Page 317 - Carbonate Facies in Geologic History

304 Shoaling upward Shelf Cycles and Shelf Dolomitization

Platform Cycles with Intense Diagenesis

A third type of upward shoaling cycle is seen on major limestone banks or
platforms where rapid carbonate buildup has created considerable bathymetric
relief and where these buildups have been exposed to frequent sea-level fluctua-
tion. Such cyclic beds were termed Laferites by Fischer (1964) who emphasized
their fenestral fabrics. Some grainstone members of the cycles are very coarse and
contain onkoids, coated particles and grapestone lumps. Such sediment may
result from deposition and early diagenesis in warm hypersaline water of moder-
ate circulation and from conditions of intermittent exposure. In general such
sequences are distinguished from the wide shelf cycles not so much by dominance
of certain textural types as by paleogeographic position and diagenesis. The
impressive diagenetic effects could result from abrupt and sustained drops of sea
level. At least in the Lofer example much of the sedimentary record is transgres-
sive instead of regressive.
The opportunity for good drainage and splash zone cementation coupled with
strongly seasonal periods of rainfall and aridity results in such spectacular altera-
tion of these carbonates that certain sedimentary-diagenetic structures become
hallmarks of this type of cycle. Two of these sequences are well-known backreef
or bank interior strata of the Permo-Triassic banks of North America and the
Alpine region discussed in Chapter VIII. Their great similarity substantiates the
general analogy already made between these buildups.

Bank Interiors of the South Alpine Triassic

Assereto and Kendall (1971) illustrated spectacular exposures in the Val Seriana
quarries in the Bergamasc Alps north of Bergamo, Italy. Here in cycles a few
meters thick, burrowed bioclastic micrite with tiny tubules and gastropods consti-
tutes a lower phase. A middle phase consists of gastropod, onkoid, tubules, and
lithoclast-bearing wackestone, and an upper laminate fenestral dolomite mud-
stone exists. The lower unit is considered shallow lagoonal, the middle intertidal,
and the upper supratidal, affected by rigorous early diagenesis. Lithoclasts, coated
grains, pisoids, fenestrule (shrinkage pores), disarranged crusts, vadose sediment
infills, and giant polygons ("tepees") in cross section which show mud cracking
through many feet of sediment are characteristic. These features offer evidence
that strong evaporative conditions intermittent with marine deposition caused
frequent periods of drying out and abundant precipitation of calcium carbonate.
Leaching as well as desiccation and precipitation of carbonates occurred under
both marine and fresh-water vadose influence.
Bosellini and Rossi (1974) have also described similar cycles in the Dolomites,
particularly in the atoll-like interior of the Latimar Mountain group. Here the
lower subtidal phase consists of fossiliferous, burrowed, peloid and dasyclada-
cean-rich strata; the middle intertidal and supratidal member contains fenestral
stromatolitic structures with Foraminifera and ostracods, and an upper soil mem-
ber includes red vadose pisoids, sheet cracks, and tepee structures.