Geology Reference
In-Depth Information
coatings on hard substrates. Rounded glauconite grains
reflect the original shape of fecal pellets or rounding
by transport. The bulk of glauconite pellets may be clay
debris from the sea floor, that passed through the di-
gestive tract of burrowing organisms.
In thin sections glauconite grains are easily identifi-
able by (a) green, blue-green, yellowish-green or brown-
ish-green colors both in ordinary and polarized light.
(b) speckled greenish birefringence and (c) a very fine
granular texture.
Significance.
Glauconite is usually regarded as an
indicator of marine environment, relatively shallow
deposition and slow sedimentation. The mineral is of-
ten concentrated at discontinuity surfaces indicating
depositional breaks (Pl. 22/1, 2). Different mineralogi-
cal compositions of glauconites, distinguished by dif-
fractometric parameters, appear to be related to spe-
cific sequence system tracts (Amorosi 1993).
13.1.2.2 Sulfides: Pyrite
Opaque in transmitted light, pyrite is most readily iden-
tified by reflecting light owing to its characteristic yel-
low-gold appearance. Authigenic pyrites in limestones
are usually developed in the form of cubic euhedral
crystals (Fig. 13.2; Pl. 109/2).
Pyrite attracts sedimentologists, for the mineral is a
valuable indicator of chemical processes (Wilkin et al.
1996) and diagenetic stages (Hudson 1992). Fossils
preserved in pyrite are attractive for their beauty and
for morphological details revealed by pyritization (Pl.
109/3; Canfield and Raiswell 1991).
Most of the pyrite in sedimentary rocks is of diage-
netic origin, although detrital and synsedimentary py-
rite occurs, too. Authigenic pyrite commonly forms
under reducing conditions replacing organic material
or in close proximity to organic material.
Pyrite is formed in normal marine, euxinic, and fresh-
water environments. Three principal factors limit ulti-
mately how much pyrite is formed in sediments (Berner
1985): (1) the amount and reactivity toward bacterial
sulfate reduction of organic matter supplied to the sedi-
ments; (2) the amount and reactivity toward H
2
S of
detrital minerals supplied to the sediment; and (3) the
availability of dissolved sulfate. The primary factor lim-
iting pyrite formation in normal marine sediments, e.g.
those deposited in oxygenated bottom waters, is organic
matter. In euxinic environments pyrite can form at lo-
cations where no organic matter is deposited, because
H
2
S is present everywhere in the bottom waters. Pyrite
formation in euxinic basins is limited by the amount
and reactivity of detritic iron minerals. Very little py-
rite is formed in freshwater lake and swamp sediments
Any use of glauconite as an environmental proxy
should consider three points:
•
Differentiation of authigenic and detrital glauconite
grains.
Glauconites are formed coevally or non-coe-
vally with sedimentation, and intrabasinal or extra-
basinal.
Amorosi (1993) distinguished four major groups:
(1) Intrabasinal authigenic glauconites formed in situ
that have not undergone transport.
(2) Intrabasinal detrital glauconite grains transported
from submerged structural highs or derived from in
situ reworking processes.
(3) Extrabasinal detrital glauconite grains eroded from
older stratigraphic levels. Glauconite grains occur-
ring in association with terrigenous quartz may point
to this possibility (Pl. 49/1; Pl. 75/6, 8).
(4) Perigenic authigenic glauconites dispersed in an un-
consolidated sediment on the sea floor, that have
undergone transport from the area where they were
formed by tides, storms or turbidity currents.
•
Glauconite grains
are particularly common in ramp
and platform carbonates of Cambro-Ordovician and
Cretaceous age. This concentration points to specific
and possibly non-actualistic conditions controlling the
formation of the mineral.
•
Differences between modern and ancient environ-
ments.
The occurrence of glauconite cannot be used a
priori as an environmental indicator of either mid-shelf
and deeper water and/or a slow rate of sedimentation
because of striking differences between modern and
ancient glauconite-bearing limestones. Chafetz and
Reid (2000) showed that Cambro-Ordovician glauco-
nite-rich strata were formed under shallow-water to tidal
conditions.
Fig. 13.2.
Authigenic pyrite crystals
grown within layers of
cyanobacterial oncoids rich in organic matter. Note the well-
developed crystal shape. SEM photograph. Late Cretaceous:
Subsurface, Ras al Khaimah, United Arabian Emirates.
