Prepaleozoic red quartzites in the north-central United States are among the world’s oldest red beds. Not only do they constitute evidence about Proterozoic weathering and the advent of significant atmospheric free oxygen but they also provide important evidence bearing upon the sedimentary and tectonic history of North America. All of the red quartzites postdate the Penokean orogeny (1,760 to 1,860 m.y., broadly defined), and isotopic evidence permits that some could conceivably be as young as 1,200 m.y. (Keweenawan basalt eruption). Although these different quartzites may not be strictly correlative, it is probable that those south of Lake Superior (Baraboo, Sioux, Barron) fall within the narrower span of 1,450 to 1,750 m.y., here called the Baraboo interval for the best-dated sequence of sedimentation, deformation, and metamorphism. It has been suggested that their deposition probably predates a major 1,615 to 1,630-m.y. Rb-Sr resetting event, which would narrow the depositional age span considerably more. The severity of deformation of the quartzites is enigmatic in terms of their maturity and present intracratonic location. Sources of the red quartzitic sediments were quartz-bearing silicic volcanic rocks, such as underlie the Baraboo Quartzite, and older sedimentary, plutonic, and minor metamorphic rocks. Predominance of aluminous kaolinite and pyrophyllite in fine red argillaceous strata and near-absence of feldspar in the sandstones suggest mature chemical weathering in the source. Without vegetation to stabilize soils, this seems possible only on a stable landscape with little topographic relief and in a warm, humid climate. Braided rivers apparently drained the hinterland and formed a dominantly sandy coastal plain. Marine transgression may have begun during deposition of the sandstones, for the youngest strata preserved, which are black slates and iron-formation-bearing dolomite overlying the Baraboo Quartzite, are considered to be marine. Deposition of a red, mature quartzose (90% to 95%) sedimentary wedge up to 2,000 m thick points to a stable, passive continental margin on the southern edge of a Proto-North American craton. Later deformation was intense and greenschist metamorphism was pervasive, implying compressive orogenesis. The hypothesis is proposed that a major, hitherto unrecognized plate suture exists beneath Iowa and Illinois. Southward subduction during or after deposition of the red quartzites caused consumption of a sea floor that lay to the south of Proto-North America and culminated in collision either of a volcanic island arc or of another continent with southern Proto-North America about 1,615 to 1,630 m.y. ago. Foliated granitic plutons 1,625 m.y. old are known in northern Kansas and Missouri, and the Mazatzal belt of Arizona and New Mexico that contains plutonic and volcanic rocks of 1,610 to 1,680-m.y. ages also seems to be related to some such event. Apparently emplacement of the anorogenic Wolf River batholith in Wisconsin (about 1,500 m.y.) and eruption of widespread rhyolites to the south (1,380 to 1,480 m.y.) postdated the collision. Whatever may be the merits of this particular hypothesis, the red quartzites provide important constraints upon concepts of North American continent-building during Proterozoic time.

Abstract Transport of sand across shelves to deeper water still poses questions in spite of the acknowledged importance of submarine canyons as accessways, especially because canyons are little recognized in the ancient record. Eocene strata in southwestern Oregon, U.S.A. contain many small-scale channels to 100 m wide and 25 m deep that acted as conduits of much sand from a sandy littoral and deltaic zone across a narrow shelf and slope to feed deeper marine turbidity currents and other gravity flows, which built deepsea fans. The middle Eocene Elkton Siltstone Member of the Tyee Formation (500-600 m thick) is transitional stratigraphically from thick-bedded, mid-fan sandy turbidites also of the Tyee Formation beneath to the coal-bearing deltaic Coaledo Formation above. Foraminifers suggest depths of upper bathyal at the base to inner neritic at the top of the generally fine-grained Elkton; megafauna is extremely sparse although trace fossils are common. Some channels are filled with laminated mudstone-siltstone identical with surrounding material. Many other channels, however, are filled with massive to faintly parallel-laminated and rarely graded light-colored sandstone lacking fauna; spectacular mudstone intraclast conglomerate lenses are associated. At least one small channel levee is identifiable. Sedimentary structures in the channel sands suggest gravity-flow transport and considerable post-depositional deformation. Rare thin Bouma T a and T ab graded beds in the slope mudstones attest to occasional overbanking or levee breaching by gravity flows. Symmetrical ripples and hummocky cross stratification at the top of the Elkton, together with changes in foraminifers, indicate a shoaling trend. In sharp contrast, overlying Coaledo sandstones are coarser, show large-scale cross bedding, much of it contorted, and contain abundant wood, coal, zones of shallow-marine megafossils, and trace fossils. These deposits occur in a series of coarsening-upward cycles that reflect episodic shoreline progradation. Formerly we interpreted the channels as short delta distributary extensions onto the shelf. Recent investigations suggest channels formed deeper and farther from delta fronts, apparently as an array of sea gullies crossing the shelf and slope toward deeper water where they fed sand to deepsea fans. Modern slopes probably have many such channels that are not resolvable by conventional profiling techniques.