Page 283 - Carbonate Facies in Geologic History

270 Reef Trends and Basin Deposits in Late Jurassic Facies

TEXTURES
~ Dicera. (ance.lral rudi.11 Fa.ciculale carol. Io7l Rounded
and
~ brachiopod. e.g . Thecounilia ~ b i oclQSh
~ Encrusting i rregula r r:;:::::] ShUly, plaly G"-:::l Bioclastic
l...ZJ carol and red algae L:::::l Thomnalt.ria l...:::..:::J debris
~ Head -like carol , I ~l lilhi'lid .pong .. 1-.LI Mieri,e
~ Stylina motri.
.
e
.
g
Fig. IX-5. Idealized Upper Jurassic patch reef or reef knoll showing upwards changes in
biological sequence and growth forms of corals. Note change from sheety to dendroid to
massive encrusting, and from sponge to corals to red algae and Diceras. Matrix changes from
micrite at base to rounded grainstone at top

Several backreef facies are equally well-defined and, owing to regression, are
widely exposed both behind the upper Oxfordian reef ramp and stratigraphically
higher above and in front of it. Behind the platform chalky limestone (St. Ursanne
unit) overlies and grades into the pisolitic sediments. Patch reefs occur in these
fine-grained chalky beds. Their corals show an interesting ecologic sequence dis-
cerned by Ptimpin (1965). Lower beds contain compact corals such as 1hamnas-
teria followed by fasciculate forms such as Calamophyllia and Cladocora. Between
the corals red algae such as Ptychochaetetes and large Solenopora occur. Higher in
the reef are found large branching corals such as Latomaeandra and 1hamnasteria
gracilis. Reef growth ends with large head-like corals of Stylina and the red alga
Solenopora. Some irregular encrusting corals may also cap the sequence. Many
thick-walled mollusks, such as Diceras, Cardium, Nerinea, lived in the reef. The
same sequence of coral growth may be discerned in Oxfordian patch reefs in the
Yonne Valley in the southeastern Paris basin at Merry-sur-Yonne (Fig. IX-5).
Strata known as Vorbourg beds, form another backreef sequence of light gray,
dense, homogeneous mudstone and wackestone, containing scattered onkoids.
The upper Vorbourg beds are laminated algal mat tidal flat beds with windblown
quartz silt and highly resistant trace minerals such as zircon, rutile, and tourma-
line. Marl intercalations contain freshwater ostracods and charophyte oogonia
and indicate the presence of old lakes, ponds, and salinas. Black pebble breccias
and hard grounds mark sites of exposure or very slight deposition. More argilla-
ceous backreef strata (Natica marls) overlie the Vorbourg unit. They contain
some ferruginous oolite and calcarenite with local algal mats. Some normal ma-
rine, open circulation organisms are found. These include snails: N atica, N erinea;