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AAPG Bulletin


Volume: 77 (1993)

Issue: 11. (November)

First Page: 1874

Last Page: 1903

Title: Thermal Maturity Patterns in Alaska: Implications for Tectonic Evolution and Hydrocarbon Potential

Author(s): Mark J. Johnsson (2), David G. Howell (3), Kenneth J. Bird (3)


Nearly 10,000 vitrinite reflectance and conodont color alteration index determinations from sedimentary rocks in Alaska were used to produce a thermal maturity map of rocks exposed at the surface and to evaluate subsurface thermal maturity relations in the Colville and Cook Inlet basins. Rocks exposed at the surface of the Tertiary interior basins and in the Aleutian forearc and backarc basins uniformly are of very low thermal maturity, indicating that these basins are at or near maximum burial, have seen little uplift and exhumation, and are probably thermally immature with respect to hydrocarbon generation. In contrast, many sedimentary basins show elevated levels of thermal maturity at the surface, with the highest values at basin margins. This geometry suggests a patt rn of greater uplift along basin margins, possibly reflecting isostatic readjustments as crustal loads are removed by erosion.

We investigated thermal maturity relations in three sedimentary basins (Colville, Cook Inlet, and Kandik) in more detail. Thermal maturity patterns in the Colville basin are broadly asymmetric, indicating systematic differential uplift ranging from a minimum of no uplift in the north (Point Thomson area) to 9-13 km of uplift and exhumation in the central Brooks Range; even greater uplift further to the south is indicated by the presence of greenschist facies and higher grade metamorphic rocks. This pattern may reflect the deflexing of the lithosphere subsequent to the principal episode(s) of crustal convergence and thickening. These patterns further suggest a similar thermal history for the proximal Colville basin and the northern foothills belt, suggesting the possibility of hydrocar on accumulations in the foothills. Thermal maturity isograds within the Brooks Range cut major thrust faults, indicating that maximum burial postdated the principal phases of thrusting. In contrast, isograds in the foothills belt to the north are warped broadly by local structure, indicating continued north-south shortening subsequent to maximum burial. Such deformation could have remobilized hydrocarbons in early traps. A broad southward extension of thermally immature rocks in the central portions of the foothills belt suggests relatively young east-west shortening (parallel to the strike of the orogene), a feature that to date has not been included in regional tectonic syntheses. Alternatively, the thermal maturity pattern could be explained by tectonically unrelated episodes of uplif in the eastern and western parts of the Brooks Range. In the Cook Inlet basin, vitrinite reflectance isograds also are indicative of relatively greater uplift at the basin margins than at the basin center, which appears to be presently at its maximum burial depth. Uplift in the Cook Inlet basin may reflect compression along the faults bounding the basin. Relatively high thermal maturity along the western margin of the basin also may reflect magmatic heat sources from the Alaska Peninsula-Aleutian volcanic arc. The Seldovia arch, a major structural feature of the

End_Page 1874------------------------------

basin, does not appear to deform vitrinite reflectance isograds, implying that deformation on that structure ceased prior to maximum burial. In the Kandik basin, a thermal maturity anomaly (thermally mature younger rocks in fault contact with thermally immature older rocks) provides clues to the nature and timing of east-west thrusting. Mesozoic foreland basin deposits associated with thrusting buried Paleozoic rocks of the easternmost part of this fold-and-thrust belt to relatively shallow depths, driving potential hydrocarbon source rocks into the oil-generation window. The western foreland basin deposits were overridden by advancing thrusts, and tectonically buried to as deep as 10 km. These disparate thermal domains are juxtaposed along the Glenn Creek fault, which may represent a terrane boundary in east-central Alaska.

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