67 Omnipresent Ichnology: Traces of Life From Mountain Slopes to the Deep Sea

Tuesday, 21 March 2017
1:30 PM - 5:30 PM
Lawrence Welk Room, Mezzanine Level

DIAGNOSTIC GEORADAR SIGNATURES OF BRACHYURAN BURROWS, SAN SALVADOR ISLAND, THE BAHAMAS: IMPLICATIONS FOR CARBONATE NEOICHNOLOGY AND BEYOND

Paper number: 67-2
Program: Theme Sessions
Time: 1:50 PM – 2:10 PM

Authors

Abstract

Bioturbation in supratidal carbonate substrates has only recently received attention from neoichnologists, relying primarily on traditional ground-based techniques. Our study is the first high-resolution geophysical investigation of three decapod species inhabiting adjacent coastal biotopes on San Salvador Island. Extensive (>1.5 km) 800 MHz georadar surveys captured a diverse suite of >120 burrows of ghost (Ocypode quadrata), blackback (Gecarcinus lateralis) and blue (Cardisoma guanhumi) land crabs. Site-specific post-processing protocols were used to establish characteristic attributes that distinguish unfilled burrows from other discordant anomalies based on signal polarity structure. Variably oriented 2D and quasi-3D (~0.5λ) images allow the diagnostic signature of G. lateralis burrows to be distinguished from those of O. quadrata based on: 1) higher density of point-source diffractions (0.9-1.9/m vs 0.01-0.06/m) and 2) shallower inclination angle (21-46◦ vs >60◦). C. guanhumi burrows are differentiated by: 1) larger diameter (>10 cm); 2) presence of end chamber (where not attenuated by saline groundwater), and 3) low electromagnetic signal velocity (9 cm/ns vs 11-12 cm/ns) in organic-rich muddy substrate. Our study establishes GPR as a viable non-invasive subsurface imaging technique, with potential implications for: 1) identification of shifting ichnocoenoses, as exemplified by intense hurricane impacts (2015/2016); 2) recognition and mapping of similar biogenic structures in buried or lithified carbonates and, 3) quantification of a near-surface ichnofabric index and the dual porosity/permeability structure of prospective hydrocarbon reservoirs. Furthermore, the basic attributes of subsurface visualization can be readily extended to other mesoscale biogenic structures in evaporite and siliciclastic media.

Geological Society of America Abstracts with Programs. Vol. 49, No. 2

doi: 10.1130/abs/2017NE-289866

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