Terrestrial Palaeoecology and Global Change

Chapter 6. See-level fluctuations
183
bituminous limestones as intermediates and with the mm-thick lamellae of black shale
sequences left from post-sedimentary decalcification.
Shallow-water to paralic Cretaceous deposits occur on submerged oceanic plateaux,
rises and aseismic ridges. In the North Atlantic, shallow-water mid-Cretaceous facies
and subaerially weathered basalts occur on the J-anomaly Ridge, Newfoundland Sea
Mounts and elsewhere (Tucholke & Ludwig, 1982; Sullivan, 1983), occasionally even in
the axial zone of Mid-Atlantic Rise (Bonatti & Crane, 1982). Bryozoan limestones are
dragged from the Bahamas escarpment at a depth of 3100 m (Uchupi & Milliman, 1971).
In the South Atlantic, the littoral mid-Cretaceous deposits are recorded on the Walvis
Ridge. After a period of non-deposition they were covered by the shallow-water Maastrichtian
limestones. Similar deposits also occur on the Rio Grande Rise and São Paulo
Plateau (Hekinian, 1972; Humphris & Thompson, 1983; Schaffer, 1984).
In the central Indian Ocean, the Late Cretaceous andesites of Ninetyeast Ridge are
topped by shallow-water limestones and lignites (Hekinian, 1974; Sclater & Fisher, 1984).
On the Broken Ridge and Kerguelen to the south, the Late Cretaceous shoal carbonates
are truncated by a pre-Eocene erosion surface (Guilfy, 1973; Dosso et al., 1979; Mutter
& Cande, 1983).
Shallow-water Cretaceous facies are widespread over the Pacific rises and plateaux,
including the Obrutchev Rise in the north, Magellan Rise in the central area and Manihiki
Plateau to the south (Kauffman, 1974; Thiede et al., 1981; Pushcharovsky et al., 1984).
Actually, all the positive oceanic structures presently submerged to about 2500 m
were at shallow depths or above sea level in the Cretaceous. Summarily, these data
indicate an about 2500 m subsidence of oceanic floor since the Cretaceous. By inference,
the isostatically compensated oceanic crust were at depths of 1500 – 3500 m in the
Cretaceous against 4000 – 6000 m at present.
Cretaceous basalts are widespread over oceanic depressions piling as ridges along
the then active fault zones. The huge amount of basaltic material, mostly of a tholeiitic
composition (but not necessarily produced over the mid-ocean ridges, for there are examples
of such “intraplate” basalts: Batiza et al., 1980) is in itself evidence of a general
upheaval – extension on a sphere – and rifting of oceanic crust.
These considerations give us a crude idea of hypsometric steps for a thalassocratic
epoch, with a wide area, no less than 55% of the earth’s surface, at altitudes from +200
m to -200 m, a restricted highland area at +500 m to +1500 m, and oceanic depressions
at -1500 m to -3500 m. Such a distribution is conveyed by a gently sloping hypsometric
curve like that of the Moon, a slow rotating planet (Fig. 79).
A thalassocratic situation like in the Cretaceous thus implies a convergence of mean
hypsometric levels for the isostatically compensated oceanic and continental lithospheric
domains resulting in shallow oceans and low land. A divergence of these levels generates
the alternative situation, with deep oceans and elevated continents. Remarkably,
rifting of cratonic areas (trap magmatism) marks a culmination of regressive trends at
both the Permian/Triassic and Cretaceous/Tertiary boundaries (IX.1).