Page 271 - Carbonate Facies in Geologic History

258 Reef Trends and Basin Deposits in Late Jurassic Facies

Regional Settings

Several major paleogeographic provinces are recognized: (1) the European shelf
divided into several islands and basins, (2) a now missing North Atlantic source
area of terrigenous clastics, (3) a shelf margin of carbonate buildups stretching
west to east across southern Europe just north of (4) the Tethyan geosyncline
whose Alpine nappes contain deep sediments of the continental margin, (5) the
North African-Arabian Shield, (6) the thick miogeosynclinal platform areas of
southeastern Europe and a southern carbonate shelf in Algeria, Tunisia, Iraq and
Arabia, and (7) the starved and evaporite basins bordering the Zagros geosyncline
to the east of the Arabian Shield.
The generalized maps of Figs. IX-l and IX-2 include all the Late Jurassic. The
Late Jurassic began in Europe with the Callovian-Oxfordian transgression which
coincided with an influx of fine terrigenous material. Starved basins with thin,
nodular, ammonite-bearing argillaceous limestone developed in the Mediterra-
nean Tethyan trough, while dark ammonite-bearing marls formed widely in the
earliest part of the Late Jurassic throughout central and western Europe. Ox-
fordian basins in the Middle East were also starved of sediment at this time.
Neritic, oolitic and coralline facies are present in the Lower Oxfordian principally
in the northwestern European areas. Reefs began developing in the Jura Moun-
tains area late in Oxfordian time. Somewhat later (during Kimmeridgian) the
amount of clay and silt across the European shelf diminished and in central
Europe and the Middle East, much pure carbonate sediment formed.
The thickest carbonate strata formed around the northern and eastern perim-
eters of the Tethyan trough and to some degree along the northern and eastern
edges of the North Africa-Arabian shield on the opposite side of the opening
geosyncline. This neritic carbonate facies continued its development to the end of
Jurassic time and the Tithonian reefy beds are widely regressive into the geosyn-
cline, particularly to the east. At the same time, in northwestern Europe salt basins
and fresh-water lakes with clay and silt deposits were forming. It is as if the whole
of western Europe had tilted slowly and steadily up and gently warped in the
process. The eastern areas sustained marine conditions to the end of the Jurassic.
The interior of the Tethyan trough is now preserved in fragments in the Mediter-
ranean borderlands and islands. These consist of carbonate platforms and sea-
mount facies as well as the persistent starved basin radiolarite sediments. Major
subsidence in many areas continued throughout the Jurassic with block-faulting
as the Tethyan trough opened (Bernoulli, 1972).
Some of the areas of special sedimentation are mentioned below.

Evaporites

Late Jurassic time is one of the major evaporite periods in earth history. Through-
out this time basins of the European warped shelf and those marginal to the
North African-Arabian Shield, became periodically isolated and evaporite depo-
sition occurred. Some of the basins were shallow, ephemeral lakes and lagoons;