Page 31 - Petroleum Geology

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geology because rising relative sea level can lead to increased rates of sediment
accumulation and, perhaps, increases in the biomass of the seas over sedi-
mentary basins.
It is important to realise that the definition of present-day sealevel is
far from simple, and that the mean ocean surface (part of the geoid) departs
significantly from the surface of an ellipsoid. Figure 1-4 shows the departure
of the geoid from its best-fitting ellipsoid, the contours being the elevation
of the geoid in metres above or below (-) the ellipsoid. Two features of this
map are particularly important: first, there is no obvious relationship be-
tween the shape of the geoid and the surface features of the Earth; and se-
condly, migration of the geoid relative to the surface features of the Earth
could give rise to sea-level changes of several tens of metres in both senses,
rises and falls, without changes in the volume of ocean water.
The main causes of sea-level changes, confining ourselves to those that
operate on a time and space scale that can be of significance to the strati-
graphic record, are: (1) changes in the volume of sea water; (2) changes in
the volume and shape of the ocean basins; (3) changes in the axis of rotation
of the Earth; and (4) migrations and changes of shape of the geoid.
The first two lead to eustatic changes of sea level and present no great con-
ceptual difficulties. The retention of water on land during ice ages has long
been recognized as a cause of eustatic falls of sea level, with a rise when the
ice melts. The last two do not lead to eustatic changes according the Sues2
definition because the changes will not be in the same sense over the world,
let alone the same height. For these to be important processes, the rate of
polar wandering and of geoid migration must be such that adjustment of the
crust lags behind the instantaneous changes in the surface of the seas. The
last cause listed, changes in the shape of the geoid relative to the geographical
features, has only been recognized in the last decade as a possible cause of
sealevel changes, largely due to the work of Morner (e.g. 1976) on geoid
maps determined from artificial satellite and surface gravity measurements.
All four processes may operate at the same time, and the local changes of
relative sea level are the net result of the various processes operating and any
local changes due to relative subsidence or uplift.
Despite the difficulties of detail, it is axiomatic that a world-wide change
of sealevel, whether eustatic or not, will leave its record in all sedimentary
basins that were active and developing at that time. This axiom has its
corollary: those periods of geological history that show a word-wide ten-
dency for transgression or regression are likely to be periods of eustatic sea-
level changes.
It has been known for about a century (at least since 1888 when Suess
discussed “the Cenomanian transgression”) that the Cretaceous was a period
of world-wide transgressive tendency. It is now known that the transgressions
were not strictly synchronous around the world. They were earlier in Austra-
lia, South Africa and South America; later in north-west Europe and interior