Alexandre N. Bandini

Catalogue of Mesozoic Radiolarian Genera. Part 2: Jurassic-Cretaceous

ABSTRACT The catalogue of Mesozoic radiolarian genera is a revision of all described genera with re-illustration of their type species. This project was organized under the auspices of the International Association of Radiolarian Paleontologists (Inter-Rad), and was carried out by the Mesozoic Working Group. This is the second of two contributions, this one devoted to the Jurassic-Cretaceous period. It contains 581 genera with re-illustration of their type species. This part shares 30 genera in common with the Triassic catalogue, most of which arose in the Carnian, Norian and Rhaetian. The sharp difference manifested between the Triassic fauna and the Jurassic-Cretaceous fauna is so evident that it justifies two independent catalogues. A comparable division between the Jurassic and Cretaceous could not be justified however, because of the similarity of the fauna, and by the greater number of genera crossing the Jurassic-Cretaceous boundary which is three times that for the Triassic-Jurassic boundary. A distinct characteristic of Jurassic-Cretaceous genera is the high number of nomina dubia (up to 131), contrary to the low number in the Triassic interval. This reflects, in part, the influence of Haeckelian taxonomy in earlier research on Jurassic-Cretaceous faunas prior to the application of SEM techniques.The Mesozoic Working Group has carefully reviewed and re-examined the taxonomy of all available genera, their family assignment and stratigraphic ranges. Following careful comparisons, 91 genera were declared as synonyms. The review has noted 26 homonyms which were duly notified to their corresponding authors, and were corrected previous to the publication of this catalogue. In spite of this effort, unfortunately nine homonyms still remain. Two invalid nominal genera and two nomina nuda are also reported. The systematic revisions have validated 341 genera for the Jurassic-Cretaceous interval. At the end of this catalogue 24 additional photographs are presented as support for those genera having a poor original illustration of the type species.

Late Cretaceous to Miocene seamount accretion and mélange formation in the Osa and Burica Peninsulas (Southern Costa Rica): episodic growth of a convergent margin

Abstract Multidisciplinary study of the Osa and Burica peninsulas, Costa Rica, recognizes the Osa Igneous Complex and the Osa Mélange – records of a complex Late Cretaceous–Miocene tectonic–sedimentary history. The Igneous Complex, an accretionary prism (sensu stricto) comprises mainly basaltic lava flows, with minor sills, gabbroic intrusives, pelagic limestones and radiolarites. Sediments or igneous rocks derived from the upper plate are absent. Four units delimited on the base of stratigraphy and geochemistry lie in contact along reactivated palaeo-décollement zones. They comprise fragments of a Coniacian–Santonian oceanic plateau (Inner Osa Igneous Complex) and Coniacian–Santonian to Middle Eocene seamounts (Outer Osa Igneous Complex). The units are unrelated to other igneous complexes of Costa Rica and Panama and are exotic with respect to the partly overthickened Caribbean Plate; they formed by multiple accretions between the Late Cretaceous and Middle Eocene, prior to the genesis of the mélange. Events of high-rate accretion alternated with periods of low-rate accretion and tectonic erosion. The NW Osa Mélange in contact with the Osa Igneous Complex has a block-in-matrix texture at various scales, produced by sedimentary processes and later tectonically enhanced. Lithologies are mainly debris flows and hemipelagic deposits. Clastic components (grains to large boulders) indicate Late Eocene mass wasting of the Igneous Complex, forearc deposits and a volcanic arc. Gravitational accumulation of a thick pile of trench sediments culminated with shallow-level accretion. Mass-wasting along the margin was probably triggered by seamount subduction and/or plate reorganization at larger scale. The study provides new geological constraints for seamount subduction and associated accretionary processes, as well as on the erosive/accretionary nature of convergent margins devoid of accreted sediments.

Catalogue of Mesozoic radiolarian genera. Part 1: Triassic

ABSTRACT The Catalogue of Mesozoic radiolarian genera is a revision of all described genera with re-illustration of their type species. This project was organized under the auspices of the International Association of Radiolarian Paleontologists (Inter-Rad), and was carried out by the Mesozoic Working Group. This contribution (Part 1), deals with the Triassic period only. There are 381 known Triassic radiolarian genera. Most have been published since the 1970s following the introduction of scanning electron microscopy (SEM), which enabled good pictures for most type species. For this reason the Triassic period is characterized by a very low number of nomina dubia (only 14), contrary to the higher number in the Jurassic-Cretaceous interval. The Mesozoic Working Group has carefully reviewed and reexamined the taxonomy of all available genera, their family assignment and stratigraphic ranges. Following careful comparisons, 73 genera were declared synonyms. The review has also detected 11 cases of homonymy that were duly notified to their authors, and were corrected previous to the publication of this catalogue; unfortunately one homonym still remains. Two invalid nominal genera are also reported herein. The systematic revisions have validated 282 genera for the Triassic, and of these only 30 genera cross the Triassic-Jurassic boundary. At the end of the catalogue 15 additional photos are presented as support for those genera having a poor original illustration of the type species.

Turonian Radiolarians from Karnezeika, Argolis Peninsula, Peloponnesus (Greece)

Near Karnezeika a roughly 140 m thick Upper Cretaceous section consists of interbedded pelagic limestones, cherts and coarse polymict breccias including ophiolites and shallow water limestones. At the base, pink pelagic limestones rest on deeply altered and fractured Lower Jurassic Pantokrator Limestone. This first pelagic facies is dated as middle Turonian, based on planktonic Foraminifera. Over 100 m of coarse ophiolite-carbonate breccias, interpreted as a channel or canyon fill in a pelagic environment, document the erosion of the Late Jurassic nappe edifice along the Cretaceous Pelagonian margin. Above these breccias, we mesured 16 m of principally pink and red pelagic limestones and radiolarian cherts, in which we recovered well-preserved radiolarians discussed here. In this interval, the presence of planktonic Foraminfera allows to state a late Turonian to Coniacian age. More than 40 radiolarian species are described and figured in this work. The radiolarian chronostratigraphy established by 10 different authors in 11 publications was compared for this study and used to establish radiolarian ranges. This exercise shows major discrepancies between authors for the radiolarian ranges of the studied assemblage. Nevertheless, a Turonian age can be stated based on a synthesis of cited radiolarian ranges. This age is consistent with the age based on planktonic foraminifera. In combining the ages of both Radiolaria and planktonic Foraminifera, the studied samples can be restricted to the late Turonian. However, the discrepancies of published radiolarian ranges call for an urgent, major revision of the Late Cretaceous radiolarian biochronology. The integration of planktonic foraminifera with radiolarians may greatly enhance biochronologic resolution in sections where both groups occur.

Early Jurassic to early Late Cretaceous radiolarians from the Santa Rosa Accretionary Complex (Northwestern Costa Rica)

In the circum-Pacific ophiolitic belts; when no other biogenic constituents are found; radiolarians have the potential to provide significant biostratigraphic information. The Santa Rosa Accretionary Complex; which crops out in several half-windows (Carrizal; Sitio Santa Rosa; Bahia Nancite; Playa Naranjo) along the south shores of the Santa Elena Peninsula in northwestern Costa Rica; is one of these little-known ophiolitic melanges. It contains various oceanic assemblages of alkaline basalt; radiolarite and polymictic breccias. The radiolarian biochronology presented in this work is mainly based by correlation on the biozonations of Carter et al. (2010); Baumgartner et al. (1995b); and O’Dogherty (1994) and indicate an Early Jurassic to early Late Cretaceous (early Pliensbachian to earliest Turonian) age for the sediments associated with oceanic basalts or recovered from blocks in breccias or megabreccias. The 19 illustrated assemblages from the Carrizal tectonic window and Sitio Santa Rosa contain in total 162 species belonging to 65 genera. The nomenclature of tectonic units is the one presented by (Baumgartner and Denyer; 2006). This study brings to light the Early Jurassic age of a succession of radiolarite; which was previously thought to be of Cretaceous age; intruded by alkaline basalts sills (Unit 3). The presence of Early Jurassic large reworked blocks in a polymictic megabreccia; firstly reported by De Wever et al. (1985) is confirmed (Unit 4). Therefore; the alkaline basalt associated with the radiolarites of these two units (and maybe also Units 5 and 8) could be of Jurassic age. In the Carrizal tectonic window; Middle to early Late Jurassic radiolarian chert blocks associated with massive tholeitic basalts and Early Cretaceous brick-red ribbon cherts overlying pillow basalts are interpreted as fragments of a Middle Jurassic oceanic basement accreted to an Early Cretaceous oceanic Plate; in an intra-oceanic subduction context. Whereas; the knobby radiolarites and black shales of Playa Carrizal are indicative of a shallower middle Cretaceous paleoenvironment. Other remnants of this oceanic basin are found in Units 2; 6; and 7; which documented the rapid approach of the depocentre to a subduction trench during the late Early Cretaceous (Albian-Cenomanian); to possibly early Late Cretaceous (Turonian).