Geology Reference
In-Depth Information
Fig. 7.21.
Sparite
(sparry calcite) represents carbonate cement or products of neomorphic changes. The former have been
termed
orthosparite
, the latter
pseudosparite
. The thin-section photograph displays various early and late diagenetic stages
of a lithoclast/bioclast rudstone deposited near a reef. The numbers indicate syn- and postdepositional diagenetic alterations
affecting the skeleton of a dasyclad green alga (
Macroporella
sp.) and its infillings as well as the carbonate precipitated in
interparticle pore spaces. 1: Early diagenesis starts with the micritization of the walls between the algal branches (preserved
as pores 2). 2: Infilling of algal pores with micrite (now preserved as microspar) and small peloids. 3: Microbial encrustation
of the algal surface. 4: Transformation of the originally aragonitic pore walls to calcite. 5: Infilling of the algal interior by
sedimentary grains, some of which are covered by thin marine cement crusts composed of bladed calcite crystals. 6: These
grains are incorporated within originally micritic microbial crusts formed in the algal interior and now preserved as microspar.
7: The alga is overgrown by a complex biogenic crust. 8: The interior of the alga is partly filled with larger bioclasts. 9: The
boundary between the infilling and the remaining pore space is marked by an irregular microbial crust. 10: Large interpar-
ticle spaces between (dark) reef-derived lithoclasts are occluded by abundant shallow-marine radiaxial calcite cements
(orthosparite). 11: Late diagenetic processes are responsible for the formation of coarse calcite spar (
pseudosparite
), prob-
ably of burial origin. 12: Recrystallization causes coarsening of the walls of algal pores. 13: Tectonic shear is indicated by
breaking and dislocation of the radiaxial cement crusts and by 14: a thin calcite-filled veinlet crossing cements and alga. 15:
Neomorphic alterations enlarge crystals within the calcite cements. Sparite-producing diagenesis took place under marine
near-subsurface conditions (points 4, 5, 10) and burial conditions (points 11, 15). Middle Permian: Straza near Bled, Slovenia.
Scale is 2 mm.
apparently floating within sparite. The boundary be-
tween sparitic and micritic areas is often poorly de-
fined and can crosscut micritic grains. Larger crystals
are often irregularly intergrown. A common neomor-
phic fabric is
microspar
characterized by a crystal size
between about 5 and 30
m (see Sect. 4.1.3). Coarse
neomorphic fabrics originating from aggrading recrys-
tallization and consisting of calcite crystals larger than
30
m or so are called
pseudosparite
(Folk 1965).
Meta-
sparite
(Kovacs 1987) designates a similar fabric that
exhibits indications of metamorphic processes (e.g.
granoblastic texture, complex twinning).
7.8 Dolomitization and Dedolomiti-
zation
The first part of this chapter deals with the descriptive
criteria of dolomites seen in microfacies thin sections.
The lack of undisputed modern analogues for ancient
dolomites is the core of the dolomite problem and the
reason for several competing dolomitization models that
are briefly summarized in the second part of this chap-
ter. The third part discusses dedolomitization phenon-
ema.
