Geology Reference
In-Depth Information
shore basins where the carbonates intercalate with ba-
sinal shales;
•
in basins with pelagic carbonate sedimentation.
Lateral transitions from siliciclastic to carbonate
facies have been described occurring over different
distances from the shoreline, depending on the climate
and precipitation rates (Roberts 1987; Fay et al. 1992).
The strong influx of siliciclastic sediment into the low-
latitude near-shore carbonate Nicaragua shelf can be
followed for up to 20 km. In contrast, under the condi-
tions of an arid climate (Red Sea) and low precipita-
tion siliciclastic influx is restricted to periodic flash
floods affecting the near-shore carbonate environments
only within distances of tens of meters or a few kilo-
meters; transitions between siliciclastic and carbonate
depositional environments are abrupt. Relatively
gradual transition patterns confined to a distance of a
few kilometers from the shoreline have been observed
in the moderately humid climate of the low-relief coast
of Tanzania.
Fig. 16.18.
Mixed carbonate-siliciclastic sediment.
The thin
section shows a rock consisting of calcareous ooids and ter-
rigenous quartz grains. The latter form the nuclei of many
ooids or occur dispersed in the micritic matrix. The angular
quartz grains are moderately sorted. The sample (a sandy
allochem micrite; see Sect. 8.5) comes from massive to coarse
bedded sediments interfingering with coral reef limestones
and also with near-coastal sandstones and calcareous sand-
stones, and was deposited in an inner shelf environment. Late
Jurassic (Early Kimmeridgian): Trevijano, Sierra de la Dem-
anada, Province Soria, Spain. After Benke et al. (1981).
The siliciclastic and volcaniclastic influx into envi-
ronments with carbonate production differs with regard
to the intensity, grain size and effects on salinity and
nutrients (Wilson and Lokier 2002):
•
Tropical/equatorial, humid areas
are characterized
by high-continuity clastic influx, clay to cobble clast
size (often clay to sand), normal to low salinity, and
high nutrient contents. In siliciclastic-dominated envi-
ronments, regional carbonate production can be pro-
moted by structural or depositional antecedent topog-
raphy, shelf currents, changes in clastic distribution
pathways and relative sea-level change (Wilson 2002).
•
Subtropical arid areas
exhibit moderate and episodic
clastic influx, silt to boulder clast size, normal to high
salinities, and low nutrient contents.
•
Volcanic shoreline-attached areas
are affected by
high to very highly continuous and punctuated clastic
influx, clay to boulder clast size, normal to low salini-
ties, and high nutrients.
•
Volcanic influx on distal parts of isolated platforms
is low to high-punctuated, clast size corresponds to clay
to cobble, salinity is normal, and nutrients range be-
tween low and high.
Modern low-latitude regions with mixed carbonate-
siliciclastic sedimentation are known from the north-
ern Puerto Rico shelf (Pilkey et al. 1988), Southwest
Florida (Holmes 1988), the northern Great Barrier Reef
(Flood and Orme 1988; Belperio and Searle 1988;
Lacombe and Woolfe 1999), the Gulf of Suez and the
Gulf of Aqaba, Red Sea (Friedman 1988; Roberts and
Murray 1988), and along the coast of Tanzania (Fay et
al. 1992). Examples of high-latitude mixed siliciclas-
tic-skeletal carbonate sediments have been described
from southern Australia (James et al. 1992) and New
Zealand (Gillespie and Nelson 1997).
Mixed siliciclastic and carbonate deposits occur
• on shelves on which land-derived siliciclastic sedi-
ments are trapped in nearshore and inner shelf envi-
ronments and contemporaneous deposition of carbon-
ate sedimentation takes place in subtidal settings (e.g.
eastern Nicaragua coast and Great Barrier Reef);
• on siliciclastic shelves on which detrital carbonates
are deposited, supplied by the erosion of uplifted car-
bonate rocks in the hinterland (source mixing model;
Mount 1984);
• at the margins of carbonate shelves where the shelves
interfinger with deep-water shales. In these settings,
storms periodically carry carbonates out into distal off-
Effects of siliciclastic input on shallow-marine car-
bonate production.
Siliciclastic or volcaniclastic input
into an area of carbonate production has a number of
potential effects on carbonate producers:
• Physical covering or surrounding of the organisms,
often resulting in the death and burial of the biota.
