Abstract

SAGASTI, Guillermina. La sucesión rítmica de la Formación Agrio (Cretácico inferior) en el sur de la provincia de Mendoza, y su posible vinculación con Ciclos de Milankovitch. Rev. Asoc. Argent. Sedimentol. [online]. 2000, vol.7, n.1-2. ISSN 1853-6360.

Sequences deposited in pelagic and hemipelagic environments commonly develop a striking small-scale cyclicity in which beds of shally character (shale or marl) alternate at regular intervals with beds of higher carbonate content (marl or limestone). The coupling of a less calcareous and a more-calcareous bed is termed "bedding couplet" (Fischer & Schwarzacher, 1984; Einsele& Ricken 1991). Einsele & Ricken (1991) suggest that the major processes forming marl-limestone alternations and shale-carbonate alternations include variations in carbonate productivity, terrigenous dilution, carbonate dissolution, and redox conditions of the bottom waters. These processes have been related to orbital driven climatic variations or Milankovitch cycles. Rhythmically arranged stratigraphic sequences are widely distributed in space and time. There are many described examples of Cretaceous age sequences from North America and Europe. However, few studies have been carried out in the Southern Hemisphere, and particularly in South America. The lower Cretaceous succession of the Neuquén Basin, Argentina, (Agrio Formation) constitutes an excellent example of a rhythmic succession. Previous work refereed to the cyclicity of this units was carried out by Spalletti et al., 1990. More recently Spalletti et al., (in press) mentioned the presence of high frequency cycles probably related to climatic changes in the Loma La Torre section of the Upper Member of the Agrio Formation (north of Neuquén province). This contribution provides the first cyclostratigraphy analysis of the Agrio Formation based on outcrop data. Deposition of the Agrio Formation (late early Valanginian - early lower Barremian) began with a marine incursion following major regression in the earliest Valanginian. Accumulation occurred in a wide, semistarved distal ramp setting, where periods of catch-up carbonate deposition alternated with periods of finegrained clastic aggradation, resulting in a strongly rhythmic succession being shales and micritic limestones its major components. This facies arrangement characterises the Lower member of the Agrio Formation. This depositional scenario continued trough the late Hauterivian and early lower Barremian. However, these conditions were punctuated by a short episode of shallowing that caused an abrupt basinward shift of the depositional systems. During this time shales and sandstones of the Avilé member were deposited in the centre of the basin. In spite of the drastic changes implied by the lowstand facies distribution, the area was reflooded and fine-grained carbonates and clastic of the Upper member of the Agrio Formation were deposited over the whole extent of the Neuquén Basin. The Upper Member of the Agrio Formation was studied in two sections (arroyo Cienaguitas and Río Seco Cinta Roja, see Fig. 4) selected from the north of the Neuquén Basin (Mendoza province). The arroyo Cienaguitas section (138.8 m thick) is situated about 40 km from Malargüe city; the Río Seco Cinta Roja section (98.8 m thick) is located about 150 km south from Malargüe city, in the northern edge of the Cara Cura range (Fig. 5). Fine-grained sediments deposited in a low-energy, distal ramp environment compose the entire sedimentary succession at both localities. The lithology is characterised by alternations of carbonate shales, marlstones, marly limestones and micritic limestones. The rhythmic bedding is the main attribute of the Upper Member of the Agrio Formation and it is clearly revealed in outcrops (Fig. 6). Definition of the bedding cycles is based on the lithologic alternation of a relatively light coloured carbonate-rich end member (carbonate-rich hemicycle), and a relatively dark coloured clay-rich end member (clayrich hemicycle). Visual identification of these hemicycles in outcrops is facilitated by variations in bed colours, bed thickness, and induration/weathering profile (mainly a function of the CaCO₃ content) Carbonate-rich hemicycles include "impure" carbonates such as limestones and marly limestones (CaCO₃, 90 to 68 %). Bed colour range from light grey to bluish grey. Study of the microfacies reveals a dominance of mudstone over wackestones textures. Massive fabric dominates, but some laminated beds are also present. Some pyrite partially replaces shells as well as it is finely disseminated throughout beds. Shell material, scattered in carbonate-rich units includes pelecypods (lucinidae), gastropods, and Inoceramus bivalve shells and shell fragments, radiolarians, calcispheres, and sponge spicules (Fig. 7). Some of the limestone beds have Thalassinoids trace fossils. Carbonate-rich beds typically are 0.10 to 1.00 m thick in the Arroyo Cienaguitas sections, and 0.03 to 0.40 m thick in the Río Seco Cinta Roja section. Average values are 0.32 and 0.15 respectively (Table 1). Clay-rich hemicycles include marlstones and calcareous shales (CaCO₃, 65 to 42 %). Bed colour ranges from dark olive grey to dark grey and the fabric is laminated. Clay-rich hemicycles tend to be thicker than the corresponding carbonate-rich beds. In the Arroyo Cienaguitas section the average thickness is 1.03 m (with a minimum value of 0.01 m and a maximum of 5.65 m). In the Río Seco Cinta Roja section the average thickness is 0.71; with a minimum value of 0.07 m and a maximum of 4.98 m (Table 1). Bedding couplets have been defined from the base of the clay-rich hemicycle to the top of the overlying carbonate-rich hemicycle. Grouping of the couplets in bundles was made according to variations in couplet thickness observed throughout the sections (Fig. 9). The groups are generally characterised by a thinning upward trend. Figure 8 shows the thickness distribution of the carbonate-rich hemicycles, clay-rich hemicycles and bedding couplets. An excellent correlation between the clay-rich hemicycles and the couplet thickness can be observed. This trend seems to reflect changes in siliciclastic input superimposed on relatively constant fluxes of CaCO₃. In a first stage the individual couplet duration was estimated using average sedimentation rates calculated using the ratio between the thickness of the section and its duration in million of years, obtained with ammonites stratigraphy. According to Aguirre Urreta & Rawson (1997), the Upper Member succession comprise the Spitidiscusricardii, Crioceratites schlagintweiti and Crioceratitesdiamantensis zones, and the lower part of the Paraspiticeras groeberi zone (Fig. 10). In the studied sections the lower Hauterivian-early lower Barremian age was determinate with ammonites and nannofossils. determinate with ammonites and nannofossils. The Gradstein et al., (1996) time scale was used to determine an age-thickness relationship for the Upper Member in the arroyo Cienguitas and Río Seco Cinta Roja sections. The error ranges of the stage boundaries define minimum and maximum limits for duration of the studied interval. Calculation of the average bulk sedimentation rate yields a value of 45 m/Ma in the arroyo Cienaguitas section, and 32 m/Ma in Río Seco Cinta Roja section. This value represents an effective sedimentation rate (Park and Herbert, 1987) which is not corrected by compaction and non-deposition. Based on Gilbert (1895) couplet duration was calculated using the mean sedimentation rate. In the arroyo Cienaguitas section as well as in Río Seco Cinta Roja section, most of the cycles (70 %) represent time intervals between 10 and 30 ky This values may be related to the near 20 ky preccessional signal (Fischer, 1993; Gale, 1998). In a second stage Fourier spectral analysis was applied to the couplet thickness series of each sequence. In the arroyo Cienaguitas spectra consistent peaks appear in the 378 ky; 126-103 ky range; 69 ky and 47-45 ky range (Fig. 11 A). In the Río Seco Cinta Roja spectra major peaks are in the 875 ky; 319 ky; 125-117 ky range; 77,5 ky and 65 ky (Fig. 11 B). Some of these peaks closely match orbital periods, for example the 378 ky peak (and perhaps the 319 ky one) may correspond to the near 400 ky long eccentricity signal; the 126-103 ky and 125-117 ky peaks may represent the 97-123 short eccentricity signal; finally the 47-45 ky peaks may be assigned to the 41 ky obliquity signal. However there are others peaks do not match any known orbital periods (77,5 ky; 69 ky and 65 ky), raising the question whether other, up to now unidentified, cyclic forcing agents have not been recognised, or whether some of the orbital periods have changed and interacted in unresolved ways.

Keywords : Rhythmicity; Milankovitch cycles; Spectral analysis; Lower Cretaceous; Neuquén Basin.

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