**6. Discussion**

### **6.1 Correlation with grain size characteristics of high-purity quartz sand**

The grain size and its distribution are two of the four basic characteristics of high purity quartz sand (chemical purity, grain size, grain size distribution and particle shape) [1] and an important factor which directly affects the equilibrium melting of high purity quartz sand and a material control index to reduce the defects of quartz glass [13]. Therefore, Shi Bin et al. established a set of grain size parameter evaluation system for high purity quartz sand [14]. The grain size parameters of typical highpurity quartz sand are as follows: average grain size (Mz) = 3.125Φ; Standard deviation (σ) = 0.274; Partial degrees (SK) = 0.000. The grain size parameters of samples (ZOZ) are as follows: average grain size (Mz) = 2.64Φ; Standard deviation (σ) = 0.33; Partial degrees (SK) = 0.36. The comparison results show that the grain size, grain size distribution and grain size curve of quartz sand in the Member 1 of Suotoushan Formation reach or approach the index of high quality and high purity quartz sandstone. Considering the common grain size characteristics of foreshore deposits, the quartz sandstone of the Member 1 of Suotoushan Formation should be the most ideal high purity quartz sandstone.

### **6.2 Relationship between sea level rise and high purity quartz sandstone**

The Qingmen Formation in the lower strata of Suotoushan Formation is dominated by gray-green, yellow-green and yellow-gray mudstone, intermixed with sandy mudstone, siltstone, argillaceous limestone and limestone. The fossils of normal shallow sea creatures are abundant, mainly brachiopods and corals. Trilobites, ammonites and bambusitic stones are found in the shallow sea shelf environment [7]. The Member 1 of Suotoushan Formation shoreline quartz sandstone is obviously lower than Qingmen Formation shallow sea shelf deposition, which records an event of sea level decline and shallow sea shelf uplift and exposure. Extensive shelf exposure can lead to quartz sand sorting, transformation and redeposition, which may be one of the reasons for the high structural and compositional maturity of quartz sandstone in the Member 1 of Suotoushan Formation. The Member 1 of Suotoushan Formation in the study area is gray medium fine quartz sandstone, with almost no argillaceous interlayer, indicating that the sedimentation of sedimentary basement and sedimentary filling were in a

long-term equilibrium state, and the sedimentary environment was always in the foreshore zone, which was conducive to the full sorting and enrichment of quartz sand. The Member 2 of Suotoushan Formation is mainly composed of thin bedded sandstone and mudstone, which is estimated to be littoral zone-inner shelf deposition, indicating that the sea level begins to rise and the weakening of hydrodynamic conditions is not conducive to quartz sand sorting and enrichment. Although the sea level of the Member 3 of Suotoushan Formation fell again during the sedimentary period, the sedimentary environment was not stable and the medium thickness bedded quartz sandstone was not developed, which was not conducive to the formation of high purity quartz sandstone deposits. Chemical analysis results (**Table 2**) also prove that the purity of quartz sandstone in the Member 1 of Suotoushan Formation is higher than that in the Member 3 of Suotoushan Formation.
