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What Does Ophiomorpha irregulaire Really Look Like?

Leaman, Mary *1; Boyd, Chris 1; McIlroy, Duncan 1
(1) Earth Sciences, Memorial University of Newfoundland, St. John's, NF, Canada.

Ophiomorpha irregulaire is a poorly understood and under-characterised ichnotaxon compared to other ichnospecies of Ophiomorpha. Debate surrounds its ichnotaxobases and palaeogeographic distribution (Bromley and Pedersen, 2008; McIlroy et al., 2009). In core, Ophiomorpha irregulaire is classically characterised by its thin “spikey” mud pellets which form the burrow lining. It is much more difficult to determine the horizontal “meander maze” which forms part of the ichnospecific diagnosis of the type material (Frey et al., 1978).

Considering the importance of correctly identifying Ophiomorpha irregulaire in ichnofabrics that commonly constitute petroleum reservoir intervals, a redescription of the type material would be an extremely effective tool in identifying the trace fossil in core and outcrop (McIlroy et al., 2009). The aims of this research are to increase the confidence in recognizing O. irregulaire through improved characterization of the morphology from type locality material with regard to the meander maze and its pelletal morphology.

Samples were collected from the type location of Book Cliffs, Utah and the Juncal Formation, California, to create high resolution three dimensional morphological models of the trace fossil by utilising a serial grinding method (after Bednarz and McIlroy, 2009). Samples were cast in plaster and clamped into a milling machine programmed to precisely grind off increments of 0.2 mm on the Z-axis. After each grind a photo was taken of the rock face and subsequently reground. This process was repeated until the whole rock was ground. Photos were stacked into Adobe Photoshop where the Ophiomorpha irregulaire burrows were selected and separated out into new images. The selected burrow images were reconstructed into a three dimensional morphological model using VG Studio. From the 3D model a variety of morphological features were observed, that have hitherto been under-appreciated both in cross section and in hand specimens.

Key morphological features were identified from the 3D models which can be linked to core slab faces and outcrops. This creates a more complete understanding of Ophiomorpha irregulaire, and greatly aids recognition of this common trace fossil in core sections. By comparison of the morphological features from the two localities, it has been proven that O. irregulaire exists outside the Cretaceous Western Interior Seaway (contra Bromley and Pedersen, 2008) and is globally distributed.

 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California