The Geologist Who wrote our physical geology book, Stephen Marshak

Hello all,

This morning I went to a series of talks on Perspectives on Orogenic Evolution, Dating Brittle Faults and Mylonitic Shear Zones, Bending Mountains, and Assembling Supercontinents I: A Session to Honor the Career of Ben Van Der Pluijm where Stephen Marshak gave a talk on cleavage and kinks of the baraboo syncline.   A great talk from a geologist who started my career in the fall of 2014 as geologist with the book essentials of geology as a guide to Dr DeAngelis's physical geology class.  There's a lot of information on this particular talk so I hope you enjoy.

The Baraboo Syncline of Wisconsin is a doubly-plunging, south-vergent, regional-scale fold with a slightly overturned north limb and a gently north-dipping south limb. This Proterozoic-age structure involves Baraboo Quartzite, consisting of greenschist-grade, ultramature quartzite interlayered with phyllite. Mesoscopic structures developed during regional folding include F1parasitic folds related to regional flexural slip, and S1 cleavage whose intensity and orientation correlates with the concentration of phylllosilicates. In phyllitic layers, S1 is roughly axial planar to F1. At several localities, especially in thicker phyllitic layers of the south limb, S1 cleavage is deformed by S2asymmetric crenulation cleavage and by F2 kink bands. F2 kinks become folds with rounded hinges where thin quartzite layers are involved. S2cleavage dips in a direction opposite to S1 (i.e., is antithetic), and F2 folds verge in a direction opposite to the parasitic mesoscopic F1 folds formed by out-of-the-syncline shear (e.g., F2 verge down-dip on the south limb; local exceptions represent conjugate F2 kinks). High-resolution digital photography allows us to trace S1 microlithons across several S2crenulation domains, and to trace S2 domains into hinge surfaces of F2 kink bands/folds. These observations emphasize that the overall sigmoidal shape of crenulated S1 microlithons is due to the intersection of the microlithon with four micro-scale kink hinges. We interpret S2 and F2 to be manifestations of late-stage subsimple shear within phyllitic layers during progressive development of the Baraboo Syncline. Specifically, out-of-the-syncline flexural-slip causes S1 cleavage in the phyllite to rotate into an orientation that allows slip on S1 domains to initiate. Local resistance to this slip, along with lateral confinement of the phyllitic layers, triggers initiation of contractional antithetic kink bands. Larger ones become F2 folds, and smaller ones become S2 crenulations. Progressive deformation locally overturns F2 limbs. Shortening across subhorizontal S2 domains accommodates overall thinning of phyllitic layers, thereby amplifying the effects of subsimple shear in these layers.

Authors

• Stephen Marshak 
  • University of Illinois
• M. Scott Wilkerson 
  • DePauw University
• Joshua DeFrates 
  • University of Illinois

-chris