USC Sequence Stratigraphy: Exercise in construction of Chronostratigraphic chart or diagram

Chronostratigraphy

Chronostratigraphic chart based on an exercise by Jerry Baum.

This page, and related links to movies and illustrative cross-sections, is focused on how to analyze the component layers and system tracts of sequences and relate them to time. The exercise below involves the generation of a Chronostratigraphic chart and is based on an identical exercise created by Jerry Baum. It forms the subject of web site movies linked above that follow changing geological cross-sections tracking evolving sedimentary geometries responding to sea level change. The exercise itself is focused on how the component layers and system tracts of sequences can be analyzed and related to time. This type of analysis involves using either seismic and/or geologic cross-sections and the building of a chronostratigraphic chart. Chronostratigraphic charts, also called Wheeler diagrams after the geologist who initially formalized this time-stratigraphy concept in 1958, display both the horizontal distribution of the component sedimentary layers of a sequence and any significant hiatuses in sedimentation. This diagram is derived from sedimentary successions and is used to show the time relationships of both the depositional systems and system tracts, and their relationship to surfaces of non-deposition (Emery et. al, 1996).

The basic units of the charts are "chronosomes", horizontal ribbons that represent sedimentary rock units bounded by time planes. Chronostratigraphic charts are best constructed from interpreted seismic sections and help understand how sedimentary sections develop through time. The horizontal axis of the chart matches the horizontal dimension of the seismic section and vertical axis represents time (see the above figure and the movie).

Exercise to construct a chronostratigraphic chart:
The following section describes how to construct a chronostratigraphic chart. You should first download the exercise pdf file of a pseudo-seismic section.

Steps for extracting information from a seismic section to build a chronostratigraphic chart:
1. Carefully interpret the pseudo-seismic section by identifying and marking where reflector terminations intersect seismic surfaces. Identify the type of reflector terminations (onlap, downlap, toplap, and/or truncation).

2. Identify the system tracts and their bounding seismic surfaces (sequence boundaries (S.B.), the first transgressive surface, ravinement surfaces, condensed sections, and maximum flooding surfaces). Number the interpreted seismic reflectors in order of deposition, starting from oldest (numbered 1) to youngest (top reflector) as in the linked movie.

3. Transfer the numbered reflectors to a time-scale:

a. The horizontal line matches the length of the pseudo seismic section or distance, with SP refering to "Shot Points" numbered 10 through 240. The vertical axis on the lower part of the diagram represents an arbitrary time line. The numbered time intervals, 1 through 30, are assumed to be of equal duration.
b. Transfer the horizontal dimension of the interpreted reflectors, starting with oldest (numbered here as 1), to the bottom of the time chart. Draw this to match the horizontal length of the equivalent reflector. Mark up properties related to that reflector (type, geometry, facies, system tract info, SB type, etc., within the chronosome).
Continue this process in order of deposition for all remaining reflectors, as in the linked movie .

4. The void space on the chart now represents areas of non-deposition, erosion, or condensation of the sedimentary section with thicknesses below the resolution of the seismic.

Baum's Chronostratigraphic Exercise as a QuickTime Movie

Click for Baum's solution:

Chronostratigraphic-chart
Stratigraphic+Chronostratigraphic chart

(click on Help on left frame for assistance in viewing files).

References
Emery, D., Myers, K. J., Sequence Stratigraphy, 1996, published by Blackwell Science Ltd., p. 297.

Wheeler, H.E. 1958. Time stratigraphy. American Association of Petroleum Geologists, Bulletin, v. 42, p. 1047-1063.

Last Revised: October 13, 2005