FACIES ANALYSIS AND COMPARISON OF JURASSIC, QUATERNARY, AND MODERN CARBONATE LAKE AND SPRING DEPOSITS OF TEN MILE GRABEN, EAST CENTRAL UTAH

Ten Mile Graben, Utah, provides the unique opportunity to document cold-spring deposits within the same fault-controlled spring system, from actively precipitating deposits to progressively older rock units (< 400 ka). These carbonates are found near Green River, Utah, as a series of actively flowing springs and fossil tufa terraces. An upper Jurassic limestone unit has also been discovered nearby, forming cliffs (3-10 m thick) that cap the very top of the Brushy Basin Member (148 Ma) in an area ~10 km². The Brushy Basin Member (upper Morrison Formation) is primarily volcaniclastic shales deposited under a semi-arid climate in a fluvio-lacustrine setting. The limestone described herein is the first and only major carbonate unit (> a few centimeters) to be documented within this member.

The purpose of this study is to investigate the microfacies characteristics of this Jurassic carbonate unit and compare features, environmental interpretations, and diagenetic histories with those of the Quaternary and modern spring carbonates. This research is part of a larger study investigating the diagenesis of terrestrial carbonates and their associated biosignatures, to aid in future investigations of ancient life on Earth and the search for evidence of past life on Mars. Springs and spring-fed lakes are ideal sites for biosignature research since they host diverse microbes, are possible locations of early evolution of life on Earth, and have been proposed as ancient depositional environments on Mars. Preliminary interpretations favor the hypothesis that the Jurassic limestone was deposited beneath a laterally restricted spring-fed lake. It appears to be mostly massive in outcrop with some intervals of nodular columns (interpreted as lacustrine pinnacles) and laminated limestones (interpreted as microbialites). Thin section microscopy reveals features that indicate that lake levels changed through time, leading to deposition of lacustrine carbonate facies (e.g. ostracod/charophyte wackestones, microbial silica-rich laminated boundstones) and calcrete paleosol facies (e.g. rounded quartz sand micritized wackestones). Modern and Quaternary tufa terraces host facies including carbonate microbial laminated boundstones and veins of crystalline fan cementstones.