Abstract: The Utility of Small Scale Sedimentary Structures in Shales

Juergen Schieber

Although quite inconspicuous or even invisible in outcrop, small scale sedimentary structures in shales contain information about depositional conditions and history of a shale. When polished slabs and petrographic thin sections are examined, many seemingly drab shales show a large range and variability of sedimentary features. Examples ranging from Proterozoic to Eocene demonstrate how small scale sedimentary structures can be used to detail sedimentary conditions. Among the most common structures are thin silt layers. These may for example indicate deposition by density currents (grading, fading ripples), storm reworking and transport (graded rhythmites), wave winnowing (fine even laminae with scoured bases), and bottom currents (silt layers with sharp bottom and top).

Gradual compositional changes between e.g. clay and silt dominated laminae, are another commonly observed feature, and are suggestive of continuous (although slow) deposition, possibly from deltaic sediment plumes and shifting nepheloid flows. Upon close inspection shales may also reveal clay-filled mud cracks, brecciation due to desiccation, and sands or conglomerates that consist entirely of shale particles. The latter can for example form as a result of soil erosion (pedogenic particles), erosion of cracked mud crusts, and submarine scouring of mud substrates by strong currents. Biologic agents may produce microbial laminae and protection of mud surfaces from erosion, or may manifest themselves as bioturbation and destruction of primary fabrics. However, in many instances sufficien proportions of primary features survive bioturbation, and in many instances observation of bioturbation features indicates substrate firmness, event deposits (escape traces), and rates of deposition.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994