Angular Unconformity Research Articles

LEONARDO DA VINCI’S AND NICOLAUS STENO’S GEOLOGY

Anniversaries for the two founding fathers of geology occurring in the same year prompted a comparative evaluation of how the two contributed to establishing the basic principles of the discipline. To do so, passages from their publications, codices and manuscripts have been quoted directly. The Stenonian principles (‘original horizontality’, ‘original continuity’, and ‘superposition of individual strata’) are present in Leonardo’s notebooks amazingly formulated, using similar wording when studying the same area more than 150 years earlier. Also, Stenonian priority in naming and explaining geological concepts and processes (e.g., faulting, folding, angular unconformity, relative chronology) are mirrored in Leonardo’s writings and pictorial works. While Steno enjoys priority in stepwise restoration of the geological history of a given region, Leonardo was the first to construct a 3D geological profile representation and geomorphologic maps. Lastly, the paper focuses on diverging stances of the two savants about the Noachian Deluge and the age of the Earth. Already 500 years ago, Leonardo had solved the question of marine fossil remains of organic origin found in the mountains implying the possibility of deep geologic time in a statement of ‘eternalism’. 350 years ago, Steno solved the same question in a different way in which he retained a basic role for the Deluge and assumed a short age for the Earth by focusing mainly on short-lived sedimentary and geomorphologic processes.

Fluvial interpretations of stratigraphic surfaces across Upper Triassic to Lower-Middle Jurassic continental red beds northeastern Mexico

This work broadens the understanding of previous interpretations from Late Triassic-Early-Middle Jurassic reds beds in northeastern Mexico from the Huizachal Group. Proposed bounding surfaces in El Alamar, La Boca, and La Joya formations are informal. The surfaces are here assigned allostratigraphic hierarchies for genetic interpretations. The formations are continental and provide no sea-level markers to tie the surfaces to the marine base level, but these fluvial-alluvial environments produce various scales of traceable scour surfaces.Upper Triassic-Middle Jurassic sandstones were analyzed at ten localities. Sedimentary profiles provide details for two facies associations and two main facies, which share 20 sedimentary lithofacies. The documentation of fluvial architecture considers sedimentary structures and paleocurrents in the sand bodies to features of five fluvial styles driving the depositional system, including mixed debris flow and braided river systems, gravelly or sandy meandering river, ephemeral sand-bed meandering river, and ephemeral sheet-flood distal sand-bed river.This model provides greater detail for more accurate stratigraphic correlation to sedimentary conditions under which these rocks were deposited, as it allows identifying four major bounding surfaces that can be tied to fluvial processes of amalgamation, aggradation, degradation, or flooding. The exercise estimates provisional regional tracts in continental deposits with no apparent impact by the marine base level. Amalgamation surfaces are underneath laterally extensive sheets of amalgamated channel-belt architectural elements, the contacts with underlying surfaces are disconformable, but locally are angular unconformities. The amalgamated beds also include sediment-gravity flows deposited such as channelized debris flows. Aggradation underlies and binds volcaniclastic rocks and facies with common mass wasting deposits from creep or debris flows, as well as overland flow and gullying over broad alluvial plains forming upward-coarsening sequences. It locally records the rapid burial of landscapes by mass wasting, distinguishing it from typical valleys that are cut by normal channel processes. Degradation binds incised valleys that confine channels and floodplains, and unconformably overly fining-upward sequences. Flooding marks sediment accumulation formed by overbank flows in low-energy environments.

LWD DATA INVERSION FOR STRUCTURE WITH ANGULAR UNCONFORMITY

Non-structural traps and reservoir flanks are characterized by angular unconformities. Angular unconformity between dipping formation and sub-horizontal oil-water contact is common in the North Sea fields. This paper presents an approach to real-time inversion of LWD resistivity data for the scenario with angular unconformity. The approach utilizes artificial neural networks (ANNs) for calculating the tool responses in parametric surface-based 2D resistivity models. We propose a parametric model with two non-parallel boundaries suitable for scenarios with angular unconformity and pinch-out. Training of ANNs for this parametric model is performed using a database containing samples with the model parameters and corresponding tool responses. ANNs are the kernel of 2D inversion based on the Levenberg-Marquardt optimization method. To demonstrate applicability of our approach and compare with the results of 1D inversion, we analyze Extra Deep Azimuthal Resistivity tool responses in a 2D synthetic model. It is shown that 1D inversion determines either the position of the oil-water contact or dipping layers structure. At the same time, 2D inversion makes it possible to correctly reconstruct the positions of non-parallel boundaries. Performance of 2D inversion based on ANNs is suitable for real-time applications.

Origin and evolution of unconformities in a continental rift basin: a case study from the Lishu Depression in Songliao Basin, China

A variety of unconformity types are commonly developed during the rifting of a continental basin. Identifying their origin and evolution is a prerequisite to understanding the complex tectono-sedimentary evolution of the basin and to evaluate the basin’s oil and gas resources. Based on the 2D/3D seismic, geophysical logging, and subsurface lithological data, the unconformities within the rift layer of the Lishu Depression in the Songliao Basin are identified as disconformities, nonconformities, and four “types” of angular unconformities. Their evolution and origin are analyzed by associating the unconformities with the basin’s tectonic evolution. The results show that the rifting of the Lishu Depression was not a continuous process. Instead, several regional tectonic episodes influenced the basin’s evolution. Episodic compressional events and a phase of extension along the main faults determined the basin configuration and sedimentary fill characteristics of different sub-rifting stages. The tectonic history and related paleogeomorphology changes are the dominant factors in the formation of these different unconformity types. During the rifting process, the type and distribution of unconformity varied. In the initial rifting stage, the basin developed a nonconformity and four types of angular unconformities as the result of both regional uplift and fault block tilt. Rotation and tilting of the fault blocks during the intensive rifting resulted in two types of angular unconformities. Finally, in the recession rifting stage, a different form of angular unconformity and disconformity developed, mainly as a result of the regional uplift and subsequent subsidence.

Late- to post-Variscan tectonics and the kinematic relationship with W–Sn vein-type mineralization: evidence from Late Carboniferous intramontane basins (Porto–Sátão syncline, Variscan Iberian belt)

Many studies have constrained that late-Variscan buckling produced the arcuate geometry of the Ibero-Armorican belt. Nonetheless, debate remains on the associated geodynamic framework. Poorly studied Late Carboniferous intramontane basins offer an excellent framework to decipher the timing and kinematics of the late- to post-Variscan tectonics. Understanding the latter also helps constrain the structural emplacement mode of contemporaneous W–Sn–Nb–Ta–Li mineralization. In Iberia, the Porto–Sátão syncline is an example of such a Late Carboniferous intramontane basin. We present a structural analysis of the syncline, its basement and the associated W–Sn deposits. The regional structure is dictated by the Alcudian angular unconformity, caused by Cadomian tectonics (575–555 Ma) and separating tilted Ediacaran and subhorizontal Lower Paleozoic formations. Superimposed Variscan deformation led to F 1 –F 3 folds with steep and gentle plunges, respectively. The late-orogenic D 3 fabric is locally affected by post-orogenic F 4 kink folds and an S 4 crenulation cleavage. W–Sn-bearing vein systems occur along granite-hosted cone sheets, or exploit cross-fold joints associated with the F 3 and F 4 fold generations, revealing a close kinematic relationship between granite-related mineralization and the late- to post-Variscan deformation style. This structural history is interpreted as a plate-scale geodynamic change from Late Carboniferous north–south (D 3 ) to Early Permian WNW–ESE (D 4 ) convergence.

Reconnaissance Of Kenny Hill Formation Outcrops In The Rawang Area, Northern Selangor, Malaysia

New outcrops of Paleozoic meta-sediments northwest of Kuala Lumpur expose the deformational effects of the Late Paleozoic-Mesozoic collisions between various Gondwana-derived continental fragments as they amalgamated to form the core of SE Asia. Over a duration of 6 months, beginning in August 2020, we conducted field trips within northern Selangor to new laterally extensive outcrops for field observations, structural mapping and to measure and log the stratigraphic section. This paper focuses on Upper Paleozoic Kenny Hill Formation outcrops in northern Selangor. The most studied is the heavily weathered Jalan Rawang-Bestari Jaya (JRBJ) outcrop, which is characterised by a steeply dipping (southwest), upward-coarsening succession of sandstones and shales interpreted as a system of ephemeral fluvial channels possibly related to Gondwana glaciation. Concretions within bedding planes and fractures were possibly formed around organic material. Less than 4 km to the east, the Scientex development has excavated fresher outcrops of the same rocks dipping to the NE. Metamorphic lineation is not present in either outcrop location. In addition, a monocline is exposed at outcrop location number 3 nearby. Finally, at Bukit Botak, 14 km to the southwest, a system of westward verging thrust faults, back thrusts and normal faults can be viewed and an angular unconformity or decollement marks the contact between the Upper and Lower Paleozoic. These laterally extensive outcrops are rare and are quickly subject to intense tropical weathering, the encroachment of jungle vegetation and urban development. Historic mapping and prior stratigraphic, structural, and petrographic studies have been conducted in the area, but these relied on poor exposures. As suburban development escalates in the area, we hope that new outcrops, featuring multi-dimensional views of these formations, such as the four described in this paper, will complement the earlier work.

Neoproterozoic stratigraphy of the Southwestern Basement Province, Svalbard (Norway): Constraints on the Proterozoic-Paleozoic evolution of the North Atlantic-Arctic Caledonides

The Arctic archipelago of Svalbard, Norway, plays an important role in tectonic reconstructions of the Cambrian–Devonian Caledonian orogen in the North Atlantic and circum-Arctic regions. To elucidate the Proterozoic geological history of the Southwestern Basement Province of Svalbard and place its origin and displacement history in the context of regional tectono-stratigraphic events, we present new geological mapping, stratigraphic assignments, carbonate carbon (δ13Ccarb) and strontium (87Sr/86Sr) isotope chemostratigraphy, and detrital/igneous zircon U-Pb geochronology coupled with Hf isotope geochemistry from Neoproterozoic rocks exposed in southwestern Spitsbergen. The examined metasedimentary successions include a Tonian mixed carbonate-siliciclastic succession (the Nordbukta/Deilegga groups) that is regionally truncated by a Cryogenian (<650 Ma) angular unconformity (the Torellian unconformity), the age of which is confirmed herein by field relationships and re-analysis of previously reported detrital monazite data. New detrital zircon U-Pb data from the Nordbukta Group yield ca. 1030–1170, 1280–1400, 1430–1510, and 1580–1680 Ma age populations that are typical of North Atlantic (e.g., North and East Greenland, Scotland, and the Scandinavian Caledonides) Mesoproterozoic-Neoproterozoic sedimentary successions. Overlying Cryogenian–Ediacaran strata of the Sofiebogen, Dunderbukta, and Kapp Lyell (Bellsund) groups record rift-related sedimentation and host chemostratigraphic data that provide ties to the ca. 640(?)–635 Ma Marinoan snowball Earth glaciation. Detrital zircon U-Pb data from the Cryogenian Slyngfjellet, Jens Erikfjellet, and Konglomeratfjellet formations yield similar age spectra to the underlying Nordbukta and Deilegga groups, while younger Ediacaran strata of the Höferpynten and Gåshamna formations record a prominent shift to bimodal ca. 1850 and 2850 Ma U-Pb age populations. Integrating these data with previously published geochemical and geochronological datasets from Svalbard and other circum-Arctic Mesoproterozoic–Neoproterozoic successions, we propose that the Southwestern Basement Province represents a composite terrane composed of multiple peri-Baltican (Wedel Jarlsberg-Oscar II, Berzeliuseggene-Eimfjellet, and Prins Karls Forland terranes) and peri-Laurentian (Hornsund terrane) crustal fragments that were likely initially juxtaposed and/or amalgamated during the Caledonian orogeny. These terranes were subsequently complexly deformed and modified during the younger Devonian-Carboniferous Ellesmerian and Paleogene-Eocene Eurekan orogenic events that affected western Svalbard.

Meso to Neoproterozoic polyphase rifting and tectonic inversion: The example of the São João del Rei region at the southern border of the São Francisco Craton, Brazil

The up to 3000 m thick Mesoproterozoic and Neoproterozoic sedimentary succession exposed at the southern border of the São Francisco Craton, São João del Rei region (southern Minas Gerais State, Brazil), records the stacking of five sedimentary basins. These basins evolved due to multiple rifting episodes related to the southernmost Espinhaço rift system and were deformed around 600 Ma during the evolution of the Ribeira Orogen. These basins are the 1- São João del Rei Basin with quartzite units with maximum depositional age of ca. 1.5 Ga preserved in a half graben associated to the younger rift phases, 2- Carandaí Basin with rift related fault bounded debris flow deposits with maximum depositional age of ca. 1.38 Ga, and a sag carbonate infill recording the post-rift thermal relaxation, 3- Prados Basin, an NE-SW aulacogen linked to the passive margin Andrelândia Basin (<1.04 Ga) and 4- a foreland basin of the Brasiliano Orogeny with early syn-collisional plagioclase wacke deposits with maximum depositional age of ca. 650 Ma. The present paper discusses the role of the faults at the São João del Rei region in the evolution of these basins and the subsequent orogenic deformation. Five phases of fault activity were recognized. The first one is recorded by a few E-W faults and a single N–S fault and associated growth strata within the São João del Rei Basin. The second phase is recorded by activity of NE-SW, E-W and minor NW-SE faults during the Carandaí rifting (1.53–1.38 Ga). The main faults are the NW-SE Lenheiro and São João del Rei faults and the E-W São José Fault. Minor NW-SE faults are interpreted as release faults associated with the last two faults. NE-SW mafic dikes occurring along the faults suggest precursor/coeval magmatism. The third phase is characterized mainly by the reactivation of the NW-SE Lenheiro Fault leading to the development of the NW-SE Prados aulocogen. The fourth phase is inferred to be a reverse reactivation of NW-SE faults related to the NW tectonic transport direction of the Ribeira Orogen. The NW verging D1 folds and steeply SE dipping cleavage contribute to this interpretation. However, the reverse reactivation was insufficient to overcome the cumulative previous normal throw. During the fifth phase NE-SW faults were reactivated as dextral shear zones. Subhorizontal stretching lineation and mylonitic foliation occur along the Lenheiro Fault for at least 60 km. These structures are related to the E-W compression that generated other NE-SW regional dextral shear zones, such as the Três Corações and Caxambu shear zones during the late stages of the Brasiliano Orogeny. Structural data also reveals that during the Carandaí and/or Prados rifting, prior to the orogenic contraction, rotational oblique tilting in what was to become the northern limb of the Lenheiro Anticline generated the difference between the trends of the Beta axis and L1 in the northern limb of the anticline in relation to the other structural domains. This also explains the ENE-WSW trend of the Lenheiro Anticline contrasting with the NE-SW trend of the Águas Santas Syncline and of the other D1 regional folds south of the studied area. A forward model based on a cross section through the Águas Santas Syncline reveals that the NE-SW Lenheiro Fault acted as a major listric fault responsible for the development of a rollover anticline and the half-graben in which the São João del Rei basin deposits are preserved. Rotation and partial erosion of the hanging wall resulted in the deposition of the Carandaí diamictites in angular unconformity on the São João del Rei basin deposits. The model indicates 41% of total extension with a cumulative displacement of 4.2 km produced by the Carandaí and Prados rifting phases. The minimum shortening of the orogenic compressive phase is estimated to be about 11% with 1.57 km of displacement. These values may be underestimated since the model does not take into account parasitic folds, crystalplastic deformation and volume loss produced by layer parallel shortening and metamorphic reactions.

Angular Unconformity of the Late Quaternary Strata in the Hetao Basin, North of the Ordos Block (West China): Timing and Its Tectonic Implications

Following the uplift of the Tibet Plateau and the continuous subduction of the Pacific Plate, graben faulting began to appear around the Ordos Block in the Cenozoic. The Hetao Basin is a Cenozoic rift basin between the Ordos Block and the Yinshan Mountains, and Late Quaternary sedimentary strata, which have lacustrine facies, are widely distributed inside this basin. However, the evolution of the Hetao Basin and its related fault systems has been debated for a long time due to the lack of tectonic evidence. In this study, four sections named Haolaigou, Bianqianghao, Huhesala, and Hazigai are selected along the north margin of the Hetao Basin. With the lithology and structural analysis of the Upper Pleistocene series in these sections, two new angular unconformities are found within the 10 m thick sedimentary sequence of the lacustrine sediments. Based on the dating results, we speculate that these two upper and lower angular unconformities are formed between 33 ka BP and 40 ka BP, and 60 ka BP and 80 ka BP, respectively. The angular unconformities also provide tectonic constraints for the latest and ongoing tectonic activity in the Quaternary. This tectonic movement begins at around 80 ka BP and causes two different unconformities of the lower strata with varying degrees of deformation (tilt) but also leads to the final death of the ancient lake.

Early and Middle Jurassic tectonically controlled deposition in the High-Tatric succession (Tatricum), Tatra Mountains, southern Poland: a review

The High-Tatric succession of the Tatra Mountains represents the Tatricum domain of the Central Western Carpathians, which in the Jurassic was located on the southern margin of the incipient and expanding Vahic Ocean – a branch of Western Tethys. This paper describes the various depositional consequences of extensional tectonic activity as it impacted on sedimentation in the High-Tatric succession of the Tatra Mountains during the Early and Middle Jurassic. Evidence of such impacts on depositional style and facies development are present within the Dudziniec, Smolegowa and Krupianka formations, in all the High-Tatric tectonic units. These impacts also include erosional surfaces and sedimentary gaps separating particular formations, commonly associated with minor angular unconformities. The Lower Jurassic, pre-Bajocian, Dudziniec Formation of the Kominy Tylkowe (autochthonous) Unit is developed in mixed carbonate-clastic facies. The occurrence and proportion of sand-dominated and carbonate-dominated facies, as well as their thickness differences, were controlled by syndepositional tilt-block tectonics, taking place both in depositional and in neighbouring source areas. The Smolegowa and Krupianka formations (Bajocian–Bathonian) occur in all High-Tatric tectonic units, but in the Czerwone Wierchy and Giewont units they are represented mainly by laterally discontinuous bodies of crinoidal limestone of very limited thickness. The preservation of these deposits only in some areas, as well as their thickness reductions, are effects of differentiated subsidence and uplift of isolated blocks taking place in an extensional regime. Moreover, the Krupianka Formation abounds in condensed facies with ferruginous crusts and stromatolites – a feature characteristic of rapidly drowning ocean margins. Deposits of the Dudziniec, Smolegowa and Krupianka formations are also preserved as infills of extensive systems of neptunian dykes penetrating mainly the Triassic substrate, which is yet another classic symptom of synsedimentary extension. The dominant influence of tectonics on sedimentary development ceased with the onset of deposition of the Raptawicka Turnia Formation in the Callovian.

Tectonic Switch From Triassic Contraction to Jurassic-Cretaceous Extension in the Western Tarim Basin, Northwest China: New Insights Into the Evolution of the Paleo-Tethyan Orogenic Belt

We use stratigraphic, sedimentological, and borehole data and seismic profiles from the western Tarim Basin to document its Mesozoic tectonic evolution. A nearly 60-km-wide, Triassic fold-and-thrust belt along the southwestern margin of Tarim Basin is unconformably overlain by a Jurassic-Cretaceous sedimentary sequence along a regional angular unconformity. The Lower-Middle Jurassic strata consist mainly of an upward-fining sequence ranging from terrestrial conglomerates to turbidite deposits, which represent the products of an initial rift stage. Palaeocurrent analyses show that sediments for these rift deposits were derived from the paleo-Kunlun and paleo-Tienshan Mountains to the southwest and northern, respectively. The overlying Upper Jurassic-Cretaceous series consist of coarse-grained, alluvial fan to braided river deposits in the lower stratigraphic member, and lagoonal mudstones and marine carbonates in the upper member. These finer-grained rocks were deposited in a subsiding basin, indicating that a significant change and reorientation in the drainage system should have occurred within the basin during the Early Cretaceous. The western Tarim Basin evolved from a syn-rift stage to a post-rift stage during the Jurassic-Cretaceous. A post-orogenic stretch developed due to the evolution of the Paleo-Tethyan orogenic belt in Central Asia is a likely geodynamic mechanism for this major tectonic switch from a contractional episode in the Triassic to an extensional deformation phase in the Jurassic-Cretaceous.

Late Neogene evolution of the Peruvian margin and its ecosystems: a synthesis from the Sacaco record

The highly productive waters of the Humboldt Current System (HCS) host a particular temperate ecosystem within the tropics, whose history is still largely unknown. The Pisco Formation, deposited during Mio-Pliocene times in the Peruvian continental margin has yielded an outstanding collection of coastal-marine fossils, providing an opportunity to understand the genesis of the HCS ecosystem. We present a comprehensive review, completed with new results, that integrates geological and paleontological data from the last 10 My, especially focusing on the southern East Pisco Basin (Sacaco area). We discuss the depositional settings of the Pisco Formation and integrate new U/Pb radiometric ages into the chronostratigraphic framework of the Sacaco sub-basin. The last preserved Pisco sediments at Sacaco were deposited ~ 4.5 Ma, while the overlying Caracoles Formation accumulated from ~ 2.7 Ma onwards. We identified a Pliocene angular unconformity encompassing 1.7 My between these formations, associated with a regional phase of uplift. Local and regional paleoenvironmental indicators suggest that shallow settings influenced by the offshore upwelling of ventilated and warm waters prevailed until the early Pliocene. We present an extensive synthesis of the late Miocene–Pleistocene vertebrate fossil record, which allows for an ecological characterization of the coastal-marine communities, an assessment of biodiversity trends, and changes in coastal-marine lineages in relation to modern HCS faunas. Our synthesis shows that: (i) typical endemic coastal Pisco vertebrates persisted up to ~ 4.5 Ma, (ii) first modern HCS toothed cetaceans appear at ~ 7–6 Ma, coinciding with a decline in genus diversity, and (iii) a vertebrate community closer to the current HCS was only reached after 2.7 Ma. The genesis of the Peruvian coastal ecosystem seems to be driven by a combination of stepwise transformations of the coastal geomorphology related to local tectonic pulses and by a global cooling trend leading to the modern oceanic circulation system.

Characteristics of late Miocene Lithofacies - Paleogeography in the Southeast Region of Hanoi Depression

The characteristics of late Miocene lithofacies in the southeast region of Hanoi depression have been revealed on the basis of sedimentary evolution in relation to sea level change and tectonic movement. During late Miocene, global sea level change has created one depositional sequence and three sedimentary systems tract. The lowstand systems tract (LST) is characterized by 5 rhythms of alluvial lithofacies. These rhythms are represented by rough, humoc seismic wave fields. This environment is not favorable condition for coal formation. The transgressive systems tract (TST) is characterized by 6 transitional lithofacies rhythms. Each lithofacies rhythm consists of 4 facies: the tidal flats sand facies of the bay, the mud facies of the river mouth lagoon, the coastal marshy mud facies creating coal and the bay greenish-gray clay facies. The pacing process involves changes in the local sea level caused by tectonic lift. Each tectonic subsidence phase takes place at a very slow velocity, so it is compensated for fine-grained sediment creating marshy mud facies to develop mangroves on a large scale. It is a prerequisite to create thick coal seams distributed near the end of each rhythm. A part of highstand systems tract (HST) are eroded due to the improved folding process, creating an angular unconformity boundary with Pliocene-Quaternary sediments. However, after reconstracting of deformated section, it was clear that this systems tract had only one rhythm including 2 facies: prodelta mud facies and alluvial fan sandstone facies.&#x0D; Keywords: Lithofacies-paleogeography, sedimentary systems tract, sedimentary rhythm

Early Cretaceous (∼138–134 Ma) Forearc Ophiolite and Tectonomagmatic Patterns in Central Tibet: Subduction Termination and Re‐initiation of Meso‐Tethys Ocean Caused by Collision of an Oceanic Plateau at the Continental Margin?

AbstractThe Bangong‐Nujiang Suture Zone (BNSZ) in central Tibet represents relics of the Meso‐Tethys Ocean that once separated the Lhasa and Qiangtang terranes. When and how this ocean closed has been a matter of debate, and the cause of the uncertainty stems primarily from a lack of consensus as to whether Early Cretaceous ophiolite exists within the BNSZ. New geochronological and geochemical data presented here identify ∼138–134 Ma MORB‐like and OIB‐type mafic rocks in the Kangqiong area of the western BNSZ. These data, together with the spatially associated peridotite and marine sediment, provide the first firm evidence for the presence of Early Cretaceous ophiolite within the BNSZ. Our integrated study of geological, mineralogical, and geochemical data suggests that the Kangqiong Ophiolite formed in a forearc setting related to the subduction re‐initiation of the Meso‐Tethys Ocean. By combining the present results with a synthesis of tectonomagmatism in central Tibet, including the existence of an Early Jurassic oceanic plateau within the Meso‐Tethys Ocean, the magmatic lull (∼145–130 Ma) in the Qiangtang Terrane, an angular unconformity between Lower Jurassic oceanic rocks and Upper Jurassic shallow‐marine strata, and the compositional differences and migration of magmatism before and after the magmatic lull, we propose a new model of the evolution of the Meso‐Tethys Ocean. This model proposes that the collision of an oceanic plateau with the Qiangtang continental margin caused the termination of Meso‐Tethyan subduction during the Late Jurassic and subsequent re‐initiation behind the oceanic plateau during the Early Cretaceous.

Geodynamic evolution of the Tunisian margin during the Albian–Cenomanian: structural evidence of the Austrian orogenic phase and the early tectonic inversion of the Tunisian Atlas

The Zebbag and Fahdene formations outcrop onshore Tunisia and provide an excellent opportunity to test models of the tectonosedimentary evolution of this region during the Albian–Cenomanian. A NW–SE compressive stress regime resulted in shortening of the Tunisian margin and this compressional tectonism defines the Austrian phase described in the surrounding margins. This event is not widely documented, but regionally extensive tectonism is suggested by NE–SW thrusting and folding, which produced an angular unconformity, active halokinetic diapirs and transpressional NW–SE pull-apart basins. The observed compressional deformation can be considered as a precursor to the Alpine Orogeny and led to the inversion of palaeoblocks inherited from Tethyan Jurassic and Lower Cretaceous rifting. A late Albian–Cenomanian onset of compressional deformation along the Tunisian margin may be intimately related to the drift of Africa with respect to Europe and to opening of the Atlantic Ocean.

Middle Jurassic orogeny in the northern North China block

Multiple shortening took place during the Mesozoic in the periphery of the North China block (NCB), a major tectonic grain of East Asia, but no consensus exists as to the timing, intensity, and causes of individual events. This work focuses on the first phase shortening in Jurassic, which represents the initiation of Yanshanian orogeny, by investigating the structural evolution of the Yanshan belt in the middle segment of the northern NCB. The first-phase shortening used to be regarded as a weak compressive event for it is recorded by parallel or low-angle unconformities between Middle and Upper Jurassic strata in some places. Our recent field investigations, however, identified some pronounced angular unconformities separating the Upper Jurassic strata from the underlying highly folded units in the northern Yanshan belt. The new observations demonstrate that the first-phase shortening was actually a strong contractional event. The co-existence of parallel and angular unconformities in the southern and northern Yanshan belt can be explained by different thrusting processes. It is argued that lateral variations of initial basement dips must have played a crucial role in controlling structural evolution of the overlying pre-Middle Jurassic sedimentary succession. It is shown that the first-phase thrusting and folding system was thin-skinned and predominantly verge northwestward. Deformational intensity also weakened toward the north and northwest. The first-phase shortening is constrained to occur in a short time span from 170 to 165 Ma based on new radiometric ages of strata above and below the unconformity, and presumably took place simultaneously in the northern NCB. The first-phase shortening was likely driven by horizontal tectonic push from the southeast. It is suggested that low-angle subduction of the paleo-Pacific plate might be responsible for the Middle Jurassic orogeny in the northern NCB and other parts of East Asia.

Constructing large-scale complex aquifer systems with big well log data: Louisiana model

This study introduces a lithofacies modeling method to construct a complex Louisiana stratigraphy model up to depth of 900 m using more than 114,000 well logs. Lithofacies intervals in one well log can be as many as 76. Louisiana comprises a number of interconnected aquifer systems and distinct geological features, such as angular unconformities, domes, and faults. The method contains processes of tessellation, discretization, translation, and interpolation to accommodate these geological features. First, the model domain is tessellated into tiles. Formations have the same dip direction within the same tile, but may have varying dip angles. Each tile is then discretized into computational cells. The data of well logs in and around a tile are translated to a non-dipping domain. A tile-lithofacies model for each tile is then constructed by interpolating well log data in the non-dipping domain. Tile-lithofacies models for all tiles are generated concurrently through parallel computing. Finally, the tile-lithofacies models are back-translated to the dipping domain and are assembled to form a complex lithofacies model. The method was applied to construct a Louisiana stratigraphy model as deep as 800 m. The model revealed two different depositional patterns in southeastern Louisiana likely due to depositional dissimilarity of the ancestral Mississippi River and Tennessee River. The major recharge zones appear to be in northwestern, southeastern, and west-central Louisiana. These recharge zones represent essential outcrop zones for Louisiana's principal aquifers. The model confirmed hydraulic connections of the Mississippi River and the Red River to their underlying alluvial sands. Hydraulic connections among different aquifer systems were also revealed. The Louisiana lithofacies model offers valuable information to improve current knowledge about Louisiana's geology and demonstrates a method potential for large-scale lithofacies modeling.

Arc‐parallel extension in preparation of the rotation of southwest Japan: Tectonostratigraphy and structures of the Lower Miocene Ichishi Group

AbstractOpening of the Japan Sea back arc basin was accompanied by extensional tectonics in the drifting southwest Japan arc. Various trends of Early Miocene grabens in the arc suggest multi‐directional rifting, which necessarily involved strike‐slip components of some of basin‐margin faults. However, such components are not well understood. In this work we conducted a field survey in the Early Miocene Ichishi basin on the northern side of the Median Tectonic Line, central southwest Japan. We found that the basin was a compound of grabens that were formed along normal and sinistral strike‐slip faults, the latter of which had northeast–southwest trends. The block faulting in this phase produced basement highs between sub‐basins, which were filled with the lower part of the Ichishi Group. We found a low‐angle angular unconformity at a middle horizon in the group, with which we define the upper and lower part of the group. The upper part onlapped both the basement highs and the lower part. It means that the transtensional basin formation ceased sometime between 18 and 17.5 Ma in the Ichishi area. The Ichishi basin turned subsequently into a sag basin subsided due to normal faulting probably along the Nunobiki‐sanchi‐toen fault zone. The transtension and the basin sag were driven by ENE–WSW extensional stress. This arc‐parallel extension produced grabens various areas including Ichishi in the Early Miocene. The extensional deformation was eventually localized to the deep rift along the Fossa Magna to make the lithosphere under southwest Japan decoupled from that under northeast Japan. The decoupling allowed the rapid rotation of southwest Japan from ~17.5 Ma. The cluster of those grabens around the Ise bay probably determined the southeastern margin of the Kinki triangle.

Velocity anomalies and out-of-plane reflections in Barnicarndy 1, Canning Basin

Barnicarndy 1 was drilled as a stratigraphic well and penetrated 2585m of clastic sedimentary rocks overlying an angular unconformity composed of Proterozoic dolomite basement near SSE end of the eponymous graben (previously known as the Waukarlycarly Embayment) of the Canning Basin. The post-drilling data exhibited several types of anomalies and inconsistencies, including ultra-slow velocity of the vertical seismic profile (VSP) within the top 210m, substantially high velocity from 210 to 1390m, poor synthetic–seismic correlation, and discrepancies between the prognosis and actual depths. The VSP shows that the uppermost sedimentary section has an average velocity of less than 1000m/s, which is unrealistically slow and probably caused by a systematic time delay in the data acquisition. Such a delaying effect was removed when calculating the VSP interval velocity, which had reasonable consistencies with sonic velocity and lithological packages. The consistencies verify that the sections from 210 to 1390m have high velocities that are beyond the range of offset wells at equivalent depths and do not follow an expected trend along most of the well trajectory. Quantitative analyses of multiple synthetic correlations suggest that the depth discrepancies in the prognosis are not related to pre-drilling seismic interpretation, as the two-way time picks for major boundaries mostly match what has been penetrated in Barnicarndy 1. Two thirds of the depth error for basement prognosis comes from the unexpected high velocity in the shallow formations. The velocity anomalies indicate that the Barnicarndy Graben possibly has a complex history of tectonic movement, and thick sections could have been eroded during the Late Triassic. The remaining discrepancy in depth prognosis for the basement is related to the out-of-plane seismic reflection near the SSE end of the graben. The steeply dipping basement causes out-of-plane issues for 2D seismic data, and the Kidson seismic survey images the shallower basement reflected from further south, rather than what is vertically below the survey route.

A Discussion on the Characteristics of Tectonic Transformation and Its Mechanism of 2nd Episode of Zhu-Qiong Movement in Zhu 1 Depression, South China

Based on the detailed interpretation of high-accuracy 3D seismic data, the characteristics of regional unconformities, fracture systems, sedimentary filling and paleo-stress field in Zhu 1 depression were comprehensively analyzed. The results showed that 2nd episode of Zhu-Qiong movement was an important tectonic transformation that occurred during rifting, basin structure patterns above and below the regional angular unconformity caused by 2nd episode had distinct differences. The orientations of those basin-controlling faults changed from NE, NEE to EW, NWW, which indicated that the paleo-stress field veered from NNW extension clockwise to sub-NS extension and basin structures patterns transformed from NE, NEE strong rifting to sub-EW, NWW weak rifting. The depocenter showed seesaw-like migration from S to N. And the sedimentary systems transformed from semi-deep-deep lacustrine facies to braided river delta facies developed in shore-shallow lacustrine. Combined with its tectonic recombination of periphery plates that period, the mechanism of above tectonic transformation had direct correspondence to the readjustment of the Indian plate and the Pacific plate. The southward subduction of the pro-to-south China Sea is the direct cause of this tectonic transformation.