FERNANDEZ Diana Elizabeth
congresos y reuniones científicas
Dolomite cementation in a tidally influenced system; the uppermost Agrio Formation, Neuquén Basin, Argentina
Puerto Madryn
Congreso; Tidalites 2015 - 9 th International Conference on Tidal Sedimentology; 2015
Institución organizadora:
Asociación Argentina de Sedimentología
The Lower Cretaceous Agrio Formation was defined by Weaver (1931) and it represents the last extensive transgression from the palaeo-Pacific ocean which spread throughout almost the entire Neuquén Basin. The Upper Hauterivian-Lower Barremian, mixed siliciclastic-carbonate ramp system, known as the Agua de la Mula Member (Leanza & Hugo, 2001) provides excellent exposures of tidal deposits associated with dolomitic beds. These beds exhibit in many cases diagnostic features which permit their identification in the field; they form major erosion-resistant ledges and are characterized by red to brown weathering color (sensu Munsell color system). In such cases they contrast in color with the interbedded light gray to pale yellow fine-grained sandstones, pale olive to dark gray illitic-rich mudstones and pale yellow to grayish brown bioclastic to oolitic wackestones, packstones and grainstones. Even though dolomite-rich beds (up to 75 % by weight of dolomite) related to intertidal and subtidal facies were previously well described in the uppermost part of the studied succession at Mina La Continental area (Tunik et al., 2009), no geochemical information was provided. In this work we present some data about the isotopic signatures (delta18O and delta13C) from dolomitic beds of this interval, present not only in outcrops proximal to Mina La Continental (e.g. Cerro Rayoso), but also at Bajada del Agrio, Agua de la Mula, Mina San Eduardo as well as at Loma La Torre localities. Previous works in the uppermost part of the Agua de la Mula Member at Bajada del Agrio (Fernández & Pazos, 2012) and Cerro Rayoso areas (Pazos et al., 2012) have documented the interplay between normal marine to marginal marine siliciclastic-carbonate deposits. At Bajada del Agrio, the uppermost 65 m show five main facies, from wave-influenced deposits: (1) oolitic-skeletal bar;(2) transgressive pelites and coquinas (bioclastic conglomerates); (3) progradational coarsening-upward sandstones, interpreted as subaqueous fluvial discharges in the way of hyperpycnal flows; to tide-dominated ones: (4) heterolithic deposits with IHS and channelized bodies; and (5) heterolithic and carbonatic deposits (Fernández & Pazos, 2012). In this locality, dolomitic beds (0.20-0.40 m in thickness) were found in tide-dominated deposits where dolomite, occurring as cement filling the interparticular space and as partial replacement of bioclasts, is mainly associated with oolitic-bioclastic packstones/floatstones. According to quantitative analyses by X-ray diffraction dolomite content ranges from 15 to 20 % by weight. Also, at Agua de la Mula locality, oolitic-bioclastic grainstones to packstones (0.30 m thick) with symmetrical to slightly asymmetrical wave ripples and containing 30 to 50 % by weight of dolomite were found in the uppermost 40 m. These dolomitic beds are interbedded with decimetre-scale mud-dominated heterolithic deposits which grade into laminated illitic-rich mudstones with intercalations of massive, calcareous siltstone to fine-grained sandstone beds and bioclastic packstones/floatstones. Soft-sediment deformation structures similar to convolute lamination occur in sandstones bodies which are laterally discontinuous in thickness (0.10-0.40 m). Oscillation and combined-flow ripples with opposite paleocurrent directions were documented in fine-grained sandstone beds and when the original depositional fabric has not been totally disrupted by bioturbation, very thin cycles (millimetre-scale) consisting of silt and sand-rich laminae grading up into a clay-rich laminae are observed. At Cerro Rayoso and Mina La Continental areas Pazos et al. (2012) recognized that the uppermost 30 m consist of two intervals: (i) a lower mixed carbonate-siliciclastic tidally-dominated interval; and (ii) an upper wave-influenced siliciclastic interval. The lower interval is characterized by several fining upward cycles, starting with relatively thin dolomitized oolitic grainstones to packstones deposited under subtidal conditions but with evidence of subaerial exposures (dinosaur tracks) and replaced upwards by intertidal deposits. The later consist of sandy limestones that grade up to sand-dominated heterolithic deposits containing ripples with opposite paleocurrent directions and mud drapes, capped in some cases by mud-cracked claystones and laminites that represent supratidal deposits. The upper interval consist of a relatively thick sharp-based amalgamated sandstone package with current and wave ripples as well as combined-flow ripples, that ends with green sandstones and shales deposited in extended flats with evidence of subaerial exposure, as shown by flat-topped ripples (Pazos et al., 2012). At Mina La Continental, dolomitized subtidal and intertidal facies have been interpreted as the result of the mixing of waters, continental and marine, which promoted the early dolomitization of the marine cements and ooids concerning a shallow subtidal dolomitization model (Tunik et al., 2009).Optical petrography and SEM observations support an authigenic origin for the dolomite and two types are differentiated based on crystal size and shape in relation to host rock and dolomitization processes. Type I occur as a bimodal crystal size population with a minor fraction of isolated euhedral crystals of 50-70 μm and a major fraction of tightly packed, subeuhedral crystals under 40 μm in size. Type II occurs as an unimodal crystal size population (under 30 μmin size) of tightly packed anhedral to euhedral crystals displaying a mosaic texture. There is a broad consistency in the stable isotope data from dolomite-bearing samples, with delta13C values of +0.32 to -0.89 VPDB and delta18Ovalues of -3.98 to -8.52 VPDB. According to delta13C and delta18O values obtained from oyster shells (cf. Aguirre-Urreta et al., 2008; Lazo et al.,2008), fluctuations in palaeosalinity were likely due to dilution from normal marine water, caused primarily by variations in rainfall and continental runoff, even near hyperhaline conditions were reached in some levels towards the top of the Agua de la Mula Member (Lazo et al., 2008). Also, the presence of miliolids, which may indicate hypersalinity in tidal-flat deposits at Bajada del Agrio (Fernández & Pazos, 2012) agrees with data obtained by Lazo et al. (2008). If dolomite cement precipitated from relatively low meteoric water/rock ratio conditions, a relatively high average of delta18O values are expected, although under such conditions a wide range of delta18O values are also associated with bacterially mediated sulfate reduction processes (Chafetz et al., 1999). Moreover, the incursion of meteoric fluids is commonly invoked to account for low delta18O values in both, early carbonate cements in limestones as well as in shoreface sandstones (cf. Taylor & Gawthorpe,2003). Taking into account the lack of a penetrative thermal overprint altering the isotope composition during burial diagenesis, it is likely that values presented here were controlled by eogenetic conditions at the time of deposition.Therefore the negative delta18O values obtained from type I and II dolomites probably reflect precipitation from meteoric pore-water conditions and related to mixing of continental and marine waters. Nevertheless, partial re-equilibration of the delta18O due to recrystallization and overgrowths during burial diagenesis cannot be totally discarded.In addition, for the same interval, the information obtained from oyster shells is mostly indicative of normal salinity conditions (Aguirre et al., 2008; Lazoet al., 2008). The relatively more negative delta18O values obtained in the present work would indicate freshwater influence. These preliminary results allow us to envisage a regional dolomitization pattern, possibly related to high-frequency relative sea-level changes as a control mechanism on the early dolomitization, operating over tidal facies in the studied interval.