LATE HOLOCENE HIGHSTANDS OF GREAT SALT LAKE AT LOCOMOTIVE SPRINGS, UTAH

Locomotive Springs marsh complex, on the northern shore of Great Salt Lake, is built on and incised into an extensive plain underlain by marl of Pleistocene Lake Bonneville. Geomorphology, stratigraphy, fossils, and chemistry are combined to evaluate Holocene lake incursions, some of which can be chronologically constrained by dating organic marsh deposits. The marl plain is overlain by the ~10 ka (all ages are AMS uncalibrated ¹⁴C before present) Gilbert shoreline deposits (~4260 ft) and widespread marsh deposits with ages of 9.6 to 8.7 ka. Beach deposits of ooids and carbonate fragments lie on the early Holocene marsh deposits at ~4233 ft and form a complex of beaches ~4220 to~4215 ft altitude. The ~4233 beaches postdate underlying 9.4 ka marsh deposits and therefore are not part of the Gilbert lake system. The lower beaches, and platforms of carbonate mud that extend below them, are interbedded with organic muds dated between ~4.2 and ~1.5 ka. Our preliminary interpretations indicate the ~4220-ft beach complex was constructed during one or two events, between 4.2 and 2.1 ka. These lake inundations deposited two sand beds offshore from the beach. As many as three more sand beds apparently indicate more lake inundations. Above these sand beds lie ooid and carbonate mud platforms. A platform at ~4218 ft records four major lake depositional events with two intervening organic deposits. The carbonate muds are younger than 2.1 ka; apparently, as many as two lake deposits formed between 2.1 and 1.5 ka and two after 1.5 ka. Lake levels did not exceed 4220 ft during the last 500 years, based on preservation of an eolian silt cap and hearth. This interpretation of lake levels corresponds well with ¹⁸O data from Great Salt Lake cores, and suggests that late Holocene lake highstands were greatest between roughly 4.2 and 2.1 ka, lower from 2100 to 500 yr, and lower yet during the Little Ice Age. All lake episodes were probably of short duration, with long periods of marsh sedimentation between brief lake inundations.