U-PB AGES AND HF-ISOTOPES OF ZIRCONS IN ASH BEDS AND RE-OS ISOTOPES OF THE EAGLE FORD FORMATION OF SOUTH TEXAS

We analyzed Re-Os isotope systematics in kerogen rich shales in a proprietary core of the Eagle Ford Formation in South Texas, LA-ICPMS U-Pb ages and Hf isotopes in volcanic ash beds as well as surface exposure ash beds in the Boquillas Formation. The zircons in three of the contiguous cores, separated on the surface by 140 miles from the Middle Eagle Ford, give ages 93.2±1.66 Ma, 94.13±1.25 Ma and 93.7±1.9 Ma, from West to East, yielding a consistently uniform Cenomanian-Turonian age of deposition of the ash beds. The top and the bottom of the Eagle Ford ash beds of these cores give ages from 85.76 to 93.7 Ma, suggesting duration of 9 Ma for the Eagle Ford deposition. Zircons from 7 ash beds in the surface exposures yield ages between 84.63 Ma to 90.91 Ma, implying much younger ages for these samples than the C-T age of proper Eagle Ford. These data clearly indicate that the ash beds in surface to sub-surface correlation are not correct and should not be used for stratigraphic correlations of these Boquillas Formation rocks with the sub-surface Eagle Ford.

Although the major element and trace elements of the ash beds suggest their origin as a result of arc-derived volcanism, the ε_Hf(T) of most of the ash bed zircons yield values between 0 to +8 averaging around +3, clearly of a mantle component in the host magmas of the zircons.

6 Lower Eagle Ford and 4 Middle Eagle Ford kerogen rich rocks show characteristic high Re-Os ratios; Re concentrations range from 7560 to 17701 ppt and Os concentrations vary from 37.5 to 932.1 ppt, yielding ¹⁸⁷Re/¹⁸⁸Os of 155.5 to 1537.4. The ¹⁸⁷Os/¹⁸⁸Os ratio in these rocks range from 2.7233 to 5.3529. In a Re-Os isochron diagram only three whole rock samples yield a reasonable isochron of 76±13 Ma with an initial ¹⁸⁷Os/¹⁸⁸Os ratio of 2.52±0.22. Other seven samples show scattering and do not define reasonable isochrons. All together, the Re-Os data suggest mobility of the kerogen-rich layers within the analyzed 100ft range of this core.

The initial ¹⁸⁷Os/¹⁸⁸Os at 93 Ma, the time of deposition of the Eagle Ford of the ten whole rock samples, range between 2.466 to 5.111 with a majority of the samples lying between 2.46 and 3.62 indicating a continental crustal origin of the Os without taking into account the sea water Os-isotopic composition at 93 Ma. The contrasting inference from Hf and Os-isotopes need to be resolved by future work.