Geology Reference
In-Depth Information
not limited to lagoons or restricted bays in arid climates,
but can also originate from salinar solutions derived
from salt diapirs underlying platforms and reefs (Reitner
1986). Authigenic quartz and feldspars are usually re-
stricted to micritic limestones.
tals varies greatly (long diameter about 30 to 300
m).
Diagenetic processes which could mobilize silica from
within carbonate sequences are pressure solution of sili-
cates, replacement of silicates by carbonates, and clay
mineral diagenesis connected with the conversion of
smectite clay minerals to illite.
Authigenic feldspars
Idiomorphic crystals, euhedral outline, carbonate in-
clusions and specific optical criteria are characteristic
for authigenic feldspars. Their mineralogy appears to
be determined by the composition of the host sediment.
The most common replacement feldspar is albite. Only
a few reports of authigenic K-feldspars are known (e.g.
Buyce and Friedman 1975)
.
Euhedral
albite
crystals occur isolated or in asso-
ciation with authigenic quartz. The feldspar crystals are
double terminated and many of them have long, well-
developed prismatic crystal faces. The size of the crys-
Glauconite
This hydrous potassium-iron-alumo-silicate mineral
today forms exclusively in marine environments and is
found in carbonate as well as in siliciclastic sediments.
In modern oceans glauconite occurs between 50 and
500 m and is abundant in mid-shelf to upper slope set-
tings at depths between 50 and 300 m.
Criteria.
The mineral is typically found as rounded
aggregates, as pellets consisting of very fine-grained
scaled particles, as authigenic mineral within skeletal
grains (e.g. echinoderms, foraminifera) or as surface
Plate 109 Common Authigenic Minerals in Limestones
The plate displays some examples of authigenic non-carbonate minerals that are common in limestones: Pyrite
(-> 1-4), quartz (-> 5, 6) and gypsum/anhydrite (-> 9). Aspects of silicification are shown in -> 7 and 8.
1
Pyrite:
Framboidal pyrite in residual clay seams of stylolites. The rounded aggregates consist of many tiny euhedral
crystals. The size distribution of framboidal pyrite is a measure of redox conditions within sediments (Wilkin et al. 1996).
Compare Fig. 13.1. SEM photograph. Late Cretaceous: Subsurface, Ras al Khaimah, United Arab Emirates.
2
Pyrite
: Well-crystallized pyrite in residual clay seams of stylolites. Compare Fig. 13.2. SEM photograph. Same locality as
-> 1.
3
Pyritization of fossils.
Monaxon sponge spicula. Parts of the siliceous spicula is replaced by pyrite, other parts by calcite.
Late Jurassic: Northern Alps. Roman mosaic stone, Kraiburg am Inn, Bavaria, Germany.
4
Pyrite.
Scattered pyrite crystals in a dolomitic mudstone. Same locality as -> 1.
5
Silicification.
Authigenic quartz crystals in a lagoonal lime mudstone. Scattered 'hexagonal' replacement quartz crystals
are not uncommon in fine-grained micritic limestones, and have been regarded as indications of hypersaline fluids (see
Box 12.7). SEM photograph. Same locality as -> 1.
6
Silicification.
The authigenic quartz crystals exhibit inclusions (arrows) of biogenic structures pointing to an early diage-
netic replacement of the limestone. Middle/Upper Jurassic: Argelita north of Valencia, Spain.
7
Timing of silicification.
The reef-building fossil
Zondarella communis
Keller and Flügel (stromatoporoid or stromatolite)
is characterized by dome-shaped growth layers. The layers consist of densely-spaced discontinuous laminae (arrows)
separated by micritic layers that have been replaced by blocky calcite (C) and euhedral dolomite crystals (D). Silicifica-
tion (S) took place subsequent to dolomitization within solution voids. Silicification (chertification) of carbonate rocks,
subsequent to dolomitization is a common feature (Whittle and Alsharhan 1994). Replacement of calcite by chert may
occur at a much more rapid rate than that of dolomite (indicated by floating dolomite within chertified matrix). Ordovi-
cian (San Juan Formation, Late Arenigian): Las Lajas, Precordillera, western Argentina.
8
Silicification of rotaliid foraminifera:
The silicification of the aragonitic shells took place subsequent to the redeposition
of the shallow-marine bioclasts in deeper parts of a ramp, probably under shallow burial conditions. Early Tertiary:
Southern Galala Basin, Egypt.
9
Evaporite-dolomite couplets formed within a sabkha environment.
Alternating
layers consisting of fine-grained dolomite
(black) and gypsum. Requirements for the formation of supratidal sabkhas: (1) hot and arid climate, (2) sea-marginal
location with adequate supply of saline water to the sabkha to develop evaporites, (3) supply of terrigenous mud, (4)
protection from winds, and (5) low tidal range, allowing the groundwater level to be maintained within the zone of
capillary rise. Crossed nicols. Early Tertiary (Eocene): Paris Basin, France.
-> 8: Courtesy of J. Kuss (Bremen)
