Page 52 - Carbonate Facies in Geologic History

The Effect of Differential Subsidence during Deposition 39

.-,"",c:::::.===- EXPAND I 16 BANK

1. NORMAL BASINAL PYROCLASTIC SEDIMENTS
CIPIT LIMESTONE, EXOTIC BLOCKS
DETRITAL CARBONATE OF FOREREEF TALUS
MASSIVE DOLOMITE OF BANK

~ ~m~
1 2 3 4

,..,..~--= PROFILE OF BANK WHICH EXPANDED
AND THEN SHRANK

[:?gl ~
~ ~
2

Fig. II-17. Upbuilding, outbuilding, and inbuilding of carbonate banks in Dolomites of South
Tyrol, Italy as discerned from foreslope beds, after Leonardi (1967, Figs. 154 and J 55)

Some of the greatest carbonate banks have mainly built upward; in areas of
geosynclinal subsidence, sedimentation often just manages to keep pace. The EI
Abra Cretaceous of central Mexico and the Middle Triassic of the Dolomites are
notable examples (Fig. II -17).
Rarely is there found a continuously transgressive record of carbonate facies.
The best documented is that of the Helderberg Group of New York State (La-
porte, 1969) developed in beds marginal to the Appalachian geosyncline where
clear evidence exists of westward inundation followed by a seaward progradation
back into the geosyncline to the east.
Usually carbonate production keeps up with relative sea-level rise. Transgres-
sive reefs forming a continuous stratigraphic unit are essentially unknown, but the
pinnacle form developed on some offshore banks indicates that in some basins
subsidence has overcome sedimentation. Generally, some cause other than a too-
rapid subsidence or sea-level rise ends the carbonate building process. These
causes might include a rapid, but brief, sea-level drop preceding the next marine
incursion, terrigenous influx, climatic change, or restriction ofthe basin to evapor-
itic conditions.