THIN-BEDDED FACIES ANALYSIS OF FINE-GRAINED PARASEQUENCES: CENOMANIAN-TURONIAN TUNUNK SHALE MEMBER, HENRY MOUNTAIN REGION, UTAH

Muddy formations in the Cretaceous Seaway of North America comprise a major part of the stratigraphic record of the ancient seaway, but are relatively poorly understood. This study examines the Cenomanian-Turonian Tununk Shale Member within the Mancos Shale Formation in south-central Utah. The Tununk shale is approximately 173 m thick and contains around 29 upward coarsening distal parasequences. Detailed bed-scale measured sections include grain size, lithology, sedimentary structures, fossils, and ichnology that were used to interpret depositional processes. Three parasequences located in the lower muddier Tununk and one parasequence located within the middle sandier Tununk, suggest an overall progradational succession and were examined in more detail.

The lower parasequences exhibits siltier-upwards facies successions that consist of 97% dark-coloured, fossiliferous, mudrocks with abundant starved current rippled silty beds and a bioturbation index (BI) of 0 to 1. Paleocurrents were observed to be unidirectional towards the SE. Fossils consists of Inoceramid shell fragments and fish scales. About 3% very fine hummocky cross stratified (HCS) sandstone layers cap each parasequence. This is interpreted as an oxygen deficient offshore shelf environment; however, the observation of HCS suggests deposition above storm wave-base. Paleocurrent data suggest deflection of turbidity currents towards the south due to geostrophic shelf currents.

The upper parasequence is comprised of 68.5% mudstone and 31.5% very-fine sandstone. It showed an overall sandier-upward succession of interbedded mudstones and very-fined grained sandstones with abundant wave rippled beds and combined-flow rippled beds. The BI is 3 to 4. Wave ripple crests are dominantly west-northwest and east-southeast, suggesting storm waves coming from the north-northeast. The parasequence is interpreted as a progradational storm-dominated inner shelf.

The progradational succession from a muddy lower Tununk to a relatively sandy middle Tununk could suggest the progression from a Highstand Systems Tract to a Lowstand Systems Tract. This study reveals the importance of thin-bedded facies analysis especially within historically poorly understood muddy rocks in ancient systems in determining depositional processes.