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Unformatted text preview: Unfolding the attitude of cleavage surfaces results in an unimodal distribution of poles to cleavage that is still 14
degrees away from vertical. Thus, some gentle folding had already taken place by the time cleavage started to
develop (mild folding during late Campanian times). Upper Cretaceous rocks crossed by cleavage, however,
constrain cleavage formation to before Paleogene times. After latest Cretaceous times, these early fabrics of
deformation were passively rotated by younger structures.
Deformed external molds of ammonite shells preserved in black shale of the lower part of the Villeta Group were
used as strain markers. The axial ratio ranges between 1.0 and 2.3, with an average of 1.5. After rotation of axial
orientations to remove the tilt of bedding, an ENE preferred orientation of the long axes of the ellipses is obtained,
which are generally parallel to the trend of cleavage. Axial orientations for the molds indicate E-W and SE-NW
shortening directions. Magnitude of axial ratios is related to distance to faults....
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- Winter '14