GEOLOGIJA 39, 119-131 (1996), Ljubljana
Ambrus Beds - Important Key for Interpretation of Neocomian
Paleogeography, Sea-Level Changes, Depositional Setting and
Tectonics in Suha Krajina Area (Slovenia)
Ambruške plasti in njihov pomen za interpretacijo
neokomskih paleogeografskih, evstatičnih in tektonskih razmer
na območju Suhe krajine (Slovenija)
Stevo Dozet
Geološki zavod Ljubljana
Inštitut za geologijo, geotehniko in geofiziko
Dimičeva 14, 1000 Ljubljana, Slovenija
Key-words: carbonate sediments, lithology, genesis, paleogeography. Lower
Cretaceous, Outer Dinarides, Central Slovenia
Ključne besede: karbonatni sedimenti, litologija, geneza, paleogeografija,
spodnja kreda. Zunanji Dinaridi, osrednja Slovenija
Abstract
The lower part of the Lower Cretaceous stratigraphie sequence at Ambrus,
composed of limestones, dolomitic limestones, dolomites, intertidal breccias and
overlying heterogeneous carbonate breccias is described in this paper On the basis
of stratigraphie order, fossil contents as well as the well-determined fossiliferous
underlying Upper Malm and overlying Barremian carbonate sediments, the for-
mation denominated the Ambrus beds was ascribed to the Neocomian. Fenestral
limestones, laminated limestones and dolomites containing stromatolites, mud-
cracks and numerous erosion and oxidation surfaces are interpreted as tidal storm
deposits. The shallow water sediments, the breccias and the subaerial exposures,
evidenced by relatively common karstification phenomena, are considered to be
connected with eustatic sea-level variations and epeirogenetic movements. The
lithology and areal distribution of these deposits indicate a sedimentation in a rel-
atively symmetric, rather shallow lagoon. The breccias composed of various poorly
sorted angular and subangular limestone and dolomite fragments, as well as of
calcific and dolomitic cement, have been attributed to tidal and cliff breccias. The
majority of the breccia fragments indicate a pretty shallow water and subaerial
sedimentation conditions.
Kratka vsebina
Najnižji del spodnjekrednega sedimentnega zaporedja pri Ambrusu sestavljajo
apnenci, dolomitizirani apnenci, dolomiti in heterogene karbonatne breče. Glede
na statigrafsko lego, fosile in s fosili dobro dokazane spodaj ležeče zgornje-
malmske plasti ter zgoraj ležeče barremijske sedimente, smo formacijo, ki smo jo
poimenovali Ambruške plasti, uvrstili v Neokom. Fenestralni in laminirani apnen-
120
Stevo Dozet
ci in dolomiti s stromatoliti, izsušitvenimi porami ter številnimi erozijskimi in
oksidacijskimi površinami so nastali v plimskem območju. Nastanek plitvovodnih
karbonatnih sedimentov, breč in nadplimskih tvorb s številnimi pojavi zakraso-
vanja povezujemo z evstatičnimi in epirogenetskimi premikanji. Litološka sestava
in razširjenost teh sedimentov na površini kažeta na sedimentacijo v plitvi laguni.
Breče, ki so sestavljene iz različnih slabo sortiranih oglatih do slabo zaobljenih
apnenčevih in dolomitnih drobcev, vezanih s kalcitnim in dolomitnim cementom,
smo uvrstiU med plimske breče in breče strmih obrežij (cliff). Drobci kamnin v
brečah kažejo na plitvovodne in nadplimske pogoje sedimentacije.
Introduction
The purpose of this preliminary v^ork is to describe the carbonate formation,
denominated the Ambrus beds that builds up the small hills Kamni vrh, Stražarjev
vrh and Mali vrh at Ambrus. The most part of the data from the study area were
obtained by detailed geological mapping for the Geological map of Slovenia on the
scale of 1:50.000. This study is based on field mapping data and thin-section analysis.
Facies characteristics of the Ambrus sedimentation have been considered, with the
Fig. 1. Location sketch map of investigated area
SI. 1. Položajna karta raziskanega ozemlja
Ambrus Beds - Important Key for Interpretation of Neocomian ...__121
Previous Work
Šribar (1966) described the Jurassic sedimentary rocks between Zagradec and
Randol in the Suha Krajina area. On the basis of microfossils and the stratigraphie
position she divided the Jurassic stratigraphie sequence into the Lower and Middle
Liassic, the Upper Liassic-Dogger, the Lower Malm and the Upper Malm. After styd-
ing all the aspects of the occurrences of abberant tintinnids she found (1979) that
abberant tintinnids from the Dinarides of the South Slovenia do not indicate the
Valanginian stage. She attributed the transitional beds between Jurassic and the
Lower Cretaceous to Berriasian.
Buser (1979) mapped the Ribnica Map sheet area and divided the Upper Malm
carbonate rocks of the area into the lower (Oxfordian and Lower Kimmeridgian) and
upper part (Upper Kimmeridgian and Tithonian). The author subdivided the Lower
Cretaceous beds into the following units: Valanginian, Hauterivian+Barremian, Apt-
ian+Albian+ +Cenomanian.
According to Buser (1989) the Outer Dinarides underwent a differentiation due
to the formation of the Slovene trench, and the originally uniform area was dissected
into two minor platforms, the Julian and the Dinaric one.
Savie and Dozet (1985) described the general geology of the Delnice Map sheet
area dividing the Lower Cretaceous succession as follows: Berriasian, Valanginian,
Hauterivian, Barremian, Lower Aptian, Upper Aptian, Lower Albian and Upper
Albian. To the Neocomian they attributed the limestone-dolomite-chert breccia.
Dozet (1990) subdivided the Jurassic stratigraphie sequence of the Kočevje and
Gorski Kotar area into 5 cenozones and 3 subzones by algae, foraminifera and
pelecypods. Furthermore, 5 cenozones and 3 subzones are recognized in the Lower
Cretaceous sequence by algae and foraminifera.
Strohmenger and Dozet (1991) studied the stratigraphy, facies developments
and geochemistry of the Jurassic carbonate rocks in Suha Krajina. The field studies
showed that at least the uppermost part of Dogger was not deposited.
purpose to explain the geological events of the Suha Krajina area (Fig. 1). In order to
ascertain the geologic significance of the discontinuous breccia body, we have
attempted to date the breccia exposures along the road that leads to Kamni vrh, and
to reconstruct its shape, size, composition, depositional conditions and environment.
In addition to this, we wanted to find out, whether the brecciation phenomenon was
related to tectonic activity. The study of the Ambrus formation has a great impor-
tance not only for the stratigraphy of the study area, but also for better explanation
of the Lower Cretaceous paleogeography. On the other hand, the breccias provide an
important key to the depositional setting during the Neocomian. An attention is
given to the Ambrus breccia constituents, differentiating fragment and cement types,
as well as to the underlying and overlying sedimentary successions, providing in this
way a more general information on the brecciation process and evolution. As a mat-
ter of fact, the carbonate breccias reflect a specific and relatively longstanding phase
in the geologic history of the considered area.
The carbonate rocks are classified using Folk's (1959) practical pétrographie
classification of limestones and Dunham's (1962) classification of carbonate rocks
according to depositional textures. The present work has been performed with sup-
port of the IGGG-Geological survey of Ljubljana.
122_Stevo Dozet
The underlying strata
The strata occurring immediately bellow the Ambrus beds are composed of
micritic, biomicritic, biointrasparitic, oncolitic and stromatolitic limestones, as well
as early and late diagenetic dolomites and carbonate breccias that reflect the shallow
subtidal, intertidal and supratidal cycles. The carbonate rocks enumerated above are
more or less recrystallized and dolomitized. According to the texture they mostly
belong to calcirudites and calcarenites. The Malm beds of the Suha Krajina area con-
tain coated grains of algal origin that were formed in periodically and intermediately
agitated water in a shallow subtidal environment. Some data suggest that a part of
oncolites were deposited in a water of greater turbulence (Dozet, 1995b). The Upper
Malm succession is composed predominantly of Clypeina and Tintinnina limestones
and dolomites intercalated with carbonate breccias. Sedimentological features with-
in the majority of limestones and dolomites point at shallow-subtidal, intertidal and
supratidal depositional settings. The carbonate breccias prevalently occur in the
uppermost part of the Malm stratigraphie sequence.
Occurring in numerous stratigraphie levels of the Malm sedimentary succession at
Korinj, the breccias have been named the Korinj breccias (Dozet&Strohmenger,
1996). Five genetically different breccias have been distinguished. The polymict karst
breccia is interpreted to be the lateral equivalent of the Lower Malm/Upper Malm
bauxite horizon (Strohmenger, 1988). Emplacement of carbonate breccia into the
Malm sedimentary succession is a characteristic feature in certain places of the Outer
Dinarides region, the Logatec plateau and especially in the Suha Krajina area (Dozet,
1995a). Mudstone breccia-conglomerate originated by desiccation. Textural and
structural characteristics of some breccias indicate their intertidal and supratidal
formation (tidal breccias). Dolomitic, limestone-dolomitic and limestone talus brec-
cias were formed by accumulation and consolidation of talus at the base of coastal
cliffs. Some talus breccias have been formed in relation to submarine synsedimentary
faulting.
The uppermost part of the Malm sedimentation is characterized by rhythmic sedi-
mentation of light gray, micritic and laminated limestones and dolomites. The strati-
graphic sequence described above belongs biostratigraphically to the Upper Malm
Clypeina jurassica cenozone (Dozet, 1994, 1996).
The overlying strata
The strata which overlie the Ambrus breccias and which consist of platy and
stratified (5 - 45 cm) limestones, dolomitic limestones and dolomites, sporadically
intercalated with thin layers of small-scale intraformational carbonate breccias,
belong to the Barremian. The limestones include fine, medium and thick-bedded,
gray, dark gray and black, more or less bituminous micrites, pelmicrites, bioin-
Stratigraphie Position
The Neocomian Ambrus beds can be followed in a some kilometres wide area
north of Ambrus. The treated beds are about 250 metres thick. They lie between the
Upper Malm and the Upper Barremian beds.
Ambrus Beds - Important Key for Interpretation of Neocomian ... _123
The Ambrus Beds
Berriasian
The lower part of the Ambrus sedimentary succession consists of limestones,
dolomitic limestones, dolomites, dedolomites and tidal carbonate breccias. Lime-
stones are commonly more or less early and late-diagenetically dolomitized. Conse-
quently, all transitional types between limestone and dolomite can be observed in the
Ambrus stratigraphie sequence. Limestones were formed in shallow marine environ-
ments, namely: lagoon, subtidal, intertidal and back reef. According to the structure
and the texture the limestones belong to micrites, dismicrites, as well as stromatolitic
and intraclastic limestones. Most typical and important are Favreina and Salpingo-
porella limestones. The limestones are commonly more or less karstified and contain
at some places residual materials. Erosive, rarely stylolitic contacts and surfaces can
be found, too. Further on, the lower part of the Ambrus stratigraphie sequence which
chronostratigraphically belongs to Berriasian consists of alternating late diagenetic,
early diagenetic, structureless and stromatolitic- dolomites, as well as dedolomites.
Early diagenetic dolomite is light gray to white, stratified (30-80 cm), fine-grained,
cryptocrystalline to microcrystalline, laminated and stromatolitic, containing ostra-
cods, rare gastropods, oncoids, shrinkage pores, fecal pellets and bird's-eyes. Late
diagenetic dolomite is chiefly dark gray to brownish gray, coarse-grained (saccha-
roidal), internally structureless, occurring in lenticular or irregular bodies. Small-
scale brecciation and the limestone breccia become apparent at the Lower
Malm/Lower Cretaceous contact and within the beds at the base. According to its
origin it belongs to the tidal type of breccias. The strata occurring immediately bel-
low the breccias reflect shallow intertidal and supratidal cycles. Tidal breccias, origi-
nated due to desiccation, are overlain by fenestral and laminated limestones of pre-
dominantly algal origin, and karstified limestones with residual materials. Stroma-
tolitic dolomites, locally characterized by gastropod remains, are sometimes interca-
lated within structureless dolomites. At the top of the described sedimentary succès-
trasparites, biosparites and stromatolites. Among biomicrites and biosparites pre-
dominate the foraminiferal and algal ones. The main constituents of the foraminifer-
al limestones are benthic foraminifera, miliolids and algae Dasycladaceae. Rhythmic
sedimentation of limestones can be often seen in this interval of the Lower Creta-
ceous stratigraphie sequence. Rhythms of pelsparite-stromatolite are very common
and typical. Also characteristic is a rhythmic alternation of early diagenetic and late
diagenetic dolomites. From the biostratigraphical point of view the described succes-
sion of limestones and dolomites belongs to the Salpingoporella muehlbergii (Lorenz)
cenozone (Šribar, 1979). The gradational contact of the breccia and overlying strata
is characterized by a development of alternations of up to 0.35 metres thick, light-
brown coloured dolomites and equally thick, dark gray mudstones, locally with
bird'S-eyes. The dolomite beds decrease in thickness from bottom to top. In fact, these
dolomite beds are chiefly composed of coarse-grained structureless and finer-grained
stromatolitic dolomites. The stromatolites within breccia testify to high-intertidal
and supratidal conditions. Above this alternating succession the unit is mainly com-
posed of mudstones. On the basis of field and sediment-petrographical data the over-
lying carbonate strata (Barremian) are interpreted to be chiefly originated in a shal-
low lagoon.
124_Stevo Dozet
Neocomian (Valanginian to Lower Barremian)
Ambrus Breccias
General description of breccias
The Neocomian heterogeneous carbonate breccias come to light in the Kamni vrh-
Stražarjev vrh-Mali vrh area at Ambrus. They build the upper part of the Ambrus
beds with an average thickness of 150 metres. The Ambrus breccias (Fig. 2) are
prevalently massive. In the lower part of the breccias an unclear stratification can be
seen. Better stratified are breccias from the upper part of the Neocomian lithologie
column. The size of limestone and dolomite fragments varies considerably. In the
lower part of the breccia unit the fragments are on an average 3 cm to 6 cm and in the
upper one from 0.5 cm to 3.5 cm in size. The fragments are bound together by calcitic
and dolomitic cement and some clayey matrix. Since some carbonate fragments con-
tain the Malm microfossils and due to the Barremian dasycladaceans in the overlying
strata, the breccias chronostratigraphically correspond to Neocomian. In spots in the
upper part of the breccia column a thick-bedded, white biomicrite and biopelmicrite
with algae Clypeina? solkani occur, which confirms the Neocomian age of the brec-
cias. According to the composition and sedimentological properties of the breccias we
can conclude that after the Upper Malm important paleogeographic and depositional
changes took place. These changes are supposed to be a consequence of epeirogenetic,
eustatic and tectonic movements which caused the differentiation of up to this time
relatively uniform environment. These movements caused faulting, uplifting, vertical
displacements, local dry land and, accordingly, new paleogeographic disposition
between the land and the sea. On the basis of morphological characteristics, the size,
the shape, the poor sorting of breccia constituents, and with regard to the type of
cement, we come to conclusion that during the deposition of the breccias there were
great differences with respect to the base level of erosion.
The section Brezovšče-Kamni vrh at Ambrus was investigated in the field, howev-
er, the description of the breccias given here is based on numerous outcrops in the
entire study area. About 30 thin sections from the type locality section and 20 thin
sections from other outcrops were studied under the pétrographie microscope. The
maximum thickness of the breccias at the type locality on Kamni vrh at Ambrus is
150 m, but elsewhere it can be extremely reduced. The contact with the underlying
Berriasian rocks is pretty sharp and it is about parallel to bedding. The contact of the
breccias with the overlying Barremian limestones and dolomites is in some places
sharp, but locally a gradual transition of the breccias into the overlying carbonate
succession can be seen. The dark gray to black heterogeneous carbonate breccias
consist of fragments varying in dimensions from pieces of almost half a metre in
length to the smallest ones of microscopic dimensions. On the basis of the fragment
size the breccias can be divided in two parts: the coarse-grained (5-55 cm) lower part
and the finer-grained (0.5-3.0 cm) upper one. The Ambrus breccias are mostly mas-
sive carbonate sediments. However, the upper part of the breccia complex is often
more or less clearly stratified. Moreover, carefull examination of the fragment
Sion v^^hite and darle gray mudstones occur. An increase in bed thickness towards the
upper contact is present. The mudstones above the Lower Berriasian stratigraphie
sequence represent a short transgressive trend. At the top of the transgressive strata
mudstones with bird-s-eyes and stromatolites occur.
Ambrus Beds - Important Key for Interpretation of Neocomian ...
125
Fig. 2. Geologic column of the Ambrus beds in the Suha Krajina area
1 Bedded limestone; 2 Bedded coarse-grained dolomite; 3 Stromatolitic dolomite; 4 Massive
breccia; 5 Bedded breccia; 6 Clypeina jurassica Favre; 7 Aberrant tintinnids; 8 Gastropods; 9
Favreinas; 10 Algae (generally); 11 Clypeina Ž solkani Conrad & Radoičić; 12 Dasycladaceae;
13 Benthic foraminifers; 14 Stromatolites
SI. 2. Geološki stolpec Ambruških plasti na območju Suhe krajine
1 Plastnati apnenec; 2 Plastnati debelozrnati dolomit; 3 Stromatolitni dolomit; 4 Masivna breča;
5 Plastnata breča; 6 Clypeina jurassica Favre; 7 Velike tintinine; 8 Polži; 9 Favreine; 10 Alge
(splošno); 11 Clypeina ? solkani Conrad & Radoičič; 12 Dazikladaceje; 13 Bentonske fora-
minifere; 14 Stromatoliti
126_Stevo Dozet
Petrography of the breccia fragments
The Ambrus breccias are composed of limestone, dolomite and silica fragments.
The fragments are rather variable from millimetre to half a metre-sized. Larger frag-
ments occur more frequently in the lower part of the breccia. Angular and subangu-
lar fragments are the main constituents of the breccias. Some fragments, especially
from the lower part of the breccia interval, contain calcite and/or dolomite veins.
Normally, the breccias have a heterogeneous and rather chaotic fabric. Namely, the
breccia which is locally associated with tectonics, consists of a mixture of large and
small blocks of irregular shape and different age and very fine-grained, tightly
packed material accumulated in a state of total disorder. It is oligomictic with
dolomite fragments as the most abundant rock type.
Limestones fragments are common in the upper part of the breccia complex. The
predominant limestone type of fragments is white lime mudstone containing small
algae and ostracods. Other mudstone fragments contain rare mollusc remains. The
mudstones are interpreted as lower intertidal to shallow subtidal muds. The exis-
tence of broken ostracods suggests that some of these bioclasts were redeposited.
Algal laminated mudstone fragments, consisting mainly of dense accumulation of
pellets, are also common. The algal laminated mudstone is interpreted as having been
deposited in the intertidal zone or possibly in a shallow subtidal environment. Lime-
stone fragments are usually subordinate constituents of the Ambrus breccias, but in
spots they become even predominant. The limestone fragments of the Ambrus brec-
cias belong to the following structural and textural types: black, greyish black, dark
gray, dark brownish gray, brownish gray, gray, moderate gray, light gray and white
micrites, biomicrites, biopelmicrites, biosparites, biointrasparites, intramicrites,
intrasparites, intrasparrudites, as well as grained, stromatolitic, fenestral and
dolomitic limestones.
Dolomite fragments are predominant (60-85 %) particles in the Ambrus breccias.
Three dolomite types are present in the fragments, namely: brownish gray, dark
brownish gray and grayish black dolomicrite, dolosparite and stromatolitic dolomite.
Gray dolomicrite was recognized in the lower breccia unit: The dark brown idiotopic
dolosparite fragments predominate in the lower breccia unit. Dolomite in crystalline
form almost always occurs together with calcitized and dolomitized mudstones or is
interbedded with grainy limestones. It consists of subhedral to euhedral dolomite
crystals. This dolomite rarely retains original sedimentary textures. In outcrop it is
often intensively altered by surface weathering. Fragments composed solely of stro-
matolitic dolomite also exist. These dolomites normally retain original sedimentary
textures. Here and there gastropod sections, ostracod shells, as well as fecal pellets
and algal skeletons can be observed in the stromatolitic dolomite. All three dolomite
types constitute continuous beds in the lower part of the Ambrus beds, i.e. in the sed-
imentary succession immediately underlying the cliff breccias, as well as the
dolomites in the Upper Malm dolomite-limestone succession.
lithology show that many of the larger displaced fragments still maintain a relative
orientation approximately parallel to the stratification giving the breccias of the
lower part an apparent stratification, too. On the other hand lenses of very light gray
to white, thick-bedded Clypeina? solkani limestone can be observed in the upper part
of the Lower Cretaceous brecciated lithologie interval.
Ambrus Beds - Important Key for Interpretation of Neocomian ■..__12T_
Breccia cement and matrix
The cement in Ambrus breccias belongs to fine-grained calcite, dolomite, rarely
matrix. In spots, coarser sparry crystals originated as a result of crystallization of
fine-grained miciritic carbonate can be observed. Occasionally, only coarsely
crystalline calcite (sparry calcite) cement fills the space among fragments. Since the
matrix itself is often brecciated, it clearly postdates the lithification of the
microsparitic matrix. As already said, the two breccia units were differentiated on
the basis of the grain size and the material filling the interstices among grains. In the
lower breccia unit the sparry calcite cement is preceded by a finelly crystalline cal-
cite. Moreover, the clayey fine-grained interstitial material contains individual clasts
and dolomite crystals. The dolomite crystals commonly are abraded and/or broken,
being in any case formed before brecciation. Some smaller mudstone fragments,
incorporated in the mud, are completely recrystallized. Larger cavities may be partly
sediment-filled or in part spar-filled which created geopetal fabrics. In fact, the sedi-
ment forms the floor, and the sparry calcite the upper part of the cavity.
Genesis
The Ambrus beds represent an important key to the depositional setting during
Neocomian time intervals as well as to major tectonic events in the Suha Krajina area.
The occurrence of stromatolites, mud-cracks, erosion surfaces, karstification phenom-
ena, accumulations of insoluble residue, breccias, rhythmical alternations of highly
dolomitized and laminated lime mud in the lower part of the Ambrus beds, lead us to
conclusion that the finely stratified dolomites are supratidal deposits. The lack of fos-
sils as well as the structural characteristics suggest current-agitated conditions. Fur-
thermore, fine stratification of the intertidal and supratidal mud deposits is per-
formed by storms that transport and deposit marine sediment into the intertidal and
supratidal setting. It should be noted that the fragmentation of the newly formed sed-
iment occurred at shoaling and temporary withdrawal of water by dessication and
mud-cracking. Irregular dried-out and mud-cracked polygons have been broken into
angular fragments, being later deposited and lithified together with remained lime
mud. The lithologie composition, the scarity of fauna as well as textural and structur-
al characteristics of the breccias indicate their intertidal and supratidal formation.
The main characteristics of the cliff breccias are angular to subangular carbonate
clasts of uniform or polymict composition, a very dense packing of clasts, a very poor
sorting without grading or bedding, and different-sized fragments. It should be
emphasized that this all speaks in favour of hypothesis that the breccias were formed
by accumulation and consolidation of rock fragments derived from cliffs i.e., a high,
very steep to overhanging face of rocks rising above the shore, usually produced by
physical disintegration, chemical decomposition, faulting and erosion. The talus has
been chiefly formed by gravitational falling of loose fragments, and their accumula-
tion and consolidation at the foot of the described escarpments or steep walls.
Silica fragments occur only sporadically in the studied breccia. They are present
exclusively in the middle part of the upper breccia unit and belong to chert. The sili-
ca fragments are usually more than 5 centimetres in size.
128_Stevo Dozet
Paleogeographic Interpretation
According to the composition and sedimentological properties of the breccias, we
can conclude that after the deposition of the Upper Malm beds important paleogeo-
graphic and depositional changes took place. The quoted changes, which are sup-
posed to be the consequence of epeirogenetic, eustatic and tectonic movements,
caused the differentiation of up to this time relatively uniform environment. These
movements and forces caused faulting, uplifting, vertical displacements, local dry
land and, accordingly, new paleogeographic disposition of the land and the sea.
On the Jurassic/Lower Cretaceous boundary some parts of the Suha Krajina area
were subjected to tectonic movements as a consequence of the Late Kimmerian oro-
genetic phase (Dozet, 1989), resulting in an uplifting and rupturing of individual
parts of the bottom of the sea. The new formed land gave the material for origin of
the heterogeneous limestone-dolomite sedimentary breccias. A new paleogeographic
picture between the land an the sea developed. The Neocomian carbonate breccias
that originated predominantly in shore regions are genetically closely related to the
mentioned paleogeographic changes.
On the basis of morphological characteristics, the size, the shape, the poor sorting
of breccia constituents, and with regard to the type of cement, we come to conclusion
that during the deposition of the breccias, there were great differences with reference
to base level of erosion. The coast region was characterized by steep, often vertical
and even overhanging cliffs and with a strong water dynamics, but the transport was
very short. With regard to the composition of the coarse-grained carbonate rocks it is
evident that then being land, as source area of rock material, was composed exclu-
sively of carbonate rocks. At new conditions of sedimentation carbonate breccias
were formed.
Fossils and Age
The lov^er part of the Ambrus beds contains numerous Favreina salevensis Parejas
which are almost always rock-forming, further on Salpingoporella annulata Carozzi,
Thaumatoporella parvovesiculifera (Raineri), Verneuilinidae and Textulariidae. Fos-
sils have not been found in the matrix of the breccias so far Consequently, we can
describe biofacies and define the age of the Ambrus breccias on the basis of the fossil
association from the rock fragments. The biofacies can be referred to the Clypeina?
solkani Conrad & Radoičić zone, which is typical for the carbonate platform shallow
marine environment of the Neocomian age. Considering the constituent breccia frag-
ments, their fossil contents, the stratigraphie position as well as the underlying and
overlying fossiliferous carbonate sediments the breccias are of Valanginian, Hauteri-
vian and very probably of the Lower Barremian age.
During the Neocomian the restricted lagoon was developed in the study area.
Detrital influx within the area studied was insignificant, suggesting that the sur-
rounding dry land was built of carbonate rocks. The lithologie composition, structur-
al and textural characteristics of the sediments suggest that these sediments were
formed in a shallow-marine setting. The stratification of breccias is poor, what is
comprehensible, since strata in these depositional conditions could not be formed.
Ambrus Beds - Important Key for Interpretation of Neocomian .■._       129
Depositional environment of the Berriasian beds (Lower Ambrus Beds)   '"^
A poor faunal diversity i.e., a low number and a low quantity of species, as well as
stromatolites, bird's-eyes, numerous local erosion surfaces, common occurrences of
marine and meteoric cements, dolomitization, common occurrences of karstified car-
bonate rocks, clearly indicates shallow subtidal to intertidal and even shortstanding
periodical supratidal setting during Berriasian as well as Valanginian, Hauterivian
and very probably the Lower Barremian.
Deposition of the brecciated rocks (Ambrus breccias)
The brecciated Neocomian carbonate succession at Ambrus was deposited in a
very shallow restricted lagoonal setting. The predominance of the shallow intertidal
to supratidal carbonate fragments i.e., dolomites, algal laminates and mudstones
with birds-eyes within the breccia indicates an overall restricted shallow lagoonal
setting. Bellow the breccia in the Ambrus-Kamni vrh type section the pétrographie
evidence suggests that the restricted lagoon periodically changed into dry land.
The first small brecciation could have resulted from small-scale periodic seasonal
dessication under evaporative conditions. The lower breccia part underwent two
major phases of forming. The first probably occurred soon after deposition during a
short period of emergence. The existence of an emergence period between the lower
and upper breccia part is resumed from the observation that the mud matrix is
absent in the upper breccia part. The carbonate groundmass material may originate
from unlithified mud, occurring at the top of the lower breccia unit, or may relate to
mud infiltration at the initial stages of sedimentation of the upper breccia unit. After
the emergence period sedimentation resumed with deposition of mudstones. The
dolomite beds become less prevalent through time and finally disappeared.
Characteristic appearances within the breccia such as veined fragments, re-brec-
ciated breccia fragments and the presence of two groundmass generations (micro-
crystalline and sparry calcite cement) exhibit at least two phases of formation of
breccia. Initial formation of breccia occurred soon after deposition during a short
period of emergence. During this stage the original carbonate beds locally became
veined and fractured. Carbonate mud infiltrated into interstices between the frag-
ments. Finally, the breccia fragments were cemented by sparry calcite.
Conclusions
During the Upper Triassic, Jurassic and Cretaceous periods the study area was a
part of the Dinaric Carbonate Platform.
As far as tectonics is concerned, in the Jurassic and the Lower Cretaceous periods
the Dinaric Carbonate Platform was not subjected to any stronger tectonic move-
ments (Savie, 1973; Bus er, 1979, 1980, 1989; Dozet, 1989, 1994). Consequently in
the Upper Malm and the Lower Cretaceous a relatively continuous sedimentation
occurred, interrupted with periodical, short-lived local interruptions, as a conse-
quence of intensified epeirogenetic movements of the Carbonate Platform. These
movements caused periodical land forms especially at the time between the Lower
and the Upper Malm as well as between the Upper Malm and Neocomian.
130_stevo Dozet
The Malm and the Berriasian sedimentation was, generally speaking, a result of
three different modes of sediment forming: 1) - land setting, 2) - changeable shallow
marine and subaerial setting and 3) - shallow marine setting.
The beds between fossiliferous underlying Upper Malm Tintinnina and Clypeina
limestones and overlying Barremian Dasycladacea limestones at Ambrus have been
named the Ambrus beds.
The lower part of the Ambrus beds, belonging to Berriasian, is interpreted as a
shallow water deposition with considerable degree of subaerial exposures. The pre-
dominance of laminites, mudstones with birdseyes and shallow intertidal to suprati-
dal carbonate fragments indicates that the treated sequence was originally formed in
a shallow restricted lagoonal setting.
The upper part of the Ambrus beds is occupied by heterogeneous carbonate brec-
cias.
In the breccia interval the heterogeneous dolomite breccias are predominant, the
limestone-dolomite breccias are less extended, whereas pure limestone breccias are
rare.
Chronostratigraphically, the Ambrus-breccias are of Valanginian, Hauterivian,
and probably of the Lower Barremian age, since in their composition the fragments
of dark gray to black carbonate fragments, being common in Barremian, are predom-
inant.
From the biostratigraphic point of view the Ambrus beds are included in the
Clypeina? solkani Conrad & Radoičić zòne (Neocomian).
It seams acceptable to conclude that the Neocomian heterogeneous breccias dis-
cussed above, composed of the Upper Berriasian, Valanginian, Hauterivian and very
probably the Lower Barremian fragments, could not be exposed by erosion alone.
The reason can be found in tjae increasingly more intensive movements of the sedi-
mentary basis from the Berriasian to the Upper Barremian.
Interruption in the stratigraphie record without larger angular discordance testi-
fies to the statement that from the Upper Malm, Berriasian respectively, to the Upper
Barremain predominantly epeirogenetic movements with local disturbances existed,
contributing to the origin of the Ambrus breccias.
Finally, our main concept is, the heterogeneous Ambrus breccias with limestone
and dolomite constituents are products of intensified epeirogenetic and tidal move-
ments, eustatic sea-level changes, erosion as well as local synsedimentary tectonics.
Ambrus Beds - Important Key for Interpretation of Neocomian .■.__131.
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