ABSTRACT

The Mesoproterozoic to Neoproterozoic Big Cottonwood Formation of north-central Utah contains some of the oldest known ( 900 Ma) examples of cyclic tidal rhythmites. Despite mild metamorphic overprinting, there is excellent preservation of sedimentary structures. The thick formation (4.8 km) has been previously interpreted as a shallow-water, intracratonic basin deposit. Five distinct facies are recognized. Two quartz arenite facies are dominated by dune (meter-scale) cross-bedding recording westward flow, but distinguished 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 hundreds to thousands of meters long). Channeled beds have scoured bases and coarse-grained lags or rip-up clasts. Stacked channel beds form upward-fining successions up to 50 m thick. Three distinct argillite facies contain different structures and cyclicities, and are also characterized by color. Dark, laminated argillites contain abundant heterolithic rhythmites (with thick couplets up to 1 cm per lamina), syneresis cracks, flame structures, internal truncation scours, intraformational blocks/clasts, and diagenetic pyrite. The other two argillite facies commonly occur overlying channelized quartzites in large-scale upward-fining successions. A transitional argillite composed of thin intertidal beds grades upward into mud-cracked argillite of massive to weakly bedded intertidal to supratidal beds characterized by wave ripples and abundant mud cracks. Important diagnostic tidal features recognized in the Big Cottonwood Formation include: (1) heterolithic tidal rhythmites, (2) current ripples with crests rounded by backflow, (3) sigmoidal bundles, and (4) abundant clay-draped reactivation surfaces. Other structures 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 argillites with heterolithic rhythmites indicate subtidal deposition in tidal channels. Sand-filled tidal channels are represented in stacked quartzite beds that grade to transitional argillite and thin successions of mud-cracked argillite. Thicker units of mud-cracked argillite may represent deposition 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 unrecognized Mesoproterozoic to Neoproterozoic tidal deposition in north-central Utah.