Confluence scours in braided rivers occur where channel threads join together, producing erosional relief that may be considerably deeper than average channel depth. Based on studies of the continental-scale Ganges-Brahmaputra river system, it has been observed that the maximum depth of confluence scours by autocyclic processes may reach up to four to five times the average depth of the incoming channels. Considering the possibility of such massive scours in an ancient fluvial system, it was argued that allogenically produced incised valleys at sequence boundaries should only be properly defined in ancient systems if the erosional relief is more than five times average channel depth.Based on cross-sectional geometries, a number of confluence scour fills were interpreted on well-exposed fluvial outcrops in a Cretaceous-age compound incised-valley system in the Notom delta complex of the Ferron Sandstone Member, Utah. Crosssectional observations from outcrops and comparisons with modern rivers reveal that confluence scours have diagnostic fill facies (single set of large steep foresets) and do not produce multistory sand bodies. A confluence scour fill produces a singlestory body in which a fifth-order scour is filled with unit bar foresets, which in turn are overlain by a fourth-order surface capped by compound bar deposits. The story thickness in the confluence scour does not represent the average channel depth because confluence scours allow preservation of the deepest parts of channels as well as fully preserved abnormally thick stories. Therefore, we argue that interpretation of incised valleys in an ancient system associated with a sequence boundary should not be based on the depth of the erosional surface versus the number of average preserved channel stories. Rather, it should be defined by the erosional relief that is significantly deeper than the thickest fully preserved stories, which in a braided stream are likely to represent confluence scour fills.

We investigated the interaction among basin-bounding faults, basin fi ll, and geomorphic features of the southern Gar Basin, one of only two known releasing double-bend basins along the Karakoram fault, to better understand their structural evolution and role in basin development. The southern Gar Basin is bounded by an ~44-km-long, N20°Wstriking central fault segment fl anked by two N40°W-striking segments that parallel the regional strike of the Karakoram fault system. The central fault segment is composed of a system of strike-slip and normal faults that young basinward and incorporate basin fi ll in their uplifted footwalls. The oldest faults along the extensional portion of the bend are dominantly strike-slip, and they strike ~15°W from the main strike of the Karakoram fault. Basin fi ll is broadly folded about a NNWtrending axis and can be explained by E-SEdirected slip along a listric normal fault. Cross sections across the basin and associated faults suggest the geometry is best described as an extensional fl ower structure. Forward structural modeling of the intrabasinal faults shows that the system has accommodated ~8 km of east-west extension. We interpret the bend to have formed from linking of R and P shears into a through-going principal displacement zone. At shallow levels in the crust (low confi ning pressures), R shears are exploited; at deeper levels, these faults merge with the principal displacement zone, forming the extensional fl ower structure geometry. We estimate that the shear zone is 50-35 km wide based on the aerial distribution of P and R shears. Restoration of R shears on the west and east sides of the Gar Valley indicates ~55 km of right-lateral separation along the Karakoram fault, which is a minimum slip estimate for the Karakoram fault system.

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Field mapping in the Yukon delta region of western Alaska, combined with laboratory analysis of sediment and Landsat imagery, has provided insights into the role of climate and tectonics on delta ic processes on high-latitude continental sh~lves. The climatic and tectonic influences on sediment type, in combination with the role of river and sea ice in controlling patterns of sediment transport and deposition, suggest that the Yukon delta may provide a model for deltaic sedimentation in an ice-dcrninated environment.

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