Great Salt Lake

Great Salt Lake (—4800 km2, 8 m deep) is a remnant of the 300-m deep Lake Bonneville (70,000 km2). Seismic profiling began with sparker and air-gun surveys in 1968-69 to investigate basinal tectonics (Mikulich and Smith, 1974). These surveys identified major fault traces and Quaternary fill tilted gently eastward. They mapped the extent of a mirabilite bed buried beneath 8 m of soft muds (Fig. 8); this bed was drilled earlier during a causeway construction and dated at >11,600 14C B.P. by bulk carbonate radiocarbon. During Amoco oil exploration in the 1970s, the lake was surveyed extensively with deep-penetration seismic sources.

In conjunction with an expedition in 1979, 127 km of 3.5-kHz profiling was used to site and correlate piston cores (Spencer et al., 1984). These profiles (Fig. 8) showed a distinctive late Quaternary seismic stratigraphy with a strong, uniform reflection (LB) found 2-8 m subbottom, and correlated with the Lake Bonneville stage. Post-Bonneville sediment packages show subparallel, and some irregular reflections with cut-outs, lensing, or buried hummocky surfaces (Great Salt Lake series) that are consistent with generally shallow, saline lake sediments identified in cores. The profiles suggest lower to mid-Holocene low stand(s), yet undated, that correlate with bioherms rooted on a planation surface

FIGURE 8 (A) Example of a 3.5-kHz profile (13:04) from Great Salt Lake illustrating midlake section from southern basin (1) neotectonic faulting of older lake beds (OL horizon) beneath a Lake Bonneville maximum (reflection LB); (2) Holocene layered bedding (horizon GSL) above reflection LB; (3) a mid-Holocene, low-stand planation (reflection h) correlates with biohermal mounds (BH1, BH2) overlying fault traces. (B) Profile transition into north basin showing (1) Carrington Island-Promontory ridge with a carbonate-cemented biohermal pavement (mid-Holocene?) over parallel reflections of horizon GSL and OL below reflection LB. (2) A major fault runs along the northern ridge margin with evidence of disrupted bedding from strong recent movements. (3) The northern basin is characterized by a thick layer of (lower Holocene?) mirabilite (NaSO4-10H2O) buried beneath ca. 7-8 m of Holocene sediments (GSL [Great Salt Lake] series). The insert map shows intrabasin profile locations A and B, locations of faults identified by Mikulich and Smith (1974) marked by heavy dotted lines, and an outline of the northern mirabilite basin in light dashes.

FIGURE 8 (A) Example of a 3.5-kHz profile (13:04) from Great Salt Lake illustrating midlake section from southern basin (1) neotectonic faulting of older lake beds (OL horizon) beneath a Lake Bonneville maximum (reflection LB); (2) Holocene layered bedding (horizon GSL) above reflection LB; (3) a mid-Holocene, low-stand planation (reflection h) correlates with biohermal mounds (BH1, BH2) overlying fault traces. (B) Profile transition into north basin showing (1) Carrington Island-Promontory ridge with a carbonate-cemented biohermal pavement (mid-Holocene?) over parallel reflections of horizon GSL and OL below reflection LB. (2) A major fault runs along the northern ridge margin with evidence of disrupted bedding from strong recent movements. (3) The northern basin is characterized by a thick layer of (lower Holocene?) mirabilite (NaSO4-10H2O) buried beneath ca. 7-8 m of Holocene sediments (GSL [Great Salt Lake] series). The insert map shows intrabasin profile locations A and B, locations of faults identified by Mikulich and Smith (1974) marked by heavy dotted lines, and an outline of the northern mirabilite basin in light dashes.

overlain by a meter or so of parallel lake beds that mirror the modern, smooth, flat bottom. A link with fault locations suggests the interaction of groundwater inflow and the formation of single constructional mounds, 20 m to ca. 80 m wide.

While not extensive, the profiles in the south basin cross numerous closely spaced faults with 0.5- to 1-m scale displacements. Some scarps break the surface, and others are visible only by the detection of drape folds beneath the LB reflection (>18,000 years B.P.; Fig. 8A). The general fault orientation runs parallel to Antelope Island, and at two sites it conforms to surface traces of deep faults F7 and F8 defined by Mikulich and Smith (1974). A major fault oblique to this trend, the Carrington-Promontory Fault (CPF), shows major recent disruption (Fig. 8B). When lake levels were about 4 m lower than they are today, the north and south basins could have been separated by a nearly emergent Carrington-Promontory Ridge covered in a veneer of a polygonal biohermal pavement (Fig. 8B).

Penetration north of the CPF was stopped ca. 6.75 m subbottom by a high-impedance, horizontal reflection M that is interpreted as a massive mirabilite salt bed described by Eardley (see Spencer et al. 1984). It thins to the north and west where OL reflections again are visible below the salt layer. Above reflection M is a 2-m series and reflection TM, which was hit by core GSL 79-09, confirming mirabilite salt and aragonitic mud interlay-ers. A controversy continues about whether this salt layer may potentially be seismic evidence of aridity during the Younger Dryas chronozone.

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