VARIATIONS IN LACUSTRINE ENVIRONMENTS PRESERVED IN STROMATOLITES

Despite the commonly-held wisdom perpetuated in most textbooks, stromatolites are not in-and-of-themselves microbial mats. Rather, they are layered sedimentary deposits produced via microbial activity in interaction with the environment (if the biologic definition is employed) and subsequently modified by diagenesis, where actual mat components may not be preserved at all in the resultant structure. Even in the best of circumstances, the resultant structure is not a fossilized layered mat but a record of the localized amalgam of microbial-environment (LME) interactions. Lacustrine stromatolites may have the potential to record greater variability in LME interactions relative to the marine environment because lakes are much more susceptible to environmental forcing (climate, chemical changes, etc.) versus typical tropical marine carbonate platforms. For example, lacustrine stromatolites are commonly strongly banded versus marine forms, likely representing the stronger LME interactions present in the lacustrine systems. Thus, when examining lacustrine stromatolites as “evolutionary mileposts” or “environmental dipsticks,” the pendulum would likely swing toward dipsticks in lacustrine systems.

In this study, the microstucture of cyclic stromatolite beds (10s of centimeters thick in cycles of several meters) in the LaClede Bed of the Green River Formation (Eocene, Wyoming, USA) was studied using outcrop and cores near Fort LaClede. The microstructure varies considerably throughout individual stromatolite domes and between successive beds. Microstructures are dominated by micritic undulose laminae of medium inheritance and micro-bladed cement layers normal to the underlying surface but other structures occur both gradationally and alternating with the dominant forms.

It is inferred that the variations in stromatolites as lacustrine ‘environmental dipsticks’ reflect changes driven by lake level fluctuation and consequent evaporation-precipitation cycles. Non-stromatolitic, massive to brecciated limestone layers also occur in lieu of stromatolite in some cycles, reflecting the importance of carbonate precipitation regardless of stromatolite formation.