Quartz crystallinity index: New quantitative evidence for biogenic silica of the Late Ordovician to Early Silurian organic-rich shale in the Sichuan Basin and adjacent areas, China

Abstract

Quartz crystallinity index (QCI) was used to reflect the crystallisation of silica in the Late Ordovician Wufeng (WF) and Early Silurian Longmaxi (LM) Formation shale, as well as the airborne volcanic ash-derived silica in the Lucaogou Formation tuffaceous shale, to distinguish the two types of silica. The silica in different graptolite biozones exhibited different crystallisation. The WF2–3, LM1–4 graptolite biozones showed obviously lower QCI values than the LM5–9 graptolite biozones and the Lucaogou Formation samples. The graptolite organisms played the role of adsorption, fixation, and precipitation in silicon accumulation and enrichment in stratum. The biogenic origin caused the poorest quartz crystallisation in WF2–3 and LM1–4 graptolite biozones samples. The airborne volcanic ash-derived silica in the Lucaogou Formation tuffaceous shale exhibited relatively poor quartz crystallisation because of weaker diagenesis intensity. Generally, although the WF2–3 and LM1–4 graptolite biozones underwent strong diagenesis and contained a small amount of detrital quartz, the silica still exhibited lower QCI values than the airborne volcanic ash-derived silica in the Lucaogou Formation tuffaceous shale. The biogenic silica crystallisation was much poorer than that of the airborne volcanic ash-derived silica. QCI is an effective quantitative index to demonstrate the biogenic silica in the organic-rich and silica-rich shale.