Travel Reference
In-Depth Information
number of new terms have been coined to describe all the manifestations of
sea level change: high stands (times of sea water high levels), low stands (the
opposite), onlap (a landward migration of underwater deposits as the sea
slowly floods a continental surface), offlap (the opposite), transgression (a
slow flood of continental regions caused by rising sea level), and regression
(the opposite). The field of sea level change remains one of the most inten-
sively studied disciplines of geology, because it has been so useful in finding
oil. It is also useful in determining both time and the character of past envi-
ronments.
A quantum leap in our understanding of sea level change came about
in the 1960s and 1970s with the inttoduction of "seismic stratigraphy." Ge-
ologists discovered that by tracing the path of shock waves moving through
rock, they could observe structures deeply buried under the earth's surface.
When charges of dynamite are exploded above a buried pile of sedimentary
rock and the reflections of these shock waves are followed through the rock
with recording instruments, very detailed three-dimensional pictures of the
stratigraphy emerge. Using this methodology, the geologists recognize
onlap and offlap deposits in sediment now deeply buried. The oil compa-
nies have found (and still find) a great deal of oil in this way. Over many
years, scientists also noticed that in many different parts of the world the
sea level curves—diagrams illustrating sea levels through time—seemed to
match up.
But do the various sea level curves really match up, or are we seeing dif-
ferent curves in every individual ocean basin? To find out, you need excel-
lent time control correlation matching the various sedimentary rocks that
record the sea level changes—in the other words, you need to be able to rec-
ognize the age of sediments in widely disparate tegions. Without great time
control, the acceptance or rejection of global sea level curves could not be
completed. By some strange coincidence, the latter part of the Cretaceous
seems to hold as much oil as, or even more oil than, rocks of any other age.
Yet for reasons outlined in the first several chaptets of this topic, the sea level
curves for the latter part of the Cretaceous Period have been among the least
