Geology Reference
In-Depth Information
7.4.4.3 Cold-Water vs. Warm-Water
Diagenesis
tation may be common in some temperate and cold-
water carbonates (Rao 1981, 1993, 1994; Rao and Adabi
1992; James et al. 1992, 1994; Freiwald 1998).
In contrast to warm-water reefs, Mg-calcite radi-
axial cements appear to dominate in cooler waters
(Beauchamp 1994; Nelson and James 2000). Dolomite
is rare, known from the cool temperate water of the
Coorong, South Australia, and locally present in New
Zealand.
Carbonate minerals are predominantly High-Mg cal-
cite (and minor aragonite) in shallow warm temperate
carbonates and High-Mg to Low-Mg calcites in cool
temperate carbonates. Mineralogical composition is
controlled by water temperatures and water depths.
Carbonate saturation of seawater in non-tropical set-
tings is less pronounced or even undersaturated (Opdyke
and Wilkinson 1990). Diagenesis, therefore, is predomi-
nantly destructive (Alexandersson 1978). In compari-
son with tropical shallow-marine carbonates the diage-
netic potential is low and sediments may remain es-
sentially unlithified for a long period of time. Never-
theless, intraskeletal and intergranular carbonate cemen-
7.4.5 Diagenetic Pathways and Patterns
Pore waters and their products, including cement and
porosity evolve over time resulting in short- and long-
term diagenetic sequences and stages. Changes over
Plate 35 Diagenetic Pathways and Spatial Variations
A detailed study of diagenetic spatial variations is necessary for understanding porosity development over time
and the highly complex diagenetic history of carbonate rocks. Instructive examples of these processes were
presented by Schroeder (1988). Spatial variations depend on the individual characteristics of grains and cements
(mineral and chemical composition, ultra- and microstructure, surface properties), collective characteristics of
sediments and sedimentary rocks (component mix due to ecological and transport selection, fabrics as deter-
mined by primary sedimentary and skeletal structures), synsedimentary to early diagenetic processes (internal
sedimentation, bioturbation, bioerosion), early to late diagenetic processes (cementation, dissolution, fracturing,
decomposition of organic matter), hydrology (wave and tide ranges, sea- and ground-water levels, mixing zones),
as well as substrate mobility and solubility. Many of the causal factors are controlled by biologic, meteorologic
and oceanographic factors. The samples shown in this plate illustrate a few variations in cements and neomor-
phic alterations as well as in grain preservation, both between and within particles. Diagenetic pathways are
highly complicated. Therefore attributing the depicted diagenetic features to specific diagenetic environments
has been restricted to the samples described here and should not be too generalized.
1
Cementation
in a Holocene beachrock from Uyombo, Kenia. The photograph shows inherited aragonite spherulitic ce-
ment (IAC) within an intraskeletal pore of a coralline red alga (CA) and fibrous marine Mg-calcite palisade cement (MC)
surrounding carbonate particles but not the quartz grains (Q). Meteoric sparry calcite cement (SC) fills the remaining pore
spaces. Crossed nicols.
2
Particle alteration
in a Holocene beachrock from Uyombo, Kenia. A fragment of a mollusk shell exhibits a variety of
modes of preservation/alteration: The original fabric is partly preserved and rimmed by a micrite envelope. A secondary
solution pore is partly empty and partly filled by blocky sparry calcite cement. Interparticle pores are filled with fibrous
Mg-calcite cement. CA: Coralline alga; Q: quartz. Crossed nicols.
3
Modern Bermuda algal cup reef rock
. Adjacent skeletal pores within barnacles are filled completely by aragonite needle
cement (1), by a basal layer of micrite and subsequent Mg-calcite cements with calcified algal filaments (2, 3), or by Mg-
calcite cement on calcified algal filaments and subsequent aragonite needle cement (4). Crossed nicols.
4
Branching reef coral
Procladocora tenuis
. The corallite is filled by micrite (1), fine-grained sediment (2), sparry calcite
(3) and neomorphic calcite (4). The Paleocene coral knobs studied passed through a sequence of diagenetic environments
ranging from marine to meteoric and back to marine, on to shallow burial conditions at various depths, and then back to
the surface. In the course of this development various portions of the corals were affected at different times by neomor-
phism, replacement, dissolution, cementation, and/or fracturing. As a result of this history, various incomplete diagenetic
sequences occurred in spatial proximity, many of which can be recognized in a single thin section. Infilling of intraskeleton
pores with fine-grained sediment (2) and micrite (1) occurred in the marine environment. This micrite may be a sedimen-
tary or diagenetic product. The sparry calcite mosaic (3) represents neomorphically altered Low-Mg calcite cements
(indicating meteoric processes). The large crystals (4) with irregular boundaries and relicts of septa are caused by neomor-
phic alterations, probably in the burial environment. Crossed nicols. Paleocene: Bir Abu El Husein area, southern Egypt.
-> 1-4 Schroeder (1988)
