Reinterpreted history of latest Pleistocene Lake Bonneville

Abstract

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.

Enhanced development of lacustrine microbialites on gravity flow deposits, Great Salt Lake, Utah, USA

“…The origin of the Bonneville Flood is still under discussion and it is possible that regional seismicity was involved (Janecke and Oaks, 2011). Earthquakes are frequently at the origin of debris and hyperconcentrated density flows (e.g. the 1929 Grand Banks earthquake and associated debris flow; Piper et al, 1988) through the generation of liquefied silt and fine-sand layers caused by excess pore pressure created by seismic waves.…”

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“…The easily eroded, lacustrine tuffaceous Salt Lake Formation and some smaller fault blocks of pre-Tertiary rocks are truncated by a sloping erosional pediment surface, which is overlain by a thin pediment cap, overlying and inset alluvial-fan deposits in some areas, and later loess (DeVecchio et al, 2003;Janecke and Oaks, 2011;Thackray et al, 2011;this study). The thickness of the alluvial material on and adjacent to the pediment cap varies unpredictably due to cut-and-fi ll processes near paleochannels and due to aggradation near active streams, such as Marsh Creek, after formation of the pediment and pediment cap (e.g., DeVecchio et al, 2003).…”

“…Either these fi ne materials mantle underlying coarser fl uvial sediment, or the river system transported mostly fi ne sediment. The latter is expected along an outlet stream downfl ow from the large sand to mud-dominated deltas of the Bear River (Janecke and Oaks, 2011). Water wells through the deltas of the Bear River consistently encounter abundant silt and clay, as well as some sand and minor gravel (Janecke and Oaks, 2011).…”

“…The lower Provo shoreline is especially well expressed on the Weston delta, where it forms a step down to the east of ~30 ft (9 m) from the top of the smooth Provo delta that formed during the stable lake at the main Provo shoreline (Janecke and Oaks, 2011;Fig. 8).…”

“…However, during occupation of the two Provo shorelines, Lake Bonneville was restricted fi rst to south of the Swan Lake terrain and then to south of the Twin Lakes horst block. The deltas of the Bear River, the large southernmost area, are the topic of a separate paper (Janecke and Oaks, 2011).…”