Facies Architecture and Sequence Stratigraphy of Fine-Grained Lacustrine Deltas Along the Eastern Margin of Late Pleistocene Lake Bonneville, Northern Utah and Southern Idaho, David R. Lemons and Marjorie A. Chan

This study documents the effects of changing lake level (limnostasy), tectonics, sediment yield, and basin physiography on the facies architecture, sequence stratigraphy, and reservoir quality of three fine-grained deltas deposited along the eastern margin of late Pleistocene Lake Bonneville. Analysis of facies architecture indicates that the Weber and Spanish Fork deltas were strongly wavemodified because they were situated along openly exposed portions of the shoreline. The Bear River delta, nestled in a relatively isolated northeast arm of the lake, records both fluvial and wave processes. These fine-grained deltas were fed by low-gradient rivers that drained large regions that were sparsely glaciated, whereas other contemporaneous, coarse-grained “Gilbert” deltas were fed by steep-gradient rivers that drained local source areas that were strongly influenced by glaciers. Limnostasy, tectonics, and sediment yield were similar for all three fine-grained deltas, implying that the most inf luential forcing parameter on sequence stratigraphy is basin physiography. Basin physiography (specifically ramp length and accommodation) most strongly controlled the external and internal geometry of the lowstand systems tract of each delta. Potential hydrocarbon reservoir quality (e.g., grain size and sorting) is largely a product of 635 AAPG Bulletin, V. 83, No. 4 (April 1999), P. 635–665. ©Copyright 1999. The American Association of Petroleum Geologists. All rights reserved. 1Manuscript received October 28, 1996; revised manuscript received December 11, 1997; final acceptance July 27, 1998. 2Exxon Exploration Company, P.O. Box 4478, Houston, Texas 772104478; e-mail: [email protected] 3Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112. This project was funded by NSF grant EAR 9303519. A research grant from the Geological Society of America supported radiocarbon dating. We thank Mark Milligan, Kerry Barker, and Matt Rees for field assistance. Constructive criticisms by W. B. Harris, K. Kelts, R. G. Stanley, and Kevin Bohacs much improved the original manuscript. Facies Architecture and Sequence Stratigraphy of Fine-Grained Lacustrine Deltas Along the Eastern Margin of Late Pleistocene Lake Bonneville, Northern Utah and Southern Idaho1 David R. Lemons2 and Marjorie A. Chan3 drainage basin size and stream gradient. Relative differences in ramp lengths seemed to determine thickness and lateral continuity of delta front deposits. Relative differences in accommodation appeared to determine the internal geometry of the delta front deposits, especially in the lowstand systems tract. These deltas can serve as analogs for lacustrine exploration and production where many forcing parameters typically are unknown.