Abstract

The Three Forks Formation, composed of intrashelf deposits, is the chronostratigraphic equivalent of the greater Palliser Formation (Wabamum Formation in the subsurface, western Canada) ramp and shelf deposits. These sub-equatorial Famennian (Late Devonian) formations represent deposition on one of the most extensive epicontinental carbonate systems. Three main environments constitute the Three Forks Formation intrashelf deposits described as facies associations (FA): 1) storm-influenced shallow shelf, 2) arid shallow shelf, and 3) storm-influenced mudflats.

Storm-influenced shallow shelf (FA 1) deposits contain three main facies: F1, distorted claystones that contain a moderate abundance of mobile feeding traces and syneresis cracks; F2, laminated mudstones; and F3, dolomudstones that include mobile feeding traces, escape burrows, and opportunistic suspension-feeding colonies. These facies demonstrate abundant sharp layer contacts which imply scouring, high-depositional-rate sedimentary structures, and wave-formed features. Rare facies of FA 1 include: F4, brecciated to distorted mudstones, syndepositional deformation through dewatering, evaporite precipitation and dissolution, and bioturbation; and F5, thin-bedded quartz sandstones, representing preserved and unmixed floods into the shallow basin. Arid shallow shelf (FA 2) deposits consist of: F6, mosaic anhydrites, which imply periods of restriction and evaporation at a basinal scale. Storm-influenced mudflat (FA 3) facies include: F7, rarely preserved, seiche-deposited thin laminated mudstones; F8, abundantly preserved clast-supported breccias; and F9, massive or intraclastic mudstones. These deposits represent a spectrum of strength and influence of mudflat storm surge that locally transported mudstone clasts derived from FA 1.

Mechanical sedimentary structures and trace fossils in all facies indicate a depositional system dominated by storm-generated waves and currents in both the shallow intrashelf and mudflats. The intrashelf was also influenced by marine and freshwater incursions to form a mixed carbonate, evaporite, and siliciclastic system. The results of this study contribute to the global knowledge of epicontinental systems, specifically in the storm-influenced shallow intrashelf and mudflats.