Differential Tectonic Subsidence Research Articles

Depositional architecture, evolution and controlling factors of the Miocene submarine canyon system in the Pearl River Mouth Basin, northern South China Sea

Submarine canyon system in the Miocene Pearl River Mouth Basin, northern continental margin of the South China Sea, records the process of sediments transported from shelf margin to advanced abyssal plain and comprises important petroleum reservoirs in the basin. Based on integrated analysis of seismic, well logs and core data, the sequence framework, depositional architecture and controlling factors of the submarine canyon system are well documented. As a composite sequence bounded by regional unconformity, the Zhujiang Formation (CS1) can be divided into 5 sequences (CS1-1-CS1-5), and the lower CS1-2 and CS1-3 are the main strata that the submarine canyon system developed. The submarine canyon system can be divided into five segments (A-E) by their distinct geomorphology and depositional architectures. From Segment A to E, the channels in the submarine canyon system transform from “V” shaped single channel, “U” shaped single channel, “gull-wing” shaped channel-levee complex, “W” shaped branch channels to mound shaped distal composite channel and front splay deposits. This is accordantly consistent with sedimentary process of the channels transitional from erosional, erosional-aggradational, aggradational and then lateral migrational. The changing interaction of multi-source sediment supply, differential tectonic subsidence and relative sea level change are considered to be the major control on the development and evolution of the submarine canyon system. The differential tectonic subsidence since the Late Oligocene produced the adequate slope angle, sufficient accommodation and distinct basin geomorphology for the development and segmentation of the submarine canyon system. Slope-gully to slope fan deposits fed by the shelf margin delta systems from the Paleo-Pearl River and the northeastern Dongsha Uplift converged into the submarine canyon system, providing abundant sediments for its development. Relative sea level changes, especially the two regional regressions with abundant sediment supply have made great influence on the evolution of the submarine canyon system.

Late Cretaceous palaeogeography of NE Bohemian Massif: diachronous sedimentary successions in the Wleń Graben and Krzeszów Brachysyncline (SW Poland)

This paper provides a new sedimentological and palaeogeographic interpretation of the Late Cretaceous (late Cenomanian to early Coniacian) shallow-marine succession exposed in the Wlen Graben and in the Krzeszow Brachysyncline (NE Bohemian Massif, Sudetic Block). These two tectonic subunits are outliers of the North Sudetic and Intra-Sudetic synclinoriums, respectively, and contain relics of the diachronous sedimentary succession of a seaway linking the Boreal and Tethyan marine provinces during the Late Cretaceous. Results of sedimentological study and facies analysis show that the late Cenomanian sedimentation within this corridor was dominated by strong in situ reworking of the pre-Cretaceous bedrock driven by storm waves and possibly tidal currents. In the latest Cenomanian, siliciclastic sedimentation was followed by the deposition of offshore-transition to offshore muddy calcareous facies in the Krzeszow area, while the deposition of coarse-grained siliciclastic facies continued in the Wlen area. The nearshore clastic belt in the latter area changed into a wider strait dominated by offshore sedimentation in the early Turonian, whereby the interconnected Wlen and Krzeszow passages evolved into a uniform strait of fully-marine sedimentation. In the middle Turonian, the southern part of the strait became progressively filled with coarse-grained siliciclastic material supplied from the east. At the end of the late Turonian and in the early Coniacian, the strait funnelled bi-directional tidal currents along its axis. The youngest Cretaceous strata in the Wlen area comprise erosional relics of the early Coniacian shoreface to offshore-transition deposits. The younger Cretaceous deposits are unpreserved, probably eroded during the post-Santonian(?) tectonic inversion of the Wlen–Krzeszow strait. Despite the relatively small distance between the two relic parts of the strait, the shallow-marine succession reveals distinct diachroneity on a regional scale, reflecting an interplay of eustatic changes, differential tectonic subsidence and clastic sediment supply. This paper presents the first detailed sedimentological logs from the two study areas, with new findings of fossil fauna and flora, proposes a new palaeogeographic interpretation and discusses the influence and development of the source areas for the Cretaceous Sudetic Wlen–Krzeszow marine strait at the NE fringe of the Bohemian Massif.

Sedimentary environments controlled by tectonics and induced differential subsidence: A perspective in the Permian Liangshan and Qixia Formations, northwestern Sichuan Basin, China

Based on field observation, core description and well logging analysis, the tectonic-sedimentary framework of the Liangshan and Qixia Formations in the northwestern Sichuan Basin, China is deeply discussed. Two long-term sequence cycles were identified, denoted as LSC1 and LSC2, respectively. The sequence stratigraphic framework was established, suggesting the Liangshan Formation to be not isochronously deposited. Paleogeomorphy before deposition of LSC1 was reconstructed by the impression method. LSC1 was featured by thin, low-energy shoal deposits in the high topography, and thick inter-shoal sea and open sea deposits in the low topography. Meanwhile, paleogeomorphy before deposition of LSC2 was reconstructed using the residual thickness method, which was demonstrated to have primary high-energy, thick shoal deposits in the high topography, and thin inter-shoal and open sea deposits in the low topography. The results show that differential tectonic subsidence has already taken place during the Qixia Period, and thus the Dongwu Movement should occur earlier than previously expected. Meanwhile, pre-depositional paleogeomorphy has obvious controlling effects on the sequence stratigraphic filling and sedimentary facies distribution. Results of this study were expected to provide practical guidance to fine characterization of the sedimentary evolution process and prediction of high-quality reservoir distribution.

Structural control, seismic expression, and types of reefs of rifting margins in the late Permian Changxing Age, Sichuan Basin, China

A total of 3 intracratonic rifts were developed during the Changxing period in the Sichuan Basin, and organic reefs developed in the periphery areas of the rifts. Outcrops, drilling, and 2D and 3D seismic data covering the entire basin were used to study the structure of the periphery areas of the intracratonic rift formed in the Late Permian Changxing Period, and seismic responses for the organic reefs were also used. The organic reefs in the periphery areas of the rifts can be divided into two categories: accretionary platform margin reefs and progradational platform margin reefs. The external morphology and internal structure of the organic reefs are described. In addition, the organic reef genesis sites in the periphery areas of the rifts in the Changxing period are analyzed for both the tectonic aspects and depositional background. Studies suggest that transfer structures were developed on both sides of the rifts in the Changxing period. The superimposed degree of the fault systems in the equidirectional transitional zones and the equidirectional accommodation zones had important impacts on the topography and structural relief of the rift. The differential tectonic subsidence, the uplifting and subsidence of the sea level, and environmental impacts (monsoon and ocean currents) were major factors controlling the evolution, external morphology, and internal structure of the organic reefs on the periphery areas of the rifts. The synsedimentary activities of the basement rifts controlled the variations in growing spaces and thicknesses of the organic reefs. The structural segmentation effect caused by the transfer structures that developed on both sides of the rifts caused progradation in the margin of the platform. Sea level and environmental changes also affected the time-space migration and internal sedimentary texture of the organic reefs as well as the vertical superposition pattern of the reefs in the Changxing Formation.

Conodont biostratigraphy and depositional history of the Darriwilian (Middle Ordovician) Pygodus serra Zone, western Argentina

The stratigraphic record of the Pygodus serra conodont Zone in the Cuyania terrane of western Argentina is discussed in this contribution. Three well-known sections were sampled in Precordillera and the San Rafael Block. The studied successions are composed mainly by clastic rocks with variable amount of carbonate. The lower part of Ponón Trehué Formation of the San Rafael Block and the La Cantera Formation of the Eastern Precordillera are composed of conglomerate and represent the input of coarse clastic deposits to the Cuyania basin. The Los Azules Formation, in Los Amarillitos section of the Central Precordillera, has a massive sandstone bed with carbonate nodules in a section largely of black shale. Key conodonts recovered from these sections indicate a correlation to the E. robustus and E. lindstroemi subzones of the Pygodus serra Zone of the upper Darriwillian Stage (Middle Ordovician Series). The species in the Ponón Trehué Formation are almost all the same as those in the Precordillera sections. Correlation of the clastic sedimentary successions between the three sections indicates that vertical facies changes were not controlled by eustasy. More likely, they were controlled by differential tectonic subsidence.

Shelf-edge trajectory and sediment dispersal in a lacustrine setting: A case study from Qinnan Depression, Bohai Bay Basin, China

The Oligocene succession of Dongying Formation in Qinnan depression in Bohai Bay Basin has preserved well-imaged lacustrine prograding shelf-edge scale clinoforms, providing a good opportunity to study the relationship between shelf-edge trajectory, stratal stacking patterns and sediment dispersal, and to investigate the effect of controlling factors on sediment dispersal shelf-edge trajectories in a lacustrine setting. 3D seismic volumes were used for defining seismic units, bounding surfaces, shelf-edge trajectories and sediment dispersal. Four seismic units have been subdivided, and four types of shelf-edge trajectories and the associated stratal stacking patterns have been recognized. Greater volume of sandstone in deep-water area tends to develop in seismic units with descending shelf-edge trajectory angles and tends be linked to lower shelf-edge trajectory angles. Differential tectonic subsidence rates along the shelf and along-strike variation in sediment supply were prime factors controlling lateral variability in trajectory, progradation and sediment dispersal. The evolution of shelf-edge trajectory can be subdivided into three stages. Local climate might play a major role in controlling lake-margin evolution during the first two stages, when dry hot weather foster basin floor fan deposition and descending trajectories, and humid cold weather hindered the development of basin floor fan and forms ascending trajectories. While differential tectonic subsidence could have the dominate role during the third stage, when climate signal can hardly be recognized and lateral variability in trajectory angles and sediment supply is most obvious.

The connectivity of reservoir sand bodies in the Liaoxi sag, Bohai Bay basin: Insights from three-dimensional stratigraphic forward modeling

The distribution of sand bodies is complex in small half-graben rift basins influenced by differential tectonic subsidence. The characterization of reservoir sand bodies remains half quantitative because the depositional processes of the sand bodies are poorly understood and quantitatively investigated. Based on wells and seismic data, a three-dimensional stratigraphic forward modeling is conducted to quantitatively characterize the depositional evolution of reservoir stratigraphy and to analyze the connectivity of sand bodies as well as its controlling elements in Liaoxi Sag, Bohai Bay Basin. We integrate Controlling factors for sequence development including initial bathymetry of basement, subsidence, lake level change, sediment supply, and sediment transport Dionisos program to establish a 3D reservoir-sequence model for the middle part of the third member of Eocene Shahejie Formation. The model matches the wells data of the oilfield, such as lithologies, gamma ray, and resistivity, making it credible to further predict sandstone distribution. The sedimentary process of reservoir sand bodies appear four stages. In stage 1, two prototypes of delta-front sand bodies developed in the northwest. In stage 2, both the western delta-front sand bodies and eastern slope-fan sand bodies were widely developed. In stage 3, the delta-front sand bodies developed in large scale, whereas the slope fans were smaller. In stage 4, only a slope-fan sand body developed in the southeast, whereas lacustrine shale dominated in the rift basin.During the depositional evolution process, the western sand bodies were not connected to the eastern sand bodies. This disconnection is a function of source supply and sustained slope sinking caused by basin differential subsidence. We tested the interplay of tectonic subsidence and sediment supply through the stratigraphic modeling. Under the conditions of bilateral source supply, the limited number of sand sediments from western delta front could not be transported to the eastern structure high. Nevertheless, under assumptions of one source supply and strong water flow, the plentiful sand sediments could be carried onto the eastern structurally elevated position. Therefore basin differential subsidence and bilateral supply of source are the two controlling factors for the sand distribution in small half grabens. In addition, we found an interesting phenomenon that sand ratio in rifted basin center increases with subsidence decreasing. However, it will be a constant (upper limit) when subsidence declines to a threshold value. The threshold of “subsidence slope” (vertical subsidence divided by length of slope) varied with climate and sedimentary environment. In warm climate and delta environment, the threshold value is 2.3°.

Morphologies and depositional/erosional controls on evolution of Pliocene-Pleistocene carbonate platforms: Northern Carnarvon Basin, Northwest Shelf of Australia

The detailed morphologies, evolution and termination of latest Neogene tropical carbonate platforms in the Northern Carnarvon Basin (NCB), on the passive margin of the Northwest Shelf (NWS) of Australia, defined based upon mapping using 3D seismic images, reveal the history of local/regional oceanographic processes, fluctuations in relative sea-level and changing climate. Cool-water carbonate deposition, dominant during the early-middle Miocene, was followed by a siliciclastic influx, which prograded across the NWS beginning in the late-middle Miocene, during a period of long-term global sea-level fall. The resulting prograding clinoform sets, interpreted as delta lobes, created relict topographic highs following Pliocene termination of the siliciclastic influx. These highs constituted multiple favorable shallow-water environments for subsequent photozoan carbonate production. Resultant platform carbonate development, in addition to being a response to cessation of siliciclastic influx and the existence of suitable shallow-water substrate, was also influenced by development of the warm-water Leeuwin Current (LC), flowing southwestward along this margin.Four flat-topped platforms are mapped; each platform top is a sequence boundary defined by reflection onlap above and truncation below. Successive platforms migrated southwestward through time, along margin strike. All platforms exhibit predominantly progradational seismic geometries. Mapped tops are ≥10km wide. Seismic evidence of karst on three of four platform tops, e.g., v-shaped troughs up to 50m deep and ~1km wide, and broader basins with areas up to 20km2, suggests episodic subaerial exposure that may have contributed to platform demise. Platform 4, the most recent, is unique in having interpreted biohermal build-ups superimposed on the progradational platform base. The base of these interpreted patch reefs now lies at a water depth of ~153m; therefore, we suggest that these reefs developed post-LGM (~21Ka); Subsequent reef drowning occurred, possibly as a result of accelerated subsidence and turbidity associated with formation of adjacent sediment drifts and fluctuations in strength of the LC during the late Pleistocene. Progressive drowning and termination of platforms from northeast to southwest along strike may have resulted from unknown combinations off climate factors, supplemented by differential compaction of the deltaic substrate and/or differential tectonic subsidence caused by mantle induced downwelling.

Fluvial Facies Architecture and Sequence Stratigraphy of the Tertiary Duchesne River Formation, Uinta Basin, Utah, U.S.A.

Continental sequence stratigraphy in upstream fluvial environments can be complex due to the interplay of source tectonics, local and global climate change, and topography. The Tertiary Duchesne River Formation, Utah, U.S.A., represents the last stage of intermontane basin fill in the Uinta Basin. It is surrounded by sediment source mountain ranges of the Uinta Mountains to the north and the Sevier Fold-Thrust Belt (FTB) to the west. Excellent basin-scale exposure of the Duchesne River Formation allows the vertical and lateral characterization of facies architectures in the upstream environments. The four members of the Duchesne River Formation are distinctive lithological units. The lower three members comprise a typical upward-fining unconformity-bounded fluvial sequence from the basal coarse-grained unit into the overlying fine-grained units. The fourth uppermost coarse-grained member records the onset of another upward-fining cycle. The sequence stratigraphy of these unconformity-bounded upward-fining cycle(s) was primarily tectonics-driven due to uplift of the Uinta Mountains. The Duchesne River Formation records a distinct change in fluvial–lacustrine styles between the western and eastern part of the basin, demonstrating the variable allogenic controls of tectonic subsidence and discharge related to the local climate and source-terrain input within the basin. Specifically, the western degradational fluvial system with a high net-sandstone-to-gross-thickness ratio of the basal member was controlled by high discharge due to a wet climate and two source-terrain inputs (i.e., Uinta Mountains and Sevier Fold-Thrust Belt), whereas the eastern aggradational fluvial system with a low net-sandstone-to-gross-thickness ratio was controlled by low discharge due to a dry climate and single source terrain input (i.e., Uinta Mountains). The development of local lacustrine environments on the alluvial plain of the third member, present in the western Uinta Basin, was controlled by differential tectonic subsidence in the basin. The Duchesne River Formation preserves a valuable example of an upstream fluvial sequence, and demonstrates how internal facies architectures at the basin scale were dictated by allogenic controls.

Along-strike sequence stratigraphy across the Cretaceous shallow marine to coastal-plain transition, Wasatch Plateau, Utah, U.S.A.

Current sequence stratigraphic models deal with stratal packages almost exclusively along depositional-dip. A high-resolution outcrop example detailing sequence stratigraphic architecture along depositional-strike is largely lacking. This outcrop study provides a detailed sequence stratigraphic correlation along depositional-strike for a length of ~50km. Moreover, this study focuses on the facies and stratigraphic relationships particularly at the very transition between coeval marine and nonmarine strata, which is relatively underdocumented.Within the Mesaverde Group of central and eastern Utah, the Upper Cretaceous shallow-marine Star Point Sandstone and paralic Blackhawk Formation are relatively well studied along depositional-dip in the Book Cliffs, but the nature of their spatio-temporal transition particularly along depositional-strike in the NNE-SSW trending Wasatch Plateau remains poorly documented. Facies-scale to stratigraphic-scale data were gathered from ten outcrop “windows” along the eastern margin of the Wasatch Plateau. The vertical and lateral transition between marine and nonmarine strata varies in complexity within the study area. In the southern part of the study area near the central Wasatch Plateau, the shallow-marine Star Point Sandstone passes stratigraphically upward into the nonmarine Blackhawk Formation in one simple transition without any intercalation of marine and nonmarine strata. In the northern part of the study area, however, where the Star Point Sandstone to Blackhawk Formation transition is complex, aggradationally stacked shallow-marine sandbodies (i.e., parasequences) taper and completely pinch-out within coastal-plain mudstones in a paleo-landward direction over short distances (<1km), representing potential stratigraphic traps. These marine sandbodies are intercalated with coeval nonmarine strata within the Blackhawk Formation. Fourth-order sequence boundaries and flooding surfaces were correlated along the lower Campanian depositional-strike.The results reveal along-strike undulations in the early Campanian depositional topography, which was characterized by alternating incised valleys, interfluvial topographic highs with mature soil development, and shoreline embayments. In particular, the variable along-strike transition-complexity from shallow-marine to coastal-plain strata is the result of differential tectonic subsidence, where the transition-complexity increases and the stacking pattern is more aggradational towards an early Campanian depocenter in the north of the study area. This study reveals that differential subsidence is the key in controlling along-strike variability of sequence stratigraphy and that correlation of sequence stratigraphic surfaces are far more challenging in nonmarine strata than in adjacent marine strata.

Drowning of the Triassic Yangtze Platform, South China, By Tectonic Subsidence Into Toxic Deep Waters of An Anoxic Basin

Abstract The Yangtze Platform drowned and was buried by pelagic facies and siliciclastic turbidites in western Guizhou Province during the Late Triassic, Carnian. The uppermost platform facies of the Ladinian Yangliujing Fm. consists of peritidal cyclic carbonate. Ladinian margin facies of the Longtou Fm. consist of grainstone and lenses of coral–Tubiphytes algal boundstone indicating high-energy shoals and patch reefs. The drowning horizon is laterally variable; it is either a sharp surface or a gradational shift to dark, nodular-bedded lime mudstone and wackestone of the Zhuganpo Fm. The contact lacks phosphatized or glauconitized hardgrounds that would indicate drowning by excess nutrient flux. Uppermost platform carbonates have a tropical photozoan biota and lack siliciclastic content indicating neither climate cooling nor siliciclastic flux played a role in drowning. The Zhuganpo Fm. contains a dominantly pelagic biota of ammonoids, planktonic bivalves, pseudoplanktonic crinoids, conodonts, and the marine reptile Keichousaurus. Rare bioturbation and benthic biota in the lower part indicate dysoxic conditions with an upward shift to anoxic conditions. The overlying Wayao Fm. is a pyritic, laminated, black argillaceous lime mudstone that grades to black shale with an exclusively pelagic fauna and marine reptile lagerstattes indicating anoxic deep-marine sedimentation. Syndepositional faults played a significant role in the evolution of the western sector of the Yangtze Platform and controlled three local, backstepped accommodation cycles in the Zhenfeng area. Faults that developed during the last accommodation cycle tip out at the drowning horizon and include a flower structure upon which a pinnacle reef developed while the rest of the platform drowned. Lateral variability in the drowning horizon and thickness of the post-drowning pelagic facies suggest that differential tectonic subsidence caused the platform to sink into deep water along faults. Magnetic susceptibility and paleomagnetic-reversal correlations demonstrate that the western sector of the platform drowned while shallow-marine, mixed carbonate siliciclastic sedimentation continued in the eastern sector which later terminated in shallow water by increasing siliciclastic flux. Starved black shale horizons in the basin indicate recurrent bottom-water anoxia. Elevated trace-metal concentrations in the pelagic facies of the Zhuganpo and Wayao formations indicate dysoxic to anoxic conditions and enhanced preservation of organic matter. Tectonic subsidence likely submerged the western sector into deep, toxic waters of the basin, killing benthic carbonate production. Previous studies have implicated the Carnian pluvial event as a cause of drowning of the Yangtze Platform and the shift to siliciclastic turbidites of the Laishike Fm., but the Yangtze Platform drowned earlier than western Tethys platforms and siliciclastics were already rapidly infilling the basin in the Anisian and Ladinian, indicating that the events are probably unrelated. Siliciclastics of the Laishike Fm. that overlapped and buried the platform only after the basin was filled are better interpreted as a response to tectonics than to a shift in climate.

Tectono-stratigraphy of Lower Cretaceous Tanan sub-basin, Tamtsag Basin, Mongolia: Sequence architecture, depositional systems and controls on sediment infill

Tanan sub-basin is an active-fault bounded basin. The spatial distribution and temporal evolution of depositional systems were significantly influenced by tectonics. Fault movement and stages of basin development controlled the subsidence rates and the potential for erosion and the rate of sediment supply. Distinct stages of rift evolution during the early Cretaceous can be recognized, namely the early syn-rift, rift climax and late syn-rift stages. Three types of lacustrine sequence, consisting of distinctive depositional systems, are distinguished: (1) the early syn-rift sequences (SQ1 + SQ2), which are composed mainly of fan delta and shallow lacustrine depositional systems; (2) the rift climax sequences (SQ3) which developed in response to rapid and differential tectonic subsidence rates, and consist of fan delta, deep lacustrine and sublacustrine fan depositional systems; and (3) the late syn-rift sequences (SQ4) which are comprised of braided-delta and shallow lacustrine depositional systems. Each of the three lacustrine sequence architectures stands for a particular stage of basin fill and reflects variable rates of basin subsidence. Within each sequence, depositional systems and their stacking patterns are interpreted to have been a function of the interaction between tectonics and sediment supply. Differential subsidence across the basin, related to rotation of fault blocks, resulted in the formation of distinct paleomorphologies in different structural settings. These settings were fault-scarp zones controlling the development of fan-deltas, fault-terrace zones controlling the development of fan-delta and sublacustrine fans, half-graben dip-slope zones controlling the development of braided river and braided deltas, and intra-basinal fault-break zones controlling the development of sublacustrine fans. During the late syn-rift stage, active tectonism, displacement on the boundary faults had ceased. At this stage the depositional systems and their stacking patterns were dominantly related to the sediment supply rates, and not to tectonic activity.

Marine sediment records and relative sea level change during late Pleistocene in the Changjiang delta area and adjacent continental shelf

Optically stimulated luminescence (OSL) dating and microfossil analysis were conducted on late Quaternary samples from two boreholes in the Changjiang delta area. Successions from 12 collected boreholes revealed lithology, stratigraphy and sedimentary facies of the Changjiang delta area and continental shelf of the East China Sea (ECS). These data were used to determine the chronology of two transgressions during marine isotope stage (MIS) 5 and MIS3, interpret the sedimentary facies and mechanisms of the distinctive transgression pattern, and reconstruct relative sea level of the study area during late MIS3. Fluvial gravelly sediment deposition prevailed with limited marine influence during MIS5, while muddy and sandy deposition occurs along with widespread coastal facies during mid- and late MIS3 in the Changjiang delta area. This suggests that the study area was dominated by high relief and local rivers developed with large freshwater and sediment discharge during MIS5. Therefore, marine micro-faunas may not have survived due to dominant freshwater process or strong erosion has scoured away the marine records of MIS5. Tectonic subsidence is indicated to occur at a rate of ∼1 m/ka since MIS5 and lower the Changjiang coastal area to be invaded by seawater during MIS3. The altitude of late MIS3 transgressive sediments decreases from the inner delta plain seaward gradually, reflecting differential tectonic subsidence of the study area. Abrupt sea level rise occurred, and relative sea level is proposed as −5 m during late MIS3.

Graben width controlling syn-rift sedimentation: the Palaeogene southern Upper Rhine Graben as an example

Eocene to Early Oligocene syn-rift deposits of the southern Upper Rhine Graben (URG) accumulated in restricted environments. Sedimentation was controlled by local clastic supply from the graben flanks, as well as by strong intra-basinal variations in accommodation space due to differential tectonic subsidence, that in turn led to pronounced lateral variations in depositional environment. Three large-scale cycles of intensified evaporite sedimentation were interrupted by temporary changes towards brackish or freshwater conditions. They form three major base level cycles that can be traced throughout the basin, each of them representing a stratigraphic sub-unit. A relatively constant amount of horizontal extension (ΔL) in the range of 4–5 km has been estimated for the URG from numerous cross-sections. The width of the rift (Lf), however, varies between 35 and more than 60 km, resulting in a variable crustal stretching factor between the bounding masterfaults. Apart from block tilting, tectonic subsidence was, therefore, largely controlled by changes in the initial rift width (L0). The along-strike variations of the graben width are responsible for the development of a deep, trough-like evaporite basin (Potash Basin) in the narrowest part of the southern URG, adjacent to shallow areas in the wider parts of the rift such as the Colmar Swell in the north and the Rhine Bresse Transfer Zone that delimits the URG to the south. Under a constant amount of extension, the along-strike variation in rift width is the principal factor controlling depo-centre development in extensional basins.

Impact of differential tectonic subsidence on isolated carbonate-platform evolution: Triassic of the Nanpanjiang Basin, south China

The Nanpanjiang Basin of south China contains four exceptionally well-exposed, isolated Triassic carbonate platforms. Detailed mapping of two-dimensional transects and description of stratigraphic sections allow the reconstruction of facies architecture, sequence stratigraphy, and evolution of the platforms. Biostratigraphy, magnetic-susceptibility profiles, and volcanic-ash horizons allow chronostratigraphic correlation and, thus, a basinwide evaluation of mechanisms controlling platform evolution. A comparison of platform architecture demonstrates that southerly platforms have substantially greater thickness, backstepping geometry, pinnacle development, and earlier drowning that resulted from greater tectonic subsidence proximal to a probable convergent margin along the southern perimeter of the basin. Felsic volcanics thicken southward and contributed to the termination of the southernmost platform, indicating the development of a volcanic arc along the southern margin of the South China tectonic block. The northernmost isolated platform had greater longevity and lesser accumulation and lacks backstepping and pinnacle phases of development. Basin-margin intertonguing relationships, or lack thereof, demonstrate that earlier siliciclastic influx into the basin to the south and concurrent starved-basin conditions to the north impacted the evolution of platform-margin geometries. Comparative analysis of platform evolution shows that the timing and rates of tectonic subsidence controlled the timing of platform termination by drowning, backstep geometries, pinnacle development, and overall platform thickness. The timing of siliciclastic basin fill dictated differences in platform-margin geometries such as slope angle, relief above basin floor, and the presence or absence of basinward platform progradation. Despite the dramatic differences in platform architecture, eustatic sea level fluctuations imparted a basinwide sequence-stratigraphic signal.

Synsedimentary faults and amalgamated unconformities: Insights from 3D-seismic and core analysis of the Lower Triassic Middle Buntsandstein, Ems Trough, north-western Germany

The Late Permian/Early Triassic succession of the Central European Basin (CEB) was repeatedly affected by the tectonic pulses associated with the earliest phases of Tethyan and Arctic–North Atlantic rifting. Effects of the differential tectonic subsidence are particularly well recorded by unconformities, which form widespread sequence boundaries. Such unconformities are most obvious in areas occupied by fault-controlled intra-basinal highs (swells). In that areas, stratigraphic loss may comprise entire Lower and Middle Buntsandstein formations and in places remnant Middle Buntsandstein successions directly rest on Permian strata. Analysis of 3D-seismic data and well logs combined with high-resolution sedimentological logging of drillcores at the western margin of the Ems Trough (NW Germany) reveals details of synsedimentary tectonic control on sequence development. Early Triassic extensional faulting of basement blocks provided stepwise addition of accommodation space for continental sequences by growth faulting along north–south oriented fault zones blocks on the flanks of the East Netherlands High. This process is most evident during the development of the Hardegsen Unconformity, which is characterised by an amalgamation of succeeding unconformity surfaces in areas of structurally controlled intrabasinal highs.

Plate-boundary earthquakes and tsunamis of the past 5500 yr, Sixes River estuary, southern Oregon

Eleven plate-boundary earthquakes over the past 5500 yr have left a stratigraphic signature in coastal wetland sediments at the lower Sixes River valley in south coastal Oregon. Within a 1.8 km2 abandoned meander valley, 10 buried wetland soils record gradual and abrupt relative sea-level changes back in time to ;6000 yr ago. An additional, youngest buried soil at the mouth of the Sixes River subsided during the A.D. 1700 Cascadia earthquake. Multiple lines of evidence indicate that tectonic subsidence caused soil burial, including permanent relative sea-level rise following burial, lateral continuity of buried soil horizons over hundreds of meters, diatom assemblages showing that sea level rose abruptly at least 0.5 m, and sand deposits on top of buried soils demonstrating coincidence of coseismic subsidence and tsunami inundation. For at least two of the buried soils, liquefaction of sediment accompanied subsidence. The 11 soil-burial events took place between 300 and ;5400 yr ago, yielding an average recurrence interval of plateboundary earthquakes of ;510 yr. Comparing paleoseismic sites in southern Washington and south coastal Oregon with the Sixes River site for the past 3500 yr indicates that the number and timing of recorded plate-boundary earthquakes are not the same at all sites. In particular, a Sixes earthquake at ;2000 yr ago lacks a likely correlative in southern Washington. Therefore, unlike the A.D. 1700 Cascadia earthquake, some Cascadia plate-boundary earthquakes do not rupture the entire subduction zone from southern Oregon to southern Washington. In the lower Sixes River valley, the upperplate Cape Blanco anticline deforms sediment of late Pleistocene and Holocene age directly above the subduction zone. Differential tectonic subsidence occurred during two of the plate-boundary earthquakes when a blind, upper-plate reverse fault, for which the Cape Blanco anticline is the surface fold, slipped coseismically with rupture of the plate boundary. During these two earthquakes, sites ;2 km from the anticline axis subsided ;0.5 m more than sites ;1 km from the axis.

Neoproterozoic Chuar Group (∼800–742Ma), Grand Canyon: a record of cyclic marine deposition during global cooling and supercontinent rifting

Abstract Chuar Group sediments were deposited in a marine cratonic basin synchronously with marine deposition in other basins of western North America and Australia. The top of the Chuar Group is 742 Ma; thus, this succession offers a potentially important record of global climatic and tectonic changes during the mid-Neoproterozoic — a critical time leading to possible global glaciation during supercontinent rifting. Chuar Group cycles and sequences may record glacioeustatic fluctuations during the climatic transition into the Sturtian Ice Age (∼750–700 Ma), as well as extension related to the dispersal of Rodinia. The mid-Neoproterozoic Chuar Group (1600 m thick) is predominantly composed of mudrock with subordinate meter-scale beds of dolomite and sandstone. Facies analysis leads to the interpretation of a wave- and tidal-influenced marine depositional system. Diagnostic marine features include: (1) marine fossils and high local pyrite content in the mudrock facies; (2) mudcracked mud-draped symmetric ripples and reverse flow indicators in the sandstone facies; (3) facies associations between all facies; and (4) no unequivocal terrestrial deposits. Chuar facies stack into ∼320 dolomite- and sandstone-capped meter-scale cycles (1–20 m thick) and non-cyclic intervals of uniform mudrock facies (20–150 m thick). Nearly all cycles have mudrock bases indicating subtidal water depths. Dolomite-capped cycles shallow to peritidal environments (peritidal cycles), some indicating subaerial exposure of varying degrees (exposure cycles). Sandstone-capped cycles shallow to peritidal environments with no evidence of prolonged exposure (peritidal cycles). Non-cyclic intervals represent the deepest water depths (∼10s of meters). Peritidal cycles and non-cyclic intervals dominate the lower and middle Chuar Group. In the upper Chuar Group, the percentage of exposure cycles increases, as does the thickness of individual cycles and non-cyclic intervals, and the degree of subaerial exposure in the cycle caps. Correlation of cycles across 10 km shows that many cycles are laterally continuous and some vary in thickness. Some thickness changes are coincident with local and regional extensional structures. These cycle trends reflect a resolvable mixture of tectonic and climatic controls. Cycles are interpreted to be high frequency and glacioeustatically-controlled based on comparison of cycle character and thickness to Phanerozoic examples, and lateral continuity of many cycles, respectively. Cycles in the lower and middle Chuar Group show characteristics suggestive of low-amplitude sea-level changes similar to meter-scale cycles from global greenhouse climates in the Phanerozoic. Upper Chuar Group cycles show characteristics suggestive of moderate-amplitude sea-level changes similar to Phanerozoic greenhouse–icehouse transition climates, indicating an increase in continental ice volume. Variability in cycle thickness and lateral pinching of some cycle caps is controlled by variable tectonic subsidence, evidenced by thickening trends coincident with active rift-related structures, and also by variability in sediment supply. The presence of dolomite caps, in this otherwise siliciclastic succession, is interpreted to be due largely to rapid changes in climate related to short-term glacioeustatically-controlled sea-level changes. Four crude lithostratigraphic sequences (150–775 m thick) are defined based on dolomite-poor to dolomite-rich stratigraphic intervals. Long-term changes in cycle cap-lithology may be driven by long-term changes in climate, similar to short-term glacioeustatic controls on meter-scale cycle cap lithology. Large-scale thickness variability in these sequences is due to local differential tectonic subsidence.

Seismic Evaluation of Differential Tectonic Subsidence, Compaction, and Loading in an Interior Basin

Quantitative analysis of depth-converted reflection times defines long-term differential motion across individual structures in a central Appalachian interior basin known as the Rome trough. Differential motion decreases exponentially with time. Rotation about a hinge defining the trough's west margin reached approximately 37% of total displacement in about 63-78 million years (m.y.). Displacement across the trough's faulted east margin occurred more rapidly and reached 37% of the total in 13-51 m.y. A major fault in the interior of the trough developed rapidly with 37% of total displacement reached in from 16 to 23 m.y. Longer term rotation across the west margin may be due to its participation in the overall subsidence of the craton during the Paleozoic. The time spanned by the formation of the East-Margin and Interior faults was restricted to the Cambrian in the northern part of the area, but to the south, movement along the East-Margin fault continued through the Middle Ordovician.

Peat accumulation on a drowned coastal braidplain: the Mullins Coal (Upper Carboniferous), Sydney Basin, Nova Scotia

The Mullins Coal lies within the braided-fluvial South Bar Formation. The lateral extent (15 km) and thickness (2 m) of the coal suggest that controls for peat accumulation were allogenic. Marine inundation of the distal braidplain, as indicated by the high-sulphur content of the coal and the occurrence of agglutinated foraminifera in associated shales, caused ponding of freshwater near maximum marine transgression, with accumulation of thick paralic peat across a stable platform of sandy fluvial sediments. Highstand parasequences with thin capping coals were subsequently incised beneath a sequence boundary as the braidplain readvanced. The Mullins Coal thins and splits in association with muddy bayfill deposits eastward towards the Glace Bay Syncline, a long-lived palaeotopographic element related to fault-bounded basement blocks. Although relative sea-level change controlled the stratigraphic position of the precursor peats, differential tectonic subsidence and/or compaction modified their extent, thickness and quality.