FACIES MODELING OF STROMATOLITE HORIZONS IN THE BIWABIK IRON FORMATION OF MINNESOTA

Stromatolites provide one of the records of the earliest life on earth, dating back to 3.5 Ga. The exploration for new forms of life on Earth and other planets has re-energized the study of stromatolites. However, there have been questions to whether stromatolites are formed through biogenetic processes or form abiogenetically.

This research uses facies analysis to assess the biogenicity of stromatolites in the Paleoproterozoic Biwabik Iron Formation of Minnesota. Twelve proprietary cores were logged at the centimeter scale and petrographic thin-sections were created of key lithofacies. The core analysis yielded eleven separate and distinguishable lithologic units: septarian nodules composed of hematite with syneresis cracks ranging in size from 0.5 – 4.2 cm long; peloid wackestone of microcrystalline hematite peloids; flat-pebble conglomerate composed of chert and 0.5-3 cm diameter magnetite clasts; stromatolites of multiple morphologies; silt/mudstone with sub-millimeter parallel beds; 1-3 cm diameter oncoids composed of a lutitic nucleus surrounded by irregular laminae; ooid grainstone composed of largely superficial and well-sorted ooids with nulceii composed of hematitic red chert and calcite; wavy bedded taconite; sandstone and siltstone with abundant magnetite grains with both cross and parallel-laminations; matrix supported conglomerate of sub-rounded to rounded clasts of hematite, magnetite, quartz, calcite, 1 mm to 4 cm in diameter; auto brecciated fabric composed of calcite, quartz, red chert, and magnetite; and sandstone with rounded to sub-rounded, medium to coarse, moderately to well sorted grains. Lithofacies were correlated in Petrel (Schumberger) software to create 3D models of possible depositional environments. Lithofacies are both laterally continuous and pinch-out. The lithofacies are consistent with shallow water environments conducive to microbial existence and the eventual creation of stromatolites.