TSK 11 Göttingen 2006 Wittek et al.
 The importance of lithologi-
 cal heterogeneity of the On-
 aping Formation for under-
 standing post-impact defor-
 mation of the Sudbury Im-
 pact Structure, Canada
 Poster
 Andrea Wittek1 Ulrich Riller2 Lutz
 Hecht2
 The suevitic Onaping Formation over-
 lies the layered Main Mass of the
 1.85Ga Sudbury Igneous Complex
 (SIC) of the Sudbury Impact Structure,
 Ontario. The Formation consists of
 four Members, namely from top to
 bottom, the Black, the Green, the
 Gray and the Basal. Post-impact NW-
 SE shortening during the Penokean
 Orogeny (ca. 1.9–1.75Ga) affected the
 Onaping Formation and led to the
 lobate shape of the SIC in plan view.
 In order to investigate the possible fold
 origin of the NE-lobe of the SIC, a
 field-based structural analysis of the
 Onaping Formation was conducted in
 the Frenchman Lake area. The analysis
 is based on structural measurements
 at 580 stations and encompasses the
 orientation of mineral shape fabrics as
 well as their intensity. In addition to
 these quantities, lithological variation
 and metamorphic overprint of the On-
 aping Formation was examined. Special
 attention was paid thereby to the Green
 Member since previous workers stated
 that it forms a continuous unit at the
 base of the Black Member.
 Individual Members of the Onaping For-
 1 Freie Universität Berlin, Institut für Ge-
 ologische Wissenschaften, Malteser Strasse
 74-100, 12249 Berlin 2 Humboldt-Univer-
 sität Berlin, Institut für Mineralogie, Invaliden-
 strasse 43, 10115 Berlin
 mation differ in mineral composition,
 size and abundance of clasts as well as
 in color and texture of the matrix, the
 latter one of which ranges from cryp-
 tocrystalline to coarse-grained as well as
 in the content of felsic minerals. Over-
 all, grain size of the matrix as well
 as size and content of clasts increase
 from the top of the Onaping Forma-
 tion towards its base. The Green Mem-
 ber is well apparent by the presence of
 angular mineral aggregates, composed
 chiefly of chlorite. Therefore, these
 aggregates became known as ‘chloritic
 shards’ and possibly represent original
 fragments of melt particles. In the
 Frenchman Lake area, the Green Mem-
 ber forms a 70–100m thick, discontinu-
 ous band, whereby the strike of its con-
 tacts at surface varies highly. By con-
 trast, intra-formational lithological con-
 tacts of the lower units of the Onaping
 Formation are rather straight.
 Microscopic inspection of samples from
 the Black, Green and Gray Members
 shows that mineral shape fabrics are
 chiefly defined by chlorite and epidote.
 This points to post-impact deformation
 under lower greenschist-facies metamor-
 phic conditions. Deformation of the On-
 aping Formation varies greatly with po-
 sition. This is indicated by the ori-
 entation and intensity of planar min-
 eral shape fabrics. Fabric intensity
 was visually estimated and is based on
 the shape-preferred alignment of matrix
 minerals and clasts. It varies as a func-
 tion of the mineralogical composition
 and grain size of the matrix, as well as
 in distance to the NE-lobe. More specif-
 ically, fabric intensity increases towards
 the top of the Onaping Formation sug-
 gesting that mechanical strength during
 deformation of the Formation increased
 toward its lower contact. This is corrob-
 1
Wittek et al. TSK 11 Göttingen 2006
 orated by the variation in strike of intra-
 formational lithological contacts, which
 are straight at the base but highly irreg-
 ular towards the top of the Formation,
 and may well account for the discontin-
 uous nature of the Green Member.
 An increase in shape fabric intensity is
 also apparent towards the east. Simi-
 larly, the strike of planar shape fabrics
 becomes more uniform in this direction,
 i.e., NE–SW, which is axial-planar to
 the acute bisectrix of the NE-lobe. This
 may indicate that the effect of folding
 of the SIC on the fabric development
 in the Onaping Formation decreases to-
 wards the west. The planar shape fab-
 rics also display a progressive change
 in their overall strike from NE-SW in
 the NE–lobe to ENE–WSW towards the
 west of the lobe. This change in strike
 can be explained by the curvature of
 the fold axis of the NE-lobe. Despite
 the strong heterogeneity of deformation
 in the Frenchman lake area, the orien-
 tation in, and gradients in intensity of,
 planar shape fabrics in the Onaping For-
 mation are consistent with a fold ori-
 gin of the NE-lobe, which formed un-
 der lower greenschist-metamorphic con-
 ditions.
 2