GRAIN SIZE DISTRIBUTIONS AND MINERALOGY OF COATED GRAINS, DETRITAL SAND AND SILT WITHIN THE ∼1.9 GA MESABI IRON RANGE, MINNESOTA

Due to their biological significance and preservation potential, Precambrian chemical sedimentary rocks such as iron formations (IF) represent unique information archives used to decipher questions relating to early life and it’s habitat. The ~1.9 Ga Mesabi Iron Range, Minnesota constitutes a well preserved (sub-greenschist facies) IF interpreted as a shallow marine shelf depositional sequence based on the presence of high-energy sedimentary structures and transported chemical sedimentary grains. Specific water depths and energy requirements of grain formation and transport are critically missing from current genetic models for granular sediment within IFs. Here we present preliminary 2D physical size and shape data for individual transported sedimentary grains (of chemical and detrital origin) to inform current depositional models. We measured grain sizes (reported as median = D₅₀), perimeter, area, roundness (R), and sphericity (S) for 868 detrital grains and 659 transported chemical sedimentary grains in 3 distinct, texturally coherent depositional layers in genetically linked samples (MIR-15-00, MIR-15-01) using Fiji software. Sample MIR-15-00 contains transported fine-sand-sized (n = 7; D₅₀ = 2.75 φ), moderately poorly sorted (0.85 φ range), equant (S = 0.72) chemical sedimentary grains of greenalite composition deposited with very poorly sorted (2.27 φ range), very equant (S = 0.79) silt-sized non-coated detrital quartz (n = 152; D₅₀ = 4.42 φ). Within sample MIR-15-01, grains are ferruginous, tangential concentric ooids where organic coatings encase single or multiple detrital quartz crystals that serve as nuclei. Individual internal detrital quartz grains are silt sized (n = 709; D₅₀ = 4.13 φ), extremely poorly sorted (4.36 φ range), and on average very equant (S = 0.79). External measurements of coated grains are very fine sand-sized (n = 618; D₅₀ = 3.11 φ), very poorly sorted (3.15 range), and very equant (S = 0.82). Results indicate that chemical sedimentary grains of iron-silicate composition are larger than associated non-coated detrital quartz grains, and closer in size to coated detrital grains, implying potential density differences at the time of deposition. Future work will focus on comparison to computational results based on recent experimental work on granular sediment.