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Recognition of Deltaic Environments from Small Samples
Petrographic analyses of small samples from the modern Burdekin Delta, Australia and the Upper Caseyville Delta, Pennsylvanian of Illinois, yielded compositional and textural data that are environmentally diagnostic. The Burdekin, a tidal dominated delta, is comprised dominantly of first cycle detritus with a mean size of quartz of 0.74 mm, and which are relatively immature (43.77% quartz, 16.9% feldspar, 29.9% rock fragments). Four distinct environments or groups of environments may be segregated using multiple discriminant function analysis on compositional and textural data: (1) fluvial channel and tidal channel (37.2% quartz, 43.2% rock fragments, 0% matrix, 1.10 mm mean size of quartz); (2) natural levee (46.0% quartz, 12.2% rock fragments, 10.4% matrix, 0.30 mm mean size of quartz); (3) beach-dune (49.0% quartz, 21.3% rock fragments, 0% matrix, 0.52 mm mean size of quartz); and (4) low tide flat (56.5% quartz, 16.0% rock fragments, 0% matrix, and 0.35 mm mean size of quartz).
The Upper Caseyville Delta, Illinois, was fluvially dominated and is comprised of rocks with a mean size of quartz of 0.26 mm, and which are mature (73.5% quartz, 1.6% feldspar, 14.2% rock fragments). Three environments may be segregated using multiple discriminant function analysis on compositional and textural data: (1) distal and distributary-mouth bar (82.4% quartz, 9.6% rock fragments 4.6% matrix, 0.34 mm mean size of quartz); (2) distributary channel (67.6% quartz, 20.6% rock fragments, 8.6% matrix, 0.31 mm size of quartz); and (3) interdistributary open bay (71.8% quartz, 12.5% rock fragments, 10.1% matrix, 0.15 mm mean size of quartz). In both deltas, compositional and textural differences between environments appear to be a function of variation in local sedimentary processes and process intensity.
Petrographic analyses of samples obtained from sidewall cores or cuttings could provide a geologist with quantative and environmentally diagnostic data useful in petroleum exploration.
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