Microfacies of the Triassic limestones in the Izvorul Malului klippe (Rarău Syncline, Transylvanian Nappes, Eastern Carpathians, Romania)

Abstract

The Transylvanian Nappes belongs to the Central – East – Carpathian Nappes System (the Dacides Medianes) which forms. The Transylvanian Nappes have a superior position in the Carpahian tectonic system that favored their fragmentation in the obduction process and slow gravitational decollement. This process makes difficult to establish the exclusively Mesozoic Transylvanian
sedimentary series, especially because the majority of the litostratigraphic members occur only as isolated klippe in the Hauterivian-Aptian wildflysh filling (the superior formation
belonging to the Bucovinian Nappe) of the Rarău Syncline.
The allochtonous sedimentary succesion of the Transylvanian Nappes is almost exclusively represented by pelagic carbonate deposits. The amazing fossil diversity offered
mainly by the klippes of the Rarău Syncline facilitated the reconstruction of the Triassic lithological column which contains all stratigraphical terms confirmed by a rich paleontological material. The Upper Triassic carbonate deposits cropp out in few metric (Piatra Zimbrului, Popii Rarăului) or submetric blocks (the klippes on the Cailor, Măceş, Izvorul Malului brooks,
on the springs of the Timon brook etc.) occuring in the Rarău Syncline. The studied limestone klippe is located on the left side of the Izvorul Malului brook, about 2,5 – 3 km up from his confluence with the Moldova river. The klippe which is almost
totally exploited consists of few white and gray limestone submetric blocks with Halobia. Microcrystalline carbonates are represented by muddy sediments accumulated in lowenergy
quiet waters on the sea floor. The sediment consists of skeletal debris and unattached
precipitates or of attached non-sketetal precipitates. The last two cases corespond to the
autochthonous organomicrites and are characterized by common peloidal fabric. The
formation of peloids requires low or moderate rates of sediment input.
Tethyan Carnian, Norian and Rhaetian carbonate platform are characterized by specific
benthic foraminiferal assemblages. They have proved to be extremely useful in recognising
facies zones. The distribution was controlled by substrate conditions and salinity, nutrient
requirements, specific life behavior, the topography of depositional environment, water depth
and light.
The microscopic analysis of the carbonate klippe made on 30 thin sections allowed the
differentiation of several microfacies types whose micropaleontological content covers the
Upper Triassic time. 

The Carnian microfacies are: biomicrites with pelmicrites and sparites. The biomicrites
contain a fossil assemblage consist of few foraminifera, alga, sponges, incertae sedis, (e.g.
Agathammina austroalpina Kristan-Tollmann & Tollmann, Trochammina alpina Kristan,
Glomospirella aff. facilis Ho, Endoteba ex. gr. obturata Brönnimann & Zaninetti, Glomospira
sp., Solenopora simionescui Dragastan, Solenopora cassiana Flügel, Cryptocoelia zitteli Ott,
Spiriamphorella carpathica gemerica Borza & Samuel), microgastropods etc.
The Norian – Rhaetian microfacies, according to their frequency, are: crinoid
biosparites, biomicrites and pelmicrites. The foraminiferal assemblage contains a few species:
Agathammina austroalpina Kristan-Tollmann & Tollmann, Variostoma catilliforme Kristan-
Tollmann, Trochammina alpina Kristan, Glomospira inconstans Michalik, Glomospirella
friedli Kristan-Tollmann, Glomospirella sp., Endoteba ex. gr. obturata Brönnimann &
Zaninetti, Diplotremmina sp.
Aerobic conditions are indicated by a gastropod-ostracod-foraminifera community and
a monotypic bivalve assemblage.
Diagenetic microfacies criteria are cements, neomorphic alterations, dissolution
features and fracturing. Calcite cements occur predominantly within the shell interior (marine
radial fibrous cement and late diagenesis recrystallized blocky cement). The limestones were
affected by near-surface and marine diagenesis (inversion of aragonite, radial fibrous cement)
and burial diagenesis (coarse blocky calcspar cement).
Microfacies features prove the depositional settings of the limestones of the Izvorul
Malului klippe: flat platform top within euphotic zone sufficiently connected with the open sea
to maintain salinity and temperature close to that of the adjacent sea; shallow-water (water
depths a few meters to ten of meters). The studied limestones are the inner platform
carbonates. (open marine and restricted area of the inner platform).