Alan Moro

Stratigraphy and paleoenvironments of rudist biostromes in the Upper Cretaceous (Turonian-upper Santonian) limestones of southern Istria, Croatia.

Abstract Rudists are well preserved in the Upper Cretaceous limestones of southern Istria, where they flourished within different shallow-water environments during the Turonian to upper Santonian. In order to determine paleoenvironments, a vertical succession of the rich rudist assemblages has been investigated and described within sequence stratigraphy framework. The Turonian to upper Santonian sediments of southern Istria, in the sense of sequence stratigraphy, comprise one complete third-order sequence and the beginning of the transgressive systems track (TST) of the next sequence. Lower Turonian pelagic limestones were deposited on a foundered carbonate platform, and represent the beginning of the TST. Limestones of the late TST-lowstand systems track (LST) were deposited on the inner shelf, and the early TST of the next sequence was deposited on the outer shelf. All the rudists which appeared after the early Turonian disappearance and partial extinction were elevators. The genera Durania and Praeradiolites thrived in bouquets, which may coalesce to form thickets in inner shelf environments of the late TST-LST. Their growth depended completely ( Durania ) or partly ( Praeradiolites ) on net sediment accumulation. The environment in which Durania and Praeradiolites thrived was subtidal and deep enough to be out of reach of emersion. Within the same depositional environment, these genera were later replaced vertically by Radiolites . Individuals with slim, cylindrical shells, and wide growth lamellae, have full contact within the thickets, in which their growth depended little, if at all, on net sedimentation rate. Also, Radiolites occupied the shallower parts of the subtidal zone with a commissure rim rising above the sediment surface. Renewed deepening in the late Santonian resulted in the outer shelf environment of the next TST. Gorjanovicia , together with rare Vaccinites , flourished after the Radiolites . Gorjanovicia differed from Radiolites only in possessing more pronounced vertical ribs. In their slightly deeper subtidal environment with pelagic influxes, Gorjanovicia thrived in bouquets. Loosely packed Gorjanovicia gave Vaccinites an opportunity to grow among them. The environmental setting of the southern Istria rudist biostromes and their correlation with other shallow-water deposits in the Mediterranean region suggests that a sequence stratigraphy approach can be applied to correlate rudist biostratigraphy on a regional, or even inter-regional, scale.

Upper Cretaceous Peritidal Deposits of Olib and Ist Islands (Adriatic Sea, Croatia)

Upper Cretaceous carbonate deposits of Olib and Ist islands are characteristic of peritidal sediments. They consist of shallow subtidal deposits alternating with intertidal laminites (shallowing-upward cycles). Subtidal beds with micritic matrix predominate over peritidal sediments. Determination of the micro- and macrofauna revealed two distinctive assemblages: one of Middle to Upper Cenomanian and the other of Upper Turonian to Lower Santonian age. The relatively high proportion of subtidal over intertidal sediments (subtidal/intertidal ratio 2.72) indicates that the Middle to Upper Cenomanian beds were deposited during a fall in the third order relative sea-level curve (late HST to LST). Sediments of the next carbonate sequence (Upper Turonian to Lower Santonian) with a higher proportion of the subtidal over intertidal sediments (subtidal/intertidal ratio 4.57) indicate deposition during relative sea-level rise (TST) and highstand (HST). Senonian limestones are overlain by sediments of Lower Lutetian age.

Decastronema barattoloi (DE CASTRO), characteristic fossil of the Palaeocene and the Eocene peritidal sediments from the Adriatic carbonate platform

The renewed deposition in the Palaeogene and the oldest part of the non-interrupted Palaeocene succession is characterized by the presence of Decastronema barattoloi (DE CASTRO), cyanobacterial tubes originally described in the Apennines. The reinterpretation of limestones from several sections located in the Kras region (NW part of the AdCP: Basovizza, Dolenja vas, Sopada, ?ebulovica) confi rmed the presence of these microfossils in peritidal sediments of Danian age (SBZ 1). The Cuisian (Late Lower Eocene) sediments, from eastern Istria (Brnjci section), Cres Island (Koroma?na Cove section) and Ravni kotari (Benkovac–Korlat section) contain individuals of this species in great numbers. The cyanobacterial tubes are scattered in laminated, fi ne grained mudstones and wackestones, immediately above the Cretaceous sediments (occasionally above bauxites or breccias). The Decastronema-bearing sediments pass upward into the Foraminiferal limestones of Cuisian age (SBZ 11, based on conical agglutinated foraminifera and alveolinids), allowing the age determination of the cyanobacterial remnants. The Palaeocene specimens are minute (up to about 180 ?m long), thick walled tubes that occur with the index fossil Bangiana hanseni DROBNE. The Eocene forms accompanied by ostracods, pelecypods, and miliolid and rotaliid foraminifera are segmented tubes, up to 400 mm long and usually thin walled.

Upper Turonian–Santonian slope limestones of the Islands of Premuda, Ist and Silba (Adriatic Coast, Croatia)

Upper Turonian-Santonian limestones at three island locations (Ist, Silba and Premuda) in the southwestern part of the Adriatic carbonate platform, record slope deposition based on their sedimentological and palaeontological characteristics.These Upper Cretaceous successions consist of three vertically superimposed lithotypes: (1) pelagic mudstonespackstones, (2) laminated pelagic wackestones-packstones, and (3) bioclastic floatstones-rudstones to packstonesgrainstones with fossils of shallow marine organisms.According to the proximity of the shallow water carbonate platform interior the depositional setting of the slope deposits could be identified as relatively more proximal or distal. The proximal part is characterized by non laminated pelagic limestones with resedimented bioclastic limestones, while the more distal parts have both laminated and nonlaminated pelagic limestones with rare resedimented bioclastic limestones. The resedimented bioclastic limestones represent slope apron deposits. Locally, at Premuda Island, the slope apron includes blocks of laminated pelagic limestones. The depositional environments of the Ist and Premuda profiles could be interpreted as of more distal origin, while those of the Silba profile represents a more proximal part of the slope.

A NEW DINOSAUR TRACKSITE IN THE CENOMANIAN OF ISTRIA, CROATIA

The new locality with dinosaur footprints has been discovered in the port of Karigador village in Istria. The site is situated in peritidal limestones of late Cenomanian age. The track-bearing horizon is bioclastic wackestone-packstone deposited in a subtidal environment. A single trackway which consist of 28 pairs of footprints and a group of 4 track pairs was registered at the site. Trackways belong to quadrupedal dinosaurs and are assigned to sauropods. Oval-circular shaped footprints represent the pedal prints and horseshoe-semicircular shaped ones represent the manus prints. The trackway is of narrow-gauge type with outwardly rotated manus and pedal prints. The average length of the pedal prints is 33 cm what indicates the length of the individual of app. 10 m. The preferred gait of the individual was a normal walk, taking normal strides with a speed of around 3 km/h. The trackmaker was a non-titanosaurian sauropod. Together with the other late Cenomanian sauropod localities in Istria (Fenoliga and Ladin Gaj), the Karigador site represents an example of sauropod tracks in a carbonate platform environment which can be assigned to Brontopodus ichnofacies.

THE RUDISTS OF SOUTHERN ISTRIA - AN EXAMPLE OF ENVIRONMENTALLY INDUCED SUCCESSION WITHIN SANTONIAN LIMESTONES

Rudist bivalves thrived commonly in the shallow carbonate platforms of the Cretaceous Tethyan realm. The presence of two vertically separated Rudist assemblages along two well preserved Upper Cretaceous sections of cape Mrlera (southern part of the Istrian peninsula) is interpreted as environmentally induced faunal replacement. The first succession begins with pelagic limestones, followed by a mixed radiolitid-hippuritid assemblage, and by monogeneric floatstones-biostromes of radioliotids. The lower pelagic unit represents relatively open marine conditions while the uppermost radiolitid unit originated under more restricted marine conditions.Radiolitid floatstones with rare Gorjanovicia bouquets represent the beginning of the second succession. A mixed radiolitid-hippuritid assemblage, characterised by radiolitid floatstones where rare large Vaccinites individuals occur, replaces the radiolitid assemblages in vertical succession. In both sections, sea level changes were the most important factor causing the replacement of rudist assemblages. Slight differences between the two successions could be attributed to differences in submarine topography. Radiolitids and hippuritids might have occupied different biotopes within subtidal environments of the Adriatic carbonate platform. Hippuritids preferred more stable and deeper subtidal environments. By contrast, radiolitids preferred the shallowest parts of subtidal areas. Therefore, these two major rudist groups may be used for determination of different paleoecological conditions.

New paleomagnetic results from imbricated Adria: Ist Island and related areas

For the purpose of this work samples for palaeomagnetic analysis were taken from Upper Cenomanian and Lower Senonian shallow water limestones, as well as from Senonian pelagic limestones from both Ist and the surrounding islands. This area belongs to Imbricated Adria, which is characterized by gently folded and faulted strata with a Dinaridic (NW–SE) trend. An exception is Premuda island where the beds are strongly folded and are subvertical. A total of 96 samples were drilled from 10 localities distributed between eight islands. The samples were then subjected to standard palaeomagnetic laboratory analysis and statistical evaluation. Eventually, six localities yielded statistically well-defined palaeomagnetic directions, which were shown pre date the folding in age. The overall mean palaeomagnetic direction obtained for the study area, characterizing the Cenomanian–Early Senonian time period had a Declination of 334°, Inclination=+46°, with statistical parameters k=188, a 95 =4.9°, defining a palaeomagnetic pole at l (N)=63°, f (E)=254°, d p=4.0°, d m=6.2°. This was compared with palaeomagnetic directions obtained for rocks of similar ages from stable Istria and the Kvarner islands. As the three palaeomagnetic directions are statistically identical, we conclude that there was no significant relative movement between the three areas after the Early Senonian. The palaeomagnetic declination for the study area, which characterizes the post-Early Senonian rotation of the Adriatic microplate, is the same as the declination for the Pannonian–Pontian age group from the South Pannonian basin. As the palaeomagnetic signals in both cases are primary, the results of the present paper not only support the conclusion that the rotating Adriatic microplate triggered rotations in the South Pannonian basin, but also suggest that the Adriatic platform did not change its orientation between the late Cretaceous and the Early Pontian.

Mineral and Chemical Composition of Rudist Valves from Upper Cretaceous Limestones of Southern Istria, Croatia

Turonian to late Santonian limestones of southern Istria are rich in rudist remains. The main aim of this study was to determine the mineral and chemical composition of 22 samples of rudist valves belonging to the genera Durania, Praeradiolites, Radiolites, Gorjanovicia and Vaccinites, and to explain any observed differences. X-ray diffraction and chemical analyses showed that in all the analysed samples, the primary constituents of rudist shells, aragonite and low-Mg calcite, were transformed into diagenetic low-Mg calcite. The strontium concentrations of all the analysed shells correspond to pelagic bulk rock samples altered by diagenesis and are significantly lower than those for biological calcite. Observed differences in the chemical composition of diagenetically altered rudist shells belonging to different rudist genera, namely a slightly higher content of Sr in specimens of Gorjanovicia and Durania and lower concentrations of Mg in shells of Vaccinites are most probably the result of different shell structures and original mineral constituents.

A Proposal for Taxonomic Re-Evaluation of Vaccinites Species from the Santonian Limestones of Southern Istria (Croatia)

Rudists, especially members of the family Hippuritidae, were common dwellers on the Adriatic Carbonate Platform during the Late Cretaceous. Morphological elements (ligamental ridge, teeth, pillars, length of the contour around the inner margin of the outer shell layer and the ratio between this length and the distance between the sutures of the pillars) from the transverse shell sections permit different species to be defined within the family. Transverse sections of Vaccinites specimens from the Santonian limestones in southern Istria show different values of the angle between the teeth and the ligament ridge, between the ligamental ridge and the E pillar (P2), and of the ratio between the length of the contour and the pillar suture distance. Cluster analyses (Ward’s and Unweighted pair–group average methods) using measured elements, allowed the definition of three species groups from seventeen species at the beginning of study.