Microbial Mat-Related Structures: Laboratory Experiments and Observations from the Mesoproterozoic Belt Supergroup, Southwestern Montana, U.S.A.

• Author / Creator

  Kovalchuk, Olga

• Siliciclastic sediments become biostabilized when benthic bacteria penetrate into the pore spaces and secure grains with filaments and extracellular polymeric substances, forming a microbial mat. The cohesive microbial cover changes typical sedimentary dynamics and leads to formation of diverse microbial mat-related structures. While modern communities are mostly restricted to stressed environments, in the Precambrian through to Ordovician time microbial mats dominated aquatic environments, which is reflected by the abundance of mat-related structures found in strata of this age. Modern day structures are well documented and provide analogs for fossil mat-related structures, but some missing links still exist. Crack-like features are the most common, but also the most puzzling structures. While some are interpreted as shrinkage cracks that form due to desiccation, synaeresis or intrastratal degradation and dewatering of the biomat, others are interpreted as loading structures or remnants of buckled mats. To better understand the influence of microbial mats on desiccation, experiments were carried out using clay-rich and clay-poor substrata. It was observed that the water-rich microbial mat shrinks substantially with desiccation, but its heterogeneous distribution results in an irregular crack network, made up of composite radiating cracks with coiled margins. The shallow penetration of bacteria into the substratum, especially the clay-rich sediment, limits its influence on cracking. The microbial mat modified the orthogonal clay-induced cracks by withstanding crack propagation and by producing curled-up crack polygon margins. The strata of the upper Missoula Group and Spokane Formation (Ravalli Group) of the Mesoproterozoic Belt Supergroup in southwestern Montana were observed to contain well preserved and reoccurring mat-related structures. Various cracks, were examined in detail, including thin section and scanning electron microscopy analyses. Some cracks intersect in 120o or 90o junctions, providing evidence for shrinkage, while others show soft sediment deformation features and deformed laminae within the crack fill that are the most consistent with an active infill. Other structures were interpreted as kinneyia wrinkles, nodular surfaces and remnants of gas domes. Thin veneers made up of anastomosing argillaceous, carbonaceous and iron-rich fibers were observed to be associated with mat-related structures and interpreted as biolaminae that were formed by the trapping and binding action of ancient bacteria. In addition to mat-related structures, four instance of bilobate structures were found on two bedding surfaces of strata that are currently mapped as the upper Missoula Group. Such structures are interpreted as probable metazoan traces, such as the Aulichnites-like trace made by an early bilaterian mollusc-like animal.

• Subjects / Keywords

  • Microbial mats
  • Microbial mat related structures
  • Diastasis cracks
  • Siliciclastic sedimentology

• Graduation date

  Spring 2017

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/R38P5VN5D

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.