GEOLOGIJA 31, 32, 395-401 (1988/89), Ljubljana
UDK 552.5.551.763.781(497.12)=20
Diagenetic features of limestones from the Cretaceous/Tertiary
boundary of SW Slovenia (Yugoslavia)
Roman Koch
Institut für Paläontologie, Loewenichstr. 28, D-8520,
Erlangen, BRD
Abstract
The Upper Cretaceous limestones show commonly syntaxial overgrowth on
echinoid fragments, geopetal fillings with vadose silt, gravitational cements,
neomorphic altered miliolids, and microsparite and pseudosparite. AU features
are indicative of a strong early freshwater diagenesis. In the Kozina beds minor
hints for a freshwater diagenesis are present. In general a trend from freshwater
influenced limestones to normal marine limestones can be observed. Birdseyes
occur commonly with dedolomite. Additionally molds, vugs, and geopetal micro-
sparitic silt is present. The Miliolid limestones generally were formed under
normal marine lagoonal conditions. Only very locally hints for a weak early
diagenetic freshwater influx are given. Scarce syntaxial overgrowth on echinoid
fragments and some clear fine granular cements can be observed. Microcodium
was commonly formed in local areas of short time emersion.
Introduction
The recent analyses v^^ere carried out on material from D el valle (1985) and
Delvalle and Buser (1990) v^ho studied the microfacial development of limesto-
nes from the Cretaceous/Tertiary boundary in SW Slovenia in detail. They analysed
three profiles near the villages of Kozina, Materija, and Slivje by microscopic and
geochemical means. From their samples representative limestones of Upper Cretace-
ous, Early Paleocene (Kozina beds), and Late Paleocene age (Miliolid limestones)
v^ere selected. Delvalle and Buser (1990) characterise the facial development of
these beds as follov^^s:
Upper Cretaceous: Shallow subtidal, environment with local intertidal zones. The
water energy was low (micrites), but locally higher (sparites). Some rudist mounds
were present. Bauxitisation occurred during longer periods of emersion. Bauxitic
material in the limestones indicates a proximity of land. A longer period without
sedimentation followed which was ended by the deposition of the overlying Kozina
beds.
Early Paleocene (Kozina beds): The Kozina beds have a polygenetic character.
They comprise different facies types. Nevertheless a general trend can be observed
396 Roman Koch
Diagenetic features
The aim of the recent study is to reveal the diagenetic features of these limestones
and to demonstrate their facies indication.
Upper Cretaceous limestones
The most prominent diagenetic features of limestones in the three profiles analy-
sed are (1) syntaxial overgrowth on echinoid fragments, (2) the filling of molds with
vadose silt, (3) relic textures of gravitational cements, (4) partly and nearly comple-
tely altered (neomorphosed) miliolids, and (5) commonly microsparite.
Syntaxial overgrowth on echinoid fragments (plate 1, A) occurs
commonly. The rims are 20 to 250 џт thick ahd reveal a replacement texture.
According to Walkden and Berry (1984) and Koch and Schorr (1986) they are
interpreted to be of early diagenetic meteoric origin. Their formation was possible
due to the dissolution of neighbouring carbonate material and a subsequent cementa-
tion of the mold by a large calcite crystal which has partly a replacing character.
Syntaxial overgrowth is only developed around echinoid fragments which have no
micritic envelopes.
Geopetal fillings with vadose silt (plate 1, B) consist of fragments of
crystals with a size of 20 to 100 џт. They are generally floating in a micritic to
microsparitic matrix. Probably this internal sediment represents a mixture of two
sediment types. The crystals were formed in a supratidal environment and trans-
ported into the intertidal zone where they were deposited together with marine
micrite. The micrite jvas recrystallized subsequently. This type of sediment is compa-
rable to a mixture of crystal silt (supratidal and terrestrial environment) and marine
micrite (subtidal) as described by Aissaoui and Purser (1983).
Gravitational cement (plate 1, B) occurs only locally on the roofs of molds.
It can be recognized due to the fine crystal size and is interpreted to be the result of
an aggrading neomorphism of a former meteoric vadose gravitational cement (Mül-
ler, 1971). It consists of xenotopic crystals of 20 to 70 џт size. The rest of the molds is
filled by coarse-granular calcite. It is interpreted to be primarily of meteoric vadose
origin and to be overprinted by meteoric phreatic conditions.
Neomorphic altered miliolids (plate 1, C and D) occur commonly. They
are replaced by clear, greyish microsparite or pseudosparite partly or nearly comple-
tely. The miliolids are dark grey or brownish where no replacement had occurred.
There they reveal the original texture. The brownish colour indicates the presence of
a certain amount of organic material. When this material was oxidized completely,
the formation of microsparite took place and the colour turned to light grey. The
'shelter-effect' of the organic material (Hall & Kennedy, 1967; Kemper
& Koch, 1982) disappeared.
characterising the development from a highly freshwater influenced environment to
normal marine conditions in the upper part of the Kozina beds.
Late Paleocene (Miliolid limestones): Normal marine subtidal to intertidal condi-
tions are referred to favourable conditions for marine organisms. Locally areas of
emersion existed where plants grew commonly. During these periods Microcodium
was formed.
Diagenetic features of limestones from the Cretaceous/Tertiary boundary 397
The intraparticle pores of the miliolids are filled by fine to mediumcrystalline
calcite (20 to 120 џт size). Both features indicate an early diagenetic freshwater
influx. Nearly complete alteration of the miliolids by a longer reaching freshwater
diagenesis resulted in a complete transformation and recrystallization of the mili-
olids.
Microsparite and pseudosparite (plate 1, D) occurs where an early
freshwater diagenesis altered the primary micritic matrix of the limestones comple-
tely. In these limestones all miliolids are recrystallized too.
Early Paleocene limestones (Kozina beds)
The Kozina beds show layerwise abundant "birdseyes" and locally molds and
vugs which were created by enhanced freshwater influx. In some vugs geopetal
fillings of microsparite silt occur.
"Birdseyes" (plate 2, A and B) according to Shinn (1968) are indicative for an
intertidal environment of deposition. Additionally degassing structures can occur.
Many of these "birdseyes" in the recent study are similar to molds enlarged by
solution of the surrounding micrite. Now they are filled by sparry calcite. A closer
view reveals angular shapes of former crystals indicating that the vugs are filled now
by dedolomite (plate 2, B). The former dolomite is replaced completely by very fine
grained calcite crystals of different optical orientation. This structure is described by
Evamy (1967) and De Groot (1967) as the result of near surface diagenetic
processes resulting in dedolomitization.
Geopetal microsparitic silt (plate 1, C and D) shows a slightly pelletoidal
structure. It is composed of finely recrystallized micrite containing some coarser
crystals and is therefore similar to vadose crystal silt. Due to the very fine crystal size
it is considered to be another type of vadose silt and therefore indicative for
supratidal conditions. The relict pore space in the vugs is completely cemented by
coarse granular calcite.
Late Paleocene (Miliolid limestone)
The miliolid limestones show a normal marine fauna content with abundant
benthic foraminifers. One of the most striking features is the abundance of Microco-
dium. This indicates soil forming processes within several periods of the Late
Paleocene combined with the growth of plants. Regardless of these hints for times of
emersion diagenetic indications of freshwater influences are only weak. The most
prominent features are (1) the syntaxial overgrowth on echinoids, (2) clear, fine
granular cements, and (3) the occurrence of Microcodium.
Syntaxial overgrowths on echinoid fragments (plate 3, A) are very
thin and very scarce. Therefore they can not be taken as proof for freshwater influx in
the Miliolid limestones. These thin overgrowths were probably formed by direct
replacement of the surrounding micritic matrix only. Compared with the packstones
(plate 1, A) where large overgrowths are developed in the recent wackestones only
a very low flow rate of freshwater was possible. Therefore no stronger solution can be
taken into account.
Clear, fine granular cement (plate 3, B) occurs in molds and in intrapar-
ticle pores of some foraminifers and gastropods. These cements commonly show
growth forms perpendicular and subperpendicular to the substrate and can be
sometimes observed in the form of granular rim cements. The single crystale have
398_Roman Koch
a size of 10 to 40 |xm. Granular rim cements can be formed by an early freshwater
diagenesis (meteoric phreatic) overprinting former marine cements (probably Mg-
calcite) immediately after the formation of the first marine bladed crystals. At this
time marine phreatic isopachous rim cement was not yet formed. This indicates only
local and short time freshwater influxes - probably in form of freshwater lenses.
Some crystals show the shape of "dog-tooth" cements which are interpreted to be of
meteoric phreatic origin too.
Microcodium (plate 3, C and D) occurs enriched layerwise. A closer look to the
boundaries of the Microcodium aggregates towards the micritic matrix demonstrates
the growth form and the type of enlarging processes of Microcodium. The first, outer
rims at the contact to the micrite consist of recrystallized micrite (plate 1, D).
Whereas the micrite is composed of tiny crystals of 5 to 15 џт size, the microsparites
are enlarged to crystals of 10 to 20 џт. Further crystal growth results in pseudospa-
rite (40 to 60 џт size), and finally in the large crystals characteristic for Microco-
dium. These large crystals have a length of up to 300 џт and a diameter of up to
200 \im at the outer contact. This processes are probably induced by the influence of
pore waters delivered from the centre of Microcodium throughout. By this process
expanding forms can be created.
References
Aissaoui, D.M. & Purser, B. H. 1983, Nature of origins of internal sediments in
Jurassic limestones of Burgundy (France) and Fnoud (Algeria). Sedimentology, 30, 273-283,
Amsterdam.
De Groot, S.J. 1967, Experimental dedolomitization. J. Sed. Petrol., 37, 1216-1220,
Tulsa.
Delvalle, D. 1985, Mikrofazielle Untersuchung der Kreide/Tertiär-Grenze in NW Jugo-
slawien (Slowenien). Unveröffentl. Dipl. Arbeit, Universität Heidelberg.
Delvalle, D. & Buser, S., 1990, Microfacies analysis of limestone from the Upper
Cretaceous to the Lower Eocene of SW Slovenia (Yugoslavia). Geologija, 31, 32. (1988/89).
Evamy, B.D. 1967, Dedolomitization and the development of rhombohedral pores in
limestones. J. Sed. Petrol., 37, 1204-1215, Tulsa.
Hall, A. & Kennedy, W.J. 1967, Aragonite in fossils. Proc. Roy. Soc. London, B 168,
377-412, London.
Kemper, E. & Koch.R. 1982, The preservation of aragonite and its significance for the
dark claystones of the Upper Aptian and the Lower Albian, Geol. Jb., A 65, 259-271, Hannover.
Koch, R. & Schorr, M. 1986, Diagenesis of Upper Jurassic sponge-algal reefs in SW
Germany. In: J. H. Schröder & B. H. Purser (eds.). Reef Diagenesis. Springer Verlag,
Berlin, Heidelberg, 224-244, Berlin.
Müller, G. 1971," Gravitational " cement : an indicator for the vadose zone of the subaerial
diagenetic environments. Johns Hopkins Univ. Stud. Geol., 19, 301-302, Baltimore.
Shinn, E. A. 1968, Practical significance of birdseye structures in carbonate rocks. J. Sed.
Petrol., 38, 215-223, Tulsa.
Walkden, G. M. & Berry, J. R. 1984, Syntaxial overgrowth in muddy crinoidal limesto-
nes: cathodoluminescence sheds a new light on an old problem. Sedimentology, 31, 251-267,
Amsterdam.
Diagenetic features of limestones from the Cretaceous/Tertiary boundary
399
Plate 1
Diagenetic features of Upper Cretaceous limestones
(A) Syntaxial overgrowth of echinoid fragments are common and well expressed
(B) Vadose crystal silt occurs as geopetal filling in a mold. At the roof of the mold granular
gravitational cements occur which are characterized by their very fine crystal size
(C) Miliolids are partly neomorphosed to microsparite. Obviously the inner part of the
miliolid chamber walls are more susceptible for recrystallization
(D) Limestone completely altered to microsparite and pseudosparite. Only relics of mili-
olids can be observed
400
Roman Koch
Plate 2
Diagenetic features of Early Paleocene limestones (Kozina beds)
(A) "Birdseyes" and degassing structures are common. Some of these structures are molds
and vugs enlarged by solution and a dolomitization-dedolomitization process
(B) Detail of microphotograph A reveals local angular outlines of former dolomite crystals
(arrows) which are now dedolomitized and consist of fine calcite crystals of different optical
orientation
(C) Larger vug created by solution of a former biogenic component. The vug is filled with
geopetal microsparitic silt
(D) Detail of microphotograph C shows microsparite with some coarser crystals and
pelletoidal structure. Large, blocky calcite crystals occur in fractures cutting through the vug
and sparry calcite
Diagenetic features of limestones from the Cretaceous/Tertiary boundary
401
Plate 3
Diagenetic features of Late Paleocene limestones (Miliolid limestone)
(A) Echinoid spine with only very thin syntaxial overgrowth
(B) Miliolid with only very weak recrystallization. The intraparticle pores within the
miliolid show fine to medium crystalline calcite cements which reveal an orientation perpendi-
cular and subperpendicular to the substrate. These cements are interpreted to be former Mg-
calcite rim cements which were altered by a freshwater diagenesis
(C) Limestone rich in Microcodium
(D) Transition zone between Microcodium and surrounding micritic matrix. The micrite is
first altered to microsparite and pseudosparite. Subsequently the recrystallization to the very
large crystals characteristic for Microcodium occurs