Flexural Margin Research Articles

Deciphering hydrogeochemical evolution in the multilayered Ilhas-São Sebastião aquifer system, Brazil: Implications for groundwater resources management

This study examines the hydrogeochemical processes shaping groundwater quality in the Ilhas-São Sebastião aquifer system, situated at the interface of the Central and Southern Recôncavo basins in the densely populated area near the Brazilian metropolis Salvador. Analysis of 71 groundwater samples reveals distinctive hydrogeochemical compositions in aquifers. In the São Sebastião aquifer, alkalis (Na+ + K+) and strong acids (Cl− and SO42−) prevail. Furthermore, a moderate correlation of Na+–Cl-marks an evolution from Mg–Ca–HCO3 to Mg–Ca–Cl and Na–Cl facies. In contrast, the Ilhas aquifer displays a Na+>Ca2+>Mg2+>K+ relationship for cations and HCO3− > Cl− > SO42− > CO32− for anions and a recharge-discharge trajectory from the Mg–Ca–HCO3 to the Ca–Na–HCO3 facies. Additionally, it presents greater mineralization and dispersion of physicochemical parameters, especially around sub-basin depocenters. Its hydrogeochemical signature is characterized by robust correlations between TDS and EC, and between these parameters and SO42−, HCO3−, Ca2+, and Mg2+, complemented by moderate correlations of EC with Na+ and Cl−. Bivariate Gibbs diagrams and ionic ratios indicate silicate weathering and ion exchange as the primary geochemical processes controlling solute concentrations in both aquifers. However, in the Ilhas aquifer, a subordinate contribution from reverse ion exchange is indicated by weak (TDS–Na+, TDS–K+, Na+–Ca2+, K+–Ca2+) and positive TDS–Ca2+ and TDS–Mg2+ correlations. Conversely, negative chloroalkaline indices and the moderate Na+–Cl- correlation indicate that reverse ion exchange processes are mostly absent in the São Sebastião aquifer. Instead, both chloroalkaline imbalance reactions and silicate weathering contribute equally to the observed geochemical patterns. Groundwater geochemical signatures indicate recharge on flexural margins, active water-rock interaction in large depocenters, and mixing of hydrogeochemical facies between aquifer units. These insights contribute to a comprehensive understanding of groundwater evolution, crucial for effective water resource management in the region.

Sedimentary provenance in continental rifts: U–Pb detrital zircon, Nd and Sr isotopes and lithogeochemistry of the Eocene alluvial sandstones of the Resende Basin, SE–Brazil

Multiproxy analysis of detrital minerals is a powerful tool for unravelling the source-to-sink evolution of syn-rift sedimentary basins that represent important depositional settings during major rift events. The Cenozoic Resende half-graben in SE Brazil is a classic example of a syn-rift basin developed under oblique extension, which preserves evidence of tectonic episodes and palaeoclimatic conditions during the opening of the Atlantic. Sediment in the basin was sourced from the surrounding basement rocks, which to the faulted margin consists of older Proterozoic orthogranulites and orthogneisses, whilst to the flexural margin is characterised by Neoproterozoic passive margin paragneisses. Cretaceous intra-plate alkaline stocks crop out at the western edge and in the central-south basement of the basin and represent a distinct provenance fingerprint. This study uses new detrital zircons U–Pb ages combined with previously published elemental and Nd and Sr isotopic data to characterise the sedimentary provenance of Eocene alluvial deposits and to understand how sediment sourcing has changed throughout the evolution of the basin. U–Pb detrital zircon analyses from samples collected across the Resende Basin are presented herein. Samples yield a spread of detrital age spectra from the Archaean to the Cretaceous. Precambrian zircons range from ca. 3228 Ma to ca. 542 Ma. Cretaceous zircons yield ages between ca. 138 Ma and ca. 67 Ma. The sediment source location is interpreted by comparing the yielded detrital zircon spectra with the age spectra of zircons from the basement rocks. The dominance of detrital zircons yielding ages of ca. 1500 Ma and ca. 1000 Ma is interpreted to be sourced from the Neoproterozoic basement rocks to the flexural border of the half-graben. Cretaceous zircons from samples with whole rock εNd40Ma > −10 are found in both the western and eastern portions of the half-graben but not in the central portions. It is interpreted that an along-axis fluvial system was responsible for a small increase in the compositional maturity of the Resende sandstones from west to east, resulting in the progressive dilution of Cretaceous alkaline igneous detritus along the basin axis. Preserved metamorphic rims in zircon grains indicate low transport abrasion, suggesting that Cretaceous zircons in the eastern depocentre could be associated with other small proximal sources. U–Pb zircon analyses and Nd and Sr isotopes provide reliable insights regarding sediment provenance and transport processes, whilst element geochemistry supplies evidence for hydraulic sorting effects. Consequently, the data represents a more robust estimation of the provenance patterns of the Resende Basin, showing chemical and geochronological differences that support the division of the basin into three depocentres (western, central, and eastern) connected by a longitudinal fluvial system sourced mostly by sediments coming from the south-western flexural border. This study highlights the usefulness of using multiple provenance proxies to unravel the complex development of rift basins, assessing multiple source terrains and the variable degrees of mixing in sedimentary basins.

Diagenesis of continental carbonates linked to the evolution of the flexural margin of the Triassic Sorocayense-Hilario rift sub-basin, Argentina

The lithostratigraphic units that conform the Sorocayense Group and fill the Sorocayense-Hilario sub-basin represent an alluvial-fluvial-lacustrine succession with significant volcanic supply during its deposition. They are namely the Cerro Colorado del Cementerio, Agua de los Pajaritos, Monina, Hilario and El Alcázar formations and present several carbonate levels. The genesis, diagenesis, and main controlling factors on continental carbonates present a means to understand basin evolution through the study of their petrography and chemical elemental composition through cathodoluminescence techniques. We have identified six microfacies: a) homogeneous micrite, b) bioclastic micrite, c) dolomicrite, d) laminated micrite, e) oncolitic packstone, and f) sparitic carbonate. Among these microfacies, six cementation and alteration phases have been identified: a) micritization, b) mechanical compaction, c) calcitic cementation, d) sparitic cementation, e) microsparitic cementation, and f) chemical compaction. This analysis allowed establishing a chronology of the diagenetic modifications undergone by the carbonates. Results support diagenesis was controlled mainly by tectonics showing major impact in carbonate facies identified at the flexural margin of the rift. The effect would have been linked to exertion of a paleohydrological effect favoring lateral meteoric water migration through faulting. The presence of dolomite in some of the microfacies is linked to the presence of montmorillonite as the dominant Mg-rich-phyllosilicate in clay assemblages most likely acting as the potential source. In turn, Mg is more likely to be replaced by Mn leading to luminescent carbonate microfacies.

Contrasting thermal evolution of the West African Equatorial and Central Atlantic continental margins

The landscape of the West African continental margins is the product of tectonic, thermal and surface processes acting in concert during and following the breakup of Gondwana. Central Atlantic opening was marked by the emplacement of the Central Atlantic Magmatic Province (CAMP) and continental breakup proceeded through Late Jurassic and Early Cretaceous divergent tectonics while opening of the Equatorial Atlantic was dominated by early and mid-Cretaceous transform movement. The onshore erosional response to these events is poorly constrained yet is a crucial component of our understanding of topographic evolution and sediment transfer across continental margins. We present new apatite fission-track (AFT) data for 24 samples from Guinea and 11 samples from Ivory Coast, and thermal histories from inverse modelling. Our data and thermal histories show the following: the thermal effect of the CAMP across Guinea and Ivory Coast; rapid cooling along the coast during the early to mid-Cretaceous related to erosion of short-wavelength rift-shoulders; moderate cooling across longer wavelengths reflecting a pattern of erosion across flexural margin upwarps; and low cooling rates from the start of the Cenozoic to present day, consistent with low magnitudes of erosion inferred by onshore geomorphological data. We present our results alongside the published regional AFT dataset and draw inferences on the thermal and tectonic evolution of the onshore margin.

An experimental study of reinforced concrete beamcolumn joint with partially high strengthened longitudinal bar

It is necessary to generate the yield position away from the column face to minimize damage to the beam-column joint during a large earthquake in a reinforced concrete building. The same efficacy can be realized using partially high strengthened rebar. The number of longitudinal bars can be calculated for a bending moment smaller than the column face, reducing their number compared to the conventional bar-arrangement method. This paper describes reinforced concrete an interior beam-column subassemblage tests using this rebar as the longitudinal bars of beams and a column. The beam yield hinge was formed at a position apart from the column face, and the damage to the beam-column joint was less than the conventional bar-arrangement method. Additionally, the good performance was obtained if the bending strength of the column was large, even if the shear capacity margin of the beam-column joint was small. The column-beam flexural strength ratio and shear capacity margin at the beam-column joint need to be set with consideration of their relationship.

Cenozoic uplift of the Fátima Basin, NE Brazil

The Fátima Basin is a small Cretaceous interior basin that was formed within the continental Borborema Province in northeastern Brazil. Its formation was related to the opening of the southern Atlantic Ocean. This study investigates the hypothesis of post-rift tectonic reactivation within this basin through an integrated set of geophysical (aeromagnetic), surface (outcrops), and subsurface (boreholes) data to model the tectono-stratigraphic framework of the basin. An analysis of several morphometric parameters, including the slope index, relief aspect, stream profiles, asymmetry factor (AF), stream-gradient index (SL), and valley floor width-to-height ratio (VF) was used to identify possible tectonic adjustments related to post-rift stress fields. The integration of geophysical and subsurface data showed that the basin possesses a half-graben geometry with the main basin axis trending NE-SW. Two families of topographic lineaments were identified: 1) NE-SW, corresponding to the main shear zones and faults of the Pre-Cambrian basement, and 2) NW-SE, representing faults and fractures in the adjacent basement and the sedimentary cover. A digital elevation model revealed that the maximum topographic elevation was 950 m, associated with deposits of the Paleozoic Tacaratu Formation located over the hinge line of the flexural margin of the basin. The AF values in the basin ranged between 25 and 76, and first- and second-order SL index anomalies were frequent, especially in the faulted border, the southeast sector, and the northeast sector. Analyses of the main river channels revealed VF values of around 1, which indicate active incisions in the river valleys. We suggest that the basin was subjected to differential uplift during the Cenozoic, which strongly affected the southeast and northeast sectors. Based on the exhumation of the Tacaratu Formation deposits, we estimate that the basin experienced approximately 500 m of uplift that influenced the erosion of Cretaceous deposits and adjacent basement rocks.

The hidden sedimentary basin underneath the Quaternary volcanic unit in Bogor and Kendeng area

The Java Island is an active volcanic arc that experiences several volcanism episodes, which gradually changes from South to North from the Late Oligocene to Pleistocene, following the subduction of the Australian plates underneath the Eurasian plates. During the Eocene, the southern and northern part of Java was connected as one passive margin system with the sediment supply mainly comes from Sundaland in the north. The compressional tectonics creates a flexural margin and a deep depression in the central axis of Java Island and acts as an ultimate deep-sea depocenter in the Neogene period. In contrast to the neighboring Northwest and Northeast Java Basins in the Northern edges of Java Island, the basin configuration in the East-West trending depression in median ranges of Java (from Bogor to Kendeng Troughs) are visually undetected by seismic due to the immense Quaternary volcanic eruption covers.Five focused window areas are selected for this study. A total of 1,893 Km sections, 584 rock samples, 1569 gravity and magnetic data, and 29 geochemical samples (rocks, oil, and gas samples) were acquired during the study. Geological fieldwork was focused on the stratigraphic unit composition and the observable features of deformation products from the outcrops. Due to the Paleogene deposit exposure scarcity in the Central-East Java area, the rock samples were also collected from the mud volcano ejected materials in the Sangiran Dome.The distinct subsurface configuration differences between Bogor and Kendeng Troughs are mainly in the tectonic basement involvement and the effect of the shortening on the formerly rift basin. Both Bogor and Kendeng Troughs are active petroleum systems that generate type II /III Kerogen typical of reduction zone organic material derived from transition to the shallow marine environment. The result suggests that these basins are secular from the neighboring basins with a native petroleum system specific to the palaeogeographical condition during the Paleogene to Neogene periods where the North Java systems (e.g., Northwest and Northeast Java Basin) was characterized by oxidized terrigenous type III Kerogen.

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.

Cordilleran Foreland Tectono-Sedimentary Element, Canadian Northern Interior Plains

Abstract Upper Jurassic–Paleocene siliciclastic strata comprise the Cordilleran Foreland Tectono-Sedimentary Element of Canada's northern Interior Plains. These strata record four major packages deposited on top of Paleozoic shelf strata on the NW margin of the Canadian craton. These packages are a Jurassic interval interpreted to record deposition associated with extension at the Arctic Ocean margin, a Lower Cretaceous, dominantly marine, interval deposited on the flexural margin of the foreland basin, and two Upper Cretaceous intervals of west-to-east progradational marine and non-marine strata deposited on the orogenic margin. The full succession has been affected by Cordilleran deformation within the Mackenzie Plain, Franklin Mountains and Colville Hills. Organic-rich shale is documented from Lower and Upper Cretaceous successions but these strata only reach thermal maturity in deeper parts of the basin, close to the Canadian Cordillera. Potential reservoirs exist within sandstone-dominated intervals throughout the succession, although some locally lack a top seal. One natural gas discovery has been reported from Upper Cretaceous sandstone of the Little Bear Formation at the Stewart D-57 well in the southeastern Mackenzie Plain. Oil sourced from Upper Cretaceous shale is reported from the Mackenzie Plain East Mackay B-45 well.

Tectono-stratigraphic evolution of a cretaceous intracontinental rift: Example from Rio do Peixe Basin, north-eastern Brazil

The Rio do Peixe Basin is part of a set of aborted rifts with NE-SW trend along the NE region of Brazil. This axis was originated in response to the stress caused by the fragmentation of Pangea during the Mesozoic, and effectively installed during the Cretaceous event that defined the Brazilian equatorial margin. The detailed structural framework allowed the recognition of the Brejo das Freiras fault zone which presents NE-SW trend and, locally, ENE-WSW. This fault zone, strongly influenced by the basement fabric, deformed the layers and originated drag folds, fault-bending folds, and fault-propagation gentle folds. The half-graben fault margin developed alluvial fans, triggered by tectonic activity and geologically instantaneous creation of accommodation space. At the central portion it predominates a lacustrine sedimentation, while fluvial systems are responsible for carrying sedimentary supply from the flexural margin towards the depocenter. Since tectonics controls sedimentation on rifts, the tectonic systems tracts correspond to an arrangement of contemporaneous depositional systems associated with basin evolutional stages. In the syn-rift sequence, corresponding to Rio do Peixe Group nine stratigraphic units were recognized, bounded by flooding surfaces and drowning unconformities which record cycles of expansion and contraction of the lacustrine system. The tectonic systems tracts related to the initial stages of the rift installation were identified, followed by moments of high and low tectonic activity. The seismic and the well data support both the tectono-sedimentary evolution of the Brejo das Freiras Sub-Basin and the seismic-stratigraphic model proposed in this work. Different correlations between accommodation and sedimentation along a half-graben basin are expected in moments of tectonic activity and quiescence. The use of seismic attributes and filters improved the visualization and extraction of geological features, in both, structural (using pseudo-relief and curvature attributes) and stratigraphic interpretations (RMS amplitude and cosine phase). The understanding of spatial and temporal distribution of depositional systems responsible for a basin filling allows predicting potential petroleum systems.

Lacustrine carbonate tufa facies of Winnemucca Dry Lake Basin, Nevada, U.S.A.

ABSTRACT Lacustrine carbonate tufa deposits are common in present-day lakes and dry pans of the western United States, and large-scale deposits (> 100 m high) are found throughout the subbasins of Pleistocene Lake Lahontan. This study presents a depositional model for very well exposed tufa in Winnemucca Dry Lake, a subbasin of Lake Lahontan, that incorporates new observations of tufa growth over length scales of 10–4–102 m. Tufa depositional facies are defined on the basis of outcrop morphology and texture. Deposits were mapped using satellite imagery and field observations. Tufa facies and volumes were quantified for seven tufa exposures across the basin using digital outcrop and elevation models from aerial images acquired from a small uncrewed aerial system (sUAS). Tufa thin sections were examined using transmitted-light petrography and scanning electron microscopy and combined with measurements of porosity and permeability to define small-scale facies characteristics. Both porosity and permeability are highly variable across textures; average values for both (ϕ = 29%, k = 5.5 D) indicate that all tufa types may exhibit excellent reservoir properties. The age and distribution of these facies across the basin are directly linked to hydroclimate and variations in lake level. The most important controls on tufa distribution at the basin scale are basin hydrology and pathways of groundwater inflow. Groundwater flow into the basin is largely concentrated along the western flexural margin along the contact between volcanic and volcaniclastic bedrock and alluvial sediments, rather than concentrated along the border fault margin, in contrast to other models which predict strong fault control of tufa occurrence. Microbially influenced tufa textures and morphologies are the most volumetrically significant tufas in the basin, composing between 77% and 100% of tufa volume at individual exposures; these are inferred to form during times when lake waters were warmer and levels higher, while physico-chemical processes dominate during early tufa formation, and generally in colder waters and under conditions of lower lake level. Deposition of tufas is a result of combined physical, chemical, and biological factors that are directly related to the basin geology and hydroclimate; however, the importance of each controlling factor is highly variable both spatially and temporally, complicating the development of effective and predictive depositional models. This case study describes tufa deposition intrinsically linked to basinal hydroclimatic histories, and understanding these relationships may assist in predicting volumes, physical properties, and stacking patterns of petroleum reservoir facies in lacustrine basins.

Quantitative interpretation of seismic attributes for reservoir characterization of Early-Middle Miocene syn- and post-rift successions (Songkhla Basin, Gulf of Thailand)

Abstract The major hydrocarbon reservoirs in the Gulf of Thailand, including the Songkhla Basin, are deposited throughout the syn- and post-rift section (Late Eocene-Recent), with the main reservoirs occurring in the Early Miocene to Middle Miocene section. These reservoirs are predominantly fluvial channels with complex lateral and vertical geometries and distributions particularly as point bars, overbank deposits, and sand- or mud-filled channels. This study characterizes the reservoir architecture and depositional environment of Songkhla Basin from late syn-rift to post-rift, during the Miocene, by integrating the seismic attributes (RMS, similarity, and spectral decomposition), well log interpretations, and well biostratigraphic data. Two distinctive channel characteristics and patterns were interpreted: (A) N-S oriented meandering channel complexes, aligned sub-parallel to the western boundary faults, developed during the late syn-rift (upper Early Miocene), (B) NE-SW oriented long, narrow fluvial channels with tidal and marine influence, developed in the post-rift phase I (Middle Miocene). These channel orientations reflect changes in the basin architecture passing from the late syn-rift stage to the post rift. Syn-rift channel orientation was controlled by topography created by motion on the western boundary fault system, while during the post-rift stage channel orientations were affected by mild inversion on the boundary fault system and thermal subsidence, which resulted in an eastward shift in the basin depocentre towards the flexural margin.

Contrasting stress fields on correlating margins of the South Atlantic

The “passiveness” of passive continental margins across the globe is currently under debate since several studies have shown that these margins may experience a variety of stress states and undergo significant vertical movement post-breakup. Of special interest is the South Atlantic, because the bounding continents have very different recent geological histories, with Africa experiencing continental rifting whereas South America is influenced by subduction on the Pacific side. It is not clear to what extent the Atlantic continental margins are subject to the same stresses and vertical motions as the main continents. To address this problem, we performed a paleostress analysis of two originally adjacent areas, i.e. NW Namibia and SE/S Brazil. Both areas are covered by the ~133-Ma-old Paraná-Etendeka extrusives that were emplaced shortly before or during the onset of the Atlantic rifting. Thus, the volcanics serve as a time marker for syn- or post-rift deformation. Collected fault slip data in the volcanics reveal remarkable differences between the two correlating areas. NW Namibia was dominated by extension in ENE-WSW and SW-NE directions, and by minor strike-slip movement with NW-SE directed compression. SE/S Brazil was mostly affected by strike-slip faulting, with compression oriented E-W and SW-NE. Similar fault systems appear widespread across SE Brazil and may be the combined result of flexural margin bending and the Nazca plate subduction. The results of NW Namibia differ from known compressional stress tensors in western South Africa, post-dating 90Ma. The south-western African continental margin may thus have experienced a spatially variable stress history. Our results show that the tectonic evolution of the continental margins of the South Atlantic is not passive and that both margins vary significantly in structural style and stress fields, indicating that variable plate boundary forces play a major role in margin evolution.

The flexural margin, the foredeep, and the orogenic margin of a northern Cordilleran foreland basin: Cretaceous tectonostratigraphy and detrital zircon provenance, northwestern Canada

Reconstructions of the Albian to Campanian foreland basin adjacent to the northern Canadian Cordillera are based on outcrop and well log correlations, seismic interpretation, and reconnaissance-level detrital zircon analysis. The succession is subdivided into two tectonostratigraphic units. First is an Albian tectonostratigraphic unit that was deposited on the flexural margin of a foreland basin. At the base is a shallow marine sandstone interval that was deposited during transgressive reworking of sediment from cratonic sources east of the basin that resulted in a dominant 2000–1800 Ma detrital zircon age fraction. Subsequent deposition in a west-facing muddy ramp setting was followed by east-to-west shoreface progradation into the basin.Near the Albian–Cenomanian boundary, regional uplift and exhumation resulted in an angular unconformity at the base of the Cenomanian–Campanian tectonostratigraphic unit. Renewed subsidence in the Cenomanian resulted in deposition of organic-rich, radioactive, black mudstone of the Slater River Formation in a foredeep setting. Cenomanian–Turonian time saw west-to-east progradation of a shoreface-shelf system from the orogenic margin of the foreland basin over the foredeep deposits. Detrital zircon age peaks of approximately 1300 Ma, 1000 Ma, and 400 Ma from a Turonian sample are consistent with recycling of Mississippian and older strata from the Cordillera west of the study area, and show that the orogen-attached depositional system delivered sediment from the orogen to the foreland basin. A near syndepositional detrital zircon age of ca. 93 Ma overlaps with known granitoid ages from the Cordillera. After the shelf system prograded across the study area, subsequent pulses of subsidence and uplift resulted in dramatic thickness variations across an older structural belt, the Keele Tectonic Zone, from the Turonian to the Campanian.The succession of depositional systems in the study area from flexural margin to foredeep to orogenic margin is attributed to coupled foreland propagation of the front of the Cordilleran orogen and the foreland basin. Propagation of crustal thickening and deformation toward the foreland is a typical feature of orogens and so the distal to proximal evolution of the foreland basin should also be considered as typical.

Young rift kinematics in the Tadjoura rift, western Gulf of Aden, Republic of Djibouti

The Tadjoura rift forms the westernmost edge of the westerly propagating Sheba ridge, between Arabia and Somalia, as it enters into the Afar depression. From structural and remote sensing data sets, the Tadjoura rift is interpreted as an asymmetrical south facing half‐graben, about 40 km wide, dominated by a large boundary fault zone to the north. It is partially filled up by the 1–3 Myr old Gulf Basalts which onlapped the older Somali Basalts along its shallower southern flexural margin. The major and trace element analysis of 78 young onshore lavas allows us to distinguish and map four distinct basaltic types, namely the Gulf, Somali, Goumarre, and Hayyabley Basalts. These results, together with radiometric age data, lead us to propose a revised volcano‐stratigraphic sketch of the two exposed Tadjoura rift margins and to discriminate and date several distinct fault networks of this oblique rift. Morphological and statistical analyses of onshore extensional fault populations show marked changes in structural styles along‐strike, in a direction parallel to the rift axis. These major fault disturbances are assigned to the arrest of axial fault tip propagation against preexisting discontinuities in the NS‐oriented Arta transverse zone. According to our model, the sinistral jump of rifting into the Asal‐Ghoubbet rift segment results from structural inheritance, in contrast with the en échelon or transform mechanism of propagation that prevailed along the entire length of the Gulf of Aden extensional system.

Sedimentation styles and variability of organic matter types in the Triassic, non-marine half-grabens of west Argentina: implications for petroleum systems in rift basins

ABSTRACT Extensional relaxation due to the collapse of the active margin of Gondwanaland during the Triassic led to rapidly subsiding, fault-bounded half-grabens in west Argentina. The Cuyo rift basin was the largest of these fault-bounded troughs. Two linked asymmetrical half-grabens have been identified in the Cuyo basin: Cacheuta in the south and Las Penas-Tamberias in the north. Their stratigraphy exhibits a classic tripartite internal organization with a basal alluvial and fluvial section followed by a lacustrine interval which in turn is overlain by fluvial deposits. The basin fill in both half-grabens shows significant lateral thickness variations that reflect the contrasting subsidence rates on the fault and flexural margins. The lacustrine shales in the Cacheuta half-graben have an average total organic carbon (TOC) content of 4%, locally reaching 20%, dominated by type I, amorphous, algal organic matter and high hydrogen index (HI) values. The shales are associated with parasequences in river-dominated deltas. Oils derived from these source rocks are waxy and with low sulphur content. The oil shales are associated with sandstones arranged in parasequences deposited in river-dominated Gilbert-type deltas. This interval in the Cacheuta half-graben can be assigned to a slightly overfilled to balanced-fill lake type. In Las Penas-Tamberias, the dominant source rock facies in the lacustrine section is made up of calcareous shales with oil-prone, type I(II) kerogen and TOC values up to 13% and high HI values. The presence of gammacerane and b-carotane, common in saline conditions, is conspicuous. The presence of oolitic and bioclastic grainstones and microstromatolitic limestones on the ramp margin and clastic facies on the border fault suggests a slightly underbalanced to balanced lake type. The Cuyo rift basin branches to the NE into the Ischigualasto-Villa Union half-graben. Lacustrine shales along the fault margin of this half-graben are dominated by type III, gas-prone organic matter with TOC values up to 4% and low HI values. Parasequences with a strong progradational stacking pattern and steep front slopes are interpreted as mouth bars in a Gilbert-type delta. These characteristics are consistent with an overfilled lake basin type where sedimentation rate exceeds subsidence rate. The Triassic rift system of west Argentina shows the gamut of lacustrine source rocks that, combined with the analysis of diagnostic associated facies, allow the discrimination of lake basin types and their influence in the resulting hydrocarbon phase.

Facies Architecture of Flexural Margin Lowstand Delta Deposits in Lake Edward, East African Rift: Constraints from Seismic Reflection Imaging

Abstract Lake Edward is single half-graben rift basin unique in the East African Rift System as a result of its connection to Lake George via the Kazinga Channel, a paleo–river valley which is incised into the flexural margin of the lake. Using recently acquired normal-incidence seismic reflection data and thirty-three lake-bottom sediment samples, we examine the consequences of this hydrologic connection for understanding depositional processes and facies development along the flexural margin of Lake Edward. Three depositional sequences were defined in the study area based on top and bottom discordances of seismic reflections. Climatically induced changes in lake level appear to have forced sequence development. Sequence I deposits indicate that highstand conditions prevailed earlier in the Pleistocene. Evidence for high-frequency lake-level change comes from lowstand delta clinoforms in two separate stratigraphic intervals on the flexural margin. An older lowstand event occurred during Sequence II time and reduced lake level by at least ~ 37 m. A younger lowstand event occurred during the deposition of Sequence III and reduced lake level by ~ 18 m. Clastic sedimentation on the flexural margin of Lake Edward is strongly modified by inflow from the Kazinga Channel and Lake George during periods of lowered lake level. We suggest that the presence of an incised valley at the margin of the rift controls the stratigraphic development of clinoforms diagnostic of lowstand deltaic sedimentation. The morphology and orientation of these clinoforms suggest that discharge from Lake George during arid periods utilized the Kazinga Channel to transport sediment toward the exposed flexural margin of Lake Edward. Dipping reflections in sigmoid-oblique clinoforms suggest that these flows were capable of transporting sediment loads that included coarse grain sizes. Deposition by this process occurred several times during the late-Quaternary history of Lake Edward, and represents an important modern analogue for linked incised valley-lacustrine delta sequences in the stratigraphic record.

Acoustic stratigraphy of Bear Lake, Utah–Idaho—Late Quaternary sedimentation patterns in a simple half-graben

A 277-km network of high-resolution seismic-reflection profiles, supplemented with a sidescan-sonar mosaic of the lake floor, was collected in Bear Lake, Utah–Idaho, in order to explore the sedimentary framework of the lake's paleoclimate record. The acoustic stratigraphy is tied to a 120 m deep, continuously cored drill hole in the lake. Based on the age model for the drill core, the oldest continuously mapped acoustic reflector in the data set has an age of about 100 ka, although older sediments were locally imaged. The acoustic stratigraphy of the sediments below the lake indicates that the basin developed primarily as a simple half-graben, with a steep normal-fault margin on the east and a flexural margin on the west. As expected for a basin controlled by a listric master fault, seismic reflections steepen and diverge toward the fault, bounding eastward-thickening sediment wedges. Secondary normal faults west of the master fault were imaged beneath the lake and many of these faults show progressively increasing offset with depth and age. Several faults cut the youngest sediments in the lake as well as the modern lake floor. The relative simplicity of the sedimentary sequence is interrupted in the northwestern part of the basin by a unit that is interpreted as a large (4 × 10 km) paleodelta of the Bear River. The delta overlies a horizon with an age of about 97 ka, outcrops at the lake floor and is onlapped by much of the uppermost sequence of lake sediments. A feature interpreted as a wave-cut bench occurs in many places on the western side of the lake. The base of this bench occurs at a depth (22–24 m) similar to that (20–25 m) of the distal surface of the paleodelta. Pinch-outs of sedimentary units are common in relatively shallow water on the gentle western margin of the basin and little Holocene sediment has accumulated in water depths of less than 30 m. On the steep eastern margin of the basin, sediments commonly onlap the hanging wall of the East Bear Lake Fault. However, no major erosional or depositional features suggestive of shoreline processes were observed on acoustic profiles in water deeper than about 20–25 m.

Preferential deposition and preservation of structurally-controlled synrift reservoirs: Northeast Red Sea and Gulf of Suez

ABSTRACT An integrated GIS-based play evaluation, which incorporates restorations of the North Red Sea and Gulf of Suez, has helped to identify potentially prospective areas in the Northeast Red Sea associated with point-sourced synrift sandstone reservoirs. The three largest synrift Gulf of Suez fields (Belayim Land, Belayim Marine, and Morgan) are located along major fault-transfer zones that optimized the conditions for the deposition and preservation of thick point-sourced sands adjacent to extensive hydrocarbon source kitchens. Belayim Land and Morgan fields contain stacked submarine fan, delta, and alluvial fan systems that developed during the deposition of the Miocene Rudeis, Kareem, and Belayim-South Gharib formations, respectively. This continuous, point-sourced sedimentation is indicative of stable drainage and by inference, a stable eastern border fault system. We attribute this stable border fault system to a stress heterogeneity related to the pre-existing Najd Shear Zone, and polarity reversals in upper-plate transport direction. Tectonic restorations indicate that the North Red Sea, like the Gulf of Suez, should have reservoir facies deposited in similar structural positions, but preservation is a significant risk due to additional crustal extension. Although crestal block erosion remains a great concern for reservoir preservation, seismic mapping indicates that block size along the coastal region and inboard areas are similar to the Gulf of Suez. This suggests that most of the strain may have been accommodated along the warmer axial portion of the rift where weaker crustal rheology exists. Landsat mapping of the Northeast Red Sea border fault system has found a high degree of variability in structural styles. The southern Yanbu-Jeddah and Umm Luj-Al Wajh sub-basins are bound by listric, down-to the west-southwest border faults, separated by suture-controlled accommodation zones. To the north, the Midyan-Ifal sub-basin is located along the Miocene flexural margin, and is structurally more complex. Northwesterly-trending (Najd Shear Zone) planar faults are overprinted by a strong northeasterly (Aqaba) trend, such that transpressional and transtensional features exist. Although structurally complex, the offshore northern flexural margin has been determined to have the best potential for localized, second-generation, thick, synrift sediments similar to that of the Gulf of Suez.

Lacustrine basin hydrocarbon exploration -current thoughts

Although much of the world's petroleum resource-base is associated with marine systems, regionally lacustrine petroleum systems are important. Individual accumulations may exceed several billion barrels. In each of these cases the oil is derived from a lacustrine source rock and may be produced from either nonmarine or marine reservoir rocks. The purpose of this paper is to describe the factors that control lacustrine source rock development and the nature of lacustrine reservoirs. Lacustrine oils display different physical and chemical characteristics than their marine counterparts. These differences can be related to the nature of their precursor material. Although the nature of the products are different, the geochemical threshold criteria for defining source rocks in both settings are the same because of common expulsion requirements. Commercially significant lacustrine systems require the presence of large, long-lived lakes. Such lake settings are tectonic in origin and restricted to climatic settings where precipitation exceeds evaporation. Within these large lake systems three primary factors determine source rock potential and quality. These factors are primary productivity level, organic preservation potential, and matrix sedimentation rate, which controls the dilution of preserved organic matter. Source rock potential is maximized where both productivity and preservation potential are maximized and sedimentation rate is minimized. To some degree these factors can compensate for each other. Hydrocarbon reservoir potential within lacustrine basins is partially impacted by overall tectonic setting. Within extensional settings, transport distances tend to be limited, with much of the sediment being transported away from the basin. The sediments delivered to the lake are poorly sorted and sedimentologically immature, commonly resulting in poor reservoirs due to both primary properties and their susceptibility to diagenesis. Within rifts better reservoirs tend to develop along platform or flexural margins. Stacking of reservoirs is important in lacustrine systems but baffles and barriers are often present between individual sand units. These barriers form as a result of lake level fluctuations. In compressional settings transport distances tend to be longer, resulting in more mature, better sorted sediments leading to higher quality reservoirs. These reservoirs typically develop in fluvial-deltaic and wave-dominated shoreline settings. Lacustrine carbonate reservoirs are locally important. These carbonates tend to develop during lake level lowstands and are dependent on diagenesis (dissolution and karstification) for porosity and permeability development. Lacustrine reservoirs are often stratigraphically and areally limited and display low individual well production rates. Within ‘pure’ lacustrine systems exploration opportunities appear to be often restricted by either reservoir presence or quality (i.e., production rates). The best exploration opportunities in lacustrine basins appear to be associated with hybrid systems where a lacustrine source and marine reservoir exist.