Abstract

Sandy and gravelly lacustrine deltas deposited along the eastern margin of late Pleistocene Lake Bonneville record the complex interplay of changing lake level, tectonics, sediment yield, and basin physiography. In contrast to contemporaneous gravelly “Gilbert” deltas influenced by steep gradients of locally sourced rivers, some with extensively glaciated drainage basins, sandy deltas resulted from lower gradients of regionally sourced rivers with sparsely glaciated drainage basins.

The sandy Weber and Spanish Fork deltas are wave-influenced, situated along openly exposed portions of the shoreline. The sequence stratigraphy of the Weber River delta closely resembles that of a passive margin. The sequence stratigraphy of the Spanish Fork delta was strongly affected by lack of accommodation and the smaller size of the delta. The sandy Bear River delta, nestled in a relatively isolated northeast arm of the lake, was influenced by both wave and fluvial processes. The sequence stratigraphy of the Bear River delta was heavily influenced by its long, low-gradient ramp geometry.

Two end member gravelly delta systems are distinguished: (1) topset-dominated deltas at the Bonneville level comprising the transgressive/highstand systems tract and (2) foreset-dominated deltas at the Provo level comprising the lowstand systems tract. Where valley glaciers extended to the canyon mouths, extensive horizontal subaerial outwash gravels were deposited at the Bonneville level. Where valley glaciation was limited to the mid to upper reaches of the canyon, horizontally stratified subaqueous gravel was deposited to form a Bonneville transgressive delta. A catastrophic 100-m drop in the lake level, followed by a climate-driven regression, caused reworking of Bonneville-level deltaic sediments and the development of gravel foresets in the lowstand systems tract.

Accommodation, as controlled by both rapid lake level fluctuation and basin physiography, played the largest role in the resulting sequence stratigraphy for all deltas of this study. This is manifested in the variability of lowstand systems tracts of the sandy deltas and the horizontal topset/basinward-dipping forset geometry of the coarse-grained deltas. Allocyclic controls of climate, sediment supply, and tectonism were more important than autocyclic changes in producing each delta’s stratigraphy.

Sequence boundaries do not coincide with individual deep lake cycles. As an alternative way of packaging these sediments, allostratigraphy is a reasonable and descriptive way to divide such basin-fill successions. However, it lacks the predictive potential of sequence stratigraphy.