Final answer:
The purity of quartz in the St. Peter Sandstone is due to prolonged geological processes like physical sorting and chemical weathering that occur in high-energy environments, which selectively remove less durable minerals over time.
Step-by-step explanation:
The process responsible for the purity of quartz in the St. Peter Sandstone from the Tippecanoe Sequence involves continued weathering and redeposition of sand-sized mineral grains. Initially, as part of the early Earth's igneous rocks, these grains underwent chemical transformation and were transported by water or wind. Over geological time scales, repeated cycles of erosion and sedimentation filtered out less durable minerals, leaving behind a high concentration of resilient quartz grains. The St. Peter Sandstone being nearly pure quartz indicates that it has undergone extensive natural sorting and winnowing processes. These processes usually take place in high-energy environments like beaches or river channels, where the force of water or wind can separate lighter minerals and rock fragments from the denser quartz grains.Sequence stratigraphic analysis suggests that the St. Peter Sandstone was formed during a transgressive-regression cycle. Transgression phases typically lead to the deposition of finer sediments, while regression might see the reworking of these deposits, further purifying the sandstone. Alongside physical sorting, chemical weathering contributes to the breakdown of less stable minerals. The resulting quartz sandstone, with its robustness and resistance to dissolution, represents a geologically sorted and mature sedimentary rock.In conclusion, the high quartz purity in the St. Peter Sandstone is a result of prolonged geological processes, including physical sorting and chemical weathering, that occurred during early Paleozoic transgressive-regression cycles.