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The AAPG/Datapages Combined Publications Database

Four Corners Geological Society

Abstract


Canyonlands Country, Eighth Field Conference, 1975
Pages 245-252

The Abajo Mountains: An Example of the Laccolithic Groups on the Colorado Plateau

Irving J. Witkind

Abstract

Eight laccolithic groups are irregularly scattered across the Colorado Plateau. In these, three different laccolithic types have been recognized: a simple planoconvex (mushroomlike) laccolith; an asymmetric laccolith, somewhat like half a planoconvex laccolith; and a satellitic laccolith, a tonguelike mass attached to a parent stock. The satellitic laccoliths radiate from a parent stock, much like spokes from the hub of a wheel, to form a cluster. In several, but not all, of the laccolithic groups, these clusters form igneous centers, each of which appears as a distinct mountain mass. In all laccolithic groups the dominant igneous rock is diorite porphyry, which locally grades into quartz diorite porphyry and granodiorite porphyry.

The Abajo Mountains represent a typical laccolithic group composed of four igneous centers. At two of these centers, East and West Mountains, the parent stock and its satellitic laccoliths are exposed. At the other two, Shay Mountain and the Johnson Creek dome, a parent stock is inferred at each location, but erosion has not cut deeply enough to expose it.

In all, 31 laccoliths have been identified in the Abajo Mountains. The total volume of igneous rock emplaced is estimated to be 5.2 cubic miles (about 22 km3).

Two grabens flank the Abajos; the northeast-trending Shay graben cuts across the northwest flank of the mountains, and the east-trending Verdure graben cuts across the south flank. The grabens appear to be unrelated to the mountains and were probably formed before the laccoliths were emplaced.


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