Abstract This field trip highlights the current understanding of the tectonic assemblage of the rocks of the Central Appalachians, which include the Coastal Plain, Piedmont, and Blue Ridge provinces. The age and origin of the rocks, the timing of regional deformation and metamorphism, and the significance of the major faults, provide the framework of the tectonic history which includes the Mesoproterozoic Grenvillian, Ordovician Taconian, Devonian to Mississippian Neoacadian, and Mississippian to Permian Alleghanian orogenies.

ABSTRACT This field trip is an overview of Precambrian terranes in northeastern Minnesota using some of the most illustrative and accessible exposures—the term "classic" refers to the fact that many were the basis for earliest geologic study of the Precambrian and continue to be exemplary. The geology is presented in the context of major orogenic, rifting, and meteorite impact events during evolution of the North American continent. The Archean rocks are the products of three periods of orogenesis: the ca. 2695 Ma Shebandowanian orogeny that created major folds and thrust stacks; the ca. 2680 Ma Minnesotan orogeny that produced regional transpressive fabrics, folds, and metamorphism to greenschist-amphibolite grade; and a third event that produced localized faulting and folding of earlier structures and fabrics. The Sheban-dowanian may represent collision of the Wawa subprovince with the composite Superior superterrane to the north. The Minnesotan can be attributed to oblique collision of the Minnesota River Valley subprovince with the Superior superterrane. Structures bounding major components of the Superior Province are locally inferred to be thrust faults that formed during terrane assembly. Their vergence and offset histories are derived from seismic surveys in Minnesota and extrapolation from Lithoprobe and NATMAP transects in adjacent Canada. The Paleoproterozoic rocks are the products of three orogenic and rifting events, reflecting continued continental growth at Geons 18, 17, and 16. Mesoproterozoic rocks result from Geon 11 continental rifting, producing volcanic and sedimentary rocks of the Keweenawan Supergroup and plutonic rocks of the Midcontinent Rift Intrusive Supersuite.

Abstract Microbial earths are communities of microscopic organisms living in well-drained soil. Unlike aquatic microbial mats and stromatolites, microbial earths are sheltered from ultraviolet radiation, desiccation, and other surficial hazards within soil cracks and grain interstices. Currently, such ecosystems are best known in small areas of unusually cold, hot, or saline soils unfavorable to multicellular plants and animals. During the Precambrian, microbial earths may have been more widespread, but few examples have been reported. This review outlines a variety of features of modern microbial earths that can be used to distinguish them from aquatic microbial mats and stromatolites in the fossil record. Microbial earths have vertically oriented organisms intimately admixed with minerals of the soil, whereas microbial mats are laminated and detachable from their mineral substrate as flakes, skeins, and rollups. Microbial earths have irregular relief, healed desiccation cracks, and pressure ridges, whereas microbial mats have flexuous, striated domes, and tufts. Microbial earths form deep soil profiles with downward variations in oxidation, clay abundance, and replacive nodular subsurface horizons, whereas microbial mats form as caps to unweathered, chemically reduced sedimentary layers. Microbial earths develop increasingly differentiated soil profiles through time, whereas microbial mats build upward in laminar to domed increments. Microbial earths are found in nonmarine sedimentary facies, whereas microbial mats form in lacustrine, floodplain, and marine sedimentary facies. Microbial mats and stromatolites are known back to the oldest suitably preserved sedimentary rocks in the 3458 Ma Apex Chert and 3430 Ma Strelley Pool Formation (respectively) of the Pilbara region of Western Australia. The geological antiquity of microbial earths extends back to 2760 Ma in the Mount Roe paleosol of the Hamersley Group near Whim Creek, Western Australia.

ABSTRACT The 7–9-km-thick North Shore Volcanic Group (NSVG) constitutes the volcanic products of the 1.1 Ga Midcontinent Rift System in northeastern Minnesota. With close physical, chemical, and volcanological analogies to Tertiary-to-modern Iceland, these flows accumulated in a gradually subsiding basin over a mantle plume centered beneath modern Lake Superior between 1108 and 1094 Ma. They are essentially undeformed, except for local faulting and disruption associated with hypabyssal intrusions. Geochemically the NSVG is bimodal, dominated by basalts and rhyolites, but includes a complete tholeiitic Fe-enrichment suite that ranges from primitive olivine tholeiite through transitional basalt, basaltic andesite, andesite, and icelandite to rhyolite. The mafic magmas were partial melts of the plume and lithospheric mantle, variably modified by crystal fractionation in crustal chambers and by crustal interaction. Many, but not all, of the rhyolites were derived largely from partial melting of Archean crust. The volcanic rocks were erupted subaerially, primarily from fissures, though there is some evidence for central volcanoes. Some of the rhyolites are very large and widespread, and were emplaced as high-temperature lavas and rheoignimbrites that crystallized primary tridymite. During their accumulation and subsidence, these plateau volcanics were subjected to burial/hydrothermal metamorphism, resulting in secondary mineral associations that range from greenschist (epidote-chlorite-albite±actinolite) to zeolite (thomsonite-scolecite-smectite) facies. This field trip will allow participants to examine outcrops throughout the stratigraphic section of the NSVG, including structural relations, volcanology, geochemical diversity, burial metamorphism, and associated hypabyssal intrusions.