Abstract

A density model of the upper crust of Wyoming, computed on structural and petrographic evidence without prior reference to gravity data, when compared with the observed gravity field, demonstrates that: (1) Existing measurements and theory of change of sedimentary rock density with depth of burial are satisfactory for gravity interpretation for local rock types and the slower rates of sedimentation in the area. (2) Gravity stations established over sedimentary basins are generally as satisfactory as those on basement rock outcrops for depicting regional Bouguer gravity trends. (3) The Laramide deformation was mainly germanotype. The regional gradient in Bouguer gravity across Wyoming has increasingly negative values to the southwest. Local variation in the Bouguer anomaly field is related mainly to the low density Cretaceous and Cenozoic sediments of the basins, and to a lesser degree to the Laramide structural blocks. Interpretation of the gravity field indicates that the blocks are bounded by near-vertical faults extending into the upper mantle, and that the uplifted blocks are undercompensated (i.e., rootless). In order to lead to a reasonable depiction of isostasy for the region, plots of mean Bouguer anomalies versus mean elevation must be made over 2 degrees X2 degrees squares, or larger, because of the variation in structure and composition of the crust.

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