A model is proposed whereby a Miocene Colorado River precursor canyon, deeper than 600 m, formed on the western Hualapai Plateau by headward erosion along a strike-valley drainage. Basin and Range faulting of the margin of the Colorado Plateau initiated canyon formation. This canyon was occupied by a long narrow lake, and the surface of the lake was at or above the level of the Hualapai Limestone. Such a hypothesized lake would have trapped any coarse sediment derived from the surrounding basin at the head of the lake, well upstream from the Grand Wash Trough. The drainage area feeding into the lake would have included the Hualapai Plateau and the combined ancestral drainages of Kanab and Cataract Creeks. This >13,000 km 2 basin has been dominated by surface exposures of Paleozoic carbonates since at least late Eocene time and generates no more than 1%–2% of the runoff associated with the modern (predam) Colorado River discharge. Such a carbonate-dominated, sediment-deficient basin would supply carbonate-rich runoff to the structural depocenter in the Grand Wash Trough, possibly explaining the upward transition to the Hualapai Limestone facies in late Miocene time. The upstream canyon delta produced in this proposed model could have been removed by the Pliocene-Pleistocene integration and younger incision of the more powerful, modern Colorado River.

ABSTRACT About 17,000 yr ago, Glacial Lake Maumee breached the Fort Wayne Moraine, sending an unimaginably large torrent of meltwater down the upper Wabash River Valley (UWRV). The Maumee Megaflood (MM) may have lasted only a few weeks, but it scoured out a deep trough along the main stem of the river, radically lowering regional base level in what amounts to a geological instant and imposing a strong disequilibrium on a landscape that continues to experience major geomorphic, environmental, and ecological adjustments. In Huntington and Wabash Counties, the central part of the trough is engorged in resistant, Late Silurian reef-associated and inter-reef rocks, producing the largest natural bedrock exposure in heavily glaciated northern Indiana. Unlike the immature, deranged drainage pattern that characterizes most of the glaciated region, streams adjacent to the UWRV form well-integrated drainage networks that exhibit features and processes more typical of high-relief bedrock areas, such as steep fall zones with prominent, lithologically controlled knickpoints, canyons, large terraces, falls and cascades, and a variety of bluff and hillside morphologies and associated groundwater phenomena. The exceptional exposures and diverse landscape of this region have attracted well over a century of interest from geomorphologists and glacial geologists, sedimentologists, stratigraphers, and paleontologists, as well as hydrogeologists, anthropologists, ecologists, and geoscience educators. Among other firsts, the organic origin of fossil reefs in the southern Great Lakes was definitively established in the UWRV, as was the occurrence of convulsive meltwater outbursts during deglaciation of the Laurentide Ice Sheet; likewise, the first direct Mississippi River–Great Lakes connection was also established here by early voyageurs. Today, the region is a popular destination for both nature tourism and history buffs, due in no small part to the burgeoning number of geologically inspired natural areas and historical sites. This field trip traces the MM from its outlet at Fort Wayne, through the bedrock gorge of the upper Wabash River, to the confluence with the late Tertiary Teays Bedrock Valley, with major emphasis on how the depositional framework and diagenetic history of the Late Silurian reef archipelago continue to reverberate in the modern geomorphic response of the valley to Pleistocene events. The first three stops focus on the Wabash-Erie Channel, which acted as the principal outlet of Glacial Lake Maumee and whose underlying geologic characteristics controlled the overall incision history of the MM. Several stops in the Wabash bedrock gorge and Salamonie Narrows will examine the handiwork of this flood, which created the spectacular klintar, or pinnacle-like reefs, of the UWRV, within a landscape that early geomorphologists likened to the scablands of eastern Washington. There, we will see world-class exposures of the fossilized Late Silurian reefs and how their organic framework and diagenesis are controlling the ongoing adjustment of the UWRV landscape and its streams to the convulsive changes imposed by the MM. Stop 9 will showcase the elusive Teays Bedrock Valley and its complex pre-Wisconsin fill, where it converges with the modern river and has been partially exhumed by a major tributary, and offers a study in contrasts between the bedrock-controlled landscapes of earlier stops and an equally steep one excavated entirely into unconsolidated deposits. After a brief stop at the iconic Seven Pillars landmark, the trip concludes at the spectacular Pipe Creek Jr. Quarry, which features several km of tall exposures through the Late Silurian carbonate complex, a late Neogene sinkhole deposit, and the overlying Pleistocene section.