A railroad causeway across Great Salt Lake, Utah (GSL), has restricted water flow since its construction in 1959, resulting in a more saline North Arm (NA; 24%-31% salinity) and a less saline South Arm (SA; 11%-14% salinity). Here, we characterized microbial carbonates collected from the SA and the NA to evaluate the effect of increased salinity on community composition and abundance and to determine whether the communities present in the NA are still actively precipitating carbonate or if they are remnant features from prior to causeway construction. SSU rRNA gene abundances associated with the NA microbialite were three orders of magnitude lower than those associated with the SA microbialite, indicating that the latter community is more productive. SSU rRNA gene sequencing and functional gene microarray analyses indicated that SA and NA microbialite communities are distinct. In particular, abundant sequences affiliated with photoautotrophic taxa including cyanobacteria and diatoms that may drive carbonate precipitation and thus still actively form microbialites were identified in the SA microbialite; sequences affiliated with photoautotrophic taxa were in low abundance in the NA microbialite. SA and NA microbialites comprise smooth prismatic aragonite crystals. However, the SA microbialite also contained micritic aragonite, which can be formed as a result of biological activity. Collectively, these observations suggest that NA microbialites are likely to be remnant features from prior to causeway construction and indicate a strong decrease in the ability of NA microbialite communities to actively precipitate carbonate minerals. Moreover, the results suggest a role for cyanobacteria and diatoms in carbonate precipitation and microbialite formation in the SA of GSL.

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A sinuous zone of gently southeast-dipping low-angle Tertiary normal faults is exposed for 100 km along the eastern margins of the Anaconda and Flint Creek ranges in southwest Montana. Faults in the zone variously place Mesoproterozoic through Paleozoic sedimentary rocks on younger Tertiary granitic rocks or on sedimentary rocks older than the overlying detached rocks. Lower plate rocks are lineated and mylonitic at the main fault and, below the mylonitic front, are cut by mylonitic mesoscopic to microscopic shear zones. The upper plate consists of an imbricate stack of younger-on-older sedimentary rocks that are locally mylonitic at the main, lowermost detachment fault but are characteristically strongly brecciated or broken. Kinematic indicators in the lineated mylonite indicate tectonic transport to the east-southeast. Syntectonic sedimentary breccia and coarse conglomerate derived solely from upper plate rocks were deposited locally on top of hanging-wall rocks in low-lying areas between fault blocks and breccia zones. Muscovite occurs locally as mica fish in mylonitic quartzites at or near the main detachment. The 40Ar/39Ar age spectrum obtained from muscovite in one mylonitic quartzite yielded an age of 47.2 + 0.14 Ma, interpreted to be the age of mylonitization. The fault zone is interpreted as a detachment fault that bounds a metamorphic core complex, here termed the Anaconda metamorphic core complex, similar in age and character to the Bitterroot mylonite that bounds the Bitterroot metamorphic core complex along the Idaho-Montana state line 100 km to the west. The Bitterroot and Anaconda core complexes are likely components of a continuous, tectonically integrated system. Recognition of this core complex expands the region of known early Tertiary brittle-ductile crustal extension eastward into areas of profound Late Cretaceous contractile deformation characterized by complex structural interactions between the overthrust belt and Laramide basement uplifts, overprinted by late Tertiary Basin and Range faulting.

Protodolomite was detected in sediments from Great Salt Lake (GSL), Utah, United States, that have no history of elevated temperature or pressure, conditions that are thought to promote dolomitization of sedimentary carbonates. Protodolomite was abundant in a non-oolitic sediment core from the South Arm (SA) of GSL at < 17% salinity but was rare in an oolitic sediment core collected from the North Arm (NA) of GSL at > 26% salinity. Protodolomite was also abundant in a non-oolitic NA sediment hand sample yet was absent in a nearby oolitic sediment hand sample in locations that likely receive allochthonous nutrients. Protodolomite was not detected in benthic photosynthetic mats from the SA. However, the mats comprised aragonite with halite and minimal calcite; benthic photosynthetic mats do not form in the NA. To begin to identify potential controls on the formation of protodolomite in the SA and NA of GSL, the composition and abundance of 16S rRNA gene transcripts, carbon cycling activities, and porewater geochemistry of the sediment cores were characterized. Transcripts affiliated with a dominant halophilic, heterotrophic sulfate-reducing bacterium were detected in the uppermost sections of the SA core and their abundance was positively correlated with rates of acetate oxidation/assimilation and concentrations of sulfide. Differences in the quality of organic matter between the SA and NA cores, as indicated by carbon to nitrogen ratios, indicate fresh deposition of photosynthetic biomass at the SA sediment core site but not in the NA sediment core site. Sediment grains from the SA core exhibit micrometer-sized euhedral protodolomite crystals that were not detected in the NA core. Collectively, these observations suggest that deposition of photosynthetic biomass drives the development of a sharp, anoxic lens of heterotrophic sulfate reduction. Sulfide, in turn, may promote dehydration of Mg 2+-water complexes and primary protodolomite nucleation and growth. The scarcity of dolomite in the NA sediment core may result from constraints imposed by a combination of extreme hypersalinity and depositional environment on phototrophs and sulfate reducers, their activities, and the thermodynamics of protodolomite formation.

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