DEFORMATIONAL STATE OF A PALEOPROTEROZOIC SEAFLOOR EXHALATIVE-COMPLEX EXPOSED AT PACTOLA DAM, SOUTH DAKOTA BLACK HILLS

A lower (biotite)-grade metamorphic sequence of pelitic and basaltic beds crop out for 400m adjacent to the Pactola Dam spillway in the east-central Black Hills. The exposure is interpreted as products of a volcanic vent, characteristic of seafloor rifting. In addition to a more-massive and porphyritic (now porphyroblastic) basalt flow, the original lithologies include tuffaceous greywacke, sulfidic muds and carbonaceous sulfidic mud interbedded with chert. Correlation with gabbroic sills of similar tectonic affinity in the region suggests ages ranging between about 1900 Ma to 2500 Ma. The Pactola exposure provides a 3-D perspective on dominant large scale(10s of meters)to small scale (several centimeter) folds. These are steeply plunging and typically classed as regional F₃, which warp foliation and bedding of N to NW strike. Local evidence alone does not suggest the character of third-phase folding, although there is some indication of refolded earlier tectonic features. The more impressive structures observed include shear zones with northerly dips and thrust-like top to the east and southeast motion. Associated with the pervasive shearing are discreet to diffuse sections of phyllonitic rock, twisted F₃ fold axes and lineations, complex quartz-carbonate veins, non-penetrative crenulations with sub-horizontal axes, and static-porphyroblast development at high angles to earlier foliation. This later metamorphism produced euhedral biotite, chlorite, actinolite and pyrite. Similar observations in the eastern Black Hills may be attributed to a broad regional “hydrothermal” event, possibly associated with the 1760-1715Ma evolution of Harney Peak granitic magma.