SEQUENCE STRATIGRAPHIC ANALYSIS OF LATE CRETACEOUS (TURONIAN) TUNUNK SHALE MEMBER OF THE MANCOS SHALE FORMATION, SOUTH-CENTRAL UTAH: PARASEQUENCE STYLES IN SHELFAL MUDSTONE STRATA

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/shale-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 second-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 varied 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. 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). Based on parasequence stacking patterns, the 50 parasequences can be grouped into 11 parasequence sets, and 4 sequences, and key sequence stratigraphic surfaces can be identified. The 3rd-order depositional sequences recorded in the Tununk Shale are interpreted to be dominantly controlled by eustatic sea-level changes on the basis of their close correspondence with global sea-level cycles. The average duration of parasequence-scale facies variations within the Tununk Shale is calculated to be about 40 k.y., strongly suggesting 4th-order Milankovitch-scale climatic cyclicity.