Re-evaluation of depositional environments of the Neslen Formation from subsurface and surface data, Book Cliffs, Utah

Brianne Spear and Mark Kirschbaum

The Upper Cretaceous Neslen Formation contains two coal-bearing members divided stratigraphically by an interval that includes the Thompson Canyon and Sulphur Canyon Sandstone beds, which are composed of several continuous, well-sorted sandstones that are observable for miles on outcrop. These beds were previously interpreted as a brackish-water valley-fill succession, but core and outcrop studies now show the interval to contain a high-diversity trace-fossil assemblage including Teredolites, Diplocraterian, Arenicolites, Teichichnus, Planolites, and Ophiomorpha indicating marine influence. Hummocky cross stratification and symmetrical ripples in the sandstones indicate wave influence. The coastal-plain successions are composed of cross-bedded to rippled, lenticular sandstone and heterolithic strata, mudrock, and coals. Lenticular units are interpreted as channels and have paleocurrents to the northeast and east. Channels have an overall decrease in size and energy based on the occurrence of cross beds to ripples and an increase in burrowing in a down-depositional-dip direction. Down dip they have low diversity of traces and wavy to streaky bedding, both indicating a possible tidal influence. This downstream decrease in energy and size of channels and increase in tidal influence suggest a revised interpretation for the Neslen as distributive fluvial systems rather than as a tributary system. Instead of an interpretation as a valley-fill succession for the Thompson Canyon and Sulphur Canyon Beds, this interval may instead represent the flooding and reworking of a distributive fluvial system during sea level transgression.

AAPG Search and Discovery Article #90156©2012 AAPG Rocky Mountain Section Meeting, Grand Junction, Colorado, 9-12 September 2012