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Chronostratigraphy
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: Steps
for extracting information from a seismic section to build a chronostratigraphic
chart: 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:
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
on Help on left frame for assistance in viewing files). Wheeler, H.E. 1958. Time stratigraphy. American Association of Petroleum Geologists, Bulletin, v. 42, p. 1047-1063.
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of a pseudo-seismic section.
