Regional Unconformity Research Articles

Paleogeographic evolution and vertical motion of the central Lesser Antilles forearc since the Early Miocene: A potential driver for land fauna dispersals between the americas

Phylogenetic studies of present-day terrestrial organisms suggest that faunal dispersals between South America and the Greater Antilles may have occurred during the Cenozoic through the Lesser Antilles. However, because of the lack of geological data to unravel the areas that may have emerged along the Lesser Antilles trench, the migration paths used by their ancestors remain unknown. Here, we present novel paleogeographic maps of the central Lesser Antilles (extending from Guadeloupe to Martinique islands) which are built on the basis of onshore and offshore stratigraphic correlations (50 seismic lines, biostratigraphy of 9 dredged and 29 field samples, six sedimentary logs). We find that repetitive episodes of uplift and drowning have occurred in the central part of the Lesser Antilles during the Neogene. Offshore, the Marie-Galante Basin comprises three sedimentary megasequences that deposited between: (i) the Oligocene and Early Miocene, including the extinct arc, (ii) the Middle and Late Miocene and (iii) the latest Miocene and Holocene. These sediments infill a NNW-SEE trending forearc rift that opened during the Early Miocene. The megasequences are separated by subaerial regional unconformities that affect the rift shoulders. Onshore, we show that the lower part of the carbonate platform in Guadeloupe and La Désirade has deposited during the late Messinian. In Martinique, we refine the age of the carbonate deposits belonging to the extinct arc to the Chattian-Burdigalian, and evidence a major subaerial unconformity corresponding to the Middle Miocene. We propose that between Anguilla and Martinique, from north to south, large archipelagos, which are now drowned, have existed during the early Middle Miocene and the latest Miocene. We suggest that during the Miocene, the Lesser Antilles may have been used as a pathway for land-faunal dispersals from South America.

A Discussion on the relationship between prominent unconformities on the SCS shelf margins and the end of seafloor spreading in the South China Sea

In an effort to reconcile different data sources (seafloor spreading, seismic and gravity images, well calibrations, outcrop studies) in the South China Sea (SCS), we reviewed the unconformity records, in particular in the context with the shut-off of seafloor spreading in the SCS. With respect to the start of spreading, there is a consensus: ca. 34-41 Ma. Recent data infer an end of spreading, near to the magnetic Anomaly 5 = ca. 15.5 Ma (Langhian age). In Northwest Borneo, it is suggested that this event is coeval with the Deep Regional Unconformity (DRU) in Sabah and Brunei, and the Mid-Miocene Unconformity (MMU) in Sarawak. The MMU is also recognized in offshore Vietnam and Palawan, on the Western and Southeastern margins of the SCS, respectively. The MMU/DRU may constitute the border between active margin and passive margin deposits within the marine SCS sub-basins. The progradation of clastic shelves post-MMU/DRU may have hampered growth of bioherms whilst creating prolific sandstone reservoir sequences offshore Northwest Borneo and Vietnam.

Paleoenvironments and stratigraphy of the Valanginian–Hauterivian carbonates of the Arabian Platform: Implications for the recognition of the Weissert Event on the Southern Tethys

On the Arabian Platform, the Valanginian Weissert Event is associated with the sudden drowning of a major north-easterly prograding carbonate platform located on the southern margin of the Tethys Ocean. This was associated with a sharp back-stepping of the main facies belts to the south-west, bringing shallow marine carbonates into the previously restricted platform top. Integrated detailed facies analysis, biostratigraphy (including nannofossil and palynology) and carbon isotopes have been used to reconstruct the evolution of the carbonate shelf during the Valanginian–Hauterivian in Saudi Arabia. During the early Valanginian the study area was dominated by the deposition of dolomitized clay-prone microbial intertidal mudstones, which passed basinwards into low-energy inner ramp limestones. These facies encompass the initial stage of a 3rd order transgressive trend above the regional Lower Valanginian Unconformity. They are sharply overlain by Lithocodium/Bacinella-dominated floatstones, indicating the maximum flooding of the platform during the late Valanginian. These deposits systematically increase in thickness and abundance towards the platform margin and are associated with a broad positive excursion in δ13C (c.+2‰ V-PDB), corresponding to the Weissert Event. The presence of abundant Lithocodium/Bacinella in strata deposited during the major flooding of the platform is consistent with higher nutrient levels and increased upwelling during the late Valanginian. The Valanginian–Hauterivian boundary is marked by a regional unconformity identified through palynology and benthic foraminifera assemblages, and locally confirmed by carbon isotopes. The combination of carbon isotope data with detailed palynology and micropaleontology provided a stratigraphic resolution in line with ammonite biozones published in the western Tethys. The Valanginian–Hauterivian continental palynoflora, described for the first time on the Arabian Platform, is dominated by primitive conifer pollen such as Classopollis suggesting a rather arid climate in agreement with published reconstructions. In addition, the occurrence of various fern and bryophyte spores suggests an increase in humidity along coastline areas, associated with higher continental input into inner ramp settings, therefore affecting the carbonate factory.

The Late Cretaceous tectono-sedimentary evolution of northern Poland – A seismic perspective on the role of transverse and axial depositional systems during basin inversion

This paper describes a new tectono-sedimentary model of the Late Cretaceous evolution of the northern, peripheral part of the Polish Basin, strongly influenced by inversion tectonics and fed by transverse and parallel depositional systems. The study area is located in northern Poland, in the boundary zone between the southeastern marginal part of the East European Craton and the northeastern margin of the Mid-Polish Anticlinorium, a regional inversion structure that runs northwest-southeast across the entirety of Poland. Traditionally, a “layer-cake” geological model, mainly based on borehole data, has been employed to characterize the regional depositional architecture of the Upper Cretaceous succession. Our new interpretation is based on regional geo-seismic transects, including the unique, regional high-resolution PolandSPAN™ seismic survey, calibrated by deep wells. The upper Albian – Upper Cretaceous succession was subdivided into five regional seismic stratigraphic units. Within these units, seismic facies and corresponding lithofacies representing siliciclastic and carbonate-siliceous depositional systems are described. The relative role of transverse and parallel depositional systems, including bottom currents, was recognized. Bottom current activity was documented by numerous, previously unknown seismostratigraphic features such as changes in lateral thickness, unconformities, and incisions. Two recognized contour current systems (N–S and NW–SE) operated along the edges of inversion structures that were uplifted in the Late Cretaceous – Palaeogene. A very significant rearrangement of this part of the Polish Basin, highlighted by a regional unconformity mappable over the entire study area, took place in the Campanian – Maastrichtian.

Catastrophic craton destruction via wholesale lithosphere delamination

Abstract The nuclei of continents, manifested as cratons, are the most long-lived parts of Earth’s lithosphere. However, ancient cratons in some areas can be substantially destroyed through mechanisms that are not fully understood. We used experimentally calibrated geobarometers to calculate the equilibrium pressures of mafic magmas in the North China craton, which directly constrain the evolving depth of the lithosphere-asthenosphere boundary beneath the craton through time. We show that the lithospheric thickness of the eastern part of the craton decreased from ~200 km to ~35 km in the Early Cretaceous. This intense destruction took place within a short time interval of ~10 m.y., at least locally. Following this destruction, the lithosphere gradually rethickened and stabilized as the upwelling asthenosphere cooled and formed a juvenile lithosphere. We suggest that this catastrophic lithosphere thinning resulted from wholesale lithosphere delamination. As a consequence of this catastrophic loss of thick mantle roots, the eastern part of the North China craton may have undergone rapid crustal rebound and surface uplift, as recorded by the regional unconformities formed between 130 and 120 Ma in the destructed area.

Hypogenic origin of paleocaves in the Ordovician carbonates of the southern Tahe oilfield, Tarim basin, northwest China

The Lower-Middle Ordovician paleocaves at the depths of over 6000 m in the southern Tahe Oilfield have been important objectives of petroleum production in the Tarim Basin. 3D seismic data, drilling well data, image logs, and cores were integrated to delineate the development characteristics, spatial distribution, and morphology of paleocave reservoirs in the Tahe Oilfield. The petrographic, geochemical features and fluid inclusion measurements of the cave and fracture-filling calcite in the Tahe Oilfield were subsequently performed. The formation mechanism of paleocaves in the southern Tahe Oilfield was revealed through the comparative analysis of paleocaves in the north and south of the Tahe Oilfield. The paleocaves in the northern and southern Tahe Oilfield are extensively distributed 0–150 m and 160–350 m away from the regional unconformity, respectively. Different from the diverse paleocave morphologies including single cave passages, anastomotic mazes, and branchwork passages in the northern Tahe Oilfield, the paleocaves in the southern Tahe Oilfield are dominated by isolated chambers and rising shafts. Additionally, almost all the cave and fracture-filling calcite cements in the northern Tahe Oilfield coexist with gray-green sand and clay. While, the cave-filling calcite cements are relatively clean, without any terrigenous minerals. Furthermore, the calcite samples in the northern Tahe Oilfield show δ13C and δ18O values varying from −5.90‰ to −0.39‰, and from −13.47‰ to −7.23‰, respectively. The calcite cements in the southern Tahe Oilfield present similar δ13C values with the Ordovician limestone samples and relatively more negative δ18O values in a range between −15.02‰ and −12.29‰. Compared with the calcite samples in the norther Tahe Oilfield, the cave-filling calcite in the southern Tahe Oilfield shows a narrower range of 87Sr/86Sr ratios varying from 0.709342 to 0.709689. In particular, large amount of two-phase aqueous inclusions observed in the calcite from the southern Tahe Oilfield display relatively higher homogenization temperatures ranging from 90.8 °C to 205.9 °C. These significant differences on the paleocaves between the north and south of the Tahe Oilfield suggest that the paleocaves in the south are overprinted by both gravity-driven deep circulating meteoric water during the early Hercynian and the ascending thermal water possibly occurring from the second episode of the middle Caledonian to the early Hercynian. The paleocaves in the southern Tahe Oilfield are interpreted to be generated by hypogene karst under the constraints of strike-slip fault system and lithostratigraphic sequence in the Ordovician in confined settings. This study is an important supplement to the previous Ordovician meteoric karst model in unconfined zones of the Tahe Oilfield. It indicates that the stratabound isolated paleocaves developed in the lower submember of the Upper Yingshan Formation in the southern Tahe Oilfield have significant exploration potentials.

Sedimentary architecture and glacial hydrodynamic significance of the stratified Oak Ridges Moraine, southern Ontario, Canada

High‐quality subsurface data provide new insights into the formation of Oak Ridges Moraine (ORM), an ~80 km3 sequence of stratified meltwater deposits resting >200 m above adjacent Lake Ontario. The ORM sedimentary succession comprises a three‐part regional architecture: (i) ~north–south channel sand–gravel; (ii) channel‐capping rhythmites; and (iii) east–west ridge sediments. The ORM depositional sequence overlies a regional unconformity with a cross‐cutting channel network resulting from ~north–south meltwater floods that transitioned progressively (falling stage) from a ~NNE to ENE flow direction (parallels Lake Ontario depression). Seismic profiles delineate the channels and channel fill characteristics of bank‐to‐bank channel sedimentation of thick gradational gravel–sand–mud sequences. Channel‐capping mud (~100–236 rhythmites) within multiple channels beneath the ORM landform mark a widespread interval of low‐energy, seasonally controlled subglacial pond deposition. During this quiescent period ice‐sheet thickness adjusted to flood‐induced stretching/thinning and re‐profiled slopes. New ice gradients led to east–west flow and deposition of the overlying third element, a sequence of high‐energy confined esker–fan sediments along ORM ridge. Close, sequential timing (~329 varve years) of channel, basin and ridge‐forming architectural elements supports naming this assemblage the ORM formation. Proposed ORM floods are analogous to Icelandic jökulhlaups based on the size, geometry and sedimentology. The observed rhythmite interval between flood events represents a short period (~236 years) of regional meltwater storage prior to east–west ORM flooding. The ORM channel and overlying esker‐fan sediment ridge represent two closely timed meltwater drainage events rather than formation by coalescing ice streams. The scale and timing of the ORM flood events are linked to rapid sea‐level rise, ~13.5 ka BP. This high‐resolution ORM sedimentological record may provide insights into depositional and glaciogenic controls of other large, stratified moraines. The ORM data indicate deposition in response to hydrodynamic events (outbreak floods, re‐profiled ice) rather than direct climate forcing.

Application of paleo-structural and seismic sequence stratigraphy analysis to determine the potential plays within the Proterozoic sequence of the Ganga Basin, India

Gas discovery in Nohta-2 well from Proterozoic Rohtas plays in Vindhyan Basin, south of Ganga Basin has been a significant lead to suggest that Proterozoic sediments underneath Ganga Basin could be prospective. With this in view, the Govt. of India (GOI) acquired 1350 LKM (Line kilometer) of 2D regional seismic lines to image the Proterozoic sediments for the first time within the basin under National Seismic Program (NSP). The present study is carried out to understand the paleo structures, the nature of the sedimentary sequences, and the trapping mechanism to further visualize the prospectivity of the Proterozoic sequence within the basin. The paleo-tectonic and seismic sequence stratigraphy analysis reveals that the Proterozoic sediments were deposited in the passive margin setting in shallow marine and tidal conditions with several progradations and retrogradation cycles. Precisely, it is noticed that the Proterozoic sediments experienced two stages of progradations and retrogradations. That was followed by a long non-depositional hiatus (>500 Ma) spanning from the Paleozoic to Mesozoic ages which led to a regional unconformity throughout the basin as well as other local unconformities and hydrocarbon traps within the Proterozoic sediments. Four giant structural traps (>300 km2) and one combination trap (>500 km2) are identified within Neo-Proterozoic to Meso-Proterozoic sequence and are expected to be prospective. However, with more exploration efforts and additional data, these traps may lead to a paradigm shift in exploration strategy and discoveries in near future within the basin.

Sequence-paleogeography and coal accumulation of the Late Carboniferous – Early Permian paralic successions in western Shandong Province, northern China

The coal-bearing strata of the Late Carboniferous to the Early Permian in western Shandong Province, northern China, were deposited within a range of paralic facies associations spanning from marine offshore carbonate shelf to fluvial and delta, which constituted a general paralic cratonic setting. In this paper, the sedimentary characteristics, sequence stratigraphy, paleogeography, and coal accumulation of the Benxi, Taiyuan and Shanxi formations of the Moscovian to Sakmarian age are studied by using data of outcrops, borehole cores, and geophysical loggings. A total of 14 different lithofacies are identified, with lithologies including sandstones, siltstones, mudstones, and bioclastic limestones, and grouped into fluvial, delta, tidal bar-lagoon, and offshore shelf facies associations. Furthermore, the key sequence boundaries represented by regional unconformity, incised valley erosional surface, switching surface of transgressive direction, stratigraphic color change surface marking the abrupt change of environment, and paleosol horizon have been recognized. Based on these sequence boundaries, three third-order sequences (S1, S2, S3 in ascending order) and corresponding lowstand, transgressive, and highstand systems tracts are subdivided. S1 corresponds to the Benxi Formation and the lower part of Taiyuan Formation, S2 corresponds to the middle and upper parts of Taiyuan Formation, and S3 corresponds to the Shanxi Formation. The lithofacies paleogeography maps of three sequences are reconstructed based on the contours of a variety of lithological parameters including strata thickness, limestone thickness, limestone percentage content, coarse (sandstone)/fine (siltstone and mudstone) ratio, and total coal thickness. S1 and S2 were developed with the offshore carbonate shelf paleogeographic units and the tidal bar – lagoon paleogeographic units, and S3 was developed with delta and fluvial units. In general, the western Shandong experienced transgression in the S1, transgression direction switching between S1 and S2, transgression again in S2, and then regression in the S3 during the Late Carboniferous to the Early Permian. Most significant coal accumulation occurred in the S3, followed by S1 and then S2. The coal accumulation centers of S1 were mainly distributed in Liaocheng, Feicheng, Jining regions in the west of the western Shandong where the tidal bar and lagoon were developed, those of S2 were mainly distributed in Tai'an, Jining and Tengzhou regions in the central part of the western Shandong, where the lagoon and tidal flat were developed, and those of S3 were mainly distributed in Heze, Jining, Tengzhou and Zaozhuang regions of the southern part of the western Shandong where the delta environment was developed. Overall, the high accommodation setting of S1 and S2, represented by the Benxi and Taiyuan formations, favors thick coal accumulation in the early TST, while the low accommodation setting S3, represented by the Shanxi Formation, favors thick coal accumulation in the middle and late TST. The research results of this study can be used in the evaluation of the favorable areas for the exploration and development of the deep-buried coal and CBM resources in western Shandong Province.

The amalgamation of Gondwana: calcite twinning and finite strains from the early–late Paleozoic Buzios, Ross, Kurgiakh and Gondwanide orogens

Abstract Orientated carbonate (calcite twinning strains; n = 78 with 2414 twin measurements) and quartzites (finite strains; n = 15) were collected around Gondwana to study the deformational history associated with the amalgamation of the supercontinent. The Buzios orogen (545–500 Ma), within interior Gondwana, records the high-grade collisional orogen between the São Francisco Craton (Brazil) and the Congo–Angola Craton (Angola and Namibia), and twinning strains in calc-silicates record a SE–NW shortening fabric parallel to the thrust transport. Along Gondwana's southern margin, the Saldanian–Ross–Delamerian orogen (590–480 Ma) is marked by a regional unconformity that cuts into deformed Neoproterozoic–Ordovician sedimentary rocks and associated intrusions. Cambrian carbonate is preserved in the central part of the southern Gondwana margin, namely in the Kango Inlier of the Cape Fold Belt and the Ellsworth, Pensacola and Transantarctic mountains. Paleozoic carbonate is not preserved in the Ventana Mountains in Argentina, in the Falkland Islands/Islas Malvinas or in Tasmania. Twinning strains in these Cambrian carbonate strata and synorogenic veins record a complex, overprinted deformation history with no stable foreland strain reference. The Kurgiakh orogen (490 Ma) along Gondwana's northern margin is also defined by a regional Ordovician unconformity throughout the Himalaya; these rocks record a mix of layer-parallel and layer-normal twinning strains with a likely Himalayan (40 Ma) strain overprint and no autochthonous foreland strain site. Conversely, the Gondwanide orogen (250 Ma) along Gondwana's southern margin has three foreland (autochthonous) sites for comparison with 59 allochthonous thrust-belt strain analyses. From west to east, these include: finite strains from Devonian quartzite preserve a layer-parallel shortening (LPS) strain rotated clockwise in the Ventana Mountains of Argentina; frontal (calcite twins) and internal (quartzite strains) samples in the Cape Fold Belt preserve a LPS fabric that is rotated clockwise from the autochthonous north–south horizontal shortening in the foreland strain site; Falkland Devonian quartzite shows the same clockwise rotation of the LPS fabric; and Permian limestone and veins in Tasmania record a thrust transport-parallel LPS fabric. Early amalgamation of Gondwana (Ordovician) is preserved by local layer-parallel and layer-normal strain without evidence of far-field deformation, whereas the Gondwanide orogen (Permian) is dominated by layer-parallel shortening, locally rotated by dextral shear along the margin, that propagated across the supercontinent.

Unconformity generation and the shift from storm‐dominated to tide‐dominated processes in a Jurassic retroarc foreland basin: Insights from ichnology

AbstractAn enigmatic transition from the storm‐dominated, offshore to lower shoreface deposits of the Redwater Shale Member (Sundance Formation) to the overlying mixed tidal and aeolian Windy Hill Sandstone (Morrison Formation) in the Oxfordian of the North American Western Interior has long been a source of intrigue. Previously proposed drivers include the progradation of a large, tide‐dominated delta onto a storm‐dominated shelf, a complete reorganisation of the basin's hydrodynamics and climate, or the development of a regional unconformity (termed the J‐5). In south‐eastern Wyoming, the Redwater Shale is characterised as an offshore to distal shoreface deposit with glauconitic siltstones and sandstones punctuated by coquinoid and sandy tempestites and hosting a Cruziana Ichnofacies. The Windy Hill Sandstone, a time‐transgressive, sand‐rich, intertidal succession with classic Pteraichnus and stressed Skolithos Ichnofacies, sharply overlies the Redwater Shale and records an abrupt basinward shift in facies that accompanied at least tens of metres of sea‐level fall. New, detailed sedimentological, ichnological and architectural data collected across this transition in the study area provide fresh insights into the depositional history of these units and demonstrates the existence locally of a composite J‐5 unconformity. The unconformity developed as tectonically driven, prograding shoreline trajectories of the Redwater Shale gave way to degrading trajectories of the Windy Hill Sandstone, leading to a forced regression and formation of a regressive surface of marine erosion. The sharp juxtaposition of intertidal flat facies (Pteraichnus Ichnofacies) directly upon offshore to lower shoreface deposits (Cruziana Ichnofacies) is the key to recognising the unconformity and proves the value of the previously underutilised ichnological data.

Episodic postglacial deltaic pulses in the Gulf of Cadiz: Implications for the development of a transgressive shelf and driving environmental conditions

Abstract The postglacial sea-level rise after the Last Glacial Maximum provided ideal conditions to study the transgressive sedimentary response to sudden shelf flooding driven by different rates of sea-level rise. In this study, a high-resolution seismic stratigraphic interpretation and sedimentological analysis were conducted on data from the northern Gulf of Cadiz continental shelf (SW Iberian Peninsula), in order to: 1) understand the succession of sedimentary processes during each shelf flooding episode and 2) explore the significance of variable rates of sea-level rise, sediment fluxes, and climatic conditions on the development of postglacial deposits. Four backstepping seismic postglacial transgressive units (PTUs; 4 to 1 from oldest to youngest) that are linked to the retreating mouth of the Guadiana River were interpreted. Together, these seismic units display a wedge-shape geometry, are located over the inner to middle shelf, and overlie a regional unconformity formed during the Last Glacial Maximum. Each PTU can be divided into several sub-units with distinctive seismic facies that have a similar stratigraphic organization. Each PTU contains lower sub-units that are composed of low-angle tangential-oblique clinoforms. The clinoforms are locally topped by a channelized sub-unit. The distal and/or lateral parts of the clinoforms are occasionally buried by sheet-like semitransparent subunits. The uppermost sub-units are present over the proximal and central parts of each seismic unit and are also sheet-like. PTUs can also be subdivided and described sedimentologically. Fine-grained sands with intercalated silty layers dominate the lower part of each PTU (lower clinoform sub-units). The upper part of each PTU (upper sheet-like sub-units) is characterized by reworked facies, composed of highly fragmented bioclasts within a mixture of silt and coarse to medium sand. Finally, mud deposits occur as a sediment drape over the PTUs. The internal structure of each PTU reveals several phases of development under a general process of transgressive submergence in which both coastal and marine deposits were formed and eventually preserved. The initial phase involved the development of coarse-grained deltas in shallow water, which were locally eroded by a network of distributary channels. In a transitional phase, the infilling of distributary channels and the offshore export of fine-grained sediments is related to a change in sediment sources, possibly triggered by enhanced hydrodynamic processes. The final phase involved the reworking of fluvio-deltaic sediments by shoreface processes to generate a sediment sheet. Age correlation with a suite of postglacial sea-level curves indicates that the formation of the postglacial transgressive deposits is bracketed between 14 ka and 9 ka. The studied deposits are related to a period of reduced sea-level rise, culminating in the Younger Dryas event (two oldest PTUs), and to phases of enhanced sea-level rise, such as Meltwater Pulse (MWP) 1B (two youngest PTUs). In spite of high rates of sea-level rise over MWP-1B, each PTU exhibits progradation and preservation of much of the delta. The preservation of progradational deltaic units is likely caused by increased sediment supply during progradational pulses. We suggest that those pulses of enhanced sediment fluxes during MWP-1B were strongly driven by the overall climatic conditions in the southwest of the Iberian Peninsula, probably resulting from enhanced rainfall runoff during humid periods and scarce land vegetation cover.

Stratigraphic record of continental breakup, offshore NW Australia—Discussion

AbstractReeve et al. (2022) address the stratigraphic record of continental breakup by focusing on a set of stratigraphic unconformities from a proximal sector of the NW Australian continental margin, inboard from the Exmouth Plateau. They suggest that such unconformities can potentially document a well‐defined three‐stage process: end of the syn‐rift phase, formation of a wide continent‐ocean transition zone (COTZ) and generation of ‘true’ Penrose‐type oceanic crust. We counterargue that continental breakup is a protracted event that can only be understood via seismic‐ and chronostratigraphic correlations of strata, and their composing sequences, across and along rifted margins. Tying proximal stratigraphic unconformities to magnetic anomalies outboard from the study area in Reeve et al. (2022) is open to question. In parallel, we suggest that age resolutions of ca. 1 Ma are not achievable using the micropaleontological data presented in Reeve et al. (2022), with an important reworking of microfossil assemblages potentially occurring during the erosional process forming local and regional unconformities. Our discussion addresses these points in more detail.

Sequence paleogeography and coal accumulation model in the fluvio-lacustrine rift basin: The Lower Cretaceous of the Huhehu Sag of Hailar Basin, Inner Mongolia (NE China)

Significant volumes of coals can be preserved in fluvio-lacustrine rift basins but prediction of occurrence and geometry within a sequence-stratigraphic framework is always a challenge. The Huhehu Sag is a continental rift basin in Hailar Basin of NE China, with the Damoguaihe and Yimin formations of Lower Cretaceous as the main coal-bearing strata. In this study, the sedimentary facies, sequence stratigraphy, paleogeography, and coal accumulation characteristics in the Huhehu Sag are analyzed. Data used includes boreholes, geophysical well logs and seismic profiles. The coal-bearing strata are mainly composed of conglomerates, sandstones, siltstones, mudstones, and coals. Alluvial fan, braided fluvial, fan delta, braided fluvial delta and lacustrine depositional systems are identified. Coal seams were mainly formed in lacustrine and delta depositional systems. Six third-order sequences were subdivided according to sequence boundaries represented by regional unconformities, basal surfaces of incised valley fills, facies-reversal surfaces, and abrupt color changes related to the base level changes. Sequence 1 corresponds to the Tongbomiao Formation formed in the early syn-rift phase, Sequence 2 corresponds to the Nantun Formation formed in the syn-rift climax phase, Sequence 3 and 4 span the Damoguaihe Formation, with Sequence 3 formed in the syn-rift climax phase and Sequence 4 formed in the late syn-rift phase, and Sequence 5 and 6 span the Yimin Formation which were formed in the post-rift stage. All six sequences were developed with littoral-sublittoral lake, delta front and delta plain paleogeographic units. Sequence 1 to Sequence 2, Sequence 3 to Sequence 4 and Sequence 5 to Sequence 6 are of progradational patterns, while Sequence 2 to Sequence 3 and Sequence 4 to Sequence 5 display retrogradational features. From Sequence 1 to Sequence 4, the syn-rift led to the thickening of strata as well as the coal seams towards the eastern side of the sag. In Sequence 5 and 6, post-rift thermal sagging caused the wide distribution of the strata with the depositional centers as well as the thick coals being distributed in the central belt of the sag. Coals were mainly developed in the transgressive systems tract (TST) and highstand systems tract (HST) of Sequences 4 and 5. The favorable paleogeographic units for thick coal accumulations were mainly littoral-sublittoral lacustrine and delta. The Huhehu Sag is interpreted as an overfilled basin, we further divided the overfilled basin into three subtype basins by accommodation space, including the low, moderate, and high accommodation overfilled basins. In the low accommodation overfilled basin, the relatively thick coals were developed near the maximum flooding surface, while in the moderate accommodation overfilled basin, the relatively thick coals were developed in the lower-middle part of TST and the middle part of HST. These results could be used to predict the preferred depositional environments and favorable areas for coal and coalbed methane exploration in the fluvio-lacustrine rift basins.

Seismic stratigraphic framework and seismic facies of the Aptian Pre-salt Barra Velha Formation in the Tupi Field, Santos Basin, Brazil

The discovery of giant oil accumulations in the Pre-salt (sag) section of the Santos Basin has attracted huge interest in the lacustrine carbonate deposits of the Aptian Barra Velha Formation. Although for the significant volume of publications on the Pre-salt section, there is still limited understanding of its tectono-stratigraphic evolution, and lack of a coherent published stratigraphic framework for the Tupi Field, the largest and most productive Pre-salt accumulation. This work aims to define a seismic-stratigraphic framework in the Aptian Pre-salt interval of the Tupi Field, using 19 seismic lines, and photomicrographs of thin sections from 13 wells. Four seismic units (SU1, SU2, SU3, and SU4) and two seismic facies (SFA and SFB) were identified. The depositional pattern of the seismic units is of a continuous expansion of the accommodation space, although with significant erosion at the top of SU4. SFA is composed mostly of calcite spherulites and Mg-clays mud, which constitutes the background of lacustrine sedimentation preferentially formed and/or preserved in relatively deeper areas. SFB corresponds to crusts of fascicular calcite shrubs preferentially precipitated in relatively higher areas, and to intraclastic deposits of reworked spherulites and shrubs. The Pre-Alagoas unconformity represents the base of the study interval. The top is characterized by a regional unconformity which represents a temporal gap between the youngest of the units (SU4) and the start of the evaporitic deposits of the Ariri Formation. No additional unconformities were identified within the Barra Velha Formation in the Tupi Field, characterizing a continuously expansive deposition on a flat, tabular and onlapping pattern upon the Outer High, as a product of continuous accommodation creation, under low tectonic activity. The predominant post-depositional configuration, together with the continuity and onlap patterns of seismic units onto a remanent high, support a shallow evaporitic lake model, rather than a deep lake with microbial platforms for the Tupi Field area.

Chronology of Early to Mid-Pleistocene sediments in the northern North Sea: New evidence from amino acid and strontium isotope analyses

Sediments deposited during glacial-interglacial cycles through the Early to Mid-Pleistocene in the North Sea are chronologically poorly constrained. To contribute to the chronology of these units, amino acid racemization (AAR) and strontium (Sr) isotope analyses have been performed on samples from four shallow borings and one oil well along a transect in the northern North Sea. D/L Asp (aspartic acid) values obtained through reverse-phase liquid chromatography in the benthic foraminiferal species Elphidium excavatum is focused on because of consistent results and a good stratigraphic distribution of this benthic species. For the Early Pleistocene, an age model for the well 16/1–8, from the central part of the northern North Sea based on Sr ages allows for dating of the prograding wedges filling the pre-Quaternary central basin. A regional calibration curve for the racemization of Asp in Elphidium excavatum is developed using published ages of radiocarbon-dated samples and samples associated with the previously identified Bruhnes/Matuyama (B/M) paleomagnetic boundary and a Sr age from this study. Based on all the available geochronological evidence, samples were assigned to marine oxygen isotope stages (MIS) with uncertainties on the order of 10–70 ka. Sr ages suggest a hiatus of <2 million years (Ma) possibly due to non-deposition or low sedimentation between the Utsira Formation (Pliocene) and the Early Pleistocene. An increase in sedimentation rates around 1.5 ± 0.07 Ma (∼MIS 51) may partly be due to sediment supply from rivers from the south-east and partly due to the extension of ice sheet around 1.36 ± 0.07 Ma from the Norwegian coast to the central North Sea. A possible basin-wide glaciation occurred around 1.1 Ma (∼ MIS 32) (upper regional unconformity/top of unit Q4 in this study), resulting in erosion and regional unconformity. Two interglacials in the Norwegian Channel have been dated: the Radøy Interglacial to 1.07 ± 0.01 Ma (possibly MIS 31, the ‘super interglacial’), and the Norwegian Trench Interglacial to 0.50 ± 0.02 Ma (possibly MIS 13). A massive till unit identified at the same stratigraphic level in all shallow borings may partly represent an extensive MIS 12 glaciation. This study shows that the combined use of amino acid racemization data and Sr isotope chronology can refine the chronological ambiguities of Quaternary North Sea sediments related partly to the impact of glacial processes.

Seismo-Stratigraphic Model for the Urban Area of Milan (Italy) by Ambient-Vibration Monitoring and Implications for Seismic Site Effects Assessment

In this paper, we present the work carried out to characterize the spatial variability of seismic site response related to local soil conditions in the city of Milan and its surroundings, an area with ∼3 million inhabitants and a high density of industrial facilities. The area is located at the northwestern end of the Po Plain, a large and deep sedimentary basin in northern Italy. An urban-scale seismo-stratigraphic model is developed based on new passive and active seismic data, supported by the available geological data and stratigraphic information from shallow and deep vertical wells. In particular, 33 single-station and 4 ambient-vibration array measurements are acquired, together with 4 active multichannel analyses of surface waves (MAWS). To estimate the resonant frequencies of the sediments, the horizontal to vertical spectral ratio technique (HVSR) is applied to the ambient-vibration recordings, whereas to determine the Rayleigh-wave dispersion curves from the passive array, the data are analysed using the conventional frequency-wavenumber, the modified spatial autocorrelation and the extended spatial autocorrelation (ESAC) techniques. The array data are used to determine the local shear wave velocity profiles, VS, via joint inversion of the Rayleigh-wave dispersion and ellipticity curves deduced from the HVSR. The results from HVSR show three main bands of amplified frequencies, the first in the range 0.17–0.23 Hz, the second from 0.45 to 0.65 Hz and the third from 3 to 8 Hz. A decreasing trend of the main peaks is observed from the northern to the southern part of the city, allowing us to hypothesize a progressive deepening of the relative regional chrono-stratigraphic unconformities. The passive ambient noise array and MASW highlight the dispersion of the fundamental mode of the Rayleigh-wave in the range 0.4–30 Hz, enabling to obtain detailed Vs. profiles with depth down to about 1.8 km. The seismo-stratigraphic model is used as input for 1D numerical modelling assuming linear soil conditions. The theoretical 1D transfer functions are compared to the HVSR curves evaluated from both ambient noise signals and earthquake waveforms recorded by the IV. MILN station in the last 10 years.

Re-examination of the Base Miocene Unconformity in west Sabah, Malaysia, and stratigraphic evidence against a slab-pull subduction model

This review chiefly examines data on the regionally important Base Miocene Unconformity , confirming its age and expression in west Sabah, then integrates this with data on the tectono-stratigraphic development of the broader Sabah to Palawan region. The two unconformities described (the Base Miocene Unconformity and the younger Deep Regional Unconformity) were both produced when deformation abruptly stopped, leaving erosional surfaces that were then transgressed. The ages of these two events are c. 24 and 12–13 Ma respectively, and the areas affected by the prior uplift and erosion can be mapped. The results are incompatible with widely cited plate tectonic models that describe closure of a postulated Proto-South China Sea [PSCS]. These prior models had assumed plate drift ended with collision at plate docking over a PSCS suture in about mid Early Miocene times. However, stratigraphic data shows that Early Miocene times were a period of quiescence across Sabah, with mostly mild subsidence . It is therefore proposed that plate drift in the SCS could not have been linked to any type of subduction process anywhere around western Sabah. The absence of any plate docking or any Oligocene through Middle Miocene compression around northern Palawan is a further anomaly to the old subduction based tectonic model.

Mid-Miocene to recent tectonic evolution of the Punta Stilo Swell (Calabrian Arc, southern Italy): An effect of Calabrian Arc migration

The development of Punta Stilo Swell (PSS), a submerged lobate-shaped promontory located in the Crotone-Spartivento Basin, is inferred to be closely linked to the Calabrian Arc kinematics since mid-Miocene onward. The latter consists of an alternation between long-term phases of subsidence related to forward migration of the Calabrian accretionary complex, and short-term compressional-transpressional phases, characterized by episodes of uplift and formation of regional unconformities, in a context of stopping or slowdown of Calabrian accretionary complex migration. In particular, the study area is supposed to have respectively experienced phases of compressional and extensional tectonics during late Messinian and early Pliocene, which likely produced a reactivation of inherited compressional and extensional structures. Late Pliocene/Pleistocene transtensional tectonics is documented along the offshore extension of the Soverato-Lamezia Fault Zone (SLFZ), in the N sector of the PSS. The resulting physiographic modifications associated with such compressional, extensional and transtensional tectonics is believed to have controlled the accumulation of the thick Plio-Pleistocene succession of the PSS. The northern continental slope is inferred to be the result of late Pliocene/Pleistocene transtensional tectonics occurred along the SLFZ, whereas the shelf break is considered the result of an early Pliocene extensional tectonics. The considerable steepness (>10°) of the southern and eastern continental slope of the PSS is associated with the late Messinian compressional tectonic activity and is marked by the occurrence of slope failure-associated events.

Insights on deepwater turbidite sedimentary features from outcrops in onshore Northwest Sabah, Malaysia

This paper discusses findings of sedimentary features observed from deepwater outcrops in Northwest Sabah. The observations were based on 7 outcrop localities of Lower Oligocene – Upper Oligocene sections around Klias and Kudat Peninsulas, and Kota Kinabalu, covering Temburong, Belait, West Crocker, and Kudat formations. Temburong Formation (Lower Oligocene-Lower Miocene) was observed at Menumbok (Stop 1) and Tanjung Aru (Stop 2). It comprises interbedded shale and sandstone turbidite sequence representing deposition in deepwater setting. At Batu Luang (Stop 3) the deepwater turbidite Temburong Formation is observed overlain unconformably by stacked fluvial channel deposits of Belait Formation (Middle to Upper Miocene). Here, the overlying fluvial deposits comprise predominantly conglomerates which are believed to represent basal Belait Formation lies over regional unconformable surface correlatatable to Deep Regional Unconformity (DRU). Deepwater West Crocker Formation (Lower Oligocene-Lower Miocene) was observed at King Fisher-Sulaman, Kota Kinabalu (Stop 4), composed of predominantly thick sandstone interbedded with shale. Meanwhile, Sikuati Member of the Kudat Formation (Lower Miocene) was observed at Pantai Sikuati (Stop 5) and Bukit ‘V’ (Stop 6) outcrops. It consists of interbedded sandstone and shale of turbidite successions. At Tanjung Simpang Mengayau (Stop 7) thick sandstone interbedded with thin shales of Kudat Formation was observed. In general, the Temburong Formation at Stops 1 and 2 show Bouma sequence of Ta-Tb-Te successions, but relatively thicker Te at Stop 1 suggesting a prolonged mud suspension deposit compared to Stop 2. At Stop 3, the deepwater Temburong Formation was subjected to a substantial tectonic uplift and massive erosion associated with DRU. Further northwards at Stop 4 West Crocker Formation exhibits thicker Ta successions which implies deposition within a slightly higher energy regime as compared to that of Temburong Formation at Stop 3. Meanwhile, at Stops 5 and 6 different depositional energy levels are shown in the lower and upper stratigraphic intervals. In the lower interval deposition took place within a higher energy condition as shown by thick Ta. It was followed by gradual decreasing depositional energy regime in the upper interval as shown by predominance Tc-Td-Te. Meanwhile, at Stop 7 a thick massive graded sandstone overlain by thin mudstone bed was observed which implies a prolonged Ta succession followed by short-term Te of mud suspension deposits.