ABSTRACT The Baraboo District includes an exceptional array of outcrops that have provided geological enlightenment to students and professionals, alike, for 150 years. In the late nineteenth century, several fundamental structural principles were developed here, such as criteria for determining stratigraphic facing and the significance of cleavage-bedding relations. More recent studies of deformational features in the folded Baraboo Quartzite, such as crenulation cleavage and quartz fabrics, have yielded insights into the kinematics of folding in the District and the significance of regional tectonics in the context of the Proterozoic assembly of North America. Additional petrologic, geochemical, and isotopic studies have established the age of the Baraboo Quartzite (≤1700 Ma), identified a Paleoproterozoic weathering profile, confirmed the supermature composition of the Baraboo Quartzite, established the presence of geon 14 hydrothermal alteration, and elucidated the Proterozoic tectonothermal evolution of the District, all of which bear importantly on Proterozoic tectonic, atmospheric, and climatic conditions in the southern Lake Superior region. By Late Cambrian time, the Baraboo Quartzite was a ring of islands, which was abutted by spectacular conglomerates deposited by tropical storms. These were surrounded by more distal sandstones and were eventually buried by Ordovician dolomite and sandstone. During the field trip, we will visit eleven localities, which have been selected to illustrate the key geological features of this North American classic.

The Waterbury dome, located in the Rowe-Hawley zone in western Connecticut, is a triple window exposing three terranes: parautochthonous or allochthonous peri-Laurentian rocks in its lowest level 1, allochthonous rocks of the Rowe-Hawley zone in its middle level 2, and allochthonous cover rocks, including Silurian-Devonian rocks of the Connecticut Valley Gaspé trough, in its highest level 3. Levels 1 and 2 are separated by the Waterbury thrust, a fault equivalent to Cameron's Line, the Taconic suture in southwestern New England. Relict mesoscopic folds and foliation in levels 1 and 2 are truncated by a dominant D 2 migmatitic layering and are likely Taconic. U-Pb zircon crystallization ages of felsic orthogneiss and tonalite, syntectonic with respect to the formation of S 2 , and a biotite quartz diorite that crosscuts level 2 paragneiss are 437 ± 4 Ma, 434 ± 4 Ma, and 437 ± 4 Ma, respectively. Level 3 nappes were emplaced over the Waterbury dome along an Acadian décollement synchronous with the formation of a D 3 thrust duplex in the dome. The décollement truncates the Ky + Kfs-in (migmatite) isograd in the dome core and a St-in isograd in level 3 nappes, indicating that peak metamorphic conditions in the dome core and nappe cover rocks formed in different places at different times. Metamorphic overgrowths on zircon from the felsic orthogneiss in the Waterbury dome have an age of 387 ± 5 Ma. Rocks of all levels and the décollement are folded by D 4 folds that have a strongly developed, regional crenulation cleavage and D 5 folds. The Waterbury dome was formed by thrust duplexing followed by fold interference during the Acadian orogeny. The 40 Ar/ 39 Ar ages of amphibole, muscovite, biotite, and K-feldspar from above and below the décollement are ca. 378 Ma, 355 Ma, 360 Ma (above) and 340 (below), and 288 Ma, respectively. Any kilometer-scale vertical movements between dome and nappe rocks were over by ca. 378 Ma. Core and cover rocks of the Waterbury dome record synchronous, post-Acadian cooling.

Orogenesis within the New England Appalachians has classically been regarded as occurring discontinuously even though the collision of plates driving it was essentially continuous for 200 million years from the Taconic through the Alleghanian orogenies. Structural, metamorphic, and age data obtained from the cores of porphyroblasts reveal a near continuous history of tectonism that is partitioned within and between outcrops as well as regionally. Very prolonged deformation and metamorphic histories predate the foliation parallel to bedding, and the oblique matrix foliations only reflect brief increments of the uplift path of these rocks back to the earth's surface. The matrix shows none of the structural effects of the path down into the crust. This deepening path is revealed by the sequences of foliations that developed about regionally consistent successions of foliation intersection axis trends preserved within porphyroblasts (FIAs). Indirect coupling between plates throughout the period of collision resulted in horizontal shortening accompanied by subvertical foliation development, followed by crustal instability, collapse, and the formation of subhorizontal foliation, repeated over and over until orogenesis ceased. These cycles repeat on time scales as short as 100,000 to 500,000 years, but because of partitioning of the deformation, only the weakest rocks preserve much of this history. Shifting directions of relative plate motion every 5 to 30 million years also results in easily deformed rocks being protected by more competent ones, with none of them seeing the total history. Furthermore, if the bulk composition is not suitable for porphyroblast growth, none of this history will be recorded. The recurring role of gravitational collapse and the variable scale of partitioning of this type of deformation is obscured by repeated reactivation of the bedding parallel foliation in multiply deformed rocks containing porphyroblasts in the New England Appalachians. It is also obscured by the lack of topographic relief relative to total crustal thickness.