Geology Reference
In-Depth Information
Fig. 2.8.
Map exhibiting the
occurrence of selected tropical and non-tropical shelf carbonates
. There are, of course, much
more records (see maps provided by Wilson 1975, Rao 1996, and James 1997). The sites shown in the figure were selected
with regard to their importance in comparative facies analyses. For numbers see Tab. 2.1. Base map after Nybakken (1993).
Subzones are differentiated according to surface
water temperatures (and bottom water temperatures in
the polar zone) as well as the latitudinal range (Fig.
2.9). The boundaries between the zones and subzones
are transitional rather than absolute.
Some trouble exists in the precisely defining of sub-
zones and the use of the terms warm, temperate, cool
and cold, particularly in the context of facies studies.
Warm-temperate and cool-temperate subzones are
rather well-defined (Briggs 1974) in contrast to the sub-
tropical zone. Some authors consider this zone to be an
equivalent of the warm-temperate zone; others define
the subtropical one as a zone which borders the trop-
ics. Here the climate is intermediate in character be-
tween tropical and temperate, though more like the
former than the latter.
Modern shelf carbonates and reefs are not limited to
low-latitude tropical and subtropical warm-water en-
vironments, but also occur in temperate cool-water, and
even polar cold-water 'non-tropical' settings of mid-
to high-latitudes. Non-tropical settings can be loosely
defined as lying poleward of the present limits of her-
matypic corals reefs, or beyond the mean annual sur-
face-water isotherm of about 20 °C, today typically near
the 30° latitude. Carbonate production also takes place
along a transitional spectrum of environments between
tropical and non-tropical settings depending on ocean
circulation and upwelling patterns. The northern, cen-
tral and southern parts of the Gulf of California, Mexico,
offers a unique possibility for studying carbonate pro-
bonates are forming in all latitudes and at warm, cool
and cold water temperatures. Modern non-tropical car-
bonates occur in tidal flats, on shelves and platforms as
well as in the deep sea, similar to tropical shelf carbon-
ates. Tropical carbonates are relatively well understood
and are, therefore, discussed rather briefly in this chap-
ter. The following text concentrates chiefly on non-
tropical shelf and reef carbonates, because microfacies
analysis will have a high potential in the future for rec-
ognizing and evaluating ancient temperate and cool-
water limestones.
2.4.4.1 Latitudinal Zonation and Diagnostic
Criteria of Tropical and Non-Tropical Carbonates
On the basis of mean surface ocean temperatures and
overall organism distribution, three major biogeographi-
cal zones can be established whose boundaries are
roughly parallel to latitudes: Tropical, temperate and
polar (Fig. 2.8). Surface waters in the tropical zone are
warm throughout the year, but in the temperate zone
warm only in the summer.
The
thermocline
(a zone of rapid temperature change
between warm surface waters and cooler deeper wa-
ters) is a permanent feature of the tropical zone, occurs
seasonally in temperate waters and is absent in polar
zones since the ocean surface is covered with ice in
winter and solar radiation is low in summer.
Text continued on p. 34
