South-Central Section - 51st Annual Meeting - 2017

Paper No. 27-5
Presentation Time: 3:10 PM


BASU, Asish R., Department of Earth and Environmental Science, University of Texas at Arlington, 500 Yates St, 107 Geoscience Building, Arlington, TX 76019 and CHAKRABARTY, Puloma, Earth & Environmental Sciences, University of Texas at Arlington, 500 Yates St., Geoscience Bldg, Room 107, Arlington, TX 76019,

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 εHf(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.