APATITE FISSION-TRACK ANALYSIS OF OLIGOCENE STRATA IN SOUTH TEXAS, USA - TESTING ANNEALING MODELS

Analysis of apatite fission-track data from forty-seven surface and subsurface samples of fine-grained sandstone to pebbly conglomerate from the Oligocene Catahoula (Gueydan), Frio and Vicksburg Formations documents the response of fission-track ages and track-length distributions to simple post-depositional thermal histories characterized by monotonic heating. Data from outcrop samples document that the apatite component was derived from source areas characterized by rapid cooling through the temperature range from approximately 120-degrees to 50-degrees-C during the early Tertiary. The apatite population is characterized by a log-normal chlorine distribution (mean=0.3 wt%, sigma=0.3 wt%) consisting predominantly of low-Cl apatites (80% is less-than-or-equal-to 0.5 wt%). Mean track lengths from core samples decrease systematically with increasing depth and temperature. The decrease in apatite fission-track ages with increasing depth and temperature is less systematic due to the scatter in inherited ages at the time of deposition. No tracks were observed in apatite from samples at temperatures above 130-degrees-C. Mean track-lengths for the South Texas data agree well with those for a similar data set from the Otway Basin, south-eastern Australia, for present-day temperatures of < 70-degrees-C. At higher temperatures, mean lengths are 1 to 9 mum longer than those observed from the Otway Basin. This discrepancy is interpreted to reflect up to 40-Ma-longer residence times at maximum burial temperatures for the Otway Basin samples. Standard deviations about mean lengths are also lower than observed in the Otway Basin study, consistent with the wider variation of Cl contents (mean=0.8 wt%, sigma=0.7 wt%) that characterize apatite from the Otway Basin. Recently published apatite fission-track annealing models predict significantly different mean track length vs. present-day temperature profiles for thermal histories believed to be representative of those experienced by sampled South Texas strata. Because of the large uncertainties attributed to present-day temperatures (+/- 10-degrees-C), the predicted mean length vs. temperature trends for these models essentially fall within or on the uncertainty envelope for the observed trend. Residuals of predicted minus observed mean lengths suggest that the previously published Carlson composite parameter model. using an initial mean length of 15.8 mum instead of 16.5 mum, or the Crowley et al. model for track-length reduction down to approximately 11 mum, provide the best estimates of mean track-length reduction over geologic time scales for near end-member F-apatite populations.