The Paleoecology and Geomorphology of Holocene Deposits of the Southern Malad River, Box Elder County, Utah

Widespread Lake Bonneville sediments have been modified by river aggradation and degradation associated with Holocene fluctuations in the Great Salt Lake. Exposures of exceptionally abundant and well-preserved molluscan deposits in the Bear River Valley, Utah, allow detailed paleoenvironmental reconstruction of Holocene environments. The exposed basal unit consists of largely unfossiliferous deltaic silts and clays deposited during Lake Bonneville time ( roughly 11,000 - 13,000 yr B. P.). An unconformity representing at least 2000 yr separates the deltaic material from overlying highly fossiliferous stream sands. Eight species of molluscs, comprising a single community, occupied this low energy stream environment at 7690 ± 270 14C yr B. P. A second unconformity separates these sands from a dark brown silt unit deposited by a river-associated environment, most likely an over-bank marsh, at 2420 ± 135 14C yr B. P. Nine species of molluscs, comprising 3 communities, were present in this environment. Analysis of size-frequency distributions, percentage of pelecypod valves, preservation, and orientation of the shells that were present in each environment suggests that the 7690 ± 270 14c yr B. P. fossil assemblage has been only slightly altered by biostratinomic processes. The younger assemblage has also been altered, with the size-frequency curves of the smallest gastropods displaying normal distributions. Geomorphic and stratigraphic data from the Malad River show that water levels in the Great Salt Lake twice rose and fell significantly during the Holocene epoch. The oldest rise, to an altitude of at least 1288 m, occurred before 7690 ± 270 yr B. P., perhaps in response to a worldwide period of climatic cooling. This high-stand was followed by a fall of lake level roughly corresponding to the classic Hypsithermal Interval, about 7000 - 5000 yr B. P. A second rise occurred by 2420 ± 135 yr B. P., when the Great Salt Lake rose to approximately 1286 m. During this second rise, the Malad River overflowed its levees and later, as the Great Salt Lake receded for a second time, the river was captured by a headward-cutting tributary of the Bear River. The regional distribution of the fossiliferous deposits was controlled by the time at which capture occurred. Capture of the Malad channel by the Bear River occurred after the last fossiliferous sediments were deposited; thus no fossils are found downstream from the point of capture