ABSTRACT Supermature siliciclastic sequences were deposited between 1.64 Ga and 1.59 Ga over a broad swath of southern Laurentia in the Archean, Penokean, Yavapai, and Mazatzal Provinces. These siliciclastic sequences are notable for their extreme mineralogical and chemical maturity, being devoid of detrital feldspar and ferromagnesian minerals, containing the clay mineral kaolinite (or its metamorphic equivalent, pyrophyllite), and having a chemical index of alteration >95. Such maturity is the result of a perfect confluence of tectonic and climatic conditions, including a stable continental crust with low topographic relief (the Archean, Penokean, and Yavapai Provinces ca. 1.70 Ga), a warm humid climate, an elevated level of atmospheric CO 2 , and relatively acidic pore fluids in the critical zone. The weathered detritus was transported and deposited by southward-flowing streams across the Archean, Penokean, and Yavapai Provinces, ultimately to be deposited on 1.66 Ga volcanic and volcaniclastic rocks in the Mazatzal continental arc along the southern margin of Laurentia.

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.

ABSTRACT This field trip examines a sequence of ejecta and deformed substrate resulting from the 1850 Ma meteorite impact. An impact origin for the Sudbury structure in Ontario has long been accepted, but knowledge of the corresponding ejecta was limited to fall-back breccia in the relict crater at Sudbury. The more distant ejecta blanket was discovered only recently near Thunder Bay, Ontario, and later in other parts of the Lake Superior region. Known informally as the Sudbury impact layer (SIL), it occurs at and near the stratigraphic top of Paleoproterozoic iron-formation. The impact-related deposits in the western Lake Superior region include (1) autochthonous material interpreted to be seismically folded and shattered iron-formation and carbonate rocks (breccia), overlain by (2) strata composed largely of allochtho-nous material (ejecta) derived in part from target rocks, and (3) irregular layers that appear to be mixtures of locally and distally derived material. Definitive microscopic evidence of an impact origin includes the occurrence of accretionary lapilli, ash pellets, spherules, devitrified glass, and quartz fragments marked by planar deformation features. The SIL exhibits extreme lithologic variability from place to place within each exposure area and between exposure areas. Nevertheless, the stratigraphic relationships that are presented by these exposures can be used to devise a sequence of deformation and depositional events that is largely consistent with experimental and empirical evidence of impact processes. This field trip will demonstrate that the stratigraphic arrangement of facies in the SIL has important temporal implications for understanding mechanisms of ejecta delivery and deposition.

ABSTRACT This field guide examines the evidence for multiple readvances of the Superior lobe, as well as the morphological and sediment record of glacial lakes in the western Lake Superior basin. During each readvance of the Superior lobe, ice went a shorter distance, reached a lower elevation, and laid down a finer-grained till due to incorporation of proglacial lake sediment. There are three distinct tills, which are correlated to three readvance phases: the St. Croix/Automba, Split Rock, and Nickerson. A red clay typically caps the stratigraphy at lower elevations in the basin. This clay may be a fourth till associated with a late readvance, perhaps equivalent to the Marquette phase in eastern Lake Superior. Alternatively, the red clay may be lacustrine. At issue are the potential hydraulic connections between glacial Lake Agassiz and the Atlantic Ocean during, and after, the Younger Dryas, because a readvance would fill the western Superior basin with ice and prohibit eastern Lake Agassiz drainage. Additional stops highlight the strandline and sediment record of the youngest glacial lake phase (glacial Lake Duluth), including the inspection of Lake Superior sediment cores that are archived at the National Lacustrine Core Repository in Minneapolis. The goals of the selected stops are to underscore the current understanding of the late glacial history of the western Superior basin and to provide new insights to spark discussion.

ABSTRACT The deglaciation history of northeastern Minnesota and northwestern Ontario is outlined using geological and ichthyofaunal evidence from the continental to local scales. Both published and new data indicate the existence of eastern outlets drawing from pre-and-early Agassiz lakes. Part of the Arctic watershed was impounded between the Duluth Complex highlands and the retreating Rainy lobe. These outlets had their flows routed through the “Keating Complex” and the “Gunflint Arrow Lakes Corridor.” Discharges around the Duluth Complex’s northeast limb reached the Superior basin along Superior lobe ice, then exited down existing pre-glacial river valleys and a prominent, “valley-type” topographic bench between Hovland and Grand Marais, Minnesota.

A systematic pattern of beach ridges forming strandplains commonly fills embayments in the Great Lakes of North America. Ground penetrating radar (GPR) and vibracore results define a common preserved architecture inside beach ridges. Comparing the preserved architecture with a conceptual model of beach-ridge development explains the conditions responsible for their development and preservation. Great Lakes beach ridges are a product of a positive rate of sediment supply and a multidecadal fluctuation in lake level. Many shoreline behaviors occur throughout the development of a beach ridge, but not all successions originally formed by these behaviors are preserved. Beach ridges are stratigraphically separated by concave lakeward-dipping ravinement surfaces extending at depth below beach-ridge crests to the ground surface in adjacent landward swales. These surfaces are formed during rapid rises in water level, where previously laid deposits erode, forming a base for the beach-ridge core. As the rate of rise decreases and the water-level elevation approaches a highstand, the core of the ridge is built by vertical aggradation. Subsequent deposits build lakeward during progradation when water levels become stable, protecting the core from being eroded during future rapid rises in water level. Dune sand deposits on beach-ridge cores are stabilized by vegetation, and swales are commonly filled with organic material.