Well exposed, glacially polished iron formation outcrops near the town of Mellen, Wisconsin. There, units form a portion o the western Gogebic iron range which strikes east-west through the area. Iron formation exposure of this calibre is unusual in the Lake-Superior region and provides an opportunity to investigate physical and chemical sedimentary controls on accumulation of this type of sediment.
A 40 x 40 m section of iron formation composed of trough cross-stratified chert grainstones and parallel laminated slaty iron formation was mapped in detail. The sequence is composed of three distinct stratigraphic units. The first comprises 12 m of parallel laminated slaty iron formation with small chert lenses appearing at the bottom of the section. The second is a 15 m thick succession of large trough cross-stratified grainstone lenses, some extending laterally 10-15 m. The third unit lies immediately above the grainstone succession and is composed of 8 m of slaty iron formation. Paleocurrent data is dominantly unidirectional from the north.
Slaty iron formation is dominated by parallel laminated beds 2-5 mm in thickness. In regions where slaty iron formation coexists with small grainstone lenses the characteristic parallel lamination gives rise to wavy bedding. Ripple lamination is present in the lowest stratigraphic unit. Many beds are both physically and chemically graded. Variations in the distribution of Al, Mn, Fe and Mg are seen within individual beds and throughout the entire span of the section. Mineralogically, the dominant phases are hematite, magnetite and chert with minor amounts of greenalite, apatite and manganese oxide. A few of the largest magnetite grains possess cores composed of ilmenite.
The trough cross-stratified chert grainstone lenses are composed of rounded chert and hematite grains ranging in size from 0.1 to 2 mm. Many chert grains are coated with hematite, suggesting rolling by bottom currents through iron-rich muds. The matrix is dominantly chert & calcite with traces of stilpnomelane. Lenses range from 2 to 40 cm thick and may extend laterally 0.70 to 15 m. The smallest grainstone lenses appear at the base of the section within slaty iron formation. The largest lenses are found in the succession of trough cross-stratified grainstones. Rip-ups of slaty iron formation, 1-3 cm in length, are common at the base of the largest lenses (30 cm thick, 8-11 m wide) at the top of the succession.
The section is characterized by a coarsening then fining upwards sequence in which muddy shelf deposits pass through ripple laminated fine sand into a succession of cross-stratified dunes, then back into muddy shelf deposits. Both offshore bar complexes and tidally influenced, storm enhanced shelf deposits possess sedimentary structures similar to those described. The thin ripple lamination in slaty iron formation in the lowermost slaty unit may represent sands moved by storm produced currents. The dominantly unidirectional paleocurrent pattern is also indicative of storm enhanced currents. Slumping is suggested throughout the sequence by the presence of extensive convolute bedded slaty iron formation.