Late Devonian Source Rocks in the Eastern Us - Not So Deep, Not So Quiet, Rarely Anoxic
Late Devonian black shales of the eastern US are in facies and carbon content (up to 20% TOC) quite representative of a wide variety of source rocks. If one views the Late Devonian as a rapid succession of oceanic anoxic events, one might expect that these black shales should preferentially have accumulated in quiet and deep water under anoxic conditions.
In the Appalachian and Illinois Basins, these black shales show erosional discontinuities that break the supposedly continuous succession into stacked sequences and parasequences. At the bed scale, scoured surfaces, muddy tempestites, graded rhythmites, and cross-laminated mud ripples attest to reworking by storm waves and bottom currents. In thin section scours and truncations occur at the lamina scale. Bioturbation, though subtle, is widespread. Collectively this suggests water depths and water column dynamics incompatible with long term anoxic conditions.
Pyrite framboid diameters suggest largely dysoxic conditions as well. At the lamina scale, variability of framboid diameters matches that observed in carbonaceous muds from the Santa Barbara Basin (SBB). Just like the SBB muds, these Devonian shales also contain agglutinated benthic foraminifera, even in those units that are deemed most “anoxic” on the basis of geochemical proxies. Because these organisms require at least some oxygen to survive, any anoxic interludes must have been short-lived. The SBB is a basin with suboxic bottom waters and has a redox interface that hovers around the sediment-water interface. Given that such conditions seem to have sufficed to produce Late Devonian source rocks, many of the so called OEA’s in the rock record may have required less exceptional circumstances than previously believed.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009