3D-imaging of foraminifera by X-ray microtomography

Abstrac

Rapid vertical motions and formation of volcanic arc gaps: plateau collision recorded in the forearc geological evolution (Costa Rica margin)

The collision of bathymetric features with modern convergent margins has been investigated with the full range of tools used in geosciences. Hence, a comprehensive picture exists of the characteristic effects of collision events on the evolution of convergent margins. In contrast, much less studies documented past collisions of bathymetric features with convergent margins, as colliding features were generally lost to subduction. The arc-trench system of southern Central America provides modern and past textbook examples of active margin interaction with incoming bathymetric reliefs. Here we propose a synthesis which combines basin and terrane analysis of the forearc of northern Costa Rica and takes up the challenge of documenting past episodes of plateau accretion to the active margin. As illustrated in modern examples, our study shows that kilometric uplift of the overriding plate and termination of the volcanic arc activity are the most profound effects of colliding/accreting oceanic plateaus. Kilometric uplift of the forearc is documented by short-lived (~ 3 m.y.) occurrences of shallow-water deposits in an overall deep-water forearc record. These shallow deposits contain material reworked from underlying sedimentary and basement lithologies. The development of spatial gaps in arc volcanism is deduced from the transition from arc-derived turbidites to pelagic sediments. Eventually, end of the collision event is evidenced by the subsidence of the whole forearc to deep-water environments. Basin subsidence is accompanied or followed by renewed volcanic arc activity and coeval arc-derived sedimentation, which may occur 1–7 m.y. after plateau collision. These past episodes of plateau accretion are archetypal for the following reasons: a) they may be studied in outcrop, whereas most of the modern collisions of plateaus largely occur underwater; b) no tectonic or metamorphic imprint has significantly complicated the forearc geological record; c) the colliding feature and the sedimentary response to its collision are both preserved in the forearc geology; d) they may be used as analogues for any setting where a bathymetric feature is suspected to have caused rapid forearc uplift and cessation of the volcanic arc activity

Late Oligocene Larger Foraminifera From Nosara (Nicoya Península, Costa Rica) and Indward (Carriacou, Lesser Antilles), Calibrated by 87sr/ 86sr Isotope Stratigraphy

The Late Oligocene first occurrence of Miogypsina gunteri Cole and Miogypsina tani Drooger is verified in the light of larger foraminiferal assemblages from Nosara (Nicoya Peninsula, Costa Rica) and Windward (Carriacou, Grenada, Lesser Antilles). At Windward, they co-occur with planktonic foraminifera and nannofossils studied by earlier workers. 87Sr/86Sr isotope ratios were measured in calcite of larger foraminifera at both sites to determine independent absolute ages of the two outcrops. Late Oligocene shallow water formations unconformably overlie Paleocene-Eocene distal turbidites and siliceous shales in the outcrops in Costa Rica. The shallow-water lithostratigraphy includes near shore volcanic sandstones that alternate with sandy bioclastic limestones. The latter have yielded rich assemblages of larger foraminifera that have been studied in oriented sections, SEM for split material and polished rock thin sections for transmitted light and cathodoluminescence (CL) observation. The association of larger foraminifera includes Heterostegina antillea Cushman, Miogypsina tani Drooger, Miogypsina gunteri Cole, Miogypsina c.f (Miolepidocyclina) panamensis (Cushman), Miogypsina sp., Lepidocyclina (nephrolepidina) vaughani Cushman, Lepidocyclina yurnagurensis Cushman, Lepidocyclina undosa Cushman. At Windward, scattered outcrops of the Belvedere Formation expose mass flow deposits rich in larger foraminifera associated with an early late Oligocene (NP24) planktonic foraminifa assemblage. Planktonic foraminifera and nannofossils recovered in rocks slightly downsection indicate an upper Middle Oligocene age. Larger foraminifera species collected just north of Windward include Lepidocyclina undosa Cushman and Miogypsina gunteri Cole. The 87Sr/86Sr ratio was measured in 11 specimens of megalosphaeric Lepidocyclina spp., mechanically extracted from 2 rock samples collected at Punta Peladas (Costa Rica) Ratios range from 0.709088 to 0.708196, which correspond to a model age of 27.4 to 24.94 Ma. Very similar ratios, ranging from 0.708150 to 0.708167 were obtained from Windward Village (Carriacou). All measured 87Sr/86Sr ratios fall within a Chattian (Late Oligocene) age. Nannofossils, planktonic foraminifera and identical 87Sr/86Sr ratios from Punta Peladas and Windward clearly indicate a Chattian (Upper Oligocene) first occurrence of Miogypsina gunteri and Miogypsina tani and confirm this age range for the other larger foraminifera cited above. The first occurrence of Miogypsina gunteri in the lower Aquitanian and Miogypsina tani in the upper Aquitanian as proposed by European workers cannot be confirmed.

Priabonian (upper Eocene) larger foraminifera from the Helvetic Nappes of the Alps (Western Switzerland): new markers for Shallow Benthic zones 19-20

Here, we revise and update the biostratigraphy of larger foraminiferal assemblages in three sections of the Priabonian Sanetsch Formation in the Helvetic Nappes of the Western Swiss Alps: The Sex Rouge (SE) and the Sanetsch Buvette (SA) sections in the Wildhorn Nappe Complex, and the Col des Essets (ETS) section in the most external Morcles Nappe. In the SE and SA sections, the Tsanfleuron and most of the Pierredar Limestone members of the formation are assigned to SBZ 19 (early Priabonian), while the upper- most part of the formation is assigned to SBZ 20 (late Priabonian). In the external ETS section the entire Sanetsch Formation contains as- semblages characteristic of SBZ 19, suggesting an earlier, middle-late Priabonian onset of the hemipelagic Stad Formation (“Globigerina Marls”). Since it was established in 1998, the Shallow Benthic Zones (SBZ), a biozonation based on larger foraminifera, has been a useful tool in the biostratigraphy of the Paleogene. Biozonation proposals for the late middle-late Eocene are based mainly in biometrical subdivi- sion of lineages of nummulitids and orthophragmines, which requires measurements in oriented sections of isolated specimens. Here, we define previously unreported taxa from the Sanetsch Formation, which are considered characteristic for the Priabonian. They are easy to identify in random sections and thus useful biostratigraphical markers. We also describe a new orthophragminid genus, Virgasterocylina n. gen. (Orbitoclypeidae) characterized by the presence of rods, radial thickenings of calcite along ribs; a new species of Rotorbinella, R. epardi n. sp., and a new genus and new species of difficult suprageneric attribution, Sanetschella indeprensa n. gen., n. sp. We add the new taxa to the larger foraminiferal association characterizing the Priabonian (SBZ 19–20). The revision of the literature, together with our own sample collections revealed that these new taxa occur in Priabonian rocks from different basins in the western Tethys. Virgasterocylina n. gen. also occurs in the Caribbean bioprovince in the middle and upper Eocene. In the western Tethys, Virgasterocylina ferrandezi is subdivided into two subspecies, V. f. ferrandezi (Özcan and Less) and V. f. lessi n. ssp., which characterize the SBZ 19 and 20 biozones respectively

Foraminifera

Capítulo de libro-- Universidad de Costa Rica. Centro de Investigaciones en Ciencias del Mar y Limnología, 2009. Este documento es privado debido a limitaciones de derechos de autorEighty-four species of benthic and one species of planktonic Foraminifera, classified under 40 genera and 34 families reported for Costa Rica are listed in this paper. These lists are based on literature data and ongoing studies. All (except for four species from the Caribbean) are reports from the Pacific Ocean, and most are from offshore or have no specific indication of where in Costa Rica the Foraminifera were collected. Of the other Central American countries there is little information except from Panama. More research is needed on Foraminifera, since they may be a predominant group in some areas and ecosystems, for example the meiofauna of Caño Island, and much more research is need on planktonic Foraminifera.Vicerrectoría de Investigación, Universidad de Costa Rica (Projects 808- 92-237; 808-96-601). CONICIT, Costa Rica, proyecto 90-326-BIDUCR::Vicerrectoría de Investigación::Unidades de Investigación::Ciencias Básicas::Centro de Investigación en Ciencias del Mar y Limnología (CIMAR