David Krantz

University of Toledo

This activity is designed for a required course in Stratigraphy for undergraduate majors.

In previous exercises, students have described cores (lithologic description, with basic interpretation of what major lithologic changes could indicate), and have interpreted lithologic changes from a core transect across a barrier island into the back-barrier lagoon (either from actual cores or from logs of cores collected previously).

This activity is about two-thirds of the way through the course, following discussion of depositional systems and facies models, and as an introduction to the basic concepts of sequence stratigraphy.

Discussed in lecture prior to the exercise:
Factors affecting relative sea level
eustatic sea level, sediment supply, tectonic movement
the result: transgression, stillstand, or regression
prograding and retrograding units
General concept of depositional environments and lithofacies
Walther's Law and relation between lateral and vertical facies changes
General process of barrier-island transgression

Concepts incorporated into the exercise:
Primary facies associated with a barrier island and shoreface
characteristic geometries and bedding of facies
vertical and lateral facies relations
Components of a seismic sequence
bounding surfaces
shoreface ravinement—process and result
Accommodation space and preservation potential

Interpreting deposition and erosion from a seismic profile
Visualization of a reconstructed map view of a barrier-island depositional system Visualization of 3-D structure of a sedimentary sequence from 2-D profiles
Visualization of 3 dimensions plus time, as the transgression proceeds
Iterative interpretation, and evaluating multiple working hypotheses

Working in groups.
My Stratigraphy course is a University Writing Across the Curriculum course, so students will be required to write a brief report with description and interpretation of the seismic sections that they work on.

The transgressive coastal sequence, as a fundamental concept in stratigraphy, will be explored by the students in a hands-on activity based on a set of high-resolution seismic profiles collected in the shoreface off Assateague Island, Maryland and Virginia. Small groups of 2-3 students will identify primary surfaces, such as the ravinement surface and sequence boundaries, and major sedimentary facies, such as offshore shoals, flood-tidal deltas, and tidal inlets, in a set of shore-parallel and shore-perpendicular lines. The exercise begins with factors controlling relative sea level and leads into accommodation space and preservation potential.

Each Task has a specific product—an interpreted section, facies diagram, isopach map, or other synthesis. Points are assigned to each Task, and points are accumulated for the entire exercise.

If the product of a task is worth 20 points, 10 pts are given for meeting the minimum (generally following directions and creating a product), 5 more pts assigned for thoroughness and accuracy of interpretation, and a final 5 pts for quality of work.

Points may be deducted for a task that is turned in late or sloppily thrown together, for formatting errors such as no labels, or for an unclear or disorganized written explanation.

The instructor will review the students' work at regular intervals as tasks are completed, and lead a discussion by the class of the results to that point. The later tasks require reasonably correct interpretations for the initial tasks. In cases that a group is substantially off track, the instructor may redirect the line of thinking/interpretation.