12.6: Measuring Stratigraphic Sections

In order to use stratigraphic tools provided by lithostratigraphy, biostratigraphy, etc., sections of strata have to be measured. This critical work is done primarily in the field. The stratigrapher plans a field outing, beginning with researching relevant background information collected from geologic maps of an area academic and state geologic survey publications. Direct exploration of brand new stratigraphic sections created by human or natural processes, such as cuts in a hillside for new roads or new stream cuts caused by flooding, are also excellent means to examine strata. Once the preliminary library or fieldwork is done, detailed fieldwork can begin. This includes measuring sections, photography, sample collection, and sketching. Sometimes, sections are measured at a meter-scale, sometimes at a centimeter-scale or finer, depending on the research questions being considered. Data is recorded at intervals, relative to a known datum and a detailed stratigraphy will emerge through the drawing of stratigraphic columns. However the measurements are taken, using consistent techniques from one site to the next is critical toward providing a solid stratigraphic model that is useful for drawing conclusions about the data and for generating predictions to be tested.

Measuring sections on multiple scales can be very important. At the time of deposition, sedimentary units are much thicker than they are once millions of years of compaction and lithification have taken place. Sedimentation rates can vary over time and compaction is one secondary effect that begins to remove some of the original information from a particular page in the stratigraphic record. While the original thickness of the unit can be a matter of some speculation, it is usually possible to produce a good estimate. This can give us an idea of the rate of erosion, climate, and other factors working at the time of deposition. Because compaction also leads to burial and, with burial, heating, we can obtain information about these secondary processes from fossils and other sedimentological evidence.

The analysis of fine layering within a unit of rock or entire stratigraphic column can be described as microstratigraphy. At these scales of analysis, from sub-millimeter to perhaps up to a meter of scale, it is possible to describe environmental events and processes occurring within rather small time scales. The Tonoloway Formation of West Virginia preserves packages of alternating gypsum crusts resulting from periodic evaporation events. While we cannot be absolutely certain about the timescales involved with each layer of limestone and gypsum crust, we can surmise that it is possible that such deposition represents time intervals of days to years, or perhaps seasonal changes over the course of a single year. Other microstratigraphic evidence could help constrain this further.

Microstratigraphic record of Silurian tidal flat carbonates, including salt casts, in the Tonoloway Formation, West Virginia. What can we learn from such fine layering? What can we read from the packages of sediment here that are no more than 2-3mm thick after compaction and lithification?

Studying stratigraphic layering is most often done at a larger scale, by describing strata on the order of decimeters on up to perhaps thousands of meters. These larger patterns of deposition can represent a few million to tens of millions of years. Providing a broader, larger, view of events occurring within a basin over larger spans of time allows for a deeper understanding of the behavior of the basin as a whole. These behaviors would include being able to study the tectonic processes occurring or major changes of sea level at a variety of scales. As space in a basin fluctuates and, eventually, a basin infills, this large-scale record provides the story of how deposition took place. Just as important, this story provides direct evidence for developing models of tectonic change for mountain belts, changes in paleogeography, or changes in ancient climates.

A larger stratigraphic record of the Devonian Foreknobs Formation in West Virginia, along Corridor H. This roadcut, created by the human activity of building a road, documents just one section of this formation and its alternating sequences of sandstone, siltstone, and shale.