Tidal sedimentology and estuarine deposition of the proterozoic Big Cottonwood Formation, Utah

Mesoproterozoic to Neoproterozoic Big Cottonwood Formation of north-central Utah contains some of the oldest known (similar to 900 Ma) examples of cyc lic tidal rhythmites, Despite mild metamorphic overprinting, there is excel lent preservation of sedimentary structures. The thick formation (4.8 km) h as been previously interpreted as a shallow-water, intracratonic basin depo sit, Five distinct facies are recognized. Two quartz arenite facies are dom inated by dune (meter-scale) cross-bedding recording westward flow, but dis tinguished by different large-scale geometries: (1) thick, tabular bodies ( 10-20 m thick by 600+ m long) and (2) channeled beds (0.3-0.3 m thick by hu ndreds to thousands of meters long). Channeled beds have scoured bases and coarse-grained lags or rip-up clasts, Stacked channel beds form upward-fini ng successions up to 50 m thick. Three distinct argillite facies contain di fferent structures and cyclicities, and are also characterized by color. Da rk, laminated argillites contain abundant heterolithic rhythmites (with thi ck couplets up to 1 cm per lamina), syneresis cracks, dame structures, inte rnal truncation scours, intraformational blocks/clasts, and diagenetic pyri te. The other two argillite facies commonly occur overlying channelized qua rtzites in large-scale upward-fining successions, A transitional argillite composed of thin intertidal beds grades upward into mud cracked argillite o f massive to weakly bedded intertidal to supratidal beds characterized by w ave ripples and abundant mud cracks. Important diagnostic tidal features re cognized in the Big Cottonwood Formation include: (1) heterolithic tidal rh ythmites, (2) current ripples with crests rounded by backflow, (3) sigmoida l bundles, and (4) abundant clay-draped reactivation surfaces. Other struct ures that corroborate the tidal interpretation include: (1) flaser bedding, (2) mud cracks, and (3) mud-draped wave ripples. The sedimentary structures and genetic sequence relationships recognized in this study suggest deposition in a tide-dominated estuary, The laminated a rgillites with heterolithic rhythmites indicate subtidal deposition in tida l channels. Sand filled tidal channels are represented in stacked quartzite beds that grade to transitional argillite and thin successions of mud crac ked argillite. Thicker units of mud cracked argillite may represent deposit ion in tidal flats with periodic exposure. Tabular sheet quartzites suggest deposition as sand sheets near the mouth of the estuary. Our documentation of tidal rhythmites and estuarine deposition suggests previously unrecogni zed Mesoproterozoic to Neoproterozoic tidal deposition in north central Uta h.