Abstract:
This map forms part of the Montana State Geological Map.
The Ennis 1:100,000 quadrangle lies within both the Laramide (Late Cretaceous
to early Tertiary) foreland province of southwestern Montana and the
northeastern margin of the middle to late Tertiary Basin and Range province.
The oldest rocks in the quadrangle are Archean high-grade gneiss, and granitic
to ultramafic intrusive rocks that are
... as old as about 3.0 Ga. The gneiss
includes a supracrustal assemblage of quartz-feldspar gneiss, amphibolite,
quartzite, and biotite schist and gneiss. The basement rocks are overlain by a
platform sequence of sedimentary rocks as old as Cambrian Flathead Quartzite
and as young as Upper Cretaceous Livingston Group sandstones, shales, and
volcanic rocks.
The Archean crystalline rocks crop out in the cores of large basement uplifts,
most notably the "Madison-Gravelly arch" that includes parts of the present
Tobacco Root Mountains and the Gravelly, Madison, and Gallatin Ranges. These
basement uplifts or blocks were thrust westward during the Laramide orogeny
over rocks as young as Upper Cretaceous. The thrusts are now exposed in the
quadrangle along the western flanks of the Gravelly and Madison Ranges (the
Greenhorn thrust and the Hilgard fault system, respectively). Simultaneous with
the west-directed thrusting, northwest-striking, northeast-side-up reverse
faults formed a parallel set across southwestern Montana; the largest of these
is the Spanish Peaks fault, which cuts prominently across the Ennis quadrangle.
Beginning in late Eocene time, extensive volcanism of the Absorka Volcanic
Supergroup covered large parts of the area; large remnants of the volcanic
field remain in the eastern part of the quadrangle. The volcanism was
concurrent with, and followed by, middle Tertiary extension. During this time,
the axial zone of the "Madison-Gravelly arch," a large Laramide uplift,
collapsed, forming the Madison Valley, structurally a complex down-to-the-east
half graben. Basin deposits as thick as 4,500 m filled the graben.
Pleistocene glaciers sculpted the high peaks of the mountain ranges and formed
the present rugged topography.
Compilation scale is 1:100,000. Geology mapped between 1988 and 1995.
Compilation completed 1997.
Review and revision completed 1997.
Archive files prepared 1998-02. 
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