Organic Facies and Palynofacies of Late Jurassic "Lemeš Facies" in Croatia
Snježana BLAŽEKOVIĆ1, Georg KOCH2, Jeronim BULIĆ3, Anđa ALAJBEG4, Jasmin SMAJLOVIĆ5, Vladimir
Faculty of Science, Zvonimirova 8, 10000 Zagreb, Croatia ([email protected])
Institute of Geology, Sachsova 2, 10000 Zagreb, Croatia
Croatian Natural History Museum, Demetrova 3, 10000 Zagreb, Croatia
INA-Strategic Development, Research and Investment, Lovinčićeva bb, 10000 Zagreb, Croatia
Geotechnical Faculty, Halerova 7, 42000 Varaždin, Croatia
INA-Research and Laboratory Testing Dept., Lovinčićeva bb, 10000 Zagreb, Croatia
The Lemeš facies ("Die Lemeš-schichten", FURLANI, 1910) was deposited during the Kimmeridgian and
Tithonian in the central part of the Adriatic platform where the tectonic movements with an approximate SW-NE
orientation formed a relatively shallow intraplatform trough (VELIĆ et al., 2002). The palaeogeographic pattern during
that time span is characterised by different facies developments as a consequence of gradual progradation of reefal and
peri-reefal facies followed by oolitic facies resulting in the final infilling of the intraplatform trough and reestablishment of peritidal facies (VELIĆ, 1994). Due to this facies development, the Lemeš facies is a diachronic facies
ranging through the Kimmeridgian and Early Tithonian. The part of the trough in the area of Lika and northern
Dalmatia was connected with the open Tethyan realm, and therefore comprises deposits with more pelagic influences.
These deposits consist of bedded cherty limestones with radiolarians and ammonites intercalated with bentonite layers
and organic rich shales or marls.
For the purposes of the organic facies and palynofacies study, samples were collected in Lika and northern
Dalmatia from following localities mapped as Lemeš facies: Poštak Mountain (Dimići and Mijina Glava), Koreničko
Vrelo, Korenica-Milanović Draga, Srb-Brotnja, Knin-Rastićevo, Kijevo-Kozjak, Vrlika-Lemeš, Vrlika-Maovice,
Vrlika-Dabar and Muć-Bulići.
Most of the analysed samples were found to have TOC greater than 2.0 percent and hydrogen indices in the
range 530 to 638, i.e. they were found to be good petroleum source rocks. Kerogen type was primarily amorphous.
Biomarker analyses were performed for the bitumenes extracted from selected outcrop samples. Steranes (mass
fragmentograms m/z 217 and 218), monoaromatic steroids (mass fragmentogram m/z 253), triaromatic steroids (mass
fragmentogram m/z 231) and hopanes (mass fragmentogram m/z 191) were studied. According to the biomarker
parameters, the studied organic matter was derived mostly from the algal biomass enriched by bacterial biomass. This is
consistent with the palynofacies features and the composition of the organic matter under consideration. These high
quality petroleum source rocks were deposited in a marine environment at low redox potential, characterised as anoxic
to suboxic one which is in accordance with the high portion of lipid-rich amorphous organic matter. Biomarker maturity
parameters as well as the fluorescence features of the isolated kerogen showed early to mid mature stage of the related
organic matter. It indicates that the related strata were buried deeply during a long geological past at rather elevated
temperature resulting in thermal stress recognisable in a decent maturity level.
Palynological composition of the analysed samples shows that they contain almost entirely marine derived organic
matter. Marine derived palynodebris consists mostly of liptodetrinic amorphous organic matter (95-100%) (Fig. 1-8).
Marine palynomorphs are represented by varying concentrations of prasinophycean phycomata (Fig. 1, 5, 6), organic
walled dinoflagellate cysts (Fig. 2, 3, 4, 7) and acritarchs. Land derived palynomorphs are represented by very low
concentrations of cheirolepidiacean pollen (Fig. 8). Spores and other types of sporomorphs have not been identified.
Land derived palynodebris occurs in traces. Inclusions of sulphides have not been observed.
Palynofacies features of the analysed samples indicate deposition in a oxygen-depleted, relatively shallow, restricted
depositional setting (TYSON, 1995; BATTEN, 1996). Varying values of the ratio prasinophyts / dinoflagellate cysts
suggest variations in open sea influence, sea-level fluctuations and/or synsedimentary tectonics as well as
environmental changes resulting in variable cyst production of dinoflagellate communities. These conditions enabled
formation of a palynofacies dominated by marine derived liptodetrinitic AOM, most of it being of phytoplankton, algal
and bacterial origin. The lack of sulphide inclusions is consistent with depositional sets of carbonate platforms where
the supply of heavy metal ions is rather limited. The composition of organic matter clearly indicates a depositional set
devoid of terrestrial organic matter input.
Palynological kerogen of the analysed samples is characterised as Type II-S which is consistent with kerogen of sourcerocks which originate on carbonate platforms. Fluorescence properties suggest early to mid mature stage of the related
organic matter.
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FURLANI, M. (1910): Die Lemeš-schichten. Ein Beitrag zur Kenntnis der Juraformation in Mitteldalmatien. Jahrb.
Geol. Reichanst., 60 (1), 67-98.
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Carbonate Platform. Mem. Soc. It., 57 (2002), 141-151.
Transmitted white light
Magnification: 400X
Incident UV light cut with 530 nm filter