Page 92 - Carbonate Sedimentology and Sequence Stratigraphy

CHAPTER 6

Fundamentals of sequence stratigraphy

INTRODUCTION smaller units and seismic data. These authors defined a
depositional sequence as a “stratigraphic unit composed of
PRINCIPLES AND DEFINITIONS a relatively conformable succession of genetically related
strata and bounded at its top and base by unconformities
Careful definitions and consistent terminology have been or their correlative conformities”. The definition by Vail et
a hallmark of sequence stratigraphy from the outset. By al. (1977) is in essence a geometric one and therefore well
now, however, the field suffers from an inflation of terms. I suited for seismic interpretation. It contains no statement
will introduce and use only the most essential ones. Others on scale in time or space. The definition furthermore avoids
are explained elsewhere and can be recovered via the index. any reference to the origin of a depositional sequence even
The discussion in this chapter assumes that the following though Vail et al. (1977) clearly expressed their conviction
principles hold. that sea level fluctuations were the dominant control.
➤ Sedimentary bodies and thus all physical stratigraphic Sequence boundaries were defined by Vail et al. (1977, p.
units are of finite extent - they are lenses in the most 53) as “observable discordances ... that show evidence of
general sense of the term. erosion or nondeposition with obvious stratal terminations
➤ Surfaces of erosion or non-deposition are also of lim- but in places they may be traced into less obvious paracon-
ited extent. On the flat basin floors, the ultimate limits formities recognized by biostratigraphy or other methods”.
are set by the principle that siliciclastic sediment mass This definition, too, is essentially based on geometric rela-
is conserved. Basins continually receive sediment and tionships and again abstains from any statement on scale
the water column cannot hold this material for any ge- in time or space. Vail et al. (1977, p. 55) add that the hia-
ologically significant time. As dissolution of siliciclastic tus at the unconformity generally is on the order of a mil-
sediment is negligible, the bulk must come rest on the lion to hundreds of millions of years. The definition of se-
basin floor. Thus, erosion in part of the basin must be quence boundary also lacks a statement on the origin of the
compensated by increased sedimentation in others. For unconformity (except for the reference to erosion and non-
carbonate material, the conservation of mass does not deposition).
hold for the abyssal sea floors but it remains a reason- Van Wagoner et al. (1988, p. 39) added an important
able assumption for neritic and bathyal settings. genetic qualification to the definition of sequence and se-
➤ The sediment record is dissected by hiatuses at all ge- quence boundary. They follow Vail et al. (1977) in defin-
ologically relevant scales. The assumption of a “com- ing a depositional sequence as an unconformity-bounded
plete section” is an (often useful) idealization. succession of strata but then proceed to re-define unconfor-
➤ The sediment record at any given locality reflects the mity as “a surface ... along which there is evidence of sub-
history of relative sea-level movements. The history of aerial erosional truncation (and, in some areas, correlative
eustasy requires either evidence of a world-wide phe- submarine erosion) or subaerial exposure”. This definition
nomenon (such as orbital signals), or a global stack of has been used in several key publications (e.g. Posamentier
records plus constraints from geodynamics. Geody- et al., 1988, p. 110; Emery, Myers et al.,1996, p. 24). Van
namic input is necessary because crust and mantle de- Wagoner et al.’s definition makes for an internally consis-
form under the changing water load and therefore the tent concept: all sequence boundaries are related to relative
same eustatic signal is recorded differently in different falls of sea level and thus sequences are predominantly con-
regions (Watts, 2001). trolled by sea-level fluctuations. In spite of this, I recom-
mend not to use this unconformity definition in sequence
Sequence and sequence boundary stratigraphy because it is at odds with the traditional us-
age of the term unconformity and faces grave difficulties in
When Sloss (1963) formally introduced the concept of de- practical application. Van Wagoner et al.’s definition of un-
positional sequences, he defined sequences as “rock strati- conformity is much more restrictive than the generally ac-
graphic units ... traceable over major areas of a continent cepted connotation of this old and widely used term (for in-
and bounded by unconformities of interregional scope”. stance Bates and Jackson, 1987). Unlike the geometric fea-
Vail et al. (1977) adjusted this definition for work with tures used by Vail et al. (1977), the criterion of subaerial ex-

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