Page 302 - Carbonate Facies in Geologic History

Middle Jurassic Cycles of the Paris Basin 289

25km

ARGILl. oo' MLD IHflEEfIlItI;·~·
CALCI-SILT j .;g@
CRINOID-CORAL-BRYOZOA SAND c::::::::::::::
PURE OOLITIC SAND
MIXED OOLITIC-BIOCLAST. SAf.() mmnm
"BAHAMITH" SAND ~
CORAL BIOSTROME
PELLETOIOAL SAND ~
CALLOVIAN
ONCOIOAL GRAVEL __
MUDDY PELLETOID. SAND c:::::r CYCLE
co' MUD c:::::::J
HARD GROUND ~
Fig.X-8. North side of low relief Callovian bank, southeast Paris basin. For additional
explanation see Fig. X -7. Courtesy of B. Purser (1972)

of very considerable post-Jurassic subsidence. To the south lay the Rub al Kali
basin, an immense but more shallow subsiding area which accumulated about
5000 m of post-Permian sediment (Fig. IX-2).
Late Jurassic beds were laid down during sporadic marine advances and
retreats at the time of a major transgression over the periphery of the shield.
Calcareous sandstone is seen in places along the edge of Late Jurassic outcrops in
inland central Arabia, indicating proximity to shoreline west of the present Tu-
waiq escarpment. Essentially, however, terrigenous sediments are absent across
the Late Jurassic shelf and sediments are pure carbonate and evaporites, except in
the two basins where dark, argillaceous limestone predominates. Tectonic move-
ments intensified during the Late Jurassic and the basins became more strongly
differentiated and probably starved of sediment in Oxfordian time. Basinal strata
of this age are merely reduced sections of dark shale and limestone although shelf
equivalents are transgressive, fossiliferous limestones. In latest Jurassic time
(Kimmeridgian-Tithonian), perhaps owing to a change of climate, gradually in-
creasing marine restriction resulted in the characteristic carbonate-evaporite
Arab zone cycles over the shelf areas. Progressive evaporite conditions formed a
terminal Jurassic anhydrite sheet (Hith Formation of Arabia) and final marine
retreat into the basins formed salt deposits, a stratigraphic situation comparable
to the Mississippian Charles Formation of the Williston basin.
Description of the cycles: There are about eight discernible cycles in the Late
Jurassic of the Arabian Hasa coast, but four prominent ones, high in the sequence,
have long been designated Arab zones A through D from top down. Each consists
of a lower unit of bioclastic lime mudstone-wackestone with normal marine