Print this pageDo Depositional Cycles Fall into Discrete Order Classes?
SCOTT, R. W.
A common practice in sequence stratigraphic analysis is to divide a
basin-fill section into a hierarchy of depositional cycles. What is new about
this long-time practice are the inferences that successive cycle orders have
specific durations and processes of formation. However, two current
classifications have been proposed and are not in agreement.
The first question to be posed is: Do natural classes of cycle scale durations exist? The hypothesis that natural classes exist is readily testable by statistical methods. But this has not been done because accurate techniques of dating cycles have not been widely used. Graphic correlation is a quantitative technique that determines the relative rate of sediment accumulation by correlating a section to a standard time scale.
A sample of Cretaceous depositional cycles defined by sequence stratigraphic criteria have been correlated into a Mid-Cretaceous composite standard data base. "Third-order" Albian cycles average 1.74 m.y. in duration, Cenomanian cycles average 1.07 m.y., and Turonian cycles average 0.59 m.y. long. Higher order cycles in these sections are in the frequencies of climatic Milankovitch bands. The durations of anoxic events defined by the inflection points on deltaC{13} excursions vary with time. The Aptian/Albian events, OAE 1a, b and c, respectively, were 7.56 m.y, 3.08 m.y. and 820 k.y. long, and the Cenomanian/Turonian event, OAE 2, was 500 k.y. long.