Diagenetic origin of quartz silt in mudstones and implications for silica cycling

Abstract

Mudstone--the most abundant sedimentary rock type, composed primarily of clay- or silt-sized particles--contains most of the quartz found in sedimentary rocks. These quartz grains, which are chemically and mechanically resistant and therefore preserve their characteristics well, have long been considered to be derived from the continental crust. Here we analyse quartz silt from black shales in the eastern USA, dating back to the Late Devonian period (about 370 million years ago), using backscattered electron and cathodoluminescence imaging and measure oxygen isotopes 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 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 about 120 Myr ago and molecular evidence for a much earlier appearance between 266 or even 500 Myr ago. Moreover, if many other mudstone successions show similarly high proportions of in situ precipitated--rather than detrital--quartz silt, the sedimentary record in mudstones may have been misinterpreted in the past, with consequences for our estimates of palaeoproductivity as well as our perceptions of the dynamics and magnitude of global biogeochemical cycling of silica.

Sedimentary and residual gas geochemical characteristics of the Lower Cambrian organic-rich shales in Southeastern Chongqing, China

“…The quartz content of marine sediments has been used to determine the distance to the shoreline (Blatt and Totten, 1981). Schieber et al (2000) analyzed quartz silt from Late Devonian black shales in the New Albany Basin, eastern USA, using backscattered electron and cathodoluminescence imaging (BSE and SEM-CL). Schieber proposed that the majority of the quartz silt in the Late Devonian black shales did not originate from the continental crust.…”

Geochemical and lithological controls on a potential shale reservoir: Carboniferous Holywell Shale, Wales

“…Scanning electron micrographs indicate the presence of laminar organic matter and also abundant silt-grade detrital quartz ( Figure 12D). Whilst it is difficult to differentiate detrital and diagenetic quartz without cathodoluminescence data (Milliken et al, 2012;Schieber, 2000), textural evidence suggests that some quartz grains may be recrystallized biogenic silica ( Figure 12E). Lower Holywell Shale samples from River Terrig contain 0.6 -3.4 wt % TOC (Table 1).…”

A Diagenetic Study of the Wolfcamp Shale, Midland Basin, West Texas

“…Schieber et al, 2000). An early quartz event formed overgrowths around detrital quartz grains and chert filled pores in the mudstones.…”