Page 300 - Carbonate Facies in Geologic History

Middle Jurassic Cycles of the Paris Basin 287

PotRIS 01. flEWS
•
PARIS
BASIN

c:::::J PELLET -ONCOID SAND-MJD Fig. X-6. General facies at
IIIIIIIllIl OOLITIC-BIOCLAST. SAND upper surface of Callovian
c=:J CALCI-SILT. ARGILL. MUD cycle outlining a low relief
bank, southeast Paris basin.
TOP OF CALLOVIAN CYCLE Courtesy of B. Purser (1972)

across. This impressive oolite is overlain by a thick-bedded, light-colored, peloi-
dal grapestone unit, with mudstone to packstone texture, and with large algal
onkoids. Hard grounds begin to appear in this upper unit and the top of each
cycle consists of a widespread surface showing oxidized and blackened particles,
large oysters plastered along it, and in places, evidence of early submarine cemen-
tation which has prohibited stylolitization and prevented later compaction. The
oolitic and bioclastic grainstone of Callovian age may be traced around the
outline of a large, low carbonate bank which existed over a weak structural high.
The wide sheet-like grainstone distribution indicates outward diachronous migra-
tion of the shoal sands and construction of a wide flat bank with gently sloping
ramps at the edges. It had no well-defined shelf margin and only 20 m of relief. It
is about 200 km across. Figures. X-5 and X-6 are maps by B. H. Purser of the
Jurassic outcrops and configuration and facies of this low buildup. Figures X-7
and X-8 diagram the facies through this buildup at a very high vertical exaggera-
tion of 2500 X. Types of microfacies can be illustrated from traverses across this
bank and in vertical sequences in anyone of the cycles (Purser, 1972). They are:
1. Well-bedded, argillaceous, silty lime mudstone- oflbank on the shelf.
2. Well-bedded bioclastic peloidal calcisiltite packstone-grainstone- oflbank
on the shelf. Standard microfacies types 2 and 9.
3. Bioclastic oolitic grainstone- just off the bar at the edge of the bank.
Standard microfacies type 11.
4. Oolite grainstone- bars at bank edge. Standard microfacies type 15 ..
5. Bioclastic onkoidal coated particle packstone. Standard microfacies
type 13.
6. Peloidal slightly bioclastic wackestone- bank interior. Standard micro-
facies 17 and 19.