Distinctive far-traveled fluvial sediment of the lower Colorado River fills 20 paleovalleys now stranded by the river downstream of Grand Canyon as it crosses the Basin and Range Province. These sediments resulted from two or more aggradational episodes in Pliocene and Pleistocene times following initial incision during the early Pliocene. Areview of the stratigraphic evidence of major swings in river elevation over the last 5 m.y. from alternating degradation and aggradation episodes establishes a framework for understanding the incision and filling of the paleovalleys. The paleovalleys are found mostly along narrow bedrock canyon reaches of the river, where divides of bedrock or old deposits separate them from the modern river. The paleovalleys are interpreted to have stemmed from periods of aggradation that filled and broadened the river valley, burying low uplands in the canyon reaches into which later channel positions were entrenched during subsequent degradation episodes. The aggradation-degradation cycles resulted in the stranding of incised river valleys that range in elevation from near the modern river to 350 m above it.

Abstract The lower Pennsylvanian strata in Indiana, Illinois, and western Kentucky rests unconformably on Mississippian to Devonian deposits within the midcontinent Illinois Basin. These deposits fill a complex drainage network of sub-Pennsylvanian paleovalleys with as much as 140 m of shale, sandstone, coal, and mudstone. Paleovalley widths are variable ranging from a kilometer to several kilometers wide and trend roughly northeast to southwest. Timing of valley incision is difficult to constrain and may have been diachronous. Filling of the southern reaches of the drainage system commenced during earliest Pennsylvanian while incision may have continued in the more northern (upland) reaches in Indiana. The paleovalleys progressively filled from south to north during an overall transgressive sea-level rise. During sea-level rise, these valleys became the sites for estuarine deposition. Two relatively small paleovalleys (1 to 1.5 km wide) have been studied in detail in south-central Indiana. Lithofacies can be mapped down the valleys for several kilometers by using subsurface and outcrop data. Within these valleys, conglomerate and conglomeratic sand-dominated inner (upper) estuarine, mud-dominated estuarine central basin, and sand-dominated outer (lower) estuarine deposits can be identified. Central basin sediments overlie fluvial deposits. Direct evidence of tidal influence within the central basin deposits can be recognized by the presence of intertidal rhythmites that exhibit well developed neap-spring cycles. Outer estuarine deposits overlie the central basin deposits and accumulated during the final stages of valley-filling.