Basin Research - Volume 33, Issue 5, 2021

Increasing evidence suggests that supergene exotic copper deposits were emplaced during periods of geomorphic quiescence and pulses of humidity in arid environments. We tested this idea in the Centinela Mining District in the Atacama Desert (northern Chile). We collected 14 sand samples at depth (up to 110 m) in two open‐pit mines (Central Tesoro and Mirador) exposing Miocene sediment, and located in the El Tesoro Basin, which hosts two exotic copper‐rich orebodies. We inverted the ¹⁰Be and ²¹Ne concentrations by using a two‐box model (IMIS, inversion of multi‐isotopes in a sedimentary basin) composed of an eroding source of sediment and a depositional sedimentary basin, and by selecting denudation and sedimentation rate histories that can explain our data. The ages found demonstrate that the two exotic orebodies were deposited during a narrow period between 14 Ma (10 Ma younger than previously thought) and 9.5 Ma, when an ignimbrite covered the sedimentary sequence. The dated lower exotic copper orebody was deposited during or just before a sharp decrease in the sedimentation rates (from >100 to 0.5–5 m/Ma), which is consistent with published sedimentological and carbonate isotopic data in this district. This confirms the idea that exotic deposits form during a quiescence of the geomorphic activity. Nevertheless, our model suggests that the back‐ground denudation rate providing sediment to these basins between ca. 14 Ma and ca. 9.5 Ma was surprisingly high (>250 m/Ma) for such an arid environment. These denudation rates can be explained by a relatively rapid local back‐scarp retreat providing most of the sediment to these basins and possibly a wetter climate compared to the present. Then, during the period 10–7 Ma, the denudation rates decreased to >50 m/Ma. This decrease may correspond to a local progressive decrease in the slope of the surrounding hills, or to a progressive aridification, or a combination of both phenomena.

Cosmogenic nuclides ¹⁰Be and ²¹Ne measured in a sedimentary basin records the denudation evolution. Around 9.5–14 Ma, thigh denudation rates (>250 m/Ma) were recorded, possibly linked to a scarp retreat. Between 7 and 10 Ma denudation rates were moderate (>50 m/Ma) and very low since then (<10 m/Ma).

Subsurface salt flow can deform overlying strata and influence contemporaneous sedimentary systems. Studying salt‐sediment interactions is challenging in the subsurface due to poor imaging adjacent to salt, and in the field due to the dissolution of halite. Discrete Element Modelling provides an efficient and inexpensive tool to model stratigraphy and deformation around salt structures, which is advantageous over other modelling techniques as it realistically recreates brittle processes such as faulting. Six 2D experiments were run representing 4.6 Myr to determine the effect of salt growth on syn‐kinematic stratigraphy. Halokinetic deformation of stratigraphic architecture was assessed by varying sediment input rates through time. Results show the realistic formation and evolution of salt‐related faults which define a zone of halokinetic influence ca. 3 times the width of the initial diapir. Outside of this, early diapiric and syn‐kinematic stratigraphy are undeformed. Within this zone, syn‐kinematic strata are initially isolated into primary salt withdrawal basins, onlapping and thinning towards the salt‐cored high. In most models, syn‐kinematic strata eventually thin across and cover the diapir roof. Thinning rates are up to six times greater within 350 m of the diapir, compared to further afield, and typically decrease upwards (with time) and laterally (with distance) from the diapir. Outputs are compared to a subsurface example from the Pierce field, UK North Sea, which highlights the importance of considering local fluctuations in diapir rise rate. These can create stratigraphic architectures that may erroneously be interpreted to represent increases/decreases in sedimentation rate. Exposed examples, such as the Bakio diapir, northern Spain, can be used to make inferences of the expected depositional facies, below model resolution. Our models aid the prediction of sedimentary unit thickness and thinning rates and can be used to test interpretations arising from incomplete or low‐resolution subsurface and outcrop data when building geological models for subsurface energy.

We use a Discrete Element Model (DEM) to determine the effect of salt growth on stratigraphic architecture for different sedimentation rates and patterns. Halokinetic modulation is up to six times more intense within 350 m of the diapir, compared to further afield; halokinetic influence is shown to reduce laterally and temporarily.

3D modelling is a fundamental tool to visualize and understand the history of sedimentary basin filling and to reconstruct the geobody architecture. Spatial distribution of discontinuity surfaces and geobody characteristics provide valuable information on the factors controlling the sedimentary evolution of basins. Several Neogene‐Quaternary basins of central‐western Italy are controlled by extensional and strike‐slip tectonics and characterized by travertine deposition, related to hydrothermal fluids rising up along discontinuities and fractured carbonate bedrocks. This study presents the 3D modelling results of the quarry area within the tectonically controlled Acque Albule Basin (Tivoli, Central Italy) that hosts the Pleistocene Lapis Tiburtinus travertine. The 3D reconstruction of the different surfaces bounding the travertine units shows a complex architecture composed of depressions, reliefs and channels as predominant morphological elements related to four different depositional environments (subaqueous, palustrine, slope and travertine channel). The reconstructed surface maps highlight the presence of laterally migrating, E–W‐oriented lens‐shaped geometries, with a drainage system persistent through time oriented towards the southern part of the study area in the direction of the Aniene River, bordering the Acque Albule Basin in the South. The Lapis Tiburtinus travertine developed in an area of 28 km², accumulated in a system composed of sub‐basins (approximately 1–2 km² wide) with subaqueous conditions interconnected by a hydrographic network, controlled through time by fluctuations of the Aniene River base level. Based on the results obtained, base‐level fluctuations of the Aniene River, related to glacio‐eustatic sea‐level oscillations of the last 115 kyrs associated with alternation of humid and arid climatic conditions, arise as the most important factor affecting the architecture of the travertine geobodies.

The Lapis Tiburtinus travertine developed in a system of sub‐basins with subaqueous conditions interconnected by a hydrographic network controlled through time by fluctuations of the Aniene River base level

Thick salt deposits allow the rapid growth of supra‐salt carbonate platforms. Subsidence analysis allows to distinguish between tectonic subsidence and the down‐building of carbonate mini‐basins.

The subsidence analysis study presented by Strauss et al. (Basin Research, 2020, doi: 10.1111/bre.12500) for Triassic carbonate platforms located in the eastern Northern Calcareous Alps shows that salt expulsion allowed for the growth of thick isolated depocenters (>1.5 km) at rates faster than that tectonic subsidence alone can provide. In this answer to a comment by Schollnberger (Basin Research, 2021, doi: 10.1111/bre.12549) we discuss in detail the rationale behind our subsidence model and the assessment of the initial salt thickness. Excellent biostratigraphic control as well as precise knowledge of paleo‐bathymetry for most of the sedimentary record allowed to setup the subsidence model with well‐defined boundary conditions.

The evolution of the subsidence in the Billefjorden Trough shown by symmetrical early syn‐rift phase followed by asymmetrical rift climax phase. The late syn‐rift reorganization phase is highlighted by the progressive cessation the faults on the dipslope and deformation focusing in the central part of the basin.

Fault‐controlled mixed siliciclastic‐carbonate‐evaporite depositional systems exhibit distinct sensitivity to tectonic and eustatic controls that are expressed in the sedimentary architecture. In the Upper Carboniferous Billefjorden Trough (Svalbard, Norway), up to 2,000 m of a warm and arid climate syn‐rift basin fill comprises such depositional systems, documented in this study with traditional field techniques supported by helicopter‐ and ground‐based LIDAR models. The basin evolved from siliciclastics‐dominated red beds and paralic units that filled a symmetrical basin, to a rift climax half‐graben with alluvial fans entering the basin along relay ramps of the master fault zone (Billefjorden fault zone). Faults located in the hanging wall dip‐slope prevented the progradation of coarser material to the eastern part of the basin. Later, structural reorganisation in the dipslope led to the cessation of easternmost faults with deformation focusing along one major lineament (Løvehovden fault zone) antithetic to the master fault zone. The basin subsidence became more symmetrical, with main central depocentre and shallower platforms near the basin flanks. Footwall anticlines from faults displacement gradients were sensitive to periodical exposure and recorded dissolution breccias and footwall synclines preserved evaporites coupled with shallow marine siliciclastic deposits. Concurrently, thick gypsum/anhydrite deposits in the basin centre reflect glacio‐eustatic lowstands, whereas evenly thick carbonate deposition characterises highstands. While most analysis of syn‐rift basin fill is based on siliciclastics deposits, we here demonstrate the complexity of tectonism versus eustatic sea level changes in a mixed carbonate‐evaporite syn‐rift deposits. Tectonic influence is ascribed to the deposition of alluvial fans that prograded from the master fault towards the basin centre. On the dipslope glacio‐eustatic signals outperformed tectonic influence on deposition. Sea level lowstands promoted deposition of red sabkha mudstones and gypsum/anhydrite, salinas evaporites or dissolution breccias, interbedded with highstand carbonate beds.

Field examples of squeezed diapirs linked by steep reverse faults allow us to assess their three‐dimensional geometry and evolution, and the lateral transition from diapirs to linking faults. The results can aid geoscientists evaluating three‐way traps against salt or faults, helping to predict style and abundance of structures that are below seismic resolution.

The interactions between salt diapirs, thrust welds and thrusts in contractional belts are poorly understood due to, first, the inability of seismic data to distinguish between thrusts and welds or resolve associated sub‐resolution deformation, and second, the paucity of good field examples. The Warraweena area in the Northern Flinders Ranges of South Australia contains examples of Neoproterozoic to Early Cambrian squeezed diapirs linked by steep reverse faults formed during the Delamerian Orogeny. Benefiting from good field exposures, we use geological mapping, cross‐section construction and conceptual structural models to assess the three‐dimensional geometry and evolution of the structures, the lateral transition from diapirs to linking faults and the variability of associated meso‐ and small‐scale deformation. Three discrete diapirs consist of narrow outcrops of Callanna Group megabreccia (Willouran in age) up to 5‐km long. Their diapiric origin is confirmed by local development of caprock, steepening of flanking strata in composite halokinetic sequences and reworked diapir and roof debris in adjacent strata. The surrounding rocks display only background levels of small‐scale deformation. In contrast, the linking faults show no evidence of precursor diapirism, have fault‐related anticlines up to 100s of m in wavelength in their hanging walls, and an associated increase in small‐scale deformation (i.e. millimetre to metre scale folds, fractures and shear fabrics). The transitions from diapirs to faults occur within less than 200 m as short thrust welds at the diapir terminations. The exposed structures are analogous to those found on the subsurface of other salt basins such as the Gulf of Mexico and the South Atlantic conjugate margins. The results of this work can aid geoscientists evaluating three‐way traps against squeezed diapirs, welds or faults, and can help them to predict the style and abundance of both halokinetic and small‐scale structures that are below seismic resolution.

Field evidence of the interaction between extensional and reverse‐reactivated Mesozoic normal faults.

In this study, we conducted an integrated analysis supported by previous regional and mesoscopic field observations, new U–Pb chronological data of synorogenic deposits and thermochronological data from syncontractional granitic rocks. The objective is understanding the main mechanism and timing of crustal uplift of the western Central Andes on the modern flat‐slab segment of northern Chile. The first‐order structural styles identified in several basins of northern Chile consisted mostly of large reverse‐reactivated Mesozoic normal faults, which formed large kilometer‐scale inverted anticlines and localized doubly verging basement reverse faults depending on the degree of tectonic inversion. The observations indicate that crustal shortening experienced in the region was distributed along with the pre‐orogenic half‐graben structures of the Triassic to Jurassic basement. The wide distribution of Upper Cretaceous–Paleocene synorogenic deposits over the syn‐rift Mesozoic deposits along inverted structures in both the Coastal and Frontal cordilleras indicate that the Andean orogenesis in the region was initiated during this period. Our field and geochronological interpretations suggest that basin inversion of the ancient Mesozoic half‐graben structures was frequently accompanied by the emplacement of Upper Cretaceous and Paleocene intrusive granitic bodies hosted in the core of the anticline and syncline folds. Their crystallization ages correlated with those reported by the synorogenic deposits, thereby suggesting that both basin inversion and magmatism occurred simultaneously. The thermal history of the intrusives also indicates that they were rapidly exhumed at 53‒57 Ma, possibly during the final episodes of the basin inversion.

Sediment gravity flows transport large volumes of sand and clay minerals into submarine systems, however, knowledge about grain‐coating clay mineral formation and its role in preserving reservoir quality in deep marine settings is poorly documented. In this paper we present evidence that suggests the observed chlorite coating in the turbiditic sandstones of the Agat Formation originate from a precursor clay coating that was emplaced prior to sediment remobilization from a shallow marine setting.

Sediment gravity flows transport large volumes of sand and clay minerals into submarine systems, which store some of the world's major reserves of oil and gas. However, knowledge about grain‐coating clay mineral formation and its role in preserving reservoir quality in deep marine settings is poorly documented. Here we present a case study on the Agat Formation, a deep marine deposit interpreted as a series of turbidites, using a multimethod approach including petrographical, petrophysical and sedimentological data. This study investigates the occurrence and origin of chlorite coating and demonstrates how extensive chlorite coating substantially affects reservoir quality. The presence of green marine clay pellets suggests an initial shallow marine origin and sedimentological evidence reveals that the sediments were later remobilized by gravity flows and deposited at their present location. We suggest that the precursor clay coating was emplaced prior to sediment remobilization because of the presence of clay coating on grain contacts and all detrital components, the continuous nature of coating and the lack of clay bridges between the grains. Therefore, the origin of chlorite coating in deep marine environments may be recognized using the characteristic properties of inherited precursor clay coating. Chlorite coating thickness varies between an upper and lower sand unit, with an average of ca. 4.5 µm and ca. 24 µm, respectively. Permeability is significantly reduced in the interval with exceedingly thick chlorite coating but shows only a subtle decrease in helium porosity. This study enlightens the importance of crucially evaluating porosity in sandstones with thick chlorite coating using a multimethod approach. The results from this study can be useful in future exploration endeavours in the area and in other deep marine systems with a similar setting worldwide.

Representative seismic section showing the variation in fault geometry and the dominance of horst and graben conjugate pairs in the layer bound fault system.

Polygonal fault systems, sometimes termed layer‐bound faults, are extraordinary features of many fine‐grained sedimentary successions and have been described in a significant number of sedimentary basins over the last two decades. Their formation represents an important mechanism by which fine‐grained sediments compact often resulting in a variety of complex patterns for which several controlling factors have been proposed. Here, three‐dimensional seismic data from the North West Shelf of Australia are used to interpret previously undescribed characteristics of layer‐bound fault systems where systematic horst and graben structures are the dominant structural style. Conjugate fault pairs, which form the horsts and grabens, frequently have a systematic spacing with graben‐bounding faults exhibiting a spacing of half that of the horst‐bounding faults. This systematic spacing of fault pairs indicates, (a) the presence of a mechanically weaker layer at the base of the fault system and (b) that the horizontal shortening required by the volume loss due to compaction can be accommodated without reaching saturation with respect to fault intensity. Furthermore, topological analysis indicates that areas with different patterns also have different intersection and branch characteristics, and these differences suggest that the growth of layer‐bound faults is not explained by a single model. The findings have implications for the genesis and growth of layer‐bound fault systems and the potential for cross‐stratal fluid flow.

An understanding of the evolution of foreland basins improves our knowledge of how mountain belts have grown and helps us to decipher events which may not be preserved in the orogen. The infilling of the north Pyrenean retro‐foreland basin (Aquitaine Basin, France) during the main exhumation of the Pyrenees and its corresponding thermal history have not been fully investigated. We applied apatite fission track (AFT) and (U‐Th‐Sm)/He (AHe) methods coupled with inverse thermal modelling on both the detrital Eocene (47 to 33 Ma) syn‐orogenic Palassou conglomerates of the eastern part of the Aquitaine Basin and basement samples from the North Pyrenean Zone and the Axial Zone of the Pyrenees. Apatite crystals were separated from granitic cobbles found in the conglomerates. AFT ages for detrital samples range from 27 ± 2 to 43 ± 4 Ma, and AHe ages from 13 ± 1 to 76 ± 5 Ma. For in situ massifs AFT ages range from 35 ± 2 to 90 ± 17 Ma and AHe ages from 39 ± 2 and 80 ± 5 Ma. AFT ages for detrital samples are close to deposition ages, whereas AHe ages are older and younger than deposition ages and show a partial thermal resetting due to burial. A detailed analysis of the ages obtained and thermal histories derived from modelling shows that ages reflect (a) exhumation from 70 to 55 Ma revealed by a long stay in the partial retention zone (PRZ), (b) a Palaeocene–Eocene cooling in the Pyrenees, (c) a post‐depositional episode of moderate heating of the sediments in the basin represented by partially reset young AHe and AFT ages compared to deposition ages and (d) an early to mid‐Miocene final exhumation of the basin deposits as evidenced by young AHe ages and geological constrains. These results reflect a common event with the south Pyrenean foreland basin that is characterized by high piedmont aggradation from the late Eocene to the Miocene. The aggradation of sediments is possibly connected with well‐known high elevation low relief surfaces in the core of the Pyrenees and followed by a Miocene exhumation event that is already observed on the southern flank. However, the timing of the aggradation and exhumation events could be different between the north and the south. Erosion occurred most probably during the early to mid‐Miocene in the north and during the late Miocene–early Pliocene in the south.

This paper provides new detrital low temperature thermochronology data from the eastern part of the Aquitaine Basin (the Corbières region). Thermal modelling of the obtained results allowed the determination of the thermal history of the samples from their source (the Pyrenees) to the basin (the Corbières region).

The Himalaya, the Earth's largest active orogen, produces a deep but relatively unexplored foreland basin by loading the Indian Plate. Newly available two‐dimensional seismic data (ca. 5,180 line km) spanning 900 km of the Nepali lowlands allow mapping and interpretation of several regional subsurface markers in two‐way‐travel time and estimated depth. Isopach maps for the major intervals allow us to interpret the interplay between basement structure, flexure, and faulting within the Ganga Basin. The Indian continental lithosphere beneath the foreland basin contains basement ridges oriented at high angles to the thrust belt. These basement structural highs and intervening depressions, tens to hundreds of kilometres wide, influenced deposition of the Precambrian Vindhyan strata and overlying Paleozoic to Mesozoic successions. The overlying Miocene to Quaternary foreland basin shows along‐strike thickness variations across the basement features. Because the foreland basin sediments were mainly deposited in an alluvial plain close to sea‐level, accommodation, and therefore thickness, was predominantly controlled by subsidence of the Indian Plate, providing evidence that the basement features controlled foreland basin development. Subsidence varied in time and space during Neogene basin development. When combined with flexural modelling, these observations imply that the subsidence history of the basin was controlled by inherited lateral variations in the flexural rigidity of the Indian Plate, as it was translated northward beneath the Himalayan Orogen. Basement features continue to play a role in higher levels of the thrust belt, showing that basement features in a down‐going plate may produce non‐cylindrical structures throughout orogen development.

Conceptual cartoon showing along‐strike thickness variations in the Ganga Basin. Not to scale. Foreland basin fill shown in green. Relative subsidence rates are shown schematically by black (faster) and grey (slower) arrows.

Tectono‐stratigraphic evolution of the central‐southern Apennine foreland basin system. The shape and migration rate of the foreland basin is constrained by a new high‐resolution regional dataset of Sr‐isotope dating for the base of the time‐transgressive shallow‐water carbonate unit at the bottom of the foreland basinal megasequence sealing the forebulge unconformity.

The Apennines are a retreating collisional belt where the foreland basin system, across large domains, is floored by a subaerial forebulge unconformity developed due to forebulge uplift and erosion. This unconformity is overlain by a diachronous sequence of three lithostratigraphic units made of (a) shallow‐water carbonates, (b) hemipelagic marls and shales and (c) siliciclastic turbidites. Typically, the latter two have been interpreted regionally as the onset of syn‐orogenic deposition in the foredeep depozone, whereas little attention has been given to the underlying unit. Accordingly, the rate of migration of the central‐southern Apennine fold‐thrust belt‐foreland basin system has been constrained, so far, exclusively considering the age of the hemipelagites and turbidites, which largely post‐date the onset of foredeep depozone. In this work, we provide new high‐resolution ages obtained by strontium isotope stratigraphy applied to calcitic bivalve shells sampled at the base of the first syn‐orogenic deposits overlying the Eocene‐Cretaceous pre‐orogenic substratum. Integration of our results with published data indicates progressive rejuvenation of the strata sealing the forebulge unconformity towards the outer portions of the fold‐thrust belt. In particular, the age of the forebulge unconformity linearly scales with the pre‐orogenic position of the analysed sites, pointing to an overall constant migration velocity of the forebulge wave in the last 25 Myr.

This study uses 2D and 3D seismic reflection and borehole data from the south‐central Campos Basin to characterise salt‐tectonic structural styles and related evolution of salt and overburden structures. Variations in dip angle and direction in the base‐salt surface define base‐salt ramps, which delineate the boundary between the External High and the External Low. The distribution of the interpreted salt and overburden structures define three domains of thin‐skinned, salt‐detached deformation: extensional—subdivided into subdomains E1 and E2—contractional, and multiphase. Base‐salt relief caused local variations in salt flux, affecting the regional domains of deformation, controlling or influencing the types of generated salt and overburden structures, and their evolution through time and space.

Classic models of gravity‐driven salt tectonics commonly depict kinematically linked zones of overburden deformation, characterised by updip extension and downdip contraction, separated by a weakly deformed zone associated with downdip translation above a relatively smooth base‐salt surface. We use 2D and 3D seismic reflection and borehole data from the south‐central Campos Basin to show that these models fail to adequately capture the complex range of structural styles forming during salt‐detached gravity‐driven deformation above a rugose base‐salt surface. In the Campos Basin, the base‐salt is defined by broadly NE‐trending, margin‐parallel, generally seaward‐dipping ramps that have up to 2 km of structural relief. We define three domains of overburden deformation: an updip extensional domain, an intermediate multiphase domain and a downdip contractional domain. The multiphase domain is defined by large, partly fault‐bounded, ramp‐syncline basins, the stratigraphic record of which suggest ca. 28 km of seaward gravity‐driven translation of salt and its overburden since the end of the Albian. We also identify three main types of salt structures in the multiphase domain: (a) contractional anticlines that were subjected to later extension and normal faulting; (b) passive‐to‐active diapirs that were later extended and widened, and which are bound on their landward margins by landward‐dipping, salt‐detached normal faults and (c) reactive (extensional) diapirs that were subsequently squeezed. We argue that this multiphase deformation occurs because of basinward translation of salt and its overburden over complex base‐salt relief, consistent with the predictions of physical models and several other seismic reflection data‐based studies. Critically, these complex local strains overprint margin‐scale patterns of deformation.

We have identified seven mechanisms by which mobile shales can pierce their roofs. The operative piercement mechanism depends on mobile‐shale viscosity, roof strength and stress state. For mobile shales at depths of several kilometres, three mechanisms are possible: fracture piercement, thrust piercement and ductile “piercement.” However, injection up fractures and faults appears to be the dominant mechanism by which mobile shales rise towards the surface. In this process, mobile shales behave similar to magmas rising through the Earth's crust. Nearer the surface, a wider range of piercement mechanisms becomes possible: passive piercement, reactive piercement, active piercement and erosional piercement. These mechanisms all have salt‐tectonics analogues. Although shale tectonics and salt tectonics share common piercement mechanisms, in many cases the resulting structures are different. This is because near‐surface mobile shales can have much lower viscosities than salt. Mobile shales that reach the surface extrude very rapidly, in many cases leading to caldera collapse of the underlying shale chamber. This instability in the near‐surface means that long‐term, stable growth of passive shale diapirs is unlikely, in contrast with the behaviour of salt. A key question in seismic interpretation of mobile‐shale structures is whether large‐volume mobile‐shale diapirs exist. We show that both active piercement and ductile “piercement” can create such structures. Both of these mechanisms create steeply upturned beds on diapir flanks, which are diagnostic. However, active shale diapirs appear to be rare, and ductile “piercements” are not documented. We therefore suggest that large‐volume shale diapirs should be interpreted with caution on seismic data.