Page 318 - Carbonate Facies in Geologic History

Bank Interiors of the South Alpine Triassic 305

." .
~.:
' •• •• 1 •
. . . . '. Groin.fon • . wock •• ton.,
c ... Cl biola abundant. mar. olga.,
:¥
;; .' .... ' ~ m.golodont clam. and
Go ... "iif' / oth.r mollu.k.,
•
V'" ~Q QQ .chinod.rm., Fig. X-i8. Diagrammatic repre-
o onkolit •• roth.r than sentation of Lofer cyclothem,
.., algal .tromatolit •• principally developed in near
~
~l basal, argillaceous member,
:> reef Dachstein Formation. A,
If)
representing reworked residue
of weathered material (red or
'
. . ,.' .~ . .': green), commonly confined to
Dalomi~c algal mob and eru.I., cavities in underlying lime-
coo mud crock,. f.ne •• rol fab ric, stone. B, intertidal member of
] re.tric •• d biota . "loferites" with algal mats and
;;
-< ] __ /'0'01 cano L, r.d or gr •• n motri~ abundant desiccation features.
C, subtidal "megalodont lime-
----~-¥""t:I DISCONfORMITY
)...,---- V. in. extend ing down. stone" member, with cavities
V Solulion coyili •• fill.d produced by desiccation and
by red or gr •• n matrix solution during succeeding
drop in sea level. From Fischer
(\964, Fig. 7)
The cyclic Lofer series of Sander (1936), Schwarzsacher (1948), and Fischer
(1964) in the Dachstein limestone displays the same rock types in a sequence
which varies somewhat depending on position in the regional facies. In the North-
ern Limestone Alps the Norian Dachstein facies with megalodonts and onkoids
was deposited in large subsiding carbonate banks scattered within the Alpine
trough. In central Austria northward and westward from Lofer towards Inns-
bruck, the bank facies is 1000-1500 m thick. In these directions the more bank-
edge Dachstein facies passes stepwise into the more restricted Hauptdolomit
which is distinguished from the former by the abundance of dolomitized intertidal
and supratidal deposits and by less of the megalodont-bearing grainy limestone.
The Hauptdolomit extended toward the Vindelician shoreline which lay north of
the Alpine thrust belt and into the evaporites of the Keuper basin.
Fischer's important study (1964) of the Lofer facies resulted in the discernment
of repetitive sequences such as that diagrammed in Fig. X-18. These are character-
istic of immediate backreef or bank-edge deposits. The sequence consists of a
conglomeratic boundary zone of red soil breccia marking a disconformity. Asso-
ciated with this is fracture-filling marine sediment in places with nests of brachio-
pods and crinoid debris (A unit). A thin intertidal laminite (B unit) follows and
grades in most places into an upper thick lagoonal grainstone-packstone member
(C unit) with onkoids, coated particles of dasycladaceans, large megalodonts in
place, gastropods, and variable amounts of micrite matrix (Plates IX B, XII B).
The C unit has generally a sharp top showing erosion, minor karsting, fracturing,