Lower Turonian at the Breaza-Comarnic Highway Bridge Pierfeet (Prahova Valley)
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ACTA PALAEONTOLOGICA ROMANIAE V. 5 (2005), P. 87-93 LOWER TURONIAN AT THE BREAZA-COMARNIC HIGHWAY BRIDGE PIERFEET (PRAHOVA VALLEY) Eugen Tiberiu DABU1 Abstract: The present work is studying especially the planktonic foraminifera of the Lower Turonian to confirm its presence in this area. The study of planktonic foraminiferal association revealed the presence of Helvetoglobotruncana helvetica population – fossil zone marker for the Lower Turonian. Besides this taxon other species such as Praeglobotruncana oraviensis and Dicarinella hagni appear, confirm the Lower Turonian age. Keywords: Cretaceous, Turonian, planktonic foraminifera, Prahova Valley. Rezumat: Lucrarea se ocupă în mod deosebit cu studiul foraminiferelor planctonice ale Turonianului inferior, în vederea confirmării prezenţei acestuia în zonă. Studiul asociaţiei de foraminifere planctonice a pus în evidenţă prezenţa populaţiei de Helvetoglobotruncana helvetica – fosil de zonă pentru Turonianul inferior. Asociat acestui taxon mai apar şi specii ca Praeglobotruncana oraviensis şi Dicarinella hagni, ce confirmă vârsta Turonian inferior. Cuvinte cheie: Cretacic, Turonian, foraminifere planctonice, Valea Prahovei. INTRODUCTION From a micropaleontological viewpoint it is well known that Rotaliporids, Hedbergellids and In the studied area of the Prahova Valley (Fig. Globotruncanids are the commonest planktonic 1), from a lithological viewpoint, the lowermost foraminifera families; out of these Rotaliporids and Upper Cretaceous deposits (Cenomanian – Lower Globotruncanids have the most systematical and Turonian), are transgressive upon the Upper biostratigraphical importantance for Cenomanian - Aptian and are represented by grey marls which Maastrichtian while Hedbergellids have the same alternate with green-grayish or blackish importance for Hauterivian – Lower Maastrichtian. sandstones. Interbedded reddish and violaceous At the Cenomanian/Turonian boundary a marls appear in the upper part of Cenomanian – decrease of planktonic foraminifera is noticed, Turonian, confirming the change of anoxic event which even today raises some questions and with low oxidic environment. The Turonian deposits debates. However, the bicarinal species of present a great facies variation, generally Marginotruncana and Dicarinella which appeared characterized by sandstone-marls interbedded with at the end of Cenomanian created some marker sideritic calcareous marls, lens of reddish species for Turonian - Santonian interval, micaceous clay or even thin cineritical layers. The reddish calcareous marls facies, between Fig. 1. Geological map of the Breaza-Comarnic area Dâmboviţa and Ialomiţa Valleys, appearing in (after the map sheet L-35-XXVI, Tîrgovişte, Cenomanian, continues with small variations up to 1:200.000). Triangle marks the position of the Senonian. The farther you go to the East the more sampled sections changes are noticed; thus Cenomanian begins with reddish calcareous marls, which gradually become grey-blackish calcareous marls. On the other hand, Turonian is characterized by grey sandish marls interbedded with red clay. Comparing numerous outcrops of Cenomanian – Turonian boundary we found that a consistent ritmicity characterized all of them along this interval, these being enriched in macrofossils, microfossils, as well as nannofossils. The Cenomanian – Lower Turonian deposits in the Eastern Carpathians (southern part) contain a very rich micropaleontological assemblage of benthonic and planktonic foraminifera. However, due to space shortage and because the Prahova Valley is strongly tectonized, this work approaches only the planktonic foraminifera study from Lower Turonian, thus confirming its presence in the studied area. 1 Bucharest University, Faculty of Geology and Geophysics, Paleontology Laboratory, 1, B-dul Nicolae Bălcescu, sector 1, Bucharest, [email protected] Eugen Tiberiu DABU excepting the monocarinal and planoconvexed descriptions made by Robaszynski, Caron and forms of Helvetoglobotruncana, which indicate the EWGPF (1979), on the following, we will try to presence of the H. helvetica zone from Lower present a morphological and systematical Turonian. description of the most important genera of The most specific aspect of Cenomanian and planktonic foraminifera, as follows: Whiteinella Lower Turonian is represented by genus of PESSAGNO 1967, Praeglobotruncana Rotalipora cushmani, Rotalipora reicheli and BERMUDEZ 1958, Dicarinella PORTHAULT 1970, Helvetoglobotruncana helvetica, where the first Marginotruncana HOFKER 1956 and ones appear under reworked forms at the Helvetoglobotruncana DALBIEZ 1955. lowermost Lower Turonian. PALEONTOLOGICAL FEATURES METHODS Planktonic Foraminifera The samples were collected from three sections The Suprafamily Rotaliporaceae Sigal: of the same outcrop from Prahova Valley, on the known only from Middle Jurassic (Upper Bajocian) Breaza - Comarnic highway pierfeet (Fig. 1). The to Upper Cretaceous (Maastrichtian), planktonic land and laboratory researches were carried out in foraminifera have a trochospiral test while the several stages and the species described latter on, primary aperture is extraumbilical - umbilical, being are from a micaceous sedimentary breccia (with bordered by an apertural lip; some secondary quartz and mica elements) interbedded within sutural apertures on the umbilical side can be green-graysh marls layers (Fig. 2). observed at the complex forms. Tests are The samples were prepared and studied by trochospiral (low to high), some of them having standard techniques (boiling, washing-up, sieving, carinal structures, non-perforated peripheral broiling, drawing and photo taking) at the bands, and rows of pustules on peripherical side. Paleontological Laboratory from Bucharest Chambers are smooth to strongly ornated (with University, under the direct supervision of polygonal structures or meridional rows of professor dr. doc. Theodor Neagu – member of the pustules). Romanian Academy. The Family Hedbergellidae (K1ht - K2ma): By studying the planktonic foraminifera from a includes foraminifera with trochospiral tests, a sample we usually came to the conclusion that primary interiomarginal, extraumbilical-umbilical some forms could be identified as members of aperture with a proeminent apertural lip. The species described in the specialized literature, former apertural lips are still visible, without each of them having distinct morphological forming relict apertures or secondary sutural characteristics. However, according to earlier apertures. The most significant genera of this family are Hedbergella, Whiteinella, Fig. 2. Synthetic lithological column, Breaza- Praeglobotruncana, Helvetoglobotruncana, Comarnic area, with the position of the samples Dicarinella, quite important for Hauterivian - Maastrichtian interval. Whiteinella archaeocretacea Interval Range Zone Definition. This zone has been defined as the Interval Range Zone, ranging from the last occurrence of Rotalipora cushmani Morrow, to the first emergence of Helvetoglobotruncana helvetica Bolli (Caron, 1985; Sliter, 1989). Stratigraphic Position. Upper Cenomanian– lower Turonian. Author. Bolli (1966), = Praeglobotruncana gigantea Zone. Remarks. Besides a species diversification of Dicarinella, this zone includes a low-diversity assemblage represented by rare specimens of Hedbergella and Whiteinella and by a zonal marker scarcity. This low-diversity assemblage may be related to the widespread deposition of organic- rich sediments linked to the Oceanic Anoxic Event (Sliter, 1989; Robaszynsky et al., 1990; Premoli- Silva and Sliter, 1994; Venkatachalapathy and Ragothaman, 1995). In the Boreal realm, other authors have conferred an early Turonian age on this zone 88 Lower Turonian at the Beaza-Comarnic highway bridge pierfeet (Prahova Valley) (Caron, 1985; Venkatachalapathy and the Cenomanian and Turonian are commonly Ragothaman, 1995). established by using ammonites, inoceramid This zone expands over the bivalves, planktonic foraminifera and calcareous Cenomanian/Turonian boundary. The W. nannofossils (Birkelund et al., 1990). archaeocretacea Zone in the present study The planktonic foraminiferal zones in pelagic corresponds to the transition from the shallow- facies have been assigned the following marine facies to the hemipelagic and pelagic ones, chronostratigraphic equivalencies: Whiteinella being characterized by strong changes in the fossil archaeocretacea Upper Cenomanian to Lower associations. Its base is concordant with the Turonian; Dicarinella hagni remaining part of the disappearance of most large benthic foraminifera; Lower to Upper Turonian and in the lower part of this zone, there is a scarcity of Helvetoglobotruncana helvetica Middle Turonian fossils mainly due to the predominance of (Soto-Jaramillo, 1981). intertidal–supratidal facies with common sub-aerial Helvetoglobotruncana Reiss, 1957: exposure features for all the sections from the Helvetoglobotruncana helvetica type Bolli, 1945, aforementioned outcrop. shows a plan-convex test, flattened on the spiral side and highly inflated on the umbilical side. At Helvetoglobotruncana helvetica Total Range the same time, on the peripherical side even a keel Zone (Plate I, fig. 13 – 18) could be present. The umbilicus is covered by a Definition. Total Range Zone of system of portici with accessorial infralaminal Helvetoglobotruncana helvetica GANDOLFI. apertures, the umbilical side being covered with Stratigraphic Position. In this study, the numerous pustules. respective zone belongs to the