QUARTZ SILT IN THE SEDIMENTARY RECORD-A DIAGENETIC ORIGIN

Mudstones are the most common sedimentary rock types, and contain most of the quartz found in the sedimentary record. They are comprised mostly of silt and/or clay particles, are chemically and mechanically resistant, and have long been considered to have been derived from the continental crust. Quartz silt from black shales in the eastern USA, dating back to the Late Devonian period (about 370 million years ago), has been examined using backscattered and cathodoluminescent scanning electron microscopy; oxygen isotopes have been studied with an ion probe. Our results indicate that up to 100% of the quartz silt in our samples does not originate from the continental crust. Instead, it appears to have been precipitated early in diagenesis in algal cysts and other pore spaces, with silica derived from the dissolution of opaline skeletons of planktonic organisms, such as radiolaria and diatoms. Transformation of early diatoms into in situ quartz silt might explain the time gap between the earliest fossil occurrences of diatoms at about 120MYr ago, and molecular evidence for a much earlier appearance between 266 and 500 Myr ago. Moreover, if many other mudstone successions show similarly high proportions of in situ precipitated rather than detrital quartz, the sedimentary record in mudstones may have been misinterpreted in the past, with certain consequences for estimates of paleoproductivity as well as preceptions of the dynamics and magnitude of global and biogeochemical cycling of silica.