Page 111 - Geology of Carbonate Reservoirs

92 STRATIGRAPHIC PRINCIPLES

in which they formed. For example, lagoonal facies, tidal - flat facies, or reef facies
are common terms in ordinary practice. More formal procedures require that facies

names reflect only rock properties, not environmental terms. A name such as “ intra-
clastic, skeletal packstone facies, ” for example, does not have explicit environmental
significance. This distinction is made to avoid subjective or interpretive terms in

facies names, but in common practice geologists do not always make these distinc-
tions. It is important to recognize that facies are considered in this book to be two -
dimensional (2D) features that may not represent all of the variations in sedimentary
character that can occur in one environment through some span of time until the
original facies is replaced by a new one as the environment shifts in response to
changes in relative sea level. Some workers will not agree with this usage, but for
teaching purposes, it is valuable to consider all the variations in fundamental rock
properties in one environment as a depositional succession, not as one facies. If all
variations in rock properties in a beach – dune complex comprised a facies, for
example, then each variation (lower shoreface, middle shoreface, upper shoreface,

and dunes) would become a subfacies. I find it simpler to work with depositional
successions that include the usual variations in one vertical “ package. ”
Depositional succession (also called depositional sequence) is therefore defi ned
as the vertical array of distinctive lithological and biotic characteristics at a single
site such as a borehole or an outcrop. Because the vertical dimension is included,
depositional successions only occur in the rock record; modern sedimentary facies
are two - dimensional “ blankets ” that represent a single, short time. The word “ suc-

cession ” is defined as the process of following in order or in sequence. Succession,
in the geological sense, describes the vertical array of rock properties at a location

over an unspecified, but generally short and continuous time span. A short time span
in this case can be on the order of 6000 years or less, as illustrated by the Holocene
successions that have accumulated in the various environments on carbonate
platforms since sea level reached its present height. It is critical that successions
represent essentially unbroken timelines, because gaps in stratigraphic continuity
represent unknown amounts of time and time – rocks in the sedimentary record.
Clearly, the Holocene successions will have different thicknesses, textural trends,
sedimentary structures, and constituent compositions, depending on the environ-
ment in which they were deposited. Finally, the term succession is used partly to
distinguish successions from standard microfacies and to avoid confusion with

sequence , as defined by Sloss (1963) , namely, “ rock stratigraphic units of higher rank

than group, megagroup, or supergroup, traceable over major areas of a continent and
bounded by unconformities of interregional scope . ”
In the following pages, idealized depositional successions are illustrated as
sketches that resemble borehole cores arbitrarily about 10 m long (the length of a
standard core barrel), showing the typical sedimentary characteristics for each envi-
ronment on ramps and shelves. Thicknesses of the successions depend on accom-
modation, time, and rate of sedimentation, but the ideal successions are rock units
without time connotation. Depending on relative sea - level history, accommodation,
and depositional rates, the successions can migrate landward or seaward with
advancing or retreating relative sea level. For example, Holocene barrier islands on
the northeastern coast of Yucat á n may be about 10 m thick, representing deposition
during only a few thousand years since present sea level was reached. However,
where sediment production and accumulation are less than on barrier islands, such