Page 48 - Carbonate Facies in Geologic History

Natural Construction of Carbonate Ramps-Platforms 35

Transgression
c
Shorel ine 2 Shorel ine I
I

j, O::::::C:::~~:----""":"-i"'-""::"':7=~:::-::=::::::::::.Sea L ..... ,
Hin~e

a

Zone of Clast ic Zone of Optimum Zone of Dimini'hed
tnfluK Carbonate Production Carbonate Product ion
r---~-,v ~ A~ ______ _
"""»> Seo Level
.,. ~ ~
b

Fig. II-12a and b. Creation of shelf to basin topography from Meissner (1972, Figs.4 and 5),
with permission of West Texas Geological Society. (a) Stratigraphic model showing surfaces
of deposition related to a transgression caused by differential subsidence. Rate of subsidence
is assumed to be greater than rate of deposition. (b) Stratigraphic model showing the creation
of shelf to basin topography caused by differential rates of carbonate sedimentation. Little or
no subsidence following a rapid initial transgression is assumed

The belt of maximum carbonate accumulation on a planar surface is apt to
occur removed from land, down the paleoslope, and paralleling ancient coast
lines, but in water shallow enough to be within the zone of maximum biological
productivity. Consequently, the mass of organic carbonate grows most rapidly
along preferred positions on the upper part of any gentle seaward slope. Contin-
ued submergence of a positive block, plus the fact that organic buildup and
sediment accumulation, when once started, is normally very rapid, results in the
formation of a line of shelf margin organic buildups (Fig. 11-12). Concurrent fill-in
of the lagoon, between the shelf margin and land, occurs by restricted marine or
evaporite deposits, and eventually a sloping ramp or level platform is constructed
surrounding the positive element.
Not much sediment accumulates simultaneously in the area downslope from
the shelf margin, and as the platform buildup occurs, and relative sea level rises,
the water offshore becomes relatively deeper. The normal result is a basin sur-
rounded by carbonate ramps or platforms which built out from the positive
elements rimming the original depression and possess certain special facies down
the slope into the basin (Fig. 11-13). The basin seaward of the platform remains
relatively deep for three reasons: (1) it may be the site of more rapid subsidence;
(2) it is commonly starved of sediment, receiving only pelagic, calcareous and
siliceous organic debris, and some fine argillaceous matter, either wind blown or
brought in from distant rivers; and (3) such argillaceous sediment compacts to a
higher degree than pure carbonate shelf or shelf margin deposits. The latter are