Vertical variations in planar lamination of marine shale: Elucidating hydrodynamic changes during the Ordovician–Silurian transition on the Upper Yangtze Block

The hydrodynamics and their evolution on the Upper Yangtze Block during the Ordovician–Silurian transition period remain unclear. The present study is an assessment of how regional and global events may have influenced the hydrodynamic evolution based on a planar lamination investigation of the shales from the Upper Yangtze Block. Analyses of large thin sections and argon-ion polished thin sections using field emission-scanning electron microscopy (FE-SEM) showed that there are four types of planar lamination, namely, silty graded planar lamination (SGPL), silt–clay graded planar lamination (SCGPL), silt–clay interlaminated planar lamination (SCIPL), and paper-like planar lamination (PPL). SGPL is formed by turbidity current with a flow speed less than 15 cm/s. SCGPL is formed by turbidity currents with a flow speed less than 15 cm/s for normal grading type and 15–25 cm/s for alternating grading type. SCIPL has a continuum of sparsely spaced type, closely spaced type, and alternating type, which is formed by bottom current with an increasing flow speed from 15 to 25 cm/s to above 25 cm/s. PPL can be divided into normal grading and composite grading types. The former is formed by vertical settling, while the latter is formed by bottom current with a flow speed of 5–15 cm/s. Vertically, types of planar lamination varied from SGPL to PPL and then SCIPL manifesting the waxing and waning of flow speed with a positive excursion at graptolite biozone Metabolograptus extraordinarius (WF4) and a negative excursion at graptolite biozone Persculptograptus persculptus (LM1). The sudden decrease in flow speed across Linxiang and graptolite biozone Paraorthograptus pacificus (WF3) and the subsequent progressive increase from graptolite biozone Akidograptus ascensus (LM2) to graptolite biozone Demirastrites triangulatus (LM6) and to graptolite biozone Stimulograptus sedgwickii (LM8) during deposition of the Ordovician–Silurian transition succession on the Upper Yangtze Block were linked to the bulge uplift, the rapid subsidence, and the relaxation controlled by the Kwangsian orogeny. In contrast, the positive excursion at WF4 and the negative excursion at LM1 were strongly controlled by the Hirnantian Glaciation and global warming, respectively.