Abstract This field guide describes four exposures of glacigenic sediment along the Missouri River bluffs east of Omaha, Nebraska. Field trip stops include Loveland, Iowa, which is the type section of the Loveland Silt and Pisgah Formation (Illinoian and Early Wisconsinan loess) and Crescent Quarry, which exposes the type Nebraskan till. Additionally we will examine core samples of the Kennard Formation, a new stratigraphic unit consisting of multiple pre-Illinoian loesses. We also present recent results on the pre-Illinoian till stratigraphy in the Missouri River valley region. A variety of evidence indicates that the present location of the Missouri River valley originated sometime between deposition of the youngest (pre-Illinoian) till and the (Illinoian) Loveland Silt. The spatial distribution of the youngest pre-Illinoian till further suggests that this reach of the Missouri River (along the Iowa-Nebraska border) was established as an ice-marginal stream along the western terminus of the last pre-Illinoian glaciation.

ABSTRACT Circa 12 Ma, there was a fundamental reorganization of magmatism and tectonics in the Mojave Desert, California, USA, from basaltic to rhyolitic fields associated with extensional tectonics to dispersed basaltic monogenetic fields associated with the northwest- or east-striking strike-slip faults. The broad zone of strike-slip faults associated with the San Andreas transform margin stretched east to Arizona, but the high slip-rate central part that is known as the Quaternary eastern California shear zone is the locus of the post–12 Ma volcanism. We compiled a literature review of 29 basaltic fields, conducted detailed and reconnaissance study of several fields, and conducted geochemistry on many. Most of the volcanic fields are near or cut by the long fault systems, but more importantly, in nearly two-thirds of the fields that have scoria cones, the cones are <1 km from one of these faults. Eighteen volcanic fields have geochemistry data, and of the 441 analyses of major elements, 286 are new U.S. Geological Survey data that include comprehensive trace elements. The data are used to compare fields and determine the compositional variations during the formation of each field. Comparisons between fields show that several nearby volcanic fields have similar geochemical compositions and trends compared to distant fields; we suggest that six magmatic clusters formed, some with 200–300 k.y. duration. Spatial patterns change over the 12 m.y. span, with early fields forming in the north and south and the youngest in a central area from Pisgah to Amboy fields. The volume of magma also changed, most notably with a sixfold increase ca. 4 Ma followed by a 2–3 m.y. quiescence. In addition to the fracture control provided by strike-slip faults as magma conduits, we advance arguments for shear melting in the mantle along the most active faults, with secondary controls of local tectonic interactions, to explain spatial and temporal patterns in the fields.