Page 120 - Geology of Carbonate Reservoirs

SEQUENCE STRATIGRAPHY 101

successions on maximum flooding surfaces and condensed intervals), sequence
stratigraphy (divides successions at unconformities and their correlative conformi-
ties — a system in which time value of the sequence is fundamental), and parase-
quence stratigraphy (recognizes shallowing - upward successions characterized by

flooding surfaces at their upper boundaries). These methods have in common that
(1) they recognize cyclicity in the rock record, (2) they depend on establishing a
time - stratigraphic framework, and (3) they focus on the natural boundaries that
delineate stratigraphic successions. These boundaries are unconformities, disconfor-
mities, conformities, and fl ooding surfaces.
Cyclical sedimentation results when relative sea - level changes systematically
from high to low to high, and so on. Relative sea - level change can be caused by
changes in global ice volume (glacioeustatic change) and by tectonic processes of
regional or local scale. Some points for argument among geologists are the causes
of relative sea - level change, the extent to which sea - level change is global (eustatic)
or regional (tectonic), and what mechanisms govern the periodicity of sea - level
change (e.g., changes in basin volume, changes in global ice volume, and forces
related to variations in the Earth ’ s orbit). The periodicity of sea - level change deter-

mines the order or time rank of the cycle. First - order cycles span 200 – 300 Ma and
are caused by major plate tectonic movements that may open basins or break up

continents. Depositional onlap and offlap at cratonic scale are produced by these
cycles. Second - order cycles have durations of 10 – 50 Ma and are related to changes

in ocean basin volumes by tectonism, changes in global ice volume, or both. Second -
order depositional sequences may be hundreds to thousands of meters thick. Third -
order cycles are thought to be driven by changes in ice volume; they represent
relative sea - level changes on the order of 50 m or less. High - frequency, climatically

driven cycles of 20 – 400 ky are thought to be caused by periodic fluctuations in the

Earth ’ s orbital characteristics referred to as Milankovich cycles. The amplitude of
relative sea - level change in these cycles may range from 100 m to 10 m depending

on whether the sea - level change happened during “ icehouse ” (extensive continental
glaciation) or “ greenhouse ” (limited continental glaciation) times, respectively.
The natural boundaries that delineate sequence - stratigraphic successions are
breaks in the rock record produced by erosion or nondeposition. Unconformities
are surfaces of erosion or nondeposition that represent gaps in time and that usually
have discordant relationships with bedding above and below. Disconformities also
represent breaks in continuity of deposition, but disconformities are surfaces that
are parallel with beds above and below. Conformities, following the defi nition in
Van Wagoner et al. (1988) , are surfaces that separate younger beds above from older
ones below without evidence of erosion or nondeposition and along which no sig-
nificant hiatus is indicated. They may also define marine flooding surfaces as surfaces

that separate older from younger strata and across which there is evidence of abrupt
deepening. Maximum flooding surfaces and condensed intervals separate the

transgressive phase from the highstand phase of a stratigraphic sequence.
We have already discussed the time ranking of cycles in sea - level change. The
spatial scales of observation in seismic and sequence stratigraphy range in area from
entire platforms to meter - scale flow units within fields. First - order sequences may

be thousands of meters thick and occupy entire basin margins. Each systems tract,
the lithogenetic association that formed on a platform during lowstand, trans gressive,
or highstand relative sea level, may incorporate many facies or ideal depositional