SEPM Strata

condensed section

A thin marine stratigraphic interval characterized by very slow depositional rates (<1-10 mm/yr) (Vail et al., 1984).

A condensed section often consists of hemipelagic and pelagic sediments, starved of indigenous materials, and deposited on the middle to outer shelf, slope, and basin floor during a period of maximum relative sea-level rise and maximum transgression of the shoreline. Because the clastic sedimentation rates are starved of the sediment from the continental margin, the skeletal remains of pelagic fauna preferentially accumulate to form condensed layers of fauna. These condensed layers of fauna are often used as paleomarkers of time. Examples of condensed pelagic fauna used in this way include graptolites, conodonts, goniatites, ammonites and foraminifera. Initially the condensed sections are seen to accumulate in more distal slope and basinal settings, but as the shoreline backsteps landward during the sea level rise of a transgression so the condensed sections may gradually expand in their coverage to include not only the basin but all of the slope and part of the shelf as well (T.S. Loutit, et al., 1988).

The condensed sections are often used as markers during sequence stratigraphic analysis. This is because commonly the upper layer of the transgressive systems tract forms a condensed section which is associated with the mfs where it is overlain by the downlapping highstand systems tract. Sometimes the transgressive surface marking the base of the trangressive systems tract is immediately overlain by a condensed section that is in turn capped by the mfs. High gamma signals in well logs are often associated with condensed sections; the radioactive elements that cause this signal were sequestered by the organic matter (often cyanobacteria and phytoplankton) that accumulated preferentially as the clastic input from the basin margin decreased.

In conclusion condensed sections and sequences, their condensed pelagic fauna and high gamma ray signal form common time markers for sequence stratigraphic analysis and are equated with maximum flooding surfaces.

References
Loutit, T.S., Hardenbol, J., Vail, P.R., Baum, G.R., 1988, Condensed sections: the key to age-dating and correlation of continental margin sequences. In: Wilgus, C.K., Hastings, B.S., Kendall, C.G.St.C., Posamentier, H.W., Ross, C.A., Van Wagoner, J.C. (Eds.), Sea Level Changes––An Integrated Approach, vol. 42. SEPM Special Publication, pp. 183–213.
Vail, P.R., Hardenbol, J., Todd, R.G., 1984, Jurassic unconformities, ChronoStratigraphy and sea-level changes from seismic stratigraphy and BioStratigraphy. In: Schlee, J.S. (Ed.), Interregional Unconformities and Hydrocarbon accumulation, vol. 36. American Association of Petroleum Geologists Memoir, pp. 129–144.