Page 144 - Geology of Carbonate Reservoirs

DEPOSITIONAL ENVIRONMENTS AND PROCESSES 125

shelves have high rates of carbonate productivity and high taxonomic diversity
because the rim - forming process is a feedback loop in which favorable environmen-
tal quality promotes biological productivity that generates a reef rim. A fundamen-
tal characteristic of rimmed and open shelves is that the shelf - slope breaks have
persistent and distinctive facies changes paralleling the slope break. This distin-
guishes both rimmed and open shelves from distally steepened ramps, which do not
have facies changes that parallel abrupt changes in slope. Facies changes do not
occur at slope changes on distally steepened ramps because the distal steepening
occurs in water deeper than that at which fair - weather waves and currents interact

with the bottom to influence sedimentation. Distal steepening on ramps occurs
below the greatest depth at which the carbonate factory operates. Generally, the
sediments that accumulate around deep - water slope changes on distally steepened
ramps are allochthonous mudstones and wackestones. In strong contrast, slope
breaks on tropical, shallow - water rimmed shelves are dominated by high - energy
deposits. Shelf rims build nearly to sea level and act as barriers to incoming oceanic
waves and currents. The rims are buffeted by breaking waves and strong currents
so that only the most resistant reef structures can withstand the pounding. Coarse
rudites usually accumulate on the seaward sides of rims, and grain - rich successions
accumulate in the immediate lee of the rims. Rim - forming organisms do not appear
to thrive in temperate climates nor do temperate seas favor the precipitation of
aragonite; consequently, there are probably no temperate rimmed shelves. Another

platform configuration with a slope break is the open shelf. When one compares
modern high - energy open shelves such as the Lacepede shelf of Australia with low -
energy rimmed shelves like the one in South Florida, it is immediately clear that
great differences exist in where and how much sediment accumulates around the
open shelf and rimmed shelf slope breaks. These differences exist because the
dynamics of the respective hydrologic regimes are so different. It is not enough to
identify the slope break and attendant facies changes that distinguish shelves; it is
also necessary to determine if the environment of deposition was high or low energy,
and if it was a tropical or temperate regime.

5.2.8 Depositional Rock Properties in Slope -Break Successions
Slope - break successions are asymmetrical or polar deposits. That is, grainy facies,
carbonate gravels, and massive reef growth characterize the seaward side of the
slope break, facing the prevailing winds and incoming oceanic waves and currents.
Lower - energy facies accumulate on the leeward side of the slope break. If reefs
develop at the slope break, growth patterns of the reef - building organisms also

reflect the polarity of the environment. Massive skeletal forms develop on the high -
energy, windward side of shelf - margin reefs and more delicate skeletal forms develop
in the protected zones behind the reef. Spur and groove features, also known as
buttresses and chutes, may mark the windward side of some framestone reefs. These
gaps in the reef help diffuse and absorb incoming wave and current energy. The area
behind the shelf - edge reef trend is usually blanketed by bioclastic grainstones and
packstones derived from the breakdown of reef skeletal structures, or from the
benthic organisms that lived on and near the reefs. In tropical environments the
detrital grains may be oolitically coated. The modern White Bank shoal of South
Florida is a bioclastic sand – wave (grainstone) complex that extends about 2 km