ARCHITECTURE OF A TIDE-DOMINATED SHOREFACE SYSTEM IN THE LOWER CRETACEOUS KOOTENAI FORMATION, WEST-CENTRAL MONTANA

Integrated long-distance, high-resolution photography and detailed outcrop analysis were used to document the architecture of a tide-dominated, low wave-energy, shoreface lithosome along the Missouri River Gorge near Great Falls, Montana.

The lithosome (~18 m thick) consists of five vertically stacked, widespread, tabular units and scattered channel bodies. Internally, the tabular units (~2-4 m thick) coarsen upwards and the architecture of a representative unit includes a thin basal mudstone or heterolithic interval changing upward to a thicker bedded, amalgamated, fine- to rarely medium-grained sandstone. However, the full vertical succession of five units are stacked in a net upward-coarsening succession in which the basal unit is mudstone dominated and successive units contain less mud. Sand fractions are quartzose, very well rounded, and very well sorted. Rhythmic bedding is characteristic and ranges from horizontal to convex on the tidal bar scale (dm wavelength). Scattered medium-scale trough cross-stratification and thin channel-shaped lenses (~1-15 m wide) indicate episodic current activity. Widespread erosional surfaces separate the units, as well as occur internally, causing localized discordances between units or the amalgamation of sandstone-dominated units. The channel bodies are relatively large (50-100's m wide, 5-10 m thick), contain heterolithic fill similar in composition to the tabular units, and transect one or more of the tabular units, documenting the coexistence of cross-shore channels.

Diagnostic structures of tidal origin include mud couplets, neap-spring lamination sets, flaser-to-lenticular bedding, amalgamated ripple beds separated by reactivation surfaces, rhythmically alternating ripple beds and parallel-laminations, mm-scale clay drapes, and lateral sets of small-scale ripple bundles. Small-scale wave-ripples (~ 4 cm spacing) are present on some surfaces and hummocky cross-stratification is exceptionally rare. Trace fossils include Arenicolites, Ophiomorpha (very rare), Macaronichnus, Planolites, Piscichnus, horseshoe crab-like resting traces, and bivalve crawling traces indicating marine to slightly brackish conditions. Bioturbation index typically ranges between 3 and 6. A Glossifungites ichnofacies caps the succession.