Page 98 - Carbonate Sedimentology and Sequence Stratigraphy

CHAPTER 6: FUNDAMENTALS OF SEQUENCE STRATIGRAPHY 89

SEQUENCE Fig. 6.6.— Lapout patterns in a se-
TOPLAP BOUNDARY quence of the standard model. Note
that besides the sequence boundaries
onlap there are downlap and toplap patterns,
i.e. unconformities in common geologic
onlap
language, within the sequence. After Vail
(1987). (Reprinted by permission of the
AAPG whose permission is required for
further use).
TRUNCATION
SEQUENCE
BOUNDARY DOWNLAP
DOWNLAP SURFACE APPARENT
TRUNCATION

Fig. 6.7.— Lapout patterns in carbon-
ate sequences are more varied than in
siliciclastics particularly because there
4 strike section 8 are many localised centers of high pro-
duction, such as reefs, and localised ar-
eas of erosion, such as inter-reef chan-
nels. Particularly characteristic is the el-
evated platform margin that may simul-
taneously prograde landward and sea-
ward. Numbers refer to characteristic
2 situations: (1) karst buried by marine
3 9
4
sediment, (2) lagoonal patch reefs or
mounds, (3) prograding backreef apron,
1 5 (4) bioherms at platform margin, (5)
6 7 slope clinoforms, (6) slope clinoforms
downlapping on basin floor, (7) onlapping
basin-floor sediments, (8) incisement of
shelf edge at sequence boundary, (9)
shelf-margin incision within sequence.
After Handford and Loucks (1993), mod-
ified.

gram represents pelagic or hemipelagic sediments whose The geometry of systems tracts leads to characteristic stra-
sedimentation rate is sufficiently low that they frequently tal patterns in seismic profiles and large outcrops (Fig. 6.6;
are not resolved by seismic data. It is important to note that Fig. 6.7). It should be noted that the lapout patterns indi-
this does not automatically imply that these sediments show cated in this scheme refer to seismic lapout and that seismic
evidence of biostratigraphic condensation, i.e. fossils of sev- lapout does not necessarily imply a genuine unconformity
eral biozones in one layer (Heim, 1934); nor does it necessar- in outcrop or cores (see sections on unconformities, p. 90f
ily imply field-geologic evidence of starved sedimentation and pseudo-unconformties p. 127 f).
such as hardgrounds or authigenic minerals in the form of
Fe-Mn oxide crusts, phosphate crusts or glauconite grains Stratigraphic time lines and seismic reflections
(Flügel, 2004, p. 211-216).
Field geology of continental interiors was the cradle of
For the sequence stratigrapher the term “condensation” sequence stratigraphy but what makes it such an impor-
is broader than for the biostratigrapher or the sedimentol- tant tool at this day and age is its immediate applicabil-
ogist. The condensed facies in sequence stratigraphy in- ity in seismic interpretation. In fact, most of what here is
cludes the facies with evidence of biostratigraphic or sedi- called the standard model of sequence stratigraphy was in-
mentologic condensation mentioned above but the term also troduced under the heading “seismic stratigraphy” by Vail
includes normal pelagic or even hemipelagic facies of the et al. (1977). The connection between seismic reflections and
ocean basins with sedimentation rates of 10 µ/y or higher time lines (or time surfaces) in the sediment record is of piv-
(Loutit et al. 1988, p. 186). otal importance in this context and merits some comment.