Page 252 - Carbonate Facies in Geologic History

Organic Composition of the Dolomite Banks 239

Organic Composition ofthe Dolomite Banks
Because of extensive and rather coarse dolomitization, the organic makeup and
petrographic character of these masses is only just now being understood. A
better understanding of the rock is actually obtained from studies in the Northern
Limestone Alps where more limestone is present. However, not all the Dolomites
have suffered this common disease of carbonates. Notably the Marmolada and
Latimar are still limestone.
(As an aside, it is interesting that the mineral Dolomite was named for its discoverer,
Count Dolomieu and that the mineral gave its name to these impressive mountains. Nature
lovers, other than mineralogists, may be surprised that the circumstances should have not
been the other way around.)
Although in the past the Dolomites frequently have been considered to be
composed of coral reefs, estimates found in Leonardi (1967) are that about 50% of
the bulk may be algae, 18% mollusks, 17% corals, and 6% foraminifera. Even
these figures compiled by experienced modern researchers are hard to evaluate
because dolomitization has obscured so much. Furthermore, the coarse, fibrous,
veinous calcite (Evinospongia) appearing in the "reef" mass has been accepted in
past years as an organism and probably also vadose pisolites have been confused
with the alga Sphaerocodium. Corals that are present vary from bank to bank in
importance with the algae. Those described and figured by Leonardi (1967), Ihe-
cosmilia and Koilocenia are branching, ramose, and tubular, some with tubules
only a few mm in diameter. These occur commonly in micrite. The colonies are on
the order of only a meter or so in size. Commonly they appear bedded as bio-
stromes. Traverses made mainly by D.Rossi (Leonardi, 1967) across the Cattinac-
cio and Sasso Lungo show corals all the way across the bank (Fig. VIII-IS). In
some other areas, such as the Sella and Sciliar, corals seem more prevalent at the
bank edges and on the upper foreslopes. Much coral material is seen in some
bank-derived blocks in the higher basinal sediments. The peloidal micrite matrix,
the scarcity and dendroid growth habit of the coral, and the recognition of the
widely adaptable 1hecosmilia (found in both backreef and forereef of the Northern
Limestone Alps) are facts arguing strongly against any type of wave resistent coral
reef rim for the Dolomite banks despite the steep slopes into the basin. Algal
content is hard to estimate but much sediment-binding algae do exist. The micritic
limestone Cipit blocks of the basin-fill, which were derived from bank edges,
contain abundant evidence of crustose irregularly laminate algae(?), foraminifera,
and the organism Tubiphytes (Plates XXIV B, XXV A). Sparse and isolated mol-
lusks, including some large gastropods occur at bank margins. Brachiopods (com-
monly Waldheimia) occur, but bryozoans are not reported, and foraminifera and
radiolarians are rare. Bulbous forms of hydrozoans or stromatoporoids are re-
ported but are not commonly seen, perhaps owing to poor preservation in
dolomite.
Crinoids are found on outer foreslope deposits in the Sciliar and Sasso Lungo
Banks. Small mounds of crinoid particles occur in places. Crinoid remains are
common in the Cipit blocks and cidarid echinoids also are present, indicating
abundant echinoderms on the open sea sides of the banks.
Published traverses across the banks, by Rossi and Leonardi, show ubiquitous
stromatolites (Leonardi, 1967). The alleged "Cyanophyte algal balls" (Sphaero-