Abstract

Precambrian arkose and arkose conglomerate that contrast sharply with the commonly fine-grained units of the Belt Series crop out in a narrow zone from the Bridger Range on the east to the Highland Mountains on the west in southwestern Montana. The name LaHood Formation is satisfactory for these beds, but the formation is clarified and redefined, and a principal reference section is introduced to indicate more complete, more accessible exposures. The names North Boulder Group and East Gallatin Group should be abandoned.

The LaHood represents a long time-stratigraphic interval. Its intertonguing relationships with typical fine-grained Belt formations, and data regarding pre-Flathead (Middle Cambrian) truncation of Belt strata, permit correlations that show that LaHood equivalents range from oldest known Belt up to mid-Greyson. These correlations suggest relationships between rocks of the Philipsburg district and the type Belt that are quite different from those of other authors.

The LaHood formed in a partly fault-controlled eastward embayment of the Belt geosyncline. The detritus was derived from a rugged source area of older Precambrian metamorphic rocks to the south. It was dumped by torrential streams into the differentially subsiding marginal part of the Belt sea and was redistributed to more stable bottom areas by slumping and turbidity currents.

Sedimentary characteristics, regional truncation and local angular unconformities beneath the Flat-head Quartzite indicate regional northward tilting, broad folding, and faulting during and after deposition of Belt sediments but before the Flathead Quartzite formed.

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