Clastics Outcrop

Book Cliffs, Utah and Colorado

Introduction to high frequency parasequences interpretation from outcrops:from outcrops of clastic Rocks:  Click on thumb nails to expand images.
There are four exercises linked to this section. They are designed to introduce geologists to interpret the depositional setting of the clastic sediments of the exposed in outcrop. The exercises are based on measured sections from the "Book Cliffs" described by Van Wagoner et al 1999. They are focused on parasequences identified in the up-dip Spring Canyon Member of the Blackhawk Formation and the down-dip Mancos shale.

In this well exposed Book Cliff escarpment a large eastward prograding clastic wedge of sands and shales can be correlated regionally by tracking the individual parasequences. These are used to interpret and so trace the evolution of the Campanian shoreline of the Uinta and Piceance basin of Colorado and Utah (Figures 1 and 2).


Figure 2. Location of the Book Cliffs at the Utah/Colorado border.

This sedimentary section was uplifted with little tectonic deformation of the the some 6000 m in the Post-Cretaceous. This section provides a unique opportunity to study high-frequency clastic sequences and their systems tracts, and includes the sediments of fluvial to estuarine, coastal and shallow marine depositional systems (Figure 3).


Figure 3. Foreground of Mancos shale and background of the Blackhawk and Castlegate Formations shoreline sands.

Chronologically evolving cross-section with 3D perspective and detail of clastic response to varying sea level.
Systems Tract Evolution.
Movie of response of shoreline clastic systems formed by shoaling upward parasequences with finer downdip deeper water facies at their base and seaward, while coarser better-sorted facies are towards parasequence tops and landward.

The parasequences of clastics (as with carbonates) are studied since they provide relative time control to the section being studied and can be correlated as the fundamental buidling blocks of a hydrocarbon reservoir (reservoir Rock, seal and source Rock). Just as in carbonates parasequences of clastics are bounded by synchronous surfaces that envelope this layered wedge of sediments that were deposited synchonously but vary in their character updip and downdip as they responded to their depositional setting. Using the exercises provided in the section that follows it is possible to identify these surfaces and using Walther's Law see how the effects of changes in sea level have led to vertical and lateral changes in the facies of the parasequence.

High-frequency "cycle" or parasequence
Exercises provided on the site demonstrate that the high-frequency "cycle" or "parasequence" is the smallest set of genetically related facies deposited during a single base-level cycle. It has been established that cycle boundaries mark the turnaround from base-level fall to base-level rise (a period of time during which sea level rises from a highstand position, through a lowstand, and returns to a highstand). Cycles can be correlated and mapped updip and downdip across multiple facies tracts and include multiple vertical facies successions (VFS).  The sediments of individual cycles are considered to be contemporaneous and are therefore chronostratigraphic units (Kerans & Tinker, 1997 and Mitchum & Van Wagoner, 1991). One of the commonest manifestations of a parasequence is the shoaling upward cycle, with finer deeper water facies at their base and coarser better-sorted facies towards their top.
These exercises based on the outcrops of the Book Cliffs are linked below.
Exercise 1: Introduction to parasequence identification on the basis of the lithologies within outcrops. One section is considered and used to identify a vertical set of parasequences within a measured section.
Exercise 2: Three measured sections are correlated using a combination of changes in grain size and depositional facies to identify the major stratigraphic surfaces and parasequence stacking patterns that are tied from section to section.
Exercise 3:
This exercise involves the correlation of twelve measured sections in the Book Cliffs. You should make a regional sequence stratigraphic interpretation of facies geometries by establishing the lithofacies, identifying major surfaces, identifying parasequences and tieing these together.
Exercise 4: This exercise (created and kindly donated by Jennifer Aschoff of the Colorado School of Mines) introduces the application of Walther's Law and the identification of non-Waltherian shifts. The exercise uses some of Van Wagoner et al's (1990) measured sections, and a facies table fabricated by Jen to match with these sections.

References related to the Book Cliffs
For more detailed discussion of high frequency sequence analysis of the Book Cliff escarpment on which this exercise is based, refer to:

Van Wagoner, J.C., Mitchum, R.M., Campion, K.M., and Rahmanian, V.D. (1990), Siliciclastic Sequence Stratigraphy in Well Logs, Cores, and Outcrops. American Association of Petroleum Geologists, Tulsa, 55p.

Angela Coe, Dan Bosence, Kevin Church, Steve Flint, John Howell and Chris Wilson, (2003), "The Sedimentary Record of Sea-Level Change", published by Open University, Milton Keyynes, MK7 6AA, United Kingdom, 285p.

Henry W. Posamentier and George P. Allen, (1999), "Siliciclastic sequence stratigraphy - Concepts and Applications", published by the Society of Economic Petrologists and Paleontologists, 216 pages.

Kirschbaum, Mark A. and Robert D. Hettinger, (2002), "Variations in systems tract Architecture and Accomodation Space in Upper Campanian strata, Eastern Book Cliffs, CO and UT", U.S. Geological Survey, Denver CO, 80225 Poster.

Book Cliffs sequence stratigraphy - Papers
This is a link to a page that lists some of the literature on the Book Cliffs sedimentary record and its sequence stratigraphic character in response to base level change.
Sunday, July 24, 2016
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