A COMPREHENSIVE BIOSEDIMENTOLOGICAL CHARACTERIZATION OF MICROTERRACETTES THAT FORMED IN THE PRESENCE OF CALOTHRIX-DOMINATED MATS IN QUEEN’S LAUNDRY, YELLOWSTONE NATIONAL PARK, USA

Siliceous hydrothermal spring deposits are of interest in the search for ancient and extraterrestrial evidence of extremophile bacterial life. Such deposits, known as siliceous sinters, display distinctive fabrics indicative of their formation in the presence of microbial communities. The focus of this research is the distinct lithofabric that forms in association with Calothrix-dominated microbial mats at Queens Laundry, a silica-depositing hot spring in Yellowstone National Park. Calothrix are sheathed, filamentous cyanobacteria (~10-12μm diam.) that occur where microterracettes form at the distal end (temperature ≤40°C) of the hot spring drainage apron. Microterracettes are stepped terraces of siliceous sinter and microbial mats. Two microterracette sample types were collected, one inside and one outside the main outflow channel of Queen’s Laundry hot spring. Over a 4 year period of time, the outermost sample collection area was always observed to be dry, having formed at least prior to 2009. Microscopic observations revealed two types of microterracette profiles: high relief and low relief. High relief microterracettes have a vertical relief at the surface greater than 5 mm and contain prominent shrub-like structures within the microterracette profile. Low relief microterracettes have a vertical relief at the surface of less than 3 mm. The high relief microterracettes are characterized by a distinct succession of lamina: successive structures in overlying lamina appear increasingly domical and overall lamina height decreases upward. In addition, each lamina in the microterracettes were observed to have a capping silica deposit that produces a distinct white band in the siliceous sinter record of the hot spring. The capping silica deposit, which records the accretionary profile of the deposit, preserves detailed morphological characteristics of the shrubs and other structures. The capping silica remains intact through decades of time. The combination of shrubs with overlying successive domical structures, decreased lamina height, and a vertical relief profile greater than 5mm in a siliceous sinter are important in identifying these types of organosedimentary structures in older diagenetically altered siliceous hot spring deposits.