Geology Reference
In-Depth Information
• Increased suspended matter in the water column will
result in a reduction of light levels. Turbidity will re-
strict the type and composition of communities present
and reduce the water depth at which different assem-
blages occur.
• The increase of siliciclastic or volcaniclastic sedi-
ments can cause changes in water chemistry, such as
reduced salinities, variations in pH, and an increase in
nutrients. Salinity and pH changes may have a detri-
mental effect on organic carbonate production.
Ancient reefs and siliciclastic/pyroclastic sedimen-
tation:
Reefs formed in environment influenced by si-
liciclastic influx are known all over the Phanerozoic,
but became more frequent from the Late Triassic. Coral
reefs occur in different siliciclastic settings in the Ju-
rassic (Leinfelder et al. 2002). Because rudists could
cope with sediment-loaden and turbid waters (Steuber
et al. 1998; Mitchell 2002), Cretaceous rudist reefs also
occur in stressed siliciclastic environments. The num-
ber of coral reefs formed in mixed carbonate-siliciclas-
tic environments increased during the Tertiary.
Pyroclastic sedimentation can partially or com-
pletely surpress growth rates of reefs, depending on
the amount and grain size of pyroclastic sediment in
the water column. Turbidity caused by the suspension
of volcanic ash reduces light penetration and strongly
discourages the growth of calcareous algae and corals.
Fine ash will plug up filter-feeding reef organisms.
Reefs and siliciclastic sediments.
Many reef organ-
isms are adapted to nutrient-poor conditions. The high
input of nutrients leading to eutrophication may cause
altered skeletal growth rates and changes in biotic com-
position, and finally the decline and death of reefs. Ter-
rigenous sedimentation, however, must not completely
hinder the development of reefs, but may control the
position and geometry of reefs and modify ecologic
zonations (Roberts 1987; Santisteban and Taberner
1988; Acevedo et al. 1989; Carbone et al. 1999). Mod-
ern reefs can survive in areas of significant terrigenous
influx and turbidity. For example, deeper-water coral
reefs are able to grow despite periodic blanketing by
fine siliciclastic sediment introduced during times of
heavy rains (Acker and Stearn 1990). Corals reefs are
not uncommon in some coarse-grained deltas (see
McPherson et al. 1987).
The effect of terrigenous input on coral reefs is de-
scribed by a model based on the relative net rates of
terrigenous framework and carbonate non-framework
sediment accumulation and and/or removal (Woolfe and
Lacombe 1999).
Island-arc carbonates and volcaniclastic sediments.
Associations of limestones, volcaniclastic strata and
volcanics have been described from several island-arc
carbonates and tectonic terranes. These limestones oc-
cur as (1) carbonate platforms around volcanic edifices
(Soja 1990), (2) gravity-displaced blocks embedded
within basinal sediments (Blome and Nestell 1991),
and (3) small autochthonous limestone lenses enclosed
by deltaic volcaniclastic rocks (Watkins 1993; Beltramo
et al. 2001).
16.6.1.3 Describing CarbonateSiliciclastic
Sediments: Practical Advice
Understanding carbonate-siliciclastic successions re-
quires field work focused on geometrical relationships
and lateral and vertical lithologic changes, paleonto-
logical studies using fossils as sensitive indicators of
environmental changes, and evaluation of the compo-
sition of the sediments.
16.6.1.2 Ancient Mixed CarbonateSiliciclastic
Environments
The formation of carbonate platforms and reefs
within siliciclastic settings requires combinations of
adaptive strategies by carbonate-producing organisms
and sheltering mechanisms that protect the organisms
from unfavorable influences. Sheltering mechanisms
include an elevated position (e.g. basement uplift)
within areas dominated by siliciclastic sedimentation,
longshore currents that screen off suspended siliciclastic
grains, local subsidence traps, or sea-level rise result-
ing in the reduction of siliciclastic influx and favoring
carbonate deposition (Blair 1988; Brachert 1992;
Garcia et al. 1993; Philip 1993; Leinfelder 1994; Okhravi
and Amini 1998; Sanders and Höfling 2000; Khetani
and Read 2002).
Description.
Microfacies data are valuable in com-
positional analysis. In thin sections look for:
• Matrix and carbonate grains (categories, frequency,
distribution).
• Fossils. Many organisms are adapted to siliciclastic
influx. Adaption is reflected by the specific morpholo-
gies and composition of the assemblages as illustrated
by benthic foraminifera (Kumar and Saraswati 1997),
corals (Carbone et al. 1999), and bivalves (Best and
Kidwell 2000).
• Siliciclastic grains (mineralogy, size, angularity/
roundness, texture, abundance).
