--> Facies Characteristics and Sequence Stratigraphy of the Upper Cretaceous Tununk Shale Member, Henry Mountains Region, Utah

AAPG Annual Convention and Exhibition

Datapages, Inc.Print this page

Facies Characteristics and Sequence Stratigraphy of the Upper Cretaceous Tununk Shale Member, Henry Mountains Region, Utah

Abstract

Lower-Middle Turonian strata of the Tununk Shale Member of the Mancos Shale Formation were deposited along the western margin of the Cretaceous Western Interior Seaway during the Greenhorn second-order sea level cycle. Two stratigraphic sections were measured near Hanksville, Utah, in order to examine facies characteristics and variations within the Tununk Shale. Within each section, sedimentological data including lithology, grain-size, bedding thickness, sedimentary structures, paleocurrent direction, bioturbation characteristics, and amount and types of fossil fragments were documented. Relatively unweathered samples were collected at a spacing of approximately 1 m for detailed facies analysis on polished slabs, petrographic analysis in thin sections, and SEM analysis. In addition, lamina/bed-scale variations in physical and biologic attributes of three continuous coarsening-upward successions (i.e. parasequences) within one regional section have been documented to quantitatively determine the relative proportion of sedimentary structures generated by different depositional processes/events, which can be related to depositional setting. Initial results indicate that the total thickness of the Tununk Shale in this area ranges from 170 to 220 m, and that it consists of at least 48 parasequences. Each parasequence ranges from 1.2 to 13.5 m in thickness, suggesting Milankovitch-scale relative sea level variations (104-105 years duration). Parasequences are characterized by an upward increase of silt/sand and indications of storm-wave reworking, and a decrease in the proportion of shell fragments towards the top. Based on parasequence stacking patterns, the 48 parasequences can be grouped into 11 parasequence sets, and 3 sequences. Parasequence sets show a progradational stacking pattern characterized by an increasing proportion of storm-influenced facies (from 66% to 93%), suggesting deposition in progradational storm-dominated inner shelf environments during lowstands of sea level. Vertical facies variations within the Tununk Shale are interpreted to reflect transgression and regression of a storm-dominated offshore mud blanket. The high-frequency sea level cycles marked by parasequence sets and sequences in the Tununk shale are superimposed on the second-order Greenhorn cycle, probably due to the combined effects of glacio-eustasy, episodic tectonism, and climate change.