Page 256 - Carbonate Facies in Geologic History

Later Triassic Deposits in the Dolomites 243

fragments. These beds constitute the La Valle or Wengen Formation. The litera-
ture describes them as "pseudoflysch" because of their characteristic monoto-
nous, even, thin bedding. Certainly the strata were deposited below wave base. A
few ammonites and Daonella are present. Exotic blocks derived from the banks
rarely occur in them. The volcanic activity clearly postdates the maximum reef
growth. There is no interfingering of the two facies although in places the volca-
nics cover the banks. Dikes and some major volcanic vents (e.g., in the Fassa
Valley) cut through the buildups in places as well as fill in the interbank areas.
The explosion craters contain blocks of Ladinian limestone showing that their
age is late in the time of carbonate development.
The basin fill between the banks is formed by the St. Cassian beds. The strata
have the same flysch-like bedding as the Wengen-La Valle formation, but are
more calcareous, less tuffaceous and grade upward into varicolored marls and
limestone with local conglomerates of volcanic pebbles. Some reworked tuff is
also present. The strata are most noted for the presence of numerous blocks of
bank-derived limestone, the Cipit boulders. These exotic, rounded boulders, up to
a meter or so across, are rarely dolomitized and offer the best view of the original
composition of the bank margins. Most are micritic and contain abundant
crinkly-laminate algae(?), Tubiphytes, and other encrusters, small dendroid corals,
and crinoid debris. The blocks contain many vugs lined with drusy cement and
filled with internal oxidized silty ferruginous sediment.
The banks and interbank sediments of the western Dolomites are covered by a
widely distributed sheet of argillaceous, dolomitic, and evaporitic strata, the
Raibl, varying in thickness from about ten to several hundred meters. This upper
Carnian marker is found also in the Northern Limestone Alps. It carries a mollus-
can fauna and much Sphaerocodium. It represents a time of clastic influx, general
marine regression and widespread development of shallow lagoons. In the eastern
Dolomites the Raibl is thick and bears evaporites. This is significant because
evaporite (CaS04) deposition during its formation may well have caused the high
Mg content of refluxing fluids which dolomitized the underlying Ladinian banks.
This dolomitization is clearly secondary, and to some extent controlled by bed-
ding and original sediment type. Finer-grained sediments are preferentially do-
lomitized. Corals are dolomitized but crinoids resisted the replacement process.
Lithification of the strata had not been completed when dolomitization occurred
because obvious permeability control on the process was exerted at several levels
of magnitude. Peripheral areas of the banks are less dolomitized than the interi-
ors. Certain levels in the foreslope beds are more dolomitized indicating preferen-
tial fluid migration. The Cipit blocks in the basin-fill sediment are rarely dolomi-
tized showing that the process was not contemporaneous with bank formation
but somewhat later. Apparently impermeable basin sediments protected the ex-
otic blocks from the dolomitizing fluids. Large banks whose tops were covered by
penecontemporaneous volcanics or by particularly thick and argillaceous Raibl
Formation are now still limestone. The dolomitizing fluids passed not only
through the Ladinian but also into the underlying Anisian Serla Formation.
Significantly, the strata next overlying the Raibl constitute an equally wide-
spread sheet of tidal flat and lagoonal dolomites. The Norian Dolomia Principale
or Hauptdolomit is about 250 m thick in the western Dolomites but thickens to