Geology Reference
In-Depth Information
Current 3-D modeling incorporates the basic pro-
cesses of carbonate accumulation and diagenesis in re-
lation to sea-floor fluctuations, basement subsidence,
wave-base erosion and transport of sediment. Common
input parameters are initial bathymetry and sea-floor
geometry, sea-level changes, changes in paleoenviron-
mental conditions (e.g. salinity, temperature), and pa-
leoclimate. 3-D modeling of carbonate basins has be-
come a very successful technique in reservoir model-
ing, particularly where the geological and physical res-
ervoir parameters show extreme heterogeneities (e.g.
Vahrenkamp and Grötsch 1994). Modeling of the rela-
tionships between reservoir lithofacies and petro-
physical criteria reflected by logs requires the differ-
entiation of well-strained lithofacies units. The consid-
eration of microfacies types is an important tool in de-
fining these units.
with supratidal microbial laminites, sometimes with
inland freshwater marsh deposits, coal, or siliciclastics.
Lagoonal facies. Developed behind barrier com-
plexes. Mainly bedded limestones with micritic or
peloidal lime mudstones, or cherty burrowed skeletal
packstones to mudstones, sometimes with local patch
reefs of colonial metazoans. Minor, thin interbeds of
fenestral or cryptalgal carbonates reflect periods of shal-
lowing of the lagoon to tide level.
Shoal-water sediments consisting of banks, reefs and
ooid/pellet shoals. Occurring as shallow-ramp skeletal
banks or lime-sand shoals, or as shelf-edge skeletal reefs
and lime sands that may pass gradually downslope into
deep ramp facies. On deeply sloping platform edges,
the facies passes downslope into foreslope and slope
deposits adjacent to deep shelves or basins.
Deep shelf and ramp facies. Nodular, sometimes
cherty bedded skeletal packstone and wackestone with
abundant whole fossils and diverse open-marine biota.
The limestones may have upward-fining and storm-
generated beds. Water depths range from some tens to
more than 100 m. The depositional base is largely be-
low the fair-weather wave base, but may be influenced
by storms.
Slope and basin facies. Foreslope and slope depos-
its adjacent to steeply sloping platforms have abundant
breccias and turbidites interbedded with periplatform
lime muds and terrigenous muds. Adjacent to most
ramps, slope and basin deposits are thin-bedded peri-
platform lime muds or terrigenous muds with gener-
ally only few sediment-gravity flow deposits. Paleo-
zoic basinal deposits are commonly shales, with car-
bonate content increasing toward the platform. Meso-
zoic and Cenozoic basinal deposits may be pelagic lime-
stones or shales. The slope and basin floor can be an-
oxic and lacking benthic organisms. These deposits are
laminated and non-burrowed. Oxic slope and basin de-
posits are burrowed and fossiliferous.
14.1.2 Basic Elements of Carbonate Facies
Models
Basic elements used in carbonate facies models describ-
ing transects from shallow-marine to deep-marine set-
tings are
• changes in the relief of the sea bottom expressed by
differently dipping shelf slopes and the shallow-water
or deeper-water position of a distinct break in the mor-
phology of the shelf,
• vertical boundaries affecting the sea bottom, and rep-
resented by high and low tide levels, the fair-weather
wave base and the storm wave base, and
• lateral differences in the composition of sediments
and benthic biota allowing facies zones to be distin-
guished.
These basic elements have been discussed in Chap. 2
of this topic.
14.1.2.1 Common Facies Belts
14.1.2.2 Common Depositional Patterns
Most models used in interpreting ancient shallow-
marine to deep-marine carbonate rocks consider five
major facies belts along a shoreline-to-basin transect
that include platforms or ramps and continue via a slope
into basins (Read 1995):
Sedimentation on shelves often results in meter-scale
shallowing-upward sequences , because the accumula-
tion rates commonly outspace the combined rates of
subsidence and sea-level rise (James 1984; Osleger and
Read 1991). Consequently, the tops of these sequences
show signs of intertidal to supratidal exposure, or they
are eroded. Such small-scale cycles can be stacked to
form larger sequences displaying transgressive and re-
gressive trends. Stacking of sequences is a characteris-
tic criterium of many ancient platforms. Larger-scaled
sequences are commonly attributed to third-order sea-
Tidal flat facies. Generally arranged in cyclic, shal-
lowing-upward units. Sequences in arid climate exhibit
burrowed to non-burrowed lagoonal limestones over-
lain by intertidal microbial and algal laminites, su-
pratidal evaporites, or eolian-fluvial clastics. Humid
sequences are burrowed subtidal-intertidal limestones
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