Basin Research - Volume 36, Issue 1, 2024

The Guadalquivir Basin is the foreland basin of the Betic Cordillera (S Spain). Closest to the orogen, several thrust‐top basins evolved during the Late Miocene in the central part of the cordillera. Here, we study the Upper Miocene deposits in five of these satellite basins: Montefrío, Iznájar‐Cuevas de San Marcos, Antequera, Bobadilla Estación and Teba, in order to (1) update the stratigraphic framework, (2) infer a depositional model, (3) establish the relationship between sedimentary record and tectonic context and (4) reconstruct the palaeogeography of the area during the Late Miocene. Upper Miocene sediments mostly consist of mixed carbonate‐terrigenous deposits. Facies characterization allows inferring a sedimentary model corresponding to a ramp with foreshore deposits changing to a shoal belt offshore in the inner ramp. Swaley and hummocky cross‐stratified deposits formed in the transition to the middle ramp, and plane parallel carbonate beds in the distal middle‐outer ramp. Factory facies, dominated by rhodoliths and bryozoans, also occur in the middle‐outer ramp environments. Silts and marls formed in the deepest outer ramp and basin settings respectively. Breccias accumulated at the toe of palaeocliffs and conglomerates and massive coarse sands were deposited in fluvio‐deltaic systems. Conglomerates and sands were also reworked as gravity flows and redeposited offshore. Local facies include rudstones‐grainstones displaying large‐scale trough‐cross bedding formed in a strait in Montefrío, and marls with chalky carbonates deposited in a shallow marine, sheltered lagoon with hydromorphic soils in Bobadilla Estación. The study basins evolved in an N‐S compressive tectonic context responsible of the emersion of the main Betic reliefs. Concomitantly, E‐W and ESE‐WNW extension originated the main depocentres. The influence of the tectonic activity on the sedimentary infills is indicated by the presence of synsedimentary deformations and several diachronic unconformities, which are younger westward. Tectonism, in turn, also controlled the palaeogeographic evolution during the late Tortonian‐early Messinian interval.

Summary chart of the stratigraphic framework of the study sub‐basins in the wedge‐top depozone of the Guadalquivir Basin compare with olistostromic deposits emplaced in the foredeep depozone. The stratigraphic division of the Upper Miocene deposits in the foredeep (at the right part of the figure) is based on different authors: MO.19: Martínez del Olmo (2019); Sie.96: Sierro et al. (1996); Led.00: Ledesma (2000). Unc‐1 to Unc‐4: Unconformities. Ol‐1 to Ol‐3: Olstostrome emplacements. Thick black lines indicate the final emersion of the different areas.

Continental margins that exhibit high terrigenous input are generally located near deltas that are capable of transporting large quantities of sediments into the basin. However, in rare cases, high terrigenous sedimentation occurs in regions deprived of major riverine systems where the sedimentary pathway is enigmatic. One such case is the Neogene of the Pelotas Basin of Brazil and Uruguay, adjacent to the La Plata River mouth. Since the Miocene, anomalous sedimentation formed a giant progradational wedge, the Rio Grande Cone, one of the largest submarine fan‐like features on Earth. To understand the Neogene evolution of the margin and the origins of the Rio Grande Cone, here we present a seismic‐stratigraphic framework based on well‐logs and 2D seismic data. Three depositional environments are identified: (1) on the shelf, upper Miocene to Pliocene fluvial channels delivered sand deposits on the mud‐dominated shelf; (2) on the slope, sediment instability resulted in structural deformation and multiple phases of mass transport deposition and (3) on the lower slope and basin floor, large contourite drifts formed by sediment reworking. We classify the Rio Grande Cone as a megaslide complex, due to its depositional and structural setting. Local deltaic systems were active on the shelf in the Neogene, but the limited size of their paleo‐drainage systems in comparison to the volume of sedimentation in the margin suggests that an additional sedimentary pathway existed. In this sense, the demise of an epicontinental sea over the La Plata Basin during the Neogene likely enabled the input of large volumes of fine sediments into the margin, via the La Plata plume water. We suggest that the desiccation of this epicontinental sea and the intensification of ocean currents since the middle Miocene explains the anomalous Neogene terrigenous influx into the SW Atlantic margin.

The Neogene Evolution of the Pelotas Basin (South Atlantic) was marked by the interplay of anomalous sediment input, sea‐level changes, slope instability and the intensification of bottom currents which led to the establishment of submarine megaslides (e.g., Rio Grande Cone) and widespread contourite drifts.

Three plausible pathways (white lines) to Zama Upper Miocene minibasin depocenter from interpreted shelf‐margin entry point. Upper Miocene well‐based net sandstone contours are superimposed on an Upper Miocene interval thickness map derived from seismic analysis. Reconstructed coeval shelf‐margin is shown by the red line. The blue line is the location inferred for Palaeo‐Rio Grijalva. Grey‐lined polygons denote present‐day locations of allochthonous salt bodies. Plausible pathways follow trends in interval and sandstone thicknesses.

The Late Miocene source terrane tectonic history in the southern Gulf of Mexico Basin, as informed by detrital zircon geothermochronology data, supports a detailed regional palaeogeographic reconstruction from palaeoshoreline to the deepwater Zama minibasin of the Sureste salt basin. Seismic mapping points to a trio of pathways that converge upon two entry points into the Zama minibasin, illuminating how sediment gravity flows transit a complex seascape defined by shallow salt bodies. Consideration of empirical scaling relationships within and between segments of this sediment dispersal system allows for testable predictions of Upper Miocene submarine fan‐runout lengths over basin exploration areas. Distances from the reconstructed shelf‐margin to the Zama wells vary around 100 km, an increase of 20% over a straight‐line distance as flows likely navigated around extant salt stocks, walls and sheets. This 100‐km fan length is about 40% of the reconstructed minimum palaeo‐river length, within predicted ranges for smaller source‐to‐sink systems in tectonically active areas (25 to 50%). The estimated fan‐runout distance can be extended even further basinwards, considering the contemporaneous passage of the mobile Chortis block along the Tonala shear zone, expanding the Palaeo‐Rio Grijalva drainage network during the Tortonian. These Late Miocene deepwater systems linked to the Palaeo‐Rio Grijalva differ substantially from onshore Mexico‐sourced turbidity flows feeding into the axis of the north‐trending Veracruz Trough. Textural data from wells here suggests these systems were less effective at larger grain transport and sorting. Local (intrabasinal) variations are also evident within the Zama minibasin, as well data (image logs and cores) indicate that axially oriented sediment gravity flows involved fewer high‐density turbidities, depositing lower net‐to‐gross sandstones and thicker shales than those flowing transverse to the basin axis from a southeastern basin entry point. These interpretations will guide both local exploitation of these economic resources and could also support future exploration for analogous salt‐influenced deepwater reservoir systems in the Sureste basin and globally.

The preexisting structures that developed in the basement and subsalt strata play a key role in the salt structural deformation in the Kuqa Depression, Tarim Basin. The characteristics of preexisting structures and their controls on the salt structure are investigated via the latest three‐dimensional seismic data and numerical modelling. The results show that the preexisting structures that developed in the Kuqa Depression mainly consist of basement faults, palaeouplifts, subsalt slopes and early passive salt diapirs. Basement faults are mainly distributed in the Kelasu and Qiulitag structural belts and control the position of development and deformation style of the Miocene compressive salt structure. The differences in styles and reactivation degrees of basement faults lead to great diversity in the salt structure. The palaeouplifts mainly include the Wensu, western Qiulitag, Xinhe and Yaha‐Luntai palaeouplifts. The original sedimentary range and later deformation space of the salt layer are limited by the palaeouplift, resulting in strong salt thrusting in the Awate sag in the western part of the Kuqa Depression. The heterogeneous spatial distribution of the palaeouplift promoted the development of regional strike‐slip transform belts. Subsalt slopes are located mainly on the northern edge of the western Qiulitag low uplift and block the southward flow of the salt, causing the salt to form salt domes; the size of these domes is closely related to the subsalt slope. Early passive salt diapirs mainly developed in the Quele and Bozidun areas of the western Kuqa Depression, and they were preferentially active during the compression period, inducing the formation of a piercement salt nappe. Numerical modelling revealed that the preexisting structure strongly controlled the stress–strain distribution during the deformation of the salt structure. The spatial distribution heterogeneity of the basement structure is an important factor in the structural zonation along the north–south strike and segmentation along the west–east strike in the Kuqa Depression, as well as an important inducer of the piercement salt structure.

Different salt structural deformation models controlled by preexisting structures in the Kuqa Depression. (a) In the Wensu segment, the sedimentary range and deformation space of the salt layer are restricted, resulting in the strong thrust of the subsalt faults and piercement of the salt. (b) In the Quele segment, progradational sedimentary load induces a passive salt diapir. Influenced by the blocking of the steep subsalt slope and the reactivation of the preexisting salt diapir, a piercement salt wall was formed on the right edge of subsalt slope. (c) In the Dabei segment, a salt dome was formed at the northern edge of the western Qiulitag palaeouplift due to the blocking of the steep subsalt slope. (d) In the Keshen segment, the subsalt slope and preexisting salt diapir are not developed, and finally, two salt domes are formed at the top of the basement faults in the Kelasu and Qiulitag structural belts.

Enriched sands close to the point with the maximum fault throw as domonstrated by RMS amplitude

Layer‐bound polygonal fault systems (PFS) are a prevalent feature in fine‐grained sediments across many continental margin basins worldwide, yet their origin remains enigmatic. In this study, we report on the structural characteristics of polygonal faults recently discovered in Middle Miocene mudrocks of the Yinggehai Basin, northern South China Sea. Our data reveal that the polygonal arrays of normal faults, which comprise master faults and minor synthetic/antithetic faults with complex tiers, exhibit either straight or curvilinear traces with frequent orthogonal intersections, forming a highly interconnected fault network. We observe several sub‐circular to elliptical‐shaped depressions that lie above the faulted interval and are filled with syn‐deformation deposits, with the long axis of these depressions aligned sub‐parallel to the structure contour lines. Our findings suggest that the polygonal faults emerged during the sediment deposition and compaction preceding the deposition of overlying sediments. The faults were created through the nucleation of penecontemporaneous faults due to the overloading of sandy sediments onto unconsolidated clays, followed by the propagation of the faults along with continuous sediment deposition. The cessation of fault propagation coincided with the termination of sedimentation in the faulted interval. Additionally, the local horizontal stress anisotropy resulting from topographic‐gravitational effects may have played a crucial role in the development of polygonal faults. Our study provides novel insights into early sediment deformations in the northern South China Sea region and sheds light on the timing and genesis of PFS.

Modelledstratigraphy reconstructed using Bayesian inversion. The top panels show the reconstructedstratigraphy (panel a) and a wheeler diagram of this model output (panel b). Thethree other panels show the original and reconstructed values of: 1) flexure,thermal subsidence, and total subsidence (panel c); 2) sea‐level change (paneld); and 3) sediment supply variations (panel e). The greater variability in theposterior distribution of sediment supply values relative to those for sealevel suggests that the development of passive continental margin stratigraphic architecture is particularlysensitive to sea‐level variation.

We produced a 10 Myr synthetic stratigraphic section using a forward stratigraphic model that generates marine deltaic stratigraphy over geological timescales. We recursively fit the model using a Bayesian inversion algorithm to test: (1) if it could be accurately reconstructed; (2) if the parameters used to create it could be recovered; and (3) the sensitivity of the model output to given model parameters and the attendant physical processes. The original synthetic stratigraphic section was produced with cyclical sea‐level variations of 40 and 30 m with 2.4 and 10 Myr periods respectively. Sediment was also supplied cyclically, in 2.4 and 10 Myr cycles with amplitudes of 30 and 80 tons/100 kyr, respectively, varying from a mean of 232 tons/100 kyr. Parameter values were sampled to fit the model using a Markov chain Monte Carlo algorithm, resulting in a ±5 m (1σ) variation between the experimental output and the original. Sea level varied by ±7 m (1σ) within the posterior distribution of parameters. As a result, both the 10 Myr and 2.4 Myr sea‐level cycles could be extracted from the original output. The variation in sediment supply was approximately ±38 tons/100 kyr (1σ) and, as a result, only the larger long‐term supply variations could be accurately recovered in refitting the model. The variation in thermal, flexural and total subsidence across those parameter sets is less than ±10 m (1σ). The original section experienced 150 m of total subsidence at the depocentre. Our results demonstrate the distinct and interpretable imprint of sea level and subsidence on continental margin stratigraphy can be quantified. Moreover, we conclude that sea‐level change produces a defined effect on the geometries of stratigraphic architecture, and that techniques applied for the purpose of delineating sea‐level variation from continental margin strata have a well‐founded conceptual basis.

Polygonal fault systems (PFS) are developed in many sedimentary basins, and their formation, growth, and ultimate geometry have been widely studied. The geometry and growth of PFS forming under the influence of regionally anisotropic stresses, however, are poorly understood, despite the fact these structures may serve as key paleo‐stress indicators that can help reconstruct the tectonic and stress history of their host basins. We here use high‐quality 3D seismic reflection data and quantitative fault analysis to determine the geometry and evolution of a PFS in the Qiongdongnan Basin (NW South China Sea), and its possible relationship with the geological and stress history of the basin. The PFS is dominated by two intersecting NNW‐to‐N‐ and E‐striking fault sets, which initiated in the Early Miocene. The dominant fault strike at the structural level at which the faults nucleated and where strain is greatest (i.e., Lower Miocene) is close to NW–SE. However, at the top and bottom of the PFS tier faults strike NNW–SSE, thereby defining a very slight vertical, clockwise rotation of strike. Based on the observation that the host rock is flat‐lying, it is unlikely that basin‐tilting perturbed (i.e., δ2 ≠ δ3) the otherwise radially isotropic stress field that typically characterize PFS. Likewise, diapirs that punctuate the host rock and that are spatially related to the PFS appear not to control fault geometry. We instead infer that the PFS geometry reflects a combination of local isotropic and regional, extension‐related tectonics stress affecting the Qiongdongnan Basin during the Early Oligocene to Middle Miocene. Regional studies suggest that during this time, extensional stresses in eastern Qiongdongnan Basin rotated clockwise from roughly NNW to N; we noticed the rotation of strike of the PFS, within which the vertical change in fault strike being relatively minor. Our study determines the timing of polygonal fault growth within the Qiongdongnan Basin and the associated geometry, highlighting the key role played by regional and local stresses.

The polygonal faults are dominated by two intersecting NNW‐to‐N‐ and E‐trending fault sets of the Qiongdongnan Basin, NW South China Sea, which initiated in the Early Miocene. The geometry of the polygonal faults was mainly influenced by isotropic stresses in the Qiongdongnan Basin during the Early Oligocene to Middle Miocene.

Model depicting the northward expansion evolution of the Jiaolai Basin in East Asia during the Early Cretaceous period. The left column illustrates the new stratigraphic framework as presented in this study. The grey arrows in the diagram indicate the primary provenance directions, with the size of the arrows representing the relative contribution of each source.

The Jiaolai Basin, situated in the northern Sulu orogenic belt along East Aisa continental margin, preserves evidence of the extensional events in East Asia and the post‐orogenic evolution of the Sulu orogenic belt during the Cretaceous period. In this study, multiple provenance analyses were employed to reconstruct the source‐to‐sink system of the Laiyang Group within the Jiaolai Basin. These studies reveal a history of northward expansion dictated by two significant rift events. During the early Early Cretaceous period (ca. 135–121 Ma), the Zhucheng and Gaomi sags in southern region developed initially. Subsequently, in the late Early Cretaceous period (ca. 120–113 Ma), the Laiyang sag in northern region emerged. Furthermore, these sags were fed by independent source‐to‐sink systems in their early stages but eventually shared a similar source‐to‐sink system towards the end of the Laiyang Group deposition (ca. 113 Ma). The provenance analysis results indicate that ca. 121 Ma, ultrahigh‐pressure rocks in the northern segment of the Sulu orogenic belt experienced rapid exhumation, while those in the southern segment might have remained concealed until ca. 113 Ma. The two rift events in East Asia, coupled with the alteration in the direction and magnitude of extension documented in the Jiaolai Basin, suggests that trench retreat and the change in subduction direction from E–W to NW–SE of the Izanagi plate played a principal role in driving the extensional events in East Asia during the Early Cretaceous. Our findings imply that the change in Izanagi subduction direction may have occurred ca. 121 Ma.

Integrated stratigraphic‐compositional studies on alluvial successions provide a valuable tool to investigate the provenance of detritus in multi‐source systems. The Po Plain is an intermediate sink of the Po‐Adriatic source‐to‐sink system, fed by rivers draining two orogens. The Alps are characterized by extensive outcrops of plutonic‐metamorphic and ultramafic rocks to the north‐west and of Mesozoic carbonates to the east (Southern Alps). The Northern Apennines, to the south, are dominated by sedimentary successions. The Po River flows from the Western Alps to the Adriatic Sea, interacting with a dense network of transverse tributaries that drain the two orogens. Stratigraphic, sedimentological and compositional analyses of two 101 and 77.5 m‐long cores, recovered from the Central Po Plain, reveal the stacking of three petrofacies, which reflects distinct provenance and configurations of the fluvial network. A South‐Alpine sedimentary input between MIS 12 and MIS 10 is testified by petrofacies 1, characterized by carbonate‐ and volcanic‐rich detritus from rocks exposed in the Southern Alps. A northward shift of the Po River of more than 30 km is marked by a quartz‐feldspar and metamorphic‐rich detritus (petrofacies 2), similar to modern Po River sands. This dramatic reorganization of the fluvial network likely occurred around MIS 9–MIS 8 and is possibly structurally controlled. A further northward shift of the Po River and the onset of Apennine sedimentation in the Late Holocene is revealed by petrofacies 3, rich in sedimentary lithics from the Apennine successions. The results of this study document how compositional analysis, if framed in a robust stratigraphic picture, may provide clues on the evolution of multi‐source alluvial systems.

Inferred palaeogeography of the central Po Plain, deduced from compositional variations of core sands, during: (a) MIS 12 and MIS 10 (petrofacies 1). (b) MIS 5–4 (petrofacies 2). (c) MIS 2 (petrofacies 2). (d) The early Holocene (petrofacies 2). (e) The late Holocene (petrofacies 3).