LAMINATION-SCALE GEOCHEMISTRY OF MICROBIALITES: INVESTIGATING THE COMBINED EFFECTS OF PRIMARY MINERALOGY AND DIAGENESIS IN MICROBIALITES FROM THE UPPER CAMBRIAN AND LOWER TRIASSIC OF THE WESTERN US

Microbialites are important for understanding the history of Earth’s changing life and environments. Although macroscopic features of stromatolites have received extensive study, considerably less research has focused on lamination-sale differences in elemental and isotopic composition with the goal of understanding the role of diagenesis in the preservation of geochemical signals. Here we report preliminary results from three microbialite samples—one from the Lower Triassic and two from the Upper Cambrian—that reveal lamination-scale differences in diagenetic alteration resulting from primary lithologic features.

The Early Triassic microbialite was collected from the Virgin Limestone Member of the Moenkopi Formation, Nevada, USA. These limestone stromatolites are characterized by alternating dark and light brown laminations. The light brown layers have lower Sr/Mn ratios (~2), lower δ¹³C compositions (average -1.5 ‰ VPDB), elevated abundances of quartz, and increased secondary porosity. In contrast, the darker layers show heavier δ¹³C compositions (average +0.4 ‰ VPDB), higher Sr/Mn ratios (~7), an absence of quartz, and lower porosity. Differences in depositional lithology likely led to increased diagenetic alteration in the light brown laminations compared to the dark brown laminations.

Two Late Cambrian microbialite samples were collected from the Hellnmaria Member of the Notch Peak Formation, Utah, USA. These dolomitic samples exhibit poor stromatolitic lamination consisting of alternating light brown and dark black layers. In both samples, δ¹³C compositions are uniform between layers and fall between 0.3‰ and 0.9‰ VPDB. Sr/Mn ratios are also uniform between the layers (~0.5). It is possible that laminations were lithologically similar to begin with such that dolomitization altered both the light brown and dark black laminations to a similar extent.

Although preliminary, our results highlight the impact of primary lithology on diagenetic alteration of microbialites. Similarly, they highlight the importance of the impact of diagenesis on paleoenvironmental proxies in microbialites. Further study of lamination-scale geochemistry will allow for a better understanding of paleoenvironmental conditions of microbialite formation and preservation.