Shallow-burial diagenetic realm of Upper Jurassic carbonate ramp: La Manga Formation, Neuqu¨¦n Basin, Argentina

PALMA, R.M.; KIETZMANN, D.A.

Hammamet

Simposio; IGCP 506 Fifth Symposiums; 2008

Faculty of Sciences of Bizerte (FSB), Faculty of Sciences of Tunis (FST)

The outcrops of La Manga Formation located close to Bardas Blancas area in the southof Mendoza Province allow a reconstruction of the shallow sedimentary environment ofan Oxfordian carbonate ramp, developed in the Neuqu¨¦n basin in west-central ofArgentina. La Manga Formation is composed of greyish wackestone/packstone (outerramp), greyish grainstone/packstone (middle ramp), light grey grainstone andsubordinate packstones (shoal), grey and white coralline limestones (inner rampmargin), bioclastic, and peloidal wackestones-packstones from inner ramp. Thesedimentary record shows a hierarchical sequence stratigraphic organization, whichimplies sea-level changes of different frequencies (see Palma et al., this symposium).Several aspects of diagenesis were investigated including micritization, cement types,cathodoluminescence and stable isotopes. The earliest diagenetic process is representedby micritized peripheral areas of bioclasts and an early isopachous to bladed calcite.This cement where found lines skeletal and non skeletal particles. Their fibrous habitand non-luminescent features are interpreted to be of early phreatic marine originsuggesting oxidizing conditions. Other varieties of cements are granular calcite andsyntaxial cement. The granular calcite cement that either completely fills the dissolvedaragonitic shells of molluscs and interparticles pores suggesting a freshwater phreaticorigin... This cement display an early non-zoned dull luminescence, followed by zoned,bright luminescence and late dull luminescence of the cavities fillings. Each zonerepresents the precipitation of calcite from pore waters with different chemicalcompositions, from reducing to slightly reducing, and reducing conditions during ashallow burial meteroric-phreatic diagenetic environment. The transition to the shallowburialdiagenetic realm is indicated by a typical non-bright-dull cathodoluminescencesequence. Syntaxial calcite cement on echinoid fragments shows a similar luminescencepatterns. Granular calcite cement has a ¦Ä18 O value that ranges between -2.6 %0 to -6.8%0 PDB and ¦Ä13 C values from -2.50%0 to -4.12%0 PDB. These values are consistentwith meteoric phreatic diagenesis. Physical compaction processes include deformationand breakage of particles, while chemical compaction includes both pression-solutionand microstylolites development. Results strongly indicate that the diageneticenvironment changed from marine phreatic through meteoric-phreatic, interrupted by aperiod of emergence.