Abstract

Synthesis of the available stratigraphic data on the Baikal basin sediments exposed around the lake and their correlation with offshore lake sediments and with onshore sections in the Baikal Foredeep allows a new perspective of the Baikal rift history. The basin sediments on the Baikal shore comprise three tectonic–lithological–stratigraphic complexes (TLSC) which correspond to three seismic stratigraphic sequences (SSS) in the lake sediments and to three complexes in the Baikal Foredeep.

The oldest unit, TLSC-1, has a particular lithology being deposited in an environment which never repeated in the later history of the area. This proves the validity of its lithostratigraphic correlation with Masstraichtian–Early Oligocene sediments of the Baikal Foredeep constrained by biostratigraphy. Further support comes from isotope dating, paleontology, and other evidence.

Unlike seismostratigraphy-derived models, SSS-1 is correlated in the new model with TLSC-1 rather than with the Tankhoi Formation (which actually represents TLSC-2), and the onset of rifting is placed at the Late Cretaceous–Paleogene rather than at the Oligocene (or Miocene). Thus, the Baikal rifting began prior to the India–Eurasia collision, and the first rifting pulse originally had other causes. This inference agrees with fission-track apatite thermochronology indicating Cretaceous ages of samples from the Barguzin rift basin in the northeastern flank of the rift system.

Rifting developed successively in three different tectonic settings and was driven by different geodynamic mechanisms at each stage. First it was a passive response to Late Cretaceous–Eocene distributed continent-wide extension in Asia (purely passive rifting). At the second stage spanning Late Oligocene–Early/Late Pliocene time, the area was subject to compressive impact from the India–Eurasia collision, which propagated from the southwest since the Eocene and reached the region about 30 Ma ago to take control over its geodynamics (conventionally passive “impactogenic” rifting). Finally, the Pliocene–Quaternary evolution has been driven by extension from a local source associated with hot mantle material rising to the base of the rifted crust (active rifting).

The major rifting stages are further subdivided into substages: two substages in the second stage with the boundary at ~ 10 Ma (Middle–Late Miocene) and three substages in the third stage, with boundaries at 1.0–0.8 and 0.15–0.12 Ma. The stages and substages of rifting are separated by events of tectonic activity and stress reversal when additional compression produced folds and shear structures. The events that mark the stage boundaries show up as gaps, unconformities, and deformation features in the deposition patterns.

Thus, the three units of synrift sediments composed of eluvium, early molasse, and late molasse, respectively, were deposited during preorogenic, early orogenic, and postorogenic stages of rifting driven by passive (first purely passive and then “impactogenic”) and active mechanisms.

The new model of the Baikal rift history agrees with data obtained by different other methods.

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