Abstract
Lower Pahranagat Lake (LPAH) is a shallow alkaline carbonate lake located in south-central Nevada. Using a 4m core spanning 2300yr, Hickson et al. (2016) established LPAH as a potential analogue for ancient microbialites in the Horse Spring Formation (HSF) located in the Lake Mead region. Our current goal is to look for signatures of modern microbialites in the LPAH sediments, and characterize the depositional environment in which they formed. To do this, we retrieved two approximately 13m long cores from LPAH in 2016. These cores extend our record to approximately 5500yr based on previous radiocarbon age-dating done by Wigand (1997). Here we report our findings on the sedimentology, sediment fabrics, and mineralogy of this extended core record. We did facies analyses on our cores including visual core descriptions, smear slide microscopy, thin section petrography, and XRD and SEM. We defined four major facies of the cores: seed pod, blue-grey mud, massive, and indistinct laminae. These facies are characterized by carbonate muds with different textures and amounts of gyrogonites. Gyrogonites are common throughout the core. Their presence represents a higher, more permanent lake level. We also found two other less prominent facies: distinct laminae facies and organic-rich facies. The distinct laminae facies are very distinct 1-2mm thick laminations of carbonate mud, and are only seen in about 10cm of the cores. The laminae could be microbial mats, indicating the formation of stromatolites. The organic-rich facies ( approximately 38% TOC) is found in the lowermost 50cm of the core and indicates a distinct environment. Carbonate accounts for up to 80% of the sediments. To better characterize the carbonate muds of the core, we did XRD analyses and found mainly high-Mg calcite and dolomite, with low-Mg calcite being less prominent. An interesting finding was establishing the presence of microbial signatures through a new face-freezing technique, in which we made thin sections of the wet sediment and preserved the fragile fabrics of the core. We looked at the thin sections under petrographic microscope for textures that were similar to the textures seen in ancient stromatolites and thrombolites. Finding and characterizing microbial signatures in our cores can help us make inferences about how the ancient microbialites found in the HSF formed and their environment.