ABSTRACT The most common and widespread sedimentary facies of Pleistocene Lake Bonneville, in the eastern Great Basin of North America, is marl, which consists of a mixture of fine-grained endogenic calcium carbonate that precipitated in the epilimnion of the lake and then settled onto the lake floor and mixed with fine-grained clastic sediments. Primary sources of clastic sediment were inflowing rivers, wave activity in shore zones, and ice rafting. The thickness of deposits in cores and outcrops is largely dependent on the proportion of clastic sediment, although the rate of endogenic calcium carbonate precipitation probably also varied temporally and spatially. Net sediment-accumulation rate in the marl, as measured in outcrops and cores, ranges from a low of 4 cm/1000 yr, in the middle of the lake basin far from sources of clastic input, to over 100 cm/1000 yr near clastic-sediment sources. Underflow deposits, derived from higher-density river water loaded with suspended sediment, are thick and extensive near the mouths of major rivers that drained glaciated mountains. Net sediment-accumulation rates in suspended-load underflow deposits were much greater than those in contemporaneously deposited marl. The largest underflow-sediment accumulations, which have a fan shape in plan view, have been referred to as deltas (as at the mouths of the Sevier, Provo, Weber, and Bear Rivers). True Gilbert-type deltas composed of gravel, with topset, foreset, and bottomset beds, are uncommon in the basin. Variability in the sedimentary characteristics of the Bonneville deposits is determined by geomorphic factors, such as wave energy, composition of surficial material in the shore zone (e.g., resistant bedrock vs. unconsolidated alluvium), slope, and proximity to river mouths and active shore zones.

Abstract This chapter summarizes the tectonic events that have affected the region of the Arabian Intrashelf Basin and the development of the intrashelf basin. Precambrian–Infracambrian fault systems provided a structural framework, which was later reactivated during the Late Paleozoic. Further development along the structural trends continued in the Triassic and Early Jurassic with the development of an Early Jurassic tectonically controlled intrashelf basin. The accommodation space was filled with Dhruma Formation carbonates by the early Bathonian, resulting in a broad and fairly flat platform. A crucial factor is how suppressed tectonism was during the Mid- and Late Jurassic. The intrashelf basin developed on the broad, tectonically stable Tethyan passive margin continental shelf 200--300 km distant from the Tethyan outer shelf edge. During the Mid- and Late Jurassic, this tectonic stability provided the foundation for a broad, stable Tethyan continental shelf region at least 1000 × 1200 km in area, far removed from siliciclastic sources, within which the huge Arabian Intrashelf Basin formed and the sequences of carbonate rocks and evaporites that created the Jurassic hydrocarbon system were deposited. Tectonism during the Mid- to Late Jurassic evolution of the Arabian Intrashelf Basin initially included only moderate subsidence. There was subtle uplift along some of the structural trends in the Late Jurassic and uplift and westwards structural tilt along the Tethys oceanic margin. Relatively stable tectonism continued after the Jurassic and was a major factor in the accumulation of the vast reserves of Jurassic sourced oil. Tectonic stability prevented major faulting and structural compartmentalization of the basin features, which provided large areas for hydrocarbon migration as the large and broad anticlines developed into the huge structural traps. The lack of major faulting limited the movement of burial fluids, preserving the early formed porosity and the regionally extensive seals. The present day structures primarily developed from the Late Cretaceous to Miocene as the Tethys Ocean closed.

ABSTRACT High-resolution sand petrography and heavy mineral analyses help to frame U-Pb age and Hf isotope data from zircon grains, integrated in turn with geochemical data from detrital apatite, rutile, garnet, and monazite, and with Raman spectroscopy data from detrital amphibole, pyroxene, and epidote-group minerals. This multitechnique approach, including stream-profile analysis, was used to characterize components of the sediment flux and define erosion patterns across the Lhasa block, a complex continental arc terrane caught in the Himalayan collision. Litho-feldspatho-quartzose detrital modes and hornblende-dominated heavy mineral assemblages suggest that the majority (four fifths) of the sand bed load in the Lhasa River catchment is derived from erosion of granitoid batholiths. Gravel composition, however, is markedly different and dominated by volcanic pebbles in the trunk river, as in all of its four major tributaries, testifying to an order-of-magnitude difference in apparent erosion rates between granitoid batholiths and arc lavas. This marked contrast, partly explained by wide exposures of granitoid rocks in the rugged Nyainqêntanglha Range characterized by active incision, is notably amplified by the high sand-generation potential of granitoid rocks, which, in contrast to dense joint blocks of andesitic lavas, tend to disintegrate to sandy grus upon weathering. Sedimentary strata, making up a good half of exposed rocks, are also underrepresented in sand bed load, suggesting selective mechanical breakdown of nondurable shale/slate grains. This exposes a serious bias affecting estimates based on sand only, and it highlights the necessity for taking into account the entire size spectrum from mud to gravel in order to improve the accuracy of sediment budgets. Provenance analysis should involve multiple methods applied to multiple minerals, rather than be based solely on a single rare mineral, even if it is exceptionally laden with potential provenance information, such as zircon. We here divide arc-derived suites into those eroded from undissected arcs, in which nearly continuous volcanic cover is present, and those from dissected arcs, in which cogenetic plutons are widely exposed from erosional unroofing. —Dickinson and Suczek (1979, p. 2175)

ABSTRACT Streams in the Midwest of the United States have experienced major changes in their watersheds since European settlement that have altered sediment loads, runoff, nutrient concentrations, and the abundance of woody debris. Moreover, the near extirpation of keystone species such as beaver, and the construction of dams and impoundments (e.g., milldams, causeways, reservoirs, small ponds, etc.), have had impacts on the entrainment of sediments, the connectivity between tributaries, main channels, and floodplains, and channel form. As stream restoration efforts increase, how do we restore streams to their ‘natural’ state? Can streams restored to a pre–European settlement condition maintain equilibrium under current land use? Here we examine the impact of post-European settlement changes to a small (432 km 2 ) watershed in southwestern Ohio that is largely representative of rural watersheds in the Midwest. We examine the impact of nineteenth-century milldams, report the results of a 21-year study of nutrient and sediment concentrations in the upper portion of the watershed during a shift from conventional to conservation tillage, and assess the potential impact of the return of beavers on stream sediment and nutrient concentrations. Our objective is to understand how streams have been impacted by humans over the past 250 years, and to identify strategies for ‘restoring’ streams in the Midwest.