Structural
Styles In A Deformed
Butler, Robert W.H.1, Henry
Lickorish2, William D. McCaffrey1, Jake Lockley3,
Jamie Vinnels1 (1) University of Leeds, Leeds, United Kingdom (2)
Rock Deformation Research, Calgery (3) School of
Earth and Environment, University of Leeds, Leeds, United Kingdom
Thrust zones are notoriously difficult to
image seismically. Does the transparency of such zones result from inadequate
seismic resolution of simple, discrete faults or does it represent complex
structure? Here we present a field study using seismic scale outcrops (km x km)
to explore the nature of contractional deformation
developed in multilayer turbidite sandstones. Our
examples come from the Champsaur sandstones of the
French Alps. These late Eocene-early Oligocene tubidites
lie in the footwall to a major detachment thrust sheet (Embrunnais-Ubaye
Nappe). They are in turn involved in the thrusting.
Yet in all cases deformation includes folding. The importance of folding vs thrusting varies from site to site. In the eastern part
of the area there are almost no thrusts within the turbidite
sequence. Deformation is accomplished by asymmetric folding. This style
generates km-long panels of gently dipping strata, connected by forelimbs of
greater structural complexity. Given the problems of imaging forelimbs such
structures are likely to be (mis)
interpreted in the subsurface as narrow fault zones. At outcrop, the
macroscopic forelimbs themselves contain trains of subsidiary folds that vary
in tightness, frequency and wavelength depending up on the stacking pattern of turbidite sandstones. In general, short wave-length folds
and correspondingly greater structural complexity relate to thinner bed
packages. These studies indicate that folding, related to stratigraphic
variations, may be a significant component of subsurface structural styles.
While fold interpretations may be difficult to resolve from seismic data alone,
dip-metre logs from well penetrations offer useful
indicators. Correct assessment of fold styles is important for predicting
reservoir connectivity in the subsurface, especially for contractional
systems in deep water settings.
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California