Geology Reference
In-Depth Information
water agitation and low sedimentation rates = strong
cementation (e.g. reef front) and (b) low agitation, high
sedimentation rates and substantial deposition of fine-
grained sediment = low cementation (e.g. back reef).
Active cementation in reefs seems to be confined to
the outer few feet of the sediment. Water energy and
circulation control the intensity of cementation, local
permeability controls the degree and type of cement on
a small scale. Wave pumping and biological activity
have a direct bearing on the formation of reef cements.
Another important factor influencing cementation and
mineralogy of cements is the size of framework pores.
Small pores can trap sediments, ultimately producing
cemented cavity fills and cement crusts. In large pores
fine-grained sediment will be quickly flushed through
the voids and cement will be precipitated (James et al.
1976; Marshall and Davies 1981; Marshall 1983). Reef
cementation takes place just centimeters beneath the
sea floor; the framework reefs are commonly lithified
within one-half meter or less. Substrate control is a com-
mon feature as demonstrated by the precipitation of ara-
gonite cements on aragonitic corals, and Mg-Calcite
Plate 33 Marine and Meteoric Carbonate Cements
The plate exhibits typical carbonate cements formed in marine environments (-> 1, 3, 7, 8), in the marine-
vadose zone (-> 2, 3) and in meteoric environments (-> 4 ). Note open, usually interparticle porosity in all
samples.
1
Circumgranular bladed High-Mg calcite cement
(arrow) surrounding conspicuously well-rounded skeletal grains (fora-
minifera
-
F; coralline algae - CA). Note open interparticle pores (P). Roundness/sphericity values correspond to the field
D, N and E of the Krumbein visual chart (Fig. 4.30). Bladed cements often form isopachous rinds and constitute the first
generation of marine cements. Shape and orientation of substrate crystals may strongly control crystal habits (Wilson and
Palmer 1992).
Marine-phreatic environment.
Bermudas.
2
Thin isopachous microcrystalline calcite cement
(black arrows) is developed around various bioclasts (coralline algae -
CA, miliolid foraminifera - MF, micritized bivalve shell - BS, open pores - P). Interparticle pores are filled with skeletal
debris and peloids. Note the low sphericity and medium roundness of the red algal fragments (fields L, P, R of the
Krumbein chart, Fig. 4.30). White grains are airborne quartz. Interparticle pores appear black.
Marine-vadose environ-
ment.
Pleistocene eolian dune: Carthage, northern Tunisia. Crossed nicols.
3
Thick bladed equidimensional cement
(white arrow) on skeletal grains (foraminifera - F), formed in a marine-vadose
environment and thin, poorly developed Mg-calcite cements (black arrow) on black lithoclasts (black pebbles; see
Sect. 4.2.8). No cements are present on the bivalve shell (BS). Open interparticle pores (P) filled with resin.
Marine-
vadose environment.
Beachrock: Holetown, Barbados.
4
Meteoric-vadose cements.
Grains are connected by
meniscus cement
(black arrow) causing pore rounding, and
pendant
cements
(white arrows), formed on droplets beneath grains within the
meteoric-vadose environment
. Pendant (or gravita-
tional) cements are absent on the upper surface of the grains but thicken down the sides to the undersurface. Note the good
preservation of calcitic bivalve fragments (BF) without micrite envelopes, in contrast to the poor preservation of origi-
nally aragonitic shell fragments (SF) exhibiting micrite envelopes around casts and filling with coarse calcite (CC).
Interparticle pores (P) are still open. Holocene: Belmont, Bermudas.
5
Partly dolomitized foraminiferal grainstone
with
Amphistegina
. Matrix and Mg-calcite skeletal grains are replaced by
anhedral, finely crystalline dolomite mosaics (appear black), whereas the radial hyaline tests of the foraminifera are
preserved as calcite. Arrows point to isopachous, bladed to fibrous dolomite cements developed in interparticle voids.
Geochemical data suggest that seawater of slightly variable temperature and/or slight degrees of evaporation, and en-
riched in bicarbonate originating from the oxidation of methane, was the principal agent for the replacement dolomitiza-
tion (Machel and Burton 1994). Late Pleistocene (fore-reef facies): Golden Grove, Barbados.
6
Partly dolomitized Amphistegina limestone
exhibiting large secondary vuggy porosity. The large dissolution void is filled
with thin isopachous bladed dolomite cement (white arrows) and occluded by recrystallized coarse, sparry calcite cement
(black arrows). Skeletal grains are coralline algae (CA) and foraminifera (F). Same locality as -> 5.
7
Marine-phreatic isopachous carbonate cement
(arrow) with irregular surface, surrounding bioclasts.
Syntaxial overgrowth
on echinoderm fragment (right). The dredge sample comes from sediment deposited on a seamount exposed to seawater.
Diagenesis of seamount sediments is highly complex and includes cementation, transformation of carbonate phases,
phosphatization, silicification and mobilization of Fe-Mn oxides (Grötsch and Flügel 1992). Early diagenetic cements of
the drowned platforms are rarely preserved but exhibit a still open interparticle porosity (black). Cretaceous (Albian reef
facies): Darwin Guyot, northwestern Pacific.
8
Isopachous rim cement
composed of bladed crystals (arrows), growing within the mold of a dissolved aragonitic shell
(AS). Open moldic porosity is preserved due to the micrite envelope (ME) coating. Primary calcitic mollusk shells (CM)
show remnants of microstructures. Cretaceous (Late Albian): Charly Johnson Guyot, northwestern Pacific.
