Nora G. Cabaleri

Influence of a biohermal belt on the lacustrine sedimentation of the Cañadón Asfalto Formation (Upper Jurassic, Chubut province,Southern Argentina)

The Upper Jurassic Canadon Asfalto Formation (Canadon Asfalto Basin, Patagonia Argentina), consists mainly of carbonate deposits accumulated in hydrologically closed lakes, which were especially sensitive to rainfall changes. The lacustrine carbonate sedimentation also interplayed with volcanic episodes recorded by tuffs and lavas, as observed in different basin sectors. These lakes probably underwent warm, alternating humid-subarid and arid conditions that resulted in spreading and shrinkage cycles of the closed water bodies. In the Cerro Condor area, carbonates were deposited as part of a 500 m long and 39 m thick microbial biohermal body that extended over 5,5 km2, overlying a hard basalt substratum. This bioherm ridge acted as a physiographic barrier that controlled the sedimentation in the surrounding lacustrine zones, whose environments ranged from shallow and deep littoral to eulittoral (including microbial patch reefs) and palustrine. A hydrologically isolated portion of the lacustrine basin evolved into a pan lake where widespread carbonateevaporite sequences developed.

The First Record of the Family Fushunograptidae (‘Conchostraca’, Spinicaudata) from the Cañadón Asfalto Formation (Upper Jurassic), Patagonia, Argentina

Abstract. A description is provided for Wolfestheria smekali gen. and sp. nov., a new member of the family Fushunograptidae, from the “Estancia La Sin Rumbo” locality in the Puesto Almada Member of the Cañadón Asfalto Formation. Assignment to the family Fushunograptidae is supported by evidence from detailed scanning electron microscope studies of this species. An assessment of the complex carapace ornamentation is also provided, as W. smekali exhibits an array of ornamentations that cover the entire carapace surface. These consist of irregular, thick radial lirae with numerous thin cross-bars on the dorsal middle-upper third of the carapace, changing to straight, thick radial lirae with fewer cross-bars restricted to the upper half of the growth band in the ventral third of the carapace, with the interspaces between the radial lirae wider than the lirae themselves, and with the radial lirae terminating in the upper part of the growth bands, where they then enlarge to form a triangular shape upon contact with the following growth line. These characteristics allow us to compare W. smekali with the eosestheriid genera Yanjiestheria Chen and Abrestheria Wang, as well as with the other fushunograptids from the Upper Jurassic to Lower Cretaceous of China such as Cratostracus Huang and Qinghaiestheria Wang. W. smekali is also comparable to fushunograptids and related forms from the Jurassic system of Argentina. The occurrence of W. smekali correlates biostratigraphically with the “Eosestheriopsis dianzhongensis fauna” of China, as well as with other Jurassic faunas from Africa and Europe. This correlation supports a Late Jurassic age for these fossiliferous assemblages (Puesto Almada Member, Cañadón Asfalto Formation) from the province of Chubut, Argentina.

Facies de patch reefs en la Formación Picún Leufú (límite Jurásico/Cretácico) en la región de Zapala, Cuenca Neuquina

Patch reefs facies in the Picun Leufu Formation (Jurassic/Cretaceous boundary) in the Zapala region, Neuquen Basin, Argentina. In the locality of Cerrito Caracoles, situated in the nearby of Zapala city, in the western central region of the Neuquen province, Argentina, the basal part of the Picun Leufu Formation (Jurassic/Cretaceous boundary) crops out. The facies and microfacies association analysis points out to an inner shelf margin environment, with development of well preserved patch reefs. The description of these bodies (coral and algae bafflestone, bryozoan bafflestone and algal boundstone), and of a low relief mound reef (skeletal algal packstone) are presented in this paper. The associated facies are represented by low energy deposits (mudstone), bar complexes (sandy skeletal packstone and skeletal packstone) and channels (skeletal intraclast floatstone and intraclast skeletal ooidal floatstone).

Chemical taphonomy and preservation modes of Jurassic spinicaudatans from Patagonia: a chemometric approach

Abstract. Spinicaudatans (‘clam shrimps’) are small branchiopod crustaceans enclosed in a chitinous bivalved carapace that is often the only preserved element in the fossil record. However, few studies have analyzed the preservation of these carapaces, which have been found in continental facies from the Devonian to the present. The aim of this study was to contribute to a better understanding of the chemical preservation of fossil spinicaudatan carapaces, and it focused on spinicaudatan carapaces of the Cañadón Asfalto Formation from the Jurassic of Argentina. Semiquantitative energy-dispersive X-ray spectrometry (EDS) analysis provided elemental composition data that were interpreted using principal component analysis (PCA). The results showed a complex chemical mode of preservation for spinicaudatan carapaces. In some parts, EDS spectra of the specimens exhibit peaks of calcium, phosphorous, aluminum, and fluorine, representing the retention of original carapace material with some diagenetic recrystallization. Certain zones of the carapace show low-intensity peaks of the elements mentioned, while silicon and oxygen peaks (from the rock matrix) become the dominant spectral signals. These modes of preservation modify the interpretations and observations of the ornamentation of the carapace, which are used as taxonomic features. Our results suggest that specific diagenetic processes play a fundamental role in the preservation of spinicaudatans.