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