Page 30 - Petroleum Geology

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petroleum reservoir rocks): regressions tend to accumulate potential reser-
voir rocks on top of potential source rocks. We shall refine these generaliza-
tions later.
Dominant trends must be distinguished from episodic reversals, for there
are usually transgressive episodes in dominantly regressive phases of sedimen-
tary basin development, and vice versa. Consider for a moment such a trans-
gressive episode. It may result from a change in the energy patterns, so that
sands accumulate in areas where muds were accumulating, and muds accumu-
late where sand was accumulating. Such an episode, due perhaps to changing
courses of distributary channels in a large delta, may be quite local. Changes
on a larger scale could result from a changing geography along a coastline,
perhaps with deltas changing their positions, or rivers finding new courses to
the sea. Temporary climatic changes could result in a reduction of sediment
supply, and episodic accelerations of subsidence could also lead to transgres-
sive episodes. There could also be sea-level changes.

EUSTATIC* SEA-LEVEL CHANGES

There is ample evidence, long recognized, from raised beaches and terraces,
and submerged forests, that sealevel has not remained constant relative to
the land during the Cenozoic, so the Principle of Uniformitarianism requires
us to suppose that changes have taken place throughout the history of the
oceans and continents. The evidence of marine borings in the columns of a
Roman market in Puzzuoli on the Bay of Naples, noted by Lye11 (1875, v. 2,
p. 164, and illustrated as the Frontispiece to Vol. l), indicates that the time
scale for relative sea-level changes of some metres can be quite short, and
tide-gauge records show significant trends and departures from trends over a
few decades (Gutenberg, 1941). We must, however, seek to distinguish be-
tween local and regional changes of sealevel, and the world-wide changes
that are called eustatic. This is a typical geological problem with many vari-
ables and no absolute datum. It is an important problem for petroleum

* Eustutic was coined by Suess (1888, Vol. 2, p. 680): “Um nun Vorgange dieser Art
naher zu verfolgen, trennen wir von den verschiedenartigen Veranderungen, welchen die
Hohe des Strandes unterworfen ist, solche ab, welche annahernd in gleicher Hohe, in posi-
tivem oder in negativem Sinne uber die ganze Erde sich aussern, und bezeichnen diese
Gruppe von Bewegungen als eustatische Bewegungen”. In the Sollas’ translation (Suess,
1906, Vol. 2, p. 538): “. . . we must commence by separating from the various other
changes which affect the level of the strand, those which take place at an approximately
equal height, whether in a positive or negative direction, over the whole globe; this group
we will distinguish as eustatic movements. ” (Sollas’ italics).
Subsequent definitions usually include a cause, which is undesirable. When a cause can
be identified, it can be indicated. There are almost certainly several causes of eustatic
changes of sea level.