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!

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 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.

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Current interest in global sea level change was stimulated by the Deep Sea Drilling (DSDP) and CLIMAP programs and by the development of seismic sequence stratigraphy during the 1960s and 1970s. With “eustatic” now on the lips of climatologists, marine geophypsicists, tectonicists, and petroleum explorationists, as well as stratigraphers, there is a tendency to assume that eustasy is a new concept, whereas it has a long and complex history of ups and downs. This symposium volume presents much of that history. Eustasy has ultimate roots in the flood myths of several ancient civilizations and in seventeenth century sacred theories, which sought geologic evidence of Noah’s Flood. Then eighteenth century neptunism postulated a one-way eustatic fall to explain all rocks of the crust, while competing plutonism postulated uplift of land instead. It was Agassiz’s 1840 glacial theory that nurtured our modern concept because of the implication of lowered sea level during an ice age, as first noted by MacLaren in 1842. Surprisingly, however, it was not until Daly’s 1934 Changing World of the Ice Age that glacial eustasy became firmly established. The term eustatic was coined by Suess (1888) for global changes of sea level, which he attributed to cyclic oceanic subsidence due to cooling and contraction of the earth. In 1898 and 1909, Chamberlin proposed a diastrophic (tectonic) control of sea level as a cause of periodic universal unconformities. Both eustasy and cyclicity then became so in vogue that by the 1930s and 1940s theories of global rhythms of everything were rampant. This era gave us Grabau’s Pulsation theory, Haarman’s oscillation theory, Stille’s global orogenic cycles, Umbgrove’s Pulse of the Earth and Symphony of the Earth, and Carboniferous cyclothems. In the 1950s, a reactionary continuist dogma arose, which retarded the acceptance of sequence stratigraphy first proposed by Sloss, Krumbein, and Dapples in 1948. It was the development of seismic stratigraphy by Vail and associates in the 1960s that firmly established sequence stratigraphy. Now, with rediscovery of Milankovitch cycles as a result of DSDP and CLIMAP, we have reinvented the wheel of global cyclicity, for which rhythmic eustatic change has provided the axle.

The diminution of the sea according to de Maillet represents the fall portion of an endlessly repeated complete and cosmic eustatic cycle during which water and ashes were exchanged between celestial bodies. This concept, concerning both the geological and biological sciences, is a remarkable achievement for an early eighteenth-century career diplomat and traveler. Furthermore, his ingenious approach to the history of the earth, according to which the sea—during thousands of millions of years of eustatic fall—was entirely responsible for all the physiographic, lithologic, and structural features of the earth’s crust, made him an unusual forerunner of eustasy, generalized transformism, marine geology, and sedimentology.

The Austrian geologist Suess was the person who introduced the concept of eustasy, distinguishing two types of movement, caused by different processes. Negative movements, involving lowering of sea level, were caused by spasmodic subsidence of the ocean floor as a consequence of global contraction. Positive movements, involving rise of sea level, were more continuous and caused by the displacement of seawater by ocean-floor sedimentation. Suess’s eustatic interpretation was disputed by later scholars. Haug, thus, maintained that transgressions on the continents correlated with regressions in the geosynclines, and vice versa, and Haarmann argued for contemporary up and down movement of landmasses, with rises and falls of sea level a secondary consequence. However, the German geologist, Stille, was a confirmed eustasist, arguing that the major movements of the strandline had affected all the continents in much the same sense at the same time. Stille claimed that a series of relatively brief global “orogenic periods” increased the total continental area and caused general regression of the sea. Stille was the first to produce a eustatic curve for the Phanerozoic, but publication of this had to await a paper by Umbgrove shortly before the Second World War. Another Dutchman, Kuenen, was a pioneer in the use of the hypsographic curve to attempt a crude estimate of the amount of sea-level change resulting from a given change in area of land flooded by epicontinental seas. He rejected Suess’s explanations of positive and negative eustasy, preferring with Umbgrove an underlying mechanism bound up with mantle processes. After the Second World War a reaction set in against Stille’s geotectonic ideas, but eustatic studies were given support by oceanographic studies that suggested a plausible mechanism for long-term eustatic changes, and more detailed stratigraphic work across the world, which supported the reality of eustasy. Modern work concentrates on applying the concepts of sequence stratigraphy.