The Significance of Macrobioturbation and Cryptobioturbation in the New Albany Shale for the Interpretation of Depositional Histories
Differing morphologies of trace fossils in the Selmier and
Camp Run Members of the Late Devonian New Albany Shale in southern Indiana
suggest differences in the water content of sea floor muds. The Selmier and
Camp Run Members of the New Albany Shale are composed of greenish-gray to olive-gray
shale interbedded with brownish-black shale. Both members show abundant bioturbation
with the most common genera being Zoophycus and Chondrites. Hand
sample analysis of the
bioturbation
shows that macrobioturbation in the Selmier
Member is most prevelant in the greenish-gray to olive-gray shale where it
completely obliterates bedding. Shallow tiering of smaller-sized burrows is
observed in the brownish-black shale. Mantle and swirl structures are the most
common trace fossil observed.
Bioturbation
in the Camp Run Member is also most
prevalent in greenish-gray to olive-gray shale beds. Deeper tiering of large Zoopyhycus and Chondrites burrows is observed in brownish-black shale beds. The
relative paucity of discrete trace fossils and the abundance of mantle and
swirl structures suggests a water content of ~70% for the sea floor muds during
deposition of the Selmier Member. A greater abundance of discrete trace fossils
in the Camp Run Member may indicate less early depositional burrowing (mantle
and swirl) due to oxygen restriction, and later emplacement of discrete trace
fossils in muds of lower water content. The macrobioturbation observations of
discrete traces, deeper tiering, and recognition of spreiten suggests that
shale of the Camp Run Member experienced
bioturbation
for a shorter time
interval after deposition when compared with the Selmier Member. An important
part of the story missing is the distribution of trace fossils that are
difficult to see because of lack of color contrast as well as the pervasiveness
of cryptobioturbation. Cryptobioturbation is defined here as the
bioturbation
of small organisms that causes diffuse fabric disruption and indistinct
boundaries between laminae. Ongoing studies of the Selmier Member and Camp Run
Members using CT scans will allow us to better recognize and quantify cryptobioturbation in hand sample and arrive at a better assessment of
bioturbation
history and intensity. These data will also allow differences in
bioturbation
to be put into a context of fluctuating oxygenation levels and
changes in sediment accumulation dynamics.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California