Stratigraphy as a Badly-Edited Tape

“The stratigraphic record is like a badly edited tape.” Discuss.

The rock record is incomplete. Between limited exposure, erosion, and recycling, only patches are preserved, and deposits of the same age are no longer connected. Particular lithographies can be paired by matching sedimentary properties by correlating outcrops and unit geometry, correlate geophysical and geochemical properties, and using depositional and plate tectonic frameworks to build local depositional histories. However, this leads to a layer-cake vision of the world, homogeneous layers simultaneously deposited around the world.

Modern deposits have a limited extension, with rock type diagnostic of local environment, not age. Facies are rocks with specific characteristics with limited lateral extent. Time lines cut across facies boundaries, with rock-bodies thinning out laterally and vanishing, or grading in to each other. A pinch out of a rock body is a particularly common stratigraphic trap. Rapid changes in shoreline results in a jagged stratigraphic section, leading to inter-tonguing pinch outs of particular interest to petrologists. During transgressive episodes, erosion obliterates the deposited facies from retrogressive episodes, making preservation unlikely unless subsidence is extremely rapid. The result is that the stratigraphic record is asymmetric, with more records of transgressive than regressive cycles.

Base level erosion is the point below which erosion cannot occur. The ultimate base level is ocean basins, where sediment accumulates, but local conditions can create temporary base levels at higher elevations. Similarly, base level aggregation is the point above which aggregation of sediments cannot occur. Grain size and environment vary this level greatly, but preservation of any lithography about this point is very rare, the original rocks are eroded and transported to lower elevations. These concepts lead to discontinuities in location of sedimentation, farther breaking the myth of continuous sedimentation. Oscillation in base levels can lead to sporadic sedimentation or small-scale unconformities.

Unconformities can also come from other temporal breaks. Angular unconformities, nonconformities, disconformities, and paraconformities are uncomformaties with different lithographic and geometric relationships between overlaying strata.

With all these missing sections and spotty sedimentation, geologists are left trying to patch together short, local sections into a coherent history. Outcrops are rarely extensive enough for continuous tracing, but strong lithographic similarity, position in sequence, uncomformaties (including global sequences), structural features, and evidence of deformation or metamorphism can all be used for correlation. Like an edited tape, pieces are missing; like a badly edited tape, pieces are randomly missing, potentially with key sections utterly destroyed.

This entry was posted in Geoscience and tagged , . Bookmark the permalink.

Leave a Reply

Your email address will not be published. Required fields are marked *