewhere within the Iberian and other peri‐Atlantic regions and matches a transgressive to regressive change in eustatic sea‐level curves, indicating that it is related in part to a global event.     

Coeval versus reciprocal mixed carbonate–siliciclastic deposition, Middle Devonian Baltic Basin, Eastern Europe: implications from the regional tectonic development

The Middle Devonian Narva succession in the Baltic Basin represents a significant turnaround in the history of the basin. The detailed study of core and outcrop sections and the three‐dimensional correlations across the Baltic Basin reveal a carbonate‐dominated, mixed retrogressive succession, overlain by a siliciclastic‐dominated, progradational succession. The palaeogeographic reconstructions show how the shallow, tide‐influenced basin expanded from south‐west to north‐east and, later during the transgression, also to the north, south and east. The transgressive portion of the basin fill is dominated by carbonate‐rich sabkha and supratidal to intertidal deposits on the basin margins, and subtidal carbonates in the basin centre. Siliciclastic material was derived by tidal currents and storm waves from the south‐west through a tidal inlet and flood‐tidal delta complex. This initial transgressive phase is characterized by the lack of subsidence or even episodic uplifts in the northern/north‐western part of the basin margin, shown by convergence of timelines and the thin (30 m) transgressive succession. In contrast, on the southern margin, the facies associations stack vertically into a 70 to 80 m thick succession, indicating significantly higher subsidence rates. The upper part of the transgressive phase indicates subsidence across the whole basin. The upper, progradational portion of the basin fill is dominated by coarse, siliciclastic, tide‐influenced deltaic deposits that rapidly prograded from north‐west to south‐east. This detailed study on the Narva succession shows that siliciclastic and carbonate deposition was coeval and that mixing occurred at different temporal and spatial scales. The mixing was controlled by grain‐size, volume and location of siliciclastic input rather than relative sea‐level changes as suggested in widely used reciprocal mixing models. It is suggested that the forebulge of the Scandinavian Caledonian fold‐and‐thrust belt migrated to the north‐western margin of the Baltic Basin during the earliest Eifelian, as indicated by the lack of subsidence and probable uplift in the northern/north‐western margin during the early transgressive phase. The forebulge migration ceased although the forebulge had already started to subside during the later stages of the transgressive phase. The deltaic progradation is interpreted to be associated with the orogenic collapse and uplift in the Scandinavian Caledonides that caused the erosion of the foreland basin fill and the coarse sediment transport into the Baltic Basin.     

The Late Jurassic Tithonian, a greenhouse phase in the Middle Jurassic–Early Cretaceous 'cool' mode: evidence from the cyclic Adriatic Platform, Croatia

Well‐exposed Mesozoic sections of the Bahama‐like Adriatic Platform along the Dalmatian coast (southern Croatia) reveal the detailed stacking patterns of cyclic facies within the rapidly subsiding Late Jurassic (Tithonian) shallow platform‐interior (over 750 m thick, ca 5–6 Myr duration). Facies within parasequences include dasyclad‐oncoid mudstone‐wackestone‐floatstone and skeletal‐peloid wackestone‐packstone (shallow lagoon), intraclast‐peloid packstone and grainstone (shoal), radial‐ooid grainstone (hypersaline shallow subtidal/intertidal shoals and ponds), lime mudstone (restricted lagoon), fenestral carbonates and microbial laminites (tidal flat). Parasequences in the overall transgressive Lower Tithonian sections are 1–4·5 m thick, and dominated by subtidal facies, some of which are capped by very shallow‐water grainstone‐packstone or restricted lime mudstone; laminated tidal caps become common only towards the interior of the platform. Parasequences in the regressive Upper Tithonian are dominated by peritidal facies with distinctive basal oolite units and well‐developed laminate caps. Maximum water depths of facies within parasequences (estimated from stratigraphic distance of the facies to the base of the tidal flat units capping parasequences) were generally <4 m, and facies show strongly overlapping depth ranges suggesting facies mosaics. Parasequences were formed by precessional (20 kyr) orbital forcing and form parasequence sets of 100 and 400 kyr eccentricity bundles. Parasequences are arranged in third‐order sequences that lack significant bounding disconformities, and are evident on accommodation (Fischer) plots of cumulative departure from average cycle thickness plotted against cycle number or stratigraphic position. Modelling suggests that precessional sea‐level changes were small (several metres) as were eccentricity sea‐level changes (or precessional sea‐level changes modulated by eccentricity), supporting a global, hot greenhouse climate for the Late Jurassic (Tithonian) within the overall ‘cool’ mode of the Middle Jurassic to Early Cretaceous.     

A sequence stratigraphic model for an intensely bioturbated shallow-marine sandstone: the Bridport Sand Formation, Wessex Basin, UK

The Bridport Sand Formation is an intensely bioturbated sandstone that represents part of a mixed siliciclastic‐carbonate shallow‐marine depositional system. At outcrop and in subsurface cores, conventional facies analysis was combined with ichnofabric analysis to identify facies successions bounded by a hierarchy of key stratigraphic surfaces. The geometry of these surfaces and the lateral relationships between the facies successions that they bound have been constrained locally using 3D seismic data. Facies analysis suggests that the Bridport Sand Formation represents progradation of a low‐energy, siliciclastic shoreface dominated by storm‐event beds reworked by bioturbation. The shoreface sandstones form the upper part of a thick (up to 200 m), steep (2–3°), mud‐dominated slope that extends into the underlying Down Cliff Clay. Clinoform surfaces representing the shoreface‐slope system are grouped into progradational sets. Each set contains clinoform surfaces arranged in a downstepping, offlapping manner that indicates forced‐regressive progradation, which was punctuated by flooding surfaces that are expressed in core and well‐log data. In proximal locations, progradational shoreface sandstones (corresponding to a clinoform set) are truncated by conglomerate lags containing clasts of bored, reworked shoreface sandstones, which are interpreted as marking sequence boundaries. In medial locations, progradational clinoform sets are overlain across an erosion surface by thin (<5 m) bioclastic limestones that record siliciclastic‐sediment starvation during transgression. Near the basin margins, these limestones are locally thick (>10 m) and overlie conglomerate lags at sequence boundaries. Sequence boundaries are thus interpreted as being amalgamated with overlying transgressive surfaces, to form composite erosion surfaces. In distal locations, oolitic ironstones that formed under conditions of extended physical reworking overlie composite sequence boundaries and transgressive surfaces. Over most of the Wessex Basin, clinoform sets (corresponding to high‐frequency sequences) are laterally offset, thus defining a low‐frequency sequence architecture characterized by high net siliciclastic sediment input and low net accommodation. Aggradational stacking of high‐frequency sequences occurs in fault‐bounded depocentres which had higher rates of localized tectonic subsidence.     

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.     

Sequence development of Late Cenomanian–Turonian carbonate ramps, platforms and basins in Israel

The Cenomanian–Turonian carbonate-dominated lithofacies of Israel reflect a complex interplay between tectonics, sea-level change, and palaeoecology. Improved correlation based on revision of the bio- and chronostratigraphic framework has enabled the establishment of a sequence-stratigraphic model comprising five sequences delineated by four sequence boundaries, in the Late Cenomanian–Early Coniacian interval. The Late Cenomanian–Turonian succession begins with prograding, highstand, carbonate-platform deposits of the first sequence. Interruption of progradation and drowning of this platform took place within the Late Cenomanian guerangeri Zone (=the vibrayeanus Zone in Israel), resulting in a drowning unconformity which is regarded as a Type 3 sequence boundary (labelled CeUp). The drowning is attributed in part to extinctions in the rudist-dominated biofacies (e.g., Caprinidae), which led to reduced carbonate production and enhanced the impact of the sea-level rise. Similar drowning of Tethyan platforms around the C/T boundary has been linked to the establishment of coastal upwelling and consequent eutrophication. Outer ramp hemipelagic facies (Derorim and the Lower Ora formations) replaced the platform carbonates, thickening substantially southwards in the Eshet-Zenifim Basin of southern Israel. Along the ancient continental slope (Mediterranean coastal plain) evidence of this drowning is obscured by submarine erosion, while in central and northern Israel the drowned section is represented by condensation or a hiatus, reflecting an elevated, sediment-starved sea-floor. A carbonate platform dominated by rudistid shoals (‘Meleke’ Member; Shivta Formation) was re-established in the Judean hills and northern Negev during the middle part of the Turonian coloradoense Zone (local zone T4). Later, during kallesi Zone times (T7), the platform facies prograded southwards towards the Eshet-Zenifim intra-shelf basin. The drowning succession and overlying resurrected carbonate platform are topped in central and southern Israel by a pronounced Type 1 sequence boundary (Tu1) between the kallesi (T7) and ornatissimum (T8) zones (Middle Turonian). In central Israel and northern Negev the sequence boundary is overlain by lowstand deposits of the ‘Clastic Unit’ and by the transgressive and highstand inner to mid-ramp deposits of the Nezer and Upper Bina formations. In the southern Negev the sequence boundary is overlain by lowstand and transgressive systems tracts of mixed carbonates, siliciclastics, and localized evaporites (Upper Ora Formation), and then by mid to inner ramp carbonates of the Gerofit Formation. The latter represents a very high rate of accumulation, indicating rapid, continued subsidence balanced by platform growth. The Tu2 sequence boundary of the Late Turonian is expressed in the southern Negev by a shift from inner ramp carbonates of the Gerofit Formation to outer ramp chalky limestones of the Zihor Formation, indicating localized drowning. The succeeding Co1 sequence boundary again indicates localized drowning of the prograding highstand deposits of the Zihor Formation (‘Transition Zone’) overlain by Lower Coniacian transgressive deposits of the upper part of the Zihor Formation. All of these third-order sequences are expressed in southern Israel, where the rate of subsidence was in balance with sea-level fluctuations. In contrast, the Judean Hills and eastern Galilee areas have a more incomplete succession, characterized by hiatuses and condensation, because of reduced subsidence. More distal areas of continuous deep-water deposition in western Galilee and the coastal plain failed to record the Middle Turonian lowstand, while a longer term, second-order sequence spanning the entire Late Cenomanian–Early Coniacian interval, is present in the Carmel and Yirka Basin areas.     

Sequence stratigraphic framework for mixed aeolian,peritidal and marine environments: Insights from the Pennsylvanian subtropical record of Western Pangaea

Due to difficulties in correlating aeolian deposits with coeval marine facies, sequence stratigraphic interpretations for arid coastal successions are debated and lack a unifying model. The Pennsylvanian record of northern Wyoming, USA, consisting of mixed siliciclastic–carbonate sequences deposited in arid, subtropical conditions, provides an ideal opportunity to study linkages between such environments. Detailed facies models and sequence stratigraphic frameworks were developed for the Ranchester Limestone Member (Amsden Formation) and Tensleep Formation by integrating data from 16 measured sections across the eastern side of the Bighorn Basin with new conodont biostratigraphic data. The basal Ranchester Limestone Member consists of dolomite interbedded with thin shale layers, interpreted to represent alternating deposition in shallow marine (fossiliferous dolomite) and supratidal (cherty dolomite) settings, interspersed with periods of exposure (pedogenically modified dolomites and shales). The upper Ranchester Limestone Member consists of purple shales, siltstones, dolomicrites and bimodally cross‐bedded sandstones in the northern part of the basin, interpreted as deposits of mixed siliciclastic–carbonate tidal flats. The Tensleep Formation is characterized by thick (3 to 15 m) aeolian sandstones interbedded with peritidal heteroliths and marine dolomites, indicating cycles of erg accumulation, preservation and flooding. Marine carbonates are unconformably overlain by peritidal deposits and/or aeolian sandstones interpreted as lowstand systems tract deposits. Marine transgression was often accompanied by the generation of sharp supersurfaces. Lags and peritidal heteroliths were deposited during early stages of transgression. Late transgressive systems tract fossiliferous carbonates overlie supersurfaces. Highstand systems tract deposits are lacking, either due to non‐deposition or post‐depositional erosion. The magnitude of inferred relative sea‐level fluctuations (>19 m), estimated by comparison with analogous modern settings, is similar to estimates from coeval palaeotropical records. This study demonstrates that sequence stratigraphic terminology can be extended to coastal ergs interacting with marine environments, and offers insights into the dynamics of subtropical environments.     

Facies partitioning and sequence stratigraphy of cool-water, mixed carbonate-siliciclastic sediments (Upper Miocene Guadalquivir Domain, southern Spain)

During the late Miocene, the Guadalquivir Basin and its satellite basin, the Ronda Basin, were under Atlantic cool-water influence. The aim of our study is to develop a sequence stratigraphic subdivision of the Ronda Basin fill and to provide models for the cool-water carbonates. The Upper Miocene of the Ronda Basin can be divided into three depositional sequences. Sequence 1 is early Tortonian, Sequence 2 late Tortonian to earliest Messinian, and Sequence 3 Messinian in age. The sediments were deposited in a ramp depositional system. Sequence 1 is dominated by conglomerates and marls. In Sequence 2 and Sequence 3, carbonate deposits dominate in the inner ramp whereas siliciclastics preferentially occur in the middle and outer ramp. Bryomol carbonate sediments occur in all sequences whereas rhodalgal carbonates are restricted to Sequence 3. In bays protected from siliciclastic influx, rhodalgal deposits formed under transgressive conditions. A bryomol factory occurs in zones of continuous siliciclastic supply. This distribution results from facies partitioning during the flooding of the Ronda Basin, which has a rugged and irregular relief. Embayments were protected from siliciclastic influx and provided regions with less hydraulic energy.     

Anatomy of a cyclically packaged Mesoproterozoic carbonate ramp in northern Canada

Carbonates in the upper member of the Mesoproterozoic Victor Bay Formation are dominated by lime mud and packaged in cycles of 20–50 m. These thicknesses exceed those of classic shallowing-upward cycles by almost a factor of 10. Stratigraphic and sedimentological evidence suggests high-amplitude, high-frequency glacio-eustatic cyclicity, and thus a cool global climate ca. 1.2 Ga.The Victor Bay ramp is one of several late Proterozoic carbonate platforms where the proportions of lime mud, carbonate grains, and microbialites are more typical of younger Phanerozoic successions which followed the global waning of stromatolites. Facies distribution in the study area is compatible with deposition on a low-energy, microtidal, distally steepened ramp. Outer-ramp facies are hemipelagic lime mudstone, shale, carbonaceous rhythmite, and debrites. Mid-ramp facies are molar-tooth limestone tempestite with microspar-intraclast lags. In a marine environment where stromatolitic and oolitic facies were otherwise rare, large stromatolitic reefs developed at the mid-ramp, coeval with inner-ramp facies of microspar grainstone, intertidal dolomitic microbial laminite, and supratidal evaporitic red shale.Deep-subtidal, outer-ramp cycles occur in the southwestern part of the study area. Black dolomitic shale at the base is overlain by ribbon, nodular, and carbonaceous carbonate facies, all of which exhibit signs of synsedimentary disruption. Cycles in the northeast are shallow-subtidal and peritidal in character. Shallow-subtidal cycles consist of basal deep-water facies, and an upper layer of subtidal molar-tooth limestone tempestite interbedded with microspar calcarenite facies. Peritidal cycles are identical to shallow-subtidal cycles except that they contain a cap of dolomitic tidal-flat microbial laminite, and rarely of red shale sabkha facies or of sandy polymictic conglomerate. A transect along the wall of a valley extending 8.5 km perpendicular to depositional strike reveals progradation of inner-ramp tidal flats over outer- and mid-ramp facies during shoaling. The maximum basinward progradation of peritidal facies coincides with a zone of slope failure that may have promoted the development of the stromatolitic reefs.The sea-level history of the Victor Bay Formation is represented by three hectometre-scale sequences. An initial flooding event resulted in deposition of the lower Victor Bay shale member. Upper-member carbonate cycles were then deposited during highstand. Mid-ramp slumping was followed by late-highstand reef development. The second sequence began with development of an inner-ramp lowstand unconformity and a thick mid-ramp lowstand wedge. A second transgression promoted a more modest phase of reef development at the mid-ramp and shallow-water deposition continued inboard. A third and final transgressive episode eventually led to flooding of the backstepping ramp.Overall consistent cycle thickness and absence of truncated cycles, as well as the high rate and amount of creation of accommodation space, suggest that the periodicity and amplitude of sea-level fluctuation were relatively uniform, and point to a eustatic rather than tectonic mechanism of relative sea-level change. High-amplitude, high-frequency eustatic sea-level change is characteristic of icehouse worlds in which short-term, large-scale sea-level fluctuations accompany rapidly changing ice volumes affected by Milankovitch orbital forcing. Packaging of cyclic Upper Victor Bay carbonates therefore supports the hypothesis of a late Mesoproterozoic glacial period, as proposed by previous workers.     

Carbonate ramp-to-deeper shale shelf transitions of an Upper Cambrian intrashelf basin, Nolichucky Formation, Southwest Virginia Appalachians

The Nolichucky Formation (0–300 m thick) formed on the Cambrian pericratonic shelf in a shallow intrashelf basin bordered along strike and toward the regional shelf edge by shallow water carbonates and by nearshore clastics toward the craton. Lateral facies changes from shallow basinal rocks to peritidal carbonates suggest that the intrashelf basin was bordered by a gently sloping carbonate ramp. Peritidal facies of the regional shelf are cyclic, upward-shallowing stromatolitic carbonates. These grade toward the intrashelf basin into shallow ramp, cross-bedded, ooid and oncolitic, intraclast grain-stones that pass downslope into deeper ramp, subwave base, ribbon carbonates and thin limestone conglomerate. Ribbon limestones are layers and lenses of trilobite packstone, parallel and wave-ripple-laminated, quartzose calcisiltite, and lime mudstone arranged in storm-generated, fining upward sequences (1–5 cm thick) that may be burrowed. Shallow basin facies are storm generated, upward coarsening and upward fining sequences of green, calcareous shale with open marine biota; parallel to hummocky laminated calcareous siltstone; and intraformational flat pebble conglomerate. There are also rare debris-flow paraconglomerate (10–60 cm thick) and shaly packstone/wackestone with trace fossils, glauconite horizons and erosional surfaces/hardgrounds. A 15-m thick tongue of cyclic carbonates within the shale package contains subtidal digitate algal bioherms which developed during a period of shoaling in the basin. Understanding the Nolichucky facies within a ramp to intrashelf basin model provides a framework for understanding similar facies which are widely distributed in the Lower Palaeozoic elsewhere. The study demonstrates the widespread effects of storm processes on pericratonic shelf sedimentation. Finally, recognition of shallow basins located on pericratonic shelves is important because such basins influence the distribution of facies and reservoir rocks, whose trends may be unrelated to regional shelf-edge trends.     

Three‐dimensional modelling and sequence stratigraphy of a carbonate ramp‐to‐shelf transition,Permian Upper San Andres Formation

This study highlights three‐dimensional variability of stratigraphic geometries in the ramp crest to basin of mixed carbonate–siliciclastic clinoforms in the Permian San Andres Formation. Standard field techniques and mapping using ground‐based lidar reveal a high degree of architectural complexity in channellized, scoured and mounded outer ramp stratigraphy. Development of these features was a function of location along the ramp profile and fluctuations in relative sea‐level. Deposition of coarse‐grained and fine‐grained turbidites in the distal outer ramp occurred through dilute and high‐density turbidity flows and was the result of highstand carbonate shedding within individual cycles. In this setting, high‐frequency cycles of relative sea‐level are interpreted on the basis of turbidite frequency, lateral extent and composition. Submarine siliciclastic sediment bypass during lowstand cycles resulted in variable degrees of siliciclastic preservation. Abundant siliciclastic material is preserved in the basin and distal outer ramp as point‐sourced lowstand wedges and line‐sourced early transgressive blankets. In mounded topography of the outer ramp, siliciclastic preservation is minimal to absent, and rare incised channels offer the best opportunity for recognition of a sequence boundary. Growth of mounded topography in the outer ramp began with scouring, followed by a combination of bioherm construction, fusulinid mound construction and isopachous draping. Intermound areas were then filled with sediment and continued mound growth was prevented by an accommodation limit. Mound growth was independent of high‐frequency cycles in relative sea‐level but was dependent on available accommodation dictated by low‐frequency cyclicity. Low‐angle ramp clinoforms with mounded topography in the outer ramp developed during the transgressive part of a composite sequence. Mound growth terminated as the ramp transformed into a shelf with oblique clinoform geometries during the highstand of the composite sequence. This example represents a ramp‐to‐shelf transition that is the result of forcing by relative sea‐level fluctuations rather than ecologic or tectonic controls.     

重庆老龙洞二叠系-三叠系界线地层的海平面下降事件

老龙洞的二叠系-三叠系界线地层剖面许多学者做过研究,关于其海平面变化问题的争论一直没有解决。本文的研究首次在该剖面上发现了明显的侵蚀面,说明曾经有海平面下降事件。对沉积环境的解释也有新的认识。该剖面在长兴期钙质海绵灰岩之上沉积了一套开阔台地相的沉积,其上是1.4m 厚的局限台地相沉积,具有斑点状构造。此层顶部出现一个波状起伏的侵蚀面。侵蚀面上下的岩性不同。此面之上是0.8m 厚的浅水的局限台地相沉积,具有树枝状的构造和外貌。此层顶面是一个更加明显的侵蚀面,起伏高差达到0.3m 以上。侵蚀面上下的岩性截然不同,以及其上岩石层理与侵蚀面的斜交关系,以及侵蚀面的形态把侵蚀面同缝合线区分开来。这个侵蚀面代表一次显著海平面下降事件之后的一次较长时间的出露和风化剥蚀。侵蚀面之上是透镜状分布的灰色的层状的小腹足类的颗粒岩,再上是一薄层灰黄色的泥粒岩,含丰富的小双壳类。此层以上是青灰色的薄板状的泥质灰泥岩,舍丰富的0.3～3mm 大小的、内部重结晶的同生角砾。     

Depositional processes of the Zhushadong and Mantou formations (Early to Middle Cambrian), Shandong Province,China: roles of archipelago and mixed carbonate–siliciclastic sedimentation on cycle genesis during initial flooding of the North China Platform

To understand the depositional processes and environmental changes during the initial flooding of the North China Platform, this study focuses on the Lower to Middle Cambrian Zhushadong and Mantou formations in Shandong Province, China. The succession in the Jinan and Laiwu areas comprises mixed carbonate and siliciclastic deposits composed of limestone, dolostone, stromatolite, thrombolite, purple and grey mudstone, and sandstone. A detailed sedimentary facies analysis of seven well‐exposed sections suggests that five facies associations are the result of an intercalation of carbonate and siliciclastic depositional environments, including local alluvial fans, shallowing‐upward carbonate–siliciclastic peritidal cycles, oolite dominant shoals, shoreface and lagoonal environments. These facies associations successively show a transition from an initially inundated tide‐dominated carbonate platform to a wave‐dominated shallow marine environment. In particular, the peritidal sediments were deposited during a large number of depositional cycles. These sediments consist of lime mudstone, dolomite, stromatolite and purple and grey mudstones. These shallowing‐upward cycles generally resulted from carbonate production in response to an increase of accommodation during rising sea‐level. The carbonate production was, however, interrupted by frequent siliciclastic input from the adjacent emergent archipelago. The depositional cycles thus formed under the influence of both autogenetic changes, including sediment supply from the archipelago, and allogenic control of relative sea‐level rise in the carbonate factory. A low‐relief archipelago with an active tidal regime allowed the development of tide‐dominated siliciclastic and carbonate environments on the vast platform. Siliciclastic input to these tidal environments terminated when most of the archipelago became submerged due to a rapid rise in sea‐level. This study provides insights on how a vast Cambrian carbonate platform maintained synchronous sedimentation under a tidal regime, forming distinct cycles of mixed carbonates and siliciclastics as the system kept up with rising relative sea‐level during the early stage of basin development in the North China Platform.     

鄂西—湘西北地区二叠纪栖霞期岩相古地理

对鄂西—湘西北地区多个沉积剖面的地层及沉积相进行了详细分析,结果表明,该区二叠纪栖霞期至茅口初期主要为内克拉通碳酸盐岩缓坡环境,发育内缓坡相、中缓坡相、外缓坡相和盆地相.内缓坡相以厚层至块状生物碎屑石灰岩为主,生物颗粒以绿藻和底栖有孔虫为主,缺乏高能沉积的生物颗粒.中缓坡相以中厚层含生物碎屑颗粒石灰岩以及厚层灰泥石灰岩...     

Source-controlled carbonates in a small Eocene half-graben lake basin (Shulu Sag) in central Hebei Province, North China

Carbonate deposits, which unconformably overlie the Palaeozoic bedrocks, extensively occur in the base of the Tertiary lake succession in the half‐graben Shulu Sag, central Hebei Province, North China. This study focuses on the basal carbonate successions on the hinged western slope. Based on seismic, borehole and core data, nine facies are identified in the carbonate successions, and are further grouped into five facies associations: mid‐proximal alluvial fan, distal alluvial fan, fan fringe, moderately deep lake and deep lake. The first two facies associations constitute alluvial fans formed by debrisflows at the edge of lake and are dominated by mounded‐ to lobate‐shaped, matrix‐ to clast‐supported carbonate rudstones with minor calcretes in the lowermost rudstone units and basinward increase in interfingering with lacustrine carbonate facies. The fan fringe, moderately deep lake and deep lake associations are dominated by pebbly carbonate arenites (or rare carbonate arenites), calcisiltite‐calcilutites, and varve‐like calcilutites, calcareous shales and oil shales, respectively. Widespread occurrences of fine‐grained limestone packages containing varve‐like organic‐rich laminations, minor authigenic glauconite and pyrite, and planktonic and plant fossils suggest a meromictic, anoxic deep lake under a semi‐humid to humid climate, probably with a connection to marine basins. Similarities in lithology and fossil assemblages (e.g. trilobites) of lithoclasts with those of the Mid‐Upper Cambro‐Ordovician bedrock carbonates suggest that the clastic and dissolved carbonate loads were sourced from this Lower Palaeozoic catchment, and shed off the surrounding highlands into the basin. These carbonate facies associations represent the lake lowstand and transgressive deposits of the basal third‐order sequence (Ia) in which the highstand deposits are composed of lacustrine siliciclastics. During the lake lowstand stage (or initiation of basin‐filling) under an intermediate climate, carbonate alluvial fans occurred mostly subaerially at the bottom of the hinged slope with a narrow, shallow lake zone basinwards, and locally were perched within the palaeovalley on the mid‐upper slope. During the transgressive (deepening) stage under a semi‐humid to humid climate, carbonate alluvial fans became smaller in size and episodically stepped backwards upon the slope, with greatly expanded and deepened lake. Nevertheless, the carbonate system was switched to an exclusively siliciclastic system during the highstand stage. The exhumation and erosion of the Mid‐Lower Cambrian bedrock dominated by siliciclastics was probably the cause due to further uplift of the drainage basin. All these facts indicate that the carbonate deposition in the Shulu Sag was mostly controlled by the interactions of tectonics, climate and provenance.     

Facies and facies association of the siliciclastic Brak River and carbonate Gemsbok formations in the Lower Ugab River valley,Namibia, W. Africa

The Neoproterozoic Zerrissene Turbidite Complex of central-western Namibia comprises five turbiditic units. From the base to the top they are the Zebrapüts Formation (greywacke and pelite), Brandberg West Formation (marble and pelite), Brak River Formation (greywacke and pelite with dropstones), Gemsbok River Formation (marble and pelite) and Amis River Formation (greywacke and pelites with rare carbonates and quartz-wacke).In the Lower Ugab River valley, five siliciclastic facies were recognised in the Brak River Formation. These are massive and laminated sandstones, classical turbidites (thick- and thin-bedded), mudrock, rare conglomerate and breccia. For the carbonate Gemsbok River Formation four facies were identified including massive non-graded and graded calcarenite, fine grained evenly bedded blue marble and calcareous mudrock. Most of these facies are also present in the other siliciclastic units of the Zerrissene Turbidite Complex as observed in other areas.The vertical facies association of the siliciclastic Brak River Formation is interpreted as representing sheet sand lobe to lobe-fringe palaeoenvironment with the abandonment of siliciclastic deposition at the top of the succession. The vertical facies association of the carbonate Gemsbok Formation is interpreted as the slope apron succession overlain by periplatform facies, suggesting a carbonate slope sedimentation of a prograding depositional shelf margin.If the siliciclastic–carbonate paired succession would represent a lowstand relative sea-level and highstand relative sea-level, respectively, the entire turbidite succession of the Zerrissene Turbidite Complex can be interpreted as three depositional sequences including two paired siliciclastic–carbonate units (Zebrapüts-Brandberg West formations; Brak River–Gemsbok formations) and an incomplete succession without carbonate at the top (Amis River Formation).     

Allogenic and autogenic controls on facies and stratigraphic architecture of the Lower Jurassic Mashabba Formation,Gebel Al-Maghara,North Sinai,Egypt

The Lower Jurassic Mashabba Formation crops out in the core of the doubly plunging Al-Maghara anticline, North Sinai, Egypt. It represents a marine to terrestrial succession deposited within a rift basin associated with the opening of the Neotethys. Despite being one of the best and the only exposed Lower Jurassic strata in Egypt, its sedimentological and sequence stratigraphic framework has not been addressed yet. The formation is subdivided informally into a lower and upper member with different depositional settings and sequence stratigraphic framework. The sedimentary facies of the lower member include shallow-marine, fluvial, tidal flat and incised valley fill deposits. In contrast, the upper member consists of strata with limited lateral extension including fossiliferous lagoonal limestones alternating with burrowed deltaic sandstones. The lower member contains three incomplete sequences (SQ1-SQ3). The depositional framework shows transgressive middle shoreface to offshore transition deposits sharply overlain by forced regressive upper shoreface sandstones (SQ1), lowstand fluvial to transgressive tidal flat and shallow subtidal sandy limestones (SQ2), and lowstand to transgressive incised valley fills and shallow subtidal sandy limestones (SQ3). In contrast, the upper member consists of eight coarsening-up depositional cycles bounded by marine flooding surfaces. The cycles are classified as carbonate-dominated, siliciclastic-dominated, and mixed siliciclastic-carbonate. The strata record rapid changes in accommodation space. The unpredictable facies stacking pattern, the remarkable rapid facies changes, and chaotic stratigraphic architecture suggest an interplay between allogenic and autogenic processes. Particularly syndepositional tectonic pulses and occasional eustatic sea-level changes controlled the rate and trends of accommodation space, the shoreline morphology, the amount and direction of siliciclastic sediment input and rapid switching and abandonment of delta systems.     

前陆盆地层序地层学研究简介

前陆盆地层序地层学是将层序地层学理论应用于构造活动的前陆盆地分析的一个特例。前陆盆地三级层序成因并非受全球统一的海平面变化控制，而是与盆缘造山带区域本报特约记者运动、盆内沉积作用和相对海平面变化的联合作用有关，代表了前陆分地一个成盆期的不同发育阶段。层序界面是相对海平面下降和区域构造隆的联合作用面。在盆地演化的不对称沉降阶充填阶段，邻造山带区为低水位浊积扇沉积层序；远离造山带区，低水位体系域不发育     

Tectonic, eustatic and environmental controls on mid-Cretaceous carbonate platform deposition, south-central Pyrenees, Spain

Cenomanian–Turonian strata of the south‐central Pyrenees in northern Spain contain three prograding carbonate sequences that record interactions among tectonics, sea level, environment and sediment fabric in controlling sequence development. Sequence UK‐1 (Lower to Upper Cenomanian) contains distinct lagoonal, back‐margin, margin, slope and basin facies, and was deposited on a broad, flat shelf adjacent to a deep basin. The lack of reef‐constructing organisms resulted in a gently dipping ramp morphology for the margin and slope. Sequence UK‐2 (Upper Cenomanian) contains similar shallow‐water facies belts, but syndepositional tectonic modification of the margin resulted in a steep slope and deposition of carbonate megabreccias. Sequence UK‐3 (Lower to Middle Turonian) records a shift from benthic to pelagic deposition, as the shallow platform was drowned in response to a eustatic sea‐level rise, coupled with increased organic productivity. Sequences UK‐1 to UK‐3 are subdivided into lowstand, transgressive and highstand systems tracts based on stratal geometries and facies distribution patterns. The same lithologies (e.g. megabreccias) commonly occur in more than one systems tract, indicating that: (1) the depositional system responded to more than just sea‐level fluctuations; and (2) similar processes occurred during different times throughout sequence development. These sequences illustrate the complexity of carbonate platform dynamics that influence sequence architecture. Rift tectonics and flexural subsidence played a major role in controlling the location of the platform margin, maintaining a steep slope gradient through syndepositional faulting, enhancing slope instability and erosion, and influencing depositional processes, stratal relationships and lithofacies distribution on the slope. Sea‐level variations (eustatic and relative) strongly influenced the timing of sequence and parasequence boundary formation, controlled changes in accommodation and promoted platform drowning (in conjunction with other factors). Physico‐chemical and climatic conditions were responsible for reducing carbonate production rates and inducing platform drowning. Finally, a mud‐rich sediment fabric affected platform morphology, growth geometries (aggradation vs. progradation) and facies distribution patterns.     

The role of tectonism in sequence development and facies distribution of Upper Oligocene cool-water carbonates: Coromandel Peninsula, New Zealand

The Torehina Formation is part of a cool‐water carbonate succession of Oligocene age in New Zealand that crops out on the Coromandel Peninsula, North Island. It contains two major transgressive sequences that record successive onlap of a once emergent landmass. The first sequence records marine flooding of non‐marine to marginal marine fan delta/estuarine facies, followed by deepening upward and formation of a low‐energy, deep (100+ m) muddy carbonate ramp. The capping sequence boundary is characterized by differential uplift and varies considerably in its character over a small (9 km²) area, varying from a burrowed glauconitic firmground to an erosional hardground to an undulatory marine contact to a palaeokarst with < 25 m relief. Sequence 2 sediments, which overlie the palaeokarst with minor (< 10°) angular unconformity, are clayey, marine (offshore) siltstones, whereas open‐marine limestones of equivalent age overlie the other boundary types with no apparent angular discordance. The siliciclastics could either represent lowstand channel deposits or may define interbank deposits contemporary with adjacent carbonates. Palaeogeographic restriction of palaeokarst and sequence 2 siliciclastics identifies a structural corridor oriented strike‐parallel to the adjacent Harauki Graben, which began to develop by this time. Palaeogeographical differences in the character of the basal limestone facies of sequence 2 also occur. These differences identify variation in accommodation during initial stages of deposition imposed by previous differential movement of fault blocks. As a result, relatively warm‐water (20 °C) Amphistegina‐bearing limestones in one area contrast with co‐existing deeper water, silty foraminiferal (benthic > planktic), echinoderm and bivalve limestones in another. This variation disappears upsection, which suggests that initial bathymetric differences were eliminated with renewed rise in sea level, yielding deeper water inner‐shelf sediment facies followed by the accumulation of still deeper, but higher energy, outer‐shelf bivalve and bryozoan facies. The sequence architecture of the Torehina Formation is controlled by tectonism, both long‐term subsidence and short‐term differential uplift. This arose as a result of increasing tectonic activity throughout proto‐New Zealand during the Late Oligocene. In such a system, local and regional variation in tectonism among adjacent basins can impose subtle to marked differences in the timing of sequence boundaries and the character of basin‐fill patterns.