EARLY ORIGIN OF SHALE FABRIC BY GRAVITATIONAL COMPACTION OF FLOCCULATED CLAY

The strongly oriented microfabric common to many organic-rich shale deposits is normally explained in terms of burial-induced collapse of high-porosity flocculated clay. Still, the depth at which this microfabric, so crucial to considerations of fluid migration and entrapment, forms is not well understood. Microscopic analysis of fissile black shale beds exposed at the bottom of the Upper Devonian Hanover shale of western New York State suggests that (1) rapid collapse of clay floccules that comprised the water-rich, laminated carbonaceous sediment began soon after sedimentation, perhaps within 10 cm of the sediment-water interface, and (2) the planar microfabric was largely in place by the time the organic-rich sediment was bioturbated by Planolites-making animals, probably not much more than 25 cm burial depth. Analysis of post-bioturbation strain recorded by flattened traces and finite compaction strain of the local Upper Devonian sequence as measured by differential compaction of black shale around early formed carbonate concretions in the Dunkirk black shale suggests that ~18% compaction strain (33% of the finite compaction strain) took place within the upper 25 cm or so of the sediment column, probably by occlusion of large voids in the organic-rich clay. Post-bioturbation compaction (~37% compaction strain) of the black shale likely occurred by flattening of the planar fabric and decay of small pores. Backfilled sediment in the Planolites traces, however, experienced minimal post-bioturbation grain reorientation; instead, compaction strain appears to have been limited to the decay of pore volume accompanied by little rotation of sediment particles. This study demonstrates that the open fabric of bioturbated sediment is preserved throughout its entire mechanical compaction history and points to the important role of depositional environmental conditions (anoxic vs. dysoxic or oxic) on the migration of formation fluids in shale-dominated sequences.