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 The relative importance of tectonics, climate, base level and source lithology as the primary controls on the evolution of alluvial fans is highly debated. This study examines the role of upstream catchment characteristics on the evolution of alluvial megafans by examining three Quaternary fans (the Kalahrud, Zefreh and Mughar fans) along the flanks of the Kohrud Mountain Range in central Iran. These fans formed in a tectonically active basin under arid to semi-arid climatic conditions. The key differences between the evolutionary trends of these fans are that their catchments are underlain by different bedrock types and they have different catchment shapes and outlet characteristics. The catchment of the Kalahrud fan is in a sedimentary terrain with limited sediment supply, whereas the bedrock lithologies of the Zefreh and Mughar fans are fractured and weathered igneous rocks. However, the evolution of the Mughar fan is also controlled by the tilting of the catchment towards a wide apex and lateral shifting in the catchment outlet/fan feeder channel position. These variables resulted in relatively large-scale incision in the Kalahrud and Mughar fans that is absent in the aggradational trend of the Zefreh fan. Upstream lithological and structural controls are the dominant drivers behind the development and evolution of alluvial megafans.