Geology Reference
In-Depth Information
and Stanton 1996) are down-ramp sequential distribu-
tion of depth-sensitive organisms (see Fig. 14.12), well
preserved and unabraded fossils, beds and bed sets that
are traceable along ramp slopes for tens of kilometers,
and that are also traceable through mounds growing on
the ramps, geopetals in original position, and lack of
sedimentary structures indicative of transport. Calci-
siltite is abundant in outer ramps, characterizing a par-
ticular microfacies type (RMF 1). The origin of these
calcisiltites is enigmatic. Abrasion in storm-influenced
ramp parts and transport to deeper ramp parts may be
one of the sources in Paleozoic ramps. Pelagic fallout
contributed to the formation of calcisilite and allomi-
crite of Mesozoic and Cenozoic ramps. Skeletal min-
eralogy might have had some influence on the devel-
opment of ramps that are more abundant in calcite-
dominated times with maxima in the Cambrian-Ordovi-
cian, Mississippian and Jurassic than in aragonite-domi-
nated times.
isms by bottom currents is indicated by fossils that are
uncommon in the depositional environments where they
are found (e.g. larger foraminifera in outer ramp set-
tings).
Ooid accumulations originate in three ramp settings:
(1) in inner ramps near shorelines and at coasts (here
also contributing to eolian sand dunes), (2) in mid-ramp
and inner ramp settings forming shoals and thick grain-
stone sheets; and (3) as grains having been transported
from mid-ramps to outer ramps and deposited in thin
tempestite beds intercalated with lime mudstones and
wackestones, marls or shales. Most of the spherical
ooids correspond to concentric or micrite ooids. Parau-
tochthonous spherical and non-spherical radial ooids
may occur in inner ramps. Relations between water
energy, ooid size and sorting and the thickness of ooid
cortices are indicators of transport mechanisms (see Fig.
4.25 and Fig. 4.26).
The dominant grain types of carbonate ramps are
skeletal grains and ooids, followed by peloids and in-
traclasts. Other grain types are quantitatively of minor
importance (cortoids, oncoids), or are very rare or to-
tally absent (pisoids, aggregate grains). Abundant ag-
gregate grains are characteristic of carbonate platforms
but missing in ramp settings.
Peloids occur as fecal pellets in inner ramps and as
mud peloids in outer ramps. Cortoids are present in
current-washed sand shoals of inner ramps, but appear
to have not the same importance as in rimmed plat-
form carbonates. Oncoids occur in inner and outer
ramps. Oncoids from shallow and deep environments
differ in nuclei, organisms contributing to the cortices,
lamination type, and the composition of the encruster
association. Oncoids built by cyanobacteria and algae
are present in inner ramps; those built by red algae
(rhodoids) in inner ramp, but more frequently in outer
ramp settings. Micrite oncoids are common in outer
ramps and basins.
Common skeletal grains in outer ramps are calci-
microbes, smaller foraminifera, sponge spicules, bryo-
zoans, brachiopod and mollusk as well as arthropod
shells, serpulids and echinoderms. Shallow inner ramp
sediments exhibit similar bioclasts in comparison to the
normal marine and restricted platform interior facies
of platforms: Calcimicrobes, benthic smaller and larger
foraminifera, calcareous green algae, mollusks, ser-
pulids, ostracods and echinoderms. These shells are also
major constituents of storm and current induced skel-
etal shoals in mid-ramp and innerramp settings.
Organisms taking part in the formation of mud
mounds, reef mounds or patch reefs are microbes, some
green and red algae, sponges including archaeocyathids,
stromatoporoids, coralline and siliceous sponges, tabu-
late and scleractinian corals, and some pelecypods (rud-
ists). Burchette and Wright (1992) suggest that the main
sediment producers both in organic buildups and in
level-bottom communities have migrated from outer
and mid-ramp settings to inner ramp settings since the
Late Jurassic.
Taphonomic and preservation criteria including
shape, rounding, breakage, and size and sorting of fos-
sils are good indications of transport and allochthonous
deposition. Out-of-habitat transport of benthic organ-
Intraclasts may occur in all ramp settings, but are
abundant in distally steepened ramps (RMF 9) within
debris flows accumulating near to the outer ramp slopes.
These intraclasts are micritic, poorly sorted, angular to
subrounded grains embedded within a micrite matrix.
Sedimentary mass flow breccias derived from ramp
slopes are monomict, with medium to high matrix con-
tents and clast-supported fabrics. The decimeter- to
meter-thick breccia beds have sharp boundaries.
Important depositional fabrics in differentiating in-
ner, mid- and outer ramps are bioturbation and lamina-
tion. Burrows occur all over the ramps, but are particu-
larly abundant below the wave base and below the storm
wave base. Mid-ramp and outer ramp burrows exhibit
differences in density, spatial distribution and ichnotaxa
association. Burrows that have not caused major break-
age of shells, and burrows that have produced exten-
sive breakage are proxies for the intensity of burrow-
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