By Michael D. Vanden Berg, Utah Geological Survey
The Sedimentary Record, Vol. 17, No. 1, March 2019
Two recent events have put Great Salt Lake (GSL) in northern Utah at the forefront of microbialite research. First, massive oil accumulations were discovered in the mid-2000s in offshore South Atlantic “pre-salt” deposits of Cretaceous lacustrine carbonates, including purported microbialites. Petroleum geologists working the pre-salt reservoirs fanned the globe looking for analogs to better understand lacustrine systems and the unique highly permeable and porous deposits called microbialites. At about the same time, GSL experienced record low levels not seen since the early 1960s, exposing one of the world’s largest Holocene accumulations of lacustrine microbialites. As a result, GSL quickly became a must visit locale for petroleum geologists. In light of this new international interest, researchers have sought to better understand GSL microbialites―their age, formation mechanisms, distribution, and relationship to other lake facies. This paper provides an introduction to the basic morphology of these unique structures and how local environmental conditions, as well as periods of exposure and erosion, contribute to growth location, grouping, shape, size, orientation, and internal structure. Several other research groups are exploring other important aspects including mineral precipitation mechanisms (Bouton et al., 2016; Pace et al., 2016), biogeochemistry/microbiology (Lindsay et al., 2016; Baxter, 2018), and possible age of formation and paleoenvironmental record (Newell et al., 2017; Vennin et al., 2019).
GSL is the remnant of Pleistocene (32-12 ka) Lake Bonneville, which covered 52,000 km2 of northwestern Utah as well as small parts of northeastern Nevada and southeastern Idaho (Gwynn, 1996). Lake Bonneville first retreated due to a catastrophic flood into the Snake River Plain, but then the changing climate (warmer and drier) further reduced its size, leaving behind present-day, hypersaline GSL. GSL averages 121 km long and 56 km wide, covering 4100 km2 , and fills the lowest depression in the terminal Bonneville basin (Fig. 1). The volume of water in the lake varies both annually and seasonally depending on catchment precipitation, whereas water loss is primarily due to evaporation (~3600 hm3 per year; Gwynn, 1996). GSL surface elevation has fluctuated nearly 6 m over recorded history (since 1847), with a long-term elevation average of ~1280 m (4200 ft) above mean sea level (Fig. 1, inset). GSL is shallow, maximum depth is ~10 m, and has broad lowgradient shorelines (Fig. 1). These shallow nearshore areas are favorable for microbialite formation but are also subject to exposure as lake levels fluctuate. In the late 1950s, a gravel-filled railroad causeway was constructed across the lake, isolating the north arm from the rest of the lake (Fig. 1). With none of the four major rivers entering the north arm, the salinity climbed to 24-26% (near halite saturation), whereas the salinity of the south arm is 12-14% and probably more representative of Holocene conditions. Post-Bonneville Holocene lake level fluctuations are poorly understood (Murchison, 1989), but measured lake level records reach back to 1847 (Fig. 1, inset). With some exceptions, it is generally assumed that Holocene (since ~12 ka) and historic lake level fluctuations were similar in magnitude and frequency, notwithstanding the anthropogenic influences that have contributed to the more recent low lake level (Wurtsbaugh, 2016). One exception may be the warm/dry period during the mid-Holocene Climatic Optimum (~8-6 ka), in which the lake might have dropped to 6 m below the historic average (Murchison, 1989; Steponaitis, 2015). Two previous decade long periods where lake levels receded below 1278.6 m (4195 ft), exposing the GSL microbialites, were initiated in 1935 and 1960 (Fig. 1, inset). Eardley (1938) provided the earliest definitive work on “algal bioherms” and associated deposits, including the importance of bacteria in their formation. Carozzi (1962) and Post (1980) described GSL “algal biostromes” and the precipitation of calcium carbonate by “blue-green algae,” and Halley (1976) investigated the textural variations within GSL “algal mounds.” As a result of the more recent low lake levels, Lindsay et al. (2016) researched the living microbial communities and their abilities to survive in a hypersaline environment, while Baskin (2014) attempted to characterize the lake-wide distribution and depth of GSL microbial “bioherms.” In addition, Chidsey et al. (2015) and Della Porta (2015) looked more closely at GSL microbialite characteristics and facies associations. Moreover, a possible older generation (~12 ka) of GSL microbialites are present at higher elevations (1281.7-1284.7 m, 4205-4215 ft; not further discussed). Examples include the well-lithified microbialites, with associated multimeter-scale travertine mounds, near Lakeside (Homewood et al., 2018) and the heavily eroded remnants of microbialites near Rozel Point (Chidsey et al., 2015).