Basin Research - Volume 28, Issue 4, 2016

Changes in sandstone and conglomerate maturity in tectonically active basins can be considered either as the product of climatic change or of tectonic restructuring of the feeder drainage system. Besides these regional controls, changes in the configuration of local sources can expressively affect basin fill composition. The Early Cretaceous fluvial successions of the Tucano Basin, a rift basin in northeastern Brazil related to the South Atlantic opening, contain one such case of abrupt change in maturity, marked by the passage from pebbly sandstone and conglomerate rich in quartz and quartzite fragments (Neocomian to Barremian São Sebastião Formation) to more feldspathic pebbly sandstone and conglomerate bearing pebbles of varied composition (Aptian Marizal Formation). Systematic analysis of stratigraphic and spatial variation in palaeocurrents and composition of pebbles and cobbles from both units, integrated with the recognition of fluvial and alluvial fan deposits distribution, revealed an abrupt decrease in maturity during the passage from the São Sebastião Formation to the Marizal Formation. This change is explained by exhumation of basement rocks and erosional removal of originally widespread Silurian to Jurassic sandstone and conglomerate units which were a major source of reworked vein quartz and quartzite pebbles to the São Sebastião Formation. Basin border faults activation during the deposition of the Marizal Formation caused adjacent basement uplift above the local erosional base level at the basin borders, whereas during the São Sebastião Formation deposition, the basin border fault scarps probably exposed mineralogically mature sedimentary units. The proposed model has important implications for interpreting changes in sediment maturity in rift basin successions, as similar results are expected where activation of basin border faults occurs after the erosional removal of older sedimentary or volcanic units that controlled syn‐rift successions composition.

Piggyback basins developed at the mountain fronts of collisional orogens can act as important, and transient, sediment stores along major river systems. It is not clear, however, how the storage and release of sediment in piggyback basins affects the sediment flux and evolution of downstream river reaches. Here, we investigate the timing and volumes of sediment storage and release in the Dehra Dun, a piggyback basin developed along the Himalayan mountain front in northwestern India. Based on OSL dating, we show evidence for three major phases of aggradation in the dun, bracketed at ca. 41–33 ka, 34–21 ka and 23–10 ka, each accompanied by progradation of sediment fans into the dun. Each of these phases was followed by backfilling and (apparently) rapid fan‐head incision, leading to abandonment of the depositional unit and a basinward shift of the active depocentre. Excavation of dun sediment after the second and third phases of aggradation produced time‐averaged sediment discharges that were ca. 1–2% of the modern suspended‐sediment discharges of the Ganga and Yamuna rivers that traverse the margins of the dun; this sediment was derived from catchment areas that together comprise 1.5% of the drainage area of these rivers. Comparison of the timing of dun storage and release with upstream and downstream records of incision and aggradation in the Ganga show that sediment storage in the dun generally coincides with periods of widespread hinterland aggradation but that late stages of dun aggradation, and especially times of dun sediment excavation, coincide with major periods of sediment export to the Ganga Basin. The dun thus acts to amplify temporal variations in hinterland sediment supply or transport capacity. This conceptual model appears to explain morphological features of other major river systems along the Himalayan front, including the Gandak and Kosi Rivers, and may be important for understanding sediment flux variations in other collisional mountain belts.

The Austral Basin (or Magallanes Basin) in southern Argentina is situated in a highly active tectonic zone. The openings of the South Atlantic and the Drake Passage to the east and south, active subduction in the west, and the related rise of the Andes have massively influenced the evolution of this area. To better understand the impacts of these tectonic events on basin formation to its present‐day structure we analysed 2D seismic reflection data covering about 95 000 km² on‐ and 115 000 km² offshore (Austral ‘Marina’ and Malvinas Basin). A total of 10 seismic horizons, representing nine syn‐ and post‐ rift sequences, were mapped and tied to well data to analyse the evolution of sedimentary supply and depocenter migration through time. 1D well backstripping across the study area confirms three main tectonic stages, containing (i) the break‐up phase forming basement graben systems and the evolution of the Late Jurassic – Early Cretaceous ancient backarc Austral/Rocas Verdes Basin (RVB), (ii) the inversion of the backarc marginal basin and the development of the foreland Austral Basin and (iii) the recent foreland Austral Basin. Synrift sedimentation did not exceed the creation of accommodation space, leading to a deepening of the basin. During the Early Cretaceous a first impulse of compression due to Andes uplift caused rise also of parts of the basin. Controlling factors for the subsequent tectonic development are subduction, balanced phases of sedimentation, accumulation and erosion as well as enhanced sediment supply from the rising Andes. Further phases of rock uplift might be triggered by cancelling deflection of the plate and slab window subduction, coupled with volcanic activity. Calculations of sediment accumulation rates reflect the different regional tectonic stages, and also show that the Malvinas Basin acted as a sediment catchment after the filling of the Austral Basin since the Late Miocene. However, although the Austral and Malvinas Basin are neighbouring basin systems that are sedimentary coupled in younger times, their earlier sedimentary and tectonic development was decoupled by the Rio Chico basement high. Thereby, the Austral Basin was affected by tectonic impacts of the Andes orogenesis, while the Malvinas Basin was rather affected by the opening of the South Atlantic.

The late Palaeozoic Cumberland Basin of Nova Scotia and New Brunswick (eastern Canada) developed as a strike‐slip basin in the aftermath of the Middle Devonian Acadian Orogeny. Following deposition of thick salt during the middle Viséan (middle Mississippian), this basin mainly accommodated fault‐controlled continental deposits during the late Viséan, which generated halokinesis from clastic loading. The Mississippian halokinetic history of this basin is cryptic, as it was severely distorted by subsequent tectonic and halokinetic overprints. After minor structural restoration, the study of upper Viséan minibasin units in wide coastal sections and deep wells allowed a fairly detailed reconstruction of the Mississippian halokinetic setting to be made. Paleoenvironments and depositional settings in the western part of the basin include sectors that were proximal to three fault‐bounded source areas and characterized by alluvial fan systems transitioning laterally into gravelly to sandy braidplain environments. More central areas of the basin were characterized by tidal flats transitioning laterally into shallow marine environments. Because of halokinesis, the marine body was eventually forced to subdivide into three separate salt expulsion minibasins. Although late Viséan marine incursions were short‐lived in the rest of eastern Canada due to ongoing glacioeustatic variations, there are sedimentologic and stratigraphic lines of evidence for the long‐lasting entrapment of restricted marine bodies in salt expulsion minibasins of the western Cumberland Basin. In one minibasin that was characterized by especially high accommodation rates, NE of Hopewell Cape (New Brunswick), the proximal conglomerates and marine carbonates of a fan‐delta setting transition laterally into thick sulphate over a short distance, away from freshwater inputs from the source area. The vertical continuity of the latter sulphate succession suggests that this entrapped evaporitic basin was cut‐off from significant marine influxes, even at times of glacioeustatic highstands. This is in contrast with salt expulsion minibasins in open marine shelf settings, which always remain open to global marine transgressions and regressions.

Unique vertical fluidization structures from the Lower Cretaceous Athabasca Oil Sands Deposit in Western Canada are described. The multi‐metre long structures resulted from ascending Devonian karst aquifer waters injected into the overlying unconsolidated sands of the McMurray Formation (Aptian). The dissolution removal of 100–200 m of Middle Devonian salt beds caused the collapse and fragmentation of the 200 m of Upper Devonian limestones underlying the Cretaceous sands. Hypogene karst aquifer waters fingered upwards along the faults and fractures. Disintegrated muddy wall rock sourced hydroplastic mud flows along fractures in the limestone beds below the floor of the central Bitumount Trough. These dykes widened and lengthened collapse‐induced Upper Devonian fault blocks that differentially subsided due to salt dissolution. Conduits plugged by these mud flows diverted over‐pressured aquifer waters upwards along remaining open pipes, some of which vented on the trough floor. Aquifer waters injected into the McMurray sand and mud beds accumulated on the trough floor generated several types of collapse‐induced fluidization structures: (1) stronger aquifer flows mobilized sands into tens‐of‐metres high sand dykes consisting of multiple vertical pipes of fluidized sand contorted around and intertwined with fragmented muddy wall rock; (2) smaller aquifer jets resulted in 1–2 m high sand‐rich pillars on the trough floor; and (3) narrower high‐pressure jets flowed muddy waters along multi‐metre long, 3–5 cm wide, vertical pipes that cross‐cut and wrapped around fragmented in situ beds. The velocity gradient between these narrow but multi‐metre long water pipe flows and the ambient velocity in the surrounding sediments caused water infiltration to radiate outwards. This plugged porosity along the outer wall of the pipe, diminished drainage into the wall rock, and stabilized concentric growth ring depositions along the length of the pipe. These pipe fills appear as unique striped ribbon fabrics when viewed in longitudinal section.

In the East Coast Basin (ECB), an active convergent margin of the North Island, New Zealand, the smectite‐rich Eocene Wanstead Formation forms an effective regional seal, creating high overpressure in the underlying Cretaceous through Palaeocene units due to disequilibrium compaction. This study examines the evolution of pore pressure and porosity in Hawke Bay of the ECB based on stepwise structural reconstruction of a stratigraphic and structural framework derived from interpretation of a regional two‐dimensional seismic line. This framework is incorporated into a basin and petroleum system model to predict the generation, distribution, and dissipation of overpressure, and examine the influence of faults, erosion, structural thickening, and seal effectiveness of the Wanstead Formation on pore pressure evolution. We find that natural hydraulic fracturing is likely occurring in sub‐Wanstead source rocks, which makes it a favourable setting for potential shale gas plays. We use poroelastic modelling to investigate the impact of horizontal bulk shortening due to tectonic compression on pore pressure and the relative order of principal stresses. We find that shortening modestly increases pore pressure. When 5% or greater shortening occurs, the horizontal stress may approach and exceed vertical stress in the last 4 Myr of the basin's history. Shortening impacts both the magnitude and relative order of principal stresses through geological time. Due to the overpressured nature of the basin, we suggest that subtle changes in stress regime are responsible for the significant changes in structural deformational styles observed, enabling compressional, extensional, and strike‐slip fault regimes to all occur during the tectonic history and, at times, simultaneously.