Page 278 - Carbonate Facies in Geologic History

Alpine Basin-Slope Microfacies 265

J 15. Limestones with very thin tests or shells of bivalves and brachiopods; also termed
"filamentous micrites." Halobia, Halollela, Posidonia (Bisotra) are pelagic bivalves with
extremely thin tests. Such organisms accumulated in great abundance forming shell
coquinas (lumachelles) in micrite matrix on the bottoms of Mesozoic basins. They are
an especially distinctive deposit in Triassic and Jurassic geosynclinal strata. A pelagic
variety of Standard microfacies 8. (Plate XXIX C).

J 16. Radiolarites. These irregularly thin-bedded (ropy or nodular) red, purple and green
strata are presumed to be derived from the very slow deposition, at J!;reat depth, of
siliceous and calcareous tests. Much early mutual replacement of Si0 2 and CaC0 3 took
place. The opaline silica is altered to cristobalite, chalcedony, and crystalline quartz.
Matrix is generally clay-silica; any calcareous matrix was silicified in diagenesis. The
radiolarians which formed the bulk of the original sediment are usually poorly pre-
served. A microscopic species of unknown biological affinity, Globochaete alpina, is
commonly recognized in these deposits as well. The colors derive from trace amounts of
Fe and Mn undiluted by the normally high carbonate or clay content. The radiolarite is
generally only a few tens of meters thick, the product of basin starvation, below compen-
sation depth for most carbonate. In places it must have been deposited in troughs with
steep sides, for breccias and clastic turbidites with silt, shale and lithoclastic limestone
microbreccia may be interbedded with it. Flute casts and graded bedding may occur in
such strata, formed as the extraneous clastic material moved into the troughs from
swells in the geosyncline. Massive mud-flow breccias some meters thick may also be
present, disturbing the even, thin bedding below them by plowing, loadcasting, and
channeling (Plate XXIX A).
J 17. Red nodular to conglomeratic limestone, rote Knollenkalk, or Ammonitico Rosso. Red
conglomerate with nodular clasts usually in a micritic matrix (occasionally with spar
cement between them). Bedding is thin and wavy to nodular. Microstylolites and ferro-
manganese crusts commonly occur at clast boundaries but some boundaries between
clasts and matrix are vague. Many clasts appear to be only slightly displaced, others are
more clearly transported. Early diagenesis and sediment flow is indicated. Redeposited
sediment in slumps and turbidites are recognized (Plate XXIX B).
Laminated crusts attributed by some authors to stromatolitic algae(?) coat some peb-
bles. Burrowing structures of Chondrites and Zoophycos types are reported. The fauna is
dominantly pelagic, although allochthonous brachiopods, crinoids, and mollusk re-
mains occur. Calcareous microplankton such as Saccocoma, Globochaete and Stomios-
phaera may be present. In some places almost all calcareous microplankton is absent
within the nodules, as if dissolved at an early stage----even during deposition. Lime mud-
steinkerns of ammonites are prevalent. Aragonitic shells are not preserved even as
calcitic replacements, and careful petrographic study indicates that this type of shell
material was dissolved away before burial. Molds of ammonites are corroded at former
sea water-substrate surfaces, now forming irregular bedding surfaces resembling hard
grounds. Commonly, crusts of ferro-manganese carbonate occur at these levels. Such
evidence indicates very slow deposition in relatively deep water. Slow deposition is
confirmed by the thinness of such strata, generally only 25 m or so; stratigraphic con-
densation is indicated by the numerous ammonite zones such strata bear.
Several possibilities exist for the cause of the nodular fabric and its depth of formation.
Suggestions listed below are not necessarily mutually exclusive.
a) Solution of crusts to form nodules on very deep ocean floors-below present com-
pensation depth for aragonites which is about 4000 m. Hollmann (1962) and Garrison
and Fischer (1969) termed this process "subsolution".
No one can be certain that the Jurassic compensation depth for carbonates was the
same as that in the present oceans.
b) Concretionary growth of nodules of high Mg calcite concomitant with aragonite
solution in shallow buried sediment-hence segregation of lime in a somewhat marly
substrate. Water depth within the photic zone, not more than a few hundred meters
(H.C.Jenkyns, personal communication).