FOSSILIFEROUS CONCENTRATIONS ? UNIQUE BIO-EVENTS IN A SECOND ORDER LOWSTAND WEDGE OF THE LOWER CRETACEOUS (NEUQUÉN BASIN, ARGENTINA)

TOSCANO, AGUSTINA G.; ANDRADA, MARIEL A.; LUCI, LETICIA; CATALDO, CECILIA S.

Viena

Congreso; Annual Meeting of the Paläontologische Gesellschaft; 2021

Paläontologische Gesellschaft

The Neuquén Basin (west-central Argentina) encompasses a nearly continuous UpperTriassic?lower Paleogene sedimentary record including marine and continentalsiliciclastics, carbonates and evaporites. Here, an early Valanginian marine successionrepresenting a second order lowstand wedge (Mulichinco Formation) is studied atCerro La Parva locality, in which several exceptional study cases are registered. Themost remarkable of these can be found within the third order TST; it consists of 13-mthick, laterally extensive oyster mass occurrences (OMOs) of the genus Ceratostreon.They are up to 2,5 km wide and represent the maximum expression of OMOs recordedbasinwide in equivalent stratigraphic levels, in localities up to 50 km apart from eachother. These were interpreted as composite biogenic concentrations of intercalatedbioherms and autobiostromes. The extraordinary proliferation of oysters represents apopulation burst bio-event, which according to geochemical analyses, was triggeredby lowered salinity and high primary productivity. Below the OMOs, 20 m above thebase of the third order LST, a lenticular monospecific nerineoid assemblage isrecorded, representing a mixed-origin concentration (within-habitat wave-reworking oflocally abundant shells). Up-section, above the OMOs in the third order HST severalindividuals of the glypheidean lobster species Atherfieldastacus rapax are preservedin incomplete reworked calcareous nodules in shales and shell beds. The abundanceof lobsters in these two beds was interpreted as event-concentrations caused by stormreworking. Further up-section, soft-bottom dwelling serpulids are found encrusted bysmaller serpulids and Ceratostreon. Oysters eventually covered the serpulids, formingmasses that coalesced laterally to constitute at least three simple biogenicconcentrations up to 30 cm-thick. Each of these fossil concentrations represent uniquebio-events, most of which are not recorded elsewhere in the basin, reflecting particularconditions that are currently under research. Their integral study will allow us tounderstand the development of benthic communities along the second order LST.