High-resolution study of a preserved fluvial-to-marine succession for refinement of regional palaeogeography: example from the Neslen Formation, Utah, USA

Reconstructing palaeo-shoreline position in ancient successions is important for improved understanding of the dynamic sedimentology of the fluvial-to-marine transition zone (FMTZ); this has implications for reconstructing regional palaeogeography, and also applied value in predicting characteristics of down-dip reservoir units. The FTMZ can extend 10s to 100s of kilometres up-dip from the shoreline, though the full extent – from shallow-marine to fully fluvial strata – cannot necessarily be traced uninterrupted through the rock record. Here, we analyse an up-dip portion of an ancient preserved FMTZ in the Campanian Neslen Formation (Floy to Sagers Canyons, Book Cliffs, Utah) to estimate the approximate distance to the contemporaneous shoreline for different parts of the stratigraphy.One-hundred-and-six vertical sedimentary logs, totalling 3000 m, and 194 stratigraphic panels collectively detailing a 20 km-wide stratigraphic window have been used to undertake detailed facies and ichnological analyses in a part of the succession oriented perpendicular to the trend of the palaeoshoreline. The relative position of the shoreline through time has been tentatively established by assessing changes in marine influences, and sand-body geometry and type. Inferences of channel slope and bank-full depth have enabled estimation of backwater lengths (part of the river where the streambed drops below sea level) of 20 and 40 km for different intervals of the stratigraphy. Mapping the position of the shoreline onto the stratigraphy reveals additional coastline complexity not revealed by analysis of facies belts alone. This approach has identified a large marine-influenced embayment present in the late Campanian, previously thought to have been infilled by the time of deposition of the Neslen Formation. The novel multi-faceted approach employed to constrain the distance to the shoreline is important for provision of a refined regional palaeogeographic reconstruction.