STRAIN ANALYSIS OF CONGLOMERATIC IRON-FORMATION WITHIN THE NEMO SHEAR ZONE, BLACK HILLS SD

The Black Hills of South Dakota expose rocks deformed during the suturing of the Wyoming and Superior Provinces. The final continent-continent suturing, local D₃, is interpreted from NW-striking shear zones and related folding. These structures deformed supracrustal and basement rocks during left-lateral strike-slip, NE-side up transpression. The Nemo shear zone is a similarly oriented, km-wide structure in the NE Black Hills. Two miles southeast of Nemo village it parallels the Estes unconformity deforming both Nemo group rocks below and the overlying Estes Conglomerate. This project describes shear sense and quantifies strain preserved in the Benchmark Iron-formation, the uppermost unit of the Nemo group, and compares it to other D₃ shear structures.

This research mapped three NW-trending lenses of iron formation within the shear zone adjacent and below the Estes unconformity. The iron-formation contains ~80% Fe-oxide matrix, dominantly specularite, separating discontinuous cm-scale quartzite lenses elongate parallel to the NW-striking, near vertical shear fabric, giving the unit a banded appearance. Locally, areas in the iron formation appear massive with angular and nearly equi-dimensional quartzite clasts. These areas are interpreted to represent the unsheared protolith, and the quartz lenses in the “banded” component are interpreted to be sheared quartzite clasts. This interpretation allows for a quantitative strain analysis using the sheared quartzite clasts as strain markers.

Assuming no area loss, quartzite lenses viewed on horizontal exposures record stretch (S) of 0.6 and 1.75 in their short and long axes, respectively. In vertical exposures, long axes average S≈3, however S ranges from 1.75 to >4. Short axes in this orientation have S≈0.3. These data support significant flattening; however, stretch is greater in the vertical than horizontal. Asymmetry of sigma shaped quartzite lenses and cm-scale folding of lenses in both horizontal and vertical exposures supports NE-side up and left-lateral strike-slip shear movement. This shear displacement combined with the strong flattening component support the interpretation that the Nemo shear zone is associated with other NW-striking D₃ structures formed under transpression during the final suturing of the Wyoming and Superior provinces.