Recognizing extinction events and determining their cause at the Triassic/Jurassic (T/J) transition and near the Pliensbachian–Toarcian (Lower Jurassic) boundary is a field of growing interest. We ...provide arguments for these events through a literature based new evaluation of coral diversity from Triassic to Dogger and a new palaeobiogeographical map. The T/J extinction of corals is clearly related to the breakdown of reef environments. Origination curves show that Hettangian (the lowest Jurassic stage) was not only a survival phase but already rather a recovery phase. Post‐extinction evolution of reefs and their survival only in the northernmost margin of the Tethys support the hothouse hypothesis for the T/J extinction event. During Pliensbachian, many new taxa appear, but mostly solitary corals, not really framebuilders. Many of these taxa do not occur anymore during the following stages. The new increase in diversity is related to the development of Bajocian (Middle Jurassic) reefs.
The fissility is the ability of some rocks to split along relatively smooth surfaces parallel to the bedding. This property observed mostly in fine-grained rocks is particularly expressed in ...outcrops, where rocks are subjected to weathering processes. Most authors associate the fissility to the abundance of clay minerals and their orientation parallel to the bedding. The horizontal fabric can be promoted by depositional conditions such as sediment composition, quantity of total organic carbon content (TOC) and depositional mechanisms, giving rise to a primary fissility. Alternatively, the alignment of platy minerals can be linked to the burial history of the rock, by processes such as mechanical compaction or secondary mineral growth, resulting in a secondary fissility. The present study aims to identify the main controls of fissility development at the micro- and macroscopic scale in rocks of the Vaca Muerta Formation exposed in the Cerro Mulichinco area and in a 121-meter-long core extracted from a well within the Neuquén Basin. In outcrops, fissility is related to fine-grained laminated facies with low carbonate content, revealing the strong control exerted by lithology. The TOC measurements allow establishing a positive correlation between organic matter content and fissility intensity. Moreover, the analysis of the transgressive-regressive cycles shows that fissility is higher around the maximum flooding surfaces. Regarding their mechanical characteristics, the different interfaces observed in core are classified into first and second-order, the last one including fissility planes. Some of these interfaces evolve from potential (partially open) to effective (totally open) discontinuities in response to changes of stress conditions during the core extraction and due to the stress relaxation through time: weeks (T1), months (T2) and years (T3) after extraction. The time evolution of the effective core discontinuities points out rock intervals that are variably broken and core segments that remain intact. The Drying Alcohol Discontinuities (DAD) methodology reveals potential discontinuities within apparently intact core segments. By using this technique, a 4-class index is established as a proxy for fissility degree. When integrated with geological, petrophysical and geomechanical data, this index enables characterizing the main mechanisms controlling rock fissility that express through discontinuities promoting the loss of competence of a rock. Consequently, this mechanical property is considered to influence the efficiency of hydraulic fracture in shale reservoir completion.
Based on a dense 2D seismic reflection dataset and information from 8 exploration wells, we reinterpreted the stratigraphic evolution of the Colorado Basin. The basin is located on the continental ...shelf and slope within 50 to 2250m of bathymetry. The total sediment fill can be up to 16,000m. Seismic-to-well log correlations provide a chrono-stratigraphic framework for the interpreted seismic sequences. We show that the Colorado Basin records the development of a Permian pre-rift period, a Triassic/Jurassic to Early Cretaceous rift phase and a Lower Cretaceous to Tertiary drift phase. This passive margin represents the evolution of lithospheric extension from active rifting to the thermal subsidence/drift stage. Several Cretaceous to Cenozoic slumping episodes were identified and related to progradation of the sequences and sediment build-up in the slope, as well as to the development of seaward dipping extensional faults.
Assuming an accumulation time span of 25Ma for the syn-rift succession, which corresponds to the duration of extensional phases preceding the final break-up of the South Atlantic, sedimentation rates are in the order of 118m/Ma. By Late Cretaceous, the Gondwana break-up had stopped in almost all areas and post-rift thermal subsidence dominated the basin development. During the early phase of subsidence, high sedimentation rates of up to 122m/Ma are recorded. An important decrease of sediment supply (30m/Ma) occurred until the end of the sag phase during the deposition of the Colorado Formation and sedimentation took place in elongated depocenters with oceanward progradation. A second phase of increased sediment supply takes place during the Oligocene and Miocene, resulting in the development of large basinward prograding sedimentary wedges. This event seems to be associated to the interaction between continental dynamics, subsidence, and increased continental erosion during climate shift from greenhouse to icehouse conditions.
► Sedimentation rate record the evolution of the margin. ► Cretaceous–Cenozoic MTDs driven by slope sediment build-up & seaward-dipping faults ► Shelf progradation up to latest Cretaceous followed by aggradation until Oligocene ► Sediment supply increase during Oligo-Miocene caused basin-wide prograding wedges. ► South American uplift drives regression and low sedimentation rates during Eocene.
This 3D structural model of the Colorado Basin provides new insights into the crustal geometry of the basin and its evolution in relation with the Argentine passive margin. Three NW–SE segments ...(oblique to the N30°E-trending margin) structure the basin. The oldest infill is generally thought to be coeval with the rifting of the South Atlantic margins in Late Jurassic–Early Cretaceous. This coeval development of the Colorado Basin and of the passive margin is still under debate and gives rise to several hypotheses that we investigate in the light of our observations.
We propose that reactivation of inherited structures is predominant in the evolution of the Colorado Basin: (1) the Western segment follows the continental continuation of the Colorado transfer zone; (2) the Central segment consists in the continental continuation of the Tona deformation zone; (3) the Eastern segment is superimposed over the Palaeozoic Claromecó Basin. In addition to the 3 segments, the Central High, separating the Central segment to the Eastern segment, corresponds to the Palaeozoic Sierras Australes Fold Belt.
The direction of extension responsible for the South Atlantic opening cannot explain the syn-rift infill and thinning of the basin. The structural analysis shows two phases of syn-rift deformation with different directions. Thus, we suggest that the Colorado Basin and the South Atlantic margin are not coeval but that a first extensional event, probably oblique, predates the extension responsible for the South Atlantic opening.
This event is then followed by the formation of the N30°-trending distal margin and the reactivation of Palaeozoic N70°-trending faults occurs under the NW–SE opening of the South Atlantic. This two-phase evolution is consistent with the fault chronology and the two directions of thinned crust observed in the distal margin.
•The Colorado Basin is not entirely linked to the South Atlantic opening.•Two different phases of extension are observed, the first is probably oblique.•The Paleozoic structural inheritance controls the localisation of the deformation.
This study documents for the first time the occurrence of Shanbeipollenites proxireticulatus Schrank in levels of the Vaca Muerta Formation, Neuquén Basin, Argentina. It represents the first record ...of this species in high latitudes of the Southern Hemisphere. The Argentinian specimens are associated with useful palynostratigraphical indicators which suggest an early Valanginian age for the palynoflora yielding S. proxireticulatus. Therefore, this finding constitutes the first record from the Cretaceous. This taxon is related to the orders Cycadales/Bennettitales/Ginkgoales. During the Cretaceous in Patagonia cycads and bennettitaleans groups show marked adaptations to warm and strongly seasonal dry climate periodically influenced by volcanic activity. The paleogeographic distribution of the records of S. proxireticulatus, symmetrically located on both sides of the equator, together with its relationship with the climate zones or Biomes, suggest that the parental plants of this pollen grain thrived under warm climate and probably stressed paleoenvironmental conditions. Since the studied sample is located near 1000 m below the Intravalanginian unconformity, and older age (i.e. late Berriasian) cannot be discarded.
The detailed 3D geometry of normal fault planes is described and analysed using datasets from outcrop studies (2D), seismic surveys (3D) and analogue models (4D). Different geometric configurations ...of simple isolated normal faults are studied by reference to processes of normal fault propagation. When a normal fault propagates without interacting with other fault zones, the entire border of the principal plane displays characteristic connected secondary structures. These secondary structures cause bifurcations of the principal fault terminations. The along-strike terminations of the principal plane display typical bifurcation configurations (‘ear geometry‘). The orientation of the bifurcations depends on the vertical direction of propagation (downwards and/or upwards). The along-dip terminations display en échelon secondary fault planes linked to the principal plane and are described as ‘lobate geometry’. A 3D genetic model of isolated normal fault geometry is proposed with a new general terminology for the secondary structures. When two isolated normal faults propagate towards each other and overlap, the two principal planes connect up via a relay fault. The resulting geometry is a longer fault exhibiting a characteristic undulation with two inactive branches.
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