Page 116 - Geology of Carbonate Reservoirs

ANATOMY OF DEPOSITIONAL UNITS 97

on low - to - moderate - energy platforms will be accompanied by rather monotonous,
mud - dominated depositional successions (wackestones and mudstones). Platforms
with the same bathymetry and depth but with a very high - energy environment may
be swept clean of detrital sediment, leaving a bare rock surface as on the high -
energy platform off southwestern Australia (James et al., 1992 ).
Variations in bathymetry can cause unexpected facies to occur in the usually
monotonous neritic, wackestone – mudstone terrain. If bathymetric highs or lows are

present on the otherwise monotonous flats, then facies variations will be present in
and around the highs and lows that are not the same as the ordinary mudstones or
wackestones. Grain - dominated facies or even reefs may be present on the highs and

be surrounded by the more monotonous neritic flats and thick sections of mudstones
may fill in low areas. In fact, bathymetric highs may have been so shallow that they

were exposed as islands or they may have been shallow shoals surrounded or blan-
keted by grain - rich facies that have fundamental rock properties nearly identical to
shelf - edge or barrier island grainstone belts. Without knowing that a grainstone or
reef was deposited on an isolated high within a broader neritic zone, it would be
diffi cult to tell the difference from a rock sample alone between it and an identical
succession deposited on a shelf edge or in the subtidal segment of a strandplain
succession. In such cases, seismic data or extensive subsurface geological data are
invaluable aids to help differentiate between local anomalies and regional trends in
facies character.

Studies of modern and ancient platforms generally confirm that standard depo-

sitional successions can be associated with specific depositional environments on
ramps, rimmed shelves, and open shelves. For example, shoreline grainstone succes-
sions are typical of ramps but they are absent on rimmed shelves because rimmed
shelf shorelines lack breaking waves and strong currents required to winnow and
concentrate grainstones. Stronger hydrological conditions occur at the shelf - slope
break on rimmed shelves, well away from the shoreline. Grainstones may occur
along the shorelines of some open shelves depending on the power of the surf and

on the supply of grains to be deposited. Tidal flats and lagoons are common on
rimmed shelves and where they occur behind detached barrier islands on ramps.
Tidal flats are mud - dominated systems that require sheltered or restricted circula-

tion that occurs behind barrier islands and on nearshore zones far inboard from the
agitated environment of the slope break.
The shallow subtidal environment is common to all depositional platforms, as is

the basinal environment. Oceanography textbooks define the neritic environment
as the zone that extends from below mean low tide to a depth of 200 m, but that
depth is below the principal zone of carbonate production virtually everywhere. In

fact, 200 m is about the maximum depth of the Persian Gulf, where it represents the
basinal environment. For our purposes the shallow subtidal or neritic environment
is the zone inhabited by an abundant and relatively diverse benthic fossil population
that contains mainly photozoans (perhaps including reef organisms), and that is
bounded on the landward side by tidal flats or beaches and on the seaward side

abruptly by slope breaks (shelves) or gradationally by outer - ramp environments
(ramps).
The shelf - slope break is present only on open and rimmed shelves — by defi nition.
Distally steepened ramps may have changes in slope, but those changes are not