The Times of Sand: Applying Cosmogenic 21Ne to Examine the Brief
Exposure Times of Quartz Sands throughout Sedimentary Cycles
Abstract
The sedimentary cycle of quartz sand is comprised of the weathering of
source rock, followed by erosion, deposition, then burial, and in some
cases, lithification. Over geological timescales, quartz sands undergo
several sedimentary cycles, with sandstones retaining evidence of
previous cycles in various mineralogical and geochemical traces. These
sedimentary cycles can span across timescales of hundreds of millions of
years, with some sands holding evidence of the earliest rocks in Earth’s
history (e.g., Australian and Kalahari deserts). However, we have little
knowledge of the timescales that sands spend exposed at the Earth’s
surface and how these compare to the timescales of sedimentary cycles.
Unlike the more commonly used cosmogenic nuclides, cosmogenic
21Ne is stable and can be used to study rates of
surface processes in the deep geological past. We use cosmogenic
21Ne to quantify exposure times at the surface from
Lower Cretaceous and Miocene outcrops (Kurnub and Hazeva formations) and
one active fluvial system (Colorado River). Although not devoid of
drawbacks and limitations (i.e., diffusion out of the quartz crystal and
high levels of non-cosmogenic 21Ne is quartz), we are
able to constrain exposure times at the surface to
<106 years. Compared to the overall
timescales of sedimentary cycles, evaluated using detrital zircon
provenance data, exposure times at the surface are shorter by over an
order of magnitude. These results demonstrate how in the context of
sedimentary cycles, quartz sands spend the greater part of the time
buried in fluvial systems sedimentary units (e.g., fluvial terraces,
fans, and deltas) or at sedimentary basins. In comparison, the time that
these sands spend at the surface during weathering, erosion, and
deposition is strikingly brief.