Page 99 - Geology of Carbonate Reservoirs

80 STRATIGRAPHIC PRINCIPLES

Rimmed Shelf Distally Steepened Ramp
Shallow
Subtidal
Beach
Slope Shallow to Deep
Break Subtidal
Slope Toe Distal
Steepening
Basin Basin

Figure 4.3 Isolated platforms may be shelves or ramps but rather than being attached or
detached in close proximity to the mainland, isolated platforms are completely surrounded
by oceanic depths. In other words, isolated platforms are islands. There is no size limit for
isolated platforms but common usage dictates that individual atolls and pinnacle reefs are
not included as isolated platforms. Platforms must be large enough to exhibit a continuous,
lateral array of standard depositional successions across their submerged surfaces.

of individual ramp or shelf depositional surfaces in stacked successions clearly
requires knowledge about time equivalence among the individual surfaces, espe-
cially where facies have migrated laterally and vertically. Facies may migrate up or
down depositional platforms as they follow rising or falling relative sea level. If
facies are correlated only on the basis of lithology, then they will cut across time
surfaces, a condition known as diachroneity . One of the objectives in sequence
stratigraphy is to identify time surfaces and the depositional units that are bounded
by them.

4.1.1 Rimmed and Open Shelves
Rimmed shelves are shallow marine platforms that have pronounced slope breaks
some distance from shore. The slope break is marked by rim - forming reefs and
banks that may consist of rigid reefs and mounds, as on the South Florida shelf that
extends seaward from the Florida Keys. Shelf rims may consist of submarine dunes
and grainstone “ sand waves, ” as on the Great Bahama Banks. Such rims are con-
structional; that is, they were formed by depositional processes that shaped the shelf
edge. In other cases, shelf rims may be inherited from periods of destructional
exposure and weathering (Purdy, 1974 ). Regardless of their origin, shelf rims inter-
act with the hydrologic regime. Incoming waves, tides, and currents from the open

sea are refracted, reflected, or translated by the rims so that the shelf interior is
shielded from vigorous water movement except during the most severe storms. This
situation can only exist in water shallow enough for the rim to interact constantly
with inbound, fair - weather waves and currents. In Florida and the Bahamas, the
rimmed margins are partly or totally exposed at low tide. The depth to which reefs
or grainstone banks can interact with waves and currents is determined by wave
climate and rim bathymetry. Oolite grainstone banks and hermatypic coral reefs do
not form rims in water deeper than a few meters. For example, the average maximum
water depth on the Florida and Bahama shelves is about 10 meters.

Rims on rimmed shelves serve as baffles, or “ sieves, ” to filter and reduce the
power of incoming oceanic waves, tides, and currents. Geologists commonly use the
term “ high energy ” to describe settings in which wave power is great or in which