Rift Sequence Research Articles

Chapter 4. Evidence of Working Petroleum Play Systems: Oil Seeps and Slicks

Maturation and accumulation of economic quantities of hydrocarbons are a common sedimentary component of continental rift basins around the world, both ancient and modern. Thus, groundwater is not the only fluid that may migrate along major rift faults away from basinal depocentres. De-risking petroleum exploration in rift basins requires demonstrating the presence of working petroleum play systems and the presence of leaking hydrocarbons to the surface is a primary way of demonstrating they are operating. Oil seeps have been known in Lake Albert since 1925 and the Kingfisher, Mputa-Waraga and Jobi-Rii petroleum play systems are due to go into full oil production. Lake Edward, however, has remained a virgin frontier exploration area. Tufa-travertine limestones along the eastern rift bounding fault of the Rwenzori Mountains have yielded organic residues which GCMS analyses indicate have a mature, oil-like distribution. Biomarkers suggest a marine shale source rock, the origin of which is at odds with both oils from Lake Albert and generation within the present EARS. A Lake Edward Slick Survey also sampled oil on the water surface that has the same biomarker signatures as the tufa-travertine-derived residues. Together they suggest maturation from a deeper pre-Neogene (?Mesozoic–Paleogene) rift sequence.

Chapter 3. Release of Geothermal Energy: Hot Springs and Tufa-Travertines

Heat and groundwater flow through a rift basin are an integral part of its geodynamics, but predicting what is happening below the surface is often difficult due to a lack of direct information. Field observations on the occurrence, subaerial distribution, temperature and geochemistry of freshwater springs may help to form an idea of groundwater flow through the basin and what it is, or has been, flowing through. Hot springs in the Albertine Rift are common and their occurrence is directly linked to deep-seated main rift bounding faults, or major intra-basinal fault intersections. The majority of Lake Edward and Lake Albert active, or paleo-springs, are also associated with precipitation of localised tufa-travertine limestones. The cooler tufas may contain calcitised plant roots, leaf imprints and freshwater gastropods. Active scavenging of uranium can be demonstrated in the algae and cyanobacteria that inhabit active spring mouths and corresponding tufa-travertines are depleted in radioactive K, U and Th elements. The source for concentrated HCO3- in groundwater at depth, needed to precipitate limestones at the surface, remains problematic. However, REE+Y geochemistry of the tufa-travertines suggest end-member sources of either carbonatites or marine limestones, indicating the possibility of a pre-Neogene rift sequence beneath the Albertine Rift.

Tectonostratigraphic framework and provenance of a Mesoproterozoic rift succession: An example from the Espinhaço Supergroup, SE Brazil

During the Mesoproterozoic era, the amalgamation of the Rodinia supercontinent witnessed numerous intraplate rifting events far-field induced by “Grenvillian-type age” orogenic systems. The intimate connection between tectonics and sedimentation enables the reconstruction of these intraplate rifting cycles, linking them to accretionary events along craton margins. This is essential for understanding the paleotectonic history of continents during the Mesoproterozoic era. In southeastern Brazil, the Espinhaço Supergroup documents long-lasting superimposed rifting events that extended to the eastern border of the São Francisco paleocontinent from the Paleoproterozoic to the Mesoproterozoic era. Three unconformity-bound rift sequences (∼1.77–1.7 Ga; ∼1.57–1.5 Ga; and ∼ 1.19 Ga) have been identified within the Espinhaço Supergroup, terminated by sag-marine deposits of the Conselheiro Mata Group. In the southern extension of the southern Espinhaço range (Cipó range), a detailed sedimentological and stratigraphic study was conducted on a rift-related sequence with unknown stratigraphic positioning. This sequence is bounded at the base by eolian sediments of the Galho do Miguel Formation of the Southern Espinhaço Supergroup and at the top by Ediacaran-Cambrian carbonates and pelites of the Bambuí Group. Examination of the tectonically-induced depositional controls in the provenance signatures involved stratigraphic surveys coupled with U-Pb geochronological data of detrital zircons. The lower section of the rift-related sequence transitions eastward into marine sediments of the Conselheiro Mata Group within the Santa Rita Formation, now identified as the Lower Member. With a maximum depositional age of 1660 ± 32 Ma, this section directly overlies the Galho do Miguel Formation by an erosive unconformity, comprising syn-rift coarse-grained deposits from alluvial and fluvial systems within an inverted half-graben structure. The Lower Member of the Santa Rita Formation provides evidence of the Mesoproterozoic rifting event associated with the Conselheiro Mata Group. The marine sediments of the Undefined sequence have a maximum depositional age of 1462 ± 19 Ma, overlying the Lower Member of the Santa Rita Formation with an erosive unconformity. The stratigraphic position of this sequence remains speculative, possibly linked to a later extensional phase of the Mesoproterozoic Conselheiro Mata rifting event or one of the two continental rifting stages leading to the Neoproterozoic Macaúbas Group.

The origin and tectonic evolution of the late Neoproterozoic rift basin in the Tarim Craton, NW China

The Cryogenian-early Paleozoic succession in the Tarim Craton is generally regarded as a continuous rift sequence that followed the prolonged superplume-related breakup of Rodinia. However, the rift architecture and origin of the late Neoproterozoic basin are poorly known, but these are important for the understanding of the tectonic evolution of the basin and for the evaluation of the hydrocarbon potential. This paper aims to elucidate the rift architecture, distribution, origin and evolution of the late Neoproterozoic Tarim Basin by integrating outcrop investigations with well and seismic data. The late Neoproterozoic magma-rich active rifting primarily occurred in the northeastern part of the basin, whereas other areas experienced slightly younger magma-poor passive rifting. The Ediacaran succession overlaps in the northern basement uplift to show a pre-Ediacaran unconformity occurs in the northern Tarim Craton, while a sub-Cambrian unconformity traverses the entire Tarim Craton and presents a distinct truncation in the southern basement uplift. A tiered architecture exists in which narrow, deep Cryogenian grabens are overlain by a broad Ediacaran depression. Two NE-striking rift systems developed in the northeastern and southwestern Tarim. It can be inferred from the compiled data that the late Neoproterozoic rift basin evolution in the Tarim Craton can be divided into four stages: early Cryogenian rifting, late Cryogenian fault-graben, Ediacaran intracratonic depression, and end-Ediacaran regional uplifting. These results suggest that documented Neoproterozoic retreating subduction-related slab failure/detachment processes led to both active and passive rifting, which contributed to the diverse continental rift architecture and evolution of the late Neoproterozoic Tarim Basin.

Rifting of the upper Yangtze carbonate platform: Insight from middle Permian carbonate olistostromes in the NE Sichuan basin, SW China

Rifting of the upper Yangtze carbonate platform in the middle Permian gave rise to Kaijiang-Liangping (KL) and Deyang-Wusheng (DW) rifts in the NE Sichuan basin, SW China. These rifts are well-endowed in gas, hosting eight out of nine discovered gas fields in the Sichuan basin. However, the initiation time and geodynamics driving the rifting are poorly understood, which restricts petroleum evaluation and exploration in the KL and DW regions. Our field surveys found that the basalt and underlying volcano-carbonate olistostrome in Huayingshan Mountain (Mt.) in the NE Sichuan basin are the interlayers within the limestone of the middle Permian Maokou Formation (Fm.). Stratigraphically, the Maokou Fm. and the late Permian sedimentary rocks above it form a mega-sequence from rifting to subsidence. Within the Maokou Fm., the rifting sequence comprises the basalt, volcano-carbonate olistostrome (tephra-rich polymictic breccia limestone) and limestone oligomictic breccia of the slope facies. A new data set composed of stratigraphic, petrologic, geochemical, isotopic and zircon U-Pb analyses shows that the basalt is sub-alkaline basalt and displays OIB-like geochemical signatures. The tephra block is basaltic tuff with abundant lath-shaped plagioclase crystals. Parent magma of the basalt originated from low-degree partial melting of the slightly enriched asthenospheric mantle at the garnet-spinel peridotite transition zone. The basalt and volcano-carbonate olistostrome reveal syn-rifting magmatism in ca. 267 ± 3 Ma. The KL and DW rift basins developed on the substratum of the upper Yangtze carbonate platform, and were triggered by asthenosphere upwelling during the Wordian stage.

Unconformity-bounded rift sequences in Terreneuvian–Miaolingian strata of the Caledonian Highlands, Atlantic Canada: Reply

Unconformity-bounded rift sequences in Terreneuvian–Miaolingian strata of the Caledonian Highlands, Atlantic Canada: Reply

Unconformity-bounded rift sequences in Terreneuvian−Miaolingian strata of the Caledonian Highlands, Atlantic Canada: Comment

Unconformity-bounded rift sequences in Terreneuvian−Miaolingian strata of the Caledonian Highlands, Atlantic Canada: Comment

The Boipeba member in the Jatobá Rift (northeastern Brazil): Occurrence, facies characterization and palaeoenvironmental implications

Fluvio-aeolian and lacustrine depositional systems are commonly associated with the initial sedimentary processes in rift-type basins, typically indicative of relatively calm tectonic settings. Fluvio-aeolian sandstones from the Boipeba Member of the Aliança Formation have been documented in well-drilled samples from the Recôncavo and Tucano basins, with thicknesses decreasing progressively from south to north. These units constitute the Brotas Group, encompassing deposits from the Jurassic period (Dom João local Brazilian stage). However, due to the absence of any records within the Jatobá Basin up until now, its occurrence in this basin remains uncertain. Based on facies analysis and depositional architectural elements, the fluvio-aeolian system of the Boipeba Member can be categorized into four associations (1) medium to low energy fluvial channels, (2) floodplains, (3) dunes and (4) sand sheets. Palaeocurrent data have indicated a variable paleoflow direction ranging from SW and NW, which is expected for the rift-onset phase fluvial system. Within the broader context of the Recôncavo-Tucano-Jatobá Rift System (RTJ), five distinct fluvial and/or aeolian units can be identified: the Tacaratu Formation (Syneclisis), the Boipeba Member (the basal unit of the Aliança Formation), the Sergi Formation (Early Rift), the São Sebastião Formation (Rift), and the Marizal Formation (Post-Rift). The Boipeba Member is the first unit of the Early Rift Sequence in the RTJ, deposited in a fluvio-aeolian environment until it evolves to the exclusively lacustrine deposition of the Capianga Member (the upper unit of the Aliança Formation). The subsequent recurrence of the fluvio-aeolian cycle is evident in the Sergi Formation. Faciological and petrographic analyses of outcrops in drainage channels in the Tacaratu municipality, located in the southeastern part of the Jatobá Basin, were performed. The rock formations appear as whitish-gray sandstones, typically thin to medium, subrounded, and consistently sorted. Some samples exhibit clay matrix and contain feldspars. Notably, the presence of planar cross-stratifications and a distinct upper contact with sediments of lacustrine origin provide diagnostic features indicating the Boipeba Member. Based on the facies, depositional system and palaeocurrent analyses, the deposits of the Boipeba Member in the Jatobá Basin were interpreted as resulting from fluvial channel sedimentation, followed by subsequent aeolian reworking. This process transitioned gradually to a lacustrine infilling, exemplified by the Capianga Member. Therefore, the Aliança Formation within the Jatobá Basin signifies the complete deposition of a fluvio-aeolian and lacustrine system, exhibiting both of its members, as observed in the other basins of the RTJ.

КРЕЙДОВИЙ РИФТОГЕНЕЗ У ГЕОЛОГІЧНІЙ ІСТОРІЇ УКРАЇНСЬКОГО СЕКТОРУ ЧОРНОГО МОРЯ

Within the Ukrainian sector of the Black Sea, the Albian-Cenomanian rift faults formed three large rift basins, which consisted of a system of grabens and half-grabens. One of the sublatitude rift basins occupied the modern Karkinit Trough, Krylov-Zmiiny Uplift, Gubkin Ridge and the Sulina Depression on the Odesa Shelf. The second sublatitude basin occupied the modern Tetyaev High, Sorokin Trough, Marine Continuation of the Crimean Folds and, apparently, the Crimean Mountains. The third rift basin extended from the northwest to the southeast and encompassed the Euxinian Graben, Andrusov Ridge, Eastern Black Sea Basin, and Shatskiy High. The greatest depth of the base of the rift sequence is observed in the central part of the Western Black Sea Basin, where it is 15.5–16.5 km. In the Eastern Black Sea basin, the maximum depth is 12.5–13.5 km. The shallowest depths of the base of the rift sequence are characteristic of the Kalamit Swell and the Marine Continuation of the Crimean Folds. During the Cenozoic phases of compression, almost all rift (half)grabens underwent intense folding and vertical movements from several hundreds of meters to 4–5 km. At the same time, most of the rift faults underwent partial or complete inversion.

Origin of the Songpan-Garze Terrane on the northeastern Tibetan Plateau, eastern segment of the Tethyan tectonic domain: A Middle Permian−Early Triassic intracontinental rifting system

The initiation of a continental rift or the birth of an oceanic basin, and their long-term evolution, are challenges in the understanding of the evolution of regional and global tectonics. Rifts develop within the interior or margins of plates, essentially as extensional structures, and they have been studied widely since the discovery of plate tectonics. The eastern segment of the Paleo-Tethys traversed central Asia, its structural features, timing, geological setting, and geodynamic mechanisms are poorly understood. The Songpan-Garze Terrane (SGT) on the northeastern Tibetan Plateau has been regarded as either a remnant of the Paleo-Tethys Ocean or a continental rift basin. Different interpretations of the SGT lead to various tectonic reconstructions between Gondwana and Laurasia. Here, sedimentology, geochemistry, and geochronology have been applied to the southern and northern parts of the SGT to reconstruct the initiation of a Permian rift system. In this area, for the first time, field investigation reveals that the SGT is characterized by a continuous succession of Permian−Lower Triassic terrestrial-neritic deposits and ocean island basalt (OIB)-type basalts, and that it developed as an intracontinental rift basin in the Yangtze Block. Zircons from gabbros with OIB-type geochemical signatures intruded in the southern SGT yield U-Pb ages of 263−254 Ma, constraining the timing of the initial rifting. Proterozoic felsic rocks collected from the southern SGT yield U-Pb ages of 761−865 Ma, and they are petrologically and geochemically comparable to Neoproterozoic basement rocks along the western margin of the Yangtze Block. The Middle−Late Permian volcano-sedimentary sequences of the SGT and their geochemistry and geochronology are comparable to rift sequences in the Panxi region of the western Yangtze Block. Thus, it has been proposed that the SGT originated as an intracontinental rift within the Yangtze Block during the Middle Permian−Early Triassic, and that it continued to deepen as a rift basin through the Middle−Late Triassic.

The effect of tectonic subsidence on the evolution of depocenters and development of the Mundaú sub-basin, Brazilian Equatorial margin

Located on the Brazilian Equatorial Margin, the Mundaú Sub-Basin is the only offshore hydrocarbon-producing region of the Ceará Basin. The origin and evolution of the Mundaú Sub-basin are a consequence of the rupture of the Pangea supercontinent during the Early Cretaceous, which led to the opening and development of the Equatorial Atlantic Ocean. This area has been considered a new exploration frontier, promoting research to be developed in recent years. However, the interactions between transtensive events and spatial-temporal distributions of tectonic subsidence have been poorly studied. To cover this gap, fifty-seven wells and approximately 5800 km of 2D seismic lines were analyzed to understand these events. From these data, stratigraphic and structural interpretations were performed throughout the sub-basin, enabling the construction of 43 pseudo-wells. Using descompactation of the wells and pseudo-wells using the backstripping method, it was possible to reconstruct seven tectonic phases for the Sub-Basin, which correspond to the rift sequence (RP01, RP02, and RP03), the transition sequence (BP01), and the drift sequence (DP01, DP02, and DP03). Four sets of faults of three zones of the three zones (WZ, CZ and EZ) proposed for the Mundaú Sub-Basin were also analyzed. The faults are distributed in two orthogonal families, formed in two distinct events. Older R and P faults, respectively trending NWW-SEE and NNESSW, formed during RP01 (first event). Younger R and P faults, respectively trending NNW-SSE and NEE-SWW, formed during RP02 (second event). Tectonic reconstruction revealed the diachronic formation of two depocenters, one in the West Zone (WZ) during RP01, and the other in the Central Zone (CZ) during RP02. Despite the intense tectonic activity in WZ and CZ, the East Zone (EZ) showed low subsidence rates and little development during the entire rift phase. In turn, BP01 showed faster subsidence rates when compared to RP03, indicating tectonic influence even during the breakup phase. Finally, the drift phase showed the lowest subsidence rates, spatially distributed in an opposite pattern to the rift phase. From these results, a rotation of the Principal Displacement Zone (PDZ) was identified, with initial parallel motion vectors, forming the WZ depocenter and generating the first fault family during RP01; after RP02, motion vectors were oblique, forming the second depocenter in CZ and generating the second fault family. Significant thermal control associated with magmatic events was identified during the drift phase. This study also describes the main structures formed along with the overlap of parallel and oblique PDZ displacement.

Unconformity-bounded rift sequences in Terreneuvian‒Miaolingian strata of the Caledonian Highlands, Atlantic Canada

The Cambrian syn-rift strata preserved in western Avalonia provide a distinctive example of how unconformity-bounded sequences are diachronous throughout proximal to marginal rift branches. Terreneuvian‒Miaolingian third-order sequences of the Caledonian Highlands, New Brunswick, Canada, reflect a complex interplay among syn-rift tectonic events, denudation pulses, and sea-level fluctuations. Unconformably overlying the early, rift-related volcanosedimentary Coldbrook Group (ca. 560‒550 Ma), the Ratcliffe Brook, Glen Falls, Hanford Brook, and Forest Hills Formations can be subdivided into two transgressive systems tract (TST)‒highstand systems tract (HST) sequences (each ∼10 m.y.) and an incomplete TST sequence that are separated by stratigraphic gaps. They reflect uplift and tilting events affecting the basement, transgressive and drowning surfaces, and condensed sections. Arid to semi-arid climatic episodes are supported by the excellent preservation of mafic to felsic volcanic clasts in non-marine breccias and conglomerates, which are derived from the Ediacaran basement, and the local precipitation of marine gypsum through the evaporation of pore fluids. Early Miaolingian episodes of microbial/shelly carbonate production preserved precipitates of coeval evaporite (gypsum pseudomorphs after drusy mosaics of calcite) and ikaite (glendonitic, star-shaped aggregates and crusts). Both minerals, traditionally considered to be indicators of contrasting climate conditions, potentially co-occur in temperate-water substrates recording high rates of microbial activity. The early rift phases preserved in the western Avalonian rift transect comprise stepwise uplift and unroofing of rift shoulders, which are related to diachronous, angular discordances and paraconformities bounded by syntectonic slope-apron deposits. Facies homogenization was attained during Miaolingian times as a result of generalized flooding, sealing of paleotopographies, and blanketing with monotonous offshore-dominant shales.

The Tucano Basin (Cretaceous, Brazil) – a world-class example of an aborted rift system with anomalous depth

The structural and stratigraphic characteristics of the central portion of the Tucano Basin, a Cretaceous continental rift basin in northeast Brazil, was investigated, using post-stack 2-D reflection seismic, gravity and well data. Methodology was steered by the principles of seismic stratigraphy, targeting at an adequate identification of stratal terminations and on the mapping of structural features, where a negative flower structure was interpreted. The identification of erosional truncations led to a stratigraphic framework of seven intra-rift unconformities, forming eight third-order rift sequences, while the literature on the Tucano basin recognizes only two depositional sequences forming the rift sedimentary infill. In order to study the abnormal depth of Central Tucano depocenter, gravity data were processed and a spectral analysis was performed, which located the depocenter at approximately 16.8 km, a rare configuration for a rift basin. Gravity models were created in order to verify the seismic interpretation and the depth found by the spectral analysis; and to provide a view of the basin in a regional scale, analysing its crustal architecture. The modelling confirmed the abnormal depth and also indicated a severe crustal thinning under Central Tucano Basin, where the Moho was interpreted around 24 km and a major crustal necking was observed. The models introduced the geometry of the lower continental crust and indicated the presence of a high-density layer (3000 kg/m3) at its base, which this work suggests to be a result of magmatic underplating. The paper closes discussing the role of tectonics and basement inheritance over the Central Tucano Basin structural framework and commenting on the mode of extension for the Recôncavo-Tucano-Jatobá rift system.

Early Evolution of the Adelaide Superbasin

Continental rifts have a significant role in supercontinent breakup and the development of sedimentary basins. The Australian Adelaide Superbasin is one of the largest and best-preserved rift systems that initiated during the breakup of Rodinia, yet substantial challenges still hinder our understanding of its early evolution and place within the Rodinian supercontinent. In the past decade, our understanding of rift and passive margin development, mantle plumes and their role in tectonics, geodynamics of supercontinent breakup, and sequence stratigraphy in tectonic settings has advanced significantly. However, literature on the early evolution of the Adelaide Superbasin has not been updated to reflect these advancements. Using new detrital zircon age data for provenance, combined with existing literature, we examine the earliest tectonic evolution of the Adelaide Superbasin in the context of our modern understanding of rift system development. A new maximum depositional age of 893 ± 9 Ma from the lowermost stratigraphic unit provides a revised limit on the initiation of sedimentation and rifting within the basin. Our model suggests that the basin evolved through an initial pulse of extension exploiting pre-existing crustal weakness to form half-grabens. Tectonic quiescence and stable subsidence followed, with deposition of a sourceward-shifting facies tract. Emplacement and extrusion of the Willouran Large Igneous Province occurred at c. 830 Ma, initiating a new phase of rifting. This rift renewal led to widespread extension and subsidence with the deposition of the Curdimurka Subgroup, which constitutes the main cyclic rift sequence in the Adelaide Superbasin. Our model suggests that the Adelaide Superbasin formed through rift propagation to an apparent triple junction, rather than apical extension outward from this point. In addition, we provide evidence suggesting a late Mesoproterozoic zircon source to the east of the basin, and show that the lowermost stratigraphy of the Centralian Superbasin, which is thought to be deposited coevally, had different primary detrital sources.

Multi-scale tectono-stratigraphic analysis of pre- and syn-rift sequences in the Rio do Peixe Basin, NE Brazil

Intracratonic rift basins developed in Northeastern Brazil during the Early Cretaceous, recording the continental rifting stage that preceded the development of the passive margins along the South Atlantic. The Rio do Peixe Basin is a remarkable example of this set of basins. It comprises four well-exposed NE-to E-W-trending half-grabens, the larger of which, the Brejo das Freiras and Sousa Half-grabens, being connected by a complex rift-related structural high, the Santa Helena Step. The basin also represents a unique case in the region where Lower Devonian strata underlies the Lower Cretaceous Rift Sequence, as a remnant pre-rift Paleozoic basin. This work covers part of the Sousa Half-graben and the Santa Helena Step, combining outcrop, wells and 3D seismic data to improve analysis of the tectono-stratigraphic framework of both the pre- and syn-rift sequences. The Lower Devonian Sequence comprises a deltaic system, which delta front to prodeltaic deposits characterize a transgressive systems tract, followed by delta plain deposits of a regressive systems tract. The base of the Lower Cretaceous Rift Sequence is defined by a low-angle unconformity recognizable at both surface and subsurface. This sequence displays interfingered contacts between its coeval units and is subdivided into four tectonic systems tracts related to the beginning of extension and periods of high and low tectonic activity. The southern termination of the Brejo das Freiras Fault performs an important depositional control in the Santa Helena Step, which at this place didn't developed high escarpments and acted as source area for fluvial fans. In the strike ramp and flexural margin of the Sousa Half-graben, fluvial distributary systems predominate and the basement contact is characterized by NE- and E-W-trending oblique slip normal faults. Finally, our study emphasizes the association of the structural framework with the Neocomian NW-SE rifting extension responsible for the South Atlantic opening.

Sequence stratigraphy and organic geochemistry: An integrated approach to understand the anoxic events and paleoenvironmental evolution of the Ceará basin, Brazilian Equatorial margin

An organic geochemical investigation combined with sequence stratigraphy was performed in the Ceará Basin, an offshore basin located in Northeastern Brazil. The information available from 30 well logs (gamma-ray, resistivity, density), besides geochemical (TOC, and pyrolysis indexes) and isotopic (δ13C) data, aided the preparation of a dataset for this study. The application of sequence-stratigraphic methods helped classify and correlate seismic and organic facies. Four key petroleum source-rock units were identified, from the oldest to the youngest: (1) Mundaú Formation – top of the Rift Sequence (Berriasian-Aptian); (2) Paracuru Formation – Breakup Sequence (Aptian-Albian); (3) Itapajé Member of the Ubarana Formation – Continental Drift Sequence (Albian-Turonian), and (4) Uruburetama Member of the Ubarana Formation – Continental Drift Sequence (Turonian-Maastrichtian). The geochemical characteristics of the Mundaú Formation (high total organic carbon (TOC), hydrogen index (HI), relative hydrocarbon potential (RHP = (S1 + S2)/TOC)) point to a typical transgressive sequence. Six transgressive-regressive (T-R) cycles were recognized in the entire Paracuru Formation. The best geochemical marker is related to the top of the Paracuru Formation. This stratigraphic unit can be correlated to a major anoxic event and is the best source rock of this basin. Evaporitic facies found in this top section, maximum RHP values (anoxic conditions), and maximum flooding surfaces related to transgressive events characterize this interval. Moreover, the wide spatial cover of organic-rich rocks, carbon isotopic data, and the recognition of favorable characteristics for anoxia in other basins of the Equatorial Margin are suggestive of the Aptian-Albian Oceanic Anoxic Event (OAE-1b) occurring in the Ceará Basin. The Ubarana Formation represented by the Uruburetama Member and the Itapajé Member yields the least promising source rocks. However, high TOC values suggest the occurrence of the late Cenomanian–early Turonian Oceanic Anoxic Event (OAE-2), when organic-rich strata started to deposit in deep-water regions. The predominance of a regressive interval in the Uruburetama Member points to oxic or sub-oxic conditions. Additionally, the correlations between the Brazilian Equatorial Margin and its African counterpart, and the organic geochemical characterization allied to the definition of depositional systems for these regions proved to be useful for oil exploration.

The Barreirinhas Basin internal architecture and the evidence of transform movements along the Romanche Fracture Zone, Brazilian Equatorial Margin

The Barreirinhas Basin, located in the central part of the Brazilian Equatorial Margin, was originated by Pangea breakup during Early Cretaceous time. The Romanche Fracture Zone played an essential role during the entire tectono-stratigraphic evolution of the Barreirinhas Basin. This fracture zone divided the basin in two sub-basins to the south and north with distinct structural style printed on the basin internal geometry. This study is aimed to investigate how the tectonic and sedimentary evolution in the onshore, shallow and deep-water Barreirinhas Basin, were influenced by the Romanche Fracture Zone. Eight 2D seismic sections, subjected to seismic attributes to enhance the seismic stratigraphic horizons and structural features, and supported by five well profiles, allowed analyzing the basin infill, major faults, and the spatial relationship with the fracture zone. The basin evolution and the tectono-stratigraphic architecture are marked by structures from rift to drift stages. The rift sequences are composed of four seismic units (U1-U4) delimited on top by the horizon R4. The sag and drift sequences are composed of six seismic units (U5-U10) delimited between horizons R4-R10. The structural architectures are dominated by transtensional regimes, such as negative flower faults, tilted blocks, and wipeout zones, whereas transpressional tectonics is also present by reverse faults and positive flowers, which are fingerprints of Romanche Fracture Zone. Especially in the southern sub-basin, the transform tectonic activity deformed the entire basin from Aptian to Holocene. These structures correspond to those in Africa conjugate basins, where recurrent dextral strike-slip plate motions caused transtensional and transpressional deformations in the sedimentary pile since separation of South America and Africa. Asymmetric basin architecture, intensity and asynchrony of tectonic regimes distinguish the Atlantic Equatorial basins.

Mineralogy and origin of the alkaline Nsungwe Formation tuffs of the Rukwa Rift Basin, southwestern Tanzania

The incipient phases of continental rift development remain enigmatic, as early rift sequences are rarely exposed, or poorly preserved in the geologic record. Here, we present petrographic and geochemical analyses of heavy mineral separates from pervasively weathered volcanic horizons intercalated within syn-rift sedimentary deposits from the Rukwa Rift Basin, southwestern Tanzania. These Oligocene (25–26 Ma) ashfalls represent the first magmatic products associated with the initiation of rifting in the western branch of the East African Rift System (EARS). In this work, we combine detailed single crystal petrography with in-situ geochemistry to characterise an alkaline volcanic mineral assemblage consisting of titanite, phlogopite, fluorapatite, clinopyroxene, Ti-rich andradite, sanidine, pyrochlore, nepheline, cancrinite, Fe-sulphide (pyrrhotite), calcite, Na-REE‑carbonate and peralkaline silicate melt. These minerals are significantly enriched in trace elements, particularly Ba, Sr and rare earth elements (REE), with REE + Y contents consistent with derivation from peralkaline phonolitic melts. Placed in context of EARS development, these tuffs display the hallmarks of explosive alkaline‑carbonatite volcanism in their resistant minerals and inclusions; the nature and occurrence of which is consistent with early rift progression elsewhere in the EARS. Importantly, we showcase the depth of extractable petrographic information preserved in extensively weathered volcanic ash horizons deposited into an active continental rift basins. Such an approach may have significant implications for recognising early volcanogenic sequences in continental rift zones worldwide, but particularly in other segments of the western branch of the EARS.

Seismic Stratigraphy of Camamu Basin, Northeastern Brazil

The Camamu Basin is located at the northeastern Brazilian coastline and has significant hydrocarbon potential in both shallow and deep water settings. However, despite an already operating productive gas field, the basin is not well known. Herein, a regional stratigraphic interpretation of the rift, the transitional and the drift megasequences is reported, based on a data set of 152 post-stacked 2D seismic lines, 1 3D seismic cube and 34 wells with eletro-logs. The study revealed that the rift megasequence is much more complex than previously thought. In the southern region of the Camamu Basin, five rift sequences were mapped, while in the northern region, in the area of the regional tectonic lineament known as the Salvador Transcurrent Zone (STZ), seven rift sequences were recognized. This difference suggests a tectonic control during the rifting process, because the northern region is intensely affected by shear stress induced by transcurrent tectonics of the STZ during the crustal breakup. For the post-rift or "transitional" phase associated with thermal subsidence installed after the rift, as well as for the drift succession, the tectonic control exerted by Salvador's Transcurrent Zone is not detectable and the tectono-sedimentary evolution of the basin follows the general pattern of the Brazilian marginal basins.

Seismic attributes and analogs to characterize a large fold in the Taranaki Basin

The Taranaki Basin lies in the western portion of New Zealand, onshore and offshore. It is a Cretaceous rift basin that is filled with up to approximately 10 km thick deposits from marine to deepwater depositional environments from the Cretaceous (approximately 93 ma) to the Neogene (approximately 15 ma). This basin underwent important tectonic events that resulted in large-scale features such as faults and folds and the deposition of turbidites such as channels and channel belts. These features easily are recognizable in seismic data. When analyzing the offshore 3D Pipeline data set, we recognized a peculiar fault-like feature with large-scale dimensions (approximately 15 km long and approximately 1 km wide) within the sequence. The alignment was perpendicular to the direction of deposition in the basin (southeast–northwest) as identified by previous studies and subparallel to the main structures in the area (southwest–northeast). We interpreted the seismic character of the funny-looking thing (FLT) likely as (1) a fault, (2) a fold, or (3) a large channel belt within the basin. We use seismic attributes such as coherence (Sobel filter), dip, cosine of phase, and curvature to characterize this feature geomorphologically. The geologic background of the area and analog settings aided in understanding and distinguishing the nature of this large structure. Monocline examples in seismic data are rare to find, and we want to show how to avoid misinterpretations. Geological feature: Fault-bend fold or large-amplitude fold (possibly monocline) Seismic appearance: Large, discontinuous, high-variance feature Alternative interpretations: Fault, fold Features with a similar appearance: Fault, fold, wide straight channel belt (time or horizon slice) Formation: Rift sequence of the Taranaki Basin Age: Eocene Location: Taranaki Basin, Western offshore New Zealand Seismic data: Provided by New Zealand Petroleum and Minerals Contributors: Karelia La Marca, Heather Bedle, Jerson Tellez; School of Geosciences; University of Oklahoma, Norman, OK, USA Analysis tool: 3D reflection seismic, geometric seismic attributes