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Lower Cretaceous Transgressive Offshore Sandstone Bodies (Mulichinco Formation, Neuquén Basin, Argentina): Outcrop Analogues for Tide-Built Sand Ridges?

Schwarz, Ernesto *1
(1) Centro de Investigaciones Geologicas, Universidad Nacional de La Plata and CONICET, La Plata, Argentina.

This study integrates sedimentology, ichnology, taphonomy, and palaeoecology of Mulichinco Formation strata in central Neuquén Basin (Argentina) to describe and interpret sharp-based sandstone bodies developed in a ramp-type marine setting. These bodies are sandwiched between finer grained siliciclastics beneath and thin carbonates above. The underlying sediments comprise progradational successions (2-10 m thick) from offshore mudstones to offshore-transition muddy sandstones, occasionally grading into lower-shoreface sandstones. The surfaces capping the regressive siliciclastics are flat and regionally extensive, and are demarcated by skeletal concentrations and a Glossifungites Ichnofacies suite. They are also marked by cobble-size, sandstone rip-up clasts, with incrustations and borings. These surfaces are interpreted as composite discontinuities, cut during a relative sea-level fall and remodeled during initial transgression (SB/TRS). The overlying transgressive sandstone bodies are 3-7 m thick, > 4 km long and about three times longer than wider. They are composed of clean, fine- and very fine-grained sandstones with little lateral changes in grain-size. Cross-stratification and cross-lamination are common, typically with smaller-scale structures and finer grain size towards the top. Large-scale, low-angle (5-8°) inclined stratification is also common, deeping at ~ 30° with respect to body elongation and dominant currents. These sand bodies are interpreted as tide-built offshore sand ridges. Intense burrowing is typical at the top of each unit, suggesting an abandonment stage. Final deactivation favored colonization by epibenthic-dominated communities and the formation of condensed skeletal-rich limestones during the latest transgressive conditions. As partial reworking of pre-existing ridges occurred during this stage, the sandstone bodies are considered the remnants of the transgressive offshore accumulations.

The inferred tide-built offshore Mulichinco sandstone bodies have ~ 85% of net sand and they lack mud-prone facies. Their remaining volume is represented by non-reservoir cemented skeletal sandstones (sometimes occurring as internal inclined surfaces) that could produce reservoir compartmentalization. The lessons learned from these transgressive bodies could help recognizing similar units world-wide, as well as providing insights into characterization of reservoir analogues (e.g. Western Interior and offshore Java).


AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California