The ore-yielding main iron-formation of the Cuyuna district is strati-graphically overlain by approximately 300 feet of titaniferous sedimentary rocks, and a field test for titahia has been an effective supplement to customary stratigraphic studies. The titanium is in leucoxene.
The sedimentary rocks of the district are highly folded, and the main iron-formation serves as the reference for relative stratigraphic position. The strata older than the main iron-formation are light-gray argillite and siltstone with local lenses of quartzite near the upper contact; the older strata contain about 0.5 percent titania. The main iron-formation is a persistent layer of chemically precipitated ferruginous sediments that contain about 0.2 percent titania. The strata younger than the main iron-formation are gray or black argillite, partly carbonaceous, partly ferruginous, and are transitional into lenses of lean upper iron-formation. At the base of the younger strata, 250-350 feet of argillite and slate contain 1-3.7 percent titania and average 2.0 percent. This titaniferous argillite is similar to argillite higher in the younger sequence which contains less than 1.5 percent titania and averages about 1 percent. There are weathered rocks interpreted as tuffs and basalt flows locally interlayered near the bottom of the titaniferous zone. The basalt contains 1-2 percent titania. The high titania content of the 250-350-foot zone suggests a volcanic origin; the argillite probably is reworked basaltic pyroclastics. A few titaniferous bodies have been found in rocks below, within, and above the main iron-formation and thin titaniferous argillaceous layers occur within the main iron-formation.
