Page 276 - Carbonate Facies in Geologic History

Microfacies in Shelf and Shelf Margin in Europe 263

fabric. These became filled with sparry calcite. In some areas fecal pelleted mud became
rapidly hardened and formed peloid sands. On areas subjected to periodic flooding and
rapid drying (e.g., natural levees) lamination is graded with pellet sand overlain by finely
laminated fenestral muds, in layers about a centimeter thick. Standard microfacies 19.
J 2. Large foraminiferal grainstones and packstones, commonly with grains encrusted by
algae. Foraminifera occur with rounded and worn particles in clean, washed, lime sand
as well as in muddy sediment. Parurgonia, N autiloculina, opthalmids, Pseudocy/ammina
Anchispirocyclina, Trocholina are common genera; pure carbonate sediment with these
large foraminifera probably represents deposition in ponds, lagoons, and associated
shoals in areas of warm water with restricted circulation. Other forms such as Alveo-
septa occur in terrigenous, marly limestone probably representing brackish to fresh
water deposits. Trocholina forms a particularly resistant particle, being of robust, cham-
bered, and thick-walled construction and is common in grainstone and oolitic sparites.
Standard microfacies 18.
J3. Onkoidal-pisolitic (Mumienkalk) limestone; light-colored, commonly of variable tex-
ture, grainstone to wackestone. These algal biscuits are unusually large (several centime-
ters) and their lime-mud coatings may bear multitudes of algal filaments and Girvanella
tubules. They are closely related in microstructure to Sphaerocodium. Such sediment
occurred in restricted marine shoals and lagoons where currents were strong enough to
overturn lightweight fragments. Modern channels with moderate water movement in
intertidal and shelf-margin shoals contain living blue-green algal onkoids which resem-
ble the Mumienkalk particles. The Jurassic onkoids, however, were generally hard when
emplaced, and in this more resemble fresh-water algal biscuits. A good marine analog to
such hard algal particles has not been found except in the red algal nodules. Standard
microfacies 22 (Plate XV A).
J 4. Oolite grainstone with multiple coated ooids; nucleii of shell bioclasts and pelletoids.
The excellent sorting of absolutely mud-free sediment indicates accumulation on bars
and shoals with high energy-tidal currents which move sediments to and fro almost
continuosly. Such well-formed ooids more commonly characterize shelf-margin posi-
tions than restricted marine shoals or belts of longshore sand sedimentation. Standard
microfacies 15.
J 5. Bioclastic grainstone-packstone. Brachiopod, bryozoan, echinoderm, coral, and mollus-
can debris occurring in grainy sediment associated with patch reefs and shoals are
scattered across shelves. Most of the particles are worn and coated and in places ooids
and lithoclasts are present with them. The fauna indicates normal marine salinity and
open circulation. Standard microfacies 11.
J 6. Coralline facies-boundstone. Both fasciculate and platy encrusting forms of corals make
up patches of solid reef framework, along with the heavy-shelled pelecypod Diceras. The
sediment between the coral heads generally consists of worn, coated, and rounded
bioclasts of varied types indicating normal marine, open-water salinity and circulation.
Standard microfacies 7 (framestone).
J7. Reefy limestones constructed mainly of spongiomorph hydrozoans. These organisms
much resemble the Paleozoic stromatoporoids. Presumably such organisms are more
tolerant of warm water and greater salinity than the corals. Standard microfacies 7
(framestone).
J8. Sponge facies with algal and foraminiferal (tuberoid) encrustations (Hiller, 1964). These
are well known from the Middle Malm of Schwabia and Lower Kimmeridgian Treucht-
linger Limestone of Franconia. Large mounds with micritic matrix were formed in
quiet and deeper water by sponges in the Late Jurassic. Standard microfacies 7 (batne-
stone).
J 9. Solnhofen Plattenkalk. This is extremely fine-grained, lithographic lime mudstone with
fine calcite mosaic micrite. Grains of 3-4 microns are welded together. Many coccolith
remains can be seen with the Scanning Electron Microscope. Such beds of limestone,
Flinze, a few cm thick, are separated by thinner marls, Fiiule. The bedding is almost
perfectly planar and regular with the exception of local slumps. Preservation of whole
fossils and lack of benthos or burrow structures indicates deposition below wave base in