Structural features of the lower Permian salt-bearing series and the character of salt-tectogenesis manifestations in the Caspian Depression. Paper II

The article presents the analysis of the inner structure of the salt-bearing series within the Caspian Depression. Certain salt-bearing complexes of various ages, compositions and plastic properties are distinguished. Their structural relations and the features of salt-tectogenesis manifestation are evaluated. Up to the present no subsalt uplifts have been confirmed in the Russian sector of the Caspian Depression and no HC fields have been discovered. Therefore, any search for additional criteria for predicting the salt-bed structure from the character of salttectogenesis manifestation is conductive to predicting the zones of subsalt structure development over the Caspian Depression area.

Structural features of the lower permian salt-bearing series and the character of salt-tectogenesis manifestations in the northern and northwestern margins of the Сaspian depression. Paper 1

The detailed evaluation of the accumulated geological-geophysical data, particularly on the inner structure of the saltbearing series in the margins of the Caspian Depression, the mechanism of salt-tectogenesis manifestation, its reflection in the structure of the salt table and association with the structural inhomogeneities of the subsalt bed is presented. The authors attempt to acquire additional indirect criteria for predicting the structure of the subsalt bed by means of analyzing the structure of the salt-bearing series and the character of salt-tectogenesis manifestation. The problem solution is based on the complex analysis of the data from drilling and seismic prospecting.

Hooked on salt: Rethinking Alpine tectonics in Hallstatt (Eastern Alps, Austria)

Permian salt in the Hallstatt and neighboring salt structures of the Eastern Alps (Austria) crops out along with Triassic deep-water deposits that are at odds with the surrounding Triassic platform carbonates. The traditional interpretation of this juxtaposition is that the salt bodies were emplaced in the Late Jurassic as gravity nappes onto the carbonate platforms, in what has been considered to be the earliest orogenic event in the Eastern Alps. Here we describe for the first time a world-class outcrop of halokinetic sequences in Triassic platform carbonates flanking the Hallstatt diapir. Combining this with other outcrop evidence, we prove that the Hallstatt diapir grew passively during the Triassic, surrounded by carbonate platforms, and extruded to the seabed during the Jurassic. The development of the Hallstatt diapir in a platform setting disproves its emplacement as a gravity-driven nappe, proves the relevance of salt tectonics in the Mesozoic development of the Eastern Alps, and challenges the existence of a Late Jurassic Alpine orogenic event.

Structural evolution of continental and marine Permian rock salt of the North German Basin: constraints from microfabrics, geochemistry and U–Pb ages

Analyzing the dynamics of microstructural response on natural deformation in rock salt, we present microfabric, EBSD, geochemical and U–Pb data, obtained from Permian salt formations of the Kiel-Honigsee salt wall in Northern Germany. The samples were recovered from deep drillings, which penetrated through an overturned rock salt sequence of both Rotliegend and Zechstein deposits. The bromide concentration in halite indicates a continental and marine origin for the Rotliegend and Zechstein deposits, respectively. Despite intense deformation, relics of early diagenetic fabrics are still preserved. Deformation of the impure Rotliegend rock salt was accommodated by pressure solution and hydrofracturing as is indicated by the microfabrics and bromide concentration in halite. Fractures in siliciclastic domains were filled with fibrous halite and deformed by subgrain rotation recrystallization (SGR). Fluid-rich Zechstein rock salt, on the other hand, was deformed by formation of subgrains and grain boundary migration (GBM). The distribution of mineral phases and fluids had a significant impact on the fabric evolution and on strain localization. U–Pb dating of carbonate phases of the Rotliegend sequence yielded Permian depositional ages and Jurassic to Cretaceous deformation ages, the latter related to diapiric ascent. The combination of results traces a dynamic evolution of the rock fabric inside the diapir structure driven by locally active deformation processes that can be correlated with early stages of halite deposition and diagenesis and syntectonic fabric reorganization related to diapirism in an extensional setting.

Evaporite controls on basin-inversion structures in the southern Danish Central Graben, North Sea – a 3D seismic-data study

<p>During Cretaceous and Paleogene times, tectonic shortening caused mild basin inversion of earlier rifting depocentres in the Danish Central Graben. This exerted an important control on the thickness variations and geometries of e.g. the Late-Cretaceous and Danian Chalk Group. Structural highs formed by inversion and especially Permian-salt movements, host important hydrocarbon reservoirs in the sector. Earlier researchers have linked basin inversion in the North-Sea area to Alpine deformational phases and the onset of seafloor spreading in the North Atlantic.</p><p>The objective of this 3D seismic-data study is an analysis of the relationships between basement (sub-salt) faults, salt movements, and salt-cover deformation, as well as fluid migration near and within inversion structures.</p><p>We find that the northeastern margin of the larger inverted area generally has a thick-skinned style. Here, reverse reactivation of the rift-bounding master fault is coupled between the strata above and below the salt. Oppositely, the southwestern margin has a thin-skinned style. Here, buttressed hangingwall folds sit above reverse faults detaching into even thin evaporite sequences. The strike of this cover-fault trend mimics that of the underlying basement faults, although they dip in opposite directions. A triangle-zone model explains how sub-salt shortening (reactivation of major basement faults) can be balanced to the shortening observed in the sedimentary cover. As the current thickness of Permian salt increases and mobile-salt structures become predominant towards the south, the effects of basin inversion grow difficult to distinguish from those of halokinesis.</p><p>Interestingly, the shallow crests of inversion folds, especially along the southwestern margin, host groups of smaller normal faults. These formed to some degree during inversion, indicating that local extensional tectonism (crestal collapse) took place during the overall shortening. We conclude that the shallow parts of the folds experienced forced bending rather than buckling during folding.</p><p>A significant number of hydrocarbon reservoirs sit within basin-inversion structures. Potentially, this work can increase our understanding of deformation within these and similar structures.</p><p><em>Acknowledgements: We thank the Centre for Oil and Gas – DTU (DHRTC) for funding and supporting this project and for providing data. We also thank Schlumberger and Eliis for providing seismic-interpretation software.</em></p>