Onshore groundwater spring carbonate mounds to lacustrine microbialites, the perplexing record of a transitional Great Salt Lake carbonate shoreline at Lakeside, Utah

Non‐marine carbonates at Lakeside (Utah) are the depositional record of transitional shoreline palaeoenvironments around 1,285 m, as lake elevation fell from Lake Bonneville levels to low‐stand Great Salt Lake levels of 1,284–1,278 m. Lakeside carbonates provide a rare example of onshore groundwater spring deposits connected to lacustrine microbialite reefs. Clotted fabrics and mineralized bacteriomorphs suggest microbial influence both in groundwater spring and littoral carbonates. Thrombolitic clotted aragonite and dendritic clotted intermediate magnesium calcite may document two distinct microbial ecosystems each with a specific mineralogy. Radiocarbon dates from interior to outer layers of a 20 cm littoral microbialite suggest several thousand years of domal growth. Climate fluctuations may be recorded in littoral deposits by the alternation of ooids (increased salinity, arid conditions) with ostracods (fresher water, humid conditions). Successive higher frequency cycles of sedimentation, exposure, microkarst and early diagenesis were cemented by vadose and phreatic very high magnesium calcite and non‐stoichiometric dolomite. Several layers of littoral and beach deposits accumulated during longer lacustrine transgressive–regressive cycles. Clastic detritus and lack of known initial U and Th concentrations could explain the lack of correlation between widely scattered U‐Th ages and closely grouped 14 C dates. Calibrated ages from radiocarbon dating may have been increased by long residence time of groundwater in bedrock, since 87 Sr/ 86 Sr signatures do indicate possible littoral‐zone mixing of lake water with groundwater from outcropping and subsurface Mississippian Great Blue Limestone. The perplexing Lakeside carbonate system is the result of interaction between microbial and chemical processes during sedimentation and early diagenesis.