Palaeogeography and Palaeoenvironments – A Multifield Examination of the Devonian-Permian Evolution of the Dneipr-Donets Basin

Context is everything. Not all thick sands pay out and not all thin sands are poorly productive. It is important to understand a basin's palaeogeographical drivers, the resultant palaeoenvironments and their constituent sedimentary architecture. Development of a depositional model can be predictive with respect to the magnitude of accessible pore space for potential development. We present a multi-field study of the Dneipr-Donets basin. Over 600 wells were studied with >4500 lithostratigraphical picks being made. Over 7500 sedimentological picks were made allowing mapping of facies bodies and charting shifts in facies types. A facies classification scheme was developed and applied. The Devonian-Permian sedimentary section records the creation, fill, and terminal closure of the Dneipr-Donets Basin:Syn-rift brittle extension (late Frasnian-Famennian): intracratonic rifting between the Ukrainian Shield and Voronezh Massif formed a NW-SE orientated trough, with associated basaltic extrusion. Basin architecture consists of rotated fault blocks forming graben mini-basins. Sedimentation is dominantly upper shoreface but sand packages are poorly correlatable due to the faulted palaeotopography.Early Post-rift thermal subsidence (Visean-Lower Bashkirian): the faulted palaeotopography was filled and thermal subsidence drove basin deepening. Cyclical successions of offshore, lower shoreface and upper shoreface dominate. Sands are typically thin (<10m) but can be widely correlated and have high pore space connectivity.Mid Post-rift: the Bashkirian (C22/C23 boundary), paralic systems prograde over the shoreface. Changes in vertical facies are abrupt due to a low gradient to basin floor. Deltaic and fluvial facies can produce thick amalgamated sands (>30m), but access limited pore space because they are laterally restricted bodies.Terminal post-rift (Mykytivskan): above the lower Permian, the convergence of the Kazahkstanian and Siberian continents began to restrict the Dnieper-Donets basin's access to open ocean. The basin approached full conditions and deposition was dominated by evaporite precipitation, with periodic oceanic recharge. Ultimately, this sediment records the formation of Pangea. The successions examined were used to construct a basinal relative sea level curve, which can be applied elsewhere in the basin. This can be used to help provide palaeogeographical context to a field, which in turn controls the sedimentary architecture.

Ombilin Basin as Inverted Oblique Rift in Barisan Mountains, Sumatra: Considerations on Subsidence Mechanisms and Fault Development

Ombilin Basin is a NW-SE inverted oblique rift which is currently being part of Barisan Mountains in western Central Sumatra. Regarding its current position, Ombilin Basin can be one of the windows to see the evolution of Barisan Mountains since Paleogene. Two schools of thought, namely rift basin and pull-apart basin, have been established to explain the evolution of Ombilin Basin. This paper aims to present another perspective on the evolution of Ombilin Basin based on subsidence mechanisms and fault development. This study integrated remote sensing and subsurface interpretations. Remote sensing interpretation took the role to delineate surface fault lineaments using digital elevation model, while subsurface interpretation dealt with log and seismic interpretations, subsidence analysis, and palinspatic reconstruction. Fault lineaments derived both from remote sensing and seismic interpretations were combined to construct structural framework of the basin. Subsidence analysis generated geohistory and backstripped tectonic subsidence charts. Palinspatic reconstruction illustrated structural configurations through time. This study figured out that Ombilin Basin went through fault-controlled subsidence in Middle Eocene – Late Oligocene and thermal subsidence in Early Miocene – Late Pliocene. Each subsidence mechanism was terminated by an uplift. Subsidence mechanisms in Ombilin Basin represented the criteria of rift basin in terms of amount and rate of tectonic subsidence, duration of subsidence, and contribution of thermal subsidence. On the other hand, fault development captures extensional and strike-slip components during rifting and development of flower structures during inversion of the basin. Oblique rifting operates when dominant extensional component works together with strike-slip component. Therefore, subsidence mechanisms and fault development are in agreement to regard Ombilin Basin as inverted oblique rift.

The Scoresbysund rifted margin composite TSE offshore central East Greenland

The little explored central East Greenland margin contains thick sedimentary accumulations confined within the Scoresbysund Basin. The geological evolution of the area distinguishes from other parts of East Greenland. Even so, resemblances with the prospective basins onshore and offshore farther north probably exist, and the margin may hold a real petroleum potential. The Scoresbysund Rifted Margin Composite Tectonic-Sedimentary Element delineates the oldest part of the Scoresbysund Basin. It formed through multiple phases of rifting, volcanism, uplift and thermal subsidence between Devonian and Miocene time. The development of the composite tectonic-sedimentary element concluded with the latest Oligocene or early Miocene continental break-up of the Jan Mayen microcontinent and East Greenland. The Scoresbysund Rifted Margin Composite Tectonic-Sedimentary Element contains approximately 4 km of Eocene-lower Miocene fan-delta deposits that accumulated during down-faulting along the East Greenland Escarpment and farther seawards intercalate with basalts. The fan-delta deposits rest on Paleocene basalts that most likely cover Paleozoic-Mesozoic strata. Equivalent to onshore, the deeply buried section probably include source rock and reservoir intervals of Carboniferous, Permian and Mesozoic age. Together with the major fault structures existing in the western part of the area, this may form the basis for a working petroleum system.

PALEOSTRIPv1.0 – a user-friendly 3D backtracking software to reconstruct paleo-bathymetries

We present PALEOSTRIPv1.0, a MATLAB open-source software designed to perform 1D, 2D and 3D backtracking of paleo-bathymetries. PALEOSTRIP comes with a Graphical User Interface (GUI) to facilitate computation of sensitivity tests and to allow the users to switch on and off all the different processes and thus separate the various aspects of backtracking. As such, all physical parameters can be modified from the GUI. It includes 3D flexural isostasy, 1D thermal subsidence and possibilities to correct for prescribed sea level and dynamical topography changes. In the following we detail the physics embedded within PALEOSTRIP and we show a few applications on a drilling site (1D), a transect (2D) and a map (3D), taking the Ross Sea (Antarctica) as a case study. PALEOSTRIP has been designed to be modular and to allow user to insert their own implementations.

Early Cretaceous syn- to post-rift evolution of the Mentelle Basin on the southwest Australian rifted margin (IODP Expedition 369 Sites U1513–U1516)

<p>The Mentelle Basin is a large and deep-water sedimentary basin located on the southwest Australian rifted margin. The basin lies west of the Perth Basin, east of the Naturaliste Plateau and south of the Perth Abyssal Plain. The rifted margin formed when the Greater Indian plate separated from the Australian-Antarctic plate during the Jurassic to early Cretaceous. Based on seismic reflection data, several km thick sediments infilling the basin have been interpreted. However, due to lack of geological and geophysical data, the basin has not been studied enough to understand its evolution. In 2017, International Ocean Discovery Program (IODP) Expedition 369 drilled four sites, U1513–U1516, in the Mentelle Basin and recovered important cores including late Jurassic to Early Cretaceous sections. At Site U1515 on the eastern margin of the basin, drilling penetrated below the seismically imaged breakup unconformity into the middle Jurassic to earliest Cretaceous syn-rift strata. Holes at Site U1513 on the western margin cored the syn-rift volcanic sequence, the Hauterivian to early Aptian volcaniclastic-rich sandstone sequence spanning the syn- to post-rift phase, and the Aptian to Albian post-rift claystone sequence. Drilling at Sites U1514 and U1516 in the central part reached the Albian post-rift sequence. Using a combination of shipboard and post-expedition data, we interpret the lithological, paleontological and geochemical characteristics of the syn- to post-rift sequences. The results allowed us to reconstruct the Early Cretaceous stratigraphy, tectonics, paleo-environment, and basin evolution of the Mentelle Basin. During the syn-rift phase, the middle Jurassic to lower Cretaceous non-marine sediments were deposited in the eastern Mentelle Basin, while volcanic rocks were emplaced in the western part. The 82 m thick volcanic sequence consists of alternating basalt flows and volcaniclastics with dolerite dikes, which indicate multiple volcanic eruption events in subaerial to shallow water environments. It was overlain by the 235 m thick volcaniclastic-rich sequence consisting of massive or laminated sandstone layers, deposited in shelf to upper bathyal depths. The deposition period spans the syn- to post-rift phase of the basin but decreasing sedimentation rate and shallow marine setting suggest that the post-rift thermal subsidence did not immediately follow the final continental breakup. We interpret that the delayed thermal subsidence was likely to be induced by adjacent mantle plume activities. Deep marine claystone sequences blanketing most of the basin indicate Aptian to Albian post-rift thermal subsidence.</p>

Seismic and borehole-based mapping of the late Carboniferous succession in the Canonbie Coalfield, SW Scotland: evidence for a ‘broken’ Variscan foreland?

Local seismic and borehole-based mapping of the Carboniferous Pennine Coal Measures and Warwickshire Group successions in the Canonbie Coalfield (SW Scotland) provides evidence of repeated episodes of positive inversion, syn-depositional folding and unconformities. A Duckmantian (Westphalian B) episode of NE–SW transpression is recognized, based on onlapping seismic reflector geometries against NE-trending positive inversion structures and contemporaneous NNE-trending syn-depositional growth folding. The basin history thus revealed at Canonbie is at variance with generally accepted models in neighbouring northern England that imply subsidence was due to post-rift thermal subsidence during late Carboniferous times. A late Westphalian–Stephanian unconformity recognized within the Warwickshire Group succession signifies NW–SE, c. 10% local basin shortening during a time of major shortening in the late Carboniferous Variscan foreland, contradicting suggestions that maximum Variscan shortening had negligible impact on Carboniferous basins in northern Britain. Local inversion structures appear to have strongly influenced local late Westphalian–Stephanian depocentres. In this respect, the Variscan foreland at Canonbie may have resembled a ‘broken’ foreland system. Variations in crustal rheology, fault strength and orientation, and mid-crustal detachments are suggested to have played important roles in determining strain localization and the nature of Westphalian–Stephanian depocentres in the Canonbie Coalfield.

Cenozoic sea-level and cryospheric evolution from deep-sea geochemical and continental margin records

Using Pacific benthic foraminiferal δ18O and Mg/Ca records, we derive a Cenozoic (66 Ma) global mean sea level (GMSL) estimate that records evolution from an ice-free Early Eocene to Quaternary bipolar ice sheets. These GMSL estimates are statistically similar to “backstripped” estimates from continental margins accounting for compaction, loading, and thermal subsidence. Peak warmth, elevated GMSL, high CO2, and ice-free “Hothouse” conditions (56 to 48 Ma) were followed by “Cool Greenhouse” (48 to 34 Ma) ice sheets (10 to 30 m changes). Continental-scale ice sheets (“Icehouse”) began ~34 Ma (>50 m changes), permanent East Antarctic ice sheets at 12.8 Ma, and bipolar glaciation at 2.5 Ma. The largest GMSL fall (27 to 20 ka; ~130 m) was followed by a >40 mm/yr rise (19 to 10 ka), a slowing (10 to 2 ka), and a stillstand until ~1900 CE, when rates began to rise. High long-term CO2 caused warm climates and high sea levels, with sea-level variability dominated by periodic Milankovitch cycles.

Crustal Thickness and Composition of the São Paulo Plateau and Florianópolis Ridge, SE Brazilian Margin

<p>The São Paulo Plateau (SPP) and the Florianópolis Ridge (FR), located on the Santos segment of the SE Brazilian margin in the South Atlantic, are large positive bathymetric features with a combined lateral dimension of approximately 500 km. An important question is whether they are underlain by thinned continental crust or by anomalously thick magmatic crust. Each hypothesis has implications for the breakup of the South Atlantic and the evolution of the overlying saline Santos basin.</p><p>Integrated quantitative analysis consisting of gravity inversion, RDA (residual depth anomaly) analysis and flexural subsidence analysis has been applied to a deep long-offset seismic reflection line running NW-SE across the SPP and FR. Gravity inversion predicts crustal basement thicknesses in the range of 12 to 15 km for the SPP and FR, deceasing to 7-8 km thickness at the extreme SE end of the profile. The SPP and FR are separated by a region of thinner crust approximately 80 km wide. Thinning factors from subsidence analysis for SPP and FR are typically between 0.6 and 0.7.</p><p>RDA values close to zero and a thinning factor of 1 were obtained for the region with 7-8 km thick crust at the SE end of the profile which are all consistent with normal oceanic crust rather than previously interpreted exhumed mantle. This oceanic crust is highly tectonised and corresponds to the location of the Florianópolis Fracture Zone.</p><p>Flexural backstripping and reverse thermal subsidence modelling were performed to calculate palaeo-bathymetry at breakup and give 2.5 km below sea level at the SE end of the profile consistent with this region being oceanic crust. Flexural subsidence analysis applied to base salt shows that the observed base salt subsidence requires a component of syn-tectonic subsidence as well as post-rift thermal subsidence, and that the salt was deposited while the crust was still thinning.</p><p>Joint inversion of time seismic reflection and gravity data to determine the lateral variation in basement density by comparing seismic and gravity Moho in the time domain gives a basement density under the SPP and FR of between 2600 and 2700 kg/m<sup>3</sup>. The same method gives a basement density of 900kg/m<sup>3</sup> for the oceanic crust at the SE end of the profile. The FR basement in the NW shows a basement density similar to that of the SPP while in its SE the basement density is much higher approaching 2950 kg/m3.  We interpret the relatively low basement densities of the SPP with respect to that of oceanic crust as indicating a continental rather than magmatic composition. A similar analysis to determine basement density applied to the Evain et al. (2015) seismic refraction profile in the same location also gives a SPP basement density that supports a continental composition.</p>

Movements of thick evaporites on the flanks of a mid-ocean ridge: the central Red Sea Miocene evaporites

<p>Thick evaporites ("salt") were deposited in the South and North Atlantic, and Gulf of Mexico basins, in some parts deposited onto the flanks of nascent oceanic spreading centres.  Unfortunately, knowledge of the history of evaporite movements is complicated in such places by their inaccessibility and subsequent diapirism.  This is less of a problem in the Red Sea, a young rift basin that is transitioning to an ocean basin and where the evaporites are less affected by diapirism.  In this study, we explore the vertical movements of the evaporite surface imaged with deep seismic profiling.  The evaporites have moved towards the spreading axis of the basin during and after their deposition, which ended at the 5.3 Ma Miocene-Pliocene boundary.  We quantify the evaporite surface deflation needed to balance the volume of evaporites overflowing oceanic crust of 5.3 Ma age, thermal subsidence of the lithosphere and loss of halite through pore water diffusion, allowing for isostatic effects.  The reconstructed evaporite surface lies within the range of estimated global sea level towards the end of the Miocene.  Therefore, the evaporites appear to have filled the basin almost completely at the end of the Miocene.  Effects of shunting by terrigenous sediments and carbonates near the coast and contributions of hydrothermal salt are too small to be resolved by this reconstruction.</p>