Tectonic Drift Versus Climatic Variations: Rhodoliths as Indicators of Limits Between Tropical and Nontropical Sedimentary Conditions: Examples from Pacific Miocene

Francoise G. Bourrouilh-Le Jan

Modern examples show that rhodoliths or red algal nodules are forming around the 18°C winter isocline and that huge amounts of these red coralline algae are living and accumulating in the subtidal zones, from -60 m to sea level, of temperate seas, such as the English Channel and Rockall.

In the Pacific Ocean, several high carbonate platforms, so-called "uplifted atolls," show uniform, extended, and thick accumulations of rhodoliths. These accumulations have been recognized in the Solomon Islands (Rennell) and in the Loyalty Islands (Mare and Lifu, New Caledonia), but also in the Vanuatu (Vila), in the Austral Archipelago (Rurutu), where their age can be proved or estimated as middle Miocene. They are also mentioned in the literature on the Emperor Rise (northwest Pacific).

On other high carbonate islands, such as Makatea (Tuamotu), red algae and rhodolith formations appear at the top of a sedimentary pile of lower Miocene coral accumulation. The same observations and perhaps the same age can be said for Nauru (central Pacific).

Such a wide distribution, from the east to the west part of the Pacific Ocean and between the tropics, seems to be due to climate variations during the Miocene, more than tectonic drift due to oceanic spreading.

Temperate conditions shown by this shallow platform sedimentation, just under the coral growth conditions, seem to be confirmed by isotopic studies on pelagic and benthic Foraminifera and could confirm the existence of climate variations affecting the surface water of the Pacific in an extensive area that does not consider the presence of trenches, arcs, and ridges.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.