Tidal Rhythmites in the Upper Cretaceous Neslen Formation, Utah, USA: Their Implications for the Sedimentology and Stratigraphic Architecture of Tidal-Fluvial Channel

Upper Cretaceous Neslen Formation in the Tusher Canyon, Utah is dominated by multiple stackings of tidal-fluvial channel deposits that consist of inclined heterolithic stratification (IHS). Each channelized unit is 3 - 10 m thick and has an upwardly fining succession with a sharp and erosional base and a gradational top of coaly mud. IHS has variable dips ranging from 2 and 5 degree and consists of rippled fine to medium sandstone and interlaminated siltstone to mudstone.

On the basis of facies association and stratigraphic occurrence, four types of tidal rhythmites (TR) are identified within the IHS. TR is composed of mostly siltstone and less commonly very fine sandstone, exhibiting neap-spring tidal cycles and diurnal inequalities. TR is either planar laminated or rippled. Ripples are commonly unidirectional and migrating updip of IHS. Type 1 is composed of laminated TR that alternates with non-cyclic rippled sandstone, occurring near the base of channel. Rippled sandstone has a highly variable thickness and geometry with an erosional base. Type 2 consists of alternating rippled sandstone and laminated mudstone, wherein the former represents spring tides and the latter neap tides. Type 2 is common in the lower to upper part of IHS unit. Type 3 is defined by rhythmically climbing ripple lamination (RCRL), exhibiting rhythmic changes in cross-laminae thicknesses. Type 3 is mainly present at the top of IHS unit. Type 4 is made up of laminated siltstone that is either planar or inclined, occurring in the middle to upper part of IHS.

Neslen Formation is dominated by rhythmic tidal deposition in the subtidal (Types 1 and 2) and intertidal (Types 3 and 4) point bars of tidal-fluvial channels. The best-developed TR is associated with abandoned channel-fills. Common presence of updip migrating ripples on the IHS suggests that deposition occurred in a highly sinuous and actively migrating channel, where mutually evasive tidal current is well established. Tidal deposition is frequently interrupted by river flooding events in the proximal tidal-fluvial channel, which typifies the lower part of Neslen Formation.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California