Page 28 - Petroleum Geology

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*.....+.
.*.**-t
..*.. *....*

..**If
*
*
.
I
.
.
***.**.**
**I++**+
I****t**.***
+*****..***~
Fig. 1-3. Block diagram showing accumulation of transgressive sequence (above) and trans-
gressive sequence followed by regressive sequence (below). Neritic facies, dashes; neritic-
paralic, fine dots; paralic-terrestrial, coarse dots.
facies migrate seawards, the development is regressive. Let us clarify the
terms and concepts involved in transgressions and regressions because they
are vital to an understanding of geology in general and petroleum geology in
particular. There are advantages in taking regressions first.
“Regression” is defined as a lowering of sea level relative to the land. When
this happens, the shoreline and the associated environments and facies mi-
grate in a seaward direction, with an extension of the land area. This is the
basic concept. A second concept is derived from the first in the following
manner. If the mass of sediment supplied to an area requires more energy for
its dispersal than is available, baselevel rises and sediment accumulates com-
mensurately. The accumulating sediment tends to extend the land area, and
the facies also tend to migrate seawards, or progrude. There is thus a subtle
difference between the two aspects of regression, but a difference of con-
siderable importance geologically. Both can result stratigraphically in a se-
quence that shows sediments of shallower-water facies overlying those of
deeper-water facies, or terrestrial over marine, provided sediment accumu-
lates. The important difference between the two types of regression is that