Biostratigraphy of a Paleocene–Eocene Foreland Basin boundary in southern Tibet

AbstractThis study of the Paleocene–Eocene boundary within a foreland basin of southern Tibet, which was dominated by a carbonate ramp depositional environment, documents more complex environmental conditions than can be derived from studies of the deep oceanic environment. Extinction rates for larger foraminiferal species in the Zongpu-1 Section apply to up to 46% of the larger foraminiferal taxa. The extinction rate in southern Tibet is similar to rates elsewhere in the world, but it shows that the Paleocene fauna disappeared stepwise through the Late Paleocene, with Eocene taxa appearing abruptly above the boundary. A foraminifera turnover was identified between Members 3 and 4 of the Zongpu Formation—from the Miscellanea–Daviesina assemblage to an Orbitolites–Alveolina assemblage. The Paleocene and Eocene boundary is between the SBZ 4 and SBZ 5, where it is marked by the extinction of Miscellanea miscella and the first appearance of Alveolina ellipsodalis and a large number of Orbitolites. Chemostratigraphically, the δ13C values from both the Zongpu-1 and Zongpu-2 Sections show three negative excursions in the transitional strata, one in Late Paleocene, one at the boundary, and one in the early Eocene. The second negative excursion of δ13C, which is located at the P–E boundary, coincides with larger foraminifera overturn. These faunal changes and the observed δ13C negative excursions provide new evidence on environmental changes across the Paleocene–Eocene boundary in Tibet

Biostratigraphy of a Paleocene–Eocene Foreland Basin boundary in southern Tibet

This study of the Paleocene–Eocene boundary within a foreland basin of southern Tibet, which was dominated by a carbonate ramp depositional environment, documents more complex environmental conditions than can be derived from studies of the deep oceanic environment. Extinction rates for larger foraminiferal species in the Zongpu-1 Section apply to up to 46% of the larger foraminiferal taxa. The extinction rate in southern Tibet is similar to rates elsewhere in the world, but it shows that the Paleocene fauna disappeared stepwise through the Late Paleocene, with Eocene taxa appearing abruptly above the boundary. A foraminifera turnover was identified between Members 3 and 4 of the Zongpu Formation—from the Miscellanea–Daviesina assemblage to an Orbitolites–Alveolina assemblage. The Paleocene and Eocene boundary is between the SBZ 4 and SBZ 5, where it is marked by the extinction of Miscellanea miscella and the first appearance of Alveolina ellipsodalis and a large number of Orbitolites. Chemostratigraphically, the δ13C values from both the Zongpu-1 and Zongpu-2 Sections show three negative excursions in the transitional strata, one in Late Paleocene, one at the boundary, and one in the early Eocene. The second negative excursion of δ13C, which is located at the P–E boundary, coincides with larger foraminifera overturn. These faunal changes and the observed δ13C negative excursions provide new evidence on environmental changes across the Paleocene–Eocene boundary in Tibet

Progressive Indosinian N-S deformation of the Jiaochang structure in the Songpan-Ganzi fold-belt, Western China.

Integrated field data, microstructural and three-dimensional strain analyses are used to document coaxial N-S shortening and southward increase in deformation intensity and metamorphism at the Jiaochang structure. Two episodes of deformation (D1,D2) with localized post-D2 deformation have been identified in the area. The first deformation (D1) episode is defined by a main axial-plane of parallel folds observable on a micro- to kilometer-scale, while the second episode of deformation (D2) is defined by micro-scale metamorphic folds, associated with E-W oriented stretching lineation. These processes are the result of Indosinian tectonism (Late Triassic to Early Jurassic) characterized by nearly coaxial N-S compression and deformation. This is indicated by E-W trending, sub-parallel to parallel foliation (S1, e.g. axial-plane of folds, and S2, i.e. axial-plane of metamorphic folds, crenulation cleavage) and lineation (L1, e.g. axis of folds, and L2, i.e. stretching lineation, axis of metamorphic folds and B-axis of echelon lens). Most of the porphyroblasts and minerals (e.g. pyrite, biotite) show two growth phases with localized growth in the third phase (muscovite). The progressive D1-D2 structure is widespread in the south of the Jiaochang area, but only D1 structure crops out at the north. The strain intensity (γ), compression ratios (c%) and octahedral strain intensity (εs) are similar across the Jiaochang structure (i.e., γ ≈ 1.8, c ≈ 27%, εs = 0.9), showing a broad range of Flinn values (K = 0.77 to 7.57). The long-axis orientations are roughly symmetric between two limbs of the structure. Therefore, we suggest that the architecture of the Jiaochang structure has been controlled by coaxial N-S shortening and deformation (D1-D2) during the Indosinian tectonic epoch, with insignificant post-D2 deformation

Geochemistry of intercalated red and gray pelagic shales from the Mazak Formation of Cenomanian age in Czech Republic

Pelagic red and gray shales are intercalated within the lower part of the Mazak Formation of Middle Cenomanian age in Czech Republic. A detailed geochemical study of major, trace and rare earth elements and carbon isotopic compositions of organic carbon has been conducted on sixteen red and gray shales. The data suggest that the shales were most likely accumulated in well-oxygenated bottom waters with very limited organic matter supply and consisted of marine organic matter mixed with minor amounts of terrestrial organic matter. The shales were deposited below CCD in one of the tectonic troughs developed along northern margin of the western Tethys. Similar geochemical covariances of major, trace and rare earth elements for the shales suggest palaeoceanographic conditions and provenance during their deposition. The most probable cause for the variation of redox bottom conditions in the mid-Cretaceous deep ocean was periodic changes in the concentration of dissolved oxygen in bottom waters, due to changes in deep water circulation and processes driven climate changes

The Early Cambrian Mianyang-Changning Intracratonic Sag and Its Control on Petroleum Accumulation in the Sichuan Basin, China

The older and deeper hydrocarbon accumulations receive increasing attention across the world, providing more technical and commercial challenges to hydrocarbon exploration. We present a study of an asymmetrical, N-S striking intracratonic sag which developed across the Sichuan basin, south China, from Late Ediacaran to Early Cambrian times. The Mianyang-Changning intracratonic sag is ~50 km wide, with its steepest part in the basin center. In particular the eastern margin shows its greatest steepness. Five episodes in the evolutions of the sag can be recognized. It begins in the Late Ediacaran with an uplift and erosion correlated to Tongwan movement. Initial extension occurred during the Early Cambrian Maidiping period, when more strata of the Maidiping Formation were deposited across the sag. Subsequently, maximum extension occurred during the Early Cambrian Qiongzhusi period that resulted in 450–1700 m thick Maidiping-Canglangpu Formations being deposited in the sag. Then, the sag disappeared at the Longwangmiao period, as it was infilled by the sediments. The intracratonic sag has significant influence on the development of high-quality reservoirs in the Dengying and Longwangmiao Formations and source-rock of the Niutitang Formation. It thus indicates that a high probability for oil/gas accumulation exists along the intracratonic sag, across the central Sichuan basin

Geologic and structure map of the Jiaochang area in the Songpan-Ganzi fold-belt.

<p>All stereonets are lower-hemisphere equal area projection, showing the S1 foliation. The solid circles and triangles in the stereonets stand for F1 hinge and L1 intersection lineation, respectively.</p

Schematic representation of pre-, inter-, syn, and post-tectonic prophyroblast and mineral growth, in which the D₁,D₂ and D₃ correspond to the multiphase deformations of foliation.

<p>Pyrite is divided into pre- to syn-D₁ and pre- to syn-D₂. Biotite is divided into syn- to post D₁, and syn-D₂. Chlorite and garnet are pre- to post-D₂, and pre- to syn-D₂, respectively. Furthermore, muscovite is divided into three growth phases, syn-D₁, and syn-D₂ and syn-D₃.</p

Three-dimensional strain data in the Jiaochang structure.

<p><i>Notes</i>: e₁,e₂, e₃ represent the relative magnitude of principal strains; K-Flinn value; ε_s –magnitude of natural octahedral strain; γ –strain intensity; c–compression ratio; e₁/e₃-axial ratio of three-dimensional strain ellipsoid; Bedding-sample’s bedding orientation; e₁′, e₂′, e₃′ represent the orientations of principal axis in geographic reference frame; è is the intersection angle between the orientation of the principal axis and the sample bedding.</p

Diagram of strain pattern and mechanism of formation of orogen curvature.

<p>a) Oroclinal bending, strain ellipse remains the same but varies in orientation without undergoing tangential extension (after Ries and Shackleton <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0076732#pone.0076732-Ries1" target="_blank">[26]</a>); b) Simple shear along boundary, strike-slip shearing along one limb of orogen curvature causes stretch and deformation increase toward the boundary fault (after Marshak <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0076732#pone.0076732-Marshak1" target="_blank">[7]</a>); c) Differential transport, oroclinal bending and tangential extension resulted from differential transport to the foreland. It could accommodate axis rotation and strain decrease toward the foreland (after Marshak <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0076732#pone.0076732-Marshak1" target="_blank">[7]</a>); d) Finite strain in Jiaochang, showing tangential extension and strain increase toward the apex.</p