|mhorn||Дата: Monday, 19.07.2010, 22:57 | Сообщение # 1|
|Deenen M.H.L. (2010) A new chronology for the late Triassic to early Jurassic // Geologica Ultraiectina, Volume 323. 216 pp. |
Throughout geological history of life, a small number of mass extinctions have forever changed the path of evolution. The two main mechanisms that account for these dramatic events are asteroid impacts and massive volcanic eruptions of Large Igneous Provinces (LIP’s). The interplay between mass-extinctions, LIP emplacement, extra-terrestrial causes, and perturbations in the carbon cycle belongs to the most significant, fascinating and hotly debated issues within earth sciences. Massive flood basalts of the Central Atlantic Magmatic Province (CAMP) have been emplaced roughly contemporaneously with the end-Triassic mass-extinction (~201 Ma), one of the “Big Five” extinction events in Earth’s history. However, a causal relationship has been subject of debate. Here, we develop a high resolution time frame, using a combination of stratigraphic dating tools, to link the major events recognized in the marine realm to the reference time frame established in the continental realm for a time span of > 10 million years. We show that the initial volcanic phase in the northern CAMP province is marked by multiple volcanic pulses that all occurred within a very short (~100 kyr) time span just preceding the Triassic-Jurassic boundary and exactly coinciding with the end-Triassic mass extinction. Additionally, the combination of paleomagnetism and cyclostratigraphy provides important information on the geochemical signatures of the volcanic CAMP pulses. Our time control indicates that these signatures appear to be remarkably homogeneous for time-equivalent pulses over very large distances (>1000 km). We show that the geochemical trace-element ratios provide a new powerful stratigraphic correlation tool for the time period around CAMP emplacement. We furthermore use our time frame to improve the geological time scale for the late Triassic and early Jurassic. Correlation of events recognized in the marine realm to the continental reference frame where time control is derived from astronomically forced climate cycles provides us with durations of the various stages. We constrain the Norian to 15.5 Myr, the Rhaetian to ~9 Myr and the Hettangian stage to 1.8 Myr.
Middle Jurassic - Lower Cretaceous ammonites & aptychi