Tectonic, sea-level, and climatic controls on Late Paleozoic sedimentation in the western basins of Argentina

Tectonic activity, sea-level changes, and the climate controlled sedimentation in Late Paleozoic basins of western Argentina. The role of each factor is investigated from the geologic record of the Río Blanco and Paganzo basins using three hierarchical orders of stratigraphic bounding surfaces. First-order surfaces correspond to regional unconformities, second-order ones to local unconformities with a lesser regional extent, and third-order surfaces represent locally extended sedimentary truncation. Using this methodology, the Carboniferous–Permian record of the Paganzo and Río Blanco basins may be divided into two megasequences, four sequences, and 12 stratigraphic sections. Megasequences are bounded by regional unconformities that result from tectonic events important enough to cause regional paleogeographic changes. Sequences are limited by minor regional extension surfaces related to local tectonic movements or significant sea-level falls. Finally, stratigraphic sections correspond to extended sedimentary truncations produced by transgressive events or major climatic changes. Sequence I is mainly composed of marine deposits divided into basal infill of the basin (Section 1) and Tournaisian–Visean transgressive deposits (Section 2). Sequence II is bounded by a sharp erosional surface and begins with coarse conglomerates (Section 3), followed by fluvial and shallow marine sedimentary rocks (Section 4) that pass upward into shales and diamictites (Section 5). The base of Sequence III is marked by an extended unconformity covered by Early Pennsylvanian glacial sedimentary rocks (Section 6) that represent the most important glacial event along the western margin of Gondwana. Postglacial deposits (Section 7) occur in the two basins and comprise both glaciolacustrine (eastern region) and transgressive marine (central and western regions) deposits. By the Moscovian–Kasimovian, fluvial sandstones and conglomerates were deposited in most of the Paganzo Basin (Section 8), while localized volcanic activity took place in the Río Blanco Basin. Near the end of the Carboniferous, an important transgression is recorded in the major part of the Río Blanco Basin (Section 9), reaching the westernmost portion area of the Paganzo Basin. Finally, Sequence IV shows important differences between the Paganzo and Río Blanco basins; fluvial red beds (Section 10), eolian sandstones (Section 11), and low-energy fluvial deposits (Section 12) prevailed in the Paganzo Basin whereas volcaniclastic sedimentation and volcanism dominated in the Río Blanco Basin. Thus, tectonic events, sea-level changes and climate exerted a strong and complex control on the evolution of the Río Blanco and Paganzo basins. The interaction of these allocyclic controls produced not only characteristic facies association patterns but also different kinds of stratigraphic bounding surfaces.     

A paleoclimatic review of southern South America during the late Paleozoic: A record from icehouse to extreme greenhouse conditions

This paper provides a review of the Late Mississippian to Permian paleoclimatic history for southern South America based on lithologic indicators, biostratigraphic information, and chronostratigraphic data. The region is divided into three major types of basins: 1. Eastern intraplate basins (e.g., Paraná Basin), 2. Western retroarc basins (e.g., Paganzo Basin) and 3. Western arc-related basins (e.g., Río Blanco Basin). Four major types of paleoclimatic stages are recognized in these basins: 1. glacial (late Visean–early Bashkirian), 2. terminal glacial (Bashkirian–earliest Cisuralian) 3. postglacial (Cisuralian–early Guadalupian), and 4. semiarid–arid (late Guadalupian–Lopingian). The glacial stage began in the late Visean and continued until the latest Serpukhovian or early Bashkirian in almost all of the basins in southern South America. During the Bashkirian–earliest Cisuralian (terminal glacial stage), glacial deposits disappeared almost completely in the western retroarc basins (e.g., Paganzo Basin) but glaciation persisted in the eastern basins (e.g., Paraná and Sauce Grande Basins). A gradual climatic amelioration (postglacial stage) began to occur during the earliest Permian when glacial deposits completely disappeared across all of South America. During this interval, glacial diamictites were replaced by thick coal beds in the Paraná Basin while north–south climatic belts began to be delineated in the western basins, which were likely controlled by the distribution of mountain belts along the Panthalassan Margin of South America. Towards the late Permian, climatic belts became less evident and semiarid or arid conditions dominated in the southern South America basins. Eolian dunes, playa lake deposits, and mixed eolian–fluvial sequences occur in the Paraná Basin and in the western retroarc basins. Volcanism and volcaniclastic sedimentation dominated along the western margin of South America at that time. The stratigraphic record obtained in southern South America supports a long duration transition from icehouse to extreme greenhouse conditions.     

Tectono-stratigraphic evolution of the Upper Jurassic–Neocomian rift succession,Araripe Basin,Northeast Brazil

The rift succession of the Araripe Basin can be subdivided into four depositional sequences, bounded by regional unconformities, which record different palaeogeographic and palaeoenvironmental contexts. Sequence I, equivalent to the Brejo Santo Formation, is composed of fluvial sheetflood and floodplain facies association, while Sequence II, correspondent to the lower portion of the Missão Velha Formation, is characterised by braided fluvial channel belt deposits. The fluvial deposits of Sequences I and II show palaeocurrents toward SE. The Sequence III, correspondent to the upper portion of Missão Velha Formation, is composed of fluvial sheetflood deposits, which are overlain by braided fluvial channel deposits displaying a palaeocurrent pattern predominantly toward SW to NW. Sequence IV, equivalent to the Abaiara Formation, is composed of fluvio–deltaic–lacustrine strata with polimodal paleocurrent pattern. The type of depositional systems, the palaeocurrent pattern and the comparison with general tectono-stratigraphic rift models led to the identification of different evolutionary stages of the Araripe Basin. Sequences I, II and III represent the record of a larger basin associated to an early rift stage. However, the difference of the fluvial palaeocurrent between sequences II and III marks a regional rearrangement of the drainage system related to tectonic activity that compartmentalised the large endorheic basin, defining more localised drainage basins separated by internal highs. Sequence IV is associated with the renewal of the landscape and implantation of half-graben systems. The high dispersion of palaeocurrents trends indicate that sedimentary influx occurs from different sectors of the half-grabens.     

The late Paleozoic El Imperial Formation,western Argentina: Glacial to post-glacial transition and stratigraphic correlations with arc-related basins in southwestern Gondwana

The El Imperial Formation of the San Rafael Basin records a succession of depositional environments during the latest Mississippian to earliest Permian that span before, during, and after the glaciation of west central Argentina. At the base of the formation, a restricted marine environment is recorded in mudstone containing marl and rippled and deformed sandstone beds. This unit, or sequence 1, is incised by a deltaic facies association composed of cross-bedded sandstone and conglomerate that form at least 5 stacked Gilbert deltas. The deltaic facies association grades upward into the glacially-influenced facies association, made up of stratified diamictite, mudstone with dropstones, and massive deformed sandstone, indicating deposition by wet-based tidewater glaciers that calved icebergs into the basin, with contributions from mass movement processes. The glacially-influenced facies association is overlain by mudstone and horizontally laminated and cross-bedded sandstone of the post-glacial open marine facies association, recording post-glacial transgression followed by relative sea level fall. The deltaic, glacially-influenced, and post-glacial open marine facies associations comprise sequence 2. Sequence 2 is incised by conglomerate of the upper fluvial member, or sequence 3.The strata of the El Imperial Formation are correlated to those of the other arc-related basins of western Argentina: Río Blanco, Calingasta–Uspallata, and Tepuel. A Bashkirian transgression and fluvial incision in the El Imperial Formation correlate with events in the Río Blanco and Calingasta–Uspallata Basins to the north, whereas glaciation continues to the south in the Tepuel Basin through the Early Permian. The deviating stratigraphic record of the Tepuel Basin may be the result of its higher latitudinal position during the Pennsylvanian–Early Permian and higher altitude due to either tectonic convergence of the Patagonian microplate or convergence along the Panthalassan margin of southwestern Gondwana.     

沉积盆地的层序和沉积充填结构及过程响应

现代层序地层学的理论发展,把沉积过程纳入到地质演化的时空框架中并与地球的多旋回或节律演化结合研究,形成了一套带有革命性的、在等时地层格架中研究沉积作用的新方法,成为了油气资源等沉积矿产预测勘探的重要工具。沉积盆地的沉积充填可划分出与各级沉积旋回相对应的层序地层单元。追踪对比由不整合面或不整合面及其对应的整合面为界的高级别层序地层单元建立的区域性等时地层格架,对盆地构造古地理再造和油气勘探战略性研究至关重要;追踪四、五级等低级别层序地层单元和体系域建立的高精度层序地层格架,可为重点区域或区带的沉积体系和储集体的沉积构成和分布等的解剖提供精细的地层对比基础。依据沉积基准面的变化,从层序内水进到水退的沉积旋回中可划分出正常水退沉积、强制性水退沉积、水进沉积及垂向加积等成因沉积类型。海相或湖相盆地中三级层序地层单元内均可较好地划分出低位、水进、高位及下降体系域。盆地构造作用、气候变化、海、湖平面升降过程对层序发育的控制作用及沉积响应研究,一直是层序地层学或沉积地质分析领域的研究热点。沉积盆地的层序地层序列演化是盆地地球动力学过程的总体响应。层序地层学把盆地古构造、古地理的变迁纳入到统一的地球演化系统中研究,形成了与区域地球演化史或盆地动力学演化相结合的重要研究领域。多旋回盆地或叠合盆地中多期次的构造变革导致了多个区域性不整合面所分隔的多个构造层序的叠加。注重构造—层序地层的结合分析,揭示盆地的层序地层序列与多期盆地构造作用的成因联系,是构造活动盆地或大型叠合盆地沉积地质演化和油气聚集规律研究的关键。盆地构造作用,如前陆盆地多期次的逆冲挠曲沉降和回弹隆起的构造作用、多幕裂陷过程、多期构造反转等与重要不整合及区域性沉积旋回或层序的形成密切相关;而由气候变化引起的海或湖平面变化是控制高频沉积旋回或低级别层序发育的主要因素。在构造活动盆地中,构造坡折带对沉积体系域和沉积相的发育分布具重要控制作用。     

Facies analysis and sequence stratigraphy of neoproterozoic Platform deposits in Adrar of Mauritania, Taoudeni basin, West Africa

The Neoproterozoic and Palaeozoic Taoudeni basin forms the flat-lying and unmetamorphosed sedimentary cover of the West African Craton. In the western part of this basin, the Char Group and the lower part of the Atar Group make up a 400-m-thick Neoproterozoic siliciclastic succession which rests on the Palaeoproterozoic metamorphic and granitic basement. Five erosional bounding surfaces of regional extent have been identified in this succession. These surfaces separate five stratigraphic units with lithofacies associations ranging from fluvial to coastal and fluvial-, tide-, or wave-dominated shallow marine deposits. Owing to their regional extent and their position within the succession, the erosive bounding surfaces correspond to relative sea-level falls, and accordingly the five stratigraphic units they bound represent allocyclic transgressive–regressive depositional sequences (S₁–S₅). Changes in the nature of the deposits forming the transgressive–regressive cycles reflect landward or seaward shifts of the stacked sequences. These successive relative sea-level changes are related to the reactivation of basement faults and tilting during rifting of the Pan-Afro-Brasiliano supercontinent 1000 m.y. ago. The stromatolite bearing carbonate-shale sequences which form the rest of the Atar Group mark the onset of a quiet period of homogeneous subsidence contemporaneous with the Pan-African I oceanization 800–700 m.y. ago.     

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.     

Three orders of regional sea-level changes control facies and stacking patterns of shallow platform carbonates in the Maestrat Basin (Tithonian-Berriasian,NE Spain)

Middle Tithonian-Berriasian shallow platform carbonates of the Maestrat Basin (Salzedella and Montanejos sections, NE Spain) are stacked in sequence stratigraphic units of different orders. Higher-order sequences (parasequences and subunits) have a shallowing or deepening-shallowing evolution. They have been related to the short-term eccentricity and precession cycles. Major facies changes and stacking pattern of parasequences reveal the presence of two 2nd-order sequences. The Lower Sequence is middle Tithonian-mid early Berriasian in age. The Upper Sequence extends up to the mid-late Berriasian. It is suggested that local subsidence changes along with regional sea-level changes controlled the long-term evolution of accommodation in the Maestrat Basin. Facies evolution, stacking pattern and sharp lithological changes have allowed the definition of five 3rd-order sequences in the Lower Sequence in Montanejos. The transgressive deposits are characterised by the progressive absence of the restricted lagoon facies, and the presence of deepening-upward intervals in the parasequences. The highstand deposits display an increase in siliciclastics and a progressive predominance of restricted lagoon facies. Some of the 3rd-order sequence boundaries match the sequence boundaries identified in other European basins and may be related to sea-level falls (induced by the long-term eccentricity cycle) enhanced during periods of long-term loss of accommodation.     

Lower Cretaceous provenance in the northern Austral basin of Patagonia from sedimentary petrography

The northern Austral basin from Patagonia is characterized by an Early Cretaceous (Barremian–Albian) coarse-grained regressive sequence. These littoral to continental deposits conform a 150 km long basin cropping out along the Southern Patagonian Andes between 47 and 48°S. The basin fill consist of basal deltaic sandstones with interbedded shales and limestones from the Río Belgrano Formation, topped by up to 350 m of fluvial conglomerates and reworked tuffs of the Río Tarde and Kachaike formations. This continental depocenter represent a major geodynamic and paleoenviromental change from the underling marine Río Mayer Formation. In this study we analyze the tectonic setting and provenance during deposition of the coarse-grained sequence using sedimentary petrography of 37 thin sections in four stratigraphic profiles covering the northern basin. Our dataset indicates mainly a recycled orogenic sandstones provenance, in agreement with potential surrounding basement sources.     

Genesis of sandstone-type uranium deposits along the northern margin of the Ordos Basin,China

Sandstone-type U mineral resources are among the important sources for nuclear energy.The U deposits in the Ordos Basin in China form part of the northern segment of the sandstone-hosted Central Asian Uranium MegaProvince.Two types of mineralizations are recognized in this basin: "phreatic permeable type" and "interlayer permeable type",both exhibiting features equivalent to roll-front subtypes.The "interlayer permeable type" is widely accepted as the dominant mineralization type for sandstone-type uranium deposits within large-scale basins,also designated as the "interlayer oxidation zone type",based on the horizontal color zoning model representing changing redox conditions.Here we synthesize data from several drill holes within the Ordos Basin,which suggest that major Mesozoic tectonic movements controlled the evolution of the sedimentary system in the basin.These tectonic movements contributed to the formation of three angular unconformities and four parallel unconformities as inferred from the stratigraphic relationships.In addition,other features such as vertical color zoning,paleo-channel controlled tabular or lentoid ore bodies(without roll-type) and a group interlayer horizontal zoning of altered minerals are also documented.Sequence stratigraphic analysis indicates that the Ordos Basin generally witnessed four cycles of water level variations during Mesozoic.During the variations,three high water level and three low water level events were recorded.Biological characteristics imply that the Ordos Basin went through multiple arid to humid climatic evolutions during Mesozoic.Combining the newly documented features with some novel concepts on the hydrodynamic mechanism for supergene ore-forming fluids,we propose a metallogenic model which invokes the importance of tectonic movements and water level fluctuations to explain the genesis of uranium deposits along the northern margin of the Ordos Basin.     

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     

层序地层学研究进展

近代沉积地质学研究的对象和目的早已超越了沉积岩石学、相分析和沉积环境演替的经典研究，而是把发生沉积作用的堆积场所──盆地．放在全球沉积地壳演化的时空坐标上，通过正负沉积记录寻求有成因联系的沉积和构造综合体在地质历史中各种事件的铸记，恢复它在全球古板块和古地理的位置．进而探索沉积地壳演化和地球起源的证据。在科学技术高速发展的今天．地学领域也具有明显的时代特征：一是地质学中各学科的专业化、计算机化和定量化；另一特征则是多学科的渗透化。其中层序地层学研究在近代地学领域中具有里程碑意义的重大进展是追踪对比全球沉积记录和全球古地理再造的重要研究途径。     

Marine transgression and regression in Miocene sequences of northern Pegu (Bago) Yoma,Central Myanmar

Neogene strata of the northern part of the Pegu (Bago) Yoma Range, Central Myanmar, contain a series of shallow marine clastic sediments with stratigraphic ages ranging from the Early to Late Miocene. The studied succession (around 750 m thick) is composed of three major stratigraphic units deposited during a major regression and four major transgressive cycles in the Early to Late Miocene. The transgressive deposits consist of elongate sand-bars and broad sand-sheets that pass headward into mixed-flats of tidal environments. Marine flooding in transgressive deposits is associated with coquina beds and allochthonous coral-bearing sandy limestone bands. Major marine regressions are associated with lowstand progradation of thick estuary point-bars passing up into upper sand-flat sand bodies encased within the tidal flat sequences and lower shoreface deposits with local unconformities. The succession initially formed in a large scale incised-valley system, and was later interrupted by two major marine transgressions in the generally regressive or basinward-stepping stratigraphic sequences. Successive sandbodies were formed during a sea-level lowstand and early stage of the subsequent relative rise of sea level in a tide-dominated estuary system in the eastern part of the Central Myanmar Tertiary Basin during Early to Late Miocene times.     

Early Jurassic palaeoenvironments in the Surat Basin,Australia – marine incursion into eastern Gondwana

Interpretations of palaeodepositional environments are important for reconstructing Earth history. Only a few maps showing the Jurassic depositional environments in eastern Australia currently exist. Consequently, a detailed understanding of the setting of Australia in Gondwana is lacking. Core, wireline logs, two-dimensional and three-dimensional seismic from the Precipice Sandstone and Evergreen Formation in the Surat Basin have been used to construct maps showing the evolution of depositional environments through the Early Jurassic. The results indicate the succession consists of three third-order sequences (Sequence 1 to Sequence 3) that were controlled by eustatic sea level. The lowstand systems tract in Sequence 1 comprises braidplain deposits, confined to a fairway that parallels the basin centre. The strata were initially deposited in two sub-basins, with rivers flowing in different orientations in each sub-basin. The transgressive systems tract of Sequence 1 to lowstand systems tract of Sequence 3 is dominated by fluvio–deltaic systems infilling a single merged basin centre. Finally, the transgressive and highstand systems tracts of Sequence 3 show nearshore environments depositing sediment into a shallow marine basin. In the youngest part of this interval, ironstone shoals are the most conspicuous facies, the thickness and number of which increase towards the north and east. This study interprets a corridor to the open ocean through the Clarence–Moreton Basin, or the Carpentaria and Papuan basins, evidence of which has been eroded. These results challenge a commonly held view that eastern Australia was not influenced by eustasy, and propose a more dynamic palaeogeographic setting comprising a mixture of fluvial, deltaic and shallow marine sedimentary environments. This work can be used to unravel the stratigraphic relationships between Mesozoic eastern Australian basins, or in other basins globally as an analogue for understanding the complex interplay of paralic depositional systems in data poor areas.     

Facies analysis of a Toarcian–Bajocian shallow marine/coastal succession (Bardas Blancas Formation) in northern Neuquén Basin,Mendoza province,Argentina

Strata of the Bardas Blancas Formation (lower Toarcian–lower Bajocian) are exposed in northern Neuquén Basin. Five sections have been studied in this work. Shoreface/delta front to offshore deposits predominate in four of the sections studied exhibiting a high abundance of hummocky cross-stratified, horizontally bedded and massive sandstones, as well as massive and laminated mudstones. Shell beds and trace fossils of the mixed Skolithos-Cruziana ichnofacies appear in sandstone beds, being related with storm event deposition. Gravel deposits are frequent in only one of these sections, with planar cross-stratified, normal graded and massive orthoconglomerates characterizing fan deltas interstratified with shoreface facies. A fifth outcrop exhibiting planar cross-stratified orthoconglomerates, pebbly sandstones with low-angle stratification and laminated mudstones have been interpreted as fluvial channel deposits and overbank facies. The analysis of the vertical distribution of facies and the recognition of stratigraphic surfaces in two sections in Río Potimalal area let recognized four transgressive–regressive sequences. Forced regressive events are recognized in the regressive intervals. Comparison of vertical distribution of facies also shows differences in thickness in the lower interval among the sections studied. This would be related to variations in accommodation space by previous half-graben structures. The succession shows a retrogradational arrangement of facies related with a widespread transgressive period. Lateral variation of facies let recognize the deepening of the basin through the southwest.     

Sedimentary response to a tectonically induced sea-level fall in a shallow back-arc basin: the Mulichinco Formation (Lower Cretaceous), Neuquén Basin, Argentina

This study examines the sedimentary response to a tectonically driven relative sea‐level fall that occurred in the Neuquén Basin, west‐central Argentina, during the late Early Valanginian (Early Cretaceous). At this time the basin lay behind the emergent Andean magmatic arc to the west. Following the relative sea‐level fall, sedimentation was limited to the central part of the Neuquén Basin, with the deposition of a predominantly clastic, continental to shallow marine wedge on top of basinal black shales. This lowstand wedge is called the Mulichinco Formation and consists of a third‐order sequence that lasted about 2 Myr and contains high frequency lowstand, transgressive, and highstand deposits. Significant variations in facies, depositional architecture, and internal organization of the sequence occur along depositional strike. These variations are attributed mainly to tectonic and topographic controls upon sediment flux, basin gradient, fault tilting, and shifting of the depocentre through time. These controls were ultimately related to asymmetrically distributed tectonic activity that was greater towards the magmatic arc in the west. The superposition of fluvial deposits directly upon offshore facies provides unequivocal evidence for a sequence boundary at the base of the Mulichinco Formation. However, the Mulichinco sequence boundary is marked by shallow, low erosional relief and widespread fluvial deposition. The surface lacks prominent valleys traditionally associated with sequence boundaries. This non‐erosive sequence boundary geometry is attributed to the ramp‐type geometry of the basin and/or rapid uplift that limited stratigraphic adjustment to base‐level fall. Significant along‐strike facies changes and a low‐relief sequence boundary are attributes that may be common in tectonically active, semi‐enclosed basins (e.g. shallow back‐arc basins, foreland basins).     

A contribution to regional stratigraphic correlations of the Afro-Brazilian depression – The Dom João Stage (Brotas Group and equivalent units – Late Jurassic) in Northeastern Brazilian sedimentary basins

The Dom João Stage comprises an interval with variable thickness between 100 and 1200 m, composed of fluvial, eolian and lacustrine deposits of Late Jurassic age, based mainly on the lacustrine ostracod fauna (although the top deposits may extend into the Early Cretaceous). These deposits comprise the so-called Afro-Brazilian Depression, initially characterized as containing the Brotas Group of the Recôncavo Basin (which includes the Aliança and the Sergi Formations) and subsequently extended into the Tucano, Jatobá, Camamu, Almada, Sergipe, Alagoas and Araripe Basins in northeastern Brazil, encompassing the study area of this paper. The large occurrence area of the Dom João Stage gives rise to discussions about the depositional connectivity between the basins, and the real extension of sedimentation. In the first studies of this stratigraphic interval, the Dom João Stage was strictly associated with the rift phase, as an initial stage (decades of 1960–70), but subsequent analyses considered the Dom João as an intracratonic basin or pre-rift phase – without any relation to the active mechanics of a tectonic syn-rift phase (decades of 1980–2000). The present work developed an evolutionary stratigraphic and tectonic model, based on the characterization of depositional sequences, internal flooding surfaces, depositional systems arrangement and paleoflow directions. Several outcrops on the onshore basins were used to build composite sections of each basin, comprising facies, architectural elements, depositional systems, stratigraphic and lithostratigraphic frameworks, and paleocurrents. In addition to that, over a hundred onshore and offshore exploration wells were used (only 21 of which are showed) to map the depositional sequences and generate correlation sections. These show the characteristics and relations of the Dom João Stage in each studied basin, and they were also extended to the Gabon Basin. The results indicate that there were two main phases during the Dom João Stage, in which distinctive sedimentary environments were developed, reflecting depositional system arrangements, paleoflow directions were diverse, and continuous or compartmented basins were developed.     

An alternative stratigraphic scheme for the Sarawak Basin

The present stratigraphic scheme for the Tertiary of Sarawak (Ho, K.F., 1978. Stratigraphic framework for oil exploration in Sarawak. Bulletin of the Geological Society of Malaysia, 10, 1–14), which subdivides the entire sedimentary succession into eight sedimentary cycles, is based on the genetic sedimentary cycle concept. Each cycle starts with a transgressive basal part, followed by a regressive unit, which is in turn overlain by the basal transgressive unit of the next cycle. The limitation of the present scheme is, however, in its general applicability; for instance, when one tries to identify the equivalents of marine surfaces within non-marine sediments. This is critical for some areas in Sarawak where the sediments were deposited predominantly within lower coastal plain to upper coastal plain environments, normally barren of foraminifera. Among other inadequacies of the scheme is the lack of basin-wide association between the cycle boundaries and seismic reflectors. In practice cycle boundaries, picked on the composite well logs, often do not agree with the seismic pick. The scheme is, however, well accepted and continues to be used. The alternative stratigraphic scheme for the Sarawak Basin, which will be discussed in this paper, is based on sequence stratigraphic concepts, with the utilisation of unconformity or its correlative conformity as the stratigraphic boundary. This scheme has been generated as the result of a programme of basin-wide seismic mapping of the Sarawak Basin. Seven regional unconformities within the Tertiary sedimentary sequences have been mapped. The sedimentary units between the unconformities can be recognised as ‘Sequences’. The oldest unconformity is that between the basement (Belaga Formation) and the overlying Tertiary sediments, mainly of Late Oligocene age. Since the sediments in the basin are mainly of Tertiary age, the oldest unit of the succession is referred to as the Tertiary One Sequence (T1 S). The next younger sequence is called Tertiary Two Sequence (T2 S) and so on. The proposed scheme, which is based on a more conventional and multi-disciplinary approach, can be understood and appreciated, not only by palaeontologists, but by all members of exploration and production teams. The scheme should be able to facilitate prediction of stratigraphic levels, not only after the well has been drilled, but also prior to drilling.     

成因层序地层学的回顾与展望

回顾了以成因地层层序为基础的成因层序地层学的形成、发展与研究现状,对成因地层层序及其内部构成、高分辨率成因地层层序、成因地层层序的旋回性、非海相成因地层层序、成因地层层序与沉积物堆积速率等主要观点作了简略评述,并结合我国陆相沉积特征对成因层序地层学未来研究前景作了初步展望。     

Stacked fluvial and tide-dominated estuarine deposits in high-frequency (fourth-order) sequences of the Eocene Central Basin, Spitsbergen

Eighteen coastal-plain depositional sequences that can be correlated to shallow- to deep-water clinoforms in the Eocene Central Basin of Spitsbergen were studied in 1 × 15 km scale mountainside exposures. The overall mud-prone (>300 m thick) coastal-plain succession is divided by prominent fluvial erosion surfaces into vertically stacked depositional sequences, 7–44 m thick. The erosion surfaces are overlain by fluvial conglomerates and coarse-grained sandstones. The fluvial deposits show tidal influence at their seaward ends. The fluvial deposits pass upwards into macrotidal tide-dominated estuarine deposits, with coarse-grained river-dominated facies followed further seawards by high- and low-sinuosity tidal channels, upper-flow-regime tidal flats, and tidal sand bar facies associations. Laterally, marginal sandy to muddy tidal flat and marsh deposits occur. The fluvial/estuarine sequences are interpreted as having accumulated as a series of incised valley fills because: (i) the basal fluvial erosion surfaces, with at least 16 m of local erosional relief, are regional incisions; (ii) the basal fluvial deposits exhibit a significant basinward facies shift; (iii) the regional erosion surfaces can be correlated with rooted horizons in the interfluve areas; and (iv) the estuarine deposits onlap the valley walls in a landward direction. The coastal-plain deposits represent the topset to clinoforms that formed during progradational infilling of the Eocene Central Basin. Despite large-scale progradation, the sequences are volumetrically dominated by lowstand fluvial deposits and especially by transgressive estuarine deposits. The transgressive deposits are overlain by highstand units in only about 30% of the sequences. The depositional system remained an estuary even during highstand conditions, as evidenced by the continued bedload convergence in the inner-estuarine tidal channels.