Thrombolitic stromatolites of the Cambrian Gallatin Formation: significance of well-preserved doppelgangers

Abstract

Microbialites constitute the some of the earliest records of life on Earth, but diagenetic processes often blur or erase a clear signal of the depositional accretionary processes that control their morphological intricacies. This study examines the role of sedimentation, metazoan skeletal clasts, and depositional environment on the accretion of well-preserved stromatolitic-thrombolitic microbialites from the Upper Cambrian Gallatin Formation of western Wyoming collected from the eastern flank of Rendezvous mountain. Microbialites exhibit dome shaped decimeter-scale columns that occur as part of shallowing upward parasequences deposited during the late Cambrian marine transgression recorded throughout the Cordilleran of Laurentia. Data derived from thin section point counts, clast size to depositional angle comparisons, and chromatic mesosequencing (CMS) suggests: i) coarse-grained agglutinated textures are not restricted to modern day microbialites, ii) ancient microbialites were not restricted to biologically exclusive environments. The presence of trace fossils, ooid clasts and complete fossil segments within micritic microfacies suggests metazoan bioturbation, proximal fossil origin, or in situ necrolysis. The presence of allochthonous clasts in precipitated microspar and micrite suggests an active integration of the grains by microbial mat communities. Evidence of biogenicity includes the incorporation of very fine to medium sand sized grains at high angles as well as the presence of Girvanella, and Renalcis in micrite and microsparitic laminae. The textural details provided herein suggest coarse-grained microbialites may be more common in the Paleozoic than previously thought and has important implications for using recent insights into modern coarse-grained microbialites to interpret the paleoecology and taphonomy of microbialites from the early Paleozoic.