Page 115 - Geology of Carbonate Reservoirs

96 STRATIGRAPHIC PRINCIPLES

The seven standardized successions presented in this book are designed to allow
for some variability. For example, the beach – dune succession may be attached or
detached from the mainland. Detached beach – dune successions occur as barrier

islands that may have updip equivalent lagoon and tidal - flat successions. Lagoonal
successions typically consist of burrowed, wavy bedded mudstones and wackestones
with low - diversity biota. In some cases, storm washover fans may be incorporated

in the lagoonal succession. Tidal flats may or may not have extensive evaporite
deposits like those in the Trucial Coast sebkhas. Tidal flats may or may not have

extensively developed channels in the intertidal zone. Shallow subtidal successions
may or may not include patch reefs and associated grainstones; slope - break succes-
sions on rimmed shelves may consist of reef trends along the break or of grainstone
buildups with few, if any, reefs. Slope - toe successions may include slumps, debrites,
grain flows, or coarse turbidites, and basinal successions may appear as microlami-

nated zones, rhythmites, or turbidites. Seven generalized environmental zones were
chosen for simplicity, brevity, and especially to reduce the number of possibilities
one must identify from unknown successions in borehole cores or “ lith logs ” from
cuttings. Some environmental zones produce more complex patterns of sedimenta-
tion than others. In those cases, it is necessary to have “ supplemental ” successions
to provide more complete coverage of the range in depositional style. The seven
basic environmental sectors are: (1) the shoreline environment consisting of beaches
with or without dunes and that may be attached to or detached (as a barrier island)

from the mainland; (2) the intertidal or tidal - flat environment updip from lagoons
(on ramps) or shallow subtidal waters (on shelves); (3) the lagoonal environment
behind barrier islands detached from the mainland and consisting of strike - parallel,
elongate islands with beaches plus or minus dunes; (4) the shallow subtidal environ-
ment; (5) the slope - break environment that characterizes shelves; (6) the slope and
slope - toe environments that comprise the slope below the shelf - slope break and the
base, or toe of slope below the break; and (7) the basinal environment that extends
from the toe of slope on shelves to the greatest basinal depths on ramps or shelves.

This is a greatly simplified system by design. Basinal environments on ramps may
encompass environments extending from outer neritic to bathyal regimes. It is not
possible to predict every variation in facies characteristics that may occur on any
given platform within any given environmental zone. That notwithstanding, years of
personal experience and extensive review of the literature have convinced me that
this simplified system enables one to identify depositional textures, sedimentary

structures, biota, and constituent components common to specifi c depositional suc-
cessions, to associate the successions, or models, with environmental zones or posi-
tions on ramps or shelves, and to make reasonable predictions about the types of
laterally equivalent successions in updip and downdip environmental cells.
Most of the ideal successions are associated with narrowly defined locations on

platforms and with implicit constraints on variability of environmental conditions
that characterize each zone, or sector. Limited variability in environmental condi-
tions suggests limited variability in lithological characteristics of the associated
depositional successions. Some of the environmental zones are not so narrowly
defined and may have rather widely varying lithological characteristics depending

on platform geomorphology, bathymetry, and hydrological regime. For example,
monotonous bathymetry and depth in the subtidal zone (the neritic environment)