Significance of Combined Vitrinite Reflectance and Fission-Track Studies in Evaluating Thermal History of Sedimentary Basins: An Example from Southern Israel

Abstract

Vitrinite reflectance and fission-track age determinations (apatite, zircon, and sphene) from three deep boreholes in the Har HaNegev area, southern Israel, have been integrated for reconstruction of the thermal history. Drill holes, Ramon 1, Makhtesh Qatan 2, and Kurnub 1, each sited in the core of a separate breached anticline (“makhtesh”), penetrated a discontinuous Early Permian to Mesozoic sedimentary succession unconformably overlying a Precambrian arkosic and igneous complex. Post-Early Permian burial history was reconstructed from borehole data and regional stratigraphy.

The relationship between isoreflectance contours and bedding indicates that coalification predated the regional basal Cretaceous unconformity. Vitrinite reflectance profiles together with reconstructed burial curves have been used to calculate time-temperature models. These models reveal that the coalification profiles measured in the deeper part of the sedimentary section reflect a relatively short, intense Jurassic thermal event: for Ramon 1 and Makhtesh Qatan 2, 45° to 55°C/km over a period of approximately 10 to 25 my, and for Kurnub 1, 65° to 75°C/km over a period of <5 my. “Freezing” of the coalification process since at least Early Cretaceous time implies decrease of postcoalification temperatures. This cooling prevailed despite additional burial and reflects decay of the thermal gradient since Early Cretaceous time.

Fission-track dates of apatites are younger than their host strata and are interpreted as thermally reset ages. In Ramon 1, maximum paleothermal gradients derived from the apatite ages considerably constrain maximum possible postcoalification thermal gradients derived from vitrinite reflectance alone. In Makhtesh Qatan 2 and Kurnub 1, however, maximum paleothermal gradients derived by coal rank and fission-track measuremerits are in good agreement. In all three boreholes, apatite data independently reflect regional decay of the thermal gradient, at least through Cenozoic time. Furthermore, the ages suggest that the present-day relatively low thermal gradient (approximately 20°C/km) has prevailed in the study area since at least Early Oligocene time. Those ages obtained from samples within 150 m of total depth of the drill holes indicate cooling from the zone of total track annealing under the Early Oligocene depth-thermal gradient regime and thus provide fixed points in time on the regional cooling curve.

Zircon fission-track ages probably record cooling resulting from major regional Early Carboniferous uplift prior to Permian deposition. The fact that these ages have not been significantly reset limits the maximum possible Permian to present-day paleotemperatures to approximately 200°C ± 50°C in the section investigated. Sphene ages also record the effects of pre-Permian thermal history but of higher temperature.

The integration of vitrinite reflection and fission-track measurements provides a powerful means of thermal history analysis. The dual approach, using two parameters with different reaction kinetics, on varied lithologies in different parts of the section, yields complementary data and considerably improves constraints in developing paleothermal models.

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