DETRITAL ZIRCON PROVENANCE STUDY OF THE LATE JURASSIC BOSSIER SANDS, EAST TEXAS SUBSURFACE

The Bossier sands are a series of Jurassic stacked sand layers within the Bossier shale found in the subsurface of East Texas. Along with the related Haynesville Formation, these sands act as a natural gas reservoir and are an important natural resource. The Bossier sands were deposited and reworked as a result of a series of transgressions and regressions during the Jurassic Period, accompanying the establishment of normal marine conditions in the just-opened Gulf of Mexico. These deposits represent several depositional environments including bar sands, estuarine deposits, river deltas and beach and channel sands. We carried out the first study of detrital zircon U-Pb ages for Bossier sands. We separated detrital zircons from 5 wells (4 sets of cuttings and 1 core) and determined U-Pb ages of 393 individual zircon grains using LA-ICP-MS at the University of Arizona. Ages given by 206Pb/238U were generally used for zircons with measured ages <900Ma, while 206Pb/207Pb ages were used for zircons with measured ages of >900Ma. These results reveal two dominant age peaks: 32.5% have Early Neoproterozoic -Late Mesoproterozoic U-Pb ages (950-1360 Ma ‘Grenville’), and 27% of the zircon grains have U-Pb ages in the Pennsylvanian-Ediacaran age range (300-680 Ma ‘Ouachita’). Other U-Pb age peaks include approximately 8.5% of the grains in the Jurassic-Permian age group (150-300 Ma), 10.5% in the Middle Mesoproterozoic (1360-1500 Ma), 14% in the Late Paleoproterozoic-Early Mesoproterozoic (1500-1900 Ma), and 7.5% were Mid-Paleoproterozoic or older (>1900 Ma). We conclude that the dominant provenance for the Bossier sands was from local sources in the Ouachita and Grenville orogens. This suggests that flanking regions of Texas, Oklahoma, and Arkansas were highlands during the Jurassic time, shedding detritus into rivers feeding Bossier depocenters.