Delineating silica sources in Cretaceous marine sedimentary systems

Identifying biogenic silica in mudstones provides critical insights into paleoproductivity, paleoenvironments, biogeochemistry, sequence stratigraphy, and geomechanical properties. Other silica sources, such as rivers, wind, ash, hydrothermal fluids, and diagenesis of clay minerals, complicate identification of biogenic silica. Traditionally, petrographic observations or geochemical proxies such as excess Si versus Al have been used to identify biogenic silica. More recent approaches use Si–Zr relationships, where a negative correlation suggests biogenic input and a positive correlation indicates detrital sources. However, more complex patterns can emerge, highlighting additional influences that need elucidation.

This study evaluates the Si–Zr proxy by investigating its variability across diverse depositional settings in the Cretaceous Western Interior Seaway (WIS). We analyzed 2751 samples from 15 drill cores and one outcrop spanning Aptian–Maastrichtian strata from Texas to Alaska. These samples include 56 bentonites to assess ash influence. Samples were selected using specific constraints: Cretaceous age, consistent analytical methods, low thermal alteration, and broad latitudinal, depositional environment, and lithofacies coverage. Elemental concentrations were measured by inductively coupled plasma optical emission spectroscopy and mass spectrometry (ICP-OES/MS). Total carbon, total organic carbon, and programmed temperature pyrolysis were also acquired to relate source rock properties to silica sources.

The results reveal that while the Si–Zr proxy effectively identifies biogenic silica in many cases, lithofacies, depositional environment, and diagenesis additionally influence Si and Zr relationships. Bentonites occupy a limited region of the Si–Zr concentration space, allowing differentiation of mixed ash from biogenic and detrital silica. We further show which other elements reliably separate biogenic and detrital quartz, and we use the Si–Zr framework to highlight lithofacies associations with critical minerals. By delineating regions of the Si–Zr crossplot, and validating interpretations with lithofacies assignments, this work refines the Si–Zr proxy and improves understanding of silica sources in sedimentary systems.