Page 159 - The Petroleum System From Source to Trap

152 Jordan and Wilson

Figure 7 .12. Block diagram of
carbonate lithofacies patterns
across an idealized carbonate
shelf during dominantly
aggradational stages of sedi·
mentation associated with
transgressive systems tracts
{TSTs). (See Figure 7.9 for
lithofacies legend.)

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63

INNER MIDDLE-OUTER SHELF BASIN
I SHELF I

Tamabra Limestone from the Poza Rica field of eastern SEQUENCE STRATIGRAPHY
Mexico (Enos, 1985).
A variation of the inner middle outer shelf pattern The theoretical basis of sequence stratigraphy takes
occurs on oceanic atolls (Figures 7.10 and 7.110). Here, into account constructive and destructive interference
the dominant facies belt is middle shelf, with a narrow among tectonic subsidence, eustatic sea level changes,
rim of outer shelf deposits outlining the atoll. Only small, and sediment accommodation space and accounts for
localized occurrences of inner shelf settings might be trends in relative sea level change. The concepts of
represented on small islands. Reef types include the sequence stratigraphy as applied to carbonate rocks
main barrier reef of the outer shelf and numerous steep­ (Shaw, 1964; Irwin, 1965; Sarg, 1988; Schlager, 1992) are
sided patch reefs in the central lagoon of the atoll. based on the lateral correlation of coeval lithogenetic
A similar diagrammatic approach is used to show units that are separated by one of two kinds of unconfor­
aggradational lithofacies patterns (Figures 7.12 and 7.13). mities: type 1, extending out into the basin, and type 2,
Here, the profile is a homoclinal ramp that gently slopes restricted mainly to inner shelf settings. Correlations are
basinward (with dips commonly less than 1 '-2 '),with made on shelf to basin profiles across carbonate shelves
numerous reversals of water depth (or dip) occurring at and atolls that exhibit one of two main types of deposi­
localized shoals and patch reefs. In this profile, shorelines tional profiles: ramps or drop-offs (Wilson, 1975).
are generally not smooth but rather digitate or barred. A shelf to basin profile shows the general concepts of
Extensive tidal flat deposits are formed on a gently sequence stratigraphy in time and depth (Figure 7.14).
dipping coast. Grainstone shoals or reefs form above any Within this framework, time-equivalent inner , middle ,
submarine topographic expression: faults, salt or shale and outer shelf, slope, and basinal environments of
diapirs, paleohighs formed as erosional remnants, or deposition can be recognized, and associated lithofacies
older reefs. Water depth and substrate type are critical to can be predicted for various systems tracts. With this
facies development, and slight changes affect facies approach, it is evident that the best developed barrier
patterns, resulting in complex lithofacies maps. In reef sequences can be expected in shelf-margin wedges
addition, on ramp profiles, it is difficult to distinguish (SMWs), in highstand system tracts (HSTs) where reefs
easily between middle and outer shelf settings. In the can "stack up," and in lowstand wedges (LSWs). Patch
proximal parts of the basin and in lower slope settings, reefs occur across middle shelf environments in all
pinnacle reefs may occur in a belt subparallel to the basin systems tracts and in outer shelf settings in transgressive
margin. They are steep sided and have narrow halos of system tracts (TSTs). Empirically, it appears that ramp
reef-derived material around them. Pinnacles are charac­ profiles can evolve into drop-off profiles, but not vice
teristically surrounded by and encased in basinal shales versa, and that they tend to be related to TSTs and to
(or evaporites) that may provide source and seal. early stages of HSTs.