--> Stratigraphic Heterogeneity in Fine-Grained Lacustrine Deltas in Pleistocene Lake Bonneville, Northern Utah and Southern Idaho, by D. R. Lemons; #90986 (1994).

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Abstract: Stratigraphic Heterogeneity in Fine-Grained Lacustrine Deltas in Pleistocene Lake Bonneville, Northern Utah and Southern Idaho

David R. Lemons

The Weber River (northern Utah) and Bear River (southern Idaho) delta systems were deposited on the eastern margin of Lake Bonneville. Both deltas consist primarily of fine-grained quartz sand and clay, with very little coarse material. The Weber River delta is wave-influenced in contrast to the Bear River delta, which is fluvial influenced. Both were deposited during the transgressive phase of Lake Bonneville (30-15 ka). The dioric nature of their drainage basins is partly responsible for their mode of deposition and geometry by providing fine-grained source material.

Facies architecture and detailed stratigraphy (80-m vertical sections) were constructed based on river and highway outcrop exposures. Sequence stratigraphy suggests that several episodes of delta progradation occurred with intervening periods of rapid lake level rise. The Weber River delta system exhibits at least two cycles with 10-25 m of lacustrine clays separating 3-10-m-thick deltaic sands. However, sediment influx was high enough during transgressive

events that lacustrine clays were never deposited directly at the river mouth.

Highest permeabilities are associated with the tabular-bedded, current-rippled, and wave-rippled sandy facies for both the Weber and Bear River deltas. However, rapid vertical and lateral facies changes introduce a high degree of heterogeneity.

Recognition of stratigraphic heterogeneities in these wave and fluvial-influenced lacustrine delta systems provides an exploration analog to other deltaic and lacustrine reservoirs where lake histories and basin geometries are not as well constrained.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994