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Miscellaneous Field Studies Map MF-2426

Geologic Map of the Bonners Ferry 30' x 60' Quadrangle, Idaho and Montana

By Fred K. Miller and Russell F. Burmester


In this report, Bonners Ferry quadrangle refers to the Bonners Ferry 1:100,000-scale quadrangle, as distinguished from the Bonners Ferry 7.5' quadrangle. The Bonners Ferry quadrangle spans the northernmost part of the Idaho panhandle, extending about 3.5 km into Montana; its northern limit is the United States-Canada boundary.

The bedrock geology of the Bonners Ferry quadrangle consists of sedimentary, metamorphic, and granitic rocks ranging in age from Middle Proterozoic to Eocene. Bedrock units include rocks of (1) the Middle Proterozoic Belt Supergroup, (2) the Middle Proterozoic Deer Trail Group, (3) the Late Proterozoic Windermere Group, (4) miogeoclinal or shelf facies lower Paleozoic rocks, and (5) Mesozoic and Eocene granitic rocks.

The Belt Supergroup, a thick sequence of argillite, siltite, quartzite, and impure carbonate rocks up to 14,000 m thick is found in two noncontiguous sequences in the quadrangle: (1) the Clark Fork-Eastport Sequence east of the Purcell Trench and (2) the Newport Sequence in the hanging wall of the Newport Fault. Only the two lowest Belt formations of the Newport Sequence are found in the Bonners Ferry quadrangle, but these two units are part of a continuous section, which extends southwest of the map area to the town of Newport.

Belt Supergroup rocks of the Clark Fork-Eastport Sequence are separated from those of the Newport Sequence by the Newport Fault, Priest River complex, and Purcell Trench Fault. Some formations of the Belt Supergroup show differences in thickness and (or) lithofacies from one sequence to the other that are greater than those predicted from an empirical depositional model for the distances currently separating the sequences. These anomalous thickness and facies differences suggest that contraction on structures separating the sequences was greater than Eocene extension associated with emplacement of the Priest River complex. In addition to these two Belt sequences, probable Belt Supergroup rocks are present in the Priest River complex as highly metamorphosed crystalline schist and gneiss.

Northwest of the Newport Sequence of the Belt Supergroup is the Deer Trail Group, a distinct Middle Proterozoic sequence of argillite, siltite, quartzite, and carbonate rocks that is lithostratigraphically similar to the Belt Supergroup but separated from all Belt Supergroup rocks by the Jumpoff Joe Fault. Rocks of the Deer Trail Group are pervasively phyllitic and noticeably more deformed than most of the rocks in either of the Belt Supergroup sequences. Lithostratigraphically the Deer Trail Group is equivalent to part of the upper part of the Belt Supergroup (Miller and Whipple, 1989). Differences in lithostratigraphy and thickness between individual Deer Trail and Belt units and between the Deer Trail and Belt sequences as a whole indicate that they were probably much farther apart when they were deposited.

The Windermere Group is a lithologically varied sequence of coarse-grained, mostly immature, clastic sedimentary rocks and volcanic rocks up to 8,000 m thick. It is characterized by extreme differences in thickness and lithofacies over short distances; these differences are interpreted as due to syndepositional faulting associated with initial stages of continental rifting in the Late Proterozoic. Strata of the Windermere Group unconformably overlie only the Deer Trail Group and are nowhere found in depositional contact with Belt Supergroup rocks.

Paleozoic rocks in the Bonners Ferry quadrangle are represented only by a thin, fault-bounded remnant preserved within the Clark Fork-Eastport Belt Supergroup sequence.

Mesozoic granitic rocks underlie at least 50 percent of the Bonners Ferry quadrangle. Most fall into two petrogenetic suites, hornblende-biotite bodies and muscovite-biotite (two-mica) bodies. Both suites are represented in the mid-crustal Priest River complex and in the higher level plutons that flank the complex; by far the majority of plutons in the Priest River complex are two-mica bodies.

Tertiary rocks are restricted to a single small stock, numerous hypabyssal dikes that are too small to show at the scale of the map, and cataclastic rocks related to the Newport Fault. Quaternary deposits include unconsolidated to poorly consolidated glacial, alluvial, glacial-lacustrine, and landslide units.


File Name
File Type and Description
File Size
PDF version of the readme file that explains how to use the digital database.
356 Kb
bferry_met.html HTML file of the FGDC-compliant metadata
72 Kb
Text file of the FGDC-compliant metadata
52 Kb
Compressed tar file of the digital database for this map
7.1 MB
Compressed (gzip) PostScript file used for plotting a paper copy of the map
9 MB
PDF file of Description of Map Units for the Bonners Ferry geologic map
632 Kb
PDF file of map sheet that can be used for viewing map in a browser, as well as for plotting
3.7 MB

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Maintained by: Michael Diggles
Last modified: May 25, 2005 (mfd)