Late Triassic through Early Cretaceous detrital zircon separated from Lower Cretaceous sedimentary strata provides a record of arc magmatism that is not obscured by products of the mid- to Late Cretaceous surge, which dominate the exposed Sierra Nevada batholith. Matching U-Pb age-probability maxima to U-Pb dates of exposed arc plutonic rocks provides confirmation that the detrital zircon was sourced in the erupting and eroding Sierra Nevada arc. These data suggest that magmatic productivity in the southwestern arc increased steadily through the Middle Jurassic, from an Early Jurassic lull through the Late Jurassic. The detrital-zircon record documents an original footprint of the Early Cretaceous arc extending from its current exposure in the western Sierra Nevada foothills northwestward into the eastern Sacramento Valley, where its relatively mafic roots are presumably buried beneath younger sedimentary strata infilling the Great Valley. The sparse record of Late Triassic magmatism preserved in the analyzed intra-arc and forearc deposits likely reflects greater separations in both time and space between the Early Cretaceous basins and the Triassic arc. Analysis of an atypically dense sample set from the Goldstein Peak Formation intra-arc basin deposits, in conjunction with new data from the Lower Cretaceous Gravelly Flat Formation and published data from other Lower Cretaceous forearc strata of the Great Valley Group, suggests that an even greater density and broader geographic distribution of detrital-zircon samples are needed to more completely reconstruct the record of arc magmatism.

Abstract Groundwater-fed wetlands are common features of many lake margins, and they may leave characteristic deposits that record hydrologic conditions at their time of formation. Geomorphic and sedimentologic observations of wetland complexes on the margins of Lakes Eyasi, Makat (Ngorongoro Crater), and Natron, Tanzania, show that clastic, chemical, and biological processes in the wetlands produce discernible lithofacies variation. Five distinct sets of lithofacies are identified: sands and gravelly sands (S and Sg), silty to sandy muds (Ms), bioturbated muds (Mb), alkaline muds (Ma), and organic-rich rootmats, peat, and clay (R, P, Co). Several wetland depositional subenvironments can be defined on the basis of lithofacies associations, including drainage channels (facies S, Sg, Mb, Ms), marshes (R, P, Co, Mb), hippopotamus flats (Co, Mb, Ms), pools (S, Ms, Mb), and alkaline flats (Ma, Co). Sediment supply, aqueous geochemistry, activity of large mammals, and composition and distribution of vegetation are the primary controls on sedimentation. Sediments in these subenvironments are generally thin (< 1 m), but they are thicker where large mammals like Hippopotamus amphibius generate deep (> 1.5 m) bioturbation zones. Recognizable lithofacies are found up to ∼ 1 km basinward from the sources of spring waters, with down-gradient sediment fining. Very strong evaporatively driven geochemical gradients are seen as well, with increasing salinity and alkalinity downstream and laterally. Wetland sediments are laterally restricted to only a few tens of meters, and they abruptly interfinger with sediments from adjacent fluviolacustrine environments. In all three basins, these Recent deposits overlie a regressive disconformity or hiatus that signifies a drop in lake level, recording the onset of regional aridity during the late Holocene. Such wetland sediments can be important paleohydrological indicators, generally indicating a fluctuation that led to subaerial exposure and colonization of lake flats by wetland vegetation.