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Show The Puddle Valley Inflow Bar Dorothy Sack, Dept. of Geography, Ohio University Puddle Valley is a 400- km2 closed drainage basin located near the center of the Lake Bonneville basin. Relief in the comparatively small drainage basin ranges from 1316 m on the valley floor to 2019 m in the Lakeside Mountains along the valley's eastern boundary. Puddle Valley is completely surrounded by ( inset into) the Bonneville basin and for most of the last deep- lake cycle it was an integrated subbasin of Lake Bonneville, containing an arm of the great lake ( Gilbert, 1890; Currey et al., 1984). Shoreline evidence reveals that an independent Lake Puddle ( Currey, 1980) occupied the valley at least briefly after its re- isolation from Lake Bonneville ( Sack, 1995), but today there is no naturally occurring perennial or intermittent surface water in the valley. The lowest point on Puddle Valley's drainage divide lies in the unnamed pass through which the paved highway extends in the northeastern part of the valley. The modern elevation of the pass ( 1362 m) is below the modern local elevations of the Bonneville ( 1625 m), Provo ( 1501 m), and Stansbury ( « 1380 m) shorelines, and above the post- Bonneville Lake Puddle level ( 1338 m) ( Currey, 1982; Sack, 1995). Puddle Valley was incorporated into the Bonneville basin when transgressing Lake Bonneville flowed in from the north over this pass, into Puddle Valley. The inflow event is marked on the Puddle Valley side of the threshold by a 1.2- km long spillway that slopes to the south and by a distinctive landform, first noted by Currey ( 1980), that is preserved just beyond the southern end of the spillway. The Lake Bonneville inflow feature in Puddle Valley is a large- scale, tongue- shaped bar on which are found giant current ripples. The gravel bar, which is 1.5 km long and up to 1.2 km wide, consists of a stoss- side pan bordered by a continuous lateral and lee- side rim. Along the highway roadcut, the moderately asymmetrical giant current ripples range in height from 0.2 to 5.2 m, in length from 6 to 271 m, and in vertical form index ( L/ H) from 15 to 71. The ripples consist of gravel cross beds which dip 9° to 24° towards Puddle Valley. The A- axis of clasts sampled from 14 sediment pits dug in the bedforms averaged 10 cm, with the A- axis of the overall ten largest clasts averaging 35 cm. The surface of the bar complex is overlain by Lake Bonneville marl, clastic lacustrine fine- grained sediments, and postlacustrine eolian sandy silt. The dimensions of the current ripples, size of constituent clasts, and presence within the bedforms of rip- up fragments of pre- Bonneville lacustrine marl ( R. M. Forester, 1992, pers. comm.), tufa, and beachrock indicate that inflow was a high velocity, sudden event that eroded material from the threshold. Because evidence of the Stansbury shoreline complex, which formed between about 22 and 20 ka ( Oviatt et al., 1992), is found in Puddle Valley ( Sack, 1995), the valley must have become part of the Bonneville lake basin before, or early in, Stansbury shoreline time. Using the Lake Bonneville hydrograph ( Oviatt et al., 1992) and the Puddle Valley threshold elevation, corrected for postlake hydroisostatic rebound ( Currey and Oviatt, 1985), it is estimated that Lake Bonneville may have spilled into Puddle Valley and created the inflow bar as early as 25.8 ka, and that the two basins re- isolated no later than about 12.2 ka. References Cited Currey, D. R., 1980, Coastal geomorphology of Great Salt Lake and vicinity: Utah Geological and Mineral Survey Bulletin 116, p. 69- 82. Currey, D. R., 1982, Lake Bonneville: Selected features of relevance to neotectonic analysis: U. S. Geological Survey Open- File Report 82- 1070, 31 p. Currey, D. R., Atwood, G., and Mabey, D. R., 1984, Major levels of Great Salt Lake and Lake Bonneville: Utah Geological and Mineral Survey Map 73, scale 1: 750,000. Currey, D. R., and Oviatt, C. G., 1985, Durations, average rates, and probable causes of Lake Bonneville expansions, stillstands, and contractions during the last deep- lake cycle, 32,000 to 10,000 years ago, in Kay, P. A., and Diaz, H. F., eds., Problems of and Prospects for Predicting Great Salt Lake Levels: Salt Lake City, University of Utah Center for Public Affairs and Administration, p. 9- 24. Gilbert, G. K., 1890, Lake Bonneville: U. S. Geological Survey Monograph 1, 438 p. Oviatt, C. G., Currey, D: R., and Sack, D., 1992, Radiocarbon chronology of Lake Bonneville, eastern Great Basin, USA: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 99, p. 225- 241. Sack, D., 1995, The shoreline preservation index as a relative- age dating tool for late Pleistocene shorelines: An example from the Bonneville basin, U. S. A.: Earth Surface Processes and Landforms, v. 20, p. 363- 377. |
