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The Significance of Macrobioturbation and Cryptobioturbation in the New Albany Shale for the Interpretation of Depositional Histories

Riese, David *1; Schieber, Juergen 1
(1) Department of Geological Sciences, Indiana University, Bloomington, IN.

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