Detailed Anatomy of Parasequences in Mudstone-Dominated, Allegedly Anoxic Settings: Examples from the Cretaceous Mowry Shale and Associated Formations, Western U.S.

Bohacs, Kevin ¹; MacQuaker, Joseph ²; Lazar, Ovidiu R.¹; Jonk, Rene ¹; Hemmesch, Nikki ³; Cassel, Elizabeth J.^4
1 Upstream Research Company, ExxonMobil, Houston, TX.
² Department of Earth Sciences, Memorial University of Newfoundland, St. John, NF, Canada.
³ Geology & Geological Engineering, Colorado School of Mines, Golden, CO.
⁴ Geological and Environmental Sciences Department, Stanford University, Stanford, CA.

Parasequences are subtly but distinctly expressed even in dominantly fine-grained strata. In the Cretaceous Mowry Shale and associated formations of the western U.S., parasequences can be recognized through systematic changes in lithofacies, grain size, sedimentary structures, body and trace fossils, and geochemistry.

In relatively proximal, mudstone-dominated areas, parasequences are composed at their base of medium to dark brown claystones and silty mudstones and at their top of sandy mudstones and muddy sandstones. Mm to cm-thick beds of very fine sandstone become more abundant towards the top of the parasequences. Sandstone beds are dominated by wave ripples and commonly have erosional bases and fining-upward trends. Within the parasequence, discontinuous, curved or wavy non-parallel and parallel laminae are common. Bioturbation is present throughout the parasequence and includes horizontal and inclined burrows (e.g., Planolites, Phycosyphon). Vitrain and fusain are common. Total organic carbon (TOC) content is generally less or around 1%, whereas the hydrogen index (HI) is less than 50 mg HC/g C. These observations are interpreted to record an increase in bottom energy and oxygen level and suggest that clastic dilution ultimately controlled the character of fine-grained rocks in proximal areas.

In relatively distal, mudstone-dominated areas, parasequences start in dark-gray claystones with planar-parallel laminae, sporadic bivalve shell fragments, and sporadic to scattered fish scales. Grain size increases upward and, at the top of the parasequence, medium to dark gray silty mudstones and siltstones prevail. Silty beds, dominated by current bedding, with erosional bases and fining-upward trends are more common towards the top of the parasequence. Horizontal to subhorizontal burrowing (e.g., Phycosyphon, Helminthopsis, Planolites) increases in intensity upward and the top of parasequences can be churned. TOC content ranges from 1% to 4 %, whereas the HI ranges from 100 to 300 mg HC/g C. These observations suggest a decrease in clastic dilution and an increase in the role that organic production played in the formation of mudstones in distal areas.

This study indicates significant variation in parasequence attributes as a result of the interplay among physical, biological, and chemical processes. Capture of this variation contributes to a successful characterization of mudstone successions for hydrocarbon systems analysis.

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009