Diagenetic outlines in the Danian transgresion carbonates, North East Patagonia (Rio Negro, Argentina).

MATHEOS, S.; RAIGEMBORN MARIA SOL; BORYA, AILEN

Alghero, Italia

Otro; 27th Meeting of Sedimentology; 2009

IAS

The diagenesis of Danian carbonates from the north east Patagonia region (east of Rio Negro Province) was studied formed in cool-water, shelf environments. Dolomitization, dedolomitization and cementation are the most importance processes that included. The thickness of the carbonate sequence (Roca Formation) is about 5 to 9 meters and lies discordantly above the Jurassic vulcanite of Chon Aike Formation and it is covered by Tertiary tuffs deposits or quaternary deposits. The sedimentation is characterized by a relatively well developed shoreface which parts are mainly composed of cross-bedded massive to granular fosiliferous limestones and in less content fine bioclastic sand rich in glauconite. The stratofabric results of tabular type, with wedging beds in the upper part. The dominant texture is coarse-grained bioclastic packstone  and granistone with fossils (bivalves, echinoderms, bryozoans, ostracods and foraminifers). In general, the fossils are cemented by radiaxial calcite; as an exception, the bivalves are totally to partially silicificated. The abundance of broken fossils would indicate a highly energy environment. To the top of the limestone sequence, the size of the fossils increases and the faunistic diversification decreases. On the tidal flat, the presence of rather thin bioturbation traces (Thallasinoides) would indicate conditions of instability during the development of this ichnofacies, as well as a rather moderate energy with low sedimentation rates. At the final stage, shallow marine, coastal prevail, marked by the abundance of entire shells, cross bedding and an increment in the Fe concentration. In the sequence, there has been identified a selective and differential dolomitization which consists of planar-s subhedral dolomite rhombohedrons nearly completely ferruginous (between 1800 to 3000 ppm Fe+2) with straight to slightly stepped crystal faces, which display a homogeneous to weak undulatory extinction. The dolomite crystals commonly have a cloudy core with a clear rim and is characterized by unimodal (bimodal locally) crystal size (150-600 ìm) and occurs as mosaics/patches in the matrix of the carbonate rocks. Some of the rhombohedrons are in parts dedolomitizated. Under CL, the dolomite shows alternating dull and moderate bright orange-reds bands (3-5 stages) whith 800–1500 ppm Mn+2 and contents between 1200-1500 ppm Fe+2, generally with a bright thin rim where the crystal faces the pore space. Interfaces between zones may be planar, embayed or truncated represented by small nucleated which give a nearly dull cathodoluminescence. The selective dolomitization and dedolomitization, which occurred in the calcareous package may evidence a mixture of pore and meteoric water influences. Two types of cementation were determined by cathodoluminescence analysis: a) marine cement dominated by isopachous rinds of fibrous to bladed spar, often precipitated sequentially in that order, and b) “blocky type”, obliterating the residual porosity composed of LMC, typical of a mixed water environment; has a higher content of activator elements, even inside the esqueletal fossils but not into the shells (not luminescence), showing moderate uniform orange luminescence, which reveals values of 600 to 900 ppm Mn+2 and <1550 ppm Fe+2.