Tectonic Tilting and its Effect on Facies Stacking Patterns and Stratal Surface Formation: Sobrarbe Formation, Ainsa Basin, Spain
M. Hall, R. Gawthorpe, T. Dreyer, and A. Taylor
The Middle Eocene Sobrarbe Formation is a shallow marine succession representing the final stages of delta progradation into a piggyback basin. An integrated approach, using sedimentology, ichnology and palaeontology to identify key stratal surfaces, has been used to define seven fourth-order sequences within a succession of third-order sequences. The fourth-order sequences diverge basinwards, where they are ultimately truncated by a large collapse scar, and converge landwards. A complete range of facies is superbly exposed, allowing unequivocal tracing of stratal surfaces over several kilometres.
A high-frequency succession of discrete, tilting episodes, about a notional, non-static, strike-parallel axis, resulted in a landward-converging stratal architecture consisting of progressive unconformities and composite surfaces. In the proximal setting, close to the tilt axis, the regressive surface of erosion cannibalises the preceding sequence.
It is clear that the principal control on stratal architecture was tectonic. The regressive surfaces of erosion mark the onset of new tilting episodes and represent periods of cannibalisation during relative sea-level fall. Development of sharp-based shorefaces took place during falling or early-rising relative sea-level. Transgressive events are easily recognised as calcite-cemented, non-erosive surfaces with thick bioclastic accumulations (dominated by Nummulites) and multiple lag events. Thick deposits of mud represent the transgressive and highstand systems tracts, the updip shoreface deposits of which are not preserved. Owing to the inherent tectonic heterogeneity, preservation potential of surfaces and sediments varies considerably along the basin axis.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California