Detailed study of post-Miocene stratigraphy in southeastern Virginia reveals at least 13 formations which show six Pliocene (?) and Pleistocene cycles of emergence and submergence, with maximum submergent sea levels near +45, +45, +20, +25, +15, and 0 feet, respectively. The newly established stratigraphic framework disproves earlier interpretations of "terrace-stratigraphy" and sea level chronologies.
Geologic, geomorphic, and geophysical analyses of landforms, sediments, and geologic structures document the complex history of pluvial Lake Bonneville in northern Cache Valley, NE Great Basin, and shows that the outlet of Lake Bonneville shifted ~20 km south after the Bonneville fl ood. The Riverdale normal fault offsets Bonneville deposits, but not younger Provo deposits ~25 km southeast of Zenda, Idaho. Rapid changes in water level may have induced slip on the Riverdale fault shortly before, during, or after the Bonneville fl ood. Although other processes may have played a role, seismicity might have been the main cause of the Bonneville fl ood. The outlet of Lake Bonneville shifted south from Zenda fi rst 11, then another 12 km, during the Provo occupation. The subsequent Holocene establishment of the drainage divide at Red Rock Pass, south of Zenda, resulted from an alluvial fan damming the north-sloping valley. Weak Neogene sediments formed sills for the three overfl owing stages of the lake, including the pre-fl ood highstand. Field trip stops on fl ood-modifi ed landslide deposits overlook two outfl ow channels, examine and discuss the conglomerate-bearing sedimentary deposits that formed the dam of Lake Bonne ville, sapping-related landforms, and the Holocene alluvial fan that produced the modern drainage divide at Red Rock Pass. The fl ood scoured ~25 km of Cache and Marsh Valleys, initiated modest-sized landslides, and cut a channel north of a new sill near Swan Lake. Lake Bonneville dropped ~100 m and stablilized south of this sill at the main, higher ~4775 ± 10 ft (1456 ± 3 m) Provo shoreline. Later Lake Bonneville briefl y stabilized at a lower ~4745 ± 10 ft (1447 ± 3 m) Provo sill, near Clifton, Idaho, 12 km farther south. An abandoned meandering riverbed in Round Valley, Idaho, shows major fl ow of the large Bonneville River northward from the Clifton sill. Field trip stops at both sills and overlooking the meander belt examine some of the fi eld evidence for these shorelines and sills.
Geologic, geomorphic, and geophysical analy ses of landforms, sediments, and structures in northern Cache Valley, USA, document a revised history of fl ooding and recession of Lake Bonneville, the world's premier pluvial lake. Crosscutting relationships suggest that the Riverdale fault produced a surfacerupturing earthquake ~25 km southeast of Zenda shortly before, during, or after the Bonneville fl ood, as well as possible younger surface ruptures. Thus fl uctuating stresses and pore pressure induced by changing lake levels may have triggered a large earthquake that, in turn, triggered the Bonneville fl ood. The fl ood scoured ~25 km of Cache and Marsh Valleys and activated landslides during its ~100 m incision to a new outlet near Swan Lake, Idaho. One to two thousand years of steady outfl ow produced the main ~4775 ± 10 ft (1455 ± 3 m) Provo shoreline, ~10 m above the commonly accepted altitude. Later Lake Bonneville oscillated below the main Provo shoreline, incised the Swan Lake outlet, rose and paused briefl y at a new lower Provo sill (4745 ± 10 ft [1446 ± 3 m], P9?) 23 km south of Zenda, before reverting to a closed-basin condition. Correlation to the Blue Lake chronology of Benson et al. suggests that aridity during the Heinrich 1 event activated the lower Provo sill ~15.9 ka. An abandoned, meandering riverbed, north of the lower Provo sill, records a large northward fl owing river. The Great Basin's modern divide at Red Rock Pass formed in the Holocene when a small alluvial fan fi lled the dry bed of this paleoriver.For permission to copy, contact editing@geosociety.org
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