Geology Reference
In-Depth Information
Fig. 14.3. Carbonate Ramp Model of a homoclinal ramp, modified from Burchette and Wright (1992). The differentiation
into an inner, mid- and outer ramp can be applied both to homoclinal and distally-steepened ramps (see inset). Sediment
texture, grain size, and biotic criteria depend on the water energy at the sea bottom differing above the fair-weather wave
base, between the fair-weather wave base and the storm wave base, and below the storm wave base. The slopedip is strongly
exaggerated. Not to scale. Ramp lengths vary between some tens and some hundreds of kilometers.
Box 14.2. Carbonate Ramp Model (Burchette and Wright 1992). Criteria of inner, mid- and outer parts of ramps.
Inner ramp
The inner ramp comprises the euphotic zone between
the upper shoreface (beach or lagoonal shoreline) and the
fair-weather wave base.The sea floor is almost constantly
affected by wave action. The zone is dominated by sand
shoals or organic barriers and shoreface deposits. The shal-
low inner ramp may consist of (1) a beach barrier-tidal
delta complex with lagoons and tidal flats behind (back-
ramp), or (2) fringing sand banks and shoal complexes
with intertidal and supratidal flats, but no lagoons behind,
or (3) a strandplain of linear beach ridges with depres-
sions.
Characteristic sediments are carbonate lime sand bod-
ies formed in agitated, shallow subtidal shoreface areas
above the fair-weather wave base. The sands consist pre-
dominantly of ooids or various skeletal grains, usually fora-
minifera, calcareous algae, or mollusks. Peloids may be
common in places. Storms contribute to the formation of
extended sheet-like sand bodies and sand beaches that may
grade into eolian dunes. Offshore storm surges transport
shoreface sands to deeper, outer ramp settings. Organic
buildups in inner-ramp environments are biostromes and
small patch reefs characterized by low-diversity biota (e.g.
corals, rudists, oysters). Frequent limestone types are grain-
stones and packstones.
Back-ramp sediments originate in peritidal settings
similar to those of inner platforms (comprising mudstones,
bindstones and wackestones), and in restricted lagoonal
areas (mudstones, wackestones, packstones).
Thick oolitic and bioclastic sand shoals are common.
Storm-related features are graded packstone, grainstone
beds, hummocky cross-stratification, and tempestite cou-
plets. Skeletal grains exhibit signs of transport.
Fair-weather phases are represented by burrowed sedi-
ments dominated by lime mud or terrigenous mud form-
ing lime mudstones and marls. Much of the fine-grained
sediment might be caused by lateral sediment transport in
offshore zones or by transport from the shoreline to mid-
and outer ramp areas. Mid-ramp deposits are often thicker
than coeval inner ramp deposits. Organic buildups are rep-
resented by pinnacle reefs and mounds.
Outer ramp
The outer ramp is the zone below normal storm wave
base. Water depths vary between tens of meters and sev-
eral hundreds of meters. The zone is characterized by low-
energy allochthonous and autochthonous carbonates, and
hemipelagic sedimentation. Little evidence of direct storm
reworking exists, but various storm-related deposits (e.g.
graded distal tempestite beds) may occur. Common litho-
facies types are bedded, fine-grained limestones (argil-
laceous lime mudstone and wackestone) associated and
interbedded with marl or shale beds. Calcisiltite matrix is
abundant. Biota comprise normal marine diverse benthos,
sometimes associated with plankton and nekton. Benthic
organisms include foraminifera, sponges, bryozoans, bra-
chiopods, mollusks, and echinoderms. Algae may be rep-
resented by red algae. Burrows are common. In deeper outer
ramp settings restricted bottom conditions may develop.
Common organic buildups are mud mounds.
Mid-ramp
The mid-ramp is the zone between fair-weather wave
base and the storm wave base. Water depths reach some
tens of meters. The bottom sediment is frequently reworked
by storm waves and swells. The sediments reflect varying
degrees of storm influence depending on the water depth
and bottom relief. Intraclast and breccia beds may be com-
mon.
The slope break of distally steepened ramps is usually
located in a position around the mid- or outer ramp bound-
ary or within the outer ramp. Deposition of slope-derived
material may dominate proximal to the break.
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