Page 112 - Geology of Carbonate Reservoirs

ANATOMY OF DEPOSITIONAL UNITS 93

as the shallow subtidal environment on the Yucatan ramp, the postglacial sediment
blanket varies in thickness from just a few centimeters to about 1.4 m (Logan et al.,
1969 ). The shallow subtidal Holocene blanket on the Florida shelf varies in thickness
from zero (bare Pleistocene rock) near the shore at Key Largo to about 5 m in the
White Bank sand wave buildup (Enos and Perkins, 1977 ). These variations in sedi-

ment thickness reflect accommodation, sediment source, and mechanisms of sedi-
ment dispersal and retention. Sedimentary successions have stratigraphic contacts
that mark a beginning and an end, and the successions may be cyclically repeated
as time progresses. There may be a variety of rock properties in the vertical
dimension, but the variations in those rock properties are generally regular and
predictable.

Ideal depositional successions, like two - dimensional facies, are defined on funda-
mental rock properties and biota. They differ from facies in being explicitly linked
to depositional environments, in the emphasis placed on their vertical dimensions,
and by exhibiting rock properties that can vary widely, but predictably, from base
to top of the succession. The beach – dune example mentioned earlier consists of
several subenvironments and their characteristic deposits — dunes, upper shoreface,
middle shoreface, and lower shoreface. Those deposits are stacked vertically to form
an ideal beach – dune or barrier island depositional succession. Each of the subenvi-
ronments is associated with a set of characteristic rock properties. If the beach – dune
complex builds seaward (regressive barrier) then the horizontal arrangement of the
subenvironments is repeated in the vertical (Figure 4.4 ) according to Walther ’ s rule
(Walther, 1894 ). This vertical array represents all of the subenvironments in the
barrier complex and comprises a standard depositional succession .

4.4.2 Environmental Subdivisions and Standard
Depositional Successions

Ideally, facies are defined objectively based on their fundamental rock properties
regardless of depositional environment. However, Wilson (1975) interpreted asso-
ciations of rock properties, biota, and depositional environments to defi ne nine
standard microfacies that represent nine environmental subdivisions of an ideal
platform (Figure 4.8 ). Application of the standard microfacies concept to the rock
record enables one to interpret environments of deposition directly from rock and
fossil characteristics at any location on the standard platform and to predict the
locations and characteristics of other facies on the platform. This method of inter-
preting depositional environments in carbonate rocks is commonly practiced today
and it can be a powerful tool for predicting the locations of reservoir rocks. The
concept of standard microfacies was a major advance in our understanding of the
relationships between environment, sedimentary characteristics, and biota. Wilson ’ s
(1975) method provides reliable results as long as it is recognized that his idealized
platform is a rimmed shelf. Note that beaches, dunes, or barrier islands that typify
ramp shorelines do not normally occur on rimmed shelves and are not included in
Wilson ’ s nine standard microfacies.
Standard microfacies are two - dimensional (2D) blankets that represent the sedi-
mentary cover on a platform sector during a short time (maybe a few hundreds to
thousands of years — a “ blink ” in geological time). As time and sedimentation con-
tinue, the sedimentary blankets evolve into stacked assemblages (successions), each