Geology Reference
In-Depth Information
Fig. 15.2.
Diagnostic features of paleocaliche.
One of the most significant features of caliche seen in thin sections is their
inhomogeneous texture and the lack of fossils. Characteristic criteria shown in this picture are the breccia-like texture,
nodular fabrics (N), clotted fabrics with micritic peloids (PEL), grains coated by variously thick micrite (MC), and spar-filled
wrinkled microcracks (white arrows). The latter occur as straight or wrinkled cracks, or surround the nodules (circumgranu-
lar cracking). The coatings surround alveolar structures (A; irregular pores infilled with sparite and separated by intercon-
necting micritic bridges; interpreted as root structures). Black arrows point to rhizogenic structures. The rock is strongly
fractured. Early Tertiary (Ilerdian, Paleocene): Serraduy, Tremp Basin, Southern Pyrenees, Spain. After Eichenseer and
Luterbacher (1992).
and expansion processes in soils induced by seasonal
drying/wetting cycles.
•
Microcodium
(Pl. 128/4). Aggregates of 0.1-1 mm
long prismatic petal-like sparry calcite crystals, with a
central cavity in each one. The cavity is filled with dark
material. The calcite crystals are radially arranged
around an axis, or are grouped into irregular masses
and crusts. Interpreted as calcified mycorrhizae, a sym-
biotic association between soil fungi and cortical cells
of plant roots, or as bacteria or algae (see Sect. 10.2.6.2).
•
Alveolar structures
(Fig. 15.2).
Tubular to irregular
pores infilled with sparite and separated by straight or
various sinuous interconnecting bridges of micrite. The
bridges consist of tangentially or randomly oriented
micrite and/or microsparite. The pores may contain
arcuate micritic septae (alveolar-septal structure). In-
dividual pores reach 1 mm in width and 20 mm in
length. Interpreted as entwined rhizoliths, resulting
from the calcification of root hairs.
(4) Root-related features
Rhizoliths,
Microcodium
, calcified root hairs, pedo-
tubules, and alveolar structures can be combined un-
der the term 'root-related structures'. Examples of car-
bonate rocks with evidence of the former activity of
plant roots forming
'rhizogenic calcretes'
(Wright et
al. 1995) are known from various Phanerozoic time
intervals, particularly from the Late Paleozoic, Juras-
sic, Cretaceous and from the Tertiary and Quaternary.
•
Root structures (rhizoliths)
occur as root casts or
rhizoconcretions. Rhizogenic calcretes are formed by
the calcification on, in, or around roots of higher plants.
Root casts
consist of tubular branched voids (pedo-
tubules; Pl. 128/5) of decayed roots infilled with fine
sparite. They are generally less than 1 mm in diameter.
Rhizoconcretions
are characterized by concentric ac-
cumulations of tangential to random fibers of micrite.
They are typically 0.5 to 2 mm in diameter, often sur-
rounding root casts or alveolar structures. See Klappa
(1980) for a classification of root structures.
(5) Breccia structures
•
In-situ brecciation
(Fig. 15.2, Pl. 128/6). Micrite
fragmented in place, forming a polygonal fracture net-
work infilled with sparite. Infilled micrite polygons are
millimeters to several centimeters in size. Brecciation
