Abstract

The Pleistocene Bear River delta is located in northern Cache Valley, north of Preston, Franklin County, Idaho where the Bear River emptied into Lake Bonneville. Steep cliff faces north of the river near the hamlet of Riverdale expose approximately 110 m of fine-grained sediment of the Bonneville Alloformation. The sediment was deposited between elevations 1400 and 1510 m, at least 40 m below the Bonneville Shoreline (1550 m). These elevations, combined with recent refinement of the timing of the Bonneville lake-cycle, suggest that the delta formed during the middle part of the rapid transgression of the lake, from ~20 to ~15.5 ka. This suggests a rather astounding punctuated sedimentation rate of 2.75 m per 100 years.

Application of the methods of sequence stratigraphy to the Bear River delta reveals twenty-one parasequences, grouped into parasequence sets and five stratigraphic sequences (6th-order, 1-2 ka) of the climatically forced transgressive phase of the lake-cycle. These are overlain, above the major unconformity formed during the Lake Bonneville flood, by a 6th sequence containing gravel of the regressive phase of the lake-cycle. The several sequence boundaries within the transgressive systems tract likely represent both lake level oscillations (falling-lake events) documented elsewhere and autocyclic lobe-switching during construction of the delta. Highstand systems tract strata, formed when the lake occupied the Bonneville shoreline (15.5 to 14.5 ka), are largely absent from the study area near Riverdale, since at this time the shoreline had transgressed 50 km up the Bear River canyon in Oneida Narrows, to southern Gem Valley.

The fall of the lake to the Provo shoreline during the Bonneville flood, and subsequent lower base levels, produced gravel strath terraces (lowstand systems tract) of the regressive phase of the Bonneville Alloformation, which are incised into underlying fine-grained sediment. In sequence stratigraphic terminology, these regressive phase deposits are separated from the underlying transgressive and highstand deposits by a sequence boundary that represents fall in base level and a diachronous period of erosion.