Molly F. Miller1, Trent A. McDowell2, Yu Shyr3, Noel Kemp4, John Isbell5
(1) Vanderbilt University, Nashville, TN
(2) Vanderbilt university, Nashville, TN
(3) Vanderbilt University
(4) Tasmanian Museum and Art Gallery, Hobart, Australia
(5) University of Wisconsin, Milwaukee, Milwaukee, WI
The amount and style of bioturbation is different in freshwater vs. marine deposits. We compared the amount of bioturbation on bedding planes and vertical surfaces of fluvial (channel and subaqueous floodplain) and lacustrine facies in rocks of Permian-Jurassic age exposed in the Shackleton Glacier area, Antarctica, and Colorado Plateau to that in Permian marine rocks in Tasmania and in marine, high-energy sandstones, as reported in the literature.
Amount of bioturbation in freshwater facies is low. Most vertical surfaces observed (94%) showed no bioturbation (NB). The percentage of unbioturbated (NB) bedding planes of all freshwater facies is 65% but differs significantly between facies (p<0.001; Fisher's Exact Test [FET}); bioturbation is least in channel and greatest in lacustrine deposits. The Antarctic fluvial deposits are more bioturbated than the lower paleolatitude Colorado Plateau deposits, suggesting climatic control of the amount of bioturbation.
Amount of bioturbation on both vertical and bedding surfaces of Permian marine sandstones in Tasmania is statistically significantly greater (p<0.001; FET) than that in the freshwater deposits. All observed surfaces of marine rocks were bioturbated, and 34% had the highest level of bioturbation (60-100%) vs. only 0.2% of the freshwater deposits. The data from Tasmania are limited, but the high level of bioturbation is consistent with that reported in lower Paleozoic marine sandstones deposited in high energy environments (only 33% NB; 14% were 60-100% bioturbated) (Droser and Bottjer, 1990). The marine bioturbation contrasts with the low bioturbation in high energy fluvial channel deposits (99% NB), and underscores the potential utility of bioturbation in distinguishing between marine and freshwater deposits.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado