Sequence Stratigraphic Analysis of Late Cretaceous Tununk Shale Member of the Mancos Shale Formation, South-Central Utah: Parasequence Styles in Shelfal Mudstone Strata

Abstract

Despite the wide application of sequence stratigraphic concepts to coarse-grained siliciclastic deposits in nearshore settings, high-resolution sequence stratigraphic studies have seldom been attempted in fine-grained (mudstone-dominated) sedimentary successions deposited in more distal hemipelagic to pelagic settings. In order to examine how facies variability can be incorporated into sequence stratigraphic frameworks of mudstone-dominated successions, detailed stratigraphic and sedimentological analysis were conducted in the Tununk Shale Member of the Mancos Shale Formation of south-central Utah through a combination of field work and petrographic methods.

The Tununk Shale was deposited during the Greenhorn 2nd-order sea level cycle over a time span of about 2.5 million years. Vertical variations in lithofacies types and sedimentary facies characteristics indicate that the depositional environments of the Tununk Shale shifted laterally from outer-shelf to lower shoreface environment, with sediment accumulation rates ranging from 2.5 cm/k.y. to over 10 cm/k.y., respectively. At least 50 parasequences can be identified in the Tununk Shale, which can be further grouped into 11 parasequence sets (i.e. system tract), and 4 sequences. The thickness of parasequences ranges from 0.4 to 12.5 m and averages 3.6 m. Each parasequence shows coarsening-upward via increases in silt and sand content, thickness and lateral continuity of lamina/beds, and abundance of storm-generated sedimentary structures. Variations in bioturbation styles within parasequences are complex, though abrupt changes in bioturbation intensity and/or diversity commonly occur across parasequence boundaries (i.e. flooding surfaces). Within each depositional sequence, parasequences in the highstand system tract (HST) exhibits the highest average thickness, followed by parasequences in the lowstand and transgressive system tract (LST and TST). This trend is interpreted to be caused by the lowest sedimentation rate during transgression and different preservation potential between HST and LST (higher erosion potential during lowstand).

Sequence stratigraphic analysis reveal a hierarchy of sea-level cyclicity recorded in the Tununk Shale, which is dominantly controlled by the interplay of tectonic, eustatic, and climatic cycles. Development of a detailed sequence stratigraphic framework enables prediction of the distribution and characteristics of sedimentary facies within mudstone-dominated successions.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018