Basin Research - Volume 32, Issue 3, 2020

Stratigraphic architecture, lithofacies, provenance, detrital zircon mixture modeling, and paleocurrent analysis elucidate Paleogene tectonic activity and basin development in north‐central New Mexico. Depocenters from this area comprised a portion of the headwaters of a dynamic transcontinental river system that exerted a first order effect on sediment character and volume received by the northwestern margin of the Gulf of Mexico. During the middle Eocene depocenters in north‐central New Mexico captured Laramide province‐derived sediment, which starved the Gulf of Mexico of this detritus, and permitted westward incursion of Appalachian‐derived sediment.

The El Rito and Galisteo depocenters in north‐central New Mexico archive tectonically‐driven Paleogene drainage reorganization, the effects of which influenced sedimentation along the northwestern margin of the Gulf of Mexico. Although separated by ~100 km and lacking depositional chronology for the El Rito Formation, the two aforementioned New Mexican depocenters are commonly considered remnants of a single basin with coeval deposition and shared accommodation mechanism. Detrital zircon U‐Pb maximum depositional ages indicate that the El Rito and Galisteo formations are not coeval. Moreover, stratigraphic thickness trends and mapping relationships indicate different accommodation mechanisms for the Galisteo and El Rito depocenters; tectonically‐induced subsidence versus infilling of incised topography, respectively. The regional unconformity that bounds the base of both the El Rito and Galisteo formations is a correlative surface induced by local tectonic activity and associated drainage reorganization in the early Eocene, and was diachronously buried by northward onlap of fluvial sediments. Detrital zircon distributions in both depocenters indicate increased recycling of Mesozoic strata above the unconformity, but diverge upsection as topographic prominence of local basement‐involved uplifts waned. Sediment capture in these depocenters is coeval with deposition in other externally‐drained Laramide basins. Further, it corresponds to a period of low Laramide province‐derived sediment input and replacement by Appalachian‐sourced sediment along the northwestern margin of the Gulf of Mexico during a basin‐wide transgression. This illustrates the potential effect that pockets of sediment storage within the catchment of a transcontinental drainage system can have over the sedimentary record in the receiving marine basin.

Zooms of the key, most distinctive types of structures associated with complex, multiphase deformation and salt flux variations observed in different domains in the study area: (a) salt‐cored bucklefolds; (b) collapsed folds; (a‐b) transition from buckle‐folds into collapsed folds over extensional hinge at the crest of the Tupi Sub‐High; (c) reactive diapirs; (d) reactive diapirs nucleating onto salt‐cored buckle‐folds; (e) passive diapirs; (f) squeezed diapirs; (g) fold‐injection; (h) thrust‐piercement; and (i) multiphase diapirs.

Base‐salt relief influences salt flow, producing three‐dimensionally complex strains and multiphase deformation within the salt and its overburden. Understanding how base‐salt relief influences salt‐related deformation is important to correctly interpret salt basin kinematics and distribution of structural domains, which have important implications to understand the development of key petroleum system elements. The São Paulo Plateau, Santos Basin, Brazil is characterized by a >2 km thick, mechanically layered Aptian salt layer deposited above prominent base‐salt relief. We use 3D seismic reflection data, and physical and conceptual kinematic models to investigate how gravity‐driven translation above thick salt, underlain by complex base‐salt relief, generated a complex framework of salt structures and minibasins. We show that ramp‐syncline basins developed above and downdip of the main pre‐salt highs record c. 30 km of Late Cretaceous‐Paleocene basinward translation. As salt and overburden translated downdip, salt flux variations caused by the base‐salt relief resulted in non‐uniform motion of the cover, and the simultaneous development of extensional and contractional structures. Contraction preferentially occurred where salt flow locally decelerated, above landward‐dipping base‐salt and downdip of basinward‐dipping ramps. Extension occurred at the top of basinward‐dipping ramps and base‐salt plateaus, where salt flow locally accelerated. Where the base of the salt layer was broadly flat, structures evolved primarily by load‐driven passive diapirism. At the edge of or around smaller base‐salt highs, salt structures were affected by plan‐view rotation, shearing and divergent flow. The magnitude of translation (c. 30 km) and the style of salt‐related deformation observed on the São Paulo Plateau afford an improved kinematic model for the enigmatic Albian Gap, suggesting this structure formed by a combination of basinward salt expulsion and regional extension. These observations contribute to the long‐lived debate regarding the mechanisms of salt tectonics on the São Paulo Plateau, ultimately improving our general understanding of the effects of base‐salt relief on salt tectonics in other basins.

This study constrains the sediment provenance for the Late Cretaceous–Eocene strata of the Ager Basin, Spain, and reconstructs the interplay between foreland basin subsidence and sediment routing within the south‐central Pyrenean foreland basin during the early phases of crustal shortening using detrital zircon (DZ) U‐Pb‐He double dating. Here we present and interpret 837 new DZ U‐Pb ages, 113 of which are new DZ (U‐Th)/He double‐dated zircons. U‐Pb‐He double dating results allow for a clear differentiation between different foreland and hinterland sources of Variscan zircons (280–350 Ma) by leveraging the contrasting thermal histories of the Ebro Massif and Pyrenean orogen, recorded by the zircon (U‐Th)/He (ZHe) ages, despite their indistinguishable U‐Pb age signatures. Cretaceous–Paleocene sedimentary rocks, dominated by Variscan DZ U‐Pb age components with Permian–Triassic (200–300 Ma) ZHe cooling ages, were sourced from the Ebro Massif south of the Ager Basin. A provenance shift occurred at the base of the Early Eocene Baronia Formation (ca. 53 Ma) to an eastern Pyrenean source (north‐east of the Ager Basin) as evidenced by an abrupt change in paleocurrents, a change in DZ U‐Pb signatures to age distributions dominated by Cambro‐Silurian (420–520 Ma), Cadomian (520–700 Ma), and Proterozoic–Archean (>700 Ma) age components, and the prominent emergence of Cretaceous–Paleogene (<90 Ma) ZHe cooling ages. The Eocene Corçà Formation (ca. 50 Ma), characterized by the arrival of fully reset ZHe ages with very short lag times, signals the accumulation of sediment derived from the rapidly exhuming Pyrenean thrust sheets. While ZHe ages from the Corçà Formation are fully reset, zircon fission track (ZFT) ages preserve older inherited cooling ages, bracketing the exhumation level within the thrust sheets to ca. 6–8 km in the Early Eocene. These DZ ZHe ages yield exhumation rate estimates of ca. 0.03 km/Myr during the Late Cretaceous–Paleocene for the Ebro Massif and ca. 0.2–0.4 km/Myr during the Eocene for the eastern Pyrenees.

A provenance shift occurred during the Early Eocene in the Ager Basin. A sediment source region in the southern foreland dominated by Variscan granitic basement of the Ebro Massif is interpreted for the Cretaceous ‐ Paleocene strata. This changes to a source region in the northern fold‐and‐thrust belt for Early Eocene strata, eroding recycled Mesozoic strata with plutonic and metamorphic Paleozoic basement exhumed during the early stages of Pyrenean shortening.

In the early stages of margin development when minibasins are relatively thin and do not interact with the base‐of‐salt relief, the strain pattern on the salt‐detached slope is expected to be relatively simple. In contrast, later in margin development, when minibasins are thick enough such that their bases weld to the rugose base‐of‐salt, they become obstructed. During the latter stage a more complex strain pattern develops, with shortening strains typically developed immediately upslope of each obstructed minibasin and an extensional breakaway developed immediately downslope. In this paper, we propose that minibasin obstruction exerts a first‐order control on the mobility of minibasins and the pattern of supra‐salt strain.

Salt‐detached gravity gliding/spreading systems having a rugose base‐of‐salt display complex strain patterns. However, little was previously known about how welding of supra‐salt minibasins to the sub‐salt may influence both the downslope translation of minibasins on salt‐detached slopes and the regional pattern of supra‐salt strain. Using a regional 3D seismic reflection data set, we examine a large salt‐stock canopy system with a rugose base on the northern Gulf of Mexico slope, on which minibasins both subside and translate downslope. Some minibasins are welded at their bases and others are not. We suggest that basal welds obstruct downslope translation of minibasins and control regional patterns of supra‐canopy strain. The distribution of strain above the canopy is complex and variable. Each minibasin that becomes obstructed modifies the local strain field, typically developing a zone of shortening immediately updip and an extensional breakaway zone immediately downdip of the obstructed minibasin. This finding is corroborated by observations from a physical sandbox model of minibasin obstruction. We also find in our natural example that minibasins can be obstructed to different degrees, ranging from severe (e.g., caught in a feeder) to mild (e.g., welded to a flat or gently dipping base‐of‐salt). By mapping both the presence of obstructed minibasins and the relative degree of minibasin obstruction, we provide an explanation for the origin of complex 3‐D strain fields on a salt‐detached slope and, potentially, a mechanism that explains differential downslope translation of minibasins. In minibasin‐rich salt‐detached slope settings, our results may aid: i) structural restorations and regional strain analyses; ii) prediction of subsalt relief in areas of poor seismic imaging; and iii) prediction of stress fields and borehole stability. Our example is detached on allochthonous salt and where the base‐of‐salt is rugose, with the findings applicable to other such systems worldwide (e.g., Gulf of Mexico; Scotian Margin, offshore eastern Canada). However, our findings are also applicable to systems where the salt is autochthonous but has significant local basal relief (e.g., Santos Basin, Brazil; Kwanza Basin, Angola).

Investigation of a >6‐km‐thick succession of Cretaceous to Cenozoic sedimentary rocks in the Tajik Basin reveals that this depocentre consists of three stacked basin systems that are interpreted to reflect different mechanisms of subsidence associated with tectonics in the Pamir Mountains: a Lower to mid‐Cretaceous succession, an Upper Cretaceous–Lower Eocene succession and an Eocene–Neogene succession. The Lower to mid‐Cretaceous succession consists of fluvial deposits that were primarily derived from the Triassic Karakul–Mazar subduction–accretion complex in the northern Pamir. This succession is characterized by a convex‐up (accelerating) subsidence curve, thickens towards the Pamir and is interpreted as a retroarc foreland basin system associated with northward subduction of Tethyan oceanic lithosphere. The Upper Cretaceous to early Eocene succession consists of fine‐grained, marginal marine and sabkha deposits. The succession is characterized by a concave‐up subsidence curve. Regionally extensive limestone beds in the succession are consistent with late stage thermal relaxation and relative sea‐level rise following lithospheric extension, potentially in response to Tethyan slab rollback/foundering. The Upper Cretaceous–early Eocene succession is capped by a middle Eocene to early Oligocene (ca. 50–30 Ma) disconformity, which is interpreted to record the passage of a flexural forebulge. The disconformity is represented by a depositional hiatus, which is 10–30 Myr younger than estimates for the initiation of India–Asia collision and overlaps in age with the start of prograde metamorphism recorded in the Pamir gneiss domes. Overlying the disconformity, a >4‐km‐thick upper Eocene–Neogene succession displays a classic, coarsening upward unroofing sequence characterized by accelerating subsidence, which is interpreted as a retro‐foreland basin associated with crustal thickening of the Pamir during India–Asia collision. Thus, the Tajik Basin provides an example of a long‐lived composite basin in a retrowedge position that displays a sensitivity to plate margin processes. Subsidence, sediment accumulation and basin‐forming mechanisms are influenced by subduction dynamics, including periods of slab‐shallowing and retreat.

Platform carbonates diagenesis in salt basins could be complex due to potential alterations of fluids related and non‐related to diapirism. This paper presents the diagenetic history of the Hettangian to Pliensbachian platform carbonates from the Tazoult salt wall area (central High Atlas, Morocco). Low structural relief and outcrop conditions allowed to define the entire diagenetic evolution occurred in the High Atlas diapiric basins since early stages of the diapiric activity up to their tectonic inversion. Precipitation of dolomite and calcite from both warmed marine‐derived and meteoric fluids characterised diagenetic stages during Pliensbachian, when the carbonate platforms were exposed and karstified. Burial diagenesis occurred from Toarcian to Middle Jurassic, due to changes of salt‐induced dynamic related to increase in siliciclastic input, fast diapir rise and rapid burial of Pliensbachian platforms. During this stage, the diapir acted as a physical barrier for fluid circulation between the core and the flanking sediments. In the carbonates and breccias flanking the structures, dolomite and calcite precipitated from basinal brines, whereas carbonate slivers located in the core of the structure, were affected by the circulation of Mn‐rich fluids. The final diagenetic event is characterised by the income of meteoric fluids into the system during uplift caused by Alpine orogeny. These results highlight the relevant influence of diapirism on the diagenetic modifications in salt‐related basins in terms of diagenetic events and involved fluids.

The hydrodynamics of rivers approaching a receiving basin are influenced by the onset of backwater conditions that give rise to decelerating reach‐average flow velocity and decreasing boundary shear stress. These changes occur across a spatial gradient over which decreasing sediment transport capacity triggers morphodynamic responses that include sediment deposition at the transition from uniform to nonuniform flow. As a consequence, the channel width‐to‐depth ratio and bed sediment grain size decrease downstream. While nonuniform flow and associated morphodynamic adjustments have been investigated in modern fluvial–deltaic systems, the impacts to fluvial–deltaic stratigraphy remain relatively unexplored. This represents an important unresolved gap: there are few contributions that link morphodynamic response to nonuniform flow, impacts on sediment deposition and influence on the rock record. This study uses a numerical model to explore how variable channel hydraulics influence long‐term (1000s years) patterns of sediment deposition and development of stratigraphy. The model results indicate that: (a) nonuniform flow propagates upstream beyond the backwater transition that is traditionally estimated with a basic backwater length scale relationship. (b) Base‐level fluctuations, especially rising, enhance the impact of nonuniform flow. (c) Sediment deposition shows large spatio‐temporal variability, which ultimately contributes to unique stacking patterns of fluvial–deltaic stratigraphy. (d) Nonuniform flow imparts spatial variation in flow depth, channel bed slope and sediment grain size over the delta, and these signatures are potentially preserved and recognizable in the rock record.