Characterization and controls of high-frequency sequences in the Lower Cretaceous Neuquén depocenter (Argentina). From margin to basin

SCHWARZ, E.

Congreso; II Congreso Latinoamericano de Geología y XV Uruguayo de Geología; 2001

Asociación Uruguaya de Geología

This contribution analyses the depositional behaviour and sequence stratigraphy of the marine Mulichinco Formation, particularly the ca. 150 meters belonging to the Karachaschiceras attenuatus subzone (early Late Valanginian). We characterise the transgressive (TDs) and regressive deposits (RDs) along a low relief, semi-enclosed depocenter, and we establish the controls in the generation of high frequency sequences (HFSs). Six sedimentological sections located along proximal, medium and distal settings of the basin were used in this study. We analysed and interpreted facies and shellbeds, which in turn led to definition of depositional systems and sequence framework. Key surfaces (especially transgressive and flooding surfaces) were recognised from shellbed type and/or drastic facies changes. Transgressive deposits in proximal settings are characterised by thick wave- and tide-dominated upper shoreface to lower shoreface siliciclastic facies. Allochthonous conglomeratic shellbeds placed at the base of TDs mark the transgressive surface, while parautochthonous shellbeds at their top announce the flooding event. In intermediate regions the TDs also start with conglomeratic shellbeds, but grade upwards into lower shoreface and transition zone terrigenous facies. In these areas, but under relatively deeper conditions or higher rate of sea level rise, the TDs are only formed by carbonate parautochthonous massive shellbeds. TDs in distal settings are always composed of relatively thin calcareous deposits, being either parautochthonous or in situ shellbeds accumulated in transitional to offshore environments. Regressive deposits in proximal areas include wave-dominated lower shoreface to transition zone facies. In middle parts of the depocenter, RDs attend maximum thickness and range from lower shoreface to offshore deposits. Thick offshore shales prevail towards basinal environments and true RDs are difficult to identify. Studied sequences (2 to 22 metres) have been generated by high frequency sea level fluctuations. Although regional correlation of each HFS is difficult, 18 to 24 sequences have been defined across the basin. As estimated time span for the ammonite subzone is between 0.5 and 1 Ma, the duration of each sequence ranges from 20 to 55 Ka. Therefore, since Milankovitch precession and obliquity cycles are within the same time span, we infer that high frequency sea level fluctuations are closely related to variations of insolation, which in turn are controlled by orbital changes. A higher order vertical stacking including 3 to 6 TD- or RD-dominated sequence associations is recorded. This long-term cycle suggests the overprint of a sea level curve of lower frequency and higher amplitude on the small scale one. The results of this study contribute to predict internal architecture and time span of HFSs along a low relief restricted marine depocenter. Findings coming from well calibrated Cenozoic HFSs of alike geological settings (e. g. Pleistocene Wanganui Basin of New Zealand), show remarkable similarities with those encountered here. Hence, we propose that methodology and results may be assessed to outcrop and subsurface successions with poor biostratigraphic resolution developed in high paleolatitude areas, as the Nequén and others southwestern Gondwana basins.