ABSTRACT The original location and tectonic setting of the prominent Paleocene dike swarm in the British Isles are reconstructed for a “tight fit” of the North Atlantic region prior to any Cenozoic opening of the ocean basin between Greenland and Europe. The present-day northwest-southeast–oriented swarm originally trended toward southern Greenland and the locations of magmatic rocks of comparable age along the eastern and western margins of Greenland and approximately the position of the Iceland hotspot at 70–60 Ma in a “fixed hotspot” model. This raises the possibility that the northeast-southwest–oriented extensional stress field in which the dikes and associated central igneous complexes were emplaced may have been generated by impingement on the base of the lithosphere by a rising plume beneath present-day West Greenland. It is speculated, on the basis of seismic tomography and three-dimensional modeling, that the Paleocene igneous activity in the British Isles may have resulted from flow of a hot “finger” of upper mantle outward from the plume, perhaps controlled by preexisting lithospheric structures and the distant location of a second Paleocene volcanic province in central Europe.

Abstract With its wide variety of geological features and phenomena packed into a small area, the Baraboo of south-central Wisconsin is among the most visited parts of the Midwest by geology students. This guidebook, the first comprehensive look at the area in decades, covers the spectrum of geological features present in the area, and it is useful as a teaching tool. An exceptional outdoor classroom, the Baraboo area contains a spectrum of geology, including excellent examples of geomorphology, glacial geology, structural geology, petrology, stratigraphy, and sedimentology. Ages of the strata range from 1.7-billion-year-old Precambrian to the Quaternary. The area has been studied for about a century, but it still holds surprises for professionals and students alike.

Abstract The Proterozoic Baraboo Quartzite was laid down by braided rivers and in near-shore marine environments on the southern edge of Paleo–North America during a period of tectonic stability following a prolonged interval of orogenic activity. The exceptional chemical maturity of the Quartzite points to warm, wet, tropical climate conditions, and its distinctive maroon to pink and purple color marks it as one of the earliest “red beds” on Earth. Detrital zircons from the base of the Quartzite constrain its depositional age to be younger than ca. 1710 Ma. The Quartzite is overlain by two other units known only from subsurface exploration: the Seeley Slate, interpreted as a shallow marine deposit, and the Freedom Formation, which has the physical and mineralogical characteristics of classic Superior-type iron formations but is younger than any of these by at least 150 m.y. The folding event at Baraboo was previously thought to have occurred at ca. 1630 Ma, based on indirect regional arguments. However, a growing number of 40 Ar/ 39 Ar ages in the range of 1450–1480 Ma have been obtained from occurrences of muscovite in hydrothermal veins in the Quartzite and from tectonic cleavage surfaces in the Seeley Slate, and these suggest a possible connection with the Wolf River Batholith igneous interval. The remarkable topographic relief that existed in the Baraboo Range at the time of the late Cambrian marine transgression, one billion years after the folding event, is another aspect of the regional geologic history that remains incompletely understood.

Abstract A field trip to the Baraboo District provides an amazing opportunity to teach (or learn) many important aspects of structural geology. For example, students can define the regional-scale shape of the Proterozoic-age south-verging Baraboo Syncline from data on bedding attitudes and facing indicators and then can compare this shape to a digital elevation model of the district to see relationships between the dip of a stratigraphic unit and the width of its outcrop belt. Key outcrops of the Baraboo District, which we describe in detail, allow students to identify and sketch mesoscopic tectonic structures (joints, spaced and phyllitic cleavage, veins, faults, slip lineations, parasitic folds, boudinage, crenulation cleavage, and kink folds) and to interpret the kinematic significance of these structures. Students will leave Baraboo with a clear image of how progressive crustal shortening can be accommodated under lower-greenschist conditions.

Abstract The Baraboo Quartzite contains numerous well-preserved sedimentary structures that enable interpretation of environments of deposition. Included are various types of cross-stratification, reactivation surfaces, and tidal bedding. The bulk of the Baraboo was deposited under tidal influence. The remainder represents deposition in a braided stream system. The origin of the tremendous volume of quartz sand remains somewhat unknown.

Abstract Cambrian strata in Wisconsin compose a sheet of mostly marine sandstone, with minor dolomite, deposited during the fluctuating advance of the North American epeiric sea. Sedimentary features and fossils indicate that deposition took place in both shallower, current-dominated regimes and deeper quiet-water settings swept by episodic storm surges. The sand sheet surrounds inliers of Precambrian rocks in the Baraboo area. The Baraboo inliers are remnants of an elliptical ring of islands in a subtropical shallow sea, which were gradually buried by Cambrian and Ordovician sediments. Spectacular conglomerates composed of red quartzite clasts accumulated around the islands, which were pounded repeatedly by waves that we presume to have been generated by tropical storms. Paleomagnetic evidence places Cambrian Wisconsin in the southern tropics. Boulders up to 1.5 m in diameter are well rounded whereas larger ones (up to 8 m) are not. This suggests the possibility of estimating the magnitude of the Cambrian storm waves using knowledge from modern oceanography and from wave trough experiments by coastal engineers. Such analysis suggests waves necessary to tumble quartzite boulders 1.5 m in diameter were of the order of 7–8 m high at their point of breaking. Such magnitudes are not uncommon today during storms on many modern rocky coasts.

Abstract The surficial geology of the Baraboo area is very important because it includes the transition from a glaciated region to the Driftless Area. The eastern portion of this area was glaciated as part of the Green Bay lobe of the ice sheet in this area. The terminal moraine is present and is characterized by sandy till. No valid information that substantiates glacial activity exists west of the city of Baraboo. The Driftless Area includes a site of the earliest Wisconsin habitation.

Abstract This road log is different than most in a variety of ways. It is similar in that the stops are numbered in a certain order. That is because each stop must have some identification and numbers are the simplest and easiest to follow. Mileage is provided between stops, not in a cumulative fashion. This makes it easy to arrange the stops to suit the specific leader(s) and students. Those who use this field guide can choose to visit the stops in any order that they wish. The complete trip is designed to take two full field days, but stops can be visited in any fashion that suits the wishes and schedule of the group. There are a few alternate stops that may be used in addition to or in lieu of some of the regular stops. The estimated time necessary to spend at each stop is indicated in the log to help in organizing your trip. The total estimated time of the combined stops is ~12–14 hours. This does not include any travel time or lunch stops so that leaders can develop their own plans. Unless indicated in the figure caption, all figures herein are those of the co-authors. The trip starts at the intersection of Wisconsin Highway 33 with Interstates I-90 and I-94 (Appendix Figure A1.) Enjoy!