Basin Research - Volume 26, Issue 5, 2014

The Tarim Basin in western China formed the easternmost margin of a shallow epicontinental sea that extended across Eurasia and was well connected to the western Tethys during the Paleogene. Climate modelling studies suggest that the westward retreat of this sea from Central Asia may have been as important as the Tibetan Plateau uplift in forcing aridification and monsoon intensification in the Asian continental interior due to the redistribution of the land‐sea thermal contrast. However, testing of this hypothesis is hindered by poor constraints on the timing and precise palaeogeographic dynamics of the retreat. Here, we present an improved integrated bio‐ and magnetostratigraphic chronological framework of the previously studied marine to continental transition in the southwest Tarim Basin along the Pamir and West Kunlun Shan, allowing us to better constrain its timing, cause and palaeoenvironmental impact. The sea retreat is assigned a latest Lutetian–earliest Bartonian age (ca. 41 Ma; correlation of the last marine sediments to calcareous nannofossil Zone CP14 and correlation of the first continental red beds to the base of magnetochron C18r). Higher up in the continental deposits, a major hiatus includes the Eocene–Oligocene transition (ca. 34 Ma). This suggests the Tarim Basin was hydrologically connected to the Tethyan marine Realm until at least the earliest Oligocene and had not yet been closed by uplift of the Pamir–Kunlun orogenic system. The westward sea retreat at ca. 41 Ma and the disconformity at the Eocene–Oligocene transition are both time‐equivalent with reported Asian aridification steps, suggesting that, consistent with climate modelling results, the sea acted as an important moisture source for the Asian continental interior.

The North Sea giant sand injectite province (NSGSIP) is the global type area for large‐scale sandstone intrusion complexes. Despite decades of research on the NSGSIP, this paper presents the first detailed case study in which all aspects of the intrusion process have been constrained, including fluid and sediment sources, injection timing and driving mechanisms. The study describes and analyses high‐amplitude discordant amplitude anomalies within the Oligocene succession in the eastern North Sea, which are interpreted as large‐scale brine‐saturated sand injectites. Potential feeder conduits extending from the top of the Paleocene Lista Formation to the base of the injectites indicate that the source sand was located within the Lista Formation; possibly deposited in a distinct valley cut into the top of the Chalk Group. The geometry of the observed injectites ranges from a basal sill with wings to V‐shaped and conical; their dimensions range from 300 to 3700 m in width and up to 150 m in height. In all cases, a significant deformation of the overburden is observed. The study area is located in the Ringkøbing‐Fyn High area above the basement high separating two smaller Paleozoic half‐grabens. During the Oligocene, rapid and significant differential loading occurred. We interpret that the injectites formed due to remobilization of the source sand facilitated by overpressure caused by differential loading combined with a possible influx of fluids from the deeper succession. The case study has with its assessment of the full injection system, implications for the understanding of subsurface remobilization processes and furthermore for oil and gas exploration in the eastern North Sea.

In this study, we integrate 3D seismic reflection, wireline log, biostratigraphic and core data from the Egersund Basin, Norwegian North Sea to determine the impact of syn‐depositional salt movement and associated growth faulting on the sedimentology and stratigraphic architecture of the Middle‐to‐Upper Jurassic, net‐transgressive, syn‐rift succession. Borehole data indicate that Middle‐to‐Upper Jurassic strata consist of low‐energy, wave‐dominated offshore and shoreface deposits and coal‐bearing coastal‐plain deposits. These deposits are arranged in four parasequences that are aggradationally to retrogradationally stacked to form a net‐transgressive succession that is up to 150‐m thick, at least 20 km in depositional strike (SW‐NE) extent, and >70 km in depositional dip (NW‐SE) extent. In this rift‐margin location, changes in thickness but not facies are noted across active salt structures. Abrupt facies changes, from shoreface sandstones to offshore mudstones, only occur across large displacement, basement‐involved normal faults. Comparisons to other tectonically active salt‐influenced basins suggest that facies changes across syn‐depositional salt structures are observed only where expansion indices are >2. Subsidence between salt walls resulted in local preservation of coastal‐plain deposits that cap shoreface parasequences, which were locally removed by transgressive erosion in adjacent areas of lower subsidence. The depositional dip that characterizes the Egersund Basin is unusual and likely resulted from its marginal location within the evolving North Sea rift and an extra‐basinal sediment supply from the Norwegian mainland.