Arkell.1956.Jurassic..

DIASTROPHISM 641
presented by the Jurassic system, the third proposition (p. 638) may be
restated as follows: 3. 'Orogenies within the mobile belts do not
synchronize with transgressions outside the mobile belts'. Rather, the
transgressions tend to follow orogenies. Table 27 shows that the Callovian
transgression is an exception, and this prompts enquiry whether there
was not somewhere a Bathonian orogeny still to be discovered ? Perhaps
the 'Matmatian' folding in southern Tunisia (p. 282) may turn out to be
of this age. The Bathonian is missing or peculiar in facies in many parts
of the world, and even where it is physically transgressive it is lithologically
and palaeontologically in 'regressive facies' (Cutch, Burma,
Egypt, S. Tunisia). There is here a paradox requiring explanation.
Perhaps the transgressive beds (probably all late Bathonian) are merely
precursors of the Callovian transgression.
Haug strove to find in the transgressions over continental areas accommodation
for water expelled from the geosynclines by orogenic compression
and uplift. Stille's conclusion, that the main mountain-building periods
in geologic history were periods of regression outside the mobile belts,
while obviously true, aggravates the problem if too many of the oceans
are supposed to be continents, as on Haug's maps. Stille postulated
'asylums' for the regressed waters in deeps within the mobile belts. If,
however, as was shown in Chapter 27 to be probable, the Pacific was an
ocean much as now, all the water drained from the continental margins
could be taken care of by modest depression of parts of the Pacific floor.
The volume of water in the Pacific (it has more than half the water
surface of the earth) is so huge that comparatively small movements of its
floor could account for all the observed transgressions and regressions.
That movements took place in the Pacific hemisphere as in the other
hemisphere there is no reason to doubt. Tertiary to recent movement
over large areas is proved by the coral reefs.
So far as our knowledge goes at present, it does not point to any master
plan of universal, periodic, or synchronized orogenic and epeirogenic
movements. The events were episodic, sporadic, not periodic. There
was no 'pulse of the earth'.
Different regions of the earth had different histories, but some spasms
in the mobile belts were great enough to affect very large areas. Perhaps
a strong spasm in one part of a mobile belt would touch off others at points
of weakness or mounting unbalance in distant parts of the globe. Movements
under the huge water reservoir of the Pacific could have repeatedly
caused world-wide changes of sea-level, but the effects of these would vary
according to the relief in different parts of the world and might often be
modified by local land movements.
It appears that tectonic events were controlled by magmatic and
compressional processes which went forward with ever-shifting emphasis
in both time and place, as the crust of the earth gradually adapted itself
to the shrinking interior.
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