Geology Reference
In-Depth Information
5.1.6 Nodular Fabrics
processes (Pl. 21/3). Boundaries between nodules and
matrix are inconspicuous or distinct. Distinct bound-
aries are often marked by a dark clay-rich seam (Pl.
21/2). Isolated nodules exhibiting black ferruginous and
manganese coatings often represent 'hardground clasts'.
Nodule size (largest diameter) and orientation may be
of interest in the discussion of a possible transport, e.g.
along a slope relief.
Nodular limestones are usually mud-supported. The
matrix corresponds to a mudstone or a wackestone, of-
ten with small bioclasts. Variations in the color of ma-
trix and nodules are due to differences in the amount of
non-carbonates, Fe-hydroxides, sulfides, and organic
carbon.
Bedded
nodular limestones exhibit a conspicuous fab-
ric characterized by centimeter- and decimeter-sized,
often 'rounded' nodules floating within a usually mi-
critic matrix. The overall appearance sometimes re-
sembles a conglomerate.
Descriptive criteria of nodular fabrics
Criteria important for the discussion of the origin
of nodular fabrics relate to nodules, matrix, and spatial
arrangement of nodules. Carbonate/clay ratio, grain size
distribution, and burrowing are additional criteria.
Nodules are described with regard to their compo-
sition, shape, boundaries and size. Composition (mi-
crofacies, fossils within and at the periphery of the nod-
ules) can be the same as or different from that of the
surrounding matrix. Many nodules appear 'rounded'
to 'subrounded'. 'Subangular' and 'angular' nodules
may indicate reworking and redeposition, these nod-
ules may form conglomerates or even breccias. Appar-
ent rounding, however, can also be caused by solution
Origin of nodular limestone fabrics
The origin of the nodular fabric has been explained
by diagenetic, sedimentary and tectonic processes. Di-
agenetic processes include solution processes as well
as cementation and nodule growth within the sediment.
Sedimentary models underline the role of transport and
redeposition. Tectonic explanations favor the forma-
Plate 21 Nodular Fabrics
Nodular fabrics of limestones exhibit scattered or densely packed nodules within a matrix of like or unlike
character. Nodules are generally cm-sized and often have 'rounded' outlines. Contacts between nodules and
matrix or between adjacent nodules are indistinct (indicating bioturbation and diagenetic overprint) or sharp
(indicating mechanical transport and sorting of nodules, pebbles or lithoclasts) and sometimes accentuated by
darker seams (caused by pressure solution). Many nodular limestones exhibit striking red colors which have led
to the term 'Rosso Ammonitico' for pelagic and hemipelagic red and variously colored limestones in the Alpine-
Mediterranean region (Farinacci and Elmi 1981). Red nodular limestones often are associated with condensed
pelagic limestones and resedimented beds and occur on submarine rises (structural highs and volcanic ridges).
Nodular fabrics are common in deeper-marine and slope environments (e.g. Devonian 'griotte facies' and
Tournaisian 'calcaire noduleux' of France, Jurassic 'Ammonitico rosso' facies of the Alpine-Mediterranean region)
but also can be formed in shallow-marine and non-marine settings.
The Early Jurassic nodular Adnet limestone of Salzburg (Austria) shown on this plate is explained by early
diagenetic partial cementation and later compactional pressure solution (Pl. 139; Böhm 1992).
1
Red nodular limestone.
Initial stage of nodule formation, interpreted as local and partial cementation of the lime mud.
Note the indistinct boundaries (arrows) between the nodules. Nodules correspond to bioclastic wackestones. Skeletal
grains are echinoderms and brachiopod shells. Kiefer-Bruch, Adnet near Salzburg, Austria.
2
Nodular limestone.
Boundaries are marked by dark clay seams (arrows) resulting from incipient pressure solution. The
sediment is a fine-grained bioclastic wackestone. Larger fossil fragments are brachiopods (B) and bivalve shells. Scheck-
Bruch, Adnet, Austria.
3
Nodular fabric
becomes more pronounced by pressure solution. Stylolitization produces an iden-supported fabric (see Pl.
37/7). Adnet, Austria.
4
Red crinoid-shell wackestone
. A nodular fabric is formed by redeposited hardground intraclasts (arrow, see Pl. 139/10).
Kiefer-Bruch, Adnet, Austria.
5
Beginning nodule formation
explained as partial cementation within isolated areas of the bioclastic lime mud. Early
Jurassic: Wolfgrub-Bruch, Adnet, Austria.
6
Typical nodular limestone.
Darker nodules are red-colored due to Fe-oxides and a higher organic content. Note the high
degree of rounding. Adnet, Austria.
